Hydroponics and Environmental Bioremediation: Wastewater Treatment (Springer Water) 3031532570, 9783031532573

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Springer Water

Nitish Kumar   Editor

Hydroponics and Environmental Bioremediation Wastewater Treatment

Springer Water Series Editor Andrey G. Kostianoy, Russian Academy of Sciences, P. P. Shirshov Institute of Oceanology, Moscow, Russia Editorial Board Angela Carpenter, School of Earth and Environment, University of Leeds, Leeds, West Yorkshire, UK Tamim Younos, Green Water-Infrastructure Academy, Blacksburg, VA, USA Andrea Scozzari, Institute of Information Science and Technologies (CNR-ISTI), National Research Council of Italy, Pisa, Italy Stefano Vignudelli, CNR—Istituto di Biofisica, Pisa, Italy Alexei Kouraev, LEGOS, Université de Toulouse, Toulouse Cedex 9, France

The book series Springer Water comprises a broad portfolio of multi- and interdisciplinary scientific books, aiming at researchers, students, and everyone interested in water-related science. The series includes peer-reviewed monographs, edited volumes, textbooks, and conference proceedings. Its volumes combine all kinds of water-related research areas, such as: the movement, distribution and quality of freshwater; water resources; the quality and pollution of water and its influence on health; the water industry including drinking water, wastewater, and desalination services and technologies; water history; as well as water management and the governmental, political, developmental, and ethical aspects of water.

Nitish Kumar Editor

Hydroponics and Environmental Bioremediation Wastewater Treatment

Editor Nitish Kumar Department of Biotechnology Central University of South Bihar Gaya, Bihar, India

ISSN 2364-6934 ISSN 2364-8198 (electronic) Springer Water ISBN 978-3-031-53257-3 ISBN 978-3-031-53258-0 (eBook) https://doi.org/10.1007/978-3-031-53258-0 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

Preface

Bioremediation is the use of biological interventions for mitigation of the noxious effects caused by pollutants in the environment. It is an excellent strategy for a range of environmental protection applications. It has emerged as a compelling alternative to traditional cleanup techniques that rely on plants and the microbes that live on them to remove, contain, or neutralize environmental pollutants. The remediation method uses a variety of hydroponic designs, including floating plants, as well as particular planting layouts. To eliminate organic matter, nitrogen, phosphorus, and other pollutants, the approach combines physical, chemical, and biological treatment procedures. The treatment components in the form of vegetation, filter beds, and microorganisms contribute both directly and indirectly for the removal of pollutants from wastewater. Bioremediation of wastewater by hydroponic techniques is recommended as decentralized wastewater treatment and reuse. Hydroponic systems, which utilize plants which are grown in a nutrient solution without soil, are expanding and raising great interest in commercial and scientific community. They are engineered systems designed and constructed to utilize the natural processes involving macrophytes, media, and the associated microbial assemblages to assist in treating wastewaters. This is a relatively new approach in wastewater treatment by which a variety of emergent macrophytes are grown hydroponically on top of floating platform with their roots developing freely into the flowing wastewater. The roots provide a support medium for attached microbial growth which participates in the treatment process. The present book “Hydroponics and Environmental Bioremediation” has been designed to provide a basic understanding with regards to the use of hydroponic system in bioremediation. Gaya, India

Nitish Kumar

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Acknowledgments

Thanks to all the authors of the various chapters for their contributions. It had been a bit of a long process from the initial outlines to developing the full chapters and then revising them in the light of reviewer’s comments. We sincerely acknowledge the author’s willingness to go through this process. I also acknowledge the work and knowledge of the members of our review panels, many of which had to be done at short notice. Thanks to all the people at Springer Nature, The Netherlands, especially Ms. Margaret Deignan and Mr. Ambrose Berkumans with whom we corresponded for their advice and facilitation in the production of this book. I am grateful to my family members Mrs. Kiran (Wife), Miss Kartika Sharma and Laavanya Sharma (Daughters), and parents for their incredible and selfless support all the time. Gaya, Bihar, India

Nitish Kumar

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Contents

1

Hydroponics: A Significant Method for Phytoremediation . . . . . . . . . Pratyush Kumar Das, Khusboo Sahu, Bikash Kumar Das, Bidyut Prava Das, and Patitapaban Dash

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Bioremediation of Wastewater Using Hydroponics . . . . . . . . . . . . . . . Prasann Kumar and Debjani Choudhury

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Sustainable Approach for Agriculture and Environmental Remediation Using Hydroponics and Their Perspectives . . . . . . . . . . Rishi Mittal and Santosh Bhukal

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Applications of Hydroponic Systems in Phytoremediation of Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sayon Mukherjee, Sabyasachi Koley, Dibyajyoti Panda, Gorantla Prathap Reddy, Biswajit Pramanik, and Sandip Debnath

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Environmental Remediation Using Hydroponics . . . . . . . . . . . . . . . . . 115 Abhijit Kumar, Gunjan Mukherjee, and Saurabh Gupta

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Hydroponic Removal of Organic Contaminants from Water . . . . . . . 143 Prasann Kumar and Debjani Choudhury

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Harnessing the Power of Plants in Hydroponics for Wastewater Treatment and Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Faten Dhawi

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Removal of Heavy Metals From Contaminated Water Using Hydroponics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 Partha Chandra Mondal, Shreosi Biswas, Biswajit Pramanik, and Sandip Debnath

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Hydroponic: An Eco-friendly Future . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 Prasann Kumar and Joginder Singh

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Contents

10 Hydroponic Root Mats for Wastewater Treatment: A Review . . . . . 269 Vikanksha, Arun Kumar, and Jatinder Singh 11 Soilless Cultivation of Plants for Phytoremediation . . . . . . . . . . . . . . . 297 Abhijit Kumar, Gunjan Mukherjee, and Saurabh Gupta 12 Effect of Bio-Sorptive Removal of Heavy Metals from Hydroponic Solution: A Review . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 Nagma Khan, Baby Tabassum, Mohammad Hashim, and Asma Hasan 13 Hydroponics Phytoremediation: An Overview . . . . . . . . . . . . . . . . . . . 361 Prasann Kumar and Shipa Rani Dey 14 Hydroponics Removal of Wastewater’s Contaminants . . . . . . . . . . . . 397 M. Liliana Cifuentes-Torres, Leopoldo G. Mendoza-Espinosa, and J. Gabriel Correa-Reyes

Editor and Contributors

About the Editor Dr. Nitish Kumar is a Senior Assistant Professor at the Department of Biotechnology, Central University of South Bihar, Gaya, Bihar, India. Dr. Kumar completed his doctoral research at the Council of Scientific and Industrial Research–Central Salt and Marine Chemicals Research Institute, Bhavnagar, Gujarat, India. He has published more than 70 research articles in the leading international and national journals, more than 20 book chapters and 7 books with Springer and Taylor and Francis. He has a wide area of research experience in the field of Agriculture and Crop improvement, and Microbial and Environmental Biotechnology. Dr. Kumar is a recipient of the Young Scientist Award from the Science and Engineering Research Board (SERB) in 2014. He has received many awards/fellowships/projects from various prestigious government organizations like CSIR, DBT, ICAR, and SERBDST, BRNS-BARC, among others. He is a reviewer for various International Journals and serves as an associate editor of the journal Gene (Elsevier).

Contributors Santosh Bhukal Department of Environmental Science and Engineering, Guru Jambheshwar University of Science and Technology, Hisar, India Shreosi Biswas Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi, India Debjani Choudhury Department of Plant Pathology, School of Agriculture, Lovely Professional University, Phagwara, Punjab, India M. Liliana Cifuentes-Torres Corporación Autónoma Regional de Cundinamarca, Cundinamarca, Colombia

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J. Gabriel Correa-Reyes Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California (Autonomous University of Baja California), Baja California, Mexico Bidyut Prava Das Sailabala Women’s Autonomous College, Cuttack, Odisha, India Bikash Kumar Das Centre for Biotechnology, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India Pratyush Kumar Das Centre for Biotechnology, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India Patitapaban Dash Centre for Biotechnology, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India Sandip Debnath Department of Genetics and Plant Breeding, Palli Siksha Bhavana (Institute of Agriculture), Sriniketan, West Bengal, India Shipa Rani Dey Department of Agronomy, School of Agriculture, Lovely Professional University, Phagwara, Punjab, India Faten Dhawi Agricultural Biotechnology Department, College of Agricultural and Food Sciences, King Faisal University, Al-Ahsa, Saudi Arabia Saurabh Gupta Department of Microbiology, Mata Gujri College, Fatehgarh Sahib, Punjab, India Asma Hasan Department of Zoology, Toxicology Lab, Govt. Raza P.G. College, Rampur, Uttar Pradesh, India Mohammad Hashim Department of Biochemistry, Mohammad Ali Jauhar University, Rampur, Uttar Pradesh, India Nagma Khan Department of Zoology, Toxicology Lab, Govt. Raza P.G. College, Rampur, Uttar Pradesh, 244901 India; Mahatma Jyotiba Phule Rohilkhand University, Uttar Pradesh, Bareilly, 243005 India Sabyasachi Koley Department of Soil Science and Agricultural Chemistry, Institute of Agriculture Sciences, Banaras Hindu University, Uttar Pradesh, Varanasi, India Abhijit Kumar University Institute of Biotechnology, Chandigarh University, Gharuan, Punjab, India Arun Kumar Department of Horticulture, School of Agriculture, Lovely Professional University, Phagwara, Jalandhar, Punjab, India Prasann Kumar Department of Agronomy, School of Agriculture, Lovely Professional University, Phagwara, Punjab, India

Editor and Contributors

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Leopoldo G. Mendoza-Espinosa Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California (Autonomous University of Baja California), Baja California, Mexico Rishi Mittal Department of Environmental Science and Engineering, Guru Jambheshwar University of Science and Technology, Hisar, India Partha Chandra Mondal Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi, India Gunjan Mukherjee University Institute of Biotechnology, Chandigarh University, Gharuan, Punjab, India Sayon Mukherjee Department of Soil Science and Agricultural Chemistry, Institute of Agriculture Sciences, Banaras Hindu University, Uttar Pradesh, Varanasi, India Dibyajyoti Panda Department of Soil Science and Agricultural Chemistry, Institute of Agriculture Sciences, Banaras Hindu University, Uttar Pradesh, Varanasi, India Biswajit Pramanik Department of Genetics and Plant Breeding, Palli Siksha Bhavana (Institute of Agriculture), Sriniketan, West Bengal, India Gorantla Prathap Reddy Department of Agronomy, Institute of Agriculture Sciences, Banaras Hindu University, Uttar Pradesh, Varanasi, India Khusboo Sahu Centre for Biotechnology, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha, India Jatinder Singh Department of Horticulture, School of Agriculture, Lovely Professional University, Phagwara, Jalandhar, Punjab, India Joginder Singh Department of Botany, Nagaland University, Nagaland, India Baby Tabassum Department of Zoology, Toxicology Lab, Govt. Raza P.G. College, Rampur, Uttar Pradesh, India Vikanksha Department of Horticulture, School of Agriculture, Lovely Professional University, Phagwara, Jalandhar, Punjab, India

Chapter 1

Hydroponics: A Significant Method for Phytoremediation Pratyush Kumar Das , Khusboo Sahu , Bikash Kumar Das , Bidyut Prava Das , and Patitapaban Dash

Abstract The wastewater released from multiple points and nonpoint sources is increasing the magnitude of water pollution over the years. The consumption of contaminated surface and groundwater is responsible for the occurrence of health ailments of living organisms and loss of diversity in aquatic ecosystems. This chapter is designed with the objective to trace the role of wetlands in the effective phytoremediation of wastewater. The hydroponics based phytoremediation technique is a novel technique for the degradation of sewage, effluents, and agricultural runoff water using aerobic and anaerobic phases in a regular sequence. The wide acceptance of this alternative method is for minimal sludge formation during wastewater treatment with possible recovery of resources. This technology is gaining popularity for use at community levels due to the minimal involvement of skilled personnel, the least financial constraint, and limited chances of secondary pollution. Further study on this aspect is required to ameliorate the efficiency of wastewater phytoremediation. Keywords Aquatic ecosystems · Hyper-accumulator · Hydroponics · Phytoremediation · Pollution · Wastewater

1.1 Introduction The recent practices of technological innovations and operations to meet anthropological demands on a daily basis have escalated the problem of water pollution. Water pollution is accelerating the occurrence of eutrophication, disruption in energy flow, and loss of biodiversity in natural and manmade aquatic ecosystems. To maintain water quality, sanitation standards, and public health, it is required to promote the P. K. Das · K. Sahu · B. K. Das · P. Dash (B) Centre for Biotechnology, Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha 751003, India e-mail: [email protected] B. P. Das Sailabala Women’s Autonomous College, Cuttack, Odisha 753001, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_1

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remediation of aquatic ecosystems. Environmental degradation has decisively caused a bad effect on the health of human beings like significant disorders in exposed respiratory, neurological, circulatory, and dermal systems. The development of carcinogenesis and genomic mutations of exposed organisms have proved to be lethal and threatened the survival of those organisms. The generation of pollutants due to the overexploitation of natural resources disturbs sustainable development (Sharma et al., 2020). It is highly essential to maintain environmental health along with the required economic growth (Das et al., 2021b). The hydroponic growth of suitable hyper-accumulators in specially designed wetlands is an alternative in situ technique to reduce the toxicity of harmful pollutants in wastewater before being released into the open environment (Tai et al., 2019). During this soil less growth, the roots of the plants are exposed to nutrients rich water (Sarah, 2017). It is one of the essential techniques to reduce the gap between real and possible amelioration of environmental health at a wider perspective from local to global scale. In this hydroponics based wastewater treatment, the system can be maintained easily by quick monitoring of nutrient deficiency symptoms in plants. The plants growing under this system are quick indicators of toxicity caused by this wastewater (Cuba et al., 2015). It is one of the easy and fast processes for the growth of hyper-accumulators, as the plants are producing biomass in quick succession due to the absence of nutrient limitations (Lages Barbosa et al., 2015). Amidst water as a scarce natural resource, it is treated as a boon for the phytoremediation of wastewater because of its water use efficiency.

1.2 Global Water Pollution Status The maintenance of water quality status is a prime concern for sustainable growth and development (Brack et al., 2017). Anthropogenic activities are increasing over few decades and are responsible for the contamination of water resources by harmful chemicals and microbial populations (Alam et al., 2009). The contamination of aquatic systems by heavy metals persists for a long period of time and is responsible for the disruption of natural biogeochemical cycles (Chabukdhara & Nema, 2012). The accumulated heavy metals get biomagnified in exposed organisms through the food web and express adverse effects on those organisms (Uluturhan & Kucuksezgin, 2007). Water pollution came into prominence during the last part of the twentieth century and is responsible for types of human ailments and environmental degradation. Improvement in wastewater treatment has been essential to protect environmental health and make it a sustainable one. World Economic Forum (WEF) ranked it among the top ten global risks in the recent past. Effective wastewater treatment is required for achieving sustainable development, and effective functioning of freshwater ecosystems. Almost 80% of sewage and effluents are released into water bodies

1 Hydroponics: A Significant Method for Phytoremediation

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without treatment. It is responsible for approximately 33% loss of global biodiversity and up to 5% of gross domestic product (UNESCO, 2021). The loading of pathogen-rich organic wastes in aquatic ecosystems is one of the major contributing factors for the degradation of water quality on a global scale (WWAP, 2018). It is a common factor in developing countries due to the lack of technological infrastructure and skills required for wastewater management (Global Health, 2020).The improper water treatment status deprived 26% of the world’s population of safe drinking water and each year many children are dying due to waterborne diseases (Hipsey & Arheimer, 2013). The drinking of unhygienic water and improper sanitation is responsible for more than 1.5 million deaths (WHO, 2011). The global population is expected to attain 11.2 billion figures by 2100 (UN, 2017). The booming population and increase in the release of sewage, effluents, and runoff water through agricultural fields raise the toxicity of surface waters of rivers across the globe (Conserve Energy Future, 2020). The degradation of water quality over the years is making freshwater a scarce natural resource. The dumping of >40 million litters of wastewater per day into water resources is responsible for severe public health issues in ecosystems. The severity of issues can be realized from the suffering of 38 million people by waterborne diseases and the death of more than 2 million people globally (Global Health, 2020). Water pollution is not only responsible for diseases and death of human beings; it also expresses its adverse effects on national and international economic conditions. It leads to a loss in gross domestic production, a lowering of agricultural revenues, and ultimately economic stagnation (Global Health, 2020). Effective management of water resources is becoming a difficult task due to the rapid growth of the population and allied marketing sectors. It is pushing the development towards water stressed conditions (NIH, 2010). Without adequate preventive measures, the freshwater ecosystems quality may degrade further in the coming days. The population growth coupled with urbanization, globalization, increase in slums, overexploitation of natural resources, and expansion of industrial and agricultural activities has the possibility of squeezing access to available freshwater resources. The intensified agriculture systems are consuming approximately 2 million tons of chemicals per annum as fertilizers and pesticides, globally (De et al., 2014). The return of agricultural runoff/over irrigated water to water bodies increases the chemical toxicity of aquatic ecosystems. The discharge of effluents from industrial sources loads approximately 300–400 megatons of pollutants per annum into aquatic ecosystems. The discharge of raw sewage adds a billion gallons of waste per day (Conserve Energy Future, 2020; McCarthy, 2016). The excessive use of agrochemicals and runoff from agronomical practices are polluting the surface water and groundwater resources (Babu et al., 2022). The excessive use of fertilizers and the flow of runoff water through barren agricultural lands are responsible for surface water pollution (Gržini´c et al., 2023). After the implementation of various natural and manmade control measures to reduce the heavy metals contaminations of aquatic systems, the concentration of

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metals still remains high in the aquatic ecosystems (Paller & Littrell, 2007).The hydroponics based freshwater wetland aquaculture is a possible option in this context to reduce the toxicity of contaminated aquatic systems.

1.3 Hydroponics and Plant Growth The hydroponic growth of selected aquatic hyper-accumulators on the surface water of natural and constructed wetlands has the possibility to minimize wastewater toxicity. The growth of aquatic plants under hydroponic conditions is dependent on the nutrient status of wetlands. The presence of the requisite amount of macronutrients, micronutrients, vitamins, and carbohydrates in the growing medium is essential (Murashige & Skoog, 1962) to maintain the healthy growth of plants for optimal hyper-accumulation of organic and inorganic wastes from wastewater. The least is the possibility of limitations of any nutrients in wetland ecosystems (Sharma et al., 2018). Hence, less is the chance to see the impairment of organelles and metabolic deterioration of plants applied for the purpose of phytoremediation. It is a possible dynamic technique to reduce the toxicity and waste load of discharged effluents, sewage, and agricultural runoff water. In this system, selected aquatic hyper-accumulators are allowed to grow in wetlands containing wastewater. In this soil free remediation method, aquatic toxicity management can be done effectively in a short period of time. The hyperaccumulators engaged for this purpose can be able to do effective management of life cycle and simultaneous waste management quickly in small wetlands (Hughes, 2017).

1.4 Hydroponics as a Sustainable Phytoremediation Technique Phytoremediation is a sustainable and eco-friendly technique (Nemali, 2022) to limit the concentration and dynamics of pollutants present in wastewater. The hyperaccumulators used for phytoremediation have the ability to uptake and accumulate a selective group of pollutants in biomass (Roosens et al., 2003). The bioaccumulation ability of plant species used for the purpose even shows pollutant specific variation.

1 Hydroponics: A Significant Method for Phytoremediation

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1.4.1 Selection of Plants for Hydroponics The application of suitable plant species under hydroponics decides the efficacy of phytoremediation (Rezania et al., 2015). The application of those plants individually or in consortium under hydroponics can effectively do the phytoremediation of pollutants present in wastewater. The hyper-accumulators used for the purpose show a wide range of variations relating to the types of elements getting bio-accumulated (Table 1.1). The selection of suitable plant species is of prime concern to make hydroponics an effective method of phytoremediation. The rate of biomass production in hyperaccumulating plants, tolerance to fluctuation in environmental parameters, tolerance to targeted pollutants, networking of roots/rhizoids in aquatic systems, and the rate of extraction of pollutants from wastewater are certain important criteria for hyperaccumulators selection (Das et al., 2021a). The tolerance of each wetland plant species to pollutants is not the same in all cases. The pollutant-specific tolerance of plant species can show variation with the changes in the chemical form of pollutants (Fig. 1.1). Phytoremediation using hyperaccumulators is a slow process but promising for the remediation of a wide range of chemicals in elemental or compound forms. It is conducive to the remediation of metals, metalloids, and non-metals from aliphatic and organic groups (Hu et al., 2019; Sharma et al., 2015; Singh & Singh, 2017). The rate of biomass formation in tolerant hyper-accumulators may remain unaltered under stress conditions. The presence of chelators like EDTA increases the mobility of pollutants during phytoremediation. It is not free from constraints as biomass reduction in hyper-accumulators is extremely common in the presence of chelators (Liphadzi & Kirkham, 2006). The physiological processes of hyper-accumulators show differential stress responses while exposed to pollutants during wetland growth. It depends upon the specific activities of hyper-accumulators like absorption, translocation, and bioaccumulation. The uptake of pollutants is influenced by the synergistic effect of plant-microbes interactions at the rhizospheric region (Singh & Singh, 2017).The promising hyper-accumulators are following specialized mechanisms to selectively uptake a specific pollutant (Kamnev & van der Lelie, 2000).

1.4.2 Types of Hydroponics 1.4.2.1

Wick or Passive System

It is a cost-effective and small-scale method of plant growth that can operate for a short duration (Lee & Lee, 2015). The basic principle of this technique is to transport the water from the surroundings (exterior system) to plants (interior system) using the

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Table 1.1 List of promising aquatic hyper-accumulators for phytoremediation of heavy metals in aquatic ecosystems Plant name

Family

Elements

References

Ceratophyllum demersum L.

Ceratophyllaceae

Pb, Cd, Cu, Fe, Ni, Zn, Mn, Co

El-Khatib et al. (2014), Al-Ubaidy and Rasheed (2015), Borisova et al. (2014), Kamel (2013)

Myriophyllum spicatum L.

Haloragaceae

Pb, Cd, Co, Cu, Ni, Zn

El-Khatib et al. (2014), Kamel (2013)

Utricularia gibba L.

Lentibulariaceae

Cr

Augustynowicz et al. (2015)

Lemna minor L.

Araceae

Cd, Pb, Zn, Cu

Basile et al. (2012)

Elodea canadensis Michx

Hydrocharitaceae

Cd, Pb, Zn, Cu

Basile et al. (2012)

Leptodictyum riparium (Hedw.) Warnst

Amblystegiaceae

Cd, Pb, Zn, Cu

Basile et al. (2012)

Potamogeton alpines Balb

Potamogetonaceae

Cu, Fe, Ni, Zn, Mn

Borisova et al. (2014)

Scirpus mucronatus L.

Cyperaceae

Ni

Marbaniang and Chaturvedi (2013)

Rotala rotundifolia (Buch.-Ham. ex Roxb.) Koehne

Lythraceae

Ni

Marbaniang and Chaturvedi (2013)

Myriophyllum intermedium Haloragaceae DC

Ni

Marbaniang and Chaturvedi (2013)

Eicchornia crassipes (Mart.) Solms

Pontederiaceae

Cd, Co, Cu, Ni, Pb, Zn

Kamel (2013)

Lemna gibba L.

Araceae

Cd, Co, Cu, Ni, Pb, Zn

Kamel (2013)

Phragmites australis (Cav.) Poaceae Trin. ex Steud

Cd, Co, Cu, Ni, Pb, Zn

Kamel (2013)

Typha domingensis Pers

Typhaceae

Cd, Co, Cu, Ni, Pb, Zn

Kamel (2013)

Azolla pinnata R.Br

Salviniaceae

Cu, Cr

Wani et al. (2017)

Pistia stratiotes L.

Araceae

Ag, Cd, Cr, Cu, Hg, Pb, Zn

Wani et al. (2017)

Myriophyllum heterophyllum Michx

Haloragaceae

Cd

Wani et al. (2017)

Potamogeton crispus L.

Potamogetonaceae

Cd

Wani et al. (2017)

capillary function of roots and fibers (Ferrarezi & Testezlaf, 2016). It is not suitable for effective management of phytoremediation as it is not favouring recycling of water during treatment (Fig. 1.2).

1 Hydroponics: A Significant Method for Phytoremediation

(a)

(d)

(b)

(e)

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(c)

(f)

Fig. 1.1 Glimpses of aquatic plants used for phytoremediation of heavy metals during hydroponics. (a) Lemna minor L. (b) Elodea canadensis Michx. (c) Eicchornia crassipes (Mart.) Solms (d) Lemna gibba L. (e) Typha domingensis Pers. (f) Pistia stratiotes L.

Growing Plant Soilless absorbing fibrous medium Wire mesh Wastewater reservoir

Fig. 1.2 Cost effective wick or passive system of hydroponics

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P. K. Das et al. Sensor for turn OFF pump Growing plants Aeration chamber Soilless absorbing fibrous medium Fine wire mesh Sensor for turn ON pump Drainage points Wastewater reservoir Screening (Foot) valve for water pumping

Pump

Fig. 1.3 Recycling of water in deep-water culture hydroponics

1.4.2.2

Deep-Water Culture

It consists of a porous platform for the growth of plants. Uninterrupted pumping and aeration are required for this process to continue (Hoagland & Arnon, 1950). In this technique, roots are completely exposed to nutrient solutions. This system can be used for recycling water in a closed system (Fig. 1.3). This system can be operated for a long duration with the monitoring and control of nutrient-rich solution parameters like pH, temperature, dissolved oxygen, and dissolved carbon dioxide (Benton Jones, 2005).

1.4.2.3

Nutrient Film Technique (NFT)

It is a common technique used in closed circulation. In this technique plants are allowed to grow on supporting media and roots are not completely in contact with the oxygenated nutrient solution (Fig. 1.4). It allows only a thin layer of oxygenated nutrient solution to remain in contact with the roots of plants (Morgan, 1999).

1 Hydroponics: A Significant Method for Phytoremediation

Pipe for pumping wastewater

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Growing plants Aeration chamber Nutrient film

Drainage pipe Wastewater reservoir (Closed system) Filtration unit Pump

Wastewater reservoir

Sterilization chamber

Fig. 1.4 Common nutrient film technique of hydroponics

1.4.2.4

Media Bed Based

This technique uses a substrate filled tank that is poured in and drained out of nutrient solutions at regular intervals (Buwalda et al., 1993). The common occurrence of bacterial and fungal diseases on roots is one of the limitations of this process (Fig. 1.5).

1.4.3 Sources of Nutrients for Hydroponics Nutrient management is essential for the successful maintenance of plant species under hydroponics (Santiago-Aviles & Light, 2018). The growth of plants can be perfect in the presence of nine macronutrients like carbon, hydrogen, oxygen, phosphorus, potassium, nitrogen, sulphur, calcium, and magnesium, and eight micronutrients like iron, zinc, boron, chlorine, copper, cobalt, manganese, molybdenum (Khan et al., 2020). The standard composition of nutrients for the perfect growth of plants under hydroponics very much complies with the formulations framed by Hoagland and Arnon (1938). The proportion of nutrients to be maintained in a hydroponics system is determined by the types of plant species used for the purpose.

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Pipe for pumping

Growing plants Aeration chamber Tank for substrate

Drainage pipe Pump

Wastewater reservoir

Screening (Foot) valve

Fig. 1.5 Filling and drainage in media bed based hydroponics

1.4.4 Mechanism of Phytoremediation Hydroponics is a beneficial method of nutrient uptake from the liquid phase through the submerged roots of plants (Kumar & Saini, 2020). The emerging problem of land scarcity for the operation of phytoremediation is giving emphasis to hydroponicsbased phytoremediation. In static hydroponics (micro-scale), there is no flow of wastewater but in dynamic hydroponics (pilot and field scale), the flow of wastewater is observed in open and closed circulation. For better remediation of wastewater, the closed circulation is followed as recycling of treated water is possible in this system for a requisite number of times (Fig. 1.6). The operation of hydroponics-based wastewater remediation needs pre and primary treatments before the entry of wastewater through an inlet channel into the hydroponics system. During this secondary treatment, soilless plant growth (hydroponics) is allowed in the oxic zone of the tank to uptake pollutants from the wastewater as nutrients or as an aid to nutrients. The uptake of pollutants from wastewater present in the tank is proportional to the gross phyto-assimilation ability in the oxic zone of that system. The decrease in oxygen concentration and movement from the oxic to the anoxic zone of the hydroponics system inhibits the root growth of plants present in that system. It is better to confine phyto-assimilation based phytoremediation of plants to the oxic zone of the hydroponics system otherwise, it requires oxyfertigation (Trejo-Téllez & Gómez-Merino, 2012). The aquatic phyto-assimilation by plants in the eutrophic region of the hydroponics system has been done effectively in the presence of sunlight, carbon dioxide, water, and nutrients. The phyto-assimilation can run consistently until an associated

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Fig. 1.6 Wastewater treatment by closed circulation hydroponics

factor is a limiting factor. The carbon dioxide released from microbial catabolism can reduce the possibility of treating carbon dioxide as a limiting factor. The release of oxygen as a byproduct during phyto-assimilation is helpful for microbial growth and catabolism in that system, as it requires the presence of a high cascade of oxygen for its smooth operation. The residues formed during the degradation of pollutants present in wastewater subsequently get deposited in the anoxic zone of that hydroponics system. In the anoxic zone present at the depth of the hydroponics system pollutants present in wastewater can get degraded to elemental form with the help of processes like hydrolysis, acidogenesis, acetogenesis, and methanogenesis (Themelis & Ulloa,

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2007). The monomers of polymeric wastes formed by hydrolysis enter the acidogenesis to form organic acids. The organic acids formed in this step can release protons to change the pH of the surroundings but in the presence of acetogens these organic acids are converted to chemical forms that can get readily utilised by methanogenic bacteria (Weiland, 2010). The methane produced in this step can be extracted carefully, as it has the socio-economic value of biogas. The water and carbon dioxide formed by methanogenesis can get consumed by the phyto-assimilation process in the hydroponic system. At the hydroponics based phytoremediation stage, monitoring of selected water quality parameters is required to decide the fate of treated wastewater. With an unsatisfactory value of parameters, the water can be recycled through the hydroponics systems the number of times as required. After the secondary treatment, if desired, the water can be allowed to pass through the tertiary treatment stage for removal of excessive quantities of pollutants like solid particles, soluble nitrogen, phosphorus or suspended pathogens. The phytoremediation of pollutants using hydroponics-based aquaculture system helps in the elimination, toxicity conversion, and bioaccumulation of toxic agents present in aquatic bodies (Kanwar et al., 2020). The possible paths for its channelization, from the open reservoir of water to the closed reservoir of plant systems include absorption, retention, and translocation in plant tissues. Besides, biodegradation is already established as an important method for the reduction of toxic pollutants in ecosystems. These hydroponics based wetland systems show insignificant differences in the magnitude of phytoremediation during ON/OFF conditions of oxygen supply (Chen & Xie, 2018). The phytoremediation of inorganic and organic pollutants including heavy metals has given due importance to the role of plant and microbes symbiotic association established at the rhizospheric region of plants (Das et al., 2023). Using the root exudates from hyper-accumulating plants, the stimulated microbes hasten the degradation of pollutants at the rhizospheric region (Glick, 2014). The presence of microbes in a hydroponics system improves the availability of nutrients to the plants present in that system through the conversion of chemical forms of ingredients like phosphorus and nitrogen (Gopi et al., 2020). Besides, these microbes can be instrumental in keeping the surrounding pH within the range, which is optimal for plant growth. The organic acids produced by these microbes can alter the surrounding pH of the system (Yang et al., 2018) and make the pollutants available for phytoremediation by plants. The microbes present in the phytoremediation systems can allow active and passive biosorption. In passive biosorption, the metals can get attracted by the functional groups present on the surfaces of the bacterial cells including dead surface cells (Fomina & Gadd, 2014). Many microbial species including the Bacillus have expressed metabolic efficiency under hydroponics (Gül et al., 2008).The presence of beneficial microorganisms in the rhizosphere of plants improves the phytoremedial ability of plants (Das et al., 2022a). The conducting strands in roots, stems and leaves of plants are used for the purpose of phytoremediation. The differential accumulation of pollutants occurs in plant parts

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Phytovolatilization

Phytodegradation

Rhizofiltration Phytostabilization

Fig. 1.7 Types of phytoremediation in the hydroponics system

during phytoremediation. The pollutants concentration in plants used for phytoremediation shows a positive correlation with the concentration of pollutants in immediate surroundings (Das et al., 2022b).The transpiration of water from leaves develops diffusion pressure deficits and sucks the water and pollutants present in the root cells (Tangahu et al., 2011). Such uptake of pollutants by aerial plant parts from the root system is a passive process. It leads to the flow of pollutants from hydroponics systems to plants using water potential gradients (Sumiahadi & Acar, 2018). The excessive pollutants entered into the plant systems have the possibility to get consumed in the plant’s metabolic cycles. In plant tissues with low metabolism, it is accumulated in vacuoles and cytosols (Ali et al., 2013). The hydroponic-based systems allow the occurrence of multiple types of phytoremediation (Fig. 1.7).

1.4.4.1

Rhizofiltration

In this technique roots of selected plants are the sites for the remediation of water pollutants. The translocation of pollutants can take place from wastewater by the hydroponic plant systems. The plant roots of hydroponic systems are the site for the adsorption or absorption of pollutants from wastewater. The extraction of pollutants from wastewater depends upon the functional ability of hyper-accumulating plants and the tolerance of those plants to the available water pollutants. The optimal pH and temperature in the growing medium help the plant to develop its root system (Adilo˘glu, 2018).

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Phytostabilization

The outcome of this technique is the stabilization/immobilization of pollutants from wastewater in hydroponic-based plant root systems (Ali et al., 2013). It involves the processes of adsorption, absorption, or precipitation of pollutants. It can improve the functioning of natural aquatic ecosystems.

1.4.4.3

Phytodegradation

The rapid transformation of water pollutants can take place inside the hydroponically grown plants by shortening the pollutant’s residence time and hastening the turnover time. It involves bioaccumulation and subsequent catabolic degradation of pollutants in tissues of hyper-accumulators. It may get assistance from the inducible or constitutive enzymes involved with plant’s metabolism (Adilo˘glu, 2018)

1.4.4.4

Phytovolatilization

It helps in the remediation of water-soluble pollutants present in wastewater using the active or passive diffusion mechanism of hydroponically grown plants. It is beneficial with the involvement of plants having developed root systems (Ali et al., 2013).

1.5 Factors Influencing Hydroponics Based Phytoremediation 1.5.1 Light The presence of light is required for photosynthetic assimilation and photophosphorylation of hyper-accumulators applied for the purpose of phytoremediation in hydroponic systems. The oxygen released as a byproduct is favourable for the respiration of bacteria present in the system. The light intensity required for the purpose generally ranges between 5 × 104 and 7 × 104 lux. The availability of optimal intensity of light favours the synthesis of biomass in hyper-accumulators and can promote phytoremediation. The decrease in light intensity can suppress the carbohydrate assimilation of plants (Das et al., 2009) in the hydroponic system. The decrease in the availability of photons can lead to the loss of photosynthetic pigments like chlorophyll in plants (Das et al., 2008). It is not only responsible for the required photosynthesis of plants but it is instrumental in determining its niche through temperature regulation (Yang et al., 1990).

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1.5.2 Water Use Efficiency The optimal availability of water is essential for the smooth functioning of phytoremediation using hydroponics. It is highly efficient in the utilization of water as it consumes about 10% of water as compared to its growth under geoponics. It may be due to the presence of those plants in hydrated conditions (Nemali, 2022). It is a beneficial attribute for the occurrence of phytoremediation under challenging conditions like acidic, alkaline, or water stress conditions. Hence, plants under hydroponics are least affected by conditions like drought and flood during growth and metabolism (Vasdravanidis et al., 2022). It is possible to grow several plants using this technique (Sardare & Admane, 2013).

1.5.3 Aeration Aeration is highly essential for the proper development of root systems of plants under hydroponics (Raviv et al., 2008). The maintenance of low density in hydroponic systems favours the gaseous exchange between the hyper-accumulators and the surroundings. The ratio of dissolved oxygen to dissolved carbon dioxide in the hydroponic system determines the net rate of anabolic and catabolic processes being possible in the system. It is highly essential for the success of phytoremediation. The symbiosis maintained between green plants and bacteria present in the system is fruitful for the successful operation of phytoremediation.

1.5.4 Temperature The optimal surrounding temperature for smooth functioning of plants under controlled conditions is 21–27 °C (Kawasaki & Yoneda, 2019). The wider variation in temperature affects the growth and development of plants (Shimizu, 2007). The reduced biomass loss can adversely affect the phytoremediation of pollutants.

1.5.5 Electrical Conductivity (EC) The variation in the composition of nutrients changes the EC value of the hydroponic system. This value is very important for determining the success of the phytoremediation process as it controls the water uptake ability of plants from their surroundings. There is a possibility of a decrease in the phytoremediation ability of plants with EC > ECt (threshold value). The Solanum lycopersicum L. plants limit the water uptake with hike in EC values above 4–6 dS m−1 (Cuartero & Fernández-Muñoz, 1998).

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1.5.6 pH It is one of the parameters influencing hydroponics based phytoremediation of pollutants. The optimum pH for growth and function of plant hyper-accumulators are species specific (Othman et al., 2019). The nutrients and pollutant concentration of the hydroponics system determines its pH. A pH between 5.5 and 6.5 is normally good for hydroponics (Waters et al., 1970).

1.5.7 Size of Remediation Pond This phytoremediation technique is preferred to be used in large sized remediation ponds. It can provide adequate aeration and limited interference from nutrient fluctuations.

1.5.8 Carbon Dioxide Concentration Carbon dioxide is essential for the synthesis of carbohydrates in plants using dark assimilation. The increase in the availability of carbon dioxide is able to proportionately increase the carbon assimilation in plants following Blackman’s principle of limiting factor (Boretti & Florentine, 2019).

1.5.9 Nutrient Management The high biomass growth of hyper-accumulators under hydroponics may be due to the optimal concentration of nutrients and water (Sharma et al., 2018). The flow of nutrient rich water in large remediation ponds is required to minimize the effects of nutrient fluctuation. Regular monitoring of the nutrient level of water in the remediation tank is essential. It helps in the optimized maintenance of remediation tanks on a sustainable basis (Ben-Yaakov & Ben-Asher, 1982).

1.5.10 Hyper-Accumulation Ability of Plant Species It shows a wide range of variation in relation to changes in chemical forms of pollutants and the plant species used for the purpose of phytoremediation. The phytoremediation ability of plant species is linked with the specific pollutants (Roosens et al., 2003).

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1.6 Opportunities and Challenges The recently used physical and chemical methods for the detoxification of pollutants have financial and technological constraints. Those methods are proven to be expensive or responsible for secondary environmental pollution. Besides, many processes are found to be technologically ill-defined or beyond the acceptance of the public for pollution control purposes. On the basis of comparison, the application of hydroponics based aquatic plant systems for the removal of metals from effluents, sewage, and agricultural runoff water was found to be free from pre-discussed constraints. The use of aquatic micro and macrophytes, preferably the hyper-accumulators for bioaccumulation of numerous toxic pollutants can ameliorate the detoxification of wastewater without any significant secondary pollution. It is a green technology based cost effective possible remediation process. It is least affected by interference from soil parameters and free from seasonal fluctuations. The better is the possibility of phytoremediation using hydroponics as the biomass yield and nutrients accumulations are high in plant biomass under hydroponics compared to geoponics (Ramteke et al., 2019). It is a significant attribute of the application of hydroponics for the phytoremediation of sewage and effluents. Hydroponics based wastewater remediation is a membrane-based technique capable of cleaning our environment to a certain extent and restoring the environmental parameters within an optimal range. The plants used for this purpose can adsorb, assimilate, absorb, or accumulate selective pollutants in hydroponics based systems. These systems hold good for low cost remediation of inorganic and organic toxic chemicals including heavy metals present in wastewater (Lasat, 2002; Salt et al., 1998). The pollutants present in wastewater may be assimilated by selective plant parts as macro or micronutrients during hydroponics. The detoxification of other chemical pollutants can be done by aquatic hyperaccumulators through rhizofiltration, phytostabilization, phytodegradation, and phytovolatilization (Kanwar et al., 2020). It is an eco-friendly and affordable treatment technique for in situ detoxification of wastewater from multiple sources. It causes the least secondary disturbances in aquatic ecosystems during the treatment of organic and inorganic compounds including hazardous metals (Hu et al., 2019). It can be operated by the common people of the community with the least interference from the skilled personnel (Nwosisi & Nandwani, 2018). Hydroponics can act as a secondary treatment method for the treatment of wastewater in sewage treatment facilities. The waste treatment facilities along with the treated water will also produce some secondary wastes in the form of sludge and gases (Fig. 1.8). The sludge due to its characteristic properties can be used as a component in biofertilizers. It can also be used for land filling purposes or as a feed in aquaculture. The gases released after treatment of the wastewater mostly contain methane, and carbon dioxide that can be used for heating and generation of electricity. The treated water can be used for multiple purposes depending on its final quality. Water with A quality can be used for drinking purposes while B quality water

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can be used for purposes like cleaning, washing, and bathing. C-quality water can be used for drinking purposes but only after proper treatment. D-quality water can be used for aquaculture and the lowest grade of water (Quality E) can be used for irrigation purposes and cooling of heavy machines. The shortage of land and variation in soil parameters are constraints for wastewater remediation. It requires the remediation processes to be carried out in a small space with the least interference from edaphic factors. In the present condition, the hydroponics method is found to be promising, as the remediation has to be done in a controlled condition with the least interference from edaphic factors, seasonal variation, and nutrient loss (Okemwa, 2015). This process can be operated on a small piece of land with effective water and nutrient management (Treftz & Omaye, 2016). The active or passive metabolism of plants is intimately connected with the phytoremedial ability of targeted plants. The metabolism of plants under hydroponics conditions is better as compared to its performance on soil (Goenka, 2018) as the least interference is expected from nutrient limitations. The effective maintenance of aquatic ecosystems leads to healthy growth and metabolism of targeted plants (Qureshi, 2017), an essential feature for the successful achievement of environmental phytoremediation. Despite having enormous advantages, this technique is not completely free from challenges. It is a relatively slow process as compared to the remediation of toxicants by other competitive physical, chemical, and biological processes (Khalifa & Alkhalf, 2018). It is also not free from financial constraints at the start-up stages. The marginal section of people specifically in developing countries with weak economic conditions, adopts low-cost techniques for meeting daily requirements. To increase the acceptance of this phytoremedial technique, it is essential to keep the involvement of initial and maintenance costs at a minimal level. Getting the availability of land resources for phytoremediation is another challenging task in rural and mountain belts. The situation may turn complicated with the speedy growth in population. To get out of this complicacy it is required to design and develop well-composed aquatic wetlands as a unit structure for hydroponics based phytoremediation. This process of phytoremediation needs the least interference from skilled personnel at maintenance stages. But, the unavailability of skilled personnel at the operational site during the start-up stages is another limiting factor as it can reduce the efficiency of outcomes from phytoremediation. Improper counselling at preliminary stages can ruin the system and its efficiency.

Fig. 1.8 Resource recovery from wastewater using hydroponics based phytoremediation

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1.7 New Trends of Hydroponics Based Environmental Remediation The hydroponics based wastewater treatment is a beneficial technique to bring back significant volumes of water into utilizable forms with the economy of time and space. The initial report on the use of hydroponics as a wastewater treatment technique was noticed in 1993 (Furukawa & Fujita, 1993). Many improvements have been done after it for the removal of wastes from sewage, effluents, and agricultural wastewater. A novel application of hydroponics based growth of aquatic plants is to extend the area required for phytoremediation. It is possible by utilizing vertical spaces for this purpose of phytoremediation like window farming (Gentry, 2019). It can manage to a certain extent the shortfall of land for the purpose of phytoremediation. Land unsuitable for cultivation can be used for this hydroponics based phytoremediation (Vasdravanidis et al., 2022). The use of this technique at the community level or unit domestic sector can improve the quality of sewage released from those units. The window phytoremediation technique is a microscale phytoremediation technique to upgrade the quality of the environment in an urban lifestyle. The accomplishment of this technique can comply with the basic provisions of pollution control, the 3Rs, like reduce, reuse and recycle, in wastewater management. The nutrient-rich sewage water can provide better growth and biomass yield of those plants applied for the purpose under hydroponics even that may be 14 times of output under geoponics (Touliatos et al., 2016). The active/passive uptake of salts and water by the hyper-accumulators in its aquatic rhizosphere may be the possible reason behind the high biomass yield. It is supported by the earlier findings of Sharma et al. (2018).

1.8 Conclusion and Future Prospects The ubiquitous presence of pollutants in water and soil is responsible for multiple living health disorders. Expansion in wastewater treatment can raise the possibility of reuse of treated polluted water even under water stress conditions. It will cause the volumetric reduction of released polluted water and also be able to do easier handling of waste load released from household, industrial, and agricultural sectors. The hydroponics based phytoremediation looks promising at the time of squeeze in available land resources. This process basically depends upon the smooth and effective operation of metabolic cycles of wetland hyper-accumulators. It is an intensive technique for uninterrupted phytoremediation. In the forthcoming days, it may be able to reduce the pressure on conventional geoponics to a significant extent during phytoremedial processes. It may act as a bridge to reduce the gap between the requirement and availability of land resources for the purpose of phytoremediation of environmental pollutants. Further research on this aspect is essential to get amelioration

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of hydroponics-based remedial efficiency in wetland ecosystems. The healthy growth of selected plant species with the effective metabolism of targeted plants is essential for the successful remediation of polluted ecosystems. Acknowledgements The author(s) acknowledge the encouragement and support provided by the Siksha ‘O’ Anusandhan (Deemed to be University), Bhubaneswar, Odisha for this publication.

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Chapter 2

Bioremediation of Wastewater Using Hydroponics Prasann Kumar and Debjani Choudhury

Abstract Rapid industrialization and urbanization have led to a significant increase in wastewater generation, presenting a critical environmental challenge. Traditional wastewater treatment methods often fail to remove contaminants, especially recalcitrant organic compounds and heavy metals. In recent years, bioremediation using hydroponics has emerged as a promising and sustainable approach to tackle wastewater pollution. Hydroponics, a soilless cultivation technique, involves the growth of plants in nutrient-rich water solutions. This method offers several advantages for wastewater treatment, such as enhanced nutrient uptake, efficient water utilization, and minimal land requirements. The plant roots act as a natural filter, facilitating the removal of pollutants through adsorption, precipitation, and microbial degradation. The success of hydroponic systems for wastewater bioremediation relies on carefully selecting plant species and optimizing various factors, including pH, temperature, nutrient concentration, and hydraulic retention time. Certain plant species, known as hyperaccumulators, can accumulate high levels of heavy metals, thereby aiding in their removal from contaminated water. Integrating hydroponics with advanced technologies, such as biofilm reactors and microbial fuel cells, can further enhance the treatment efficiency and promote the conversion of pollutants into valuable resources. These combined approaches facilitate the removal of organic contaminants through plant–microbe interactions, enzymatic reactions, and microbial transformations. Implementing hydroponic systems for wastewater bioremediation helps purify water and offers additional benefits, including biomass production, carbon sequestration, and aesthetic improvement. Furthermore, this approach promotes the reuse of treated water for various non-potable applications, conserving freshwater resources and contributing to a circular economy. Bioremediation of wastewater using hydroponics represents a promising and sustainable solution to address growing water pollution concerns. This approach combines the natural abilities of plants and P. Kumar (B) Department of Agronomy, School of Agriculture, Lovely Professional University, Phagwara, Punjab 144411, India e-mail: [email protected] D. Choudhury Department of Plant Pathology, School of Agriculture, Lovely Professional University, Phagwara, Punjab 144411, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_2

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microorganisms, providing an efficient and eco-friendly method for wastewater treatment. Future research efforts should focus on optimizing system design and plant selection and exploring the potential for scaling up these systems to meet the demands of large-scale applications. Keywords Microorganism · Wastewater · Hydroponics · Heavy metals · No Povety · Zero Hunger

2.1 Introduction Due to increased industrial and agricultural activities and increased population, the environment has an impact. Because of the introduced chemicals, wastewater, wastes, and toxic substances into the air, soil, and water make unsafe for unpotable water and drinking, human habitation, contamination of crops, and food unsafe for consumption. These impacts adversely affect human, animal and plant health (Yang et al., 2019). The presence of toxic metals and low pH indicate the impact of wastewater. Some industries like coal and mining and palm oil mills’ effluents are generating wastewater that must be treated as it is causing severe ecological impacts. The reports of various studies highlighted the use of physical, chemical and biological bioremediation methods, which are expensive and laborious. There are various sorption processes of bioremediation, but those technologies are not fit for application in rural areas due to their complications and high operating costs. Due to the increased concentration of several organic and inorganic contaminants creates toxicity, bioaccumulation and biomagnification due to its prolonged presence in human beings and animals (Keshavarzifard et al., 2019). Hazardous substances like aromatic hydrocarbons, dyes, metal nanoparticles, radionuclides, pharmaceutical products, petroleum hydrocarbons, heavy metals, pesticides etc. are present in the aquatic ecosystem, which creates toxicity to the organisms present in the aquatic environment (Mishra & Bharagava, 2016; Yadav et al., 2017). These hazardous substances are due to overconsumption of minerals, synthetic chemicals, industrial effluents, fossil fuels, chemical fertilizers and other anthropogenic activities. Due to the presence of these chemicals, the aquatic environment is also at greater risk (Borgwardt et al., 2019). Bioremediation is a process where the pollutants causing effects on the environment are mitigated through biological interventions. To protect the environment, this approach is convenient. This technique has become an attractive replacement to the conventional methods of environmental protection where the plants and the microorganisms related to the plants are used for environmental contaminants. This technique is a practical approach as it is costless and environmentally friendly (Ojoawo et al., 2015). Due to high bioremediation potential, the macrophytes and the rhizospheric microorganisms are gaining importance. Through this technique, the plant and the microorganisms degrade, detoxify, or reduce the pollutants into less toxic substances.

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This remediation technique follows different hydroponic growing systems like floating plants in constructed wetlands. This method uses various techniques for removing organic and inorganic contaminants. This technique helps in direct or indirect removal of contaminants from wastewater. Hydroponic techniques for bioremediation of wastewater are recommended as a decentralized way of treating wastewater and its reuse. In this system, the plants grown without soil in a nutrient solution are gaining interest and expanding in the scientific community (Ulrico & López-Chuken, 2012). These are engineered techniques, designed and constructed, where the macrophytes, media and the associated microbes assemblages are used to treat wastewater through natural processes. This is a relatively new approach to treating wastewater in which various floating, submerged and emergent macrophytes are grown hydroponically (Newete & Byrne, 2016). In this chapter, the growth of hydroponics and the removal of pollutants through hydroponics are described in detail.

2.2 Research Gap on Bioremediation of Wastewater Using Hydroponics Research Gap: Although the application of hydroponics for wastewater bioremediation has shown promising results, there is a significant research gap in understanding this approach’s long-term effects and scalability. Specifically, there is a need for further investigation into the following areas: 1. Long-term performance and sustainability: While short-term studies have demonstrated the effectiveness of hydroponics in removing contaminants from wastewater, there needs to be long-term monitoring and assessment of system performance. It is crucial to understand the system’s stability, resilience, and overall sustainability over extended periods of operation to ensure its reliability as a wastewater treatment solution. 2. Optimization of system design: The design parameters of hydroponic systems, such as plant selection, nutrient formulation, hydraulic retention time, and system configuration, require further optimization. Research should focus on identifying the most suitable plant species, their specific roles in pollutant removal, and the ideal conditions for nutrient uptake and biomass production. Additionally, exploring the potential integration of complementary technologies, such as biofilm reactors or microbial fuel cells, could enhance the treatment efficiency and overall system performance. 3. Scale-up and practical implementation: Most studies on hydroponic bioremediation have been conducted at laboratory or pilot scales, limiting their practical applicability. There is a need to explore the challenges and feasibility of scaling up hydroponic systems for real-world wastewater treatment scenarios. Factors such as cost-effectiveness, land requirements, and integration with existing wastewater

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treatment infrastructure must be thoroughly investigated to assess the potential for large-scale implementation. 4. Evaluation of treated water quality and reuse potential: The treated water quality from hydroponic systems must be comprehensively evaluated to ensure its suitability for various reuse applications. Assessing the removal efficiency of different classes of contaminants, including recalcitrant organic compounds and heavy metals, is essential. Furthermore, understanding the potential risks associated with residual pollutants and by-products generated during bioremediation is crucial for assessing treated water reuse’s safety and environmental impact. Addressing these research gaps will provide valuable insights into the long-term performance, optimization, scalability, and practical implementation of hydroponicsbased bioremediation systems. This knowledge will contribute to developing more efficient and sustainable strategies for wastewater treatment, facilitating the transition towards a circular economy and a cleaner environment.

2.3 Possible Solutions to Address the Research Gap on the Bioremediation of Wastewater Using Hydroponics 1. Experimental Design: Design comprehensive and well-controlled experiments to investigate the effectiveness of hydroponics in wastewater bioremediation. Consider different variables such as plant species, nutrient concentrations, wastewater types, and treatment durations to provide a more thorough understanding of the process. 2. Nutrient Uptake Studies: Research to determine the optimal nutrient uptake capacity of different hydroponic plants for specific contaminants commonly found in wastewater. This information can help identify the most suitable plant species for efficiently removing specific pollutants. 3. Long-Term Monitoring: Perform long-term monitoring studies to assess the stability and sustainability of hydroponic systems in treating wastewater. Monitor factors such as plant growth, nutrient depletion, contaminant removal efficiency, and system performance over an extended period to evaluate the long-term effectiveness and challenges of the method. 4. Comparative Studies: Conduct comparative studies between hydroponics and other wastewater treatment methods to evaluate the advantages and disadvantages of using hydroponics in terms of treatment efficiency, cost-effectiveness, energy consumption, and environmental impact. This can help position hydroponics within the broader context of wastewater treatment options. 5. Optimization Techniques: Investigate optimization techniques for enhancing hydroponic wastewater treatment. Explore strategies such as adjusting pH levels, nutrient ratios, and plant growth parameters to improve removal efficiency and maximize the use of available resources.

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6. Plant–Microbe Interactions: Investigate the role of microbial communities in hydroponic systems for wastewater treatment. Explore the potential symbiotic relationships between plants and beneficial microorganisms to enhance pollutant removal and system performance. 7. Economic Feasibility: Assess the economic feasibility of implementing hydroponic systems for wastewater treatment on various scales. Consider initial setup costs, operation and maintenance expenses, and potential revenue generation from harvested biomass or by-products. 8. Field Trials: Conduct field trials to validate the laboratory findings and evaluate the real-world applicability of hydroponics in different wastewater treatment scenarios. Assess the performance of hydroponic systems in diverse environmental conditions and wastewater compositions to ensure practical feasibility and reliability. 9. Environmental Impacts: Assess the potential environmental impacts of hydroponic systems used in wastewater treatment. Investigate any possible release of contaminants or pollutants from the harvested biomass, disposal of residual wastewater, or adverse effects on soil or groundwater quality. 10. Knowledge Sharing and Collaboration: Encourage knowledge sharing, collaboration, and interdisciplinary research among scientists, engineers, environmentalists, and policymakers. Foster partnerships between academia, industry, and governmental organizations to promote the exchange of information, resources, and expertise for advancing research in hydroponic wastewater treatment. Implementing these solutions can help bridge the research gap and provide valuable insights into the optimization, efficacy, and practical application of hydroponics for wastewater bioremediation.

2.4 Types of Water Pollution Toxic substances: Pesticides, industrial compounds, the release of petroleum products through transport, oil spills, heavy metals such as Cu, Zn, Ni, Cd, Cr, Pb, etc., and excess organic matter, such as sewage or manures, are the great contributors to pollution. These compounds enter into the water source and cause toxicity. The release of excess organic matter in the water body increases the number of decomposers. These microorganisms use oxygen for rapid growth, leading to oxygen depletion and limiting oxygen availability to aquatic organisms. These lead to the death of aquatic organisms, which are again broken down by decomposers, leading to further oxygen level depletion. Accumulating phosphates and nitrogen in the aquatic environment leads to another type of organic pollution. Due to the high level of these nutrients, algae and plants are rapidly growing in the water. This led to the enormous decay of those plants and algae and lowered the level of oxygen, which is suffocation for the organisms in the aquatic ecosystem. This overall method is known as eutrophication (Table 2.1).

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Table 2.1 Type of water pollution Type of water pollution

Description

Common sources

Causes

Mitigation strategies

Nutrient pollution

Excess levels of nitrogen and phosphorus in water bodies lead to excessive algae growth and harmful algal blooms

Agricultural runoff, sewage treatment plants, industrial discharges, fertilizers

Excessive use of fertilizers, manure runoff, inadequate wastewater treatment, soil erosion

Implement best agricultural management practices, such as precision farming, buffer zones, and nutrient management plans. Upgrade wastewater treatment plants to remove nutrients

Sediment pollution

Excessive soil erosion and sedimentation in water bodies, resulting in reduced water clarity and habitat degradation

Construction sites, mining activities, agricultural practices

Poor construction practices, deforestation, improper land management, erosion of farm activities

Implement erosion control measures like sediment basins, silt fences, and vegetative buffers. Adopt sustainable land management practices

Chemical pollution

Contamination of water by synthetic and organic chemicals, including pesticides, pharmaceuticals, and industrial pollutants

Industrial discharges, improper waste disposal, chemical spills, wastewater discharges

Industrial activities, improper disposal of chemicals, agricultural runoff, accidental spills

Properly dispose of hazardous substances, promote safer chemical alternatives, implement advanced wastewater treatment technologies, and enforce industrial discharges and waste management regulations

Heavy metal pollution

Presence of toxic heavy metals, such as lead, mercury, cadmium, and arsenic, in water bodies

Industrial discharges, mining activities, improper waste disposal

Industrial processes, mining activities, improper disposal of electronic waste, atmospheric deposition

Implement strict regulations on industrial wastewater discharges, promote cleaner production methods, improve waste management and recycling practices, and remediate contaminated sites (continued)

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Table 2.1 (continued) Type of water pollution

Description

Common sources

Causes

Mitigation strategies

Thermal pollution

Increased water temperature due to human activities leads to reduced oxygen levels and detrimental impacts on aquatic ecosystems

Power plants, industrial cooling systems, urbanization

Discharge of heated water from power plants, industrial processes, removal of shade and vegetation along water bodies

Implement cooling technologies in power plants, reduce energy consumption, restore riparian vegetation, and promote water-efficient industrial practices

Oil and Petroleum Spills

Accidental release of oil and petroleum products into water bodies, causing severe ecological damage and harming aquatic organisms

Oil spills from tankers, offshore drilling operations, transportation accidents

Oil tanker accidents, pipeline leaks, oil extraction and refining activities

Improve safety measures in transportation and extraction, enhance spill response capabilities, promote renewable energy sources, and enforce strict regulations on offshore drilling

Microbial pollution

Presence of disease-causing microorganisms, such as bacteria, viruses, and parasites, in water bodies

Sewage discharges, animal waste runoff, contaminated drinking water sources

Inadequate wastewater treatment, improper sanitation practices, agricultural runoff, malfunctioning septic systems

Upgrade sewage treatment systems, promote sanitation and hygiene practices, manage animal waste properly, and monitor water quality regularly

Radioactive pollution

Contamination of water by radioactive substances, which can have long-term health and environmental impacts

Nuclear power plants, uranium mining, radioactive waste disposal

Accidental leaks or spills from nuclear facilities, improper disposal of radioactive waste

Implement stringent safety protocols in nuclear facilities, improve radioactive waste management practices, develop alternative energy sources, and remediate contaminated sites (continued)

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Table 2.1 (continued) Type of water pollution

Description

Common sources

Causes

Mitigation strategies

Plastic pollution

Accumulation of plastic debris in water bodies, posing threats to marine life and ecosystems

Improper waste disposal, littering, plastic manufacturing and packaging industries

Improper waste management, inadequate recycling facilities, littering, stormwater runoff

Promote recycling and circular economy practices, reduce single-use plastics, implement effective waste management systems, and raise awareness about plastic pollution

Agricultural runoff

Contamination of water by pesticides, fertilizers, and animal waste from agricultural activities

Excessive use of pesticides and fertilizers, improper manure management

Runoff from agricultural fields, improper storage and application of pesticides and fertilizers, inadequate management of animal waste

Implement integrated pest management practices, promote organic farming, adopt precision application techniques, and develop and enforce agricultural runoff management plans

Source Based on the review of literature

2.4.1 Thermal Pollution Thermal pollution is a process of water quality degradation due to increased or decreased water temperature due to industrial plants. This process is also known as “Thermal enrichment”. These industries uptake the water from water sources and then release it back to the source. These increased or decreased temperatures cause disastrous effects on the aquatic environment changing oxygen content, thus reducing the life of aquatic animals.

2.4.2 Transport Pollution Pollution through transport has become a severe problem causing adverse effects on the environment and human health, including global warming, noise pollution, air pollution, and water quality degradation. The water quality is degraded by chemicals, fuels and other particles dumped from vehicles, airport terminals, trucks, and trains. Mainly due to marine transport and activities like oil spills from cargo vessel accidents, garbage etc., cause an impact on the marine environment. Additionally, wastes like plastic and metals can linger on the water surface for a long time, causing severe effects on the marine environment and disturbing the ecosystem.

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2.4.3 Natural Pollution Natural pollution is also known as ecological pollution, which occurs when nature plays a role in causing pollution by releasing chemicals, organic compounds and effluents from industries. These types of pollution occur not due to human activities but by nature. For example, volcano eruptions are a source of ecological pollution. When large dead animals like cows, deer, and buffaloes drain due to floods, it increases the amount of organic matter in the water. Increased siltation rate of the waterway after a landslide increases the runoff of water sediments.

2.5 Sources of Water Pollution The most crucial water pollution sources are industrial effluents, wastes from domestic works and agricultural wastes. The other sources include marine dumping, oil spills, atmospheric deposition, eutrophication and radioactive wastes.

2.5.1 Domestic Sewage This type of sewage occurs due to household activities. Due to these activities, there is an increase in organic materials generated through vegetable and food wastes and inorganic materials, such as heavy metals wastes, nitrates and phosphates from soaps and detergents.

2.5.2 Industrial Effluents Manufacturing and processing industries release different organic and inorganic wastes and toxic chemicals. Some of the pollutants from various industrial sources are cadmium, Lead, mercury, arsenic, oils, nitrates, phosphates, etc. Food and chemical processing industries and other industries like thermal plants, leather processing industries, distilleries etc., pollute the water systems. Due to dye industries, it changes the colour of water, making it unable to use. The mine sites, industries and hazardous waste sites release radioactive or toxic materials, an essential source of water pollution. There was a death report of 1,800 people in 1932 due to Minamata disease from consuming fish containing methyl mercury released from the chemical factory Chisso Corporation (Ulrico & López-Chuken, 2012). Due to the release of sulphur dioxide from burning fossil fuels into the atmosphere. The Sulphur dioxide reacts with water creating acid rainfall which contains sulphuric acid. This H2 SO4 falls into

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different water sources and lowers the pH of the waterway, affecting plants and the entire food chain. This may lead to the leaching of heavy metals from the soil into the water.

2.5.3 Agricultural Waste The different sources of industrial wastes include manures, slurries, pesticides and runoffs. These compounds are particularly hazardous to life in lakes, rivers, ponds and streams where there is a buildup of toxic substances over some time. The runoffs from these agricultural fields cause water pollution to nearby water sources. The seepage of pesticides, fertilizers, and water enters the soil, causing groundwater pollution. The phosphates and nitrates from the fertilizers in water lead to eutrophication.

2.5.4 Heavy Metal Pollution in Water Water pollution employing heavy metal has become a severe problem in industrial areas and, therefore, a global issue that needs serious attention. The heavy metals identified in wastewater are Cu, Cd, Ar, Cr, Zn, and Hg. These metals have become a significant threat to public health because of their accumulation, persistence and biomagnification in the food chain. These in water are due to industrial discharges, infiltration of groundwater and residential dwellings. Due to direct and indirect human activities like urbanization, industrialization and anthropogenic activities, these compounds are accumulated within the environment. The metals can persist in the environment and accumulate in the biological chain, causing acute and chronic diseases. Due to the uptake of these heavy metals, there are severe toxic effects on living organisms like cancer development, reproductive and nervous system damage, reduced growth and development and in the extreme, it may lead to the death of the organism (Ulrico & López-Chuken, 2012).

2.6 Effect of Pollutants on the Aquatic Ecosystem 2.6.1 Pesticides Various pesticides like organophosphorus, carbamates, synthetic pyrethroids etc., are used in agriculture and daily activities. These compounds create contamination of groundwater which then flows to different water sources like ponds, lakes, and rivers that finally reach the ocean (Lalithakumari, 2011). Insecticides like synthetic pyrethroids and β-cypermethrin have greatly threatened aquatic organisms’ survival.

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These chemicals create serious threats like developmental disorders, neurotoxicity and reproductive failures (Zhang et al., 2011). Due to excess nitrogen and phosphorus fertilizer, the excessive blooming of algal biomass takes place, which causes hypoxia and loss of species diversity in the sea. These compounds accumulate in the fatty tissues of fish and then pass on successive trophic levels.

2.6.2 Heavy Metals Heavy metals are compounds that harm living beings. These compounds include arsenic, cadmium, lead, mercury etc. Bioaccumulation and biomagnification is the major problem associated with these compounds. The combinations are carcinogenic and highly toxic and exert undesirable effects on developing fish embryos, delay hatching, retard growth and development or sometimes cause death (Zhang et al., 2012). These compounds also disrupt endocrine (Hontela et al., 1996) and growth hormones and inhibit estrogen receptors (Guével et al., 2000). Due to cadmium exposure, Ital-Itai disease occurs in women and is characterized by anaemia, renal dysfunction and severe osteoporosis in women. In contrast, mercury is a potent neurotoxin that causes minamata diseases.

2.6.3 Crude Oil Crude oil is a combination of 20,000 chemical compounds which occur naturally in the environment. Various anthropogenic activities like incomplete combustion of petroleum products and fossil fuel results in the emission of petroleum hydrocarbons which cause adverse effects in the environment, causing cancer, immune-modulatory effects, and mutagenesis on various living beings like human, animals and plants.

2.6.4 Dyes and Paints There are more than lakhs of commercial dyes, like acidic, basic, reactive etc., that are present, and these are synthetic chemicals and recalcitrant (Campos et al., 2001). These dyes are used in different substrates such as foods, cosmetics, plastics, cloths, and textile industries. Around 50% of dyes are released in industrial effluents, which ultimately reach the oceans, causing very adverse effects on aquatic life. Even in small amounts, these dyes and paints reduce the sunlight penetration to the seas, prevent gas exchange, are carcinogenic and mutagenic, and potently toxic (Abdelkader et al., 2011; Sampath et al., 2012).

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P. Kumar and D. Choudhury

2.6.5 Polycyclic Aromatic Hydrocarbons These compounds adversely affect the environment. They deplete the ozone layer affecting the earth’s heat balance, adding acidic pollutants to the environment, and reducing visibility. Most of these compounds are teratogenic, mutagenic, and carcinogenic. These PAH compounds are present as a polished layer on the surface of the water bodies, which blocks the sunlight and prevents gaseous exchange, which affects photosynthesis and respiration (Yeung et al., 2011). There are also reports of various genotoxic, endocrine, physical and psychological effects on human beings due to the presence of these compounds in the atmosphere.

2.6.6 Plastic Plastic is the most notorious xenobiotic, adversely affecting the entire ecosystem. Over 80% of the plastic debris on water bodies comes from land-based sources (Sheavly & Register, 2007). Plastic debris in less amount ingested by aquatic organisms can cause growth inhibition, behavioural disorders, reproductive dysfunction, mortality, and reduced viability (Green et al., 2016; Nasser & Lynch, 2016; Sussarellu et al., 2016).

2.6.7 Nitroaromatic Compounds Nitro group of chemicals are essential compounds for producing many pesticides, explosives, dyes, pharmaceuticals, plasticizers, as solvents. Nitroaromatic compounds have versatile qualities due to the presence of the nitro group. These compounds are essential for many industries due to their persistence, stability, and toxicity, but they becomes hazardous when released into the environment polluting the aquatic environment. These compounds degrade to form more toxic and harmful compounds creating carcinogenic and mutagenic effects on aquatic organisms (Venulet & Van Etten, 1970).

2.6.8 Pharmaceuticals Pharmaceuticals in massive amounts are produced and consumed by humans and animals worldwide. These products in the form of parental compounds or active ingredients are consumed and spread in the environment, employing pharmaceutical

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39

industries, effluents from sewage treatment plants, or runoff water from agricultural lands. These pharmaceutical products create adverse effects on the non-target organisms (Furuhagen et al., 2014).

2.7 Treatment of Water for Removal of Heavy Metals Several methods of removal of heavy metals from water have been developed based on chemical and microbiological processes. There are various physicochemical methods for removing heavy metals based on chemical precipitation and coagulation, followed by sedimentation, extraction, microfiltration, ion exchange, flotation, reverse osmosis, etc. However, these techniques are high in cost and require high cleanup standards. Due to the less effectiveness of these traditional methods of removing heavy metals, they have led to the search for economical and straightforward processes to remove heavy metals (Salt et al., 1995). This technique involves growing hydroponics in wastewater which involves removing the metals by precipitation, adsorption or precipitation. In this method, higher terrestrial and aquatic plants are grown, leading to pollutants being buried in particular areas.

2.7.1 Bioremediation Through Hydroponics Bioremediation is a sustainable strategy to attenuate the negative impacts of effluents and maintain the aquatic environment. It is a process where hydroponics, macrophytes (Thakur et al., 2023) or microorganisms (Watanabe, 2001) can degrade, transform, or remove hazardous compounds from the wastewater. Macrophytes have a remarkable capacity to uptake nutrients and other compounds for their medium of growth, lowering the concentration of pollution from a targeted waterbody (Dhote & Dixit, 2009). This method is cost-effective and can be quickly executed. The main focus of research on phytoremediation technique is the use of plants to a. remove or extract perilous heavy metals from water or soil and b. to degrade organic contaminants, usually in prom with root rhizosphere microorganisms. This technique is appealing as it is low in cost and aesthetically pleasing to the public compared to traditional remediation strategies. Macrophytes degrade or remove pollutants using fundamental mechanisms of phytodegradation, phytostimulation, phytovolatilization, phytoextraction, phytostabilization, hemofiltration or rhizofiltration described in Tables 2.2 and 2.3.

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P. Kumar and D. Choudhury

Table 2.2 Mechanism of phytoremediation (Ulrico & López-Chuken, 2012) Mechanism

Description

Phytodegradation

This method is also known as phytotransformation, where the plants break down the organic contaminants through a metabolic process that occurs within them or surrounding the plants through the effects of the compound produced by the plant and degrade the organic contaminant into less simple molecules

Phytostimulation

This process is also known as rhizostimulation, in which the organic contaminants are a breakdown in the rhizospheric area in water or soil through microbiome activity enhanced by plant roots. The microorganisms digest or degrade the pollutants like pesticides, solvents, and hydrocarbons into harmless products and gain energy and nutrition for their growth and survival

Phytovolatilization

In this process, the plants take the pollutants and, through transpiration, release them into the environment. Some pollutants pass through leaves and volatilize or evaporate into simpler compounds

Phytoextraction

This method is also known as phytoaccumulation, in which metals are uptaken from soil by the roots of plants to the upper part of the plant and accumulate there. Then the plants are grown for some time, harvested and burned completely. The procedure must be repeated as necessary so that the level of contaminants comes down to a limit which is not harmful

Phytostabilization

In this method, inorganic contaminants are immobilized by using certain plant species through adsorption, absorption and accumulation by the roots of plants in soil or groundwater

Phyto or rhizofiltration

In this method, terrestrial or aquatic plants absorb, concentrate and precipitate the pollutants to their roots from polluted aqueous sources

Sources Based on the review of literature

2.8 Classification of Macrophytes Macrophytes can be broadly divided into three primary forms, which remove pollutants from the water column and sediment depending on their deployment (Newete & Byrne, 2016) (Table 2.4). a. Floating: These macrophytes occupy the water’s surface. They may be accessible floating or rooted, e.g. Duckweeds, frogbits and water lilies. b. Submerger: These macrophytes grow below the water surface and may be anchored to the substrate, e.g. Hornwort c. Emergent: These macrophytes occupy waterbody margins and are rooted into the substrate but have significant shoot growth above water level, e.g. Reedmace and common reed.

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Table 2.3 Different types of hydroponic systems and their description Hydroponic system Description Nutrient Film Technique (NFT)

In this system, the roots are coated with a nutrient solution and is an ideal growing system. This technique can be used for specific crops because a widespread adaption has not occurred and also there is a risk of spreading many root-borne diseases. The chamber is prepared in such a way that the tray is placed at an angle so that the nutrient solution flows through the plants and drains through the drainpipe and a new nutrient solution is constantly being pumped into the high end of the tube. NFT is a hydroponic system where a thin film of nutrient-rich water flows continuously over the roots of the plants. In wastewater bioremediation, NFT systems can be used to grow plants that take up pollutants from the water as it passes over their roots

Deep Water Culture (DWC)

DWC is a hydroponic system where plants are suspended in a nutrient solution with their roots submerged in the oxygenated water solution. In wastewater bioremediation, DWC systems can be utilized to grow plants that absorb contaminants directly from the water column, aiding in the purification process. In this method, the plants are typically grown in netted pots or baskets and the roots dangle into the nutrient solution absorbing water, nutrients, and oxygen as needed. When the roots get direct contact with the nutrient solution the plant growth becomes vigorous. There are some advantages to this technique which allow faster growth of the plant, higher yield harvest, and low maintenance technique as there are no soil-related pests, diseases, insects and no weed competition

Floating Raft System

Plants are grown on floating platforms in a raft system, with their roots submerged below. The rafts are filled with wastewater. The plant roots dipped in wastewater where they can absorb the nutrients and contaminants from the water as it passes through their root systems. Floating raft systems can be employed in wastewater treatment to promote the uptake of pollutants by plants and improve water quality. It is a new and promising concept for the remediation of wastewater to improve surface water quality where there is vegetation on artificial floating platforms

Aeroponics

Aeroponics is a hydroponic technique where plants are grown in an air or mist environment without a growing medium. In wastewater bioremediation, aeroponic systems can be used to grow plants that are misted with nutrient-rich water-containing pollutants. The plants absorb the nutrients and contaminants through their exposed roots. It falls under the category of hydroponics where water is employed in aeroponics to deliver nutrients to the plants

Vertical Tower Systems

Vertical tower systems utilize vertically stacked planting containers or towers to grow plants hydroponically. These systems are suitable for limited spaces and can be employed for wastewater bioremediation by allowing the plants to uptake contaminants from the nutrient solution as it circulates through the system. Here the nutrient-rich wastewater is fed from the top and collected at the bottom (continued)

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P. Kumar and D. Choudhury

Table 2.3 (continued) Hydroponic system Description Ebb and Flow System

In an ebb-and-flow system, plants are periodically flooded with a nutrient solution and then drained. It is also known as a flood and drain hydroponics system that uses a water pump periodically to flood plant roots with nutrient-rich contaminated water and then drain the water away. Depending on the needs of the plants this process is repeated multiple times throughout the day. This cyclic flooding and draining allow the plants to absorb nutrients and contaminants from the water. Ebb and flow systems can be adapted for wastewater bioremediation by supplying the plants with wastewater and allowing them to remove pollutants during the flooding phase. This system is known for its low initial investment cost

Wick System

Wick systems are simple hydroponic setups where a wick is used to transport the nutrient solution to the plant’s root zone. The plants draw up the solution through capillary action. Although less commonly used for bioremediation, wick systems can grow plants that take up pollutants from the nutrient solution. This system is very simple and consists of four basic components a. growing solution b. reservoir for the nutrient solution c. growing medium and d. wicks. Here the growing containers are kept a short distance above the reservoir and wicks are placed so that it will draw the nutrient solution up and from the reservoir and release it into the growing medium. This helps in absorption and makes it available to the roots of the plants

Drip Irrigation System

Drip irrigation systems deliver nutrient-rich water directly to the plant’s root zone through drip emitters. These systems can be adapted for wastewater bioremediation by providing contaminated water to the plants, allowing them to absorb pollutants as the water drips onto their roots. It is a versatile system where the plants to be fed are kept singly in separate containers or the plants can be grown in large trays. The drip emitters can be kept in a large container that is connected to a pump placed into a reservoir holding a nutrient solution and through drip emitters the solution is directly delivered to the roots of the plants. A timer is also set as the roots need time to dry in between nutrient flows which allows roots to absorb oxygen

Sources Based on the review of literature

2.8.1 Macrophytes Used in Aquatic Phytoremediation Macronutrients: Nitrogen is uptaken and sequestered by the macrophytes in nitrate and ammonia, while phosphorus is uptaken in phosphate. The macrophytes that produce huge biomass or faster growth rates are some of the most effective phytoremediators of nutrients (Kennen & Kirkwood, 2015). For example, Typha latifolia (60–110), Lemma sp. (6–26 t/ha/yr), and Eichhornia crassipes (8–61 t/ha/yr) (Gumbricht, 1993). Emergent macrophytes such as Cyperus spp. and Canna spp. remove the highest ammonia between 74 and 100%. Polygonum hydropiperoides, Lolium multiflorum and T. latifolia showed the highest total phosphorus removal of 81–90% (Table 2.3). Floating macrophytes like Pistia stratiotes, Lemma gibba and E. crassipes shows efficient nutrient removal. L. gibba removes 100% NO3 and

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Table 2.4 Different types of Macrophytes and its description Classification

Description

Based on the Growth Form Emergent Macrophytes

These macrophytes have their roots submerged in the water or mud, while their stems and leaves extend above the water surface. Examples include cattails, bulrushes, and reeds

Submerged Macrophytes

These macrophytes are fully submerged in water and do not emerge above the water surface. They typically have long, slender leaves. Examples include eelgrass, pondweeds, and water milfoil

Floating Macrophytes

These macrophytes float on the water surface, with their roots dangling beneath. They may have leaves that float or floatation devices such as air-filled bladders. Examples include water lilies and duckweed

Based on Life Cycle Annual Macrophytes

These macrophytes complete their life cycle within a year, from germination to reproduction, and then die off

Perennial Macrophytes

These macrophytes have a life cycle lasting for multiple years. They can persist and reproduce for several growing seasons

Biennial Macrophytes

These macrophytes have a life cycle that spans two years. In the first year, they establish vegetatively; in the second year, they flower, produce seeds, and die off

Based on Habitat Freshwater Macrophytes

These macrophytes inhabit freshwater environments such as lakes, ponds, rivers, and wetlands. They are adapted to survive in low-salinity conditions

Marine Macrophytes

These macrophytes inhabit marine or saltwater environments such as oceans, seas, and estuaries. They can tolerate higher salinity levels

Based on the Taxonomic Group Algae

Algae are diverse photosynthetic organisms that include both microscopic and macroscopic species. They can be classified into green algae, brown algae, and red algae

Mosses

Mosses are non-vascular plants that lack true roots, stems, and leaves. They typically grow in moist environments and can form dense mats

Ferns

Ferns are vascular plants with leaves, stems, and roots. They reproduce via spores and are commonly found in wet habitats

Flowering Plants

Flowering plants, also known as angiosperms, are vascular plants that produce flowers and fruits. They are the most diverse macrophytes and include various families and species

Source Based on the review of literature

82% NH3 while E. crassipes removes 92% NO3 and 81% NH3 . Total phosphorus removal by these two species is also effective. Due to the difficulty of cultivating and harvesting and lower biomass, submerged plants have received less attention than emergent plants (Du et al., 2017). Total nitrogen and phosphorus removal rates by Myriophyllum aquaticum and Ceratophyllum demersum are more than 41%. Most

44

P. Kumar and D. Choudhury Volatilize into atmosphere

Emerged vegetation

Submerged vegetation Floating vegetation

Phytoextraction and phytostabilization

Free floating Algae

Pollutants

Pollutants

Pollutants uptake by different vegetation

Degraded pollutants

Fig. 2.1 Different types of Macrophytes involve in aquatic phytoremediation

submerged species are rooted in sediment and remove nutrients through foliar absorption. So, they have a dual ability to remove nutrients from water and sediments (Kuiper et al., 2017) (Fig. 2.1; Table 2.5). Competition, predation and developmental stage (Quilliam et al., 2015) are the biotic factors and temperature, light, pH, and nutrient load are (Ansari et al., 2014)the abiotic factors which influence the phytoremediation potential of a macrophyte. NO3 − removal efficacy by E. crassipes increased between 100 and 300 mg/l of concentration but decreased between 400 and 500 mg/l (Ayyasamy et al., 2009). In another experiment, the net maximum accumulation of Total N and P by Nasturtium officinale and Oenanthe javanica occurred at 22 °C but deteriorated afterwards. (Hu et al., 2010). Understanding the accumulator’s performance under different environmental conditions and their capacity to remove contaminants is necessary to optimize species selection. Metals: Macrophytes can remove metals from water and sediments. For removing metals, hyperaccumulators are the most appropriate (Ali et al., 2013). Due to widespread industrial effluents and ecological risks, the search for hyperaccumulator species has become a primary focus in reducing the pollutants released by industries (Van der Ent et al., 2013). Due to sedimentation and adsorption of clay particles, metal bioavailability can be reduced. Various studies were conducted with an elevated metal concentration on synthetic solutions to assess the potentiality of hyperaccumulators to remove metal pollutants. Different species can uptake various types of metals, indicating the benefits of some of the macrophyte species in phytoremediation (Table 2.6). Macrophytes considered hyperaccumulators with high biomass-producing capacity are the free-floating plants like Lemna minor, P. stratiotes, E. crassipes

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Table 2.5 Table of commonly used macrophytes in aquatic phytoremediation Macrophyte name

Scientific name

Types

Phytoremediation potential

Contaminants targeted

Water Hyacinth

Eichhornia crassipes

Free Floating

High

Heavy metals, organic pollutants

Duckweed

Lemna spp.

Free Floating

High

Nitrogen, phosphorus, heavy metals

Reed

Phragmites australis

Emergent

High

Heavy metals, organic pollutants

Cattail

Typha spp.

Emergent and submerged

High

Organic pollutants, nutrients

Water Lettuce

Pistia stratiotes

Free Floating

Moderate

Heavy metals, organic pollutants

Watercress

Nasturtium officinale

Submetged

Moderate

Nitrogen, phosphorus, heavy metals

Water Pennywort

Hydrocotyle spp.

Free Floating

Moderate

Organic pollutants, nutrients

Water Spinach

Ipomoea aquatica Free Floating

Moderate

Heavy metals, organic pollutants

Waterweed

Elodea spp.

Submerged

Low

Nutrients

Hornwort

Ceratophyllum spp.

Submerged Free floating rootless

Low

Nutrients

Parrot feather watermilfoil

Myriophyllum aquaticum

Emergent and submerged

High

Nitrogen and Phosphorus removal

Umbrella plant and

Cyperus spp and Canna spp

Emergent

High

Ammonia

Swamp smartweed and false water pepper

Polygonum hydropiperoides

Emergent

High

Phosphorus removal

Italian Raygrass

Lolium multiflorum

Emergent

High

Phosphorus removal

Source Based on a review of literature

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P. Kumar and D. Choudhury

Table 2.6 Macrophytes and metal removal from water and sediments Macrophyte

Metal removal capability from water

Metal removal capability from sediments

Water Hyacinth

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Duckweed

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Water Lettuce

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Water Fern

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Water Velvet

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Watercress

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Cattails

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Indian Mustard

Lead, Zinc, Copper, Nickel, Chromium

Lead, Zinc, Copper, Nickel, Chromium

Sunflowers

Lead, Zinc, Copper, Nickel, Chromium

Lead, Zinc, Copper, Nickel, Chromium

Vetiver Grass

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Spartina Grass

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Typha (Cattail)

Lead, Zinc, Copper, Nickel, Chromium

Lead, Zinc, Copper, Nickel, Chromium

Azolla

Lead, Zinc, Copper, Nickel, Chromium

Lead, Zinc, Copper, Nickel, Chromium

Lemna (Duckweed)

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Water Milfoil

Lead, Zinc, Copper, Nickel, Chromium

Lead, Zinc, Copper, Nickel, Chromium

Water Primrose

Lead, Zinc, Copper, Nickel, Chromium

Lead, Zinc, Copper, Nickel, Chromium

Giant Salvinia

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Pennywort

Lead, Zinc, Copper, Nickel, Chromium

Lead, Zinc, Copper, Nickel, Chromium

Water Spinach

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Bacopa

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel (continued)

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Table 2.6 (continued) Macrophyte

Metal removal capability from water

Metal removal capability from sediments

Spirodela (Giant Duckweed)

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Eichhornia (Water Hyacinth)

Cadmium, Lead, Zinc, Copper, Nickel

Cadmium, Lead, Zinc, Copper, Nickel

Source Based on a review of literature

and the species from genera Salvinia (Table 2.7). A report of L. gibba concentrating 14,000 mg/kg dry wt. of Cd while E. crassipes to concentrate 10,000 mg/kg of Zn (Mkandawire et al., 2004). T latifolia and C demersum are also considered potential bioremediation of metals. If the toxicity of the metal concentration is high, it becomes a limitation for the macrophytes to uptake metals (Landesman et al., 2011). The detoxification mechanism also allows the macrophyte species to avoid adverse effects which may occur due to metals (Deng et al., 2004). It was observed that more than 50% of the metals like Mg, Mn, Cd, Ca, and Fe recovered in the roots of P. stratiotes were attached externally, indicating the plant’s ability to maintain the internal tolerance level (Lu et al., 2011). Newete and Byrne (2016) also reported ability of macrophytes to remove metals depends on the type of root; for example, macrophytes with fibrous root systems can remove metal due to their large surface area. For the uptake and accumulation of metals, physicochemical factors along with light, temperature, salinity and pH are also essential to influence bioremediation performance (Rai, 2009) (Table 2.7).

2.9 Organic Pollutants Organic pollutants, synthetic compounds which contain carbon, are environmentally persistent and potentially toxic. Solvents, pesticides, pharmaceuticals and personal care products are the product that comes under organic pollutants (El-Shahawi et al., 2010). Many studies have been conducted to know the phytoremediation potential of macrophytes to remove organic pollutants. Notably, 95% of the 2,4,5-trichlorophenol was removed by Lemna minor, whereas 25% and 8% of isoproturon and glyphosate, respectively, were removed by this inferior plant. There was a report of 81% of ethion removal by E. crassipes which is relatively high. 66% and 50% removal of DDT was observed in Spirodela oligorrhiza. In comparison, 76% and 82% removal were observed in M. aquaticum (Gao et al., 2000). Removal efficacy of OP, PP- DDT by Elodea Canadensis is reported to be 48–89% (Gao et al., 2000; Garrison et al., 2000). For removal of phenols from water L. gibba, L. minuta and P. crispus have been reported to be very efficient. However, the efficiency of P. crispus in removing PAHs, pyrene and phenanthrene is less (Meng et al., 2015). Various reports of organic products are mentioned in Table 2.8.

48 Table 2.7 Macrophytes in removing heavy metals from water

P. Kumar and D. Choudhury

Macrophytes

Heavy metals

Lemma gibba

As, Ni, Cd

Eichhornia crassipes

Cd, Cr, Cu. Hg, Ni, Zn

Spirodela polyrizha

As

Salvinia natans

Cr, Zn

Salvinia cucullata

Cd, Pb

Pistia stratiotes

Hg, Cu, Cr

Azolla cariliniana

As, Cr, Cu, Hg

Azolla filiculoides

Cd, Cr, Ni, Pb, Zn

Spartina patens

Cd

Spartina alternaflora

As

Scirpus maritimus

As

Phragmites australis

As, Hg

Pharlaris arundinaceae

Mn, Ni, Fe

Elodea densa

Hg

Typha angustifolia

Pb

Typha latifolia

As, Ni, Cu, Zn

Glyceria maxima

Zn, Cu

Limnocharis flava

Cu, Pb, Zn, Hg, Fe

Hydrilla verticillata

Cu, As

Potamogeton pectinatus

Cd, Cu, Mn, Pb, Zn

Myriophyllum spicatum

Co, Cu, Mn, Pb, Zn

Potamogeton natans

U

Ceratophyllum demarsum

Cr, Pb

Ceratophyllum submersed

Ni

Source Based on a review of literature

2.10 Microbial Pollutants The presence of microbial pollutants in water is indicated by various macrophytes grown within Constructed wetlands; therefore, based on planting types, the examples were mentioned. Most studies observed that microbes remove pollutants from water in constructed planting systems via physical, chemical and biological mechanisms. A year-long study was conducted in 12 constructed wetlands, reporting 95–97% faecal coliform removal and 93–95% faecal streptococci removal (Karathanasis et al., 2003). Another study reported a Removal report of 94%, 87% and 94% of Salmonella, Shigella and Vibrio, respectively (Makvana & Sharma, 2013). The removal rate of Salmonella and E. coli was more than 98% compared to unplanted control treatment and treatment containing T. latifolia, Cuyperous alternifolius and Cyperus papyrus. In unplanted experiments, the removal rate was maximum compared to planted ones,

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Table 2.8 Aquatic plants in the removal of organic contaminants Organic contaminants

Aquatic plants species

Pesticides Ethion, pentachlorophenol, cyhalothrin„ dicofol

Eichhornis crassips

2,4,5-trichlorophenol (TCP), Phenol

Lemma gibba

Phenol

Lemma minuta

Glyphosate, 2,4,5-trichlorophenol (TCP), isoproteuron, halogenated phenols

Lemma minor

Aniline and Phenol

Raygrass

Chlorobenzenes, Organochlorine and organophosphorus compounds (PP-DDT and OP-DDT)

Spirodela oligorrhiza

Perchlorate, simazine, HCA, CT, OP, PP- DDT,

Myriophyllum aquaticum

Phenol

Potamogeton crispus

Chlorobenzenes, organochlorine and organophosphorus compounds,

Ceratophyllum demersum

DDT, Phenanthracene, organochlorine, and organophosphorus compounds chlorobenzenes

Elodea Canadensis

DDT, Carbon tetrachloride (CT), Hexachloroethane (HCA)

Elodea

Oryzalin (herbicide)

Pontaderia cordata

Phenanthracene

Scirpus lacustris

Isoxaben, oryzalin

Pontederia cordata

Ethion

Eichhornia crassipes

Atrazin, lambda-cyhalothrin

Ludwigia peploides, Junsus effuses

DDT(OP, PP-DDT)

Myriophyllum aquaticum, Spirodella oligorrhiza

Phenol, PAH (Phenenthrene and pyrene)

Potamogeton crispus

Flazasulfuron, copper sulfate, dimethomorph,

Elodea canadensis, Cabomba aquatica Lemma minor

2,4-D

Lemna minor

Chlorpyrifos

Lemna minor, Pistia stratiotes

Chlorpyriphos and nutrients

Nymphaea alba, Phragmite australis, M. verticillatum

2,4-D, Ethion, sodium tartrate, malathion, KHP,

Eichhornia crassipes

Endosulfan γ–HCH DDTs, Aldrin,

Phragmites, Typha Ceratophyllum demersum,

Pharmaceuticals and personal care producs (PPCP) Estrone, 17, beta-estradiol, 17alpha-ethinylestradoil

Phragmites australis

Caffeine, Diclofenac, Naproxen, Carbamazapine

Scirpus validus

Ibuprofen, Diclofenac, Naproxen, Carbamazapine Typha angustifolia (continued)

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P. Kumar and D. Choudhury

Table 2.8 (continued) Organic contaminants

Aquatic plants species

Triclosan, methyl triclosan and triclocarban

Pontederia cordata, Sagittaria graminea, Typha latifolia

sulfamethazone (SMZ), Carbamazepine (CB), sulfadiazine (DIA), sulfamethoxazole (SMX), ibuprofelinn (IB), and triclosan (TRI)

Pistia stratiotes, Eichhornia crassipes

Ibuprofen, clofibric acid (CA) and carbamazepine Typha spp. CB, declofenac, IB and naproxen

Typha angustifolia

Oxytetracycline (OTC) and tetracycline (TC),

Pistia stratiotes Myriophyllum aquaticum,

Ciprofloxacin (CIP), fluoroquinolones (FQs) and norfloxacin (NOR)

Rhizophora apiculata Acrostichum aureum L.,

Propyl parabens (PrP) and methyl parabens (MeP)

Lemna minor. Landoltia punctate

Ibuprofen, fluoxetine, and triclosan

Lemna minor

Atrazine

Lythrum salicaria, Iris pseudacorus, Acorus calamus

DYES Black B (dye) and Red RB

Eichhornia crassipes

Malachite green, Triphenylmethane dyes (crystal violet and malachite green), Azo dye (AB92),

Lemna minor

Source Based on a review of literature

from which it can be suggested that plants provide minimal additional benefits for removing biological pollutants (Kipasika et al., 2016). An experiment conducted where Lemna sp. treated ponds against untreated ponds showed that the removal rate of E. coli was higher in untreated ones facilitated by UV light water exposure and subsequent photodegradation and microbial die-off (Ansa et al., 2015). However, in another study, gravel beds planted with P. australis were observed to remove E. coli more quickly when compared with untreated soil beds, which may be due to root exudates released by P. australis (Decamp & Warren, 2000). The variability obtained between treated and untreated experiments suggests that different mechanisms of microbial pollutant removal become dominant for each treatment system. Within the unplanted system, it is likely that photodegradation and oxygenation from UV light play an essential role in removing microbial pollutants (Ansa et al., 2015). Conversely, biological and chemical may become important within planted systems. For example, P. stratiotes provide a structural habitat facilitating protozoa presence, increasing predation on Salmonella sp. (Awuah, 2006). Conversely, protozoa predation seemed negligible in the Spirodela polyrhiza planting system. Microbial biofilm growth is facilitated by an increased surface surrounded by a root zone which is thought to be a structure for crucial removal for bacterial

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adsorption and predator microbial proliferation (Decamp & Warren, 2000). Therefore, smaller grasses like Festuca arundinacea may have limited microbial removal potential compared to T. latifolia (Decamp & Warren, 2000). Further research investigating macrophytes’ ability for microbial pollutant removal from water is merited. In developing countries, direct deployment of macrophytes is highly beneficial for pathogen removal, where low-cost bioremediation could provide accessible water treatment. Of the few experiments investigating the macrophyte potential for Removing microbial pollutants outside of Constructed wetlands, 72% E. coli reduction was observed in FTWs planted with P. australis and Canna indica (Saeed et al., 2016). However, during high E. coli loading times, induced by experimental shock phases where hydraulic loading increased between 5 and 14 fold to stimulate high magnitude and low-frequency discharge events, the E. coli removal was reduced significantly to 6% and 45%. The hydraulic retention time is also essential for the pathogen’s survival and die-off (Reinoso et al., 2008) (Table 2.9).

2.11 Issues in Utilizing Invasive Macrophytes In bioremediation, the most effective phytoremediators have faster growth rates and high biomass accumulation. The macrophytes with such traits are often considered invasive; given their rapid colonization potential, they can quickly compete with native macrophytes (Chambers et al., 2008). Azolla filiculoides and Hydrocotyle ranunculoides are invasive species in the UK that can clog waterways and causes severe ecological impacts on various flora and fauna (Schultz & Dibble, 2012). Controlling these invasive plants causes an economic impact, estimated at £1.7 billion per annum (The Great Britain Non-native Species Secretariat, 2015). So there is a significance between using invasive plant species and their management (Rodríguez et al., 2012). In many cases, the complete removal of these invasive species like E. crassipes is not possible. So it is more appropriate to exploit these macrophytes as part of integrated management strategies which help control the spread of these species, remove nutrients and metals from water, and harvest the biomass for economic profit. There is a report of invasive detrimental Zebra mussels (Dreissena polymorpha) (Matsuzaki et al., 2009) to stabilize the clear water state of shallow lakes by filtering phytoplankton and removing harmful cyanobacteria species (Gulati et al., 2008) (Table 2.10). Invasive species present in water bodies may be targeted for harvesting after a periodical regrowth for phytoremediation (Xu et al., 2014). However, various other factors must be considered, including the macrophyte contaminants to avoid transfer to other water bodies and the most appropriate harvesting techniques through engineering systems (Rodríguez et al., 2012). The deployment of invasive species in freshwater systems for phytoremediation is inappropriate and illegal, which may

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Table 2.9 Microbial pollutant harmful impact and mitigation measures Microbial pollutant

Harmful impacts

Mitigation measures

Bacteria

– Gastrointestinal illnesses, such as diarrhea, vomiting, and abdominal cramps. – Infections, including urinary tract infections, skin infections, and respiratory tract infections. – Transmission of typhoid, cholera, and dysentery

– Regular monitoring and testing of water sources for bacterial contamination. – Proper treatment of water using disinfection methods, such as chlorination, UV irradiation, or ozonation. – Implementing and maintaining proper sanitation practices to prevent faecal contamination of water sources

Viruses

– Various illnesses, such as gastroenteritis, – Effective water treatment hepatitis, respiratory infections, and methods like disinfection meningitis. (chlorination, UV treatment, – Outbreaks and epidemics of viral etc.) to inactivate viruses. diseases in communities. – Ensuring proper hygiene practices, including – Increased morbidity and mortality rates handwashing and sanitation, to prevent viral transmission. – Public education and awareness campaigns on safe water handling and consumption

Protozoa

– Intestinal illnesses, including – Proper filtration and cryptosporidiosis and giardiasis, with disinfection of water to symptoms like diarrhoea, abdominal remove or inactivate pain, and dehydration. protozoan pathogens. – Long-lasting health effects, especially in – Enhanced monitoring and immunocompromised individuals testing for protozoan contamination in water sources. – Educating communities about the risks and promoting safe water practices, such as boiling or using water filters (continued)

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Table 2.9 (continued) Microbial pollutant

Harmful impacts

Mitigation measures

Fungi

– Respiratory problems, including allergies, asthma, and fungal infections. – Exposure to mycotoxins can cause various health issues, such as liver damage, respiratory illnesses, and neurological effects

– Prevent and control water leaks and moisture intrusion to avoid fungal growth in buildings and water systems. – Proper maintenance and ventilation to prevent dampness and mould growth. – Regular cleaning and disinfection of water storage tanks and systems. – Mold remediation in water-damaged environments

Algae

– Toxic algal blooms can produce harmful toxins called cyanotoxins, which can cause liver damage, respiratory issues, neurological effects, and skin irritations. – Ecological impacts like oxygen depletion, fish kills, and disruption of aquatic ecosystems

– Monitoring and early detection of algal blooms through regular water sampling and analysis. – Implementing water treatment processes capable of removing or inactivating algal toxins. – Nutrient management to reduce excessive nutrient inputs that contribute to algal growth. – Promoting watershed protection and managing pollution sources

Other Microorganisms

– Infections and diseases caused by helminths (parasitic worms) and prions. – Health impacts can vary depending on the microorganism involved

– Proper sanitation practices to prevent helminth contamination, including safe waste disposal and hygiene education. – Rigorous testing and screening measures for blood and tissue products to prevent prion transmission. – Public health surveillance and awareness campaigns on specific microorganisms and associated risks

Source Based on the review of literature

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Table 2.10 Invasive Macrophytes and its possible role Issue

Description

Ecological impact

Invasive macrophytes can have significant negative impacts on ecosystems. They can outcompete native species, leading to a loss of biodiversity. They can alter habitat structure, water flow, and nutrient cycling, affecting the overall ecosystem balance

Water quality

Some invasive macrophytes, such as water hyacinth or hydrilla, can cause water quality issues. They can create dense mats on the water’s surface, blocking sunlight and reducing dissolved oxygen levels, which can harm fish and other aquatic organisms

Economic costs

Invasive macrophytes can have substantial economic costs. They can clog waterways, hindering navigation and affecting recreational activities like boating and fishing. They can also interfere with irrigation systems, impacting agriculture and water management

Human health concerns

Certain invasive macrophytes, such as giant hogweed or poison ivy, can harm human health. They may cause skin irritations, allergies, or even severe reactions. Direct contact or inhalation of allergenic or toxic compounds can lead to health problems

Infrastructure damage

Invasive macrophytes can damage infrastructure, such as dams, bridges, and water intakes. Their dense growth can block water flow and increase sedimentation, potentially leading to the deterioration of structures and increased maintenance costs

Impacts on fisheries

Invasive macrophytes can impact fisheries by altering fish habitats, reducing available food sources, and changing the structure of aquatic communities. This can reduce fish populations and affect the livelihoods of those dependent on fisheries

Control and management

Effectively controlling and managing invasive macrophytes can be challenging. Traditional methods like manual removal, herbicides, or mechanical control may have limitations and unintended ecological consequences. Developing effective management strategies is crucial

Spread and range expansion

Invasive macrophytes can spread rapidly and expand their range, often outcompeting native species. Human activities, water movement, and climate change can contribute to their dispersal, making containment and eradication efforts difficult

Legal and regulatory Dealing with invasive macrophytes often involves legal and regulatory challenges. Identifying responsibility for control, coordinating efforts among different stakeholders, and complying with regulations related to their management can be complex and time-consuming Research and monitoring

Understanding invasive macrophytes’ biology, ecology, and impacts requires ongoing research and monitoring. This includes studying their life cycles, spread patterns, and identifying effective control methods. Monitoring is crucial for early detection and rapid response

Source Based on the review of literature

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cause damage to the ecosystem and cause long-term effects. In such circumstances, non-invasive or native plants should be employed. Macrophyte Planting Systems: Macrophyte planting systems are effective planting strategies that facilitate phytoremediation from water. Appropriate macrophyte plant systems have been developed a. Constructed wetland b. Wild macrophyte harvesting c. Floating treatment wetlands. Constructed Wetlands. CWs have been used for treating various effluents, including sewage, urban stormwater, drainages, stormwater treatments, and wastewater polishing (Vymazal, 2011). This system also has potentiating wastewater. Constructed wetland is categorized into Free water surface flow (FWSF) or Sub-surface flow (SSF) (Dhir, 2013). FWSF wetland system contains emergent, floating and submerged macrophytes where the plants are grown in shallow ponds or lagoon waters in sandy or organic soils, which allow the contaminated water to flow through the stems of emergent macrophytes for taking maximum pollutants and UV degradation (Kadlec, 2009). In the SSF wetland system, emergent macrophytes are grown ove of stone and gravel mat substrate, enabling able water to come in direct contact with the roots of plants or rhizomes (Vymazal, 2011). The average SSF-constructed wetland system is 100 times smaller than FWSF constructed wetland system (Kadlec, 2009). FWSF is a more common bioremediation system used in Australia and North America, while the SSF system is more commonly used in Europe as the availability of land is more limited (Vymazal, 2011). SSF Wetland systems are used more frequently to enhance the concentration of organic material derived biologically, as indicated by lowering the BOD and COD from wastewater (Vymazal & Kröpfelová, 2009).CWs are the most up-to-date form of deployment of macrophytes within aquatic phytoremediation (Kennen & Kirkwood, 2015). However, these system types can require high investment costs and are restricted to the source of pollutants before the water enters a natural waterway. This restricts the application of constructed wetland method in treating water pollutants from diffused sources. This method can treat contaminants, including COD, BOD, N and P (Kadlec & Wallace, 2009). This system varies in level of engineering and design for their development. SSF requires more construction and management, whereas SSF systems are low-tech gravity-fed (Kadlec & Wallace, 2009). In both systems, high investment is required in construction and operation. Sediments can be clogged into the CW system, impacting the system’s functioning. According to design guidance, the SSF system requires more area (5–10 m2 per person) for adequate water purification (Tilley et al., 2014). Therefore the land required should be more for its construction. Various industrial-based observations and literature show that water treatment and polishing is constructed wetland’s primary purpose. Bioremediation through aquatic plants is a promising technology for the treatment of wastewater with the operational point source-based CW system, but given limitations of this system include investment costs, lack of focus on the ecosystem, and lack of application for diffused pollutants.

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2.12 Wild Macrophyte Harvesting Harvesting of existing wild macrophytes from the water bodies can also be a phytoremediation strategy, relying on the timely removal of biomass of macrophytes in order to manage waterborne pollutants like P and N (Huser et al., 2016). A study on urban shallow water showed that harvesting 3,600 kg dry weight of E. Canadensis led to 16.4 kg removal of P from the system (Bartodziej et al., 2017). However, the removal cost is higher than chemical flocculation treatment (Bartodziej et al., 2017). Macrophyte harvesting is often carried out in various lakes and waterways to relieve navigation, drainage or recreational problems rather than for phytoremediation purposes. In other studies, it was observed that nutrient removal through macrophyte harvesting does not reduce nutrient load (Carpenter & Adams, 1977), estimating the plants harvesting only 1.4% of total phosphorus loading (Peterson et al., 1974). Harvesting macrophytes is a crucial in-water nutrient management technique to determine the base balance of nutrient input/output and removal capacity of plants and identify the need for best practices as part of an integrated management strategy. The harvesting methods are essential for harvesting wild macrophytes, e.g. Hand or mechanical Removal using specialized boats equipped with ranking and cutting apparatus (Quilliam et al., 2015). Hand removal is time and labour-intensive, while mechanical methods include rapid and extensive removal, but it is non-selective, which can lead to high turbidity and impact the aquatic environment (Habib & Yousuf, 2016). Floating Treatment Wetlands: FTW is one of the developed novel ecological engineering phytoremediation techniques. The hypothesis of this system is the highly productive emergent macrophytes. For example, T. latifolia is planted within a growth medium in which the roots of this plant help in rhizofiltration, phytoextraction and phytodegradation (Kiiskila et al., 2017). In FWT, the water-soluble contaminants are uptaken by the roots within the water column only, although the pollutants bound to the sediments can be physically filtered using plant roots from the water column (Tanner & Headley, 2011). Recently the FTWs have gained attention and, in various literatures, have been referred to as artificial floating islands (AFI), floating plantbased systems (FPBS), integrated ecological floating beds (ICFB) and hydroponic root mats (HRM) (Yeh et al., 2015). FTWs have been studied for their capability to remove nutrients, but attempts have been taken to assess pathogen, heavy metal and phytoplankton removal (Jones et al., 2017; Kiiskila et al., 2017). FTWs are conducted in microcosms, mesocosms and pilot trials within lagoons (Chang et al., 2014; Headley & Tanner, 2008; Kiiskila et al., 2017; Ladislas et al., 2013; McAndrew et al., 2016; Nichols et al., 2016). The experiment polluted water, which included store water, river water, lake water, domestic wastewater, sewage effluents, refinery wastewater, acid mine drainage and livestock effluents (Abed et al., 2017; Kiiskila et al., 2017; Li et al., 2012; Zhu et al., 2011). To know the effectiveness of FWTs, mesocosm experiments are the most prominent so far (Chen et al., 2016), although there are few reports of field studies are also present in which it is demonstrated that TN and TP concentrations can be

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reduced polluted Chinese reduced (Zhao et al., 2012). For supporting the growth medium, netting material or foam, peat, soil, cotton and coir fibre are generally used in which macrophytes are grown (Pavlineri et al., 2017; Yeh et al., 2015). The growth medium helps support macrophytes, provides nutrition, and stimulates microbial activity to remove pollutants (Tanner & Headley, 2011). By transplantation of seedlings, cuttings or whole plants, macrophytes can be established (Ning et al., 2014; Yang et al., 2008). Through these planting techniques, it is easy to harvest the biomass from the frame, while in direct planting, whole plants need to be removed from the sediments. The quick and simple method of growing plants in FTW helps remove pollutants from plant biomass (Bartodziej et al., 2017) (Table 2.11). Another FTW planting method, the hybrid method, has been developed to enhance the removal of pollutants and ecosystem restoration (Guo et al., 2014; Li et al., 2010; Lu et al., 2015). This system integrates a new layer beneath the floating platform containing submerged macrophytes (Guo et al., 2014; Li et al., 2010). The frames of FTW photovoltaic solar panels have been attached to give power to the submerged aerator to enhance oxygenation in the vicinity of plant roots and the microorganisms associated with roots, thus helping degrade nutrients (Lu et al., 2015). So these hybrid method helps in the removal of pollutants from the water column (Guo et al., 2014; Li et al., 2010). With the complexity of FTW design, there is an increase in cost and maintenance and removal efficacy of the pollutant, but this may lead to a decrease in user uptake. Increasing maintenance cost creates a barrier to uptake by stakeholders like land managers, farmers, and government organizations, looking for low-cost, low-maintenance treatment options, especially in developing countries. It has been rarely discussed in various literature about the poor design and maintenance of FTWs. If the biomass is not harvested and removed continually or if the water birds get attracted to FTWs, they release egg shells and excreta into the water, inputting nutrients and microbial contaminants (guanotrophication). In watercourses, the placement of FTWs gives full consideration as water birds and recreational users may also use the target water body. In small water bodies, the FTWs potentially slow the velocity of water, which may conflict with farming interests where good drainage is required.

2.13 Conclusion Globally, the contamination of water systems has become a severe concern and removing these contaminants has become a pivotal task. These contaminants threaten soil, water and the environment and cause health problems for humans and animals. Conventional methods of bioremediation are costly and create devastation to the environment. Therefore low-cost and environmental technologies are needed, which can be done through hydroponics. So Removing these contaminants can be done through different processes like photodegradation or phytovolatilization. Pistia stratiotes, Trapa natans, Lemma minor, and Elodea Canadensis are plants which can be

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Table 2.11 Comprehensive table of wild microphytes harvesting Aspect

Description

Purpose

Wild macrophyte harvesting refers to collecting macrophytes from natural habitats for various purposes. These may include commercial uses such as food, fodder, medicinal applications, bioremediation, biofuels, construction materials, or scientific research

Species diversity

Wild macrophyte harvesting can affect species diversity. Overharvesting specific macrophyte species can lead to declining populations, disrupting ecological balance and potentially impacting other organisms that depend on them for habitat or food sources

Ecosystem impact Harvesting macrophytes can have ecological consequences. Removal of macrophytes from aquatic ecosystems can alter nutrient cycling, water clarity, and habitat structure, affecting the overall health of the ecosystem and potentially disrupting the food web and other ecological processes Sustainability

Sustainable harvesting practices are crucial to ensure the long-term viability of wild macrophyte populations. Sustainable approaches may include selective harvesting, rotation of harvest sites, and adherence to harvesting quotas or regulations to prevent overexploitation and promote species recovery

Habitat disturbance

Harvesting activities can cause habitat disturbance, particularly without proper planning and management. Physical disturbances like trampling, removal of substrate, or alteration of water flow patterns can impact the structure and function of aquatic habitats

Legal and regulatory

Wild macrophyte harvesting is often subject to legal regulations and permits. Depending on the location and species involved, specific rules and restrictions may be in place to ensure sustainable harvesting practices and prevent the illegal collection of protected or endangered species

Economic considerations

Wild macrophyte harvesting can have economic implications for local communities and industries. It can provide income and livelihood opportunities for harvesters and support local economies. Additionally, it may contribute to developing value-added products or alternative industries

Monitoring and research

Monitoring and research are essential for understanding the impact of wild macrophyte harvesting and developing appropriate management strategies. This includes monitoring harvest levels, studying the ecological effects, and investigating sustainable harvesting methods and techniques

Invasive species risk

Care must be taken to prevent the unintentional introduction or spread of invasive macrophytes during harvesting activities. Harvesting equipment, boats, or vehicles can be vectors for invasive species, posing risks to other ecosystems if not properly cleaned or disinfected

Cultural and traditional uses

Wild macrophyte harvesting may have cultural and traditional significance for local communities. These practices may be deeply rooted in cultural heritage and can contribute to preserving traditional knowledge and cultural identity. Care should be taken to respect and involve local communities in decision-making processes

Source Based on the review of literature

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grown to remove organic and inorganic contaminants in water. Most of the macrophytes have good growth and produce vast amounts of biomass and, therefore, may be used for energy production such as biogas, biofuels etc. Therefore, applying hydroponics to bioremediate these pollutants may also help manage global climate change and energy crises. Acknowledgements We would like to express our sincere gratitude to the Department of Agronomy, School of Agriculture, Lovely Professional University for their support and assistance throughout the writing. The Department’s commitment to academic excellence and research has been instrumental in completing this endeavour. Author’s Contribution The authors of this work have made significant contributions to the research project/study. Each author has participated sufficiently in the research project/study, made intellectual contributions, and is responsible for the work’s accuracy and integrity. The authors have collaborated closely, ensuring the completion of this work through collective effort, expertise, and dedication.

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Chapter 3

Sustainable Approach for Agriculture and Environmental Remediation Using Hydroponics and Their Perspectives Rishi Mittal and Santosh Bhukal

Abstract Agriculture and the use of land were inextricably linked to human existence in the natural world during the evolution of society. Human involvement was not limited to the manufacturing of agricultural products but additionally included the creation of many different kinds of industrial-made items, mining, and the use of fossil fuels as a conventional technique of generating energy. However, increased urbanization has contributed to the loss of the natural world over the last several years. The demand for the advancement of new agricultural methods is constant and continuously a warm topic, especially in light of the many challenges that traditional farming faces. These problems have been solved by the novel advancement technique known as hydroponics. Hydroponics is a technique of cultivating crops that do not require soil and they grow the plant using solutions of minerals and nutrients in an aquatic solvent. Hydroponic cultivation has also been utilized for environmental remediation. Hydroponic systems are designed to eliminate pollutants from water or soil and are used to grow plants. The present study has focused on hydroponics systems for sustainable agriculture and environmental clean-up. In this article, we also discuss the current challenges for hydroponics and their future opportunities. Keywords Hydroponics · Energy footprint · Sustainable Agriculture · Wastewater Remediation

3.1 Introduction The 1.1% annual increase rate of the global human population, or roughly 84 million people, makes it more challenging to preserve biodiversity and optimization of land area. By 2030, developing countries’ arable land would rise by a net 120 million hectares (12%), from 956 to 1076 million ha, according to FAO (Bruinsma et al., R. Mittal · S. Bhukal (B) Department of Environmental Science and Engineering, Guru Jambheshwar University of Science and Technology, Hisar Haryana 125001, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_3

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2003). A growing list of environmental issues, such as climate change, greenhouse gas emission, and forest loss are to be predicted as more living space, food, and water are needed for agricultural development (Ramankutty et al., 2018). In addition to issues with human health, Kinnunen et al. (2020) believe that agriculture and declining food production will also be impacted. Due to rising erosion, compaction, degradation, declining topographical conditions, etc., readily available and most advantageous support matrix soil in agriculture will be significantly affected, which will decrease agricultural production. Hydroponic agriculture has gained popularity recently since it promises to grow terrestrial plants without using soil as a growing medium by merely exposing their roots to a nourishing liquid (Khan, 2018). The use of nutrients in soilless technology comes from a variety of sources, including artificial nutrient solutions, wastewater, manure, chemical fertilizers, etc. Hydroponics is a sustainable and best method to address the scarcity of land, water, enhance the productivity of different crop species, eliminate pollutants from water or soil, and assist in meeting the issues posed by climate change. Lages Barbosa et al. (2015) claim that compared to traditional agriculture, hydroponic systems may use ten times less area and provide yields that are over eleven times higher. Additionally, hydroponic systems emit less gas (0.11 kg CO2 equivalent) than soil farming (0.24 kg CO2 equivalent). Hydroponics also has a considerable potential to conserve water. Other benefits of hydroponic growing systems include their ability to be grown in cities, their ability to grow year-round, their ability to use fertilizers effectively, and their ability to regulate the entire process (Martinez-Mate et al., 2018).

3.2 Background Information and Importance of Environmentally Friendly Agriculture The definition of “sustainable agriculture” is the holistic integration of ecological processes, bioprocesses, physical activities, chemical processes, and socio-economic sciences to develop new farming practices that are both safe and environmentally benign. By protecting and maintaining all of its natural resources such as preserving soil fertility, protecting surface and underground resources, developing renewable energy sources, and looking for ways to adapt farming practices to climate change sustainable agriculture is a process by which agro farming can sustain itself over an extended period of time. Agro-farming must take social groupings and the environment’s sustainability into account. Remediation is the process of removing harmful substances from areas that have been tainted by human activity, such as industrial production, mining operations, or commercial enterprise. The process of remediation consists of a thorough series of processes, including detection, investigation, assessment, selection of corrective

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action, real cleanup, and site reconstruction. These steps are all included in the process. The following is a list of the usual remediation methods that are utilized in a variety of contaminated sites: – Remediation of soil. – De-sedimentation. – Remediation of surface and groundwater.

3.3 An Overview of Hydroponics The origin of the word “hydroponics” is from the two Greek words ‘hydro’ and ‘ponos.’ ‘Hydro’ stands for water and ‘ponos’ stands for labour. In 1929, a professor from California Dr. Gerikea, used the term hydroponics for the first time as he began the process of turning what had previously been a laboratory technology into a method that could be used in the real world to cultivate plants. There were sustainable commercial farms across America, Asia, Africa, and Europe till the 1950s. The United States Army used the soilless technique to cultivate fresh food during World War II for soldiers who were stationed on desolate islands in the Pacific. A boom in hydroponic gardening was observed in 1990, with applications in space initiatives, large-scale production, vertical farming, and polar and desert plant growth (Debangshi, 2021).

3.4 Components and Materials Used in Hydroponics 3.4.1 Water Hydroponics relies on water purified by reverse osmosis (RO), to supply salts to the plant, which depends on the type and developmental stage of crops.

3.4.2 Nutrients One of the cornerstones of gardening of plants, whether in soil or in soilless culture, is giving the plant everything, it needs to thrive. There are a lot of chemical components essential for plant growth and harvesting. Plants are grown in water that is rich in nutrients and oxygen instead of soil. Managing the nutrient content of the water used in hydroponic systems is essential (Srivani & Manjula, 2019). In order to supply the plant roots with soluble versions of the required mineral components, water, and oxygen, a hydroponic fertilizer solution is used. The biological decomposition of organic materials in the soil breaks it down into fundamental nutrients that plants

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can use. The roots then absorb the dissolved nutrients in the water. To supply crops with a balanced diet, the soil’s constituent parts must be in peak condition. To flourish, plants require a whopping 17 separate elements. Nitrogen, Phosphorus, Potassium, Oxygen, Carbon, Sulphur, Calcium, Hydrogen, and Magnesium are the big nine because plants need so much of them to flourish. To a lesser extent, our bodies also need the other eight elements classified as macronutrients, including Molybdenum, Chlorine, Manganese, Iron, Cobalt, Zinc, Boron, and Copper. Hoagland and Arnon (1938) laid the groundwork for the vast majority of nutritional remedy proposals. A nutrition solution’s price, purity, and solubility may differ greatly from grade to grade (pure, technical, food, or fertilizer) (Arnon, 1938). Nutrient solutions can be easily prepared by adding water to pre-mixed formulations, which many small firms prefer. A larger number of facilities are required to prepare the solution. A crucial role in the development and growth of plants is played by nutrients, including edibles like fruits and vegetables. Therefore, proper nutrition administration plays a significant role in determining product quality (Abou-Hadid et al., 1995).

3.4.3 Electrical Conductivity (EC) The maximum allowable EC for cultivable soil is 4 dS/m. The values of EC of the various media used in hydroponics technology may vary, this variation is basically dependent on their sources and admixtures. EC values of organic substrates (OS) are greater when compared to inorganic substrates. According to Abad et al. (2002), peat has the highest EC value of all the organic substrates (1.065 dS/m), and subsequently peat moss (0.706 dS/m). According to Cuartero and Fernández-Muñoz (1998), tomato plants have considerably limited water uptake at EC levels exceeding 4–6 dS/m.

3.4.4 Media An ideal cultural medium must be capable of supplying plants with the most water availability while simultaneously making certain that plant roots can receive plenty of aeration. We can also say that the ratio between macro-porosity, which comprises the pores that are filled with air and do not retain water at all, and microporosity, which comprises pores that can hold water after saturation, must be balanced. Materials having high porosity (ideally 70%) and with the proper ratio of micropores and macropores will ensure better water-to-air ratios. For crops cultivated in compact containers, it is estimated that the total pore space will be 85% of the volume like fast pots.

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3.4.5 Light Light is a crucial factor that affects photosynthesis, photorespiration, and photoperiodism, all of which have an impact on the growth of plants. The ideal light intensity for various greenhouse plants is between 50,000 and 70,000 lx. It is essential for the development of crops. The presence of light, nutrients, temperature, and carbon dioxide affect how quickly plants can produce oxygen through photosynthesis. It takes a large amount of energy to transform carbon from the linked state it shares with oxygen in CO2 gas into the uncoupled one it resides in when it is part of carbohydrates. As a direct consequence of this, the light’s energy is absorbed by the carbohydrate and put to use. The rate of photosynthesis slows down when there is less light available, which has an effect on the rate of growth. Even at greater ideal intensities, the damage to the chloroplasts causes the growth to slow down once more. In photoperiodism, a plant’s response to the day-night cycle, light plays a second role. Numerous reactions, including leaf form, stem elongation, flowering, and other responses, are governed by the temperature differences between the daylight and nighttime periods. In the winter, light is frequently a constraint.

3.4.6 Bulk Density It has a variety of effects on the selection of media. By adding heavy mineral components to the mixture, such as clay, sand, or tuff, high bulk density can be attained. As opposed to that, high-intensity greenhouse crops that are often watered need a medium with low BD (like Gravel has high aeration and BD), since they can be susceptible to oxygen (O2 ) deficit if air-filled porosity and hydraulic conductivity are not high. Low BD media are easier to mix and carry than high BD ones.

3.4.7 Temperature While changes in temperature have no effect on light-dependent reactions of photosynthesis, they do alter the light-independent reactions. Enzymes are responsible for catalyzing these processes. When temperatures approach those at which enzymes perform at a level of efficiency that is optimal for them, the overall rate of reaction increases. It roughly doubles for every 10 °C as the temperature rises. The development and photosynthetic activities of crops are both significantly influenced, to a great degree, by temperature. After achieving an optimum temperature, the rate starts to decrease as enzymes are denatured and continue to diminish until it stops. It enhances the growth of crops by either enhancing or diminishing various processes shown by crops like transpiration, respiration, and photosynthesis. For the majority of vegetables, the highest activity is attained between 21 and 27 °C, which is the daytime temperature in a greenhouse.

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3.4.8 Carbon Dioxide The rate at which carbon is incorporated into carbohydrates in the light-independent process accelerates as a concentration of CO2 rises. So, until it is limited by another factor, the rate of photosynthesis normally increases. As the rate of fixation is reached its peak level, photosynthesis eventually approaches a plateau and speeds up when concentrations of CO2 rise. The major reason is that the average concentration of CO2 is just about 0.04%. As a result, it is conceivable to assert that there is a direct correlation between CO2 concentration and photosynthetic rate.

3.4.9 Relative Humidity As the quality of crops is affected by controlling the relative humidity (RH) within a greenhouse. Most plants typically experience RH between 60% and 75%. An increase in atmospheric humidity enhances photosynthesis and growth, and high RH levels boost the rate of photosynthetic growth. The final total cucumber production was favourably correlated with daytime RH, while early yield, leaf area, stem length, and fresh and dry weight were unaffected by high humidity. However, because pathogenic spores mostly grow at high RH, excessive RH is also bad for the growth of crops (Zhen et al., 2016).

3.5 Various Hydroponic Systems and Methods Different types of hydroponic systems (Fig. 3.1) are described below.

3.5.1 Nutrition Film Method (NFT) In these systems, the crops are grown in gullies while the fertilizer solution is pumped throughout the reservoir. The roots of crops are kept wet by a thin layer of fertilizer solution. The nourishment solution should ideally be accessible to the bottoms of the roots. Similar to a stream delivering nutrients into a line, it works similarly. This innovation uses a pump to continuously feed the plants nutrients, doing away with the need for a timer. Dr. Allan Cooper invented the NFT in the late 1960s at the Glasshouse Crops Research Institute in Littlehampton, England (Winsor, 1979); since then, the same organization has achieved significant improvements. Crops with vast root systems that can reach far into the water can be effectively produced utilizing this system. A Liter of feed is delivered to the majority of NFT channels every minute. It is essential that roots always have moisture because they are not

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Different Hydroponic Systems

• Nutrition Film Method (NFT)

• Drain System/ EBB Flow

• Drip System

• Floating Raft System

• Aeroponics System

• Deep Flow Technique

Fig. 3.1 Different hydroponic system

in a growing media. The fertilizer solution is mixed in a primary reservoir before being cycled through the channels and returned to the reservoir. Numerous shortterm crops, like lettuce, green vegetables, herbs, and onions are suitable for NFT. Longer-term crops like tomatoes and cucumbers are frequently cultivated utilizing larger NFT channels throughout the world. As there is no requirement for soil or a growing medium, the crop develops cleanly without the need for washing, enabling farmers to simply enjoy harvests (Al-Tawaha et al., 2018). This is another advantage of the process.

3.5.2 Flow and Drain System For 5–10 min before the drainage of the solution, an area is filled with a flow of nutrients. To store the nutrient combination, a reservoir is utilized. This system is not typically used in industrial systems but is generally utilized in recreational systems (Sheikh, 2006). In this approach, the soil is typically made up of larger clay pebbles, rock wool, or perlite, which is used to grow the plant roots.

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3.5.3 Drip Systems Long-term crops are grown using this system, which is generally utilized in spot hydroponic techniques. This technique is utilized to supply nutritional solutions to the crops. This technique is configured to operate for roughly ten minutes each hour, which depends on the plant’s developmental stage and the amount of light available (Sharma et al., 2018). In order to provide the plants with fresh nutrients, water, and oxygen, the drip cycle is employed to clean the growing medium. Because the system offers all they require, plants don’t need soil. Drippers at the bases of each plant convey nutrition solution on a timer in a drip irrigation hydroponic system. The used nutritional solution may flow back to a reservoir or off as rubbish based on whether these mechanisms are recovery or non-recovery technologies.

3.5.4 Deep Flow Technique (DFT) or Pipe System As implied by the name, nutrient solution that is deeper than 2–3 cm flows through PVC pipes with a diameter of 10 cm that is attached to plastic net pots holding plants. The fertilizer solution running through the pipes comes into contact with the bottoms of the plastic planting containers. It is possible to arrange the PVC pipes in a single plane or in a zigzag pattern depending on the types of crops planted. The zigzag method efficiently utilizes the available space but works best with crops that develop slowly (Pramono et al., 2020). The single-plane approach shouldn’t be used with crops that are tall or short. In plastic net pots that are screwed into PVC pipe holes, plants are grown. The net pots can either be planted with carbonized rice husk, old coir dust, or a combination of the two. In place of net pots, very little cups made of plastic featuring holes at the bottom and sides could be used. Whenever the recycling solution enters the stock tank, it oxygenates the nutritional solution. There should be a slope with a drop of 1 cm every 30–40 cm for the fertilizer solution to flow more readily through the PVC pipes. This system can be placed as part of CEA in open spaces or secure structures (Jensen et al., 2018). The additional hydroponic techniques are the Deep-Water Culture, Surface Trench or Trough Techniques, as well as the Root Dipping and Capillary Action.

3.5.5 Aquaponics Systems In this technique, hydroponic production and fish farming are integrated (Maucieri et al., 2018). Through plant grow beds, nutrient-rich effluent from the fish tanks is delivered. Aquaponics depends on the development of a robust bacterial community (Joyce et al., 2019). The beneficial microbes present naturally in air, soil, and water

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convert ammonia to nitrate, which agricultural crops readily uptake. It is assumed that in this technique, the consumption of nitrate and other nutrients by plants contributes to the water’s purification.

3.5.6 The Floating Raft Systems Jensen and Collins (1985) independently devised a system in 1976 for growing several heads of lettuce or other leafy crops on a floating raft of inflated plastic. Nowadays large-scale industrial facilities are common and popular in Japan. According to Sweat et al. (2004), the floated technologies employ a floating-raft or mat system, which involves floating Styrofoam rafts that have holes in them on top of the water that is rich in nutrients. Short-season, shallow-rooted crops that thrive in highmoisture environments, such as lettuce, basil, and watercress, function well with this system. The term “dynamic root floating techniques” (DRFT) is another name for this technology.

3.5.7 Aeroponics Systems This technique is a more modern and high-tech way to cultivate plants hydroponically. Nutrients and moisture are provided to the plants through a spray, and the plants are permitted to float in the surrounding air with the roots exposed (Kumari & Kumar, 2019). A timer enables the pump to release a new stream of mist (made of water) at regular intervals of a few minutes each. The pump must always be operating properly, just like with the NFT, because even a little hiccup might affect the roots to dry out. In general, low-leafy crops like lettuce and spinach are good candidates for this strategy. The two most widely used aeroponic hydroponic techniques are the Fog Feed Technique and Root Mist Technique (Khan et al., 2020). Because a mist is easier to control than a liquid under zero gravity, NASA has paid particular attention to aeroponic techniques. The design of this system consists of an A-frame with boards on either side, with plant plugs on each side, and a mister in the area between the boards. With plant plugs, a circular, large-diameter PVC pipe is positioned vertically. Despite the fact that it is an uncommon method of manufacturing and growth. Hydroponic farming with computers, electronics, and automation. Due to the fast-paced development of electronics and the expanding market, people now have access to cutting-edge equipment and instruments that were previously only available in wellequipped research facilities and laboratories. This technological advancement has benefitted agricultural engineering generally, whether through the creation of new machinery or the conversion of equipment that has previously been developed for use in other production sectors for agricultural usage. The creation of an automated system that could manage pH and conductivity online using software and a webcam, even with the significant 24-h difference in greenhouse temperature. He also revealed

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how the system regulates the pH and nutritional concentration of the fluid used to irrigate the hydroponic lettuce automatically by adjusting the opening and closing of its solenoid valves and feeding solutions containing acids, bases, and nutrients. To assess the nutritional quality of lettuce produced, agronomic traits such as the quantity of fresh and dry matter in the aerial part, the quantity of dry matter in the root, the total number of leaves, and the number of leaves larger than ten centimetres, as well as chemical traits such as levels of macro and micronutrients, were compared to conventional soil grown lettuce as the reference. It claimed a “intelligent” addition of nutrients that would improve productivity while maintaining the product’s nutritious value. A new computer-operated analytical platform that can be utilized easily to identify crucial nutrients in hydroponic growing systems was originally disclosed by Rius-Ruiz et al. (2014). To autonomously condition, calibrate, and clean a multiprobe of solid-contact ion-selective electrodes (ISEs), the liquid-handling system uses low-cost components (such as a peristaltic pump and solenoid valves), which are covertly computer-controlled. These carbon nanotube-based ISEs provide excellent mobility, robustness, and ease of maintenance and storage. A real-time, automated method for preparing and delivering fertilizer that supports soilless tomato production has been created by Neto et al. (2014). The Penman–Monteith model’s estimates of transpiration and electrical conductivity measurements of the leachate concentration served as the foundation for the control strategy (Lizarraga et al., 2003). Under greenhouse circumstances, tomato cultivation in the sand substrate was used to assess the effectiveness of the fertigation system. The average tomato fruit’s total soluble solids were 4.50 Brix, whereas the commercial crop’s output was 4.74 kg/m2 . In designed control system resulted in a tomato crop with a water use efficiency of 17.94 kg/m3 . It took 44.42 L of fertilizer solution to create 1 kg of tomato fruits. The suggested approach proved effective in regulating the concentration of the prepared nutrient solution, minimizing environmental issues associated with wastewater disposal, and enhancing the efficiency of fertilizer and water resources.

3.6 Hydroponics System in Sustainable Agriculture and Environmental Remediation 3.6.1 Hydroponics and Environmental Remediation Traditional wastewater treatment methods are very expensive and need a lot of energy both during operation and investment (Bhukal et al., 2022; Mittal et al., 2023). The majority of urban cities in developing nations do not have appropriate municipal wastewater treatment systems in place. Due to its effects on environmental degradation and issues with human health, improper municipal wastewater treatment and disposal management is a substantial reason for worry. In the majority of developing nations, water contamination has been identified as the primary risk factor for public health (Ferronato & Torretta, 2019). In order to prevent the depletion of water

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resources, it is crucial to employ a variety of techniques that can recover water and nutrients found in wastewater while following recycling. In the field of wastewater management, alternative treatment technologies that take advantage of ecological benefits are presently gaining ground. In order to control the long-term impacts on water resources, it is most crucial to apply policies involving the design of sanitation systems linked to crop production (Magwaza et al., 2020). In order to merge these two areas of research, municipal wastewater treatment, and agricultural production, the optimization of nitrification, denitrification, and absorption rates must make way for the maximization of nutrient recycling rates (phosphorus and nitrogen) (Richa et al., 2020). Decentralized systems and resource recovery techniques are examples of such interventions They not only produce environmentally friendly effluents but also work effectively to reduce the cost of sanitation mechanisms, making them readily accessible (Yadav et al., 2022). In an effort to improve the treatment of organic pollutants, techniques focusing on organic removal, reuse of water, removal of nutrients, recovery, and reusing have recently been developed. These systems emphasize the reuse of organic wastewater’s solid and liquid fractions. To enhance the separation of solid and liquid components from wastewater, pour-and-flush toilets, pit latrines, urine diversion toilets, and decentralized wastewater treatment systems (DEWATS) are being deployed. These systems can be used to recover and process water, nutrients (N, P, K), soil moisturizers, biofuel, and biogas following treatment. The solid portion is often utilized for composting and can be further processed into compost and Latrine Dehydrated Pasteurised (LaDePa) pellets, which are beneficial wastebased fertilizer products. On the other hand, the liquid portion can be converted into struvite, a phosphorus-rich fertilizer, and nitrified urine concentrate, a source of nitrogen (Chapeyama et al., 2018). Compared to nitrogen and phosphorus, magnesium is often present in wastewater or urination in much lower amounts. In order to maximize the crystallization of struvite, a supply of magnesium is supplied (Massey et al., 2010). Common precipitants used in lab research include magnesium chloride and magnesium oxide (Wilsenach, et al., 2007). According to Shu et al. (2006), struvite is a phosphate fertilizer that can support agricultural production systems. Magnesium and nitrogen are also present in substantial amounts (Massey & Hartley, 2009). There may be environmental and financial benefits from using these items in agricultural crop production. These advantages result from municipal wastewater’s fertilizer-like properties in terms of nutrients and organic matter. The literature has documented improvements in crop growth and yield when treated municipal wastewater is used for irrigation. In this regard, numerous research has shown that treated effluents can enhance the development and productivity of various crops, including cotton, vegetable crops (Zavadil, 2009), and wheat. Even though employing these items offers numerous financial advantages. A decline in the soil’s composition and nutritional value can have an impact on the growth and development of plants, fruit quality, crop yields, and soil structure. Additionally, the largest sources of risk to the general public’s health may result from ecosystems becoming contaminated by chemicals like minerals, toxic metals, and nutrients. Using hydroponic systems to reuse municipal wastewater in agriculture can be a viable solution to lessen adverse effects on the environment as well as the health of individuals in general (Kumar &

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Cho, 2014). These techniques have been widely employed to produce a variety of fruits and vegetables. In soilless technology, plants grow with their roots submerged in a nutrient solution or are kept alive by an inert substrate in tanks that are filled with specially formulated nutrient solutions. As a result, adopting such systems encourages the treatment of urban wastewater in natural settings and is a widespread practice in wastewater management, particularly in small towns or communities. In a hydroponic treatment of wastewater facility, agricultural crops are grown in effluent that is rich in nutrients (Egbuikwem et al., 2020). This procedure is regarded as a crucial part of a wastewater treatment facility and is in charge of removing the majority of nutrients, particularly nitrogen. According to several pilot studies, hydroponic wastewater treatment is preferable to traditional approaches in some cases. Similar to artificial wetlands, hydroponic systems use a combination of chemical, physical, and biological processes along with interactions between media, plants, and microorganisms to remove pollutants from municipal wastewater. However, it has been discovered that hydroponic wastewater treatment plants are a more cost-effective and environmentally beneficial solution than built wetlands. According to Vaillant et al. (2003), built wetland treatment systems use plants with little commercial value to remove contaminants from wastewater as opposed to hydroponic processes. It is affordable, needs less space, and can be used onsite as a wastewater technology. According to many research studies adopting a hydroponic system for wastewater treatment has certain financial advantages. The comparison of the costs and benefits of using hydroponic technology vs traditional wastewater treatment was based on factors such as the amount of labour, resources, commodities generated, and consumption value that each method required. Despite the fact that these studies discovered significant differences between the two systems, the hydroponic wastewater treatment technique required more initial capital investment, labour costs, and energy expenditures. The cost–benefit analysis varies, nevertheless, according to the location, climate, and societal knowledge it affects the wastewater treatment on environmental pollution. For instance, the cost of cooling the growth environment will be higher for a system placed in a hot climate. Nitrogen and phosphorus belong to the nutrients that are essential in urban wastewater treatment. Nitrogen exists in organic and inorganic forms, such as Total Kjeldahl Nitrogen (TKN), ammonia nitrogen, and nitrate-nitrogen. Hydroponic systems with various plant species have been found to have removal efficiencies for nitrogen and phosphorus ranging from 47 to 91% (Bawiec, 2019). This system’s ability to extract nutrients is dependent on the interaction of different kinds of bacteria, root systems, stone, water, and lighting. All of this contributes to the removal of pollutants from municipal wastewater, either directly or indirectly. The plant root system affects the living conditions of microbes present in municipal wastewater by producing different substances increasing the biological treatment of wastewater using denitrification and nitrification processes (Nihorimbere et al., 2011). In addition to eliminating nutrients, combining municipal wastewater with hydroponic systems can help with biomass production for high-value-added crops. This has the possibility of improving food security and economic creation in poor communities. Because it reduces the health concerns associated with contact with wastewater for workers, harvested crops, and consumers, this technology is especially

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well suited for agricultural reuse of treated municipal wastewater (Bos et al., 2010). When edible crops are cultivated utilizing a hydroponic system, pathogen concerns related to wastewater reuse have reportedly decreased. The ability to use a variety of techniques, such as water culture, drip irrigation technology (DIT), and nutrient film technology (NFT), is attributed to assisting this strategy in minimizing pathogen contamination. When compared to field applications where alternative irrigation systems, including spray irrigation, are typically utilized, are capable of lowering risks (Oliveira et al., 2021). In contrast to tomatoes grown in nutrient solutions that had been injected with pathogens, there were no indications of pathogen contamination in hydroponically produced tomatoes cultivated with NFT, according to Guo et al. (2002). However, if the treatment of wastewater is to be linked with a hydroponic system, those results emphasize the importance of wastewater pre-treatment, the irrigation system picking, the hydroponic method, and the selection of crops. The development and implementation of hydroponic systems for municipal wastewater reuse is a multidisciplinary effort that incorporates elements from environmental, mechanical, and civil engineering design as well as plant-related genetics and biochemistry, according to Norström et al. (2003). A typical hydroponic wastewater treatment system includes primary and secondary treatment, as well as the reuse or recycling of wastewater that has been used for hydroponically grown crops. According to Yang et al. (2015), the hydroponic system stage is also known as a tertiary treatment step. Municipal wastewater is often treated physically in septic tanks to remove settleable contaminants and provide partial anaerobic treatment. The ability of the septic tank to perform partial anaerobic treatment prevents suspended contaminants from entering the secondary-treatment system (Rajasulochan & Preethy, 2016). Municipal wastewater treatment utilizing septic tanks results in effluents with high pathogen levels that need additional treatment using secondary treatment techniques. Pathogens can be effectively removed utilizing sand-based filters, activated sludge anaerobic digestion, and anaerobic baffled reactors, which are employed as secondary treatment methods. A secondary treatment stage, on the other hand, is defined as a biological process because it involves the utilization of microorganisms’ capabilities to consume municipal wastewater materials to provide the energy needed for microbial digestion and biomass synthesis (Leung et al., 2000). However, the composition of the wastewater to be treated influences the removal approach used in the secondary treatment stage. In the hydroponic system/tertiary treatment stage, a greenhouse is added on top of the biological treatment stage, increasing the environment for plants to thrive in the growth beds. A greenhouse’s primary goal is to generate ideal climatic conditions for plant growth by protecting crops from adverse weather conditions such as very high or low temperatures, copious amounts of precipitation, and strong winds. While using municipal wastewater in the greenhouse, it is also critical to provide irrigation, ventilation, and a comfortable temperature range (Manoharan, 2020). Additional naturally existing resources like energy, sunlight, heat, and carbon dioxide (CO2 ) are consequently necessary to boost productivity, particularly during the colder winter months, and to permit year-round production in locations where it would otherwise be impractical. Plants are frequently grown in pots in hydroponics and placed on benches or held in place by a grid that covers the basins. There

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is enough space between the benches for employees to access and exit the system while it is in operation. The root systems of the plants change the living conditions for the microorganisms in the grow beds, which affects how well they perform in nitrogen removal activities for wastewater going through the biological treatment stage. According to IPCC (2013), the capacity of a biological process to reduce greenhouse gas emissions, such as nitrous oxide, is a requirement for sustainability in a municipal wastewater treatment system using plant growth systems. According to Shine et al. (2005), Over a 100-year period, this gas has a global warming potential of 265 times more than CO2 . During the biological nitrogen removal process from wastewater, which is carried out by ammonia-oxidizing bacteria and incomplete denitrification, nitrous oxides accumulate in municipal wastewater. According to research by Kampschreur et al. (2009), among the variables that affect the volume and unpredictability of nitrous oxide accumulation in a wastewater treatment plant are the system’s design and operational parameters. According to Hussain et al. (2022), these elements are to blame for the microbial community’s normal operation in the wastewater treatment system. A number of factors, including dissolved oxygen (DO), COD: N ratio, pH, and indirect parameters such as temperature, salinity, heavy metals, and other toxic compounds, are reported to be accountable for the optimal functioning of hydroponic wastewater systems due to their impact on metabolic pathways such as enzymes responsible for the process of nitrification and denitrification. To evaluate the impact of system layout on nitrous oxide emissions, three distinct secondary wastewater treatment system types (anoxic, aerobic, and anaerobic tanks) were used. The fluctuation was stated to have an effect on the ecology and physiology of the microbial community in the wastewater treatment facility (Narihiro & Sekiguchi, 2007). Similar nitrogen loss is reported to occur in hydroponic wastewater treatment systems via denitrification and nitrifier denitrification, with substantial N2 O emissions (0.6–2.0%). The most crucial characteristic in hydroponic wastewater systems is DO, which affects N2 O emissions and nitrogen loss. Through nitrifier denitrification, low DO levels in aerobic zones have been linked to an increase in N2 O production by ammonia-oxidizing bacteria (Luo et al., 2022). These findings imply that optimizing the facility’s aeration regimes will raise DO levels and be taken into consideration when designing a hydroponic system for treating municipal wastewater. Continuous aeration decreased the rate of nitrous oxide emission, according to earlier research conducted at both the laboratory and full-scale levels (Bhatia et al., 2004).

3.6.2 Hydroponic Crop Production For hydroponic crop production, open and closed drainage systems are employed (Fig. 3.2). However, due to their ability to prevent the flow of toxins into the environment, closed systems are typically preferred for wastewater treatment. This is one of the primary reasons why a hydroponic system is recognized as one of the

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most efficient and environmentally friendly solutions for reusing wastewater in agriculture (Sapkota et al., 2019). In closed hydroponic systems, the same fertilizer solution is continually circulated, and the nutrient concentration is measured and altered in response to the needs of the plants. In contrast, with open hydroponic systems, each irrigation cycle entails the introduction of a new fertilizer solution. The huge quantity of nutrients that are often released into the local environment by the drainage water from open hydroponic systems raises significant environmental concerns. According to research studies, water from drains cycling can help keep hydroponic wastewater affordable while also reducing the environmental impact of the agricultural and wastewater treatment industries (Pan et al., 2007). Recirculating drainage water reduced the amount of water needed to irrigate cucumber plants grown hydroponically by 33%. The drained water also had 59% nitrogen, 25% phosphorous, and 55% potassium more than the first treatment. In a similar vein, Christie (2014) investigated the impact of recycling the nutrient solution used in drainage from the conventional soilless culture in order to reduce environmental contamination. In his experiment, two nutrient systems one that can be restored and reused and the other that must be discarded after each use were employed to grow lettuce. In comparison to the control system, the recycled nutrient solution utilized about 42% less water, 23% less KH2 PO4 , 57% less KNO3 , 58% less MgSO4 , and 58% less micronutrients. These figures illustrated the possibility of nutrient recovery and cost-saving production through the recycling of drainage water. Free drain systems, on the other hand, caused a significant loss of water and nutrients, which is what causes ground and surface water to become contaminated (Tan et al., 2011). Results by Incrocci et al. (2006) demonstrated that when roses were grown utilizing substrate culture, open hydroponic systems indicated an annual loss of 2123 m3 /ha of water and 1477 kg/ha of nitrogen. However, the closed system was effective in lowering nutrient input (35%), as well as water utilization (21%). The productivity and quality of tomatoes cultivated in both closed and open systems did not significantly differ, according to a literature review by Incrocci et al. (2010). The ability of a closed hydroponic system to use a lower concentration of nutrients, allowing the system to cultivate without leaching, is what gives the system its nutrient use efficiency. Closed systems effectively use nutrients and water, but they also require constant pathogen, pH, and EC management. Because ion intake varies over time or between ion species (for example, differential phosphate and nitrate uptake), specific ion concentrations must be determined to maintain a proper nutritional medium. Wastewater hydroponic systems can be divided into two main categories: (a) Solution culture, which includes the NFT, passive sub-irrigation, the wick system, and continuous drip systems; (b) Media-filled systems, which include hydroponic plant growth systems, aeroponic plant growth systems, flood and drain systems, static culture systems, and anaerobic culture systems.

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Fig. 3.2 Schematic process of hydroponic wastewater treatment

3.6.3 Hydroponic Waste Water System Primary or secondary-treated wastewater from crop plants is pumped into a tank at the top of the NFT channels for gravity feed in a closed-loop system (Rababah et al., 2000). As they grow, the plant’s roots are submerged in the effluent solution that is gravity-fed into the NFT channel. The NFT system can provide passive oxygen transfer, which requires no direct energy for oxygenation and results in high levels of oxygen in plant roots (Guo et al., 2017). Because the grow bed cannot support a significant number of roots due to potential recirculating flow restriction, NFT is only suitable for crops with modest biomass production. In terms of clogging, the floating-raft type is preferred over NFT because it provides enough surface area for plant roots to freely absorb nutrients from the water without impeding the water channel. The plant production space is appropriately proportioned, and floating-raft hydroponics, which uses floating polystyrene/plastic mess/bamboo sheets for plant support, can provide adequate biofiltration. Due to the complexity of both systems for wastewater treatment in terms of clogging and nitrification, a sedimentation tank and a biofilter are often added as part of the treatment system for the removal of solids and nitrification, respectively (Kong et al., 2016). On the other hand, mediafilled systems are regarded as the most straightforward for wastewater treatment since they use growth medium (clay beads, stones, and rock wool) for nitrification without the requirement for separate biofilters. Using gravel and an ebb flow system, a removal effectiveness of 48% for nitrogen was also recorded (Gebeyehu et al., 2018). The contribution of the binding sides present in gravel and the development of biofilm on the surface of the gravel are credited with the ability of the gravel and ebb flow system to remove nitrogen. There were also substantial differences in the yield of the plants utilizing soil and soilless media (perlite) when gerbera

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plants were irrigated with wastewater effluents and regular fertilizer solutions (Vaillant et al., 2004). When compared to the regular nutrient solution, the yield was reduced by 44% when fertilized with wastewater effluents. Furthermore, compared to surface drip irrigation, subsurface drip irrigation dramatically increased flower production across all substrates and water sources. Similar to this, there were 10% and 14% decreases in the overall quantity and weight of marketable tomato fruits on plants that were watered with wastewater effluent and hydroponic fertilizer solution, respectively. Huber et al. (2009) conducted a large-scale experiment to compare the developed system to traditional rock wool for growing cucumbers, and the results revealed that the developed system’s plants grew at a significantly faster relative rate than plants grown in traditional Rockwool. The use of Datura innoxia plants in a commercial hydroponic system resulted in effluents that met legal discharge standards and had decreased levels of total suspended particles, biochemical oxygen demand, and chemical oxygen demand after 24 h of wastewater treatment. According to the scientists, when treated wastewater effluents were administered by drip irrigation for gerbera plants using both soil and soilless media, the number of marketable flowers was reduced by 21% when compared to irrigation with standard nutrient solution. There were no noticeable differences in the fruit output of single-cluster tomatoes when the ebb and flood hydroponic system was used instead of black paper mulch as a growth medium. These findings imply that the efficacy of wastewater hydroponic systems for supplying nutrients for plant growth varies depending on the type of growth medium and irrigation system employed; therefore, priority should be given to the substrate selection in order to prevent potential problems with the normal operation of hydroponic wastewater treatment systems in the future. Crop species have a significant part in the hydroponic system’s ability to treat municipal wastewater, not only by directly absorbing nutrients or encouraging microbial activity, but also by helping to ensure that this type of technology is adopted and used in urban settings. However, there is no approach accessible to choose plants that are ideal for hydroponic wastewater systems because of the uniqueness of this system. In the majority of studies looking at the use of wastewater in agriculture, vegetable or ornamental crops were used. Due to their quick development cycles, vegetables including tomatoes, cucumbers, and lettuce are frequently utilized for soilless production. This allows for better control and standardization of the agricultural process. Local availability and products that meet consumer expectations are also chosen from a market perspective. Some of the factors utilized for crop selection include crop quality qualities like nutritional aspects, flavour, colour, shelf life, and disease resistance (Brummell & Harpster, 2001). Although using treated wastewater to grow vegetables in hydroponic systems has been shown to have many advantages, there have also been reports of decreased shelf life and crop quality, which can restrict the adoption of these technologies. According to recent research in commercial agricultural production, wastewater mixed with hydroponic technology significantly lowered the risks of fresh products being contaminated by microbes. The decrease can be a result of less irrigation water coming into contact with the plant’s edible sections. In a small-scale soilless technique, lettuce grown with diluted

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secondary UV-treated grey water effluent demonstrated an acceptable level of E. coli (1000/100 mL) on plucked leaves, but severe infections on roots.

3.7 Advantages and Disadvantages of Hydroponics Prior studies have shown that hydroponic farming has a number of advantages over other traditional agricultural systems (Fig. 3.3). In general, hydroponic farming encourages ecologically benign practices and outperforms conventional open-field farming in terms of commercial food production. Producing high-quality crops is one of its benefits. Soilless crops are uniform in size, texture, clean, good tasting, and devoid of pesticide residue. According to research findings, consumers were more willing to pay more for “hydroponic tomatoes” grown in greenhouses as opposed to “open-field” tomatoes because they were pesticide-free. Crops produced hydroponically have a higher mineral content than plants grown in soil [59]. According to Liu and Xu (2018), hydroponics is a CSA system that is independent of the weather. Farmers discovered that hydroponic crop production gives the possibility of year-round crop production because it is not dependent on rainy seasons and is not discouraged by the presence of drought circumstances. Furthermore, hydroponics enables vertical farming, which produces high harvests in tiny spaces or in places with infertile soils, as opposed to the traditional agricultural technique, which required farmers to cultivate vast tracts of fertile land in order to produce large harvests. Due to the lack of available arable land, it is an excellent urban farming technique. According to Gharooni et al. (2014), hydroponics allows for the growth of crops in non-arable locations. Vertical farming, which enhances crop yield per unit area through vertical crop cultivation methods, can be used to apply this farming system in locations with non-fertile soils. Another benefit of adopting hydroponics is that, in contrast to soil farming, there are no pests or diseases that are transmitted through the soil. Hydroponics’ controlled environment, the absence of soil in cultivation, and the utilizing insect traps in both indoor and outdoor systems help reduce the demand for pesticides by deterring pests like white flies. One advantage of hydroponics farming is that pest and disease occurrence is constrained. This offers a singular opportunity for seed production in a controlled environment with few pests and diseases. Some farmers who were growing vegetables in fully automated greenhouses said they had control over the environment of the plants by keeping an eye on factors like pH, temperature, electrical conductivity (EC), and humidity. The climatic conditions in the greenhouse environment can be managed with hydroponic farming. Other benefits of hydroponic farming include the absence of weeding, the ability to earn money from selling produce and mentoring new farmers, the availability of fresh produce, the need for little care during growth, and the ability to produce extra food for domestic consumption.

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Fig. 3.3 Advantages and disadvantages of hydroponic system

The largest complaint about hydroponic farming is the hefty investment expenditures needed to put up this sophisticated technology, particularly for fully automated greenhouse farms (Kaloxylos et al., 2012). These expenses include building a greenhouse, fertilizer charges, energy installation fees, hydroponics equipment including PVC pipes and net cups, as well as systems for monitoring the climate. Energy costs are a major consideration when artificial lighting, such as Light Emitting Diodes (LED) lights, is regarded as important for stable output (Mubaid et al., 2019). One of the things that drives up the cost of hydroponics is its reliance on power (Bhandari et al., 2015). Another drawback of hydroponic farming is the need for sufficient technical knowledge, which keeps farmers from utilizing the technique. For instance, understanding the precise quantity of nutrients needed for a specific crop, how to combine them, in what proportions, and recycling. The majority of respondents claimed to have learned about hydroponic farming online, which is consistent with the high percentage of educated research participants. The need for technical knowledge for hydroponics, such as: maintaining pH and EC (electrical conductivity), is one of the challenges of hydroponics. A problem with maintaining the EC, pH, and temperature of the nutrient solution, as well as crop damage in the event of system failure, was noted by farm operators who used high-end technology such as climate control systems. According to Specht et al. (2014), hydroponics can fail or be improperly managed and harm crops. They also stated that hydroponics is not sustainable if improperly managed. The implementation of the farming technique is further hampered by the lack of suitable ideas or breakthroughs regarding the use of alternative, locally accessible materials for hydroponic farming. As an illustration, consider using buckets or bottles instead of PVC pipes to cultivate hydroponic crops. Another issue with hydroponics is the lack of sufficient selection of organic fertilisers for hydroponics in agricultural supply stores. The lack of a wide range

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of organic fertiliser substitutes, bias from the community towards hydroponic food, and the system’s necessity for the promptness to prevent crop or system failure are additional difficulties faced by hydroponics.

3.8 Indian Hydroponics Market Size In India, hydroponic farming is only beginning to take off. India’s farming practises are essentially conventional. As a result, there is a limited market for this form of farming in urban centers. The forward-thinking farmers in India today are using this creative and unique farming method. India has a very broad application for hydroponics due to the country’s indiscriminate population growth and decreasing quantity of arable land. Because of this, it is challenging to produce enough staple foods to feed an expanding population. Farmers can soon find a solution to the issue of arable land availability by utilizing hydroponics techniques. This could be the beginning of a new green revolution. In addition, a major advantage of this growth of the Indian agricultural system is that it will lighten the burden on farmers. Hydroponic farms use less land and water, and the crops develop much more quickly than in conventional farms. There won’t be any hunger due to the abundance of food produced via hydroponic gardening. The use of hydroponic farming will lessen weed growth and pest and disease attack. As a result, fewer pesticides and herbicides will be used. As a result, there won’t be much environmental damage or cultivation expense. Agriculture in India is being impacted by climate change factors such as hailstorms and unseasonal rain. In contrast, a hydroponic farming system uses an artificial atmosphere to allow plants to grow under controlled conditions.

3.8.1 India’s Hydroponics Industry In India, hydroponics was first used by an English scientist named William Frederick Gericke. In the Kalim pong region of West Bengal, he founded a laboratory with J. Shalto Duglas. Hydroponics Bengal System is the title of another book he has authored on the subject. When he arrived back in England, the work came to an end. Genuine hydroponic farming in India was first offered as the “Pet Bharo project” in 2009 by the Institute of Simplified Hydroponics, which is located in Bangalore. This organization is a division of Optimus Interweave, which is based in Australia. Principally responsible for the project’s overall concept is Mr. C.V. Prakash. Growers of vegetables, fruits, and herbs in urban and rural areas of India are the target audience for this project, which aims to disseminate soilless farming methods that are both inexpensive and easy to master. They offer products for establishing hydroponics, consulting services, and training. They have trained numerous faculty members from various Indian institutions of agriculture and horticulture, including:

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3.8.2 Senior Professionals and Specialists The four Indian firms listed below are setting the standard for indoor farming by implementing cutting-edge agricultural practices.

3.8.2.1

Upcoming Farms

Future Farms, a company based in Chennai, has created practical and affordable farming kits to support hydroponic systems that protect the environment while producing cleaner, fresher, and healthier food. It emphasizes using precision agriculture and rooftop farming to be environmentally sustainable. The company creates domestic systems and solutions that are effective and inexpensive, constructed from high-quality, food-grade materials.

3.8.2.2

Agritech Letcetra

The first indoor hydroponic farm in Goa, Letcetra Agritech produces high-quality, pesticide-free veggies. In its 150 square feet, the farm in Goa’s Mapusa currently produces approximately 1.5–2 tonnes of leafy vegetables, including various types of lettuce and herbs. square meters. This firm was created by hydroponic farmer turned software engineer Ajay Naik.

3.8.2.3

Inventions from Bit-Mantis

With its IoT solution Green-Sage, Bengaluru-based Bit-Mantis Innovation, an IoT and data analytics firm, enables both private individuals and commercial herb producers to produce fresh herbs all year long. A micro-edition kit called The Green Sage makes effective use of water and nutrients by using hydroponic techniques. Two trays are included so that microgreens can be grown at the user’s convenience.

3.8.2.4

Junga Green and Fresh

Infra Co Asia Development Pte and Junga Fresh N Green, an agricultural technology firm, have partnered. Ltd. (IAD) to create techniques for hydroponic farming in India. The development of a 9.3-hectare hydroponic-based agriculture facility at Junga in Himachal Pradesh’s Shimla district served as the project’s launchpad. In order to establish cutting-edge farms in India, Junga Fresh N Green has partnered with Westlands Project Combinatie (WPC BV), a leading Dutch agricultural technology business. The objective is to develop a hydroponic farming system that produces

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farm-fresh vegetables with consistent quality, minimal to no pesticide use, and that are neither affected by soil nor by weather conditions. In addition to the projects mentioned above, numerous additional people are engaged in automated hydroponic work. Such concepts are also funded by the Indian government.

3.9 Indian Prospects of Hydroponics for the Future The traditional farming system currently and possibly cannot supply the world’s food needs. So, it is the need of the hour to find out a new agricultural technique that promotes plants to grow more quickly and sustainably. Additionally, the overuse of herbicides and insecticides has made the ground less fertile, which has forced farmers to switch to soilless farming. Rivers provide the majority of the water used for agriculture, yet as industrialization advances, toxic waste is thrown into these rivers. As a result, they get contaminated with heavy metals and other contaminants, making it impossible to use the water for conventional farming. One will therefore need to switch to hydroponics, which uses 80–90% less water. The Indian government is actively encouraging the use of hydroponic farming, which has led to a surge in its popularity. The government has funded R&D into the creation of new hydroponic farming methods and tools. Automated systems and climate controllers are being developed to make farming more productive and less expensive. Farmers who are interested in implementing hydroponic farming methods can also apply for government incentives and grants. Hydroponic gardening is also gaining popularity in India. Hydroponic farming is gaining popularity as word spreads about its environmental friendliness, increased crop yields, and higher-quality produce. Hydroponic farming is becoming more popular and easier to implement thanks to the proliferation of new instruments and technologies.

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Chapter 4

Applications of Hydroponic Systems in Phytoremediation of Wastewater Sayon Mukherjee, Sabyasachi Koley, Dibyajyoti Panda, Gorantla Prathap Reddy, Biswajit Pramanik , and Sandip Debnath

Abstract Demands from the agricultural sector to provide the rising population’s needs for fresh produce, together with population growth and urbanization, have severely strained the world’s natural water resources. Water is thus no doubt one of the most important resources in the modern era. Aquatic weeds which are difficult to completely eradicate, are typically considered as a global threat to both humans and the aquatic ecosystem. Nevertheless, several studies and research have demonstrated their importance in the field of wastewater phytoremediation, whether in created wetlands, open ponds, or hydroponic systems in a cost-effective and environmentally acceptable manner with little to no sludge waste. Phytoremediation is the employment of plants to clean up contaminants or lower their bioavailability and using this technique to treat wastewater is referred to as wastewater phytoremediation. Plant body parts come in direct contact with contaminants are considered as the active surface area for phytoremediation. Hydroponics, a soil less method of growing plants in vertical farming, has a promising and economical role in phytoremediation of wastewater and heavy metal by the modest space requirements. Most of the hydroponic studies only feature a single plant species, either rooted or free floating. Grasses having rapid growth rate and large root biomass are preferred. Though foliar surface, root and shoot system have their own mechanism in reducing the contaminants, a key approach of improving the effectiveness is to increase their surface area which in turn provide additional areas for microbes to grow, absorb and take up nutrients. In addition to its use in organic food and ornamental plant culture, the value of hydroponically produced plants in toxicological investigations is firmly established. An S. Mukherjee · S. Koley · D. Panda Department of Soil Science and Agricultural Chemistry, Institute of Agriculture Sciences, Banaras Hindu University, Uttar Pradesh, Varanasi, India G. P. Reddy Department of Agronomy, Institute of Agriculture Sciences, Banaras Hindu University, Uttar Pradesh, Varanasi, India B. Pramanik · S. Debnath (B) Department of Genetics and Plant Breeding, Palli Siksha Bhavana (Institute of Agriculture), Visva-Bharati, Sriniketan, West Bengal, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_4

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alternative sustainable solar energy source or devices can be employed to power the hydroponic systems in order to get good results at a reasonable price. Keywords Agriculture · Heavy Metals · Hydroponics · Nutrients · Phytoremediation · Wastewater Management

4.1 Introduction As far as 8 billion individuals worldwide are thought to exist on scarce supply of water each year, and it is believed that roughly 500 million people live in areas with a shortage of water round the year (Mekonnen & Hoekstra, 2016). Therefore, it is a global duty to save water while creating various tactics for overcoming the problems associated with the lack of it. For the reason of the way they build up in biological entity, metals are immensely harmful to aquatic and terrestrial life due to their extreme durability. The last couple of decades witnessed several detrimental effects of metals on both plants and animals (del Carmen et al., 2022). Metals are cleansed from wastewater discharges using a variety of techniques, including sorption, chemical precipitation, ex-osmosis, ion exchange, electrocoagulation, as well as biological treatment, although having a bunch of setbacks such as less profit, use of meticulous resources, metal specificity and production of numerous secondary debris (Mishra & Tripathi, 2008). Following this context, recuperating water via biological processes i.e., phytoremediation has been recognized to be a viable method of supplementing freshwater. The treated effluent from household sewage disposal facilities is safe for home and agricultural applications, and it is frequently noticed not to be an economic but also a promising approach to boost the supply of fresh water. Phytoremediation has emerged as an innovative strategy which utilizes the capability of specific kinds of vegetation, such as aquatic plant species, in order to eliminate, decay, metabolize, or even restrain a broad spectrum of toxins from the terrestrial or aquatic environments (Purakayastha & Chhonkar, 2010). Underwater weeds are often regarded as an ever-present hazard to human being and aquatic habitats due to difficulty in complete eradication. Nonetheless, numerous studies and research investigations have demonstrated whether they can be useful in the realm of wastewater phytoremediation, especially in hydroponic systems with an ecologically sound and inexpensive way. On account of fast development, excellent biomass production, substantial adaptability, high accumulative capacity, aquatic macrophytes pose a significant importance with regard to wastewater phytoremediation (Lu et al., 2011). Superfluous contaminants identified in city waste and industrial wastewater are revealed to be absorbed by aquatic vegetation at the secondary or tertiary levels of water purification process. As a result, treated wastewater meets International Standard Requirements (ISR) with a wide range of utilities. Although the prospective advantages of this method in sewage water reclamation, phytoremediation is restricted to the superficial region to root depth of plants, and it is unable to avert pollutant leaking and ultimately entering groundwater using phyto-based rehabilitation schemes. The

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hazardous compounds from polluted soil have an impact on the endurance of plants. Phytoremediation strategies employing plants are gaining popularity in recent times and are regarded as efficient techniques of sterilizing water and soil toxins. Multiple species of plants have shown considerable capacity for phytoremediation. Various monocots, dicots, as well as grasses and trees fall under this category (Raju et al., 2010). Global development and modern scientific technologies are not only solving the current issues but also raising some immense challenges for future. Anthropological and industry-sourced waste water is becoming very much difficult to manage due to the concern regarding environmental protection and conservation of biodiversity. These waste water carry heavy metals in suspended and dissolved forms (Li et al., 2023). In the modern era, rising anthropogenic activities had increased heavy metals concentration several folds than that of the natural (Zou et al., 2022). Heavy metals pose a potential threat to numerous ecosystems due to a number of features such as non-biodegradability, persistency and ecotoxicity (Hu et al., 2016). Their concentration in waste waters is also a budding threat for not only the underwater ecosystem but also for human health (Jha et al., 2023). This non-biodegradability results in immobilization of these metals, which ultimately can reduce their threats towards the environment. The current physicochemical methods used for remediation comes with a series of adversities like high expenses, huge time consumption along with environmental hazards. So, phytoremediation technique is a major emerging technology in the direction of immobilizing these hazardous elements (Jadia & Fulekar, 2009).

4.2 Hydroponic Phytoremediation Systems and Treatments of Wastewater Hydroponics is a method of plant cultivation which does not necessitate soil (Aires, 2018). Under the framework of biological processing of wastewater, seedlings are grown in wastewater loaded with nutrients in hydroponics. This method has been recognized as a crucial phase in phytoremediation of wastewater, involving plant roots for the elimination of surplus nutrients (Norström et al., 2004). As a consequence, the use of such structures supports the use of native ecosystems for handling of wastewater and subsequent bioremediation techniques. Hydroponic farming systems might be classified depending on nutrient supply to root zone of plants (Hussain et al., 2014). Furthermore, they ought to be categorized in two distinct types: hydroponic systems devoid of substrate use, like raft systems and nutrient film technique (NFT), and another employing substrate, assuring root anchorage, which creates an environment for the association of microorganisms (Magwaza et al., 2020). Ignatius et al. (2014) employed Mexican mint through NFT hydroponic set up to investigate potential rhizo-filtration of lead (Pb) in multiple dosages. That particular investigation revealed that plants can acquire Pb in their roots instead of transferring

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it onto their leaves and foliage. Metal poisoning in hydroponic cultivation systems is currently being assessed implementing Cd, Cu, Ni, As, Fe, and Se (Garousi et al., 2016), exhibiting sorption of these components, as well as the efficacy of phytoremediation in ornamental and medicinal plants. On the other hand, Chanu and Gupta (2016) reported that water spinach not only possesses the ability to separate lead from wastewater but also to store this inside its root system. Findings showed that elimination got quicker in roots owing to outer layer adsorption, which includes mechanisms like quelation, ion exchange, and selective absorption (Cifuentes-Torres et al., 2021). The physiological aspect of the elimination mechanism comprises intracellular ingest, vascular deposition, along with sprout translocation onto shoots (Kumar et al., 2017). Phytoremediation entails the environmentally friendly and effective usage of green vegetation to uptake/absorb contaminants using roots alongside translocate towards the apical portions of the foliage. Phyto refers to plant while remedium represents to clean (Sharma et al., 2015). The plant species utilized in phytoremediation ought to be probably native, possess rapid growth, possess a developed rooting system, accumulate huge biomass, be able to accustomed to different ecosystems, exhibit an elevated degree of tolerance, as well as capable to retain toxins in their above-ground portions (Rezania et al., 2016). However, Parmar and Singh (2015) demonstrated that the lengthy removal process is a drawback of phytoremediation that might be overcome by combining a number of different phytoremediation approaches.

4.3 The Historic Journey of Hydroponics Hydroponics represents a means of production that has been used by various civilizations for a long time. Vegetation in hillsides had previously been nourished using water flow of Euphrates River in Babylon (605–562 BC). In Tenochtitlan, a technique known as “chinampas” employed by the Aztecs close to 40 AD. These “chinampas” are artificial islands that float above water and have roots that are in close proximity with the seawater (González & Valladares, 2014). Chinampas generate 40,000 t yr−1 of flowering plants and vegetables at current rates (Arano, 2007), and because of their cultural importance, FAO designated them as a Globally Important Agricultural Heritage System (FAO, 2017). Since 1984, programmes funded by the FAO have supported the exploration of hydroponics technology in Colombia and its introduction in 13 other nations. The first study on the application of hydroponics as a substitute method for removing nutrients from sewage had been published in 1993 (Furukawa & Fujita, 1993).

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4.4 Aquatic Phytoremediation Communities An approach based on plant communities offers the chance to improve the extraction of both singular and composite contaminants. Results from the studies on phytoremediation using plant communities were encouraging (Turker et al., 2016). According to earlier researches, plant polycultures can also lower biochemical oxygen demand (BOD) and have a greater capacity to extract heavy metallic elements. However, Turker et al. (2016) found that native emergent monocultures were more effective at removing boron (B) from mine effluent than polycultures, while nitrogen dioxide (NO2 ) extraction followed the contradictory pathway. According to abovementioned findings, proper combinations of plants were required for specific pollutants/contaminants, and further investigation aimed at finding these combinations would be beneficial for maximizing phytoremediation’s effectiveness. Diverse functional roles of the community are one of significant factors to take into account when assembling appropriate plant combinations. Higher speciation diversity in a plant community has been shown to enhance nutrient extraction, even though polycultures implementing four or more species didn’t exhibit any extra advantages (Geng et al., 2017). Although the significance of identification of particular species in explaining nutrients extractive variation is a recurring theme in these studies, particular plant species admixture can be more effective for the same. Consequently, building a suitable combination of plant community depends upon the selection of individual plant with distinctive potential in creating complementarity in boosting species exuberance. Notably, the factor of plant competition needs to be prioritized also in this regard (Zhang et al., 2007). Enhanced bioaccumulating and degrading ability, absorbability towards contaminants in problematic environment, higher potential to produce adequate biomass enduring pollution are some pre-requisites for aquatic phytoremediation. Three distinct kinds of plants i.e., free-floating, emergent, and underwater (submerged), have been employed for aquatic phytoremediation. On the water’s surface, freefloating plants are visible. Plants submerged in water continue to be submerged. Emergent plants while growing above the water surface, plants are rooted underwater. The plant genera used in aquatic phytoremediation are listed in the Table 4.1 (Dhir, 2013). The cleanup of contaminants from surface level water and repair of impaired water bodies such as streams, rivers, ponds, and lakes are accomplished through the use of aquatic phytoremediation, a phytotechnology. Although aquatic plants are grown to get rid of contaminants from both surface water column as well as sediment (Newete & Byrne, 2016); they may also be positioned close to a small source or in waters where diffuse pollution is problematic (Lu et al., 2011). The pollutant removal and degradation procedures through aquatic phytoremediation specifically employ several aquatic plants (i.e., pteridophytes, freshwater adopted angiosperms, and ferns) (Rai, 2009).

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Table 4.1 List of various plant genera used in aquatic phytoremediation Types of vegetation

Genus

Free floating

Remediated contaminants Metals

Organic pollutants

Radionuclides

Lemna

As, Ni, Cd

TCP, Phenols

Tc-99, La-140, – Co-60

Spirodela

Cr, As

p,p’-DDT, o,p’-DDT, – Chlorobenzene

Eichhornia

Cu, Cr, Ethion, Zn, Cd, Pentachlorophenol, Arsenate, Dicofol, Cyhalothrin Arsenite, Ni, Hg

–

Azolla

Pb, Cu, Cd, Zn, Hg, Ni, As, Cr

–

Co-60, Cs-137 –

Pistia

Cu, Cr, Hg

–

–

Submerged Ceratophyllum Pb, Ni, Chlorobenzenes, Arsenite, Organochlorine, Arsenate, Organophosphates Cr

Emergent

Explosives

– –

–

Co-60, RDX, TNT Cs-137, Sr-89, P-32, Cs-134

Potamogeton

Cu, Pb, Cd, Mn, Zn

Phenols

Cs-137, U-238, Sr-90

RDX, TNT

Myriophyllum

Cu, Ni, Co, Zn

Perchlorate Simazine, p,p-2 DDT, o,p-2 DDT, HCA

Hydrilla

As, Cu

–

Elodea

Hg

Chlorobenzenes, Am-241, Phenanthracene, Cs-137, Sr-90 Organochlorine, Organophosphates, Carbon tetrachloride, DDT, HCA

HMX, RDX

Typha

Cu, Ni, As, Zn

–

–

RDX, TNT

Phragmites

Hg, As

–

–

TNT

Scirpus

Hg, As

Phenanthracene

–

RDX, TNT

Spartina

Cu, Pb, As, Hg, Zn, Cr, Al, Fe, Se, Cd

–

–

Phalaris

–

–

–

RDX, TNT, HMX –

–

RDX, TNT (continued)

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Table 4.1 (continued) Types of vegetation

Genus

Remediated contaminants Metals

Organic pollutants

Radionuclides

Explosives

Sagittaria

–

–

–

RDX, TNT

Pontederia

–

Oryzalin

–

a

b

c

d

e

f

Fig. 4.1 Photograph examples of Lemna (A), Nymphaea (B), and Hydrocharis (C), Ceratophyllum (D) and Phragmites (E) and Typha (F)

The three primary growth forms of aquatic plants are floating, submerged, and emergent. Water surface is occupied by floating aquatic plants, which can be freefloating or rooted and belong to genera like Lemna (duckweed) (Fig. 4.1), Nymphaea (water lilies) (Fig. 4.1, and Hydrocharis (frogbit) (Fig. 4.1). Heavy metals are actively transported in free-floating aquatic plants through the roots, where they are transferred to various sections of the plant body. The direct contact of the plant body with the pollutant medium is connected with passive transfer. Heavy metals accumulate mostly in the higher portions of the plant body during passive transport. Duckweed is a free-floating aquatic plant that floats on the surface of motionless, slow-moving water. This plant is in the Araceae family, however it is commonly placed in the subfamily Lemnoideae. This family of free-floating plant species includes five genera, including (1) Wolffia, (2) Wolffiella, (3) Spirodela, (4) Lemna, and (5) Landoltia, with over 40 species recognised. Duckweed plants are effective in phytoremediation due to their ability to develop in polluted sites with varied pH, temperature, and nutrient levels. They can eliminate heavy metals, contaminants, pesticides, and nutrients from agricultural runoff, sewage, and industrial wastewater.

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Duckweed also inhibits algae and fungi growth, diminishing nitrogen by taking up ammonia and denitrification and improving water quality. Despite submerged aquatic plants having no roots, Ceratophyllum (hornwort) (Fig. 4.1) grows and develops primarily beneath the surface of water and eventually gets attached to substrate present. The cell wall’s polyglalacturonic acid and negatively charged cutin and pectin polymers of cuticle cause mineral sucking inward. Because of this inwardly increased charged density, positive metal ions move. They possess removal of heavy metals from water and sediments. Vallisneria spiralis and water mint are well known for their ability to accumulate Zn, Cr, Fe, Cu, Cd, Ni, Hg and Pb. Emergent aquatic plants, such as Phragmites or common reed (Fig. 4.1), and Typha or reedmace (Fig. 4.1), grow significantly above the water surface while still occupying the margins of water bodies. These plants are often found on submerged soil with a water table less than 0.5 m below the earth. The accumulation of HMs in emergent plants differs by plant; some have the ability to bio-concentrate most of the metals in below ground-level roots from water and sediments, while others disperse the burden of metals in aerial portions. The roots of common reed (Phragmites australis) bear the majority of the heavy metal burden. Heavy metal sequestration and detoxification occur at the cellular level in these plants. Depending on the application procedures, these various growth forms help remove pollutants from the sediment as well as the water column (Newete & Byrne, 2016). Aquatic plants by absorbing macro- and micro-nutrients required for their overall development from the growth culture can lower the level of pollutants in a targeted water body significantly (Dhote & Dixit, 2009). In contrast to submerged aquatic plants, which can also use their stem tissue as a major water column removal pathway, emerging and floating aquatic plants take use of their roots for the same (Dhote & Dixit, 2009). According to McAndrew et al. (2016), the performances of the aquatic vegetations and mechanisms related to pollutants elimination vary from each other depending on the type and site of contaminant deposition.

4.4.1 Aquatic Phytoremediation of Macronutrients The available form of nitrogen (N) for aquatic plants is reportedly ammonium (NH4+ ) and nitrate (NO3− ), and the same for phosphorus (P) is phosphate (PO3− ). The higher biomass accumulating as well as growing ability of aquatic plants makes them extremely efficient phytoremediators (Kennen & Kirkwood, 2015). As instances, some floating aquatic plants such as Eichhornia crassipes (Fig. 4.2) and Lemna gibba reveal excellent potential to extract nutrients from water bodies. The former species can extract up to 81% NH4 + and 92% NO3 − from water, while the releasing ability of the later is little bit higher for NH4 + and NO3 − i.e., 82% and 100%, respectively, although both are equally competitive to remove total P. In this same parameter, submerged plants can be placed at a slightly lower position when compared to the floating one. for example, Cyperus (Fig. 4.2) and Canna (Fig. 4.2) possess some of the

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a

b

c

d

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Fig. 4.2 Photograph of Eichhornia crassipes (A), Cyperus sps (B), Canna sps(C) and Myriophyllum aquaticum (D)

highest NH4+ removal efficiencies, ranging from 100 to 74% respectively, proving submerging species to be significantly attentive for phytoremediation. According to Du et al. (2017), this may be due to the challenges involved in cultivating and harvesting submerged aquatic plants as well as the potential lower biomass produced. Potential targets for N and P elimination with a rate of > 41% include Ceratophyllum demersum and Myriophyllum aquaticum (Fig. 4.2) (Guo et al., 2014). The majority of submerged aquatic plant species have the ability to absorb nutrients through their leaves as their roots are emerged in the sediment portion. Subsequently, this provides those plants the opportunity to extract nutrients both from water as well as sediments. Given the multitude of factors that could impact aquatic plants’ ability to remove pollutants, it is wise to understand the effectiveness of several major macrophyte accumulators in different environmental conditions as a way to optimize selection of species.

4.4.2 Aquatic Phytoremediation of Metal Micronutrients Hyperaccumulators are best suited for aquatic phytoremediation of metallic elements, and aquatic vegetation can also remove micronutrients from water and sediments (Ali et al., 2013; Rai, 2009). Since many aquatic plant species have the potential to uptake various kinds of metallic elements, some specific aquatic plant species may

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be highly effective in aquatic phytoremediation. Aquatic plants, especially the freefloating one, have often been mentioned as hyperaccumulators of biomass. According to reports, L. gibba can concentrate Cd up to 14,000 mg/kg of the dry weight, while E. crassipes has the potential to concentrate Zn up to 10,000 mg/kg. The main factor preventing aquatic plants from absorbing metals is the exorbitant toxicity exhibited by the targeted metal pollutants (Landesman et al., 2011). The fibrous rooting system of aquatic species can purge heavy metals to a very high extent because of greater surface area coverage, as stated by Newete and Byrne (2016). Several abiotic elements like light, temperature, salinity, and pH have also been reported to affect phytoremediation activity, making both physical and chemical factors significant for metal absorption and accumulation (Rai, 2009).

4.4.3 Aquatic Phytoremediation of Organic Pollutants Hazardous chemicals known as organic pollutants (OPs) endanger both human health and the planet’s ecosystems. Some OPs accumulate and grow more potent in living things as they move up the food chain. They are not only widely dispersed throughout the entire environment, but also anthropologically toxic. OPs can be removed using aquatic phytoremediation. For example, Spirodela oligorrhiza (Fig. 4.3), a potent phytoremediator aquatic species, can extract two different isomers of DDT (viz. p,p -DDT and o,p -DDT) at 50% and 66%, respectively. On the other hand, the same for Myriophyllumaquaticum was accordingly estimated as 82% and 76% (Gao et al., 2000). In addition, another plant species Elodea canadensis (Fig. 4.3) can eliminate maximum of 89% p,p–DDT compound from water (Garrison et al., 2000). Due to their ability to oxygenate water due to the presence of rhizome along with extensive rooting system, Typha latifolia can be marked as an advantageous species for the aforesaid (Makvana & Sharma, 2013). The third world countries, in recent times, are witnessing several lab- and field-based approaches for OP removal through aquatic plants due to lower expenditure and dynamic variability.

a

b

Fig. 4.3 Photograph of Spirodela polyrrhiza (A), and Elodea canadensis (B)

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4.4.4 Aquatic Phytoremediation of Microbial Pollutants The chemical and biological effects of plant physiological activities play pivotal roles in remediation. For instance, Pistia stratiotes (Fig. 4.4) encourages the protozoan existence by facilitating their structural habitat, which ultimately increases Salmonella predacity on the host (Awuah, 2006). Awuah and Fosu (2014) highlighted that extraction mechanisms are likely to be associated with the underground morphology like spreading rhizome and root system, which could provide better habitat for the microbes. On the other hand, protozoan predation seemed to have smaller impact in planted systems with Spirodela polyrhiza (Fig. 4.4). Greater microbial biofilm growth is made possible by an expanded root zone surface area. It is reportedly both a crucial extraction structure for the bacterial adsorption as well as microbial growth. Small grass species like Festuca arundinacea (Fig. 4.4) can eliminate a very small number of microbial pollutants than that of the larger emergent plants like Typha latifolia. Hence, further studies are definitely required to determine the type of aquatic species, microbial contaminants, and wetlands.

4.5 Removal of Pesticides and Toxic Substances Through Phytoremediation Technique: Pesticides are the poisonous chemicals used in agricultural practices to eliminate crop pests including insects, rodents, weeds, pathogenic fungus, etc. Massive consumption may cause bioaccumulation and magnification of those compounds in water bodies, soil as well as in animal or human bodies (Mohany et al., 2011). Surface water infused with pesticide residues is the major source of worry since they can endanger the aquatic ecosystem, including humans at a huge aspect. Pesticides of notably two groups, namely organochlorines and organophosphates, follows run-off to infiltrate freshwater bodies (Karunya & Saranraj, 2014). On the basis of subjects on which pesticides will work, it can be insecticide (act against insects), fungicide against fungal diseases, acaricides for controlling ticks and mites, nematicides for

a

b

c

Fig. 4.4 Photograph of Pistia stratiotes (A), Spirodela polyrhiza (B) and Festuca arundinacea(C)

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nematodes, molluscicides and rodenticides against molluscs and rodents, respectively (Rani et al., 2017). Based on the chemical conformations, it can also be categorized into organochlorines, organophosphates, carbamates, and substituted urea, among which the first one can be considered as the most dangerous one from the point of environmental concern. Low concentration pesticides are present all throughout the world, according to a group of experts (Moschet et al., 2014), although a variety of cleanup measures may be implemented to reduce the pesticide concentration below threshold level in order to protect the environment along with human health. Innumerable approaches have been followed from physical, biological and chemical approaches for eliminating pesticide residuum (Li et al., 2010; Rani et al., 2017). Further to mention, the biological therapy is the most recommended one because of its lower expense than the rest.

4.5.1 Phytoremediation: A Novel Technique The plant-sourced decontamination technologies used for depleting the adverse effects of pollutants through employing a diverse range of natural or transgenic plants are generally termed as ‘phytoremediation’. This cost-effective eco-restoration technique, is actually an alternative to other damage-prone engineering methodologies for the soil. This technique basically entails the production of potent pesticide or metal accumulator genetically engineered plants with the ability to absorb the toxic chemical through various metabolic activities in order to repair the damaged sites. These methods are widespread throughout various habitats as well as pollutants, which include numerous in situ removal, stabilization or degradative measures (Table 4.2).

4.5.1.1

Phytostabilization

This is actually an in-situ deactivating procedure, which remediate the soil, precipitates, and sludges. The word ‘stabilization’ here means the restriction of mobility of the contaminants in the rhizospheric region of plants using root system. This procedure is actually designed aiming to limit the pollutants around the rhizosphere. Several plants along with their symbionts are responsible for this stabilization. As instance, some metal-enduring grasses like Agrostis tenuis (Fig. 4.5) cv. Goginan Table 4.2 Comparison of various technologies used in phytoremediation Technologies used

Reaction on contaminants

Contaminants types

Vegetations

Phyto-stabilization

In situ retention

Organometals

Continual coverage

Phyto-degradation

In situ attenuation

Organics

Maintained coverage

Phyto-volatilization

Removal

Organometals

Preserved coverage

Phyto-extraction

Removal

Metals

Repeated harvesting

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a

103

b

Fig. 4.5 Photograph of Agrostis tenuis(A) and Festuca rubra (B)

and cv. Parys, and Festuca rubra (Fig. 4.5) cv. Merlin are involved in maintaining the stabilization parameters in mine tailing (Kaushik, 2015). Furthermore, arbuscularmycorrhizal fungi or shortly, AMF, which are the symbiotic fungal association of more than 80% terrestrial plants, also sequestrate these toxic metallic compounds through glomalin present in the hyphal secretion. Thus, this fungal association can be considered as a potent source of phyto-stabilization. Certain plants with chelate forming ability in their root zone are able to reduce the toxicity or availability of the contaminants by forming metallic complexes like several organic acids, phenolic compounds, siderophores, and many more. Further to mention, the plants in the wet areas produce such enzymes which can bring about some conformational change in heavy metals i.e., the trivalent form of chromium is transformed into the pentavalent one [Cr (III)→Cr(V)], which is equivalently less toxic. Also, plant-associated bacteria generate some biosurfactant in order to increase contaminant absorption. In brief, the benefits of this technique include the elimination of the requirement for hazardous material/biomass disposal, and its effectiveness when quick immobilization is necessary to safeguard ground and surface waters.

4.5.1.2

Phytofiltration

Pollutants can get stabilized in both naturally occurring as well as man-made wetlands via phytofiltration. This is primarily accomplished by rhizofiltration, in which metals are precipitated inside the rhizosphere. Rhizofiltration is typically utilized to purify diverse aquatic domains (Ensley, 2000). Oxygen elusion through the aerenchymatous tissues of the roots of marshland plants causes metal-plaques, into which several metals and metal oxides gets precipitated. These plaques on roots function as the key accumulator for active Fe2+ ions, increasing plant tolerance to other hazardous metals. In a study comprising of sunflower (Helianthus annuus), Indian mustard (Brassica juncea), tobacco (Nicotiana tabacum), rye (Secale cereale), spinach (Spinacia oleracea), and maize (Zea mays) suggested that sunflower possessed the optimum potential to eliminate Pb from water. Other hydroponic studies conducted by Sasmaz

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et al. (2017) reported that Limno charisflava Buchenau, and Lemna minor were able to filtrate Cd, and other heavy metals amalgamated with organic pollutants from contaminated water through rhizo-filtration, respectively.

4.5.1.3

Phytoextraction

Phytoextraction takes use of plants’ tendency to collect contaminants in aboveground, harvestable biomass. The biomass is harvested repeatedly in this procedure to reduce the concentration of contaminants in the soil. Phytoextraction can be either continuous (using metal hyperaccumulating plants or fast-growing plants) or induced (using agents to improve metal bioavailability in the soil). Continuous phytoextraction is based on some plants’ capacity to progressively incorporate contaminants (mostly metals) into their biomass. According to Mitton et al. (2016), phytoextraction-mediated pesticide absorption acts an excellent phytoremediator for specific pesticides like Endosulfan. Notably, there are a huge number of plants (over 400 species) which can escape or tolerate the toxicity even after hyperaccumulating these metallic compounds, although most of them are able to deal with single metal, which comprises of > 1% of their dry biomass. For example, up to 3.8% nickel (Ni) hyperaccumulation was reported in Berkheya coddii was reported in a previous study. The genes associated with the membrane transporters like copper transporters (COPT1) and zinc transporter (ZNT1) get overexpressed while the excessive cumulation takes place. Above all, this hyper-cumulation also coexists with several constraints like slow development, inadequate generation of biomass, etc. Some external factors play significant roles behind the entire process of phytoextraction including the production of adequate biomass, and the contaminants frequency in the produce, due to which rapidly growing plants overcome the rest with a much better potential of phytoextraction. Both AMF and plant growth promoting rhizo-bacteria (PGPR) are inoculated to enhance plant biomass. Among them, AMF is not only associated with the depletion of metallic biomass in the aerial parts of plants, but also with the absorption of As, Pb and Ni from soil. Therefore, reducing AMF activity with certain soil fungicides has enhanced metal build-up as well as biomass accumulation in plants. Several artificial acidic chelating compounds like ethylenediaminetetracetic acid (EDTA), ammonium sulphate (NH4 SO4 ), etc. can be recruited to enhance metal bioavailability to plants. These synthetic chelating chemicals are responsible for both metal absorption and their translocation from source to sink. The application of chelates is crucial, and it should ideally take place during the peak of biomass production. Growing maize (Zea mays) on Pb-contaminated soil treated with 10 mmol kg−1 EDTA revealed the efficiency of the compound. This resulted in substantial Pb accumulation (1.6% of shoot dry weight) and promoted Pb transfer from the roots to the leaves (Meers et al., 2008). Some disadvantages associated with employing synthetic chelates in phytoremediation stem from increased metal solubility in the soil. As a result, the danger of metal movement through the soil profile and into groundwater

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rises. However, treating polluted soil ex situ in a limited region with an impermeable surface might be a viable option. In addition, using modest dosages of synthetic chelates on a regular basis minimizes the likelihood of metal migration.

4.5.1.4

Phytodegradation

Plants and microorganisms are used in phytodegradation to absorb, metabolise, and breakdown organic contaminants. Plant roots are utilised in conjunction with microorganisms in this strategy to detoxify soil polluted with organic chemicals. It is sometimes referred to as phyto-transformation. By generating enzymes, certain plants may cleanse soil, sludge, sediment, and ground and surface water. Organic substances such as herbicides, insecticides, chlorinated solvents, and inorganic pollutants are used in this method (Pivetz, 2001). The breakdown of organic pollutants within plant tissue is known as phytodegradation. Plants generate enzymes that aid in breakdown, such as dehalogenase and oxygenase. It appears that both the plants and the related microbial populations play an important role in pollutant attenuation. It is defined as the degradation or breakdown of organic pollutants by plant-driven internal and external metabolic processes (Prasad & Freitas, 2003). Populus spp. trees have been employed effectively in phytodegradation of hazardous and resistant organic substances (Kaushik ed., 2015). Rhizo-degradation is the transformation of organic pollutants into less hazardous compounds inside the rhizosphere by soil microbial biodegradation. This process is aided by root exudates (organic compounds) that support soil microbial populations. A special bacterium inoculum can be applied to contaminated soils to speed up the process. Bacterial inoculum comprises strains with the required metabolic activity to breakdown the pollutants of interest. Inoculating plants with genetically modified bacteria strains that digest a specific pollutant has yielded encouraging results. Additionally, populations of soil bacteria can be boosted through a process known as bio-stimulation. This includes, for example, changing the soil’s nitrogen and pH levels in order to enhance bacterial populations. Suresh et al. (2005) discovered that the plants Cichorium intybus and Brassica juncea are effective in degrading DDT. Both plants’ hairy root cultures enhanced absorption and decomposition of DDT. Pesticide absorption and phytodegradation by Eichhornia crassipes in water bodies can be exploited as a possible, cost-effective, and alternative biological technique. Other possible benefits of this plant include cost effectiveness, increased storage capacity, and less chemical use. However, the pyrethroid removal efficacy of E. crassipes and P. strateotes was shown to be much greater than that of organochlorine (Riaz et al., 2017).

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Table 4.3 List of various phytoremediation methodologies followed for various pesticides Pesticides

Plants used

References

Butachlor

Triticum vulgare

Yu et al. (2003)

Dimethoate, malathion

Nasturtium officinale

Al-Qurainy and Abdel-Megeed (2009)

Atrazine

Acorus calamus

Wang et al. (2012)

Lindane

Jatropha curcas L

Abhilashet al. (2013)

DDT

Solanum lycopersicum, Helianthu sannuus, Glycine max, Medicago sativa

Mittonet al. (2014)

Azoxystrobin

Plantago major

Roman et al. (2012)

Endosulfan

S. lycopersicum, H. annuus, G. max, M. sativa

Mittonet al. (2016)

4.5.1.5

Phytovolatilization

This term generally refers to the entire process of transpiring some volatile pollutants i.e., mercury (Hg) and selenium (Se) after uptaking them from disturbed soils. Hence, a plant with higher evapotranspiring or conversion ability is the most recommended one for this process. For example, hybrid poplar can transform trichloroethylene into volatile liquid chlorinated acetates and gaseous carbon dioxide (CO2 ). Some genetically modified plants are developed to volatilize metals like Se by converting them into dimethylselenide [Se (CH3 )2 ]. In high-selenium environments, Bañuelos et al., (2000) observed that some plants could change Se into dimethylselenide and dimethyldiselenide. The major pitfall regarding this method is the uncontrollable spread of these heavy metals into the environment due to evapotranspiration. Here, Table 4.3 represents various plant-mediated remediation methods of pesticides.

4.6 Elimination of Heavy Metals Through Phytoremediation Technique Cleaning up polluted region requires phytoremediation to integrate with multiple disciplines, bringing together the fields of soil chemistry, soil microbiology, and plant physiology. Numerous pollutants like heavy metals, chlorinated solvents, petroleum hydrocarbons, organo-phosphatic insecticides, and surfactants have been subjected to marginal fields and/or laboratory experiments using phytoremediation (Khan et al., 2004). Some higher plant species can assimilate heavy metals at very high concentrations in their tissues without exhibiting that much toxicity (Magwaza et al., 2020). If the biomass and metal content of these plants are sufficient to accomplish remediation within an acceptable amount of time, they can be significantly employed to disinfect

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soil or wastewater from heavy metal contamination. The plants must undergo three of the following steps: (1) extraction of significant quantity of heavy metals into the roots, (2) translocation of those metals from source to sink, (3) generation of respectable amounts of plant biomass for this cleaning procedure. Remedial plants of around 45 families either possess the detoxifying capacity or withstand increased metal concentrations that have built up in their shoots. As earlier, there are same types of methods followed by plants for remediating these heavy metals.

4.6.1 Phytoextraction Through this method, the plant roots-extracted metals are transferred to shoot, followed by harvesting of both the parts to get rid of soil/ wastewater borne pollutants. The expense ratio of this method to the traditional one is reported to be 1:10 per hectare. Being a low impact method, it is much more beneficial to the environment (Licht & Iserbrands, 2005).

4.6.2 Phytostabilization Phytostabilization or in situ deactivation is the remediation of both soil and sludges. In this process, roots of the plants are utilized for lowering the bioavailability of pollutants in the soil through the reduction of their mobility by precipitation, absorption, reduction of metallic valence, etc. The plants follow the succeeding steps for inducing stabilization: (1) reduction in the amount of percolating water leading to accumulation of toxic leachates, (2) prevention of exposure to the polluted soil; and (3) detaining of spreading of contaminated metals such as As, Cd, Cr, Cs, Cu, Pb, Zn and many more through inhibiting soil erosion (Raskin & Ensley, 2000).

4.6.3 Rhizofiltration Contaminants available in various habitats of water are remediated by this technique. The heavy metals in the water sources get filtered in the rhizospheric zone of plants of both habitats i.e., aquatic and terrestrial (Ensley, 2000). Among all the plants used in a rhizofiltration study, Indian mustard has shown superior performance by removing Pb from water sample with a significant a bioaccumulation coefficient of 563 (Raskin & Ensley, 2000).

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4.6.4 Phytovolatilization Through evapotranspiration, plants emit mercury (Hg) through stomata at the maximum rate after absorbing from soil. The conversion of the pollutants into less toxic elemental Hg in this process overpasses others, although the recycling of this Hg to waterbodies through the production methyl mercury (CH3 -Hg) by some anaerobic bacteria creates a dilemma to this (Jadia & Fulekar, 2009). In a lab-based experimentation, genetically modified Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants were able to transform Hg (II) to Hg (0) for volatilization using mercuric reductase.

4.7 Advantages of Hydroponics: i. Microorganisms and plants, all work together to bioremediate wastewater in hydroponic systems through several biological, chemical, and physical processes. ii. This is basically an ecofriendly and economically viable technology. iii. Hydroponic systems encourage the growing of valuable crops and plants in wastewater cleanup processes. iv. Hydroponic wastewater treatment remedies are less expensive and may be implemented on-site in a short space. v. The technology has demonstrated multiple benefits against field crop cultivation, including plant growth rates that are 30–50% higher than terrestrial plants produced at identical circumstances. Furthermore, hydroponicsassociated wastewater management and recycling schemes dismiss the need of area towards crop cultivation while minimizing the probability of soil degradation owing to salt and solid depositions caused by effluent renewal. vi. Increased energy consumption is associated with hydroponics than that of the rest traditional greenhouse producing techniques.

4.8 Constraints Behind the Application of Hydroponics i. Hydroponics must be constantly monitored. Sensor systems can be used to improve oversight, but the individual accountable must have technical expertise and proficiency in computer handling. ii. Carvalho et al. (2018) investigated the financial feasibility of hydroponic techniques in Brazil, and discovered that hydroponic gardening of leafy and spicy veggies displayed excellent economic sustainability so represent an alluring substitute for food production. iii. The initial and ongoing costs are higher than in traditional agricultural systems.

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iv. Once the plants get infected by microorganisms that reside in the effluent system, infection spreads quickly and encompasses the whole system, destroying the vegetation. v. To conduct the procedure, one should be familiar with the number of units to be generated as well as the outcome of the investment. vi. The availability of trained individuals in the location wherein the entire system is supposed to be implemented is critical.

4.9 Conclusion and Future Directions The environmental pollution caused by the application of various pesticides in the field and heavy metal contamination in the water bodies require a necessary and immediate measure mediated by plants i.e., phytoremediation. Other rehabilitative and restorative protocols (both physical as well as chemical) followed against these contaminations bring in several significant limitations including irreversible modifications to the basic soil properties, high expenditure, diminishing microflora communities, subsidiary production of secondary pollutants, etc. On the other hand, the strategies of phytoremediation are not only ecologically safe but also economically viable, comparatively than the rest. Although this solar-induced technology has gained a lot of public acceptance, the research on the same involving several interdisciplinary approaches is still going on towards a better improvement, till date. These multidisciplinary studies regarding soil biology, soil microbiology, soil chemistry along with plant biology and physiology are, right now, broadly accepted throughput the scientific communities across the world in order to develop a high-level knowledge for implementing in this newly emerging field. Further researches are genuinely needed for introducing many more species of both aquatic and terrestrial habitat, which can more efficiently remove the pesticide residues and metallic exudates from the governing water bodies. Moreover, advancement in molecular perspective implicating the characterization of proteins associated with membrane transport and vacuolar sequestration are still in progress to understand the proper mechanisms to enhance phytoremediation. Eventually, this progress will lead to the establishment of phyto-mining designed for extracting metals from poor quality ores. Therefore, phytoextraction can be indicated as a mass-marketable technique in future, which can bring out the advantages of both phytoremediation as well as phyto-mining.

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Chapter 5

Environmental Remediation Using Hydroponics Abhijit Kumar, Gunjan Mukherjee, and Saurabh Gupta

Abstract Soil is widely acknowledged as the optimal medium for promoting plant growth. Besides providing physical support to plants, soil serves as a source of nutrients. However, several factors can hinder plant growth, including the presence of disease-causing microorganisms, depletion of soil fertility, erosion of topsoil, and inadequate soil drainage. Additionally, challenges such as the availability of abundant water resources, a larger land area for cultivation practices, and an adequate labor force pose difficulties for crop production in conventional agricultural fields. The situation becomes even more challenging in urban areas where space for cultivation is limited. Moreover, variations in geographic and topographic factors further restrict the availability of suitable land for cultivation. Consequently, soilless cultivation methods, such as hydroponics, have emerged as a promising solution. Keywords Hydroponics · Nutriculture · Wasteland · Phytoremediation · Hyperaccumulator · Phytoextraction

5.1 Introduction Environmental pollution has become a pressing global concern, with various pollutants adversely impacting ecosystems and human health. Contaminated soil, in particular, poses significant challenges, as it affects the growth of plants and can potentially contaminate the food chain. Traditional methods of environmental remediation often involve costly and time-consuming processes. However, hydroponics, a soilless cultivation technique, has emerged as a promising alternative for environmental remediation. Hydroponics involves growing plants in nutrient-rich water solutions, providing precise control over nutrient uptake and minimizing the reliance A. Kumar (B) · G. Mukherjee University Institute of Biotechnology, Chandigarh University, Gharuan, Punjab, India e-mail: [email protected] S. Gupta Department of Microbiology, Mata Gujri College, Fatehgarh Sahib, Punjab, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_5

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on soil quality (Hoagland & Arnon, 1950). This method offers several advantages for remediating contaminated soil, including the ability to target specific contaminants, increased plant growth rates, and reduced leaching of pollutants into the environment. In this article, we will explore the potential of hydroponics as an effective tool for environmental remediation. We will discuss the advantages of this technique, its applications in phytoremediation, and the various mechanisms through which it can contribute to the removal and detoxification of contaminants from soil. Additionally, we will examine case studies and success stories that demonstrate the viability and effectiveness of hydroponics in environmental remediation. The United Nations Food and Agriculture Organisation (FAO) has projected that the global population will reach approximately 9 billion people by 2050, marking a significant milestone in human history (FAO, 2018). This population growth will necessitate a substantial increase in food production, estimated to be around 70% higher than the levels recorded in 2007. Moreover, the world is witnessing a growing trend towards urbanization, with projections indicating that 75% of the global population will reside in urban settlements by 2050 (United Nations. Goal 11, 2018). Despite currently occupying only 3% of the Earth’s land area, cities consume a significant proportion of resources. They account for 60–80% of energy usage, contribute to 75% of carbon emissions, and accommodate 56% of the world’s population (World Bank. Urban Population [% of Total Population], 2018). Future estimates suggest that urban areas will experience the majority of population growth, with an anticipated increase of 2.5 billion people by 2050, surpassing the projected global population growth of 2.1 billion people (United Nations. World Urbanization Prospects: The 2018 Revision, 2018). This growth can be attributed to natural population increase, rural-to-urban migration, and the reclassification of areas as urban through annexation and transformation, leading to the expansion of urban settlements at the expense of rural regions. The combination of population growth and urbanization poses significant challenges in terms of meeting the food requirements of a growing urban population while minimizing the environmental impact. It calls for innovative approaches that optimize land use, resource efficiency, and sustainable food production methods. In this context, alternative agricultural practices, such as hydroponics and other soilless cultivation techniques, hold considerable promise for addressing the need for food production in urban settings while mitigating the strain on natural resources and reducing the carbon footprint associated with traditional agriculture. The growth of urban centers through migration and reclassification poses significant challenges for agriculture as it competes for limited resources such as soil, water, and labor. Agriculture is confronted with the task of producing more food with fewer people and less land while simultaneously addressing the impacts of climate change, protecting habitats, preserving endangered species, and maintaining biodiversity. As a critical component of the food chain, agriculture shoulders a substantial burden. Despite the challenges posed by factors like soil degradation, water scarcity, and soil pollution caused by chemical pesticides and fertilizers, open-field agriculture remains prevalent worldwide (Zárate, 2014). However, the increasing demand for

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food driven by population growth necessitates significant changes in agricultural practices to enhance efficiency and sustainability. Agriculture must embrace technology not merely for the sake of being different, but to effectively improve and meet the actual needs of consumers (De Clercq et al., 2018). To meet these evolving demands, agriculture needs to adopt innovative approaches such as precision farming, vertical farming, and soilless cultivation techniques like hydroponics. These methods can maximize productivity by optimizing resource utilization, reducing the environmental impact, and conserving valuable resources such as land and water. By integrating technology and sustainable practices, agriculture can strive towards greater efficiency, resilience, and customer satisfaction in a rapidly changing world. Because urbanisation does not appear to be slowing, it makes sense to include urban and periurban regions in the 2050 plan to feed the world with food that is healthy, inexpensive, and produced in an environmentally friendly manner. The production of food and cattle for the local people, notably in peri-urban agricultural areas near cities and towns when space is restricted and vegetative land uses are difficult to maintain (Lin et al., 2015). The promise of UA systems is that they might provide cities with both societal and environmental benefits, such as enhanced food security and the potential for less environmental impact (Armanda et al., 2019). To address the growing demand for healthy, affordable, and sustainable food in regions where arable land and water resources are rare, intensive, high-yield farming methods and technologies such as hydroponics are becoming increasingly popular. Europe and the Asia–Pacific regions are predicted to dominate hydroponic tomato cultivation by 2028 (GVR, 2021). Hydroponics provides numerous advantages over traditional farming methods. Higher yields, for example, can be obtained by utilising both the horizontal surface area and the vertical space above it. Vertical farming, which allows for greater plant density per unit area, is a viable option for meeting the daily demand for fresh, healthy produce in densely populated places. Moreover, hydroponics enables year-round crop production, eliminating the need for seasonal constraints. By avoiding the excessive use of pesticides and fertilizers that can harm the environment, hydroponics contributes to a cleaner and more sustainable farming approach. Furthermore, this method requires less land and water compared to traditional open-field agriculture, making it an attractive option for regions facing resource limitations. Through the adoption of hydroponics, farmers can achieve higher productivity, reduce environmental impact, and ensure a consistent supply of fresh produce throughout the year. As the global population continues to grow and agricultural resources become increasingly scarce, hydroponics offers a promising solution to meet the challenges of food security, sustainability, and efficient resource utilization. Hydroponics allows for careful management of the most crucial parameters for optimal plant growth with the help of smart greenhouse technologies. This saves water and chemicals while also removing potentially hazardous waste and residues (van Delden et al., 2021). Large-scale hydroponics facilities may now control the temperature, lighting, and watering of their plants using new equipment, web platforms, software, and mobile apps. These technical advancements have aided the growth of the hydroponics sector. The market is predicted to increase at a compound annual growth rate (CAGR) of 20.7% from 2021 to 2028 (GVR, 2021). However,

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feeding the global population by 2050 is not the only thing that must be done. To solve the severe concerns of present global warming and widespread pollution, comprehensive environmental and socioeconomic solutions are required. Hydroponics can help solve some of the issues with traditional open-field farming, which causes a lot of CO2 emissions and the loss of farmable area since it uses old, unsustainable technologies. Adopting sustainability in all major human activities, including agriculture, is critical for solving the challenges of sustainable food production and environmental preservation. Hydroponics emerges as a sustainable and suitable alternative for urban and periurban populations, aligning with Sustainable Development Goal (SDG) number 11 of the United Nations’ 2030 Agenda for Sustainable Development, which focuses on creating sustainable cities and communities. In contrast to the environmentally unsound practices of modern open-field agriculture, hydroponics offers a promising solution. Expanding the use of hydroponics is vital for urban agriculture (UA) and has the potential to revolutionize the food supply chain, improve societal well-being, and contribute to environmental health. To fully realize its potential, hydroponics needs to be embraced at small- and medium-scale levels. This entails technologists gaining a deep understanding of hydroponics fundamentals to develop appropriate technologies. Additionally, producers must recognize the benefits of integrating new technologies to optimize cost–benefit ratios and meet the specific needs of local customers or themselves. It is worth noting that agriculture, from its inception approximately 10,000 years ago to the present day, has been instrumental in transforming human society and driving population growth (Kremer, 1993). However, as we confront the challenges of sustainability, hydroponics offers a significant shift in agricultural practices, providing a more ecologically sound approach to food production. By advancing hydroponics and expanding its implementation across different scales, we can foster sustainable development, address food security concerns, and shape a resilient and prosperous future.

5.2 Hydroponic Cultivation Techniques In comparison to traditional gardening, hydroponics allows for the production of food without the usage of soil. Plants are cultivated in regions where their roots may easily pull nutrients from a nutrient solution, whether natural or man-made. There are numerous methods for growing food using hydroponics. Which one you use is determined on the plant, the climate in which you live, your budget, and other factors. Deep Water Culture (DWC), also known as Floating Root System (FRS), is a method of growing plants in water.

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Fig. 5.1 Drip irrigation (https://www.iasparliament.com/current-affairs/drip-irrigation-needs-fur ther-push)

5.2.1 Drip Irrigation Drip irrigation is an efficient and precise method of providing water to plants by delivering it directly to the root zone. It involves the slow, targeted application of water through a network of tubes or pipes with emitters placed near the plants (Fig. 5.1). This technique minimizes water wastage by reducing evaporation and runoff, making it a highly water-efficient irrigation system. In a drip irrigation system, water is released in small, controlled amounts, allowing it to seep into the soil gradually. This slow and localized application ensures that water reaches the plant roots directly, promoting better absorption and reducing water loss through deep percolation. The system can be customized to meet the specific water needs of different plants, enabling precise irrigation management. One of the key advantages of drip irrigation is its ability to conserve water. Compared to conventional irrigation methods, such as sprinkler systems, drip irrigation can save significant amounts of water by targeting water delivery and minimizing overspray. This makes it particularly valuable in regions facing water scarcity or restrictions.

5.2.2 Aeroponics Aeroponics is a sophisticated way of growing plants without the use of soil. Plants are grown in an air or mist environment rather than utilising soil or standard hydroponic media. The roots of plants are raised up in the air in aeroponics, and a nutrient-rich mist or fog is poured on the roots at regular intervals (Fig. 5.2). This enriches the

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Fig. 5.2 Aeroponics (https://medium.com/krishi-wise/what-is-aeroponics-farming-why-you-sho uld-care-238617517711)

environment with air and nutrients, allowing plants to grow swiftly and efficiently. Typically, plants are preserved in aeroponic systems in special containers or structures that support the roots and expose them to air. Misters or sprayers that produce a fine mist are used to apply the nutrient solution to the roots. This mist distributes essential nutrients directly to the roots, maximising nutrient utilisation and minimising waste.

5.2.3 Advantages of Hydroponics Hydroponics provides numerous advantages over traditional soil-based farming systems. First and foremost, it provides the most control over the growing environment, including temperature, humidity, and nutrient levels. This promotes plant growth and production. Because it does not require soil, hydroponics allows plants to be grown in areas where the soil is poor or there is a scarcity of cropland. Hydroponics improves water efficiency by using substantially less water than conventional farming. Water is recycled in the closed-loop system, reducing waste and safeguarding water resources. This makes hydroponics particularly valuable in locations where water is scarce or there is a drought. Another advantage is that fewer pesticides and fertilisers are required. Hydroponics’ regulated environment decreases pest and disease infestations, reducing the need for chemical interventions. This cleans and disinfects the food, and it leaves less chemical residue on the food (Pandey et al., 2009).

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5.3 Phytoremediation Aspect Phytoremediation is a new and environmentally beneficial method of cleaning up contaminated soil, water, and air by using plants. It harnesses plants’ natural powers to absorb, clean up, and stabilise contaminants to recover contaminated ecosystems. One of the most advantageous aspects of phytoremediation is that it is less expensive than traditional techniques of pollution removal. When compared to costly and energy-intensive procedures such as excavation and disposal, it may be a more cost-effective and environmentally friendly solution. Phytoremediation can be done directly where the problem is, requiring less transportation and causing less damage to the environment as a whole. Phytoremediation is also adaptable to a wide spectrum of contaminants. Different plant species absorb and alter contaminants in different ways. Some plants, for example, are good at storing heavy metals, while others are efficient at breaking out organic pollution. Phytoremediation can be utilised to eliminate specific contaminants by selecting the appropriate plant species for a given pollution condition. As a result, it is a very adaptable and successful method of cleaning up contaminated regions. Another advantage is that it may be able to last a long time. Phytoremediation devices can operate without much upkeep or energy input once they are set up. The ability of the plants to reproduce and spread improves the remediation process over time. As a result, phytoremediation can be employed on large contaminated sites that may require continual cleanup. Phytoremediation is also regarded to be beneficial to the environment. In comparison to traditional approaches, which may require the use of chemicals or excavation, phytoremediation relies on natural processes and produces the least amount of harm to the ecosystem. It can even help restore habitats, reduce pollution, and store carbon, all of which are beneficial to the overall health of the environment (Ure, 1990).

5.4 Hydroponics as an Alternative for Pot Culture Hydroponics is gaining popularity as an alternative to traditional soil-based or pot culture methods of producing plants. Unlike hydroponics, which relies on a nutrientrich water solution to provide plants with the nutrients they require to grow, pot culture includes growing plants in containers filled with soil or other growing medium. One of the primary advantages of hydroponics over pot culture is the precise control it offers over nutrient delivery. Plants in pot culture obtain their nutrients from the soil, which varies in what it contains and how much of it is there. The nutrient solution in hydroponics is meticulously crafted to contain the exact amount of everything plants require to flourish. This makes it easier for plants to absorb nutrients, resulting in healthier and stronger plants. Hydroponics also offers the benefit of increasing water efficiency. A lot of water is lost in pot culture due to draining or evaporation, making water use inefficient. Water is recycled throughout the system in hydroponics. This prevents water from being wasted and ensures that plants receive the water they

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require while conserving this vital resource. Hydroponics also offers greater control over environmental conditions including as pH, temperature, and humidity. These variables can be adjusted to provide the ideal growing circumstances for plants, resulting in faster development, larger yields, and higher quality produce. The atmosphere in pot culture is mostly determined by the soil and weather surrounding it, making it difficult to control and maintain. Another advantage of hydroponics is its potential to maximise available space. For their root systems to expand and reach nutrients in the soil, plants in pot culture require enough space. With their roots floating in a nutrient solution, plants can be cultivated in hydroponics in smaller systems. This vertical growing approach allows you to grow more plants in less space. As a result, it is an excellent choice for urban and flat farming. The potential for increased yields and enhanced resource efficiency over time can more than offset the upfront expenditures associated with hydroponic systems. Furthermore, hydroponics eliminates the requirement for soil and eliminates the risk of illnesses and pests transmitted by soil, resulting in cleaner and healthier produce (Pandey et al., 2009).

5.5 Toxic Substances Toxic substances are a major source of water pollution, threatening aquatic ecosystems and people’s health. These substances can enter bodies of water in a variety of ways, including industrial waste, farm runoff, and incorrect chemical disposal. Heavy metals, pesticides, herbicides, industrial chemicals, medications, and personal care products are just a few of the many pollutants that are toxic substances. These pollutants may harm aquatic organisms by interfering with their bodily functions and making it more difficult for them to develop, reproduce, and survive in general. Heavy metals such as lead, mercury, cadmium, and arsenic are toxic substances present in water. They can be caused by industrial activities, mining, or falling objects from the sky. These metals accumulate in aquatic organisms’ tissues. This process is known as bioaccumulation and biomagnification. This can be extremely harmful to aquatic organisms as well as people who consume contaminated fish or shellfish.

5.5.1 Thermal Pollution Thermal pollution occurs when human actions alter the temperature of natural water bodies such as rivers, lakes, and oceans in an unfavourable way. It occurs when the water temperature fluctuates significantly outside of its regular range. This has the potential to harm marine ecosystems. When manufacturers or power plants discharge heated water into neighbouring water sources, it is one of the leading causes of thermal pollution. A lot of water is required for cooling in power plants. This is

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particularly true for plants that run on fossil fuels or nuclear power. After cooling, the heated water is released back into the environment, raising the temperature of the water body it enters.

5.5.2 Natural Pollution Environmental pollution, also known as thermal pollution, occurs when chemical, organic, or thermal pollution is created by natural processes rather than by humans. It means that pollutants enter the environment as a result of natural events or processes. One example of natural pollution is increased siltation of a waterway following a landslip. When a landslip occurs, soil and rocks can disintegrate, resulting in an increased volume of sediments in runoff. These sediments can harm aquatic environments by reducing water clarity, obstructing fish gills, and disrupting the natural balance of aquatic life.

5.5.3 Sources of Water Pollution Domestic waste, industrial effluents, and agricultural waste are the most significant sources of water pollution. Other sources are oil spills, air deposition, marine dumping, radioactive waste, and eutrophication.

5.5.3.1

Domestic Sewage

Domestic sewage refers to the wastewater generated from households, including water from toilets, showers, sinks, and laundry. It contains various contaminants, including organic matter, nutrients, pathogens, and potentially harmful chemicals. Proper treatment of domestic sewage is essential to protect public health and the environment. Common sewage treatment methods include primary treatment, which involves the physical removal of solids, and secondary treatment, which utilizes biological processes to break down organic matter. Additional advanced treatment processes may be employed to further remove nutrients and pathogens. Effective management of domestic sewage is crucial for ensuring clean water resources and preventing waterborne diseases.

5.5.3.2

Industrial Effluents

Industrial effluents are wastewater or liquid rubbish created by industrial activities. Toxic chemicals, heavy metals, organic compounds, oils, and particles suspended in water are some of the pollutants detected in these effluents. The discharge of untreated

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or inadequately treated industrial effluents into the environment can cause severe water pollution and pose risks to human health and aquatic environments. Manufacturing, mining, chemical processing, energy generation, and food processing all generate a large amount of wastewater that must be treated and cleaned appropriately. However, if there aren’t enough treatment facilities or people don’t follow environmental regulations, toxic industrial effluents can enter rivers, lakes, and groundwater. Chemicals in industrial effluents vary according to the type of business and the methods utilised. Heavy metals such as lead, mercury, cadmium, and chromium are among the most prevalent pollutants detected in industrial effluents. These metals are commonly utilised in metal manufacture and mining. These metals are extremely toxic and can persist in the environment for long periods of time, posing risks to aquatic life and possibly entering the food chain. Chemical pollutants commonly found in industrial effluents include solvents, acids, alkalis, and insecticides. These compounds can harm aquatic organisms, resulting in the extinction of species, reproductive difficulties, or even death. Organic compounds present in industrial effluents include petroleum hydrocarbons and volatile organic compounds (VOCs). These compounds could contaminate drinking water sources and provide risks to water quality.

5.5.3.3

Agricultural Waste

Agricultural waste refers to the byproducts and residues created during various agricultural processes. Crop residues, animal manure, agricultural chemicals, and other organic and inorganic pollutants are all included. Agriculture is essential for producing food and keeping people alive, yet improper agricultural waste management can harm the environment and people’s health. Crop residues such as stalks, leaves, and husks are common types of agricultural waste. These residues, if not treated properly, can contribute to nutrient loss, soil erosion, and the production of greenhouse gases. Animal manure is another essential component of agricultural waste since it contains a variety of nutrients, pathogens, and odorous substances. If manure is not adequately managed and cleaned, it can enter water bodies and cause pollution and eutrophication. Agricultural chemicals, in addition to fertilisers and insecticides, can contribute to agricultural waste. Excessive and improper use of these chemicals can result in nutrient runoff, chemical contamination, and pollution of surface and subsurface water sources. Packing materials, plastic mulch, and other types of waste generated during agricultural practises can also contribute to overall agricultural waste.

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5.5.4 Heavy Metal Pollution in Water The presence of toxic metals in water bodies at quantities that exceed permissible limits is referred to as heavy metal contamination in water. Heavy metals such as lead, mercury, cadmium, arsenic, and chromium occur naturally but can become pollutants when discharged into the environment by human activities, including industrial processes, mining, agriculture, and inappropriate waste management. Heavy metals endure in aquatic ecosystems and can accumulate, causing major dangers to both human health and the environment. They can be toxic to aquatic organisms, including fish, invertebrates, and plants, even at low quantities. These metals can disrupt physiological processes, decrease reproductive capacity, and lead to aquatic life’s demise. Heavy metals can also infiltrate the food chain, hurting not just aquatic organisms but also human consumers of contaminated fish or shellfish. Human exposure to heavy metals from tainted water sources can lead to serious health problems. Long-term exposure to heavy metals in water can cause organ damage, neurological issues, developmental defects, and even cancer. Vulnerable populations, such as youngsters, pregnant women, and people with impaired immune systems, are especially vulnerable.

5.5.5 Water Treatment for Heavy Metal Removal Water treatment for heavy metal removal comprises a variety of processes and technologies for reducing or eliminating the presence of hazardous metals in water sources. The purpose of these treatment technologies is to make water safe to drink while also preventing heavy metal contamination from harming the ecosystem. Coagulation and flocculation are two commonly utilised heavy metal removal procedures. Chemicals known as “coagulants” are added to the water during this process to make the metal particles less stable and bind them together into larger pieces known as “flocs.“ These flocs can subsequently be removed using sedimentation or filtration processes. Another effective way is ion exchange. Resin compounds that are attracted to heavy metal ions are employed in this procedure to exchange them for less hazardous ions (Salt et al., 1995). This process is notably useful for removing metals such as lead, copper, and mercury. Another prominent application for activated carbon adsorption is heavy metal removal. Activated carbon has a porous structure that traps metal ions on its surface, removing them from the water. This approach is effective for removing organic and inorganic contaminants (Sarma, 2011).

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5.6 Phytoremediation Phytoremediation is a natural and low-cost method of cleaning up the environment (Sarma, 2011). To remove, degrade, or stabilise pollutants in the soil, water, and air, plants are used. It makes use of some plant species’ specific characteristics to absorb, break down, and store contaminants, which aids in the cleanup of polluted environments. Plants utilised in phytoremediation are known as “hyperaccumulators.“ These plants have an incredible ability to withstand and store significant quantities of pollutants in their tissues without suffering undue harm. They can remove contaminants from the soil or water and transport them to their above-ground portions, where they can be harvested or stored safely (Garbisu & Alkorta, 2001). Plants can purify a dirty environment in a variety of ways. Phytoextraction involves the uptake and buildup of pollutants in the plant’s shoots in order to be harvested and disposed. Heavy metals such as lead, cadmium, and arsenic can be removed with this procedure. Phytostabilization involves plants binding contaminants in the soil, making them less bioavailable and preventing them from migrating. It is very effective at preventing metals and metal-like compounds from moving about in the soil. Phytodegradation or phytotransformation involves plants breaking down organic pollutants and transforming them into less toxic or non-toxic forms via biochemical processes. Rhizofiltration employs plants with extensive root systems to filter and remove contaminants from water or waste water.

5.6.1 Response of Plants to Metal Pollution The general response of plants growing on a metal contaminated soil is categorized into the following:

5.6.1.1

Hyperaccumulators

Hyperaccumulators are a specific type of plants that can tolerate and accumulate huge levels of heavy metals in their tissues. When these plants are exposed to metal pollution, their physiology and biochemistry adapt in unexpected ways, allowing them to survive and thrive in polluted environments. Hyperaccumulators have developed specific mechanisms to deal with metal toxicity (Reeves & Baker, 2000). They possess efficient metal uptake and transportation mechanisms that allow them to absorb and store metals from the ground or water via their roots. These plants usually have higher metal uptake rates than non-hyperaccumulator species (Milner & Kochian, 2008). The specific metal transporters and ligands they possess allow for the uptake, transfer, and sequestration of metals in their tissues. Plants that store a lot of metal have evolved several mechanisms for getting rid of it in order to avoid metal toxicity. Metal chelation is accomplished by producing and storing molecules that

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bind to metals, such as organic acids, phytochelatins, and metallothioneins. Metals are bound by these molecules, which reduces their toxicity and facilitates their storage in certain cells or tissues (Ghosh & Singh, 2005).

5.6.1.2

Metal Indicators

Metal indicators, also known as metal biomarkers or biomonitoring tools, are substances, organisms, or parameters used to assess the presence and concentration of heavy metals in the environment. They serve as indicators of metal pollution and provide valuable information on the extent and impact of metal contamination. Metal indicators can include bioindicators such as plants, animals, and microorganisms that accumulate metals in their tissues, as well as physical and chemical parameters like pH, electrical conductivity, and sediment composition. By monitoring metal indicators, scientists and environmentalists can better understand and manage the risks associated with heavy metal pollution in ecosystems (Chaney et al., 2007).

5.6.2 Types of Phytoremediation Based on the underlying mechanisms, the polluted matrix, its applicability, and the nature of the contaminant, phytoremediation can be broadly classified as follows.

5.6.2.1

Phytodegradation

Phytodegradation, also known as phytotransformation, is a natural process in which plants play a significant role in the degradation, detoxification, and removal of pollutants from the environment. It involves the ability of certain plant species to absorb and metabolize toxic substances, transforming them into less harmful forms or completely breaking them down. During phytodegradation, plants utilize their natural biochemical processes to break down pollutants. They take up contaminants from the soil, water, or air through their roots, and then transport them to various parts of the plant, such as leaves, stems, and roots. Once inside the plant, the pollutants undergo chemical reactions facilitated by plant enzymes and metabolic pathways. One of the key mechanisms in phytodegradation is the production of enzymes, such as peroxidases, dehydrogenases, and oxidases, which are responsible for the breakdown of pollutants. These enzymes can modify the chemical structure of pollutants, making them less toxic or converting them into simpler compounds that can be easily degraded by microorganisms. Different types of pollutants can be targeted through phytodegradation, including organic compounds like pesticides, solvents, and petroleum hydrocarbons, as well as heavy metals and metalloids. Certain plant species, known as hyperaccumulators, have the ability to accumulate high concentrations of heavy metals in their tissues, effectively removing them from the soil.

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Phytostimulation

Plants can be stimulated or improved in their growth, metabolism, or tolerance to external stressors through the application of certain molecules or substances. Natural or synthetic compounds are used to stimulate plant growth and development, nutrient uptake, and tolerance to biotic and abiotic stressors. Some of the substances that can be utilised for phytostimulation include plant growth regulators, organic amendments, microbial inoculants, and biostimulants. These substances can enhance the physiological processes within plants, leading to increased vitality, root development, nutrient uptake efficiency, and improved overall plant health. In agricultural practises, phytostimulation can boost crop yields, crop quality, and increased resistance to pests and diseases. It can also be employed in ecological restoration efforts to enhance the growth and survival of plants in damaged or contaminated areas. Plants may benefit from the application of phytostimulation technologies in a variety of ways. Using plant growth regulators, for example, can alter the balance of plant hormones, leading to increased branching, flowering, and fruiting. Organic additions can improve soil structure and nutrition by supplying key nutrients and improving water retention. Beneficial microbial communities can be established in the rhizosphere using microbial inoculants. This aids in nutrient cycling and disease prevention. Biostimulants can accelerate a plant’s metabolism, leading to increased nutrient uptake and stress tolerance.

5.6.2.3

Phytovolatilization

Phytovolatilization is a process in which plants uptake certain contaminants from the soil or water and release them into the atmosphere in the form of volatile compounds. It involves the conversion of absorbed pollutants into gaseous forms through plant metabolism, followed by their subsequent release through the stomata in leaves. Phytovolatilization is particularly effective for volatile organic compounds (VOCs) and certain heavy metals, such as mercury. This natural process can help reduce the concentration of contaminants in soil or water, providing a potential remediation strategy for contaminated environments.

5.6.2.4

Phytoextraction

Plants are utilised in the process of phytoextraction, also known as phytoremediation or phytoaccumulation, to remove and store contaminants, particularly heavy metals, from soil, water, or other polluted media. A type of remediation used in this strategy is the ability of some plant species to absorb and retain pollutants in their tissues. The ability of plants to handle and store high quantities of contaminants is a key factor in phytoextraction. These plants, dubbed “hyperaccumulators,” have specific systems in their roots and shoots that allow them to absorb and transport heavy metals from the soil to their above-ground portions. Once the contaminants have

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been absorbed by the plants, the level of contaminants in the soil or water can be successfully reduced. Phytoextraction is a cheap and environmentally benign form of remediation because it does not involve considerable excavating or the removal of contaminated materials. One of the advantages of phytoextraction is its ability to target specific contaminants, particularly heavy metals, which are extremely harmful and persist in the environment for long periods of time. Phytoextraction can also be used to clean up old mines, industrial sites, and agriculture (Reeves & Baker, 2000).

5.6.2.5

Phytostabilization

Phytostabilization is a technique used in environmental cleaning to prevent the spread and mobility of contaminants in soil, sediment, and other polluted media. Plants are employed to immobilise or stabilise contaminants, preventing them from moving and reducing their availability for uptake by organisms or leaking into groundwater. Plants for phytostabilization are chosen for their ability to tolerate and thrive in contaminated conditions. These plants’ extensive root systems can bind and hang onto contaminants in the soil, reducing their mobility. The roots also act as a physical barrier, preventing contaminants from spreading. Unlike hyperaccumulators in phytoextraction, plants in phytostabilization may not have to store as many contaminants. They instead strive to make the environment in which the pollutants reside more stable and less mobile. Lime or organic matter may be added to the soil to promote pollutant immobilisation even more (Grimaldo & López-Chuken, 2011).

5.6.2.6

Phytofiltration

Phytofiltration is a method of water or wastewater treatment that utilizes plants to remove pollutants and improve water quality. The process involves the uptake and filtration of contaminants by plant roots, where they can be metabolized or stored in plant tissues. Phytofiltration is particularly effective in removing organic compounds, nutrients, heavy metals, and sediment from water. It is an eco-friendly and cost-effective approach that can be used in various settings, including wetlands, constructed wetlands, and phytoremediation systems. Phytofiltration helps to restore and maintain the health of aquatic ecosystems while providing a sustainable solution for water treatment.

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5.7 Rhizofiltration 5.7.1 Background Rhizofiltration is a phytoremediation technique that removes contaminants from wastewater or water using plant roots. It focuses on how contaminants are filtered out by plants’ root zones. In rhizofiltration, plants that have a strong attraction to particular toxins, such as heavy metals or organic chemicals, are chosen. These plants are known as hyperaccumulators or metal collectors. When contaminated water runs through the root system of a plant, the roots absorb and retain the contaminants. This is how water is purified (Ensley, 2000). The contaminants can be absorbed by the roots or transported to the shoots and leaves, where they can be carefully harvested and disposed of. Rhizofiltration is a focused and effective treatment technique for specific contaminants. This approach offers several advantages. It is an environmentally friendly method that does not require a lot of energy or infrastructure. Rhizofiltration has a wide range of applications, including the treatment of industrial wastewater, agricultural runoff, and the cleanup of contaminated groundwater (Candelario-Torres et al., 2009). Furthermore, it is a less expensive solution for water treatment than conventional methods. However, the efficacy of rhizofiltration is dependent on a variety of factors, including the selection of the appropriate plant species, the amount and kind of contaminants, and the overall health and growth of the plants. Site-specific conditions and continual monitoring are required for the best results (Vallini et al., 2005).

5.7.2 Rhizofiltration Technology Rhizofiltration technology is an innovative approach used in phytoremediation for the removal of contaminants from water or wastewater. It utilizes the root systems of specially selected plants to filter and treat water, targeting specific pollutants for removal. The technology involves the implementation of a constructed wetland or a specially designed system where contaminated water is passed through the root zone of specific plant species (Fig. 5.3). These plants, known as hyperaccumulators or metal-accumulating plants, have the ability to absorb and accumulate high levels of pollutants in their roots. As the water flows through the root system, the contaminants are trapped and retained by the roots. This process can effectively remove a wide range of contaminants, including heavy metals, organic compounds, and nutrients. The plants act as natural filters, reducing the concentration of pollutants in the water. Rhizofiltration technology offers several advantages. It is a sustainable and environmentally friendly method that does not require the use of chemicals or energyintensive processes. It can be applied in both industrial and agricultural settings, providing an effective solution for treating contaminated water sources (Dushenkov et al., 1995).

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Assesment of levels of heavy metal contamination in water

Risk assesment of water contaminates by heavy metals

Screening of effective macrophytes for metal accumulation in situ

Rhizofiltration of water polluted with heavy metals

Screening of terrestrial plants for metal accumulation and adaptation to hydroponic culture

Screening of effective macrophytes for metal accumulation in laboratory conditions

Fig. 5.3 Scheme of phytoremediation process for water contaminated by heavy metals. Modified from Galiulin et al. (2001)

Additionally, rhizofiltration systems can be customized and designed to meet specific site requirements and pollutant removal goals. They can be implemented as standalone treatment systems or integrated into existing water treatment infrastructure. However, the success and efficiency of rhizofiltration technology depend on factors such as the selection of appropriate plant species, proper system design, and regular maintenance. Site-specific conditions, including the concentration and type of contaminants, also play a crucial role in determining the effectiveness of the technology (Sas-Nowosielska et al., 2004).

5.7.3 Plant Species for Rhizofiltration Rhizofiltration is a phytoremediation0000 technique that utilizes the natural filtering ability of plant roots to remove contaminants from soil or water. Various plant species have been found to be effective in rhizofiltration, each with its specific characteristics and suitability for different types of contaminants. One commonly used plant species for rhizofiltration is the common reed (Phragmites australis). (Eichhornia crassipes, Mahmood et al., 2010), This tall, perennial grass has an extensive root system that can absorb and accumulate heavy metals, organic pollutants, and nutrients such as nitrogen and phosphorus. The common reed’s ability to tolerate a wide range of environmental conditions makes it a versatile choice for rhizofiltration projects. Another plant species frequently employed in rhizofiltration is the water hyacinth (Eichhornia crassipes). This floating aquatic plant has fast growth rates and large root systems that efficiently uptake and accumulate pollutants like heavy metals, organic

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compounds, and excess nutrients. Water hyacinth is particularly effective in treating wastewater and eutrophic water bodies (Romero-Núñez et al., 2011). For the remediation of petroleum hydrocarbons, certain plant species such as willows (Salix spp.) and poplars (Populus spp.) are commonly used in rhizofiltration. These fast-growing trees have extensive root systems that can reach deep into the soil, enhancing the uptake of hydrocarbons and promoting microbial degradation of contaminants. Other plant species that have shown promise in rhizofiltration include cattails (Typha spp.), sunflowers (Helianthus spp.), and vetiver grass (Chrysopogon zizanioides). These plants possess efficient pollutant uptake mechanisms and can effectively remove heavy metals, organic compounds, and nutrients from contaminated soil or water (Dushenkov & Kapulnik, 2000).

5.7.4 Rhizofiltration Using Terrestrial Plants Terrestrial plants can also be employed to perform rhizofiltration, a phytoremediation technique, effectively. Terrestrial plants have demonstrated the ability to remove pollutants from soil and groundwater, whereas aquatic plants often participate in rhizofiltration (López-Chuken & Young, 2010). The terrestrial plant Indian mustard (Brassica juncea) can be utilised for rhizofiltration. Indian mustard’s deep root system allows it to absorb heavy metals such as lead, cadmium, and chromium from the soil. These contaminants accumulate in its roots and shoots, effectively decreasing their concentration in the soil. Another plant that is frequently utilised in terrestrial rhizofiltration is the sunflower (Helianthus annuus). Sunflowers are excellent at removing heavy metals such as arsenic, lead, and zinc from polluted soil. They are good in phytoremediation because they have a large root system and a lot of biomass. Switchgrass (Panicum virgatum) is another terrestrial plant with rhizofiltration potential. It has the ability to effectively remediate polluted soil of organic pollutants such as petroleum hydrocarbons. Switchgrass is noted for its deep and strong root system, which aids in pollutant removal and soil stability. The rhizofiltration abilities of Festuca grasses, including as tall fescue (Festuca arundinacea), have also been investigated. Heavy metals such as copper and zinc can accumulate in the roots and shoots of these plants, reducing soil contamination.

5.7.5 Rhizofiltration: Recent Advances Rhizofiltration, as a sustainable and cost-effective phytoremediation technique, has seen recent advances and innovations aimed at enhancing its efficiency and applicability (Arthur et al., 2005). These advancements have focused on various aspects, including plant selection, genetic engineering, and the use of symbiotic microorganisms. One significant recent advance in rhizofiltration is the identification and

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utilization of specific plant species that exhibit exceptional pollutant uptake and accumulation capabilities (Schulman et al., 1999). Researchers have conducted extensive studies to identify plants with enhanced metal hyperaccumulation traits. These hyperaccumulators, such as Noccaea caerulescens and Arabidopsis halleri, have shown remarkable abilities to accumulate high levels of heavy metals, making them valuable candidates for phytoremediation projects. Genetic engineering has also played a role in advancing rhizofiltration techniques. Scientists have focused on enhancing the expression of metal transporters and other genes involved in pollutant uptake and accumulation. This approach aims to create plants with improved rhizofiltration capacities. Genetic modifications have been successfully carried out in model plants like Arabidopsis thaliana and tobacco (Nicotiana tabacum) to increase their ability to sequester heavy metals. Additionally, researchers have explored the use of symbiotic microorganisms to enhance rhizofiltration processes. Certain bacteria and fungi have shown the ability to promote plant growth, increase pollutant uptake, and aid in contaminant degradation. For example, mycorrhizal fungi form mutualistic associations with plant roots, enhancing nutrient and water uptake and potentially assisting in pollutant removal. Rhizobacteria can also stimulate plant growth and facilitate metal immobilization or transformation in the rhizosphere. Another recent development in rhizofiltration is the integration of electrokinetics and plant-based remediation techniques. Electrokinetics involves the application of electrical currents to enhance the movement of contaminants in the soil and their uptake by plant roots. This combined approach has shown promise in improving the efficiency and speed of pollutant removal from contaminated sites (Elless et al., 2003).

5.8 Challenges of Hydroponic Phytotechnologies 5.8.1 Plant Development and Variability Plant development and variability pose significant challenges in hydroponic phytotechnologies. Hydroponics involves growing plants in a soilless medium with a nutrient-rich solution, providing precise control over the growing conditions (LópezChuken, 2005). However, maintaining consistent plant development and managing variability can be complex. One challenge is ensuring optimal plant growth and development throughout the entire hydroponic system. Factors such as nutrient availability, pH levels, temperature, and lighting conditions need to be carefully monitored and adjusted to meet the specific needs of each plant species. Variations in these parameters can lead to inconsistent growth rates, nutrient deficiencies, or excesses, affecting the overall performance of the hydroponic system. a hydroponic system, plant development and growth can vary between individual plants or even different parts of the same plant. Variability can arise due to genetic differences, environmental conditions, or physiological factors. This variability can result in variations in plant size, yield, and nutrient uptake, making it challenging to achieve uniform

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and predictable crop production. Managing plant variability in hydroponics requires careful selection of plant varieties that are well-suited to the system’s conditions and goals. Additionally, implementing strategies such as regular monitoring, adjusting nutrient solutions, and providing appropriate lighting and temperature conditions can help mitigate variability. Another aspect of plant development in hydroponics is maintaining a balanced nutrient uptake. Nutrient imbalances can occur when certain elements are either deficient or present in excess. This can lead to stunted growth, nutrient deficiencies, or toxicity symptoms in plants. Careful monitoring of nutrient levels, pH, and electrical conductivity of the nutrient solution is crucial to ensure proper nutrient uptake and avoid imbalances (Berkelaar & Hale, 2003). Berkelaar and Hale (2003) It was advised that while using organic ligands as a reservoir of chelated metal in solution, short-term metal accumulation studies be employed to keep conditions sterile and prevent biodegradation. However, it has been demonstrated that the initial rapid metal sorption from treatment solutions does not always represent the regular absorption rate of plants growing in the treatment solutions under steady-state metal conditions (López-Chuken & Young, 2010). The mechanism of this phenomenon was discovered to be a rapid approach to a pseudoequilibrium state between root surface sorption sites and nutrient solutions. Furthermore, even when aseptic conditions were not strictly adhered to, organic matter in the solution of long-term experiments (6–8 weeks) had no effect on how the metals altered.(López-Chuken & Young, 2010).

5.8.2 Can Rhizofiltration Effectiveness Be Extrapolated to Soil Pollution? Rhizofiltration is a phytoremediation technique that filters contaminants from water using plant roots. It has been demonstrated to be a successful and environmentally acceptable solution for treating various types of water pollution. However, if you want to employ rhizofiltration to clean up soil pollution, you must consider several factors. Rhizofiltration is usually used to remove contaminants that dissolve quickly in water, such as heavy metals and organic pollution. Soil pollution, on the other hand, is made up of a complex mix of contaminants that can be solid particles, bound to soil particles, or in different phases. The way contaminants and soil particles mix can have a huge impact on how bioavailable and mobile they are, making cleanup more difficult. Several factors influence how well rhizofiltration cleans up dirt. The ability of plant roots to uptake and store pollutants from the soil is critical for plant growth (Chaney et al., 2005). Metal hyperaccumulation is a specific method that plants use to deposit huge levels of metals in their tissues. However, not all pollutants are absorbed by plants, and the effectiveness of rhizofiltration may be dependent on the specific contaminants in the soil. Soil factors can have a significant impact on rhizofiltration effectiveness. Soil pH, organic matter content, and nutrient availability can all

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influence how plants develop and how well their roots absorb poisons. Furthermore, other soil components, such as clay minerals, can alter how pollutants are absorbed and released, thereby limiting their bioavailability for plant uptake. The length of rhizofiltration treatment may vary for soil pollution as opposed to water pollution. Long-term pollution, which may have accumulated over many years or even decades, is frequently the source of soil pollution. Rhizofiltration alone may not be sufficient to significantly reduce soil pollutant levels, and further rhizofiltration remediation techniques may be necessary to get satisfactory results. Extrapolation is difficult due to the unique properties of soil, the way pollutants work, and the interactions of plants, roots, and soil particles. Rhizofiltration can be utilised in soil remediation, but other approaches should be considered and the strategy tailored to the specific soil pollution problem to ensure effective and full remediation.

5.8.3 Importance of Root Surface Area in Expressing Metal Uptake The root surface area plays a crucial role in determining the uptake of metals by plants. It serves as the primary interface between the plant and its surrounding environment, through which the plant absorbs water, nutrients, and other essential elements. The root system’s surface area directly influences the plant’s ability to acquire metals from the soil. A larger root surface area provides a greater contact area with the soil, enhancing the plant’s capacity to uptake metals. The increased surface area allows for more extensive exploration of the soil volume, increasing the chances of encountering metal ions and other essential elements (López-Chuken & Young, 2010). This is particularly important because metals in the soil are often present in low concentrations and unevenly distributed. Therefore, a larger root surface area improves the probability of encountering metal-rich soil patches. Additionally, the root surface area influences the density and distribution of root hairs, which are responsible for most of the nutrient and metal uptake. Root hairs are elongated, thin-walled outgrowths of epidermal cells that significantly increase the surface area of the roots. They possess specialized transporters and channels that facilitate the absorption of metals. A greater root surface area allows for a higher density of root hairs, resulting in an increased uptake capacity for metals. the root surface area indirectly affects metal uptake by influencing root exudation. Root exudates are organic compounds released by plant roots into the rhizosphere, the soil surrounding the roots. These compounds can influence metal speciation and availability in the soil, affecting metal uptake by plants. A larger root surface area means a greater exudation capacity, potentially altering the chemical properties of the rhizosphere and subsequently influencing metal solubility and uptake. A larger surface area facilitates greater contact with the soil, enhances the density and distribution of root hairs, and increases the capacity for root exudation. These factors collectively contribute to the

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plant’s ability to acquire metals from the soil, making root surface area an essential parameter to consider in understanding metal uptake by plants (López et al., 2005).

5.8.4 Utilization of Phytoremediation By-Products Phytoremediation, the use of plants to mitigate and clean up contaminated environments, has gained significant attention as a sustainable and cost-effective approach for environmental remediation. As plants uptake and accumulate contaminants from the soil, they can produce various by-products during the phytoremediation process. These by-products can have significant potential for utilization, providing added value to the phytoremediation process and promoting a circular economy. One of the main by-products of phytoremediation is the harvested biomass of the plants themselves. This biomass can be utilized in several ways. For instance, it can be used as a source of renewable energy through processes like combustion or anaerobic digestion, generating heat, electricity, or biofuels (Blaylock & Huang, 2000). The residual ash obtained from biomass combustion can also be rich in minerals and can be used as a fertilizer or soil amendment, returning valuable nutrients to the soil (Keller et al., 2005). In addition to biomass, phytoremediation can yield other by-products such as essential oils, organic acids, and enzymes. Essential oils extracted from plants used in phytoremediation can have medicinal or aromatic properties and can be utilized in various industries, including cosmetics, pharmaceuticals, and perfumes. Organic acids and enzymes produced by plants can have applications in bioremediation processes, wastewater treatment, or as natural additives in industrial processes. phytoremediation can promote the phytoextraction of valuable metals from contaminated soils. Once the plants accumulate metals in their tissues, the harvested biomass can be subjected to metal recovery techniques such as smelting, electroplating, or bioleaching. These processes can extract the metals from the plant biomass, allowing for their reuse or recycling in various industries. The utilization of phytoremediation by-products offers a sustainable and economically viable solution for managing and extracting value from contaminated environments. It not only helps in the remediation of polluted sites but also contributes to resource recovery and the establishment of a circular economy. By exploring and maximizing the potential of phytoremediation by-products, we can create a more sustainable and environmentally friendly approach to environmental remediation (Ghosh & Singh, 2005).

5.9 Cost Estimates Using Rhizofiltration Rhizofiltration is an innovative and cost-effective method for treating contaminated water using plants and their root systems. This eco-friendly approach harnesses the natural abilities of certain plant species to absorb, accumulate, and degrade various pollutants, including heavy metals, organic compounds, and nutrients. By utilizing

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rhizofiltration, significant cost savings can be achieved compared to traditional water treatment methods. One of the primary advantages of rhizofiltration is its relatively low cost (Dushenkov et al., 1995). Unlike conventional treatment technologies that require complex infrastructure and high energy consumption, rhizofiltration relies on the natural processes of plants, which are inexpensive to maintain. The initial setup cost mainly involves selecting appropriate plant species, setting up a system for water circulation, and establishing suitable conditions for plant growth. Once the system is in place, the ongoing operational costs are minimal, primarily limited to periodic monitoring and maintenance. Furthermore, the use of plants in rhizofiltration offers additional cost benefits. Plants can be easily propagated, making it feasible to establish large-scale rhizofiltration systems at a relatively low cost. Additionally, the plants used in rhizofiltration can be sourced locally, reducing transportation and procurement expenses. This makes rhizofiltration an attractive option, especially in regions with limited financial resources or where access to traditional water treatment infrastructure is challenging. Another cost advantage of rhizofiltration is its potential for resource recovery. Some pollutants absorbed by plants during the filtration process can be extracted and utilized for various purposes. For instance, certain heavy metal pollutants can be recovered from the plant biomass and recycled or sold, offsetting the operational costs further. This aspect of rhizofiltration not only helps in cost reduction but also promotes the circular economy and sustainability (Candelario-Torres et al., 2009).

5.10 Rhizofiltration and Sustainable Development Rhizofiltration, a form of phytoremediation, is an environmentally friendly and sustainable technology that has gained significant attention in the field of water purification. It involves the use of plants and their associated root systems to remove pollutants and contaminants from soil and water. This innovative approach not only helps in addressing pollution-related issues but also contributes to sustainable development. One of the key advantages of rhizofiltration is its ability to remove a wide range of contaminants, including heavy metals, organic compounds, and nutrients, from contaminated water sources. The plants used in this process, known as hyperaccumulators, have the capacity to absorb and accumulate pollutants in their roots, stems, and leaves. This natural mechanism effectively reduces the concentration of pollutants, making the water safe for various purposes such as irrigation or even drinking. Rhizofiltration offers several sustainable development benefits. Firstly, it provides an eco-friendly alternative to conventional water treatment methods that often rely on chemical-based processes. By harnessing the power of plants, rhizofiltration minimizes the use of chemicals and energy-intensive techniques, thus reducing the carbon footprint associated with water purification. Secondly, rhizofiltration promotes biodiversity and ecological balance. As plants play a vital role in the purification process, the presence of diverse plant species enhances the overall ecosystem. the treated water can be discharged into natural water bodies, thus supporting aquatic life and

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preserving the ecological integrity of the surrounding environment. Rhizofiltration can be easily implemented in both industrial and domestic settings. Its versatility and cost-effectiveness make it an attractive option for communities facing water pollution issues, particularly in areas where access to clean water is limited. By utilizing locally available plants, rhizofiltration can be tailored to specific regional needs, promoting self-sufficiency and empowering communities to take ownership of their water purification processes (IRC—International Water and Sanitation Centre, 2011).

5.11 Conclusions Rhizofiltration is an emerging and promising method for effectively addressing metals and inorganic contaminants commonly found at waste sites. This clean-up strategy is gaining attention due to its low cost, environmental friendliness, and unobtrusive nature, offering an alternative to expensive conventional technologies. To ensure successful rhizofiltration, careful selection of plant species is crucial. The ideal plants should exhibit rapid growth in hydroponic systems, have a high biomass, and demonstrate efficient metal uptake. One of the notable advantages of rhizofiltration is its flexibility, allowing for the use of both terrestrial and aquatic plants. Although terrestrial plants require support, they generally outperform aquatic plants in removing pollutants. Rhizofiltration holds great potential for application and should be promoted and expanded, especially in developing countries grappling with pollution challenges. Rhizofiltration systems can be implemented either in situ, such as floating rafts on ponds, or ex situ, involving specially designed tank systems. However, several challenges accompany rhizofiltration. Firstly, the pH of polluted water needs constant adjustment to optimize metal uptake. Secondly, a thorough understanding of chemical speciation and interactions among metallic species in the influent is essential. Additionally, an engineered system is required to control the influent flow rate, and in some cases, greenhouse cultivation may be necessary. Periodic harvesting and proper disposal of plants are also necessary for efficient operation. Moreover, it is important to note that metal uptake results obtained from laboratory studies may sometimes overestimate the actual performance. rhizofiltration represents a promising and cost-effective approach for remediating metals and inorganic contaminants at waste sites. Its versatility, low environmental impact, and potential application in various settings make it particularly valuable, especially in developing countries. Overcoming the associated challenges and advancing research in plant selection, pH adjustment, and influent control will contribute to the wider adoption and successful implementation of rhizofiltration as a sustainable solution for addressing contamination issues. Phytoremediation, as a multidisciplinary technology, holds significant potential for further study and advancement in various fields. By unraveling the intricate pathways and genes responsible for pollutant uptake, transformation, and sequestration in plants, researchers can identify new genetic targets. This knowledge can facilitate the

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creation of genetically modified plants with improved remediation traits, increasing their efficiency in cleaning up contaminated sites. Understanding ecological interactions, such as plant–microbe interactions in the rhizosphere: The rhizosphere, the soil zone influenced by plant roots, plays a crucial role in phytoremediation processes. Investigating the complex interactions between plants and microorganisms in the rhizosphere can provide insights into how these interactions impact pollutant remediation. This understanding can guide the design of more effective rhizofiltration programs and help mitigate associated risks. Advancements in knowledge will enable the tailoring of rhizofiltration systems to suit specific polluted water conditions. For instance, in areas with diverse contaminants, a combination of plant species with complementary remediation abilities can be selected to achieve comprehensive cleanup. Moreover, the use of native plant species during the remediation process can aid in ecosystem restoration, promoting the recovery of the natural environment (Pilon-Smits & Freeman, 2006). Acknowledgements I wish to express my deepest gratitude to my both supervisors, Professor Dr. Gunjan and Dr. Saurabh, who guided and encouraged me towards being more professional. Without their persistent guidance and support, the completion of this work would not have been possible. I would like to acknowledge the support and great love of my family. This work would not have been possible without their grateful and understanding input.

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Chapter 6

Hydroponic Removal of Organic Contaminants from Water Prasann Kumar and Debjani Choudhury

Abstract With an expanding population, food production and other developmental activities increase. These activities are the primary cause of increasing soil, water and environmental pollution. Several physical, chemical and biological methods have been developed to remove these contaminants, but these methods could be more economical and environmentally sound. Nowadays, organic pollutants like pesticides, dyes, pharmaceuticals, oils, phenols, etc., have become a severe problem causing pollution to the aquatic environment. Hydroponics is gaining importance because it is environmentally friendly and cost-effective. Many aquatic plants like Lemma minor, Eichhornia crassipes, Junsus effuses, Pontederia cordata, Potamogeton crispus, etc., can reclamation organic pollutants in water. There are various techniques like rhizofiltration, phytodegradation, phytovolatilisation, phytoextraction and phytostabilization utilizing which these plants can remove the contaminants from water. This chapter discusses macrophytes’ potential to remove organic pollutants thoroughly. Keywords No poverty · Zero hunger · Aquatic · Contamination · Macrophytes · Rhizofiltration · Phenols

6.1 Introduction Organic pollutants have become a severe concern in causing contamination in different water resources because of increasing population, transport, mining and smelting, urbanization, and industrializatio (Enyoh et al., 2018). Availability of sufficient fresh water has become a serious problem so far, therefore a proper technology P. Kumar (B) Department of Agronomy, School of Agriculture, Lovely Professional University, Phagwara, Punjab 144411, India e-mail: [email protected] D. Choudhury Department of Plant Pathology, School of Agriculture, Lovely Professional University, Phagwara, Punjab 144411, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_6

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should be adopted so that a sufficient amount of fresh water must be available. It is not easy to destroy these contaminants with physical, chemical and biological methods because these techniques are costly and difficult to apply. So, there is an alternate method of transferring these contaminants into less toxic forms (Jiang et al., 2018), which are easy to apply and low in cost. The life of the entire aquatic ecosystem i.e., plants, animals, and microorganisms can get disturbed due to contamination. Acid rains, industrialization, agricultural chemicals, heavy metals, household wastes, and organic and inorganic chemicals are the leading causes of water contamination. Lead (Pb), Cadmium (Cd), Copper (Cu), Selenium (Se), nickel (Ni), and Zinc (Zn) are the primary heavy metals pollutants that create contamination. This is a serious and dangerous issue worldwide and is still uncontrollable because of a lack of awareness and requires strict implementation of environmentally friendly policies (Eid et al., 2020). These heavy metals are the significant causes of asthma, cancer, skin disorders, dehydration, breathing problems, excretory and cardiovascular problems and depressed growth in human beings. Cadmium mainly affects the cell membrane in animals and human beings. Chromium is carcinogenic and detrimental to living organisms. These problems can be solved by the use of hydroponics, which helps in the removal of contaminants from the environment. Hydroponics are plants cultivated in nutrient-enriched media with or without any mechanical support of inert material like sand, gravel, or perlite. This technique is also known as agriculture, soilless culture, or tank farming. These green plants can be used in removing organic contaminants and are known as phytoremediation. Phytoremediation has gained attention as it is a cost-effective and complementary technology for remediation (Doran, 2009). The use of aquatic organisms like some higher plants and green algae were used to remove heavy metals, pesticides, antibiotics, detergents, nitrogen, phosphorus, and some persistent organic contaminants, including polycyclic aromatic hydrocarbons (PAHs) (Chen et al., 2014). Phytoremediation is an environment-friendly process where fast-growing plants eliminate organic contaminants from water or soil (Mahar et al., 2016). It also helps detoxify contaminants (Zhang et al., 2010). If a plant can uptake and store the contaminants in its shoots, it can be a successful phytoremediation method. Phytoremediation is divided into five subclasses (Ramanjaneyulu et al., 2017). There are certain limitations to applying phytoremediation, especially in the presence of high concentrations of organic contaminants in wastewater, which is how a plant can survive with high concentrations of contaminants (Fig. 6.1).

6.2 Contamination of Water and Its Security Surface water supports living beings and ecosystems. Due to the increase in population, freshwater availability is under pressure because living beings require safe water for survival (Heathwaite, 2010). Of the total global water budget, the freshwater resource comprises 2.5%, out of which only 0.0072% (93,120 km3 worldwide fresh water is available for different purposes like industries, energy and food production and drinking water (Lawford et al., 2013; Zimmerman et al., 2008). By 2050,

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Fig. 6.1 Pollution of the aquatic environment from different sources

crop production will increase by 100–110% to feed the growing population, which may lead to a global water shortage (Rockström et al., 2014). Due to stressors like diffuse pollution, point sources, climate change, and land-use changes, many surface waters are currently sub-optimal. Due to these stressors, water scarcity is a challenge (Berger et al., 2017). In the UK, due to diffuse pollution, there is significant pressure on water quality (Ulén et al., 2007). In other countries like China, heavy metals are the prominent cause of water pollution (Cheng, 2003). It is believed that the interaction between these stressors in space and time leads to addictive effects (Heathwaite, 2010). For example, the increase in land-use change towards intensive agriculture and the increase in storm frequency may increase the delivery of phosphorus, nitrogen and fine sediment to receiving water (Dunn et al., 2012). Various water pollutants cause pollution to the water environment, like organic pollutants due to pesticides, hydrocarbons and toxins released by algae, or inorganic pollutants like metals or synthetic fertilizers, which contain excessive amounts of N and P. However, there are multiple sources for different pollutants like nitrogen and phosphorus released from agriculture, aquaculture and urban wastewater streams. The vital organic pollutants from different sources that affect the environment inside water are summarized in Table 6.1. To improve the water quality, the management of waterborne pollutants can be done through the in situ method, which is the best management practice used to target the pollution source (Lam et al., 2011). However, the lag time required for water quality improvement and ecological recovery of receiving waters following

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Table 6.1 Key organic pollutants affecting the aquatic environment Pollutant source

Pollutant type

Example of a pollutant

Impacts

Agriculture and industry

Xenobiotics and persistent organic pollutants

Polychlorinated Biphenyls, polycyclic aromatic hydrocarbons, organochlorines, dioxins

Toxicity endocrine disrupting effects affect the reproductive system

Aquaculture and agriculture

Synthetic chemicals

Deltamethrin, glyphosate, Fenhexamid, Aldrin, Hexachlorocyclohexane, Methoxychlor, dieldrin, Chlorpyriphos, endrin, dicofol, Chlordane, endosulfan, heptachlor, lindane, dicofol, heptachlor, Toxaphene, DDT, dioxins, polychlorinated dibenzofurans

Toxicity Endocrine disrupting effects, Effects reproductive system

Aquaculture, agriculture and domestic

Personal care products, organofluorine and pharmaceuticals

Painkillers, antibiotics and hormones Endocrine disrupting effects Antibiotic resistance Destabilising microbial communities

Cyanobacterial Toxins released by and algal blooms algae

Microcystin-LR

Acute/chronic toxicity

Domestic, agriculture and aquaculture

E. coli O157 Cryptosporidium parvum

Human illness (intestinal infection)

Pathogens and parasites

mitigation may range from 1 to >50 years (Meals et al., 2010). The improvement of water quality is also delayed due to legacy effects, i.e. colonization of an area by plants and animals where there is no or little biological material (Haygarth et al., 2014). The places with residence for extended periods become the reservoirs of pollutants from where the pollutants are transferred to the water sources through rainwater and drainage systems (Meals et al., 2010). Therefore, management development should be combined with other best management practices of remediating water with high pollutants needed to improve the water quality sustainably. Their phytoremediation using hydroponics has a potential management strategy for both surface water sustainable remediation and recovery of nutrients.

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6.3 Phytoremediation: A Plant-Based Eco-Friendly Technology The United Nations Environment Program defines phytoremediation as “the efficient use of plants to remove, detoxify or immobilise environmental contaminants”. It is an environmentally friendly, advantageous technique that helps remove contaminated soil and water. It involves the roots in the absorption of pollutants and accumulates in body tissues, transforming highly toxic forms into less toxic ones. These techniques are used worldwide to clean up toxicants. The phytoremediation technique helps clean water contaminants and has gained serious attention from government and nongovernment bodies and research workers. However, since 300 years ago, these plants have been used to treat wastewater (Carolin et al., 2017). There are reports of various aquatic plants belonging to different families like Ranunculaceae, Lemnaceae, Cyperaceae, Haloragaceae, Hydrocharitaceae, Potamogetonaceae, Typhaceae, Najadaceae, Pontederiaceae, Juncaceae, and Zosterophyllaceae in accumulating organic and inorganic contaminants from water through field or hydroponic applications. Phytoremediation has become an efficient technique to remove unparalleled pollution in the aquatic world (Fig. 6.2). Identifying and screening plants showing excellent efficacy in accumulating dissolved nutrients and contaminants is the first step of phytoremediation (Lu, 2009). Especially the selection of fast-growing plants is necessary, which can be easily handled and harvested (Stefani et al., 2011). The growth development and photosynthesis ability of these plants are the other vital factors for the biological growth of the plants to sustain in the aquatic environment. The success of this system also depends on the factors related to the severity of the pollution (Jamuna & Noorjahan, 2009). Removal of organic pollutants through hydroponics is a phytotechnology through which pollutants are removed from impacted water bodies like rivers, lakes, ponds, small streams, etc. The plants can be cultured within the surface water to remove the pollutants from the water column and sediments (Newete & Byrne, 2016). They can also be deployed at the source or within the waterbodies where a problem occurs due to diffuse pollution (due to pesticides and fertiliser application, grazing of livestock and slurry storage and forest operations) (Lu et al., 2011). Macrophytes like freshwater angiosperms, ferns and pteridophytes can be used in aquatic phytoremediation to remove and degrade water pollutants (Rai, 2009). These macrophytes can uptake nutrients and other substances from the medium where they are grown, thus lowering the pollution concentration from the water bodies (Dhote & Dixit, 2009). There is various technique for the removal of contaminants from the environment are phytodegradation, phytostabilization, rhizofiltration, rhizodegradation, and phytovolatilization (Fig. 6.2) (Baker et al., 1994). Emergent and floating macrophytes primarily uptake nutrients through roots, whereas submerged macrophytes use stem tissues to remove pollutants from the water column (Dhote & Dixit, 2009). Depending

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on the types of pollutant (heavy metals, nutrients, biological and organic pollutants) and the location of the pollutant (water column, lake or sediment), specific mechanisms are required for the removal of pollutants from the water bodies (McAndrew et al., 2016; Polecho´nska & Samecka-Cymerman, 2016). Different mechanisms for the removal of pollutants from water systems by hydroponics are considered below (Table 6.2, Fig. 6.2).

Fig. 6.2 Different mechanisms for the removal of pollutants from water systems by hydroponics

Table 6.2 Different categories and mechanisms for the removal of various contaminants Contaminant category

Mechanism

Medium

Organic/ inorganic /heavy metal

Rhizofiltration/Phytofiltration

water

Inorganic/heavy metals

Phytoextraction/Phytoaccumulation Soil/water

Inorganic/heavy metals

Phytostabilization

Soil/ sediment

Organics

Phytovolatilization

Soil/sediment/water

Organic/inorganic/ microbiological Phytodegradation

Soil/sediment/water

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6.3.1 Rhizofiiltration Rhizofiltration is a type of phytoremediation in which hydroponic plants help remove contamination through the absorption, concentration, and precipitation of pollutants (Dushenkov et al., 1995). From the waste sites, contaminated water is collected and brought to the plants or plants grown in the contaminated water. The roots uptake water and convert highly toxic chemicals into less toxic forms.

6.3.2 Phytoextraction It is an economical process (Wan et al., 2016) in which fast-growing plants can be used to remove heavy metals (Pajevi´c et al., 2016). It involves two approaches: (a) natural or continuous phytoextraction and (b) chemically induced phytoextraction (Ghosh & Singh, 2005). In the natural process, the root networks remove heavy metals directed to the upper part of the plant tissues above ground level (Jadia & Fulekar, 2008). This method effectively reduces heavy metal concentration in soil by shooting and rooting without affecting soil properties. Harvested plant biomass is used for biogas production and can also be burned, which can be utilized for metal recovery. This process is known as biomining or phytomining (Bhargava & Singh, 2017). When such plants are continuously grown and harvested, it reduces contamination in the soil (Vandenhove et al., 2001). The plants used for phytoextraction must have an extended network of roots, high biomass, and rapid growth. They must be able to bear and store heavy metals.

6.3.3 Phytostabilization This is also known as phytoimmobilization or phytorestoration, a plant-based approach to dealing with soils contaminated with metals. The exudates from the roots of the plants help reduce contaminants in the soil. The contaminants bind to the roots of the plants and get stabilized, known as auto stabilization. Further, the accumulation, precipitation, and absorption of contaminants in soil and water occur in certain plant species’ roots through immobilization. This method helps eliminate organic and inorganic contaminants in the soil, sediments, and sludge media (Baker et al., 1994; Brooks et al., 1998; Cunningham & Ow, 1996). Phytostabilization is a functional plant-based approach for metals-contaminated soils (Ramanjaneyulu et al., 2017). The main objective behind this technique is to reduce the mobility of metal contaminants leaching into the groundwater and food chain (Khalid et al., 2017). Using sorption onto roots, metal contaminants are physically and chemically immobilized and fixed with different soil amendments (Wuana & Okieimen, 2011). The most efficient soil amendments to immobilize heavy metals are organic matter, phosphate

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fertilizers, clay minerals, and biosolids. The main objective behind growing these types of plants is to reduce water percolation and soil erosion, decrease contaminants migration and limit contact with contaminants (Akhtar et al., 2013). This method is ineffective because metal contaminants persist in the soil (Vangronsveld et al., 2009).

6.3.4 Phytovolatilization In this process, plants are used and consume organic contaminants. Through phytovolatilization, organic contaminants can be volatilized into the atmosphere. It is a cost-effective process that eliminates contaminants from wastewater, soils, residues and groundwater. Plants metabolize contaminants by producing compounds within plant tissues using the phytotransformation/phytodegradation process (Girdhar et al., 2014).

6.3.5 Phytodegradation In this process, enzymes released by roots or through metabolic activities within plant tissues help degrade organic contaminants. Roots absorb toxic substances, and they are converted into less toxic substances within plant tissues. In this process, soil microbial populations are sustained through root exudates. Specific microbial populations can also be added to contaminated soils to enhance this process. This is because these microbe strains have desired metabolic activity, which helps degradation of targeted contaminants. Moreover, promising results have been observed in plants inoculated with genetically engineered microbe strains that can degrade specific organic contaminants. This is an attractive alternative phytoremediation technique (Verma et al., 2008). For the removal of heavy metals from wastewater, a method has been developed for effective removal (Ali et al., 2020).

6.4 Types of Pollutants in an Aquatic Ecosystem Due to the attribution of organic, inorganic and anthropogenic materials, contamination of the aquatic environment occurs (Fig. 6.3). Various industries discharge hot water from thermal power plants, mine tailings and heavy metals, which harm aquatic environments. Agriculture wastes are divided into organic and inorganic materials (Milovanovic, 2007). Organic compounds include oils and synthetic chemicals like carbamates, organophosphates, and organochlorines group of chemicals, which includes endosulfan, Aldrin, dieldrin, trichloroethene, methoxychlor, DDT, diazinon, aldicarb, malathion, endosulfan, lindane aldicarb, carbofuran, toxaphene, carbaryl etc. Inorganic compounds contain phosphates, nitrates and many other

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Fig. 6.3 Types of pollutants in aquatic ecosystem causing severe damage

chemicals (Tiwari et al., 2019). Heavy metals like cadmium, lead, zinc, and mercury from different industries are inorganic contaminants.

6.5 Sources of Organic Pollution Organic pollutants are emitted primarily from household practices, industrial activities, industrial practices and military waste. Soil, water, fertilizer and synthetic chemicals are the main parts of agriculture. Due to pesticide use in different agricultural activities, organic pollution occurs in urban areas (Ratnakar et al., 2016). Organic pollutants are categorized into three groups: (a) nitrogen, phosphorus and oxygen compounds (b) organometallic compounds; and (c) hydrocarbons. DDT, PAHs and dioxins are the most toxic hydrocarbons. These compounds discharge into the environment and cause severe ecological toxicity. Disposing of these contaminants in pits or landfills contributes to soil and groundwater contamination. Polychlorobiphenyls (PCBs) and hexachlorobenzene (HCB) are the most common chemicals released by different industries. Chlorinated pesticides, aromatics, waste materials, incomplete combustion, and old disposed-of areas release HCBs. Rubber and plastic products contain pigments, dyes, and fluorescent lighting PCBs. Through various stationaries, fossil fuel burning, sewage and sludge, polychlorinated dibenzodioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are unintentionally released.

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6.6 Toxicity of Organic Pollution on Plants and Animals Persistent organic contaminants (POCs) are not naturally degradable and not soluble in water but are highly soluble in lipids. Humans, animals and plants get exposed to these organic contaminants due to their exposure to the environment through water and soil. These contaminants are very toxic to them. These contaminants are persistent in the environment. They contaminate at the origin and a distance from their source. Excessive pesticides hampers seed germination and crop production by hampering plant metabolic processes and photosynthesis, reducing yield. Through biomagnification and accumulation in fatty tissues, fat-soluble pesticides enter animal bodies. These pesticides remain in the food chain for a certain period, causing negative impacts like hormone imbalance, nervous and reproductive disorders, poor immune system and deformities (De Solla et al., 2007). Hydrochlorobenzene compounds are persistent and mobile, distributed throughout the environment using soil and water. They have toxic effects on humans, plants, animals and other organisms inside water. These compounds also bioaccumulate inside fishes and other aquatic organisms. When an organism is highly exposed to HCBs, it affects the reproductive system and may cause ovarian toxicity. HCBs can affect the skin, thyroid glands, bones and nervous system by inhaling polluted air near industries and ingesting contaminated foods. Prolonged exposure may lead to liver diseases and abnormal physical development in children. In 1996, polychlorinated biphenyls were human-made organic compounds first reported environmental pollutants. PCBs are released into the environment from electrical transformers, industrial and municipal wastes, and hazardous waste sites. Due to the lipophilic properties of these compounds, they tend to bioaccumulate within plants and animal tissues and persist in soil, water and air for an extended period. PCB compounds remain in sediments, water, fish, and bird tissues. PCBs enter food chains through plants, where leaves and other above-ground parts of plants accumulate these compounds (Campanella et al., 2002). Besides being carcinogenic to humans and animals, chronic exposure to PCBs can lead to endocrine, reproductive, immunological, and neurobehavioral disorders in children (Pieper & Seeger, 2008). Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) are exposed to humans through polluted air, skin contact and diet. These are pervasive in the environment and accumulate in plant parts above ground level. These compounds are carcinogenic and cause reproductive and hormonal problems and kidney and hepatic lesions when exposed chronically. Delayed lethality, thymic atrophy, gastrointestinal problems, and body weight gain and loss, occur due to acute toxicity.

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6.7 Abundance and Ecology of Aquatic Macrophytes In aquatic ecosystems, macrophytes are the most critical biotic components. Other aquatic organisms depend on these plants for their food and shelter. Aquatic macrophytes help control sediment erosion, create variation in buffer temperature, absorb pollutants, sequester nutrients, absorb pesticides and POPs, and stabilize dissolved which helps marinate a healthy ecosystem. There are seven categories of aquatic plants, which include (a) Bryophyta (mosses and liverworts), (b) Chlorophyta (green algae) (c) Cyanobacteria (blue-green algae) (d) Pteridophyta (ferns), (e) Rhodophyta (red algae), Spermatophyta (seed-bearing plants) and xanthophyta (yellow-green algae) (Chambers et al., 2008). Classification of macrophytes can be done in these groups: (a) floating hydroponics, which occupies the water surface (free-floating or rooted), e.g. Lemna (duckweeds), Hydrocharis (frogbit) and Nymphaea (water lilies) (b) submerged hydroponics, which grows below the surface water. Ceratophyllum (hornwort) (c) emergent macrophytes, which occupy water body margins rooted into substrates and shoot above water level, e.g. Typha (reedmace) and Phragmites (common reed). Due to these different growth forms, pollutants are removed from both water columns and sediments (Newete & Byrne, 2016). These plants can complete their life cycle in water and hydric soils (Gecheva et al., 2013). Local habitats, like sediment composition, nutrient supply, light availability and current velocity, determine macrophytes’ growth (Birk & Willby, 2010). However, the diversity of macrophytes is affected by human impact on land use change and hydrological dynamics (Oˇtaheˇlová et al., 2007). In many cases, it has been observed that these plants change in morphological and anatomical adaptations depending on habitats. For example, Eichhornia crassipes, Hippuris vulgaris, Potamogeton amplifolius, Nymphaea nouchali, Myriophyllum brasiliense and Equisetum fluviatile are freefloating, and the aerial leaves produced by them help to absorb CO2 directly from the atmosphere. Some species absorb high CO2 concentration through roots, e.g. Lobelia and Littorella. For easy movement of CO2 from roots to leaves, these plants produce modified transport vessels (Thomaz et al., 2009). High level of nutrients, essential and non-essential metals and pollutants results in macrophyte blooming in the aquatic ecosystem. These macrophytes are also known for their use in eutrophication and controlling pollution in aquatic environments. These plants are used as environment filters for treating wastewater because of their high growth rate and fast nutrient assimilation rate. Various studies have determined these plants’ purification potential and shown significant results that vary among plant species (e.g., Floating leaves, emergent and submerged) (Victor et al., 2016). Floating macrophytes such as Pistia stratiotes, Eichornia spp, and Salvinia spp. are used for wastewater remediation because water turbidity does not affect these plants. Additionally, these plants are easier to manage and, whenever needed, can be harvested. Some emergent plants, such as Typta spp. and Juncus spp. have the efficiency in adsorbing organic and inorganic contaminants due to their fast adsorption capacity and post-precipitation (Thomaz et al., 2009).

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6.8 Organic Pollutants Organic pollutants are compounds containing synthetic carbon that persist in the environment and are potentially toxic. Organic pollutants include solvents, pharmaceuticals, pesticides and personal care products (Table 6.1). To remove organic contaminants from the water column and sediments, hypometabolism and rhizodegradation are integral processes (Reinhold et al., 2010). In hydrophilic organic compounds, phytometabolism occurs, i.e. they can pass through the epidermis into the plant’s cells (Yamazaki et al., 2015). The hidden compounds undergo modification through chemical means using oxidation, reduction and hydrolysis, which makes them more active chemically within the cells of the plant and the less harmful metabolites are then coalesced or conjoined to sugars or amino acids or glutathione for reduction of toxicity and hydrophobicity (Geissen et al., 2015). These coalesced metabolites can become insoluble when bound within plant cells, stored in vacuoles, or excreted from the plant. Within the sediments, rhizodegradation occurs, and those hydrophobic compounds produced can serve as carbon sources for microbes where hydroponics supply oxygen to the root zone. Figure 6.3 illustrates that due to these two phytoremediation techniques, repeated harvests are unnecessary for pollutant extraction, ultimately reducing aquatic disturbance. Various reports of accumulation or uptake of organic pollutants like chlorobenzenes, organophosphorus, and organochlorine by aquatic plants are present (Table 6.3) (Ankit et al., 2020; Dhir et al., 2009; Fletcher et al., 2020). It depends on aquatic plant species, the physicochemical characteristics of pollutants and plant tissue biochemical composition on how much organic compound can be sequestered by the plants. The protonated form of contaminants is uptake passively. The contaminant protonated form is considered a species available for biotic partitioning in plants, which connect in pairs to enzymatic transformation and categorise in vacuoles (Tripathi et al., 1996). Concentration-dependent pseudo-first-order rate coefficients are required to uptake organic contaminants like halogenated phenols with hydroponics (Tripathi et al., 2003). Organic halogenated compound sequestration using plants includes physical and chemical processes. In the physical process, adsorption, absorption, and partitioning occur, while the chemical process includes reaction and complexation with membrane and cuticular components (Nzengung & Jeffers, 2001). If the plant has a lipid-rich cuticle, then the plant has the potential to sequester lipophilic organic contaminants (Gao et al., 2000a, 2000b; Tripathi et al., 1996). Aquatic plants’ exposure to organic chemicals results in the uptake and accumulation of organic chemicals followed by degradation or transformation. These processes can be oxidative or reductive, which ultimately helps form metabolites integrated with plants using covalent binding (Nzengung & Jeffers, 2001). In the case of dehalogenated compounds like hexachloroethane (HCA), dichloro-diphenyltrichloroethane (DDT), etc., other chemicals dehalogenation reaction takes place, which is a process of phytoreduction. The products of phytoreduction are assimilated to the plant tissues or either get oxidized into polar compounds. However, the concentration of reduction products is higher than oxidation for any plant species (Nzengung & Jeffers, 2001). There

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are reports of reductive transformation of HCA to perchloroethylene and DDT to corresponding DDD analogues, plant-bound fractions and unknown products (Gao et al., 2000a, 2000b; Garrison et al., 2000). For halogenated organic compounds removal from water by aquatic plants involves mechanisms in which rapid withdrawal of organic compounds takes place by partitioning to the lipophilic plant cuticles, reducing into less halogenated metabolites, by photooxidation and formation of non-toxic products formed by covalent binding within the tissues of the plants. There are different dehalogenase enzymes which catalyze the photoreduction reactions in plants such as glutathione-S-transferase and Fe-S clusters in chloroplast ferredoxin while photo assimilation processes like photooxidation and covalent bonding are the reactions which are mediated by oxidative-enzymes (Nzengung & Jeffers, 2001). Glutathione-S-transferase enzymes do herbicides detoxification by conjugating them with tripeptide glutathione. This biodegradation involves dealkylation, which metabolizes simazine herbicide into 2-chloro-4amino-6-isopropylamino-striazine or hydroxysimazine, followed by storing the end products in the vacuoles of the plants as suggested by Knuteson et al., (Knuteson et al., 2002).

6.9 Macrophyte Phytoremediation Communities There has been an ample amount of work focused on the ability of one plant i.e. a monoculture of macrophyte species, to remove single pollutants from the aquatic environment (Zhou & Wang, 2010). There is a lack of research on the role of mixed groups of macrophytes in taking and degrading various contaminants (Koelbener et al., 2008). Therefore, there is an opportunity to apply a plant community-based approach, which helps remove single and multiple pollutants from the aquatic environment. Various reports of phytoremediation studies using multiple plant communities have shown promising results (Fraser et al., 2004; Liang et al., 2011; Türker et al., 2016; Zhang et al., 2007). Proper plant combinations depend on various factors, including the community’s functional diversity. Some reports show that increasing the diversity of plant species in a plant assemblage can increase nutrient removal, but if there are more than three species, it shows no benefit (Ge et al., 2015; Geng et al., 2017). Species identity is a crucial theme in different studies because a specific combination of species can more effectively remove contaminants. The potentiality of individual species in phytoremediation helps assemble appropriate plant communities. The interaction of these plants in an assemblage is more significant. In aquatic environments, species competition may impact the community composition and their ability to remove target pollutants (Zhang et al., 2007). Sago pondweed, broadleaved pondweed, curled pondweed, and horned pondweed reduced other plant species’ biomass in a submerged hydroponic experiment (Engelhardt & Ritchie, 2001). Removal efficacy does not need to vary with species by reducing biomass. Understanding the interactions between interspecific species is necessary to enhance contaminant removal efficiency (Engelhardt & Ritchie, 2002). Community plantbased approaches provide an opportunity to build more consistent phytoremediation

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Table 6.3 Aquatic plants in the removal of organic contaminants Aquatic plants species

Organic contaminants Pesticides

Eichhornis crassips

Ethion, pentachlorophenol, cyhalothrin, dicofol

Lemma gibba

2,4,5-trichlorophenol (TCP), Phenol

Lemma minute, Potamogeton crispus

Phenol

Lemma minor

Glyphosate, 2,4,5-trichlorophenol (TCP), isoproturon, halogenated phenols, 2,4-D, Ibuprofen, fluoxetine, and triclosan

Raygrass

Aniline and Phenol (Ruan et al., 2016)

Spirodela oligorrhiza

Chlorobenzenes, Organochlorine and organophosphorus compounds (PP-DDT and OP-DDT)

Myriophyllum aquaticum

Perchlorate, simazine, HCA, CT, OP,PPDDT,

Ceratophyllum demersum

Chlorobenzenes, organochlorine and organophosphorus compounds

Elodea canadensis

DDT, Phenanthracene, organochlorine, and organophosphorus compounds chlorobenzenes

Elodea

DDT, Carbon tetrachlorine (CT), Hexachloroethane (HCA)

Pontaderia cordata

Oryzalin (herbicide), Isoxaben, oryzalin

Scirpus lacustris

Phenanthracene

Eichhornia crassipes

Ethion

Junsus effusus, Ludwigia peploides

Atrazin, lambda-cyhalothrin

Spirodella oligorrhiza, Myriophyllum aquaticum

DDT (OP, PP-DDT)

Potamogeton crispus

Phenol, PAH (Phenenthrene and pyrene)

Elodea canadensis, Lemma minor, Cabomba aquatica

Flazasulfuron, copper sulfate, dimethomorph

Lemna minor, Pistia stratiotes Nymphaea alba, Phragmite australis, M. verticillatum

Chlorpyrifos

Eichhornia crassipes

2,4-D, Ethion, sodium tartrate, malathion, KHP,

Phragmites, Typha Ceratophyllum demersum

Endosulfan γ–HCH DDTs, Aldrin Pharmaceuticals and personal care producs (PPCP)

Phragmites australis

Estrone, 17, beta-estradiol, 17alpha-ethinylestradoil (continued)

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Table 6.3 (continued) Aquatic plants species

Organic contaminants

Scirpus validus

Caffeine, Diclofenac, Naproxen, Carbamazapine

Typha angustifolia

Ibuprofen, Diclofenac, Naproxen, Carbamazapine

Pontederia cordata, Sagittaria graminea, Typha latifolia

Triclosan, methyl triclosan and triclocarban

Pistia stratiotes, Eichhornia crassipes

Sulfamethazone (SMZ), Carbamazepine (CB), sulfadiazine (DIA), Sulfamethoxazole (SMX), ibuprofelinn (IB), and triclosan (TRI)

Typha spp.

Ibuprofen, clofibric acid (CA) and carbamazepine

Typha angustifolia

CB, declofenac, IB and naproxen

Pistia stratiotes Myriophyllum aquaticum,

Oxytetracycline (OTC) and tetracycline (TC)

Rhizophora apiculata Acrostichum aureum L.,

Ciprofloxacin (CIP), fluoroquinolones (FQs) and norfloxacin (NOR)

Lemna minor Landoltia punctate

Propyl parabens (PrP) and methyl parabens (MeP)

Lythrum salicaria Iris pseudacorus, Acorus calamus,

Atrazine DYES

Lemna minor

Malachite green, Triphenylmethane dyes (crystal violet and malachite green), Azo dye (AB92)

Eichhornia crassipes

black B (dye) and Red RB

Source Dhir et al. (2009), Ankit et al. (2020), and Fletcher et al. (2020)

techniques by exploring plant species’ phenology. Culturing multiple crops offers the most consistent options for water treatment with the least susceptibility to seasonal variation (Karathanasis et al., 2003). Moreover, dynamic plant communities within phytoremediation are under research.

6.10 Factors Affecting Phytoremediation of Organic Contaminants by Using Macrophytes Light, temperature, salinity, pH, and dissolved oxygen are water’s physicochemical and ecological parameters that affect the uptake of metals, nutrients or organic pollutants. However, micronutrients, macronutrients, non-mineral nutrients and temperature are the environmental conditions which affect the biochemical composition

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of hydroponics and also the phytoremedial potential of these plants (Juneja et al., 2013; Kalacheva et al., 2002). Moreover, the energy derived from photosynthesis and released oxygen can improve elemental absorption. In phytoremediation, pH and growth of the growing medium biomass play an essential role. The growth of these phytoremediations depends on pH because it controls CO2 and essential nutrients’ availability and solubility. This affects the growth of plants. The change in pH reduces growth and metabolic inhibition. This was proved in an experiment where hydroponics performance was deficient, below five and above 10. The maximum performance was observed at 7.5 pH, which suggests that pH should be 6–9 for better growth and biomass production (Juneja et al., 2013; Shah et al., 2014). This statement was proved in an experiment on water hyacinth growing best at 5.5–7.0 pH (El-Gendy et al., 2006).

6.10.1 Temperature For influencing the growth, cell size, biochemical composition and nutrient supplies of macrophytes, the temperature is one of the most significant environmental factors. Studies were conducted to know macrophytes’ performance at different temperatures. Water hyacinth, lettuce and duckweed were grown at low temperatures, showing no growth and no pollution below 10 °C. Their survival also ceased at low temperatures, and nutrient uptake was negligible by these plants. It was observed that municipal wastewater treatment requires a temperature between 15 and 38 °C, which is suitable for their growth. Temperate and frigid areas are not suitable for most aquatic macrophytes because they are temperature sensitive.

6.10.2 Plant Species Submerged species are efficient for phytoremediation because they accumulate more contaminants than emerging species (Albers & Camardese, 1993). This may be due to degradation and plant root disappearance, i.e. Ceratophyllum demersum do not produce a profound root system, but they develop root-like appearance in modified leaves and also produce a waxy coat which inhibits absorption by epidermal cells (Yurukova & Kochev, 1994). In another study, free-floating plants uptake heavy metals more efficiently than underwater and emergent plants. However, if the growth rate is high, the removal efficiency of organic contaminants is excellent, tolerance to high concentrations of heavy metals is high, and environmental adaptability is greater (Rezania et al., 2016).

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6.11 Advantages, Disadvantages and Future Trends in Phytoremediation Phytoremediation technology is a promising and cheap technology compared to conventional physical and chemical techniques (Mwegoha, 2008). After phytoextraction the plant biomass harvested can be used for bioenergy production (Dhiman et al., 2016; Tian & Zhang, 2016; Tripathi et al., 2016). This is an emerging field of research, with limited studies in the pot and few field studies. This technique is affected by temperature, light, pH, uneven pollutant distribution, nutrients, and moisture. This technology must be tested in vast areas. The success of this technique depends on the growth and biomass production of this plant and its ability to accumulate pollutants.

6.12 Conclusion Globally, the contamination of aquatic ecosystems has become a significant concern, and removing these contaminants is crucial. In addition to causing health problems for humans and animals, these contaminants threaten soil, water, and the environment. Conventional remediation techniques are expensive and cause devastation to the environment. As a result, low-cost and environmental technologies are needed, which can be done through hydroponics. These contaminants can be removed through various processes, such as phytodegradation or phytovolatilisation. Pistia stratiotes, Trapa natans, Lemma minor, and Elodea Canadensis are plants which can be grown to remove organic contaminants from water. Most of the macrophytes rapidly grow and produce vast amounts of biomass, which may be used for energy production, such as biogas, biofuels, etc. Hydroponics may help manage global climate change and energy crises by remediating organic pollutants. Acknowledgements The authors sincerely thank the Department of Agronomy and Plant Pathology at the Lovely Professional University of Punjab for their continued support and encouragement. Conflicts: None

References Akhtar, M. S., Chali, B., & Azam, T. (2013). Bioremediation of arsenic and lead by plants and microbes from contaminated soil. Research in Plant Sciences, 1(3), 68–73. Albers, P. H., & Camardese, M. B. (1993). Effects of acidification on metal accumulation by aquatic plants and invertebrates. 1. Constructed wetlands. Environmental Toxicology and Chemistry: An International Journal, 12(6), 959–967.

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Chapter 7

Harnessing the Power of Plants in Hydroponics for Wastewater Treatment and Bioremediation Faten Dhawi

Abstract Hydroponics is a relatively new farming technology that involves growing crops or plants without the need of soil. This method has lately gained popularity due to its capacity to ensure rapid development, greater quality, and increased yields. The idea that drove the research and development of hydroponics was the necessity to solve the problems that prevent plants’ roots from efficiently absorbing water, oxygen, and nutrients. This was the driving force behind hydroponics. Significant interest has recently been shown in two innovative techniques: hydroponics, the soilless cultivation method, and environmental bioremediation, the use of living organisms to eliminate or neutralize contaminants. As water waste treatment and heavy metal contamination become increasingly pressing issues, these products provide viable solutions. These cutting-edge methods have the potential to dramatically alter how environmental toxins are handled by tapping into the energy of plants. Keywords Hydroponic · Bioremediation · Plant · Waste water

7.1 Introduction Conventional water waste treatment technologies frequently use high-maintenance facilities and lots of power to purify water. Hydroponics, on the other hand, is a sustainable option since it uses water-efficient technologies to grow plants in nutrientrich solutions. Under these conditions, plants not only survive and even thrive without soil, but they also remove harmful organic and inorganic compounds from the water supply through photosynthesis and cellular respiration. Hydroponics can be used in environmental bioremediation, which involves the use of biological processes to remove pollutants such as algae, bacteria, heavy metals, solvents, oil, and industrial chemicals from the environment as a result of human F. Dhawi (B) Agricultural Biotechnology Department, College of Agricultural and Food Sciences, King Faisal University, P.O. Box 420, Al-Ahsa 31982, Saudi Arabia e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_7

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activities (Demarco et al., 2023). Some plant species, known as hyperaccumulators, have the unusual ability to absorb and store toxic metals in their tissues. These plants are able to remove heavy metals from the environment by accumulating and storing them in their tissues at extremely high quantities. Hydroponics and environmental bioremediation work in harmony to improve the treatment of water waste and the elimination of heavy metals. Polluted water sources can be efficiently remedied by combining these methods, protecting local ecosystems while also making sure people have access to clean water. In comparison to conventional treatment procedures, which typically include the use of harmful chemicals or energy-intensive processes, this method is more sustainable and environmentally friendly. By realizing the potential of plant-based energy production, we may create a more environmentally friendly and sustainable future by efficiently mitigating environmental degradation using solutions found in nature. The hydroponic farming technique can be carried out either indoors or outdoors, but the degree of success it achieves is mostly dependent on environmental factors such as temperature, the amount of available light, and the quality of the air. The most important advantages of hydroponics are the maximizing of space, the enhancement of plant development, the reduction in the amount of water used, and the reduction in the amount of interference. Farmers and researchers have been encouraged to investigate new techniques of farming for small-scale and commercial purposes as a result of the ever-increasing population around the world, as well as the decreasing quantity of rainfall that has been observed in various regions geographically, and the severe heat waves that have been experienced in different places. Hydroponics was developed and put into practice in response to these and other issues, such as extensively contaminated soil by microorganisms (Magwaza et al., 2020). In certain other nations, such as the United Kingdom, Japan, South Africa, and Australia, there is an increasing and intense interest in hydroponics, particularly in urban towns and places. Several electronic components, such as the Internet of Things (IoT), a data storage hub, automated control of valves, sensor system, and a plants database, are utilized to assist in the maintenance of plant records, including growth rate, height, length, and other daily growth measurements. It is possible to increase crop production with the help of several different kinds of hydroponics, including aeroponics, deep-water culture, nutrient film technique (NFT), and wicking. These are all examples of complicated farming systems that use advanced technology. The utilization of these electronic components makes it possible to manage the hydroponics system properly and effectively. In addition to ensuring improved crop production, stronger yields, and food security, hydroponics can also be used for environmental bioremediation. According to a report published by the World Bank Group in 2021, the market value of hydroponics farming is expected to reach approximately USD 16 billion within the next 2–3 years. This represents an increase from USD 8.1 billion recorded in 2019 to a projected USD 13.61 billion by 2030 (Verner et al., 2021). When compared to traditional farming methods, hydroponics has been shown to require 80% to 90% less water and approximately 75% less space. Despite the significant start-up expenses associated with this method, it allows for greater land

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availability and helps reduce congestion in urban areas by minimizing the water needed for planting and maximizing space utilization (Sharma et al., 2023). Since its inception and subsequent implementation, numerous studies have been conducted to assess the effectiveness of smart-climate farming technology in environmental bioremediation. In this chapter, we will explore how a hydroponics system can be utilized as a sustainable technique for environmental bioremediation, specifically focusing on the removal of waste or heavy metals from water.

7.2 The Process of Bioremediation In an ideal world, bioremediation is a biological process that falls under the purview of the field of biotechnology. It is characterized by the utilization of living creatures, including plants, microorganisms such as fungi and bacteria, for the purpose of removing pollutants or contaminants from water, soil, or any other environment. The bioremediation process provides some sort of alternatives to the methods that are used for the management of waste, which helps in the effort to keep the environment clean (Gouma et al., 2014). The past ten years have seen the creation and emergence of novel bioremediation methodologies and techniques that can be utilized to restore contaminated habitats. These methods and techniques can be employed to clean up the environment. Because of the challenges posed by climate change, leading environmental organizations like Earth Justice and Greenpeace, as well as non-profit environmental organizations, are advocating for an environmentally friendly approach to the removal of pollutants in the environment, which is why the bioremediation process has become increasingly popular over the past few years. The breakdown and purification of water, soil, and any other environment with the assistance of plants and an all-inclusive activity of bacteria, fungus, and other microbes is the primary mechanism of action of bioremediation. The bacteria or microbes that are employed in bioremediation can either be aerobic or anaerobic depending on the complexity of the chemicals or toxins that need to be broken down as well as the degradative capabilities of the bacteria or microbes themselves. For instance, several of the aerobic bacteria, such as Norcardia, Pseudomonas, Rhodococcus, and Mycobacterium, have the ability to digest carbon-related pollutants, which can be challenging to accomplish in chloroform compounds. Bacteria, fungi, and actinomycetes play a crucial role in the removal of pollutants in water, soil, and the environment in a sustainable manner (Bala et al., 2022). This is true despite the fact that the bioremediation process has a number of elements that can operate as limiting factors. In the course of their metabolic processes, microbes including Pseudomonas putida, Pleurotus, and Aspergillus release enzymes like celluloses, amylases, catalases, and peroxidases, as well as other microbial products. These enzymes are responsible for the breakdown of dangerous pollutants, which in turn makes them less harmful (Bala et al., 2022).

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Bioremediation can be carried out either in-situ or ex-situ via a variety of approaches, depending on the region, the level of pollution, and the rules that govern the environment in question. Through microbial activities and biological reactions, microorganisms undertake the majority of the work (over 80%) in both scenarios to ensure the detoxification and degradation of poisonous substances and physical waste in the environment. This is done to ensure that the environment is not polluted. The bio-pile is an example of an ex-situ sort of bioremediation approach that is a solid-phase treatment that involves excavating and moving tainted soil or agricultural to a different site, heaping them in piles for the purpose of bioremediation. This method is an example of an ex-situ kind of bioremediation technique. The method can also be utilized for the cleaning of organic municipal trash, organic household garbage, and organic industrial waste. The microbial colony grows through the heaps and is fed through piles that are evenly distributed throughout the piles (Kapahi & Sachdeva, 2019; Sharma et al., 2023). The bio-pile solid-phase treatment ex-situ bioremediation technology is considered to be more cost-effective; nonetheless, it is time-consuming and requires a significant amount of space. The slurry-phase bioremediation technology provides an appropriate answer to the latter problem, which needs to be addressed. Given the relative slowness of the various other treatment procedures, the idea behind the slurry-phase is to speed up the bioremediation process as much as possible. In contrast to the solid-phase, a bioreactor incorporates the addition of water, nutrients, and oxygen to the contaminated soil or other kind of physical waste. Notably, the inclusion of these substances or components aids in the creation of an optimal environment for the proper functioning of microorganisms in the process of biodegrading the pollutants that are present in the soil. However, the concentration of the pollutants, in conjunction with the physiochemical features of the soil, is what defines the amount of water and nutrients that need to be introduced into the bioreactor, which, in turn, influences the rate at which the biodegradation process occurs. For the process known as intrinsic in-situ bioremediation, the microorganisms in the soil or water are left alone to carry out their own microbial activity with regard to the degradation of contaminants. On the other hand, the engineered approach calls for human intervention in the form of the introduction of genetically modified microbes to polluted places in an effort to assure a quick process of bioremediation. Another typical type of physical in-situ approach that is utilized for the removal of contaminants, particularly chlorinated compounds and heavy metals like lead in polluted groundwater is known as the Permeable Reactive Barrier, or PRB for short (Fig. 7.1). The PRB entails the installation of a subsurface barrier that is loaded with reactive materials and microorganisms including sulfate-reducing bacteria, such as Granular Activated Carbon (GAC), and Zero-Valent Iron (ZVI). When contaminated water travels through the PRB, a number of biological processes, such as absorption, degradation, and precipitation, take place, which ultimately leads to the purification and restoration of ground water (Zheng et al., 2022).

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Fig. 7.1 A schematic representation of permeable reactive barrier (Zheng et al., 2022)

7.3 Phytoremediation: A Solution for Treating Pollutants in Wastewater The use of plants to remediate polluted soil, water, or air is called phytoremediation. Phytoremediation has the potential to efficiently remove several contaminants from wastewater. According to Khan et al. (2022), Phytoremediation can be used to reduce the following types of contaminants in wastewater: 1. Heavy metals: Lead, cadmium, mercury, chromium, and arsenic are just some of the heavy metals that can be removed from wastewater by using phytoremediation. Plants diminish the quantity of heavy metals in water by absorbing them through their roots. 2. Organic contaminants: Subgroup of organic anti-infective agents such as antibiotics, Pesticides, herbicides, industrial chemicals, and petroleum hydrocarbons are just some of the organic contaminants that phytoremediation can assist remove from wastewater. Plants can metabolize or break down these pollutants, transforming them to less dangerous forms or storing them in their tissues. 3. ExtraNutrients: Discharging wastewater with high concentrations of nutrients, especially nitrogen and phosphorus, can cause eutrophication of receiving natural water bodies. By allowing plants to absorb these nutrients from the water, phytoremediation can alleviate this problem by decreasing the nutrient load and preventing dangerous algal blooms.

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4. Pathogens: Some phytoremediation plant species, such certain reeds and grasses, have antibacterial capabilities that can aid in reducing the prevalence of infections in wastewater. The pathogen concentration is decreased as the plants absorb and decompose the infections. 5. Organic matter: Phytoremediation helps clear wastewater of organic matter as well. Plants have the ability to take in and make use of organic molecules in the water, which can reduce the organic load and enhance water quality. The success of phytoremediation depends on a number of variables, including the nature of the wastewater, the concentration of pollutants, the growing conditions, and the species of plants used. It is crucial to choose the right plant species for each phytoremediation project since different plant species have different abilities to absorb and detoxify contaminants (Khan et al., 2022).

7.3.1 Mechanisms of Phytoremediation Pollution has significant effects on microbial communities and plant growth. It would cause altered microbial communities and affect the rate at which plant consumes soil resources. Pollution may also affect soil structure and cause water stress in the soil, especially in the situation of petroleum products. There is also intensified competition between microbes and plants for nutrients during phytoremediation. Remediation of soil that has been contaminated or polluted by heavy metals or petroleum is critical for ecosystem sustainability and human health. Phytoremediation helps to increase soil fertility and preserves biodiversity. Plant growth-promoting bacteria are applied in phytoremediation, and they help enhance plant growth despite the toxicity created by the contamination. They also allow increased removal of these metals at low costs (Nie et al., 2011). The behaviors of these microbes and their interactions with particular plants and other microorganisms are critical considerations when using phytoremediation to enhance plant growth and remove toxins from the soil. Plants apply different approaches or mechanisms to achieve phytoremediation and ensure the removal of pollutants from water or soil. The phytoremediation approaches are included:

7.3.1.1

Phytofiltration

Phytofiltration, also known as rhizofiltration, is a sustainable and eco-friendly water purification technique. This method involves the use of aquatic plants to absorb and store pollutants in their foliage, stems, and roots, thereby purifying the water. Aquatic plants have the unique capacity to absorb and accumulate contaminants in water bodies. They are capable of removing a variety of contaminants, including heavy metals, organic compounds, nutrients, and even pathogens. Thus,

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phytofiltration is a versatile method that can be applied to various forms of water pollution. The selection of suitable aquatic plant species is the first step in phytofiltration. Certain plants, such as water hyacinth (Eichhornia crassipes), duckweed (Lemna spp.), and water lettuce (Pistia stratiotes), have demonstrated exceptional pollutantabsorption capacities and are commonly utilized in phytofiltration systems. The extensive root systems of these plants provide a large surface area for the absorption of pollutants. Once the appropriate plant species have been selected, they are cultivated in a controlled environment, such as artificial wetlands or floating rafts. The contaminated water is subsequently circulated through these systems, exposing the plants to the contaminants. As water moves through the root zone, plants absorb pollutants through a variety of mechanisms, such as adsorption, absorption, precipitation, and microbial degradation. Depending on the specific contaminant and plant species, the pollutants may be retained and accumulated in various plant organs. Some pollutants may accumulate primarily in the plant’s leaves, while others may accumulate primarily in the roots. Heavy metals, for instance, are frequently stored in the roots of aquatic plants, whereas organic pollutants such as pesticides and herbicides can accumulate in the foliage. Once the plants have assimilated the contaminants, they can be harvested and extracted from the system. To prevent the reintroduction of contaminants into the environment, the harvested plant biomass must be disposed of or treated appropriately. Depending on the character and concentration of the accumulated pollutants, various disposal methods, such as composting, incineration, and landfills, may be employed. Phytofiltration has a number of benefits over conventional water remediation methods. It is an economical and energy-efficient method that requires minimal infrastructure and operational maintenance. In addition, it offers a sustainable and natural approach to water remediation by utilizing the inherent ability of plants to purify polluted water. The selected plant species can contribute to the overall attractiveness and ecological equilibrium of the environment, thereby enhancing the aesthetic value of water bodies. It is essential to note, however, that the efficacy of phytofiltration can be affected by a variety of factors, including the type and concentration of pollutants, the selection of plant species, water quality parameters, and system design. In order to optimize the efficacy of phytofiltration systems and ensure effective pollutant removal, it is necessary to carefully consider the aforementioned factors. In general, phytofiltration or rhizofiltration is a promising and sustainable method for treating water pollution. This technique contributes to the preservation and restoration of aquatic ecosystems by utilizing the natural abilities of aquatic vegetation to purify contaminated water (Dalvi & Bhalerao, 2013; Thakur et al., 2016).

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Phytodegradation and Phytotransformation

Phytodegradation and phytotransformation are two mechanisms that utilize plants to degrade and transform wastewater pollutants. These methods provide environmentally favorable and sustainable approaches to wastewater treatment and remediation. Phytodegradation is the process by which plants break down or degrade pollutants. Certain plant species are able to produce enzymes or stimulate microbial activity in their root zones during this process, which facilitates the degradation of organic contaminants. These plants’ roots release a variety of enzymes into the adjacent soil or water, accelerating the decomposition of pollutants into less hazardous compounds. Organic compounds such as pesticides, herbicides, pharmaceuticals, and industrial chemicals can be pollutants. In contrast, phytotransformation entails the transformation or conversion of pollutants into less toxic or more stable forms by plants. phytotransformation, unlike phytodegradation, does not involve the complete decomposition of pollutants. Through various metabolic processes, plants can alter the chemical structure of certain contaminants, resulting in the formation of less hazardous compounds. This transformation may occur in the roots, stems, foliage, or rhizosphere, the soil environment surrounding the roots where plant–microbe interactions occur. The effectiveness of phytodegradation and phytotransformation is contingent on a number of variables, including the plant species used, the specific pollutants present in effluent, and the environmental conditions. Due to their natural enzyme production, metabolic pathways, or symbiotic relationships with microorganisms, certain plant species have enhanced phytodegradation and phytotransformation abilities. Various systems can be used to implement phytodegradation and phytotransformation in effluent treatment. Typically, constructed wetland treatment systems involve the cultivation of specific plant species in shallow water-filled basins. As wastewater flows through these wetland ecosystems, plants absorb pollutants and undergo degradation or transmutation within their root zones. As effluent treatment methods, phytodegradation and phytotransformation provide numerous benefits. They are sustainable, cost-effective, and energy-efficient alternatives to conventional treatment methods. In addition, these methods can be incorporated into existing wastewater treatment systems to improve their overall efficacy and pollutant removal capacities. Importantly, the success of phytodegradation and phytotransformation relies on the proper selection of plant species, an understanding of the pollutant’s properties, and the optimization of system design. Optimal performance requires careful consideration of factors such as hydraulic retention time, plant density, and nutrient accessibility. The phytodegradation and phytotransformation are novel effluent treatment and remediation strategies. These methods provide environmentally favorable solutions for the degradation and transformation of pollutants in wastewater by utilizing the natural abilities of plants (Aminedi et al., 2020).

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Phytoextraction or Phytoaccumulation

Phytoextraction, also known as phytoremediation and phytoaccumulation, is a process in which plants are used to extract and accumulate pollutants from effluent. This strategy provides a sustainable and cost-effective method for treating effluent and removing contaminants. In phytoextraction, specific plant species are cultivated in areas impacted by wastewater or in specially designed treatment systems. Through their roots, these plants have the unique ability to absorb contaminants from the water. The pollutants may consist of heavy metals, organic compounds, nutrients, and other contaminants found in effluent. The plants function as natural “extractors” by absorbing pollutants via various mechanisms. To absorb and deposit heavy metals such as lead, cadmium, and arsenic in their tissues, plants use processes such as ion exchange, complexation, and precipitation. Organic pollutants can be incorporated through a plant’s root system or transformed by its enzymes. Depending on the nature of the contaminants and the plant species, the extracted pollutants accumulate in various parts of the plant. Some plants primarily store pollutants in their shoots (aerial portions), whereas others store them in their roots. The harvested plant biomass can then be disposed of appropriately or further processed to recover valuable materials or resources (Rezania et al., 2016; Sharma et al., 2015). As a wastewater treatment method, phytoextraction provides numerous benefits. First, it is a sustainable and eco-friendly method that relies on the natural capabilities of plants. It is applicable to both natural and manmade systems, including wetlands, floating rafts, and soil-based remediation systems. Furthermore, phytoextraction can be a cost-effective alternative to conventional effluent treatment techniques. It requires less infrastructure and energy, making it a viable option for regions with limited resources or where conventional treatment methods cannot be implemented. The selection of plants is vital to the effectiveness of phytoextraction. Certain plant species, referred to as hyperaccumulators, have a strong affinity for specific contaminants and can store them in large quantities. The plants Thlaspi caerulescens and Astragalus racemosus are known to hyper-accumulate metals such as Zinc and Nickel (Aransiola et al., 2019). These plants have developed mechanisms to tolerate and accumulate pollutants without incurring significant damage, allowing them to thrive in contaminated environments (Ali et al., 2020).

7.3.1.4

Phytostabilization

Phytostabilization is a technique that employs plants to immobilize or stabilize contaminants in effluent, thereby preventing their migration or absorption by organisms. It is an efficient and sustainable method for managing polluted water and minimizing the risks associated with contaminants. In phytostabilization, specific plant species are chosen and cultivated in wastewater-contaminated areas. These plants can tolerate high levels of pollutants

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and establish dense root systems, thereby creating a physical barrier that restricts the movement of pollutants. In addition, they release compounds that can bind or react with pollutants, thereby decreasing their mobility and bioavailability. In phytostabilization, the roots of the selected plants play a crucial function. They penetrate the soil or sediment, effectively anchoring the plants and forming a network that traps contaminants. In addition to physical entrapment, plants can also release chemical substances, such as organic acids, enzymes, and exudates, that interact with contaminants and promote their immobilization. Wastewater contaminants such as heavy metals, metalloids, and metal ions are effectively managed by phytostabilization. These contaminants are absorbed by plants through their roots and stored in their tissues, preventing their dispersion into the environment. The pollutants are sequestered in the roots or retained in the aboveground biomass, thereby reducing their potential impact on groundwater, surface water, and surrounding ecosystems. The success of phytostabilization is contingent on the meticulous selection of plants, taking into account their tolerance to the specific contaminants found in the wastewater. Some plants have a greater affinity for specific metals or contaminants, making them more suitable for phytostabilization. Additionally, the selection of plant species capable of establishing a dense and extensive root system is essential for maximizing the efficacy of the method. As a method for managing effluent, phytostabilization offers numerous benefits. It is a cost-effective and eco-friendly strategy that can be implemented in a variety of environments, including constructed wetlands, riparian buffers, and contaminated sites. It requires minimal infrastructure and upkeep, making it an attractive option for regions with limited resources or where conventional remediation techniques are impractical. Phytostabilization can also provide additional benefits such as erosion control, habitat restoration, and aesthetic enhancement of the affected areas. Vegetation contributes to the ecological equilibrium of the environment by stabilizing the soil, preventing erosion, and increasing biodiversity. It is essential to note that phytostabilization may not entirely eliminate wastewater contaminants. However, it effectively reduces their mobility and bioavailability, thereby mitigating their potential adverse effects on human health and the environment. Utilizing the ability of plants to immobilize pollutants offers a sustainable and cost-effective method for mitigating the dangers of effluent contamination. Continued investigation and application of phytostabilization techniques can contribute to the development of effective and environmentally benign wastewater management strategies (Abdallah, 2012; Eid et al., 2020).

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7.4 Harnessing the Potential of Plants for Phytoremediation Phytoremediation is an additional essential type of bioremediation that makes use of plants to detoxify and degrade pollutants in soil and water. Mineralization, volatilization, filtration, extraction, and stabilization are some of the biological processes that are involved in the process, which also includes a variety of other mechanisms (Berti & Cunningham, 2000; Sharma et al., 2023). The selection of plants for use in hydroponic phytoremediation is contingent on a wide variety of parameters, including the toxicity level of the contaminants to the plants, the root system, the above-ground biomass, and the capability of the plant to adapt appropriately to the predominantly adverse conditions for its growth and development (Yan et al., 2020). To enhance the efficiency of the phytoremediation process, it is crucial to select plant species that exhibit resistance to both insects and diseases. Plants such as hyacinth, azolla, sunflower, Indian mustard, alfalfa, and willow have shown the ability to remove pollutants from soil or water through root uptake and subsequent translocation to the aerial parts of the plant (Yan et al., 2020). By selecting plants with inherent resistance to pests and diseases, their ability to thrive and carry out phytoremediation processes is maximized. Pest and disease resistance in these plant species ensures their long-term survival and effectiveness in pollutant removal, minimizing the risk of damage or stunted growth due to insect infestations or pathogenic infections. In essence, the resistance of these selected plants to pests and diseases contributes to the overall success of phytoremediation efforts, allowing for sustained pollutant uptake and translocation. This resistance enables the plants to continue their vital role in remediation without being hindered by detrimental insect damage or disease-related impairments. Therefore, when considering plant species for phytoremediation projects, it is crucial to prioritize those that possess natural resistance to pests and diseases. This approach ensures the plants’ ability to efficiently remove pollutants, promoting successful and sustainable phytoremediation processes that contribute to the improvement of soil and water quality. According to the findings of studies and experiments carried out by academics and scientists, the best candidates for phytoremediation are plants with fibrous root systems. This can be explained by the greater contact area, which offers a larger surface area, allowing for the more efficient removal of contaminants from the soil or water. When conducting phytoremediation, it is important to make use of local plants that have adapted to growing in polluted environments. Doing so helps to increase the overall success rate of the procedure. When attempting to gain a grasp of phytoremediation, it is essential to keep in mind that all types of plants have the capacity and propensity to resist the presence of poisonous trace elements in soil and water in order to maintain their quality and achieve a faster rate of growth. This is a crucial point to keep in mind. On the other hand, some of the components of a plant’s inner structure are able to tolerate the uptake and extraction of pollutants. These components include the cell walls, plasma membrane, and other components such as organic acids and peptides (Nedjimi, 2021).

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In a study of Worku et al., (2018) they test the efficacy of bioremediation, using three distinct hydroponics treatment systems for a period of seven months utilizing waste water from a brewery in Adisababa, Ethiopia. Because of the fibrous natures of its root system, which contribute to the enhancement of the bioremediation process, Vetiver (Chrysopogon zizanioides) was chosen as the material of choice for the bioremediation project. In each of the treatments, the waste water was pumped into the growing tray by a hydraulic system at a pace of 10 cm per day while also being drained back into the reservoir after a period of 5 days. This process was repeated three times. Samples of the effluent water were collected on a monthly basis for the purpose of analyzing both the water’s quality and the pace of growth of the vetiver grass in terms of the nutrients it contained. According to the findings, it was clear that there was a significant reduction in the amount of biochemical oxygen demand (BOD) as well as chemical oxygen demand (COD), with respective reductions of 73% and 58%. The primary objective of the biological oxygen demand (BOD) and chemical oxygen demand (COD) systems in the treatment units is to supply the microorganisms with the oxygen they need to decompose the chemical compounds and physical waste that are present in the brewery waste water. Consequently, there are large removal efficiencies of nitrogen and phosphorus in the waste water, including the accumulation of nutrients by 7.3–8.3 g per kg of nitrogen during the study period, showing a huge success in the phytoremediation process. In addition, the removal efficiencies of nitrogen and phosphorus in the waste water have been measured. The findings of this study provide more evidence that vetiver grass is an effective method for cleaning polluted water, in this case wastewater from breweries, of harmful nutrients and other contaminants. In another study by Davamani et al. (2021), with the purpose of determining whether or not the hydroponics phytoremediation method is efficient in the process of removing contaminants from wastewater produced by a paper mill processing facility using Vetiver (Chrysopogon zizanioides). Because of its anatomical and physiological characteristics, vetiver grass is ideally suited for cultivation in hydroponic phytoremediation systems. Taking into consideration the presence of heavy metals in the waste water, which increases the electrical conductivity of the water, the findings of the study revealed a considerable reduction in electrical connectivity at the conclusion of the treatment process by around 39.39%. This reduction occurred despite the fact that the presence of heavy metals in the waste water increases the electrical conductivity of the water. In addition, there was a decrease of 81.19% in the total soluble salts and 56.16% in the total dissolved solids that were found in the waste water. On the other hand, the study found that the BOD in the waste water decreased by 55.68% and the COD decreased by 58.01%. This demonstrates that the biological mechanism of the microbe is highly effective in breaking down the chemical compounds and other trace elements found in the waste water. The enhanced and considerable development of the vetiver grass was demonstrated by a decrease in the nutrient element levels of nitrogen, potassium, and phosphorus. The most important finding from the research was that the amounts of lead and cadmium dropped by 65.63 and 64.29%, respectively. This was an important finding because hydroponics

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phytoremediation is designed to get rid of toxins and heavy metals that are present in wastewater. Plant species that tolerate stressful situations, especially the polluted ones, can remove pollutants or process them through different mechanisms. Essentially, these are those plant species that can carry out phytoremediation and participate in removing soil pollutants (Favas et al., 2014). Various species have been indicated as appropriate for processing and eliminating different contaminants, thus facilitating their growth. These species include Pinus pinaster, Juncus conglomeratus, Scirpus holoschoenus, Hypochaeris radicata, Alyssum serpyllifolium, Agrostis castellana, Digitalis purpurea, Juncus effusus (Figs. 7.2 and 7.3) (Favas et al., 2014). These plant species can grow in different metal mines where high contamination levels exist. Thus, they are class examples of tolerant plants to stressful conditions. Inoculation of these plants with appropriate plant growth-promoting microbes would have a significant effect on the plants. The injection would happen through seeds, roots, and soil. After microbial inoculation occurs, they will form beneficial associations with the plants, which would benefit the plants. The microbes would help degrade and remove some of the pollutants. They also would make some plant nutrients available and make the plant more tolerant of stress. They also help in producing plant growth hormones. Additionally, the inoculated microbes would promote resistance to some infections through different mechanisms (Dhawi et al., 2016, 2017; Dhawi, 2023a, 2023b).

Fig. 7.2 Pictures of various plant species have been indicated as appropriate for processing and eliminating different contaminants: Juncus conglomeratus, Pinus pinaster, Hypochaeris radicata and Scirpus holoschoenus

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Fig. 7.3 Pictures of various plant species have been indicated as appropriate for processing and eliminating different contaminants: Agrostis castellana, Alyssum serpyllifolium, Juncus effusus and Digitalis purpurea

7.5 Utilizing Plants for Wastewater Management: The Role of Constructed Wetlands Constructed wetland systems are engineered systems that imitate the natural processes of natural wetland systems to treat wastewater or precipitation. They consist of aquatic vegetation, such as reeds and rushes, as well as a variety of wetland sediments and substrates. In a constructed wetland, wastewater or stormwater is subjected to a series of physical, chemical, and biological processes that remove pollutants and enhance water quality. These are the essential components of a constructed wetland: 1. Aquatic plants serve an essential role in the ecosystem of wetlands. They promote the development of beneficial microorganisms, oxygenate the water, and offer attachment sites for bacteria and other organisms involved in the treatment process. Pollutants are more efficiently filtered and assimilated by plants with their intricate root networks. 2. Substrates and Soil: Typically, the wetland is filled with particular substrates and soil materials that facilitate the remediation procedure. These substances function as attachment sites for microorganisms that degrade pollutants. Additionally, they help retain and filter suspended particles. 3. Water Flow Management: Constructed wetlands are designed with specific water flow patterns to maximize the removal of pollutants. The water flow can be regulated through the use of weirs, obstructions, or channels to ensure optimal treatment efficiency.

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4. Microbial Activity: Constructed wetlands foster the development of various microorganisms, such as bacteria and algae, which contribute to the purification process. These microorganisms contribute to the breakdown of organic matter, nutrients, and other contaminants found in effluent. Sedimentation, filtration, adsorption, microbial decomposition, and plant absorption are used in constructed wetland systems to effectively remove pollutants. They can be utilized for domestic wastewater treatment, industrial effluent treatment, and agricultural discharge treatment. Moreover, constructed wetlands provide numerous advantages, such as cost-effectiveness, energy efficiency, the potential for habitat creation and biodiversity enhancement. It is essential to note that the design and performance of constructed wetlands can vary depending on variables such as the pollutants targeted, the hydraulic loading rates, the selection of vegetation, and the climate conditions. The optimal performance and long-term viability of constructed wetland systems are contingent upon careful planning and design (Herath & Vithanage, 2015; Sandoval et al., 2019).

7.6 The Distinction Between Constructed Wetlands and Hydroponics in Water Management Hydroponics and constructed wetlands are two distinct water management and remediation techniques. Here are the primary distinctions between the two strategies: 1. Constructed wetlands are primarily intended for water treatment and the enhancement of water quality by simulating the functions of natural wetlands. Hydroponics, on the other hand, is a technique for growing plants without soil, with the goal of optimizing plant growth and maximizing yields. 2. Focus on Plants: Constructed wetlands utilize natural wetland plants, such as reeds and rushes, whose root systems and interactions with microorganisms assist in water treatment. Hydroponics enables the cultivation of numerous plant species, with a concentration on delivering specific nutrients directly to the roots of the plants for optimal growth. 3. Water Source: constructed wetlands are used to treat effluent, stormwater, or agricultural runoff by directing polluted water into the wetland for purification. Hydroponics, on the other hand, necessitates a pure water source in order to deliver nutrients directly to the plants. Frequently, hydroponics is implemented in controlled indoor environments or with water sources formulated specifically for hydroponic systems. 4. Treatment Mechanisms: To remove pollutants from water, constructed wetlands rely on natural processes, such as sedimentation, filtration, adsorption, microbial activity, and plant absorption. Physical, chemical, and biological processes within the wetland system are responsible for the remediation. In hydroponics, the emphasis is placed on delivering essential nutrients directly to the plant’s roots in a controlled environment, thereby optimizing plant growth and increasing yields.

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5. Application: Constructed wetlands are frequently used in large-scale water treatment initiatives, such as municipal wastewater treatment, industrial effluent treatment, and the restoration of polluted water bodies. Hydroponics, on the other hand, is frequently employed in agriculture, particularly for the cultivation of high-value commodities in controlled environments, urban farming, or regions with limited arable land. Although both constructed wetlands and hydroponics are innovative approaches to water management, their primary goals and applications are distinct. Hydroponics emphasizes plant growth and maximizing yields in controlled environments, whereas constructed wetlands emphasize water treatment and ecological restoration (Magwaza et al., 2020; Messer et al., 2021).

7.7 Different Types of Hydroponic Growing Systems In an ideal world, the many varieties of hydroponics systems would be able to be categorized as either static or continuous flow systems, with the distinction being made according to the level of investment required and the nature of the intricacy of the system. Deep-water culture (DWC) and wicking systems are examples of static hydroponics systems. Examples of continuous flow hydroponic systems include nutrient film technique, aeroponics, ebb & flow, and drop systems (Fig. 7.4). Although there are several various techniques that go into hydroponics, they all have the same overarching goal, which is to ensure that the root system of the plant is able to take in water, nutrients, and oxygen in an efficient and effective manner. The type of hydroponics system to use is determined in large part by several important factors, including cost in terms of the nature of the hydroponics set-up, space available, and flexibility. Other considerations to make are reusability, drainage, and the quantity of labor that will be necessary (Gordon, 2021). Many of the components that make up the hydroponics framework may be recycled and used again in between planting cycles, which helps to keep costs down. In comparison to other traditional and conventional techniques of plant cultivation, research investigations and scientific papers have demonstrated that the utilization of hydroponics systems results in a 30% improvement in growth rate and over a 25% improvement in yields. Some of the most fundamental advantages of using a hydroponics system are that it places less stress on the environment by reducing the amount of soil degradation and water consumption. On the other hand, the system is believed to be somewhat expensive and requires more time not only in the setting up of the structure but also in the carrying out of regular monitoring and maintenance throughout the planting system. This is because the monitoring and maintenance must be performed across the entire planting system (Velazquez-Gonzalez et al., 2022). Hydroponics can be an effective method for phytoremediation of wastewater because precise control over nutrient availability and water quality is possible. Several

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Fig. 7.4 An illustration of the types of hydroponics system

factors must be taken into account when selecting a hydroponic system for effluent phytoremediation:

7.7.1 Deep Water Culture (DWC) In DWC, plant roots are suspended in a nutrient-rich solution. It is a basic and effective phytoremediation system. The plants can be supported by floating platforms or floating beds, and the water is aerated to provide oxygen to the roots. Deep water culture is one of the other methods of hydroponics that is simple to set up, but it does not have any control or regulation over the supply of nutrient-rich water (Gordon, 2021). This makes it one of the easiest methods of hydroponics overall. The plants are free to take in water, nutrients, and oxygen whenever they require it throughout the day and night because to their extensive root systems. The majority of the time, the entire root system of the plants is submerged in the nutrient-rich water that is contained in the reservoir. In order to prevent the formation of algae, which might potentially cause harm to the system, it is essential for the deep water culture system to have an adequate supply of light, which ought to be a consideration during the setting up phase. The term “deep-water” refers to the depth to which the roots of the tree extend into the reservoir, which is where it gets its name. The roots need to be suspended in the water so that they do not end up drowning. If this does not happen, the roots will be lost. In addition, air (oxygen) is introduced into the water by means of an air pump or compressor that is connected into the water in the container or reservoir for large-scale operations (Fig. 7.5).

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Fig. 7.5 Deep-water culture system

7.7.2 Nutrient Film Technique (NFT) Due to its efficient use of water and nutrients, NFT is a popular hydroponic system for phytoremediation. In a shallow, sloping trough, plants are cultivated with a thin film of nutrient-rich water flowing over their roots. This method is ideal for plants with shallow root systems. The nutrient film technique is an active system with a continuous flow that requires pumping nutrient-rich water into a growing tray that is tilted at a specific angle to allow the water to drain back to the reservoir. This approach is known as a “continuous flow active system.” It is possible to replenish both the nutrients and the oxygen by maintaining a steady flow of water from the reservoir to the growing tray and then back to the reservoirs. Because of the way that the NFT system is constructed, it works best with plants that have root systems that are either fibrous or tiny (Fig. 7.6)

7.7.3 Aeroponics In aeroponic systems, plant roots are suspended in an air or vapor environment and nutrients are delivered through a fine mist. This technique provides superb oxygenation to the roots and can be extremely water-efficient. However, it may necessitate more complex equipment and strict environmental control.

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Fig. 7.6 An illustration of the nutrient film technique (NFT) system

7.7.4 Drip System Drip Irrigation Systems can also be adapted for phytoremediation. Through a network of tunnels or emitters, nutrient-rich water is delivered directly to the plant roots in this technique. It permits precise control of water and nutrient delivery, thereby minimizing waste.

7.8 Plant Selection for Wastewater Phytoremediation When selecting plants for phytoremediation of wastewater, consider species that have a high tolerance for pollutants and can efficiently absorb and accumulate them in their tissues (Figs. 7.7 and 7.8). Some common phytoremediation plant species include: 1. Water Hyacinth (Eichhornia crassipes): Water hyacinth is highly effective in absorbing nutrients heavy metals and organic contaminants from water, including nitrogen and phosphorous compounds. It is known for its fast growth rate and ability to thrive in nutrient-rich environments (Priya & Selvan, 2017). 2. Duckweed (Lemna minor): Duckweed is a small floating plant that is excellent at absorbing excess nutrients like nitrogen and phosphorous. It grows rapidly and provides a dense surface cover, preventing the growth of algae and reducing water evaporation (Landesman et al., 2011). 3. Reed (Phragmites australis): Reed is a tall, perennial plant with strong root systems that are effective at filtering and absorbing pollutants. It can remove a wide range of contaminants, including heavy metals and organic compounds.

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Fig. 7.7 Group1: Papyrus (Cyperus papyrus), Bamboo (Bambusoideae), Azolla (Azolla spp.) and Cattails (Typha spp.): Several plant species are known for their high tolerance to pollutants and their ability to efficiently absorb and accumulate them in their tissues, making them suitable for use in wastewater phytoremediation

Fig. 7.8 Group2: Water Hyacinth (Eichhornia crassipes), Duckweed (Lemna minor), Reed (Phragmites australis)and Water Lettuce (Pistia stratiotes): Several plant species are known for their high tolerance to pollutants and their ability to efficiently absorb and accumulate them in their tissues, making them suitable for use in wastewater phytoremediation

Common reed is widely used for wastewater treatment due to its high nutrient uptake and ability to degrade organic compounds (Zhang et al., 2021). 4. Water Lettuce (Pistia stratiotes): Water lettuce is a floating plant that forms rosettes of leaves on the water surface. It absorbs excess nutrients and provides shade, helping to control algae growth. Water lettuce is known for its rapid

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growth and ability to remove pollutants. Water lettuce is an efficient nitrogen and phosphorus accumulator and can be used for nutrient removal (Nahar & Hoque, 2021). Papyrus (Cyperus papyrus): Papyrus is a tall, grass-like plant that grows in wetland areas. Its fibrous root system assists in water purification by trapping sediment and absorbing nutrients. Papyrus also provides habitat for beneficial microorganisms (Mburu et al., 2015). Bamboo (Bambusoideae): Certain species of bamboo can be used in hydroponics systems to absorb nutrients and contaminants from water. Bamboo has a fast growth rate and a well-developed root system that contributes to effective water purification (Bian et al., 2020). Azolla (Azolla spp.): Azolla is a floating fern that can efficiently remove excess nitrogen and phosphorous from water. It forms a symbiotic relationship with nitrogen-fixing bacteria, enhancing its nutrient removal capabilities. (Forni et al., 2001). Cattails (Typha spp.): Cattails are common wetland plants that excel in water purification. They have extensive root systems that absorb nutrients and contaminants while also providing habitat for beneficial bacteria. Cattails are known for their ability to absorb heavy metals and contaminants from water (Maddison et al., 2009).

1. The Importance of Plant Growth-Promoting Microorganisms (PGPMs) in Both Soil-Based and Soil-Free (Hydroponic) Growing Systems Some of the agricultural activities that are used in soil-based agriculture contribute not only to the contamination of the environment but also to the destruction of the environment, which results in soil erosion. Despite the fact that some of the elements, such as a reduction in the amount of water used, the sustainable exploitation of space, and the adoption of environmentally friendly technology, offer the difference between soil-based culture and hydroponics, soil-based cultivation is still the more common method. The employment of plant growth-promoting microorganisms, also known as PGPMs, is already common in soil-based agriculture; nevertheless, the hydroponics method of vertical farming offers tremendous opportunities for their application. PGPMs can be applied in hydroponics by the employment of a biostimulant extract, and treatment of seedlings before to planting can be accomplished through the utilization of microbial suspension, particularly in an aeroponics system (Dhawi, 2023a). Plant growth-promoting microorganisms (PGPM) have seen widespread application in soil-based agriculture over the course of several decades (Dhawi et al., 2016, 2017; Dhawi, 2023b). According to Zamioudis and Pieterse (2012), beneficial associations of PGPBs have the potential to encourage plant development through the biosorption, mineralization, and degradation of soil pollutants. Hydrocarbons, heavy metals, polycyclic aromatic hydrocarbons (PAHs), and organochlorine chemicals (solvents, pesticides) are the most prevalent types of contaminants that can be found in soil. According

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to Baikhamurova et al. (2020), the presence of these toxicants in the soil makes the ecology of agricultural soil worse, which in turn leads to the suppression of crops and vegetation. As a result of PGPR’s creation of siderophores, not only are iron ions chelated, but so are a variety of other heavy metals, such as manganese, cobalt, cadmium, mercury, lead, and chromium. According to research done by AbbaszadehDahaji et al. (2016), this process results in the formation of insoluble complexes, which reduces the mobility and toxicity of metals in the soil in the root absorption zone. According to Asad et al. (2019), it plays a role in reducing the amount of heavy metals that are absorbed by the roots. Pseudomonas and Mycobacterium are two of the most common types of bacteria that have the ability to transform and breakdown hydrocarbons, polyaromatic hydrocarbons (PAHs), PCB, organophosphates, and organochlorine. Other microbes have this ability as well. Because of the potential that these bacteria possess, they can be utilized in an active manner to develop solutions for the elimination of PAHs from their natural environments. According to Boudh et al. (2019), studying the characteristics of the microbial degradation of PAHs is necessary for ensuring the successful use of bacteria in the bioremediation of contaminated locations. Hydroponic technologies that use wastewater or recycled water, as well as those that reuse solutions from closed-cycle systems, may stand to benefit from the decomposition of potential toxicants if they recycle the solutions when the process is complete. According to Mikula et al. (2022), water reuse is a sustainable method for bringing soilless farming closer to technologies that are good to the environment. This is because freshwater is a valuable resource whose supply on the world is decreasing. Because the plant’s roots secrete compounds that slow down or impede the growth of other plants of the same species, the problem of autotoxicity also develops when the solution is repeated. This is because autotoxicity occurs when the solution is recycled. According to Vocciante et al. (2022), the biodegradation of such substances by the use of PGPR is a surefire way to overcome such difficulties. When it comes to hydroponics and vertical farming, the PGPMs can be introduced into the system in a variety of various ways depending on the kind of hydroponics system that is used (Dhawi, 2023a). Plants have a propensity to interact with microbes in a variety of different ways, which can have either a beneficial or negative effect on the growth and production of the plant in the case of crops. Plants are able to resist difficult settings such as excessive temperatures as a consequence of the interaction. In addition, plants are able to assure the cleaning up of contaminated soils, air, and water in a process known as phytoremediation. Interactions between plants and microbes (also known as PGPMs) can result in a number of beneficial side effects for the plant, including increased growth and production, increased resistance to stress, and plant detoxification. According to Arora and Chauhan’s (2021) research, the influence of PGPMs on soil-based traditional agriculture has been the subject of a number of studies. Pseudomonas bacteria, which are widespread in water and soil, have been shown to secrete exopolysaccharides in one of the studies. These exopolysaccharides help in improving root colonization, which is an aspect that is essential in the germination and growth of rice in the rice field (Sah et al., 2021).

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On the other hand, the pseudomonas bacteria, when interacting with wheat seedlings, secrete exopolysaccharides, which aid in preventing the uptake of sodium ions in saline conditions, which in turn ensures the promotion of plant growth. Hydroponics, as opposed to systems based on soil, provides an optimal and regulated environment that is conductive to the colonization and interaction of plant pathogens and pathogen-associated microorganisms (PGPMs). The composition of the plant’s root system and the use of various nutrient solutions both play important roles in determining whether or not it will be possible to use PGPMs in vertical farming and hydroponics. Notably, the most difficult aspect of using PGPMs in hydroponics is striking a balance between the increase of nutrients and the control of pathogens in the various systems (Dhawi, 2023a). This is the primary obstacle. A study that entailed farming different types of lettuce using hydroponics to evaluate the usage of substrates and a modest amount of fertilizer yielded encouraging findings that highlighted the significance of including PGPMs in terms of productivity. In the majority of soil-free systems, determining and recognizing the method of application of the PGPMs is crucial to be able to have a good impact regardless of the intended purpose of including PGPMs in the system. This is the case even if the addition of PGPMs was not done with the intention of having a positive influence. In addition to helping to maintain the health of the plants and improve the quality of the water, the incorporation of PGPMs into soil-free systems contributes to the increased productivity and long-term viability of hydroponics techniques in the agricultural industry. Using PGPMs, in particular in aeroponics and aquaponics systems, is one way to get around the problem of insufficient nutrients in hydroponics. This is especially true with regards to aquaponics. Different activities produce pollutants that contaminate the soil and make it hard for plants to survive in such environments. Metals and organic compounds are among the primary pollutants that affect plant growth. The plant–microbe interactions are critical in the process and facilitate the removal of pollutants and increase growth. In other words, the success of the process requires plants and microbes to work closely and ensure that the processing of the pollutants happens efficiently and there is increased growth of the plants. It is critical to use the appropriate methods to inoculate these beneficial microbes into the plants. Significantly, in plants that tolerate stressful conditions, inoculating plants with microbes would enhance their capacity to remove pollutants and enhance soil restoration and plant growth (Table 7.1). 2. Deep-Water Culture (DWC) System Deep-water culture is a low-cost technique of hydroponics that may be readily assembled at home. It consists of putting up a system that uses a reservoir or container, a net pod, and a pump. The nutritional solution that is stored in the reservoir holds the plant’s root systems, which are floating in the fluid. On the opposite side of the system, an air pump is utilized to continuously aerate the nutrient solution with the necessary amount of oxygen so that the plants can experience enhanced growth and development. The word “hydro” comes from the fact that water is such an important component of hydroponics gardening and farming rather than the more conventional soil-based gardening and farming. Within the DWC system, the water is put to use

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Table 7.1 The effect of different plant usage in phytoremediation in soil systems with and without microbes The effect

Used microbes

Plant

Phytoremediation: heavy metals, cadmium

The synthetic bacterium EcCMC: designed a synthetic bacterium to strengthen physical contact between natural microbes and plant roots for remodeling

Eichhornia crassipes (Yin et al., 2022)

Phytoremediation: heavy metals

Rhizobia

Legumes (Fabaceae) both wild-type and genetically modified plants (Jach et al., 2022)

Phytoremediation (Polycyclic Aromatic Hydrocarbons) and heavy metals Cd than Zn and Pb

Microbes

Medicago sativa L.; Solanum nigrum L. (Cao et al., 2022)

Production of plant biomass and improvements in soil fertility in two years

Plant growth promoting bacteria and fungi (Bacillus pseudomycoides, Bacillus firmus, Aspergillus luchuensis and Aspergillus tamarii

Indian licorice (Abrus precatorius L.), Stevia (Stevia rebaudiana B.), Chilli (Capsicum annum L.) (Mishra & Singh, 2022)

Saline-alkaline soil phytoremediation

NA

Miscanthus (Xu et al., 2021)

Phytostabilization of acidic mine spoil, immobilized the metals, hyto-excluder for Al, Fe, Cr, Zn, and Pb and phytoextractor for Cu, Ni, and Mn

NA

Pelargonium graveolens (Xu et al., 2021)

in the process of producing a nutrient solution, which is then employed as a source of nutrients for the handling plants. Because there is no continuous movement of the various elements that make up the system, the deep water culture system is regarded as being among the most prominent examples of hydroponic systems that fall under the category of static. In recent years, scientists as well as commercial farmers have shown a strong interest in the hydroponics system, with a primary focus on the deep-water culture system. This interest highlights the significance of hydroponics in contemporary agriculture, which relies heavily on technology to ensure an increased and more robust level of food production all over the world. Deep water culture is one of the other methods of hydroponics that is simple to set up, but it does not have any control or regulation over the supply of nutrient-rich water (Gordon, 2021). This makes it one of the easiest methods of hydroponics overall. The plants are free to take in water, nutrients, and oxygen whenever they require it throughout the day and night because to their extensive root systems. The majority of the time, the entire

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root system of the plants is submerged in the nutrient-rich water that is contained in the reservoir. In order to prevent the formation of algae, which might potentially cause harm to the system, it is essential for the deep water culture system to have an adequate supply of light, which ought to be a consideration during the setting up phase. The term “deep-water” refers to the depth to which the roots of the tree extend into the reservoir, which is where it gets its name. The roots need to be suspended in the water so that they do not end up drowning. If this does not happen, the roots will be lost. In addition, air (oxygen) is introduced into the water by means of an air pump or compressor that is connected into the water in the container or reservoir for large-scale operations. 3. Understanding the Cycle Aquaponics System The term “aquaponics” refers to a system that, as its name suggests, is a hybrid of “aquaculture” and “hydroponics,” which means that it is a somewhat more complicated method. Aquaponics is an expensive form of hydroponics because it combines aquaculture and hydroponics into a single comprehensive production system. This makes aquaponics more expensive than other forms of hydroponics systems. Fish are put into the reservoir, which serves as the base of the aquaponics system, while plants are cultivated on shelves that are suspended above the water. After the fish have moved on, their feces and urine, which include ammonia, hydrogen, and other nutrients, are expelled into the water, where they are collected by a water pump and distributed to the plants. Because aquaponics is considered to be a water-efficient system, it is an ideal choice for locations that struggle with water scarcity issues. Because the nutrients are obtained naturally from the fish meal and fecal matter, this method of hydroponics is considered to be an organic form of the technique. Aquaponics requires careful and effective management, primarily with regard to the following parameters: air temperature, light, PH levels, water temperature, CO2 concentrations in the water, and the concentration of micro and macro nutrients. Only under these conditions can optimal conditions be created for the growth and development of both plants and fish. Aquaponics is a system that combines hydroponics and aquaculture. Components like ammonia and carbon dioxide, for instance, might be detrimental to plants if they are discharged into the water in large enough quantities. As a consequence of this, it is imperative that the water in the reservoir be continuously replaced after a predetermined amount of time in order to guarantee appropriate regulation and management of the nutrient-rich water solution. This is done in the interest of improved plant growth and maximum crop yields. Depending on the scale of the operation, aquaponics can be utilized either for residential or commercial settings. Due to the fact that aquaponics is a relatively new technique in the realm of biotechnology and agriculture, there are not a great deal of market participants making use of the system, particularly on a commercial scale. In addition, there are a number of other variables that are responsible for justifying the absence of a big number of competitors in the market, particularly in terms of commercial production.

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4. The Efficiency and Sustainability of the Aquaponics System According to Goddek et al. (2019), the future or long-term sustainability of an aquaponics system is highly dependent on the management of the nutrient solution in terms of the amount of content of the corresponding components. These components include ammonia, nitrate, carbon dioxide, and the presence of heavy metals. In addition, the eradication of pathogens that are responsible for nutritional, infectious, and environmental disease is of the utmost importance for the continuation and success of the system. An effective management of fish health, with regard to the treatment and prevention of dieses, contributes to the reduction of the likelihood that root diseases will be transmitted to plants and will develop in those plants. Controlling factors that contribute to the success and long-term viability of aquaponics include conducting risk analyses, which include the identification of risks, risk assessments, risk management, and risk communication. The grower is able to make educated and substantial decisions regarding the operation of the aquaponics production system thanks to risk analysis, which is performed not only on pathogens but also on maintenance and other areas of the operations cycle. This allows the grower to keep the aquaponics production system running smoothly. Because aquaponics is such a complicated system, it can be challenging to identify and eradicate disease-causing pathogens that can affect both fish and plants. This can make the process of disease control more difficult. Despite this, the continuous and growing attention and focus on health, pest and disease control by farmers, scientists, and other stakeholders, which primarily relies on regular monitoring of the hazards, has contributed in the strengthening of biosecurity and biosafety in the aquaponics business. This has been one of the contributing factors. Understanding and properly managing the PH level of the water in an aquaponics system is essential to ensuring that the system is effective, regardless of the type of system that is being used. When cultivating a garden using aquaponics, a neutral PH range that falls anywhere between 6.8 and 7.2 is recommended as the best option. In addition, for an aquaponics system to operate effectively and reliably, the water temperature must be maintained at an ideal range of 22 to 29 degrees Celsius. The science of aquaponics, which represents a natural relationship and coexistence of fish and plants, involves a variety of different microbiological activities. Aquaponics was developed in the 1970s. For instance, the introduction of Plant Growth-Promoting Microorganisms (PGPM) such as decomposing bacteria assists in breaking down the unconsumed food in the water, hence releasing nutrients that are then absorbed by the plants in the growing tray (Thakur et al., 2023). On the other hand, the feeding and movement of the fish contributes to the addition of extra nutrients to the water through the fish’s feces and urine, while also guaranteeing that the water is continuously aerated. Fish are responsible for both of these processes. In the case of crop plants, it is essential to make certain that the plants receive an adequate quantity of nutrients, water, and oxygen in order to facilitate enhanced growth and improved, more robust yields. After its first implementation in Mexico, the system has subsequently been exported to a great number of other nations all over the world, including eastern China, Thailand, and Indonesia. The fish generate a significant amount of hydrogen,

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which makes the aquaponics system an excellent medium for the cultivation of plants. To the same extent that fish, through their waste and food, contribute to the provision of nutrients for the vegetables, the plants, in turn, supply oxygenated water to the reservoir, which is something that happens throughout the cycle. 5. The Synergy of Phytoremediation and the Aquaponics System: Enhancing Water Purification and Nutrient Cycling Aquaponic systems can be combined with phytoremediation techniques to improve water purification and pollutant removal. Aquaponics is a symbiotic relationship between aquaculture (fish farming) and hydroponics (soilless plant cultivation). In this system, plants play a crucial role in phytoremediation by absorbing and metabolizing water-borne contaminants, thereby contributing to water purification. The operation is as follows: In the aquaculture component of the system, fish detritus and uneaten food produce ammonia, a common pollutant in aquaculture effluent. Ammonia in high concentrations is toxic to fish, but the nitrification process can transmute it into nitrate, a less toxic form of nitrogen. Beneficial bacteria facilitate this process by converting ammonia to nitrites and then to nitrate. Despite the fact that nitrification reduces the toxicity of ammonia, excessive levels of nitrate in the water can still be hazardous to fish health. Phytoremediation comes into play at this point. Hydroponically grown plants, typically leafy greens or botanicals, are utilized in aquaponic systems. These plants absorb the nitrate-rich water from the aquaculture component and assimilate the nitrate as their primary source of growth nutrients. In doing so, the plants effectively remove nitrate from the water, thereby reducing the risk of damage to the fish (Garrick, 2021). In addition to nitrate, plants grown in aquaponic systems can aid in the phytoremediation of other contaminants. They have the capacity to absorb and degrade organic compounds, heavy metals, and various water contaminants. The plant roots provide a surface on which beneficial microorganisms can flourish, thereby contributing to the degradation of pollutants. In an aquaponic system, the combination of hydroponic plant growth and the natural filtration capabilities of plants improves the overall water quality. It is essential to select suitable plant species for phytoremediation in aquaponic systems, taking into account their pollution tolerance and ability to flourish in hydroponic conditions. Monitoring water quality parameters on a regular basis is also essential to ensuring that the system remains balanced and effective in its removal of pollutants. By incorporating phytoremediation techniques into aquaponic systems, water purification, sustainable plant cultivation, and fish production can be achieved simultaneously. This method illustrates the potential for using plants to improve the environmental sustainability and efficacy of aquaponics as a wastewater treatment and food production technique (Rajalakshmi et al., 2022; Thakur et al., 2023; Yep & Zheng, 2019). 6. Conclusion and Future Perspectives In conclusion, plants demonstrated their efficacy in removing pollutants from wastewater, which could be regarded a crucial role in the near future. Scientists can

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evaluate the function of plants in removing contaminants from wastewater, which cannot be done by microorganisms and microbes. As microorganisms have a long history of pollutant biodegradation and remediation, scientists should consider integrating them with plants to improve their performance. There are currently no reports on the applications of plants for the removal of radioactive elements; these notions could be useful for researchers. Depending on their species and growth characteristics, plants can be harvested and utilized as biomass following phytoremediation. Other than energy, the harvested plant material can be used for a variety of purposes. This includes decomposition, bioenergy production, or as a raw material for the production of bioplastics or biofuels. The harvested plant material can be used as an organic soil amendment to add organic matter and nutrients to the soil. This enhances the soil’s structure, fertility, and overall health. The plants can be composted to generate nutrient-rich compost, which can then be used as a soil amendment or fertilizer in agricultural or horticultural applications. Composting enables the recycling of plant nutrients and organic matter back into the ecosystem. If phytoremediation occurs in a constructed wetland or an ecosystem with similar characteristics, the plants can contribute to the wetland’s overall restoration efforts. They contribute to the health of the wetland ecosystem by providing habitat and aiding in water retention. Acknowledgements I extend my appreciation to Asma Abdulmohsen Aljogaiman and Mayada AbdulAziz AlRashed for their assistance in translating my ideas into a graphical representation.

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Chapter 8

Removal of Heavy Metals From Contaminated Water Using Hydroponics Partha Chandra Mondal , Shreosi Biswas , Biswajit Pramanik , and Sandip Debnath

Abstract Since numerous pollution sources considerably contribute to poor water quality, the availability of potable and high-quality drinking water is a severe global concern. Several of these pollution sources are spreading a variety of dangerous substances into different environmental and water matrices. Among the significant environmental contaminants are heavy metals. Even if there are already a number of techniques for getting rid of these toxins, the most of them are expensive and challenging to use effectively. There are, sadly, not many eco-friendly solutions that can be used to exclusively treat contaminated environments. As a result, exposure to heavy metals in water causes a variety of illnesses in people, including cytotoxicity and cardiac ailments. Currently, phytoremediation, phytoaccumulation and phytostabilization are emerging, effective and affordable technological solution used to extract or remove inactive metals and metal pollutants from contaminated water. These technologies have the potential to be economically and ecologically sound. In hydroponics, plants are grown without soil using a water-based nutrient solution. Growing medium used in this method can include aggregate substrates like vermiculite, coconut coir, or perlite. Using hyperaccumulating plants, this method can also be used to successfully remove heavy metals from contaminated water. Recent advances in biotechnology are expected to play a promising role in the development of new hyperaccumulators by transferring metal hyperaccumulating genes from low biomass wild species to the higher biomass producing cultivated species in the times to come. Keywords Heavy metals · Hydroponics · Phytoremediation · Phytoaccumulation

P. C. Mondal · S. Biswas Division of Agricultural Chemicals, Indian Agricultural Research Institute, New Delhi 110012, India B. Pramanik · S. Debnath (B) Department of Genetics and Plant Breeding, Palli Siksha Bhavana (Institute of Agriculture), Visva-Bharati, Sriniketan, West Bengal 731236, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_8

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8.1 Introduction Heavy metal contamination in water is a growing concern worldwide due to its potential impact on public health and the environment. These heavy metals can originate from both natural (erosion, weathering, atmospheric deposition) and anthropogenic (industrial discharge, agricultural runoff, landfills, sewage and wastewater) sources. Heavy metals such as lead, zinc, nickel, copper, cadmium, mercury, arsenic etc. are toxic and can accumulate in living organisms. Heavy metal pollution in water can pose significant health hazards to humans, depending on the type of metal, the level of exposure, and the susceptibility of individuals (Table 8.1). Table 8.1 Sources, prevalent form and health hazards caused by common heavy metals Heavy metals

Predominant ion/ species

Health hazard

Source

Mercury

Elemental (or metallic, Hg0 ), inorganic mercury Hg+ , methylmercury (MeHg)

Neurological and behavioural disorders (particularly in children), kidney damage, respiratory failure, and cardiovascular problems

Industrial processes such as coal-fired power plants and gold mining and certain types of fish and seafood containing high levels of mercury

Arsenic

Arsenate (pentavalent) and arsenite (trivalent)

Skin lesions, diabetes, Naturally or as a result of cardiovascular disease, and industrial processes such several types of cancer. as mining and smelting Long-term exposure to arsenic in drinking water has been linked to an increased risk of bladder, lung, and skin cancer

Cadmium

Cd2+

Kidney damage, lung cancer, and bone disease. Long-term exposure to cadmium can also cause respiratory problems and decrease bone density

Mining and industrial processes such as battery along with manufacturing and electroplating

Lead

Pb2+

Developmental delays in children, high blood pressure, anaemia, and kidney damage. Also affects the nervous system, causing symptoms such as headaches, memory loss, and irritability

Lead-based paints, old pipes, and batteries

Chromium

Cr(VI)

Exposure to hexavalent chromium (Cr(VI)) can cause lung cancer, skin irritation, and kidney damage. Long-term exposure to Cr(VI) has also been associated with increased risk of gastrointestinal cancer

Industrial processes such as electroplating and leather tanning

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Apart from the human health hazards, the heavy metals are a threat to aquatic life as well causing physiological changes, such as reduced growth rates, reproductive failure, and behavioural changes upon accumulation in the body. They have the potential for bioaccumulation and biomagnification thereby contaminating the whole food chain across various trophic levels. Last but not the least, heavy metal pollution can have economic consequences, including the loss of fisheries and tourism industries. Additionally, the cost of remediation and treatment of contaminated water can be expensive. Addressing heavy metal contamination is crucial to protecting public health and the environment. The conventional methods of heavy metal removal from water, such as chemical precipitation and ion exchange, are often expensive and produce toxic waste. Phytoremediation of contaminants in aquatic ecosystems is gaining popularity as an alternative low-cost and environmentally friendly technology for heavy metalenriched wastewater treatment (Jasrotia et al., 2017). This concept refers to the use of aquatic and terrestrial green plants to remediate heavy metals in sediment and water. Hydroponics has been used successfully to grow a variety of crops such as lettuce, tomato, cucumber, herbs and many types of flowers. Its advantages over conventional production systems are faster growth, higher productivity, easier handling, greater water efficiency (Paiva et al., 2015) and lesser use of fertilizers (Da Silva Cuba et al., 2015; Rana et al., 2011). In hydroponics, the concentration of nutrients can be controlled in the aqueous solution making it easier to observe the symptoms of nutrient deficiency or toxicity in plants (Da Silva Cuba et al., 2015). Hydroponics has proven to be useful in many fields such as in toxicological studies on the accumulation and destination of emergent contaminants in plants, the implementation of native and exotic crops with experimental, commercial or medical purposes, as well as the cultivation of traditional crops like vegetables and ornamental plants. Recently, Hydroponics has been used for removal of heavy metals from contaminated water, especially waste water. Hydroponics uses plants and their associated microbes to absorb, accumulate, and transform heavy metals into less toxic forms. The roots of plants are submerged in nutrient-rich water, providing an ideal environment for microbial growth and metal uptake. The plants can then be harvested and disposed of as hazardous waste, reducing the risk of further contamination. The reclaimed water is a rich source of other nutrients and hence can be safely used for irrigation as well. Hydroponics offers many advantages over traditional heavy metal decontamination methods such as sustainability, cost effectiveness, versatility, low maintenance and ecological safety. In this book chapter, we provide an overview of hydroponics as a method for removing heavy metals from contaminated waters. We explore the mechanisms of heavy metal removal in hydroponics, the factors affecting heavy metal uptake, and the limitations and challenges associated with this technology. Additionally, we present case studies highlighting the successful application of hydroponics for heavy metal removal in various settings. Through this chapter, we hope to emphasize the potential of hydroponics as a sustainable and effective method for addressing heavy metal contamination in water.

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8.2 Hydroponics as a Heavy Metal Removal Method Hydroponics is a method of growing plants without soil, where the plants are grown in a nutrient-rich solution that provides them with all the necessary nutrients. Hydroponics relies on the principle of hydroculture, where the roots of the plants are submerged in water and supplied with a range of nutrients, including macronutrients like nitrogen, phosphorus, and potassium, and micronutrients like iron, zinc, and copper. The nutrient solution is carefully balanced to ensure that the plants have access to all the necessary nutrients and minerals. However, not all the plants are suitable for the decontamination of heavy metal enriched water via hydroponics. The plants are preferred to have high biomass production for efficient heavy metal removal. They should also be easy to grow and fast growing in nature. The plants should be highly tolerant to the toxic heavy metals as well so they can accumulate as much heavy metals as possible. Bioconcentration factor (BCF) and Translocation factor (TF) are two more characteristics that play important role in the effective removal. The bioconcentration factor (BCF) indicates the ability of the plant to accumulate a metal in different plant parts (Saengwilai et al., 2020). BCF = CPP /Cs where CPP is metal concentration in plant part (mg L−1 ), and Cs is metal concentration in solution (mg L−1 ). The translocation factor (TF) is used to evaluate the movement of heavy metal from root to shoot (Shamshad et al., 2019) and is given by the formula: TF = Mes /Mer where Mes is metal concentration in shoots, and Mer is metal concentration in roots. BCF and TF values of more than 1 is considered as a benchmark for high removal efficiency. Apart from these two, some other criteria for judging the decontamination potential of plants are: percentage growth rate (PGR), percentage metal uptake (%MU), estimated daily intake (EDI), hazard quotient (HQ), maximum daily intake (MDI) and threshold daily intake (TDI) (Woraharn et al., 2021a). Some plant species that have been found to be effective for heavy metal removal through hydroponics include water hyacinth (Eichhornia crassipes), duckweed (Lemna minor), watercress (Nasturtium officinale), and water spinach (Ipomoea aquatica). These plant species have the characteristics mentioned above, making them ideal for heavy metal removal through hydroponics.

8.3 Types of Hydroponic Systems Used for Heavy Metal Removal Based on the type of circulation system, hydroponics can be largely classified into (i) open systems, in which the nutrient solution is applied directly to the roots and used only once, and (ii) closed systems (Fig. 8.1), in which the nutrient solution is recycled. In open system the roots are contact with the nutrient solution either occasionally or permanently but the nutrient solution is not recycled which reduces the risk of infection in plants. While in the closed system, the nutrient solution is recycled and circulated periodically which helps in controlling the water and nutrient

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a

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b

Water flow Nutrient rich water

Drainage Recirculated hydroponic solution

Waste Hydroponic solution Water pump

Fig. 8.1 Types of hydroponics based on circulation systems: a Open system, b closed system (Created in biorender; https://biorender.com)

in a more optimal way. The most commonly used hydroponic techniques are Kratky method, Deep Water Culture, Ebb and Flow method, Nutrient Film Technique, Wick/ Passive system, Drip system and Aeroponics. Among these, only a few are suitable for removal of heavy metals with their own sets of advantages and limitations (Table 8.2; Fig. 8.2).

8.4 Mechanisms of Heavy Metal Removal in Hydroponics 8.4.1 Phytoextraction Phytoextraction is the utilisation of plants to translocate and collect pollutants in their aboveground biomass by absorbing them from soil or water (Jacob et al., 2018). The potential of the plant species to sequester the chemical or metal and the intercellular mobilisation via the plant influence how much HM a plant may store (Yang et al., 2005). Heavy metal phytoextraction involves the following steps: Intake of heavy metals by plant roots, mobilisation of heavy metals in the rhizosphere, translocation of heavy metal ions from roots to aerial sections of plants, and sequestration and compartmentation of heavy metal ions in plant tissues are a few instances of heavy metal ion transport in plants (Ali et al., 2013). Heavy metal transfer through the plasma membrane of root cell membranes, xylem loading, translocation, sequestration, and detoxification at cellular levels in the entire plant are some of the stages that make up the complicated process of accumulation. Root-associated bacteria and root exudates can increase the bioavailability of pollutants during absorption, and even in hydroponic cultures, microbes can form a symbiotic relationship with the roots of the plant (Lombi et al., 2002). Effective phytoextraction requires careful plant species selection. The following traits should be present in the plant species used for phytoextraction: Heavy metaltolerant plants have the following characteristics: (i) high tolerance to the toxic effects of heavy metals; (ii) high extraction ability; (iii) accumulation of high levels of heavy metals in aboveground parts; (iv) abundant shoots and extensive roots; (v) good adaptation to the environment; (vi) high resistance to pathogens and pests; and (vii)

Advantages • It is simple and cheap; it only has a few elements, which means low set up costs, and it also means that there are fewer parts that can break • It allows to top up the nutrient solution • It has a form of aeration of the roots

Deep water culture

In deep water culture systems plants are grown in a reservoir in which the nutrient solution is aerated with a form of mechanical aeration (often in the form of an air stone and pump). Part of the plant is supported above the nutrient solution by a floating or suspended platform so that just the roots are submerged in the nutrient solution

Method name Description

Table 8.2 Different methods of hydroponics used in removal of heavy metal from water References

• The nutrient solution is Bello et al. virtually still which can (2018) become a breeding ground for pathogens (like bacteria), algae growth and in some cases even fungi and molds • A simple air pump does not provide good aeration • The growing tank needs to be emptied completely before cleaning it (continued)

Disadvantages

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Advantages • It is simple and cheap • It does not depend on technology and electricity • It recycles the nutrient solution • It auto regulates the quantity of nutrient solution according to the need of the plants • It provides good aeration • It reduces algae growth and pathogens compared with DWC, It is almost self sufficient

Wick system

A wicking system consists of a grow tray filled with an absorbent soilless grow media, a storage tank for the nutrient solution, and an absorbent wick that transfers nutrient solution from the tank to the grow media

Method name Description

Table 8.2 (continued) References

• It is not suitable for Woraharn et al. vertical gardens and (2021a) towers. It is not even well suited for multi-layer gardens • It still does not solve the problem posed by plants that need their roots to have dry spells • The wick system still has problems with algae and bacteria, and even fungi. This is because the grow tank will be humid all the time • It is not suitable for larger plants (continued)

Disadvantages

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Ebb and flow system

Advantages

Disadvantages

References

In an ebb and flow hydroponic system plants are grown in a tray that • The greatest • It is complex to set up Jha et al. (2023) is regularly flooded with nutrient solution at set times throughout the advantage is that it and run as it has many day. Nutrient solution is pumped from a reservoir into the grow tray, provides excellent components and it where the liquid is kept at a specific level (by means of an overflow aeration requires a sound drain) for a set amount of time before the pump shuts off, allowing • The nutrient solution knowledge of the crops is not stagnant the nutrient solution to drain back down the input pipe to be grown, their around the roots nutritional, watering and reducing the chances humidity needs of algae growth, or • The pump gets clogged bacteria, pathogens fairly regularly • The piping breaks and and fungi gets clogged; being in • One can change the constant use feeding and watering of plants according to their needs or the climate • It is suitable for most crops, including those that need dry spells and root crops • It can be developed vertically (continued)

Method name Description

Table 8.2 (continued)

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Advantages • It uses little water and nutrient mix. This is because the nutrient solution is continuously recycled • It is easy to inspect the roots and treat any root problem

NFT

In this technique, a thin layer with the nutrient solution is provided to the roots, which also allows for the necessary oxygen. In modern NFT systems water is pumped from a water reservoir and into a slanted PVC channel where it runs past the roots of the plants as it travels “downhill”. The nutrient solution is then collected, and in commercial systems filtered (to remove any debris) and sterilized prior to recirculation

Method name Description

Table 8.2 (continued) References

• NFT is not suitable for Bindu et al. large plants; this is (2010) because the roots will not have the support of a growing medium • Roots may block the flow of the nutrient solution • If the system breaks, the plants will end up with no nutrition and water, which may even ruin the crop

Disadvantages

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a

b Net pots with growing medium Tray Growing bed

Nutrient rich medium

Growth medium

Nutrient solution

Air stone Air pump

c

Air pump

Wick

Air stone

d

Nutrient solution

Nutrient solution

Timer Water pump

Water pump

Air stone

Air pump

Fig. 8.2 Type of Hydroponics for heavy metal removal, A Deep Water Culture, B Wick system, C Ebb and Flow, D NFT (Created in biorender; https://biorender.com)

be repulsive to herbivores to prevent heavy metals from entering in (Ali et al., 2013; Seth, 2012). The most important variables that affect a plant species’ ability to extract metals are its above-ground biomass and metal-accumulating capacity. As a result, two separate plant selection methodologies are being used: (i) the use of plants with high aboveground biomass production, which may have lower metal-accumulating capacities but overall heavy metal accumulation is comparable to that of hyperaccumulator plants, and (ii) the use of hyperaccumulator plants, which can accumulate heavy metals in aboveground parts to a greater extent (Ali et al., 2013; Robinson et al., 1998; Salt et al., 1998). Under the same circumstances, the naturally occurring heavy metal hyperaccumulator can accumulate metals at levels 100 times higher than typical non-hyperaccumulating species (Rascio & Navari-Izzo, 2011). Even though there are many hyperaccumulators that have been discovered and employed in phytoremediation of heavy metals, the majority of them have limited life spans, produce little biomass, and grow slowly, which reduces the effectiveness of phytoextraction. As an alternative, non-hyperaccumulators with high biomass production can be employed for heavy metal phytoextraction. The high biomass output can make up for the reduced phytoextraction efficiency, and the overall accumulation levels may even be higher than those of hyperaccumulators, even though they typically accumulate less heavy metals in their aboveground tissues on a mass basis (Ebbs et al., 1997; Vamerali et al., 2010; Vangronsveld et al., 2009).

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8.4.2 Phytofiltration Using plant roots (rhizofiltration), shoots (caulofiltration), or seedlings (blastofiltration) to filter polluted surface waters or waste waters is known as phytofiltration (Mesjasz-Przybyłowicz et al., 2004). Rhizofiltration is the process by which plant roots purge contaminants from an aqueous substrate, primarily metals (Dushenkov et al., 1995). Pollutants from the substrate are initially absorbed, adsorbed (Salt et al., 1998), and precipitated through a variety of interconnected physicochemical processes, including chelation, ion exchange, and chemical precipitation through root exudates, among others, at the root surface (Dushenkov et al., 1995). Heavy metals are either absorbed by the roots or adsorbed onto the root surface during rhizofiltration. Root exudates have the ability to alter the pH of the rhizosphere, which causes heavy metals to precipitate on plant roots and further reduces the transport of heavy metals into groundwater (Javed et al., 2019). Hydroponically produced plants used for rhizofiltration first grow a substantial root system in clean water before being switched to dirty water to acclimatise the plants. The plants are moved to the contaminated site after acclimation in order to remove the heavy metals. The roots are harvested and disposed of once they are saturated (Wuana & Okieimen, 2011). Plants utilised for rhizofiltration should ideally produce a lot of biomass, have dense root systems, and be heavy metal resistant. Both terrestrial and aquatic plants can be used for rhizofiltration (Hooda, 2007). From a physiological standpoint, root exudates and microbes (1) improve bioavailability before causing (2) metals to precipitate. (3) Both apoplastic and symplastic mechanisms mediate and regulate pollutant uptake. Chelation and sequestration occur mostly in the roots of the plant if symplastic mechanisms are unable to translocate the contaminant into leaves and steam. (4) Following the chelation of metals by phytochelatins and metalloteines, contaminants are either attached to the cell wall or sequestered inside the apoplast and cell vacuoles (Cobbett & Goldsbrough, 2002; Dushenkov et al., 1995; Tester & Leigh, 2001).

8.4.3 Phytodegradation Pollutants can either completely mineralize into inorganic compounds or degrade into a stable, less hazardous intermediate that is drawn to or secluded by the cell wall (Garrison et al., 2000). In the plant tissue, enzymes act as biological catalysts for breakdown (Newman & Reynolds, 2004). Increased microbial metabolic activity and proliferation, which are facilitated by plant exudates, promote rhizosphere pollution breakdown (Kuiper et al., 2004; Lin et al., 2021; McCutcheon & Schnoor, 2003; Sheridan et al., 2017). Therefore, interactions between plants and microbes constitute a crucial mechanism for achieving the breakdown of organic contaminants (Banks et al., 2003; Muratova et al., 2003; Nichols et al., 1997).

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The detailed mechanism of phytodegradation is as follows: (1) Interactions between plants and the rhizosphere aid in the breakdown of organic contaminants. (2) Bioavailability is increased by root exudates, which also boost rhizospheric activity. (3) Pollutant uptake then takes place in the roots and is mediated by apoplastic and symplastic pathways. (4) Organic pollutants can then be transferred to primarily leaves or the roots by the xylem sap. Finally, organic pollutants can be (5) metabolised in the cell to less hazardous chemicals by the action of plant enzymes (Kuiper et al., 2004; Lin et al., 2021; McCutcheon & Schnoor, 2003; Sheridan et al., 2017).

8.4.4 Phytostabilization In order to immobilise heavy metals underground and lower their bioavailability, metal-tolerant plant species are used in phytostabilization (Marques et al., 2009; Wong, 2003). This reduces the possibility that metals will enter the food chain and prevents their migration into the environment. Heavy metal precipitation or a decrease in metal valence in the rhizosphere, absorption and sequestration within root tissues, or adsorption onto root cell walls can all result in phytostabilization (Gerhardt et al., 2017; Ginn et al., 2008; Kumpiene et al., 2012). For phytostabilization to occur, the right plant species must be chosen. Plants should be tolerant of the conditions caused by heavy metals in order to meet the need of extremely effective phytostabilization. Plants should have extensive root systems because they are essential for immobilising heavy metals, stabilising soil structure, and preventing soil erosion. For plants to quickly establish a vegetative cover on a particular site, they must be able to produce a significant quantity of biomass and grow quickly. Additionally, the plant cover should be simple to maintain in outdoor settings (Berti & Cunningham, 2000; Marques et al., 2009). The polluted soil can be amended with organic or inorganic materials to increase the effectiveness of phytostabilization. By altering the pH and redox status of the soil, these soil amendments can change metal speciation and decrease the solubility and bioavailability of heavy metals (Alvarenga et al., 2009; Burges et al., 2018; Epelde et al., 2009). Phytostabilization can be helped by rhizosphere-dwelling microbes like bacteria and mycorrhiza. These microorganisms can increase the effectiveness of heavy metal immobilisation by chelating metals, generating chelators, and facilitating the precipitation process (Göhre & Paszkowski, 2006; Ma et al., 2011; Mastretta et al., 2009). They can even act as a filtration barrier to prevent heavy metal ion translocation from roots to shoots. They can also improve plant root surface and depth to assist phytostabilization (Göhre & Paszkowski, 2006).

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8.4.5 Detoxifications Heavy metal detoxification is a crucial step before phytoremediation can be used (Thakur et al., 2016). Avoidance and tolerance are typically the two defence mechanisms used by plants to combat the toxicity of heavy metals. Plants are able to keep the cellular concentrations of heavy metals below the toxicity threshold levels because of these two distinct mechanisms (Hall, 2002).

8.4.6 Avoidance The ability of plants to limit heavy metal uptake and prevent their transport into plant tissues through root cells is referred to as an avoidance strategy. Through a variety of mechanisms including metal exclusion, metal ion precipitation, and root sorption, it serves as the first line of defence at the extracellular level (Dalvi & Bhalerao, 2013). Plants initially attempt to immobilise heavy metals through root sorption or by altering metal ions when they are exposed to them. In the rhizosphere, a range of root exudates, including organic acids and amino acids, serve as a heavy metal ligand to create stable heavy metal complexes (Dalvi & Bhalerao, 2013). Some root exudates have the ability to alter the pH of the rhizosphere, which causes heavy metals to precipitate out and become less hazardous (Dalvi & Bhalerao, 2013). Exclusion boundaries between the root system and the shoot system are created through a metal exclusion mechanism to prevent the uptake and root-to-shoot transfer, thereby protecting aerial portions from hazardous heavy metals. For ex: arbuscular mycorrhizas as exclusion barrier (Hall, 2002). Another method of avoiding heavy metals is to embed them in the cell walls of plants (Memon & Schröder, 2009). Pectins found in cell walls are made up of carboxylic groups of negatively charged polygalacturonic acids that are capable of binding heavy metals. Therefore, to prevent the entry of free heavy metal ions into the cells, the cell wall functions as a cation exchanger (Ernst et al., 1992).

8.4.7 Tolerance The plants develop a tolerance strategy to deal with the toxicity of accumulating metal ions once the heavy metal ions enter the cytoplasm. By using different processes, such as the inactivation, chelation, and compartmentalization of heavy metal ions, it serves as the second line of defence at the intracellular level (Dalvi & Bhalerao, 2013.) This is mostly accomplished through chelation, which is the complexation of ligands with heavy metal ions. In the cytoplasm, there are a large number of organic and inorganic ligands that facilitate heavy metal chelation. Organic acids, amino acids, phytochelatins (PCs), metallothioneins (MTs), and cell wall proteins/

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pectins/polyphenols are among the organic molecules implicated in heavy metal ion chelation (Gupta et al., 2013; Hall, 2002; Sharma & Dietz, 2006). For example, citrate mediates the chelation of Ni in T. goesingense leaves (Kramer et al., 2000), while acetic and citric acids bind Cd in leaves of Solanum nigrum (Sun et al., 2006). By removing toxic heavy metal ions from sensitive regions of the cell where cell division and respiration take place, sequestration and vacuolar compartmentalization effectively protect against the harmful effects of heavy metals, minimising interactions between heavy metal ions and cellular metabolic processes and preventing damage to cell functions (Sheoran et al., 2010). Other than vacuoles, heavy metal ions can be compartmentalised and sequestered in places including leaf petioles, leaf sheaths, and trichomes (Eapen & D’souza, 2005; Robinson et al., 2003), where they lessen the plant’s toxicity. For instance, Zn is only transferred to Plantago lanceolata leaves in the final week before the plant sheds its leaves, and it is then taken away from the plant once the leaves have fallen (Ernst et al., 1992). High levels of heavy metals in the environment cause an increase in the accumulation of metal ions in the cytoplasm, which trigger the production of reactive oxygen species (ROS leading to oxidative stress. Oxidative stress can disrupt cellular homeostasis and result in DNA damage, protein oxidation, and inhibition of cellular processes (DalCorso et al., 2019; Huang et al., 2012). Plant cells activate the ROSscavenging machinery by inducing antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), and glutathione reductase (GR) as well as non-enzymatic antioxidant compounds such as glutathione, flavonoids, carotenoids, ascorbate, and tocopherols to deal with heavy metal-induced oxidative damage (DalCorso et al., 2019; Gupta et al., 2009; Jozefczak et al., 2012). Therefore, this anti-oxidative defence mechanism of plants is crucial in the response to heavy metal stress.

8.5 Factors Affecting Heavy Metal Removal in Hydroponics 8.5.1 pH pH is a critical factor in heavy metal removal in hydroponics because it affects the solubility and speciation of heavy metals in the water. Different heavy metals have different pH ranges at which they are most soluble, and the optimum pH for heavy metal precipitation can vary depending on the specific metal. Typically, heavy metal precipitation occurs at a pH range of 6.0 to 9.0. Khan et al. (2009) observed that for removal of heavy metals, the optimum pH was 7 and above. Ullah et al. (2015) carried out phytoremediation of heavy metals by plants and observed the optimum heavy metal removal efficiency in the alkaline pH range. However, extreme pH conditions can also affect the growth and health of plants, leading to reduced heavy metal removal efficiency.

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8.5.2 Temperature Temperature is an essential factor in heavy metal removal in hydroponics because it affects the biological and chemical processes involved in heavy metal removal. High temperatures can increase the growth rate of plants, leading to increased heavy metal removal efficiency. However, high temperatures can also affect the stability of the hydroponic system and increase the risk of pathogen growth. Low temperatures can slow down the biological and chemical processes involved in heavy metal removal, leading to reduced heavy metal removal efficiency. Schück and Greger (2023) reported in a study that the removal of Cd, Cu, Pb and Zn by Carex pseudocyperus, C. riparia, and Phalaris arundinacea improved significantly as the temperature increased from 5 °C to 25 °C.

8.5.3 Nutrient Concentration Nutrient concentration is a critical factor in heavy metal removal in hydroponics because it affects the growth and health of plants. High nutrient concentrations can lead to increased plant growth and heavy metal removal efficiency. However, high nutrient concentrations can also lead to reduced heavy metal removal efficiency due to nutrient competition between plants and heavy metals. Low nutrient concentrations can lead to reduced plant growth and heavy metal removal efficiency. Kong et al. (2020) reported that under enriched nitrogen and phosphorus conditions the accumulation of Cd increased in both shoots and roots of Salix matsudana Koidz.

8.5.4 Plant Species Plant species is an essential factor in heavy metal removal in hydroponics because different plants have different metal uptake capacities and affinities. Some plants have a high affinity for specific heavy metals, while others can tolerate high levels of heavy metals without showing any adverse effects. Choosing the right plant species for a specific heavy metal removal application is essential to maximize heavy metal removal efficiency.

8.5.5 Heavy Metal Concentration Heavy metal concentration is a critical factor in heavy metal removal in hydroponics because it affects the saturation and adsorption capacity of the growing medium and plant roots. High heavy metal concentrations can saturate the adsorption sites of the

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growing medium, leading to reduced heavy metal removal efficiency. Additionally, high heavy metal concentrations can lead to plant toxicity and reduce the growth and health of the plants, leading to reduced heavy metal removal efficiency.

8.5.6 Hydraulic Retention Time Hydraulic retention time (HRT) is a critical factor in heavy metal removal in hydroponics because it affects the contact time between the water and the growing medium or plant roots. A longer HRT can increase the contact time between the water and the growing medium or plant roots, leading to increased heavy metal removal efficiency. However, a longer HRT can also increase the risk of pathogen growth and decrease the oxygen availability in the hydroponic system, leading to reduced heavy metal removal efficiency. In a nutshell, several factors affect heavy metal removal efficiency in hydroponics, including pH, temperature, nutrient concentration, plant species, heavy metal concentration, and hydraulic retention time. Understanding these factors and their interactions is essential to design and optimize hydroponic systems for efficient heavy metal removal.

8.6 Case Studies of Heavy Metal Removal Using Hydroponics Several researches have been practised till date on the elimination of heavy metals with the implementation of hydroponics. Table 8.3 represents the execution of the aforesaid issues to date.

8.7 Recent Advances in Hydroponics In general, plants suitable for removing heavy metals often have drawbacks such as slow growth and susceptibility to toxic heavy metals. These limitations can be overcome through various techniques such as breeding, genetic engineering, and microbe-assisted removal of heavy metals. Traditional methods like plant hybridization or genetic engineering can be used to enhance the growth rate and biomass of hyperaccumulators or introduce hyperaccumulation traits to fast-growing, high biomass plants (DalCorso et al., 2019). To illustrate, Brewer et al. (1999) utilized electrofusion to combine protoplasts from the Zn hyperaccumulator T. caerulescens and Brassica napus. The resulting

Copper (Cu) Plants were grown in absence of liquid circulation

Copper (Cu), Iron (Fe), Manganese (Mn), Lead (Pb) and Zinc (Zn)

Copper (Cu), Zinc (Zn) and Manganese (Mn)

Dendrocalamus asper

Vetiveria zizanioides

Chloris gayana (Rhodes grass), Vetiveria zizanioides (Vetivar grass) and Pennisetum purpureum (Elephant grass)

Plants were grown in floating beds of Styrofoam sheets in a pot

The plants were grown under continuous flow conditions

Woraharn et al. (2021a)

Shao et al. (2019)

References

The removal of heavy metals was 97% for Zn and 89% for both Cu and Mn. Rhodes grass favoured up taking zinc, Elephant grass for copper and Vetiver grass preferred manganese

(continued)

Hassan et al. (2020)

Removal rate of heavy metals in water by vetiver grass is Hasan et al. (2017) ranked in the order of Fe > Zn > Pb > Mn > Cu. The removal of Fe was almost 96%. The heavy metal removal was found to be proportional to root length and higher plant density

Optimal removal of Cu was observed at pH 5 in presence Go et al. (2021) of 20 ppm Cu

Typha angustifolia was found as a suitable candidate for removal of both the heavy metals with bioconcentration factor >100 and translocation factor P. notoginseng > C. comosum

Heavy metals

Panax notoginseng, Chlorophytum Copper (Cu) Closed system with comosum and Calendula ofcinalis and Lead replacement of the (Pb) nutrient solution every 3–4 days

Plant species

Table 8.3 Case studies regarding hydroponics-mediated heavy metal elimination

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Heavy metals

Cadmium (Cd)

Zinc (Zn) and Cadmium (Cd)

Nickel (Ni) and Cobalt (Co)

Chromium (Cr)

Zinc (Zn) and Cadmium (Cd)

Nickel (Ni), Cadmium (Cd) and Lead (Pb)

Plant species

Salix matsudana Koidz

Sedum alfredii Hance

Hydrocharis morsus-ranae

Lemna minor L.

Heliconia psittacorum × H. spathocircinata, Echinodorus cordifolius, and Pontederia cordata

Hydrilla verticillata (L.f.) Royle

Table 8.3 (continued)

Plants were grown in 10% Hoagland solution for 21 days

The plants were grown in nutrient solution which was replaced every 3 days

The plants were grown under continuous aeration and the nutrient solution was renewed every 3 days

Plants were grown hydroponically in 1/ 50 Hoagland solution

Plants were cultivated in N enriched nutrient solution that was continuously aerated and replaced every 5 days

Each hydroponic chamber received 400 mL nutrient solution which was replaced once a week

Method/system

References

Lin et al. (2020)

Woraharn et al. (2021b)

(continued)

Hydrilla verticillata showed an accumulation pattern of Zhang et al. (2020) Ni > Cd > Pb during the experimental time period. On average, the heavy metal accumulation in the leaves was 2.92 (Pb), 1.76 (Cd) and 3.32 (Ni) times higher than that in the stems

The highest uptakes of Cd and Zn were found in H. psittacorum × H. spathocircinata (62.1% Zn2+ from 10 mg L−1 Zn solution) and E. cordifolius (27.3% Cd2+ from 2 mg L−1 Cd solution)

The addition of citric acid enhanced the Cr concentration Sallah-Ud-Din et al. in L. minor by 6.10, 26.5, 20.5, and 20.2% at 0, 10, 100, (2017) and 200 µM Cr treatments, respectively, compared to the respective Cr treatments without citric acid

The plant showed excellent ability in removing Co (up to Polecho´nska and 98.6% in solution with 5.33 µg L−1 Co) and Ni (up to Samecka-Cymerman 91.4% in solution with 57.1 µg L−1 Ni and 28.6 µg L−1 (2018) Co) from nutrient solution

The N doses of 1 to 2.5 mmol L−1 N represented optimal conditions for Zn and Cd accumulation in the shoots of S. alfredii seedlings. The accumulation in the shoots were 52.6 and 68.1% higher compared to control for Zn and Cd respectively

Enrichment with nitrogen and phosphorus increased Cd Kong et al. (2020) accumulation by 21% in shoots and 92% in roots under 50 µmol L−1 Cd, while under 100 µmol L−1 Cd increase it by 32% in xylem and decreased it by 20% in phloem

Removal efficiency

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Heavy metals

Cadmium (Cd)

Nickel (Ni), Cadmium (Cd) and Lead (Pb)

Plant species

Pontederia cordata

Phragmites australis

Table 8.3 (continued)

Pontederia cordata was able to effectively remove the heavy metal from the solution at 0.04–0.44 mM Cd2+ with high BCF and TF. However, the removal efficiency decreased with increasing concentration

Removal efficiency

Plants were grown in Phragmites australis showed 84–95% removal of the a deep-water heavy metals after 6 weeks as compared to 4–11% hydroponic systems at removal in the control heavy metal concentrations of 5 ppm

Plants were grown in ½ Hoagland solution with varying concentrations (0.04 to 0.66 mM) of Cd2+

Method/system

Bello et al. (2018)

Xin et al. (2020)

References

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hybrids, known as somatic hybrids, exhibited enhanced hyperaccumulation capability and tolerance from T. caerulescens, along with higher biomass production from B. napus. These hybrids demonstrated the ability to accumulate significant levels of Zn and Cd, showcasing the feasibility of transferring the metal hyperaccumulation trait to high biomass plants through somatic hybridization. Similarly, ethyl methanesulfonate (EMS) was employed to treat sunflowers in an experiment. As a consequence, a sunflower “giant mutant” with significantly improved heavy metal extraction ability was found. The mutant exhibited 7.5 times higher accumulation for Cd, 9.2 times for Zn, and 8.2 times for Pb compared to control plants. Genetic engineering presents another promising approach to enhance the effectiveness of heavy metal removal. Desired traits such as fast growth, heavy metal tolerance, and hyperaccumulation can be identified and isolated from potent species, then introduced into other plants. Genetic engineering offers a faster alternative to traditional breeding for imparting desirable characteristics to a target plant. However, the process of introducing multiple genes responsible for high accumulation and heavy metal tolerance can be complex and time-consuming. Additionally, gaining approval for field testing genetically modified plants is challenging in certain regions due to concerns about food and ecosystem safety. On the other hand, plant-associated microbes play a crucial role in increasing root volume and proliferation, leading to enhanced heavy metal uptake by plants. Arbuscular-Mycorrhizal Fungi (AMF) are particularly important in this context as they expand the effective surface area of roots through hyphal proliferation, ultimately promoting heavy metal uptake. The effectiveness of phytoremediation can also be enhanced with the support of plant growth promoting (PGP) bacteria. These bacteria employ various mechanisms such as siderophores, organic acids, biosurfactants, biomethylation, and redox processes to convert metals into forms that are readily available and soluble for biological processes. Furthermore, PGP bacteria possess characteristics that promote plant growth, including phosphorus solubilization, nitrogen fixation, iron sequestration, and the synthesis of phytohormones and ACC deaminase. These attributes not only improve plant growth and increase plant biomass, but also contribute to the overall enhancement of phytoremediation (Ullah et al., 2015).

8.8 Limitations and Challenges of Hydroponics for Heavy Metal Removal Hydroponics exhibits considerable potential as a methodology for the remediation of heavy metal contamination; however, its widespread adoption and efficacy necessitate addressing several limitations and challenges. Foremost among these limitations is the economic feasibility of implementing and sustaining hydroponic systems tailored for heavy metal removal. The establishment of hydroponic setups incurs substantial costs, coupled with the requirement for ongoing maintenance tasks such as

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nutrient replenishment and pH regulation. Moreover, the expenditure associated with procuring plant materials and providing the necessary energy to maintain optimal growth conditions poses a significant impediment to the broader implementation of hydroponics for heavy metal remediation. Technical challenges also arise with respect to hydroponics for heavy metal removal, including the imperative of ensuring appropriate growth conditions for plants encompassing temperature, pH, and nutrient levels. Additional difficulties involve guaranteeing optimal hydraulic retention times and averting clogging of the growth medium. The design and construction of hydroponic systems can further compound complexities, necessitating specialized expertise for their successful implementation. Operational challenges manifest in the form of ongoing monitoring obligations, encompassing the health of the plants and the water quality within the hydroponic system. Maintenance activities such as cleaning and nutrient replenishment prove time-consuming and financially burdensome. Furthermore, challenges arise pertaining to the disposal of plant materials and water utilized in hydroponic systems. Public perception of hydroponics for heavy metal removal presents an additional challenge. Scepticism may surround the utilization of plants for water remediation, with concerns regarding the efficacy and safety of consuming plants grown in contaminated water. Consequently, educational initiatives and outreach efforts are indispensable to enhance public awareness and comprehension concerning the safety and effectiveness of hydroponics as a viable method for heavy metal removal. Addressing these challenges necessitates the continual pursuit of research, innovation, and collaborative endeavours encompassing diverse sectors and stakeholders.

8.9 Future Directions and Conclusions Future research endeavours in hydroponics for heavy metal removal should prioritize the optimization of system design and operation tailored to different plant species, heavy metal types, and concentrations. Furthermore, there is a pressing need to explore the development of cost-effective and sustainable nutrient sources specifically tailored for implementation in hydroponic systems. Research investigations should also emphasize a comprehensive understanding of the long-term implications of heavy metal accumulation in plant tissues and its consequential impact on both food safety and nutrition. Recent advancements in hydroponic technology, encompassing the utilization of automation, sensors, and machine learning, present significant potential to enhance system efficiency, diminish operational costs, and augment plant growth rates. Additionally, the exploration of novel materials and design approaches holds promise for more efficient and effective removal of heavy metals within hydroponic systems. Sustained research endeavours in hydroponics for heavy metal removal assume critical importance in combatting the escalating issue of heavy metal pollution in

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water resources across the globe. Hydroponics proffers a sustainable and economically viable methodology for water remediation that can be implemented at various scales, ranging from small-scale domestic systems to large-scale industrial installations. Hydroponics represents a promising avenue for the elimination of heavy metals from contaminated water, characterized by its potential for achieving high removal efficiencies, incurring low operational costs, and exerting minimal environmental impact. The mechanisms underlying heavy metal removal within hydroponic systems encompass adsorption, absorption, precipitation, ion exchange, and microbial degradation. Nonetheless, the comprehensive adoption and efficacy of hydroponics for heavy metal removal necessitate addressing a multitude of limitations and challenges, inclusive of economic feasibility, technical intricacies, operational hurdles, and public perception. In conclusion, sustained research and innovative approaches in hydroponics for heavy metal removal play a pivotal role in realizing the full potential of this methodology for facilitating sustainable water remediation. With the advent of novel technologies and methodologies, hydroponics possesses the capability to emerge as an indispensable tool in addressing the mounting predicament of heavy metal pollution in water resources worldwide.

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Chapter 9

Hydroponic: An Eco-friendly Future Prasann Kumar and Joginder Singh

Abstract Hydroponics, an innovative method of cultivating plants without soil, has gained significant attention as a promising solution for sustainable and ecofriendly agriculture. This abstract explores the concept of hydroponics as a means to shape a greener future. Traditional farming practices often rely on vast amounts of arable land, excessive water usage, and harmful chemical fertilisers, resulting in environmental degradation. In contrast, hydroponics offers an alternative approach by utilising nutrient-rich water solutions to provide plants with essential elements for growth. By eliminating the need for soil, hydroponics reduces water consumption by up to 90% and minimises the risk of soil erosion and nutrient runoff. The controlled environment in hydroponic systems enables precise management of temperature, light, and nutrient levels, resulting in accelerated plant growth and higher yields than traditional farming methods. Hydroponics allows for year-round cultivation, allowing growing crops in regions with unfavourable climates or limited arable land. The ecofriendly nature of hydroponics extends beyond water conservation and efficient land use. This method also eliminates the need for chemical pesticides, as the controlled environment reduces the risk of pests and diseases. Closely monitoring nutrient solutions in hydroponic systems enables targeted application, reducing fertiliser waste and minimising the release of harmful substances into the environment. Hydroponics has found applications in various settings, ranging from small-scale indoor gardens to large-scale commercial operations. Its versatility makes it suitable for urban agriculture, where limited space and contaminated soil pose significant challenges. Hydroponics can be integrated into vertical farming systems, maximising land utilisation and reducing transportation costs associated with long-distance food distribution. Hydroponics presents a promising path toward an eco-friendly future in agriculture. Its resource-efficient nature, reduced reliance on chemicals, and adaptability to diverse environments make it an attractive option for sustainable food production. As P. Kumar (B) Department of Agronomy, School of Agriculture, Lovely Professional University, Phagwara 144411, Punjab, India e-mail: [email protected] J. Singh Department of Botany, Nagaland University, Nagaland, India e-mail: [email protected]; [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_9

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further research and technological advancements continue to enhance the efficiency and affordability of hydroponic systems, their widespread adoption can transform the agricultural landscape, contributing to a greener and more sustainable planet. Keywords No poverty · Zero hunger · Agriculture · Sustainability · Efficiency · Hyfroponics

9.1 Introduction The global agricultural sector faces numerous challenges pursuing sustainable and eco-friendly food production (Tan et al., 2023). Conventional farming practices often rely on extensive land use, excessive water consumption, and the application of chemical fertilisers and pesticides, resulting in environmental degradation, soil erosion, and water pollution. As the world’s population continues to grow and the demand for food rises, alternative farming methods that minimise these negative impacts are becoming increasingly crucial (Bhatt et al., 2023; N. Chen et al., 2023; Sahoo & Kumar, 2023; R. Singh & Arora, 2023). Hydroponics, a soil-less cultivation technique, has emerged as a promising solution for achieving a more sustainable and eco-friendly future in agriculture. Hydroponics involves growing plants in a nutrientrich water solution, allowing them to obtain essential minerals directly rather than extracting them from the soil (Etesami et al., 2023; Iyyappan et al., 2023; Omidvari et al., 2023; Shrivastava et al., 2023; Wang et al., 2023; Yuan et al., 2023). This method offers several advantages over traditional farming, including reduced water usage, increased crop yields, and precise control over environmental factors such as temperature, light, and nutrient levels. However, despite the potential benefits, hydroponics has yet to be widely adopted globally, indicating a research gap in understanding and addressing the challenges associated with its implementation. One critical research gap in hydroponics is the development of cost-effective and scalable systems suitable for different agricultural contexts (Krishnamoorthy et al., 2023; Lamnatou & Chemisana, 2023; Mukherjee et al., 2023; Numan et al., 2023; Sadek et al., 2023). While hydroponics has been successfully implemented in controlled environments such as greenhouses, exploring its feasibility in larger-scale operations and open-field conditions is necessary. The affordability of hydroponic systems remains a concern for small-scale farmers and resource-limited regions, hindering widespread adoption. Research focusing on optimising the design and reducing the initial investment and operational costs of hydroponic systems could bridge this gap and encourage broader implementation (Ganesapillai et al., 2023; Husaini & Sohail, 2023; Krishnamoorthy et al., 2023; Levin & Paltseva, 2023; Mahawar et al., 2023; Sadvakasova et al., 2023). Another research gap lies in the environmental sustainability aspects of hydroponics. While it offers water efficiency benefits compared to traditional farming, the energy requirements for maintaining controlled environments and producing nutrient solutions must be considered. Evaluating and minimising the energy footprint of

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hydroponics systems through renewable energy sources, improving nutrient recycling techniques, and exploring sustainable nutrient formulations warrant further investigation. The potential impact of hydroponics on biodiversity and ecosystem services requires careful examination. As hydroponics typically involves the exclusion of soil and the use of synthetic materials, the ecological consequences, such as the loss of soil microorganisms and changes in nutrient cycling dynamics, must be thoroughly studied (Ain et al., 2023; Ganesapillai et al., 2023; Kamilya et al., 2023; Sehar et al., 2023; D. Singh, 2023; Zuluaga et al., 2023). Incorporating strategies to enhance biodiversity within hydroponic systems, such as integrating companion planting or exploring alternative substrates that support microbial communities, could address this research gap and contribute to the overall sustainability of the approach. While hydroponics holds immense potential for revolutionising the agricultural industry and fostering an eco-friendly future, several research gaps must be addressed to ensure its successful and sustainable implementation. By focusing on developing cost-effective and scalable systems, minimising the environmental impact, and considering biodiversity conservation, researchers can provide valuable insights and solutions to promote the widespread adoption of hydroponics. Closing these research gaps will contribute to building a more sustainable and resilient food system that mitigates the environmental challenges faced by traditional agriculture and paves the way for a greener future (Adelodun et al., 2022; Akhtar et al., 2021; Aley et al., 2022; S. Chakraborty et al., 2021; T. Das et al., 2022; Goud et al., 2022; Haque et al., 2023; Kotia et al., 2022; P. Kumar, Sharma, et al., 2021; P. Kumar et al., 2020; P. Kumar, Devi, et al., 2021; P. Kumar & Mistri, 2020; Kumari et al., 2022; Upadhyay et al., 2023).

9.2 Benefits of Hydroponics in Agriculture with Latest Technology Information Hydroponics offers several significant benefits over traditional soil-based farming methods as an innovative agricultural practice (Adnan Akram et al., 2023; Khan et al., 2023; Sudheer & Chattopadhyay, 2023; Xu et al., 2023; Zhang et al., 2023). With technological advancements, hydroponics has become even more efficient, precise, and accessible. Here are some key benefits of hydroponics in agriculture, along with the latest technology information:

9.2.1 Water Efficiency Hydroponics allows for precise control over water usage, resulting in substantial water savings compared to traditional farming methods. Advanced hydroponic systems, such as drip irrigation or aeroponics, utilise water recirculation and nutrient

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recovery mechanisms to minimise waste and optimise water efficiency. Furthermore, integrating intelligent sensors and automation technology enables real-time monitoring of plant water requirements, ensuring precise and targeted water delivery.

9.2.2 Increased Crop Yields Hydroponic systems provide an optimal environment for plant growth, allowing for higher crop yields in smaller spaces. The precise control of environmental factors, such as temperature, humidity, and light, combined with the optimised nutrient delivery, enables plants to reach their full growth potential (D’Auria et al., 2023; P. Das & Paul, 2023b; Ekka & Kumar, 2023; Etesami & Jeong, 2023; Mattiello et al., 2023; Sánchez-Zarco & Ponce-Ortega, 2023). Vertical farming techniques and advanced lighting systems, such as LED grow lights, maximise the cultivation area and promote year-round production.

9.2.3 Efficient Land Use Hydroponics offers the advantage of growing crops in vertically stacked layers or compact systems, efficiently utilising limited land resources. This is particularly beneficial for urban agriculture, where available space is scarce. Hydroponic systems can significantly increase the production capacity per square meter of land by utilising multi-level racks or vertical towers.

9.2.4 Reduced Chemical Dependency Hydroponics reduces the need for chemical pesticides and herbicides. The controlled environment in hydroponic systems minimises the risk of pests and diseases, reducing the reliance on harmful chemicals. Integrated pest management techniques, such as biological controls and beneficial insects, can be easily implemented in hydroponics to maintain a healthy and pest-resistant crop environment.

9.2.5 Year-Round Production Farmers can overcome seasonal limitations and grow crops year-round with hydroponics. By providing the ideal conditions for plant growth, including temperature, light, and nutrient availability, hydroponic systems eliminate the dependence on

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specific growing seasons and enable continuous production regardless of external climate conditions. This leads to a more consistent and reliable food supply.

9.2.6 Sustainable Nutrient Management Hydroponics allows for precise control over nutrient delivery, minimising fertiliser waste and nutrient runoff. The latest technological advancements in hydroponics include using sensor-based monitoring systems that measure nutrient levels in realtime, enabling accurate adjustments and targeted nutrient application. Developing sustainable nutrient formulations, such as organic or bio-based alternatives, also reduces the environmental impact of conventional chemical fertilisers (Ajibade et al., 2023; Boyaci & Reyes-Garcés, 2023; Hechmi et al., 2023; Sarker et al., 2023; A. Sharma et al., 2023; Wu et al., 2023; R. K. Yadav, Das, et al., 2023).

9.2.7 Integration with Automation and Artificial Intelligence Automation technology is increasingly integrated into hydroponic systems, streamlining operations and enhancing efficiency. Automated processes for nutrient delivery, irrigation, climate control, and data monitoring simplify the management of hydroponic farms. Artificial intelligence algorithms analyse data collected from sensors, optimising resource allocation and decision-making for crop management, improving yields and resource efficiency. With the latest technological advancements, hydroponics in agriculture offers numerous benefits that contribute to a more sustainable and productive farming system. Water efficiency, increased crop yields, efficient land use, reduced chemical dependency, year-round production, sustainable nutrient management, and integration with automation and artificial intelligence are among the advantages provided by hydroponics (Agarwal, 2023; Bakshe & Jugade, 2023; Kronrod et al., 2023; Saeed et al., 2023; B. Sharma et al., 2023; Soussi et al., 2023; Spinozzi et al., 2023; Thakur & Thakur, 2023). As technology continues to evolve, hydroponics is poised to play a vital role in shaping a more sustainable and eco-friendly future in agriculture (Table 9.1).

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Table 9.1 Mechanisms of hydroponics, along with their advantages, disadvantages, and nutrient availability Mechanism

Advantages

Disadvantages

Nutrient Film Technique (NFT)

1. Water-efficient system 1. Prone to system clogging 2. Reduced nutrient waste

2. Requires constant power supply

3. Easy access to plant roots

3. Vulnerable to power outages

Nutrient availability Continuous

4. Limited root support Deep Water Culture 1. Simple and low-cost (DWC) system

1. Risk of oxygen depletion in water

2. Provides ample oxygen to roots

2. Susceptible to algae growth

3. Easy nutrient monitoring

3. Requires aeration equipment

Continuous

4. Limited space for root growth Ebb and Flow

Aeroponics

Drip Irrigation

Wick System

Vertical Farming

1. Flexible and customisable system

1. Risk of system malfunction

Intermittent

2. Good aeration and oxygen supply

2. Requires periodic maintenance

3. Suitable for a variety of crops

3. Possibility of nutrient imbalances

1. Maximum oxygen exposure to roots

1. Requires precise misting system

2. Water and nutrient-efficient

2. Sensitive to power outages

3. Rapid plant growth and high yields

3. Prone to clogging if not maintained

1. Precise and targeted nutrient delivery

1. Drip emitters may clog

2. Easy automation and scalability

2. Requires regular monitoring

3. Reduced water and nutrient waste

3. May require a filtration system

1. Simple and low-cost setup

1. Limited to small-scale Intermittent operations

2. No power or pumps are required

2. Limited nutrient distribution

3. Low maintenance

3. Slower growth compared to others

1. Efficient land use

1. High initial investment

Intermittent

Intermittent

It depends on the system design (continued)

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Table 9.1 (continued) Mechanism

Floating Raft System

Hybrid Systems

Advantages

Disadvantages

Nutrient availability

2. Maximises crop production per area

2. Requires advanced lighting systems

3. Reduces transportation costs

3. Complex setup and management

1. Low-cost and straightforward system

1. Susceptible to disease Continuous transmission

2. Efficient nutrient delivery

2. Limited to specific crops

3. Provides good root support

3. Requires careful water quality control

1. Customisable and adaptable

1. Higher initial setup cost

It depends on the system design

2. Combines advantages 2. Requires more of different systems complex management 3. Optimal use of available resources

3. May require advanced automation

9.3 Hydroponically Grown Crops and Their Tolerance Against Abiotic and Biotic Factor Hydroponic cultivation offers unique advantages for growing crops, including the potential to enhance their tolerance against abiotic and biotic stresses. Abiotic stresses refer to environmental factors such as temperature, water availability, nutrient deficiency or excess, salinity, and light intensity, while pests, diseases, and pathogens cause biotic stresses (R. Chakraborty et al., 2023; Dong et al., 2023; W. He et al., 2023; Kaur, 2023; Mustafa et al., 2023; Okeke et al., 2023; Soussi et al., 2023; Thakur & Thakur, 2023; Verma et al., 2023). Here is a comprehensive discussion on the tolerance of hydroponically grown crops against these stresses: 1. Abiotic Stress Tolerance: 1.1. Temperature Stress: Hydroponic systems allow for precise control over temperature, mitigating the effects of extreme heat or cold. By maintaining optimal temperature ranges, hydroponic crops can exhibit enhanced tolerance to temperature stress compared to field-grown counterparts. 1.2. Water Stress: Hydroponics enables efficient water use, reducing the risk of water stress. Through precise irrigation scheduling and monitoring, hydroponic crops can maintain adequate hydration and withstand periods of drought more effectively (Geetha et al., 2023; Heitkämper et al., 2023; Lokeshkumar et al., 2023; Naveed et al., 2023; P. Sharma et al., 2023; M. Yadav, George, et al., 2023; S. P. S. Yadav, Lahutiya, et al., 2023).

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1.3. Nutrient Imbalances: Hydroponic systems offer the advantage of precise and targeted nutrient delivery. By closely monitoring nutrient solutions and adjusting their composition as needed, hydroponic crops can be supplied with optimal nutrient levels, reducing the occurrence of nutrient imbalances and deficiencies. 1.4. Salinity Stress: Salinity can harm plant growth, but hydroponic systems allow better control of salt concentrations in the nutrient solution. This enables the adjustment of nutrient solutions to minimise salinity stress and provides for cultivating salt-tolerant varieties. 1.5. Light Intensity and Photoperiod: With hydroponics, artificial lighting can be tailored to provide the optimal light intensity and photoperiod for crop growth. This ensures consistent and adequate light exposure, allowing hydroponically grown crops to withstand low-light conditions or variations in natural light availability (L. Chen et al., 2023; P. Das & Paul, 2023a; Etesami & Schaller, 2023; Samuel et al., 2023; Selvaraj & Velvizhi, 2023; P. Sharma et al., 2023; R. S. Sharma et al., 2023; Tonelli et al., 2023). 2. Biotic Stress Tolerance: 2.1. Pest Resistance: Hydroponic systems, typically grown in controlled environments, offer a reduced risk of pest infestation compared to field crops. By implementing strict biosecurity measures, such as physical barriers, integrated pest management strategies, and the use of beneficial insects, hydroponic crops can exhibit enhanced resistance against pests. 2.2. Disease and Pathogen Control: Hydroponics minimises the exposure of crops to soil-borne pathogens, reducing the risk of diseases caused by fungi, bacteria, and nematodes. By utilising sterile growing media and pathogen-free nutrient solutions, hydroponic crops can exhibit increased resistance against soil-borne diseases. 2.3. Enhanced Plant Immunity: The controlled environment of hydroponics provides an opportunity to manipulate environmental factors that can enhance the overall immune response of crops. By optimising temperature, humidity, and nutrient availability, hydroponic systems can promote the activation of defence mechanisms, improving disease resistance. 2.4. Reduced Chemical Dependency: Hydroponic systems can reduce reliance on chemical pesticides and herbicides. Integrated pest management strategies, such as biological controls, biopesticides, and resistant crop varieties, can be effectively implemented in hydroponics. This reduces the chemical load on crops and minimises the risk of developing pesticide resistance. It is important to note that while hydroponics can enhance stress tolerance in crops, it is not a foolproof solution. The specific tolerance of hydroponically grown crops will depend on various factors, including the crop species, genetic traits, environmental conditions, and the management practices implemented within the hydroponic system (Anderson & Prosser, 2023; Anekwe & Isa, 2023; Mathur et al., 2023;

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Fig. 9.1 Phytoremediation through hydroponics

Nižeti´c et al., 2023; Rajput et al., 2023; Rowan, 2023; Saw et al., 2023). Continuous research and innovation in hydroponics will further advance our understanding of crop stress tolerance and contribute to developing more resilient and productive agricultural systems (Fig. 9.1, Table 9.2).

9.4 Hydroponically Grown Salix Species and Their Interaction with Perfluoroalkyl Substance Perfluoroalkyl substances (PFAS) are a group of synthetic chemicals that have gained significant attention due to their persistence in the environment and potential adverse effects on human health and ecosystems. Understanding the accumulation and results of PFAS in plants is crucial for assessing the potential risks associated with their presence in agricultural systems (Al Mamun et al., 2023; Boamah et al., 2023; Bouadila et al., 2023; Q. He et al., 2023; Jha et al., 2023; Pinho & Mateus, 2023; Shi et al., 2023; Vinay Kumar et al., 2023; Wani et al., 2023). This section provides detailed information on the accumulation and effects of PFAS in three hydroponically grown Salix L. species, including willow (Salix spp.), commonly used for phytoremediation purposes (Table 9.3). Accumulation of PFAS in Willow Plants:

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Table 9.2 Crops, abiotic stress tolerance, mechanism and mode of protection Crop

Abiotic Stress tolerance Mechanism

Mode of protection

Tomato

Temperature, water, salinity

Heat shock proteins

Genetic modification

Lettuce

Temperature, light intensity

Antioxidant enzymes

Controlled environment

Cucumber

Water, nutrient imbalances

Osmotic regulation

Nutrient solution management

Pepper

Salinity, temperature

Ion transporters

Genetic modification

Strawberry

Water, temperature

Drought-responsive genes

Irrigation management

Basil

Light intensity

Photomorphogenesis

Artificial lighting

Spinach

Nutrient imbalances

Nutrient uptake efficiency

Nutrient solution management

Carrot

Temperature, water

Carbohydrate metabolism

Irrigation management

Beans

Salinity, water

Ion homeostasis

Nutrient solution management

Kale

Temperature, light intensity

Photosynthetic efficiency

Controlled environment

Broccoli

Water, nutrient imbalances

Antioxidant defense system

Nutrient solution management

Peas

Salinity, temperature

Osmotic adjustment

Nutrient solution management

Radish

Water, temperature

Water-use efficiency

Irrigation management

Melons

Salinity, temperature

Ion transporters

Genetic modification

Cabbage

Water, light intensity

Photosynthesis regulation

Artificial lighting

Chard

Salinity, nutrient imbalances

Osmotic regulation

Nutrient solution management

Beetroot

Water, temperature

Osmotic adjustment

Irrigation management

Garlic

Temperature, nutrient imbalances

Sulfur metabolism

Nutrient solution management

Onions

Water, temperature

Bulb dormancy

Irrigation management

Herbs (e.g., Rosemary, Thyme)

Light intensity

Secondary metabolite synthesis

Controlled environment

Corn

Water, nutrient imbalances

Water-use efficiency

Irrigation management (continued)

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Table 9.2 (continued) Crop

Abiotic Stress tolerance Mechanism

Mode of protection

Potatoes

Salinity, nutrient imbalances

Ion transporters

Genetic modification

Wheat

Water, temperature

Abscisic acid signalling Irrigation management

Rice

Salinity, water, temperature

Osmotic adjustment

Nutrient solution management

Soybeans

Temperature, water

Osmotic adjustment

Irrigation management

Cotton

Salinity, temperature

Ion homeostasis

Nutrient solution management

Sunflowers

Water, temperature

Drought tolerance mechanisms

Irrigation management

Eggplant

Salinity, nutrient imbalances

Ion transporters

Genetic modification

Zucchini

Water, temperature

Osmotic adjustment

Nutrient solution management

Raspberries

Water, temperature

Antioxidant defense system

Irrigation management

Blueberries

Acidic soil, water

Soil pH regulation

Nutrient solution management

Strawberries

Salinity, water

Osmotic adjustment

Nutrient solution management

Watermelon

Temperature, water

Water-use efficiency

Irrigation management

Cantaloupe

Salinity, temperature

Ion transporters

Genetic modification

Peanuts

Water, temperature

Osmotic adjustment

Irrigation management

Almonds

Salinity, water

Osmotic adjustment

Nutrient solution management

Hazelnuts

Temperature, nutrient imbalances

Antioxidant defense system

Nutrient solution management

Pistachios

Salinity, water

Ion homeostasis

Nutrient solution management

Cashews

Temperature, water

Osmotic adjustment

Irrigation management

Macadamia Nuts

Salinity, nutrient imbalances

Ion transporters

Genetic modification

Quinoa

Water, temperature

Osmotic adjustment

Irrigation management

Amaranth

Salinity, nutrient imbalances

Ion homeostasis

Nutrient solution management (continued)

234

P. Kumar and J. Singh

Table 9.2 (continued) Crop

Abiotic Stress tolerance Mechanism

Mode of protection

Millet

Water, temperature

Drought tolerance mechanisms

Irrigation management

Barley

Salinity, temperature

Osmotic adjustment

Nutrient solution management

Oats

Water, nutrient imbalances

Drought tolerance mechanisms

Irrigation management

Flaxseed

Salinity, water

Osmotic adjustment

Nutrient solution management

Sesame

Temperature, nutrient imbalances

Antioxidant defense system

Nutrient solution management

Safflower

Water, temperature

Drought tolerance mechanisms

Irrigation management

Sugar Beets

Salinity, nutrient imbalances

Osmotic adjustment

Nutrient solution management

Sugarcane

Water, temperature

Drought tolerance mechanisms

Irrigation management

Jatropha

Salinity, water

Osmotic adjustment

Nutrient solution management

Miscanthus

Temperature, nutrient imbalances

Drought tolerance mechanisms

Irrigation management

Switchgrass

Salinity, water

Osmotic adjustment

Nutrient solution management

Moringa

Temperature, nutrient imbalances

Antioxidant defense system

Nutrient solution management

Spirulina

Light intensity

Pigment synthesis

Controlled light environment

Chlorella

Salinity, light intensity

Osmotic adjustment

Nutrient solution management

Microgreens

Temperature, light intensity

Rapid growth and harvest

Controlled environment

Mushrooms

Temperature, humidity

Fruiting body formation Controlled environment

Microalgae

Light intensity, temperature

Lipid production

Controlled light environment

Aloe Vera

Salinity, temperature

Osmotic adjustment

Nutrient solution management

Lavender

Temperature, light intensity

Essential oil synthesis

Controlled environment

Chamomile

Water, light intensity

Essential oil synthesis

Controlled environment

Ginseng

Temperature, nutrient imbalances

Phytochemical synthesis Nutrient solution management (continued)

9 Hydroponic: An Eco-friendly Future

235

Table 9.2 (continued) Crop

Abiotic Stress tolerance Mechanism

Mode of protection

Stevia

Salinity, nutrient imbalances

Osmotic adjustment

Nutrient solution management

Cacao

Temperature, water

Drought tolerance mechanisms

Irrigation management

Coffee

Salinity, nutrient imbalances

Ion transporters

Genetic modification

Tea

Water, temperature

Drought tolerance mechanisms

Irrigation management

Hops

Salinity, light intensity

Essential oil synthesis

Controlled environment

1. Uptake from Hydroponic Solution: Hydroponically grown willow plants can take up PFAS compounds from the surrounding nutrient solution. PFAS can be absorbed by the plant roots and transported to the above-ground plant tissues, including leaves, stems, and roots. 2. Translocation within the Plant: Once taken up by the roots, PFAS can be translocated within the plant through the xylem and phloem vessels. This translocation process allows PFAS to distribute throughout various plant parts, leading to their accumulation in different plant tissues (Ariyanta et al., 2023; Carneiro et al., 2023; Khoshru et al., 2023; Kour et al., 2023; Mabrouk et al., 2023; Soozanipour et al., 2023; Tawalbeh et al., 2023). 3. Plant Species and PFAS Accumulation: Different Salix species may exhibit variations in their ability to accumulate PFAS. Some species, such as Salix viminalis and Salix fragilis, have shown higher accumulation potential than others. However, the accumulation patterns may also depend on specific PFAS compounds and their physicochemical properties. Effects of PFAS on Willow Plants: 1. Morphological and Growth Effects: Exposure to PFAS can lead to morphological changes and growth effects in hydroponically grown willow plants. These effects may include stunted growth, reduced shoot and root biomass, decreased leaf area, and altered root architecture. The severity of these effects can vary depending on PFAS concentrations, exposure duration, and specific plant species (Akhtar et al., 2021; Dey et al., 2023; Haque et al., 2023; P. Kumar, Sharma et al., 2021; P. Kumar & Mistri, 2020; Kumari et al., 2022; Selwal et al., 2023). 2. Physiological and Biochemical Effects: PFAS exposure can disrupt willow plants’ physiological and biochemical processes. This can include impaired photosynthesis, reduced chlorophyll content, altered nutrient uptake and distribution, and oxidative stress due to increased reactive oxygen species (ROS) production. These effects can impact overall plant health and vitality. 3. Gene Expression and Molecular Effects: PFAS exposure can induce changes in gene expression and molecular pathways in hydroponically grown willow

Potential impact on plant structural integrity Potential impact on plant nutrient uptake Potential impact on plant growth and health

Facilitates storage of PFAS

Xylem and phloem transport of PFAS

Accumulation of PFAS in leaves

Accumulation of PFAS in stems Facilitates storage of PFAS

Accumulation of PFAS in roots Facilitates storage of PFAS

PFAS sequestration in plant tissues

Modulates PFAS bioavailability Potential impact on rhizosphere ecology

PFAS interactions with root exudates

Adsorption of PFAS to root exudate components

Passive diffusion through root hair surfaces

Enhances PFAS uptake efficiency

Binding of PFAS to cell components

Binding of PFAS to root tissues

Binding of PFAS to stem tissues

PFAS absorption by root hairs

Potential impact on root function

Possible spread of PFAS to different plant parts

Spread of PFAS throughout the plant

Potential leaching of PFAS into the environment

Environmental impact

(continued)

Altered rhizosphere microorganism dynamics

Increased PFAS uptake by the plant roots

Decreased PFAS mobility in plant tissues

Decreased PFAS availability in the surrounding medium

Increased PFAS concentration in plant root

Increased PFAS concentration in plant stem

Binding of PFAS to leaf tissues Increased PFAS concentration in above-ground biomass

Utilisation of plant vascular system

Active transport through xylem vessels

Adsorption of PFAS to cell wall components

Potential spread of PFAS to edible parts

Passive diffusion through root membranes

Mode of action

PFAS binding to plant cell walls Sequesters of PFAS in plant cell Potential impact on plant structures cell function

Reduces PFAS bioavailability in the environment

Enables long-distance transport Potential redistribution to of PFAS vulnerable parts Potential impact on plant productivity

Provides a pathway for PFAS distribution

Root-to-shoot translocation of PFAS

Potential contamination of the plant

Efficient PFAS uptake

Uptake of PFAS from the hydroponic solution

Disadvantages

Advantages

Mechanism

Table 9.3 Mechanisms in the accumulation and effects of perfluoroalkyl substances (PFAS)

236 P. Kumar and J. Singh

Advantages

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Mechanism

Impact of PFAS on Root Morphology

Impact of PFAS on Shoot Morphology

Altered shoot growth due to PFAS exposure

Altered root growth due to PFAS exposure

Reduced shoot biomass due to PFAS exposure

Reduced root biomass due to PFAS exposure

Effects of PFAS on leaf area

Chlorosis and discoloration of leaves

Stomatal closure due to PFAS exposure

Impaired photosynthetic capacity

Reduced chlorophyll content

Alteration of nutrient uptake in the roots

Table 9.3 (continued)

Potential impact on nutrient availability

Potential impact on plant pigmentation

Potential impact on plant energy production

Potential impact on plant water relations

Potential impact on plant pigmentation

Potential impact on plant photosynthesis

Potential impact on nutrient acquisition

Potential impact on plant biomass

Potential impact on nutrient acquisition

Potential impact on crop productivity

Potential impact on shoot structure

Potential impact on root structure

Disadvantages

Altered root architecture

Environmental impact

Decreased chlorophyll levels in plant tissues

Reduced photosynthetic efficiency

Decreased stomatal conductance

Yellowing or browning of leaves

Reduced leaf surface area

Decreased overall plant biomass

Decreased overall plant biomass

Stunted root growth

Stunted shoot growth

(continued)

Disruption of ion transport and Impaired nutrient uptake acquisition by the plant roots

Inhibition of chlorophyll synthesis

Inhibition of photosynthetic processes

Impairment of stomatal opening and closing

Disruption of chlorophyll synthesis and function

Inhibition of leaf expansion and growth

Inhibition of root biomass accumulation

Inhibition of shoot biomass accumulation

Inhibition of root elongation and expansion

Inhibition of shoot elongation and expansion

Alteration of shoot growth and Altered shoot architecture branching

Alteration of root growth and branching

Mode of action

9 Hydroponic: An Eco-friendly Future 237

Potential impact on plant cell function Potential impact on plant cell protection Potential impact on plant growth and development Potential impact on plant growth and development

Increased production of reactive Potential adaptation to PFAS oxygen species exposure

Potential adaptation to PFAS exposure

Disruption of antioxidant defence systems

Altered hormone regulation due Potential adaptation to PFAS to PFAS exposure exposure

Impaired plant hormone signalling

Potential adaptation to PFAS exposure

Potential impact on plant cell function

Potential adaptation to PFAS exposure

Oxidative stress caused by PFAS exposure

Potential impact on nutrient acquisition

Potential impact on plant chemical defences

Potential adaptation to PFAS exposure

Impact of PFAS on root architecture

Potential impact on plant growth and development

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

PFAS accumulation in the apical meristem

Potential impact on plant function

Effects of PFAS on secondary metabolite production

Potential adaptation to PFAS exposure

Distribution of PFAS in different plant parts

Potential impact on nutrient distribution

Disadvantages

Potential impact on plant metabolism

Potential adaptation to PFAS exposure

Altered nutrient transport within the plant

Changes in primary metabolites Potential adaptation to PFAS due to PFAS exposure

Advantages

Mechanism

Table 9.3 (continued)

Interference with hormone receptor signalling

Modulation of hormone biosynthesis and signalling

Inhibition of antioxidant enzyme activities

Enhanced generation of reactive oxygen species

Induction of reactive oxygen species production

Modulation of secondary metabolite pathways

Modulation of direct metabolic pathways

Alteration of root branching and growth

Binding of PFAS to meristem tissues

Systemic transport and accumulation of PFAS

Disruption of phloem and xylem transport

Mode of action

(continued)

Disturbed hormonal responses within the plant

Disturbed hormonal regulation within the plant

Decreased antioxidant capacity in plant tissues

Possible increase in oxidative stress within plant cells

Increased oxidative damage to plant cells

Altered production of phytochemical compounds

Altered metabolic profile of the plant

Altered root system architecture

Possible effects on plant development and growth

Uneven PFAS distribution within the plant

Altered nutrient allocation within the plant

Environmental impact

238 P. Kumar and J. Singh

Advantages

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Mechanism

Impact of PFAS on ethylene production

Modulation of gene expression by PFAS

Activation of stress-responsive genes

Regulation of detoxification pathways

Induction of glutathione-S-transferases

Altered expression of peroxidase enzymes

Activation of defense-related genes

Impact of PFAS on DNA methylation

Epigenetic changes due to PFAS exposure

Altered protein synthesis and metabolism

Table 9.3 (continued)

Potential impact on plant growth and development

Potential impact on gene regulation

Potential impact on gene regulation

Potential impact on plant stress responses

Potential impact on plant oxidative stress

Potential impact on plant detoxification

Potential impact on plant detoxification

Potential impact on plant stress tolerance

Potential impact on plant responses

Potential impact on plant growth and development

Disadvantages

Altered gene expression patterns in the plant

Enhanced plant defence mechanisms against PFAS

Altered peroxidase-mediated ROS detoxification

Enhanced PFAS conjugation with glutathione in the plant

Enhanced detoxification of PFAS in plant tissues

Enhanced stress response mechanisms in the plant

Altered gene regulation and protein synthesis in the plant

Altered ethylene-mediated processes in plant tissues

Environmental impact

Modulation of protein synthesis and turnover

(continued)

Altered protein synthesis and turnover in plant tissues

Alteration of DNA and histone Altered gene expression modifications patterns in the plant

Epigenetic modifications of DNA methylation

Induction of defense-related gene expression

Modulation of peroxidase enzyme activity

Activation of glutathione-S-transferase enzymes

Modulation of detoxification enzyme activity

Induction of stress-related gene expression

Alteration of gene transcription and expression

Modulation of ethylene biosynthesis and signalling

Mode of action

9 Hydroponic: An Eco-friendly Future 239

Potential impact on plant cell function Potential impact on plant cell function Potential impact on plant growth and development

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Impact of PFAS on calcium signaling

Altered cell membrane permeability

Changes in lipid composition in Potential adaptation to PFAS plant tissues exposure

Potential adaptation to PFAS exposure

Disruption of ion homeostasis by PFAS

Impaired cell division and growth

Reduced cell elongation due to PFAS exposure

Impaired water uptake and transport

Changes in cell wall structure and composition

Impaired lignin biosynthesis due to PFAS

Mode of action

Potential impact on plant cell structure

Potential impact on plant cell integrity

Potential impact on plant water relations

Potential impact on plant growth and development

Potential impact on plant cell function

Potential impact on nutrient acquisition

Impaired protein function and cellular processes

Environmental impact

Inhibition of lignin biosynthesis and deposition

Modification of cell wall components

Disruption of water channels and transporters

Inhibition of cell elongation and expansion

Interference with cell division processes

Modulation of lipid metabolism and synthesis

Perturbation of lipid bilayer properties

Modulation of calcium ion flux and signaling

(continued)

Altered lignin content and composition in plant tissues

Altered cell wall properties and structure in plants

Reduced water uptake and transpiration rates

Decreased cell elongation and increase in plant tissues

Reduced cell proliferation and growth in plant tissues

Altered lipid profiles in plant tissues

Altered cell membrane integrity and function

Altered calcium-mediated processes in plant cells

Interference with ion transport Impaired ion balance and and homeostasis nutrient uptake in the plant

Potential impact on protein Disruption of protein folding function and stability

Potential adaptation to PFAS exposure

Changes in protein folding and stability

Disadvantages

Advantages

Mechanism

Table 9.3 (continued)

240 P. Kumar and J. Singh

Potential impact on nutrient cycling Potential impact on nutrient cycling

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Changes in mycorrhizal associations

Impaired symbiotic nitrogen fixation

Impact of PFAS on rhizosphere Potential adaptation to PFAS ecology exposure

Potential adaptation to PFAS exposure

Disruption of plant–microbe interactions

Altered soil microbial communities

Effects of PFAS on nitrogen cycling

Impacted carbon sequestration by plant-soil interactions

Changes in enzyme activities in Potential adaptation to PFAS the rhizosphere exposure

Potential impact on nitrogen availability

Potential adaptation to PFAS exposure

Impact of PFAS on Pectin Metabolism

Potential impact on nutrient cycling

Changes in cell wall composition and properties

Environmental impact

Altered rhizosphere ecology and nutrient availability

Reduced nitrogen fixation and availability in the soil

Impaired mycorrhizal symbiosis and nutrient uptake

Impaired plant–microbe symbiosis and nutrient cycling

Disruption of nitrification and denitrification

Modulation of enzyme production and activity

(continued)

Altered enzyme-mediated nutrient transformations

Impaired carbon sequestration and soil organic matter

Altered nitrogen transformations in the soil

Shifts in microbial community Impaired soil microbial composition functions and nutrient cycling

Modulation of microbial community dynamics

Inhibition of nitrogen-fixing microorganisms

Altered mycorrhizal colonisation and functioning

Altered rhizosphere microorganism dynamics

Modulation of pectin synthesis Altered pectin content and and degradation properties in plant tissues

Inhibition of cellulose and hemicellulose biosynthesis

Mode of action

Potential impact on carbon Altered carbon allocation and storage sequestration

Potential impact on nutrient availability

Potential impact on nutrient acquisition

Potential impact on nutrient acquisition

Potential impact on plant cell adhesion

Potential impact on plant cell structure

Potential adaptation to PFAS exposure

Altered cellulose and hemicellulose synthesis

Disadvantages

Advantages

Mechanism

Table 9.3 (continued)

9 Hydroponic: An Eco-friendly Future 241

Potential impact on plant nutrient availability Potential impact on the experimental setup

Effects of PFAS on soil fertility Potential adaptation to PFAS exposure

Controlled exposure and monitoring of PFAS

Potential for PFAS uptake and removal

Sustainable and cost-effective remediation

Potential detoxification of PFAS Potential impact on plant metabolism

Bioavailability of PFAS in the hydroponic system

Remediation potential of hydroponically grown Salix spp.

Phytoremediation of PFAS-contaminated environments

Interaction between PFAS and phytochelatins

PFAS-induced changes in metal Potential adaptation to PFAS uptake and tolerance exposure

Possible implications for microbial activity

Potential adaptation to PFAS exposure

Impact of PFAS on water quality in the rhizosphere

Potential impact on plant nutrient acquisition

Potential impact on plant health and growth

Potential impact on plant growth and health

Potential impact on air quality

Potential adaptation to PFAS exposure

Volatilisation of PFAS from plant surfaces

Modulation of metal transport and sequestration

Binding of PFAS to phytochelatin peptides

Utilisation of plants to reduce PFAS levels

Utilisation of hydroponic systems for remediation

Assessment of PFAS accumulation and its effects

Impaired nutrient cycling and availability

Altered water chemistry and nutrient availability

Emission of volatile PFAS compounds

Release of PFAS into the surrounding medium

Potential impact on soil contamination

Mode of action

PFAS leaching from plant roots Potential adaptation to PFAS into the soil exposure

Disadvantages Possible effects on nutrient Inhibition of microbial cycling decomposition processes

Advantages

Impact of PFAS on soil organic Potential adaptation to PFAS matter decomposition exposure

Mechanism

Table 9.3 (continued)

(continued)

Altered metal uptake and distribution in plant tissues

Formation of stable complexes with PFAS in plant tissues

Possible transfer of PFAS from the environment to the plant

Possible transfer of PFAS from the medium to the plant

Potential risk of PFAS release into the environment

Altered soil fertility and nutrient availability

Changes in water quality in the rhizosphere environment

Possible atmospheric contamination with PFAS

Increased PFAS contamination in the soil

Decreased organic matter decomposition rates in the soil

Environmental impact

242 P. Kumar and J. Singh

Potential adaptation to PFAS exposure

Potential containment of PFAS within the plant

Potential removal of PFAS from Potential impact on plant water growth and health

Potential for removal and containment of PFAS

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Altered microbial activity due to PFAS exposure

Phytostabilisation of PFAS in plant tissues

Rhizofiltration of PFAS using hydroponically grown Salix spp.

Translocation of PFAS to the aerial plant parts

Impact of PFAS on plant reproductive success

Effects of PFAS on seed germination

Altered root-to-shoot ratio due to PFAS exposure

Changes in leaf surface properties by PFAS

Potential impact on plant interactions

Potential impact on plant growth and development

Potential impact on plant reproduction

Potential effects on plant reproductive organs

Potential impact on plant health and growth

Potential impact on plant growth and health

Potential impact on nutrient cycling

Potential effects on nutrient availability

Potential adaptation to PFAS exposure

Impact of PFAS on redox potential in the rhizosphere

Disadvantages

Advantages

Mechanism

Table 9.3 (continued) Environmental impact

Alteration of leaf surface characteristics

Modulation of resource allocation between roots and shoots

Inhibition or stimulation of seed germination

Disruption of reproductive processes

Systemic transport of PFAS from roots to shoots

Utilisation of hydroponic systems for filtration

Binding and sequestration of PFAS in plant tissues

Inhibition or stimulation of microbial activity

(continued)

Possible effects on plant–microbe and plant–insect interactions

Revised biomass allocation within the plant

Altered seed germination and early plant development

Reduced seed production or fertility

Potential spread of PFAS to edible plant parts

Possible accumulation of PFAS in the plant biomass

Reduced PFAS mobility and bioavailability in the environment

Impaired or enhanced microbial functions in the soil

Modulation of redox reactions Altered redox conditions and processes in the rhizosphere environment

Mode of action

9 Hydroponic: An Eco-friendly Future 243

Potential impact on plant water relations Potential effects on plant water status Potential impact on plant water relations

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Alteration of carbon allocation by PFAS

PFAS effects on nutrient use efficiency

Physiological plasticity in response to PFAS

Impact of PFAS on water-use efficiency

PFAS-induced changes in transpiration rates

PFAS impact on water relations Potential adaptation to PFAS in the plant exposure

Potential adaptation to PFAS exposure

Impact of PFAS on root exudation and composition

Altered hydraulic conductivity due to PFAS

Impact of PFAS on osmotic adjustment

Potential impact on plant water relations

Potential impact on plant water relations

Potential impact on plant stress tolerance

Potential impact on nutrient utilisation

Potential impact on plant growth and development

Potential effects on rhizosphere ecology

Potential impact on plant water relations

Potential adaptation to PFAS exposure

PFAS interactions with stomatal regulation

Disadvantages

Advantages

Mechanism

Table 9.3 (continued)

Modulation of solute accumulation and balance

Modulation of water movement through tissues

Modulation of water potential and osmotic regulation

Modulation of stomatal conductance and transpiration

Modulation of water uptake and transpiration

Modulation of physiological responses to stress

Modulation of nutrient uptake and utilisation

Modulation of carbon allocation to different sinks

Modulation of root exudate quantity and quality

Interference with stomatal opening and closure

Mode of action

(continued)

Altered osmotic regulation and stress tolerance in plants

Impaired water transport and conductivity in plants

Altered plant water balance and osmotic adjustment

Altered plant water loss and transpiration rates

Altered water-use efficiency in plants

Enhanced stress tolerance and resilience in plants

Altered nutrient acquisition and utilisation in plants

Altered carbon partitioning and distribution in plants

Altered rhizosphere microbial dynamics and nutrient cycling

Altered stomatal behaviour and water loss in plants

Environmental impact

244 P. Kumar and J. Singh

Advantages

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Potential adaptation to PFAS exposure

Mechanism

PFAS-mediated changes in nutrient remobilisation

Effects of PFAS on nutrient translocation

Modulation of nutrient transporter genes by PFAS

PFAS effects on nitrogen assimilation

Altered phosphorus acquisition due to PFAS

PFAS interactions with sulfur metabolism

Impact of PFAS on micronutrient uptake

PFAS-induced changes in root exudate composition

Altered root-microbe interactions due to PFAS

Changes in rhizobial associations by PFAS

Table 9.3 (continued)

Potential impact on nitrogen fixation

Potential impact on rhizosphere ecology

Potential impact on rhizosphere ecology

Potential impact on plant nutrition

Potential impact on sulfur utilisation

Potential impact on phosphorus utilisation

Potential impact on nitrogen utilisation

Potential impact on nutrient uptake

Potential impact on nutrient utilisation

Potential impact on nutrient utilisation

Disadvantages

Disruption of rhizobium-legume symbiosis

Disruption of plant–microbe symbiotic associations

Modulation of root exudate quantity and composition

Disruption of micronutrient uptake processes

Modulation of sulfur uptake and assimilation

Modulation of phosphorus uptake and transport

Interference with nitrogen assimilation pathways

Regulation of gene expression for nutrient transporters

Disruption of nutrient translocation processes

Modulation of nutrient remobilisation processes

Mode of action

(continued)

Impaired nitrogen fixation in leguminous plants

Impaired nutrient acquisition and rhizosphere functions

Altered rhizosphere microbial dynamics and nutrient cycling

Impaired micronutrient acquisition and utilisation

Altered sulfur metabolism and utilisation in plants

Impaired phosphorus acquisition and utilisation in plants

Altered nitrogen utilisation and metabolism in plants

Altered nutrient uptake and transport in plants

Impaired nutrient redistribution within the plant

Altered nutrient redistribution and utilisation in plants

Environmental impact

9 Hydroponic: An Eco-friendly Future 245

Potential impact on soil properties Potential impact on soil retention

Potential adaptation to PFAS exposure

Changes in soil pH due to PFAS Potential adaptation to PFAS contamination exposure

Potential adsorption and sequestration of PFAS

Potential disruption of soil aggregation

Impact of PFAS on soil microarthropod communities

PFAS interactions with soil mineral surfaces

PFAS impacts on soil aggregation and structure

Leaching and mobility of PFAS Potential movement of PFAS in soil through the soil

Potential impact on soil ecosystem

Potential adaptation to PFAS exposure

Altered rhizodeposition of carbon and nutrients

Potential impact on groundwater contamination

Potential impact on soil structure

Potential impact on nutrient cycling

Potential impact on carbon Modulation of carbon storage sequestration and decomposition

Potential adaptation to PFAS exposure

PFAS-mediated changes in soil carbon dynamics

Altered soil carbon cycling and storage

Altered soil nutrient transformations and availability

Impaired nutrient uptake in mycorrhizal plants

Environmental impact

Impaired soil structure and stability

Altered PFAS mobility and bioavailability in the soil

(continued)

Release and transport of PFAS Potential contamination of in soil profile groundwater and surface water

Interference with soil aggregation processes

Binding of PFAS to soil mineral surfaces

Acidification or alkalisation of Altered soil pH and soil pH nutrient availability

Modulation of microarthropod Altered soil abundance and diversity microarthropod community structure

Modulation of rhizodeposition Altered nutrient processes availability in the rhizosphere

Modulation of enzyme production and activity

Potential impact on nutrient cycling

Disruption of mycorrhizal associations

Mode of action

Effects of PFAS on soil enzyme Potential adaptation to PFAS activities exposure

Disadvantages Potential impact on nutrient acquisition

Advantages

Impact of PFAS on mycorrhizal Potential adaptation to PFAS colonisation exposure

Mechanism

Table 9.3 (continued)

246 P. Kumar and J. Singh

Potential impact on nitrogen cycling

Potential disruption of signaling Potential impact on Interference with signaling interactions plant–microbe interactions molecules and pathways

PFAS effects on nitrification and denitrification

Impact of PFAS on plant–microbe signaling

Potential effects on soil fertility

Potential impact on carbon storage in the soil

Potential impact on soil ecosystem

Potential impact on nutrient cycling

Altered soil carbon sequestration due to PFAS

Effects of PFAS on soil macrofauna communities

PFAS-induced changes in soil enzyme activities

Potential effects on soil functions

Potential implications for nutrient cycling

Potential effects on rhizosphere ecology

PFAS interactions with plant Potential impact on plant growth-promoting rhizobacteria growth and health

Potential impact on soil nitrogen availability

Potential impact on soil metabolic activity

Potential impact on carbon and nutrient cycling

PFAS-induced changes in soil respiration

Modulation of enzyme production and activity

Modulation of macrofauna abundance and diversity

Disruption of carbon sequestration processes

Modulation of rhizobacteria activity and function

Inhibition or stimulation of microbial processes

Modulation of microbial respiration processes

Modulation of microbial abundance and diversity

Potential impact on nutrient cycling

Potential impact on soil microbial communities

Environmental impact

(continued)

Altered soil nutrient transformations and availability

Altered soil macrofauna functions and nutrient cycling

Reduced soil carbon sequestration and fertility

Altered plant growth promotion and rhizosphere interactions

Altered plant–microbe communication and interactions

Altered nitrogen transformations in the soil

Altered soil carbon and nutrient dynamics

Altered soil microbial functions and nutrient cycling

Persistence and degradation of Long-lasting PFAS in soil contamination and environmental persistence

Mode of action

Effects of PFAS on soil microbial diversity

Disadvantages Potential impact on long-term contamination

Advantages

Fate and persistence of PFAS in Potential accumulation and soil persistence of PFAS

Mechanism

Table 9.3 (continued)

9 Hydroponic: An Eco-friendly Future 247

Potential contamination of groundwater sources

Potential impact on nutrient acquisition

Potential effects on rhizosphere Potential effects on ecology nutrient cycling

Potential adsorption and sequestration of PFAS

Potential impact on nutrient cycling

Potential impact on soil aggregation

Potential adsorption and sequestration of PFAS

Potential impact on nutrient cycling

Potential impact on carbon and nutrient cycling

PFAS contamination in groundwater

PFAS effects on plant–microbe mutualisms

Impact of PFAS on root exudate-mediated interactions

PFAS interactions with soil colloids and minerals

PFAS effects on soil microbial enzyme activities

PFAS-induced changes in soil structure

PFAS interactions with soil organic matter

PFAS effects on microbial biomass and activity

Changes in soil respiration due to PFAS

Potential effects on soil metabolic activity

Potential impact on soil functions

Potential impact on soil retention

Potential effects on soil functions

Potential impact on soil functions

Potential impact on soil retention

Decreased water infiltration and increased runoff

Environmental impact

Modulation of microbial respiration processes

Modulation of microbial biomass and activity

Binding of PFAS to soil organic matter

Disruption of soil structure and stability

Modulation of microbial enzyme production

Binding of PFAS to soil colloids and minerals

Modulation of rhizosphere microorganism dynamics

(continued)

Altered soil carbon and nutrient dynamics

Altered soil microbial functions and nutrient cycling

Altered PFAS mobility and bioavailability in the soil

Impaired soil structure and nutrient availability

Altered soil nutrient transformations and availability

Altered PFAS mobility and bioavailability in the soil

Altered nutrient cycling and microbial functions

Impaired nutrient uptake and plant–microbe interactions

Leaching of PFAS from soil to Groundwater groundwater contamination with PFAS

Reduction in water infiltration rates

Mode of action

Possible effects on Disruption of mutualistic plant–microbe interactions associations

Potential impact on water quality

Possible effects on water availability

Potential impact on soil hydrological processes

Impacted water infiltration due to PFAS

Disadvantages

Advantages

Mechanism

Table 9.3 (continued)

248 P. Kumar and J. Singh

Advantages

Potential impact on nutrient cycling

Potential disruption of soil biotic interactions

Potential impact on soil hydrological processes

Potential impact on nitrogen cycling

Potential impact on carbon storage in the soil

Potential contamination of surface water bodies

Potential impact on soil microbial communities

Potential impact on soil ecosystem

Potential impact on nutrient cycling

Mechanism

PFAS-induced changes in soil nutrient availability

Impact of PFAS on soil biota and ecosystem

PFAS interactions with soil water and moisture

PFAS impacts on soil nitrogen availability

Effects of PFAS on soil carbon dynamics

PFAS contamination in surface water

PFAS interactions with soil microorganisms

PFAS impacts on soil fauna diversity and abundance

PFAS effects on soil enzyme activities

Table 9.3 (continued)

Runoff of PFAS from soil to surface water

Modulation of carbon sequestration and decomposition

Alteration of nitrogen forms and availability

Altered water dynamics and availability

Modulation of soil organism abundance and diversity

Alteration of nutrient availability and forms

Mode of action

Possible effects on soil functions

Potential effects on nutrient cycling

Modulation of enzyme production and activity

Modulation of soil fauna abundance and diversity

Possible effects on nutrient Binding to microbial cell cycling surfaces and components

Potential impact on water quality

Possible effects on soil functions

Possible implications for plant nutrition

Possible effects on water availability

Potential effects on nutrient cycling

Potential implications for plant nutrition

Disadvantages

(continued)

Altered soil nutrient transformations and availability

Altered soil fauna functions and nutrient cycling

Altered soil microbial functions and nutrient cycling

Surface water contamination with PFAS

Altered soil carbon cycling and storage

Altered nitrogen availability and utilisation in plants

Impaired water availability and soil moisture retention

Altered soil biotic interactions and ecosystem functions

Altered nutrient availability and utilisation in plants

Environmental impact

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Advantages

Potential impact on soil respiration and aeration

Potential impact on soil structure

Potential impact on soil microbial communities

Potential impact on carbon sequestration

Potential impact on nitrogen cycling

Potential impact on nutrient cycling

Potential impact on soil properties

Potential adsorption and sequestration of PFAS

Potential impact on nutrient cycling

Potential impact on carbon and nutrient cycling

Mechanism

Impacted soil aeration and gas exchange by PFAS

PFAS interactions with soil aggregates and stability

PFAS effects on microbial diversity and activity

PFAS-induced changes in soil carbon storage

PFAS impacts on soil nitrogen transformations

Effects of PFAS on soil microbial respiration

PFAS-induced changes in soil pH

PFAS interactions with soil organic matter

PFAS effects on soil microbial enzyme activities

Changes in soil respiration due to PFAS

Table 9.3 (continued)

Modulation of carbon accumulation and stability

Modulation of microbial abundance and diversity

Disruption of soil aggregate stability

Restriction of oxygen availability in soil

Mode of action

Potential effects on soil metabolic activity

Potential impact on soil functions

Potential impact on soil retention

Potential effects on nutrient availability

Potential impact on soil functions

Altered soil carbon and nutrient cycling

Altered nitrogen cycling and availability in the soil

Altered soil carbon storage and organic matter dynamics

Altered soil microbial functions and nutrient cycling

Impaired soil structure and nutrient availability

Impaired soil aeration and plant gas exchange

Environmental impact

Modulation of microbial respiration processes

Modulation of microbial enzyme production

Binding of PFAS to soil organic matter

(continued)

Altered soil carbon and nutrient dynamics

Altered soil nutrient transformations and availability

Altered PFAS mobility and bioavailability in the soil

Acidification or alkalisation of Altered soil pH and soil pH nutrient availability

Modulation of microbial metabolic activity

Possible effects on nutrient Modulation of nitrogen availability transformations

Possible effects on soil functions

Possible implications for nutrient cycling

Possible effects on soil functions

Potential effects on plant health and growth

Disadvantages

250 P. Kumar and J. Singh

Advantages

Potential impact on nutrient cycling

Potential disruption of soil biotic interactions

Potential impact on soil hydrological processes

Mechanism

PFAS-induced changes in soil nutrient availability

Impact of PFAS on soil biota and ecosystem

PFAS interactions with soil water and moisture

Table 9.3 (continued)

Potential effects on water availability

Potential effects on nutrient cycling

Potential implications for plant nutrition

Disadvantages

Altered water dynamics and availability

Modulation of soil organism abundance and diversity

Alteration of nutrient availability and forms

Mode of action

Imp

Altered soil biotic interactions and ecosystem functions

Altered nutrient availability and utilisation in plants

Environmental impact

9 Hydroponic: An Eco-friendly Future 251

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Fig. 9.2 Removal of contaminants through rhizofilteration by hydroponics

plants. These changes may affect the expression of genes involved in stress responses, detoxification processes, and defence mechanisms. PFAS exposure can also influence the activity of enzymes involved in detoxification pathways, such as glutathione-S-transferases and peroxidases. 4. Phytoremediation Potential: Despite the potential adverse effects, willow plants, including hydroponically grown species, have been recognised for their phytoremediation capabilities, including the ability to uptake and accumulate PFAS from contaminated environments (Fig. 9.2). This suggests that willows may potentially mitigate PFAS pollution through plant-based remediation approaches (Adelodun et al., 2022; Aley et al., 2022; S. Chakraborty et al., 2021; T. Das et al., 2022; Goud et al., 2022; P. Kumar et al., 2020; P. Kumar, Devi, et al., 2021; V. Kumar et al., 2021; Upadhyay et al., 2023).

9.5 Use of Hydroponics in the Form of Vermifilteration Vermifiltration is a sustainable treatment method for wastewater that utilises earthworms to remove pollutants and enhance the quality of the effluent. It is an ecofriendly and cost-effective alternative to traditional wastewater treatment processes, such as activated sludge systems or chemical treatments. In vermifiltration, the earthworms act as biological agents, contributing to the breakdown and transformation of organic matter, nutrient removal, and overall water quality improvement (Adelodun

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et al., 2022; Aley et al., 2022; T. Das et al., 2022; Goud et al., 2022; Kotia et al., 2022; V. Kumar et al., 2021; Upadhyay et al., 2023). The process of vermifiltration involves passing the wastewater through a bed or filter medium inhabited by earthworms. The earthworms, typically of Eisenia fetida or Lumbricus rubellus, feed on organic matter in the wastewater, breaking it down into simpler forms. As they burrow through the filter bed, they create channels that enhance the aeration and drainage of the system, promoting the growth of beneficial microorganisms. These microorganisms further assist in organic pollutant degradation (Carneiro et al., 2023; Khoshru et al., 2023; Soozanipour et al., 2023; Tawalbeh et al., 2023). Vermifiltration offers several advantages as a sustainable treatment method for wastewater: 1. Nutrient removal: Earthworms efficiently consume organic matter and convert it into nutrient-rich vermicompost, which can be used as a valuable fertiliser for agricultural purposes. This nutrient removal reduces the potential for eutrophication in receiving water bodies. 2. Pathogen reduction: The gut microbiota of earthworms includes bacteria and other microorganisms found to have antibacterial and antiviral properties. These microorganisms reduce pathogens as the wastewater passes through the earthworm-inhabited filter bed. 3. Organic matter degradation: Earthworms play a vital role in breaking down complex organic compounds, such as proteins, fats, and carbohydrates, through their feeding and digestion processes. This natural decomposition process helps remove organic pollutants from the wastewater. 4. Improved water quality: Vermifiltration effectively reduces various pollutants in wastewater, including suspended solids, biochemical oxygen demand (BOD), and chemical oxygen demand (COD). The process enhances the overall quality of the treated effluent, making it suitable for reuse in irrigation or groundwater recharge. 5. Low energy and chemical requirements: Compared to conventional wastewater treatment methods, vermifiltration requires minimal energy input and eliminates the need for chemical additives. It relies on the natural abilities of earthworms and the associated microbial communities to treat wastewater effectively. 6. Scalability and versatility: Vermifiltration systems can be designed to accommodate various scales, from small-scale household applications to large-scale wastewater treatment plants. They can be easily integrated into existing treatment infrastructure or implemented as standalone units, providing system design and implementation flexibility. Despite its numerous advantages, vermifiltration also has certain limitations and considerations: 1. Hydraulic loading rate: The design and operation of vermifiltration systems need to consider the optimal hydraulic loading rate to prevent clogging of the filter bed and ensure sufficient contact time between the wastewater and earthworms for effective treatment.

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2. Earthworm population management: The population of earthworms needs to be carefully monitored and maintained within the vermifiltration system. Proper feeding, moisture control, and protection from extreme temperatures are essential to ensure the sustainability and longevity of the earthworm population. 3. Pre-treatment requirements: Depending on the characteristics of the wastewater, pre-treatment processes, such as screening or settling, may be necessary to remove larger solids or excessive grease and oil before entering the vermifiltration system. 4. Limited contaminant removal: While vermifiltration effectively removes organic matter and certain pollutants, it may have limitations in removing specific contaminants such as heavy metals, persistent organic pollutants, or certain pharmaceutical compounds. Supplementary treatment methods may be required for particular contaminants of concern. Overall, vermifiltration offers a sustainable and environmentally friendly approach to wastewater treatment. It harnesses the natural abilities of earthworms and their associated microbial communities to remove pollutants and improve water quality efficiently (Agarwal, 2023; Ajibade et al., 2023; Boyaci & Reyes-Garcés, 2023; Hechmi et al., 2023; Sarker et al., 2023; Thakur & Thakur, 2023; Wu et al., 2023). By integrating vermifiltration into wastewater management strategies, communities can promote resource recovery, reduce energy consumption, and contribute to a more sustainable and circular approach to water management.

9.6 Robotic System Design and Development for Vertical Hydroponics Farming System Designing and developing an automatic robotic system for vertical hydroponics farming brings together the benefits of advanced automation and vertical farming techniques (Fig. 9.3). This innovative approach optimises space utilisation, increases crop yields, and improves overall efficiency in hydroponic farming systems. The design and development of such a system require careful consideration of various components and functionalities (Adelodun et al., 2022; Aley et al., 2022; R. Chakraborty et al., 2023; Ekka & Kumar, 2023; Etesami & Jeong, 2023; Etesami & Schaller, 2023; Goud et al., 2022; Jha et al., 2023; Khan et al., 2023; Okeke et al., 2023; Shi et al., 2023; Verma et al., 2023; Wani et al., 2023; Zhang et al., 2023). 1. Vertical Farming Structure: The robotic system should be designed to fit within the vertical farming structure, which typically consists of stacked layers or towers. The form should provide ample space for plant growth and accessibility for the robotic components to perform their tasks effectively. 2. Robotic Arms and Manipulators: The robotic system will employ robotic arms equipped with manipulators or grippers to perform tasks such as seeding, transplanting, pruning, and harvesting. These arms should be capable of precise

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Fig. 9.3 Use of robotic in agriculture

3.

4.

5.

6.

7.

and coordinated movements to handle delicate plants and ensure accurate positioning. Vision Systems: Incorporating advanced vision systems, such as cameras or sensors, allows the robotic system to perceive and analyse the growth status of plants. This enables real-time monitoring of plant health, growth patterns, and potential issues, facilitating timely interventions. Automated Plant Nutrition and Watering: The system should integrate automated mechanisms for nutrient solutions and watering plants. It can involve using sensors to measure plant nutrient levels and pH and mechanical pumps or irrigation systems to deliver the precise amount of nutrients and water required by each plant. Environmental Controls: The robotic system should be able to control and monitor environmental factors, including temperature, humidity, and lighting. It ensures optimal plant growth conditions and allows for adjustments based on specific crop requirements. Data Monitoring and Analysis: Implementing a data monitoring and analysis system enables the robotic system to collect and analyse data on plant growth, nutrient levels, environmental conditions, and other relevant parameters. This data can be used to optimise crop management strategies and make data-driven decisions. Software and Control Systems: The robotic system requires sophisticated software and control systems to coordinate and manage the various robotic components. This includes programming the robotic arms, controlling the movement and actions of the robots, and integrating different subsystems for seamless operation.

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8. Safety Features: The system should prioritise safety by incorporating sensors and algorithms to detect and prevent collisions between robotic components, plants, and workers. Emergency stop mechanisms and fail-safe protocols should be implemented to minimise risks and ensure safe operation. 9. Scalability and Modularity: The design should be scalable to accommodate varying sizes of vertical farming systems. Additionally, a modular approach allows for easy customisation and expansion of the robotic system based on specific farm requirements. 10. Integration with IoT and Cloud Platforms: Connecting the robotic system to the Internet of Things (IoT) and cloud platforms enables remote monitoring, data storage, and analysis. It facilitates centralised control, access to real-time information, and potential integration with other smart farming technologies. Developing an automatic robotic system for vertical hydroponics farming requires multidisciplinary expertise, including robotics, automation, hydroponics, and agricultural sciences. Collaborations between engineers, agronomists, and mechanical specialists are crucial to ensuring the successful implementation of this advanced farming system. With continuous advancements in robotics and automation technologies, the future of vertical hydroponics farming holds the potential for increased productivity, sustainability, and food security (L. Chen et al., 2023; Heitkämper et al., 2023; Lokeshkumar et al., 2023; Naveed et al., 2023; P. Sharma et al., 2023; M. Yadav, George, et al., 2023; S. P. S. Yadav, Lahutiya, et al., 2023). The advantages and disadvantages of automatic robotic system design is described in Table 9.4.

9.7 Conclusion In conclusion, hydroponics represents a promising and eco-friendly future for sustainable agriculture. This innovative cultivation method offers numerous advantages that address the challenges faced by traditional soil-based farming systems, making it a viable solution for a world grappling with environmental concerns. Hydroponics eliminates the need for vast expanses of arable land, conserving natural resources such as water and soil. By growing plants in nutrient-rich water solutions, hydroponics reduces water usage by up to 90% compared to traditional agriculture, making it highly efficient and environmentally friendly. Additionally, hydroponic systems minimize or eliminate chemical pesticides and fertilizers, reducing the risk of groundwater contamination and decreasing overall ecological impact. The controlled environment of hydroponics allows for year-round cultivation, eliminating the dependence on seasonal changes and enabling consistent crop production. This aspect is particularly significant in regions with limited arable land or extreme climates, where traditional farming faces substantial challenges. Hydroponics enables vertical farming, allowing crops to be grown in stacked layers or urban settings. This vertical integration optimizes space utilization, enables food production in urban areas, and reduces transportation costs and carbon emissions associated with long-distance food

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Table 9.4 Automatic robotic system design with advantages and disadvantages Robotic system design

Advantages

Disadvantages

Robotic Arm

1. High precision and accuracy

1. Limited range of motion

Autonomous Mobile Robot

2. Flexibility in navigation

2. Limited load-carrying capacity

Pick and Place Robot

3. Efficient handling and sorting

3. Requires accurate object detection

Automated Guided Vehicle

4. Automated material transportation

4. Requires infrastructure modifications

Sorting Robot

5. High-speed sorting

5. May require complex vision systems

Welding Robot

6. Consistent and precise welding

6. Initial setup and programming complexity

Packing Robot

7. Increased packaging efficiency

7. Limited adaptability to diverse products

Painting Robot

8. Uniform and error-free painting

8. Requires proper calibration and setup

Inspection Robot

9. Efficient quality control

9. Complex integration with inspection tools

Cleaning Robot

10. Enhanced cleaning in hard-to-reach areas

10. Limited adaptability to different surfaces

Surveillance Robot

11. Continuous monitoring and surveillance

11. Limited functionality in complex environments

Harvesting Robot

12. Efficient and precise crop harvesting

12. Requires accurate crop detection

Surgical Robot

13. Precise and minimally invasive surgeries

13. High initial cost and maintenance

Drones

14. Quick and remote data collection

14. Limited payload capacity

Robotic Exoskeleton

15. Assists in physical tasks and rehabilitation

15. Limited mobility and agility

Robotic Prosthetics

16. Improved mobility for amputees

16. High cost and complexity

Robotic Companion

17. Provides emotional support and assistance

17. Limited natural interaction capability

Robotic Entertainment

18. Enhances entertainment experiences

18. Limited functionality and real-world application

Agricultural Robot

19. Increases efficiency in farming

19. Requires adaptation to different crops

Construction Robot

20. Speeds up construction processes

20. Limited adaptability to complex tasks

Underwater Robot

21. Explores and researches underwater environments

21. Challenging communication and power supply (continued)

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Table 9.4 (continued) Robotic system design

Advantages

Disadvantages

Robotic Wheelchair

22. Provides mobility for individuals with disabilities

22. Limited terrain adaptability

Robotic Traffic Control

23. Efficient traffic management

23. Challenges in integration with existing infrastructure

Robotic Warehouse System

24. Streamlines order fulfillment

24. High initial cost and maintenance

Robotic Security System

25. Enhances security and surveillance

25. Limited decision-making capabilities

Robotic Food Preparation

26. Fast and accurate food preparation

26. Requires specialised equipment and hygiene considerations

Robotic Material Handling

27. Automates material transportation

27. Limited adaptability to diverse materials

Robotic Parking System

28. Optimises parking space utilisation

28. Initial setup and integration complexity

Robotic Customer Service

29. Improves customer 29. Limited natural language interactions and assistance understanding

Robotic Farming System

30. Increases productivity in agriculture

30. High upfront investment and maintenance

Robotic Delivery System

31. Efficient and timely package delivery

31. Limited adaptability to diverse locations

Robotic Education System

32. Enhances interactive learning experiences

32. Limited personalisation and adaptability

Robotic Hospitality Service

33. Provides personalised guest services

33. Limited ability to handle complex requests

Robotic Waste Management

34. Streamlines waste collection and sorting

34. Challenges in handling diverse waste types

Robotic Firefighting System

35. Increases safety in fire emergencies

35. Limited adaptability to different scenarios

Robotic Exploration System

36. Explores unknown environments

36. Communication and power constraints

Robotic Retail System

37. Enhances shopping experiences

37. Limited adaptability to different retail setups

Robotic Sports Training

38. Improves athletic performance

38. High cost and limited availability

Robotic Language Translation 39. Facilitates communication 39. Accuracy and language across languages nuances challenges Robotic Musical Performer

40. Enhances live performances and entertainment

40. Limited improvisation and creativity (continued)

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Table 9.4 (continued) Robotic system design

Advantages

Disadvantages

Robotic Disaster Response

41. Assists in search and rescue operations

41. Challenges in dynamic and hazardous environments

Robotic Mining System

42. Automates mining processes

42. Requires specialised equipment and safety considerations

Robotic Weather Monitoring

43. Provides real-time weather 43. Challenges in extreme data weather conditions

Robotic Oil Spill Cleanup

44. Efficiently removes oil spills

44. Limited effectiveness in rough waters

Robotic Solar Panel Cleaning

45. Maintains solar panel efficiency

45. Challenges in cleaning large-scale installations

Robotic Waste Sorting

46. Automates waste segregation

46. Limited

supply chains. The potential of hydroponics extends beyond food production. It can cultivate medicinal plants, herbs, and ornamental plants, contributing to diverse industries while reducing the ecological impact of conventional cultivation practices. While hydroponics presents numerous benefits, it is crucial to acknowledge its limitations and ongoing research and development efforts to address them. Challenges such as initial setup costs, energy requirements, and expertise in managing hydroponic systems must be addressed for wider adoption and scalability. Hydroponics offers a sustainable and eco-friendly approach to agriculture, aligning with the growing global focus on environmental conservation and resource efficiency. Hydroponics presents a viable solution to feeding a rapidly growing global population while mitigating the impact on our planet by reducing land usage, conserving water, minimising chemical inputs, and enabling year-round production. With continued advancements and investments in research, technology, and education, hydroponics holds the potential to shape a greener and more sustainable future for agriculture. Acknowledgements We would like to express our sincere gratitude to the Department of Agronomy, School of Agriculture, Lovely Professional University, for their support and assistance throughout the writing. The Department’s commitment to academic excellence and research has been instrumental in completing this endeavour.

References Adelodun, B., Adewumi, J. R., Ajala, O. A., Ajibade, F. O., Ajibade, T. F., Akmal, M. H., Aley, P., Arif, M., Arif, M. S., Awasthi, K. K., Azeem, F., Azevedo, L. C. B., Batool, A., Bertini, S. C. B., Bhattacharya, N., Chattopadhyay, I., Chopra, M., da Silva, A. V., da Silva, M. K., … Yasmeen,

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T. (2022). List of Contributors (A. Kumar, J. Singh, & L. F. R. B. T.-M. U. C. C. Ferreira, Eds., pp. xix–xxiv). Woodhead Publishing. https://doi.org/10.1016/B978-0-323-90571-8.00028-6 Adnan Akram, H., Afzal, A., Intisar, A., Hedar, M., & Hussain, N. (2023). Chapter Eight—Advanced biomaterials for the removal of pesticides from water. In L. F. R. Ferreira, A. Kumar, & M. B. T.-A. in C. P. Bilal Environmental Management and Protection (Eds.), Recent advancements in wastewater management: Implications and biological solutions (Vol. 9, pp. 133–151). Elsevier. https://doi.org/10.1016/bs.apmp.2022.10.006 Agarwal, S. M. (2023). Go-Brown, Go-Green and smart initiatives implemented by the University of Delhi for environmental sustainability towards futuristic smart universities: Observational study. Heliyon, 9(3), e13909. https://doi.org/10.1016/j.heliyon.2023.e13909 Ain, Q. ul, Hussain, H. A., Zhang, Q., Rasheed, A., Imran, A., Hussain, S., Ahmad, N., Bibi, H., & Ali, K. S. (2023). Chapter thirteen—Use of nano-fertilizers to improve the nutrient use efficiencies in plants (T. Aftab & K. R. B. T.-S. P. N. Hakeem, Eds., pp. 299–321). Academic Press. https://doi.org/10.1016/B978-0-443-18675-2.00013-4 Ajibade, F. O., Ajala, O. A., Demissie, H., Lasisi, K. H., Ajibade, T. F., Adelodun, B., Kumar, P., Nwogwu, N. A., Ojo, A. O., Olanrewaju, O. O., & Adewumi, J. R. (2023). Chapter thirteen— Utilization of constructed wetlands for dye removal: A concise review. In L. F. R. Ferreira, A. Kumar, & M. B. T.-A. in C. P. Bilal Environmental Management and Protection (Eds.), Recent advancements in wastewater management: Implications and biological solutions (Vol. 9, pp. 227–246). Elsevier. https://doi.org/10.1016/bs.apmp.2022.11.004 Akhtar, N., Amin-ul Mannan, M., Banik, R. M., Baysal, Ö., Bera, S., Biswas, P., Christina, E., Das, T., Datta, B., Devi, P., Dey, A., Dey, S. R., Dwivedi, P., Han, J., Jayabaskaran, C., Kamalraj, S., Kaur, P., Kumar, P., Kumar, V., … Yashavantha Rao, H. C. (2021). List of contributors (A. Kumar, J. Singh, & J. B. T.-V. and M. of M. Samuel, Eds., pp. xix–xxi). Academic Press. https:// doi.org/10.1016/B978-0-12-824523-1.00028-6 Al Mamun, A., Naznen, F., Jingzu, G., & Yang, Q. (2023). Predicting the intention and adoption of hydroponic farming among Chinese urbanites. Heliyon, 9(3), e14420. https://doi.org/10.1016/ j.heliyon.2023.e14420 Aley, P., Singh, J., & Kumar, P. (2022). Chapter 23—Adapting the changing environment: Microbial way of life (A. Kumar, J. Singh, & L. F. R. B. T.-M. U. C. C. Ferreira, Eds., pp. 507–525). Woodhead Publishing. https://doi.org/10.1016/B978-0-323-90571-8.00023-7 Anderson, J., & Prosser, R. S. (2023). Investigate the potential effects of firefighting water additives on soil invertebrates and terrestrial plants. Chemosphere, 313, 137496. https://doi.org/10.1016/ j.chemosphere.2022.137496 Anekwe, I. M. S., & Isa, Y. M. (2023). Bioremediation of acid mine drainage—Review. Alexandria Engineering Journal, 65, 1047–1075. https://doi.org/10.1016/j.aej.2022.09.053 Ariyanta, H. A., Sari, F. P., Sohail, A., Restu, W. K., Septiyanti, M., Aryana, N., Fatriasari, W., & Kumar, A. (2023). Current roles of lignin for the agroindustry: Applications, challenges, and opportunities. International Journal of Biological Macromolecules, 240, 124523. https://doi. org/10.1016/j.ijbiomac.2023.124523 Bakshe, P., & Jugade, R. (2023). Phytostabilization and rhizofiltration of toxic heavy metals by heavy metal accumulator plants for sustainable management of contaminated industrial sites: A comprehensive review. Journal of Hazardous Materials Advances, 10, 100293. https://doi.org/ 10.1016/j.hazadv.2023.100293 Bhatt, P., Chaudhary, P., Ahmad, S., Bhatt, K., Chandra, D., & Chen, S. (2023). Chapter 2— Recent advances in the application of microbial inoculants in the phytoremediation of xenobiotic compounds. In D. Chandra & P. B. T.-U. P.-M. S. Bhatt (Eds.), Developments in Applied Microbiology and Biotechnology (pp. 37–48). Academic Press. https://doi.org/10.1016/B9780-323-99896-3.00013-8 Boamah, P. O., Onumah, J., Aduguba, W. O., & Santo, K. G. (2023). Application of depolymerized chitosan in crop production: A review. International Journal of Biological Macromolecules, 235, 123858. https://doi.org/10.1016/j.ijbiomac.2023.123858

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Bouadila, S., Baddadi, S., Ben Ali, R., Ayed, R., & Skouri, S. (2023). Deploying low-carbon energy technologies in soilless vertical agricultural greenhouses in Tunisia. Thermal Science and Engineering Progress, 42, 101896. https://doi.org/10.1016/j.tsep.2023.101896 Boyaci, E., & Reyes-Garcés, N. (2023). Chapter 7—Applying green sample preparation techniques to in vivo analysis and metabolomics (E. B. T.-G. A. for C. A. Gionfriddo, Ed., pp. 205–239). Elsevier. https://doi.org/10.1016/B978-0-12-822234-8.00007-X Carneiro, B., Cardoso, P., Figueira, E., Lopes, I., & Venâncio, C. (2023). Forward-looking on new microbial consortia: Combination of rot fungi and rhizobacteria on plant growth-promoting abilities. Applied Soil Ecology, 182, 104689. https://doi.org/10.1016/j.apsoil.2022.104689 Chakraborty, R., Mukhopadhyay, A., Paul, S., Sarkar, S., & Mukhopadhyay, R. (2023). Nanocomposite-based smart fertilizers: A boon to agricultural and environmental sustainability. Science of the Total Environment, 863, 160859. https://doi.org/10.1016/j.scitotenv.2022.160859 Chakraborty, S., Kumar, P., Sanyal, R., Mane, A. B., Arvind Prasanth, D., Patil, M., & Dey, A. (2021). Unravelling the regulatory role of miRNAs in secondary metabolite production in medicinal crops. Plant Gene, 27, 100303. https://doi.org/10.1016/j.plgene.2021.100303 Chen, L., Hu, Z., Chen, W., Xu, Z., Hao, C., Lakshmanan, P., Liu, D., & Chen, X. (2023). Comparative study of the effectiveness of nano-sized iron-containing particles as a foliar top-dressing of peanut in rainy conditions. Agricultural Water Management, 286, 108392. https://doi.org/10. 1016/j.agwat.2023.108392 Chen, N., Zhang, X., Du, Q., Huo, J., Wang, H., Wang, Z., Guo, W., & Ngo, H. H. (2023). Advancements in swine wastewater treatment: Removal mechanisms, influential factors, and optimization strategies. Journal of Water Process Engineering, 54, 103986. https://doi.org/10.1016/j.jwpe. 2023.103986 D’Auria, G., Nitride, C., & Ferranti, P. (2023). 3.12—Microalgae to contrast the climate change: A novel food and feed ingredient with technological applications (P. B. T.-S. F. S.-A. C. A. Ferranti, Ed., pp. 146–163). Elsevier. https://doi.org/10.1016/B978-0-12-823960-5.00024-X Das, P., & Paul, K. (2023a). A review on integrated vermifiltration as a sustainable treatment method for wastewater. Journal of Environmental Management, 328, 116974. https://doi.org/10.1016/j. jenvman.2022.116974 Das, P., & Paul, K. K. (2023b). Hydroponic rhizofiltration of dairy wastewater by Coleus Scutellarioides & Portulaca Oleracea. Journal of Water Process Engineering, 52, 103589. https://doi. org/10.1016/j.jwpe.2023.103589 Das, T., Saha, S. C., Sunita, K., Majumder, M., Ghorai, M., Mane, A. B., Prasanth, D. A., Kumar, P., Pandey, D. K., Al-Tawaha, A. R., Batiha, G. E.-S., Shekhawat, M. S., Ghosh, A., SharifiRad, J., & Dey, A. (2022). Promising botanical-derived monoamine oxidase (MAO) inhibitors: Pharmacological aspects and structure-activity studies. South African Journal of Botany, 146, 127–145. https://doi.org/10.1016/j.sajb.2021.09.019 Dey, S., Nath, S., Alam Ansari, T., Biswas, A., Barman, F., Mukherjee, S., Gopal, G., Bhattacharyya, A., Mukherjee, A., Kundu, R., & Paul, S. (2023). Application of green synthesized bimetallic nZVI-Cu nanoparticle as a sustainable alternative to chemical fertilizers to enhance growth and photosynthetic efficiency of rice seedlings. Plant Physiology and Biochemistry, 201, 107837. https://doi.org/10.1016/j.plaphy.2023.107837 Dong, M., Sun, N., & Liu, C. (2023). Bromide ion enhancing the phytodegradation of emerging phenolic pollutants and its mechanisms mediating wheat resistance to phenolic pollutants stress. Journal of Cleaner Production, 411, 137295. https://doi.org/10.1016/j.jclepro.2023.137295 Ekka, J. P., & Kumar, D. (2023). A review of industrial food processing using solar dryers with heat storage systems. Journal of Stored Products Research, 101, 102090. https://doi.org/10.1016/j. jspr.2023.102090 Etesami, H., & Jeong, B. R. (2023). Chapter 22—How does silicon help alleviate biotic and abiotic stresses in plants? Mechanisms and future prospects (M. Ghorbanpour & M. B. T.-P. S. M. Adnan Shahid, Eds., pp. 359–402). Academic Press. https://doi.org/10.1016/B978-0-323-89871-3.000 31-8

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Etesami, H., Jeong, B. R., & Raheb, A. (2023). Arsenic (As) resistant bacteria with multiple plant growth-promoting traits: Potential to alleviate As toxicity and accumulation in rice. Microbiological Research, 272, 127391. https://doi.org/10.1016/j.micres.2023.127391 Etesami, H., & Schaller, J. (2023). Improving phosphorus availability to rice through silicon management in paddy soils: A review of the role of silicate-solubilizing bacteria. Rhizosphere, 27, 100749. https://doi.org/10.1016/j.rhisph.2023.100749 Ganesapillai, M., Mehta, R., Tiwari, A., Sinha, A., Bakshi, H. S., Chellappa, V., & Drewnowski, J. (2023). Waste to energy: A review of biochar production with emphasis on mathematical modelling and its applications. Heliyon, 9(4), e14873. https://doi.org/10.1016/j.heliyon.2023. e14873 Geetha, N., Sunilkumar, C. R., Bhavya, G., Nandini, B., Abhijith, P., Satapute, P., Shetty, H. S., Govarthanan, M., & Jogaiah, S. (2023). Warhorses in soil bioremediation: Seed biopriming with PGPF secretome to phytostimulate crop health under heavy metal stress. Environmental Research, 216, 114498. https://doi.org/10.1016/j.envres.2022.114498 Goud, E. L., Singh, J., & Kumar, P. (2022). Chapter 19—Climate change and their impact on global food production (A. Kumar, J. Singh, & L. F. R. B. T.-M. U. C. C. Ferreira, Eds., pp. 415–436). Woodhead Publishing. https://doi.org/10.1016/B978-0-323-90571-8.00019-5 Haque, F., Fan, C., & Lee, Y.-Y. (2023). From waste to value: Addressing the relevance of waste recovery to agricultural sector in line with circular economy. Journal of Cleaner Production, 415, 137873. https://doi.org/10.1016/j.jclepro.2023.137873 He, Q., Yan, Z., Qian, S., Xiong, T., Grieger, K. D., Wang, X., Liu, C., & Zhi, Y. (2023). Phytoextraction of per- and polyfluoroalkyl substances (PFAS) by weeds: Effect of PFAS physicochemical properties and plant physiological traits. Journal of Hazardous Materials, 454, 131492. https:// doi.org/10.1016/j.jhazmat.2023.131492 He, W., He, B., Wu, B., Wang, Y., Yan, F., Ding, Y., & Li, G. (2023). Growth of tandem long-mat rice seedlings using controlled release fertilizers: Mechanical transplantation could be more economical and high yielding. Journal of Integrative Agriculture. https://doi.org/10.1016/j.jia. 2023.05.007 Hechmi, S., Zoghlami, R. I., Mokni-Tlili, S., Benzarti, S., Moussa, M., Jellali, S., & Hamdi, H. (2023). Chapter 6—Agricultural applications (M. Jeguirim, B. Khiari, & S. B. T.-P. T. and F. R. Jellali, Eds., pp. 223–243). Academic Press. https://doi.org/10.1016/B978-0-12-823934-6.000 11-3 Heitkämper, K., Reissig, L., Bravin, E., Glück, S., & Mann, S. (2023). Digital technology adoption for plant protection: Assembling the environmental, labour, economic and social pieces of the puzzle. Smart Agricultural Technology, 4, 100148. https://doi.org/10.1016/j.atech.2022.100148 Husaini, A. M., & Sohail, M. (2023). Robotics-assisted, organic agricultural-biotechnology based environment-friendly healthy food option: Beyond the binary of GM versus Organic crops. Journal of Biotechnology, 361, 41–48. https://doi.org/10.1016/j.jbiotec.2022.11.018 Iyyappan, J., Baskar, G., Deepanraj, B., Anand, A. V., Saravanan, R., & Awasthi, M. K. (2023). Promising strategies of circular bioeconomy using heavy metal phytoremediated plants—A critical review. Chemosphere, 313, 137097. https://doi.org/10.1016/j.chemosphere.2022.137097 Jha, A., Pathania, D., Sonu, Damathia, B., Raizada, P., Rustagi, S., Singh, P., Rani, G. M., & Chaudhary, V. (2023). Panorama of biogenic nano-fertilizers: A road to sustainable agriculture. Environmental Research, 116456. https://doi.org/10.1016/j.envres.2023.116456 Kamilya, T., Yadav, M. K., Ayoob, S., Tripathy, S., Bhatnagar, A., & Gupta, A. K. (2023). Emerging impacts of steroids and antibiotics on the environment and their remediation using constructed wetlands: A critical review. Chemical Engineering Journal, 451, 138759. https://doi.org/10. 1016/j.cej.2022.138759 Kaur, J. (2023). Chapter 24—Advances in biomedical waste management technologies (P. Singh, P. Verma, R. Singh, A. Ahamad, & A. C. S. B. T.-W. M. and R. R. in the D. W. Batalhão, Eds., pp. 543–573). Elsevier. https://doi.org/10.1016/B978-0-323-90463-6.00024-5

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Khan, S., Zahoor, M., Sher Khan, R., Ikram, M., & Islam, N. U. (2023). The impact of silver nanoparticles on the growth of plants: The agriculture applications. Heliyon, 9(6), e16928. https://doi.org/10.1016/j.heliyon.2023.e16928 Khoshru, B., Mitra, D., Joshi, K., Adhikari, P., Rion, M. S. I., Fadiji, A. E., Alizadeh, M., Priyadarshini, A., Senapati, A., Sarikhani, M. R., Panneerselvam, P., Mohapatra, P. K. Das, Sushkova, S., Minkina, T., & Keswani, C. (2023). Decrypting the multi-functional biological activators and inducers of defense responses against biotic stresses in plants. Heliyon, 9(3), e13825. https://doi.org/10.1016/j.heliyon.2023.e13825 Kotia, A., Rutu, P., Singh, V., Kumar, A., Dhoke, S., Kumar, P., & Singh, D. K. (2022). Rheological analysis of rice husk-starch suspended in water for sustainable agriculture application. Materials Today: Proceedings, 50, 1962–1966. https://doi.org/10.1016/j.matpr.2021.09.325 Kour, R., Singh, S., Sharma, H. B., Naik, T. S. S. K., Shehata, N., N, P., Ali, W., Kapoor, D., Dhanjal, D. S., Singh, J., Khan, A. H., Khan, N. A., Yousefi, M., & Ramamurthy, P. C. (2023). Persistence and remote sensing of agri-food wastes in the environment: Current state and perspectives. Chemosphere, 317, 137822. https://doi.org/10.1016/j.chemosphere.2023.137822 Krishnamoorthy, N., Pathy, A., Kapoor, A., & Paramasivan, B. (2023). Exploring the evolution, trends and scope of microalgal biochar through scientometrics. Algal Research, 69, 102944. https://doi.org/10.1016/j.algal.2022.102944 Kronrod, A., Tchetchik, A., Grinstein, A., Turgeman, L., & Blass, V. (2023). Promoting new proenvironmental behaviors: The effect of combining encouraging and discouraging messages. Journal of Environmental Psychology, 86, 101945. https://doi.org/10.1016/j.jenvp.2022.101945 Kumar, P., Devi, P., & Dey, S. R. (2021). Chapter 6—Fungal volatile compounds: A source of novel in plant protection agents (A. Kumar, J. Singh, & J. B. T.-V. and M. of M. Samuel, Eds., pp. 83–104). Academic Press. https://doi.org/10.1016/B978-0-12-824523-1.00001-8 Kumar, P., Kumar, T., Singh, S., Tuteja, N., Prasad, R., & Singh, J. (2020). Potassium: A key modulator for cell homeostasis. Journal of Biotechnology, 324, 198–210. Kumar, P., & Mistri, T. K. (2020). Transcription factors in SOX family: Potent regulators for cancer initiation and development in the human body. Seminars in Cancer Biology, 67, 105–113. https:// doi.org/10.1016/j.semcancer.2019.06.016 Kumar, P., Sharma, K., Saini, L., & Dey, S. R. (2021). Chapter 8—Role and behavior of microbial volatile organic compounds in mitigating stress (A. Kumar, J. Singh, & J. B. T.-V. and M. of M. Samuel, Eds., pp. 143–161). Academic Press. https://doi.org/10.1016/B978-0-12-824523-1. 00010-9 Kumar, V., Lakkaboyana, S. K., Sharma, N., Chakraborty, P., Umesh, M., Pasrija, R., Thomas, J., Kalebar, V. U., Jayaraj, I., Awasthi, M. K., Das, T., Oladipo, A. A., Barcelo, D., & Dumee, L. F. (2023). A critical assessment of technical advances in pharmaceutical removal from wastewater—A critical review. Case Studies in Chemical and Environmental Engineering, 8, 100363. https://doi.org/10.1016/j.cscee.2023.100363 Kumar, V., Dwivedi, P., Kumar, P., Singh, B. N., Pandey, D. K., Kumar, V., & Bose, B. (2021). Mitigation of heat stress responses in crops using nitrate primed seeds. South African Journal of Botany, 140, 25–36. https://doi.org/10.1016/j.sajb.2021.03.024 Kumari, P., Singh, J., & Kumar, P. (2022). Chapter 21—Impact of bioenergy for the diminution of an ascending global variability and change in the climate (A. Kumar, J. Singh, & L. F. R. B. T.-M. U. C. C. Ferreira, Eds., pp. 469–487). Woodhead Publishing. https://doi.org/10.1016/ B978-0-323-90571-8.00021-3 Lamnatou, C., & Chemisana, D. (2023). Photovoltaics for buildings and greenhouses: Organic solar cells and other technologies. Sustainable Energy Technologies and Assessments, 56, 103062. https://doi.org/10.1016/j.seta.2023.103062 Levin, M. J., & Paltseva, A. B. T.-R. M. in E. S. and E. S. (2023). Management of park areas, sport fields, and school yards, including golf courses and public right of ways. Elsevier. https://doi. org/10.1016/B978-0-12-822974-3.00274-3

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Lokeshkumar, B. M., Krishnamurthy, S. L., Rathor, S., Warriach, A. S., Vinaykumar, N. M., Dushyanthakumar, B. M., & Sharma, P. C. (2023). Morphophysiological Diversity and Haplotype Analysis of Saltol QTL Region in Diverse Rice Landraces for Salinity Tolerance. Rice Science, 30(4), 306–320. https://doi.org/10.1016/j.rsci.2023.02.001 Mabrouk, M., Han, H., Fan, C., Abdrabo, K. I., Shen, G., Saber, M., Kantoush, S. A., & Sumi, T. (2023). Assessing the effectiveness of nature-based solutions-strengthened urban planning mechanisms in forming flood-resilient cities. Journal of Environmental Management, 344, 118260. https://doi.org/10.1016/j.jenvman.2023.118260 Mahawar, L., Ramasamy, K. P., Suhel, M., Prasad, S. M., Živˇcák, M., Brestic, M., Rastogi, A., & Skalický, M. (2023). Silicon nanoparticles: Comprehensive review on biogenic synthesis and applications in agriculture. Environmental Research, 232, 116292. https://doi.org/10.1016/j.env res.2023.116292 Mathur, P., Chakraborty, R., Aftab, T., & Roy, S. (2023). Engineered nanoparticles in plant growth: Phytotoxicity concerns and the strategies for their attenuation. Plant Physiology and Biochemistry, 199, 107721. https://doi.org/10.1016/j.plaphy.2023.107721 Mattiello, A., Novello, N., Cornu, J.-Y., Babst-Kostecka, A., & Poš´ci´c, F. (2023). Copper accumulation in five weed species commonly found in the understory vegetation of Mediterranean vineyards. Environmental Pollution, 329, 121675. https://doi.org/10.1016/j.envpol.2023.121675 Mukherjee, P., Sharma, U., Rani, A., Mishra, P., & Saravanan, P. (2023). Chapter 12—Various methods for the recovery of metals from the wastewater. In S. K. Shukla, S. Kumar, S. Madhav, & P. K. B. T.-M. in W. Mishra (Eds.), Advances in Environmental Pollution Research (pp. 213–237). Elsevier. https://doi.org/10.1016/B978-0-323-95919-3.00007-0 Mustafa, A., Zulfiqar, U., Mumtaz, M. Z., Radziemska, M., Haider, F. U., Holatko, J., Hammershmiedt, T., Naveed, M., Ali, H., Kintl, A., Saeed, Q., Kucerik, J., & Brtnicky, M. (2023). Nickel (Ni) phytotoxicity and detoxification mechanisms: A review. Chemosphere, 328, 138574. https:// doi.org/10.1016/j.chemosphere.2023.138574 Naveed, S., Oladoye, P. O., & Alli, Y. A. (2023). Toxic heavy metals: A bibliographic review of risk assessment, toxicity, and phytoremediation technology. Sustainable Chemistry for the Environment, 2, 100018. https://doi.org/10.1016/j.scenv.2023.100018 Nižeti´c, S., Arıcı, M., & Hoang, A. T. (2023). Smart and Sustainable Technologies in energy transition. Journal of Cleaner Production, 389, 135944. https://doi.org/10.1016/j.jclepro.2023. 135944 Numan, U., Ma, B., Aslam, M., Bedru, H. D., Jiang, C., & Sadiq, M. (2023). Role of economic complexity and energy sector in moving towards sustainability in the exporting economies. Energy Strategy Reviews, 45, 101038. https://doi.org/10.1016/j.esr.2022.101038 Okeke, E. S., Chukwudozie, K. I., Addey, C. I., Okoro, J. O., Chidike Ezeorba, T. P., Atakpa, E. O., Okoye, C. O., & Nwuche, C. O. (2023). Micro and nanoplastics ravaging our agroecosystem: A review of occurrence, fate, ecological impacts, detection, remediation, and prospects. Heliyon, 9(2), e13296. https://doi.org/10.1016/j.heliyon.2023.e13296 Omidvari, M., Abbaszadeh-Dahaji, P., Hatami, M., & Kariman, K. (2023). Chapter 2—Biocontrol: A novel eco-friendly mitigation strategy to manage plant diseases (M. Ghorbanpour & M. B. T.-P. S. M. Adnan Shahid, Eds., pp. 27–56). Academic Press. https://doi.org/10.1016/B978-0323-89871-3.00020-3 Pinho, H. J. O., & Mateus, D. M. R. (2023). Bioenergy routes for valorizing constructed wetland vegetation: An overview. Ecological Engineering, 187, 106867. https://doi.org/10.1016/j.eco leng.2022.106867 Rajput, V. D., Minkina, T., Ranjan, A., Joshi, A., Kumari, A., Chauhan, P. K., Upadhya, S. K., Sushkova, S., Mandzhieva, S., & Arora, J. (2023). Chapter 9—Unraveling the role of nanoparticles and rhizosphere microbiome for crop production under stress condition. In N. S. Chauhan & S. S. B. T.-T. I. of N. on A. and S. Gill (Eds.), Nanomaterial-Plant Interactions (pp. 161–181). Academic Press. https://doi.org/10.1016/B978-0-323-91703-2.00019-1

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Rowan, N. J. (2023). The role of digital technologies in supporting and improving fishery and aquaculture across the supply chain—Quo Vadis? Aquaculture and Fisheries, 8(4), 365–374. https://doi.org/10.1016/j.aaf.2022.06.003 Sadek, N., Kamal, N., & Shehata, D. (2023). Internet of Things based smart automated indoor hydroponics and aeroponics greenhouse in Egypt. Ain Shams Engineering Journal, 102341. https://doi.org/10.1016/j.asej.2023.102341 Sadvakasova, A. K., Bauenova, M. O., Kossalbayev, B. D., Zayadan, B. K., Huang, Z., Wang, J., Balouch, H., Alharby, H. F., Chang, J.-S., & Allakhverdiev, S. I. (2023). Synthetic algocyanobacterial consortium as an alternative to chemical fertilizers. Environmental Research, 233, 116418. https://doi.org/10.1016/j.envres.2023.116418 Saeed, M., Quraishi, U. M., & Malik, R. N. (2023). Chapter 20—Advancement in mitigating the effects of heavy metal toxicity in wheat (M. K. Khan, A. Pandey, M. Hamurcu, O. P. Gupta, & S. B. T.-A. S. in W. Gezgin, Eds., pp. 313–327). Academic Press. https://doi.org/10.1016/B9780-323-95368-9.00009-6 Sahoo, T. P., & Kumar, M. A. (2023). Remediation of phthalate acid esters from contaminated environment—Insights on the bioremedial approaches and future perspectives. Heliyon, 9(4), e14945. https://doi.org/10.1016/j.heliyon.2023.e14945 Samuel, O., Othman, M. H. D., Kamaludin, R., Dzinun, H., Imtiaz, A., Li, T., El-badawy, T., Khan, A. U., Puteh, M. H., Yuliwati, E., & Kurniawan, T. A. (2023). Photocatalytic degradation of recalcitrant aromatic hydrocarbon compounds in oilfield-produced water: A critical review. Journal of Cleaner Production, 415, 137567. https://doi.org/10.1016/j.jclepro.2023.137567 Sánchez-Zarco, X. G., & Ponce-Ortega, J. M. (2023). Water-energy-food-ecosystem nexus: An optimization approach incorporating life cycle, security and sustainability assessment. Journal of Cleaner Production, 414, 137534. https://doi.org/10.1016/j.jclepro.2023.137534 Sarker, A., Masud, M. A. Al, Deepo, D. M., Das, K., Nandi, R., Ansary, M. W. R., Islam, A. R. M. T., & Islam, T. (2023). Biological and green remediation of heavy metal contaminated water and soils: A state-of-the-art review. Chemosphere, 332, 138861. https://doi.org/10.1016/j.che mosphere.2023.138861 Saw, G., Nagdev, P., Jeer, M., & Murali-Baskaran, R. K. (2023). Silica nanoparticles mediated insect pest management. Pesticide Biochemistry and Physiology, 105524. https://doi.org/10.1016/j.pes tbp.2023.105524 Sehar, S., Adil, M. F., Ma, Z., Karim, M. F., Faizan, M., Zaidi, S. S. A., Siddiqui, M. H., Alamri, S., Zhou, F., & Shamsi, I. H. (2023). Phosphorus and Serendipita indica synergism augments arsenic stress tolerance in rice by regulating secondary metabolism related enzymatic activity and root metabolic patterns. Ecotoxicology and Environmental Safety, 256, 114866. https://doi. org/10.1016/j.ecoenv.2023.114866 Selvaraj, D., & Velvizhi, G. (2023). Self-sustained semi-pilot scale Hybrid Eco-Electrogenic Engineered System for the wastewater treatment and bioenergy generation. Journal of Water Process Engineering, 51, 103474. https://doi.org/10.1016/j.jwpe.2022.103474 Selwal, N., Rahayu, F., Herwati, A., Latifah, E., Supriyono, Suhara, C., Kade Suastika, I. B., Mahayu, W. M., & Wani, A. K. (2023). Enhancing secondary metabolite production in plants: Exploring traditional and modern strategies. Journal of Agriculture and Food Research, 14, 100702. https:// doi.org/10.1016/j.jafr.2023.100702 Sharma, A., Kumar, S., & Singh, R. (2023). Formulation of Zinc oxide/Gum acacia nanocomposite as a novel slow-release fertilizer for enhancing Zn uptake and growth performance of Spinacia oleracea L. Plant Physiology and Biochemistry, 107884. https://doi.org/10.1016/j.plaphy.2023. 107884 Sharma, B., Tiwari, S., Kumawat, K. C., & Cardinale, M. (2023). Nano-biofertilizers as bioemerging strategies for sustainable agriculture development: Potentiality and their limitations. Science of the Total Environment, 860, 160476. https://doi.org/10.1016/j.scitotenv.2022.160476 Sharma, P., Bano, A., Verma, K., Yadav, M., Varjani, S., Singh, S. P., & Tong, Y. W. (2023). Food waste digestate as biofertilizer and their direct applications in agriculture. Bioresource Technology Reports, 23, 101515. https://doi.org/10.1016/j.biteb.2023.101515

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Sharma, R. S., Sharma, A., Panthari, D., Rana, A., & Som, D. (2023). Chapter 13—The future of PGPR-based plant growth promotion and bioremediation technologies. In S. Gangola, S. Kumar, S. Joshi, & P. B. T.-A. M. T. for S. A. and E. Bhatt (Eds.), Developments in Applied Microbiology and Biotechnology (pp. 229–245). Academic Press. https://doi.org/10.1016/B978-0-323-950909.00008-X Shi, Y., Wang, Z., Li, H., Yan, Z., Meng, Z., Liu, C., Chen, J., & Duan, C. (2023). Resistance mechanisms and remediation potential of hexavalent chromium in Pseudomonas sp. strain ANB15. Ecotoxicology and Environmental Safety, 250, 114498. https://doi.org/10.1016/j.ecoenv. 2023.114498 Shrivastava, A., Nayak, C. K., Dilip, R., Samal, S. R., Rout, S., & Ashfaque, S. M. (2023). Automatic robotic system design and development for vertical hydroponic farming using IoT and big data analysis. Materials Today: Proceedings, 80, 3546–3553. https://doi.org/10.1016/j.matpr.2021. 07.294 Singh, D. (2023). Chapter 17—Advances in industrial waste management (P. Singh, P. Verma, R. Singh, A. Ahamad, & A. C. S. B. T.-W. M. and R. R. in the D. W. Batalhão, Eds., pp. 385–416). Elsevier. https://doi.org/10.1016/B978-0-323-90463-6.00027-0 Singh, R., & Arora, N. K. (2023). 4.17—Bacterial formulations and delivery systems against pests in sustainable agro-food production (P. B. T.-S. F. S.-A. C. A. Ferranti, Ed., pp. 299–310). Elsevier. https://doi.org/10.1016/B978-0-12-823960-5.00064-0 Soozanipour, A., Ejeian, F., Boroumand, Y., Rezayat, A., & Moradi, S. (2023). Biotechnological advancements towards water, food and medical healthcare: A review. Chemosphere, 312, 137185. https://doi.org/10.1016/j.chemosphere.2022.137185 Soussi, Y., Bahi, H., Mastouri, H., & El Bouazouli, A. (2023). An embedded concept for sustainable building. Materials Today: Proceedings, 72, 3556–3563. https://doi.org/10.1016/j.matpr.2022. 08.307 Spinozzi, E., Ferrati, M., Cappellacci, L., Caselli, A., Perinelli, D. R., Bonacucina, G., Maggi, F., Strzemski, M., Petrelli, R., Pavela, R., Desneux, N., & Benelli, G. (2023). Carlina acaulis L. (Asteraceae): Biology, phytochemistry, and application as a promising source of effective green insecticides and acaricides. Industrial Crops and Products, 192, 116076. https://doi.org/ 10.1016/j.indcrop.2022.116076 Sudheer, C. K. A., & Chattopadhyay, I. (2023). 8—Endophytic bacteria for drug discovery and bioremediation of heavy metals. In M. Shah & D. B. T.-E. A. W. Deka Why and How (Eds.), Developments in applied microbiology and biotechnology (pp. 159–181). Academic Press. https:// doi.org/10.1016/B978-0-323-91245-7.00015-8 Tan, H. W., Pang, Y. L., Lim, S., & Chong, W. C. (2023). A state-of-the-art of phytoremediation approach for sustainable management of heavy metals recovery. Environmental Technology & Innovation, 30, 103043. https://doi.org/10.1016/j.eti.2023.103043 Tawalbeh, M., Javed, R. M. N., Al-Othman, A., & Almomani, F. (2023). Salinity gradient solar ponds hybrid systems for power generation and water desalination. Energy Conversion and Management, 289, 117180. https://doi.org/10.1016/j.enconman.2023.117180 Thakur, A., & Thakur, P. (2023). 16—Ferrite nanoparticles for agriculture-related activity. In J. Pal Singh, K. Hwa Chae, R. C. Srivastava, & O. F. B. T.-A. of N. F. Caltun (Eds.), Woodhead publishing series in electronic and optical materials (pp. 315–330). Woodhead Publishing. https://doi.org/10.1016/B978-0-443-18874-9.00005-9 Tonelli, F. M. P., Tonelli, F. C. P., & Lemos, M. S. (2023). Chapter 20—Exogenous application of phytohormones to increase plant performance under stress (M. Ozturk, R. A. Bhat, M. Ashraf, F. M. P. Tonelli, B. T. Unal, & G. H. B. T.-P. and S. R. S. M. Dar, Eds., pp. 275–285). Academic Press. https://doi.org/10.1016/B978-0-323-91883-1.00004-8 Upadhyay, S. K., Devi, P., Kumar, V., Pathak, H. K., Kumar, P., Rajput, V. D., & Dwivedi, P. (2023). Efficient removal of total arsenic (As3+/5+) from contaminated water by novel strategies mediated iron and plant extract activated waste flowers of marigold. Chemosphere, 313, 137551. https://doi.org/10.1016/j.chemosphere.2022.137551

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Verma, A. K., Chandrakant, M. H., John, V. C., Peter, R. M., & John, I. E. (2023). Aquaponics as an integrated agri-aquaculture system (IAAS): Emerging trends and future prospects. Technological Forecasting and Social Change, 194, 122709. https://doi.org/10.1016/j.techfore.2023.122709 Wang, F., Zhou, K., Fu, D., & Prasad Singh, R. (2023). Removal of fecal coliforms from sewage treatment plant tailwater through AMF-Canna indica induced bioretention cells. Ecological Indicators, 154, 110526. https://doi.org/10.1016/j.ecolind.2023.110526 Wani, M. Y., Ganie, N. A., Dar, K. A., Dar, S. Q., Khan, A. H., Khan, N. A., Zahmatkesh, S., Manzar, M. S., & Banerjee, R. (2023). Nanotechnology future in food using carbohydrate macromolecules: A state-of-the-art review. International Journal of Biological Macromolecules, 239, 124350. https://doi.org/10.1016/j.ijbiomac.2023.124350 Wu, Y., Liu, Y., Kamyab, H., Rajasimman, M., Rajamohan, N., Ngo, G. H., & Xia, C. (2023). Physico-chemical and biological remediation techniques for the elimination of endocrinedisrupting hazardous chemicals. Environmental Research, 232, 116363. https://doi.org/10.1016/ j.envres.2023.116363 Xu, J., Yang, L., & Zhou, X. (2023). A systematical review of blackwater treatment and resource recovery: Advance in technologies and applications. Resources, Conservation and Recycling, 197, 107066. https://doi.org/10.1016/j.resconrec.2023.107066 Yadav, M., George, N., & Dwibedi, V. (2023). Emergence of toxic trace elements in plant environment: Insights into potential of silica nanoparticles for mitigation of metal toxicity in plants. Environmental Pollution, 333, 122112. https://doi.org/10.1016/j.envpol.2023.122112 Yadav, R. K., Das, S., & Patil, S. A. (2023). Are integrated bioelectrochemical technologies feasible for wastewater management? Trends in Biotechnology, 41(4), 484–496. https://doi.org/10.1016/ j.tibtech.2022.09.001 Yadav, S. P. S., Lahutiya, V., Ghimire, N. P., Yadav, B., & Paudel, P. (2023). Exploring innovation for sustainable agriculture: A systematic case study of permaculture in Nepal. Heliyon, 9(5), e15899. https://doi.org/10.1016/j.heliyon.2023.e15899 Yuan, H., Liu, Q., Fu, J., Wang, Y., Zhang, Y., Sun, Y., Tong, H., & Dhankher, O. P. (2023). Coexposure of sulfur nanoparticles and Cu alleviate Cu stress and toxicity to oilseed rape Brassica napus L. Journal of Environmental Sciences, 124, 319–329. https://doi.org/10.1016/j.jes.2021. 09.040 Zhang, Y.-M., Ye, D.-X., Liu, Y., Zhang, X.-Y., Zhou, Y.-L., Zhang, L., & Yang, X.-L. (2023). Peptides, new tools for plant protection in eco-agriculture. Advanced Agrochem, 2(1), 58–78. https://doi.org/10.1016/j.aac.2023.01.003 Zuluaga, M. Y. A., Cardarelli, M., Rouphael, Y., Cesco, S., Pii, Y., & Colla, G. (2023). Iron nutrition in agriculture: From synthetic chelates to biochelates. Scientia Horticulturae, 312, 111833. https://doi.org/10.1016/j.scienta.2023.111833

Chapter 10

Hydroponic Root Mats for Wastewater Treatment: A Review Vikanksha, Arun Kumar, and Jatinder Singh

Abstract Aqueous vegetation, with its densely intertwined roots and rhizomes, produces buoyant screens in hydroponic root mats (HRMs), which are eco technological wastewater treatment methods. The water zone beneath the root mat and the base of the treatment system is subject to a preferred hydraulic flow. Devices like this may also act as a hydroponic root mats filter if the mat hits the bottom of the reservoir, allowing the hydraulic flow to be channeled immediately to the root zone. Wastewater from households, agricultural effluents, polluted rivers, lakes, stormwater, groundwater, and mine drainage with acid are only few of the water kinds that were treated with hydroponic root mats. This chapter of the book introduces the idea of using hydroponic root mats filters for wastewater treatment, both those that float and those that don’t. We compare the benefits and drawbacks of this technique with that of ponds, floating plant beds, and soil-based man-made wetlands, and offer metrics for performance. Keywords Hydroponic root mat filters · Wastewater treatment · Floating hydroponic root mats · Floating treatment wetlands · Pollutants

10.1 Introduction There has been an enormous revival in the use of ecotechnologies for treating wastewater in several nations during the past few years. These ecotechnologies demand less energy from fossil fuels intake and fewer technological gadgets. Constructed wetlands (CW) are a kind of wastewater treatment system that mimics and improve upon nature’s wetlands’ ability to remove contaminants via physical, chemical, and biological means (Miranda et al., 2021). Different types of Constructed wetlands are categorized based on the height of the water’s level and the preferred flow trajectory inside the bed; the kind of establishment of plants (emergent, rooted, Vikanksha · A. Kumar · J. Singh (B) Department of Horticulture, School of Agriculture, Lovely Professional University, Phagwara, Jalandhar 144411, Punjab, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_10

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or floated macrophyte structures); and the plant development rhythm. One kind of Constructed wetlands uses emergent water plants grown either as FHRMs (floating hydroponic root mats) on the top layer of the water or as hydroponic root mat filters (rooting-proof hydroponic root mat filters) on the liquid’s bottom (Mohanty, 2019). FHRMs are characterized by preferred hydraulic flow in the water zone below the root mat and the unrooted bottom. Water in hydroponic root mat filters is filtered as it traverses the root mat. Hydroponic root mats (HRMs) aren’t wetlands in the traditional sense, but they do blend elements of soil-free pond ecosystems and traditional soil-based cultivation with macrophytes (Weralupitiya et al., 2022). Hydroponic root mats may have cheaper expenses for investment than soil-based constructed wetlands owing to the lack of solid substrate, and since of their unique shape (with greater processing capacity), they can have a greater area of surface than lakes for adherence of pollutant-transforming microorganisms. Consequently, they are employed for the elimination of various pollutants like suspended matter, nutrients, organic contaminants, and metals, which integrate the advantages of ponds and soil-based constructed wetlands (Biswal et al., 2022). The rapid rise in the total amount of literature on such variations over the last thirty years, especially in the context of stormwater runoff treatment, is indicative of the growing curiosity in these systems. The purpose of this study is to offer an in-depth assessment of the ideas behind the design and wastewater treatment effectiveness of floating and non-floating hydroponic root mats for multiple uses and pollutants, in opposition to earlier evaluations on comparable structures that concentrated only on stormwater treatments (Chen, 2021).

10.1.1 Definition of Hydroponic Root Mats Comparable to treated lakes, hydroponic root mats feature a large body of water but unlike ponds, where phytoplankton thrives, hydroponic root mats employ halophytes to filter out pollutants. Because of their intermediary status, the language used to describe such systems varies greatly among academic publications. Human-made wetlands have been given many different names (Calabrese et al., 2021). Some of these names include “floating islands”, “artificial floating islands”, “artificial floating reed beds”, “floating mats”, “floating treatment It suggest a classification of these types of structures as hydroponic root mats (HRMs), which are further separated into floating hydroponic root mats (FHRMs) and non-floating hydroponic root mat filters (HRMFs) due to the root mat being an especially significant and indicative characteristic of these kinds of structures (Barco et al., 2021). Territories by Van Duzer consists of over 1800 sources of information in twenty different languages, and it outlines the creation and reason for flotation, different environmentally friendly elements, techniques for regulating and overseeing them in their final days and the application of floating islands for farming, conservation of wildlife, as well as to enhance water quality. Numerous nations, including Germany and Japan, have used hydroponic root mats to treat eutrophic water bodies, and the practice continues today in countries

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like China. It has been known for quite some time that they are useful for treating rainwater (Shen et al., 2021). Sewage and domestic wastewater and overflows from combined sewers have all benefited from their use. Recently, hydroponic root mats have been used to clean up groundwater polluted by chemical manufacturers. Hydroponic cultivation systems were utilized in the industrial agricultural sector during the 1970s, and the method of nutrient films was subsequently utilized for purifying domestic wastewater (Qu & Fan, 2010). The most recent model closely resembles our conception of hydroponic root mat filters. Human resource management systems have become commonplace globally, particularly in emerging economies. Floating marshes, or naturally floating wetlands (NFWs), may be found in various parts of the globe. They are made up of vascular plants that have emerged to the outermost layer of the water and are supported by a mat of living and dead roots, peat, and/or debris (Vymazal & Kröpfelová, 2008). Various hydrological, ecological, and biogeochemical events are facilitated by them in the wild. The capacity of plants to take in fertilizers and/or destroy harmful substances has made sewage treatment a growing significant industry. The use of naturally floating wetlands for purifying water goes from ancient times. Nonetheless, Russel in 1942 offered the first thorough definition, coining the word “flotant” to characterize the development of these dense root mats that support the growth of emerging macrophytes. Subsequently, van Duzer gathered the bibliography on naturally floating wetlands from almost 1800 publications in 20 languages, describing key aspects of naturally floating wetlands such development plan, basic buoyancy, biological properties, management and administration, animal habitat, and improvement of water quality (Mitsch & Gosselink, 2015). Municipal wastewater, agricultural runoff, industrial effluents, etc. are all examples of manmade input that have benefited from the use of naturally floating wetlands variations for treatment. Plants in Floating treatment wetlands are cultivated hydroponically on anchored floating mats/rafts, with roots reaching to the bottom of the system in the pelagic zones. Root networks and the microbial populations that live in and on them create a water table gradient across the pond bottom and the wastewater treatment area, which helps to trap, filter, and/or decompose pollutants (Rehman et al., 2019). Restoring eutrophic lakes in Germany and rivers in Japan were the initial business applications of Floating treatment wetlands. In the present day, their have been employed to treat hypertrophication, sewage and domestic wastewater, combined sewer overflow, polluted groundwater, stormwater, acid mine drainage, poultry effluent, piggery effluent, enhanced water with boron, contaminated river water, and enriched water with nutrients. More and more, plant-bacteria collaborations are being used in Floating treatment wetlands to clean up polluted water as efficiently as possible.

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10.2 Hydroponic Root Mat Characteristics and Operating Conditions 10.2.1 Root Growth and Usage of Plants Numerous types of emerging water plants can develop well as self-buoyant hydroponic root mats or on rafters with a view of improving water quality, and there are numerous species that have the capacity to create islands that float naturally. The primary need for hydroponic wastewater treatment facilities is the capacity to generate a thick, submerged mat. Whether kinds of plants are employed for hydroponic root mats is also dependent on the water’s makeup and the surrounding environment (Shen et al., 2021). Commonly used plant families in hydroponic CWs include Canna, Carex, Cyperus, Juncus, Phragmites, and Typha. The varieties belonging to these groups are robust and sturdy, and they have plenty of aerenchyma in their roots and rhizomes, so they may float. Plants native to wetlands may be classified into two groups based on their root structures: fibrous root plants and thicker root plants. While fibrous root plants make up the bulk of hydroponic root mats, their lower roots lifetime (40 days on average) and greater evaporation and roots oxygenation elimination rates mean that they aren’t always the best choice (Alexander, 2000). Plants with roots with fibers that are comparatively thin (3 mm) had much greater total nitrogen (TN) removal rates, whereas the total root biomass seems to be associated with the NH4 + –N removal. Fibrous root porosity was estimated to range from 10 to 33%. Variety of plant, plant age, nutritional amounts, freshwater redox circumstances, and the presence of sustaining mats or rafts are only a few of the variables that might affect root growth. It seems that the trophic condition of the water has a significant impact on root growth (Shahid et al., 2018). Carex dipsacea, Carex virgata, Cyperus ustilatus, Eleocharis acutis, and Schoenoplectus tabernaemontani all had root diameters around 24 and 48 cm; Juncus edgariae had the longest roots at 87 cm, according to research on stormwater remediation. Reports of root lengths of up to 2 m in oligotrophic conditions were made. Particularly detrimental to young plants is the buildup of sulfide generated during anaerobic circumstances in response to an elevated nitrogen loading. Root growth may be stunted by waters with an elevated toxins concentration. During a minimum of three years of development on floating rafts, the median length of the roots of Typha angustifolia used to remediate acid mine effluents was only 39 cm (Mohanty, 2019). Calamagrostis epigejos, Phragmites australis, Typha latifolia, and Juncus maritimus had the best root growth out of 18 plant species when treated with processed liquid fraction, whereas the other 14 species did not get established at all. After three years, the root width of P. australis in an floating hydroponic root mats pilot-scale system treating groundwater contaminated with benzene, methyl tert-butyl ether (MTBE), and ammonium was only around 25 cm, indicating adverse impacts of groundwater elements. Plants in floating treatment wetlands serve multiple roles in the purification of wastewater. The pond’s bottom

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is stabilized, turbulent motion of the water is decreased, sedimentation is increased, suspended matter are trapped and filtered, and a conducive environment is created for the growth of microorganisms (Headley et al., 2008). Roots in floating treatment wetlands float freely in the water’s section, allowing toxins to have immediate interaction with the root-associated microbial population, resulting in more efficient biological processes than in built wetlands. Organic debris is broken down by microbes into easily absorbed nutrients. In addition, the roots spread out in all directions, creating an immense surface to absorb nutrients. Plants also secrete organic substances (root exudates) that control denitrification and other biological processes. Nitrate reductions convert nitrates rich in nutrients water at the surface into N2 gas, which is then released into the atmosphere. In addition, plant development and expansion need the uptake of nutrients, which reduces eutrophication. In floating wetlands, it was estimated that nitrogen uptake rates for emerging plant species range from 200 to 2500 kg ha−1 per year. As a result, plants significantly reduce the transparency of water by attracting and capturing tiny, suspended particles in their root systems. In addition to enhancing sorption and sedimentation processes, bioactive chemicals produced by plants also experience physiological shifts within the water body. Some plant types, such as helophytes, add oxygen from the air to their root systems (known as the rhizosphere). Most of photosynthesis’ oxygen production occurs throughout the day. Root-exuded oxygen controls the potential for redox, which in turn controls nitrogen destiny, the oxidation of some phytotoxins, and the aerobic decomposition of organic materials by microbes (Bedair et al., 2022).

10.2.2 Microorganisms and the Root Area Numerous pollutants are exclusively or primarily converted by microbial catalysis; therefore root length is important for scientific reasons, but the specific root surface area accessible for bacterial adhesion and biofilm development is of special relevance (Fig. 10.1). Microbial communities that have greater taxonomic variety in the root zone than in open water are likely more durable and resistant to perturbations and have a broader catabolic spectrum (Griffiths & Philippot, 2013). However, our understanding of the make-up of microbial communities and their related catabolic in situ processes is, at best, sketchy. Surrounding the roots of floating macrophytes in a eutrophic reservoir, ammonia-oxidizing bacteria (AOB) and archaea (AOA) were identified, with the proportion of each of both microbial groups varying with different macrophytes (Wei et al., 2011). The proportion of both types of AOA and AOB is crucial for nitrification, and the proportion of each may be employed as a microbiological biomarker of the health of oligotrophic wetlands. Quantitative data on root areas of numerous plants in response to diverse treatments are now scenarios. A particular root surface area of 15 m2 per square meter of root mats was created by Typha after two growing months, and this figure grew to

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Fig. 10.1 Factors affecting root formation and host

114 m2 per square meter. Similarly, after a single season of development, the surface area of Eichhornia crassipes was measured to be 147 cm2 g1 while that of Pistia stratiotes was documented to be 102 cm2 g1 (Chen et al., 2016). Rhizoplane biofilm widths were much lower than that of biofilms developing on technological carrier materials, whereas high levels of microbial colonization on the surface of the roots of Glyceria maxima appeared to be connected to plant root age and vulnerability.

10.2.3 Water Depth While implementing hydroponic root mats, water depth and systemic penetration are crucial factors. Depending on the treatment goal, input changes, and the wastewater kinds, a different water level should be used in plant root mat systems. The system acts like a filter better suited to the elimination of smaller particles and dispersed contaminants when the water level is very low, preventing the buoyancy of the root mats and ensuring a much more direct contact between plant roots, wastewater, and microorganisms (Rehman et al., 2019). When the water depth of an area of water rises high enough, the root mats of plants float on the surface, and a free flow area forms underneath the mat all the way to the lake’s bottom. In the majority of situations, this method will be utilized in place of a pond to treat rainwater or to remediate water with a high concentration of coarse suspended particles that can be separated out by sedimentation. Water depth is an important factor to be considered while designing the Floating treatment wetlands. This is because of the reason that the Floating treatment wetlands system is devoid of soil/substrate and plants solely depending on their rooting structure in the free water column for nutrients acquisition (Shahid et al., 2018). For root development, at least 0.8–1 m of water depth is required for horizontal and vertical growth. However, increased water depth enhances the treatment performance of Floating treatment wetlands due to increased contact time of pollutants with roots and the microbial biofilm. Additional factors such as type of

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wastewater, treatment purpose, and inflow variations also determine the choice of the water depth. For instance, low water depth is suitable to remove fine particles and suspended solids due to increased contact between roots, wastewater, and microorganisms (Almuktar et al., 2018). On the other hand, high water depth is more suitable to remediate coarse suspended solids by sedimentation due to the formation of a free water zone below the mat down to the bottom of the water column. By adjusting the water level, it is possible to affect the treatment of pollutants in hydroponic root mats. However, it must be considered that some wetland plants are dependent on water-level fluctuations; for example, the biomass of Typha domingensis decreased by about 52% with a 45-cm water-level fluctuation, while P. australis grew best with moderately fluctuating water levels, with an average of 30 cm, functioned in a soil-based pond. Because of their remarkable shape adaptability to water-level modifications, species like Typha spp., P. australis, Scirpus spp., Juncus spp., and Phalaris arundinacea are used in stormwater wetlands, which are characterized by particularly severe water-level changes (Schulz et al., 2020). However, it is important to keep in mind that the presented investigations were conducted with varying water levels, water pieces, and hydraulic retention durations, making it challenging to generalize the effect of water depth on treatment efficiency. Earlier studies were conducted in free water depths below 0.5 m, however the present emphasis of most investigations is on water depths that are much greater than the root length (Buhmann & Papenbrock, 2013).

10.2.4 Buoyancy Natural islands that float are said to have been so solid that they were capable of carrying cattle. This self-buoyancy is influenced by environmental parameters such as temperature, seasons, plant species, age, and nutritional circumstances in natural floating wetlands and floating hydroponic root mats (Moore, 2002). The aerenchyma in many helophytes is crucial for self-buoyancy because it provides air gaps, particularly in the rhizomes. Methane and nitrogen, the end outcomes of methanogenesis and denitrification, respectively, are particularly vital for flotation because they are confined beneath the root mats. Spontaneous floating mata and floating hydroponic root mats have been studied, but quantitative evaluations of the elements that contribute to their self-buoyancy have not been conducted (Hu et al., 2009). Various floating components have been employed to assist spontaneous buoyancy; these materials serve as a platform framework to stabilize and encourage the plants, fostering their development. Nevertheless, peat is not advised due to the fact that it concludes fails to foster plant development, and creates a high own demand on oxygen, all of which might have an adverse effect on the plants (Davidson & Stebbins, 2011).

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10.3 Factors Affecting Yield and Performance 10.3.1 Plants FTWs have been established using a wide variety of plant species, including Agrostis alba, Canna flaccid, Carex lasiocarpa, Chamaedaphen calyculata, Glyceria maxima, Menyanthes trifoliate, Myrica gale, Phragmites australis, Panicum hemitomon, Pontederia cordata, Torilis japonica, Typha latifolia, T (Shahid et al., 2018). Their selection is heavily influenced by factors such as the nature of the pollution, the plant’s capacity to develop deep root systems, the water’s layout, the climate, and the accessibility of these factors in the area. Fibrous-rooted plants are used widely in floating treatment wetlands because of their high evaporation rate, release of oxygen at the root zone of the water, and overall nitrogen elimination capacity (Agarry et al., 2020). This is especially the case when dissolved oxygen levels in the wastewater are low and organic matter concentrations are high. Given their capacity to bring oxygen from the air into the rhizosphere, helophytes like Juncus effuses and P. australis are ideal. Similar success in nutrient removal was seen with A. alba, C. flaccida, P. cordata, and T. japonica. Using a biomass-based strategy rather than an area-based method resulted in higher nutrient removal rates for T. japonica in a separate study (Shahid et al., 2018). The variety of plant, the maturity of the plant, the kind of wastewater, the quantity of nutrients, the redox state of the water, the trophic level of the water, and the kind of mat all have a role in the plant’s development and subsequent biomass increase. The growth of plants, particularly when they are young, may be stunted by an excess of nutrients or by the presence of harmful substances. Root growth was measured to be 10.9 cm for Oenanthe javanica, 9.0 cm for Gypsophila sp., 6.1 cm for Gardenia jasminoides, 4.5 cm for Rohdea japonica, and 4.1 cm for Dracaena sanderiana in previous research. Due to its extensive root system, Oenanthe javanica was selected as an ideal plant for floating treatment wetlands (Gao et al., 2020). Another research found that Juncus edgariae, when used to treat storm wastewater, produces an extensive root network (root length of 87 cm). It has been shown that the biomass of certain wetland plants, such T. domingensis, may drop by as much as 52% when the water level fluctuates. Some species, such Juncus spp., Phalaris arundinacea, and P. australias, show a remarkable capacity to adjust their morphology to varying water conditions. These plants are suitable for use in stormwater ponds with significant water level variations (Técher & Berthier, 2023).

10.3.2 Temperature The effectiveness of wetlands is significantly impacted by sunlight and air temperature. That is since biogeochemical reactions driven by weather conditions are responsible for removing contaminants from wastewater. Seasonal variation has

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been regarded as a major component in the elimination of pollutants, although it is difficult to assess its impact on its own (Evans et al., 2005). Increased microbial multiplication and development of plants in the spring aid in the breakdown of pollutants, while decreased plant growth and digestion of bacteria in the autumn and winter slow the rate at which these substances are removed from the environment. A research analyzed this impact and found that nitrogen loss was greatest throughout the plants’ growth season (June–October) and smallest throughout the colder months of November–March. Several additional research found a favorable correlation among temperature and nutrient loss (Øygarden et al., 2014). It has also been suggested that temperature is not the main determinant of treatment efficacy for all contaminants. Bacteria in biological nitrification–denitrification processes are very sensitive to temperature variations; as a result, nitrogen removal is directly connected to temperature and season (Zhou & Xu, 2019). The greatest TN elimination was seen between 5 and 15 °C. Phosphorus elimination, on the other hand, seemed untouched by temperature changes, even in the range of 5–15 °C throughout the summer and fall. Root zone TOC removal and oxidation are significantly influenced by seasonal changes and plant type. At temperatures more than 35 degrees Celsius, microbial activity increases, resulting in a decrease in the wastewater’s total organic carbon (TOC) level. However, a decrease in plant efficiency from 24 to 4 °C in organic matter removal was observed. Finally, seasonality, temperature, and plant cover may all have an impact on chemical oxygen demand (COD) (Srivastava et al., 2021).

10.3.3 Aeration Increased agitation of wastewater is shown to increase the efficiency with which environmental wastewater treatment methods like wetlands remove nutrients (Sankaran et al., 2010). By creating tiny pockets of oxygen in the substrate, aeration encourages the growth of biofilm. It facilitates the aerobic elimination of ammonium and nitrate both alone and together. Research found that increasing the gas-to-water ratio in floating treatment wetlands from 0 to 5 units increased nitrate/nitrite removal efficiency from 1.7 to 33.8%. Since aeration aids in the effective discharge of oxidized nitrogen over ammonium in the wastewater, aerated wetlands have been reported to have lower CH4 fluxes than non-aerated units (Kataki et al., 2021). Because of its low toxic potential and simple elimination by denitrification in the natural system, oxidized nitrogen is preferable to ammonium. Like how an aeration system may help with COD reduction in the summer, it can do the same in the winter.

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10.3.4 Harvesting of Plants Active plant harvesting can be utilized to expedite additional nutrients removal and to limit internal nutrient cycle in floating treatment wetlands. Previous study reported that about half of the nutrients are accumulated in the shoots of wetland plants. The aerial biomass has been positively linked to the removal of nitrogen and phosphorus. As plant shoots can accumulate more nutrients during vegetative stage, it is important to observe the temporal accumulation in order to optimize the harvesting strategy (Wang et al., 2014). A study on picker weed and soft-stem bulrush recommended harvesting in July/August and in October, respectively, to obtain maximum nutrients removal. Another study also suggested harvesting in July and October for maximum removal of phosphorus and nitrogen. Interestingly, external changes in the environment such as temperature may cause the transfer of nutrients from above to below ground tissues. Therefore, harvesting should be performed when high accumulation of nutrients is observed in harvestable parts of plants in floating treatment wetlands (Xu et al., 2017). In a study conducted in Singapore, above ground tissue of Chrysopogon zizanioides and T. angustifolia were repeatedly harvested within a year and analyzed. Phosphorus and nitrogen contents in the shoots tissues were decreased when shoots acquired maturity. Therefore, it was recommended that shoots should be harvested before the culmination of growing season to achieve the maximum removal of nutrients. On the contrary, it is important to consider that harvesting increases organic carbon removal and hence may result in a potential decrease of carbon available to nitrogen processing bacteria. The early shoot harvesting, on the other hand, may decrease the nutrients removal efficiency of the plants thus reducing their ability to remobilize nutrients from the stem to the storage organs. Moreover, it is noteworthy that harvesting should not affect the plant’s health. Harvesting of the whole plant is more violent and less sustainable practices as compared to harvesting of only above mat tissues for nutrients removal. Harvesting of entire plants may lead to the complete removal of plants from floating structure and growth media, hence, increases operational and maintenance costs making this approach un-economical.

10.4 The Impact of a Plant’s Root Mat on Water Flow The efficacy of HRMs, like that of any kind of therapy device, is determined on their hydraulic features. The plant root mat has to be permeable to water in order to thrive. This may be affected by factors including the positioning of the water in respect to the prevalent wind speed and the surrounding vegetation, in addition to the connection that exists among the lake’s width, breadth, and depth. In hydroponic root mat filters, for instance, shallow basins with a water depth of 10–15 cm and oxic conditions yielded satisfactory hydraulic findings. Vegetation cover also affects the system’s hydraulic properties (Raviv et al., 2019).

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It was no difference in retention duration or effective volume between a planted and unused lake with a saturation of 2% of the entire intervention area. Full plants covering also doubled the system’s hydraulic effectiveness (the proportion of the duration of the greatest inflow concentrations to the theoretical retention time). There will be greater conditions for laminar flow and less re-suspension of deposited particulates in the free water zone among the roots and the bottom of the drainage system. Nonetheless, the root mat’s flow state won’t be a perfect illustration of plug flow because of the typical unevenness of root density and depth distribution. Hydraulic performance decreased even more when the structure was utilized as floating hydroponic root mats with a free water body above the mat, suggesting that the root density heterogeneity was to blame for the hydroponic root mat filters less-than-ideal flow behavior. Position, size, and placement configuration of the floating hydroponic root mats in the water body, along with the entrance layout, were demonstrated to affect hydraulic effectiveness. A single, big floating hydroponic root mats positioned at a length ratio of 0.125 for a side inlet and 0.25 for a centered entrance was determined to be the most efficient configuration for installing an floating hydroponic root mats in a retaining ponds (Rozkošný et al., 2014).

10.5 Vegetation Coverage’s Effect on Water Purification Methods Floating hydroponic root mats generate less favorable circumstances, particularly beneath the root mat. Reduced sunlight reaching into the water section is one of the many essential consequences that plant protection has on any type of water, such as Hydroponic root mat by comparing two floating hydroponic root mats with Carex acutiformis with various levels of plant protection (50 and 100%) with an unplanted regulate for the purification of sewage overflow water, that plant coverage not only inhibits algae growth but also has an advantageous effect on the elimination of nutrients. The rise in plant covering from 9 to 18% increased TN and total phosphorus (TP) elimination in floating hydroponic root mats used for wastewater treatment in North Carolina (Pavlineri et al., 2017). The existence of plants may influence the composition of the microbial population, which in turn can have an effect on the elimination of pollutants. Agriculture crops as well as their connected bacteria have been defined as interacting with one another; plants provide the bacteria with a unique carbon source that induces the bacteria to breakdown pollutants, and the plant associated-bacteria can synthesize a number of substances that aid the plants in overcoming anxiety, supplying the vital nutrients needed for plant growth and development, and enhancing the plant’s defense system towards pathogens (Bhattacharyya et al., 2016). In Hydroponic root mat, nonphotosynthetic microbial populations are thought to predominate over algae due to

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the restricted access to sunlight caused by the floating mat’s existence. When it comes to pollution elimination, the makeup of the heterotrophic microbial community may play a role. This community is substantially influenced by the temporal patterns of oxygen and carbon supply across the plant root mat. For microbial denitrification and sulfate elimination from low-organic-carbonloaded wastewaters, rhizodeposition goods serve as an essential component of organic matter. As of yet, the parameters that determine the size of the carbon flow from the plants or the roots to the water are not completely understood. Nevertheless, studies conducted in hydroponic cultures demonstrated just between 0.5 and 1.5% of fixed carbon is lost, whereas it is believed that 5–10% of the net carbon fixed by photosynthesis is lost for soil-based plants through root exudation. In natural floating wetlands, 23% of the shoots, 25% of the rhizomes, and 30% of the dead root are dumped annually inside the floating mat of plants known as Typha. At a rate of 0.5–1 g m2 day 1, G. maxima’s decomposing biomass was found to give electron donors for denitrification. This means that the hydroponic root mat plant coverage will affect the remediation processes for various contaminants by altering environmental conditions (light, redox, carbon, etc.) (Samal et al., 2019).

10.6 Hydroponic Root Mats, Ponds with Floating Vegetation, and Soil-Based Built Wetlands All Have Their Own Unique Characteristics 10.6.1 Hydroponic Root Mats In comparison to constructed wetlands, hydroponic root mats are more cost-effective, as well as beneficial in a number of other ways, including: allowing plants to directly absorb nutrients from the water by means of their roots; avoiding algal development because of shading; effortlessly dealing with water level fluctuation; offering physical filtration and areas for microorganisms to cling to; offering environmentally friendly value/shelter for fauna; reducing erosion and wave amplitude; and so on (Sharma et al., 2021). If silt builds up in floating hydroponic root mats, the root mat may be moved to provide room for sediment disposal. The drawbacks include a lengthy start-up phase and removal efficiencies that vary with the seasons, as is the case with other forms of constructed wetlands. Some compounds may have more severe phytotoxic impacts since there is no substratum to act as an adsorbent. Lastly, hydroponic root mats are more likely to serve as mosquito breeding grounds than soil-based constructed wetlands. Unique elimination and transformation methods can be developed and implemented depending on the pollutants included in the wastewater, as the root mats in hydroponic root mats may offer a greater amount of specific surface area for connection and the development of bacteria than different kinds of constructed wetlands. Increasing the water’s concentration in hydroponic root mat units may lengthen the

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hydraulic retention duration. Certain contaminants are more effectively removed because of this, although the amount of water beneath the root zone will increase, reducing its interaction with the biofilm developed on the roots’ surfaces (Tanner & Headley, 2011). If the duration of the doubling of important microorganisms is less than the hydraulic retention period, then the water level must be optimized corresponding to the elimination of contaminants and plant species.

10.6.2 Plants That Float on Water in Wetlands Shorter species like P. stratiotes (water lettuce) and Lemna spp. (Duckweed) are often utilized as the foundation for free-floating plant systems (Qin et al., 2016). The use of emerging macrophytes becomes increasingly prevalent in temperate areas, while the cultivation of floating plants is greater prevalent in tropical regions. This is since floating organisms are generally more susceptible to low temperatures (except for Lemna, that may develop beneath ice). While floating hydroponic root mats may be tethered to the coastline, screen frames are required in ponds with free-floating plants because the little plants (Lemna) are quickly displaced from the water body by wind (De Francesco et al., 2022). While both methods allow for the immediate assimilation of nutrients from the water body, Lemna generates only thin mats and so provides fewer targeted locations for biofilm adhesion than hydroponic root mats.

10.6.3 Constructed Wetland Ecosystems In comparison to soil-based constructed wetlands, hydroponic root mats have the distinct benefit of being capable of handling variable hydraulic intakes, as a result of the water’s depths (Fig. 10.2). However, in SSF-CWs, the main water stream makes a lot greater interaction with the surface area of the soil particles and the plant roots in which the biofilms are fixed, in contrast to SF-CWs, the the primary water stream does not come into close proximity with the roots (but does come into interaction with the stems and leaves, that could be strongly colonized with periphyton and bacteria) (Chen et al., 2016). Because plant roots may absorb toxins straight from the water instead of from the pore water of the soil, plant absorption of nutrients and other pollutants may be greater in an hydroponic root mats than in a soil-based constructed wetlands. In contrast to soil-based constructed wetlands, in which sediments must be removed through the substitution of the plants (in SFCW) or the substitute of the vegetation and the soil (in SSF-CW) or through the use of earthworms, Floating hydroponic root mats make the elimination of sediments that have collected simple (Fang et al., 2022). Furthermore, floating hydroponic root mats are not susceptible to blockage and may take wastewater with high suspended matter concentrations, while soil-based constructed wetlands need an influent with fewer than 100 mg L1 filtrable

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Fig. 10.2 Production of constructed wetland systems

particles for them to maintain operating in ranges that are appropriate. When there is no soil present, plant roots may come into touch with harmful substances more easily than they would in soil due to lower diffusion obstacles. The lack of soil means that the hydroponic root mats have a greater electrical conductivity than soil-based SSF-CWs. This has an opportunity to increase therapeutic effectiveness if diffusion speeds are a constraint in therapeutic effectiveness by facilitating the transfer of oxygen and organic molecules among roots, the aqueous phase, and biofilms.

10.7 Hydroponic Root Mat Treatment Effectiveness Assessment Several variables, including weather and water quality, affect removal efficiency. Different forms of wastewater have different arrangements, thus only certain aspects of the wastewater treatment process are covered here. Suspended particles, organic materials, nutrients (nitrogen and phosphorus), and heavy metals have been the focus of the greatest amount of research to yet.

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10.7.1 Suspended Solids Since floating hydroponic root mats have favorable sedimentation circumstances, such as a laminar flow in the open water layer above the root mat and the water body bottom, they are very efficient in removing suspended materials. Furthermore, selfrenewing root mats are readily draggable with no risk of any harm to the structures, and they are capable of absorbing and sloughing particles in a huge sediment holding capacity. In Canada, for instance, Typha floating rafts have been put in a getting pond for a coke stockpile runoff and in open mine shafts filled with wastewater from mines of pH 6 (Central Newfoundland) to remove suspended solids. Mats with a root surface area of 15 square meters per square meter were generated after two growing seasons, and each square meter of mat could hold 0.3 kg of suspended particles. Total suspended material (TSS) elimination rates ranging from 2.7 to 7.1 g m2 day 1 were recorded in hydroponic root mat systems processing household wastewater, with effluent concentrations as low as 6 mg L1 . Livestock wastewater had a better removal rate (7.1 g m2 days 1) than household wastewater, which saw a drop in TSS content from 321 to 13 mg L1 . For wastewater treatment, nonetheless, floating hydroponic root mats achieved the greatest TSS removal rates of 45 g m2 days 1, demonstrating its superiority in terms of suspended particles.

10.7.2 Organic Mass Filtering and settlement eliminate suspended organic matter, which is then decomposed by aerobic and anaerobic bacteria collectively with the absorbed organic carbon in floating hydroponic root mats. There is an upward trend between HRT and the rate at which COD and BOD are removed. As influent concentration rose, so did the rate at which BOD was removed (Chen et al., 2016). Floating hydroponic root mats may be a good option for cleaning up wastewater that contains IOCs. Rates of benzo(a)pyrene elimination in an floating hydroponic root mats and a horizontal subsurface flow (HSSF)-CW were comparable (123–355 and 60–341 mg m2 day 1, respectively). Even though MTBE is more difficult to remove than benzene, both systems achieved similar levels of elimination (between 12 and 50 mg m2 day 1 in the HSSF-CW and 7 and 51 mg m2 day 1 in the floating hydroponic root mats) (Mustapha & Lens, 2018). Benzene and MTBE emission rates of 7.24 and 2.32 mg m2 day 1 were detected, accounting for 3 and 15% of the overall elimination of benzene and MTBE in the floating hydroponic root mats, respectively. The weight balance analysis of the primary benzene removal mechanisms, encompassing a comprehensive wetland covering, revealed that rhizosphere degradation and plant absorption, respectively, achieved 76 and 13% in an HSSF-CW and 83 and 11% in an floating hydroponic root mats.

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10.7.3 Nutrients Rates of TN charge elimination may be as high as 10.64 g m2 day 1, and removal efficiency can reach as high as 92% (Li et al., 2023). Ammonia elimination rates in floating hydroponic root mats have been recorded to range from 0.11 to 4.84 g m2 day 1 with efficiency of removal ranging from 46 to 94%, whereas rates of 0.02 to 12.8 and 0.005 to 2.1 g m2 day 1 were recorded for SF-CWs and HSSFCWs, respectively. As a result, the ammonia removal capabilities of floating hydroponic root mats are more like those of HSSF-CWs than they are to those of SF-CWs. Highest clearance levels of nitrate nitrogen were found to be 3.3 g m2 day 1 for SF-CWs and 0.58 g m2 day 1 for HSSF-CWs (Sun et al., 2019). Root mats in proximity with wastewater have a greater propensity for denitrification because they give more readily accessible carbon. Greenhouse gas generation rates were not affected by the presence of floating vegetation, while denitrification and nitrogen retention probabilities were substantially greater (4.2 and 2.8 g N m2 h1 , respectively). When comparing the design structures of hydroponic root mats and other systems, it’s possible that the removal processes will be different. More oxygen needed for nitrification might be released over time by the roots and rhizomes of hydroponic root mats. It has been shown that nitrification and, in the event of a higher pH, ammonia volatilization, may both benefit from higher water temperatures than they would in soil-based systems throughout the summer. It’s important to keep in mind that evapotranspiration will be much greater in hydroponic root mats than in ponds and soil based CWs throughout the summer and in warmer areas (Jat et al., 2011). These results also reveal that at input concentrations of roughly 24 mg L1 , both the removal efficiency and removal rate of TP attained outstanding levels. Over 250 SF-CWs have been studied, and their recorded phosphorus removal rates range from 20 ug L1 to 100 mg L1 per day. Without soil, nitrogen and phosphorus elimination in hydroponic root mats rely heavily on plant uptake. Root mat entrapment, sedimentation, and osmosis are also hypothesized to have a role in the elimination of phosphorus in floating hydroponic root mats. When the hydroponic root mats are not fully developed, it can be challenging to harvest the above ground biomass, but doing so in June for maximal elimination of phosphate or September to limit phosphorus leakage because of withering is advised (Finch et al., 2014).

10.7.4 Heavy Metals Bonding to soil, sediment, and particulate matter; precipitate as insoluble chemicals; and absorption by bacteria, algae, and plants are the three primary mechanisms for the elimination of heavy metals from CWs (Guo et al., 2020). Sedimentation of suspended particles and plant absorption are the primary mechanisms for heavy metal elimination in floating hydroponic root mats (Fig. 10.1). Heavy metals may be precipitated as sulfides from microbial metabolism of root exudates. The thick plant

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root mat is thought to separate the water surface, limiting the quantity of oxygen transport into the air and maintaining anoxic circumstances within the system, which in turn triggers a decrease of metal ions such as Cr(VI), U(VI), Mn(IV), and Fe(III) (Ao et al., 2022). Due to the floating hydroponic root mats-induced decrease in sediment redox potential, Eh, more decreased water column, neutral pH, and increased supply of organic matter, metal buildup in the sediment is claimed to be larger in the summer. Roots are shown to have a significant effect on the adsorption of metallic substances and the creation of metal sulfides, and in providing an appropriate substrate for the growth of microbes owing to the discharge of organic compounds. In 1992, Canadian researchers employed cattail mats to neutralize acid mine drainage. At input loads of 0.14, 1.17, 3.27, and 2.83 g m2 day 1, the test system eliminated 88% of the Fe, 77% of the Ni, 39% of the S, and 72% of the acidity, with flow rates of about 1 L/min and a retention duration of 131 days. After 2 years of operation, the system achieved a 98 and 95% effectiveness in removing Fe and Ni, correspondingly. Root mats from four different plant species (Carex virgata, Cyperus ustulatus, J. edgariae, S. tabernaemontani) were used in mesocosm tanks in New Zealand to purify artificial rainwater with a dispersed Cu concentration of 16 mg m3 and a dissolved Zn concentration of 485 mg m3 in a batch tests. Elimination rates of Cu ranged from 3.80 to 6.40 mg m2 day 1 and Zn removal rates ranged from 25.80 to 88.80 mg m2 day 1. Parallel mesocosm tests in Belgium found that floating hydroponic root mats are more successful than ponds in removing Cu, Zn, Fe, Ni, Mn, and Pb, with surface-specific elimination levels ranging from 0.4, 2.3, 10.6, 0.3, 0.9, and 0.2 g m2 day 1. In Nantes, France, stormwater runoff from a highway portion is channeled into a large-scale floating treatment wetland for the elimination of metals that dissolve from urban waste. Ni concentration in leaves ranged from 23 to 31 g g1 dry matter, whereas Ni concentration in roots ranged from 113 to 131 g g1 dry matter (a German regulation from 2015 restricts Ni concentrations of biosolids used as soil additions to 80 g g1 dry mass). The leaf Zn concentration was 45–80 g g1 , whereas the root Zn concentration was 168– 210 g g1 . The significance of roots in heavy metal buildup is shown by the root leaf ratio, which ranges from 2.6% for Ni to 5.7% for Zn. While there was no evidence of cadmium buildup in plant tissues, all three metals were found in root biofilm, demonstrating the root mats’ filtration efficiency. In addition, radial oxygen loss of roots induces the oxidation of Fe into oxyhydroxides, which is why iron plaques are often found accumulating on the roots of wetland plants. Injecting molecular hydrogen has just been shown to considerably improve the acceleration of acid mine drainage treatments in laboratory-scale floating hydroponic root mats (Taylor, 2002).

10.7.5 Metals That Might Be Poisonous Particularly hazardous metals are removed from floating treatment wetlands by mechanisms like as adsorption, metal sulfides production, direct absorption by plants, bacteria, and algae, and trapping in the root’s biofilm. Tiny clay particles may

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bind to many potentially hazardous metals, precipitating them to the bottom of the system. Root exudates promote this precipitation by combining with water to generate sulfides and hydroxides of metals (Pat-Espadas et al., 2018). By releasing rhizodeposits including dead tissues, exudates, excretion, and lysates into the water column, roots boost association and floating of metallic substances. Absorption of soluble possibly toxic metals is enhanced by plant decomposition and the dissolution of organic molecules into the water. Plant tissues, especially the roots, are notorious for absorbing the precipitated potentially harmful metals. Juncus and Carex species cultivated under floating treatment wetlands conditions were shown to efficiently absorb Ni and Zn, as described (Guittonny-Philippe et al., 2014). Furthermore, rhizo- and endophytic bacteria have been discovered to play an important role in the cleanup of possibly hazardous metals. These bacteria can sorb metallic ions on the exterior of their cells, which increases the pace at which they are removed after being inoculated. Plants were able to take in more possibly harmful metals because of this. There has been an array of initiatives to remediate water tainted with possibly toxic metals by pairing plants with bacteria that are resistant to certain metals (Delgado-González et al., 2021). Recent research found that Brachia mutica-vegetated floating treatment wetlands were effective at removing hazardous metals (Cd, Fe, Cu, Cr, Mn, Co, Pb) and iron (79–85%) from sewage effluent. Dissolved oxygen in floating treatment wetlands facilitates the production of metal sulfide particles, which are then removed from the water. Microbes on the rhizoplane use up oxygen, which causes Cr, Mn, and Fe to be eliminated from the soil. Additionally, iron and manganese plagues arise on plant roots due to the combined influence of bacteria and oxygen. Metals like Zn and Cu are held together by these plagues (Jomova et al., 2022). Plague was responsible for the loss of 2% of Zn and 10% of Cu. The specific conductivity, pH, and dissolved oxygen levels in the water column are highly correlated with this elimination. 40 percent of Cu and 80 percent of Zn were found in the roots of plants in previous research. Wetlands collecting agriculture and roadway overflow have been studied before, and it was shown that the roots acquire almost twice as much metal as the shoots. However, absorption by plants is not a very effective mechanism for removing potentially dangerous metals from the floating treatment wetlands system (Malyan et al., 2021). Dead plant decomposition and lowered temperatures are blamed for Zn removal because they promote sorption, flocculation, and stable storage of Zn by binding to organic debris and sulfides, causing the metal to sink to the pond floor. The binding of metals with organic carbon and sulfide in wetland sediment influences metal sequestration and regulates solubility (Mangal et al., 2021). Cu is more effectively absorbed by organic stuff in the retention pond’s sediment. Cu-humic acid binding is more resilient than Zn-humic acid binding. The biota in a floating treatment wetlands pond may be somewhat harmed by the sediment’s deposited metals, but the toxicity of metals in the pond is determined on the bioavailability of the metals. Floating treatment wetlands pond was determined to have steady levels of around 80% of Cu and Zn linked to organic matter and sulfide at decreased circumstances (Irvine et al., 2023). Very little re-suspension of sediment was seen in floating treatment wetlands despite the fact that this process may influence the bioavailability of metals.

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10.8 Hydroponic Root Mats, Fundamental Layout for Treatment Methods In order to improve wastewater treatment with HRMs, the interaction among wastewater, root mat biofilms, and roots must be maximized, and the growth of the root mat must be bolstered. Plant variety is clearly a significant aspect for promoting root mat formation. Aquatic vegetation that are native to the area should be used wherever possible; this is particularly true of those with extensive roots that may create a thick veil in the water (Moore et al., 2023). Canna, Carex, Cyperus, Juncus, Phragmites, and Typha are among of the most often utilized plant genera for creating hydroponic root mats. The plants establishing the submerged mat require particular safeguarding from grazing animals at this time. That could be achieved by cultivating plants in a nursery and afterwards transferring root mats that have been successfully established to the target area. When implementing these systems on a broad scale, it is important to think about the potential for the creation of root mats; tiny frames may be required to lock plants and join them collectively. There is a great potential for diversity of species to develop in ecosystems when floating mats are present (Schweitzer et al., 2021). To maximize the relationship between the wastewater and the root mat biofilms, the water level is a further essential factor. When dealing with numerous kinds of target pollutants, the water level must be adjusted accordingly. As an example, when dealing with fluid stuffed with small particles, a high-water level is required, whereas when dealing with water containing fine particles and dissolved recalcitrant pollutants, an arrangement with linked bacteria biomass expansion is liked best. The depth of the root mat’s free water zone is proportional to the depth of the water below it. Common working depths are between 0.4 and 0.8 m. Table 10.1 provides a contrast between non-floating hydroponic root mats and floating hydroponic root mats in terms of their operating characteristics and probable application domains (Mohanty, 2019). These are the overarching characteristics of the layout: To increase the floating hydroponic root mats buoyancy and the plant’s resistance to drifts induced by wind, it may be required to utilize both floating and fixed components. Fencing may be required, particularly for young plants, while they are still developing. Primary treatment is the bare essential that must be done before proceeding. Organic wastewater particulates may collect on the roots, decreasing the transport oxygen that gets to the wastewater, while inorganic materials pose no threat. Since algae treatment is unpredictable, oxidation ponds and lagoons that produce significant quantities of algae ought not to be employed. Phosphorus effluent limitations need either pre- or post-application processing actions due to the difficulty of achieving sustained phosphorous elimination. For pretreatment, it’s important to think about the toxicity of the substances in the influent.

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Table 10.1 Comparison of floating and non-floating hydroponic root mats Properties

Non-floating hydroponic root mat filters

Floating hydroponic root mats

Height dimension

Root mat depth-dependent, Root mat and enclosed water body make up constant, and containing the two bulk reactions chambers in this factor just the one bulk response chamber

Pattern of flow

Static, as in deep horizontal flow subterranean CW (HSSF-CW); – Pollutant concentration shifts in the root mat may be created using the flow paradigm

Depends on the proportion of flow rate between the two reaction chambers and is variable

Factors typical of the treatment

Oxygen and root exudate microgradients in the treatment area; radial oxygen loss (ROL) amplitude as a determining element; a high ROL is conducive to processes of oxidation

Separating of oxygen (root mat) and anaerobic (covered water body) activities on an enormous level; Architectural similarities to non-aerated lagoon; Effluents cycling may improve treatment efficiency; Smell suppression

Reusing water

High evapotranspiration Not required for installations having a large rates may cause significant amount of submerged rainwater water loss, making wastewater treatment the priority above water reuse

Focus on water

Solitary response chamber with relationship of oxygen and root exudates especially advantageous for linked nitrification; – Minimal to medium levels of pollutants in local, regional, and commercial wastewater denitrification

System with high level of covered water body: – High-volume wastewater with low pollutant concentrations; – Sediment removal (i.e. precipitated pollutants) less feasible; – Longer hydraulic retention times; System with low level of covered water body: – High-volume wastewater with moderate to high pollutant concentrations; – Sediment removal (i.e. precipitated pollutants) feasible; – Variable hydraulic retention times and flow regime

We need to improve the hydraulics. The length ratio of 0.125 for side inlet floating hydroponic root mats and 0.25 for centered inlet floating hydroponic root mats was recommended. The suggestion was made to provide more space between the smaller hydroponic root mats. Above a 10% plant-to-water surface area ratio is preferred. Avoiding flow stratification is easier with several inlet layouts than with a single pipe entry. It is best to employ a long diffuser with many outputs of sufficient size to prevent clogging by incoming solids. The diffuser must be placed in an easily accessible area, above the sludge blanket at around mid-depth in the water body. A baffle installed at the entrance may help distribute the flow more evenly. It’s important to prevent

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short circuits in this and other situations. The plug must be adjustable to a vertical orientation. In order to get rid of the sludge that has settled to the bottom, a unique design is required. The majority of systems rely on gravity to function. Recirculation results in higher energy expenditure. High-volume, low-head propeller pumps are often used for recirculation. Because odors may form at greater load interest rates, municipal wastewater should choose organic loading rates below 11 g m2 day 1 BOD5. If the outside temperature is less than 15 degrees Celsius, the payload must be halved. Odors may develop even at modest organic loading rates in wastewater with sulphate concentrations over 50 mg L1 . In order to prevent odors and sulfide, it is recommended that the organic load be reduced to keep the effluent above +100 mV (Eh). The BOD5 content at the inlet should not exceed 200 mg L1 . Rates of recirculation of up to double the entrance flow have been shown to reduce not just odor formation but also mixing and performance issues. Feeding in increments aids in the elimination process. When dealing with municipal wastewater, hydraulic loading rates below 7 cm day 1 are the norm. The planning of the hydraulic loading is often determined by the rates of organic loading. Even though floating hydroponic root mats operating at air temperatures of 20 °C can achieve removal efficiencies of 85% for BOD and TSS and about 40% for TN with effluent qualities of BOD and TS of 30 mg L1 , evaporation and transpiration can reduce these results. Stormwater, which has a lower organic strength, may be subjected to higher hydraulic loading rates (Wu et al., 2023). The proliferation of mosquitoes and other insect vectors may pose a serious threat to operations in warmer climes. The mosquito population may be brought down to manageable levels with the use of the following management indicators: To keep oxygen levels stable, it’s important to reduce the organic load. Spraying water in the evening and harvesting more often. Use of chemical control agents (larvicides) or biological control agents (Bacillus thuringiensis, etc.) that have been authorized by the appropriate regulatory agencies. And plant harvesting helps keep plants healthy in areas where weevils and other parasites are prevalent. This is because it increases plant absorption of the nutrient phosphorus from wastewater. Land, excavation, berm construction, inlet–outlet structure, and lining are all included in the total cost of development.

10.9 Advantages of System The floating treatment wetlands may purge contaminants from water without requiring any new land to be acquired, and they can be put in any pond or preexisting reservoir requiring excavating or earth shifting. The floating treatment wetlands don’t affect the pond’s capacity for holding water since they float on the surface. Floating treatment wetlands use solar energy and natural processes to purify contaminated water in an environmentally friendly and cost-effective manner (Mfarrej et al., 2023).

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There is no special equipment needed to set up a floating treatment wetland. Furthermore, it does not need any artificial chemical input for functioning or upkeep. Consequently, this technique is a feasible and relevant method, particularly in poor countries, for the treatment of sewage and industrial wastewater due to its inexpensive capital and minimal/no operating expenses (Canfora et al., 2019). When juxtaposed with traditional wastewater treatment methods and manmade wetlands, floating treatment wetlands is a more cost-effective solution. Mat’s dangling plant roots offer a huge surface area for biofilms to form and treat wastewater. Polluted surface water, sewage, industrial, and agricultural wastewater have all been successfully treated with floating treatment wetlands. While retention ponds are good at getting rid of large and small particles, they aren’t very good at getting rid of dissolved pollutants. Constructed wetlands are effective in removing soluble pollutants and related fine particles, but they need a wide area and can’t withstand high water levels for long periods of time (Shenoy et al., 2022). Floating treatment wetlands is an effective solution to all of these problems. Floating treatment wetlands may be utilized in flood-prone locations since their buoyancy makes them immune to damage by waves and water. Likewise, floating treatment wetlands may support a variety of fish, birds, and insects in a long-term ecosystem. In addition, floating treatment wetlands may be utilized to efficiently address eutrophication issues that are prevalent in various wetland systems owing to free-floating aquatic plants such water hyacinth, water lettuce, and duckweed. The visual appeal of floating treatment wetlands is high, and it may be made even higher using blooming plants, therefore they have arisen as a potential eco-technology (García-Ávila et al., 2023). Due to their broad expanse of surface covering and lengthy retention period, floating treatment wetlands have the capacity to maintain a stable water temperature in an aquatic pond. The temperature of floating treatment wetlands outflow was found to be somewhat lower than that of natural wetland outflow in research comparing floating treatment wetlands with natural wetlands.

10.10 Drawbacks of System While floating treatment wetlands offers many benefits and can treat many types of wastewaters, it does have certain drawbacks that could restrict its usefulness and effectiveness in the real world. To keep the raft with the plants on it afloat on the water’s surface, a foundation is needed for emergence macrophytes cultivated on a float mat. Anchorage to the rafts as they float is a challenging undertaking. In addition, a certain amount of water depth must be retained in floating treatment wetlands basins to prevent plant roots from penetrating further into the sediments there. The plant’s roots in the basin will force the rafts to sink if the water level increases, as it often does during storms. The demise of macrophytes and significant degradation to the floating superstructure may result from this (Verma et al., 2020). Recreational pursuits like fishing and sailing might be hampered by the huge number of floating rafts. Additionally, plants ought to be picked on a regular basis,

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either as a whole or for their outside tissues. Some manual effort may be required for effective collection and handling of macrophytes. The floating treatment wetlands macrophytes might be harmed by invasive and unwanted plants, reducing their effectiveness (Oliveira et al., 2021). The efficacy of floating treatment wetlands has been shown for treating a wide range of wastewater, including those containing potentially hazardous metals. However, urban/agricultural runoff with an elevated level of oil and herbicides may be harmful to aquatic plants and microorganisms. Pollutant removal effectiveness in floating treatment wetlands will be reduced if macrophytes and biofilm are harmed.

10.11 The Outcomes and Directions for Further Study It is possible to manipulate the hydroponic root mats treatment efficiency by adjusting the water level, the flow velocity, the wastewater kind, and the plant variety. The hydroponic root treatment, the movement of oxygen in the atmosphere, and the variety of plants are all impacted by the water level. Throughout the hot and dry season, when water levels are at their smallest, the water temperature rises, boosting plant production and oxygen diffusion rates while lowering oxygen absorption. The discharge recycling allows for regulation of flow rates, which in turn affects hydraulic and pollutant loading (Gu et al., 2023). Because it affects the depth of the anaerobic region beneath the mat and the hydroponic root treatment, water level is a crucial variable in floating hydroponic root mats. There has not been enough study of floating hydroponic root mats to identify the complicated impacts of water level, flow rate, and loading (Bauer et al., 2021). Here must be a better understanding of how various contaminants affect these floating hydroponic root mats operational parameters. Nonfloating hydroponic root mats likewise need much more study. It seems that the root mat’s specific surface area is a crucial characteristic for characterizing microbial biofilms. The greater the root mat surface area, the greater the microbial attachment. When the mats are placed such that they contact the root proof base, the root mat acts as a filter in the system. Toxic contaminants may have a greater impact on the plants in this situation (Stefanakis, 2019). Therefore, learning more about the root mats’ composition will help with comprehending and optimizing the root mats’ eradication operations. Due to the absence of soil in the hydroponic root mats, water is lost more quickly due to evapotranspiration at higher ambient temperatures. Higher rates of gas diffusion from the water phase into the surrounding environment are achieved because of the lack of soil particles in hydroponic root mats, allowing for more efficient gas exchange. This has an opportunity to improve volatile chemical elimination effectiveness, but additional research into volatilization rates of these substances and their possible conversion at the base of the root mats is needed (Ebrahimbabaie & Pichtel, 2021).

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Here is at present no thorough and proven architectural foundation provided for the hydroponic root mats and, more specifically, to accomplish the required pollutant removal. The correlations between water level, loading rate per unit surface area, hydraulic residence duration, removal rates, and the possible effluent quality are not yet well understood, and further study is required to do so. An innovative approach for cleaning polluted water is the floating treatment wetlands system. Its pollutant-trapping techniques are comparable to those of surface-flow wetlands, but its buoyancy makes it useful for bodies of water where the water level rises and falls, such as lakes, rivers, and ponds (Breil et al., 2022). Most floating treatment wetlands plant varieties are perennials, and these plants vary in characteristics including root biomass, root length, height, and nitrogen intake. Direct absorption by plant roots, providing a habitat and nutrients for the growth of microbial biofilm, and filtering of the dirty water all contribute to plants’ important role in wastewater treatment. It has been hypothesized that macrophytes’ submerged mat accumulates more nutrients than the plants’ aerial parts. Even though floating treatment wetlands function well in both tropical and temperate settings, their effectiveness in the latter might be hindered by the cold (Nag et al., 2023). Scientist expertise, readily available floating mat material, and native plant species all play a role in the planning of floating treatment wetlands. The kind of macrophytes to be used, their nutrient uptake possibility, the form and substance of the floating mat, the quantity of the plants to be used, the method for harvesting and disposing of the plants, the hydraulic loading rate, the retention time, and the suitability of floating treatment wetlands for their use at the pilot scale for various wastewater types all need to be standardized. Organic contaminants are degraded in large part thanks to the microbes. To increase floating treatment wetlands efficacy, research on the kind of microorganisms that are specialized to certain pollutants, their ability to degrade organic pollutants, their ability to promote plant development, and their effectiveness and synergistic relationship with plants are all worthwhile pursuits (Arantza et al., 2022). To eliminate nutrients from wastewater, harvesting vegetation in floating treatment wetlands is essential. For optimal pollutant removal in wastewater treatment, additional investigation is required into whether the full plant is removed or whether just selected plant parts are harvested. There is a need for further research on issues such as optimal harvesting times, the effects of several harvests in one season, and the possibility of harvest-related harm. Most of the plants used in floating treatment wetlands are grasses, which means they might be utilized to feed animals. Additionally, more study is required to determine the long-term impact on animal health and final goods like milk and meat of consuming this collected vegetation.

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Chapter 11

Soilless Cultivation of Plants for Phytoremediation Abhijit Kumar, Gunjan Mukherjee, and Saurabh Gupta

Abstract Thisi review examines the possibility of soilless cultivation systems as a means of overcoming resource scarcity in many places of the world, such as good soil and clean water. The conventional usage of arable land is becoming more difficult, especially in light of climate change. Soilless farming systems not only allow you to save water and grow plants without soil, but they also allow you to grow food in urban locations, such as residential rooftops, near to where people dine. The review compares the uses of soilless farming systems to those of conventional farming.i It examines economic viability, sustainability, and current events in this field. The review discusses three major soilless farming systems: hydroponics, aquaponics, and vertical farming. In terms of how they affect the environment, these systems are distinguished from one another and compared to conventional cultivation techniques to the maximum extent possible. In order to set the framework for future research and practical applications, the review compares published data on the yield of hydroponic cultivation systems with soil-based cultivation methods. This research provides an overview of how profitable each strategy is. The review also compares the sustainability of the most major neutral substrates used in hydroponics to highlight their environmental effects and assist future projects in selecting the appropriate substrate. The review examines the major soilless cultivation systems and discusses the difficulties and improvements to current approaches. It seeks to provide a comprehensive image of soilless farming systems so that further research may be conducted and they can be deployed in the actual world in the future. Keywords Agriculture · Aquaponics · Hydroponics · Phytoremediation

A. Kumar (B) · G. Mukherjee University Institute of Biotechnology, Chandigarh University, Gharuan, Punjab, India e-mail: [email protected] S. Gupta Department of Microbiology, Mata Gujri College, Fatehgarh Sahib, Punjab 140406, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_11

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11.1 Introduction Phytoremediation is an innovative and sustainable approach to address environmental pollution by using plants to remove, degrade, or immobilize contaminants from soil, water, and air. It offers a cost-effective and environmentally friendly alternative to conventional remediation methods. However, the success of phytoremediation is highly dependent on the availability of suitable plant species and optimal growth conditions. Soilless cultivation, which involves growing plants without traditional soil as a growth medium, has gained significant attention in recent years for various agricultural applications (Adams, 2002). This technique offers several advantages, including better control over nutrient uptake, reduced water usage, and the ability to grow plants in urban areas or areas with poor soil quality. The integration of soilless cultivation with phytoremediation holds great promise for enhancing the efficiency and effectiveness of this remediation strategy. In soilless cultivation systems, plants are grown using alternative substrates such as peat moss, coco coir, perlite, or rockwool. Nutrients are supplied through a carefully balanced solution, often referred to as hydroponic nutrient solution (Gruda et al., 2016a; Savvas, 2003).This approach provides plants with optimal nutrient levels and eliminates the risk of contamination from pollutants present in the soil. The use of soilless cultivation for phytoremediation offers several benefits. Firstly, it allows for better control over the growth conditions, enabling the adjustment of nutrient levels to meet the specific requirements of the target contaminants and plants. Secondly, it facilitates the cultivation of plants in controlled environments, such as greenhouses or vertical farming systems, which can be tailored to maximize phytoremediation efficiency. Additionally, soilless cultivation can be easily implemented in urban areas or industrial sites where soil contamination is prevalent but the availability of arable land is limited. (Gruda et al., 2018).

11.2 Soilless Culture Systems and Equipment Soilless culture systems have emerged as an innovative and sustainable approach to plant cultivation, offering numerous benefits such as water conservation, precise nutrient control, and the ability to grow plants in urban environments. This abstract provides an overview of soilless culture systems and the equipment involved in their implementation. Hydroponics, aeroponics, and aquaponics are the primary soilless culture systems discussed in this abstract. Hydroponics involves growing plants in a nutrient-rich water solution, while aeroponics suspends plant roots in a misted environment. Aquaponics combines hydroponics with aquaculture, utilizing fish waste to provide nutrients to the plants. These systems offer unique advantages in terms of resource efficiency, space utilization, and plant health.

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11.2.1 Systems of Cultivation on Growing Media The most extensively utilised approach for economically cultivating vegetables and ornamentals without the use of traditional soil is cultivation on pathogen-free genetically modified (GM) organisms. This strategy allows for the finest possible balance of water and air availability to the plant roots. When it comes to cultivation techniques, substrate-based hydroponic systems outnumber water culture systems. The fundamental reason for this is because substrates can hold water, providing a safety net in the event that the technology fails. In addition, because they are porous, substrates allow roots to receive more air than water culture systems, with the exception of aeroponics. This characteristic improves oxygen availability to the roots, allowing the plant to grow healthier. (Van Os et al., 2002). The arrangement of cultivation systems on substrates might vary depending on the type of substrate receptor used (Van Os et al., 2008). According to Savvas et al. (2013), these sensors can be bags, pots, containers, or troughs. Despite the increased cost of utilising bags as the substrate receptor, bag culture is the most prevalent method for substrate-based cultivation. This is due to the fact that bags are simple to use and may be prepared in the same way every time. This reduces manpower expenses and installation errors. The bags are typically made of UV-resistant plastic sheets with a white outside to reflect light and prevent overheating and a black inside to prevent algae growth. To prepare for planting a new crop, it is essential to follow a specific procedure in substrate-based cultivation systems. Prior to planting, the substrate should not have any drainage holes, and it should be irrigated until it reaches saturation. This ensures that the pores of the substrate are completely filled with the nutrient solution (Savvas et al., 2013). In particular, the height of the substrate layer should be adequately chosen to ensure proper drainage and aeration, taking into account the hydraulic properties of the substrate (Heller et al., 2015). For GM with finer particles, it is generally more beneficial to use tall and narrow containers, as they allow for better performance by providing improved drainage and aeration. On the other hand, coarser GM substrates should be placed in shallower bags or channels to ensure sufficient water availability for the plants (Savvas, 2009). By considering these factors and selecting appropriate container geometries, optimal conditions for water and air exchange in the root zone can be achieved, enhancing the overall success of GM cultivation.

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11.3 Water Culture Systems 11.3.1 Fertigation Heads and Automated Control Systems Water culture systems, also known as hydroponics or nutrient film technique (NFT), are a type of soilless cultivation system where plants are grown directly in a nutrientrich water solution without the use of any solid substrate (Fig. 11.1). In this system, the plant roots are constantly submerged or partially submerged in the nutrient solution, allowing for direct uptake of water and nutrients. The water culture system typically consists of a reservoir that holds the nutrient solution, a submersible pump to circulate the solution, and a series of channels or troughs where the plants are placed (Brechner & Both, 1996). The channels or troughs are inclined to allow a thin film of nutrient solution to flow over the plant roots, ensuring continuous contact and absorption of water and nutrients. (Sonneveld & Voogt, 2009). Excess solution is collected and returned to the reservoir for recirculation, minimizing waste. One advantage of water culture systems is their simplicity and ease of operation. They require fewer materials and equipment compared to substrate-based systems, making them relatively cost-effective. Additionally, water culture systems provide precise control over nutrient delivery, allowing for optimized plant growth and increased water efficiency. However, water culture systems also present some challenges. They are more susceptible to power outages or pump failures, as the continuous flow of nutrient solution is essential for plant survival. Maintaining proper oxygenation of the root zone can be a concern, as the constant submersion of roots in water may limit oxygen availability. To address this, supplemental aeration systems or periodic oxygenation techniques can be implemented. (Savvas & Adamidis, 1999; Van Os et al., 2008). In the future, it is envisioned that the automation of fertilizer mixing in hydroponic systems will be achieved using liquid stock solutions triggered by ion selective sensors that measure nutrient concentrations in real time (Rius-Ruiz et al., 2014). Although ion selective sensors capable of such measurements already exist (Kim et al., 2013), their adoption in commercial practice is currently limited due to cost concerns and uncertainties regarding long-term reliability. The integration of online ion selective sensors for monitoring individual nutrient concentrations in the nutrient solution is particularly vital for closed hydroponic systems (Katsoulas et al., 2015). Closed systems aim to recycle and conserve water and nutrients, minimizing waste. Through continuous monitoring of nutrient concentrations, real-time adjustments can be made to maintain optimal nutrient levels for plant growth.

11.3.2 Open and Closed Soilless Culture Systems Soil-less culture systems are classified into two types: open systems and closed systems (Fig. 11.2). Each of these systems handles nutrient solutions and uses resources in a unique way. The plants in open soilless culture systems are always

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obtaining fresh nutrient solution, and any excess is either drained off or thrown away. The nutrient solution is never reused in these systems, and the plants are always supplied fresh solution. Because they do not require as many specialised technologies, open systems are simple to set up and run. However, because there is higher risk of waste, they can waste more water and nutrients than closed systems. Closed soilless culture systems, on the other hand, attempt to reuse and recycle the nutrient solution (Wohanka, 2002). The excess solution that drains from the plants is collected, cleaned, and returned to the plants. This strategy reduces the amount of water and nutrients used, making closed systems more resource efficient and better for the environment. Closed systems typically require more modern equipment, such as tanks, pumps, and filtering systems, to ensure that the nutrient solution is maintained correctly and maintains its quality over time. Closed systems require regular monitoring and regulation of nutrient levels, pH, and water quality to prevent imbalances or the development of harmful compounds. In addition, the system must be periodically cleaned and cleansed to prevent salt buildup and to maintain the ideal conditions for plant growth. Both open and closed soilless culture systems have their advantages and considerations. Open systems offer simplicity and ease of operation, while closed systems provide greater resource efficiency (Hultberg et al., 2011; Pagliaccia et al., 2007). The choice between the two depends on factors such as crop type, available resources, environmental impact, and the grower’s goals and preferences (Savvas, 2002). In long-term tomato crops, for instance, the annual leaching of nitrate nitrogen (NO3-N) through the discharged DS can exceed 380 kg per hectare per year (Morard, 1997). By reusing the nutrient solution, these excessive nutrient losses can be greatly reduced or eliminated. This not only helps to protect groundwater quality but also reduces the environmental impact associated with nutrient runoff and leaching. However, it is important to note that if the salt concentrations, particularly sodium

Fig. 11.2 Open and closed soilless culture systems

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(Na+ ) and chloride (Cl− ) ions, in the irrigation water exceed certain thresholds (typically 1–3 mmol L−1 , depending on crop sensitivity to salinity), periodic discharge of the nutrient solution becomes necessary to prevent significant yield losses. High salt levels can lead to detrimental effects on plant growth and productivity. Therefore, careful monitoring of salt concentrations and appropriate management strategies are essential to maintain optimal growing conditions and prevent salt accumulation in the root zone. The reuse of nutrient solutions in hydroponic systems on GM crops offers significant environmental benefits by reducing nutrient leaching and minimizing fertilizer usage. However, managing salt concentrations in the nutrient solution is crucial to ensure crop health and productivity. (Katsoulas et al., 2015).

11.4 Growing Media and Their Use in SCS 11.4.1 Classification of Growing Media Genetically modified organisms (GM) or “substrates” are solid materials, excluding soil, that offer improved conditions for plant growth compared to traditional agricultural soil in one or more aspects (Gruda et al., 2013). These substrates have various applications in the cultivation of high-value vegetables, ornamental plants, and in plant propagation processes like seedling and container plant production. In the horticultural industry, a combination of GM constituents and additives is commonly used. Additives in GM cultivation systems include fertilizers, liming materials, biocontrol agents, and wetting agents, which are employed to enhance plant nutrition, regulate pH levels, manage pests, and improve water absorption. The GM constituents encompass a wide range of materials, which can be either organic or inorganic in nature (Gruda et al., 2013). However, in commercial soilless production of vegetables and cut flowers, specific standalone substrates such as rockwool, perlite, or coir are predominantly utilized. These substrates can be further classified as organic or inorganic materials based on their composition and origin. In soilless cultivation systems, inorganic substrates, or GM, are primarily derived from natural sources, with just a small percentage being treated in factories before use. One well-known example is rockwool (Gruda & Schnitzler, 2000; Maher & Thomson, 1991; Raviv et al., 2004; Schmilewski, 2009), which was originally developed for use as insulation in the construction industry. Due to its light weight and ease of use, rockwool has become the most common GM for producing fruits and vegetables in greenhouses worldwide (Gruda et al., 2016b). Another inorganic GM that has been around for a long time and is highly popular in Europe is perlite. Perlite is commonly used in the Mediterranean region due of its lower cost than other possibilities. Gravel and sand were originally used in older systems due of their low porosity, however their usefulness was reduced. In addition to rockwool and perlite, several inorganic GM have been utilised in soilless cultivation systems. Pumice, zeolite, tuff, expanded clay flakes, porous volcanic rock, and vermiculite are

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a few examples. These substrates have diverse properties that make them suitable for specific crops, cultivation methods, and growing conditions (Gruda et al. 2016b). When used as raw materials or as GM constituents, both inorganic and organic constituents have benefits and disadvantages.

11.4.2 Growing Media Choice The choice of material for use as a growing medium or component depends on the specific requirements of the crop being grown. These requirements are influenced by plant biology and the applied plant technology. Additionally, the cost of the growing medium is also a factor in the decision-making process. However, it is becoming increasingly important for growing media to not only meet production needs but also be environmentally friendly and driven by consumer demand in order to address future challenges effectively (Gruda, 2012). In recent times and going forward, the evaluation of growing media is not solely based on financial success for the companies involved. Instead, life cycle assessment is being employed to classify growing media components based on their environmental impact, sustainability, and the use of “green technologies” in their production. This shift in focus aims to prioritize environmental protection and reduce the carbon footprint in horticulture. Peat substitutes, such as compost or biochar (Nemati et al., 2014), are commonly used alternatives that can significantly contribute to achieving these goals. By incorporating such substitutes, horticulture practices can reduce their negative impact on the environment (Martínez-Blanco et al., 2013; Steiner & Harttung, 2014).

11.4.3 Analyzing the Growing Media’s Performance The performance of growing media is undoubtedly the most important criterion for success. When evaluating performance, it is critical to consider the physical, chemical, and biological properties of the growing medium. These properties collectively define the efficacy and suitability of the growing medium for supporting plant growth and development. Growers may make informed decisions about which growing media to use and how to utilise it to get the most out of their plants by examining and assessing these properties.

11.4.4 Physical Properties To determine how effectively a growing medium works, consider all of its physical properties, which are critical for plant growth and development. These physical

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features reflect the structure, water-holding capacity, aeration, and overall appropriateness of the growing medium for plant roots. The texture of the growing medium, or the amount of particles of various sizes present, is a crucial physical property (Caron et al., 2015). A well-balanced layer of sand, silt, and clay particles allows water to drain while retaining adequate water for plants to use. The particle size distribution influences the porosity of the medium, which influences how much air plant roots may receive. Another essential physical property is the mass of the growing medium per unit volume. The bulk density measures this. The bulk density of the medium influences how deeply the roots can grow and how water and nutrients can pass through it. A low bulk density may result in excessive water absorption and inadequate aeration, whereas a high bulk density may limit root growth and decrease plant performance. A medium’s water-holding capacity is an important physical property that indicates how well it can hold and release water to plant roots. It is influenced by particle size, organic matter content, and the presence of pore spaces, to name a few factors. A growing medium with the appropriate quantity of water-holding capacity ensures that plants receive adequate water without being too wet or too dry. Sufficient aeration is critical for root respiration and avoiding oxygen deficiency. The air-filled porosity of the growing medium is an important physical property that influences aeration. This is the proportion of pores that contain air. It is influenced by the medium’s texture, density, and amount of organic materials (Gizas & Savvas, 2007). A well-aerated medium promotes healthy root growth and allows nutrients to be absorbed quickly.

11.4.5 Chemical Properties Analysing the chemicals in growing soil is a key step in determining how well they perform and whether they are beneficial to plant growth. These characteristics reveal a lot about nutrient availability, pH levels, and whether or not there are any toxic compounds that could impair the plant’s health and productivity. An essential chemical feature is the amount and availability of nutrients. For plants to grow successfully, growing media should include an adequate number of key macronutrients and micronutrients. Nutrient availability is influenced by factors such as pH, organic matter degradation, and the way nutrients interact with one another. Growers can determine what nutrients are in the growing medium by inspecting its chemical composition and making the required changes to ensure plants receive what they require for healthy growth. Another crucial chemical characteristic to consider is pH. It indicates how acidic or alkaline the growing medium is for plants. Varied plants require varied pH levels for optimal nutrient uptake and growth. You can tell if the pH of the growing medium is appropriate for the plants you wish to cultivate by looking at its pH. The availability of nutrients can be increased and nutrient deficiencies or toxicities avoided by altering pH levels with the use of additions. Electrical conductivity (EC) is a method of determining how many accessible salts are present in the growing medium. High EC levels can indicate that a plant has an excess of

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salt, which can be harmful to its health and growth. Monitoring and evaluating the electrical conductivity (EC) of the growing medium can assist in identifying potential salt-related issues and determining the best method to address them, such as allowing excess salts to leak out of the medium (Gruda et al., 2016b).

11.4.6 Biological Properties The biological properties of growing media are vital for understanding and controlling the intricate relationships between plants, microorganisms, and other living entities in the growing environment. These properties reveal a lot about the growing medium’s health and biological activity, which has a direct impact on plant development and production. The presence of beneficial microorganisms such as bacteria, fungus, and other soil organisms is an important biological feature. Grunert et al. (2016) These microorganisms aid in the cycling of nutrients, the decomposition of organic matter, disease prevention, and overall soil fertility. You can determine how biologically active and healthy the growing medium is by looking at the different types of microorganisms and how their populations fluctuate over time. It also allows for the discovery of potentially beneficial microorganisms that can be promoted or introduced to aid plant growth and disease resistance. The presence of plant pathogens and pests that are harmful to plants is another essential aspect of the biological properties of growing media. Pathogens and pests can cause plant illnesses, reduce food yields, and degrade crop quality. Analysing the growing medium for the presence of pathogens, nematodes, insects, or weed seeds helps identify potential dangers and promotes appropriate pest and disease management (Gruda et al., 2000, 2013; Ortega et al., 1996). For this type of research, methods such as DNA-based diagnostics, microbial culture, and bioassays can be used. When examining growing media, root health and growth are crucial biological properties to consider. The presence of healthy, well-developed root systems is critical for nutrient and water intake, anchoring, and overall plant performance. You can learn about the state of the roots and how they work in the growing medium by looking at the morphology of the roots, the presence of beneficial mycorrhizal fungi, or the presence of root diseases (Ortega et al., 1996),

11.5 Environmental Perspective To be sustainable and beneficial to the environment, horticulture must consider growing media from an environmental standpoint. Growers can make environmentally friendly selections by considering the environmental impact of growing media. There are a few essential considerations from an environmental standpoint. It is critical to determine the carbon footprint of growing media (Barrett et al., 2016). This

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entails determining how much greenhouse gas is emitted during production, transportation, and usage. Choosing low-carbon materials and production methods helps to reduce the quantity of carbon added to the climate. It is critical to investigate how resources are used. This includes investigating how raw materials are extracted, how much energy is used, and how much water is used during the production process (Barrett et al., 2016; Gruda, 2012). Natural resources can be conserved and environmental stress alleviated by reducing resource consumption and selecting sustainable solutions. Another critical component is waste management. To reduce pollution and landfill waste, a more sustainable way can be done by appropriately disposing of and reusing growing media waste. It is critical to consider how water is managed. Water can be saved and water resources protected by adopting efficient irrigation systems and water-retaining growing media (Gruda et al., 2016b).

11.6 Soilless Cultivation Types According to Shrestha and Dunn (2010), the hydroponic systems can be divided into two main categories: • Soilless cultivation, also known as hydroponics or substrate-based cultivation, refers to the practice of growing plants without traditional soil. Instead, various types of inert substrates or nutrient solutions are used to support plant growth (Fig. 11.3). Soilless cultivation offers several advantages, such as precise control over nutrient delivery, water usage, and optimal root aeration. There are several types of soilless cultivation, including:

Fig. 11.3 A basic illustration of types of soilless farming and its advantages

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• Hydroponics: In hydroponics, plants are grown directly in a nutrient-rich water solution without the use of any solid medium. The nutrient solution is carefully balanced to provide all essential elements required for plant growth. Common hydroponic systems include Deep Water Culture (DWC) (Bernstein, 2011), Nutrient Film Technique (NFT), and Drip Irrigation systems. • Aeroponics: Aeroponics is a variation of hydroponics where plant roots are suspended in the air, and nutrient solution is delivered through a fine mist or aerosol. This method allows for increased oxygen availability to the roots and efficient nutrient absorption. • Ebb and Flow (Flood and Drain): In this method, plants are grown in containers or trays filled with an inert substrate like perlite or rockwool. The nutrient solution is periodically flooded and then drained away, providing both nutrient delivery and aeration to the roots. • Drip Irrigation: Drip irrigation is a popular soilless cultivation method wherein a slow and controlled drip of nutrient solution is delivered directly to the plant’s root zone through tubes or pipes. This method conserves water and allows precise nutrient application. • Coir Substrate: Coir is a natural fiber derived from coconut husks and is used as a substrate for soilless cultivation. It provides good water retention and aeration for plant roots. • Rockwool Substrate: Rockwool is a mineral-based substrate that retains water and provides excellent aeration. It is commonly used in hydroponic and substratebased systems. • Perlite and Vermiculite Substrate: Perlite and vermiculite are lightweight, inert substrates that promote root aeration. They are often used as components in soilless mixes. • Coco Coir: Coco coir is a byproduct of coconut processing and is used as a substrate due to its excellent water retention and aeration properties. It is commonly used in container gardening and hydroponic systems. Each type of soilless cultivation has its own set of advantages and considerations. The choice of method depends on factors such as the type of crop, available resources, and specific environmental conditions. Soilless cultivation techniques offer innovative solutions for efficient and sustainable plant production, particularly in areas with limited access to fertile soil or water resources.

11.7 Plants 11.7.1 Species The selection of crops for hydroponic cultivation relies on their ability to absorb nutrients and the accessibility of data regarding specific plant species. Numerous commercial crops exhibit suitability for hydroponics.

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Hydroponics and aquaponics have done extensive study on commercial crops such as tomatoes and lettuce, which provide a high yield. According to research, both tomato and lettuce sprout at significant rates, with lettuce sprouting at up to 98% and tomato sprouting at up to 90%. Anpo et al. (2018) discovered that these crops grow at consistent rates, indicating that they can be grown in hydroponic systems. A further study found that reducing the amount of light from 200 mol m−2 s−1 to 140 mol m−2 s−1 had no effect on the yield or quality of herbs. Because herbs do not require high light conditions in their native environment (Anpo et al., 2018), this demonstrates that light requirements vary amongst plant species.

11.7.2 Limiting Factor for Plant Growth Despite the Plantae kingdom’s vast diversity, all plants share the same basic living requirements: light, carbon dioxide (CO2 ), water, and nutrients (Sadava et al., 2014). Light, CO2 in the air, and water (with nutrients dissolved in it) are all required for photosynthesis, which uses light, CO2 in the air, and water (with nutrients dissolved in it) to produce sugars (Raven et al., 2005). Aquaponics and hydroponics, like any other gardening method, are created by humans, and humans must still provide the limiting ingredients that plants require to develop.

11.7.3 Light and CO2 In addition to its role in photosynthesis, light is involved in other crucial plant processes such as photoperiodism and phototropism (Raven et al., 2005). Photoperiodism refers to the critical number of daylight hours or daylength required to initiate the flowering time in plants. Furthermore, phototropism is the phenomenon where plants exhibit movement or growth towards a light source. The controlled formation of floral buds can be triggered by manipulating specific light wavelengths, such as far-red light and blue light. However, the light wavelength necessary for bud growth initiation varies depending on the plant species (Raven et al., 2005; Sadava et al., 2014). The length of the day also affects plant growth, with some plants classified as short-day or long-night plants, requiring less than 12 h of daylight to flower, while others are long-day or short-night plants, needing more than 12 h of daylight for flowering. However, certain plants are not influenced by the length of daylight and exhibit independent flowering patterns. Manipulating the photoperiod can effectively regulate flowering, dormancy, and growth in plants. This control over the light parameter offers valuable benefits for hydroponic cultivation, saving time and expenses for farmers. For example, by subjecting a long-day plant to intermittent light during nighttime when the natural daylight duration is insufficient (less than 12 h), the dormancy of flower buds is

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disrupted. This intervention promotes the production of new flower buds, enabling the plant to flower ahead of the regular season (Raven et al., 2005). In hydroponics, artificial light sources like Light Emitting Diodes (LEDs) are commonly utilized due to their high proportion of red photons. The quality and quantity of light play a significant role in the process of photosynthesis and have a noticeable impact on nitrate reduction. Consequently, the choice of light source is crucial for ensuring effective photosynthesis and achieving successful nitrate reduction in hydroponic systems (Anpo et al., 2018).

11.7.4 Water and Nutrients Natural lakes are habitats consisting of various organisms, including animals, plants, and other living entities, both within the water and surrounding areas. A well-functioning lake ecosystem maintains a balanced nutrient cycle. However, in eutrophic lakes, there is an excessive accumulation of nutrients, surpassing what the system naturally receives. This nutrient overload leads to detrimental environmental issues such as pollution and algae blooms. Certain species of cyanobacteria, in particular, release toxins that pose risks to both human health and the overall environment. Howarth et al. (2011) discovered that these impacts directly influence the ecological condition and status of the lake ecosystem. Water plays a critical role in photosynthesis as it acts as an electron donor during the conversion of CO2 into organic compounds (Sadava et al., 2014). However, water’s significance extends beyond its involvement in carbohydrate production. It also serves essential functions in plants, such as facilitating the transportation of nutrients, ions, hormones, and other substances within plant cells and vessels. Water is central to processes like transpiration and thermoregulation in plants (Raven et al., 2005). Nutrients, on the other hand, directly impact plant growth and overall health. Any disruption in the nutrient balance can have widespread effects on the entire plant. Thus, the roles of water and nutrients are interdependent and intricate. This emphasizes the importance of water and nutrition for plants, with their relevant aspects presented here (Raven et al., 2005). Different plant species exhibit diverse nutrient requirements, with variations in the types, quantities, and ratios of nutrients they need (Sainju et al., 2003). For instance, tomatoes (Solanum lycopersicum) have distinct nutrient and pH requirements, while strawberries (Fragaria × ananassa) (Sabli, 2012). have different nutrient and pH requirements.

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11.8 Overview of Phytoremediation Phytoremediation is an environmentally friendly and cost-effective approach to mitigate soil, water, and air pollution using plants. It is a process by which plants and associated microorganisms help remove, degrade, or immobilize contaminants in the environment, ultimately restoring ecosystems and improving environmental quality. One of the key mechanisms in phytoremediation is phytoextraction, where plants accumulate and store contaminants from the soil through their roots. This method is particularly effective for heavy metals and trace elements such as lead, cadmium, arsenic, and zinc. Plants with high metal uptake capacities, known as hyperaccumulators, are often utilized for this purpose. Once the plants have absorbed the contaminants, they can be harvested and properly disposed of, thereby reducing the concentration of pollutants in the soil. Another approach in phytoremediation is rhizofiltration, which focuses on the remediation of water bodies contaminated with pollutants. Plants with extensive root systems, such as reeds and water hyacinths, are employed to absorb and filter contaminants from the water. These plants can remove various pollutants, including heavy metals, nutrients, organic compounds, and even certain pathogens. Phytostabilization is another technique in which plants are used to immobilize contaminants in the soil, preventing their migration and reducing their bioavailability. This method is particularly useful for sites contaminated with heavy metals and radioactive elements. By establishing a plant cover, the root systems bind and trap the pollutants, reducing their mobility and potential to cause harm. Additionally, phytodegradation or phytotransformation involves the ability of plants to metabolize and break down organic pollutants such as petroleum hydrocarbons, pesticides, and industrial chemicals. Through various enzymatic processes, plants can transform these toxic compounds into less harmful substances, thus detoxifying the environment. Phytoremediation has gained significant attention due to its ecofriendly nature, low cost, and potential for large-scale application. However, it is important to consider the specific plant species, site conditions, contaminant type, and concentration when implementing phytoremediation projects. Proper monitoring and management are crucial to ensure the effectiveness and long-term sustainability of phytoremediation efforts.

11.8.1 Degradation Degradation is a natural or man-made process that involves the breakdown, transformation, or conversion of various chemicals, materials, or pollutants into simpler forms. Because it occurs in multiple environmental compartments such as soil, water, and air, it can have both positive and negative effects on ecosystems and human activities. In the context of environmental pollution, the removal or transformation of dangerous chemicals, which lowers their potential impacts, is an important aspect of degradation. Different degradation processes are used depending on

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the type of pollutant and the specific environmental variables. Biodegradation is a common type of degradation in which microorganisms like bacteria and fungus use enzymatic processes to break down organic compounds into simpler molecules. Microbes consume pollutants as a source of energy and carbon in the natural environment, and this process is common. Biodegradation can successfully eliminate or reduce organic pollutants such as hydrocarbons, solvents, and insecticides. Pollutants undergo photo degradation when exposed to sunlight or ultraviolet (UV) radiation. During this process, which involves direct or indirect light interaction with the pollutant, the breakdown or transformation of the pollutant’s chemical structure occurs often. Photodegradation is especially crucial for removing organic pollutants such as volatile organic compounds (VOCs) and air pollutants.

11.8.2 Extraction Extraction is a process that involves the removal or separation of a specific substance or component from a mixture or matrix. It is commonly used in various fields, including chemistry, pharmaceuticals, food processing, and environmental remediation. The goal of extraction is to isolate the desired substance, usually for further analysis, purification, or utilization. In extraction, a suitable solvent or extracting agent is employed to selectively dissolve the target substance while leaving unwanted components behind. The choice of solvent depends on factors such as the solubility of the desired substance and its compatibility with the extraction system. Solvents can be organic solvents, water, supercritical fluids, or even complex mixtures tailored for specific applications. One common type of extraction is liquid–liquid extraction, where the target substance is transferred from one liquid phase (the source) to another liquid phase (the solvent). This method is widely used in chemical laboratories for the separation of compounds from complex mixtures. It relies on differences in solubility or partition coefficients between the target substance and the solvent to achieve the separation. Solid-phase extraction (SPE) is another extraction technique that utilizes a solid material as a stationary phase to retain and concentrate the target substance. The sample is passed through a solid phase, and the desired analyte is retained while unwanted compounds are washed away. SPE is frequently employed in analytical chemistry to prepare samples for analysis by removing interfering substances or concentrating the analyte of interest.

11.9 Phytoremediation Technologies This research gives a full review of the research on the most important water phytoremediation methods and technologies. The technologies shown here are the main, most important, or most studied types of phytoremediation. The chapter is broken up into subsections that explain what each process is and how it works. They also talk about the features of the site, the types of media that can be used, and the types of contaminants that can be effectively

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treated using that process. Also, when this information is known, the chapter describes the concentrations of contaminants at which each process works. Also, both the pros and cons of each process are carefully looked at and talked about.

11.9.1 Rhizofiltration Rhizofiltration is a phytoremediation technique that utilizes plants and their root systems to eliminate pollutants from water sources. It is particularly effective in treating water bodies contaminated with heavy metals, nutrients, organic compounds, and certain pathogens. To implement rhizofiltration, plants with extensive root systems are grown in constructed wetlands or specially designed floating platforms dedicated to water treatment. These plants, often referred to as hyperaccumulators or metallophytes, possess the unique ability to absorb and accumulate contaminants from the water through their roots. The contaminants are either stored in the plant tissues or undergo transformations within the plant. The effectiveness of rhizofiltration depends on several factors, including the selection of plant species, the concentration and nature of the contaminants, water chemistry, hydraulic conditions, and the duration of the treatment. Different plant species display varying levels of tolerance and capacity to accumulate specific pollutants. Rhizofiltration offers several advantages, including its sustainability, cost-effectiveness compared to traditional water treatment methods, and its applicability to both point source and non-point source pollution. Furthermore, it provides a natural and visually appealing approach to water remediation.

11.9.2 Hydraulic Control Hydraulic control, also referred to as phytohydraulics or hydraulic plume control, involves utilizing plants to extract and consume groundwater. This process is employed to contain or manage the movement of contaminants within an area. By actively removing groundwater through uptake and consumption, hydraulic control helps restrict the migration of pollutants and mitigate their impact on the surrounding environment.

11.9.3 Phytovolatilization Phytovolatilization is a phytoremediation process where a plant absorbs a contaminant, undergoes metabolism, and subsequently releases the contaminant or its transformed form into the atmosphere through transpiration. This process involves the uptake of contaminants by plants, their metabolic transformation within the plant, and

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subsequent release into the air through transpiration. Phytodegradation is a related process that often occurs in conjunction with phytovolatilization.

11.9.4 Riparian Corridors/Buffer Strips Riparian corridors or buffer strips are commonly implemented along riverbanks and stream edges to address and remediate issues related to surface runoff and groundwater contamination entering the water bodies. These systems serve as a means to control and prevent the downstream migration of contaminated groundwater plumes while facilitating the degradation of contaminants within the plume. The remediation mechanisms employed in riparian corridors include water uptake, contaminant uptake, and plant metabolism. Similar in concept to physical and chemical permeable barriers such as trenches filled with iron filings, riparian corridors provide a means to treat groundwater without extraction or containment. They encompass various phytoremediation processes, including hydraulic control, phytodegradation, rhizodegradation, phytovolatilization, and potentially phytoextraction. By incorporating these mechanisms, riparian corridors and buffer strips play a crucial role in mitigating and remediating pollution within aquatic environments.

11.9.5 Phytodegradation Phytodegradation, or phytotransformation, refers to the breakdown of contaminants that are either taken up by plants and undergo metabolic processes within the plant or externally degraded by compounds produced by the plants, such as enzymes. The primary mechanism involved in phytodegradation is the uptake and metabolism of contaminants by the plants themselves. Additionally, degradation may occur outside the plant through the release of compounds that facilitate the transformation of contaminants. When degradation is facilitated by microorganisms associated with or influenced by the plant root system, it is referred to as rhizodegradation. Phytodegradation encompasses both internal and external degradation processes, contributing to the overall remediation of contaminants in the environment.

11.9.6 Phytoextraction Phytoextraction is a phytoremediation process that involves using plants to remove and accumulate contaminants, particularly heavy metals, from soil, sediment, or

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water. It is a sustainable and environmentally friendly approach to remediate contaminated sites. In phytoextraction, specific plants, known as hyperaccumulators or metallophytes, are selected for their ability to efficiently take up and store high concentrations of contaminants in their tissues, predominantly in the roots and shoots. These plants have developed various mechanisms to tolerate and accumulate metals without suffering detrimental effects. The process of phytoextraction typically involves the following steps: selecting suitable plant species, preparing the site, planting the selected plants, providing necessary nutrients, irrigating as required, and allowing the plants to grow and accumulate the contaminants over a defined period of time. Once the plants have reached their desired level of contamination, they are harvested, and the contaminated plant biomass is properly managed, either through incineration, landfill disposal, or further processing for metal recovery.

11.9.7 Rhizodegradation Rhizodegradation is a phytoremediation process that involves the breakdown and degradation of contaminants in the rhizosphere, the zone of soil surrounding plant roots. It occurs through the interactions between plant roots, associated microorganisms, and the contaminants present in the soil. In rhizodegradation, plants release a variety of organic compounds, such as root exudates, enzymes, and organic acids, into the rhizosphere (Schnoor et al., 1995). These compounds serve as a source of nutrients and energy for the microbial community inhabiting the root zone. The microorganisms, in turn, utilize these compounds to metabolize and degrade the contaminants present in the soil. The process of rhizodegradation is highly dependent on the interactions between plants, microorganisms, and the specific contaminants. The root exudates attract and stimulate the growth of contaminant-degrading microorganisms, enhancing their activity and promoting the breakdown of the contaminants. This can result in the transformation of complex organic pollutants into less harmful or more readily degradable forms. Rhizodegradation offers several advantages as a phytoremediation technique. It is a natural and sustainable approach that takes advantage of the existing soil microbial community. By promoting the growth and activity of contaminant-degrading microorganisms, rhizodegradation can effectively reduce the concentration and toxicity of pollutants in the soil. (Narayanan et al., 1995).

11.9.8 Constructed Wetlands Constructed wetlands are engineered systems designed to replicate the natural processes occurring in wetland ecosystems (Cole, 1998). They are used for various purposes, including wastewater treatment, stormwater management, and the remediation of contaminated water and soil. In constructed wetlands, a combination of aquatic plants, microorganisms, and substrate materials are used to treat and purify

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water (Kadlec & Knight, 1996). The system consists of shallow basins or channels that are lined with impermeable materials to retain water. The wetland is then filled with various media, such as gravel, sand, and organic matter, which support the growth of plants and provide a substrate for microbial activity. The treatment process in constructed wetlands occurs through various physical, chemical, and biological mechanisms. As water flows through the wetland, suspended solids settle out, and organic matter is broken down through microbial degradation. The plants in the wetland take up and accumulate nutrients and contaminants through their root systems, effectively removing them from the water. Constructed wetlands can be classified into different types based on their design and intended purpose. Subsurface flow wetlands involve water passing through a gravel bed planted with emergent plants, while surface flow wetlands have water flowing over the surface of the wetland, typically supporting floating plants. Hybrid wetlands combine elements of both subsurface and surface flow systems. The advantages of constructed wetlands include their effectiveness in pollutant removal, low energy requirements, and their potential to provide habitat and aesthetic benefits. They can be cost-effective alternatives to conventional treatment methods, particularly for small-scale and decentralized applications. Constructed wetlands are also known for their ability to enhance biodiversity and support wildlife.

11.10 Soilless Cultivation Through an Intensive Crop Production Scheme Soilless cultivation, also known as hydroponics or soilless farming, is a method of growing plants without the use of traditional soil. Instead, plants are cultivated in a nutrient-rich water solution or inert medium, providing all the necessary nutrients directly to the plant roots. This intensive crop production scheme in soilless cultivation involves maximizing crop yields within a controlled environment. Various techniques and technologies are employed to optimize plant growth, such as precise nutrient management, controlled lighting, temperature regulation, and irrigation systems. In soilless cultivation, the nutrient solution is carefully formulated to provide the essential elements required for plant growth, including macronutrients (such as nitrogen, phosphorus, and potassium) and micronutrients (such as iron, manganese, and zinc). This allows for precise control over nutrient availability, ensuring optimal plant nutrition and growth. The controlled environment in soilless cultivation allows for year-round crop production, independent of seasonal variations and climate conditions. It enables farmers to optimize resource utilization by providing the ideal conditions for plant growth, resulting in higher crop yields compared to traditional soil-based agriculture. Soilless cultivation offers several advantages over traditional farming methods. It allows for efficient use of water, as the nutrient solution is recirculated, reducing water consumption. Additionally,

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since plants are grown in a controlled environment, there is a reduced risk of soilborne diseases and pests, leading to a decrease in the use of pesticides and herbicides. Soilless cultivation offers flexibility in crop selection, allowing for the cultivation of a wide range of plant species. It also enables vertical farming and stackable cultivation systems, maximizing space utilization and potentially increasing productivity per unit area. However, soilless cultivation also has its challenges. It requires a significant investment in infrastructure and equipment to create and maintain the controlled environment. Additionally, precise monitoring and management of nutrient levels, pH, temperature, and lighting are crucial for successful crop production.

11.10.1 Rough Comparison of Soilless Systems When comparing different soilless systems, several factors can be considered to evaluate their suitability for specific applications. Here is a rough comparison of some common soilless systems: • Hydroponics: Hydroponics is a widely adopted soilless system where plants are grown in a nutrient-rich water solution. It offers precise control over nutrient delivery, water usage, and environmental conditions. Hydroponics is suitable for various crops, ranging from leafy greens to fruiting plants. It allows for higher yields, faster growth rates, and can be implemented in both small-scale and largescale setups. However, it requires regular monitoring of nutrient levels and pH, as well as appropriate water management. • Aeroponics: Aeroponics takes soilless cultivation a step further by suspending plant roots in an air or mist environment, providing them with a fine nutrient mist. This system offers superior oxygenation and nutrient absorption, resulting in rapid plant growth and reduced water usage. However, aeroponics requires meticulous control of environmental factors such as humidity, temperature, and nutrient misting intervals. It is considered more technically challenging and may require advanced equipment (Sardare & Adame, 2013). • Aquaponics: Aquaponics combines hydroponics with aquaculture, creating a mutually beneficial system where plants grow in water enriched by fish waste. The plants filter the water, removing waste and providing nutrients for the fish. Aquaponics is a sustainable and closed-loop system that can produce both crops and fish. However, it requires careful management of water quality, fish health, and nutrient balance. It is well-suited for small-scale and educational settings (Zheng et al., 2015). • Vertical Farming: Vertical farming utilizes stacked layers or vertical structures to maximize space utilization. It can incorporate various soilless systems, including hydroponics or aeroponics. Vertical farming offers high crop density, efficient resource usage, and year-round production in controlled environments. However, it requires substantial investment in lighting, ventilation, and infrastructure. It is often preferred in urban areas with limited land availability.

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• Substrate-based Cultivation: Substrate-based systems use inert materials such as coco coir, perlite, or rockwool to provide physical support to plant roots while allowing nutrient absorption. These systems offer flexibility in crop selection, ease of setup, and lower initial investment compared to other soilless systems. They are suitable for both small-scale and large-scale operations. However, substrate-based systems require regular monitoring of nutrient levels, irrigation management, and potential disposal of used substrates. Each soilless system has its own advantages, considerations, and complexities. The choice depends on factors such as crop type, available space, budget, level of control desired, and technical expertise. It is essential to assess these factors and conduct thorough research to select the most suitable soilless system for a specific application.

11.10.2 Advantages of Soıl-Less Culture Soil-less culture, also known as soilless farming or hydroponics, offers several advantages compared to traditional soil-based cultivation methods (Global Footprint Network, 2019; Linet al., 2020). Here are some key advantages of soil-less culture: • Efficient Nutrient Delivery: In soil-less culture, nutrients are directly provided to the plant roots in a controlled and precise manner. This allows for optimal nutrient absorption and uptake by the plants, resulting in improved growth and higher yields. The nutrient solution can be tailored to meet the specific needs of different crops, ensuring they receive the ideal balance of nutrients for their development. • Water Conservation: Soil-less culture is highly efficient in water usage. The cultivation system recirculates the nutrient solution, minimizing water wastage compared to traditional irrigation methods. This is particularly important in regions where water scarcity is a concern or in areas where water resources need to be managed efficiently. Additionally, soil-less systems can be designed to minimize evaporation and runoff, further reducing water consumption. • Space Optimization: Soil-less culture allows for maximized space utilization. Plants can be grown vertically, using stacked layers or vertical structures, which increases the planting density and overall production capacity per unit area. This is especially advantageous in urban environments or areas with limited land availability, where space is a premium. Vertical farming techniques in soil-less culture enable the cultivation of more crops in a smaller footprint. • Reduced Dependency on Pesticides: Soil-less culture can help reduce the reliance on pesticides and chemical treatments. The controlled environment and absence of soil minimize the occurrence of soil-borne diseases and pests. Additionally, the ability to closely monitor and manage nutrient levels, pH, and environmental conditions allows for early detection of potential issues and prompt intervention, reducing the need for chemical interventions.

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• Consistent Crop Quality: Soil-less culture provides a consistent and predictable growing environment, resulting in uniform plant growth and crop quality. By eliminating the variability of soil conditions, farmers can achieve more consistent yields and better control over the overall crop production. This is particularly advantageous in commercial farming operations where consistent quality and market demand are essential. • Year-Round Production: Soil-less culture enables year-round production of crops irrespective of seasonal limitations. By providing a controlled environment, with proper lighting, temperature, and humidity control, farmers can overcome the constraints of weather and seasonal variations. This continuous production capability allows for better market responsiveness and a more stable income stream (Peters et al., 2012).

11.10.3 Lımıtatıons of Soıl-Less Culture While soil-less culture, such as hydroponics, offers numerous advantages, there are also some limitations and challenges to consider. Here are a few limitations of soilless culture: Initial Investment and Operational Costs: Establishing a soil-less culture system requires an initial investment in infrastructure, equipment, and technology. The costs associated with setting up and maintaining the system, including lighting, climate control, nutrient solutions, and monitoring systems, can be significant. Additionally, operational costs such as energy consumption and ongoing maintenance should be considered (Galli et al., 2015). Technical Expertise and Knowledge: Soil-less culture requires specialized knowledge and skills to successfully manage the system. Farmers need to understand the principles of hydroponics, nutrient management, pH control, and environmental factors. Adequate training and expertise are necessary to ensure optimal plant growth and prevent issues such as nutrient imbalances, diseases, and pests. Continuous monitoring and adjustment may be required to maintain optimal conditions. Risk of System Failures: Soil-less culture systems are dependent on technology and infrastructure, which may be prone to failures or disruptions. Power outages, equipment malfunctions, or technical issues can impact the stability and productivity of the system. Having backup systems or contingency plans in place is important to mitigate risks and prevent potential crop losses. Susceptibility to Environmental Changes: Soil-less culture systems are sensitive to environmental fluctuations, such as temperature, humidity, and light intensity. Any deviations from the optimal range can affect plant growth and crop performance. Maintaining stable environmental conditions may require additional investments in climate control systems, which can add to the operational costs. Limited Crop Selection: While many crops can be successfully grown in soil-less culture, there are some limitations. Certain crops may have specific requirements that are challenging to meet in a controlled environment. Some crops may have extensive

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root systems that are difficult to manage in a soil-less system. Additionally, certain traditional crops that rely on soil microbiota may not thrive in a soil-less culture environment. Dependency on External Inputs: Soil-less culture relies heavily on external inputs, such as nutrient solutions and substrate materials. The availability, quality, and cost of these inputs can impact the viability and sustainability of the system. A reliable supply chain for these inputs needs to be established to ensure consistent production. It’s important to note that while soil-less culture has limitations, ongoing advancements in technology and increased knowledge in the field are addressing many of these challenges. Overcoming these limitations often requires careful planning, ongoing monitoring, and continuous learning to optimize the performance of the soil-less culture system.

11.11 Soilless Culture: Concluding Remarks and Future Issues Higher yields, high production quality and the ability to have control over emission of nutrients and protection of plants has been greatly achieved. In comparison to high-tech greenhouses with low-tech greenhouses which are characterized by a mild climate and the costs of soilless systems are not always the major concerns but there are some other limiting factors too. Which hampers the production. Hence, in countries soilless culture is mainly adopted where the issue comes from the soil which becomes critical in later on stages, water resources depleting, and cause of the environmental pollution where nutrient leaching is major concern (Savvas & Gruda, 2018). These soilless systems are not merely a modern technology for the greenhouse production of vegetables and ornamentals. The innovative cropping systems are the bioregenerative life systems for production of fresh foods (Paradiso et al., 2014). Another application of hydroponics is aquaponics, which couples hydroponic production of vegetables or ornamentals with fish production, by utilizing fish excrement for crop nutrition (Tyson et al., 2011). Finally, soilless culture can be applied for urban agricultural production (Eigenbrod & Gruda, 2015). Acknowledgements I wish to express my deepest gratitude to my supervisors, Professor Dr Gunjan and Dr Saurabh, who guided and encouraged me towards being more professional. Without their persistent guidance and support, the completion of this work would not have been possible. I would like to acknowledge the support and great love of my family. This work would not have been possible without their grateful and understanding input.

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Chapter 12

Effect of Bio-Sorptive Removal of Heavy Metals from Hydroponic Solution: A Review Nagma Khan, Baby Tabassum, Mohammad Hashim, and Asma Hasan

Abstract Heavy metal removal from polluted hydroponic solutions is a key challenge in contemporary agriculture. Due to its affordability and ecological consciousness, bio-sorption, a biotechnological technique, has gained interest as an effective heavy metal expulsion technique. The study presents the findings of several investigations about various bio-sorbents for heavy metal removal in hydroponic systems, including bacteria, fungus, algae, and plant-based materials. It emphasizes bio-sorption mechanisms such as adsorption, chelation, ion exchange, and phytoremediation, which aid in binding heavy metals to the bio-sorbent surface. The review also goes through the variables influencing bio-sorption effectiveness, including pH, temperature, contact duration, and metal level. It has shown remarkable removal efficiency for different heavy metals from hydroponic solutions, including lead, cadmium, copper, arsenic, chromium, and nickel. Furthermore, bio-sorption is an appealing choice for heavy metal removal in hydroponic systems because of its low cost, convenience of use, and environmental sustainability. Furthermore, the possibility of bio-sorbent regeneration, reusability, and the long-term impacts of bio-sorption on plant development and soil health should be investigated. Finally, bio-sorptive heavy metal removal from hydroponic solutions appears to be a potential technique for controlling heavy metal pollution in agricultural systems. The assessment’s outcomes indicate that biosorption can be a successful and sustainable technology for heavy metal removal in hydroponic systems, but additional study is required to improve the procedure and investigate its long-term effects. Keywords Bio-sorption · Heavy metals · Hydroponic solution N. Khan · B. Tabassum (B) · A. Hasan Department of Zoology, Toxicology Lab, Govt. Raza P.G. College, Rampur, Uttar Pradesh 244901, India e-mail: [email protected] M. Hashim Department of Biochemistry, Mohammad Ali Jauhar University, Rampur, Uttar Pradesh 244901, India N. Khan Mahatma Jyotiba Phule Rohilkhand University, Uttar Pradesh, Bareilly 243005, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_12

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12.1 Introduction The accumulation of heavy metals has become the most significant matter affecting the environment. All metallic components that are comparatively condensed and are dangerous or toxic even at small levels are called “heavy metals.” It refers to a group of metals as well as metalloids with atomic weights higher than 4 g/cm3 . Metals that are toxic to plants like Lead (Pb), Cadmium (Cd), Mercury (Hg), and Arsenic (As) (Rodriguez et al., 2005). The origins and poisonousness of some substance ions are noted in Table 12.1. These metal ions are not biodegradable and remain in the environment for a long time. Several studies have reported that removing heavy metal ions from the effluent is critical for community health (Quansah et al., 2015). These are ubiquitous chemical contaminants that can drain into hydroponic solutions from a variety of sources, including water-based, nutritional supplements, and contaminated machinery. Over some time, these elements can build up in quantities that are harmful to plant viability. Additionally, there is a chance that eating vegetables cultivated hydroponically might expose people to heavy metals, which could have serious adverse health effects. Copper (Cu) and Zinc (Zn) are often regarded as non-toxic metals in people; however, they can have toxic consequences such as hemolysis of the arteries, liver failure, and renal failure for Cu and symptoms like nausea, vomiting, epigastric discomfort, drowsiness, and exhaustion for Zn. Hence, the creation of effective and affordable heavy metal removal techniques in hydroponics is essential for ensuring food security and environmentally friendly farming. Heavy metal concentrations are primarily carried by industrial effluent and are generated by an array of industries, which include the production of metallic materials, removal waste, metallic serving, the technique of electrolysis, removal, external finishing, electricity production, fuel production pictures, aviation, and radiation construction, among others. Therefore, it is crucial to reduce the eradication of pollutants via sewage movements. The capacity of biological systems to take up pollutants from effluent via biologically regulated or thermodynamic absorbing processes is referred to as “biosorption.” Bio-sorption, which includes the application of either living or non-living biomaterials to bind and confine heavy metal ions, is one potential strategy for eliminating heavy metals from hydroponic treatments. This process has various benefits over standard biophysical approaches, notably low expense, excellent effectiveness, and friendliness with the surroundings (Romera et al., 2006). Biomaterials are substances demonstrating absorption capabilities, such as microbial biomass, algae, fungus, and waste from agriculture debris, which have demonstrated substantial promise as heavy metal removal biosorbents (Van Ginneken et al., 2007). These biomaterials’ distinct surface properties and functional components allow them to bind heavy metal ions, thereby lowering their levels in hydroponic treatments. A scientifically practical and financially appealing alternative is biosorption. This is because certain organic compounds can maintain and accumulate contaminants in highly diluted solutions containing water. The expression “bio-sorption” indicates an individual biomolecules

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Table 12.1 Heavy metal types, sources, and their effects on living beings Heavy metal

Major source

Effects on human

Effects on plants

Effects on References microorganisms

Lead (Pb)

Cosmetic products like Surma, kohl, kajal, tiro, and tozali, metal costume jewelry, tableware, toys, soil, water, dust, paint, etc.

Diminished growth, Learning problems, kidneys damage, neuronal damage, high blood pressure, impulsiveness, sleeplessness, Alzheimer’s risk, and an inadequate ability to concentrate

Oxidative damage, chlorosis, inhibition of enzyme activity, photosynthesis, and growth are all negatively impacted

Denatures protein and nucleic acids, and prevents enzyme activity and transcription

Nagajyoti et al. (2010), Fashola et al. (2016)

Cadmium Refining, fertilizer, (Cd) Plastic, pesticide, welding, mining,

Itai-Itai, renal disorders, pulmonary and prostate tumors, lymphoma, microbial leukemia, bone disease, coughing, emphysema, headache, hypertension, vomiting, testicular shrinkage, and hypochromic anemia

Decreased seed development, chlorosis, decreased plant nourishment, inhibited growth,

Cells growth inhibition, proteins deterioration, nucleotide destruction, and which prevents the mineralization of carbon and nitrogen

Nagajyoti et al. (2010), Fashola et al. (2016), and Sankarammal et al. (2014)

Nickel (Ni)

Chest discomfort, irritation, drowsiness, dry cough, breathing difficulty, headaches, renal illnesses, pulmonary and nostril cancer, and coronary artery disease

Lower pigment concentration, inhibition of enzyme activity, and decreased absorption of nutrients by the growth

Cell membrane disruption, inhibition of enzyme activity, and oxidative damage

Fashola et al. (2016), Chibuike & Obiora, (2014), and Bashir et al., (2004)

Non-ferrous metal, paint formulation, electroplating, porcelain enameling

(continued)

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Table 12.1 (continued) Heavy metal

Major source

Effects on human

Effects on plants

Effects on References microorganisms

Mercury (Hg)

Batteries, geothermal energy, mining, paint industry, paper industry, volcanoes eruption, and rock weathering

Deafness, vision problems, decreased fertility, drowsiness gastrointestinal discomfort, gum disease, kidney issue, memory loss, pulmonary edema, lowered immunity, and scleroderma

Affects the antimicrobial system, enhances fatty peroxidation, generates gene-toxic effect, inhibits plant development, yield, nutrient consumption, homeostasis

Population decline, protein mutagenesis, cell wall disruption, and enzyme inhibition

Arsenic (As)

Mineral extraction, chemical fertilizers, sandstone settling, smelting, and atmospheric absorption

Diseases of the heart, lungs, and brain, conjunctive irritation, and malignancies of the skin

Biological disturbances, inflammation, cell surface damage, growth retardation, roots elongation suppression, metabolism interference, loss of reproduction

Decrease in Abdul-Wahab enzyme activity & Marikar, (2012), Finnegan & Chen, (2012), and Bissen & Frimmel, (2003)

Zinc (Zn)

Brass manufacturing, mining, oil refinery, plumbing

Lethargyness, metal fume fever, macular degeneration, prostate carcinoma, seizure frequency, icterus, hemorrhage, kidney and liver failure, ataxia, depression, gastrointestinal discomfort, nausea

Affects photosynthesis, slows plant development, and hurts plant biomass and chlorophyll concentration

Death, biomass decrease, growth inhibition

Ali et al. (2013), Fashola et al. (2016), and Abdul-Wahab & Marikar, (2012)

Chibuike & Obiora, (2014)

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(or biomass) tendency to accumulate and assimilate specific compounds or supplementary compounds through solutions containing water (Taiz & Zeiger, 2006). Potential metal bio sorbents include many aquatic plants, algae, and bacteria. It is regarded as the best alternative technique for cleaning contaminants out of effluents. Biosorption is an immediate method of inactive metallic retention utilizing non-growing adsorbents. It has several advantages over conventional strategies, among which are described below: (such as budget; maximum productivity; minimal mechanical and/or biological effluent; no extra nutrition demand; bio-sorbent regeneration; and metal recovery). Although the biosorption approach, which uses microorganisms to remove and regain pollutants through water solutions, has been around for a while, As a cost-effective, high-potential technology, this technique has only just grown in prominence (Vasudevan et al., 2003). Metal ions and the cellular components of living creatures interact physically to produce a variety of biological effects. Heavy metals and radioactive substances are absorbed in this process. As a result, the concept of using bioactive materials for the absorption of heavy metals has been thoroughly researched for the past 20 years. Environmental conservation is one of society’s most important concerns, and certain existing agricultural practices pose a risk. Along with other sectors, the United Nations FAO recognizes it as one of the most widespread industrial contributors to soil toxins. Erosion, The current paradigm is called into question by soil conservation for agricultural use and greenhouse gas emissions generated by farms themselves. However, there are possibilities, including hydroponic systems, a more sustainable type of farming that might be used in urban areas to make it more convenient for clients. It is an agricultural technique that grows plants in nutrient-rich water solutions rather than soil. Hydroponic farming systems have drawn a lot of interest recently as a sustainable and effective way to grow plants. In comparison to conventional soil-based agriculture, hydroponics enables optimum growth rates, greater crop yields, and decreased water use by directly supplying vital nutrients to plant roots in controlled conditions. Still, the buildup of heavy metals in hydroponic solutions is a serious problem since these hazardous contaminants can harm people’s health if ingested and inhibit the development of plants. However, there is an enhanced challenge for creative and environmentally responsible methods to eliminate heavy metals from hydroponic systems. In addition, the utilized water may be collected and recycled, and the beneficial nutrients can come from several sources, including fish manure Cultivating plants hydroponically, or without the need for soil, is a common practice. It has been reported that during the transition to the twentieth century, a pair of German botanists, namely Julius von Sachs and Wilhelm Knop, created a comprehensive inventory of essential constituents that a solution must include in order to provide enough nourishment to plants. Subsequently, this method has predominantly found use in controlled laboratory environments as a kind of agricultural practice. However, it has lately garnered significant attention as a means to cultivate crops with increased productivity while concurrently mitigating the utilization of land, water, and energy resources (Wei et al., 2016). Biotechnological methods have been effective in various fields and are intended to close such gaps. Several responses by microbes to heavy metal stress

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are being developed, which include movement through the outer layer of biosorption to the inside of cells and capture in external capsule-like structures, moisture, compounding, and redox reaction steps. It showed that they can remove contaminants from water-based solutions, particularly when the amount of metal in the wastewater is between one and twenty milligrams per liter (Shamim, 2016). Furthermore, biological metal cleaning approaches provide the versatility to manage a wide variety of physicochemical characteristics in wastewater, selectivity to eliminate just the necessary metals, and cost-effectiveness. These considerations led to a substantial study into biological metal removal techniques. The current study analyzes some of the variety of bacteria, algae, and plants, as well as their wastes, as biosorbents for the clearance of heavy metals via water; these biological bio-sorbents are very successful and dependable in the transfer of heavy metal ions from water (Table 12.1).

12.1.1 Background Numerous cultures have used hydroponics as a form of cultivation since ancient times. Plants on terraces were previously watered using water from the Euphrates River in Babylon (605–562 BC). In Tenochtitlan, the Aztecs employed a system known as “chinampas” by the year 40 AD. These “chinampas” are artificial islands that float above the water and have roots that are in direct contact with the water. Chinampas generate 40,000 t yr-1 of crops such as flowers and vegetables at the moment (Arano., 2007), and FAO classified them as a Globally Important agricultural sector Historic System due to their cultural significance (FAO, 2016). In 1993, Furukawa and Fujita released the first study on the use of hydroponics as a replacement technique for extracting fertilizers from sewage. To remediate manmade sewage generated with glucose, ammonium sulphate, and phosphoric acid, water spinach (Ipomoea aquatica) was grown on a substrate of porous concrete. Tomatoes were watered with processed effluent. 6.72 g of nitrogen and 1.86 g of phosphorus were removed from the water spinach (Boyden & Rababah, 1996) removed 77% of the Phosphorus and 80% of the Nitrogen from home wastewater before growing lettuce (Lactuca sativa) and received crops free of growth deficits.

12.1.2 Objectives An investigation of the impacts of biosorption extraction of heavy metals through hydroponic solutions is the main goal of this review of the literature. The following main research questions are what this project intends to answer: a. How do the concentration and distribution of heavy metals in hydroponic solutions change as an outcome of biosorption?

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b. What are hydroponic heavy metal removal’s ideal bio sorbents and operational circumstances? c. How does biosorption affect the functioning of the hydroponic system as a whole, including plant growth and nutrient uptake? d. How much heavy metal recovery and recycling is possible through hydroponic biosorption processes?

12.1.3 Scope This study should offer relevant information about the use of absorptive removal methods for heavy metals in hydroponic systems. By clarifying the concepts behind heavy metal biosorption, analyzing the efficiency of various bio sorbents, and calculating the total influence on hydroponic system efficiency, the study will add to the body of knowledge. The research’s findings will ultimately support the creation of long-term plans for heavy metal control in hydroponic cultivation, encouraging safer cultivation of crops and boosting the preservation of the environment.

12.2 The Exposure of Hydroponic Systems to Heavy Metals The buildup of transit heavy metals in different natural environments makes it easier for them to enter the food chain, which is dangerous for both humans and the natural world. The persistence of metallic elements and metalloids in the environment, as well as their toxicity, accessibility, and absorption potential, make them significant risk factors for the health of people, animals, and plants. Heavy metals are classified as either necessary or non-essential based on their toxicity. • Essential heavy metals are reasonably or completely safe at low amounts (Zn, Cu, Fe, and Co). • Non-essential metals are harmful even in small amounts (Cd, Pb, Hg, As, and Cr). Although required heavy metals fall within the category of micronutrients, large soil concentrations of these substances typically have harmful effects on plants (Sumalan et al., 2023). Therefore, it is crucial to create strategies for removing these pollutants from contaminated environments to maintain the ecological balance of our planet (Hassan et al., 2010). Several procedures, including biochemical processes such as chlorine treatment, electro-kinetic sorption, photochemical exchange, bioleaching, and heat-induced bioremediation activity, have been established over time to mitigate the impacts of heavy metal contamination on habitats. Recent studies have indicated that the best outcomes come from using several treatment strategies, such as mechanical exclusion, restricted movement, poison decrease, and removal. As a result, various approaches have been improved:

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Chemical-biological remediation treats heavy metal-containing wastewater in a costeffective and environmentally responsible manner (Pradhan et al., 2017). Microbialelectro-kinetic remediation uses microbial activity to create usable bioelectricity and biofuel (Logan & Rabaey, 2012). A variety of plants are capable of remediating heavy metal-contaminated soils. Therefore, a recent study suggests that Sperfoliatum, a plant with great biological flexibility, rapid development, and elevated levels of the absorption variables, translocation factors (TF), and removal efficiency (RE) (Nescu et al., 2022), fits into the hyper-accumulator pattern. In addition to all of these, using a hydroponic solution system to expose plants to heavy metals can produce superior outcomes (Sumalan et al., 2020).

12.2.1 Risks and Impacts on Plant Growth Regarding soil health and changes in the environment, soil farmers also encounter similar kinds of variances. For instance, irrespective of the growth technique utilized, poor water quality, shifts in heat, and moisture can stress crops, certainly altering their molecular composition. Studies that have compared the nutritional value of produce developed hydroponically to that produced in soil have had varying degrees of success due to these variations. Some have found no distinction between the two growing techniques, while others have found that soilless systems performed more poorly versus soil-grown systems in the amount of nutrients being examined. As shown by several studies, vegetables grown hydroponically are of higher quality than those grown in the traditional soil-based method. On the other hand, several studies have asserted that it is challenging to pinpoint the precise variations in the characteristics of crops produced in soil or hydroponically (Aires, 2018). However, it appears that all authors concur that hydroponics might be the most effective substitute if agricultural land is scarce or their varieties tend to be suitable for the targeted plant. The overall consensus among researchers appears to be that hydroponic growth can increase the number of bioactive chemicals, despite the existence of various conflicting perspectives. Recent research has demonstrated that the high bioactive component accumulation in hydroponic systems allows for enhanced nutritional quality in various highvalue fresh crops. When comparing hydroponic tomato production to that depending on the soil (Premuzic et al., 1998), discovered an increase in antioxidants, macro- and micronutrients, and other nutrients. Because this type of cultivation enables improved preservation of the picture caliber, control of turning brown and better efficiency in reducing contamination by microbes than lettuce grown in soil (Selma et al. 2012), found that the antioxidant chemicals in hydroponic systems were higher and that they were more effective at preventing microbial damage.

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12.2.2 Health and Environmental Concerns Although it may seem difficult, hydroponic plant growth has several positive impacts. Among the most prominent are:

12.2.2.1

Prolonged Season for Growth

A lengthy season for growth Plant development is impossible in cold locations with short days and icy winter days. But with a hydroponics system, the planter influences the temperature, light, and nutrient levels, allowing plants to be produced via hydroponics throughout the year. • INCREASED PRODUCTION AND GROWTH: Plants grown in hydroponic systems often grow more quickly and produce more. This is probably because of the elevated oxygen levels in the nutritional solution and the well-regulated ambient conditions. Increased oxygen levels in a plant promote root development and improve nutrient absorption. These ideal growth circumstances result in less plant stress and a more abundant crop. • MORE PLANTS PER UNIT AREA: To ensure that every plant has similar access to the soil’s relatively constrained supply of water and nutrients, certain spacing rules must be maintained while growing plants in soil. Plants may be grown closer to each other in hydroponic systems without struggling for root space because these systems offer a more nutrient-rich solution to the root zone. 12.2.2.2

Plants May Flourish Everywhere

Regardless of size or location, hydroponics systems may be readily integrated into many houses, unlike conventional farms that require outdoor space for plants.

12.2.2.3

Consuming Less Water

Even though plants produced in hydroponic systems rely largely on water, they consume 80–90% less water than plants grown in the ground. In conventional gardening, the soil is frequently watered to ensure that the root zone receives enough moisture (Devis, 2003). Only a small portion of the water that is traveling through the soil reaches the roots because it evaporates as it does so. In hydroponics, the water enters the roots right away, and evaporation losses are minimal. Water efficiency is increased further in many systems by the fertilizer solution being cycled repeatedly before it is rendered useless and thrown away.

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Reduced Pest Concern

Most growers grow plants in hydroponic systems around waist height since they are often grown on countertops, benches, tables, etc. Harvesting them is simpler since you don’t have to stoop or kneel to reach mature plants at this height. For growers who are physically unable of gardening on the ground due to poor mobility or other physical conditions, this is a significant benefit. The advantages of hydroponics are clear to perceive. But there are some clear drawbacks. As with anything, it’s critical to comprehend the negative aspects to prevent unpleasant surprises.

12.2.2.5

Costly to Establish

A hydroponics system costs more to purchase and construct than a regular garden. The kind, size, and prefabrication (or use of separate components to build a specific design) of the system that is acquired will all affect the price.

12.2.2.6

Adaptable to Shortages

The many parts of passive and active hydroponics systems, including grow lights, water pumps, aerators, fans, etc., are all powered by electricity. Consequently, the entire system will be impacted by a power loss. If a power outage in an active system is not observed by the grower, it might be harmful to the plants.

12.2.2.7

Demands Regular Oversight and Care

Compared to conventional plant cultivation, hydroponics calls for much more constant monitoring and micromanagement. All system components—lights, temperature, and various nutrient solution characteristics, including pH and electrical conductivity—need ongoing monitoring to provide a precisely regulated growth environment. To avoid accumulation and clogging, the system components must also be cleaned often, and the nutritional solution must be flushed and changed regularly.

12.2.2.8

Water-Related Illnesses

Since hydroponically grown plants are cultivated in water rather than soil, the prevalence of waterborne infections is much higher. Infections can swiftly spread across the growing system as a whole, impacting the whole collection of plants due to the system’s continual water circulation. In severe circumstances, a waterborne illness can quickly eliminate all the plants in a hydroponics system (Debnath et al., 2019).

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Plants Experience Complications Faster

The soil shields the roots from sudden temperature fluctuations, deters disease and insect invasion, and releases and absorbs nutrients regularly. Plants produced in hydroponic systems respond poorly to issues like nutrient deficits and illness much more quickly since there is no soil to serve as a buffer.

12.3 Bio-Sorption as a Heavy Metal Removal Technique The term “bio-sorption” refers to a biological material’s capacity to absorb heavy metals from wastewater via metabolically regulated or physicochemical absorption routes. An economically and physically practical alternative is provided by adsorption, it refers to the ability of specific bacterial polymers to attach to and remove heavy metals from additionally highly diluted water solutions (Petersen et al., 2005). The term “bio-sorption” refers to a biomolecule (or kind of biomass) capacity to bind particular ions or additional substances from water-based solutions (Volesky, 2007). Algal blooms, microbes, molds, and yeast cells have all been shown to be significant metal bio-sorbents (Shamim, 2016). It is said to be the best alternative technology for eliminating contaminants from effluents.

12.3.1 Process of Bio-Sorption Phytoremediation or bioaccumulation techniques have recently been recognized as unique, affordable, effective, and environmentally acceptable treatment strategies. Numerous experts have looked at the mechanisms of contaminant absorption by plants. It might be utilized to increase performance by optimizing the factors. Plant absorbents believe that the plants perform the roles of both “accumulators” and “excluders”. Although they accumulate toxins within their aerial tissue, the accumulation persists. In their tissues, the pollutants are biodegraded or transformed into inert forms. The excluders inhibit the uptake of pollutants into their tissue. Alternative, cost-efficient solutions are required because of the drawbacks of traditional metal removal techniques. Bio-sorption is described as a simple, physiologically inactive geochemical process that involves the attachment of metal ions (bio-sorbent) to the exterior of a biologically derived bio-sorbent material. Microbes made from plant substances, agricultural or commercial waste, biopolymers, and other biodegradable materials are examples of biological removal. It is a reversed, quick procedure that includes attaching ions to the functional units on the outside of the biosorbent substance in solutions of water through multiple reactions instead of burning through either anaerobic or aerobic mechanisms (Castro et al., 2004). An attribute of notliving, vacant microbes that absorb and accumulate heavy metals from the most diluted systems of water. The concentration of heavy metals, including Cadmium,

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Zinc, Cobalt, Manganese, Nickel, and lead, in metal-accumulating plant species can be a hundred to a thousand times higher than in non-accumulator species. The majority of the time, microorganisms in the plant-related environment known as the root system can transport metal ions and improve the proportion of permeable material. Even more important than their involvement in removing inorganic compounds is their role in removing organic pollutants (Erdei et al., 2005). It is a complicated process that is influenced by parameters such as cell biology and thermodynamic variables such as pH, temperature, interaction duration, ionic force, metallic quantity, metallic ion chemistry, and microbe cell wall composition (Joo et al., 2010). The term “bio-sorption” has just recently been replaced by the term “bioaccumulation” to describe bio-sorption based on cellular metabolism (Bilal et al., 2018). Precipitation and transport across the cell membrane Metal removal processes are classified as external cumulation or rainfall, cell exterior sorption or precipitation, and intrinsic gathering. The processes are classified in two ways: (a) their reliance on cellular consumption or (b) the area inside the cell whereby the metallic component is eliminated. The methods of adsorption or biological accumulation that depend on the metabolic processes of cells are absorbed and transferred across the membrane of a cell. Regarding the second criterion Extracellular deposition or dispersion is caused by movement via the cell membrane itself, while outside the cell, ion exchange, compounding, attraction to matter, and deposition are how cell dissociation or suspension is conducted. and inside the cell, it is achieved by dispersion. Simple diffusion is another fundamental mechanism seen in the majority of biosorption forms (Sao, 2014). Other elements that depend on the environmental circumstances in which biosorption occurs include pH, temperature, and the complexity of the substance, including the metal ion. In certain scenarios, mechanical adsorption, electrostatic exchange, complexity, and deposition are all involved when uranium is absorbed by an aloe vera plant substance. In an extensive procedure, some processes may occur. Accumulation of toxic substances like Cd, Ag, Pb, Ni, and others by microbes like mold, algal blooms, or bacteria (Shamim, 2016). Physiological adherence takes place on the bio sorbent’s outside, possibly with cellular surfaces when engaging with bacteria, based on electrostatic reactions, such as the strengths produced by Van der Waals connections (Chojnack, 2006). According to research, which was based on calculations of the highest absorption capacity as well as an understanding of the metallic element’s ionic diameter, A mechanical method called adsorption contained in a monolayer structure was used to eliminate Cr (III) ions via the straw of wheat and grass. Additionally, it was recently observed that the bacteria Zoogloea armiger and the algae Chiarella vulgaris can physically adsorb the copper ions. The method of ion exchange involves replacing an ion that is highly interchangeable with a different ion in the solution itself and being bound into a solid state (Gadd, 2009) that Ganoderma lucidum biosorbents As a result of the carbohydrates in their cell walls, which can interact by exchanging contrary ions, bacteria can transfer copper ions using this method (Perpetuo et al., 2011). According (Ramya et al., 2018), the cyanobacterium spirulina is capable of absorbing metals including chromium (III), cadmium (II), and copper (II) by exchange of ions.

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Fig. 12.1 Categorization of bio-sorption method

Precipitation is one of the few reactions related to metabolism-dependent biosorption, and it includes the production of impermeable material in a type of solution; however, precipitation may also occur in metabolism-independent biosorption. Precipitation occurred in metabolism-dependent biosorption as a result of the vital defense mechanism of the microorganisms in the presence of hazardous metal ion surroundings (Perpetuo et al., 2011). Precipitation results via chemical interactions with the functional units of the biosorbent cell wall and the metal ion in metabolismindependent biosorption; these reactions can involve processes such as oxidation and reduction (Ramya et al., 2018). Studies have shown that Cd, Ni, Co, and Zn tend to decrease under slightly alkaline pH conditions, where the metal charges and hydrogen ions fight against each other for biosorbent binding locations. Passing throughout the plasma membrane: A mechanism that is sometimes only observed in microorganisms is the transfer of metal ions across the membranes of cells. This process consists of two phases: the first phase, known as independent binding metabolism, involves the metal ion attaching to active sites on the microorganism’s outer membrane. The second phase, known as absorption into cells, which is influenced by oxidation, involves the metal ion being transferred within the cell by crossing the outer layer of the cell (Fig. 12.1). Although it has been noted that heavy metal ions trick cellular metal transport systems by having the same ionic radius and charge as vital metal ions, the procedure is still being worked out (Lopushniak et al., 2022).

12.3.2 Selection of Bio-Sorbent Materials The selection of the most effective biomass and bio-sorbent kinds from an extensive variety of easily available and reasonably priced biomaterials was the first significant obstacle for the absorption area. Although numerous substances with biological origins may attract heavy metals, few polymers with sufficient metal-binding capability or affinity for heavy metals are appropriate for use in complete biosorption

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procedures (Wang & Chen, 2009). It is vitally important to take the bio-sorption capacity into account when selecting a bio-sorbent for the elimination of a particular metal ion. It is advised to utilize bio-sorbents with higher bio-sorption abilities since they may be able to bio-sorb a greater amount of sorbate. Numerous evaluations investigate the use of biosorbents to cleanse wastewater as well as water that contains heavy metals (Shamim, 2016).

12.3.3 Various Forms of Biomass Agricultural items including paddy straw, outer shell of coconut, dried coffee extract, dead leaves, fur, by products of dehulling, and plant based material are just a few examples of the many varieties of biomass that have been explored for their ability to bind metal under different circumstances (Shamim, 2016). Bacteria, fungi, algae, yeast, and peat moss are among the microorganisms found in sewage sludge. Wastes from industries including Saccharomyces cerevisiae waste biomass from the microbial agitation and meal industries, and other sugars materials, etc. (Romera et al., 2006). As native biomass, the majority of these bio-sorbents fit into the following groups. The natural environments of bacteria, fungi, yeast, and algae are important suppliers of bio-sorbents (Wang & Chen, 2009). As a result of their ability to capture metals, such biosorbents can lower the number of heavy metal ions in the fluid between parts per million and parts per billion levels. Because it may occur swiftly and efficiently remove dissolved metal ions from diluted complex solutions, It is an excellent option for the processing of challenging sewage with significant amounts and low concentrations (Shamim, 2016). Table 12.2 provides a few instances of native material types that have been utilized to create microbe bio-sorbents.

12.3.4 Waste Materials from Agriculture The consumption of agricultural waste and byproducts as bio-sorbents has garnered a lot of attention in the effort to remove contaminants from wastewater. Agricultural residues, particularly those abundant in cellulose and lignin, encompass polar functional moieties such as ether, hydroxyl, carbonyl, amino, and carbonyl groups, exhibiting significant metal-binding capabilities (Hossain et al., 2012). By contributing a single pair of electrons, these molecules can form interactions with metal ions in solution (Demirbas, 2008). Agro-waste’s unique chemical composition and accessibility make it seem like a viable choice for the removal of heavy metals. According to reports, grapefruit peel can bio-sorbent Cd and nickel from aqueous solutions at rates of 42.09 and 46.13 mg/g, separately. Cu (II), Cd (II), and Pb (II) were removed from the aqueous solution using the bark powder of Acacia leucocephala, which has a bio-sorption capacity of 147.1, 167.7, and 185.2 mg/g,

Scope and extent of the research

(15 days) in the lab

In lab

Researcher

D. Liu et al. (2000)

C. Watson et al. (2003)

Hydroponic

Hydroponic

Media and absorption processes (substrate) Indian mustard (Brassica juncea) terrestrial

Copper and Salix viminalis clones (basket Nickel willow) S. triandra (Black Maul) S. burrata S. dasyclados, S. candida and S. spaceship terrestrial

Pb as Pb(NO3 )2

Pollutant or Plant species variable and quantity

Table 12.2 Water-based phytoremediation research (hydroponic)

(continued)

In addition to producing greater biomass in the greenhouse and field, The wood from the more tolerable clones contained more metals overall. Less resistant copies produced fewer seeds indoors and in the field and had epidermis with greater concentrations of copper and Nickel. The reactions of similar clones grown in the short-term hydroponic systems in the climate-controlled environment and outside showed significant parallels

Brassica juncea’s root systems, cotyledons, and shoots are all inhibited from growing when exposed to lead nitrate at a concentration of 103 M Pb2+ . Brassica juncea may transport and collect lead in its cotyledons and branches, although at much lower concentrations than it can in its vegetative parts, which are where it mostly acquires lead

Results

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Field study: contaminated soil. Medium of laboratory scale experiment: L 0.1% Hoagland solution

Laboratory and Field study: wetland-pond system (Laboratory scale: 3 days cultivating and 84 h exposure)

C. Watson et al. (2003)

Media and absorption processes (substrate)

Hydroponic

Scope and extent of the research

M.A. Rahman Laboratory et al. (2008)

Researcher

Table 12.2 (continued)

Field study: Zn, Cu, Cd, and Pb Laboratory: ZnCl2, CuCl2, CdCl2, and Pb(NO3 )2 (mixture of 20 µmol. Zn, 0.5 µmol. Cu, 1.5 µmol. Cd, and 1.5 µmol. Pb/L)

Arsenate As(V) and dimethyl arsinic acid (DMAA)

Potamogetons Natan, Lemna gibba, Alisma plantago-aquatica, Sagittaria sagittifolia, Juncus effusus, Lemna minor, Elodea canadensis Michx, Lythrum salicaria, Phalaris Arundinacea, Filipendula ulmaria (aquatic) Impatiens parviflora, Urtica dioica (terrestrial)

Duckweed (Spirodela polyrhiza) aquatic

Pollutant or Plant species variable and quantity

Aquatic plants appear to have a greater capability for metal buildup in branches than plants that grow on land. This could be related to freshwater plants’ ability to gather by sprouting out of the water’s surface. Covered and floating plants can serve as an efficient sieve in wastewater treatment if they are actively growing and gathering metals straight from the water. Overall, emerging vegetation influences the interaction of these heavy metals in the substrate. Also, because terrestrial plants may bind Cd and Zn to their roots, they can help stabilize these elements in the soil

A large quantity of arsenic is absorbed by S. polyrhiza both internally and externally, mainly arsenate contributing the most. Arsenic has been picked up by S. polyrhiza via the phosphate uptake mechanism as well as hydrophilic deposition on Fe patches on the outermost layer of the plant. The quantities of Fe ions and phosphate in the culture medium affect the plant’s absorption of arsenate but not DMAA

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separately. The process bio-sorption method involves biophysical adsorption using carboxyl, hydroxyl, and amine compounds that are immediate on the surface of the bio-sorbent.

12.3.5 Organism-Based Bio-Sorbents The capacity of microbes to survive in harsh conditions led to their utilization as bio sorbents for removing metal ions from effluent. They include of fungi, yeast, algae, and bacteria. Options for the kind of remediation to carry out are provided by experiments focusing on the utilization of dead and/or live microorganisms (Hlihor et al., 2014) However, due to the lack of nutritional requirements and the ability to monitor BOD and COD in effluents, The consumption of decaying microbes rather than living matter for metal-ion interaction has been adopted. As a result, it is affordable to use dead biomass (Rezaei, 2013). It has been widely studied how to use materials of microbial origin as bio-sorbents to remove metal ions (Tsezos et al., 2014). There are no reports of using pathogen biomass for water treatment. Many different functional groups make up the majority of microbial groups, indicating their potential as bio-sorbents.

12.3.5.1

Algae as a Bio-Sorbent

Algae are regarded as one of the most exciting forms of bio-sorbents that are being researched and developed as novel bio-sorbent materials since they have an excellent ability for sorption. and are abundant in marine environments (Shamim, 2016). Algae have minimal food needs, create vast biomass, and are both neither autotrophic, nor additional biomass and microorganisms. They do not often create harmful chemicals. Macro algae, commonly referred to as Multicellular plants known as seaweeds, can flourish in both fresh and saline water (Abdi & Kazemi, 2015). Moreover, Algae have a high capacity for bio-sorption because their cell walls are made of chitin, polysaccharides, proteins, and lipids, all of which include essential functional groups that facilitate bio-sorption (Castro et al., 2004). The bio-sorption of heavy metals by algae was shown to be primarily mediated by functional groups comprising oxygen, nitrogen, sulfur, and phosphorus (Asao, 2012). Brown algae, which are classified as macro-algae, contain cell walls that are mostly made of cellulose, acidic monomers (such as mannuronic and guluronic acids) that form complexes with light metals (such as sodium, potassium, and calcium), and polysaccharides (Ignatius et al., 2014).

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Bacteria as a Bio-Sorbents

For the bio-sorption of metals, bacteria efficiently supply amino, carboxyl, phosphate, and sulfate groups because they have polysaccharide slime layers. The majority of the time, Microbial biomass is created as leftovers of manufacturing processes or can be grown on a large scale purposely. The typical range of bacterial uptake rates is 0.23 to 0.90 milli-mole/grams. Microbes are utilized as bio-sorbents due to their tiny size, prevalence, capacity to enlarge in managed settings, and tolerance to a broad variety of different environmental factors (Shamim, 2016). The mechanism of heavy metal adsorption to the surface of bacterial cell walls typically involves two steps. The interaction of metal ions with reactive species on the cell surface occurs in the first step, and the accumulation of subsequent metal species in greater quantities occurs in the next stage. According to (Ferrarezi and Testezlaf, 2016) functional groups including oxygen, nitrogen, sulfur, or phosphorus are frequently responsible for heavy metal biosorption in bacteria.

12.3.5.3

Fungi as a Bio-Sorbent

Fungi are also regarded as affordable and environmentally acceptable bio sorbents due to their distinctive traits, such as their ease of growth, high biomass output, and simplicity of chemical and genetic alteration. Because various fungal species differ in terms of chitin, lipids, polyphosphates, and proteins, the cell walls of fungi exhibit exceptional binding characteristics. Polysaccharides and glycoproteins, which include different metal-binding substances such as amines, phosphates, carboxyls, and hydroxyls, are abundant in the cell walls of fungi. In a wide range of methods of fermentation, fungi are utilized. Several different types of ionizable sites, including phosphate groups, carboxyl groups on uronic acids and proteins, and nitrogen-containing compounds on peptides as well as on chitin, have an impact on the metal absorption capacity in the wall of fungal cells (Shamim, 2016). Greater concentrations of heavy metals can accumulate in yeasts through the bioaccumulation process than through the biosorption method. Moreover, broad biosorption is the primary route of heavy metal uptake for different fibrous fungi (Fereidouni et al., 2009). As an outcome, they’re simple to produce on a commercial basis for the adsorption of metals from an extensive amount of contaminated water bodies. Fungi are also less susceptible to changes in nutrition and other processes such as pH, temperature, and ventilation. Due to their similar existence in fibrous form, they are easy to separate using procedures such as filtering (Ramya et al., 2018).

12.3.5.4

Plant as a Bio-Sorbent

Plants were previously utilized as bio sorbents in the agriculture industry and as waste from agriculture products. Utilizing materials from plants doesn’t come at a large expense because it is a way to reuse and recycle those discarded products.

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According to (Abdi & Kazemi, 2015), the cellular matrix and/or elements linked to cellulose, such as lignin and hemicellulose, include carboxylic and phenolic groups with functions, which have the potential to be used as plant bio sorbents. According to (Baddadi et al., 2019), the carboxyl, carbonyl, and hydroxyl groups in aloe vera waste increased the binding of metals when they were utilized as bio sorbents to remove uranium and cadmium from groundwater.

12.4 Factors That Impact Bio-Sorption Capacity Bio-sorption is affected by a variety of variables (Fig. 12.2). Some of these variables are associated with biomass and metal, while others are associated with environmental circumstances. The following are the primary elements that influence the bio-sorption process:

Fig. 12.2 Factors influencing heavy metal absorption methods

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12.4.1 pH Biosorption is a process that is analogous to ion exchange; hence, biomass may be thought of as a naturally occurring ion-exchange material as it mostly contains basic and mildly acidic compounds. Thus, the pH of a solution impacts the kind of active groups for metals in biomass and their absorption; it further impacts the chemistry of metal solutions, the activity of functional groups in biomass, and the competitiveness between metallic ions. In practically every system studied, including bacteria, cyanobacteria, algae, and fungi, metal biosorption has repeatedly been demonstrated to be substantially pH-related. According to (Garousi et al., 2016), pH impacts the chemistry of metal ions and the chemistry of the functional groups of bio sorbents. When the pH is raised past the point where the high bio-sorption ability is noticed, the bio-sorption capacity frequently increases due to the generation of metal OHcomplexes, which cause metals to separate (Bilal et al., 2018). As the pH of the metal-containing concentrations is lowered from 6.0 to 2.5, the absorption of heavy metals typically decreases substantially for the majority of biomass types. There is little to no elimination of metal ions from liquids with pH levels below 2. From pH 3.0 to pH 5.0, there is an increase in metal absorption. The ideal pH level is crucial to achieving the greatest level of metal sorption, and as pH levels rise higher, this ability is going to decrease (Shamim, 2016).

12.4.2 Temperature As compared to the absorption process, bio-sorption efficiency remains unaffected within the temperature range of 20–35 °C; however, high temperatures, such as 50 °C, may increase bio-sorption in some cases, but these high temperatures may cause permanent damage to microbial living cells, resulting in decreased metal uptake (Ahalya et al., 2003a, 2003b; Goyal et al., 2003). Adsorption processes are usually exothermic; lowering the temperature increases the amount of adsorption. The parameters of the process will determine how temperature affects the biosorption process. For an exothermic biosorption process, raising the temperature would lead to a reduction in the removal of metal ions; however, this is not the case for endothermic adsorption processes (Tsezos et al., 2014). Similar to how raising temperature would improve metal ion removal in an exothermic biosorption process, the opposite is true for endothermic biosorption activities.

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12.4.3 Biomass Concentration The specific absorption is influenced by the biomass concentration in the solution. Low cell densities at an exact balance concentration allow the biomass to adsorb more metal ions than high densities. Therefore, the interplay of electrostatic charges between cells is crucial for metal absorption. Because an increase in biosorbent material content disrupts the binding sites, the specific absorption of metals is higher at lower biomass concentrations. Metal ions’ access to the active groups is impeded by high biomass content (Shamim, 2016). The selective absorption of metal ions has been found to increase at low biomass dosages (Sao, 2014; Perpetuo et al., 2011). However, utilizing a low biomass dose in complicated, polluted water would increase competition for the biosorbent’s binding site and reduce its absorption ability. If multiple metal ions exist, raising the biomass dose might reduce the competition amongst the metal ions for attaching to the binding sites. The majority of studies that have been conducted employed between 0.5 and 6.0 g/L of biomass.

12.4.4 Initially Present Metal Ions To overcome any metal transmission barrier among the aquatic and crystalline stages, the starting amount is a key driving factor (Ahmad et al., 2004). The most effective results come with lower early metallic amounts and the elimination of metals %. Due to this, the metal intake increases depending on the beginning concentrations at a certain cellulose percentage. It has been found that the metal absorption rises as the starting amount of metal increases during research examining the prospective use of a fruit peel as a biosorbent material for heavy metals (Bashir et al., 2019). The impact of raising the initial metal concentration is comparable to the lengthening of contact. Low metal concentrations might not reveal the biosorbents’ authentic maximum absorption efficiency since it’s possible that a variety of metal attachment points are unfilled. The entire capability of metal removal would be revealed by raising the quantity of metals to the point of complete overexposure with the biosorbent attachment points. Further raising the initial metal concentration will be ineffective when the biosorbent material is saturated. In the majority of reported studies, the starting amount of metals employed in experimental circumstances ranged from– 200 mg/.

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12.4.5 Additional Elements That Influence the Uptake Process As demonstrated in Figure various variables can influence heavy metal uptake mechanisms. Plant uptake performance may be considerably enhanced by understanding these parameters.

12.4.5.1

Plant Species

Species or kinds of plants are tested, and those with the best remediation qualities are chosen, plant species traits have an impact on a compound’s absorption. The selection of suitable varieties of plants that hyper abundance heavy metals and generate significant amounts of using plant matter proven crop production and control techniques are necessary for phyto extraction, which technology is to be successful (Tangahu et al., 2011).

12.4.5.2

Medium Specifications

Agronomic methods (pH correction, inclusion of chelators, fertilizers) are being developed to promote restoration. For instance, organic matter, pH, and the addition of reversible mechanical elements as vitamins and minerals and antioxidants all have an impact on how much lead plants absorb (Tangahu et al., 2011).

12.4.5.3

Include a Chelating Agent

Increased accessibility of heavy metals via the use of renewable physical and chemical components, such as chelating compounds and micronutrients as well as greater capacity of the community of microbes surrounding and within the growing medium, can both affect the absorption of heavy metals by energy-producing crops. Due to the rapid absorption of heavy metals, remediation times will be shorter and hence cheaper. The possibility of higher permeability must be considered, nevertheless, when using synthetic chelating agents (Van Ginneken et al., 2007). Chelating chemicals may encourage the leaching of heavy metal pollutants into heavy metalscontaminated soils. A chelating agent should be used in alkaline soils since heavy metal bioavailability in soils reduces at pH 5.5–6, making it desirable and maybe necessary. It was discovered that exposing seedlings to EDTA for a longer time might enhance both the overall efficiency of plant extraction and metal diffusion in the tissue of plants.

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12.5 Zone of Influence Phytoremediation in the Root Zone is of significant relevance. Pollutants may be consumed and either stored or processed inside plant parenchyma. Another vegetation enhanced method involves the disintegration of pollutants in the environment by plant chemicals released from the root system (Cruz et al., 2004). A growth in base diameter and a reduction in the elongation of roots as a reaction to diminished mobility of the dehydrated soil is a physiological response to drought stress.

12.6 Vegetative Absorption Environmental variables influence vegetative absorption. Climate influences growth substances and, as a result, root length. The root structure in nature differs from that in the protected environment. The efficiency of phytoremediation, especially phyto extraction, is dependent on the presence of a contaminant-specific hyper accumulator. Interpreting the equilibrium of mass assessments and the chemical breakdown of contaminants in plants is critical to demonstrating phytoremediation’s effectiveness. Metal absorption by plants is determined by the metal’s accessibility in its liquid form, which is influenced by the metal’s retention period as well as its association with other compounds and elements in the water’s composition (Volesky, 2007).

12.7 Standard Methods for Heavy Metals Elimination Heavy metals including Nickel, copper, Zinc, Cadmium, Chromium, Lead and Mercury are important contaminants that harm freshwater reserves owing to the outflow of enormous numbers of metal-polluted effluent through businesses (Dostalek et al. 2004). They gather in their surroundings due to their constant, nonrecyclable, and toxic behavior., including the food chain, causing major health problems Over the past few decades, numerous conventional treatment process have been introduced to address the issue of removing heavy metals from contaminated wastewater. Chemical precipitation, ultrafiltration, ion exchange, reverse osmosis, electrowinning, and phytoremediation are some of the regularly utilized technologies, and they are discussed briefly (Ramya et al., 2018).

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12.7.1 Industrial Precipitation This is the most often used technology for eliminating heavy metals from effluent factories. Metal hydroxides, sulfides, carbonates, and phosphates are formed when absorbed metal ions get precipitated by chemical substances.

12.7.2 Exchange of Ion Ion exchange depends on the reversible transfer of electrons among the liquid and solid states. An ion converter is a type of hard elastomer that may release counterions with a comparable charge in a chemically equal amount that is chemically equal while converting both cations and anions from an electrochemical solution.

12.7.3 Membrane Filtration Can filter out organic and suspended solids in addition to metal ions. A membrane is a discriminating layer with an absorbent or non-absorbent structure that is used to create contact between two homogeneous phases for the removal of contaminants of various sizes.

12.7.4 Ultrafiltration This form of purification method uses an ultra-light barrier to distinguish solvents from particle debris. The purification of copper (II), Zn (II), nickel (II), and magnesium (II) via aqueous solutions was accomplished through ultrafiltration aided by a combination of malic acid and acrylic acid, with a rate of removal of 98.8% obtained by developing macromolecular arrangements with the polymer compounds declined by the membrane’s surface (Farzan et al., 2013).

12.7.5 Reverse Osmosis This is a pressure-driven separation via membrane technique used to remove heavy metals from different industrial processes by forcing the solution to flow across a semipermeable membrane. Utilizing the polyester thin-film hybrid barrier TW301812–50, reverse osmosis has been used to remove copper (II), Nickel (II), and Zinc (II) (Hassan et al., 2009).

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Along with these traditional procedures, the removal of heavy metals through polluted water resources has also been accomplished using electrocoagulation, flocculation, electro-floatation, and electro-deposition. But all of these methods have many problems, such as inadequate metal removal, sludge production, high reagent and energy needs, metal deposit accumulation, and corrosion of membranes (Ramya et al., 2018).

12.8 Biosorbents for Heavy Metal Removal in Hydroponics The selection of the most viable biomass and biosorbent varieties among an extensive selection of readily available and cost-effective biomaterials was the first important hurdle for biosorption research. Even though numerous substances with biological origins may attract pollutants, few bio-compatibles with sufficient metal-binding ability and specificity for toxic metals are suitable for usage in their whole biosorption processes (Wang & Chen, 2009). Heavy metal removal from hydroponic solutions by biosorption means has demonstrated significant promise for a range of uses. One of the principal uses is in the cleanup of polluted hydroponic systems, where biosorption may be utilized to remove heavy metals from the nutrient solution, limiting their reception by plants and limiting their buildup in consumable plant portions (Ledrich et al., 2005). Before releasing the wastewater into the environment, heavy metals can be removed via biosorption in the treatment of industrial effluent from hydroponic systems, such as those used in greenhouses and aquaponics systems. Additionally, biosorption may be utilized in conjunction with environmentally friendly farming techniques like phytoremediation, which employs both plants and microbes to get rid of heavy metals from soil and hydroponic solutions. Table 12.2 provides a summary of some remediation studies done to remove them from polluted water and wastewater (Tangahu et al., 2011).

12.9 Different Kinds of Hydroponic Circulation Mechanisms There are two types of hydroponic systems: (a) open ones, in which the solution of nutrients is fed straight to the plants and utilized just once, and (b) closed networks, in which the nutritional source is regenerated.

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12.9.1 Systems that Are Visible In open systems, the roots of plants have intermittent or constant contact with the nutrient solution, which is not recycled or recirculated. In this setup, both the nutrient solution and the media substrate are used only once. One benefit of open systems is the ease with which the fertilizer solution is applied and as well as the decreased chance of plant infection (Jones, 2005).

12.9.2 Close Mechanisms In an enclosed structure, the plant roots are treated with a nutrient solution, which is then regularly collected and applied to the crop again. Either in a fluid medium or on a surface made of solid material, plants can grow. These substrates can be either organic or inorganic (like sand, gravel, pumice, or pulverized brick). The most significant drawback is reliance on electricity, even though both nutrients and water are handled optimally (Farzan et al., 2013).

12.9.3 Various Uncontaminated and Hydroponic System Types Deep-water culture, nutrient film technology, wick or passive structures, and media bed-based hydroponic systems are a few examples of commonly utilized varieties.

12.9.4 Wick Over an Indirect System According to (Ferrarezi & Testezlaf, 2016), the wick, or indirect process, is a low-cost method that does not require the reuse of the solution of nutrients. Instead, plants soak up the solution through an apparatus that makes use of the roots’ capillary capacity and the fibers that carry water to the plants. This technique, which can be utilized with either open or closed circulation, is mostly used in small-scale cultivation and is not advised for crops with a long lifespan (Lee & Lee, 2015).

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12.9.5 Advanced Aqua Industry The majority of the current systems were developed using the deep-water culturing technique. It comprises a platform on which the crops are grown and aeration and pumping system (Hoagland & Arnon, 1950). The roots can be immersed in the nutritious solution thanks to constant aeration. To maximize plant growth, it is necessary to regulate oxygen quantity, conductivity, and pH (Jones, 2005). Open or enclosed circulation can be used with this kind of system.

12.9.6 The Nutrient Film Method The Nutrient Film Technique (NFT), established by Allan Cooper in 1960 (Torres et al., 2020), is among the most prevalent methods in closed hydroponic systems. A thin film of the nutritional solution is applied to the bases with this method, which additionally opens up the oxygen that is needed (Morgan, 1999). Initially, the vegetation had been raised on mineral wool in translucent acrylic containers. The NFT systems have recently been upgraded; and several different supporting mediums are utilized for the growth of plants (Ghorbani et al., 2008). Open or closed circulation can be employed with this type of system; however, closed circulation seems to be more frequent.

12.9.7 Using the Media Mattress The substrate is kept in a tank or other container for this system. The crops are periodically removed after being drenched with the fertilizer solution (Göksungur et al., 2005). This system needs to be constantly monitored because a malfunction might quickly lead to the plants being dehydrated. The rapid proliferation of bacterial or fungal diseases on the plant roots poses a disadvantage (Torres et al., 2020).

12.9.8 Hydroponic Media Substrates Substrates come in a variety of varieties for hydroponic systems. A media substrate’s ability to hold water (WHC), air-filled permeability (AFP), and cation exchange capacity (CEC) are its most crucial properties. According to (Göksungur et al., 2005), the WHC/AFP is dependent on both the interstitial spaces created within the granule or fiber and the gaps created between them. The CEC measures a substrate’s ability to bind or release replaceable cations that are necessary for plant growth (Maher et al., 2008). The hydroponic system type and the nutritional, water, and air requirements

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of the plants will determine the substrate choice. Necessities about air, moisture, and nourishment (Awad et al., 2017) used a nutrient film approach in a hydroponic setup to contrast the efficiency of biochar produced from rice husks separately (RB) or in conjunction with perlite (PL) as substrates for enhancing the development of leafy greens with that of perlite. Their findings indicated that the combination of PL and RB as a hydroponic growth medium resulted in a twofold increase in the yield of leafy vegetables compared to plants cultivated solely on PL substrate. For instance, both coconut fiber and coconut peat possess a high WHC of 65.49% and AFP of 25%, ensuring adequate moisture in the medium and facilitating the absorption of nutrient solutions by plants. By minimizing water damage, they also promote effective gas exchange at the roots (Mahjoor et al., 2016).

12.10 Approaches for Hydroponic Systems and Plants’ nutritional Needs The elements that are required for the survival and evolution of plants are Carbon, Oxygen, and Hydrogen, which are gained via air and water, and different elements including nitrogen, phosphorous, potassium, calcium, magnesium, chlorine, iron, zinc, etc., which are taken by the environment. N, P, K, and Ca are commonly found in soil fertilizers since they are traditionally seen as being crucial to plant growth (FAO, 2019). Additionally, several of these essential elements might not be available as a result of poor management of farming soils (i.e., adjustments to soil composition, a single culture, and destruction of the forest) (Torres et al., 2020). Investigators established a comprehensive nutrient solution in the 1970s that clearly showed the nutritional benefits of soilless cultures and the proper development of roots. These investigations allowed for the identification of the factors that plants require to flourish (Asao, 2012). Years later (Mengel & Kirkby, 2001), proposed categorizing the essential nutrients into four classes to further comprehend their biological functions and metabolic processes. (1) The primary elements found in the organic matter of crops are carbon, hydrogen, oxygen, and nitrogen (Gabr et al., 2008). Additionally, phosphorus, boron, and silicon are absorbed through the substrate and contribute to the formation of phosphates, borates, and silica esters. Potassium, sodium, calcium, magnesium, manganese, and chlorine are absorbed as ions from the soil. Furthermore, iron, copper, zinc, and molybdenum are essential for enzyme activity and electron transfers within cells, and they are absorbed from the soil in the form of ions or chelates. According to (Asao, 2012), Hoagland and Arnon, Hewitt, Cooper, and Steine nutrient solutions have gained widespread popularity in the field of hydroponics. These solutions are changed based on the climate and form of the liquid. In hydroponic systems, salinity is a component that needs to be regularly managed because it has a significant impact on crop development. To find empirical connections among Na+ /water and Cl/water absorption ratios and the

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amounts of Na+ and Cl in the root region of melons grown in continuously hydroponic farms (Neocleous & Savvas, 2016), performed a study. This model can assist in creating a tool for managing macronutrients as well as management techniques for greenhouse melon crops in regions with low-quality water from irrigation systems. It is crucial to understand the mechanisms underlying the procedures by which nutrients are acquired and distributed throughout the various tissues. The management of NO3 or the improvements of crop quality by controlling the electric conductivity of the solution are two well-studied and tried research topics whose findings are currently widely used in soilless farming (Sambo et al., 2019). However, there are additional factors that are more challenging to control, such as nutrient fluctuations (bio-geochemical cycles) in the hydroponic solution and interactions between nutrients during their extraction procedures.

12.11 Application of Hydroponic Systems Hydroponic systems have been utilized to investigate the nutrient requirements of various crops. For example, studies have been conducted on tomatoes, chrysanthemums, rice, wheat, potatoes, tobacco, spinach, lettuce, and gerbera using hydroponics to assess their specific nutrient needs (Jaishankar et al., 2014). These experiments aimed to determine the optimal nutrient concentrations in the solution to avoid nutrient deficiencies or excesses that could cause stress. In addition to nutrient studies, physical factors such as pH, humidity, and conductivity have been examined. Furthermore, the toxic effects of heavy metals, including Cd, Cu, Ni, As, Fe, and Se, have been assessed in hydroponic systems. These studies have evaluated the absorption of these substances by plants and the efficacy of phytoremediation in agricultural products, both for ornamental and therapeutic purposes (Rahman et al., 2008). One study focused on assessing the hemofiltration of Pb using Mexican mint in nutrient film technique (NFT) hydroponic systems. The findings revealed that plants can take up Pb through their roots without significant translocation to their stems or leaves. Another study by Bedabati and Gupta in 2016 investigated the ability of water spinach to remove lead from water. They discovered that lead was stored not only in the roots but also in the stems and leaves of the plant, with the roots accumulating the highest concentration. These findings suggested that surface adsorption in the roots, involving processes such as chelation, ion exchange, and selective absorption, facilitated faster lead removal. The overall elimination process involves biological factors such as internal intake, vascular deposition, and transport to the shoots (Kumar et al., 2017; Prasad et al., 2003).

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12.12 Investigations on Energy Efficiency and Financial Sustainability Water consumption is reduced by 90%, and the area needed for crops is reduced by at least 75% when using simplified hydroponic technology (Bradley & Marulanda, 2001). Since 1984, programs financed by the FAO have researched this technology in Colombia and introduced it in 13 other nations. Since agricultural nutrients are retained and regenerated, no excess ions are discharged into the environment. In most cases, neither insecticides nor fertilizers are necessary. According to statistics from Colombia, a 40-square-meter garden with crops for business can generate $101.00 in profit per month. Alternative greenhouse cultivation methods can be more energy-efficient than hydroponics. In a study conducted by (Baddadi et al., 2019), it was discovered that hydroponic greenhouses with innovative designs offer improved energy conservation compared to traditional greenhouses. These new designs incorporate two-packed beds of latent storage energy, which effectively enhance the internal environment of the greenhouse in comparison to conventional solar heating, especially during colder periods and overnight. By implementing these advancements, energy requirements can be significantly reduced. In research conducted by Stan Hill in 1980, it was observed that heated greenhouses in the UK and Germany consume considerably more power per unit yield compared to hoop houses and openfield agriculture in warmer regions like Palestine and the Golden Bear State. These discoveries align with a current study based on life cycle assessment (LCA), which examined tomato greenhouse cultivation in Austria and revealed a carbon dioxide emission of 1.37 kg CO2 per kilogram of tomatoes, whereas unheated greenhouses in Spain and Italy showed lower emissions in comparison (Kanamarlapudi et al., 2018). The emissions for the imported products from warmer regions were half as high (0.69 kg CO2 kg-1 lycopersicum), which was similar to the Austrian production’s results. According to Stanhill 1980, In the UK and Germany, thermal conservatories demand a lot more power per unit yield than hoop houses and open-field field agriculture in warmer regions (like Palestine and the Golden Bear State). Similar results were found in the latest LCA-based study that compared tomato greenhouse cultivation in Austria (1.39 kg CO2 kg-1 lycopersicum) to material received from desert plants in Italy and Europe. The emissions for the imported products from warmer regions were half as high (0.69 kg CO2 kg-1 lycopersicum), which was similar to the Austrian production’s results (Pearce et al., 1999). The expenses, as well as the sustainability of various hydroponic substrates and the environmental benefits of various hydroponic bases, were compared in an investigation that looked at rock wool, perlite, vermiculite, peat, coconut fibers, bark, and sand. To examine the effects on the environment and the economy, a life cycle evaluation (life cycle inventory, life cycle impact assessment (LCIA), and life cycle costing (LCC)) was used. According to LCIA, perlite is the substrate that has the greatest influence. Afterward, vermiculite and stone wool are used. Investigated the financial sustainability of hydroponic systems in Brazil, where the hydroponic growing of green and sizzling crops shows good economic viability and is a tempting alternative to food

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production for farmers in the area (Battarbee et al., 1988). The farmer must exercise extreme caution before beginning hydroponics expenditures, nevertheless, as these systems demand a large initial outlay. To decide whether to proceed with the project or not, the farmer must also be aware of the number of units that must be produced as well as the expected profit. Farmers should find out whether there are any specialists in the area where they plan to conduct their work (Torres et al., 2020).

12.13 Conclusion To summarize, biological remediation is widely regarded as a developing organic, cost-effective, and environmentally acceptable solution to the complex hydrophilic approaches now used for cleaning up polluted environments. The act of adsorption, interaction of ions, combination, the process of precipitation, and transport between cells are among the phenomena that can occur during the procedure of biosorption. The absorbing ability of an adsorbent can be impacted by environmental parameters such as pH, climate, duration of contact, material dose, starting metal quantity, and others. It has been discovered that biosorption is the most cost-effective and environmentally benign way of removing heavy metals from home and industrial wastewater. It is being used as an alternative to traditional ways of removing harmful heavy metals from industrial effluents. It has various advantages, including economic effectiveness, high efficiency, chemical or biological sludge reduction, and bio-sorbent regeneration with metal recovery. The results of this study should offer relevant information about the use of absorptive removal methods for heavy metals in hydroponic systems. By clarifying the concepts behind heavy metal biosorption, analyzing the efficiency of various bio sorbents, and calculating the total influence on hydroponic system efficiency, the study will add to the body of knowledge. Acknowledgements I extend my heartfelt gratitude to my mentor, Dr. Tabassum, for her unwavering guidance and support throughout this publication. I would also like to express my profound thanks to the dedicated academics and scientists whose scholarly contributions were consulted and referenced in this publication, greatly enriching our understanding of biosorption and hydroponics. Special acknowledgment goes to MJP Rohilkhand University, Bareilly, for providing coursework facilities with plagiarism detection resources and to the Department of Zoology at Govt. Raza College, Rampur, for their essential infrastructure. These institutions have played a pivotal role in shaping our comprehension of this critical subject matter.

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Chapter 13

Hydroponics Phytoremediation: An Overview Prasann Kumar and Shipa Rani Dey

Abstract Hydroponics has emerged as a significant method for phytoremediation, providing an innovative approach to tackle environmental pollution. This paper explores the application of hydroponics in phytoremediation, focusing on its advantages and potential as a sustainable and efficient remediation technique. Through a systematic review of relevant literature, the paper highlights the fundamental principles of hydroponics and its ability to promote plant growth while enhancing the uptake and detoxification of contaminants from polluted environments. Moreover, the abstract discusses various hydroponic systems and their adaptability to diverse environmental conditions. Additionally, the role of hydroponics in improving the overall efficacy of phytoremediation processes is analysed, emphasizing its ability to reduce the time and space required for plant growth and pollutant removal. The paper also addresses potential challenges and limitations associated with hydroponics-based phytoremediation and proposes future research directions to optimize the method further. Overall, this abstract sheds light on the promising potential of hydroponics as a significant method for phytoremediation, providing a green and sustainable approach to combat environmental pollution. Keywords Agriculture · Phytoremediation · Hydroponics · Environment · Contaminants · No Poverty · Zero Hunger

13.1 Introduction Increasing environmental pollution has become a pressing global concern in recent decades, posing significant threats to ecosystems, human health, and biodiversity (Ajeng et al., 2022; Elango et al., 2022; Rane et al., 2023). Traditional remediation methods, such as chemical treatments and physical excavation, have proven P. Kumar (B) · S. R. Dey Department of Agronomy, School of Agriculture, Lovely Professional University, Phagwara, Punjab 144411, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_13

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costly, energy-intensive, and often ecologically disruptive. In light of these challenges, phytoremediation has emerged as an environmentally friendly and sustainable approach to mitigate the harmful effects of pollutants in contaminated soils and waters. Hydroponics has garnered increasing attention as a significant, effective, and efficient pollutant removal method among various phytoremediation techniques. Hydroponics is a soilless cultivation technique that allows plants to grow in nutrientrich water solutions, offering a controlled environment for root development and nutrient absorption (Chen et al., 2023; Kalni¸nš et al., 2022; Mohsin et al., 2022; A. K. Pandey et al., 2023; S. Wu et al., 2023). By harnessing the natural abilities of plants to uptake, accumulate, and metabolise various contaminants, hydroponics provides an innovative and eco-friendly alternative to conventional remediation methods. Integrating hydroponics into phytoremediation strategies enhances the overall efficiency of pollutant removal. It enables targeted treatment of specific contaminants, tailoring the approach to suit the unique characteristics of each polluted site. This comprehensive introduction explores the applications of hydroponics as a significant method for phytoremediation, delving into its underlying principles, advantages, and potential challenges. By examining the current state of research and real-world case studies, we highlight hydroponics’ versatility and promise in tackling diverse environmental pollutants (Coughlan et al., 2022; Kaur & Kaushal, 2022; D. Sun et al., 2023; Z. Sun et al., 2022; Thomas et al., 2022; Ye et al., 2022; Zhu et al., 2023). Furthermore, we will discuss the various hydroponic systems and their adaptability to different environmental conditions, allowing for the optimisation of phytoremediation efforts across varying geographies and contamination scenarios. The integration of hydroponics into phytoremediation not only facilitates the detoxification and removal of contaminants but also offers a multitude of other benefits. Hydroponic systems allow for precise control of nutrient supply, pH levels, and water availability, optimising plant growth and pollutant uptake. Additionally, growing plants in a controlled environment significantly reduces the time required for the remediation process compared to traditional in-situ phytoremediation, where plant growth is subject to unpredictable soil conditions. However, despite the considerable promise of hydroponics-based phytoremediation, some challenges must be addressed to realise its full potential. Issues such as plant selection, pollutant specificity, and scaling up from laboratory experiments to field applications demand careful consideration. Additionally, the economic feasibility and implementation of hydroponic systems on a larger scale require further investigation. This paper aims to provide a comprehensive overview of hydroponics as a significant method for phytoremediation, consolidating current knowledge and research findings to foster a deeper understanding of its capabilities and limitations. By shedding light on this innovative and sustainable remediation technique, we hope to inspire further research and practical applications that contribute to preserving and restoring contaminated environments worldwide. The ultimate goal is to advance the use of hydroponics as a powerful tool in the ongoing battle against environmental pollution, promoting a greener and healthier future for generations to come (Coimbra et al., 2022; Dai et al., 2022; Karalija et al., 2022; Muthukumaran, 2022; Niu et al., 2023; Oladoye et al., 2022; J. Wu et al., 2023; You et al., 2022).

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13.2 Research Gap in Hydroponics While the potential of hydroponics as a significant method for phytoremediation is increasingly recognised, there remains a notable research gap concerning this approach’s long-term sustainability and ecological impacts. Although hydroponics offers a controlled environment for efficient pollutant removal, there needs to be more comprehensive studies that investigate the long-term effects of sustained hydroponic cultivation on plant health, growth, and pollutant tolerance. Understanding plants’ extended physiological responses and adaptability under continuous hydroponic conditions is crucial for assessing the feasibility and practicality of this remediation technique on a larger scale. Furthermore, most existing research focuses on the uptake and removal of individual contaminants or a limited range of pollutants in hydroponic systems. However, real-world contaminated environments often contain complex mixtures of contaminants, which can interact with each other and affect plant responses differently. Exploring the synergistic or antagonistic effects of multi-contaminant scenarios on hydroponic phytoremediation efficiency is essential for developing targeted and robust strategies to address multiple pollutants simultaneously effectively. Moreover, while hydroponics-based phytoremediation has demonstrated promise in laboratory and controlled settings, there is a need for more fieldscale studies to validate its effectiveness under diverse environmental conditions (Y. Liu et al., 2023; Mazumdar & Das, 2022; Mishra et al., 2022; Nkrumah & van der Ent, 2023; Sageena et al., 2022; Sahito et al., 2022; Savio et al., 2023). Assessing the performance of hydroponic systems in real contaminated sites will provide valuable insights into the challenges and opportunities of implementing this technique in practical applications. Factors such as scalability, system maintenance, and integration into existing environmental management practices require thorough investigation to assess the long-term viability of hydroponics for large-scale remediation projects. Additionally, economic feasibility remains a critical consideration when evaluating the applicability of hydroponics for phytoremediation. The costs of setting up and operating hydroponic systems, including the necessary infrastructure, energy, and nutrient inputs, must be thoroughly assessed and compared to alternative remediation methods (Colzi et al., 2023; Hansda et al., 2022; Jiao et al., 2022; J. Liu et al., 2023; Mohsin et al., 2023; Peng et al., 2023; Vidya et al., 2022; Yadav et al., 2022). Furthermore, considering the potential revenue generation from the harvested biomass and its subsequent utilisation, economic analyses will facilitate informed decision-making and foster the adoption of hydroponics as a cost-effective remediation strategy. Addressing these research gaps will contribute to advancing the knowledge and understanding of hydroponics-based phytoremediation and pave the way for its practical implementation in real-world scenarios. By exploring the long-term sustainability, multi-contaminant interactions, field-scale applicability, and economic viability of hydroponics, future research can strengthen the evidence base for the environmental benefits and feasibility of this innovative and sustainable approach to combat environmental pollution.

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13.3 Hydroponics and Its Types and Use 13.3.1 Introduction to Hydroponics Hydroponics is a modern and innovative method of growing plants without using traditional soil. Instead, it relies on a nutrient-rich water solution to provide essential minerals and elements for plant growth. This soilless cultivation technique allows for precise control over environmental factors, leading to more efficient nutrient uptake and plant growth. Hydroponics has gained significant popularity in agriculture, horticulture, and phytoremediation due to its numerous advantages over conventional soil-based cultivation methods (Aghili & Golzary, 2023; Fernandes & Ravi, 2023; Jha et al., 2023; Majumdar et al., 2022; V. C. Pandey et al., 2022; Rabani et al., 2022; Riaz et al., 2022; P. Sharma et al., 2023).

13.3.2 Types of Hydroponics a. Nutrient Film Technique (NFT): In the NFT system, a thin film of nutrient solution flows over the roots of the plants through a sloped channel. The roots absorb the required nutrients directly from this film while the excess solution is collected and recirculated. NFT is particularly suitable for small plants with shallow root systems. b. Deep Water Culture (DWC): DWC involves suspending plant roots in a nutrient solution and submerging them continuously. Oxygen is supplied through air stones to prevent root rot. DWC is commonly used for growing leafy greens and herbs. c. Ebb and Flow (Flood and Drain): In this system, the plant roots are periodically flooded with the nutrient solution and drained to allow oxygenation. This cyclic process simulates natural irrigation patterns and is well-suited for various plant types. d. Drip System: The drip system delivers nutrient solution directly to the base of each plant through a network of tubes and emitters. This method offers precise control over nutrient delivery, making it suitable for larger plants and fruiting crops. e. Aeroponics: Aeroponics involves suspending plant roots in the air and misting them with a nutrient solution. This method maximises oxygen exposure to the roots and promotes rapid growth. It is highly efficient but requires precise control and maintenance. f. Wick System: In the wick system, a wick transports the nutrient solution from a reservoir to the plant roots. It is a simple and low-cost hydroponic method, ideal for smaller plants and educational purposes (Farooqi et al., 2022; P. Kumar, Goud, Devi, & Koul, 2022; P. Kumar, Goud, Devi, Dey, et al., 2022; P. Kumar, Koul, et al., 2022; V. Kumar et al., 2022; Mattiello et al., 2023; Srivastav et al., 2023; Takkar et al., 2022).

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13.3.3 Uses of Hydroponics a. Agriculture and Horticulture: Hydroponics has revolutionised modern agriculture by offering higher crop yields, faster growth rates, and more efficient water usage. It allows for year-round cultivation regardless of weather conditions and provides an opportunity to grow crops in areas with limited arable land. Hydroponics is especially valuable in urban farming and rooftop gardens, where space constraints are challenging. b. Indoor Gardening: Hydroponics is widely used for indoor and urban farming. It enables individuals to grow fresh produce in their homes, apartments, or offices, making it a popular choice for those interested in sustainable and organic food production. c. Phytoremediation: Hydroponics has proven to be a significant method for phytoremediation, particularly in removing heavy metals and organic pollutants from contaminated soils and water bodies. The controlled environment allows for better monitoring and management of pollutant uptake, making it an eco-friendly and efficient remediation technique. d. Research and Education: Hydroponics serves as an excellent educational tool for teaching plant biology, nutrient cycling, and environmental science. Its simplicity and visual appeal make it a valuable resource for schools, universities, and research institutions. Hydroponics is a versatile and innovative cultivation method that offers numerous advantages in agriculture, horticulture, phytoremediation, and education. With its various types of systems, hydroponics can be adapted to meet the specific needs of different plant species and environmental conditions, making it an increasingly popular choice for sustainable and efficient plant growth worldwide (Kotia et al., 2021; D. Kumar et al., 2019; P. Kumar et al., 2019; P. Kumar, Pathak, Amarnath, et al., 2018; P. Kumar & Dwivedi, 2020; P. Kumar & Naik, 2020; P. Kumar & Pathak, 2019; Naik & Kumar, 2020; Siddique, Dubey, et al., 2018; Siddique & Kumar, 2018).

13.4 Nutrient Film Technique (NFT) in Hydroponics: A Detailed Overview The Nutrient Film Technique (NFT) is one of the most widely used and popular methods in hydroponics for growing plants. Developed in the 1960s by Dr Allen Cooper, NFT is characterised by a continuous, shallow flow of nutrient-rich water over the plant roots. This thin film of nutrient solution, typically about 1–3 mm deep, is directed through sloping channels, ensuring the sources have constant access to the necessary nutrients while allowing for ample oxygenation. NFT is particularly suitable for growing small plants with shallow root systems, making it ideal for cultivating leafy greens, herbs, and certain fruiting crops (Kandpal et al., 2018; P. Kumar, Harshavardhan, Kumar, Yumnam, et al., 2018; P. Kumar, Krishna, Pandey,

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Pathak, et al., 2018; P. Kumar, Kumar, Harshavardhan, Naik, et al., 2018; P. Kumar, Pandey, Krishna, Pathak, et al., 2018; P. Kumar, Yumnam, Kumar, Misao, et al., 2018; P. Kumar & Dwivedi, 2018).

13.4.1 Components of an NFT System Channel System: The core element of an NFT system is the channel or trough where the plants are grown. These channels are usually made of durable materials like PVC or plastic, and they have a slight slope to facilitate the flow of the nutrient solution. Pump: A submersible pump circulates the nutrient solution through the channels. The pump draws the answer from a reservoir and pumps it to the highest end of the media, where it then flows down the slope, bathing the roots of the plants. Reservoir: The nutrient solution is stored in a reservoir below the channels. The pool holds a sufficient volume of nutrient solution to ensure a continuous flow to the plants. Return System: Once the nutrient solution reaches the end of the channels, it collects in a trough or gutter before being returned to the reservoir. This return system prevents the key from flowing out of the media and recirculates it efficiently. Growing Medium: Although NFT is a soilless technique, a growing medium is often used to support the plants and anchor their roots. Standard growing media in NFT systems include Rockwool, peat moss, or perlite. The growing medium provides stability and prevents the roots from blocking the nutrient flow.

13.4.2 How NFT Works In an NFT system, the nutrient solution is continuously pumped from the reservoir and allowed to flow along the bottom of the sloping channels. The plant roots, inserted through holes in the top of the media, extend into the thin film of nutrient solution, absorbing the necessary nutrients directly from the flowing solution. As the nutrient solution moves along the channel, it bathes the roots and provides them with water and nutrients. The seeds take up these nutrients, and any excess solution and dissolved oxygen drain back into the return system and then to the reservoir. This continuous circulation ensures a constant supply of fresh nutrients and oxygen to the roots, promoting optimal plant growth and health (Dwivedi & Kumar, 2011; P. Kumar et al., 2011a, 2011b, 2013, 2017; Pankaj et al., 2012a, 2012b; Pathak et al., 2017; P. C. Sharma & Kumar, 1999).

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13.4.3 Advantages of NFT Water Efficiency: NFT is highly water-efficient since the nutrient solution is continuously recirculated, minimizing water wastage compared to conventional soil-based agriculture. Nutrient Control: NFT allows precise control over the solution, ensuring that plants receive the necessary nutrients at the appropriate concentrations. Rapid Growth: The continuous supply of nutrients and oxygen to the roots promotes rapid plant growth and higher yields in a relatively short period. Space Optimization: NFT is well-suited for vertical farming or limited space environments, as the plants are grown in channels, utilising vertical space efficiently. No Soil-borne Diseases: As the plants are grown in a soilless medium, NFT significantly reduces the risk of soil-borne diseases, providing a cleaner and more hygienic cultivation environment.

13.4.4 Challenges and Considerations Root Clogging: Careful attention must be paid to prevent root clogging, especially in the channels and return system. Regular maintenance and monitoring are essential to ensure the proper functioning of the NFT system. Temperature Control: The temperature of the nutrient solution should be monitored and maintained within an optimal range to prevent root stress and potential issues with nutrient uptake. pH and Nutrient Imbalance: Monitoring the solution’s pH levels and nutrient concentrations is crucial to avoid imbalances that can lead to plant nutrient deficiencies or toxicities. Electricity Dependency: NFT systems require electricity to operate the pump continuously, making them vulnerable to power outages or disruptions. The Nutrient Film Technique (NFT) is a highly efficient and widely used hydroponic system for growing various plants. With its continuous flow of nutrient-rich water over the roots and precise nutrient control, NFT provides a favourable environment for rapid plant growth and optimal nutrient uptake. When correctly managed, NFT offers water efficiency, space optimisation, and the potential for high crop yields. However, careful attention must be given to regular maintenance, pH control, and nutrient balance to ensure the sustained success of NFT-based hydroponic cultivation (Das et al., 2022; Hidangmayum et al., 2022; P. Kumar, Sharma et al., 2021; P. Kumar & Mistri, 2020; V. Kumar et al., 2021; Paul et al., 2005; Reddy et al., 2022; Siddique, Kandpal, et al., 2018).

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13.5 Deep Water Culture (DWC) in Hydroponics: A Detailed Overview Deep Water Culture (DWC) is a popular hydroponic system that involves suspending plant roots in a nutrient-rich solution, allowing them to grow directly in the water. In this soilless cultivation method, plants are supported by floating platforms or rafts, and their roots are continuously submerged in the nutrient solution (Aley et al., 2022; Chakraborty et al., 2021; Goud et al., 2022; P. Kumar et al., 2020; P. Kumar, Devi, et al., 2021; P. Kumar, Pathak, Kumar, et al., 2018; Kumari et al., 2022; Upadhyay et al., 2023). DWC is widely used for growing leafy greens, herbs, and certain fruiting crops due to its simplicity, efficiency, and ability to promote rapid plant growth.

13.5.1 Components of a DWC System Reservoir: The heart of the DWC system is the reservoir, which holds the nutrient solution. The reservoir should be large enough to provide a sufficient solution to immerse the plant roots entirely. Air Pump and Air Stones: To ensure adequate oxygenation of the roots, an air pump is used to create bubbles in the nutrient solution. Air stones or diffusers are attached to the air pump, releasing tiny bubbles that oxygenate the water and prevent root rot. Growing Platforms: The plants are placed on floating platforms or rafts, which keep them afloat on the surface of the nutrient solution. The media may be made of materials like Styrofoam or plastic, providing buoyancy and plant support. Net Pots: Each plant is inserted into a net pot and placed in holes on the floating platform. The net pots hold the growing medium and anchor the plants, allowing their roots to extend into the nutrient solution below. Growing Medium: While DWC is a hydroponic technique, a growing medium is used in net pots to support and stabilise the plants’ roots. Standard growing media for DWC include hydroton (expanded clay pellets), Rockwool, or perlite.

13.5.2 How DWC Works In a DWC system, the nutrient solution is oxygenated by the air pump and air stones, ensuring the roots have access to a constant oxygen supply. The plants’ roots grow through the net pots and into the nutrient solution, directly absorbing the required nutrients and water. The floating platforms or rafts keep the plants afloat, allowing the roots to dangle freely in the nutrient solution. The oxygen-rich environment encourages rapid root growth and nutrient uptake, promoting vigorous plant growth. As the plants grow,

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their roots extend further into the nutrient solution, continuously absorbing the necessary elements for healthy development (P. Kumar et al., 2020; P. Kumar, Pathak, Kumar, et al., 2018; P. Kumar & Mistri, 2020; Kumari et al., 2022; Reddy et al., 2022).

13.5.3 Advantages of DWC Simplicity: DWC is straightforward to set up and maintain, making it an excellent choice for beginners or those new to hydroponics. Fast Growth: The constant supply of oxygen and nutrients promotes rapid plant growth and higher yields than traditional soil-based methods. Water Efficiency: DWC is water-efficient since it uses a recirculating system and minimizes water wastage. No Soil-borne Diseases: DWC is a soilless method; it significantly reduces the risk of soil-borne diseases, providing a cleaner and more hygienic cultivation environment. Easy Nutrient Management: DWC allows for easy monitoring and adjustment of the nutrient solution, ensuring that plants receive the required elements in proportions.

13.5.4 Challenges and Considerations Root Health: Proper oxygenation prevents root rot and ensures healthy root growth. Maintaining adequate oxygen levels can lead to plant stress and reduced yields. Temperature Control: The temperature of the nutrient solution should be carefully monitored and maintained within the optimal range to support optimal plant growth. pH Balance: Regular monitoring of the pH levels in the nutrient solution is necessary to prevent nutrient deficiencies or toxicities. Maintenance: Regular cleaning and maintenance of the system are crucial to prevent clogs and ensure the continuous flow of the nutrient solution. Deep Water Culture (DWC) is a highly effective and straightforward hydroponic system for growing various plants. Its continuous oxygenation and direct nutrient uptake by the roots promote rapid and healthy plant growth. DWC is a popular choice for both beginners and experienced hydroponic enthusiasts due to its ease of setup, water efficiency, and ability to produce high yields. However, proper maintenance, attention to oxygenation, and nutrient balance are essential to ensure the sustained success of a DWC-based hydroponic cultivation system (Kandpal et al., 2018; P. Kumar et al., 2011a, 2011b; P. Kumar, Harshavardhan, Kumar, Yumnam, et al., 2018;

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P. Kumar, Krishna, Pandey, Pathak, et al., 2018; P. Kumar, Kumar, Harshavardhan, Naik, et al., 2018; P. Kumar, Yumnam, Kumar, Misao, et al., 2018; Pankaj et al., 2012a, 2012b).

13.6 Ebb and Flow (Flood and Drain) in Hydroponics: A Detailed Overview Ebb and Flow, also known as Flood and Drain, is a popular hydroponic system that utilises periodic flooding and draining of the growing medium to provide nutrients and water to the plants. In this method, the plants are grown in a tray or container filled with a growing medium, and the nutrient solution is intermittently pumped into the tray to flood the roots. After a specific period, the nutrient solution is drained back into a reservoir, allowing the roots to access oxygen. Ebb and Flow are well-suited for various plant types, including vegetables, herbs, and flowering plants (Dwivedi & Kumar, 2011; Hidangmayum et al., 2022; P. Kumar et al., 2011a, 2011b; Paul et al., 2005; P. C. Sharma & Kumar, 1999; Siddique, Kandpal, et al., 2018).

13.6.1 Components of an Ebb and Flow System Growing Tray: The growing tray or container serves as the primary area for plant cultivation. It is usually filled with a growing medium that supports the plants and holds the roots in place. Pump: An electric pump delivers the nutrient solution into the growing tray during the flood cycle. The pump is connected to a timer or control system regulating flooding and draining intervals. Reservoir: The nutrient solution is stored below the growing tray in a reservoir. It holds a sufficient solution to flood the tray and provide the necessary nutrients to the plants. Overflow Pipe or Standpipe: To prevent overflowing the growing tray, an overflow pipe or standpipe is installed to allow excess nutrient solution to return to the reservoir during the flood cycle. Growing Medium: A growing medium, such as gravel, hydroton (expanded clay pellets), Rockwool, or perlite, is placed in the ever-increasing tray to support the plants’ roots and provide stability.

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13.6.2 How Ebb and Flow Works During the flooding phase, the pump is activated, and the nutrient solution is pumped from the reservoir into the growing tray. The ever-increasing medium absorbs the nutrient solution, and the roots of the plants take up the required nutrients. The flooding duration can vary depending on the plant’s growth stage, environmental conditions, and specific requirements. After the flooding phase, the pump is turned off, and the nutrient solution can drain back into the reservoir. The excess solution and air in the growing medium are displaced through the overflow pipe or standpipe back into the pool. The ebb and flow cycle is repeated regularly, which a timer or automated system can control. The intermittent flooding and draining provide the plants’ roots with access to water and oxygen, promoting healthy root development and preventing oxygen deprivation (Aley et al., 2022; Chakraborty et al., 2021; Goud et al., 2022; P. Kumar et al., 2020; P. Kumar, Devi, et al., 2021; P. Kumar, Pathak, Kumar, et al., 2018; Kumari et al., 2022; Upadhyay et al., 2023).

13.6.3 Advantages of Ebb and Flow Improved Oxygenation: The periodic draining of the nutrient solution allows the roots to access oxygen, preventing root suffocation and promoting healthier root systems. Flexible Nutrient Delivery: The flood and drain cycles precisely control nutrient delivery, ensuring the plants receive the right amount of nutrients and water. Reduced Water Usage: Ebb and Flow systems are water-efficient since the nutrient solution is recirculated, minimising water wastage. Adaptability: Ebb and Flow systems can accommodate various plant types, making them suitable for multiple crops and growth stages. Lower Risk of Root Diseases: As the growing medium is periodically exposed to air during the draining phase, the risk of root diseases is reduced compared to continuously submerged root systems.

13.6.4 Challenges and Considerations System Maintenance: Regular cleaning and maintenance are essential to prevent clogs and ensure the system’s proper functioning. Timing and Frequency: The frequency and duration of flooding and draining must be carefully calibrated based on plant requirements and environmental conditions. Overflow Management: Proper overflow system design is crucial to prevent overflowing the growing tray and ensure that excess nutrient solution is returned to the reservoir.

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pH and Nutrient Balance: Regularly monitoring the solution’s pH levels and nutrient concentrations is necessary to avoid imbalances that can lead to plant nutrient deficiencies or toxicities. Ebb and Flow, or Flood and Drain, is a versatile and efficient hydroponic system that allows for precise control over nutrient delivery and improved oxygenation of the roots. Its intermittent flooding and draining cycles give the plants a balanced supply of nutrients, water, and oxygen, promoting healthy growth and higher yields. Although Ebb and Flow systems require regular maintenance and careful timing, they offer a flexible and adaptable solution for various plant types. They are famous for hydroponic growers seeking effective and efficient nutrient management (Aley et al., 2022; Chakraborty et al., 2021; Goud et al., 2022; D. Kumar et al., 2019; P. Kumar, Devi, et al., 2021; P. Kumar, Goud, Devi, Dey, et al., 2022; P. Kumar, Harshavardhan, Kumar, Yumnam, et al., 2018; P. Kumar, Koul, et al., 2022; P. Kumar, Pathak, Amarnath, et al., 2018; P. Kumar, Pathak, Kumar, et al., 2018; P. Kumar et al., 2019, 2020; P. Kumar & Dwivedi, 2018; P. Kumar & Naik, 2020; Kumari et al., 2022; Naik & Kumar, 2020; Siddique, Dubey, et al., 2018; Srivastav et al., 2023; Upadhyay et al., 2023).

13.7 Aeroponics: A Detailed Overview Aeroponics is an advanced and innovative hydroponic technique that involves growing plants in a misted, oxygen-rich environment without a growing medium. In this soilless cultivation method, plant roots are suspended in the air, and a fine mist of nutrient-rich water is sprayed directly onto the roots. Aeroponics delivers nutrients and oxygen directly to the sources, promoting rapid and healthy plant growth. This precise nutrient delivery system has made aeroponics a favoured choice for cultivating various crops, including leafy greens, herbs, vegetables, and even fruiting plants.

13.7.1 Components of an Aeroponics System Misting Chamber: The core element of an aeroponics system is the misting chamber. It houses the plant roots and has a high-pressure misting system that sprays a fine mist of nutrient solution directly onto the roots. Nutrient Reservoir: The nutrient solution is stored in a reservoir connected to the misting system. The pool holds the nutrient mixture and supplies it to the misting chamber. High-Pressure Pump: An electric pump pressurises the nutrient solution, spraying a fine mist onto the roots.

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Timer or Controller: A timer or automated controller turns the misting system on and off at specific intervals to regulate the misting cycles. How Aeroponics Works: In an aeroponics system, the plant roots are suspended in the misting chamber, allowing them to hang freely in the air. The high-pressure pump is activated regularly, creating a fine mist of nutrient solution sprayed onto the roots. This mist provides the sources with direct access to water and nutrients and ensures that the roots are adequately oxygenated. After each misting cycle, any excess nutrient solution can drain away, preventing the roots from waterlogging and promoting oxygenation. This intermittent misting and draining cycle allows the roots to have access to the precise amount of nutrients they need while avoiding the risk of waterlogged conditions (Jiao et al., 2022; J. Liu et al., 2023; Vidya et al., 2022).

13.7.2 Advantages of Aeroponics Rapid Plant Growth: The direct delivery of nutrients and oxygen to the roots promotes rapid plant growth and higher yields than other hydroponic methods. Water Efficiency: Aeroponics is highly water-efficient as it uses a closed-loop recirculating system, minimising water wastage. No Growing Medium: The absence of a growing medium reduces the risk of root diseases and simplifies maintenance. Precise Nutrient Control: Aeroponics allows for precise control over nutrient delivery, ensuring that plants receive the right amount of nutrients they need. Suitable for Various Crops: Aeroponics is adaptable and ideal for various plant types, including herbs, leafy greens, vegetables, and fruiting crops.

13.7.3 Challenges and Considerations System Complexity: Aeroponics systems can be more complex to set up and maintain than other hydroponic methods. Clogging Issues: The misting system may be prone to clogging if not properly maintained, leading to uneven nutrient delivery to the roots. Root Health: Proper oxygenation prevents root rot and ensures healthy root development. High Humidity: Aeroponics systems can create a high-humidity environment, which may require additional ventilation or humidity control measures. Aeroponics is an advanced and highly efficient hydroponic system that offers precise nutrient delivery and oxygenation directly to the plant roots. Its ability to promote rapid plant growth, water efficiency, and adaptability to various crops makes it an attractive choice for modern agriculture and horticulture. Although aeroponics requires careful attention to system maintenance and root health, its increased

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yields and water conservation benefits have made it an increasingly popular method for soilless plant cultivation (Farooqi et al., 2022; V. Kumar et al., 2022; Mattiello et al., 2023; P. Sharma et al., 2023; Takkar et al., 2022).

13.8 The Wick System in Hydroponics: A Detailed Overview The Wick System is one of the most straightforward hydroponic techniques available. It is a passive system that does not require any pumps or electricity. Instead, it relies on capillary action to deliver the nutrient solution to the plant roots. The Wick System is ideal for beginners or those seeking a low-cost and low-maintenance method for growing small plants or starting seeds.

13.8.1 Components of the Wick System Growing Container: The growing container holds the plant and the growing medium. Depending on the size and type of plants being grown, they can be made of various materials, such as plastic, clay, or recycled containers. Growing Medium: A wick system uses a growing medium capable of absorbing and transporting the nutrient solution through capillary action. Common growing mediums include vermiculite, perlite, coconut coir, or a mixture of these materials. Wicks: The wicks are the essential components of this system. They are made of absorbent materials like cotton, felt, or nylon ropes. The wicks extend from the bottom of the growing container into the nutrient solution reservoir. Nutrient Reservoir: The reservoir holds the nutrient solution that will be wicked to the plant roots. It is usually placed below the growing container. Nutrient Solution: The nutrient solution contains the essential minerals and nutrients required for plant growth. It is absorbed by the wicks and transported to the plant roots.

13.8.2 How the Wick System Works In a Wick System, the wicks bridge the nutrient reservoir and the plant roots. The wicks draw the nutrient solution from the reservoir through capillary action, using the absorbent properties of the growing medium to transport it upwards to the origins of the plants. As the plants absorb the nutrient solution, the growing medium becomes drier, creating a gradient that promotes continuous wicking of the key from the reservoir. The capillary action maintains a steady supply of nutrients to the roots, ensuring

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the plants receive the essential elements for their growth and development (Kandpal et al., 2018; P. Kumar, Harshavardhan, Kumar, Yumnam, et al., 2018; P. Kumar, Krishna, Pandey, Pathak, et al., 2018; P. Kumar, Yumnam, Kumar, Misao, et al., 2018; P. Kumar & Dwivedi, 2018; Siddique, Dubey, et al., 2018).

13.8.3 Advantages of the Wick System Simplicity: The Wick System is straightforward to set up and operate, making it ideal for beginners or those new to hydroponics. Low-Cost: The Wick System is one of the most affordable hydroponic techniques since it requires no pumps or electricity. Low-Maintenance: Due to its passive nature, the Wick System requires minimal maintenance compared to more complex hydroponic systems. Suitable for Small Spaces: The simplicity and small size of the Wick System make it suitable for limited-space environments, such as small apartments or classrooms. No Electricity Dependency: Since the Wick System does not require electricity or pumps, it is not vulnerable to power outages or electrical failures.

13.8.4 Challenges and Considerations Limited Scalability: Due to its passive nutrient delivery method, the Wick System may not be suitable for growing large or heavy-feeding plants. Slower Growth: The Wick System may not be as efficient as active systems in promoting rapid plant growth and higher yields. Root Health: Proper wick selection and placement are critical to ensuring the roots receive sufficient nutrient solution without becoming waterlogged or experiencing oxygen deprivation. Nutrient Management: It may be challenging to adjust and fine-tune nutrient levels in the reservoir due to the passive nutrient delivery system. The Wick System is a simple, low-cost, low-maintenance hydroponic technique that utilises capillary action to deliver the nutrient solution to the plant roots. While it may be less efficient and scalable than active hydroponic systems, the Wick System remains an excellent option for beginners or those looking for a basic and straightforward method to grow small plants or start seeds. With proper attention to wick selection and nutrient management, the Wick System can provide a rewarding and satisfying hydroponic growing experience (Dwivedi & Kumar, 2011; P. Kumar et al., 2011a, 2011b, 2017; Reddy et al., 2022).

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13.9 Phytoremediation: A Detailed Overview Phytoremediation is an innovative and eco-friendly environmental remediation technique that utilises plants to remove, degrade, or stabilise pollutants from contaminated soils, water, and air. It harnesses the natural abilities of plants to absorb, accumulate, and metabolise toxic substances, transforming them into less harmful forms or storing them within plant tissues. This sustainable approach to cleaning up polluted environments has gained significant attention in recent years due to its cost-effectiveness, low environmental impact, and ability to restore contaminated sites to their natural state (Chakraborty et al., 2021; Dwivedi & Kumar, 2011; P. Kumar, Sharma, et al., 2021; Kumari et al., 2022; Upadhyay et al., 2023).

13.9.1 How Phytoremediation Works Phytoremediation employs various plant species, known as hyperaccumulators, that can absorb and tolerate high contaminants. These plants uptake pollutants from the soil or water through their roots and then translocate them to the above-ground parts, such as stems, leaves, and sometimes fruits. The contaminants can be subsequently removed by harvesting and properly disposing of the contaminated plant material or through natural degradation within the plant (Hidangmayum et al., 2022; V. Kumar et al., 2021; Paul et al., 2005; Reddy et al., 2022; Siddique, Kandpal, et al., 2018). There are several mechanisms through which phytoremediation can work: Phytoextraction: Hyperaccumulator plants can absorb and accumulate heavy metals and other toxic substances within their tissues. These metals can be harvested and removed from the site, effectively reducing the pollutant concentration in the soil. Phytostabilization: Certain plants can immobilise or stabilise contaminants within the soil, preventing their migration to groundwater or their uptake by other organisms. This method is beneficial for treating metal-contaminated sites. Phytodegradation: Some plants can break down organic pollutants, such as petroleum hydrocarbons and pesticides, through enzymatic processes within their root systems. Phytovolatilization: Certain plants can absorb volatile contaminants from the soil and release them into the atmosphere through transpiration, effectively reducing the concentration of pollutants in the ground.

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13.9.2 Types of Phytoremediation Rhizofiltration: This method involves using plants with extensive root systems to remove contaminants, such as heavy metals, from contaminated water or wastewater. Phytodegradation (Phytochemical Degradation): This approach utilises plants’ enzymes and metabolic processes to break down organic contaminants into less harmful substances. Phytostabilization: Plants with robust root systems are used to immobilise and contain contaminants, preventing migration to other areas. Phytoextraction: This method uses hyperaccumulator plants to absorb and accumulate heavy metals and other pollutants from contaminated soil. Phytovolatilization: Certain plants can absorb volatile organic compounds (VOCs) and release them into the atmosphere through transpiration, effectively removing them from the soil or water.

13.9.3 Advantages of Phytoremediation Sustainability: Phytoremediation is a natural and sustainable approach that does not rely on harsh chemicals or mechanical processes. Cost-Effectiveness: Phytoremediation can be more cost-effective than traditional remediation methods, particularly for large-scale projects. Environmental Benefits: Phytoremediation does not generate additional waste or byproducts; it can improve soil and water quality in the long run. Aesthetic Improvement: Phytoremediation can enhance the aesthetics of contaminated sites by introducing greenery and restoring the natural ecosystem. Community Acceptance: Local communities often receive phytoremediation due to its non-invasive and eco-friendly nature.

13.9.4 Challenges and Limitations Slow Process: Phytoremediation can be slow and unsuitable for sites with urgent remediation needs. Site-Specificity: The success of phytoremediation depends on factors such as the type of contaminant, the plant species used, and environmental conditions, making it site-specific. Limited Range of Contaminants: Phytoremediation may not be effective for all types of pollutants or highly complex mixtures of contaminants. Plant Selection: Choosing appropriate hyperaccumulator plants is crucial, and some contaminants may not have suitable plant species for effective remediation.

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Phytoremediation is a promising and eco-friendly remediation technique that has the potential to transform contaminated sites and restore them to their natural state. Using hyperaccumulator plants and their natural abilities to absorb, translocate, and degrade pollutants, phytoremediation offers a practical, sustainable, and cost-efficient alternative to traditional remediation methods. While it has its limitations and site-specificity, ongoing research and technological advancements continue to expand the applicability and success of phytoremediation in addressing environmental pollution and promoting a cleaner, healthier planet (Goud et al., 2022; P. Kumar, Sharma, et al., 2021; P. Kumar & Mistri, 2020; Kumari et al., 2022). Enzymes play a pivotal role in phytoremediation within plants. They act as catalysts, accelerating chemical reactions that break down pollutants and transform them into less harmful substances. In the context of phytoremediation, plants produce and release various enzymes into their rhizosphere—the soil region around their roots. Enzymes like peroxidases, dehydrogenases, and laccases are involved in degrading organic pollutants, such as hydrocarbons and pesticides. These enzymes facilitate the conversion of toxic compounds into simpler, non-toxic forms that plants or microorganisms can absorb or metabolise further. Furthermore, enzymes assist in the immobilisation and detoxification of heavy metals. Phytochelatins and metallothioneins are examples of metal-binding peptides synthesised by plants in response to metal contamination (Goud et al., 2022; P. Kumar et al., 2020; Kumari et al., 2022). Enzymes help produce these peptides, which bind to heavy metals and reduce their mobility and toxicity. Phytoremediation processes are often enhanced by promoting enzyme activity, including optimising soil conditions, introducing beneficial microorganisms, and utilising natural or engineered plants with higher enzyme production. In summary, enzymes are critical players in phytoremediation, facilitating the transformation and detoxification of pollutants, thus contributing to the purification of contaminated environments and promoting sustainable ecological restoration (Table 13.1).

13.10 Reactive Oxygen Species (ROS) ROS are highly reactive molecules that contain oxygen and are produced as natural byproducts of various metabolic processes within cells. Heavy metal stress can increase ROS production due to the disruption of cellular redox balance and oxidative stress (Hidangmayum et al., 2022; Paul et al., 2005; Reddy et al., 2022). Some of the common ROS produced under heavy metal stress include: Superoxide Radical (O2 ·− ): This is the primary ROS produced in response to heavy metal stress. It is generated in various cellular compartments, including chloroplasts, mitochondria, and peroxisomes. Superoxide radicals are a precursor to other ROS and can cause cellular damage.

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Table 13.1 Enzymes and their role in phytoremediation Enzyme

Function in phytoremediation

Peroxidases

Break down organic pollutants and enhance plant growth

Dehydrogenases

Assist in the degradation of organic contaminants

Laccases

Break down lignin and degrade phenolic compounds

Cytochrome P450

Metabolize organic pollutants

Catalase

Break down hydrogen peroxide and promote stress tolerance

Superoxide Dismutase

Scavenge harmful superoxide radicals

Glutathione S-Transferase

Detoxify organic pollutants and heavy metals

Metallothioneins

Bind and detoxify heavy metals

Phytochelatins

Chelate and sequester heavy metals in vacuoles

Nitrate Reductase

Convert nitrate to nitrite and promote nutrient uptake

Urease

Hydrolyze urea and improve soil pH

Cellulases

Break down cellulose for organic matter degradation

Amylases

Hydrolyze starch and enhance microbial activity

Phosphatases

Release phosphate from organic compounds

Sulfatases

Release sulfate from organic matter

Hydrogen Peroxide (H2 O2 ): Superoxide radicals can be converted into hydrogen peroxide by the enzyme superoxide dismutase (SOD). Hydrogen peroxide is a relatively stable ROS that can diffuse across cellular membranes and contribute to oxidative stress. Hydroxyl Radical (· OH): Hydroxyl radicals are highly reactive and can cause severe damage to cellular biomolecules, including lipids, proteins, and DNA. They are generated through the Fenton and Haber–Weiss reactions involving transition metals like iron and copper. Singlet Oxygen (1 O2 ): Singlet oxygen is produced by interacting excited chlorophyll molecules with oxygen. It can cause damage to cellular components and disrupt photosynthesis. Lipid Peroxides: Heavy metal stress can lead to lipid peroxidation, producing lipid peroxides. These compounds damage cell membranes and contribute to oxidative stress. Peroxynitrite (ONOO− ): Peroxynitrite can form in excess superoxide radicals and nitric oxide (NO). It is a potent oxidizing agent that can cause cellular damage and contribute to oxidative stress. Oxygen Singlet (1 O2 ): Oxygen singlet is produced during photosynthesis and can lead to oxidative damage to cellular components. The excessive accumulation of these ROS can lead to oxidative stress, which damages cellular structures, disrupts cellular functions, and contributes to the toxicity of heavy metal stress. Plant cells employ various antioxidant defence mechanisms,

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including enzymes like superoxide dismutase (SOD), catalase, peroxidases, and nonenzymatic antioxidants such as glutathione and ascorbate, to mitigate the harmful effects of ROS and maintain cellular homeostasis under heavy metal stress. The superoxide radical (O2 ·− ) is a highly reactive and unstable oxygen-derived molecule that plays a significant role in cellular physiology and signalling. It is a reactive oxygen species (ROS) produced as a natural byproduct of various cell metabolic processes, particularly during aerobic respiration and photosynthesis. Despite being potentially harmful due to its reactivity, the superoxide radical also serves as a signalling molecule in cellular communication and defence mechanisms (Kandpal et al., 2018; P. Kumar, Kumar, Harshavardhan, Naik, et al., 2018; P. Kumar, Pandey, Krishna, Pathak, et al., 2018; P. Kumar, Yumnam, Kumar, Misao, et al., 2018; Pathak et al., 2017).

13.11 Key Characteristics of the Superoxide Radical (O2 ·− ) Include Formation: Superoxide radicals are generated when molecular oxygen (O2 ) accepts an extra electron, forming a single unpaired electron. This unpaired electron makes the superoxide radical highly reactive and capable of initiating chemical reactions. Cellular Sources: Superoxide radicals are produced within various cellular compartments, including mitochondria, chloroplasts, peroxisomes, and plasma membranes. They are often byproducts of electron transport chains and enzymatic reactions involving oxygen (Kotia et al., 2021; P. Kumar, Goud, Devi, & Koul, 2022; P. Kumar, Goud, Devi, Dey, et al., 2022; P. Kumar, Koul, et al., 2022). Role in Oxidative Stress: While cells have intrinsic defence mechanisms to neutralize ROS, an excessive accumulation of superoxide radicals can lead to oxidative stress. Oxidative stress occurs when the balance between ROS production and antioxidant defences is disrupted, resulting in cellular damage to lipids, proteins, and DNA. Cellular Signalling: Despite their potential harm, superoxide radicals function as signalling molecules in cellular processes. They participate in redox signalling pathways, influencing gene expression, cell growth, differentiation, and responses to environmental stresses. Defence Mechanisms: Cells possess antioxidant enzymes like superoxide dismutase (SOD) that help neutralise superoxide radicals. SOD catalyses the conversion of superoxide radicals into hydrogen peroxide (H2 O2 ), further detoxified by other antioxidant enzymes like catalase and peroxidases. Role in Immune Response: Superoxide radicals are produced by immune cells, such as neutrophils and macrophages, as a defence mechanism against pathogens. These radicals help destroy invading microorganisms.

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Contribution to Aging and Disease: Excessive accumulation of superoxide radicals and other ROS is associated with ageing and developing various diseases, including neurodegenerative disorders, cardiovascular diseases, and cancer. The superoxide radical (O2 ·− ) is a double-edged sword in cellular biology. While it can cause damage to cellular components when present in excess, it also plays a vital role in cellular signalling, defence mechanisms, and immune responses. Balancing the production of superoxide radicals and their detoxification is essential for maintaining cellular health and overall well-being. Hydrogen peroxide (H2 O2 ) is a significant player in plants’ intricate web of responses when confronted with heavy metal stress. As a reactive oxygen species (ROS), hydrogen peroxide is a byproduct and a potent signalling molecule in cellular processes. Its role during heavy metal stress is multifaceted and pivotal in determining the outcome of plant responses to metal toxicity (Chen et al., 2023; Kalni¸nš et al., 2022; S. Wu et al., 2023; Ye et al., 2022).

13.11.1 Key Aspects of Hydrogen Peroxide (H2 O2 ) During Heavy Metal Stress Include ROS Production: Heavy metal stress, such as exposure to elevated levels of metals like cadmium, lead, or mercury, can trigger an increase in ROS production, including hydrogen peroxide. Metal-induced ROS generation occurs primarily through disruption of cellular redox balance and interference with electron transport chains. Oxidative Stress Inducer: Hydrogen peroxide contributes to oxidative stress, disrupting the balance between ROS production and antioxidant defences, leading to cellular damage. Excessive accumulation of hydrogen peroxide can harm cellular components such as lipids, proteins, and DNA. Cellular Signalling: Hydrogen peroxide is a crucial signalling molecule during heavy metal stress despite its damaging potential. It participates in redox signaling pathways that regulate gene expression, cell growth, and defence responses. Hydrogen peroxide is a secondary messenger, relaying information about the stressful environment to trigger adaptive responses. Activation of Defence Mechanisms: Hydrogen peroxide is pivotal in activating a cascade of plant defence mechanisms. It acts as a signalling molecule to induce the synthesis of antioxidants, enzymes (e.g., superoxide dismutase, catalase, peroxidases), and other molecules that help mitigate the harmful effects of ROS. Cross-Talk with Other Molecules: Hydrogen peroxide interacts with other signalling molecules, including calcium ions (Ca2+ ), nitric oxide (NO), and various hormones, to coordinate plant responses to heavy metal stress. These interactions contribute to the orchestration of complex cellular responses.

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Cell Death Regulation: Hydrogen peroxide regulates programmed cell death or apoptosis. Depending on the concentration and context, it can trigger either cell survival or programmed cell death. This regulation helps the plant decide whether to invest resources in repairing damaged cells or sacrificing them for the greater good. In summary, hydrogen peroxide (H2 O2 ) serves as a critical mediator in the intricate network of responses mounted by plants during heavy metal stress. While its accumulation can be damaging, hydrogen peroxide is crucial for initiating defence mechanisms, modulating gene expression, and orchestrating adaptive responses that help plants cope with and survive heavy metal toxicity. Balancing the concentration of hydrogen peroxide and its interaction with other molecules is essential for plants to effectively navigate the challenges posed by heavy metal stress. Singlet oxygen (1 O2 ) is a reactive oxygen species (ROS) that plays a distinctive and noteworthy role in the complex interplay of plant responses to heavy metal stress. Due to cellular processes disturbed by heavy metal exposure, singlet oxygen contributes to detrimental effects and intricate signalling pathways, ultimately influencing the plant’s ability to cope with metal toxicity (Dwivedi & Kumar, 2011; P. Kumar et al., 2011a, 2011b; Pankaj et al., 2012b; P. C. Sharma & Kumar, 1999).

13.11.2 Critical Aspects of Singlet Oxygen During Heavy Metal Stress Include ROS Generation: Heavy metal stress, often caused by pollutants like cadmium, lead, or nickel, disrupts cellular processes, including electron transport chains and photosynthesis. These disruptions lead to the generation of singlet oxygen (1 O2 ) as a byproduct, adding to the pool of reactive oxygen species within the cell. Oxidative Damage: Singlet oxygen is mainly reactive and can cause oxidative damage to cellular components, including lipids, proteins, and DNA. This oxidative damage can impair cellular functions and contribute to overall cellular stress. Photosystem Inhibition: In chloroplasts, singlet oxygen is notably produced during photosystem I (PSI) and photosystem II (PSII) reactions. Its generation within chloroplasts can damage these photosystems, impairing the plant’s ability to capture light energy and perform photosynthesis effectively. Antioxidant Responses: Plants activate antioxidant defence mechanisms to counteract singlet oxygen-induced damage. Enzymes like superoxide dismutase (SOD), peroxidases, and antioxidants like tocopherols and ascorbate help scavenge singlet oxygen and other ROS, minimizing their destructive impact. Cellular Signaling: Despite its damaging effects, singlet oxygen (1 O2 ) also acts as a signaling molecule, triggering complex cellular responses. Singlet oxygeninduced signaling pathways can modulate gene expression and activate stressrelated genes that enhance the plant’s ability to withstand metal stress.

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Cross-Talk with Other Pathways: Singlet oxygen interactions with other signaling molecules, such as jasmonic acid (JA) and salicylic acid (SA), influence the overall response of the plant to heavy metal stress. These interactions contribute to the coordination of defence and adaptive mechanisms. Programmed Cell Death Regulation: Singlet oxygen has been linked to regulating programmed cell death (PCD) pathways. Singlet oxygen-induced PCD can sometimes serve as a protective mechanism, eliminating damaged cells and preventing the spread of stress. Singlet oxygen (1 O2 ) is integral to the intricate cellular orchestra responding to heavy metal stress. While its generation can contribute to oxidative damage and cellular impairment, singlet oxygen also partakes in signalling cascades that trigger defence mechanisms, activate stress-related genes, and regulate programmed cell death (Goud et al., 2022; P. Kumar, Sharma, et al., 2021; P. Kumar & Mistri, 2020; V. Kumar et al., 2021). The balance between singlet oxygen-induced damage and its involvement in adaptive responses determines the plant’s ability to navigate the challenges of heavy metal stress. Analysing heavy metal translocation and bioaccumulation in plants involves complex interactions and factors. They encompass aspects such as metal uptake, transport, distribution, and speciation, as well as the influence of environmental conditions, plant physiology, and molecular processes. Remember that real-world situations may involve more complex interactions, and experimental validation is essential to refine these equations for specific scenarios. Programmed cell death (PCD), or apoptosis, is a highly regulated and controlled process that plays a crucial role in plant development, stress responses, and defence mechanisms. Under normal circumstances, PCD serves as a mechanism for eliminating damaged or unnecessary cells, thereby ensuring tissue homeostasis. However, heavy metal stress can disrupt this delicate balance, leading to aberrant PCD and impacting plant health and survival (P. Kumar, Harshavardhan, Kumar, Yumnam, et al., 2018; P. Kumar, Krishna, Pandey, Pathak, et al., 2018; P. Kumar & Dwivedi, 2018; Siddique, Dubey, et al., 2018).

13.12 Mechanisms of Programmed Cell Death PCD involves a series of intricate molecular and cellular events orchestrated by specific genes and signalling pathways. The process can be divided into several stages: Initiation: Heavy metal stress triggers the production of reactive oxygen species (ROS) like superoxide radicals, hydrogen peroxide, and singlet oxygen. These ROS can directly damage cellular components and activate signaling pathways that lead to PCD.

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Signalling: ROS and other stress-induced signals activate specific genes and transcription factors, such as those involved in the mitogen-activated protein kinase (MAPK) cascade. These factors initiate the expression of PCD-related genes. Execution: PCD-related genes encode proteins that carry out the actual dismantling of the cell. Enzymes like nucleases degrade nuclear DNA, proteases break down proteins, and lipases disrupt cellular membranes. Cellular Breakdown: As PCD progresses, the cell undergoes characteristic changes, including shrinkage, chromatin condensation, nuclear fragmentation, and membrane blebbing. These changes are associated with the physical and biochemical dismantling of the cell. Phagocytosis and Clearance: Surrounding or specialised phagocytic cells recognise and engulf the dying cell’s remnants. This prevents the release of harmful contents that could trigger inflammation and damage nearby cells.

13.13 Impact of Heavy Metal Stress on PCD: Heavy Metal Stress Can Influence PCD in Various Ways ROS Accumulation: Excessive ROS production due to heavy metal stress can overwhelm the cellular antioxidant defence system, leading to oxidative stress. This oxidative stress can activate PCD pathways. DNA Damage: Heavy metals like cadmium and nickel can cause DNA damage, triggering the activation of DNA repair mechanisms. If the damage is severe and irreparable, PCD may be induced to prevent the propagation of mutated cells. Inhibition of Antioxidant Defences: Some heavy metals can inhibit antioxidant enzymes, such as superoxide dismutase and catalase, further promoting oxidative stress and PCD. Alteration of Signalling Pathways: Heavy metals can disrupt key signalling pathways involved in PCD regulation, such as those mediated by calcium ions and MAPKs. Mitochondrial Dysfunction: Heavy metal stress can impair mitochondrial function, releasing pro-apoptotic factors that initiate PCD.

13.14 Consequences of Dysregulated PCD Dysregulated PCD due to heavy metal stress can have detrimental consequences: Reduced Plant Growth: Excessive PCD can lead to cell death in vital tissues, hampering plant growth and development. Increased Susceptibility to Pathogens: Dysregulated PCD can compromise the plant’s ability to defend against pathogens, making it more susceptible to infections.

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Nutrient Imbalance: PCD in root cells can affect nutrient uptake and translocation, leading to nutrient imbalances and deficiencies. Disruption of Tissue Integrity: Dysregulated PCD can disrupt tissue integrity, affecting overall plant architecture and function. In conclusion, programmed cell death is a complex process tightly regulated under normal circumstances. However, heavy metal stress can disrupt this regulation, leading to aberrant PCD and subsequent impacts on plant health and survival. Understanding the molecular mechanisms underlying PCD in response to heavy metals is crucial for devising strategies to mitigate its adverse effects and enhance plant tolerance to metal stress (Das et al., 2022; Hidangmayum et al., 2022; P. Kumar & Mistri, 2020; V. Kumar et al., 2021; Paul et al., 2005; Reddy et al., 2022; Siddique, Kandpal, et al., 2018).

13.15 Heavy Metal Transportation in Xylem Heavy metal transportation in the xylem is a critical process that influences the distribution, accumulation, and detoxification of heavy metals in plants. The xylem is a specialised vascular tissue responsible for water and mineral transport from the roots to the rest of the plant. While this transport system is primarily designed for essential nutrients, it can also facilitate the movement of heavy metals, which can have beneficial and detrimental effects on plant health (Goud et al., 2022; P. Kumar et al., 2020; P. Kumar, Sharma, et al., 2021; Kumari et al., 2022; Upadhyay et al., 2023).

13.15.1 Critical Aspects of Heavy Metal Transportation in Xylem Uptake and Root-to-Shoot Translocation: Heavy metals are taken up by plant roots from the soil solution. This uptake process involves metal ions interacting with transporters on the root cell membranes. Once inside the root cells, heavy metals can move to the xylem vessels through symplastic or apoplastic pathways. Apoplastic and Symplastic Movement: In the apoplastic pathway, metals move through cell walls and intercellular spaces, ultimately entering the xylem vessels. In the symplastic path, metals enter the root cells and move through the interconnected cytoplasmic continuum before reaching the xylem. Xylem Loading: Once heavy metals reach the xylem, they can be loaded into the vessels for upward transport. This loading process involves transporters and ion channels that facilitate the movement of metal ions from the root cells into the xylem sap. Passive Transport and Transpiration: Heavy metals are transported in the xylem sap along with water. Water from the leaves creates a negative pressure that pulls

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the xylem sap upward. This passive movement allows heavy metals to move with the water towards the shoot. Root-to-Shoot Translocation Rate: The rate of translocation from roots to shoots depends on factors such as metal concentration, plant species, physiological conditions, and the presence of competing ions. Some plants exhibit higher translocation rates, while others restrict heavy metal movement to minimise toxicity. Phloem Connection: While the xylem primarily transports water and minerals upward, there is some movement of heavy metals from the xylem to the phloem. This can influence the distribution of heavy metals to various plant parts, including developing leaves, flowers, and seeds.

13.15.2 Role of Xylem in Heavy Metal Detoxification and Accumulation The transportation of heavy metals in the xylem can have both detoxification and accumulation effects on plants: Detoxification: Some plants restrict heavy metal translocation to prevent toxic concentrations from reaching sensitive shoot tissues. This is achieved through root cell sequestration, complexation with ligands, and cell wall binding. Accumulation: Other plants, known as hyperaccumulators, have evolved the ability to accumulate high concentrations of heavy metals in their shoots. They use the xylem to transport heavy metals to above-ground tissues, where they may provide a defence against herbivores or contribute to phytoremediation efforts. In conclusion, heavy metal transportation in the xylem is a dynamic process influenced by various factors, including plant physiology, metal concentration, and environmental conditions. Understanding how plants regulate heavy metal transport and accumulation in the xylem is essential for both ecological and agricultural contexts, as it impacts plant health, metal toxicity, and the potential use of plants for phytoremediation and metal biofortification (P. Kumar et al., 2017; P. Kumar, Kumar, Harshavardhan, Naik, et al., 2018; P. Kumar, Pandey, Krishna, Pathak, et al., 2018; Pathak et al., 2017).

13.16 Impact of Heavy Metal on Mitochondrial Dysfunction The impact of heavy metals on mitochondrial dysfunction is a critical aspect of metal-induced cellular toxicity. Mitochondria are the cell’s powerhouse, responsible for generating energy through oxidative phosphorylation and participating in various metabolic processes. Heavy metal exposure can disrupt mitochondrial structure and function, leading to a cascade of events contributing to cellular damage, oxidative stress, and overall physiological disturbances.

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13.16.1 Effects of Heavy Metals on Mitochondrial Dysfunction Respiratory Chain Disruption: Heavy metals, such as cadmium, mercury, and lead, can interfere with components of the electron transport chain (ETC) in mitochondria. This disruption hinders the efficient transfer of electrons and reduces the production of adenosine triphosphate (ATP), the primary energy currency of cells. Free Radical Production: Heavy metal-induced mitochondrial dysfunction often leads to the increased production of reactive oxygen species (ROS), including superoxide radicals and hydrogen peroxide. ROS can cause oxidative damage to mitochondrial DNA, proteins, and lipids, exacerbating mitochondrial dysfunction. Loss of Membrane Potential: Mitochondrial membrane potential, a crucial indicator of mitochondrial health, can be disrupted by heavy metal exposure. Decreased membrane potential impairs ATP synthesis and ion transport across the inner mitochondrial membrane. Mitochondrial Permeability Transition Pore (MPTP) Opening: Some heavy metals can induce the opening of the MPTP, a channel in the inner mitochondrial membrane. MPTP opening can lead to uncontrolled ion exchange, swelling, and the release of pro-apoptotic factors, eventually triggering cell death pathways. Altered Calcium Homeostasis: Heavy metals can disturb calcium ion homeostasis within mitochondria. Dysregulation of calcium dynamics affects ATP production, disrupts mitochondrial membrane potential, and contributes to oxidative stress. Impaired Biogenesis and Dynamics: Heavy metals can disrupt mitochondrial biogenesis, fusion, and fission processes. This affects mitochondrial morphology, distribution, and turnover, compromising mitochondrial function. Mitophagy Inhibition: Mitophagy, the selective degradation of damaged mitochondria, is critical for maintaining mitochondrial quality control. Heavy metal exposure can inhibit mitophagy, leading to the accumulation of dysfunctional mitochondria (D. Kumar et al., 2019; P. Kumar et al., 2019; P. Kumar, Pathak, Amarnath, et al., 2018; P. Kumar & Dwivedi, 2020; P. Kumar & Naik, 2020; P. Kumar & Pathak, 2019; Naik & Kumar, 2020; Siddique, Dubey, et al., 2018; Siddique & Kumar, 2018).

13.16.2 Consequences of Heavy Metal-Induced Mitochondrial Dysfunction Energy Depletion: Impaired oxidative phosphorylation reduces ATP production, leading to energy depletion and compromised cellular functions. Oxidative Stress: ROS generated due to mitochondrial dysfunction contributes to oxidative stress, damaging cellular components and triggering various stress responses.

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Cell Death: Severe mitochondrial dysfunction can lead to apoptotic or necrotic cell death, depending on damage extent and specific signalling pathways’ activation. Organ and Tissue Toxicity: Heavy metal-induced mitochondrial dysfunction can impact specific organs or tissues with high energy demands, such as the brain, heart, and skeletal muscles. Developmental Abnormalities: In developing organisms, heavy metal exposure can disrupt mitochondrial function and contribute to developmental abnormalities. Disease Associations: Mitochondrial dysfunction resulting from heavy metal exposure has been linked to various diseases, including neurodegenerative disorders, cardiovascular diseases, and cancer. The impact of heavy metals on mitochondrial dysfunction is a complex and multifaceted process that has far-reaching consequences for cellular health and organismal well-being. Understanding the molecular mechanisms underlying severe metal-induced mitochondrial dysfunction is crucial for developing strategies to mitigate their toxic effects and protect cellular and organismal integrity.

13.17 Conclusion In conclusion, hydroponics emerges as a significant and promising method for phytoremediation, offering a versatile approach to address environmental contamination challenges. This innovative technique empowers plants to efficiently uptake and detoxify pollutants from various substrates, minimizing soil and water pollution. The controlled nutrient-rich hydroponic systems optimize plant growth and remediation performance, while the absence of soil-borne limitations widens the spectrum of applicable contaminants. However, successful implementation demands a tailored approach considering plant selection, nutrient management, and system design. As we navigate the complex landscape of environmental degradation, hydroponic-based phytoremediation stands as a beacon of hope, offering a sustainable solution to restore and preserve our ecosystems.

13.18 Future Prospects Looking ahead, the prospects of hydroponics as a significant method for phytoremediation hold immense promise. Continued research and technological advancements will likely refine and expand its application across diverse environmental contexts. Integrating cutting-edge sensors, automation, and data analytics can enhance realtime monitoring and adaptive management of hydroponic systems, optimising contaminant removal efficiency. Moreover, exploring plant–microbe synergies and genetic engineering could unlock the potential for hyperaccumulators with enhanced pollutant uptake capacities. Collaborative efforts between scientists, engineers,

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and policymakers are essential to establish guidelines, regulations, and incentives that promote the widespread adoption of hydroponic phytoremediation, ultimately leading to a cleaner and more sustainable environment for future generations. Acknowledgements We would like to express our heartfelt appreciation to the Department of Agronomy, School of Agriculture, Lovely Professional University for their unwavering support, guidance, and resources, which played a pivotal role in completing this research. Our gratitude extends to the dedicated team of authors who brought their unique expertise and perspectives to this endeavour. Their collective efforts and commitment greatly enriched the quality of this work. This collaboration exemplifies the strength of interdisciplinary cooperation and underscores the importance of teamwork in advancing knowledge and innovation.

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Kaur, N., & Kaushal, J. (2022). Screening the six plant species for phytoremediation of synthetic textile dye waste water. Materials Today: Proceedings, 71, 232–238. https://doi.org/10.1016/j. matpr.2022.08.512 Kotia, A., Rutu, P., Singh, V., Kumar, A., Dhoke, S., Kumar, P., & Singh, D. K. (2021). Rheological analysis of rice husk-starch suspended in water for sustainable agriculture application. 2nd International Conference on Functional Materials, Manufacturing and Performances, ICFMMP 2021, 50, 1962–1966. https://doi.org/10.1016/j.matpr.2021.09.325 Kumar, D., Rameshwar, S. D., & Kumar, P. (2019). Effect of intergated application of inorganic and organic fertilizers on the roots of chickpea. Plant Archives, 19(1), 857–860. https://www.scopus.com/inward/record.uri?eid=2-s2.0-85068434345&partne rID=40&md5=4d9e2d8c35a83b31a09556e919e9745f Kumar, P., Devi, P., & Dey, S. R. (2021). Chapter 6—Fungal volatile compounds: A source of novel in plant protection agents. In A. Kumar, J. Singh, & J. B. T.-V. and M. of M. Samuel (Eds.), Volatiles and metabolites of microbes (pp. 83–104). Academic Press. https://doi.org/10.1016/ B978-0-12-824523-1.00001-8 Kumar, P., & Dwivedi, P. (2018). Putrescine and glomus mycorrhiza moderate cadmium actuated stress reactions in zea mays l. By means of extraordinary reference to sugar and protein. Vegetos, 31(3), 74–77. https://doi.org/10.5958/2229-4473.2018.00076.9 Kumar, P., & Dwivedi, P. (2020). Lignin estimation in sorghum leaves grown under hazardous waste site. Plant Archives, 20, 2558–2561. https://www.scopus.com/inward/record.uri?eid=2s2.0-85090326255&partnerID=40&md5=2482a876d9ae56ac04904bd264180711 Kumar, P., Goud, E. L., Devi, P., Dey, S. R., & Dwivedi, P. (2022). Heavy metals: Transport in plants and their physiological and toxicological effects. In Plant metal and metalloid transporters (pp. 23–54). Springer Nature. https://doi.org/10.1007/978-981-19-6103-8_2 Kumar, P., Goud, E. L., Devi, P., & Koul, B. (2022). Metal pollutants in the environment. In Environmental microbiology: Emerging technologies (pp. 291–323). De Gruyter. https://doi. org/10.1515/9783110727227-012 Kumar, P., Harshavardhan, M., Kumar, P. S., Yumnam, J., Jyoti, N., Naik, M., Misao, L., Purnima, & Kumar, S. (2018). Effect on chlorophyll a/b ratio in cadmium contaminated maize leaves treated with putrescine and mycorrhiza. Annals of Biology, 34(3), 281–283. https://www.scopus.com/inward/record.uri?eid=2-s2.0-85075538775&partne rID=40&md5=aa4f2234012e98022d0cb99a02963635 Kumar, P., Koul, B., & Sharma, M. (2022). Phytoremediation of heavy metals. In Heavy metals in plants: Physiological to molecular approach (pp. 369–388). CRC Press. https://doi.org/10. 1201/9781003110576-17 Kumar, P., Krishna, V., Pandey, A. K., Pathak, S., & Siddique, A. (2018). Assessment of scavenging competence for cadmium, lead, chromium and nickel metals by in vivo grown Zea mays l. Using atomic absorption spectrophotometer. Annals of Agri Bio Research, 23(2), 166–168. https://www.scopus.com/inward/record.uri?eid=2-s2.0-85075536476&partne rID=40&md5=7a45d5d6e563d8e55e760a51d8607134 Kumar, P., Kumar, S., Harshavardhan, M., Naik, M., Yumnam, J., Kumar, P. S., Jyoti, N., Misao, L., & Purnima. (2018). Evaluation of plant height and leaf length of sorghum grown under different sources of nutrition. Annals of Biology, 34(3), 284–286. https://www.scopus.com/inward/rec ord.uri?eid=2-s2.0-85075511816&partnerID=40&md5=570ec8983a32abffaa81d117e0acae1e Kumar, P., Kumar, T., Singh, S., Tuteja, N., Prasad, R., & Singh, J. (2020). Potassium: A key modulator for cell homeostasis. Journal of Biotechnology, 324, 198–210. Kumar, P., Mandal, B., & Dwivedi, P. (2011a). Heavy metal scavenging capacity of Mentha spicata and Allium cepa. Medicinal Plants, 3(4), 315–318. https://doi.org/10.5958/j.0975-4261.3.4.053 Kumar, P., Mandal, B., & Dwivedi, P. (2011b). Heavy metals scavenging of soils and sludges by ornamental plants. Journal of Applied Horticulture, 13(2), 144–146. https://www.scopus. com/inward/record.uri?eid=2-s2.0-84860310792&partnerID=40&md5=41f3f6036e261fad9f f5a655f9aef1f5

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Kumar, P., Mandal, B., & Dwivedi, P. (2013). Combating heavy metal toxicity from hazardous waste sites by harnessing scavenging activity of some vegetable plants. Vegetos, 26(2), 416–425. https://doi.org/10.5958/j.2229-4473.26.2.106 Kumar, P., & Mistri, T. K. (2020). Transcription factors in SOX family: Potent regulators for cancer initiation and development in the human body. Seminars in Cancer Biology, 67, 105–113. https:// doi.org/10.1016/j.semcancer.2019.06.016 Kumar, P., & Naik, M. (2020). Biotic symbiosis and plant growth regulators as a strategy against cadmium and lead stress in chickpea. Plant Archives, 20, 2495– 2500. https://www.scopus.com/inward/record.uri?eid=2-s2.0-85090455388&partnerID=40& md5=e0a3e27f167a9f50afc1c5954d71b2f0 Kumar, P., Pandey, A. K., Krishna, V., Pathak, S., & Siddique, A. (2018). Phytoextraction of lead, chromium, cadmium and nickel by tagetes plant grown at hazardous waste site. Annals of Biology, 34(3), 287–289. https://www.scopus.com/inward/record.uri?eid=2-s2.0-850 75510648&partnerID=40&md5=da2abea065a69415e191244ea9e8fd78 Kumar, P., & Pathak, S. (2019). Responsiveness index of sorghum (Sorghum bicolor (L.) Moench) grown under cadmium contaminated soil treated with putrescine and mycorrhiza. Bangladesh Journal of Botany, 48(1), 139–143. https://www.scopus.com/inward/record.uri?eid=2-s2.0-850 63424662&partnerID=40&md5=7b83ecbe705e28da69bc206c34136726 Kumar, P., Pathak, S., Amarnath, K. S., Veerendra Brahma Teja, P., Dileep, B., Kumar, K., Singh, M., & Siddique, A. (2018). Effect of growth regulator on morpho-physiological attributes of chilli: A case study. Plant Archives, 18(2), 1771–1776. https://www.scopus.com/inward/record. uri?eid=2-s2.0-85060849603&partnerID=40&md5=8146fd6f6db1681549099575e6a18cdb Kumar, P., Pathak, S., Kumar, M., & Dwivedi, P. (2018). Role of secondary metabolites for the mitigation of cadmium toxicity in sorghum grown under mycorrhizal inoculated hazardous waste site. In Biotechnological approaches for medicinal and aromatic plants: Conservation, genetic improvement and utilization (pp. 199–212). Kumar, P., Sharma, K., Saini, L., & Dey, S. R. (2021). Chapter 8—Role and behavior of microbial volatile organic compounds in mitigating stress. In A. Kumar, J. Singh, & J. B. T.-V. and M. of M. Samuel (Eds.), Volatiles and metabolites of microbes (pp. 143–161). Academic Press. https://doi.org/10.1016/B978-0-12-824523-1.00010-9 Kumar, P., Siddique, A., Thongbam, S., Chopra, P., & Kumar, S. (2019). Cadmium induced changes in total starch, total amylose and amylopectin content in putrescine and mycorrhiza treated sorghum crop. Nature Environment and Pollution Technology, 18(2), 525–530. https://www.scopus.com/inward/record.uri?eid=2-s2.0-85068402600&partne rID=40&md5=dc5afa252662b7f47f02eda4a6a01554 Kumar, P., Singh, B. N., & Dwivedi, P. (2017). Plant growth regulators, plant adaptability and plant productivity: A review on abscisic acid (ABA) signaling in plants under emerging environmental stresses. In Sustaining future food security in changing environments (pp. 81–97). Nova Science Publishers, Inc. https://www.scopus.com/inward/record.uri?eid=2-s2.0-850220 15328&partnerID=40&md5=4d99a190e7c92792cec87af94a7d53ee Kumar, P., Yumnam, J., Kumar, P. S., Misao, L., Jyoti, N., Naik, M., Purnima, Kumar, S., & Harshavardhan, M. (2018). Cadmium induced changes in germination of maize seed treated with mycorrhiza. Annals of Agri Bio Research, 23(2), 169–170. https://www.scopus.com/inward/rec ord.uri?eid=2-s2.0-85075532949&partnerID=40&md5=2d08ad9891decef96ad0ad15ffa98449 Kumar, V., Dwivedi, P., Kumar, P., Singh, B. N., Pandey, D. K., Kumar, V., & Bose, B. (2021). Mitigation of heat stress responses in crops using nitrate primed seeds. South African Journal of Botany, 140, 25–36. https://doi.org/10.1016/j.sajb.2021.03.024 Kumar, V., Umrao, P. D., & Kaistha, S. D. (2022). Chapter12—Beneficial plant microbiome assisted chromium phytoremediation. In K. Bauddh & Y. B. T.-A. in M. P. of P. S. Ma (Eds.), Advances in microbe-assisted phytoremediation of polluted sites (pp. 301–346). Elsevier. https://doi.org/ 10.1016/B978-0-12-823443-3.00018-1 Kumari, P., Singh, J., & Kumar, P. (2022). Chapter 21—Impact of bioenergy for the diminution of an ascending global variability and change in the climate. In A. Kumar, J. Singh, & L. F. R. B.

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T.-M. U. C. C. Ferreira (Eds.), Microbiome under changing climate (pp. 469–487). Woodhead Publishing. https://doi.org/10.1016/B978-0-323-90571-8.00021-3 Liu, J., Zhang, D., Luo, Y., Zhang, Y., Xu, L., Chen, P., Wu, E., Ma, Q., Wang, H., Zhao, L., & Feng, B. (2023). Cadmium tolerance and accumulation from the perspective of metal ion absorption and root exudates in broomcorn millet. Ecotoxicology and Environmental Safety, 250, 114506. https://doi.org/10.1016/j.ecoenv.2023.114506 Liu, Y., Huang, L., Wen, Z., Fu, Y., Liu, Q., Xu, S., Li, Z., Liu, C., Yu, C., & Feng, Y. (2023). Effects of intercropping on safe agricultural production and phytoremediation of heavy metalcontaminated soils. Science of The Total Environment, 875, 162700. https://doi.org/10.1016/j. scitotenv.2023.162700 Majumdar, A., Upadhyay, M. K., Ojha, M., Afsal, F., Giri, B., Srivastava, S., & Bose, S. (2022). Enhanced phytoremediation of Metal(loid)s via spiked ZVI nanoparticles: An urban clean-up strategy with ornamental plants. Chemosphere, 288, 132588. https://doi.org/10.1016/j.chemos phere.2021.132588 Mattiello, A., Novello, N., Cornu, J.-Y., Babst-Kostecka, A., & Poš´ci´c, F. (2023). Copper accumulation in five weed species commonly found in the understory vegetation of Mediterranean vineyards. Environmental Pollution, 329, 121675. https://doi.org/10.1016/j.envpol.2023.121675 Mazumdar, K., & Das, S. (2022). Chapter 24—Phytoremediation of trace elements from paper mill wastewater with Pistia stratiotes L.: Metal accumulation and antioxidant response. In V. Kumar, M. P. Shah, & S. K. B. T.-P. T. for the R. of H. M. and O. C. from S. and W. Shahi (Eds.), Phytoremediation technology for the removal of heavy metals and other contaminants from soil and water (pp. 523–537). Elsevier. https://doi.org/10.1016/B978-0-323-85763-5.00020-9 Mishra, A., Singh, A. P., Takkar, S., Sharma, A., Shukla, S., Shukla, K., Giri, B. S., Katiyar, V., & Pandey, A. (2022). Chapter 10—Phytoremediation of dye-containing wastewater. In P. Sharma, A. Pandey, Y. W. Tong, & H. H. B. T.-C. D. in B. and B. Ngo (Eds.), Current developments in biotechnology and bioengineering (pp. 197–222). Elsevier. https://doi.org/10.1016/B978-0323-99907-6.00004-9 Mohsin, M., Nawrot, N., Wojciechowska, E., Kuittinen, S., Szczepa´nska, K., Dembska, G., & Pappinen, A. (2023). Cadmium accumulation by Phragmites australis and Iris pseudacorus from stormwater in floating treatment wetlands microcosms: Insights into plant tolerance and utility for phytoremediation. Journal of Environmental Management, 331, 117339. https://doi. org/10.1016/j.jenvman.2023.117339 Mohsin, M., Salam, M. M. A., Nawrot, N., Kaipiainen, E., Lane, D. J., Wojciechowska, E., Kinnunen, N., Heimonen, M., Tervahauta, A., Peräniemi, S., Sippula, O., Pappinen, A., & Kuittinen, S. (2022). Phytoextraction and recovery of rare earth elements using willow (Salix spp.). Science of The Total Environment, 809, 152209. https://doi.org/10.1016/j.scitotenv.2021. 152209 Muthukumaran, M. (2022). Chapter 19—Aquatic plant remediation to control pollution. In S. Kumar & M. Z. B. T.-B. A. to C. P. Hashmi (Eds.), Advances in Pollution Research (pp. 365–397). Woodhead Publishing. https://doi.org/10.1016/B978-0-12-824316-9.00004-5 Naik, M., & Kumar, P. (2020). Role of growth regulators and microbes for metal detoxification in plants and soil. Plant Archives, 20, 2820–2824. https://www.scopus.com/inward/record.uri? eid=2-s2.0-85090334398&partnerID=40&md5=d501ee461b077a20ac3dfe8ab537a3ee Niu, L., Li, C., Wang, W., Zhang, J., Scali, M., Li, W., Liu, H., Tai, F., Hu, X., & Wu, X. (2023). Cadmium tolerance and hyperaccumulation in plants—A proteomic perspective of phytoremediation. Ecotoxicology and Environmental Safety, 256, 114882. https://doi.org/10.1016/j.eco env.2023.114882 Nkrumah, P. N., & van der Ent, A. (2023). Possible accumulation of critical metals in plants that hyperaccumulate their chemical analogues? Science of The Total Environment, 878, 162791. https://doi.org/10.1016/j.scitotenv.2023.162791 Oladoye, P. O., Olowe, O. M., & Asemoloye, M. D. (2022). Phytoremediation technology and food security impacts of heavy metal contaminated soils: A review of literature. Chemosphere, 288, 132555. https://doi.org/10.1016/j.chemosphere.2021.132555

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Pandey, A. K., Gautam, A., & Singh, A. K. (2023). Insight to chromium homeostasis for combating chromium contamination of soil: Phytoaccumulators-based approach. Environmental Pollution, 322, 121163. https://doi.org/10.1016/j.envpol.2023.121163 Pandey, V. C., Gaji´c, G., Sharma, P., & Roy, M. (2022). Chapter 5—Adaptive phytoremediation practices for sustaining ecosystem services. In V. C. Pandey, G. Gaji´c, P. Sharma, & M. B. T.-A. P. P. Roy (Eds.), Adaptive phytoremediation practices (pp. 181–225). Elsevier. https://doi.org/ 10.1016/B978-0-12-823831-8.00008-6 Pankaj, P. K., Kumar, P., Nigam, R. C., & Mishra, P. K. (2012a). Monitoring and Surveillance of synthetic pyrethroids and organophosphate in different brands of soft drinks. Journal of Chemical and Pharmaceutical Research, 4(8), 3939–3943. https://www.scopus.com/inward/record.uri? eid=2-s2.0-84867504985&partnerID=40&md5=0078e81af00e4b6849c087c3ae5c7591 Pankaj, P. K., Kumar, P., Nigam, R. C., & Mishra, P. K. (2012b). To studies organochlorine insecticides in different brands of cold drinks. Journal of Chemical and Pharmaceutical Research, 4(8), 3934–3938. https://www.scopus.com/inward/record.uri?eid=2-s2.0-84867537578&partnerID= 40&md5=f2db0a7b022cd9e4dd96e1dcc49250a5 Pathak, S., Kumar, P., Mishra, P. K., & Kumar, M. (2017). Mycorrhiza assisted approach for bioremediation with special reference to biosorption. Pollution Research, 36(2), 329–332. https://www.scopus.com/inward/record.uri?eid=2-s2.0-85028617468&partne rID=40&md5=da296311c0f3d29ea94d21631fe01d69 Paul, A., Sharma, S. R., Sresty, T. V. S., Devi, S., Bala, S., Kumar, P. S., Saradhi, P. P., Frutos, R., Altosaar, I., & Kumar, P. A. (2005). Transgenic cabbage (Brassica oleracea var. capitata) resistant to Diamondback moth (Plutella xylostella). Peng, Y.-J., Hu, C.-Y., Li, W., Dai, Z.-H., Liu, C.-J., & Ma, L. Q. (2023). Arsenic induced plant growth by increasing its nutrient uptake in As-hyperaccumulator Pteris vittata: Comparison of arsenate and arsenite. Environmental Pollution, 322, 121168. https://doi.org/10.1016/j.envpol. 2023.121168 Rabani, M. S., Hameed, I., Mir, T. A., A. wani, B., Gupta, M. K., Habib, A., Jan, M., Hussain, H., Tripathi, S., Pathak, A., Ahad, M. B., & Gupta, C. (2022). Chapter 5—Microbial-assisted phytoremediation. In R. A. Bhat, F. M. P. Tonelli, G. H. Dar, & K. B. T.-P. Hakeem (Eds.), Phytoremediation (pp. 91–114). Academic Press. https://doi.org/10.1016/B978-0-323-898744.00006-6 Rane, N. R., Kanojia, A., Patil, S. M., Khandare, R., Kodam, K. M., & Jeon, B.-H. (2023). Chapter 20—Constructed wetland system and its engineered designs for the treatment of textile industry effluent. In S. P. Govindwar, M. B. Kurade, B.-H. Jeon, & A. B. T.-C. D. in B. and B. Pandey (Eds.), Current developments in bioengineering and biotechnology (pp. 601–626). Elsevier. https://doi.org/10.1016/B978-0-323-91235-8.00004-8 Reddy, S. S., Kumar, P., & Dwivedi, P. (2022). Heavy metal transporters, phytoremediation potential, and biofortification. In Plant metal and metalloid transporters (pp. 387–405). Springer. Riaz, U., Athar, T., Mustafa, U., & Iqbal, R. (2022). Chapter 23—Economic feasibility of phytoremediation. In R. A. Bhat, F. M. P. Tonelli, G. H. Dar, & K. B. T.-P. Hakeem (Eds.), Phytoremediation (pp. 481–502). Academic Press. https://doi.org/10.1016/B978-0-323-89874-4.00025-X Sageena, G., Khatana, K., & Nagar, J. K. (2022). Chapter 22—Biomonitoring of heavy metals contamination in soil ecosystem. In M. Naeem, T. Aftab, A. Ali Ansari, S. S. Gill, & A. B. T.-H. and T. M. in S. and P. Macovei (Eds.), Hazardous and trace materials in soil and plants (pp. 313–325). Academic Press. https://doi.org/10.1016/B978-0-323-91632-5.00019-7 Sahito, Z. A., Zehra, A., Chen, S., Yu, S., Tang, L., Ali, Z., Hamza, S., Irfan, M., Abbas, T., He, Z., & Yang, X. (2022). Rhizobium rhizogenes-mediated root proliferation in Cd/Zn hyperaccumulator Sedum alfredii and its effects on plant growth promotion, root exudates and metal uptake efficiency. Journal of Hazardous Materials, 424, 127442. https://doi.org/10.1016/j.jha zmat.2021.127442 Savio, N., Pandey, D., & Srivastava, R. K. (2023). Potentialities of plant based hybrid wetland systems for the treatment of household waste water using Canna indica, Agave americana,

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Pistia stratiotes and Tagetes erecta. Materials Today: Proceedings, 77, 217–222. https://doi.org/ 10.1016/j.matpr.2022.11.264 Sharma, P. C., & Kumar, P. (1999). Alleviation of salinity stress during germination in Brassica juncea by pre-sowing chilling treatments to seeds. Biologia Plantarum, 42(3), 451–455. https:// doi.org/10.1023/A:1002481709121 Sharma, P., Rai, S., Gautam, K., & Sharma, S. (2023). Chapter 11—Phytoremediation strategies of plants: Challenges and opportunities. In A. B. T.-P. and T. I. to E. P. Husen (Ed.), Plants and their interaction to environmental pollution (pp. 211–229). Elsevier. https://doi.org/10.1016/B978-0323-99978-6.00012-1 Siddique, A., Dubey, A. P., & Kumar, P. (2018a). Cadmium induced physio-chemical changes in roots of wheat. Vegetos, 31(3), 113–118. https://doi.org/10.5958/2229-4473.2018.00081.2 Siddique, A., Kandpal, G., & Kumar, P. (2018b). Proline Accumulation and its defensive role under diverse stress condition in plants: An overview. Journal of Pure and Applied Microbiology, 12(3), 1655–1659. Siddique, A., & Kumar, P. (2018). Physiological and biochemical basis of pre-sowing soakingseedtreatment-anoverview. Plant Archives, 18(2), 1933–1937. https://www.scopus. com/inward/record.uri?eid=2-s2.0-85060894097&partnerID=40&md5=90128f47e83ee0b1ad 44e812f7135a28 Srivastav, A. L., Patel, N., Rani, L., Kumar, P., Dutt, I., Maddodi, B. S., & Chaudhary, V. K. (2023). Sustainable options for fertilizer management in agriculture to prevent water contamination: A review. Environment, Development and Sustainability. https://doi.org/10.1007/s10668-023-031 17-z Sun, D., Zhang, X., Yin, Z., Feng, H., Hu, C., Guo, N., Tang, Y., Qiu, R., Ma, L. Q., & Cao, Y. (2023). As-hyperaccumulator Pteris vittata and non-hyperaccumulator Pteris ensiformis under low As-exposure: Transcriptome analysis and implication for As hyperaccumulation. Journal of Hazardous Materials, 458, 132034. https://doi.org/10.1016/j.jhazmat.2023.132034 Sun, Z., Dzakpasu, M., Zhang, D., Liu, G., Wang, Z., Qu, M., Chen, R., Wang, X. C., & Zheng, Y. (2022). Enantioselectivity and mechanisms of chiral herbicide biodegradation in hydroponic systems. Chemosphere, 307, 135701. https://doi.org/10.1016/j.chemosphere.2022.135701 Takkar, S., Shandilya, C., Agrahari, R., Chaurasia, A., Vishwakarma, K., Mohapatra, S., Varma, A., & Mishra, A. (2022). Chapter 17—Green technology: Phytoremediation for pesticide pollution In V. Kumar, M. P. Shah, & S. K. B. T.-P. T. for the R. of H. M. and O. C. from S. and W. Shahi (Eds.), Phytoremediation technology for the removal of heavy metals and other contaminants from soil and water (pp. 353–375). Elsevier. https://doi.org/10.1016/B978-0-323-857635.00008-8 Thomas, G., Sheridan, C., & Holm, P. E. (2022). A critical review of phytoremediation for acid mine drainage-impacted environments. Science of The Total Environment, 811, 152230. https:// doi.org/10.1016/j.scitotenv.2021.152230 Upadhyay, S. K., Devi, P., Kumar, V., Pathak, H. K., Kumar, P., Rajput, V. D., & Dwivedi, P. (2023). Efficient removal of total arsenic (As3+/5+ ) from contaminated water by novel strategies mediated iron and plant extract activated waste flowers of marigold. Chemosphere, 313, 137551. https://doi.org/10.1016/j.chemosphere.2022.137551 Vidya, C. S.-N., Shetty, R., Vaculíková, M., & Vaculík, M. (2022). Antimony toxicity in soils and plants, and mechanisms of its alleviation. Environmental and Experimental Botany, 202, 104996. https://doi.org/10.1016/j.envexpbot.2022.104996 Wu, J., Zhao, N., Zhang, P., Zhu, L., Lu, Y., Lei, X., & Bai, Z. (2023). Nitrate enhances cadmium accumulation through modulating sulfur metabolism in sweet sorghum. Chemosphere, 313, 137413. https://doi.org/10.1016/j.chemosphere.2022.137413 Wu, S., Yang, Y., Qin, Y., Deng, X., Zhang, Q., Zou, D., & Zeng, Q. (2023). Cichorium intybus L. is a potential Cd-accumulator for phytoremediation of agricultural soil with strong tolerance and detoxification to Cd. Journal of Hazardous Materials, 451, 131182. https://doi.org/10.1016/j. jhazmat.2023.131182

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Yadav, R. K., Sahoo, S., & Patil, S. A. (2022). Performance evaluation of the integrated hydroponicsmicrobial electrochemical technology (iHydroMET) for decentralized domestic wastewater treatment. Chemosphere, 288, 132514. https://doi.org/10.1016/j.chemosphere.2021.132514 Ye, D., Xie, M., Zhang, X., Huang, H., Yu, H., Zheng, Z., Wang, Y., & Li, T. (2022). Evaluation for phosphorus accumulation and removal capability of nine species in the Polygonaceae to excavate amphibious superstars used for phosphorus-phytoextraction. Chemosphere, 308, 136361. https:// doi.org/10.1016/j.chemosphere.2022.136361 You, Y., Ju, C., Wang, L., Wang, X., Ma, F., Wang, G., & Wang, Y. (2022). The mechanism of arbuscular mycorrhizal enhancing cadmium uptake in Phragmites australis depends on the phosphorus concentration. Journal of Hazardous Materials, 440, 129800. https://doi.org/10. 1016/j.jhazmat.2022.129800 Zhu, Y., Wang, Y., He, X., Li, B., & Du, S. (2023). Plant growth-promoting rhizobacteria: A good companion for heavy metal phytoremediation. Chemosphere, 338, 139475. https://doi.org/10. 1016/j.chemosphere.2023.139475

Chapter 14

Hydroponics Removal of Wastewater’s Contaminants M. Liliana Cifuentes-Torres, Leopoldo G. Mendoza-Espinosa, and J. Gabriel Correa-Reyes

Abstract As plants have the capacity to absorb nutrients, harmful metals, and developing pollutants, hydroponic systems can be employed as a treatment procedure for partially treated wastewater or reclaimed water before its release to the environment. Hydroponic systems are an alternative to stop water pollution and scarcity because of their high rates of nutrient removal from wastewater, including N, P, and K. Because they employ ecologically friendly methods, hydroponic systems are regarded as a crucial technology for food production in cities in terms of sustainability. However, since the vast majority of research on hydroponics with recycled water has been done at the laboratory scale, testing full-scale systems is important to show that it is viable. Keywords Hydroponics · Wastewater treatment · Pollution

14.1 Introduction The effect of wastewater on natural water courses will depend on several factors that include, the volume and concentration of the wastewater and its chemical and microbiological composition. Material discharged along with the wastewater includes suspended solids, organic matter, and dangerous contaminants such as toxic metals and organic compounds (Owili, 2003). The eutrophication of natural waters can build an environmental condition that favors the growth of cyanobacteria that produce M. L. Cifuentes-Torres Corporación Autónoma Regional de Cundinamarca, Cundinamarca, Colombia e-mail: [email protected] L. G. Mendoza-Espinosa (B) Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California (Autonomous University of Baja California), Baja California, Mexico e-mail: [email protected] J. G. Correa-Reyes Instituto de Investigaciones Oceanológicas, Universidad Autónoma de Baja California (Autonomous University of Baja California), Baja California, Mexico e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2024 N. Kumar (ed.), Hydroponics and Environmental Bioremediation, Springer Water, https://doi.org/10.1007/978-3-031-53258-0_14

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toxins. Contact with these toxins can produce gastroenteritis, hepatic damage, the deterioration of the nervous system, liver cancer and skin irritation in animals and humans (EPA, 2000). Water pollution has been identified as the main public health risk in most developing countries (OMS, 2016). Thus, it is necessary to combine strategies that protect natural waters by means of recycling water and nutrients from wastewater to decrease pressure on these natural resources.

14.2 The Role of Hydroponics in the Removal of Contaminants A hydroponic system can be added as a tertiary treatment to the secondary effluent of a wastewater treatment plant. By this means, the secondary effluent is reused to produce crops in the hydroponic system (Yang et al., 2015). Hydroponic systems can be either open or closed. In closed hydroponic systems, the same solution of nutrients is recycled, and the concentrations of nutrients are adjusted depending on the crop’s requirements. In open systems, new and fresh nutrients solution is added to each irrigation cycle. The drainage of open hydroponic systems generally contains significant amounts of nutrients that are discharged into the environment, which can cause contamination. Therefore, closed systems are preferred as the nutrients solution is recycled, reducing the cost of the system, its negative impact on the environment and providing for an extra step in the treatment of wastewater (Christie, 2014; Grewal et al., 2011; Van Os, 1999). In a study by Grewal et al. (2011), the recirculation of the drainage resulted in a 33% reduction of the amount of water used for the cultivation of cucumber in a hydroponic system. The drainage contained 59, 25 and 55% of N, P and K, respectively in comparison to the initial amount applied. Christie (2014) studied the effect of reusing the solution of nutrients from the drainage of traditional hydroponic systems to prevent environmental pollution. The experiments consisted in the cultivation of lettuce with a solution of nutrients that was restored and reused after each irrigation cycle and another one that was discarded after each use. The recycled solution used 42% less water, 23% less KH2 PO4 , 57% less KNO3 , and 58% less MgSO4 and micronutrients in comparison to the conventional system. This demonstrated the potential for the recovery of nutrients and the savings in costs through the recycling of the drainage. On the other hand, the operation of the open systems resulted in the discharge of large quantities of water and nutrients that can cause pollution of surface and groundwater. Plants and the microorganisms contribute towards the degradation of contaminants present in water and soil. The optimal temperatures for the denitrification and nitrification process are between 16.5–32 °C and 20–25 °C, respectively. The moderated correlation between temperature and the efficiency in the elimination of nitrogen was established by Kadlec and Reddy (2001) during the wastewater treatment from pig farms in hydroponic constructed wetlands. They found 70% removal of nitrogen

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and 45% of phosphorus during warmer periods. The optimum pH for the removal of nutrients was 6.5–8.5 for nitrification and 8–9 for denitrification. Lower pH can adversely affect both denitrification and nitrification. They concluded that optimum agronomic practices must allow for the adjustment of pH and temperature to improve the remediation of wastewater as well as the growth of plants in hydroponic systems treating wastewater. The success of the treatment of wastewater using hydroponic systems will depend on the selection of crops that can adequately use nutrients from the wastewater for its growth. The efficiency of several vegetables was studied by Haddad et al. (2012) who established a exclusion of 47 and 91% for phosphorus and nitrogen respectively, for several types of vegetables. The efficiency in the elimination of nutrients by each type of vegetables would depend on the specific nutrients demanded by each type of vegetable. Plus, according to Hirel et al. (2007), the adsorption and assimilation of nutrients and the production of biomass by vegetables will be determined by the configuration of their roots and leaves. In hydroponic wastewater treatment systems, there is commonly a combination of biological, chemical, and physical processes taking place for the elimination of nutrients. These processes include sedimentation, adsorption, microbial degradation, nitrification, and absorption by the plants (Hijosa-Valsero et al., 2010; Li et al., 2014; Matamoros & Salvadó, 2012). Plants play a crucial part in these systems by using up nitrogen, phosphorus, and other elements from the wastewater (Ko et al., 2011; Ong et al., 2010). The mechanism of the absorption of nutrients by the plants consists of a series of processes like phytofiltration, phytotransformation, phytodegradation, and phytoextraction (Tangahu et al., 2011). Through these processes, plants ooze substances through their roots that allow for the immobilization, stabilization, and bond of organic impurities, thus, causing a reduction in their bioavailability (Moreno et al., 2008). This allows for an even better treatment of wastewater, minimizing the discharge of nutrients and other pollutants to the environment. The capacity of certain types of vegetables for the elimination of contaminants such as nutrients, toxic antibiotics and metals has been studied by Vyzamal (2007), Harrington and Scholz (2010) and Ha et al. (2011). The elimination of nitrogen in hydroponic wastewater systems is undertaken mainly through sedimentation, denitrification, and adsorption by the plants. However, this last process does not represent a complete elimination process unless plants are harvested endlessly (Healy et al., 2007). The success of the denitrification and nitrification process in a hydroponic wastewater treatment system will also depend on the interaction between the roots system and the microorganisms in the biological treatment. Microorganisms will degrade the inorganic nitrogen through denitrification, transforming nitrate into nitrogen gas (Gebeyehu et al., 2018). In addition, microorganisms use nutrients by transforming them into their own biomass. Rakocy (1997) and Solanki et al. (2017) reported that, hydroponics in floating rafts consisting in plastic mesh/sheets of polystyrene/bamboo to support the plants, can produce enough biofiltration if the area of crop production is adequate. However, due to the complexity of both systems for the treatment of wastewater in terms of

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obstruction and nitrification, generally it is necessary to add a sedimentation tank and a biofilter to remove solids and ammonia, respectively (Nelson, 2008). On the other hand, systems with different types of media are simpler to operate for the treatment of wastewater as they do not need a biofilter and use common media like rocks, clay and stones, for nitrification (Zhou et al., 2015). Gebeyehu et al. (2018) described an effectiveness of removal of nitrogen (48%) applying gravel and a system with intermittent flow. This was attributed to the added capacity of the gravel to provide sites for biofilm formation. It was also observed that daisies grew significantly better with secondary effluent than with ordinary nutrients solution using soils and perlite. The use of secondary effluent contributed towards a 44% increase in production compared to a common solution of nutrients. The use of Datura innoxia in a commercial hydroponic system with wastewater resulted in effluents that complied with discharge limits and low levels of total suspended solids, BOD and COD after 24 h of treatment (Vaillant et al., 2003). The application of secondary effluent through drip irrigation for daisies using soilless and soil cultivation decreased the total number of saleable flowers by 21% in comparison to a common nutrients’ solution. However, there were no significant differences in the number of tomatoes using and closed hydroponic system with rayon: polyester 70:30 in comparison with black mulch as a growth medium. These results suggest that the efficiency of a hydroponic system with wastewater in terms of nutrients removal for the growth of plants will vary depending on the supply of nutrients and the type of irrigation used. Therefore, media selection is very important for the correct operation of these types of systems.

14.3 Removal Processes of Contaminants in Hydroponics Since from long time, wastewater has been used in irrigation as a source of nutrients in most of the arid regions in the world. Raw, partially treated or fully treated wastewater can be used for crops irrigation instead of being discharged to the environment. Though, it is essential to manage the health hazards linked with the pathogens commonly present in wastewater. Agriculture accounts for approximately 70% of the water use worldwide, so the reuse of wastewater for irrigation is very important, particularly when considering the recycling of nutrients like phosphorus and nitrogen. It may be possible to avoid detrimental effects on the environment and public health by reusing municipal wastewater in agriculture through hydroponic systems. In a wastewater treatment plant with hydroponics the plants are cultivated with wastewater rich in nutrients and it is part of the biological treatment. This method is considered critical for a tertiary wastewater treatment plant, with the goal to eliminate nutrients, especially nitrogen (Norström et al., 2003). Numerous experimental studies have shown that hydroponic wastewater systems offer advantages over conventional wastewater treatment plants (Carvalho et al., 2018; Gebeyehu et al., 2018; Osem et al., 2006).

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Just like in constructed wetlands, the mechanisms of elimination of contaminants in hydroponic wastewater treatment plants include a combination of physical, biological and chemical processes, where microorganisms and plants interact in the supporting media (Stottmeister et al., 2003; Vinita et al., 2008). However, it has been demonstrated that hydroponic wastewater treatment plants are a technology more environmentally friendly and economic that constructed wetlands.

14.4 Desirable Characteristics of Plants Used in Hydroponic Each type of crop has a specific role to play in phytoremediation and can differ in the mechanism of absorption of toxic metals, which include dispersion, osmoregulation, translocation, accumulation, and exclusion. The concentration, translocation and accumulation, of heavy metals happen in the aerial part of the plants (Bhargava et al., 2012). According to Nyquist and Greger (2007) the mechanism for the absorption of toxic metals in aquatic plants depends on the plant type and their concentration in wastewater, and it occurs through the direct absorption from the water to the surface of the plant. This procedure is followed by the active or passive transport through membranes and the small-scale absorption through roots. Jha et al. (2016) detected this mechanism particularly in floating and submerged plants due to their underdeveloped root system. The growth rate of the plant and the concentration of toxic metals in the tissues of the plant have a direct effect on the plant’s capacity to remove the toxic metals (Giripunje et al., 2015). The accumulation of toxic metals in vegetable tissue will vary depending on the type of plant: in floating aquatic plants it is through the roots while in submerged plants it is through the whole body (Jha et al., 2016). Phytofiltration is defined as the use of aquatic plants, either floating, submerged or emergent for the removal of contaminants such as toxic metals through the direct uptake by the roots of the plants (Pratas et al., 2014). Phytofiltration can be done by rhyzofiltration (the use of roots) or blastofiltration (the use of seeding) or caulofiltration (the use of sprouts). Since soil plants have longer roots than aquatic plants, they have a larger surface area for the better absorption of toxic metals. To absorb the largest number of contaminants through phytofiltration, the plants need a dense root system and to grow hydroponically. These parts of the plant can favor the bioadsorption as well as its biochemical properties (Xie et al., 2013). The efficiency of the phytofiltration will depend in great measure with the biochemical characteristics of the plants and the photosynthetic microorganisms of the surface (Ong et al., 2010). Abhilash et al. (2009) found that toxic metals removal does not interfere with the growth rate of plants through phytofiltration, although it is not a recommended method for the in-situ treatment of plants with short roots and low growth rate. For example, an ideal plant should have a high tolerance to the accumulation of metals, a large production rate of biomass and be easy to harvest.

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In aquatic plants, the adsorption and transport of contaminants, like toxic metals, is defined as its potential of bioaccumulation. The most important parameters are the concentration of contaminants in the sections of the plant that are harvested and the biomass yield of the plant (Rycewicz-Borecki et al., 2016). The bioaccumulation of metals will be greatly affected by biotic and abiotic factors like pH, temperature, and ionic species in the aquatic system (Xing et al., 2013). Vesely et al. (2011) calculated the potential of rhyzofiltration (RP) for the removal of toxic metals in an aqueous solution with the following equation:  RP (mg toxic metal m2 year)  RP =

(Cleaves × Mleaves) + (Croots × Mroots) Mtotal

 × Mplant

where C is the concentration of the toxic metal (g/L), M is the dry biomass yield (g), Mtotal is the biomass yield of leaves and roots and Mplant is the average of the plant yield (g dry weight/m2 year). The bioconcentration factor (BCF) and the translocation factor (TF) are two important aspects to evaluate the viability of any species of plant used for the phytoremediation of toxic metals. The BCF determines the absorption capacity of metals by macrophytes. The translocation of metals, on the other hand, from the roots towards other parts of the plant is called TF (for example, to rhyzoma, sprouts, leaves or stems) and is commonly used in submerged and emerging plants (Olguín & Sánchez-Galván, 2012). Massa et al. (2010) and Fawzy et al. (2012) calculated the bioconcentration factor and the translocation factor with the following equations: Metal concentration in plant Metal concentration in water Element concentration in plant Translocation factor (TF) = Element concentration in water

Bioconcentration factor (BCF) =

The capacity of the hydroponic system to eliminate pathogens in domestic wastewater was studied by Ndulini et al. (2018) for the growth of Bidens pilosa L and Amaranthus hybridus L. They reported a removal of 92.77% of fecal coliforms. A study by Ottoson et al. (2005) examined the capacity of a hydroponic system with wastewater for the removal of microorganisms. The study included water quality analysis, fecal indicators (Escherichia coli), spores of Clostridium perfrigens and somatic coliphages in raw, partially treated and fully treated wastewater. The hydroponic system reached microbial removal of 60–87%. The study concludes that adsorption and absorption through the root system contribute towards the removal of microorganisms. Because of this, if this part of the plants will be used for human consumption, special care needs to be taken, like the addition of a pretreatment system for the wastewater to eliminate pathogens before the hydroponic system and the selection of crops that are not eaten raw (WHO, 2006).

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Gebeyehu et al. (2018) also studied hydroponic systems with wastewater, in this case in Ethiopia using the plant T. Latifola to treat the effluent of breweries. The treatment units consisted of gravel and a thin layer of sand to support the roots of the plants and to provide enough surface area for the microbial activity. The hydroponic system was used as the final step for the treatment of wastewater for the reduction of BOD, suspended solids, and nutrients. The system could remove 69% NTK, 54% ammonia, 51% nitrates and 55% phosphorus. Hydroponic systems have also been proved efficient for the removal of toxic metals (Rababah & Ashbolt, 2000). The system treated primary effluent by means of a nutrients biofilm using lettuces as crops. Removal efficiencies of 85 and 92% for Ni and Cr were reached, and more than 60% removal for B, Ag and Zn. However, the system was not very efficient (less than 30% removal of As, Cd and Cu. The results demonstrated that lettuce cultivation in hydroponics using primary effluent as source of nutrients and water, was efficient at removing toxic metals from the wastewater. This demonstrates that crops that are going to be eaten raw need to have a low assimilation rate of toxic metals; the same dos does not apply for crops that are not going to be consumed raw by humans.

14.5 Environmental, Economic and Social Benefits Associated to the Use of Hydroponics for Wastewater Treatment A few studies on the economic benefits of using hydroponics for wastewater treatment have been undertaken and have provided positive results (Shalaby et al., 2008; Molinos-Selante et al., 2011). For example, Schrammel (2014) analyzed the costs and benefits of hydroponics for wastewater treatment and a conventional wastewater treatment in terms of required work effort, resources, and produced goods and value. Significant differences were detected, and the hydroponic system requires greater capital investment, labor, and energy costs (Schrammel, 2014). However, the cost benefit will vary according to location, weather, and the general environment education of the society in terms of the impact of wastewater on the environment and public health. The hydroponic system can be an opportunity to produce high quality crops with the plus of providing a treatment for wastewater, which can represent an increase in the quality of life and a decrease in the dependency of external water resources. The system can serve as a constant provider of food and water for local communities and at the same time reduces the impact of conventional agriculture practices and the use of agriculture chemicals (Rakocy et al., 2004).

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14.6 Future Research in Hydroponics for Contaminants Removal From a market point of view, it is important to consider the local availability and local consumers’ habits and preferences when selecting the crop to be produced hydroponically. Some attributes to consider are taste, color, firmness, nutritional value, shelf life and resistance to pathogens (Brummell & Harpster, 2001). As presented earlier, there are many benefits of using wastewater in hydroponic systems to produce different types of crops, especially vegetables. However, it has also been reported a decrease in the quality of the crops and its shelf life, which can limit the implementation of these systems. Therefore, it is essential that the right combination type of water/soil/irrigation is used to decrease the risk of microbial contamination in fresh products (Gomez-Lopez et al., 2005; Luedtke et al., 2003). This reduction could be caused by a lesser contact between the water for irrigation and the edible parts of the plant (Cifuentes-Torres, 2022). Thus, more research is needed in this area and in full-scale experiments to fully demonstrate the effectiveness of the technology.

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