Graphene and its Derivatives (Volume 2): Water/Wastewater Treatment and Other Environmental Applications (Materials Horizons: From Nature to Nanomaterials) [1st ed. 2023] 9819943817, 9789819943814

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Materials Horizons: From Nature to Nanomaterials

Kaustubha Mohanty S. Saran B. E. Kumara Swamy S. C. Sharma   Editors

Graphene and its Derivatives (Volume 2) Water/Wastewater Treatment and Other Environmental Applications

Materials Horizons: From Nature to Nanomaterials Series Editor Vijay Kumar Thakur, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, UK

Materials are an indispensable part of human civilization since the inception of life on earth. With the passage of time, innumerable new materials have been explored as well as developed and the search for new innovative materials continues briskly. Keeping in mind the immense perspectives of various classes of materials, this series aims at providing a comprehensive collection of works across the breadth of materials research at cutting-edge interface of materials science with physics, chemistry, biology and engineering. This series covers a galaxy of materials ranging from natural materials to nanomaterials. Some of the topics include but not limited to: biological materials, biomimetic materials, ceramics, composites, coatings, functional materials, glasses, inorganic materials, inorganic-organic hybrids, metals, membranes, magnetic materials, manufacturing of materials, nanomaterials, organic materials and pigments to name a few. The series provides most timely and comprehensive information on advanced synthesis, processing, characterization, manufacturing and applications in a broad range of interdisciplinary fields in science, engineering and technology. This series accepts both authored and edited works, including textbooks, monographs, reference works, and professional books. The books in this series will provide a deep insight into the state-of-art of Materials Horizons and serve students, academic, government and industrial scientists involved in all aspects of materials research. Review Process The proposal for each volume is reviewed by the following: 1. Responsible (in-house) editor 2. One external subject expert 3. One of the editorial board members. The chapters in each volume are individually reviewed single blind by expert reviewers and the volume editor.

Kaustubha Mohanty · S. Saran · B. E. Kumara Swamy · S. C. Sharma Editors

Graphene and its Derivatives (Volume 2) Water/Wastewater Treatment and Other Environmental Applications

Editors Kaustubha Mohanty Department of Chemical Engineering Indian Institute of Technology Guwahati Guwahati, Assam, India

S. Saran R&D and Production Good Earth Chemicals Pvt. Ltd. Hospet, Karnataka, India

B. E. Kumara Swamy Department of Industrial Chemistry Kuvempu University Shankaraghatta, Karnataka, India

S. C. Sharma National Assessment and Accreditation Council (NAAC) Bengaluru, Karnataka, India

ISSN 2524-5384 ISSN 2524-5392 (electronic) Materials Horizons: From Nature to Nanomaterials ISBN 978-981-99-4381-4 ISBN 978-981-99-4382-1 (eBook) https://doi.org/10.1007/978-981-99-4382-1 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 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 Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

Graphene oxide is one of the highly explored materials in various research areas such as environmental, energy, sensor, membrane, and biomedical application due to their distinctive properties. This book is aimed to detail the introduction and recent advancement in the preparation and synthesis of graphene, graphene oxide, reduce graphene oxide, and their composite materials. Also, it explains the various characterization techniques which are used to study the physical, chemical, optical, compositional, and morphological characteristics of the prepared graphene materials for water, wastewater, and environmental applications. This book written by renowned authors explores the novelty and importance of graphene-based materials in the field of water, wastewater, and environmental application. Hence, this book may serve as an important source of reference which provides in-depth information about the graphene-based material for environmental application which is helpful for undergraduate, postgraduate students, scholars, and scientists. We hope that all the chapters covered in this book are able to explore the interest of readers for generation of new ideas and knowledge regarding graphene-based nanotechnologies. We are extremely thankful to contributors of these book chapters who provided their novel thoughts on graphene-based research through this edited book. We are also highly thankful to Springer for their continuous support and cooperation at every stage during the production of the book. Guwahati, India Hospet, India Shankaraghatta, India Bengaluru, India

Kaustubha Mohanty S. Saran B. E. Kumara Swamy S. C. Sharma

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Contents

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Insights into Graphene-Based Materials as an Adsorbent for Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Komal Saini, Abhisek Sahoo, and Thallada Bhaskar Graphene and Its Composites for Water and Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thanigaivelan Arumugham, Abdul Hai, K. Rambabu, G. Bharath, Shadi W. Hasan, and Fawzi Banat Graphene-Based Materials in Effective Remediation of Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ragavan Chandrasekar, Das Bedadeep, Tasrin Shahnaz, Vishnu Priyan Varadharaj, Ajit Kumar, Harish Kumar Rajendran, and Selvaraju Narayanasamy Graphene-Based Nanocomposite Solutions for Different Environmental Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preetha Ganguly, Rwiddhi Sarkhel, Sandipan Bhattacharya, and Papita Das

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Application of Graphene, Graphene Oxide and Reduced Graphene Oxide Based Composites for Removal of Chlorophenols from Aqueous Media . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Subhadeep Biswas, Ankurita Nath, and Anjali Pal

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Graphene Oxide and Its Derivatives as Additives in Polymeric Membranes for Water Treatment Applications . . . . . . . . . . . . . . . . . . . 129 Krishnamurthy Sainath and Akshay Modi

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Harnessing of 2D Carbon-Based Heterostructures as a Photocatalyst Towards Wastewater Treatment . . . . . . . . . . . . . . . 151 Sujoy Kumar Mandal, Sumit Mandal, and Debnarayan Jana

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Impact of Graphene Oxide Synthesis Method on Eosin-Y Decolourization Activity of Graphene Oxide-TiO2 Nanocomposite Under UV and LED Light . . . . . . . . . . . . . . . . . . . . . . . 173 Reeti Kumar and Suparna Mukherji

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Graphene Oxide Nanocomposites for the Removal of Antibiotics, Pharmaceuticals and Other Chemical Waste from Water and Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Karan Chaudhary and Dhanraj T. Masram

10 Graphene and Its Derivatives Based Membranes for Application Towards Desalination . . . . . . . . . . . . . . . . . . . . . . . . . . . 209 Satadru Chakrabarty, Anshul Rasyotra, Anupma Thakur, and Kabeer Jasuja 11 Graphene Nanoparticles and Their Derivatives for Oil Spill Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Rupali Gautam, Abhisek Sahoo, Kamal K. Pant, and Kaustubha Mohanty

About the Editors

Prof. Kaustubha Mohanty has obtained his Ph.D. degree in Chemical Engineering from Indian Institute Technology Kharagpur and is currently working as Professor and Head of Chemical Engineering department at Indian Institute Technology Guwahati. He had two postdoctoral stints at Ben-Gurion University, Israel, and McMaster University, Canada. His key research areas are biofuels, biological wastewater treatment, membrane technology, microalgae biorefinery, and biomass pyrolysis. He has published more than 190 research papers in peer-reviewed journals and edited few books. He has developed an advanced microalgal biorefinery model that integrates wastewater treatment and high-value biofuel production. He has worked extensively on catalytic and non-catalytic pyrolysis of lignocellulosic biomass that had resulted in sustainable bio-oil production under the circular bioeconomy approach. He is Chief Editor of Journal of Chemistry; Associate Editor of The Journal of Institution of Engineers (India) Series: E; Associate Editor of Research Journal of Environmental Sciences; Editorial Board Member of Renewable Energy (Elsevier) and Biomass Conversion and Biorefinery (Springer). He is Fellow of Royal Society of Chemistry, UK, Fellow of Institution of Engineers (India), and Fellow of Indian Institute of Chemical Engineers. He is Recipient of Distinguished Alumni Award 2022 from RIT, Bangalore (Formerly MSRIT). Dr. S. Saran has obtained his Ph.D. degree in Ecology and Environmental Sciences from Pondicherry University and is currently working as Assistant General Manager, Research and Development department at Good Earth Chemicals Pvt. Ltd., Hospet, Karnataka. Previously, he was worked as Principal Project Associate at the NEERI in mercury remediation from contaminated soil project and Institutional Postdoctoral Fellow (IPDF) at Centre for the Environment, Indian Institute of Technology Guwahati, India, focused on green synthesis and characterization of metal nanocomposite for catalytic and photocatalytic applications. His key research areas are photocatalysis, catalysis, water and wastewater treatment, odor control chemicals, and waste management. Also, he has plenty of research experience in the physical, chemical, microbial, and heavy metal analysis of water, wastewater, soil, sediments, and ambient air quality monitoring and assessment. He has published more ix

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than 25 research papers in peer-reviewed journals. Presently, his research work is on development of new chemicals for odor control and wastewater treatment applications. Prof. B. E. Kumara Swamy has obtained his Ph.D. degree in Industrial Chemistry from Kuvempu University and Postdoctoral Research Associate from Southern Methodist University, Dallas, Texas, and University of Virginia, Virginia, USA. Presently, he is working as Professor of Industrial Chemistry at Department of PostGraduate Studies in Industrial Chemistry, Kuvempu University. His research areas are development of electrochemical sensor for some neurotransmitters, biosensors, nanosensors, and nanochemistry. He has published more than 282 research papers in peer-reviewed journals. He is Associate Editor in Science Letters Journal, World Research Journal of Analytical Chemistry, Academic Editor in Journal of Chemistry, and Advisory Board Member in Biopublications and worked as Guest Editor in Special Issue on Nanoparticle and Cancer Treatment. He is Fellow of American Chemical Society, Life Member of Indian Society of Analytical Scientists, SAEST Member, and ICC Member. He served as Deputy Registrar in Development Section Kuvempu University from July 2015 to September 2020, Deputy Director in IQAC, Kuvempu University, and NAAC-DVV External Expert, Government of India. He is Recipient of Prof. M. R. Gajendraghad Gold Medal, Young Scientist Award from ICC and Swadeshi Science Congress, and Distinguished Scientist Award. Presently, he is Chairman in Board of Studies in Department of Industrial Chemistry, Kuvempu University, and Research and Development Special Officer to Vice Chancellor of Kuvempu University Shankaraghatta, Karnataka, India. Prof. S. C. Sharma is an amalgamation of Eminent Academician, Researcher, and Proficient Administrator. He is Leading Scientist in the field of material science and engineering, nanoengineering and fracture mechanics. The intellectual footprints of Prof. Sharma are exemplified from more than 348 research articles and several books that he co-authored. His latest research work includes photoluminescence of nanophosphors, photoluminescence, thermoluminescence, photocatalytic studies of radioactive nanomaterials, sensors for phenolic compounds, hydroquinone, melamine, dopamine, paracetamol, folic acid, etc., display, dosimetry, and advanced forensic applications of nanomaterials. His research collaborations are with more than 40 research and academic institutions in India as well as abroad. He has ten doctoral conferments, and he was Member of State Planning Board of Government of Karnataka, Former Director of Centre for Manufacturing Research & Technology Utilization, Centre for Social Service & Skills Promotion, Principal of R V College of Engineering, Bengaluru, Vice Chancellor of Tumkur University, Vice Chairman, Karnataka State Council for Higher Education, Government of Karnataka, and Honorary Fellowship, Karnataka Science and Technology Academy, Government of Karnataka, Bangalore. He has been conferred Honorary Rank of Colonel in the National Cadet Corps by Directorate General, NCC, New Delhi. Currently, he is holding the post of Director, NAAC, Bangalore.

Chapter 1

Insights into Graphene-Based Materials as an Adsorbent for Wastewater Treatment Komal Saini, Abhisek Sahoo, and Thallada Bhaskar

1.1 Introduction Water—an essential natural resource for the prevalent flora and fauna, is on the verge of severe contamination and extermination. Rapid industrialization and urbanization are believed to be majorly responsible for various contaminants in the water bodies. So, treating wastewater with low-cost proficient techniques has become extremely important. Among all the reported proficient methods for wastewater treatment, adsorption seems to be the most efficacious as it facilitates the tuning of adsorbent surface properties depending upon the adsorbate. The plethora of literature indicates that wastewater treatment via adsorption has been tried through numerous adsorbents like fly ash, polymeric resin, and activated carbon [1]. In recent years, adsorption has now inclined toward the utilization of graphene-based materials. Graphene—a repeating unit of graphite, is a two-dimensional, single-layer, one-atom-thick carbon material [2]. It consists of hexagonally packed covalently bonded sp2 hybridized carbon atoms, and the fourth electron of these carbon atoms is delocalized over the entire layer, resulting in high electrical conductivity. Other than this, graphene exhibit various other excellent physico-chemical properties like high surface area, excellent thermal conductivity, tranquil structural modification, hardness, elasticity, and flexibility leading to K. Saini · T. Bhaskar (B) Thermo Catalytic Process Area (TPA), Material Resource Efficiency Division (MRED), CSIR-Indian Institute of Petroleum (IIP), Dehradun 248005, Uttarakhand, India e-mail: [email protected] Academy of Scientific and Innovative Research (AcSIR), Sector19, Kamla Nehru Nagar, Ghaziabad 201002, Uttar Pradesh, India A. Sahoo Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016, New Delhi, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Mohanty et al. (eds.), Graphene and its Derivatives (Volume 2), Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-99-4382-1_1

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various environmental applications like pollution detection, sensing, energy storage, contaminants removal from wastewater, and many more [3-5]. The utilization of graphene as an adsorbent for contaminants removal from wastewater seems to be a cost-intensive process. However, the efficiency of the process can be enhanced by preparing such material, which originated from the lowcost precursor and entails the properties of graphene as well, leading to the attainment of a competent process economy target. The study has already begun synthesizing such materials. For example, Wang et al. exfoliated low-defect graphene from pristine graphite using lignin as an additive. The material was then used for the preparation of anti-corrosion coatings. The coating showed outstanding corrosion resistance for carbon steel [6]. Similarly, Badri et al. successfully exfoliated a few layers of thick graphene sheets with minor structural defects using alkali lignin as a surfactant. Also, removing alkali lignin through centrifugation, annealing, and thermal treatment from the surface of exfoliated graphene was efficient [7]. It is worth noting that, in general, the degree of graphitization of the low-cost graphene material has very much deviated from the ideal graphitic carbon. Thus, this chapter has highlighted the processes which are cheap and maintained the degree of graphitization in the final material. Since the utilization of low-cost graphene-based materials in the field of adsorption is still at a preliminary stage, and thus, to have the hypothesis regarding the adsorption performance of these materials, the chapter has also entailed the physico-chemical properties and adsorption insights of graphenebased materials. Record of complete coverage of the graphene-based materials along with their respective sorbate is scarce and nearly not established for advanced studies. Thus, a meticulous approach has been adopted using a co-occurrence cluster map to determine the potent adsorbate candidate and hence the targeted wastewater stream for graphene-based materials.

1.2 Preparation Methods of Graphene-Based Materials The available graphene synthesis technique can be divided into two broad categories: bottom-up and top-down approaches. The bottom-up approach involves the use of hydrocarbon compounds as starting material, whereas the top-down involves powdered graphite as a precursor, which is exfoliated or reduced [8]. The most protruding bottom-up approaches are chemical vapor deposition, epitaxial growth, thermal pyrolysis, organic synthesis, and widely used top-down approaches include mechanical exfoliation, liquid-phase exfoliation, and chemical oxidation–reduction [5, 9]. However, based on process cost, these graphene synthesis techniques can also be categorized into cost-intensive and low-cost processes, which are discussed in detail in the subsequent section.

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1.2.1 Cost-Intensive Processes The utilization of either high-cost material as the precursor or expensive equipment for graphene preparation adds to the overall process cost. This category mainly includes synthesis techniques like mechanical exfoliation, epitaxial growth, and chemical vapor deposition.

1.2.1.1

Mechanical Exfoliation

In this technique, graphene is prepared by exfoliating graphite, i.e., layer-by-layer peeling of graphene from the bulk of graphite. In this process, the graphene is peeled off by disrupting the Van der Waal forces between the adjacent graphene through well-known methods like ball milling, sonication, and fluid dynamics [5]. These methods apply either regular or lateral forces, resulting in exfoliation. However, in some cases, these forces fragment larger graphite/graphene into smaller ones. The graphene fragmentation resulted in low surface area and reduced lateral size, whereas the fragmentation of graphite facilitates easy exfoliation and thus is more advantageous [10]. Table 1.1 illustrates the mechanics of various exfoliation methods.

1.2.1.2

Chemical Vapor Deposition

Chemical vapor deposition (CVD) involves precursor’s decomposition/vapor phase chemical reaction followed by the adsorption and deposition of gaseous atoms in the form of thin films onto the substrate [11]. The commonly used substrates are transition earth metals and ceramic materials [12]. The graphene preparation via CVD involves a wide range of hydrocarbon precursors in all three phases, viz., solid, liquid, and gas and these precursors are the source of carbon atoms. Since during CVD, precursors undergo decomposition, and thus the hydrocarbon precursor with low C-H bond dissociation energy requires low temperature and vice versa. Thus it can be concluded that the dehydrogenation ability of the hydrocarbon is the critical factor affecting graphene synthesis [13]. The commonly used gas precursors are methane [14], ethane [15], and acetylene [16]; liquid precursors are benzene [17], Table 1.1 Exfoliation mechanics Methods

Origin of exfoliation

Comments

Ball milling

• Collisions among the balls during rolling action • Shear forces

• Fragmentation dominates • High-quality and large-sized graphene

• Intensive tensile stress by Sonication micro-bubbles (liquid-phase exfoliation) • Unbalanced lateral compressive stress

• Graphene with more defects

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methanol, ethanol [18], and solid precursors are polymeric films, viz., polymethyl methacrylate, polystyrene [19]. Among the most used preparation methods, CVD is the most frequently used for graphene synthesis because of its modest setup, durable usage, and scaling-up potential.

1.2.1.3

Graphite Oxidation–Reduction

The oxidation–reduction of graphite is also one of the efficient methods reported in the literature for preparing graphene-based materials like reduced graphene oxide (rGO). In this method, firstly, graphite is oxidized, and then the graphite oxide thus obtained, is exfoliated to produce graphene oxide. The oxygen functionalities in graphene structure alter its properties like electrical conductivity, surface charge, transparency, and flexibility [20]. So, these properties are retained by further reducing the graphene oxide and finally, rGO is obtained. The reduction of graphene oxide can be attained through numerous methods like photocatalytic, thermal, and chemical reduction [21]. Chemical reduction is well practiced out of all the reported reduction methods. The commonly used reducing agents in the chemical reduction method are hydroquinone, sodium borohydride, hydrazine, and sodium hydrosulfite, but these reagents are hazardous to both living beings and the environment. Also, these reagents require the assistance of surfactants to prevent irreversible agglomeration [22]. Thus, the toxic reducing agents urge the green reductants to mitigate the ubiquitous environmental issues and to increase the process efficiency. It is worth noting that rGo prepared through this method is often confused with graphene, but it is graphene-like (absence of pristine graphene structure) due to several defects and oxygen functionalities.

1.2.2 Low-Cost Processes The synthesis of high-quality graphene-based materials at low cost is a need of an hour. The involvement of cheap and green materials during the synthesis will aid in accomplishing the low-cost aim. Thus, biomass seems to be a sustainable option for producing low-cost graphene-based materials. Graphene-based materials can either be directly produced via thermal degradation of biomass or synthesized through biomass assistance. Both methods are depicted in Fig. 1.1 and discussed in detail in a subsequent section.

1.2.2.1

Thermal Degradation of Biomass

The graphitic carbon material prepared through thermal degradation of biomass has also emerged as a low-cost process. The preparation approaches mainly involve pyrolysis and catalytic graphitization [23].

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Fig. 1.1 Low-cost preparation methods for graphene-based material’s production

Pyrolysis is a thermal degradation of biomass in an oxygen-deprived or oxygenlimited environment [24]. Graphitic carbon is generally produced via slow pyrolysis (slow heating rate (≤10 °C/min.), and longer retention time (>1 h) coupled with high temperature (up to 2500 °C)) [23]. The degree of graphitization of biomassderived carbon dramatically depends upon the pyrolysis temperature; the greater the temperature, the more the graphitization. Also, the prepared carbon has an ordered crystal structure [25], and a similar finding has been witnessed in several studies. For instance, Ru et al. [26] prepared the carbon material having a microcrystalline graphitic domain through the pyrolysis of microalgae. The pyrolysis was carried out at three different temperatures, viz., 700, 900, and 1100 °C, and it was perceived that the high pyrolysis temperature (900 °C) resulted in a high degree of graphitization. However, several studies have well documented that the carbon material obtained through biomass pyrolysis, significantly deviated from the ideal graphitic structure, even at high temperatures [27, 28]. The carbon structure thus obtained has graphite microcrystalline and amorphous zone [23]. Therefore, there is a need to find a method in which biomass-derived carbon can efficiently mutate into an ideal graphitic structure. Though the high-temperature pyrolysis produces graphitic carbon to some extent, it violates the “energy conservation and emission reduction” principles due to excessive energy consumption. So, one must look for an alternative with minimum energy consumption, and catalytic graphitization is one of them. Catalytic graphitization refers to the involvement of the graphitization catalyst in amorphous carbon [29]. In this method, the graphitization catalyst lowers the activation required for the transition from amorphous to graphitic phase and yields the graphitic carbon even at low temperatures (≤1000 °C) [30]. The significant advantage of this method is the retainment of the structural characteristic like surface functionalities of the biomass in the obtained carbon material. Catalytic graphitization mainly follows two mechanisms (i) dissolution and re-precipitation and (ii) carbide transformation and decomposition [23, 31]. The commonly used graphitization catalysts are transition metal elements and their oxides like Fe, Co, Cr2 O3, and MnO2 [29]. However, it is worth noting that the carbon material obtained through catalytic graphitization has a high degree

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of graphitization, but the porosity is minimal. So, the porosity can be improved by coupling the chemical activation with catalytic graphitization [32]. Chemical activation will promote the porosity and specific surface area of carbon via in-situ chemical, physical action, and lattice expansion caused by the activator’s metal atom insertion into the carbon lattice [9], whereas catalytic graphitization will promote graphitic structure.

1.2.2.2

Biomass-Assisted Preparation

The chemical reduction of graphene oxide to produce rGO has immense potential for industrial scalability. The reducing agents discussed in previous sections are hazardous, have cost-related issues, and compel irreversible agglomeration of GO in an aqueous solution [22]. However, the economic viability of the process can be boosted by utilizing the low-cost, highly efficient reducing agent, and biomass/ bio-organism seems to be a sustainable reductant for GO. Recently, research on biomass/bio-organism mediated reduction of graphene oxide is gaining much fame. For instance, Jiang et al. [22] utilized lignin as the reductant for synthesizing rGO. Lignin, a propyl phenol polymer and a by-product from various paper and pulp industries have the potential to reduce GO to graphene because of its antioxidant and radical scavenger ability [33]. The non-covalent interactions like hydrogen bonding, π-π interactions of lignin with rGO, and electrostatic interactions between the ionic moieties (–COO− , –SO3 2− ) of both lignin and rGO serve lignin as an excellent dispersing agent [34]. Lignin-assisted rGO was synthesized via hydrothermal carbonization of both lignin and GO. It is worth noting that the significant advantage of the process is that it produces residual lignin with enhanced electrical conductivity and degree of carbonization as well, in addition to efficient rGO [35]. Similarly, in another study [36], the reduction of GO was tried using lemon extract. The onepot reduction resulted in a high carbon/oxygen ratio and thus indicated efficient deoxygenation of GO. The reduction was mainly attributed to the various organic compounds like carotenoids, alkaloids, carbohydrates, and protein in the extract. These compounds acted as stabilizing and capping agents [37]. Much research has also begun on GO reduction via bio-organism. For instance, Ahmad et al. [38] synthesized rGO using algal extract. The reduction resulted from various functional groups and enzymes of the algal cells. The rGO obtained has reduced oxygen functionalities in comparison to that of GO. Other than these reductants, researchers have also utilized plant extract [39, 40], sugarcane bagasse [41], coconut waste [42], palm kernel shells, and oil palm leaves [43] as green reductants. Thus, it can be concluded that the seeds of commercializing the various applications of graphene-based materials can be sought at the initial stage, i.e., by adopting the low-cost preparation process.

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1.3 Graphene-Based Material(s): A Potent Sorbent Candidate for Contaminants Removal The implementation of graphene-based structures in wastewater treatment has generated much scientific attention, especially in the adsorptive removal of contaminants. To recognize the commonly employed graphene-based sorbent and their tagged contaminants, a co-occurrence cluster map was plotted, Fig. 1.2. The focus and direction of the research in the area mentioned above were interpreted based on the literature from 2015 to 2021. Eight important keywords, viz., graphene oxide, reduced graphene oxide, graphene nanoparticles, pollutant removal, adsorption, and wastewater treatment, served as the foundation for creating and visualizing this map. Node size in the map represents the frequency of the keywords used [44]. The thickness of the connection line between the two keywords indicates the co-occurrence strength, i.e., the thicker the line, the more frequently the two keywords co-appear [45]. The map shows that graphene-based sorbents often used are graphene oxide, graphene, graphene nanocomposites, aerogels, and hydrogels. Graphene oxide has been the most widely utilized among these sorbents. The cluster map also depicts

Fig. 1.2 Co-occurrence cluster map for the removal of contaminants from wastewater using graphene-based materials

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that these materials have demonstrated higher adsorption capacities toward various contaminants, viz., heavy metals and dyes. The most frequently studied dyes included malachite green and methylene blue. However, the use of graphene-based materials for pharmaceuticals and phenolics removal is least studied and appears to be a potential future area. The cluster map also shows that there has been a minimal study on the regeneration of graphene-based sorbents. Thus, the existing research must incorporate the sorbent’s regeneration study to boost the process economy.

1.4 Adsorption Insights For the continued advancement of graphene-based functional materials and their practical applications, it is essential to comprehend the adsorption insights. Thus, this section focuses on the insights of adsorption mechanism, adsorbate uptake rate, and energetics changes involved during the sorption by graphene-based materials. These revelations will aid in correlating the processes through which contaminants and adsorbents interact.

1.4.1 Adsorption Isotherm Studies The adsorption isotherm modeling often offers the quantitative benchmark for the sorption capacity of different sorbents. These models aid in the prediction of the adsorption type and mechanism. Langmuir, Freundlich, Dubinin-Radushkevich, Redlich Peterson, and Temkin are commonly employed adsorption models. However, it is worth noting that most studies focusing on contaminant removal using graphenebased materials have fitted the experimental data with Freundlich and Langmuir isotherms only, with the Langmuir isotherm being best fitted. For instance, Wang et al. utilized hybrid graphene oxide aerogel for methylene blue, and tetracycline adsorption and the process was described by the Langmuir and Freundlich model. Also, the experimental data were best fitted with the Langmuir model [46]. Similarly, Tang et al. employed amphiphilic graphene aerogel for dye removal, and the experimental data were only fitted with Langmuir and Freundlich model [47]. Table 1.2 reveals the need to employ more isotherm models and, thus, a new future direction to understand the utmost potential of graphene-based materials toward contaminants removal.

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Table 1.2 Adsorption isotherm models Graphene-based adsorbents

Adsorbate

Isotherm models employed

Best fitted model

References

Graphene

Bisphenol

Langmuir Freundlich

Langmuir

Xu et al. [48]

βcyclodextrin/graphene oxide

Methylene blue Langmuir Freundlich

Langmuir

Yang et al. [49]

Graphene oxide/cellulose nanofibrils aerogel

Methylene blue Langmuir Tetracycline Freundlich

Langmuir

Wang et al. [46]

Amphiphilic graphene aerogel

Malachite green

Langmuir Freundlich

Langmuir

Tang et al. [47]

Ionic liquid-modified graphene

Methylene blue Langmuir Freundlich

Langmuir

Gupta et al. [50]

1.4.2 Adsorption Kinetics Adsorption kinetics: a necessity for constructing adsorption units, optimizing operation conditions, and providing quality efficiency and economy [51]. In batch sorption studies, the experimental data is generally fitted with five kinetics models, viz., pseudo-first order, pseudo-second order, Elovich, intraparticle diffusion, and liquid film diffusion. Out of these models, the latter two models explain the sorbate diffusion mechanism. The rate constants, equilibrium adsorption capacity, hydrodynamic characteristics, and adsorption process at various adsorbate concentrations can all be learned via kinetic investigations based on these models. The two kinetic models that are most frequently used to describe the adsorption kinetics of graphene-based materials are the pseudo-first-order and the pseudo-second-order model. As can be seen in Table 1.3, the pseudo-second-order model suited the experimental results reasonably well for the adsorption of both inorganic and organic contaminants on graphene-based materials. For instance, the adsorption of Rhodamine B dye onto graphene-based nickel nanocomposite followed a pseudosecond-order kinetic model [52], suggesting that chemical adsorption involving valence forces through the sharing or exchange of electrons between the adsorbent and the adsorbate controls the adsorption processes. Similarly, Yan and Li [55] fabricated Fe3 O4 graphene nanocomposites via a lowcost process, for Pb (II) ions adsorption, and the experimental data best fitted with the pseudo-second-order model. Table 1.3 also demonstrates that most studies have not employed the kinetic models corresponding to the adsorbate diffusion mechanism. It is also worth noting that most studies examining graphene-based materials as sorbents have asserted that the pseudo-second-order model fits the experimental data better than other kinetic models. However, the adsorption isotherm and thermodynamic studies reveal the opposite fallouts, i.e., physisorption, and little attention has been paid to these findings. Thus, the correlation between the kinetic model’s

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Table 1.3 Adsorption kinetics models Graphene-based adsorbents Adsorbate

Kinetic models employed

Best fitted model

References

Cellulose-graphene oxide aerogel

Methylene blue Pseudo first order Pseudo second order

Pseudo second order

Joshi et al. [53]

Fe3 O4 reduced graphene oxide

As(V), Ni(II), Pb(II)

Pseudo first order Pseudo second order

Pseudo second order

Vuong Hoan et al. [54]

Biomass-derived Fe3 O4 graphene nanocomposites

Pb(II)

Pseudo first order Pseudo second order

Pseudo second order

Yan and Li [55]

Reduced graphene oxide

Ni(II)

Pseudo first order Pseudo second order

Pseudo second order

Mahmoud et al. [56]

Graphene oxide/ polyethyleneimine hydrogel

Methylene blue Pseudo first order Rhodamine ß Pseudo second order

Pseudo second order

Guo et al. [57]

Graphene oxide

Emulsified and Pseudo first dissolved order diesel oil Pseudo second order Elovich Intraparticle diffusion

Pseudo second order

Diraki et al. [58]

assumption and featured remarks of the research regarding the sorption type needs to be established.

1.4.3 Adsorption Thermodynamics Adsorption research can only be meaningful and helpful if it considers the energetic changes, and thus, it is ideal for conducting in-depth research on adsorption thermodynamics. Generally, there are two different types of thermodynamic properties, viz., directly measurable, which include temperature and equilibrium constant, and indirectly measurable properties such as Gibb’s free energy, enthalpy, entropy, activation energy, and isosteric heat of adsorption. Table 1.4 shows that wastewater treatment using graphene-based material as an adsorbent is very minimal. According to all the studies that have been done, the process is spontaneous, endothermic, and

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Table 1.4 Adsorption energetics of graphene-based materials Graphen based adsorbents

Adsorbate

Energetics of adsorption

References

Reduced graphene oxide

Malachite green Spontaneous Gupta and Khatri [59] Endothermic Increased Entropy

Tamarind reduced graphene oxide

Methylene blue Spontaneous Rajumon et al. [60] Basic fuchsin Endothermic Increased Entropy

Magnetic graphene oxide-titanate composites

Pb (II)

Not studied

Yang et al. [61]

EDTA-functionalized chitosan graphene oxide

Pb(II) Cu(II) As(III)

Not studied

Shahzad et al. [62]

Biomass-derived Fe3 O4 graphene nanocomposites

Pb(II)

Not studied

Yan and Li [55]

accompanied by increased entropy. However, a few indirect thermodynamic properties like activation energy, isosteric heat of adsorption, and thermodynamic activation parameters like activation enthalpy, activation entropy, and free energy of activation have rarely been studied. These parameters are key design factors in evaluating the performance and forecasting the mechanism of an adsorption process. Activation energy corresponds to the temperature dependence of the reaction rate. In contrast, the isosteric heat of adsorption gives an idea about the adsorption heat at a constant amount of adsorbate adsorbed. The process’s thermodynamic activation parameters must be considered to determine if the adsorption process follows an activated complex formation. Thus, further exploration is recommended to lay the foundation for the theoretical basis of sorption thermodynamics. Additionally, it is essential to note that the graphene-based material(s) fabricated using low-cost methods, particularly biomass-assisted preparation, is limited. As a result, a new future direction still needs to be formed in comprehending the adsorption insights employing these materials.

1.5 Physico-chemical Properties Accountable for Adsorption The adsorption isotherm modeling could provide the overview/basic idea, including type, mono/multi-layer formation, and favourability of sorption. However, the physico-chemical properties accountable for sorption need to be established separately.

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The graphene-based materials prepared through the cost-intensive or low-cost process have been used for numerous applications like hydrogen storage, adsorption, supercapacitors, sensors, and drug delivery [63]. However, in recent years, graphene has been well known for its adsorption application which is mainly adopted for various environmental remediation purposes like wastewater treatment. The high surface area, tunable surface functionalities, chemical steadiness, and unique graphitization are the main physico-chemical properties that contribute to its adsorption application [64]. The high surface area (up to 2600 m2 /g) of the graphene-based materials is mainly because of the smaller graphene domain size [65], whereas the extent of graphitization is attributed to the precursor used during its preparation. The surface functionalities-the tunable property of the graphene-based material, can augment the adsorption performance to a greater extent. Graphene functionalization can be divided into two categories: non-covalent and covalent. The non-covalent functionalization is the non-destructive way of functionalization as it does not disrupt the extended π conjucation of the graphene material and thus involves the change in the characteristics of the graphene-based materials without changing their chemical structure [66]. Non-covalent functionalization generally occurs via Van Der Walls, hydrophobic, and electrostatic interactions and it can also occur through immobilization of practical molecule (which imparts the desired functionalization) onto graphene material via linker molecule [64]. Covalent functionalization involves the perturbation of the extended π conjucation of the graphene material due to the conversion of one or more sp2 hybridized carbon atoms to sp3 [67]. The covalent functionalization is mainly done through two routes: (a) covalent bond formation between dienophile and C = C bonds of graphene material and (b) covalent bond formation between organic functional groups and oxygen functionalities of graphene materials [68]. Being hydrophobic, graphene materials are non-dispersible in water [69]. Also, the π-π interaction among the graphene layers sometimes leads to agglomeration/restacking and hence limits its application in wastewater treatment. However, appropriate functionalization of graphene material can inhibit non-dispersibility and agglomeration and enables the material’s applicability for broad applications. The above-discussed physico-chemical properties generally lead to the effective binding of adsorbate onto graphene material through various molecular interactions. However, the liability of particular physico-chemical properties toward the adsorption performance is discussed in subsequent sections.

1.6 Role of Physico-chemical Properties During Adsorption—Plausible Adsorption Mechanism The high surface area of graphene materials enhances the pore-filling mechanism during the adsorption process, and its graphitization further boosts the π-π interaction with adsorbate having a π electron cloud. For instance, the high ciprofloxacin

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adsorption capacity of the graphene-based material was attributed to the interaction between the π electron cloud of adsorbent and adsorbate [70]. In addition to this, the excellent adsorption performance of graphene is attributed to its tunable surface functionalities, which are the origin of various electrostatic interactions. Since adsorption is a surface phenomenon so, increasing the type and quantity of surface functionalities will significantly affect the adsorption phenomenon. For example, suppose the adsorbent has a high surface area and degree of graphitization. In that case, efficient adsorption of aromatic organic contaminants is possible. However, the adsorption performance can be further improved if the Physico-chemical properties of the adsorbent are additionally increased as it leads to an interface between the surface functionalities of the adsorbent and adsorbate through various interactions like hydrogen bonding, electrostatic, and acid–base interactions. Physico-chemical properties/functionalities of graphene-based sorbents responsible for enhanced sorption of contaminants are tabulated in Table 1.5 and the resulting sorbate-sorbent interaction is depicted in Fig. 1.3. Table 1.5 Physico-chemical properties/functionalities of graphene-based sorbents for enhanced sorption Physico-chemical properties/ functionalities of graphene-based sorbents

Interacting contaminant

Driving interaction

References

Delocalized π electron network

Aromatic

π–π

Saini et al. [71]

H-donor atoms

Aromatic

Yoshida hydrogen interaction

Arabkhani and Asfaram [72]

Organic contaminants having H-acceptor atoms

Dipole–dipole H-based interaction

Joshi et al. [53]

Aromatic

n–π

Electron deficient metal ion

Complexation

Species with electron acceptor atom

Lewis acid–base interaction

Surface charge

Opposite charged inorganic/organic species

Electrostatic interaction

Charged functionalities

Ions with similar charge

Ion exchange

Surface area

Organic/inorganic

Pore filling

Electron donor atoms

Xu and Wang [51]

Wang and You [73]

Scaria et al. [74]

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Fig. 1.3 Plausible interactions between graphene-based sorbent and organic/inorganic contaminants

1.7 Conclusions and Future Perspectives Low-cost synthesis processes have the tremendous potential to replace the conventional cost-intensive processes for graphene material synthesis. Graphene-based materials have the immense capability for contaminants removal from wastewater owing to their inherited high specific surface area and degree of graphitization. However, the adsorption performance of the graphene material can be altered/ enhanced by tuning its surface functionalities. The selection of the best adsorbent out of all the graphene-based materials depends upon the functionalities of the adsorbate and adsorbent, as their interaction will significantly enhance the adsorption performance of the adsorbent. The cluster plot indicated that the graphene-based materials are good at removing organic and inorganic contaminants. However, the removal of pharmaceuticals and phenolics is least studied. A deep insight into the adsorbate diffusional mechanism, adsorption thermodynamics, and isotherm studies using graphene-based materials has significant gaps and thus has a long way to go. These studies are key design factors in evaluating the performance of the sorbent for process scale-up and for establishing the theoretical underpinnings of the sorption process using graphene-based materials.

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Chapter 2

Graphene and Its Composites for Water and Wastewater Treatment Thanigaivelan Arumugham, Abdul Hai, K. Rambabu, G. Bharath, Shadi W. Hasan, and Fawzi Banat

2.1 Introduction Increasing water pollution issues have become more crucial as a result of deleterious environmental impacts resulting from unrestrained discharges from human, industrial, and agricultural activities [1–3]. Besides posing hazardous effects on living organisms, these discharges also create water scarcity by adversely affecting freshwater sources. To protect the available water resources, a proper remediation strategy is necessary, especially in the case of wastewater treatment. There are several technologies available to treat wastewater that utilize different principles for the separation or degradation of contaminants present in aqueous streams. Despite the wide range of technologies available for water treatment, adsorption, capacitive deionization, and membrane technology are widely adopted for practical applications due to their high efficiency, ease of use, and relative affordability [4, 5]. During the past few years, nanotechnology has been rigorously implemented in a wide range of human activities, including the process of treating water and wastewater. There has been a great deal of research conducted on the development of nanomaterials to treat different types of pollutants in wastewater. These materials include metals, metal oxides, carbon nanotubes, graphite, and fullerenes [6]. Among these materials, graphene has attracted researchers and scientists since its discovery in 2004. Due to its two-dimensional (2D) sheet-like structure, made up of sp2 hybridized carbon atoms arranged in hexagonal or honeycomb-like patterns, graphene is unique among nanoparticles. Graphene is composed of pure carbon atoms in which the T. Arumugham · S. W. Hasan · F. Banat (B) Center for Membranes and Advanced Water Technology (CMAT), Khalifa University of Science and Technology, PO Box 127788, Abu Dhabi, UAE e-mail: [email protected] T. Arumugham · A. Hai · K. Rambabu · G. Bharath · S. W. Hasan · F. Banat Department of Chemical Engineering, Khalifa University of Science and Technology, PO Box 127788, Abu Dhabi, UAE © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Mohanty et al. (eds.), Graphene and its Derivatives (Volume 2), Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-99-4382-1_2

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monolayer atoms are covalently bonded while the different monolayers are tied together through van der Waals forces. Furthermore, graphene has many unique properties, such as a highly specific surface area, outstanding thermal conductivity, exceptional elasticity and strength, and excellent mobility of the charge carrier [7]. The distinctive advantages of graphene have allowed graphene to remain at the heart of scientific research since its discovery, with its numerous achievements transformed into practical applications. Graphene derivatives can be combined with various functional composite materials to extend their practicability. Graphene can be combined with a variety of functional materials to extend their practical applications. Graphene and graphene derivatives are commonly used in a variety of applications, such as supercapacitors, fuel cells, and desalination [8–10]. Previous studies have demonstrated that graphene derivatives can amalgamate with inorganic nanostructures, organic crystals, polymers, organic frameworks, biological materials, and carbon nanotubes. The emergence and expected future growth of graphene, graphene oxide (GO), and reduced graphene oxide (rGO) have been documented in numerous research articles and patent applications over the past few years [11]. Graphene oxide (GO) is a highly oxidative form of graphene formed through the chemical exfoliation of graphite. Its graphitic back contains functional oxygen groups: carbonyl and carboxyl groups at the ends of the layers and hydroxyl and epoxy groups at the base. With its large theoretical surface area, high water solubility, and oxygen-containing surface properties, GO offers much more promise than graphene for the effective treatment of contaminants in wastewater [12]. GO’s oxygen-rich functional groups facilitate the removal of toxic metal ions and positively charged organic pollutants by electrostatic and coordination interactions. As a result of various studies, graphene and its derivatives have demonstrated impressive capabilities in adsorption, capacitive deionization, and membrane technologies for the effective treatment of aqueous pollutants and contaminants that pose a significant threat to human health and the environment. This chapter highlights recent trends in the application of graphene and its composite materials in various wastewater treatment technologies, namely, adsorption, membrane technology, and capacitive deionization. An overview of the synthesis, characteristics, applications, and separation mechanisms of graphene and composite-based working materials used for water and wastewater treatment is provided. Moreover, the challenges and outlook for the effective realization of these materials in water and wastewater treatment are also critically commented upon.

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2.2 Properties and Preparation Techniques of Graphene-Based Materials 2.2.1 Graphene, Graphene Oxide, and Reduced Graphene Oxide Nanostructured carbon materials, especially graphene, graphene oxide (GO), and reduced graphene oxide (rGO), have gained enormous attention in e-environmental (wastewater treatment) and energy fields due to their superior physicochemical properties, availability of reactive functional groups and aromatic rings [13]. Graphene is a two-dimensional (2D) sp2 hybridized carbon allotrope containing a single layer of atoms arranged in a honeycomb lattice or hexagonal nanostructure [13]. This unique carbon nanomaterial exhibited superior thermal conductivity (5000 W/m*K), higher specific surface area (up to 2630 cm2 /g), intrinsic electron mobility (250,000 cm2 /V*s), and better electrical conductivity and chemical stability [14]. Interestingly, graphene showed exceptional tensile strength and is 200 times stronger than steel [14]. Furthermore, a graphene derivative, namely, graphene oxide (GO), is enriched in oxygen-based functional groups such as carboxyl, carbonyl, epoxy, ketones, aldehydes, and hydroxyls attached on the edges or basal planes of graphene layers. During the oxidation of graphene sheets, structural changes such as defects, cracks, fragmentation, wrinkles, and disorders have been observed, which resulted in a decrease in the catalytic, adsorptive, and electronic characteristics of graphene oxide. Therefore, to retain or enhance the electrochemical performance and physicochemical characteristics, GO can be transformed into another graphene derivative known as reduced graphene oxide (rGO) by chemical or physical reduction methods [15]. The resulting rGO showed a higher porous surface and superior electrical and thermal conductivity due to the elimination of functional oxygen groups with a carbon-to-oxygen (C/O) ratio of 8:1 to 246:1 [15, 16]. The morphological aspects of the graphite flakes, GO, and rGO, and the structural illustration of graphene and its derivatives are shown in Fig. 2.1a–e.

2.2.2 Synthesis Techniques Based on these astonishing features, graphene and its derivatives (GO/rGO) are considered promising and robust materials for various demanding applications prominently in photonics, biosensors, drug delivery, adsorption, catalysis, membrane separation, and desalination by capacitive deionization, food industry, supercapacitors, batteries, etc. Therefore, large-scale production of graphene is imminent to fulfill the market demand. Generally, graphene can be synthesized by a top-down method (comprising the structural breakdown or exfoliation of graphite into graphene

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Fig. 2.1 SEM images of a Graphite, b Graphene oxide, c–d Reduced graphene oxide reproduced from [16] (Copyright 2015. Reproduced with permission from Elsevier), and e structural illustrations of graphite, GO, and rGO reproduced from [17] (Copyright 2018. Reproduced with permission from Elsevier)

sheets) or a bottom-up approach (using carbon source gas to synthesize templateassisted graphene) [18]. The following sections present a detailed discussion on the synthesis techniques of graphene and related materials. Some of the common graphene synthesis techniques used by these approaches are presented schematically in Fig. 2.2.

Fig. 2.2 Schematic illustration of the current preparation methods for graphene using top-down and bottom-up approaches [19]. (Copyright 2020. Reproduced with permission from Elsevier)

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Top-Down Approach

The effective use of graphene and its derivatives on a large-scale requires economical and environmentally friendly synthesis techniques that produce high product yield and quality. In a top-down approach (destruction method), single-, bi-, or multilayered graphene sheets were produced by the delamination or exfoliation of graphite flakes or other carbon-based materials. The deterioration of larger precursors in oneor two-dimensional graphene nanosheets was obtained by mechanical exfoliation, arc discharge, reduction of oxidative exfoliation, liquid-phase exfoliation (LPE), and unzipping of carbon nanotubes (CNTs) [20, 14]. The top-down approach for graphene synthesis is highly scalable; however, the product yield, quality, and characteristics vary with the nature of graphite flakes. Briefly, carbon-based materials can be mechanically exfoliated by either normalforce- or shear-force-based methods to synthesize graphene nanosheets. Normal force synthesis is carried out using advanced machinery of ultrasharp single-crystal diamond wedges or three-roll mill machines to produce graphene of 1.13–1.41 nm [21]. This method reduces labor costs and eradicates the need for manual operation; however, increased capital and operational costs and purification of as-developed graphene limit the adaptation of the normal-force synthesis route on a commercial scale. Furthermore, the desired quality of graphene and its nanocomposites can be obtained by exfoliating graphite flakes in a ball mill using impact and attrition (shear forces) [21]. However, the time required to complete the ball milling operation could vary between 24 and 48 h and could impede the scalability of graphene production. Furthermore, graphene can also be synthesized using asphalt- or carbon-based materials through an arc-discharge method, comprising an anode (carbon precursor) and a cathode (graphite rod) immersed either in a gas or liquid medium. The applied voltage generates a high-temperature plasma (3500–5000 °C) by dissociating the medium and resulted in the sublimation of carbon precursors. This method could be economical by using air (instead of H2 /He) or water as a medium. However, because of the high energy requirement and process control, the arc-discharge method is not applicable to industrial-scale graphene production. Graphene and its derivatives (GO/rGO) are commonly produced by the chemical (oxidative) exfoliation of graphite, followed by electrochemical reduction. Currently, the modified Hummer method is the preferred process to synthesize GO economically and environmentally friendly since it eliminates the usage of toxic chemicals such as potassium perchlorate (KClO4 ) and nitric acid (HNO3 ) [22]. The oxidation of graphite increased the distance between the layers and, hence, improved hydrophilicity because of exogenous functional groups. However, it also disrupts sp2 bonding. Therefore, the as-prepared GO should be reduced by chemical, thermal, or electrochemical techniques to restore the honeycomb lattice. Reducing agents such as hydrazine (N2 H4 ), sodium borohydride, aluminum hydride, and zinc/hydrochloric acid have been used for rGO synthesis. However, the consumption of toxic and expansive chemicals during chemical reduction and the high energy requirements for thermal reduction have attracted researchers to practice alternative sustainable reduction processes [22]. In this aspect, electrochemical reduction is deemed a

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robust, green, cost-effective, and environmentally friendly route for the successful transformation of GO into rGO. Additionally, a three-step process called liquid-phase exfoliation (LPE) was first introduced in 2008 for the production of graphene by overcoming the van der Wall forces of attraction. The LPE process consisted of the dispersion of graphite flakes in a suitable solvent, followed by exfoliation and product purification. Hernandez et al. [23] examined 40 different types of solvents for the production of graphene using the LPE technique. The nature of the solvent and sonication (power, frequency, time, temperature) parameters are the key parameters for the functionalized high-quality graphene sheets through the LPE process [23]. Single-walled carbon nanotubes (SWCNT) and multi-walled carbon nanotubes (MWCNT) were unrolled into single, bi-, or few-layered graphene nanosheets, simply known as graphene nanoribbon (GNR). Chemical treatment, plasma etching, exfoliation, and metal-catalyzed cutting techniques could be adopted to unzip the CNTs into GNR [14]. All of these methods require the utilization of expensive chemicals and precursors and hence limit the applicability of the unzipping technique for the large-scale production of graphene nanosheets.

2.2.2.2

Bottom-Up Method

Graphene and its derivatives can be synthesized by the bottom-up method, also known as the constructive technique, relying on other carbon sources instead of graphite flakes. Atomic precursors are constructively organized to produce graphene materials by chemical vapor deposition (CVD), epitaxial growth, substrate-free gasphase synthesis (SFGP), template route, and total organic synthesis [20, 14]. The graphene obtained from these methods is of high quality with some structural defects and exhibits superior physicochemical properties such as higher surface area and improved electrical performance. However, the product yield is relatively low compared to that of a top-down approach, and the graphene produced from the bottom-up method could only be employed for specific applications (demanding defect-free graphene structure). The following section briefly discusses some common techniques usually used by researchers to synthesize high-quality graphene through a bottom-up approach. Chemical vapor deposition (CVD) produces graphene sheets by dissociating hydrocarbon gases and other biomass materials on the surface of metal catalysts (Cu, Ni) at a temperature of >700 °C. Chen et al. [24] reported the synthesis of high-quality graphene that exhibits higher surface area, fewer defects, and a wellorganized interconnected structure through the CVD technique using substrates such as glass, silicon, quartz, silicon oxide, sapphire, and boron nitride. However, due to the high operational temperature, lower product yield, and post-treatment step, i.e., separating graphene sheets from substrates/catalysts hindered CVD technique for large-scale graphene production was hampered [24]. Therefore, extensive research is needed to improve the feasibility and applicability of the CVD process by linking it with thermal- and plasma-based processes. Interestingly, high-quality graphene can

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be synthesized by the thermal decomposition of silicon carbide (SiC) at a temperature of 1200–1600° C in an inert atmosphere using a muffle or tube furnace [25]. The reaction resulted in the sublimation of Si while leaving undue C atoms to stack into the sp2 hybridized network. The high energy requirement and the availability of a limited-sized SiC hindered the epitaxial process for graphene synthesis. Graphene is also synthesized by an economical route, namely, the gas-phase reaction process, without using any substrate. Briefly, an aerosol mixture of liquid ethanol and Ar gas is processed in a microwave-generated plasma reactor, resulting in a graphene synthesis by evaporating the ethanol from the aerosol mixture [26]. The yield and quality of asproduced graphene depend on the nature of the carbon precursor. However, detailed research is needed to optimize the operational parameters and to envisage the effect of carbon precursors on graphene quality. Furthermore, single-layer graphene with high throughput and good yield can be synthesized on a lamellar mesostructured silica substrate in an inert atmosphere using a pyrrole moiety as a carbon source. Graphene was also synthesized using the soft– hard template approach using cetyl trimethyl ammonium bromide (CTAB) and silica as soft and hard substrates [27]. The template-assisted technique is operated under mild conditions without using any hazardous/toxic reagents. However, the templateassisted route is not recommended for large-scale synthesis because of the tedious washing steps and the chances of template removal. A two-dimensional (2D) highquality graphene can be fabricated by a simple reaction route (total organic synthesis method) using polycyclic aromatic hydrocarbons (PAHs) as a base material. Product yield, graphene quality, and solubility highly depend on the selectivity of PAHs [28]. The manifestation of side reactions, stable dispensability, and the requirement of precisely controlling the operating parameters hindered the large-scale synthesis of graphene via the total organic synthesis approach.

2.3 Graphene and Its Composite for Adsorption Water pollution has become a serious problem due to the mixing of toxic metal ions, dyes, and other micropollutants, which is extremely harmful to human health and the environment. Dye is a photo-pollutant that poses a significant risk to all forms of life, even at low traces. Toxic and non-biodegradable heavy metals accumulate in biosystems, leading to many health complications such as emphysema, hypertension, and skeletal malformations. Therefore, a versatile technique is needed to remove toxic pollutants to ensure that water quality is protected. The adsorption technique, with its advantages such as cost-effectiveness, feasibility, recyclability, and effectiveness, has been proven to be successful in water treatment. Adsorption is a unit operation in which dissolved solute particles from solution media can be transferred to an adsorbent surface through physical or chemical interactions. Adsorbents usually possess residual surface energy due to their high surface area, which creates unbalanced forces that strongly attract water pollutants. Physical adsorption is typically carried out at low temperatures to optimize

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fast adsorption rates and consume little adsorption heat. However, the formation and destruction of chemical bonds can be significant in the chemical adsorption process. Recently, several types of new carbon-based materials (such as graphene, graphene oxide, and carbon nanotubes), metal oxides, layered double hydroxides (LDH), resins, and composites have been demonstrated to increase the adsorption ability [29]. Among them, graphene-based carbonaceous materials with a larger theoretical specific surface area (2,630 m2 g−1 ) exhibit remarkable adsorption properties to assimilate various pollutants such as heavy metal ions, dyes, and micropollutants from wastewater through electrostatic or coordinate binding forces [30]. The advantages of graphene-based materials over conventional adsorbent materials, such as nonvalent iron, iron oxide, zeolite, silica, titanium dioxide, chitosan, and polymers, are tunable surface chemistry, oxygen-based functional moieties, non-corrosive properties, and large surface area. On the other hand, the use of conventional nanomaterials with graphene as carbon-based nanocomposites or nanohybrids improves their controllable properties for environmental applications.

2.3.1 Functionalized-Graphene Composites In recent decades, nanoparticles derived from graphene and their composites have been recognized as effective adsorptive removal systems for heavy metal ions and other organic pollutants, exhibiting fast removal, high capacity, good selectivity, and reusability. Pure graphene in an aqueous medium tends to aggregate due to the strong π–π interactions and the inert surface chemistry, thus limiting the interaction with metal ions during adsorption [31]. A chemical modification or composite formation can reduce the aggregation of layers in graphene, which is essential to achieve effective interactions between graphene and heavy metal ions. Many researchers have attempted to modify graphene surfaces by complexing compounds such as sulfur-containing compounds (sulfydryls, L-cystine, cytosteamine, and mercaptobenzothiazole) [32], and nitrogen-containing compounds (triethylenetetramine, polyethylenimine, ammonia, 2-pyridinecarboxaldehyde thiosemicarbazone, 2,2' dipyridylamine, and diethylenetriaminepentaacetic acid) [33–38]. Interestingly, graphene can be selectively modified to enhance its adsorption selectivity toward metal ions. For example, the modification of the surface of graphene oxide with sulfur-containing functional groups led to Pb(II) adsorption through coordination, electrostatic interactions, cation–pi interactions, and Lewis acid–base interactions, as shown in Fig. 2.3 [39].

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Fig. 2.3 Schematic diagram of the proposed interactions between GOCS and Pb(II) [39], (Copyright 2020. Reproduced with permission from Elsevier)

2.3.2 Graphene—Macromolecular Organic Material Composites Polymers represent another organic material that can be utilized to improve both the physical and chemical properties of graphene. With the aim of biodegradability, biocompatibility, and renewability of adsorbents, some biopolymers, such as chitosan [40], carboxymethylcellulose monoliths [41], and β-cyclodextrin [42], can be better choice for the preparation of bioadsorbents. The GO-chitosan composite developed by Zhang et al. demonstrated high adsorption capacity for the removal of Cr(VI) ions from the aqueous system. Nanocomposites enriched with -OH and -NH2 groups can easily be protonated to obtain positively charged sites to create electrostatic interactions with negatively charged chrome ions at acidic pH. Therefore, the adsorption process is spontaneous, endothermic, and feasible [43]. Furthermore, the addition of polymer not only enhances the adsorption performance but also reduces the level of agglomeration in the graphene layers by acting as spacers between the layers (see Fig. 2.4) [44]. In addition to the polymer, some naturally existing macromolecular organic materials (humic acid) can enhance the adsorption capacity of graphene composites because of the large surface and many active functional groups, including quinone, enol alcohol hydroxyl, aminoc, phenolhydroxyl, and carboxyl groups of humic acids. Thus, electrostatic attraction, π –π interaction, and hydrogen bonding between the dye and bioadsorbent have resulted in approximately 59.00 mg/g dye adsorption capacity [45].

2.3.3 Graphene-Metal Oxide Nanoparticle Composites Graphene-layered materials can also serve as frameworks for accommodating functional nanoparticles such as metal oxides (Fe3 O4 , TiO2 ) [46, 47], mixed metal oxides (MgAl, CuAl, and CoAl oxides) [48], and rare earth metal oxides (Nd2 O3 , Pr2 O3 , Ce2 O3 ) [49]. This hybrid structure significantly enhances the adsorption and catalytic

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Fig. 2.4 SEM images of CS (a), GO-SH (b), and CS/GO-SH (c) [44] (Copyright 2015. Reproduced with permission from Elsevier)

properties as well. The placement or intercalation of nanoparticles can reduce aggregation by reassembling the graphene sheets, especially because they possess high adsorption sites. Interestingly, the addition of nanoparticles with magnetic properties may be advantageous for the magnetic separation of adsorbents after the adsorption process under an external magnetic field. For example, an adsorbent based on graphene oxide-magnetized iron oxide nanoparticles (GO-MNP) presented a significant removal efficiency of around 99.6% for methylene blue dye [50]. In another study, pHPZC (approximately 3.5) of Fe3 O4 /GO-based magnetic adsorbent (MGO) played a significant role in creating positive or negative charges on the surface of the adsorbents. When the pH of the solution was above 3.5, the electrostatic interaction was strong between positively charged pollutants such as Cd(II), methylene blue, whereas an anionic dye such as orange G was less likely to adsorb due to electrostatic repulsion. Therefore, MGO has the maximum sorption capacity in ultrapure water for Cd(II), methylene blue, and orange G, which were 91.29 mg/g, 64.23 mg/g, and 20.85 mg/g, respectively [51]. Different types of interactions between graphene/ metal oxide composites and pollutants such as heavy metal ions, dyes, and other organics have been reported in the adsorptive removal process, including electrostatic, covalent, and complexation (refer to Fig. 2.5) [52]. However, the lack of selectivity in metal oxide-modified graphene results in nonspecific adsorbents.

2.3.4 Photocatalytic Graphene-Based Composites TiO2 , a semi-conductor nanoparticle with a wide bandgap, is a common photocatalytic material that is used to produce graphene-based adsorbents with photocatalytic activity. In this case, the semi-conductor nanoparticles incorporated graphene composite form a heterojunction at the interface to suppress the recombination of photo-induced electron–hole pairs via a percolation mechanism, resulting in high photocatalytic performance. Photocatalytic activity of many binaries such as rGO– SnO2 [47], rGO/Ce2 O3 [49], rGO/Cu2 O [53], or ternary nanocomposites such as rGO–Fe3 O4 –TiO2 [46], rGO–TiO2 -ZnO [54], Ag-TiO2 -rGO [55] under visible and UV light irradiation has recently been reported for the removal of dye molecules and other organic pollutants. In contrast, the addition of more than the optimized

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Fig. 2.5 Adsorption mechanism for graphene-based nanomaterials

amount of rGO to the composite does not enhance its photocatalytic performance by encouraging the recombination of electron–hole pairs [56]. In addition, the Fe-incorporated graphene composites can also be used for photo-Fenton-degrading organic pollutants. Using the Fe3 O4 /TiO2 /graphene ternary composite, the methylene blue was degraded to 99% under UV light, while 94% degradation was achieved in the presence of visible light. Furthermore, the dye destruction mechanism was deduced as follows (refer to Fig. 2.6) [57].

2.3.5 3D Graphene-Based Graphene Architecture In recent years, two-dimensional (2D) graphene-based materials such as graphene oxide (GO) and reduced graphene oxide (rGO) have been used as building blocks to create three-dimensional (3D) graphene materials [58]. Compared to graphene with 2D structure, graphene with 3D structure has abundant interconnected pores and a larger surface area, which improves adsorption [59]. Various methods have been used to construct 3D structured graphene, including chemical vapor deposition [60], hydrothermal method [61], and in situ reductions [62]. Among them, in situ chemical reductions are most commonly reported, in which NaHSO3 , Na2 S, Vitamin C, HI, and hydroquinone are employed as reducing agents [63]. 2D graphene selfassembles in 3D shapes due to the interaction of many forces, such as hydrophobicity, π–π interaction, van der Waals force, electrostatic interactions, and hydrogen

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Fig. 2.6 Photo-Fenton dye degradation mechanism [57] (Copyright 2018. Reproduced with permission from Elsevier)

bonds [64]. The presence of water molecules substantially influences the durability of the matrix by affecting the interactional forces, which further leads to a restack between the rGO sheets to yield smaller surface areas. Therefore, the structural strength of 3D graphene materials has been enhanced by the addition of organic multifunctional compounds, cross-linkers, metal oxides, and metal salts. In this context, various nanoparticles, such as TiO2 [65], Fe3 O4 [66], g-C3 N4 [67], and Ag3 PO4 [68], and polymers, such as sodium alginate [69], chitosan [70], polyacrylamide [71], embedded 3D graphene composites, have been reported. For example, the synergy between adsorption–photocatalytic degradation in the 3D network Fe3 O4 @SnO2 / Ag-graphene hydrogels resulted in a maximum adsorption capacity of 14.90 mg/g for the removal of 2,4 dichlorophenol (93.8%) [72]. In particular, the 3D graphene in AgNPs/NO/graphene composites offered more electron transfer channels and thus improved the synergetic adsorption–photocatalytic performance in the degradation of methylene blue dye (see Fig. 2.7). The illumination of UV light transforms toxic dyes into safer CO2 and H2 O compounds as follows [73]: ) ( − + ZnO + hv(UV) → ZnO eCB + hVB eC B − + O2 → O.− 2 hV B + + H2 O → H+ + ·OH

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Fig. 2.7 Ag/ZnO/3D graphene structure for the removal of photocatalytic dyes [73] (Copyright 2019. Reproduced with permission from Elsevier)

·OH + MB → MB degradation (CO2 + H2 O) O.− 2 + MB → MB degradation (CO2 + H2 O)

2.4 Graphene and Its Composites for Membrane Filtration Membrane technology has become one of the most popular separation processes compared to other conventional processes such as distillation, evaporation, and crystallization [74]. Membrane material, a thin-film selective barrier, can control or allow the passage of solutes from the aqueous feed to the permeate side, according to size exclusion [75] or solution diffusion [76]. The key benefits of the membrane separation process are the compact footprint, ease of operation, minimal secondary pollution, and high reliability [77]. Nevertheless, a trade-off between membrane flux and selectivity and the fouling phenomenon limits the membrane’s potential to provide high filtration performance [78]. Although several membrane modifications have been explored, research on various types of inorganic surface modifiers, including metal oxides [79], graphene [80], carbon nanotubes [81], and metal–organic frameworks [82], has received special attention to resolve the drawbacks of membranes. During the past few decades, membrane technology has played a dominant role in wastewater treatment and desalination. Several opportunities have been presented by the revolution in nanotechnology to develop novel membrane materials with different functional properties [83, 84]. Despite many nanoparticles (silica, silver, titania, etc.), two (2D)-dimensional materials like graphene, graphene oxide (G.O) and reduced graphene oxide (rGO) have been preferentially recognized for advanced membrane separation due to their chemical stability, high thermal stability, possible porous

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Fig. 2.8 Schematic illustration of a single-layer, b lamellar structure, and c mixed matrix membranes of graphene

structure, easy dispersion in the aqueous medium, thickness in atomic scale, relatively inexpensive, and easily scalable [85, 86]. Furthermore, the nanoporous surface and 2D nanochannels of graphene-based materials make them effective desalination materials. Therefore, graphene-based membranes for pressure-driven membranes became the most familiar. A major trend in current research has been to assess ways to exploit GO to augment its performance in membrane-based water treatment using existing experimental demonstrations and simulations. Thus, the three most common ways (Fig. 2.8) to develop GO membranes are (i) a single-layer GO membrane, (ii) a lamellar GO membrane, and (iii) a mixed matrix membrane (MMM).

2.4.1 Single-Layer GO Membrane Graphene materials with an atomic-scale thickness like a single 2D layer are recognized as the thinnest barrier to separate molecular species. The existence of nanopores in the graphene layer can yield permeance properties of a higher order of magnitude than conventional membranes. Based on previous research [87, 88], substantial attention was paid to single-layered nanoporous graphene materials and their ability to desalinate. Furthermore, many computational studies based on molecular dynamics (MD) simulations preferably exhibited the possible potential of nanoporous singlelayered graphene in molecular mixture separation applications [89, 90]. For example, molecular dynamics (MD) simulation by Cohen-Tanugi et al. disclosed the incredible filtration ability of single-layered graphene in desalination as it demonstrated 2–3 times of high water permeability and high salt rejection compared to commercial reverse osmosis (RO) membrane [91].

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The well-suited fabrication technique for the preparation of single-layer graphene with a larger surface area is chemical vapor deposition (CVD) [92]. Unfortunately, while upscaling graphene using CVD, this method invariably fails to create a reliable pore (nano) size on the graphene layer [93]. As an alternative, some modern techniques such as oxidative etching [94], hydrogen plasma [88], and ion bombardment [95] have been adopted for constructing pores in sub-nanometer scale over the graphene layer matrix. However, the development of large-area graphene membranes using these modern techniques is still under development, which limits the applicability of graphene filters in industrial applications. A simple thermal/chemical reduction approach allows graphene to form with some intrinsic defects, which could serve as nanopores. Usually, the presence of functional groups in graphene oxide (GO) with a high ratio of epoxy:hydroxyl groups tends to produce bigger nanopores after reduction in the resulting reduced graphene oxide (rGO) structure. For example, Li et al. found that the magnitude of the defect in GO is mainly dependent on the composition and reduction condition of GO. However, no obvious reduction effect due to the composition and reduction conditions was obtained when GO had a low initial oxygen concentration of 17% and the resulting GO film provided a barrier property for the water molecule to pass through. In contrast, GO with an initial oxygen concentration in the range of 25–33% can support the reduction process of chemical composition and reduction conditions. Interestingly, high flux and rejection can be obtained for the resulting defective GO membrane due to the formation of pores with larger size (see Fig. 2.9) [96]. Finally, the results of the experiments greatly support the simulation results, and it seems very challenging to build the desired macroscopic single-layered graphene or GO membranes, which have a transport pathway only for water molecules.

2.4.2 Lamellar GO Membrane The single-layer graphene membrane has yet to be fully understood because of its complexity. As an alternative, a graphene-based membrane could be developed featuring a lamellar structure. Usually, the lamellar-structured G.O membrane with several nanometer thicknesses is prepared by stacking neighboring G.O sheets in tandem with an appropriate interlayer distance. During the filtration process, the interlayer nanocapillary network of the G.O membrane allows for the restricted passage of ions or molecules through the permeate. The spacing between the adjacent GO layers favors the size exclusion mechanism, dominating the membrane’s flux and rejection. For example, using the vacuum suction technique, Wang et al. [97] assembled GO layers onto PAN nanofibrous supports (GO@PAN membrane). The results revealed that graphene oxide could create a barrier layer that can be controlled on top of a PAN nanofibrous mat. With the increase in the GO thickness, the water flux decreased sharply, which can be explained by the increase in mass transfer resistance. However, the maximum water flux measured in the 34-nm-thick membrane

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Fig. 2.9 Representative rGO structures in our simulations obtained from different synthesis conditions are shown in a 3 by 3 matrix. The GO sheets’ epoxy/hydroxyl ratio and initial oxygen concentration vary in horizontal and vertical directions, respectively. The reduction temperature for the formation of these structures was set to 2,500 K. All structures are represented as balls and sticks with carbon, oxygen, and hydrogen atoms in gray, red, and white, respectively. (Reproduced from [96], copyright year: 2015, publisher: Springer)

(8.2 L·m−2 ·h−1 ·bar−1 ) was approximately 1.6 times greater than in the 33 nm thickness of the GO film (water flux 5.0 L·m−2 ·h−1 ·bar−1 ) [98]. Hagen–Poiseuille’s theory can explain such a high water flux. The hydrophilic–hydrophobic gate describes the mechanism of water diffusion through a GO layer-nanochannel model (Fig. 2.10). In the GO layer, the entire structure of the GO sheet acts as a carbon wall, and the space between the interlayers between the GO sheets is regarded as a 2D channel. Interestingly, the slip flow theory indicates that the surface and edges of GO can be considered as hydrophobic and hydrophilic channels, respectively [99]. First, this hydrophilic gate allows larger water molecules to enter the GO layer. Subsequently, the hydrophobic 2D channel causes the sliding surface of these water molecules to pass through the GO sheet quickly. However, the main drawback is the disintegration of the GO layers after swelling in the aqueous and solvent environments. To improve the mechanical strength of GO layers, three important strategies, such as (i) layerby-layer (LBL) assembly via electrostatic interactions, (ii) covalent bonding using cross-linking agents, and (iii) the addition of nanoparticles, have been approached.

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Fig. 2.10 Conceptual illustration of the hydrophilic “gate” and hydrophobic nanochannel. (Reproduced from [99], copyright year: 2016, publisher: ACS)

Because LBL is an electrostatic interaction, it partially protects the GO from swelling because of the charge screening effect. In contrast, chemical crosslinking and nanoparticle addition significantly improve membrane performance and stability. Most diamine-based cross-linking agents such as ethanediamine (EDA), o-phenylenediamine (ODA), hexamethylene diamine (HMDA), butane diamine (BDA), and p-phenylenediamine (pPDA) have been widely used to cross link the GO nanosheets with their support membranes [100]. However, the size of the crosslinking agent would influence the passage of water and ions through the GO’s interlayer spacing, resulting in a decrease permeation [101]. Sarkar et al. found that the glutaraldehyde cross-linking agent for GO—methylcellulose membrane could significantly improve the degradation temperature from 310 to 400 °C by increasing the degree of cross-linking. Furthermore, the resultant membrane had high tensile strength and Young’s modulus was due to its high degree of cross-linking [102]. The physical confinement technique is also known as the spatial confinement method, which is used in situ polymerization to laminate the GO layers to achieve a strongly stabilized structure [103]. To attain the confined interfacial polymerization inside the GO layers, well-dispersed monomer solutions were filtered through GO film. Subsequently, the trapped monomers initiate the polymerization process by reacting with each other within the lamellar structure. The continuous polymer network firmly holds the GO sheets and keeps the entire membrane from swelling, followed by disintegration. For example, Shi et al. developed a polyamide-GO reverse osmosis membrane by confined interfacial polymerization using monomers such as metal-phenylene diamine (MPD) and trimesoyl chloride (TMC). The 30 nm membrane thickness displays a high salt rejection of 99.7% and a permeance of 3.0 L m−2 h−1 bar−1 . Furthermore, prolonged stability, high chemical stability after immersion in the aqueous NaClO solution for 24 h, and low fouling tendency were acquired for the resultant polyamide-GO membrane [104]. Similarly, cross-linked

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double-network hydrogels were also employed to stabilize the stacked GO layers. The intermolecular hydrogen bond interaction between calcium alginate and polyvinyl alcohol (PVA) constrains the GO structure to exhibit low swelling in the aqueous environment [105]. Overall, compared to typical cross-linked GO membranes, this type of membrane has favorable interlayer spacing to offer comparable permeation and high salt removal capacity. Many contemporary works using CNTs, TiO2 , Ag, and zeolite have recently focused on tuning the interlayer spacing between GO layers to enhance separation performance [106–108]. Different sizes of nanoparticle inclusion can alter the stacking of GO layers with different interlayer spacing. Moreover, when charged with nanoparticles, electrostatic repulsion and physical size sieving start to dominate in the separation of charged solutes and large molecules. For example, the intercalation of nano-TiO2 demonstrated a high water flux of 137.7 L·m−2 ·h−1 ·MPa−1 than the bare GO membrane (76.2 L·m−2 ·h−1 ·MPa−1 ). The dye removal test for TiO2 -GO revealed the high rejection ability as follows: >85% for basic fuchsin (BF), >92% for methylene blue (MB), >99% for methyl orange (MO), and 99.85% for evans blue (EB) [109].

2.4.3 GO in Mixed Matrix Membrane (MMMs) The mixed matrix membrane (MMMs) is generally prepared by uniformly dispersing the inorganic additives into the polymeric matrix. These membranes tend to offer various advantages, including better permeation, high solute rejection, and immense antifouling properties. In recent years, the potential of GO-incorporated polymeric membranes in water treatment and desalination has been deeply investigated for pressure-driven membrane applications by researchers worldwide. The most widely used pressure-driven membranes are listed as microfiltration, ultrafiltration, nanofiltration, and reverse osmosis membranes that could be useful for the removal of various organic pollutants, toxic heavy metal ions, and desalination. In addition, the composite approach is considered as an alternative route for using pure GO skeleton membranes, which only allows water molecules to cross its skeleton. Interestingly, with the addition of graphene materials as fillers or modifiers, MMMs expand their pores, so water molecules can pass quickly, thereby intensifying the membrane filtration performance. Ultrafiltration is a low-pressure membrane process designated for separating and purifying a certain solution consisting of low- and high-molecular-weight substances. Due to the versatile features of GO, there is great interest in graphene-incorporated ultrafiltration membranes. As a result of the oxygen-containing functional groups on the surfaces and edges, GO retains an inherent hydrophilic character by strongly holding the water molecules through H-bonding. Thus, the addition of GO as an additive can play a major role in pore formation and controlling surface hydrophilicity. Karim et al. studied the synergistic effect of GO and the Tetronic (T904) pore former on the properties of the PES ultrafiltration membrane. Combining T904 with 0.3 wt%

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GO significantly increased water flux from 2 to 245 LMH and increased the BSA rejection from 89.6 to 93.3% [110]. Many studies have investigated the amount of GO addition to the polymeric matrix. It was claimed that the optimum loading of inherent hydrophilic GO increases the membrane porosity by increasing the thermodynamic instability in the polymer casting solution. However, when loading is more than the optimum level, the possible aggregation of GO layers increases the viscosity of the casting solution, causing better kinetic hindrance of demixing. Consequently, this kinetic hindrance effect is found to be a major cause of low porosity [111, 112]. Despite the extraordinary properties of GO, the aggregation of GO into the polymeric matrix during membrane formation deteriorates the final membrane properties. To resolve this issue, the functionalization of GO is considered one of the possible solutions. However, in addition to polymeric moieties, smaller molecules and nanoparticles could be effective due to the less steric hindrance that can offer an increase in the reaction rate of modification [113, 114]. For instance, the surface of the GO was functionalized with cysteine through a simple thiol-ene click reaction. The PES composite with cysteine-functionalized GO exhibited high hydrophilicity, negative charges, and improved ultrafiltration performance for the BSA solution [113]. Vatapour et al. designed a partially reduced graphene oxide/Ag nanocomposite (RGO/Ag) incorporating polyethersulfone (PES) mixed matrix membranes for protein removal. Up to the addition of 0.2 wt% of GO, the pure water flux and BSA rejection were dramatically increased to 429.8 (kg/m2 h) and 98%, respectively. Further loading of the nanoparticle amount greater than 0.2 wt.% negatively affected the flux behavior. By adjusting the amount of rGO/Ag membranes, the hydrophilicity, pore size, and porosity of the membrane could be easily customized. The resulting membrane exhibited reliable high antifouling behavior by exhibiting a flux recovery ratio (FRR) of 67.2%. Moreover, the synergistic effect of rGO/Ag renders the excellent antibacterial properties of the surfaces of MMMs against E. coli bacteria via the capture-killing process. Due to the effective biocidal properties of rGO/Ag against adhered microbes, the membrane surface effortlessly acquired better anti-biofouling properties [115]. The nanofiltration (NF) process lies between reverse osmosis and ultrafiltration, which exhibits a nominal molecular weight cutoff between 1,000 and 200 Da [116]. Due to its advantages, such as low energy consumption, compact design, no phase changes, and simplicity of use, nanofiltration (NF) technology has gained interest in the fields of water softening and micromolecule filtration in recent decades [117]. Despite its numerous advantages, a major limitation of NF is the flux decline due to membrane fouling, which occurs due to the unwanted adsorption and deposition of foulants onto the membrane surface or into its pores [118]. Several factors affect the fouling of a membrane, including its surface properties, such as roughness and hydrophilicity, as well as the chemistry of the feed solution and the process conditions [119]. In order to hinder the adherence of hydrophobic foulants, appropriate surface modifications with nanoparticles are believed to increase hydrophilicity [120]. A great deal of interest has recently been shown in graphene-based materials used in nanofiltration membranes [121]. For instance, Zinadini et al. embedded graphene oxide (GO) nanoplates into the PES membrane using the phase inversion technique.

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This GO-modified nanofiltration membrane exhibited a high Direct Red 16 dye rejection of 96% and high flux. Moreover, the GO sheets on the PES membrane reduced the adherence of protein molecules and increased the flux recovery ratio of 90.5% [122]. In recent decades, thin-film composite (TFC) membranes have been the dominant element in the market for nanofiltration (NF) [123] and reverse osmosis (RO) [124] membrane markets. In the TFC fabrication, the interfacial polymerization (IP) process between two monomers, such as polyfunctional amine and polyfunctional acid chloride, forms a dense aromatic or semi-aromatic polyamide layer on the support film. However, the trade-off between water flux and salt rejection is one of the biggest challenges in thin-film composite (TFC) membranes for water treatment and desalination. To improve membrane performance, 2D graphene oxide (GO) has been incorporated into polyamide layers to form thin-film nanocomposite (TFN) membranes. A large number of functional groups on the GO nanosheets indicate that they could interact with organic trimesoyl chloride (TMC) and piperazine (PIP). Furthermore, a membrane utilizing GO nanosheets and low monomer concentrations will benefit from both the nanosheets and the thin active layer, thus allowing the membrane to function under low operating pressure with the desired pure water permeance. Notably, surface-tethered GO nanosheets can offer enhanced water flux and superior antifouling properties since the abundance of functional groups within them enhances the surface hydrophilicity of the membrane. Besides their extraordinary potential for constructing functional TFN membranes, GO nanosheets also display great chemical and mechanical stabilities and good antimicrobial properties. For example, Zhao et al. developed a thin GO-TFN membrane that works under the low pressure of 4 bars. The addition of GO along with monomers facilitates interfacial polymerization at low monomer concentrations. The thickness of the active layer was remarkably decreased by about 72%, while the monomer (PIP) concentration decreased from 1 to 0.25 wt.%. Despite the thin active layer of TFN, it retained resistance properties preferentially for ions but not for water molecules. As a result, with an optimized dosage of 0.01 wt.%, GO-TFN membranes exhibited a water flux of 15.63 L/m2 h bar with the Na2 SO4 rejection of 96.56% and a rejection of MgSO4 of 90.5% [125]. The reverse osmosis (RO) process is a water purification technology that widely employs the TFC membrane to remove smaller ionic species from seawater and brackish water. Since the TFC membrane has suffered from drawbacks that include being susceptible to chlorine attack, having lower chemical stability, and biofouling, TFN membranes have emerged to protect the RO membrane from those issues. In recent years, GO materials have offered new avenues for TFN-RO membranes to conquer their limitations. Many studies have reported that the TFN-GO membrane has efficient filtration performance, fouling resistance properties, and antibacterial function. Yin et al. used GO, which has an interlayer spacing of around 0.83 nm, as a filler in the development of a TFN membrane to remove salts. In this case, the GO layers have 2D molecular channels providing an additional route in the membrane matrix for water transport, resulting in an enhancement in permeability. At membrane operation under 300 psi, the flux was improved to 59.4 ± 0.4 L/

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m2 h from 39.0 ± 1.6, while the rejection of NaCl and Na2 SO4 slightly decreased from 95.7 ± 0.6% to 93.8 ± 0.6% and from 98.1 ± 0.4% to 97.3 ± 0.3%, respectively [125]. Due to the presence of hydrophilic and negatively charged functional groups in rGO/TiO2 , the rGO/TiO2 /RO membrane revealed its superior antifouling properties by resisting protein attachment during BSA protein filtration. In addition, membrane chlorine resistance can be achieved through the possible bonding interaction between rGO/TiO2 and polyamide groups. This chemical interaction prevents chlorine attack, which can replace the hydrogen group of the amide groups on the membrane surfaces [126]. During continuous aqueous operation, bacterial growth on and within the membrane easily clogs the pores, affecting the membrane’s life and performance. In general, E. coli cells consist of thin layers of peptidoglycans, which are surrounded by the outer membrane (lipopolysaccharides), while S. aureus cells possess multiple layers of peptidoglycans and teichoic acid that provide a thick cell wall structure. While incorporating functionalized GO, such as p-aminophenolmodified GO, into the polyamide layer, the biocidal property of TFN-GO was dramatically improved against E. coli (96.78%) and S. aureus (95.26%) compared to the pristine RO membrane (4.95%; 2.48%). Due to the phenolic functional groups and the sharp edges of GO, bacterial cell walls are easily destroyed, followed by an oxidative stress reaction, resulting in cell death [127]. In addition, a great deal of attention has been dedicated to the development of TFNGO membranes for forward osmosis applications. The two major limitations of the FO membrane are internal concentration polarization (ICP) and salt leakage. The thin, highly porous, and low tortuous membrane can create a quick path from the drawn solution to the active layer during the osmotic process, thus reducing the ICP. When membranes are used in the FO process, ICP can usually be minimized by employing a membrane with a lower structural parameter S (thickness × tortuosity/porosity) value. The TFN membrane demands an adequate nanomaterial (like graphene-based materials) for the modification of the active layer (AL) and a support layer (SL) to enhance FO performance and reduce ICP. For example, the structural parameter (S, μm) was considerably reduced to 119 from 309 when GO (0.008 wt%) was added to the active layer of the TFN membranes. Compared to TFC (12.5 LMH) and commercial FO (15 LMH) membranes, water flux through the TFN membrane (34.3 LMH) increased by 174% and 129%, respectively [128].

2.5 Graphene and Its Composites for Capacitive Deionization Capacitive deionization (CDI) is an electrochemical process to remove pollutants, especially salts, from aqueous media [129]. While CDI salt removal technology is effective, inexpensive, requires less than 2 V (low voltage), and is environmentally friendly, other commercialized desalination processes such as reverse osmosis and multistage distillation are energy intensive [130]. The working performance of CDI

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is mainly dependent on electrical double layer (EDL) formation at the electrodeelectrolyte interface [131]. Unfortunately, CDI faces several major limitations in the lack of an adequate electrode material with a high electro-adsorption capacity and a high desalination rate [132]. Therefore, promising electrode materials must have the following characteristics: good conductivity, excellent capacitance, adequate pore size distribution, greater surface area, and high electrochemical stability [133]. In recent decades, carbonaceous materials (activated carbon, carbon fibers, carbon nanotubes, and graphene) [134–136], metal oxides (TiO2 , MnO2 , CuAl mixed oxide, Fe2 O3 ) [137–140], and hybrid composites (graphene/MoS2 , and rGO/HA) [141, 142] have been widely investigated. However, a great deal of attention must still be paid to the electrode’s material properties. Graphene, GO, rGO, and their associated nanocomposites have been effectively employed to treat saline water through the capacitive deionization (CDI) technique. Generally, CDI is an emerging and environmentally friendly desalination process that is effectively used to treat low saline water via electrosorption or pseudocapacitive reactions. The working principle of the CDI process is the removal/adsorption of ions on the surface of the electrodes, based on the electric double layer (EDL) formation. Oladunni et al. [143] discussed in detail the working mechanism and the associated models of the CDI technique. Briefly, the CDI setup is comprised of an electrode pair connected to a DC power supply, a flow controller, and a pump to flow wastewater into the cell. Compared to other existing water desalination technologies, such as adsorption, coagulation–flocculation, ion exchange, photocatalytic degradation, and membrane separation, CDI is the most favorable because of process simplicity, lower energy requirements, and easy regeneration ability. In the CDI process, saline water (NaCl) or ionized (toxic contaminants) wastewater is treated by flowing between two symmetrical or asymmetrical electrodes. These electrodes are connected to the positive and negative terminals of the DC power supply and work as an anode and cathode, respectively. The potential difference between the two electrodes required for the CDI process is around ~1.2 V, because higher voltages favor water electrolysis, and hence produce more ions instead of removal. These electrodes attract oppositely charged ions under the presence of an electric field, resulting in the removal of toxic constituents or water desalination. The electrodes can be regenerated by either thoroughly rinsing with DI water or simply reversing the potential. The performance of the CDI process is evaluated in terms of electrosorption capacity, removal efficiency, and specific energy consumption (SEC) [144]. Among various operating parameters, such as applied voltage, flow rate, cell geometry, configuration, and feed composition, electrode materials significantly affect CDI performance. The physicochemical characteristics, structural morphology, porosity, and electrochemical characteristics of electrode materials greatly influence the efficiency of the CDI process [144]. Ideally, the best CDI electrodes exhibit low internal resistance, high specific surface area, and pseudocapacitance, along with hydrophilicity, high stability, and recyclability [145]. Various carbon nanostructured materials, such as biochar (BC), activated carbon (AC), carbon nanofibers (CNFs), carbon cloth (CC), and graphene, have been tested for desalination applications and heavy metal removal [143]. These

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carbon materials, specifically BC, have been obtained from the pyrolysis of agricultural waste and exhibit low electrical resistivity, controllable pore size distribution, and a high specific surface area [146]. However, the poor electrochemical activity of BC-based electrodes limits their desalination, sorption performance, and heavy metal removal efficiency. To overcome these limitations, the obtained biochar should be chemically or thermally modified and/or decorated with various metal oxides, such as Fe3 O4 , MnO2 , and Ag, to increase the charge/discharge efficiency and sorption capacity. Bharath et al. [147] reported the development of a metal oxide composite framework with an activated carbon electrode for Cr(VI) reduction using the CDI technique. The developed electrodes possessed superior performance compared to the AC electrode alone, because of the synergistic electro-redox behavior of oxide nanoparticles and the high surface area/pore size distribution of activated carbon. In 2009, Li et al. [148] synthesized and tested graphene-based electrodes for the removal of various ions using the CDI technique at an applied voltage and feed flow rate of 2 V and 25 ml/min, respectively. The developed electrodes exhibited the sorption capacities of 0.45, 0.52, 0.55, and 0.62 for NaCl, MgCl2 , CaCl2 , and FeCl3 , respectively. Due to its diverse physicochemical composition and superior electrochemical characteristics, graphene and its derivatives have captured enormous attention as promising electrode materials for CDI applications and energy storage devices. Additionally, the highly lamellar interlayered structure of graphene nanosheets reduces the path for ion diffusion on the surface of the electrodes, hence resulting in an improved sorption–desorption performance in the CDI process [149]. Hence, the results demonstrated that graphene-based CDI electrodes exhibited superior desalination performance than other carbon-based materials such as activated carbon and can be effectively employed for the removal of single- and/or multivalent ions and desalination applications. Further improvements in graphene electrodes have been reported by reducing the graphene nanosheets followed by resol modification. The composite electrode (resol-modified rGO) exhibited an increased specific surface area (SSA) and specific capacitance of 406.4 m2 /g and 135.7 F/g compared to pure rGO (137.4 m2 /g and 112 F/g), respectively. Furthermore, the developed composite CDI electrode was also tested for desalination applications. The results revealed that the resol-modified rGO showed an electrosorption capacity of 1.424 mg/g, higher than that of the pure rGO electrode (0.799 mg/g) and AC electrode (0.423 mg/g) [150]. Recently, Bharath et al. [149] developed and tested asymmetric CDI electrodes based on Ni/MAX and porous rGO (pRGO) for the removal of monovalent (F− ), divalent (Pb2+ ), and trivalent (As) ions from synthetic wastewater at an applied voltage of 1.4 V. The results revealed that the CDI system based on pRGO and Ni/ MAX demonstrated an electrosorption capacity of 68, 76, and 51 mg g − 1 for F− , Pb2+ , As(III) ions, respectively, as shown in Fig. 2.11. Additionally, the developed electrodes were effectively regenerated by thoroughly rinsing with water and showed the same sorption capacity for consecutive 05 cycles. In addition, graphene nanoflakes and graphene sponges have also been synthesized and tested for desalination applications [151, 152]. The CDI performance of these materials would be improved by doping them with nitrogen [153, 154]. Additionally, the electrochemical and NaCl sorption characteristics of graphene sheets

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Fig. 2.11 a Schematic representation of the asymmetric CDI setup, b Electrosorption capacity of multivalent ions for different electrode configurations (Ni/MAX//pRGO, Ni/MAX//Ni/MAX, and pRGO//pRGO), c Relationship between time and sorption capacity of Ni/MAX//pRGO asymmetric electrodes at 1.4 V, d Effect of solution pH on ions adsorption capacity for Ni/MAX//pRGO electrodes, e Influence of applied potential (0.8–1.8 V) on ions’ adsorption capacity of Ni/MAX//pRGO asymmetric electrodes. Reprinted with permission from Ref. [149]

have been improved by synthesizing three-dimensional (3D) macroporous graphene sheets by adopting the templating technique. This will improve the electrosorption capacity by preventing intersheet restacking and facilitating ions’ diffusion of the ions between the electrolyte and the electrodes [155, 156]. The properties of the synthesized graphene sheets could be improved by functionalizing them with acids or other chemicals. For example, Liu et al. [157] functionalized 3D graphene by grafting 3aminopropyltriethoxysilane and ethylenediamine triacetic acid (EDTA), and reported that EDTA-grafted graphene nanosheets exhibited higher specific capacitance and maximum removal efficiency of more than 99% by treating Na+ and Pb2+ aqueous solution at an applied voltage of 1.4 V. The electrodes comprised of graphene and MnO2 nanorods demonstrated a NaCl removal efficiency of 93% with the associated sorption capacity of 5.01 mg/g for an operation time of 120 min at 1.2 V [158]. Xu et al. [154] reported that nitrogen-doped 3D porous graphene sponge (NGS) exhibited a better sorption capacity (8.04 mg/g) and a specific capacitance (286.6 F/g), and, therefore, could be used effectively for CDI applications. Table 2.1 illustrates the CDI performance of carbon-based electrodes and their composites for desalination applications and removal of multivalent ions from synthetic/industrial wastewater effluent. Additionally, the incorporation/impregnation of metal oxides and ferrite-based nanomaterials onto the surface of graphene and its derivatives demonstrated faradic and non-faradic capacitances and could improve the overall performance of the electrode material. For example, the incorporation of manganese ferrite (MnFe2 O4 ) into the surface of porous rGO exhibited the highest specific capacitance (237 F/g) with a

rGO

Graphene-Fe3 O4

Mn3 O4 /RGO

3D nanoporous graphene

Functionalized 3D graphene

Graphene-SnO2

rGO–polypyrrole–MnO2

2

4

5

6

7

8

EDTA-grafted 3D graphene

1

3

Electrode materials

S. no.

331

–

140

445

160

362

120

–

Specific surface area (m2 /g)

356

–

142

198.8

437

128

66

134.4

Specific capacitance (F/g)

–

–

500 NaCl

500 NaCl

1000 NaCl

300 NaCl

300 NaCl

100 Na aqueous solution

Feed concentration (mg/L)

2

1.4

1.4

1.6

1.2

1.6

1.6

1.4

Applied voltage (V)

18.5

1.49

13.72

17.1

34.5

10.3

6

–

Sorption capacity (mg/g)

–

83

98.7

–

–

–

–

98.7

Removal efficiency (%)

Gu et al. [167]

El-Deen et al. [166]

Liu et al. [165]

Shi et al.[164]

Bharath et al. [163]

Gu et al. [160]

Gu et al. [160]

Liu et al. [157]

References

Table 2.1 Comparison of electrosorption capacity among various carbon-based electrodes, specifically activated carbon, graphene, GO, rGO for desalination applications and wastewater treatment

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sorption capacity of 8.9 mg/g by treating the initial NaCl concentration of 50 mg/ L [159]. The decoration of TiO2 , ZrO2 , Fe3 O4 , Mn3 O4, SnO2, etc. in graphene and rGO improved the specific capacitance and salt adsorption capacities compared to the pure graphene sheets [160–162]. In another study, hierarchical manganese oxide (Mn3 O4 ) nanowires were impregnated on the surface of rGO to develop pseudocapacitive electrodes for water desalination using the CDI technique [163]. It was reported that the rGO and Mn3 O4 /rGO asymmetric CDI electrode systems demonstrate the maximum salt adsorption capacity (SAC) and average salt adsorption rate (ASAR) of 34.5 mg/g and 1.15 mg/g/min for the initial feed concentration and applied voltage of 1000 mg/L NaCl and 1.2 V, respectively [163]. Hence, it is concluded that the performance of a CDI process is explicitly based on the nature of the electrode materials and graphene, its derivatives (GO, rGO), and their associated nanocomposites could be promising candidates to effectively employ this desalination technology on a commercial scale. Finally, Table 2.2 provides a brief comparison of graphene and its composite-based wastewater treatment technologies in view of highlighting their advantages, perspectives, as well as challenges. Table 2.2 A comparison of graphene and its composite-based wastewater treatment technologies Technologies

Advantages

Perspectives and challenges

Adsorption process

It features a very high surface area, high thermal conductivity, exceptional flexibility, and a high degree of stiffness It is possible to remove a wide range of pollutants from waters, including dyes, organic pollutants, heavy metals, and pharmaceuticals Several O-containing groups assist in the adsorption of pollution through electrostatic attraction, hydrogen bonds, and van der Waals interactions The adsorption capacity, selectivity, recyclability, and regenerability of graphene have been improved by chemical modification and surface functionalization

A systematic investigation is needed to determine the stability and reusability of adsorbent materials by examining important indicators such as regeneration method, eluent type, and recovery time The interaction mechanism between graphene-based adsorbents and real-time pollutants is still unclear Thus, graphene-based adsorbent design and regulation, especially to increase adsorption capacity, reduce environmental risks, and investigate the interaction mechanism between adsorbents and adsorbates, are currently hot topics in this field

(continued)

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Table 2.2 (continued) Technologies

Advantages

Perspectives and challenges

Membrane separation

Water flux and rejection are high, fouling resistance is good, and antibacterial effects are present A satisfactory removal capacity for a variety of micropollutants and heavy metals A single atomic layer structure, large specific surface area, and multiple modifications make it irreplaceable for desalination

Compacted GO membrane flux attenuation is faster than conventional NF or RO membranes The integration of GO with polymers can also be challenging A better understanding of its filtration behavior in complex multi-pollutant environments is required To prevent secondary pollution, it is increasingly imperative to perform effective regeneration The wide use of GO membranes requires industrial preparation techniques

Capacitive deionization

Excellent electrical conductivity, high specific surface area, outstanding electrochemical stability, and long cycling durability

The CDI electrode performance must be optimized by understanding the effects of pore size distribution Heteroatoms (e.g., boron-, sulfur-, or phosphor-doping)-doped graphene materials need further optimization to understand their effect on the capacitance Selective electrosorption of heavy metal ions or fluoride, nitrate, and phosphate anions from aquatic resources Development of materials that permit selective storage/release of cations or anions

2.6 Conclusion and Future Perspective In this chapter, different types of graphene-based composites and their role in various water treatment applications, such as adsorption, membrane technology, and capacitive deionization, are discussed in detail. Adsorption materials based on graphene composites have made outstanding progress and demonstrated a high affinity for pollutants such as heavy metals and dyes, which enables them to achieve excellent removal efficiency. However, it is still important to address some important issues in the future to predict their practical performance: (1) designing graphene composites with high adsorption efficiency requires considerable effort; (2) a greater emphasis should be placed on selective adsorption in complex solution environments; (3) instead of complex syntheses, simpler chemical modifications should be used; (4) an in-depth investigation of nanocomposites’ stability under extreme environmental conditions is needed; (5) the implementation of a cost-effective approach is essential; and (6) ease of handling and recycling are also important. Membrane filters that incorporate graphene or its composites have favorable properties for use in water treatment. Nanocomposite membranes exhibit exceptional permeation properties as a result of their unique physicochemical characteristics. The porous graphene sheets and their arrangement during membrane fabrication

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enable selective transport across the membrane. Furthermore, the nanocomposites significantly improve membrane properties such as hydrophilicity, selectivity, and permeability. Despite much progress in this field, further investigation is needed to address the following research gaps: (1) requires a smart physical or chemical preparation method to obtain large and low-cost graphene membranes, (2) a focus should be placed on controlling the membrane pore size and interlayer spacing, (3) requires advanced characterization and simulation tools to determine the mechanism of mass transport between frictionless 2D nanochannels, and (4) real-time study of membrane efficacy is required. The excellent performance of graphene composite electrode materials for capacitive deionization (CDI) is attributable to their highly interconnected porous architecture, where electron and ion transport efficiently enhances electrosorption. In spite of the promising results in desalination, significant challenges remain. The challenges include the requirement for a suitable method for disposing of the salt solution, the need for an external power source, the dependence on the electrode material, the selfcontamination of the electrode material, and the use of low-cost and energy-optimized CDI technology. Additionally, the LCA test should examine the energy consumption and environmental impact associated with the use of graphene composites in water treatment applications.

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Chapter 3

Graphene-Based Materials in Effective Remediation of Wastewater Ragavan Chandrasekar, Das Bedadeep, Tasrin Shahnaz, Vishnu Priyan Varadharaj, Ajit Kumar, Harish Kumar Rajendran, and Selvaraju Narayanasamy

3.1 Introduction Various carbon-based materials are used in wastewater treatment processes for their porous nature and excellent functionalities present on their surface [1, 2]. Graphene is one of those carbon-based materials that is extensively used in various wastewater treatment processes [3]. Graphene and its derivatives are emerging as nextgeneration materials in wastewater treatment due to their excellent stability, high surface area, and good chemical and electronic properties. Graphene is a single-atomthick material with hybridized carbon atoms arranged in a honeycomb-like structure [4]. Graphene is used to make metal/metal oxide composites and other organic and inorganic composites owing to its outstanding charge carrier mobility at room temperature, electrical conductivity, high specific surface area, and stability [5]. In wastewater treatment, the removal of pollutants is mainly governed by surface interactions and catalysis. Surface interactions like pi–pi interaction, hydrophobic interactions, electrostatic interactions, and functionalities incorporated through chemical modifications on graphene derivatives enable the enhanced interaction of organic pollutants, charged organic, or inorganic pollutants with the composite materials based on graphene, which ultimately increases the process efficiency of the wastewater treatment methods. Graphene and its derivatives are being experimented in various wastewater treatment processes like membrane separation [6], advanced oxidation processes like sulfate radical activation [7], catalytic ozonation [8], photocatalysis [9], electrochemical oxidation [10], Fenton-like degradation [11], and in adsorption [12]. The usage of graphene as such in wastewater treatment is not preferred due to its hydrophobic nature, which restricts its dispersion in water [13]. Graphene oxide R. Chandrasekar · D. Bedadeep · T. Shahnaz · V. P. Varadharaj · A. Kumar · H. K. Rajendran · S. Narayanasamy (B) Department of Biosciences and Bioengineering, Indian Institute of Technology Guwahati, Guwahati, Assam 781039, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Mohanty et al. (eds.), Graphene and its Derivatives (Volume 2), Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-99-4382-1_3

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is a derivative of graphene that contains oxygen functional groups like hydroxyl and epoxy groups with a hydrophilic nature due to the negatively charged nature of the oxygen functional groups, which is dispersible in water but has an insulating behavior [14]. To overcome this restriction in conducting behavior of graphene oxide, reduced graphene oxide is derived from graphene oxide by chemical reducing methods. In this chapter, the application of graphene-based materials in different wastewater treatment systems, the properties of graphene-based materials governing the removal of various contaminants of emerging concern and heavy metals in different wastewater treatment systems, and the efficiency of those materials in the removal of these contaminants are discussed in detail.

3.2 Graphene and Its Derivatives Graphene, 2-D carbonaceous material that is composed of multiple six-membered structures in many patterns, is a mother of all graphitic forms. It has various characteristics such as high surface area (adsorption), mechanical strength, thermal conductivity, and electron mobility. With a higher surface area than any other material of its kind, it improves the interaction between the formed material [15]. Graphenebased composites are of interest to material scientists today because their structural achievements would lead to new applications. In the last decade, intense research and improvement in developing better graphene-based composites have taken place. The addition of graphene as a connecting material in a polymer matrix has enhanced the entire characteristics and performance of such materials, as reported by almost all of those researchers who have worked in this field [16]. The main interest in the use of graphene composites is due to their unique physicochemical properties and excellent thermal, mechanical, and electrical properties, especially their subtle physical existence [17]. These promising properties have led to considerable efforts to use graphene in various technological areas. Graphene oxide (GO) and reduced graphene oxide (rGO) are the two main graphene derivatives. GO is mainly produced by the oxidation of graphite, and rGO is generated mainly by reducing GO. A variety of functional groups are present overall in GO that are used to form the interaction between different kinds of compounds. GO has grown interest as a new, improved alternative material because of its excellent characteristics [18, 19]. The material obtained after reducing the oxidizing functional groups from GO is called reduced graphene oxide (rGO) (Fig. 3.1). rGO has a carbonto-oxygen (C/O) ratio of 8:246, which is formed by eliminating oxygen-containing functional groups of GO [20]. Graphene has sp2 hybridization among the carbon molecules in its hexagonal lattice structure. It forms a large π bond among the leftover orbital electron that can move freely in the structural plane [21, 22]. The properties of graphene and its derivatives are summarized in Table 3.1. The mechanical strength of graphene can reach up to 1100 GPa. It also possesses optical characteristics that can transmit about 97.9% visible light and have a high specific surface area of 2630 m2 g−1 [23].

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Fig. 3.1 Conversion of graphene into GO and rGO

Table 3.1 Properties of graphene, GO, and rGO Material

Electrical conductivity (S/m)

C:O ratio

Thermal (W/ mK)

Mechanical tensile strength (MPa)

References

Graphene

108

Nil

1200–2700

1.3 × 105 (monolayer)

Dasari et al. [24]

Graphene oxide

Insulator

2–4

0.21–1.45

0.5 × 103

Mehrali et al. [25], Hiew et al. [26]

Reduced graphene oxide

666.7

8–246

30–250

0.9 × 103

Dasari et al. [24]

GO is a 2-D monolayer, polydisperse polymer, that contains more oxygenous functional groups than graphene. This made GO more sophisticated than graphene, and its structure determined its properties. The structural model of GO, the L-K model, was given by Lerf and Klinowski, [27, 28] describes the randomness of the oxygen-containing functional groups (hydroxyl and epoxy) on the GO monolayer and -COOH (carboxyl) and -CO (carbonyl) were present at the edge of the monolayer. GO has mainly two regions within the structure, an oxidized aliphatic six-carbon ring region and an unoxidized benzene ring region, which depend on the random distribution and degree of oxidation on GO. However, this model is based on certain conditions and ignores the influence of the graphene source, oxidizer, and oxidation techniques. Another group studied GO nano-plates through scanning electron microscopy and concluded that GO has highly disordered regions of an oxidized and an unoxidized and also has a hole region because of overoxidation [29]. In recent years, many other GO models have been proposed, the dynamic model [30] and the binary model [31]. The formation and properties of graphene, GO, and rGO have been studied extensively, and it has been found that the structure of graphene, GO, and rGO contains a different ratio of carbon and oxygen (Table 3.1). Consequently, graphene is hydrophobic, while GO and rGO are hydrophilic. Furthermore, GO and rGO have both aliphatic and aromatic regions that help in the interaction with various

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molecules [32]. Research is going on to study more characteristics of graphene and its derivatives.

3.3 Application of Graphene and Graphene Derivatives in Various Wastewater Treatment Processes In recent decades, the presence of various toxic heavy metals, pharmaceuticals, pesticides, endocrine disruptors, and organics pollutants of anthropogenic or natural origin discharged in the water resources has become a global issue. The following sections comprehend the role of graphene and its derivatives in various wastewater treatment techniques.

3.3.1 Graphene and Its Derivatives in Membrane Technology Water is an essential resource that improves the sustainability of life on the earth. Various treatment technologies, such as adsorption, distillation, gravity separation, flocculation, chlorination, and coagulation, were used widely to treat water. But, due to various limitations on these techniques, they are incapable of producing good quality water in large quantities to meet global demands. To overcome these limitations, researchers focus on membrane filtration techniques to treat water. Membrane filtration is a space, cost, and energy-efficient technology which overcomes the challenges posed by other treatment technologies. The properties of ideal membranes were selectivity, high stability, maximum permeate flux, and less thickness of the membrane [33]. In previous decades, the membranes used in water treatment were made up of polymeric materials and inorganic compounds; however, they have issues such as high fabrication cost and permeability trade-off [34]. Due to increased efficiency and low fabrication cost needs, defined nanomaterials such as graphene and its derivatives were used to fabricate membranes. The generalized diagram of various pollutants passing through the graphene membrane is represented in Fig. 3.2. Figure 3.3 illustrates the various mechanisms involved in graphene-based membrane separation.

3.3.1.1

Toxic Heavy Metal Removal Using Graphene-Based Membranes (GBMs)

Cationic metal ions and anionic metal ions are considered to be the primary contaminants in water resources. Various industries, including steel, mining, fertilizer, metal plating, and manufacturing industries, discharge wastewater with several heavy metals (HM) such as Ni, Cd, Zn, Fe, Pb, Mn, Co, Hg, and Cr either directly or

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Fig. 3.2 Removal of various pollutants through a graphene membrane

Fig. 3.3 Various separation mechanisms of graphene-based membranes a adsorption, b size exclusion, and c electrostatic interaction

indirectly into the water streams. These heavy metals are non-biodegradable, so their removal from water is a crucial life problem in this era. So, researchers have focused on a solution to eliminate these heavy metals using graphene membranes. The graphene-based membranes for the elimination of heavy metals can be fabricated through various techniques such as layer-by-layer construction methods, filtration, casting, and coating. The elimination of HM ions through GBMs was associated with mechanisms such as size exclusion and electrostatic interaction (Fig. 3.3). Various heavy metals removal by graphene-based membranes are listed in Table 3.2.

3.3.1.2

Removal of CECs and POPs Using Graphene-Based Membranes

Traditional water treatment methods have some limitations to remove contaminants of emerging concerns from water. In order to overcome all limitations, graphenebased membranes were used to treat water-containing CECs and POPs.

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Table 3.2 Heavy metal removal by various graphene-based membranes Graphene derivatives

Heavy metal

Graphene oxide

Zn

96.85

Graphene oxide

Graphene oxide Graphene oxide

Graphene oxide Graphene oxide

Rejection (%)

Modified membrane type

References

Pb

94.57

Ni

98.86

Torlon hollow fiber modified layer by layer Graphene oxide framework membrane

Hu and Mi [35]

Hyperbranched polyethyleneimine-modified graphene oxide and ethylenediamine framework membrane

Zhang et al. [36]

Pb

95.70

Ni

96.00

Zn

97.40

Cd

90.50

As

83.65

Graphene oxide/polysulfone nanocomposite membrane

Rezaee et al. [37]

Cr

90.00

Cu

90.00

Graphene oxide impregnated mixed matrix membrane

Mukherjee et al. [38]

Rao et al. [39]

Cd

90.00

Pb

90.00

Cu

90.00

Amino-functionalized MOF/GO composite onto polydopamine-coated membrane

Graphene oxide-isophorone diisocyanate Zhang membrane et al.[40]

Cu

46.20

Cd

66.40

Cr

71.10

Pb

52.80

Graphene oxide

Cu

92.00

Polyether sulfone NF membrane modified by magnetic GO/metformin hybrid

Abdi et al. [41]

Graphene oxide

Zn

81.00

Carboxylated GO-incorporated polyphenyl sulfone NF membrane

Shukla et al. [42]

GO/ceramic-supported attapulgite composite membrane

Liu et al. [43]

Graphene oxide

Cd

74.00

As

96.00

Pb

73.00

Cr

93.00

Cu

Nearly 100

Ni

Nearly 100

Pb

Nearly 100

Cd

Nearly 100

The surface-modified polyethersulfone (PES) MF membrane was synthesized using electrostatic deposition of PEI and GO for the effective rejection of Blue Corazón reactive dye in water. In this membrane, PEI acts as the polycation, and GO acts as the polyanion. The percentage of rejection was about 90%. The modified polyethersulfone MF membranes show a small degree of fouling which can be

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Table 3.3 CECs/POPs/dyes removal by various graphene-based membranes Graphene derivatives

CECs/POPs/ Dyes

Modified membrane type

References

Graphene oxide

Blue corazol

Polyethersulfone/GO microfiltration membrane

Homem et al. [46]

Graphene oxide

Orange safranine, Bordeaux red, Twilight yellow

TiO2 - and GO-modified PES membrane

Diogo Januário et al. [47]

Graphene oxide

Direct black 38

Phosphorylated chitosan/graphene oxide NF membrane

Song et al. [48]

Reduced graphene oxide

Phenol

N-doped reduced graphene oxide membrane

Liu et al. [49]

Reduced graphene oxide

Ciprofloxacin, ofloxacin, and enrofloxacin

rGO-M-PVDF composite membrane Vieira et al. [50]

Reduced graphene oxide

Venlafaxine and phenol

rGO/PTFE membrane

Cruz-Alcalde et al. [51]

overcome by hydraulic cleaning [44]. Diogo Januário et al. modified the PES MF membrane using H2 SO4 , TiO2, and GO solutions by LBL self-assembly method. This GO-modified membrane shows a high removal rate for safranin orange (100%), Red Bordeaux (93.35%), and twilight yellow (69.98%). H2 SO4 was used to increase the hydrophilicity of the membrane, TiO2 and GO were used to improve the flux, permeability, and fouling resistance of the membrane. GO nanosheet surface-functionalized phosphorylated chitosan NF membrane was fabricated to remove direct black 38 dye from aqueous solutions. The modified membrane possessed hydrophilicity (decreasing to 41.9°), negative charge (−56.4 mV), and roughness (18.7 nm), respectively [45]. Various organic pollutants removal by graphene-based membranes are listed in Table 3.3.

3.3.2 Graphene and Its Derivatives in Various Advanced Oxidation Processes Advanced oxidation processes have demonstrated the ability to remove multiple contaminants of emerging concern due to the non-selective nature of reactive species like hydroxyl radical, superoxide radical, ozone, and hydrogen peroxide employed in those processes [52]. These processes are capable of mineralizing organic contaminants into carbon dioxide and water, the property for which they are widely preferred. In this section, the role of graphene-based materials in various advanced oxidation processes is discussed elaborately.

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Catalytic Ozonation-Driven Degradation of Contaminants of Emerging Concern and Other Persistent Organic Micropollutants Using Graphene-Based Catalysts

Ozone is a strong oxidant with an oxidation potential of 2.07 V [53]. Even though it has a strong oxidation potential, it oxidizes selectively the single bond containing organics and substituted aromatics with groups like -NO2 and -COOH, which are electron-withdrawing [54]. Interestingly, ozone can undergo autocatalysis-assisted decomposition (pH-dependent process), the rate increases with increased pH and forms non-selective and highly reactive hydroxyl radicals, which can act on a wide range of contaminants. In catalytic ozonation, catalysts are used to accelerate the initiation of a sequence of reactions that convert ozone into hydroxyl radicals even at low pH levels [55]. Graphene-based catalysts aid in the enhanced adsorption and oxidation of pollutants and also ease the electron transfer rate in redox reactions [56, 57]. Figure 3.4 represents the general mechanism of catalytic ozone decomposition by graphene derivatives. In a work done by Y Wang et al., the authors synthesized reduced Graphene Oxide (rGO) from waste graphite electrode utilizing lithium–ion battery with various degrees of defective sites and with the same level of oxygen content for the catalytic ozonation aided degradation of phenolics (4-nitrophenol, acetyl salicylic acid, and para-hydroxyl benzoic acid) and aliphatics (oxalic acid, acetic acid, and formic acid). With the help of density functional theory calculations, the authors concluded that the defects in the graphene structure played a dominant role in the decomposition of ozone and ultimately the pollutant degradation was increased. The catalytic activity increased with the increased level of defective sites. This phenomenon

Fig. 3.4 Generation of superoxide radical from ozone for the pollutant degradation by graphene derivatives

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was attributed to the modification of properties like surface reactivity, mobility of adsorbed species, and electron conductivity by the delocalized electrons in the defective sites. The surface oxygen functional groups on the graphene also played a role in the decomposition of ozone by O–O bond stretching [58]. Heteroatom-doped graphene oxide catalysts are employed for the degradation of sulfamethoxazole (a sulphonamide antibiotic) by R Yin et al. The group compared the degradation of sulfamethoxazole by reduced graphene oxide, N-doped graphene oxide, and P-doped graphene oxide and got degradation efficiencies of 83%, 95%, and 99%, respectively. The enhancement of catalysis by the heteroatom doping was related to the disruption of sp2 hybridized carbon structure and creation of new active sites for the catalytic activity by breaking the electroneutrality of the graphene oxide [22, 59]. The mechanism of radical generation in catalytic ozonation by graphene derivatives is given in the below set of equations (Eqs. (3.1)–(3.13)) [60]. π electron in the GO/rGO reacts with water molecule to form hydronium ion and hydroxyl ion GO − π + 2H2 O ↔ GO − H3 O+ + OH−

(3.1)

rGO − π + 2H2 O ↔ rGO − H3 O+ + OH−

(3.2)

Ozone reacts with a water molecule by accepting the electrons from the catalytic materials or from the pollutants to give molecular oxygen and hydroxyl ions O3 + H2 O + 2e− → O2 + 2OH−

(3.3)

The ozone is sequentially decomposed to form hydroperoxyl radical, superoxide radical, and a proton O3 + OH− ↔ HO2 . + O2 .−

(3.4)

HO2 . ↔ O2 .− + H+

(3.5)

The generated hydroxyl ion reacts with the GO/rGO to form hydroxyl groups on the surface of the GO/rGO OH− + GO/rGO ↔ HO − GO/rGO

(3.6)

Then the hydroxyl group on the GO/rGO reacts with the ozone to form hydroxyl radical and superoxide radical. O3 + HO − GO/rGO ↔ O3 . − GO/rGO + HO.

(3.7)

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O3 . + HO − GO/rGO ↔ O − GO/rGO + O2

(3.8)

O3 + . O − GO/rGO ↔ . O2 − +GO/rGO + O2

(3.9)

Carbonyl groups present on the surface of GO/rGO react with ozone to form carboxylic groups and hydrogen peroxide, again the carboxylic groups undergo oxidation and produce hydrogen peroxide and molecular oxygen. 2GO − (−HC = O) + O3 + H2 O ↔ 2GO − (−COOH) + H2 O2

(3.10)

2rGO − (−HC = O) + O3 + H2 O ↔ 2rGO − (−COOH) + H2 O2

(3.11)

2GO − (−COOH) + O3 + H2 O ↔ 2GO + H2 O2 + 2O2

(3.12)

2rGO − (−COOH) + O3 + H2 O ↔ 2rGO + H2 O2 + 2O2

(3.13)

The generated radicals and oxidant species further degrade the pollutants to give less toxic by-products, carbon dioxide and water.

3.3.2.2

Photocatalysis-Driven Degradation of Contaminants of Emerging Concern and Other Persistent Organic Micropollutants Using Graphene-Based Photocatalysts

In photocatalysis, electron–hole pairs are generated by the excitation of electrons from the valence band to the conductance band by using the energy of photons [61]. The common reactive species in a photocatalytic system are holes, superoxide radicals, hydroxyl radicals, and singlet oxygen. These reactive species either directly act upon the pollutants or are used to activate oxidants like hydrogen peroxide, persulfate/peroxymonosulfate to generate additional radicals. Conventional materials used in photocatalysis have the disadvantages of wide bandgap energy and quick recombination of electron–hole pairs which are generated through photoactivation. Figure 3.5 represents the general photocatalytic mechanism of graphene-based composites. The wide bandgap energy of these materials can be reduced by doping with a photosensitizer like graphene. Two-dimensional graphene nanosheets can be used to construct a wide range of heterojunctions like semiconductor–nanocarbon heterojunction, Z scheme heterojunction, p–n heterojunctions, Schottky heterojunctions, and Type II heterojunctions [62]. Innately graphene is a zero bandgap material with a semi-metallic nature due to the touching of bonding pi orbitals with antibonding pi* orbitals at the Brillouin zone [63, 64]. However, adding foreign atoms through chemical doping breaks the lattice structure and creates a bandgap. On the other hand, reduced graphene oxide can be directly used in photocatalysis due to the presence of oxygen atoms. The valence band of the reduced graphene oxide can

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Fig. 3.5 Photocatalytic activity of graphene-based composite in pollutant degradation

be upshifted by reducing the degree of oxidation [65]. Graphene derivatives act as electron acceptors, decreasing the electron–hole pair recombination in photocatalytic material with which it is doped. The pi-conjugation-based organic pollutants’ adsorption near the photocatalyst surface can also be regarded as an enhancement mechanism of photocatalysis by graphene derivatives [66]. Table 3.4 summarizes the efficiency of different graphene-based composites in pollutant degradation. Table 3.4 Comparisons of graphene-based composites used in photocatalysis for the degradation of emerging contaminants and organic pollutants Name of the composite

Pollutant

Pollutant conc. (mg/L)

g-C3N4/ Ag2CO3/ graphene oxide

Tetracycline

20

MIP-TiO2 /GR

Bisphenol A

Bi7 O9 I3 /RGO Bi-doped TiO2NT/ graphene rGO—SrSnO3 nanocomposites

Time (min)

Efficiency (%)

References

60

81.6%

Liu et al. [67]

4

180

67.6%

Lai et al. [68]

Rhodamine B

10

100

95.8%

Liu et al. [69]

Denosab

10

180

72%

Alam et al. [70]

Methylene blue

10

180

97%

Venkatesh et al. [71]

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Electrochemical Oxidation-Driven Degradation of Contaminants of Emerging Concern and Other Persistent Organic Micropollutants Using Graphene-Modified Electrodes

Electrochemical oxidation is a type of Advanced Oxidation Processes (AOP), whereby the degradation of pollutants is carried over by the electrodes and the electrolyte. Electrochemical oxidation consists of two different mechanisms for the deterioration of pollutant molecules, namely, direct oxidation and indirect oxidation. In the former, the pollutant molecules are degraded by the sorbed radicals over the electrode surface [72]. While in the latter, the in situ generated radicals in aqueous bulk degrade the pollutant molecules [73]. In addition to this, the reactive oxygen (ROS) and chlorine (RCS) species produced during the process also facilitate degradation [74]. Following reactions (Eqs. (3.14)–(3.19)) explain the various ways through which hydroxyl radicals are generated and their interaction with other molecules, O2 + 2H+ + 2e− → H2 O2

(3.14)

H2 O2 + OH. → HO.2 + H2 O

(3.15)

H2 O → OH. + H+ + e−

(3.16)

OH. + HO.2 → H2 O + O2

(3.17)

HO.2 + HO.2 → H2 O2 + O2

(3.18)

OH. + OH. → H2 O2

(3.19)

Because of its plethora of chemical and physical properties, graphene-based materials are used as electrodes in many applications [75]. For example, BaptistaPires and co-workers developed a graphene-based cathode (N-doped) and anode (B-doped), shown in Figs. 3.5a and d [76]. This kind of porous electrode showed higher efficiency toward pollutant removal. Graphene is also combined with other materials as a composite for electrode fabrication. Some examples include copper hexacyanocobaltate/graphene/ITO composite for levofloxacin degradation [77], lead dioxide/graphene [78], tungsten exfoliated graphite, etc. Further the enhanced production of H2 O2 , hydroxyl radicals, and low cost make them attractive in electrochemical applications [76]. Figure 3.6 represents different graphene-based anodes.

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Fig. 3.6 a B-doped graphene anode [76], b Anode made out of graphene composite, c Layered graphene anode, d N-doped cathode, e Graphene integrated with sulfur [79], f Cathode made of graphene composite

3.3.2.4

Fenton-Like Degradation of Contaminants of Emerging Concern and Other Persistent Organic Micropollutants Using Graphene-Based Composites

Due to the strong oxidation capacity of the hydroxyl radical (·OH), Fenton/Fentonlike process has been the primary focus for the degradation of toxic contaminants [80]. In comparison to other advanced oxidation processes (AOPs), Fenton/Fentonlike process has been widely studied due to its simple operation, mild conditions, and fast formation rate of ·OH [81]. Since the utilization of ·OH is vital for the Fenton process to be carried out, it is important to monitor the production and utilization of the radicals in the degradation process efficiently. Figure 3.7 depicts the involvement of graphene-based composites in Fenton-like processes. Graphene-based composites have captured the attention for their excellent properties, e.g., large surface area, mechanical stability, and abundant surface functional groups. The presence of the porous network provides enlarged sites for catalytic activities. While the Fenton-like process demands the stability of the catalyst, graphenebased composites comprised of catalysts offer the needed stability in the aqueous phase [82]. Apart from helping in the stabilization, the graphene-based materials can act as a strong electron acceptor. Its pi–pi cloud of electrons can enhance the efficiency of the Fenton-like process by attracting pollutants to its surface [83]. The detailed comparative assessment has been provided in a tabular format for a lucid understanding of the role of graphene composite in the effective degradation

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Fig. 3.7 Fenton-like reaction carried out using graphene-ferrous composite for pollutant degradation

of pollutants below. In summary, graphene-based composite can enhance efficiency in three ways: • It provides a high specific surface area, leading to the adsorption of pollutants on the surface through a strong pi–pi conjugation [84]. • Graphene-based material can act as an electron acceptor to promote the activity of the catalyst [85]. • The composite can prevent the aggregation of the stacking as well as agglomeration, thus resulting in an enhanced catalytic activity [86]. Graphene could also contribute to the photo-Fenton process. The mechanism of involvement of graphene in photo-Fenton is demonstrated in the below-stated reactions (Eqs. (3.20)–(3.28)). Hammed et al. prepared iron oxide nanoparticles (IO) coupled with GO composite for the degradation of MB [87]. The adsorption can be stated as per the following interaction on the surface of IO–Gr; IOGr − OH + OH− ↔ IOGr − O− + H2 O

(3.20)

IOGr − O − +MB+ ↔ IOGr − O− MB+

(3.21)

The role of graphene in catalytic activity in photo-Fenton process can be understood in the following reactions: IO + hv → h+ + e−

(3.22)

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    IO e− + Gr → IO = Gr e−

(3.23)

Gr(e− ) + O2 → Gr + O(·−) 2

(3.24)

MB + h+ → CO2 + H2 O

(3.25)

Fe3+ + e− → Fe3+

(3.26)

H2 O/OH− + h+ → HO.

(3.27)

HO. + MB → CO2 + H2 O

(3.28)

In this process, the excitons (electrons and holes/e− and h+ ) form in IO, and the electrons were transferred to graphene sheets and later used by O2 to form O2 − radicals. The h+ generated can oxidize and degrade MB. Fe2+ on the surface of IO– Gr30 catalyzes the decomposition of H2 O2 to produce ·OH radicals. The electrons of IO that were transferred to the graphene surface can react with the Fe3+ to form Fe2+ . The accelerated cycling of Fe3+ /Fe2+ increases the decomposition of H2 O2 into ·OH. As an alternative, photo-induced holes can react with H2 O/OH− to form ·OH radicals or directly oxidize and degrade MB. Table 3.5 summarizes the efficiency of graphene-based Fenton-like catalysts in pollutant degradation. Table 3.5 Comparisons of graphene-based composites used in Fenton-like process for the degradation of emerging contaminants and organic pollutants Name of the composite

Pollutant

Pollutant conc. (mg/ L)

Time (min)

Efficiency (%)

References

Fe3 O4 –rGO/Pt

MB

1

3

83%

Shi et al. [82]

MIL-101(Fe)/ CoFe2 O4 /GO

DtR-23

100

160

99.93%

ReR-198

100

160

99.65%

Bagherzadeh et al. [83]

IO-Gr30

MB

60

60

100%

Hammad et al. [87]

Ce0 /Fe0 –GRO

SMT

20

25

99%

Wan et al. [88]

CoFe2 O4 –rGO

AO7

10

120

90.5%

Hassani et al. [89]

AR17

10

120

84.65%

Fe3 O4 /ZnO/ graphene

MB

40

60

100%

CR

40

60

100%

GA

CIP

50

90

100%

Wang et al. [91]

Pd/nZVI/rGO

OTC

100

60

96.5%

Nguyen et al. [92]

Saleh and Taufik [90]

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3.3.3 Graphene and Its Derivatives in Adsorption Global water pollution and the availability of freshwater have been impacted by various anthropogenic activities, which leads to considerable health complications in human as well as on the ecosystem. Different hazardous materials are severely used in industries, and their uncontrolled discharge is the major issue that requires urgent mitigation. These discharges of pollutants majorly merge contaminants, and heavy metal waste accumulates in water streams and landfills. Among various methods proposed and implemented, the adsorption process is deliberated as a low-cost remediation technique using graphene and graphene-based adsorbent for a range of pollutants. Graphene is the new star candidate in adsorptive removal of toxic elements from wastewater where oxidized and chemically modified graphene derivatives are in trial process with promising results. The following sections have summarized the elimination of emerging contaminants and toxic heavy metals using graphene-based materials.

3.3.3.1

Removal of Contaminants of Emerging Concern and Other Persistent Organic Micropollutant Using Graphene-Based Adsorbents

While it comes to the remediation of contaminants of emerging concerns (CECs) as well as persistent organic pollutants, graphene-based materials have been proved to be excellent adsorbents. There have been various types of different modifications which have been performed with raw graphene to obtain aerogel, hydrogel, nanohybrid, composite materials, etc. for enhanced adsorption of organic pollutants. For elucidation purposes, model toxic dyes, both cationic, such as methylene blue (MB), Rhodamine B (RhB), Malachite Green (MG), and anionic ones, e.g., Congo Red (CR) and Methyl Orange (MO), have been taken into consideration. Li et al. have carried out polymer and rGO interaction to create amphiphilic behavior for better removal of organic pollutant. The research group managed to manufacture graphic carbon nitride (g-C3 N4 ) decorated rGO, which helped in stabilizing the rGO in water by dispersing it as adsorbents as well as preventing the aggregations of hydrophobic rGOs. In addition to these advantages, the functional groups of g–C3 N4, such as -N/-NH/-NH2, along with the structural properties of rGO, created a periodic honeycomb lattice that provided abundant binding sites for the interaction with aromatic molecules. They achieved a maximum adsorption capacity of 520 mg g−1 for cationic RhB using rGO-g-C3 N4 , proving that amphiphilic graphene-based nanocomposites can be a viable option for practical application of removal of organic pollutants [93]. Likewise, M. Yan et al. synthesized GO-polymer composite for the removal of cationic toxic dye MB. To prevent the aggregation of GO due to the strong pi–pi interaction, they incorporated lignosulfonate (LS), a polymer, to GO sheets in the presence of chitosan (CS), a crosslinking agent. This composite displayed a 3D structure with high porosity, helping

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in terms of enhanced separation efficiency. The authors ensured the materials used to be low cost and environmentally friendly by choosing LS, as it is derived from lignin polymer, which is one of the by-products in the paper industry. The GO-LS aerogel cross-linked by chitosan (GOLCA) showed a maximum adsorption capacity of 1023.9 mg g−1 for MB, posing itself as an excellent adsorbent along with its advantage of being eco-friendly, cost-effective, and recycle material [94]. Similarly, H Hosseini et al. adopted the method of modifying GO by embedding polyacrylic acid (PAA) semi-interpenetrated by carboxymethyl cellulose (CMC). With the presence of numerous carboxyl and hydroxyl groups, the nanocomposite CMC-PAA-GO showed an excellent 90% removal capacity after nine cycles for MB dye [95]. In addition to the removal of cationic dyes, graphene-based 3D composites and nanomaterials have proven their efficiency in anionic dye removal. Rong et al. prepared a versatile 3D magnetic sulfur/nitrogen-doped rGO nanomaterials (3DMSNGs). The 3D-MSNGs prepared with ferric ions as the ion source displayed a higher porous structure and larger BET surface area than other iron sources used. In addition to the porous framework and larger surface area, the increased surface hydroxyl groups laid out the platform for more binding sites for dye, and the co-doping of N and S gave enhanced chances for active sites for binding with dye molecules. With the mentioned modifications, 3D-MSNGs achieved a stunning maximum adsorption capacity of 909.09 mg g−1 for the anionic dye CR [96]. Similarly, Karaman et al. adopted the concept of modifying the surface of 3D graphene network to remediate MO. To enhance the interaction between MO and adsorbent, the Author prepared amino-functionalized graphene networks via the double-crosslinking method (3D-GNf ), which achieved 270.27 mg g−1 adsorption capacity for MO removal [97]. J. Yan and Li synthesized a graphene-based hydrogel for multifunctional purposes. It can target anionic, cationic dyes as well as heavy metals from the wastewater. The authors first incorporated the polymerization of acrylic acid (AA) and methacrylate (MMA) with magnetically modified GO (MGO) hydrogel. Later they grafted triethylenetetramine (TETA) onto the hydrogel with β-cyclodextrin (CD) embedment in the presence of citric acid (CA) as the cross-linking agent. The prepared P(AA-MMA)/MGO/CA-CD/NH2 with abundant carboxyl as well as amino groups showed the maximum adsorption capacity of 3185.16 and 3315.00 mg g−1 for cationic pollutants MB and MG, respectively, 1058.18 mg g−1 for anionic organic pollutant CR [98]. Apart from the remediation of organic pollutants, graphene-based composites have been proven as excellent adsorbents for alleviating emerging contaminants levels in wastewater. For a broad overview, antibiotics such as diclofenac (DIF), ciprofloxacin (CIF), ibuprofen (IBF); pharmaceutical compounds such as quinolone (QL), metformin (MF); plastic such as tetrabromobisphenol A (TBBPA) have been taken into consideration for their removal using graphene-based materials. Hiew et al. [99] prepared rGO aerogel (rGOA) from GO by reducing it using L-ascorbic acid, an eco-friendly and low-cost material. This reduction gave rGO stability in the aqueous environment, and the aerogel showed an excellent adsorption capacity of 596.71 mg g−1 [99]. Similarly, Sun et al. synthesized GH in one-step hydrothermal reduction method with optimizing the process through the means of

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response surface method (RSM), and they achieved a maximum removal capacity of 348 mg g−1 for the antibiotic CIP. Though GH, GA have achieved decent removal capacity for the removal of antibiotics, researchers have made modifications to obtain graphene-based nanocomposites, modified GA or GH for enhanced stability, as well as a more porous network to augment the removal efficiency [100]. Han et al. prepared graphene nanoplatelet/boron nitride composite aerogels (GNP/BNA), which comprised of boron nitride (BN) ribbon fibers. These fibers along with the structure of graphene composite (GC) displayed highly porous network and showed a maximum adsorption capacity of 99% for CIP removal [101]. Sun et al. tried the method of preparing inorganic–organic composite with amino-functionalized graphene hydrogel (NH2 -DN) cross-linked by TETA. With the achieved higher mechanical properties and porous structure, the maximum adsorption capacity of NH2 -DN was found to be 301.36 mg g−1 for the removal of CIP. To achieve more mechanical stability and higher surface area with porous network, zeolitic imidazolate frameworks (ZIFs) have been incorporated to graphene network [102]. Arabkhani et al. prepared a magnetic nanocomposite consisting of GO decorated with ZIF-8, pseudo-boehmite (γ-AlOOH), and Fe3 O4 through the means of solvothermal as well as solid-state dispersion (SSD) method. With the abundant –OH functional groups due to the presence of γ-AlOOH, and the enhanced stability achieved with the addition of ZIF-8, the nanocomposite GO/ZIF-8/γ-AlOOH demonstrated an excellent adsorption capacity of 2594 mg g−1 for the removal of DCF [103]. Khalil et al. synthesized porous graphene (PG) in a relatively low-cost manner. With abundant surface area, porous network, and the richness of functional groups present on the surface, the authors tested the efficiency of PG in the removal of six antibiotics, e.g., atenolol (ATL), carbamazepine (CBZ), CIP, DCF, gemfibrozil (GEM), and ibuprofen (IBF). It achieved over 100 mg g−1 removal capacity for all the mentioned pollutants with efficiencies more than 99% at a low dosage of PG [104]. Similar to the above-mentioned antibiotics, graphene-based materials have been recently used in the remediation of pharmaceutically active compounds. Zhu et al. first used GO to remove metformin, a pharmaceutical for type 2 diabetes mellitus. In 20 min, GO achieved 80% removal of metformin, and the efficiency remained decent till several cycles of application [105]. Kang et al. incorporated hollow carbon nanospheres (HCNSs) into GA to make HCNS/NGA composite, which displayed a porous structure consisting of nano- to micrometer pores with high specific surface area. The composite achieved 138.37 mg g−1 removal capacity for quinolone posing itself as a promising candidate for remediation of pollutants in wastewater [106].

3.3.3.2

Toxic Heavy Metal Removal Using Graphene-Based Adsorbents

Heavy metals top the list of toxic elements due to their lack of metabolization in the living body, which leads to accumulation in bone, joint, muscle, fat-causing reduced organ growth, fatal cancer, disruption of the nervous, and immune system, to name a few [107]. To date, a number of scientific reports have been reported on the removal of

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various heavy metals using graphene-based adsorbents. However, systematic documentation is as important as the work published to decipher the efficiency of heavy metals removed from wastewater to implement in an industrial setup. Thus this increasing attention is channelized to the improvement of 3D graphene-based adsorbents. A group prepared cobalt oxide graphene nanocomposite, which was modified with polypyrrole through the hydrothermal method. As reported, the microscopic technique confirmed the increased heterogeneous pores and ordered features which were then systematically checked for lead and cadmium metals and a few dyes. This adsorbent showed a high thermal stability of up to 195 °C and successfully removed the studied heavy metals with more than 90–95% efficiency at around 5–7 pH from the simulated solution. When investigated about the maximum capacity for Pb and Cd adsorption, it was found to be 780.363 and 794.188 mg g−1 for the respective pollutants [108]. Another group implemented density functional theory to analyze the adsorption of sulfur dioxide on the graphene surface. They have included raw graphene, vacancydefected graphene, titanium-doped graphene, and titanium-doped graphene with vacancies. Results indicated that due to limitation in physical adsorption with sulfur dioxide and vacancy defects of raw graphene, only a limited adsorption capacity was achieved. Whereas doping with Ti improved charge density and adsorption energy of the system and facilitated chemisorption. Besides, an impurity band showed up in the band of graphene doped with Ti structure after adsorbing SO2 , which further increased the density of states near the Fermi level and Ti-doped graphene showed the maximum adsorption of SO2 for the materials included in the study [109]. An elaborated work has been reported where Pb2+ , Cr3+ , Cu2+ , Zn2+ , Cd2+ , and Ni2+ were checked with porous carbon (graphitized) that was then oxidized to graphene oxide-like PGCO. This fabrication was done as graphene oxide made from natural graphite after exfoliation is water-soluble which makes its recovery after one cycle very difficult. All the metals adsorption followed Langmuir isotherm and found maximum capacities for Pb2+ , Cr3+ , Cu2+ , Zn2+ , Cd2+ , and Ni2+ were 377.1, 119.6, 99.1, 53.0, 65.2, and 58.1 mg/g, respectively, which was interpreted as the adsorbent’s greater functionalities of oxygenated groups. The PGCO exhibits an enhanced degree of oxidation when compared to GO, owing to 8.4% carboxyl group on the surface and an O/C molar ratio of 0.63. With the intensive chemical aggregation, the PGCO sheets form an insoluble structure that can be easily recovered from wastewater by filtration or sedimentation. Further simple acid treatment was done for successful recovery of spent PGCO, which indicated that PGCO could be a potent adsorbent to remove toxic heavy metals from wastewater [110]. Graphene oxide enriched with sand was used to mitigate heavy metals, viz., As(III), Cr(VI), Pb(II), and Cd(II) from wastewater and was found to have 87, 92, 94, and 88% adsorption capacities, respectively. For dynamic column, breakthrough time increased for Pb (10.83 h) and Cd (4.23 h) indicated the maximum efficiency in the retention of lead and cadmium [111]. In this work, three algal strains were utilized for the cellular extracts to reduce GO efficiently and were used as decontaminating agents for copper and lead. Removal percentages of (93% and 82%), (74 and 89%), and (91 and 95%) were found with

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the GO prepared with the above-mentioned three strains for Cu and Pd, respectively, within 30 min [112]. Another group also used dialdehyde cellulose-grafted graphene oxide with triethylenetetramine as a cross-linker and checked for Pb and Cu ions in simulated solution and reported very good performance. At pH 5, the maximum capacity of adsorption was found to be 80.9 and 65.1 mg/g for Pb and Cu, respectively. Further, the reusability study revealed that after four cycles the adsorption efficiency was about 77% [113]. This author group developed a green adsorbent from graphene oxide-based aerogel modified with silk fibroin. Sophisticated techniques like XPS, SEM, XRD, etc. verified well-formed pore arrangement, more oxidation groups, and a larger d-spacing after modification. The adsorbent was checked for a metal Ag+ and methylene blue dye where adsorption capacity was found up to 1322.71 mg/g for the latter with an inclination toward monolayer adsorption [114]. With less research reported on the dynamic studies on the graphene-based adsorbent, a group tested copper and manganese adsorption on SDS-modified graphene. In a fixed-bed column, the optimal adsorption capacity was 48.83 and 45.62 mg/ g for copper and manganese, respectively. Artificial neural network was implemented to optimize the experimental condition and showed that initial dosage of adsorbent impacted more with 55 and 45% on the metal intake for copper and manganese, respectively. These results are satisfactory and could be scaled up for higher performance [115]. In another study, when reduced graphene oxide (rGO) hybridized with elemental and/or magnetite, silver nanoparticles were used to remediate several heavy metals, viz., Ni(II), Cd(II), Co(II), Zn(II), Cu(II), and Pb(II) it was found that despite the inhibitory effect of silver nanoparticles and iron oxide on the rGO nanosheets, the adsorption efficiency of the metals by the nanohybrids was still high. These findings showed the probable feasibility of rGO-based nanohybrids as a potent remediation tool for heavy metal removal from wastewater [116]. In a recent work, an ice segregation-induced self-assembly technique was utilized to prepare graphene oxide-based aerogel to adsorb heavy metal ions cadmium and nickel from an aqueous solution. The primary interaction was derived to be an electrostatic attraction between GOA and the metals with maximum adsorption capacities observed to be 108.70 and 91.74 mg/g cadmium and nickel, respectively [117]. A new type of graphene oxide/carboxymethyl chitosan (GO/CMC) composite aerogel with cross-linker triphosphoric acid or glutaraldehyde was synthesized by vacuum-assisted self-assembly along with freeze-drying, and silver, copper, and lead metals were adsorbed. Adsorption diffusion was facilitated due to the availability of a cross-linked porous structure of the aerogel and –O and –N containing groups that provide sites for adsorption. Moreover, GO can resist pressure while CMC prevents fracture making a super-strong aerogel showing 368.35 and 16.77 MPa in elastic modulus and compressive strength of GO/CMC aerogels, respectively [118].

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3.4 Conclusion and Future Outlook In this chapter, detailed descriptions of the utilization of graphene-based composites for wastewater treatment have been provided. The porous nature, large surface area, mechanical stability, and rich surface chemistry of the composites make them ideal alternates for industrial wastewater remediation due to their excellent physicochemical properties. There are many types of processes that can be performed with graphene-based composites, which are not limited to one method. The materials can be used in membrane technologies to remove pollutants. Furthermore, graphene composites have proved effective in a number of AOPs, such as catalytic ozonation, photocatalysis, electrochemical oxidation, and Fenton-like degradation of toxic contaminants. There has also been a growing demand for graphene-based materials in a low-cost and efficient method like adsorption, as they have shown excellent removal efficiency and good regeneration capacity for the remediation of heavy metals, emerging contaminants, and persistent organic pollutants. There is no denying the impact graphene composites have on wastewater remediation through offering excellent features to various methods. However, when it comes to the stability and regeneration of composites, there is room for improvement to synthesize more mechanically stable or flexible materials that can further enhance efficiency, remain cost-effective, and be reused for a long time period. In addition to the use of these types of composites through a single technique, there is scope for developing composites that can be used in multiple ways, such as AOP paired with adsorption or membrane technology paired with electrochemical methods to enhance the performance of the material. It will be possible to address these issues in the future while keeping cost-effectiveness, eco-friendliness, and effectiveness in mind.

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Chapter 4

Graphene-Based Nanocomposite Solutions for Different Environmental Problems Preetha Ganguly, Rwiddhi Sarkhel, Sandipan Bhattacharya, and Papita Das

4.1 Introduction Over the past few years, widespread utilization of chemical compounds in industrial production has led to an enhancement of chemical contaminants in the ecosystem, like heavy metal ions, polycyclic aromatic hydrocarbons (PAHs), pesticides, and phthalates, which are becoming a major global concern and might also pose a serious threat to human and aquatic life. Therefore, this is of great potential and value to prepare analytical methods to effectively and quickly detect and remove different environmental pollutants. Because the environment matrix is quite complex and the concentration of the target analyte is usually too low, therefore an effective method of pre-concentration should be utilized prior to the analysis. This can be achieved by utilizing different types of polymeric nanocomposites. Currently, there has been a tremendous and continuous study on carbon-based materials for polymeric nanocomposites [1]. Many studies have focused on producing composites containing graphene utilized in diverse matrices due to its unique characteristics. P. Ganguly Department of Biochemical Engineering and Biotechnology, IIT-Delhi, Hauz Khas 110016, New Delhi, India R. Sarkhel Department of Chemical Engineering, NIT-Durgapur, A-Zone, Mahatma Gandhi Rd, Durgapur, West Bengal 713209, India S. Bhattacharya · P. Das (B) Department of Chemical Engineering, Jadavpur University, 188, Raja S. C. Mullick Road, Kolkata 700032, India e-mail: [email protected] P. Das School of Advanced Studies of Industrial Pollution Control Engineering, Jadavpur University, Kolkata, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Mohanty et al. (eds.), Graphene and its Derivatives (Volume 2), Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-99-4382-1_4

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4.1.1 Graphene The mechanical exfoliation of graphene was performed in the year 2004 by a scientific team of Novoselov. Graphene is basically two dimensional (2D) sheets of sp2 type of hybridization of carbon atoms. The graphene has a high surface area to volume ratio (2630 m2 /g). Graphene has significant potential for absorbing aromatic pollutants due to van der walls activity or π-π electron coupling [2]. Graphene is usually exfoliated from micromechanical fragmentation of graphite, or by epitaxial seeding on the silicon carbide surfaces. The dispersal of graphite in the organic solvents and chemical vapor deposition of hydrocarbons on different metal surfaces, and are other two common methods for graphene preparation. Over few conventional nanofillers that include, carbon nanotubes, carbon nano fullerene, layered silicates, metal oxides, and carbon black, Graphene is considered a good nanofiller because of its high tensile strength, thermal conductivity, surface area, flexibility, aspect ratio, and transparency. The loading of graphene within the nanocomposite determines the stability, flexibility, functionality, and chemical affinity of the nanocomposite for different applications [3]. The major disadvantage of graphene-based nanocomposites is hydrophobicity and easy agglomeration that leads to a decrease in adsorption ability of the composite. Therefore, to avoid the shortcomings, functionalization has been done. The two most common oxidized forms of graphene are graphene oxide (GO) and reduced graphene oxide (RGO). These are more popular than conventional graphene because they have reactive oxygen groups and better dispersion in water. Functionalized graphene, graphene Oxide, and reduced graphene Oxide can be homogeneously and easily dispersed in different polymer matrices [4].

4.1.2 Graphene Oxide (GO) Graphene Oxide (GO) are oxidized monolayer of graphene sheets that are generally exfoliated by chemical pre-treatment of graphite. Another common methodology to synthesize graphene oxide is the modified hammers method. This improved Hummers’ method can synthesize graphene oxide consisting of various polar functional groups, which help for mixing with other compounds of interest. GO mainly contains carboxyl, hydroxyl, and epoxides functional groups that lead to their easy dispersion in the polymeric matrices. These functional groups on the GO change the van der Waals activity and efficiently enhance the interaction between the polymeric matrix and GO, causing homogeneous dispersion of the particles [5]. On the proximal ends of the GO sheet, oxygenic functioning units have been represented in the form of epoxy or hydroxyl group, whereas fewer number of phenols, quinone, lactone, carbonyl, and carboxyl functional groups were found at the boundaries of the GO. Usually, in spite of various advantages of graphene, GO is more preferred with other composite materials due to their enormous oxygen functional units, enhancing

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Fig. 4.1 Schematic representation of graphene oxide functionalized with different molecules, nanoparticles, and polymeric compounds

covalent interactions with other types of functional groups. Various polypeptide chains are also able to be added on the surface of GO, e.g., PEG (polyethylene glycol), PVA (polyvinyl alcohol), polyallylamine, and polylysine for the synthesis of graphene-based nanocomposite. Since graphene oxide and reduced graphene oxide are negatively charged because of the presence of oxygen functional units, they can easily interact with the positively charged materials through electrostatic interactions. However, the presence of additional carboxyl and carbonyl functional groups at the boundaries of GO, makes the GO sheets highly hydrophilic in nature, enabling them to easily swell and disperse in water. Covalent modifications are most common in regard to GO sheets. Modification through amidation gives active sites of GO to molecules like amino acids, polyethylene glycol (PEG), acid pectinase, casein phosphopeptides, chitosan, polyethyleneimine, polyurethane [6, 7]. Esterification, salinization [8], and amidation are a few other approaches through which modifications in GO with different functional groups are made. Graphene oxide and its derived composites have applications in different fields such as cells, batteries, supercapacitors [9], superconductors [10], photovoltaics [11], solar cells-[12], gas sensors, photonics, laser, light emitting diodes, optoelectronics, tissue engineering, and many more (Fig. 4.1).

4.1.3 Reduced Graphene Oxide (rGO) Reduced graphene oxide (rGO) is usually fabricated via the reduction of graphene oxide by electrochemical reduction or thermal reduction. Majority of graphene-based polymeric nanocomposites identified are synthesized utilizing graphene oxide. Thermally reduced graphene oxide (TrGO) or chemically reduced graphene oxide (CrGO)

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are used in different applications as fillers such as improved dispersibility, electrical conductivity, drug delivery, improved mechanical properties, Tissue engineering, Charge separation in the solid state, and various others. The study of graphene derived composites has been disordered in various reviews either with a focus on materials like polymer, organic or inorganic compounds, or their application in energy. However, the whole review on different graphene composites, their preparation techniques, and their application in the environmental aspect in so much detail have not been reviewed to the best of our knowledge. Here in this review study, the common different graphene derived composites along with their preparation method is studied accompanied by recent progress in environmental studies.

4.2 Global Research on Graphene and Graphene-Based Nanocomposites Since its isolation by Geim and Novoselov from a block of Graphite by a mechanical exfoliation method with a scotch tape [13] Graphene has generated massive interest around the globe about its exorbitant potential. The Global Graphene market is projected to reach a value of 1.1 billion by 2025 with a compound average annual growth rate of 32% [14]. Some of the common applications for which Graphene is utilized are energy storage and generation as batteries, solar panels, and supercapacitors. For example, a group of scientists from USA and South Korea developed a LED using Graphene “superlattices” which utilized 25% less energy than ordinary bulbs and has a lifetime of 25 years [14]. Other than this Graphene is also being utilized for the synthesis of materials like super-light aerogel which can subsequently be employed for creating construction materials in future. Graphene is also utilized for the creation of a Graphene-sieve which can then be used for turning seawater into drinkable water, and for treating and storing radioactive waste materials [14]. As per a recent literature review, Graphene materials are also being utilized for cutting edge medical research such as for creating artificial retina and for the development of biosensors which can detect glucose in urine [14]. They are also being utilized for delivering drugs or for DNA sequencing, also packaging materials [14]. Also, Graphene-based materials are being used for pollution abatement such as the development of Graphene membranes which capture excessive CO2 thereby preventing their release into the atmosphere, and the development of Graphene-based filters which prevent the release of unwanted gases into the atmosphere from housing and commercial emissions. Due to the exceptional characteristics of Graphene as a material [15] it has also generated tremendous interest as a filling material in nanocomposite materials [13]. The addition of Graphene to polyacrylonitrile (PAN) improved the mechanical strength of the material. Similarly, the addition of 1% of Graphene to PMMA, resulted in a 80% increase in elastic modulus and 20% increase in elastic modulus strength. Epoxy based few layer graphene nanocomposites exhibited captivating properties for

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electronics, which could be deemed perfect for thermal-interface based material [13]. Similarly, Cheng synthesized carbon-coated SnO-Graphene sheets. The material was utilized as an anode in a Lithium-Ion battery and it showed increased storage property and cyclic performance. From the recent literatures, synthesis of an exfoliated graphene-cellulose-acetate nanocomposite exhibited high thermal stability and improved conductivity and modulus. From the study of graphene and fibrose cell, it was concluded that PVK-GO had more antibacterial property than pristine GO and PVK-GO was mostly unreactive towards the human fibroblast cell thereby opening a huge potential of the composition for health and industrial application. Addition of Graphene to ceramics can induce higher electrical and thermal conductivity, higher charge carrier propensity, and enhances various mechanical properties such as refractory, anticorrosive, antifriction, and biocompatibility properties [13]. The incorporation of ceramics to graphene decreases the ceramic’s brittle nature, increases its thermal shock resistance, and lowers its fracture toughness. This type of high temperature resistant material is routinely used as nose cap for space shuttles and ballistic equipments. For example, ZnBr2 -Graphene is used as a high temperature barrier for spaceships during their re-entry into the Earth’s atmosphere [13]. As mentioned before, since its inception Graphene has emerged as almost a “magic” material with unbound potential for both scientific and subsequently industrial applications. Indeed, a huge amount of research is being done regarding the possible utilizations of this fascinating material. As per [13] a total of 23,945 articles were found with the search word graphene. As per the Intellectual Property Office (IPO), an agency of the U.K Government, 26,000 patent applications regarding Graphene-related materials have been filed between 2005 and 2014. Among these 47% belong to China, 18% to United States of America, and 13% to South Korea. The 1.1-billion-dollar budget of the 10-year European Graphene Commission Flagship program or the 90 million dollar cost of the construction of the U. K’s National Graphene Institute at the University of Manchester is a testament to the degree of conviction that scientists of the developed nations have on the credibility of Graphene as being the new wonder material. Hence by all means Graphene and Graphenebased nanocomposite pose all kinds of exciting opportunities for the present and future generations.

4.3 Preparation Methodology for Graphene Polymeric Nanocomposites 4.3.1 Fabrication Methods Various techniques have been reported and developed to synthesize graphene-based nanocomposites such as covalent interaction, non-covalent interaction, physical deposition, solvothermal and hydrothermal, photochemical reaction, electrophoresis, and electrochemical deposition. Each technique has its own benefits and drawbacks.

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The synthesis of a specific nanocomposite depends on the adoption of the best suitable technique of preparation.

4.3.1.1

Covalent Interaction Technique

One of the usually applied fabrication techniques for graphene-based nanocomposites is the covalent interaction method. The interfacial covalent binding within the graphene or the graphene oxide surface is through amide bonding, atom transfer radical polymerization (ATRP), click chemistry, or diazonium salt [16]. Azidotrimethylsilane, p-phenyl-SO3H, perfluorophenylazide (PFPA), Ion liquids, porphyrin, and even the nanoparticles are also able to be added on the graphene surface by covalent binding. Amide binding is usually prevalent to covalently functionalize the graphene sheets. Stankovich observed that graphene-based sheets are attached through organic isocyanate by forming carbamate esters and amide with the hydroxyl and carboxyl functional units on the surface of graphene [17]. Another study has reported amine functionalized porphyrin covalently bonded with GO in DMF through amide linkage [18]. Similarly, amide ion was grafted to graphene sheets to synthesize polydisperse nanocomposite that are highly soluble in water, DMSO and DMF. Another study revealed that by alkylation of graphene oxide through amide bonding fabricated the alkylated graphene paper. Diazonium salt can be applied to covalent reaction for the synthesis of graphene polymeric nanocomposites. This reaction induced sulfonated functional review SO3 H on graphene sheets with a (1:35) ratio of S/C. The sulfonated-reduced GO was again reduced by the hydrazine reagent to eliminate the oxygenic functioning units. Another type of covalent functionalization of graphene is atom transfer radical polymerization (ATRP). The process starts from an initiator molecule accompanied by the growth of polystyrene chain on the surface of graphene with 80% grafting efficiency. The polystyrene graphene composite had upgraded young modulus (about 57%), and tensile strength (about 70%) when polystyrene is grafted with only 0.9 wt.% of graphene sheets. Click chemistry is one of the other common ways to covalently prepare graphene polymeric nanocomposites. Photoactive chromophore molecules like rutheniumphenanthroline and zinc-porphyrin can be grafted to the graphene surface by click chemistry. In this procedure, firstly the graphene surface was changed via phenylacetylene moieties that have exposed alkyne groups accompanied by clicking with the derivatives of ruthenium-phenanthroline and azide-terminated zinc-porphyrin. The nanocomposite prepared by this process represented an improved photocurrent response (Fig. 4.2).

4.3.1.2

Non-covalent Interaction

Another approach suitable for the fabrication of graphene nanocomposites is non-covalent interaction. The non-covalent interaction within the graphene sheets

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Fig. 4.2 Represent the covalent modification to fabricate graphene-based nanocomposites

and organic molecules promotes the grafting of organic molecules on the surface of graphene. Small molecular species such as polymers and surfactants can attach to the graphene surface through electrostatic, hydrophobic, and π-π stacking interaction, providing a functional way to change the surface morphology of graphene for synthesizing graphene nanocomposites. Small molecules like 1-pyrenecarboxylic acid, 1-pyrenebutyrate (PB-), sodium dodecylbenzene sulfonate (SDBS), dendronized perylene bisimides and PDI can be easily grafted on the modified graphene sheets [18]. In one of the studies by Xu et al. water-soluble graphene was obtained by adding PB because the pyrene molecule in the moiety has strong interaction (π) with the graphene oxide surface [18]. This technique was obtained by in situ chemical reduction by hydrazine on graphene sheets. Another study reported that the graphene surface can be non-covalently functionalized by aromatic acceptor and donor groups like 3,4,9,10-perylenetetracarboxylic diimide bisbenzenesulfonic acid (PDI) and Pyrene-1-sulfonic acid sodium salt (PyS). The functionalization of graphene leads to improved conductivity and overall power efficiency. Noncovalent interaction can be utilized for achieving graphene nanocomposites from polymeric compounds such as polystyrene, conjugated triblock copolymer, and polyaniline [17]. Stankovich et al. reported the synthesis of graphene/polystyrene nanocomposites by dispersion of graphene at the molecular level in the polystyrene sheet. This process was achieved by mixing isocyanate pretreated graphene with polystyrene accompanied by chemical reduction. These nanocomposites have a percolation threshold of less than 0.1%. This is the lowest one in the carbonbased nanocomposites apart from carbon nanotubes. Qi et al. [19] prepared amphilic graphene nanocomposite by a two-step process. This nanocomposite is commonly known as coil-rod coil conjugate triblock copolymer which has excellent solubility in different solvents. The graphene oxide was prepared by modified hammers methodology accompanied by chemical reduction (by hydrazine) in the presence of PEG-OPE. Excellent solubility was represented in organic solvents such as chloroform, toluene, tetrahydrofuran, methanol, dimethyl sulfoxide, ethanol, and water. The amine polystyrene can be covalently bonded to a carboxylate functionalized group on a graphene oxide surface, and this nanocomposite represents strong dispersibility in different organic solvents [20]. In another report, electron active nanocomposites with improved electrocatalytic activity were prepared by functionalizing sulfonate polyaniline (PANI) on a graphene sheet.

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Fig. 4.3 Represent the non-covalent modification to fabricate graphene-based nanocomposites with triblock polymer and BSA proteins

Metal ions can be infused non-covalently on graphene sheets to form threedimensional (3D) hydrogels or nanocomposites. An example is the hydrothermal decomposition of metal salt precursor with graphene oxide to form 3D graphene composite with metal nanocrystals. Nobel metals such as silver, gold, platinum, and rhodium on the graphene surface work as promoter sites in order to activate the assembly of porous graphene. This three-dimensional nanocomposite has represented excellent catalytic activity and selectivity (Fig. 4.3).

4.3.1.3

Solvothermal and Hydrothermal Technique

Solvothermal and hydrothermal technique has been widely utilized for the preparation of graphene polymeric composites. In this technique, a stainless-steel autoclave is utilized to perform hydrothermal or solvothermal deposition. Firstly, the precursor molecule is mixed either with graphene or graphene oxide in solution, secondly, solvothermal and hydrothermal deposition takes place. Due to high pressure in the solvothermal method, the synthesized nanocomposite has covalent connection between the deposited material and the graphene sheet. This solvothermal and

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hydrothermal techniques are versatile and can be utilized in various materials and for numerous applications [21, 22]. Solvothermal and hydrothermal methods can be applied successfully to grow metal/alloy nanoparticles (TiO2 , SnO2 , VO2 , Mn3 O4 ), complex compounds (LiFePO4 ) nanoparticles, and metal chalcogenides (SnS, ZnS, MoS2 ) on graphene surface. Generally, the metal or alloy nanoparticles were seeded on the graphene sheet by one step hydrothermal method. In one of the studies, graphene oxide/Ag nanoparticlebased nanocomposite was synthesized by hydrothermal technique, with graphene oxide and silver nitrate as the precursor molecule whereas ascorbic acid was utilized as a reducing agent [22]. Similarly, another work demonstrated the fabrication of Platinum–ruthenium (PtRu) nanoobject on graphene sheet by hydrothermal technique. The nanocomposite synthesized by this method had improved tolerance to poisoning issues and represented twice higher mass electrocatalytic activity. Metal oxides can also be seeded on the surface of graphene by solvothermal or hydrothermal techniques to fabricate graphene/metal oxide composites. Graphene oxide/TiO2 was synthesized utilizing graphene oxide with a reducing agent as glucose under solvothermal method [22]. P25-graphene nanocomposites. P25-graphene oxide composites fabricated by one pot hydrothermal technique represented higher light absorption range, efficient charge separation, and stronger adsorption of dyes. The TiO2 nanotubes and TiO2 nanosheets can form graphene-based nanocomposites by the hydrothermal reaction in alkaline solution or ethanol–water solvents [15, 23]. Moreover, along with TiO2 , VO2 nanotubes, Fe3 O4 nanoparticles, Mn3 O4 nanoparticles, NiO nanosheets, and MoO3, nanobelts can also form graphene composite by solvothermal method [24, 25]. As discussed, earlier graphene/metal chalcogenides composites can be fabricated by the solvothermal or hydrothermal treatment. Cao et al. reported the synthesis of CdS/ graphene nanocomposites by solvothermal treatment in the presence of the DMSO solvent at 180 °C. The precursor utilized in this method was Cd2 + ions along with graphene oxide [26]. In this solvothermal processing, the graphene oxide was simultaneously reduced. The CdS doping not only eliminate the assembly of graphene sheets but also the accumulation of CdS QDs. The nanocomposite thus obtained had ultrafast electron transfer from quantum dots to the graphene surface. Similarly, other metal sulfides like SnS2 can also be synthesized utilizing solvothermal treatment for their application in lithium-ion batteries [27]. Zn Se/graphene oxide nanocomposites are also synthesized by one step hydrothermal technique using graphene oxide and [ZnSe](DETA)0.5 nanobelts as the starting material [28]. The nanobelts precursor was mixed with the graphene oxide followed by hydrothermal treatment for 12 h at 180 °C. Interestingly, nitrogen doping was obtained on the graphene sheet during the hydrothermal processing. The doping resulted in better photocatalytic and electrochemical properties, and dye (methyl orange) degradation. In addition, complex compounds like Li4 Ti5 O12 , LiFePO4, and Bi2 WO6 can also be fabricated by solvothermal and hydrothermal treatment methods [21, 22] (Fig. 4.4).

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Fig. 4.4 Represent the hydrothermal technique to fabricate graphene-based nanocomposites

4.3.1.4

Electrophoresis and Electrochemical Deposition Technique

Another popular and common method to prepare graphene-based nanocomposites are electrophoresis and electrochemical deposition. Electrophoresis is a widely utilized technique to fabricate graphene-based nanocomposites. Zhu et al. studied the fabrication of graphene/carbon nanotube composite by electrophoresis. In brief, mixture containing reduced graphene and carbon nanotubes were applied a voltage of 30 V for around 30 min resulting in the formation of nanocomposite. Similarly, graphene/ activated carbon nanocomposites were also fabricated by electrophoresis method [29]. Electrochemical deposition is another convenient way to grow metal, metal oxides, and alloys on the graphene surface [30, 31]. Synthesis of the metal nanoparticles from different solvents can be achieved by applying current or potential, known as electrochemical reduction. CdSe like semi conductions can also be deposited on the graphene surface electrochemically. One of the studies reported the electrochemical deposition of ZnO nanorods on conductive reduced graphene oxide. In this treatment, firstly a constant-potential current step is employed to grow zinc oxide on the surface of reduced graphene oxide followed by nucleation of nanorods. If the conductivity of reduced graphene oxide is low then particle shaped structures are grown on them, whereas hexagonal nanorods are grown on reduced graphene oxide when its conductivity is high. MnO2 /graphene-based composites are also fabricated on textiles by electrochemical deposition methods. In a similar way, graphene nanocomposite with Cu2 O and ZrO2 can be fabricated. Metals nanoparticles (Nickel, copper, platinum) and even bimetallic nanoparticles can be seeded on graphene surface through electrochemical treatment [29]. For example, AuPd bimetallic nanoparticles were seeded on the graphene surface under constant-potential electrochemical reduction (−0.2 V) in a solvent containing PdCl2 , 0.1 M KCl and HAuCl4 . Electrodeposition can also be utilized for polymers like polypyrolle and polyaniline [32]. Feng et al. reported commercial scale fabrication of graphene/polyaniline nanocomposites by one step method. In this technique, graphene oxide was homogeneously mixed with aniline in the presence of hydrochloric acid. In the next step, washing was done to remove non-adsorbed aniline molecules, finally, graphene oxide/aniline was electropolymerized via electrochemical scanning (Fig. 4.5).

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Fig. 4.5 Represent the electrochemical technique to fabricate graphene-based nanocomposites

4.3.1.5

Photochemical Technique

Another technique to fabricate graphene nanocomposites is utilizing a photochemical process [33, 34]. Photochemical reaction can happen under light irradiation in the presence of graphene and lead to the formation of graphene composites. Graphene/ TiO2 can be fabricated by homogeneous mixing of TiO2 and graphene oxide nanoparticles under UV induced photochemical treatment. Titanium dioxide is a UV active compound, therefore upon irradiation, reduction of the graphene oxide takes place by the excited electrons from titanium dioxide nanoparticles, resulting in the fabrication of rGO/TiO2 nanocomposites [34]. Similar technique was also applied for BiVO4 -and WO3 -graphene oxide composite [35]. Moreover, metal and metal oxides can also be deposited on the surface of graphene by the assistance of irradiation [36]. A technique was developed to synthesize Pt, Ag, and Au nanoparticles on graphene surface by the photolysis of phosphotungstic acid (PTA) in order to reduce graphene oxide to graphene [33]. The photolyzed phosphotungstic acid on the surface of graphene can also be utilized to further reduce the metal precursor in situ. A two-step photochemical method can be utilized to fabricate well dispersed noble metal on graphene sheet. Firstly, graphene oxide to reduced graphene was achieved by photochemical reduction in the presence of phosphotungstate under UV irradiation. Secondly, the noble compounds or metal ions were added to the above system, resulting in the immediate formation of graphene/noble metal nanoparticles composite [37]. In another study, solution containing graphene oxide and Ag (NH3 )2 OH complex were radiated under 450 W Hg lamp to initiate the

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process of photochemical reaction to form Ag/reduced graphene nanocomposites. Another study reported the loading of silver nanoparticles on graphene oxide via photochemical method [36].

4.3.1.6

Physical Deposition and Mixing Technique

Another common and easy technique to synthesize graphene nanocomposite is direct deposition of the precursor material on graphene surface via physical depositions like atomic layer deposition [38–40] or physical vapor deposition. Atomic layer deposition technique can be utilized to physically deposit metal oxides with high- k dielectric like Al2 O3 on the graphene sheets [39]. Metal oxides were fabricated only on the defected sites or edges of graphene, whereas no deposition was shown in pristine graphene. For uniform atomic layer deposition of Al2 O3 on graphene surface, carboxylate-terminated perylene is needed on the surface of graphene. Metal nanoparticles can also be uniformly distributed on graphene surface after chemical modification by physical vapor deposition technique [41]. The study has reported that the growth on different metal nanoparticles is very different on graphene surface. The simplest method to synthesize graphene nanocomposite is physical mixing. In a method, the LiFePO4 nanoparticles were mixed with the graphene oxide by annealing process and spray drying. Finally wrapped through a graphene 3D network uniformly [40]. Commercially available LiFePO4 nanoparticles are coated by graphene via dropping DMF on graphene solution in a dropwise manner under constant stirring at 180 °C [42]. The modification by the graphene significantly improves the stability and capacity of LiFePO4 lithium batteries. Another study reported that direct mixing of graphene with conducting polymer like PEDOT: PSS enhances its quality, stability, solution-processability, and transparency [38] (Fig. 4.6).

Fig. 4.6 Represent the physical deposition technique for fabrication of graphene-based nanocomposites

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Chemical Vapor Deposition Technique

Chemical vapor deposition technique is another treatment to prepare graphene-based nanocomposites. In chemical vapor deposition technique, the graphene is applied as a substrate, resulting in the formation of graphene heterostructure with other precursor materials. Chemical vapor deposition can be utilized to grow both metal oxides and CNTs on the graphene surface [43]. Kim et al. reported the use of vapor-phase epitaxy technique to seed ZnO nanoneedles on graphene surface [44]. Graphene/ ZnO nanorod structures can be achieved by modifying the CVD seeding temperature [43]. However, ZnO/CNTs can also be seeded on graphene sheets to fabricate nanocomposites by CVD technique.

4.4 Different Environmental Applications by Functionalized Graphene Nanocomposites Environmental applications like biosensing, monitoring, and bioremediation have proved to be promising by the functionalized graphene nanocomposites. Graphene and graphene nanocomposites can be utilized for the biosensing of organic and inorganic ions, biomolecules, and microorganisms for the detection and removal of hazardous contaminants from the environment. The focus on the progress of these graphene and graphene-based nanocomposites like the removal of heavy metal ions, degradation of organic species, environmental gas sensing, and removal of bacteria and other microorganisms would be discussed in this review paper.

4.4.1 Heavy Metal Ion Detection and Removal Graphene nanocomposites prove to be an appealing option for detecting and eliminating heavy metal ions. Graphene oxide aptamer hybrids were prepared for the detection of mercuric ions using fluorescence quenching by the cross linking of graphene atoms and DNA [45]. Since the aptamer hybrids show fluorescence due to the high efficiency of graphene oxide thus enhancing its property of sensitivity. The mercuric (Hg2+ ) ions show a high sensitivity of about 30 nM for the hybrid sensors as compared to other metallic ions [45]. The recent literature survey by shows that the graphene nanohybrids are selective to silver ions with a sensitivity of about 20 nM and a detection limit of 5 nM utilizing a silver rich oligonucleotide. The biosensors show a high fluorescence activity in the river water containing Ag+ samples than in the blank river water in the absence of Ag+ ions. Due to the presence of heavy metal ions by the transfer of electrons, a micro-array technique was generated for the detection of silver, in a range of 10 μM

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as well as mercuric ions, in a range of 1 nM by photoluminescence. Photoluminescence quenching was observed for these metal ions which evaluated that higher sensitivity was observed in mercuric ions as compared to the silver ions because of the presence of aptamer hybrids of reinforced graphene atoms (Liu et al. 2013). The photoluminescent biosensor was also selective to mercuric and silver ions without showing any response to cadmium and magnesium ions. A similar study by [46] show that the quenching properties enhance by adopting a molecular beacon technique using graphene hybrids with a sensitivity of 20 nM for mercuric ions and 5.7 nM for silver aptamer. A convenient platform for the detection of heavy metal ions is the graphene field effect transistor. The field effect transistor (FET) was modified by 1-octadecanethiol to detect the presence of mercuric ions (Hg2+ ) whose detection limit was 10 ppm. Due to the high affinity for the detection of heavy metal ions, FET sensors were developed using reduced graphene oxide (RGO) intercalated with metallothionein type II protein. This FET sensor could also detect cadmium ions having a ratio of 15–20 for noise detection, xenobiotics, mercuric ions with a detection limit of 1 nM and a varying effect of noise in a ratio of 25–30, and no effect of ions like potassium, sodium, calcium, and magnesium. Similarly, for unmodified reduced graphene oxide, there was no significant change of mercuric, and cadmium ions to the field effect transistor [47]. Functionalized gold particles were developed by thermally reduced graphene oxide modified with thioglycolic acid-based FET sensors for the detection and removal of mercuric ions with a detection limit of 2.5 × 10–8 M. The sensors were used due to its advantageous properties like environment-friendly, low cost, and efficient in heavy metal ion detection [28]. Graphene nanocomposites and graphene nanohybrids have attracted scientists across the globe for itsutilization in bioremediation as well as in the removal of heavy metal ions from wastewater. Removal of arsenic ions (As3+ , As5+ ) was contributed by the fabrication of magnetite reduced graphene hybrids which initiates higher removal of arsenic ions due to high binding capacity and less aggregation with a removal of 99.9% for As3+ as compared to As5+ ions [48]. A similar study by [49] shows that the removal of chromium ions was observed by the fabrication of graphene hybrids reinforced with magnetic b-cyclodextrin with an adsorption capacity of 120 mg/g. The higher adsorption was due to the surface behavior and single layer graphene adsorption. The removal efficiency of the chromium ions after the desorption process reduced after repeating the cycle was observed to be 91% in the 1st cycle and 82% after the 5th cycle. Reduced graphene nanocomposites with polypyrolle showed a high removal efficiency of mercuric ions with adsorptive capacity of about 980 mg/g and desorption of 92.3% [48]. A study by [50] demonstrates that carbon nanotube reinforced with hybrid aerogels can be used in water purification, remediation as well as deionization of light metal salts with a desalination capacity of 633.3 mg/g. Various heavy metal ions like lead, silver, and organic contaminants like dyes can be removed using these hybrid aerogels with high removal efficiency. Graphene oxide hybrids with EDTA show higher adsorption capacity for lead ions than oxidized carbon nanotubes at 479 mg/ g at 6.8 pH. The process continued till 20 min and then ended up with regeneration using hydrochloric acid. These types of high-performance capabilities were also

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Fig. 4.7 Represents the heavy metal ion detection and removal using Graphene oxide and silver metal

noticed in GO-chitosan polymer nanocomposite which resulted in fast adsorption for metallic ions like copper and lead varying at 4–10 h. (Fig. 4.7).

4.4.2 Environmental Gas Sensing for Organic as Well as Inorganic Ions In today’s daily life, vapor pollution has been an emerging issue for industrial environments. For the vapor detection, Graphene has shown excellent performance for different gases like hydrogen, chlorine, Sulphur dioxide, and nitrous oxide with organic gases like benzene, acetone, and toluene [51, 52]. During the early time period, graphene environmental gas sensors were used for the detection of inorganic ions, and pure graphene was used for detecting nitrogen dioxide gas [52]. At a concentration level measured in ppm or ppb, reduced graphene oxide (RGO) was utilized for the detection of chemicals like hydrogen cyanide, dinitro toluene, nitrogen dioxide, chlorine, and ammonia [51]. Recent developments for the urge of different composites are been made [53]. Toxic hydrocarbon vapors like cyclohexane, ethanol, benzene, and toluene can be detected by the generation of graphene composite with ionic liquid in a layer-by-layer assembly since it possesses higher surface affinity for cyclic compounds. Palladium doped with reduced graphene oxide was prepared for the detection of nitrous oxide gas which has high selectivity and sensitivity of about 2–420 ppb concentration level. Platinum graphene nanosheet was prepared for detecting hydrogen sensing. Graphene nanocomposites reinforced with metal oxide with a sensitivity of 1 ppm

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were utilized as a photoluminescent gas sensor for detecting nitrous oxide, ammonia, and carbon monoxide.

4.4.3 Reduction and Removal of Organic Species Organic contaminants in the environment have been an eminent issue to address for the degradation and removal using various nanocomposites like graphene nanoparticles which serve the highest removal using organic species utilizing the process of photodegradation. A study was reported by to evaluate the photodegradation of methylene blue dye by Graphene oxide-TiO2 composite with efficient adsorption capacity and charge separation with a bandgap of about 3.2–3.5 eV in UV and visible light range. Graphene oxide-TiO2 composite shows better photocatalytic properties than TiO2 and TiO2 doped with carbon nanotubes which showed four times faster removal than P25 photocatalysts. The high-performance characteristics of reduced graphene oxide with carbon nanotubes have been because of the strong energy bond, porosity, and intermolecular force of attraction between the graphene sheets and carbon nanotubes. Hydrothermally prepared TiO2 -Graphene nanocomposites grown by in situ technique showed three times higher photochemical characteristics than P25 and were utilized for the purpose of dye removal using methylene blue dye [54]. Graphene-TiO2 nanocomposites can improve their photocatalytic performance by the up-conversion using silver nanoparticles and further utilized to degrade dyes like methyl orange [55]. Zinc oxide, Copper oxide, and silver doped with noble gases like chlorine, bromine is also prepared for the photocatalytic degradation of organic contaminants. Zinc oxide nanoparticle doped with graphene (about 2%) showed a high rate of photocatalytic efficiency as compared to pure zinc oxide for the methylene blue degradation. Silver when combined with chloride or bromine shows higher adsorption capacity for the degradation of methyl orange under UV irradiations with a catalytic factor of 3.5. In a similar study by reduced graphene oxide with copper ion and graphene oxide with carbon nitride have been developed to study the removal of rhodamine B dye under photocatalytic conversions.

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Graphene with iron oxide nanocomposites and iron oxide reinforced with reduced graphene nanocomposites are prepared to remove organic as well as inorganic contaminants like lead ion, 1-naphthol, and 1-naphthylamine without utilizing the photo radiation. Reduced graphene oxide composites with iron oxide had a better efficiency in removing the organic and inorganic contaminants whereas, graphene oxide nanocomposite combined with iron oxide shows better removal for lead ions due to charge separation and bandgap width [23]. Due to the presence of external magnetic field, the nanocomposite separation was very fast. In the study by [56], iron oxide nanoparticle by Fe3 O4 constituted high removal efficiency of 91% using rhodamine B dye and 94% using malachite green. Thus, Graphene oxide nanocomposites can be utilized for not only removing the organic contaminants but also hydrocarbons, and oil from wastewater.

4.4.4 Removal and Detection of Bacteria Detection and removal of Bacterial species have been done by Graphene and graphene-based nanocomposites. Due to high topographical and surface characteristics of graphene, they can be used under a vacuum pressure of 10–5 torr and beam current of about 150 Amp/cm2 [57, 58. Graphene oxide sheets were used as biocatalysts and thus the sheets impregnated with silver would be utilized for the detection of sulphate reducing bacteria by potentiometric stripping analysis. The biosensors in the potentiometers have a detection limit of 50 CFU varying between 1.8 × 102 and −1.8 × 108 CFU/ml [59]. An immunoassay blot analysis was prepared without the graphene oxide for the silver enhancement with a detection limit of 1.8 × 102 CFU/ ml. Peptide graphene hybrid nanocomposites were used at single cell level to detect bacteria [60]. Different applications of graphene-based sensors have been taken into consideration at the time of making composites such as diagnosis, sanitation and maintenance of hospital and food safety. In a similar study by [61], graphene functionalized FET sensors were used for the detection and removal of E. coli bacteria by the process of chemical vapor deposition and monitoring changes in the bacteria due to induced conductance, a detection limit of 10 CFU/ml. The biosensor also had the ability to induce the E. coli bacteria with glucose, so its metabolic activities were determined. Graphene and graphene-based nanocomposites are not only used for bacterial detection but only have antimicrobial properties so that removal of bacteria could be readily possible due to their enhanced charge transfer [1, 62, 63]. Functionalized graphene-silver nanocomposites were used as antibacterial coatings for the purpose of bacterial remediation from wastewater [64]. The coated surface of graphene nanocomposites shows an effectiveness of 90% to eliminate bacterial colonization (Fig. 4.8).

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Fig. 4.8 Represent graphene nanocomposite for the removal and detection of bacteria

4.5 SWOT Analysis for the Different Applications Mentioned Different application methods:

Strengths

Weaknesses

Opportunities

Threats

(1) Heavy metal ion detection and removal

1. Graphene and graphene-based nanocomposite helps in the easy removal of heavy metal ions 2. Adsorption efficiency is higher 3. High separation efficiency

1. Sensitivity lowers down sometimes due to high charge efficiency of graphene ions 2. Photo luminescent biosensor was selective to Ag and Hg ions showing no response to Cd and Mg ions 3. Reluctant to fluorescence in the absence of Ag ions

1. Use of aptamer hybrids helps enhancing the sensitivity and yield 2. Photoluminescence quenching proves to be an eminent option for heavy metal ion detection and removal

1. The removal efficiency of the heavy metal ions gets reduced after repeating the cycle 2. FET sensors were developed only utilizing reduced graphene oxide

(continued)

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(continued) Different application methods:

Strengths

Weaknesses

Opportunities

Threats

(2) Environmental gas sensing for organic as well as inorganic ions

1. Graphene has shown excellent performance for different gases 2. Graphene environmental gas sensors were used for the detection of inorganic ions and pure graphene was used for detecting nitrogen dioxide gas

1. High power is consumed 2. Sensor utilization time is very limited

1. Recent developments are been made for the generation of new composites 2. High surface affinity of aromatic compounds compared to aliphatic compounds

1. Graphene-based nanocomposites have a sensitivity of 1 ppm for detecting toxic gases like ammonia, nitrous oxide 2. High labor and manufacturing cost

(3) Reduction and removal of organic species

1. Graphene nanoparticles show highest efficiency for the removal of organic and inorganic contaminants 2. Graphene-TiO2 having a high bandgap energy of 3. 5 eV emerges with new photocatalytic properties with UV visible range

1. Due to strong bond and high intermolecular force of attraction, the performance characteristics somewhat lower down 2. Presence of external magnetic field reduces the bandgap energy thus reducing sensitivity

1. Graphene-TiO2 nanocomposite improves the photocatalytic performance by the elimination of organic molecules, dyes using up-conversion 2. Reduced graphene oxide when doped with iron particles induces high charge separation

1. Lack of infrastructure facilities 2. There are thermodynamic limitations and high energy loss

(4) Bacterial detection and removal

1. Graphene and graphene-based nanocomposites can be used for the detection and removal of bacteria due to the topographical surface structure of graphene 2. Graphene sheets can be used as biocatalysts for detecting sulphate reducing bacteria

1. Peptide graphene hybrid nanocomposites were used at single cell level to detect bacteria 2. Immunoassay blot analysis prepared without graphene showed less detection limit

1. Graphene functionalized FET sensors were used for the detection and removal of E. coli bacteria by the process of chemical vapor deposition and monitoring changes in the bacteria due to induced conductance 2. Functionalized graphene-silver nanocomposites were used as antibacterial coatings for the purpose of bacterial remediation from wastewater

1. Insufficient domestic demand for microbial culture and incubations 2. Uncertainty in social bodies for genetically modified bacterial strains

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4.6 Conclusion Various recent progress has been made in the field of fabrication and functionalization of graphene and graphene oxide-based nanocomposites. The nanocomposites can be utilized in different applications. The worldwide urge to clean the environment paved the partial and commercially utilize the graphene composites. Different techniques have been utilized for better preparation of these graphene nanocomposites. The techniques include covalent, non-covalent, hydrothermal, solvothermal, physical deposition, mixing, chemical vapor deposition, and electrochemical deposition method. The most commonly used ones are hydrothermal, physical deposition, and chemical vapor deposition methods. With regard to environmental perspective, graphene-based nanocomposites have represented great significance in the detection and removal of bacteria, organic and inorganic pollutants along with heavy metals. Photodegradation of the organic contaminants by graphene-based composites attracted immense interest due to the constant increase of water pollutants in the developing countries.

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Chapter 5

Application of Graphene, Graphene Oxide and Reduced Graphene Oxide Based Composites for Removal of Chlorophenols from Aqueous Media Subhadeep Biswas, Ankurita Nath, and Anjali Pal

5.1 Introduction Chlorophenols (CPs) are a class of synthetic organic compounds that are commonly encountered in many wastewater streams. It is of high industrial importance as it is often used in several industries such as plastic, pharmaceutical, pesticide, etc. There are many compounds that belong to the chlorophenol group, viz., monochlorophenols (i.e., 2-CP, 3-CP, 4-CP), di-, tri-, tetra-, and pentachlorophenol (DCP, TCP, TeCP and PCP) [1]. The solubility of these CPs is quite high in water, and they possess high toxicity towards humans and animals even at very low concentrations. These compounds are resistant to biodegradation carried out by bacteria, and their half-lives have been reported to be more than three months in aquatic environments, and several years in organic solvents [2]. CPs are used as wood preservatives, bactericides, insecticides, herbicides, and fungicides, and also they find useful applications in the manufacture of dyes and pharmaceuticals [3–6]. They are also used in paper, colorant, antiseptic, paint, and pulp industries [7, 8]. Contamination of CPs also occurs through accidental spills, hazardous waste disposal sites, municipal landfills, or storage tanks [3]. They are even accumulated in water and soil due to wastewater discharge from chemical industries where CPs are used as raw materials and intermediates [9]. 2-CP are identified as carcinogens, and PCP and 2,4-DCP are considered endocrine disruptors. Paasivirta et al. [10] revealed the emission of CPs S. Biswas · A. Pal (B) Civil Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India e-mail: [email protected] A. Nath School of Environmental Science and Engineering, Indian Institute of Technology Kharagpur, Kharagpur, West Bengal 721302, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Mohanty et al. (eds.), Graphene and its Derivatives (Volume 2), Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-99-4382-1_5

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into the environment during the chlorobleaching of pulp. 4-CP is one of the most polluting organic compounds involved in the production of various pesticides, pharmaceuticals, and dyes [11]. In 1987 USEPA listed CPs as the priority pollutants and set its upper limit in public water supplies as 0.5 mg/L [12]. Due to their highly toxic nature, it is important to eradicate them from aqueous media. The removal of CPs from water bodies by different processes such as physical adsorption, biodegradation, advanced oxidation process (AOP), enzymatic degradation, etc., is possible. The rate of biodegradation is, however, very slow. Under aerobic conditions, biodegradation occurs through the formation of catechols, while under the anaerobic condition, biodegradation proceeds via dehalogenation [13]. The application of low-cost novel adsorbents and various supported catalysts for CP degradation are also documented in the literature. Various research groups all over the world reported the applications of different new-age materials for the removal of CPs. Among them, graphene-based materials deserve special mention [2]. Graphene (G), a “wonder material,” is actually composed of 2D single-layer sheets of carbon atoms that remain organized in an sp2 -bonded lattice structure resembling that of a honeycomb [14]. It was discovered in 2004 and has been considered to be the thinnest material in the world and also identified as the simplest form of carbon. The synthesis of a very stable and monocrystalline G sheet from pyrolytic graphite by means of mechanical exfoliation was first reported by Novoselov et al. [15]. Due to the presence of polarized electron-rich and depleted sites and various functional groups on the surface, G and graphene oxide (GO) and reduced graphene oxide (rGO) find several environmental applications. Graphene-based materials possess a large surface area and strong π-π interaction, which makes them useful for the removal of various types of pollutants such as dyes, heavy metals, chlorinated aromatic compounds, etc. Moreover, rich surface chemistry, intrinsic high porosity, and exceptionally large aspect ratio have helped graphene-based composites to be used as promising materials for adsorbent and catalyst synthesis. The polyaromatic π system of G helps in reacting with organic entities present in water bodies either by π-π stacking interactions or by hydrophobic interactions. However, the hydrophobic nature of G makes it nondispersible in water, and hence the use of G composites is preferred rather than G in pure form. G, GO, and rGO belong to the same basic G family [16]. All of them are 2D laminar carbons possessing almost similar structures. In spite of being of the same chemical nature, GO, and rGO possess more surface oxygen-containing groups, surface defects, and specific edges in comparison to crystalline G. Among the oxygen-containing groups, epoxide, hydroxide, carbonyl, and carboxyl groups are mostly present in the structure of GO and rGO. Edges and defects are induced in the graphene-based nanosheets during growth, oxidation, or processing. The edges may be of zigzag and armchair types. These properties have made them more acceptable in comparison to G for wastewater treatment. Chemical modifications are often carried out to synthesize GO and rGO-based compounds. GO can be prepared by the conventional Hummers method. In 1958, two chemists, Hummer and Offeman, first reported their method, which is being used by various researchers for graphene oxide synthesis [17]. According to their method, 100 g of graphite powder, 50 g

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Fig. 5.1 Structure of a graphene and b graphene oxide [14]

of sodium nitrate (NaNO3 ), 2.3 l of H2 SO4 , and 300 g of KMnO4 are required for oxidation purposes. High-quality GO is produced by this method within a few hours. However, Hummer’s method suffers from some drawbacks. For example, toxic gases like N2 O4 and NO2 are produced during the synthesis of GO. On the other hand, rGO can be prepared by reducing GO with strong reducing agents such as hydrazine hydrate or sodium borohydride. However, hydrazine is a toxic compound, and hence several other alternatives have been tried out. Peng et al. [18] reported the synthesis of porous rGO by reducing GO under the N2 atmosphere, followed by the activation with the help of CO2 . Wang et al. [19] obtained modified GO by the addition of ammonia to GO solution, followed by heating and filtering the suspension. The structure of G and GO has been presented in Fig. 5.1. In forming the composites, the interaction of G with other moieties occurs either through covalent functionalization or non-covalent functionalization, or both. The current chapter focuses on the applications of G, GO, and rGO-based materials for the removal of CPs from water media. In the subsequent sections, the occurrence of CPs in various environmental samples and the applicability of graphene-based materials for the adsorptive and catalytic removal of CPs from aqueous media have been discussed. Then the suitability of graphene-based materials for extraction and hence quantification of CPs have been presented. Different characterization techniques used to get an idea about the properties of the composite materials are considered. The mechanisms involved in the removal process have been elaborated. Lastly, current challenges regarding applications of graphene-based materials for CP removal and hence recommendations are suggested.

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5.2 Presence of CPs in Environmental Matrices Many research groups, while carrying out their studies, have reported the presence of CPs in various environmental matrices. Lee et al. [20] carried out solid phase extraction of CPs from the landfill leachates and soil samples of southern Taiwan that were polluted by the effluent of a chemical manufacturing plant. For being lipophilic in nature, CPs tend to get accumulated on the solid soil surface. Different CPs have been quantified in the study, and the concentration of PCP has been obtained highest (21.6 μg/L) in the landfill leachate. CPs have also been found in food samples [21]. The entry occurs from food storage containers that are treated with biocides, which contain CPs. The occurrence of CPs in milk samples up to several μg/L has also been documented in the literature. Kartal et al. [22] determined CPs in the range of μg/L in fruit juice samples bought from the local market of Denizli, Turkey. Treated wastewater containing phenols may also possess CPs depending upon the degree of chlorination applied to it. The sewage treatment plant can also be a potential source of releasing CPs into the environment. Chlorine-based agents are often used to increase the brightness of the paper. Hence, CPs are the most common pollutants found in the effluent of the paper and pulp industry [23]. The level of CPs has been found to be relatively lower in sea or ocean water compared to that found in freshwater. Gao et al. [24] reported the existence of CPs in the rivers of China. Yahaya et al. [25] reported the occurrence of several USEPA-listed toxic phenolic derivatives in the Buffalo River of South Africa. In most of the sampling sites, the concentration of phenolic compounds obtained was higher than that recommended by USEPA standards.

5.3 Removal of CPs by Various Processes 5.3.1 Adsorptive Removal Fan et al. [26] studied the adsorptive removal of CPs such as 2-CP, 4-CP, 2,4- DCP, and 2,4,6-TCP by pristine G. The effect of different parameters such as contact time, pH of the CP solution, initial concentrations, temperature, etc. were examined. The equilibrium time for adsorption of CPs on G was 20 min, and the maximum adsorption capacities were obtained in the range of 88.1–175.8 mg/g for different CPs. Majority of the experimental findings matched with the Langmuir isotherm model, indicating that the adsorption of CPs on the surface of G was monolayer in nature. The thermodynamic parameters revealed the endothermic and spontaneous nature of CPs adsorption. The entropy change (ΔS°) was positive in all cases, and the values were 58.01 Jmol−1 K−1 , 57.48 Jmol−1 K−1 , 68.44 Jmol−1 K−1 , and 73.77 Jmol−1 K−1 , for 2-CP, 4-CP, DCP, TCP respectively, which demonstrated the randomness at the solid– liquid interface and the affinity of CPs towards G. Applying the one-pot carbonization

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of hyper-cross-linked polymer and glucose, Liu et al. [27] synthesized a graphenebased composite consisting of hyper-cross-linked porous carbon (GN/HCPC) and used it for the adsorptive removal of 2,4-DCP from aqueous solutions. The combination of G and hyper-cross-linked porous carbon (HCPC) into a composite resulted in the generation of a large specific surface area along with high porosity. The GN/HCPC composite followed the Langmuir isotherm model and pseudo-second-order kinetic model. The study showed a maximum adsorptive removal capacity of 348.43 mg/g, which is 43.6 and 13.6% over pure GN and HCPC. The effects of solution pH, time of contact, temperature, ionic strength of the solution, and humic acid were analyzed in the treatment process of 2,4-DCP. The secondary effluents contain dissolved organic matter (DOMs) such as humic acid, protein and fulvic acid (FA), etc., coexisting with CPs in practical water environments. These DOMs compete with the adsorbate in the adsorption process, thereby decreasing the adsorption capacity. The studies showed that the adsorption capacity of GN, HCPC, and the GN/HCPC composite decreased slightly. Thus, the results indicate that GN/HCP composite has practical application as it is capable of decreasing the secondary effluents slightly. Furthermore, GN/HCP composite achieved reusability even after five cycles of regeneration. Modi and Bellare [28] reported the adsorptive removal of 2,4-DCP using polysulfone-iron oxide/GO composite hollow fiber membranes (Psf-Fe3 O4 /cGO HFMs). The composite contained oxygen-enriched functional groups and also showed enhanced hydrophilicity. HFMs were fabricated by embedding Fe3 O4 /cGO nanohybrid of various quantities in Psf HFM. Psf-0 (0 wt.%), Psf-1 (0.25 wt.%), and Psf-2 (0.50 wt.%) were prepared by dispersing Fe3 O4 /cGO nanohybrid in a suitable solvent such as N-methyl-2-pyrrolidone (NMP). Pure water flux (PWF) is a parameter that measures the volume of water that passes through a membrane per unit of time. For determining the long-term use of a membrane, the antifouling property of the membrane was evaluated by means of calculation of the PWF of the membranes. PWF of different HFMs was measured, and it indicated that the effect of Fe3 O4 / cGO nanohybrid on PWF was positive. PWF of Psf- 0, Psf-1, and Psf-2 HFMs was obtained in the range of ~60–340 L/m2 /h. The increased values of PWF indicate that the hydrophilic and porous nature of HFMs were improved in Psf-2. Along with it the antifouling parameters, other important characteristics such as Flux recovery ratio (FRR) and flux reduction (FR) of different Psf-0 HFMs were also calculated and found lowest for Psf-0 (55.1%) and highest for Psf-2 (95.8%). The increased value of Psf-2 is attributed to the hydrophilic membrane surface of HFMs. In order to check the feasibility of the composite for long-term use, antifouling property, the pure water flux (PWF), and reusability of the modified membranes were examined for up to five cycles. Regarding adsorptive removal of 2,4-DCP from the spiked lab and lake water, the maximum efficiency for Psf-Fe3 O4 /cGO HFMs (Psf-2) reached up to 96.5 ± 1.6% and 70.5 ± 2.1%, respectively. Even up to five filtration cycles, the removal efficiency of Psf-Fe3 O4 /cGO HFMs remained almost the same, which proved its merit for practical use. Catherine et al. [29] synthesized GO nanoflakes for the adsorption of 4-CP, 2,4-DCP, and 2,4,6-TCP from an aqueous solution. GO nanoflakes were prepared by the improved Hummers method. Owing to the existence of hydrophilic functional groups on the surface, they have good water disperse

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ability. The adsorption kinetics of 4-CP, 2,4-DCP, and 2,4,6-TCP was divided into three stages: (i) film diffusion (fast diffusion), (ii) intraparticle diffusion (slow sorption), and (iii) dynamic equilibrium. From experimental investigations, it was found that the data fitted better into the pseudo-second-order kinetic model than the pseudofirst-order kinetic model, and the adsorption isotherm followed the Langmuir model better than the Freundlich model. It was reported that the van der Waals force and π-π interaction were the principal mechanisms of the adsorption of chlorophenols. Along with that, the interaction between hydrogen-bond donor and acceptor (HDA) also facilitated the adsorptive elimination of CPs by GO as it has oxygen-carrying functional groups. Soltani and Lee [6] compared the 2-CP adsorption characteristics of GO and rGO prepared by ultrasonic and conventional methods. The conventional GO and rGO were synthesized by the Hummers method by chemically oxidizing graphite and were designated as GO-C/rGO-C, whereas GO and rGO produced by fast, facile, onestep ultrasonic reduction method were named as GO-Us/rGO-Us. In the ultrasound method, severe reaction conditions such as carrying reaction at a high temperature (90 °C), investing a long reaction time (12 h), and involvement of highly toxic hydrazine were avoided in the reduction of GO into rGO. The ultrasonic method reported the synthesis of GO (30 min, 40 °C) and rGO (10 min, 50 °C) from graphite devoid of hydrazine hydrate. On the other hand, the time required for the synthesis of rGO composite in the presence of hydrazine hydrate was longer with extreme temperature requirements. In this study, the authors compared the effects of solution pH. The adsorption studies showed that between GO-C/rGO-C and GO-Us/rGOUs the latter adsorbent was more efficient, and it could completely remove 2-CP from 100 mL of aqueous CP solution (conc: 50 mg/L) in 50 min. On the contrary, the former adsorbent could remove only 40% of 2-CP. The study revealed that the maximum adsorption capacity of rGO-Us was the highest (208.67 mg/g), whereas the adsorption capacity was the lowest for GO-C (32.06 mg/g). The maximum adsorption by rGO-Us was attributed to the ultrasonic treatment, which enhanced the surface area with superior π-electron-rich matrix and oxygenated groups. Compared to the conventional method with a long reaction time of 12-24 h, the ultrasound method has a shorter reaction time in the reduction of GO into rGO. The adsorption isotherms exhibited by GO-Us and rGO-Us were nonlinear. It indicates that π–π electron donoraccepter (EDA) interaction exists between graphene-based materials and the aromatic ring of phenolic compounds, which plays an important role in the adsorption of 2-CP. In the case of GO-C and rGO-C, the isotherm fitted to the Freundlich model, which indicated the adsorption of 2-CP occurred on a heterogeneous adsorbent surface. Yan et al. [30] studied four different classes of CPs: 2-CP, 4-CP, 2,4-DCP, and 2,4,6-TCP for their removal from aqueous solutions. They synthesized magnetically modified reduced graphene oxide (MRGO) composite using the in-situ coprecipitation method. The Fe3 O4 magnetic nanoparticles embedded in the rGO enhanced its separation ability from the aqueous solution. The separation efficiency of MRGO was way more enhanced than GO and rGO. The equilibrium time for the adsorption of CPs was 10 min. Among all the CPs, TCP with the least water solubility and maximum molecular weight displayed more affinity towards MRGO. The

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adsorption of CP on MRGO is suitable under acidic and neutral conditions, whereas, in basic condition, proton dissociation occurs on CPs, and hydrophobic interactions between MRGO and CPs is weakened, thereby inhibiting adsorption of CPs on MRGO. The results showed that the adsorption followed the Freundlich isotherm model, which indicated heterogeneous and multilayer adsorption. The studies also revealed that the kinetics followed both physical and chemical adsorption mechanisms as adsorption of CPs on MRGO followed both pseudo-first and pseudo-secondorder models. The regeneration of exhausted MRGO was done using 0.01 mol/L NaOH, and after regeneration, it still worked up to five adsorption–desorption cycles for all CPs with removal efficiency higher than 90%. The synthesis of a magnetic diazonium functionalized-reduced graphene oxide (M-DF-RGO) hybrid was reported by Shen et al. [31]. The composite was prepared via a three-pot reaction and was explored for the removal of 4-CP and 2,4-DCP from water media. In the three-pot reaction, the M-DF-RGO hybrid was fabricated in three steps. At first, the rGO was synthesized via a redox reaction. The second step involved the synthesis of diazonium functionalized-RGO (DF-RGO) through a feasible chemical reaction. In the final step, Fe3 O4 particles were loaded on its surface through covalent bonding (Table 5.1).

5.3.2 Catalytic Removal In the recent era, different research groups prepared GO, rGO-based catalysts for the degradation of CPs. Qu et al. [32] prepared GO carbon nanodots (CDots) co-doped BiOBr ternary composites and applied the same for the photocatalytic degradation of 4-CP. Under visible light irradiation for 6 h, 88.9% 4-CP was removed. An interesting photocatalytic mechanism was proposed where the whole process started with the absorption of longer wavelength visible light by CDots. It was then transformed to short wavelength light which was utilized by BiOBr to produce the extra photo-generated e− /h+ pairs. Thus the whole spectrum of visible light was consumed efficiently. Liu et al. [33] reported the application of novel nano-FeO(OH)/rGO as a heterogeneous Fenton-type catalyst for the degradation of phenolic compounds. FeO(OH), having a particle size of 3 nm, got dispersed within the G aerogel, and it could efficiently activate H2 O2 for the generation of OH. to degrade phenolic compounds. Synergism between FeO(OH) and G was noticed, which resulted from the extensive electron transfer channel and the active sites of the 3D G aerogel. Within 80 min, 4-CP was completely removed. The studies showed that the degradation followed pseudo-first-order kinetics with a rate constant of 0.074 min−1 . Kumar et al. [34] prepared novel ZnO tetrapod-rGO (ZTPG) nanocomposites and applied the same for the removal of 4-CP and methylene blue dye by photocatalytic degradation. In the preparation of nanocomposite, different weight percentages of rGO were tried out to efficiently separate the hole and the electron pair and thereby improve the photocatalytic activity. Maximum mineralization was obtained as 94.8% for 4-CP under the exposure of UV light for 180 min by ZTPG-5 photocatalyst.

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Table 5.1 List of graphene-based adsorbents for CP adsorption Graphene-based adsorbent

Optimized result

Pollutant

GO-C/rGO-C and GO-Us/rGO-Us

Complete removal of 2-CP 50 mg/L of 2-CP by rGO-Us, 40% by rGO-C in 50 min. Adsorption capacity: rGO-Us (208.67 mg/g) > GO-Us (134.49 mg/g) > rGO-C (49.91 mg/g) > GO-C (32.06 mg/g)

Pristine G

The maximum adsorption 2-CP, – capacity was 88.1 mg/g for 4-CP, DCP, 2-CP, 114.2 mg/g for 4-CP, TCP 155.3 mg/g for DCP, and 175.8 mg/g for TCP in 20 min

[26]

Graphene-based hyper-cross-linked porous carbon composite (GN/ HCPC)

At a dose 0.125 g/L and 2,4-DCP pH = 6 the maximum adsorption capacity of 2,4-DCP was 348.83 mg/g

One-step carbonization

[27]

Polysulfone-iron oxide/GO composite hollow fiber membranes (Psf-Fe3 O4 /cGO HFMs)

Adsorption capacity of Psf-Fe3 O4 /cGO HFMs (Psf-2) = 38.5 ± 0.6 μg/ cm2 , 2,4-DCP removal efficiency was 96.5 ± 1.6 and 70.5 ± 2.1% from the contaminated lab water and lake water, respectively

–

[28]

GO nanoflakes

The adsorption capacity of 4-CP, 4-CP was19.7 mg/g, 2,4-DCP, 2,4-DCP was 18.75 mg/g, 2,4,6-TCP and 2,4,6-TCP was 17.92 mg/g

Hummers method [29]

Magnetically modified rGO (MRGO)

Removal efficiency of OCP = 97.87%, PCP = 99.51%, DCP = 98.32%, TCP = 99.02%

In-situ co-precipitation method

Magnetic diazonium functionalized-rGO (M-DF-RGO) hybrid

Maximum adsorption 4-CP, capacity 4-CP = 2,4-DCP 55.09 mg/g and 2,4-DCP = 127.33 mg/g at pH 6 and 25 °C. Removal efficiency=>80% after five cycles

2,4-DCP

OCP, PCP, DCP, TCP

Synthesis method

Reference

Hummers method [6] One-step ultrasonic reduction method

[30]

Three-pot reaction [31]

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Sharma et al. [35] synthesized La/Co/Ni trimetallic nanoparticles (TNPs) and GOsupported La/Co/Ni trimetallic nanocomposites (TNCs) and applied both for the photocatalytic degradation of 2-CP under the irradiation of sunlight. GO-supported nanocomposites showed higher catalytic degradation (71% in 300 min) in comparison to the pristine TNPs (57% in 300 min). The GO-supported catalyst showed higher efficiency due to the efficient charge transfer and reduced electron–hole combination. Photocatalytic degradation of 2,4-DCP under exposure to infrared radiation was carried out by the application of Cu2 (OH)PO4 /rGO nanocomposite catalyst [36]. In comparison to pure Cu2 (OH)PO4 catalyst, the catalytic activity of the Cu2 (OH)PO4 / rGO nanocomposite was found to be much higher. Moreover, Cu2 (OH)PO4 /rGO possessed higher stability. Recycling was done four times without any appreciable loss in efficiency. Moreover, the introduction of H2 O2 into the system enhanced the degradation efficiency due to the combined effect of photo-Fenton reaction and photocatalytic activity. The rate constant of 2,4-DCP degradation by Cu2 (OH)PO4 / rGO nanocomposite/H2 O2 was 6.25 times higher in comparison to that shown by Cu2 (OH)PO4 /rGO nanocomposite alone and about 10 times higher than that shown by pure Cu2 (OH)PO4 . The degradation mechanism of 2,4-DCP by Cu2 (OH)PO4 / rGO nanocomposite and H2 O2 has been proposed as follows: rGO − OCTCuI + H2 O2 → rGO − OCTCuII + HO. + OH (OCT implies octahedral sites)

H+ + OH− → H2 O 2, 4 − DCP + HO. → H2 O + CO2 + H+ + Cl− Ren et al. [37] applied Rh nanoparticles supported on rGO for the hydrodechlorination of 4-CP. The composite nanocatalyst was synthesized by means of one-pot polyol co-reduction of GO and rhodium chloride. In comparison to other supports, rGO support showed the highest efficiency in terms of conversion of 4-CP to valuable products such as cyclohexanone and cyclohexanol, having a selectivity of 23.2 and 76.8%, respectively. The functional groups present on the surface of rGO played a crucial role in the catalytic performance. Darabdhara et al. [38] synthesized Au–Pd nanoparticles decorated rGO nanosheets and applied them for the photocatalytic degradation of phenolic compounds such as phenol, 2-CP, and 2-nitrophenol. Complete degradation of CP occurred within 180 min under exposure to sunlight. Experimental data revealed that the degradation kinetics followed the Langmuir–Hinshelwood model. Moreover, the prepared rGO-based photocatalyst showed excellent stability, and it could be reused for five cycles. Deng et al. [39] explored the Pd/rGO catalyst for the hydrodechlorination of 4-CP. The rGO-supported Pd catalyst was synthesized by impregnation of polyvinyl-pyrrolidone-stabilized Pd nanoparticles on an rGO sheet prepared from the reduction of GO in the presence of hydrogen gas. Ali et al. [40] reported the application of TiO2 @rGO composite for photocatalytic degradation of 2,4,6-TCP from industrial effluents. During the degradation process, 85% COD reduction and 82% TOC reduction were achieved.

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Table 5.2 List of graphene-based catalysts for CP degradation Graphene-based catalyst

Optimized result

Mechanism

Reference

Self-assembled Nano-FeO(OH)/rGO aerogel

Complete removal of 4-CP in 80 min with a rate constant of 0.074 min−1 and 80% mineralization in 6 h

Photo-Fenton degradation

[33]

Cu2 (OH) PO4 /rGO nanocomposite

87.1% degradation after irradiation under infrared light for 6 h

Combination of photo-Fenton and photocatalytic activity

[36]

rGO nanosheet decorated with Au–Pd bimetallic alloy nanoparticles

Complete degradation of 2-CP under sunlight irradiation for 3h

Photocatalytic degradation

[38]

Pd/rGO

100% hydrodechlorination of 4-CP occurs within 100 min

Hydrodechlorination

[39]

Akageneite (beta-FeOOH)/ rGO nanocomposites

With 1 g/L catalyst dose, pH 4 in the presence of 100 mM H2 O2 , 88% 2-CP removal within 72 h

Fenton-like reaction

[42]

Pd-Fe/G catalysts

–

Electrocatalytic degradation

[43]

CuO-GO/TiO2 visible light photocatalyst

86% removal of 2-CP with a rate constant of 0.0101 min−1 at pH 5

Photocatalytic degradation

[44]

GO/CDots/BiOI

98.2% 4-CP removal

Photocatalytic degradation

[45]

H3 PW12 O40 /GR/TiO2

82.71–97.02% removal of CPs

Photocatalytic degradation

[46]

CuO/rGO

Complete degradation of 2,4,6-TCP

Advanced oxidation by [47] persulfate non-radical activation

Mesoporous graphene-Eu2 O3 /TiO2 nanocomposites

Approximately 88.5% TOC reduction in 250 min

Photocatalytic degradation

[48]

GO, and Ag engulfed TiO2 nanotube arrays

~70% 2-CP removal with an initial concentration of 10 mg/ L

Photocatalytic degradation

[49]

Qian et al. [41] explored aminated rGO (ArGO) supported heterogeneous Cu catalyst for the degradation of CPs. 3,5-Dibromosalicylaldehyde was condensed with ArGO in order to prepare the support. After that, it was subjected to complexation with copper acetate monohydrate. Next, the developed Schiff base Cu complex was utilized for persulfate activation in order to degrade triclosan. Persulfate was activated for the generation of SO4 − , which generated OH. immediately. G played a crucial role in the catalytic activity by supporting the copper catalyst and also by

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providing hydrophobic microdomains for the catalyst. Akageneite (beta-FeOOH)/ rGO composite was explored by Xiao et al. [42] for oxidation of 2-CP. Song et al. [43] reported the synthesis of Pd-Fe/G catalyst by photocatalytic reduction and consequently applied it for electrochemical oxidation of CPs. Alafif et al. [44], Qu et al. [45], and Ma et al. [46] applied promising GO and G based photocatalysts for CP degradation purposes. Du et al. [47] explored novel CuO/rGO catalysts for CP degradation by means of non-radical activation of persulfate. Myilsamy et al. [48] and Sim et al. [49] prepared visible light assisted G-Eu2 O3 /TiO2 nanocomposite and Ag and GO engulfed TiO2 nanotube arrays for oxidative removal of CP. Various graphene-based catalysts reported for CP degradation have been summarized in Table 5.2.

5.4 Application of Graphene-Based Materials for Extraction of CPs Extraction-based techniques are often utilized for detection, quantification, and removal of CPs from environmental samples when they are present in trace quantity. Due to the possession of the ultra-high specific surface area, high dispersibility, and hydrophilicity, the graphene-based materials are often exploited for this purpose. The theoretical value of the specific surface area of G has been reported as 2630 m2 /g. Liu et al. [50] utilized G for solid phase extraction (SPE) of eight CPs. Consequently, the extracted CPs were eluted using alkaline methanol for detection by HPLC equipped with a multi-wavelength detection system. Good reproducibility, along with high sensitivity, were observed under optimized conditions. Moreover, G was proved to be a superior adsorbent in comparison to other adsorbents such as C18 silica, graphitic carbon, single and multi-walled carbon nanotubes, etc. Pan et al. [51] reported the synthesis of amine-functional magnetic polymer modified GO nanocomposite and utilized it for the SPE of five CPs. The method was successfully applied to the analysis of CPs from environmental water samples, where good recovery was obtained (in the range of 86.4–99.8%). The range of linearity for 2-CP was 10–500 ng/L, while it was 5–500 ng/L for 2,4-DCP and 1–500 ng/L for PCP. The limit of quantification for five CPs was obtained in the range of 0.6–9.2 ng/L. Cai et al. [52] reported high adsorption capacity of planar graphene oxide-based magnetic ionic liquid material (PGO-MILN) towards five CPs and therefore developed a magnetic SPE-based technique for the determinations of these CPs from environmental samples. Recovery was quite good (in the range of 85.3–99.3%), with a high correlation coefficient (>0.9994) and linear dynamic range of 10–500 ng/L. The limit of detection (LOD) and limit of quantification (LOQ) of the five CPs were in the range of 0.2–2.6 ng/l and 0.6–8.7 ng/L. Liu et al. [53] applied a magnetic three-dimensional G nanocomposite for the extraction of CPs from honey samples. The graphene-based nanocomposite prepared using the vacuum freeze-dried method possessed a high specific surface area. Good linearity was obtained in the range

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of 10–1000 ng/g. In the case of spiking of CPs in the range of 100–400 ng/g, good recoveries were obtained in the range of 93.2–98.9%. Sun et al. [54] applied octadecyl-modified G (graphene-C18) for the micro solid phase extraction of CP from honey. Various parameters, such as extraction efficiency, extraction time, pH, agitation speed, etc., which influence the performance of the graphene-C18 reinforced system, were optimized. RGO functionalized with magnetic iron oxide nanoparticles, and graphitic carbon nitride was applied as an adsorbent for the extraction of CPs [55]. The composite nanomaterial was placed in a polypropylene hollow tube and utilized for the extraction of 3-CP, 2,3-DCP, 2,4-DCP, 2,4,6-TCP from cosmetic samples. After adsorption of CPs, they were desorbed with alkaline methanol, and finally, the solutions were subjected to quantification by HPLC. Recoveries from spiked cosmetic samples were quite good (80.5–104%) with a standard deviation of 500 °C in the samples of GO. The weight loss pattern observed for GO was in agreement with that reported in the literature. The weight loss percentage for the three regions for GO-1 to GO-3 is depicted in Fig. 8.6c. The higher thermal stability of GOT as compared to GO was due to fewer abundance of oxygen-containing functional groups in the former. GOT was, however, not completely devoid of oxygen-containing functional groups. Some of the functional groups containing oxygen remained even after the reduction process. The weight loss observed in the various GOT samples was ≤10% of the total weight. No sharp peaks of mass loss were observed in any of the nanocomposite samples (Fig. 8.6b). This suggests that the interaction between functionalities containing oxygen moieties on GO and TiO2 leads to an increase in thermal stability of the nanocomposites. A similar observation has also been reported by Jiang et al. [42]. FTIR analysis was conducted to reveal the functionalities associated with GO and rGO-TiO2 and to qualitatively assess the changes that occur in GO after insitu synthesis of the nanocomposite. The GO samples exhibited several characteristic absorption peaks of oxygen-containing groups associated with carbon on the GO sheets. The characteristic vibrations include C–O–C stretching vibration (1211.2 cm−1 in GO-1 and 1284.5 cm−1 in GO-2) [21, 39]. The stretching vibration peak for C-O is merged along with C–O–C in GO-2 and GO-3. C = O stretching vibration was observed in all three GO samples (1706. 2 cm−1 in GO-1, 1745.4 cm−1 in GO-2 and 1741.6 cm−1 in GO-3) [29, 35]. In GO-1, CH2 stretching vibrations were observed at 2947.0 cm−1 . The FTIR spectra of GO also displayed a broad peak centered around 3400 cm−1 which corresponds to the O–H group vibrations associated with surface adsorbed water molecules [21] as depicted in Fig. 8.7a–c.

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Fig. 8.6 TGA and DTA graphs for a GO-1, b GOT-1 and c Weight loss percent for GO-1, GO-2 and GO-3 in TGA for the temperature zones A (50 °C to 100 °C), B (150 °C to 240 °C) and C (400 °C to 570 °C)

FTIR spectra of GOT nanocomposites (Fig. 8.7d–f), also show a few oxygencontaining functionalities, implying that during the process of synthesis of the nanocomposite, the precursor GO was not completely reduced but was only partially reduced [29, 43]. These functionalities facilitated the interactions with TiO2 . The characteristic vibrations due to Ti–O-Ti were clearly observed at around 600 cm−1 [44] in all the GOT samples. It has been reported that Ti–O–C bonds are crucial for the development of an efficient photocatalyst. In a study by Wang and Zhan [9] it was reported that when P25 and GO were mechanically mixed and used for the degradation of rhodamine B dye, only low degradation of the dye was reported under

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Fig. 8.7 FTIR spectra of a GO-1, b GO-2, c GO-3, d GOT-1, e GOT-2 and f GOT-3

UV light owing to the absence of Ti–O–C bonds. This emphasizes the importance of bond formation between TiO2 and GO. Raman measurements were conducted to assess the degree of defects/ graphitization in GO and GOT and the results have been depicted in Fig. 8.8. The GO samples displayed very distinct D and G peaks around (1350 cm−1 ) and (1590 cm−1 ), respectively. The D and G bands correspond to the vibrations caused due to defects arising in the sp3 hybridized carbon and due to in-plane vibrations of sp2 graphitic carbons, respectively [38]. A slight blue shift in the D and G peaks in GO-1 was observed as compared to GO-2 and GO-3 which indicates a lower degree of crystallinity as compared to GO-2 and GO-3. Upon conversion to GOT, it was observed that the intensity of both these peaks reduced significantly and also several new peaks were obtained between 100 cm−1 to 600 cm−1 which could be attributed to the anatase phase of TiO2 . The reduction of the intensity of the peaks is indicative of the partial loss of oxygen-containing functionalities in GO causing a decrease in defects in the crystal lattice and an increase in graphitization [45]. This leads to the reconstruction of the π-π conjugation. This was crucial in the photocatalysts as this feature not only reduces the recombination of the photogenerated charges but also aids in enhancing the electron mobility during the reaction [38]. In order to further understand the behavior of the photogenerated charge carriers, photoluminescence spectroscopy was conducted for GOT-3 and the details have been reported by Kumar et al. [46]. It was found that the nanocomposite GOT-3 had lower resistance as compared to pristine TiO2 . The results corroborated well with that obtained in Raman spectroscopy and showed that the network of π-π conjugation in GOT-3 may have resulted in high electron conductivity thereby promoting charge transfer. The nanocomposite GOT-3 was a p-type semiconductor as revealed from the negative slope of the Mott-Schottky plot with a flat-band potential value (Efb ) of +1.07 V (vs

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Fig. 8.8 Raman spectra of a GO-1, b GO-2, c GO-3, d GOT-1, e GOT-2 and f GOT-3

Ag/AgCl). The electron lifetime was estimated as 1.09 × 10−2 s using the Bode phase analysis. The light-harvesting capacity and the charge separation in the synthesized nanocomposite were higher as compared to bare TiO2 . This was demonstrated by the higher photocurrent density in GOT-3 as compared to TiO2 . It was noteworthy that the dopant also enhanced the photostability of the photocatalyst. The photogenerated electrons in the TiO2 conduction band can also be transferred readily to the rGO in the nanocomposite due to the higher work function of rGO [25]. The formation of p-type semiconductor is highly beneficial as it can act as a sensitizer and enhance the photocatalytic performance of the nanocomposite in both the UV and visible regions [46].

8.3.2 Photocatalytic Degradation of Eosin-Y It has been widely reported that in visible light TiO2 shows poor photocatalysis of pollutants owing to its restricted optical sensitivity in the visible range. There are two crucial steps involved in photocatalytic degradation, firstly adsorption of eosiny on the GOT surface and secondly degradation of the dye by the action of the photocatalyst. It is reported that the nature of adsorption on GOT nanocomposite

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Fig. 8.9 Eosin-y decolourization profiles under a UV and b LED light irradiation for 4 h reaction time and initial eosin-y dye concentration of 25 mg/L

was non-covalent and was driven by the π–π stacking between the dye and aromatic regions of GO sheets [47]. Figure 8.9 shows the photocatalytic degradation of eosiny under UV and visible irradiation. Over 4 h, the photocatalytic decolourization under UV was higher than under visible irradiation. The decrease in absorption peak intensity of eosin-y at λmax = 517 nm was observed over the course of the reaction. After the end of the 1 h equilibration period in the dark, eosin-y removal due to adsorption onto the nanocomposites was 58%, 51% and 62% for GOT-1, GOT-2 and GOT-3, respectively. Thus, the initial concentration in the aqueous phase (Co) at the beginning of irradiation was similar for GOT-1 and GOT-3 and different for GOT-2. The photodegradation profiles for eosin-y under UV and LED irradiation are depicted in Fig. 8.8a and b, respectively. After UV irradiation for 4 h, the dye removal was 92% in GOT-1, 96% in GOT-2 and 100% in GOT-3, whereas the corresponding values under visible irradiation were 73%, 76% and 90%, respectively, with respect to the initial concentration (Cin ) before adsorption. The main chemical species that facilitated the decolourization of the dye from the aqueous solution are · OH and superoxide radicals as depicted for eosin-y degradation by Hossain et al. [2]. The order of decrease of photodegradation efficiency was GOT-3 > GOT-2 > GOT-1 under both UV and LED light. The superior photocatalytic performance of GOT-3 was possibly due to the higher abundance of oxygen-containing functional groups on GO synthesized by the Modified Hummer’s Method. It is reported that high degree of oxidation results in structural distortion causing a reduction in the bandgap, thereby facilitating photocatalysis [32]. The enhanced activity of GOT-2 in UV compared to GOT-1, despite the lower adsorption of eosin-y, may be attributed to the morphology of the nanocomposite which showed the presence of both nanospheres and nanorods of TiO2 as opposed to only nanospheres in GOT-1 (Fig. 8.2e). Chemical bond formation between TiO2 and graphene oxide is of utmost importance. It gives rise to

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properties, such as enhanced light absorption capability, better charge transfer characteristics and better adsorption of pollutants on the surface of the nanocomposites [9]. The presence of anatase phase TiO2 of uniform size (avg. ~ 20 nm) was highly favourable for photodegradation of eosin-y dye. Control experiments were conducted with GO-1, GO-2 and GO-3 under the presence of UV and LED irradiation. It was observed that eosin-y could not be decolourized in the presence of GO-1, GO-2 and GO-3 under both UV and LED irradiation.

8.4 Conclusion Graphene oxide was synthesized using Tours’ method, Hummers’ method and Modified Hummers’ method. It was found that Modified Hummers’ method resulted in the synthesis of GO with the highest O to C ratio, owing to an additional amount of oxidizing agent used during its synthesis. The GOT nanocomposites synthesized displayed the uniform distribution of TiO2 nanoparticles anchored on the rGO sheets. Physical and chemical characterization of GO and GOT indicated that during the synthesis of GOT, GO was partially reduced thereby increasing the thermal stability of the nanocomposites. Reduced GO also provided a number of sites for the creation of Ti–O–C bonds which were vital for photocatalysis. TiO2 nanoparticles in all the GOT nanocomposites were present in the anatase phase and the nanoparticles had an average size of 20 nm. It can be suggested with sufficient evidence that the method of GO synthesis not only affected the morphology of the TiO2 nanoparticles in GOT but also the photocatalytic performance of the GOT nanocomposites under UV and LED light. Decolourization of eosin-y was 100% and 90% under UV and LED light with GOT-3 with respect to the initial eosin-y concentration before adsorption. The combination of rGO with the semiconductor TiO2 resulted in a reduction of the bandgap of TiO2 , rendering the nanocomposite sensitive to visible light irradiation. The photo-induced electrons from the conduction band of TiO2 were transferred to the rGO sheets, which may have led to a reduction in the recombination of electron–hole pairs. The graphene oxide-TiO2 nanocomposite synthesized by modified Hummer’s method is an efficient photocatalyst for decolourization of dyes and may have good potential for water treatment. Acknowledgements Partial funding for this work was provided by a project funded by the Water Technology Initiative, Department of Science and Technology (DST), New Delhi, India (DST/TM/ WTI/2K15/101(G)). SAIF, IIT Bombay is acknowledged for providing FEG-SEM, TEM and FTIR facilities. Department of Chemistry, IIT Bombay is acknowledged for providing UV-DRS facility and MEMS, IIT Bombay is acknowledged for providing BET and XRD facilities.

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Chapter 9

Graphene Oxide Nanocomposites for the Removal of Antibiotics, Pharmaceuticals and Other Chemical Waste from Water and Wastewater Karan Chaudhary and Dhanraj T. Masram

9.1 Introduction A large quantity of antibiotics, pharmaceuticals, and other organic chemicals as pollutants are discharged into the water bodies which is the main reason behind the pollution of water bodies. These pollutants are discharged through sewage from industries, agricultural and urban areas, and water treatment plants [1, 2]. These discharges of antibiotics, pharmaceuticals, and other chemical organic pollutants are not a new problem and have been a severe problem from ancient ages, which is increasing with time along with the growth of industries, population, domestic activities, and agriculture. This pollution of water bodies will keep on increasing with the introduction of other new pollutants [3]. The pollution of water bodies has a serious negative effect on the environment [1]. As the presence of these organic pollutants even in low concentrations possess a severe risk for humans and other living beings. Therefore, the presence of pollutants in the aquatic environment has engrossed significant concern [2]. Their accumulation in the water system is the result of inefficient removal by various methods that are used for water treatment [2]. Therefore, for the removal of organic pollutants from wastewater and water bodies, several researchers worldwide are working in this area of removal and degradation of organic pollutants. As most organic pollutants are difficult to degrade naturally or through conventional wastewater treatment methods [4, 5], thus, it is the primary necessity to improve wastewater treatment methods and to develop an effective method that is simple and cost-effective for removing such organic pollutants from water bodies. K. Chaudhary · D. T. Masram (B) Department of Chemistry, University of Delhi, Delhi 110007, India e-mail: [email protected] K. Chaudhary National Forensic Sciences University, Delhi Campus, Delhi 110085, Delhi, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Mohanty et al. (eds.), Graphene and its Derivatives (Volume 2), Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-99-4382-1_9

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Several methods such as flocculation/coagulation/sedimentation, biological processes, ozonation, and sand filtration are commonly used for wastewater treatment, but these conventional methods are not designed for the removal of organic pollutants such as antibiotics, pharmaceuticals, and other chemical waste [2]. Other non-conventional methods such as reverse osmosis, adsorption, ion exchange, oxidation, and combined methods have also been applied for the removal of water pollutants [6]. Among these methods, adsorption and advanced oxidation process (AOP) are the most efficient methods for removing antibiotics, pharmaceuticals, and other chemical waste from wastewater [2]. The adsorption method is described as adhering of molecules in the fluid phase to a solid surface [6]. Adsorption engrossed great attention due to several associated advantages such as safety, short analysis time, ease to perform, material recovery, and extensiveness of different adsorbents [7]. Another method is AOP, in this method main is photocatalytic degradation of organic pollutants and this method is preferred because it is an environment-friendly method [2]. AOP also includes ozonation, Fenton reaction, chlorination, and peroxymonosulfate method, but photocatalytic degradation of pollutants is the most chosen one for its versatility, mild conditions, low cost, and effectiveness [5, 8]. In the process of the photocatalytic degradation method, reactive oxygen species (ROS) are generated from water by photogenerated electrons and holes and these ROS reacts with the organic pollutants for the degradation of pollutants [9, 10]. Further, several organic pollutants can be degraded using photocatalysts which shows their potential for wastewater treatment. Moreover, excellent stability and non-toxicity are outstanding features of photocatalysts which makes the photocatalysis method more efficient [11]. Properties displayed by carbon nanomaterials have resulted in the utilization of carbon nanomaterials in widespread applications [12–20], including biomedical and biotechnological applications [21–26]. Carbon materials have a significant position in the field of wastewater remediation [1, 27–30]. Among carbon materials, the most widely applied are graphene oxide (GO) and GO-based composite materials for wastewater treatment. GO is a two-dimensional material having a sheet structure. Moreover, GO has a basic structure similar to the parent material that is graphene but the major difference is the presence of oxygen-containing groups such as epoxy, carboxyl, hydroxyl, and carbonyl on the surface of GO [27]. Properties exhibited by GO are chemical stability, large surface area, and mechanical flexibility [7]. Due to these properties of GO, GO and GO-based composite materials are better sorbents and catalytic carriers, [31] therefore can be used as adsorbents or photocatalysts in the removal of pollutants from the wastewater. Herein, this chapter is based on the graphene oxide nanocomposites development over the last few years that have been used for the removal of antibiotics, pharmaceuticals, and other chemical waste from water and wastewater. Overall, this chapter includes a discussion on the synthetic approach of these graphene oxide nanocomposite materials and their utilization in the removal of antibiotics, pharmaceuticals, and other chemical waste pollutants.

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9.2 Synthesis of Graphene Oxide Nanocomposites In the synthesis of nanocomposites of GO for the removal of pollutants from wastewater, GO is coupled directly with metal oxide nanoparticles (NPs), mixed metal oxide NPs, and metal–organic frameworks (MOFs). Moreover, GO is chemically modified with other organic compounds and used directly or one step further this chemically modified GO is coupled with metal oxide NPs and used as a catalyst for the degradation of organic pollutants for wastewater treatment. Here, in this section, synthetic methods for some of the GO composites that have been used in the removal of pollutants have been discussed. The most used and easy method for the synthesis of GO composite is an in-situ generation of NPs on the surface of GO. Raja et al. [4] prepared rGO-HoVO4 -TiO2 in which first a suspension of HoVO4 was prepared by mixing sodium hydroxide (NaOH) to a solution of NH4 VO3 and Ho (NO3 )3 ·6H2 O and then to this suspension ethanolic solution of titanium (IV) butoxide was added and stirred. Then, GO and oxalic acid was added to the above mixture, and the mixture was hydrothermally treated and the final solid obtained was calcined to obtain the rGO-HoVO4 -TiO2 composite [4]. Xu et al. [32] initially prepared GO and reduced it by the use of sodium borohydride (NaBH4 ). Then this reduced GO was dispersed in DMF and to it FeCl3 .6H2 O and terephthalic acid were dissolved after mixing, further, the mixture was solvothermally treated in an autoclave to obtain reduced GO/ metal–organic framework composite (rGO/MIL-101(Fe)) [32]. Flores et al. also in-situ synthesized ZnO NPs over the surface of GO in which GO was mixed with a solution of Zn(NO3 )2 .6H2 O and then was treated with NaOH solution to obtain the suspension which was heated, followed by filtration, washing, and drying to get the solid of ZnO/GO [9]. Another way to prepare the composites of GO is by mixing different components and further treating them to get heterostructured composites. Liu et al. prepared a heterostructure composite in which GO, graphitic carbon nitride (g-C3 N4 ), Na2 WO4 ·2H2 O, Bi(NO3 )3 ·5H2 O and cetyltrimethylammonium bromide (CTAB) were mixed in water and ultrasonicated, and the resultant mixture was treated hydrothermally and obtained g–C3 N4 /Bi2 WO6 /rGO composite after centrifugation and drying [33]. Nawaz et al. [34] prepared a three-dimensional (3-D) composite in which TiO2 NPs and GO were mixed under sonication. Then to this mixture solution of sodium alginate was added dropwise while stirring and followed by the addition of CaCl2 which was added for crosslinking. After this mixture so obtained was freezedried and the dried gel obtained was reduced using L-ascorbic acid while heating. After reduction, the obtained hydrogel was washed and again freeze-dried to produce three-dimensional reduced GO-TiO2 /sodium alginate aerogel [34]. Composites can be synthesized by chemical modification of GO and further, they can also be coupled with NPs. Kong et al. [35] synthesized a composite in which first GO was added into the solution polymerization system and after stirring the mixture obtained GO/polyacrylic acid (PAA) hydrogel which was freeze-dried and grounded. Then this GO/PAA powder was mixed with Cd(NO3 )2 solution and then after washing was treated with Na2 S solution under constant temperature shaking conditions. The

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product so obtained was washed, freeze-dried, and grounded to obtain a powder of GO/PAA-CdS composite [35]. Umbreen et al. [36] synthesized reduced GO-based hydrogels in which GO was dispersed in water and to it sodium ascorbate was added and the mixture was sonicated to obtain the slurry. Then, the slurry was heated and a reduction reaction took place, after this reduced GO-based hydrogel was obtained which was filtered, washed, and dried [36].

9.3 Removal of Antibiotics, Pharmaceuticals, and Other Chemical Waste Pollutants Fluoroquinolone drugs remain in the environment for a long time as their degradation is not easy. Ofloxacin (OFL), which belongs to the fluoroquinolone drug is a widespread antibiotic [37]. Ehtesabi et al. [37] demonstrated the OFL pollutant removal from aqueous water by adsorption process using 3-D graphene hydrogel. This 3-D graphene hydrogel was synthesized from GO in one step by a simple method and a high liquid volumetric rate was achieved by modifying the synthesized graphene hydrogel. This graphene hydrogel exhibited a porous structure and had a surface area of 307 m2 /g. These features make this material a suitable adsorbent for OFL removal. To study OFL removal, the fluorescence property of OFL was used for its detection by a smartphone fluorometer set-up device. Results from this study revealed that this modified 3-D graphene hydrogel had 134 mg/g adsorption capacity of OFL and the adsorption process followed the pseudo-second order kinetic model. The mechanism behind efficient OFL removal was given as π-π interaction, hydrogen bonding, and electrostatic interaction between modified 3-D graphene hydrogel and OFL along with the physical adsorption effect [37]. Another type of antibiotic, tetracycline has been accumulated in the environment and can cause various potential adverse effects, therefore, removal of this potent pollutant from water is necessary. Tabrizian et al. [38] applied GO-supported bimetallic nanocomposite for the tetracycline removal from water. To prepare GOsupported bimetallic nanocomposite, Cu/Fe bimetallic nanoparticles (BNPs) were synthesized ex-situ and then immobilized on GO to prevent their aggregation. In comparison to the surface area of Fe/Cu BNPs which was 65.88 m2 /g, the surface area for GO-supported bimetallic nanocomposite was 108.62 m2 /g. When this Fe/ Cu-GO nanocomposite was applied for tetracycline removal, within 15 min greater than 97% removal was obtained in a wide pH range of 3 to 8. While, in the pH range of 5 to 7, near complete tetracycline removal was obtained. The adsorption capacity of Fe/Cu-GO nanocomposite for tetracycline was 201.9 mg/g and adsorption data fits the Freundlich model well. This Fe/Cu-GO nanocomposite was easily retrievable due to magnetic properties and was reusable for up to 5 cycles for efficient removal of tetracycline. The main force behind the removal was the adsorption of tetracycline by Fe/Cu-GO nanocomposite. Also, studies revealed the potential of Fe/Cu-GO nanocomposite for the degradation of tetracycline into small molecular

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weight products at low pH of 3.6 [38]. Oxytetracycline (OTC), another member of the tetracycline antibiotic group is also an emerging micropollutant. Prarat et al. [39] synthesized amino-functionalized silica-magnetic graphene oxide nanocomposite (A-mGO-Si) and utilized it as an adsorbent for the removal of OTC from water. The adsorption process followed the pseudo-second order kinetic model and data fitted the Freundlich isotherm model well. The removal capacity of OTC was found to be better for A-mGO-Si in comparison to materials such as GO, Fe3 O4 , mGO, and SBA-15. Further, OTC removal efficiency was high at low initial concentration. The adsorption process of OTC on A-mGO-Si was not pH dependent and at pH 5.2 the adsorption capacity was maximum. The main interactions involved in the adsorption of OTC on A-mGO-Si were π-π interaction, H-bonding, and electrostatic interaction. Furthermore, the adsorption of OTC decreased on the addition of ions (CO3 2− , Mg2+ , NO− , and Na+ ) due to competition with ions, where divalent ions inhibited OTC adsorption more than monovalent ions. At lower aqueous phase concentrations of OTC, the presence of humic acid and tannic acid resulted in decreased adsorption of OTC onto A-mGO-Si. But at a high aqueous phase concentration of OTC, the presence of humic acid and tannic acid could promote the adsorption of OTC [39]. For the removal of chlortetracycline from water, Kong et al. [35] prepared a composite of GO and cadmium sulfide (CdS) embedded polyacrylic acid (GO/PAA-CdS) and applied it as a photocatalyst under visible light irradiation. It was observed that when the amount of GO was increased, the removal ratio increased from 38.0% to 86.0% revealing the relationship between the amount of GO and photocatalyst performance. Other factors like efficient electron transfer along with GO and controlled growth of CdS NPs were attributed to enhanced photocatalytic activity. Along with this, the pH of the solution also had a significant effect, as the maximum degradation rate constant was achieved at pH 6. The ·O2− and OH· radicals generated by the catalyst mainly governed the photodegradation of chlortetracycline. Further, the catalyst was stable even after seven cycles as catalytic activity was maintained [35]. Ibuprofen, a non-steroidal anti-inflammatory drug is another emerging pollutant that at low concentrations causes negative impacts on the ecosystem. Liu et al. [33] prepared heterostructured graphitic carbon nitride/bismuth tungsten oxide/reduced graphene oxide nanocomposites (GBR) by microwave assisted hydrothermal method which was used for the decontamination of ibuprofen (IBP) through visible/ solar light from wastewater. GBR-60 which was treated for 60 min in microwave, photodegraded 93% of IBP under visible light irradiation with a degradation rate constant of 0.011 min−1 . This efficient photodegradation of IBP by GBR-60 was due to increased charge transfer efficiency, specific surface area, and higher crystallization degree. Further, for efficient photodegradation of IBP, 4.3 is the optimum pH value. Under sunlight irradiation, the photocatalytic performance of this GBR material was further enhanced and 98.6% of IBP was photodegraded. For the efficient photodegradation of IBP, hydroxyl radicals and superoxide radicals were the main active species which was revealed from electron spin resonance (ESR) studies. Liquid chromatographymass/mass spectrometry (LC–MS/MS) revealed that five intermediates are formed through three degradation pathways which were hydroxylation-demethylation, hydroxylation-decarboxylation, and direct decarboxylation. This study shows that

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such prepared photocatalysts can be used in a river water matrix for the removal of IBP under sunlight [33]. Raja et al. [4] hydrothermally prepared RGO loaded with HoVO4 -TiO2 (rGO-HoVO4 -TiO2 ) for the photocatalytic removal of ibuprofen under visible light irradiation. 95% photocatalytic decomposition of ibuprofen was achieved at pH 7. This catalyst rGO-HoVO4 -TiO2 was stable even after 5 cycles and removal efficiency slightly decreased after 5 cycles and was 86%. Further, the effects of other parameters such as concentration of ibuprofen and amount of catalyst on the photodegradation efficiency were also studied to get an optimized condition for maximum removal of the pollutant. The superoxide radicals were the main species that were photogenerated from the catalyst which was mainly responsible for the photocatalytic decomposition of ibuprofen under visible light irradiation [4]. Ciprofloxacin, which belongs to quinolone antibiotics, is also a toxic organic compound that has hazardous effects on the environment. Ma et al. [40] prepared a triple-network composite hydrogel for the removal of ciprofloxacin from wastewater. Carbon nanotubes, L-cysteine modified GO, and sodium alginate are used for the preparation of carbon nanotubes/L-cysteine@graphene oxide/sodium alginate (CNTs/L-cys@GO/SA) composite hydrogel. The concept of triple-network hydrogel is new and such hydrogels have large internal space, porous structure, and overall larger 3-D structure. These features result in the availability of more sites for the adsorption of pollutants which will provide more efficient removal of pollutants. This triple-network hydrogel had improved thermal stability, mechanical properties, and swelling ability. Further, in a weak acidic medium, CNTs/L-cys@GO/ SA hydrogel showed 181 and 200 mg/g as adsorption capacities at 25 and 15 °C, respectively. As these results indicate better results are shown by CNTs/L-cys@GO/ SA hydrogel at low temperatures [40]. Kumar et al. [3] synthesized bismuth phosphate@graphene oxide based magnetic nano-sized-molecularly imprinted polymer (BiPO4 @GO-MMIPs) which was successfully applied for the ciprofloxacin detection, removal as well as degradation. For the synthesis of this BiPO4 @GO-MMIPs, atom transfer radical polymerization process was used and different components had special functions such as N-vinyl caprolactam as a biocompatible monomer, N, Nmethylene bis-acrylamide as a crosslinking agent, ZnFe2 O4 nanoparticle as magnetic part, and BiPO4 @GO as photocatalyst. BiPO4 @GO-MMIPs displayed high selectivity toward ciprofloxacin, high adsorption capacity of ciprofloxacin, and was easily recoverable. A fluorescence study was performed for the visual detection and an electrochemical study was performed for the trace level detection of ciprofloxacin using BiPO4 @GO-MMIPs. From the fluorescence study, the linear range for sensing achieved was 39.0 to 328.0 μg/L with a detection limit of 0.40 μg/L, while, from electrochemical studies, the linear range for detection was 39.0 to 740.0 μg/L with a detection limit of 0.39 μg/L. By using BiPO4 @GO-MMIPs, ciprofloxacin was even detected in more complex matrices like blood, blood serum, and milk samples. The use of this material provided removal and degradation of ciprofloxacin in good efficiency [3]. Behera et al. [41] used a single-step hydrothermal method followed by calcination method for the synthesis of zinc ferrites [ZnFe2 O4 ] (ZFO) and RGO nanocomposite (ZFO@RGO) and applied this composite material for the degradation of ciprofloxacin. Out of the series of ZFO@RGO nanocomposites prepared, the

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ZFO@3%RGO was found to be best for the degradation studies and it degraded 73.4% of ciprofloxacin in an hour under solar irradiation which in comparison to pure ZFO was 1.67 times higher. The process of ciprofloxacin degradation followed the first-order kinetics model and the rate constant was 0.021 min−1 which was good enough and in comparison to pure ZFO was 2.3 times higher. The reason behind high photocatalytic degradation by this nanocomposite was the electron catching and channelizing property of conjugated π-systems present in RGO. In the photodegradation of ciprofloxacin by ZFO@RGO nanocomposite, ·O2− and .OH were mainly responsible [41]. Nazraz et al. [7] applied hydrothermally synthesized composite of magnesium oxide, chitosan, and GO (MgO/Chit/GO) for ciprofloxacin and norfloxacin removal from aqueous solution. The present composite had a high surface area (294 m2 /g) and pore size of 15 Å which resulted in high adsorption capacity. Adsorption isotherm data for ciprofloxacin and norfloxacin fitted the Langmuir model well and kinetic data followed the pseudo-second order kinetics. The study showed that 1111 and 1000 mg/ g were the maximum adsorption capacities for ciprofloxacin and norfloxacin, respectively. Further, for norfloxacin and ciprofloxacin, the maximum equilibrium was at over 150 and 120 min, respectively. Also, found that the composite retained an adsorption percentage above 80% after four cycles for ciprofloxacin and norfloxacin, showing the good stability of the composite [7]. For the removal of norfloxacin, Wu et al. [5] synthesized MnO2 and GO nanohybrid (MnO2 /GO) in which MnO2 needles of 200-400 nm length were evenly distributed on the surface of GO. This nanohybrid material had a large surface area and abundant oxygen-containing functionalities on the surface which was advantages in norfloxacin degradation. In the presence of 10 mM peroxymonosulfate and 0.8 g/L catalyst, more than 80% of norfloxacin was degraded within 20 min. Best degradation results were obtained in the acidic condition. Results from the quenching test confirmed the existence of ·OH, SO4 ·− and non-radical process in the degradation process of norfloxacin by nanohybrid. Also, found that nanohybrid material had excellent reusability as degradation efficiency remained high after 4 cycles. In this study, it was found that there were 14 intermediates during degradation and identified 4 possible pathways, out of which pathways considered to be the main was oxidation of the piperazine ring [5]. Rifampicin is an important antibiotic for the treatment of tuberculosis and other diseases but its high water solubility and chronic toxicity make it a pollutant of concern. Liu et al. [42] prepared a composite of bimetallic iron/palladium nanoparticles and reduced GO (rGO@nFe/Pd) in the presence of tea extract and applied this material for the removal of rifampicin from water. The 79.9% removal efficiency of rGO@nFe/Pd composite increased further in combination with Fentomoxidation up to 85.7%. In acidic condition (pH 3), rGO@nFe/Pd had the highest Fenton-oxidation ability and degradation was dominated by ·OH. The pseudo-second order kinetic model was followed by this oxidation process and 47.3 kJ/mol was the

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activation energy. Further, LC–MS analysis showed that 5,6-dihydroxy-1-oxo-1,2dihydronaphtho [2,1-b] furan-2-yl formate, 5,6,9-trihydroxynaphtho [2,1-b] furan1(2 H)-one, and (S)-5,6,9-trihydroxy-2-(3-methoxypropoxy)-2-methylnaphtho [2,1b] furan-1(2 H)-one were the main degradation products obtained after degradation of rifampicin by rGO@nFe/Pd. Rifampicin removal from the river, domestic, and aquaculture wastewater was also performed using the rGO@nFe/Pd composite and the removal efficiency obtained was 80.4, 70.2, and 77.9%, respectively, which represented the practical efficacy of the rGO@nFe/Pd composite material [42]. 2,4-dichlorophenoxyacetic acid (2,4-D) is a herbicide whose biodegradation is difficult, is toxic and its accumulation is dangerous for human and aquatic life. Tho et al. [11] synthesized reduced GO and mixed metal oxide composite (rGO/ ZnBi2 O4 ) via oxidation–reduction and co-precipitation method which was followed by heating at a higher temperature (450 °C) for the utilization as photocatalyst for the degradation of 2,4-D from aqueous solution. The catalyst containing 2% of RGO (2.0rGO/ZnBi2 O4 ) was found to be the best for the degradation studies and in a 30 mL solution, more than 90% of 2,4-D was degraded under visible light irradiation after 120 min when the amount of catalyst used was 1.0 g/L. This catalyst displayed excellent stability as no significant change in degradation rate was observed even after 4 consecutive cycles. The presence of RGO in the catalyst was said to be the reason for such improved photocatalytic activity because it is an excellent electron acceptor and mediator, which enhances electron migration and hinders electron and hole recombination [11]. Simazine, a chloro-s-triazine herbicide is another pollutant of concern and for its degradation, Flores et al. utilized a composite of zinc oxide and GO (ZnO/GO) as a photocatalyst under visible light irradiation [9]. For the degradation of simazine, the optimum pH was determined to be 2 for all samples having varied ZnO amount loading on GO. When the material was synthesized using 30 and 20 mmoles of Zn2+ ions, for such material their 40 mg was found to be most effective for catalysis. Whereas, when the material was synthesized using 10 mmoles of Zn2+ ions, for that 10 mg was found to be the most effective for catalysis. Moreover, the degradation process of simazine by ZnO/GO followed second order kinetics. In comparison to ZnO, ZnO/GO had higher photocatalytic reaction rates, and the rate for simazine degradation by 30 mmol ZnO/GO was 10 times greater than that for ZnO. Further, this composite exhibited good reusability and for 3 reaction cycles showed a constant photocatalytic activity without a conditioning cycle [9]. GO composite nanomaterials that have been discussed above along with several other nanomaterials which are used for the removal of antibiotics, pharmaceuticals, and other chemical waste pollutants have been summarized in Table 9.1.

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Table 9.1 GO composite nanomaterials for the removal of antibiotics, pharmaceuticals, and other chemical waste pollutants S.n

Material

1

Modified 3-D Ofloxacin graphene hydrogel

Material had 134 mg/g Ehtesabi et al. [37] adsorption capacity of OFL and the adsorption process followed pseudo-second order kinetic model

2

Fe/Cu-GO nanocomposite

Tetracycline

Within 15 min greater Tabrizian et al. [38] than 97% removal was obtained in a wide pH range of 3 to 8. While, in pH range of 5 to 7, near complete tetracycline removal. Adsorption capacity was 201.9 mg/g and adsorption data fits the Freundlich model well

3

g-C3 N4 /rGO hybrid loaded on nickel foam

Tetracycline

Hybrid having weight ratio of rGO to g-C3 N4 as 1:9 had the highest degradation efficiency of 90%. Even after 4 cycles, degradation efficiency was 87.5%

Wang et al. [43]

4

Sulfo-functional Tetracycline 3-D porous cellulose/graphene oxide composites

Adsorption process followed pseudo-second order kinetics and fitted Langmuir isotherm model well. The maximum adsorption capacity was 163.4 mg/g. The maximum removal efficiency was up to 99%. And was used for 10 cycles, where removal percentage slightly decreased to 86.96% from 99.98%

Wang et al. [44]

5

Aminofunctionalized silica-magnetic graphene oxide nanocomposite

Adsorption process Prarat et al. [39] followed pseudo-second order kinetic model and data fitted Freundlich isotherm model well. OTC removal efficiency was high at low initial concentration of OTC

Antibiotics

Oxytetracycline

Removal results

References

(continued)

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Table 9.1 (continued) S.n

Material

Antibiotics

Removal results

References

6

GO/PAA-CdS

Chlortetracycline

On increasing the amount of GO, the removal ratio increased from 38.0% to 86.0%. Maximum degradation rate constant was achieved at pH 6. The catalyst was stable even after seven catalytic cycles

Kong et al. [35]

7

graphitic carbon nitride/bismuth tungsten oxide/ reduced graphene oxide nanocomposites (GBR)

Ibuprofen

Photodegraded 93% of Liu et al. [33] IBP under visible light irradiation with a degradation rate constant of 0.011 min−1 . Under sunlight irradiation, 98.6% of IBP was photodegraded

8

rGO-HoVO4 -TiO2 Ibuprofen

95% photocatalytic Raja et al.[4] decomposition of ibuprofen was achieved at pH 7. Catalyst rGO-HoVO4 -TiO2 was stable even after 5 cycles and removal efficiency was 86%

9

reduced graphene oxide-TiO2 / sodium alginate aerogel (RGOT/ SA)

Ibuprofen and sulfamethoxazole

Both the contaminants Nawaz et al. [34] were removed by more than 99% within 45–90 min by the catalyst under UV-A light. At neutral pH, ibuprofen was highly photodegraded, whereas in acidic to neutral pH high photodegradation of sulfamethoxazole was observed

10

3-D RGO-based hydrogel

Naproxen, Ibuprofen and Diclofenac

Achieved excellent Umbreen et al. [36] decontamination of pollutants in acidic pH and obtained 70 to 80% removal efficiency

11

CNTs/ L-cys@GO/SA hydrogel

Ciprofloxacin

Hydrogel showed 181 and 200 mg/g as adsorption capacities at 25 and 15 °C, respectively

Ma et al. [40]

(continued)

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Table 9.1 (continued) S.n

Material

Antibiotics

Removal results

References

12

BiPO4 @GOMMIPs

Ciprofloxacin

Fluorescence study revealed the linear range for sensing achieved was 39.0 to 328.0 μg/L with a detection limit of 0.40 μg/L, while, electrochemical studies revealed the linear range for detection was 39.0 to 740.0 μg/L with a detection limit of 0.39 μg/L

Kumar et al. [3]

13

ZFO@3%RGO

Ciprofloxacin

ZFO@3%RGO was Behera et al. [41] found to be best for the degradation studies and it degraded 73.4% of ciprofloxacin in an hour under solar irradiation. The process of ciprofloxacin degradation followed the first-order kinetics model and the rate constant was 0.021 min−1

14

g-C3 N4 /RGO/ WO3

Ciprofloxacin

Degradation rate Lu et al. [45] enhanced to 85% when g-C3 N4 /RGO/ WO3 was used in comparison to that of 50% when g-C3 N4 / WO3 was used which shows the importance of RGO. Further, stronger oxidation and reduction ability to generate several reactive species due to electrons and holes transfer pathway in g-C3 N4 /RGO/WO3 resulted in such results for the degradation of pollutant (continued)

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Table 9.1 (continued) S.n

Material

Antibiotics

Removal results

References

15

MgO/Chit/GO

Ciprofloxacin and norfloxacin

Adsorption isotherm data fitted the Langmuir model well and kinetic data followed the pseudo-second order kinetics. 1111 and 1000 mg/g were the maximum adsorption capacities for ciprofloxacin and norfloxacin, respectively

Nazraz et al. [7]

16

MnO2 /GO nanohybrid

Norfloxacin

In the presence of Wu et al. [5] 10 mM peroxymonosulfate and 0.8 g/L catalyst, more than 80% of norfloxacin was degraded within 20 min. Best degradation results were obtained in the acidic condition. Also, nanohybrid material had excellent reusability as degradation efficiency remained high after 4 cycles

17

rGO/Bi4 O5 Br2

Ciprofloxacin, norfloxacin, and tetracycline

Composite containing Xu et al. [46] 1 wt.% of RGO had optimal adsorption and photocatalytic activity. Removal efficiencies obtained were 98.7%, 97.6%, and 80.7% for tetracycline, ciprofloxacin, and norfloxacin

18

ZnO/Fe3 O4 -GO/ ZIF

Sulfamethazine, metronidazole, norfloxacin, and 4-acetaminophen

Rapidly degraded Chen et al. [31] pollutants in 1 h under solar irradiation, the material is recyclable at least 10 times without obvious deactivation, degradation process followed pseudo-first-order kinetics (continued)

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Table 9.1 (continued) S.n

Material

Antibiotics

Removal results

References

19

ZnONP/GONS

Levofloxacin

Under UV light exposure, catalyst degraded more than 99% of levofloxacin

El-Maraghy et al. [2]

20

Sulphur-doped GO/Ag3 VO4

Thiram

Thiram was degraded by the catalyst to give thiourea as a product in an hour

Priyanka et al. [1]

21

rGO@nFe/Pd

Rifampicin

The 79.9% removal Liu et al. [42] efficiency of rGO@nFe/Pd composite increased further in combination with Fentom-oxidation up to 85.7%. The pseudo-second order kinetic model was followed by this oxidation process and 47.3 kJ/mol was the activation energy

22

rGO/ZnBi2 O4

2,4-dichlorophenoxyacetic acid

2.0rGO/ZnBi2 O4 was Tho et al. [11] best for the degradation studies and in a 30 mL solution more than 90% of 2,4-D was degraded under visible light irradiation after 120 min when amount of catalyst used was 1.0 g/L

23

ZnO/GO

Simazine

Optimum pH was 2, Flores et al. [9] degradation process of simazine by ZnO/GO followed second order kinetics, In comparison to ZnO, ZnO/GO had higher photocatalytic reaction rates, and rate for simazine degradation by 30 mmol ZnO/GO was 10 times greater than pure ZnO. Composite showed a constant activity for 3 cycles (continued)

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Table 9.1 (continued) S.n

Material

Antibiotics

Removal results

References

24

RGO/ MIL-101(Fe)

Trichlorophenol

At pH 3.0, 20 mM persulfate, 20 mg/L trichlorophenol and 0.5/L of composite catalyst efficient degradation was obtained and in 180 min the removal efficiency was 92%

Xu et al. [32]

25

BiVO4 /RGO

2,4-dichlorophenol

Composite having 5 Tu et al. [47] wt% GO doping, 9 h hydrothermal time and 150 °C hydrothermal temperature displayed best photocatalytic activity for the degradation of pollutant under solar irradiation

9.4 Conclusion In this chapter, we discussed the development of graphene oxide nanocomposites over the last few years that have been utilized for the removal of antibiotics, pharmaceuticals, and other chemical waste from water and wastewater. This shows graphene oxide nanocomposites are being extensively used for wastewater treatment and are proved to be efficient for this application. These composites have shown up to be a better material to compete against the problem of the water crisis. Herein, the main methods for the removal of organic pollutants from water and wastewater were adsorption, photocatalytic degradation, and their combination. Overall, this chapter provides a discussion on the synthetic approach of graphene oxide nanocomposite materials and their utilization in the removal of antibiotics, pharmaceuticals, and other chemical waste pollutants. Several graphene oxide nanocomposite materials have been summarized which show potential and efficiency for wastewater treatment.

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36. Umbreen N, Sohni S, Ahmad I et al (2018) Self-assembled three-dimensional reduced graphene oxide-based hydrogel for highly efficient and facile removal of pharmaceutical compounds from aqueous solution. J Colloid Interface Sci 527:356–367. https://doi.org/10.1016/j.jcis. 2018.05.010 37. Ehtesabi H, Bagheri Z, Yaghoubi-Avini M (2019) Application of three-dimensional graphene hydrogels for removal of ofloxacin from aqueous solutions. Environ Nanotechnol Monit Manag 12:100274. https://doi.org/10.1016/j.enmm.2019.100274 38. Tabrizian P, Ma W, Bakr A, Rahaman MS (2019) pH-sensitive and magnetically separable Fe/ Cu bimetallic nanoparticles supported by graphene oxide (GO) for high-efficiency removal of tetracyclines. J Colloid Interface Sci 534:549–562. https://doi.org/10.1016/j.jcis.2018.09.034 39. Prarat P, Hongsawat P, Punyapalakul P (2020) Amino-functionalized mesoporous silicamagnetic graphene oxide nanocomposites as water-dispersible adsorbents for the removal of the oxytetracycline antibiotic from aqueous solutions: adsorption performance, effects of coexisting ions, and natural organic. Environ Sci Pollut Res 27:6560–6576. https://doi.org/10.1007/ s11356-019-07186-4 40. Ma J, Jiang Z, Cao J, Yu F (2020) Enhanced adsorption for the removal of antibiotics by carbon nanotubes/graphene oxide/sodium alginate triple-network nanocomposite hydrogels in aqueous solutions. Chemosphere 242:125188. https://doi.org/10.1016/j.chemosphere.2019. 125188 41. Behera A, Kandi D, Mansingh S et al (2019) Facile synthesis of ZnFe2 O4 @RGO nanocomposites towards photocatalytic ciprofloxacin degradation and H2 energy production. J Colloid Interface Sci 556:667–679. https://doi.org/10.1016/j.jcis.2019.08.109 42. Liu L, Xu Q, Owens G, Chen Z (2021) Fenton-oxidation of rifampicin via a green synthesized rGO@nFe/Pd nanocomposite. J Hazard Mater 402:123544. https://doi.org/10.1016/j.jhazmat. 2020.123544 43. Wang X, Wang H, Yu K, Hu X (2018) Immobilization of 2D/2D structured g-C3 N4 nanosheet/ reduced graphene oxide hybrids on 3D nickel foam and its photocatalytic performance. Mater Res Bull 97:306–313. https://doi.org/10.1016/j.materresbull.2017.09.024 44. Wang S, Ma X, Zheng P (2019) Sulfo-functional 3D porous cellulose/graphene oxide composites for highly efficient removal of methylene blue and tetracycline from water. Int J Biol Macromol 140:119–128. https://doi.org/10.1016/j.ijbiomac.2019.08.111 45. Lu N, Wang P, Su Y et al (2019) Construction of Z-Scheme g-C3 N4 /RGO/WO3 with in situ photoreduced graphene oxide as electron mediator for efficient photocatalytic degradation of ciprofloxacin. Chemosphere 215:444–453. https://doi.org/10.1016/j.chemosphere. 2018.10.065 46. Xu M, Wang Y, Ha E et al (2021) Reduced graphene oxide/Bi4 O5 Br2 nanocomposite with synergetic effects on improving adsorption and photocatalytic activity for the degradation of antibiotics. Chemosphere 265:129013. https://doi.org/10.1016/j.chemosphere.2020.129013 47. Tu L, Hou Y, Yuan G, et al (2020) Bio-photoelectrochemcial system constructed with BiVO4 / RGO photocathode for 2,4-dichlorophenol degradation: BiVO4 /RGO optimization, degradation performance and mechanism. J Hazard Mater 389:121917. https://doi.org/10.1016/j.jha zmat.2019.121917

Chapter 10

Graphene and Its Derivatives Based Membranes for Application Towards Desalination Satadru Chakrabarty, Anshul Rasyotra, Anupma Thakur, and Kabeer Jasuja

10.1 Introduction Water shortages are threatening livelihoods all throughout the world. Factors such as burgeoning global population, pollution and climate change are only aggravating the issue [1–4]. It is estimated that approximately 20% of the world’s population lives in areas with acute water scarcity and about 33% of the population lives in areas with moderate water shortages [5]. Thus, it is imperative for the stakeholders such as the policy makers, scientists, industrialists and the general public to band together and work collaboratively in averting an impending water disaster. In the pursuit of alleviating water shortages, desalination has emerged as a viable approach. Since seawater is practically abundant, effective desalination can provide potable water at a much larger scale. Desalination is already being adopted by countries riddled with stressed water supplies, Saudi Arabia is leading the pack in this regard [6]. The current desalination capacity on a worldwide basis is around 104.7 million cubic meters per day [7, 8]. The global status of desalination capacity is shown in Fig. 10.1. Desalination is being adopted at a brisk pace throughout the world, especially in the arid regions. However, there are some systemic challenges with desalination which stifle its continual progress. One of them being the high energy requirement. The existing desalination technology is primarily run on thermal and electrical energy. Thus, desalination plants are also a big contributor towards emission of greenhouse gases [9]. At present, most desalination plants are based on reverse osmosis (RO) technology, even future plants are expected to be mainly RO based [10, 11]. S. Chakrabarty · A. Rasyotra · A. Thakur · K. Jasuja (B) Discipline of Chemical Engineering, Indian Institute of Technology, Gandhinagar, Gujarat 382355, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Mohanty et al. (eds.), Graphene and its Derivatives (Volume 2), Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-99-4382-1_10

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Fig. 10.1 Desalination capacities a Across the continents b the year-on-year increase. Reprinted with permission from [8]. Copyright Elsevier 2020

In addition to RO based desalination, another popular technique is thermal desalination, which in turn is of three types- vapor compression, multi-effect distillation and multi-stage flash distillation [12]. It is therefore necessary for strategic innovations to make desalination plants more energy efficient and eco-friendly. RO is an energy intensive process, but incorporation of nanoporous membranes promises to be more efficient than existing polymeric membranes, resulting in energy savings [13]. The discovery of graphene and subsequent research efforts in elucidating its myriad properties has provided an impetus in developing next generation membranes for RO applications focused towards desalination. This is due to graphene’s atomic scale thickness which is much smaller than conventional polymeric membranes in use today. Graphene also demonstrates greater resistance to chlorine which would benefit the membranes against rapid fouling [14]. Additionally,

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graphene is also much stronger than polyimide membranes. A derivative of graphene, graphene oxide shows antimicrobial properties, this helps in reducing biofouling of the membranes, increasing the lifetime of the membranes and reducing costs significantly [15]. Also, graphene renders itself well to be processed into membranes for nanofiltration and RO applications. Taking into consideration all these factors, it is clear that graphene is a great candidate for next generation membranes, and can help in bringing down operational costs of desalination processes. Thus, in recent years there has been a concerted effort in investigating graphene and its derivatives towards desalination approaches. In this chapter, we will provide the reader with a comprehensive glance into the research that has gone into realizing graphene and its derivatives based membranes. We will lay out the different facets of graphene that play a role in making it effective for desalination. We will also discuss about the different derivatives of graphene and how they can also be put to good use as desalination membranes. The challenges that are faced in applying graphene membranes in day-to-day usage and how they can be mitigated will also be presented. With this chapter we aspire to contribute to the growing body of literature about graphene membranes. We hope that this chapter enables the reader to understand the current scenario of desalination and how graphene membranes can play a critical role in making desalination more widespread in alleviating the water crisis facing the world.

10.2 Graphene and Its Derivatives The concept of forming a single atom thick layer from graphite known popularly today as “graphene” was first theoretically envisioned by P. R. Wallace in 1947. This term however was introduced in literature in 1986 by Boehm et al., where they combined the term graphite and the suffix -ene, referring to the polycyclic aromatic hydrocarbons [16]. In 2004, Novoselov and Geim first experimentally isolated graphene that led to their Nobel prize in 2010 [17]. Although the advent of graphene started with the experimental realization, the history of graphene derivatives–graphene oxide (GO) and reduced graphene oxide (r-GO) extends to earlier studies on the chemistry of graphene. Brodie et al., in 1859, added a slurry of graphite flakes and potassium chlorate (KClO3 ) in fuming nitric acid. He observed a change in the molecular weight of the graphite crystals formed after the reaction. The as-formed crystals were named “Graphon,” thus marking the first derivative of graphene [18]. Staudenmaier et al. in 1898 further improved the method by adding potassium chlorate in multiple aliquots throughout the reaction, thus limiting the reaction to a single vessel and providing a more practical approach to the synthesis [19]. An alternate method for synthesizing graphene oxide was developed by Hummers and Offeman, where a mixture of potassium permanganate and sulfuric acid was used to achieve the same level of oxidation as Brodie’s method [20]. These three approaches form the primary routes

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of synthesis of graphene oxide, although several modifications have been incorporated in these methods over time. The products synthesized using these methods show strong variance in their properties, depending on the oxidants used, graphite source, and reaction conditions [21]. There has been a debate over the precise structure of GO and all the models proposed are ambiguous. This ambiguity is due to the sample-to-sample variability, amorphous nature, and non-stoichiometric atomic decomposition. Several models proposed for the structure of GO are Hofmann, Ruess, Scholz-Boehm, NakajimoMatsuo, Lerf-Klinowski, and Szabo models. These different structures have been presented in Fig. 10.1 [22]. Hoffman—Holst structure has a net molecular formula of C2O with sp2 hybridization. The structure has epoxy groups on the basal plane of graphite [23]. Ruess altered the structure by introducing hydroxyl groups on the basal plane accounting for the hydrogen content of GO. The structural hybridization was altered to sp3 [24]. Scholz—Boehm, in their model, introduced a corrugated backbone substituted with regular quinoidal species. They discarded the presence of ether and epoxide groups [25]. Another model by Nakajima—Matsuo described a lattice framework forming a stage 2 graphite intercalation compound [26]. However, these lattice-based models were rejected, and the focus shifted to a non-stoichiometric amorphous model. One of the well-known models was presented by Lerf—Klinowski [27]. Lerf—Klinowski model used solid-state nuclear magnetic resonance (NMR) spectroscopy and X-ray diffraction (XRD) to characterize the material. This was the first model that used spectroscopic techniques rather than relying on elemental composition and reactivity to describe the structure [28]. The introduction of defects, impurities, structural disorders, wrinkles, and fragmentations due to the oxidation process affects the adsorption, optical and electronic properties of GO. Thus, the third derivative, r-GO was synthesized using physical or chemical reduction of GO. The reduction process eliminates the oxygenous functional groups thus, forming r-GO with a C/O ratio of 8:1–246:17. In recent years, some other derivatives—Graphyne, Graphdiyne, Graphone, and Graphane have come into the forefront due to the unique properties exhibited by them [29]. Graphyne is an allotrope of carbon having sp and sp2 bonded carbon atoms arranged in a crystal lattice [30]. It drew attention after fullerene was discovered and has been under study since 1980 [29]. Experimentally, it has been synthesized in molecular fragments [31]. It is believed to find use in different applications like transistors, nanofillers, desalination, etc. [32–34]. Graphdiyne, a variant of graphyne, contains two acetylenic linkages in each unit cell rather than one in graphyne. This makes it softer than graphene or graphyne, thus losing exceptional mechanical properties [35, 36]. The application of this material is limited to FET’s, sensors, and transistors [22, 37]. Graphone is another cousin of the graphene family. It is simply a graphene sheet with C2H stoichiometry and 50% hydrogenation. This graphene derivative stems from the fact that although zigzag edges induce magnetism in graphene sheets, they are less favorable than armchair edges that do not exhibit magnetism. To induce magnetism, hydrogenation of graphene is done, resulting in “Graphone” [38]. Although some trials have been

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performed [39], this derivative is yet to be realized experimentally. It finds application in FETs and organic ferroelectrics [40, 41]. Graphane is similar to graphone with a stoichiometry of C2H2, i.e., a graphene sheet that is 100% hydrogenated. However, the structural parameters of graphane and graphone are similar [29, 42]. Graphane finds applications in hydrogen storage [43] and biosensing [44].

10.3 Current Research Trends in Graphene Desalination Membranes Graphene is made up of carbon atoms bonded together in hexagonal configurations [17]. It is both mono- and multilayered, and is so thin that it can be classified as a twodimensional material. Graphene oxide (GO) is a less expensive and easier to make oxidized graphene derivative. Because graphene is hydrophobic and impermeable in water, it cannot be employed as a separating membrane [45]. Owing to the hydrophilic nature of GO, it is used to make nanomembranes that are impermeable to contaminants, salts, and bacteria yet permeable to water [46]. Figure 10.2 shows different arrangements of carbon atoms of graphene and graphene oxide based derivatives. Microfiltration, nanofiltration (NF), reverse osmosis (RO), ion exchange membranes, and forward osmosis (FO), are some of the membrane technologies that are relevant for water filtration and desalination [47]. The most common applications for graphene-based membranes are nanofiltration, reverse osmosis, and forward osmosis. The concept behind using GO to make membranes is to create a very thin, robust, and stable material with tiny, calibrated flow channels to give high water flow rates while collecting pollutants of various sizes down to extremely small. The benefits of graphene come from the potential for an extremely thin membrane and highly regulated patterns of holes with very small sizes and short distances between

Fig. 10.2 Structures of pristine graphene a and its graphene oxide derivativesbased on Hofmann b, Ruess c, Scholz-Boehm d, Nakajima-Matsuo e, Lerf-Klinowski f, and Szabo g models. Reprinted with permission from [7]. Copyright Elsevier 2019

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them. The nanopores in the graphene-based membrane can be engendered using ionimplantation, chemical and laser UV etching [47]. This section is included to examine the research trends in graphene and its derivatives based filtration membranes. The large literature on this is presented in Fig. 10.1, depicting the vast research interest in this domain. According to the existing literature, the boost in high permeability and selectivity established the benefit of graphene-based membranes [47]. This contribution will take a systemic approach to discuss the research trends of graphene-based membranes for water desalination, as demonstrated in Figs. 10.3 and 10.4. The current research focuses on developing membranes with superior permeability-to-selectivity using low-cost and eco-friendly materials. The recent evolution of graphene-based membranes since 2010 has projected to the next generation of porous materials, which significantly contributes to the future of membrane separation applications [48]. The porous structure is contributed by in-plane pores, interplanar spacing, and adjacent space between two layers in graphene-derived membranes [49]. The tailored pore size can be obtained by adjusting the chemical reaction duration, irradiation time and modifying the interlayer spacing with different crosslinking agents. The majority of graphene-based desalination membranes have been demonstrated as nanoporous graphene (NPG) and multilayered graphenederived frameworks [50]. Both forms function as selective layers and molecular sieves, with the size-based exclusion of unwanted solutes. Membranes based on NPG function as selective single-layer nanochannels [48]. The current challenges observed in the past decade in NPG include achieving controlled nanopore fabrication, identifying mass transport mechanisms, and producing stable membranes over a wide range of temperatures. Due to the ultralow

Fig. 10.3 Overview of the research contributions in graphene and its derivatives based filtration membranes. The number of publications from the year 2010 up to the present shows a continuous increase in research interests [Source Clarivate Web of Science]

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Fig. 10.4 Research trends in graphene and its derivatives based filtration membranes

thickness of the membrane, NPG membranes are projected to have a higher coefficient of permeability than current traditional RO membranes, with complete salt rejection for 0.45 nm diameter hydroxylated pores. The ability to make controllable sub-nanometer pores on a pristine graphene layer determines the practical use potential of a single-layer graphene membrane [48]. NPG membranes differ from graphene-based framework membranes due to differences in structure and form and additional water flow mechanisms [48, 50]. Due to the limited practicality of single-layer NPG membranes, multilayer graphene-based membranes were studied. While keeping the desalination qualities of single-layer graphene, it was revealed that multilayer graphene-based membranes might have better features by managing the layer division and offsetting the pores between the layers [50]. The research on graphene framework comprises the study of multi-layered graphene or functionalized derivatives like GO, which have a laminar structure with nanochannels of varying widths and particular surface chemistry, allowing for excellent selectivity and water permeation rate [50]. Recently, multilayer graphene-based membranes have improved desalination performance. The graphene single-layer hybrid membrane containing carbon nanotubes to give mechanical stability is one of the most promising achievements of this decade [50]. Crosslinking, compression, and temperature treatment processes can adjust the interlayer spacing in graphene-based materials, limiting hydrated ion entrance into nanochannel capillaries. The large-scale controlled fabrication of uniform pores and their distribution, at the risk of ineffective ion or pollutant sieving, is a significant problem of existing graphene-derivedmembranes [50]. Many research groups have developed and reported laboratory-scale development of adjustable pore size and controlled sieving. According to research trends, the large-area graphene-derived membranes pave the path for commercial and industrial

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advances. As a result, desalination applications necessitate careful attention to manufacturing tunable pore sizes and their optimization in graphene-derived membranes. Because of this, graphene-based membranes have a long way to go in terms of commercial feasibility [51]. In addition to basic material research, the future advancement of graphene-derived membranes necessitates a more comprehensive approach. Developing innovative desalination techniques is critical for accepting graphene, which would otherwise fail to achieve the desired enhanced cost-to-benefit ratios in industrial processes.

10.4 Applications of Graphene-Based Membranes Towards Water Desalination In the previous sections, we have apprised the reader about graphene and its derivatives and the current direction where research into graphene and its derivatives based membranes are headed. Water pollution is a major challenge facing the world today. Thus, innovations in the field of water treatment have gained momentum. One particular topic of interest is that of desalination. It is estimated that close to 4 billion people would be adversely affected due to growing water shortages all over the world [52], thus desalination can go a long way in alleviating the looming water shortage crisis. Graphene and its derivatives possess certain unique qualities which render them suitable towards desalination approaches- among these qualities, atomic scale thickness and nanopores within the 2D graphene channel allow high fluid permeability, and size-selective transport thus making graphene membranes cost/energy efficient [53, 54]. This section will elucidate on how graphene and its derivatives based membranes have been utilized for desalination efforts.

10.4.1 Graphene Monolayers as Desalination Membranes For the fabrication of advanced desalination membranes graphene monolayers as well as stacked multilayers of graphene can be used. A monolayer of graphene i.e., pristine graphene is considered as impermeable. This is on account of the formation of a dense and delocalized electron cloud from the π-orbitals which block the void within the aromatic rings of graphene [55, 56]. Simulations and experimental studies however show that incorporating nanopores into graphene sheets can overcome the impermeability issue. This allows graphene to be used as size-selective membranes [57]. In addition, electrical conductivity for graphene can reach values of up to 200 sm−1 which is a critical factor in desalination approaches. Figure 10.1 shows how nanopores can be beneficial in enhancing the selectivity and permeability [58]. Figure 10.5 shows a graphical representation of

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Fig. 10.5 A representative schematic showing how nanopores in graphene sheets help in filtering water permeability and effective rejection of salt molecules. Reprinted from [58]

graphene sheets effectively rejecting salt molecules, this can be utilized for filtration purposes. Improving graphene membrane properties relies on the addition of pores with specific sizes, functionalities and densities [59]. In one of the first studies of graphene as a sweater desalination membrane, 50 mg of functionalized hydrogen exfoliated graphene sheets were used as electrodes. When compared with non-functionalized electrodes, a marked improvement was noticed. For example, there was a 11% increase in arsenic as well as sodium removal capacities [60]. Molecular dynamics studies have shown that nanoporous graphene has a similar NaCl rejection and water flow rate when compared to conventional reverse osmosis membranes [61]. The ultrahigh water permeability of graphene is due to its atomic thickness, this is advantageous as this reduces the initial capital investment as well as the operating costs of desalination plants. The mechanism of salt rejection by monolayer graphene membranes as suggested by Thomas et al. [62] has six different facets- (1) size exclusion; (2) dehydration effects; (3) charge repulsion; (4) specific pore interactions; (5) solute interaction with pores; (6) entropic differences. A brief overview of these mechanisms indicates that the first two are important for salt rejection [14], as the diameter of salt ions is greater than the diameter of water molecules. It was found that charge repulsion also plays an important role in rejection. Presence of negatively charged functionalities at the nanopore aids in inhibiting the flux of ions such as Cl− and thus resulting in increased efficiency of the membrane [63]. Pore/solute interactions which is the fourth mechanism can notably affect the ion selectivity through the routes of over coordination and undercoordination and are correlated also with the nanopore morphology [64]. The chemical structure of the pores and their interaction with specific ions is at the core of the fifth mechanism. Studies have found that negatively charged pores facilitate greater permeation of cations. Whereas positively charged groups at the nanopores ease the passage of anions [15, 65]. The sixth and the final mechanism involves entropic differences. The free energy barrier through each nanopore plays

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a crucial role in salt rejection. Monolayer graphene can limit the possible orientations or arrangements(geometric) by which salt ions can pass through the nanopores, thereby aiding in desalinating water [64].

10.4.2 Hurdles to Overcome In the pursuit of achieving a greater degree of desalination, graphene membranes have shown stellar activity. But implementing these membranes comes with a host of challenges. Most prominent among them are- (1) production of large area as well as low-cost monolayer graphene; (2) creation and control over the formation of nanopores on graphene sheets and tuning of their size and functionalities; (3) Controlling the intrinsic and extrinsic defects that arise during the growth and transfer of graphene sheets. Researchers are constantly working to overcome these challenges. To deal with the large-scale production of graphene, one method that has gained attention is the growth of graphene on flexible copper foils by a chemical vapor deposition process at atmospheric pressure [66]. Through this method, a roll-to-roll transfer of 30-inch graphene has been realized. Pores within graphene sheets can be both intrinsically formed during the growth process or extrinsic pores incorporated via other means. In a study where graphene was grown with intrinsic pores [67]. The diffusion of molecules was studied and it was found that there was good selectivity for molecular transport. Although, this method still suffers from control over the uniformity of pore size distribution. Extrinsically pores can be included by processes such as hydrogen/ oxygen plasma etching [51, 68]. But to have even greater control over the size of the nanopores it was shown that ion and electron bombardment is more effective [69, 70]. Defects within monolayer graphene can inhibit the permeation of molecules, thus defects need to be sealed. One of the ways to achieve this has been to selectively fill the graphene monolayers with hafnia via atomic layer deposition [71]. Larger defects can be sealed by depositing nylon 6,6 [67].

10.4.2.1

Multilayer Graphene as Desalination Membranes

Monolayer graphene even with its advantages has obstacles in the way of largescale fabrication and processing. Stacked graphene oxide (GO) membranes are an attractive candidate in overcoming this very challenge. Stacked GO membranes are held together by hydrogen bonds and impart a greater degree of durability thus resulting in stable free-standing membranes [72]. Another lucrative trait of GO is that it can be easily synthesized via low-cost techniques such as ultrasonication and also possess greater thermal and chemical stability [73]. GO enables fast permeation, due to the increased capillary action that acts strongly on the ions inside graphene nanochannels. As a whole too, water flows faster through the entire GO membrane. This behavior is attributed to a few factors, namely the

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porous structure of the membrane, open spaces between adjacent GO sheets, and wrinkles and holes in the sheets [57]. Permeability of GO membranes is also affected by the thickness. Due to longer channel tortuosity, a thicker GO membrane will suffer from a low permeability [74]. The primary separation mechanism by which GO membranes bring about desalination is considered to be size exclusion [75]. Size exclusion to a great extent is governed by the interlayer spacing between GO layers. The presence of oxy functional groups increases the spacing between the sheets when compared to graphite [76]. The oxygen groups tend to cluster together and this leads to the presence of nonoxidized graphene regions. Thus, there will be the presence of pristine regions as well as oxidized domains within the GO sheets. It has been found that the interlayer spacing is greatest at the pristine regions, here the molecular transport is enhanced. The spacing is decreased at the interface between the pristine and the oxidized domains and here the transport is markedly reduced [77, 78]. It is also believed that along with size exclusion mechanism, electrostatic interactions between GO sheets and ions in water can also bring about separation. In a relevant study, it was determined that OH− ions interact with oxy functional groups such as carboxyl groups, and highly ionizing them. This leads to an increase in the electrostatic repulsion between the GO sheets which also increases the interlayer spacing. Ions therefore can permeate faster within the membrane [79]. Transition metal cations and alkali metal cations can be effectively adsorbed on to GO sheets through coordination bonds to the sp3 clusters for transition metal ions and π-interactions with sp2 clusters for alkali metal ions [80]. Figure 10.6 illustrates the different mechanisms of separation in mono and multilayer graphene [81].

Fig. 10.6 Schematic representing the separation mechanism of a monolayer graphene b multilayer graphene. Reprinted with permission from [81]. Copyright 2016 The Royal Society of Chemistry

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Hurdles to Overcome

As with monolayer graphene, so too with multilayer graphene, there are some inherent challenges that need to be addressed before utilizing these membranes in practical scenarios. These include- (1) Swelling (2) Water instability (3) low mechanical stability (4) Desalination capacity. Hydration induced swelling of GO sheets hampers the permeability. In the dry state of GO membranes, it is reported that the void spacing is around 0.3 nm [77]. This spacing is increased to about 0.9 nm when the GO is immersed in an ionic solution [75]. Thus, species with an ionic radius greater than 0.45 nm cannot permeate through the GO nanochannels. This swelling is to be controlled if effective desalination is to be achieved. Few strategies which can be used to overcome this is to partially reduce GO to shrink the hydrated functional groups and also covalently bonding GO sheets with tiny molecules which can resist hydration force [82, 83]. The low stability of GO in water is due to the presence of hydrated functional groups. The water molecules intercalate within the GO sheets rather than permeating through. This is yet another challenge that needs to be sorted. Chemical crosslinking or reduction can impart greater degree of stability to GO sheets [84]. GO sheets suffer from low durability and mechanical stability, overcoming this challenge requires GO sheets to be suitably supported. Researchers have shown that when ultrathin GO membranes are deposited on to microporous substrates such as commercially available polyvinylidene fluoride, the composite was able to withstand pressure of up to 20 bars [85]. Even after presenting themselves as favorable candidates for desalination membranes, multilayer GO membranes’ desalination capacity needs to be enhanced further for them to be commercially viable. In that regard, careful engineering of the membrane’s surface charge is required. If done correctly, it may enhance the charge selectivity close to 96% [54], while not sacrificing the superior water flux.

10.4.3 Comparison of Desalination Capacities of Different Graphene Derivatives The previous sections have in detail tried to present a general overview of graphenebased desalination membranes. Monolayer as well as multilayer graphene have both been investigated as possible desalination membranes. Both of these structures have their unique set of advantages as well as challenges. It is thus important to have an objective summary about the performance of these nanomaterials. A recent meta-analysis [86], has attempted to ascertain the efficiency of graphene and its derivatives membranes for water desalination. The study found that the effective salt rejection rate (%SAR) of graphene-based nanomaterials is in the range

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Fig. 10.7 Salt rejection efficiency as a function of the employed technology. a Graphene b Graphene oxide c Reduced graphene oxide. Adapted with permission from [86]. Copyright 2021 Elsevier

of 83.04%. The %SAR for desalination techniques where graphene and its derivatives have been applied were also analyzed and the percentages were as follows100%(distillation), 99.8%(pervaporation), and 55%(adsorption). The main findings from the analysis are summarized in Fig. 10.7.

10.4.4 Current Status of Graphene Membranes in Mainstream Desalination Processes In the preceding sections, we’ve tried to present a bird’s eye view of the research going into different graphene derivatives as potential next generation membranes. The process which is adopted for achieving desalination is equally important as that of the properties of the membranes. In this section, we will present a brief glance into the various desalination approaches where graphene-based membranes may find application. These processes are mainly categorized into- (1) Reverse osmosis, (2) Forward osmosis (3) Distillation & pervaporation, (4) Solar driven and other approaches.

10.4.4.1

Reverse Osmosis

Reverse osmosis as it stands in current times is the most dominant form of membrane process, it boasts a market share of 55% as of 2019 [87]. Yet, there isn’t a single commercial reverse osmosis product available in the market. An effective membrane must have high selectivity and permittivity, as well as be cheap, stable and resistant to fouling. This is where graphene-based membranes hit a major obstacle. Even with the advances in synthesis technologies, fabrication of pristine graphene at an industrial scale without introducing any morphological defects has proven to be almost insurmountable. Another factor is that reverse osmosis is currently reaching its thermodynamic limit with regard to energy consumption, thus further reduction of consumption using high permeability membranes is very difficult [88].

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Forward Osmosis

Graphene-based membranes have found good application in forward osmosis processes. In forward osmosis separation is driven by an osmotic pressure across a semi-permeable membrane. Various types of membranes have been realized for forward osmosis. These can be classified into different types such as blended membranes, lamellar membranes, and surface modified membranes. These are always used with porous supports. In some cases, the graphene or its derivatives are blended directly into the supports. Graphene flakes are arranged on top of each other in lamellar type of membranes. In the surface modified membranes, graphene flakes are incorporated into an active layer (such as polyimide) [61, 87].

10.4.4.3

Distillation and Pervaporation

Membrane distillation relies on purifying water by a vapor pressure gradient acting transversely through a porous and hydrophobic membrane. Recently graphene and its derivatives based membranes have seen application in membrane distillation schemes. The technology claims to purify water obtained from seas and oceans as well as industrial effluents [89]. In addition to membrane distillation pervaporation techniques are also used and this technique can also benefit from the use of graphene-based membranes. During pervaporation one of the membrane faces is in direct contact with the feed, while the other side is exposed to a vacuum where the separation of the permeate takes place.

10.4.4.4

Solar Driven and Other Desalination Approaches

Desalination is an energy intensive process. Thus, the energy source should also be taken into account before designing any scheme. Solar energy can prove to be a good candidate in this regard. Solar energy combined with graphene membranes consisting of oxidized and pristine regions have shown to accelerate water evaporation [90, 91]. A different approach namely membrane crystallization is gaining traction as an alternative desalination scheme. Here, crystal formation is sought from very highly saline feeds. A hydrophobic membrane is then necessary for separating the water [92]. Membranes based on graphene and polyvinylidene fluoride composites have already been put forward as synergistic interfaces driving membrane crystallization processes with greater efficiency [93]. In Table 10.1, we provide the reader with a brief overview of the desalination performance of various graphene-based membranes compared with some other nanomaterials.

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Table 10.1 Desalination performance of graphene-based membranes compared to other nanomaterials Membrane type

Salt rejection (%)

Water permeability

Stability

Reference

Graphene monolayer

100

106 g m−2 s−1

Stable in air

[51, 94]

Multilayer graphene

100

2 L m−2 h−1 bar−1

Stable in air

[14]

Graphyne sheet

>80

13 L cm−2 d−1 Mpa−1 Good stability

[47, 95]

MoS2 monolayer

>88

Higher than graphene monolayers

Thermal stability is good

[96]

Boron nitride nanotubes 100

0.9268 L/m2 /h

Thermal stability is good

[97]

h-Boron nitride

100

Higher than RO processes

Chemically inert

[98]

Carbon nanotubes

100

23.6 molecules/ns

Thermal stability is good

[99]

Crosslinked Mxene (Ti3 C2 Tx ) membranes

98

0.5 L m−2 h−1 bar−1

Excellent stability

[100]

10.5 Outlook In this book chapter, we examined the accomplishments of graphene-based filtration membranes for desalination. Graphene-based membranes have become essential for the water treatment process due to growing interest and research outcomes. From fabrication processes to functionalization and hybridization strategies, we highlight diverse approaches in developing graphene-based membranes. With his chapter, we aspire to contribute to the growing body of literature about graphene-derived desalination/filtration membranes. This contribution presents a systematic approach to discuss the research trends of graphene-based membranes for water desalination. The separation mechanisms of graphene-based membranes have made significant progress, and their considerable promise for desalination applications has been established. However, there are a few most critical obstacles to overcome before graphene-based membranes may be fully commercialized. Firstly, the long-term stability of graphene-based membranes is a severe challenge. Secondly, it is challenging to stabilize the practical separation of graphene-based membranes in liquid separations. Developing a novel approach for the simple synthesis of graphenebased membranes that is both economical and large-scale production is needed. It is necessary to understand better structural qualities and methods for the removal of water contaminants. To avoid environmental stress, the graphene membrane’s toxicity should also be properly investigated. More attention should be devoted to water

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permeability and membrane selectivity in long-term operation to expand the applications of graphene membranes. To fully realize the potential of graphene materials, a thorough understanding of graphene-based membranes is required.

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Chapter 11

Graphene Nanoparticles and Their Derivatives for Oil Spill Treatment Rupali Gautam, Abhisek Sahoo, Kamal K. Pant, and Kaustubha Mohanty

11.1 Introduction 11.1.1 Nanotechnology The world population is rapidly increasing, and so is growing the world’s reliance on technology. Technologists and researchers around the globe are in search of more innovative, efficient, and better materials. Here Nanotechnology comes into play. Nanotechnology is a stream that has the potential to revolutionize the way the world functions. The origin of nanotechnology can be traced back to the lecture by physicist Richard P. Feynman ‘There is Plenty of Room at the Bottom’ in 1959 [1]. It started taking concrete form in the early 1980s and gained popularity with the book ‘Engines of Creation: The Coming Era of Nanotechnology [2]. Nanotechnology is a domain that deals with nanomaterials. A nanomaterial is any particle that has at least one dimension of it in the range of 1–100 nm (10−9 m or one billionth of a meter) length scale [3]. Nanomaterials offer a tiny size and high surface area to volume ratio, which possesses unique physio-chemical properties. Additionally, materials’ mechanical, thermal, optical, electrical, and magnetic properties also get exceptionally modified at the nanoscale mainly because of the quantum effect [4, 5]. A nanomaterial can be zero-dimensional, one-dimensional, two-dimensional, or three-dimensional [6]. A zero-dimensional nanomaterial has all three dimensions R. Gautam Upstream and Wax Rheology Division, CSIR–Indian Institute of Petroleum, Dehradun 248005, India A. Sahoo · K. K. Pant Department of Chemical Engineering, Indian Institute of Technology, Delhi 110016, India K. Mohanty (B) Department of Chemical Engineering, Indian Institute of Technology, Guwahati 781039, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Mohanty et al. (eds.), Graphene and its Derivatives (Volume 2), Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-99-4382-1_11

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Fig. 11.1 Types of nanomaterials based on dimensions

in the 1–100 nm range. This includes quantum dots, Fullerenes, atomic clusters, etc. [7]. A one-dimensional nanomaterial has two dimensions in the range of 1–100 nm, while one dimension can exceed this range. Its example includes nanotubes and nanowires [8]. A two-dimensional nanomaterial has one dimension in the 1–100 nm range, while the other two dimensions can exceed this range. Its example includes Nanosheets and thin films [9]. Above this comes the three-dimensional materials where all three dimensions are above 100 nm scale. In carbon nanomaterials, buckminsterfullerene is a zero-dimensional nanomaterial, while carbon nanotube is onedimensional. Graphene is an example of a two-dimensional nanomaterial. Types of nanomaterials based on dimensionality with their common examples are shown in Fig. 11.1.

11.1.2 Origin of Graphene—A Miraculous Material Graphene is a carbon allotrope with a single layer of carbon atoms arranged in a honeycomb lattice. It is a two-dimensional nanostructure of graphite. The atoms in a graphene sheet are bonded with sp2 bonds, while in the case of graphite, graphene sheets are connected by weak Van der Waals force. Upon exfoliation of graphite, the Van der Waals force is overcome, and graphene is obtained. In general, a carbon

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atom is a diameter of 0.33 nm. This means a 1 mm thick sheet of graphite has approximately 3 million layers of graphene in it [10]. The discovery of graphene can be traced back to 2004, when Professor Andre Geim and Professor Kostya Novoselov from the University of Manchester extracted graphene from graphite using sticky tapes. This work won the Nobel Prize in Physics. At present, there are several methods and approaches to producing graphene nanomaterial. These are discussed in Sect. 11.2 of the chapter.

11.1.3 Properties of Graphene Nanomaterial Materials at the nanoscale show remarkably different properties compared to their bulk counterparts. This fact also holds for graphene nanomaterials. Graphene shows astonishing properties, listed in Table 11.1, different from any other material presently known and is popularly known as a wonder material. It is the most robust material presently known to humanity.

11.1.3.1

Physio-Chemical Properties

Graphene generally has a black appearance; however, a single layer of this nanostructure looks transparent. It is visible to the naked eye. With the thickness of one Table 11.1 Different properties of graphene nanomaterials S No

Properties

Parameters

Value

01

Physio-chemical properties

Color

Black

Structure

Single layer

Visibility

Transparent, visible to naked eyes

Thickness

One atom thick, thinnest

Chemical bond

SP2 π bonds

Chemical activity

Chemically inert, non-toxic

Nature

Lightest and strongest

Intrinsic strength

130 GPa

Young’s modulus

1 ± 0.1 TPa

Critical stress intensity factor

4.0 ± 0.6 MPa

Electron mobility

200, 000 cm2 /Vs

Electrical conductivity

106 S/m

Transparency

97.7%

Thermal conductivity

5 × 103 W/mK

02

03

04

Mechanical properties

Electro-optical properties

Thermal properties

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atom, it is the thinnest compound known to man. It has a sheet-like structure where atoms in one layer are attached with strong SP2 π bonds while two adjacent layers are attracted to each other by van der Waals force. It is chemically inert and nontoxic by nature. It has a high adsorption capacity. Its derivatives, such as graphene oxide, reduced graphene oxide, and other functionalized structures, can also interact with foreign molecules through hydrogen bonding, π- π interaction, van der Waal interaction or electrostatic force, etc., thus making them able to interact with a vast number of molecules in different applications [11].

11.1.3.2

Mechanical Properties

One square meter of graphene weighs around 0.77 mg, making it the lightest material on Earth. Even though it is the lightest and thinnest material, it is at the same time also the most pungent compound known. It is 200 to 300 times stronger than steel. It has intrinsic strength of 130 GPa and Young’s modulus of 1 ± 0.1 TPa. Its toughness is determined by the critical stress intensity factor, which is 4.0 ± 0.6 MPa [12].

11.1.3.3

Electro-optical Properties

Graphene is the best conductor of electricity at room temperature. It is a zero-band gap semimetal. Due to its 2D structure, each carbon atom in graphene connects to the other three atoms of the same plane. In the third dimension, one electron, called the pi electron, is free and is available for conduction. Thus, a layer of pi electrons is present below and above each graphene sheet that is highly mobile and supports electronic conduction. It has electron mobility higher than 200,000 cm2 /Vs and electrical conductivity of 106 S/m [13]. Graphene adsorbs just 2.3% of visible light and has transparency as high as 97.7% [14].

11.1.3.4

Thermal Properties

Graphene is also the best conductor of heat at room temperature. It is 10 times more conductive than copper. It shows thermal conductivity of 5 × 103 W/m K [15, 16].

11.1.4 Derivatives of Graphene Nanoparticles 11.1.4.1

Graphene Oxide Nanoparticles

Graphene oxide (GO) nanoparticles are an oxidized derivative of graphene. Figure 11.2 shows the structure of graphene and graphene oxide nanomaterial. GO is mono-layered graphite with a scattered aromatic region (sp2 bonds) and oxygenated

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aliphatic regions (sp3 bonds). GO nanoparticle is a highly functionalized nanomaterial consisting of different functional groups such as hydroxyl, carboxyl, epoxy, and carbonyl. The presence of oxygen explains its hydrophilic thus making it watersoluble. The magnitude of hydrophilicity depends on the level of oxidation. GO nano-sheets have a specific surface area of nearly 890 m2 g-1 , Young’s modulus of 207.6 ± 23.4 GPa, and fracture strength of ~120 MPa. Thus, it exhibits excellent mechanical properties.

11.1.4.2

Reduced Graphene Oxide Nanoparticles

One way to achieve tailor-made graphene-like performance is to reduce the graphene oxide. Figure 11.2 shows the structure of reduced graphene oxide (rGO) synthesized by reducing the graphene oxide nanomaterials. Here, some of the various functional groups present in GO are eliminated through chemical, thermal, or photo-thermal reduction method. After the elimination, the carbon-to-oxygen ratio of the product is evaluated. A higher carbon/oxygen ratio value implies a better reduction of graphene oxide. It also talks about how close the properties of the product of reduced GO to that of pure graphene will be. The higher the C/O value, the more the product will resemble graphene. Although it does not achieve an entirely pristine graphene structure, it has its application and advantages and is a popular choice in the industry. The properties of rGO change remarkably compared to that of GO. First, it turns hydrophobic because of the increase in the C/O ratio. Its surface area also increases. rGO has over 320 cm2 V−1 s−1 of mobility and over 6000 S cm−1 of electrical conductivity. With breaking strength crossing 130 GPa and Young’s modulus of about 1 TPa, it also shows strong mechanical properties.

11.1.4.3

Carbon Nanotubes (CNT)

Carbon nanotubes, also known as graphene nanotubes (GNT), are tubes made by rolling a layer of graphene sheet into a hollow cylindrical structure. A single layer of graphene seamlessly rolled into a tube makes the single-walled carbon nanotubes (SWCNTs). Multi-walled carbon nanotubes (MWCNTs) are structures with multiple layers of graphene seamlessly rolled into a concentric tubular structure. In other words, it is formed when more than one SWCNT is arranged one into the other.

Fig. 11.2 Structure of graphene, graphene oxide, and reduced graphene oxide nanomaterials

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Fig. 11.3 Structure of single-walled and multi-walled carbon nanotubes

Table 11.2 Properties of single-walled and multi-walled carbon nanotubes Nanotube

Physical properties

Mechanical properties

Electronic properties

Single-walled carbon nanotube

Rolled single layer graphene

Elastic modulus: 1000–3000 GPa

Thermal conductivity: 3000–6000 W/(m·K)

Typical diameter: 1–2 nm

Tensile strength: 50–100 GPa

Rolled two or more layers of graphene

Elastic modulus: 300–1000 GPa

Typical diameter: 7–100 nm

Tensile strength: 10–50 GPa

Multi-walled carbon nanotube

2000–3000 W/(m·K)

Figure 11.3 shows the structure of SWCNT and MWCNT. Both have distinct properties, as summarized in Table 11.2.

11.1.4.4

Other Derivatives

Apart from the above-mentioned common derivatives, graphene has other derivatives such as graphene aerogels, graphene-CNT hybrid aerogel, graphene polymer

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composites, carbon nanoribbons, etc. Derivatives of graphene can be tailor-made and functionalized to suit specific requirements.

11.2 Approach and Methods of Synthesis For the first time, graphene was obtained through multiple exfoliations of graphite using sticky tapes. The graphene thus obtained is of the highest quality; however, it is not considered suitable for bulk production. For mass production, Graphene nanoparticles can be exfoliated, synthesized, or fabricated by several techniques. These techniques can be broadly segregated into top-down and bottom-up approaches (Fig. 11.4). A top-down approach means nanoparticles are etched, exfoliated, crushed, or broken down from their bulk counterparts. On the other hand, the bottom-up approach is the reverse of the above process. Here nanoparticles are made from the growth of building blocks such as atoms, molecules, or clusters. Mechanical exfoliation, chemical exfoliation, etc., are examples of the top-down approach to graphene synthesis, while chemical vapor deposition, epitaxial growth, etc., are examples of the bottom-up approach. Figure 11.5 shows different methods of graphene nanomaterial synthesis categorized based on the approach adopted.

Fig. 11.4 Top-down and Bottom-up approaches to graphene nanomaterial synthesis

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Fig. 11.5 Different methods of graphene nanomaterial synthesis

Graphene synthesis Bottom up approach

Top down approach Micro-mechanical exfoliation

Chemical method

Chemical Vapour Deposition

Arc discharge

Ultra-sonication

Thermal

Ball milling

Plasma

Electrochemical exfoliation

Thermal Exfoliation

super acid dissolution

Supercritical fluid exfoliation

11.2.1 The Top-Down Approach to Graphene Nanoparticles Synthesis There are several methods for the synthesis of graphene nanoparticles that adopt the top-down approach. These have been discussed in detail below.

11.2.1.1

Micro-mechanical Exfoliation

In this method, external stress, longitudinal or transverse, is applied to the Highly Ordered Pyrolytic Graphite (HOPG) structure. Since graphite is a bulk form of multiple graphene layers stacked together with weak van der Waals force, the external stress with magnitude ∼ 300 nM/μm2 cleaves them into single mono-atomic graphene layers. Commonly used agents for cleaving are scotch tape, ultra-sharp single-crystal diamond wedges, etc., [17].

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Ultra-Sonication

Production of graphene by sonication through liquid phase exfoliation of graphite has made large-scale production possible. A range of solvents can be used for the dispersion of powder graphite, after which sonication and centrifugation are performed. This includes organic solvents, ionic liquids, aqueous surfactants, etc., [18]. The method results in high yield as well as high-quality graphene.

11.2.1.3

Ball Milling

Ball milling is a conventional method for nanoparticle production. It breaks graphite into graphene nanoparticle flakes using shear force. It is based on the mechanism of rotation of the ball mill pot. The grinding balls inside the ball mill pot and graphite particles roll at high speed inside the tank under a strong centrifugal force, thus exfoliating the graphite into graphene. There are two ways the milling can be performed– wet ball milling and dry ball milling. Wet ball milling uses an organic solvent as a medium, while dry ball milling uses chemically inert water-soluble inorganic salts.

11.2.1.4

Electrochemical Exfoliation

Electrochemical exfoliation is a promising method for the mass production of graphene and is gaining massive attention from the scientific community and industry nowadays. This method produces graphene nanoparticles by exfoliating graphite electrodes using an applied voltage to increase the interlayer distance [19]. This method can give a high yield and evenly formed graphene particles and be scaled up for mass production.

11.2.1.5

Other Methods of the Top-Down Approach

Apart from the significant methods mentioned above, several other methods convert graphite into graphene nanoparticles. Thermal Exfoliation uses thermal energy to exfoliate graphite into graphene, while the super acid dissolution technique uses strong acids to do the same. Another method is the transfer printing technique. Supercritical fluid exfoliation uses fluids such as carbon dioxide, ethanol, etc., at temperatures and pressures above the critical point to exfoliate graphite.

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11.2.2 The Bottom-Up Approach to Graphene Nanoparticles Synthesis Under the bottom-up approach, the graphene nanoparticles are synthesized from the molecular level. Chemical reaction among reactants results in graphene synthesis. These methods require high sophistication but produce high-quality, uniform graphene.

11.2.2.1

Chemical Method

Chemical synthesis is different from chemical exfoliation. In the chemical synthesis method, a dispersion of graphite oxide is synthesized through graphite oxidation which is followed by reduction by hydrazine. The popularly used methods for graphite oxidation are Hummer’s method, Brodie’s method, and Staudenmaier’s method. The graphene oxide reduction is carried out mainly through chemical reagent reduction, thermal reduction, and electric reduction.

11.2.2.2

Chemical Vapor Deposition

CVD is the technique of depositing material as a thin film onto substrates from vapor species through chemical reactions under certain temperatures, pressure, and gas flow. This method is a highly reliable method for producing high-quality mono or few-layer graphene on a large-scale. CVD includes a quartz reaction chamber, thermocouples, a mass flow controller, a pump, a vacuum system, a gas delivery system, an energy system, and a computer. Many substrates can be used for growing a thin graphene film on them, such as Copper (Cu), Iron (Fe), Nickel (Ni), and Stainless steel. Methane (CH4 ) and acetylene (C2 H2 ) are carbon sources. The most commonly used CVD is thermal CVD and plasma-enhanced CVD (PECVD). While thermal CVD requires a high temperature to decompose carbon sources to form graphene, PECVD plasma decomposes the source at a low temperature [20].

11.2.2.3

Arc Discharge

Another method to produce graphene from the bottom-up approach is the arc discharge method under hydrogen, helium, nitrogen, or carbon dioxide conditions. This method results in graphene nanoparticles with fewer defects than chemical methods. The different graphene derivatives, such as GO, SWCNT, MWCNT, etc., are synthesized from both top-down and bottom-up approaches. The widely used

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methods include arc discharge, chemical vapor deposition, laser ablation, and chemical synthesis. CVD is the most popular among them. The yield, properties, structure, size, and performance of the product depend highly on the method and conditions employed for their synthesis.

11.3 Oil Spill and Its Management 11.3.1 The Havoc of Oil Spill The oil spill is a type of pollution that often becomes a hazard. It is used to release a liquid petroleum hydrocarbon into the environment due to anthropogenic activities. The term is most used concerning the hydrocarbon released in marine water, mainly during crude oil production (drilling rigs, offshore storage, separation units, etc.), transportation of petroleum, ship dismantling, deliberate dumping, wars, or accident. However, it is not limited to the marine only. It also incorporates the release of liquid petroleum in inland water, coast, and land. In some rare cases, it can also occur when some natural disaster, such as a tsunami, earthquake, volcano, etc., hits crude oil production, storage, or transportation facility. In any case, the oil spill has the potential to cause huge damage to human existence and the environment. Its severity of damage can vary from low to very severe and hazardous.

11.3.2 Damage Caused by an Oil Spill The impact of an oil spill is devastating. It ruins the affected zone, claim lives, and shatters economic and ecological balance. The destruction caused is interlinked as one damage leads to another and magnifies the overall harm. For the sake of easy understanding, the damage caused by the oil spill can be arranged into four parts (Fig. 11.6). These are (i) the effect on the environment, (ii) the effect on flora fauna, (iii) the effect on humans, and (iv) economic damage. The details of the damage are listed in Table 11.3.

11.3.2.1

Environmental Damages

At the very first, oil spill causes pollution of all type. It damages shores, beaches, wetlands, marshlands, as well as inland. It creates a layer of petroleum on the ocean, causing marine pollution and toxicity. It completely covers the coast and nearby areas, causing soil pollution. Air quality is reduced due to fire, particulate matter, volatile chemicals, and fouling smell. Oil leaches out into the soil and contaminates water. Several vulnerable species get badly hit, and the entire ecosystem gets disturbed.

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Fig. 11.6 Harmful effects of oil spill

Table 11.3 Damage caused by an oil spill on the environment, humans, flora fauna Sl. No

Category

Damage caused

1

Environmental

i. Pollution ii. Complete damage to the ecosystem iii. Destruction of habitat and breeding grounds

2

Human

i. Fire hazard ii. Respiratory distress, foul smell iii. Contamination of drinking water iv. Toxicity

3

Flora fauna

i. Hypothermia ii. Toxicity to marine fish and other species iii. Inability to fly iv. Inability to see or smell, thus hindering several crucial functions

4

Economic

i. Oil wastage ii. Energy crisis iii. Expense in cleaning iv. Damage to infrastructure v. Impact on tourism vi. Impact on marine resource extraction industries vii. Fisheries viii. Compensation to victims

Bacteria that consume oil occur naturally and try to degrade the spilled oil. Since the oil is in massive amounts, these bacteria will grow enormously and replace other food web populations. Oil spills damage the mating and breeding grounds of several species and makes the place an unsuitable habitat. The oil layer formed over water restricts the entry of sunlight and reduces dissolved oxygen levels; thus, marine and aquatic ecosystems get disrupted.

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Impact on Flora Fauna

Oil spills seriously threaten the existence of flora and fauna. It causes hypothermia in birds and mammals with fur. The spilled oil covers and penetrates their plumage and fur. Oil reduces their insulating ability, makes them vulnerable to temperature fluctuation, restricts their buoyancy, chocks their mobility, and does not allow them to fly. Additionally, neither can they move to find their food nor make sufficient movement to escape their predator, thus causing death. Oil spill causes toxicity to marine and coastal life, especially fishes. Ingestion of oil damages their vital organs, causing severe dehydration and metabolic and hormonal imbalance. Due to the strong foul smell of petroleum, animals and fishes who rely on scent to navigate or find their babies cannot do so. Oil spill completely coats the plants on the shore and coast and destroys them. As it also contaminates soil and seeps to contaminate water, it damages plants even farther from where the spill occurred.

11.3.2.3

Impact on Humans’ Life and Safety

Oil spills represent an immediate fire hazard. It poses a severe safety threat and causes the loss of human lives. It also damages the infrastructure facilities such as drilling rigs, storage, separation facility, transportation facility, etc. Besides these catastrophic losses, it also induces air pollution and foul smell, thus causing respiratory distress. The petroleum components are volatile and immediately start reacting with air. They make the entire surroundings toxic. This induces a metabolic and hormonal imbalance in humans and damages the lungs, kidneys, digestive tracts, etc. Spilled oil can also contaminate drinking water supplies. They also form fine particulate matter after they evaporate into the atmosphere. These particulates can penetrate the lungs and carry toxic chemicals into the human body.

11.3.2.4

Economic Damage

An oil spill is a cause of enormous economic loss to the industry as well as the globe. The quantity of spilled oil during any oil spill incident can range from a few thousand gallons to several hundred million gallons. The total volume of oil spills was 400 million gallons in the Gulf War oil spill in 1991, 210 million gallons in the Deepwater Horizon oil spill in 2010, and 150 million gallons in Ixtoc 1 oil spill in 1979. Disastrous oil spills have continued even in recent years. In May 2020, 13,090,909 gallons of oil spilled in the Norilsk region in Russia [21]. When the Shanghai Sanchi oil tanker collision happened in 2018, it carried 101,735,065 gallons of condensate and 1,496,103 gallons of fuel oil [22]. Almost all of it leaked into the ocean, and all its crew members died, causing severe damage. Thus, a tremendous amount of petroleum is lost annually in oil spills, which means energy sources and several products derived from petroleum are lost. This hits the world severely in terms of economics.

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Oil spill incidents also impact the tourism and marine resource extraction industries. The fishing industry faces complete disruption for several months to even years. In addition, the victim of the incident needs to be compensated for their loss of life, property, and occupation. This poses an additional economic burden on the company or the government.

11.3.3 Oil Spill Management The extent of an oil spill can be huge, and its clean-up is arduous and expensive. In addition, the clean-up can take months or years, and even after many years of effort, the environment is not restored to its earlier condition. The environment is harmed, fragile ecosystems get disturbed, and some species become utterly extinct in such incidents while others struggle for survival. The loss of human life and the economy is also irreversible. It is hence necessary to manage and mitigate oil spills. The best way to manage this disaster is to avoid it from occurring. However, whenever occurred, it demands a prompt clean-up action that is quick, effective, and economical.

11.3.3.1

Traditional Methods of Oil Spill Clean-Up

Clean-up of an oil spill is difficult and tedious. Even a rigorous clean-up process can take weeks or months to clean the spill. Selection of the process and its success depends upon many factors, such as the quantity and nature of spilled oil, the water temperature, movement in the water, weather conditions, and location of the spill (mid-ocean, inland water, shore, beach, wetland, or marshland). Several methods are used for oil spill clean-up. These are summarized in Table 11.4.

Physical Methods This approach uses physical techniques to clean up the oil spill. Booms are temporary floating barriers used to keep the oil from dispersing in a vast area. Skimmers are used to collect the floating oil. The most accepted technique is the use of adsorbents. Figure 11.7 shows the working of the adsorbent for oil spill clean-up. A variety of Table 11.4 Methods of oil spill clean-up Based on dimensions

Based on approach

Two dimensional

Three dimensional

Physical

Chemical

Biological

Thermal

Membranes fabrics films meshes

Sponges aerogels

Containment booms skimming machines adsorbent materials

Dispersants solidifiers

Bioremediation

In-situ burning

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Fig. 11.7 Process of oil spill clean-up using adsorbents

adsorbents are used to adsorb oil from the location. These adsorbents can be natural or synthesized, inorganic or organic, and 2D or 3D. Some of them can be reused, and the oil can be recovered from them, while others cannot support oil recovery and can be applied only once. • Natural inorganic adsorbents: clay, glass, and sand • Synthetic adsorbent: polypropylene, polystyrene, and polyester foams • Natural organic adsorbents [23]: lignocellulose such as banana fibers, cotton, luffa orange peel, palm fibers, rice husks, wheat straw, sugarcane bagasse, sawdust, etc. Advantage—resource availability, non-toxic, biodegradable, low specific weight hence float to water, affordable [24]. Disadvantage- sorption efficiency is inferior compared to some synthetic materials, poor oleophilic/hydrophobic properties • Advanced materials: Aerogels, Nano cellulose, Nanoparticles [25]. Other advanced physical methods include the use of artificial micro- and nanomachines, nanobots, or nanomotors [26].

Chemical Methods In chemical methods, the chemical is added to the oil spill-affected area. Unlike physical methods based on collection, adsorption, and other physical processes, chemical methods use chemicals to modify the physical and chemical properties of the oil. This can be done by chemical coagulation, dispersion, or solidification. Surfactants are used as dispersants to work on the interfacial tension between oil–water interfaces. They minimize the thickness of spilled oil to reduce the damage. Solidifiers can convert oil into an inert state that facilitates oil recovery. They form a physical bond with the oil and convert it into a viscous, solid-like mass, making the oil recovery

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easier. Polymeric solidifiers are used as they can physically hold oil through the Van der Waals forces in the cavities. Examples include styrene-butadiene, polypropylene fibers, polypropylene granules, and polyethylene granules [27].

Biological Methods It is also known as bioremediation. In this method, the oil-consuming microbes are discharged into the oil spill area. By utilizing the microbial activity, the spilled oil is biodegraded into cell biomass and lighter constituents that are eco-friendly. This method is very environmentally friendly. It utilizes the natural process and forces for oil spill management. However, selecting the correct type of microbes for the process is crucial, considering the environmental condition, water movement, and nature of oil. Oil degraded this way cannot be recovered for use. Bacteria from genera Dietzia (e.g., D. papillomatosis), Nocardioides (e.g., N. deserti), Microbacterium, Pseudomonas, Micrococcus, Gordonia, Arthrobacter, and Cellulomonas have shown promising results [28]. The bioremediation process is enhanced by using nanoparticles [29].

Thermal Methods This method is also called in-situ burning. It is an efficient, rapid, and economical method of oil spill management. It requires simple technique and very minimal equipment. Firstly, the spilled oil is contained to a limited surface using fire-resistant booms. Then the oil is ignited igniter. The igniter can be surface-deployed lighters (from vessels and land) or aerially deployed ignitors (from helicopters). However, it is hazardous from an environmental perspective. It pollutes ocean water and air, jeopardizes marine life, and the spilled oil cannot be recovered as it is burnt and lost forever. The process can result in secondary fire or explosion, which can claim facilities and human life. In most oil spill cases; these methods are combined and have advantages and limitations.

11.3.3.2

Oil Spill Management Through Graphene and Graphene Derivatives Nanoparticles

Graphene and its derivatives are nanomaterials with remarkable properties. Its properties are already discussed in Sect. 1.3 of this chapter. The use of graphene and its derivatives in oil spill management is gaining popularity globally due to its low density, high specific surface area, high porosity, and ease with which its surface functionality can be tailor-made. They offer excellent mechanical performance, improved

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Table 11.5 Performance of various graphene-based adsorbents based on their adsorption capacity Adsorbent materials

Adsorbed substances

Graphene aerogel

Oils and organic solvents Adsorb 120–250 times compared to their weight

Adsorption capacities (wt/wt)

Carbon nanofibers/graphene oxide aerogel

Oils and organic solvents 120–286

CNT sponges

Oils and organic solvents 80–180

CNTs sponges

Diesel oil

56 49

CNTs sponges

Gas oil

Carbonaceous nanofiber aerogels

Oils and organic solvents 40–115

Carbon aerogels from graphene and CNTs

Oils and organic solvents 215–743

Carbon fiber aerogel made from Oils and organic solvents 50–192 raw cotton CNT-graphene hybrid aerogel

Oils and organic solvents 100–150

Carbon aerogel from winter melon

Crude oil

25

Carbon aerogel from winter melon

Gasoline

24

Carbon aerogel from winter melon

Diesel

27

Multi-walled carbon nanotubes

Different fractions of oil

120

oil clean-up capability, better oil recovery, and feasible economics. These nanomaterials have several applications in oil spill management; some of them are discussed below.

As Adsorbent Graphene-based adsorbent and their capacities have been listed in Table 11.5. As seen from the above table, Hydrophobic modified three-dimensional carbon nanofibers/Graphene oxide aerogel shows excellent results as an oil adsorbent [30]. Its performance is shown below: Adsorption. • High adsorption capacity, 120–285 times their weight • Adsorption of diverse types of oil • Short adsorption time,

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Recyclability • • • •

Highly recyclable High fire resistance hence could be recovered simply by combustion Excellent elasticity hence can be recovered by mechanical squeeze Good adsorption capacity even after 10 cycles; thus, suitable for large-scale application.

Selectivity • • • •

Ultra-low density, therefore, floats on water and acts on oil Good hydrophobicity repels water Exhibits very high selective adsorption capacity towards oil Almost no water uptake with oil.

As Raw Material for Filtration Membrane Graphene and its derivatives are used not only as sorbents but also to fabricate filtration membranes for oil–water separation. Graphene-polymer and Graphene oxide-polymer composites have shown good separation capacity [31]. The filtration membrane can be categorized as a super hydrophilic/super oleophobic membrane and a superhydrophobic membrane. In a super hydrophilic membrane, water wets the membrane quickly and makes a hydration layer on the surface of the membrane when the membrane is immersed in a mixture of oil and water. This hydration layer further prevents the oil droplets from entering the membrane, thus developing an oil-repellent tendency. As the phenomenon continues, it results in an excellent oil–water separation. The superhydrophobic membranes work with the opposite mechanism to separate oil–water.

As Separation Sheet Among the latest emerging technologies is the Gravity-based oil spill remediation technique. It uses reduced graphene oxide-Low density polyethylene (rGO-LDPE) sheets for oil spill treatment [32]. Unlike the methods mentioned above, which are based on adsorption or filtration, this method is based on gravity. The rGO-LDPE sheet is heavier than oil and lighter than water. It is placed by gravity between layers of water and oil, thus separating water and oil. The oil from above the sheet is drawn into an artificial storage facility by gravity using a tube connected to a hole in the center of the sheet. This method allows light and heavy oils to be collected regardless of the viscosity. It is an economical and quick method for oil spill remediation.

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As Autonomous Vessel This method uses the collection capacity of graphene vessels for oil spill clean-up. The graphene vessel is made by developing Graphene oxide aerogel over a copper mesh. It is placed in the oil spill-affected water. Due to its low specific gravity and high buoyancy, it floats on the water’s surface and contacts the oil layer. Its hydrophobic nature repels water away from the vessel. Owing to the oleophilic of graphene oxide aerogel, it readily interacts with oil. Both gravity and capillary forces start working on it. It selectively separates the oil and quickly adsorbs it due to the capillary effect all by itself without any external power inputs. After the oil is fully absorbed in the walls of the vessel, the oil is collected into the vessel by gravity from all sides of the vessel. The vessel thus pulls the crude oil spilled on the water’s surface and collects it inside [33]. It offers the following advantages: • • • • • •

It is autonomous Works on natural forces Requires no external power Collect oil at the rate of 20 thousand liters per square meter per hour (LMH) Very high oil selectivity (99.9% oil) Chemically stable and recyclable.

11.4 Conclusion An oil spill is a major cause of concern for the world. It damages the environment, destroys flora fauna, disturbs the ecological balance, and leads to the loss of human lives. It puts a significant burden on the global economy, results in energy loss, and puts stress on scarce resources. There have been numerous incidents of oil spills that have turned into severe hazards. Owing to all these crises, management and treatment of oil spills are inevitable. Oil spill management not only protects human lives and the environment but also helps recover the lost oil to solve the energy crisis. There are several traditional and non-traditional methods for oil spill management. This includes physical methods (use of booms, adsorbents, oil collection, etc.), chemical methods (spreading surfactants, solidifiers, etc.), thermal methods (in-situ combustion), and biological methods (bioremediation). Using different nanomaterials in these methods for oil spill management and affected water treatment is gaining popularity. Nanomaterials are materials with at least one dimension in the 1–100 nm range. They offer a large surface area to volume ratio, high adsorption capacity, ease of functionality, and improved performance. The application of nanomaterials for treating water with spilled oil has shown promising results. While there are various nanomaterials whose performance is under study, graphene nanoparticles and their derivatives

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have exhibited astonishing results in an oil spill and affected water treatment. They are the lightest and most robust materials known to date. They have unique properties such as sheet-like structure, high adsorption, low density, presence of different functional groups, ease of modification, excellent mechanical strength, electrical conductivity, etc. The various derivatives of graphene nanomaterials are applied as adsorbents, filtration membranes, separation sheets, collection vessels, etc. Their performance is a function of the components present, synthesis method, size, and concentration of nanomaterial and is dependent on the nature of spilled oil and prevailing environmental conditions. Various studies carried out in this context have suggested the applicability of graphene and its derivatives in oil spill management and water–oil separation.

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11 Graphene Nanoparticles and Their Derivatives for Oil Spill Treatment

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