Effluent Dye Removal by Microwave-Assisted Activated Carbon (SpringerBriefs in Molecular Science) 3031411447, 9783031411441

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SpringerBriefs in Molecular Science Rehab Abdelghaffar

Effluent Dye Removal by Microwave-Assisted Activated Carbon

SpringerBriefs in Molecular Science

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Rehab Abdelghaffar

Effluent Dye Removal by Microwave-Assisted Activated Carbon

Rehab Abdelghaffar Textile Industries Research Division, Dyeing, Printing, and Textile Auxiliaries Department National Research Center Cairo, Egypt

ISSN 2191-5407 ISSN 2191-5415 (electronic) SpringerBriefs in Molecular Science ISBN 978-3-031-41144-1 ISBN 978-3-031-41145-8 (eBook) https://doi.org/10.1007/978-3-031-41145-8 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 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 Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

First and foremost, my inspiration to write this book is my father (Prof. Dr. Ahmad Abdelghaffar Younis), and this book is a dedication to my father’s soul.

Preface

Green chemistry is more likely a way of thinking than a new discipline of chemistry, and it entails applying a set of principles to lessen the negative environmental impact of chemical processes and products while also contributing to long-term development, incorporating technical ways for minimizing non-renewable resource consumption and preventing pollution. Environmental chemistry is concerned with the effects of pollutants on the environment, whereas green chemistry is concerned with emerging sciences and technologies that aim to prevent the development of any waste through life cycle analysis (LCA). Green chemistry principles provide an upstream solution to many of the health, environmental, and economic issues that industrial chemicals cause. Natural colors have nowadays been replaced by synthetic dyes because they could not meet the needs of people demand, which often ends up endangering nature by polluting the environment. Chemistry and biology are considered to be used to reduce environmental impact meanwhile, something must be done to prevent dye effluents from ending up in the natural environment untreated. Although money cannot purchase happiness, it may surely assist in the preservation of our wonderful globe. Industrial wastewater green technology has been advanced to the point where it can provide solutions to prevent contaminants from escaping into the environment. Textile effluents contain a variety of substances, including dyes and other compounds. On the other hand, industrialists are frequently unwilling to invest in waste management because it will not increase their profits. As a result, the future of wastewater should be based on the concept of water reuse, which is both economically and environmentally beneficial. My objective and inspiration for writing this book are to share my work and research experience especially textile effluents over the past 8 years on finding ways to stop pollutants from seeping into the environment. I hope this book can be useful to researchers and industrialists who are tackling and facing challenges on this issue. Cairo, Egypt

Rehab Abdelghaffar

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Preface

Acknowledgments This book is made possible thanks to the continuous support of my family, I am grateful to them all for their endless support. I really appreciate the opportunity to work with and be helped by the editor and the team behind this book. Many thanks to Sofia Costa, Ravi Vengadachalam, and the Springer staff who are very supportive, responsible, and guided during the preparation of the book.

Aim of the Book

Adsorption technique is a simple operation, needs low consumption of energy, and has high efficiency with low cost as well as suitable and wide adaptability for a variety of dyes; therefore, it deserves extensive investigation and further development. Activated carbon is generated to explore the adsorption properties of the most prevalent adsorbents and is widely employed as an effective adsorbent due to its huge surface area and high adsorption capacity. Activated carbon from lignocellulosic biomass is often produced using conventional thermochemical conversion methods, which are frequently non-selective and energy inefficient. The drawbacks of activated carbon are high cost and regeneration issues that have severely limited its utilization. For that reason, Microwave-assisted pyrolysis is a cost and energyefficient technology aimed at value-added bioproducts recovery from biomass with less environmental impacts. From this point, the main goal of this book emphasizes the performance of Microwave-assisted pyrolysis in terms of product yield, characteristics, and energy consumption if compared with conventional pyrolysis. The influence of microwave activation on product regeneration identified through sophisticated techniques has been highlighted. Microwave-assisted pyrolysis accomplished at low temperatures creates a uniform thermal gradient than conventional mode. The stability, surface properties, and adsorption capacity of activated carbon were enhanced by microwave activation, thus promoting process sustainability in the energy and environment nexus. Dr. Rehab Abdelghaffar Professor at Textile Industries Research Division, Dyeing Printing, and Textile Auxiliaries Department National Research Centre Cairo, Egypt

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Contents

1 Water Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Pollution and Environmental Implications . . . . . . . . . . . . . . . . . . . . . . 1.2 Dyes wastewater—A Growing Concern . . . . . . . . . . . . . . . . . . . . . . . 1.3 Characteristics of Dyes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Treatment Techniques for Removal of Dyes . . . . . . . . . . . . . . . . . . . . 1.4.1 Physicochemical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.2 Chemical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.3 Biological Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4.4 Electrochemical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . 1.5 The Adsorption Treatment Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.6 The Commercial Activated Carbon (CAC) . . . . . . . . . . . . . . . . . . . . . 1.7 The Disadvantages of Commercially Activated Carbon . . . . . . . . . . 1.8 The Shapes, Sizes, and Functional Groups of Activated Carbons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9 The Activation Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9.1 Physical Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9.2 Chemical Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.9.3 Physicochemical Activation . . . . . . . . . . . . . . . . . . . . . . . . . . 1.10 The Pivotal Determinants for Adsorption of Effective Adsorption by High-Quality Activated Carbon . . . . . . . . . . . . . . . . . . 1.11 Activated Carbon from Natural Adsorbents . . . . . . . . . . . . . . . . . . . . 1.11.1 Types of Adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11.2 Agricultural Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11.3 Industrial Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.11.4 Bioadsorbents (Microorganisms) . . . . . . . . . . . . . . . . . . . . . . 1.12 Applications of Conventional Heating of Activated Carbon for Dye Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 1 2 3 4 4 5 5 5 6 7 8 8 9 9 11 12 13 14 14 14 15 15 16 21

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Contents

2 New Trends Using Microwave Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Microwave-Assisted Heating Technique . . . . . . . . . . . . . . . . . . . . . . . 2.2 The Disadvantages of Conventional Heating Methods . . . . . . . . . . . 2.3 Characteristics and Mechanism of Microwave Heating . . . . . . . . . . . 2.4 Microwave-Assisted Thermal Process . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 The Advantages of Microwave Techniques for the Activation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6 Activation Procedures with Microwave-Assisted Pyrolysis . . . . . . . 2.7 The Disadvantages of Microwave-Assisted Activation Process . . . . 2.8 Comparison of Traditional and Microwave Heating in the Preparation of AC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9 Different Activated Carbon Factors by Microwave Treatment . . . . . 2.9.1 Microwave Radiation Power . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.2 Microwave Activation Time . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9.3 The Characteristics of the AC Precursor . . . . . . . . . . . . . . . . 2.9.4 Microwave and Chemical Reagent Interactions . . . . . . . . . . 2.10 The Most Modeling and Optimization Approaches for Adsorption Activated Carbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.1 Response Surface Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.2 Adsorption Isotherm Models . . . . . . . . . . . . . . . . . . . . . . . . . 2.10.3 Kinetic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.11 Different Techniques for Analyzing the Surface Chemistry of ACs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.12 Surface Functional Groups of Different Activation Methods of AC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13 Applications of Microwave-Assisted Activated Carbon for Dye Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13.1 Physical Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13.2 Chemical Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.13.3 Physiochemical Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

29 29 30 30 32

3 Recycling/Regeneration of AC Using Microwave Technique . . . . . . . . . 3.1 Regeneration of Spent Activated Carbon (SAC) . . . . . . . . . . . . . . . . . 3.2 Regeneration of Spent Activated Carbon (SAC) Using Microwave Technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Regeneration Procedures with Microwave-Assisted Pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Comparison of Conventional and Microwave Heating Regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 Applications of Microwave-Assisted Activated Carbon Regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4 Miscellaneous Technologies with Microwave Heating for Producing Activated Carbon (AC) and Regeneration of Spent Activated Carbon (SAC) for Dye Adsorption . . . . . . . . . . .

53 53

32 35 35 36 36 38 38 38 39 39 40 40 41 42 42 43 43 44 47 47

54 55 56 57

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3.5 Environmental Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 3.6 Conclusions and Future Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Abbreviations

AB29 AB93 AC ACPP ACs AWAC BA BB9 BBD BET BG BN BOD CAC CC CC-AC CCD CDS Ch ChA CR Cu-AC DFP DSAC Fe-AC FZMC GA IR K LLAC MB

Acid Blue 29 Acid Blue 93 Activated Carbon Pomegranate Peels Activated Carbon Types of Activated Carbon Acacia Wood Activated Carbon Activated biosorbent Basic Blue 9 Box–Behnken designs Brunauer–Emmett–Teller Brilliant Green Basic Dye Native biosorbent Biological Oxygen Demand Commercial Activated Carbon Coconut shells Corn cob activated carbon The central composite design Carbonized Date Seeds Bituminous charcoal Chemical Activated Carbon Congo Red Dye Copper-activated carbon Dragon fruit peel Date seeds-activated carbon Magnetic Activated Carbon Functionalized Zea Mays Cob Egyptian black sand Impregnation ratio The second-order equation for a reaction rate Lemongrass leaf-activated carbon Methylene blue xv

xvi

MB-DFPAC MG MO MRAC MSA MW MWAs NOS OS400 OSAC OSMG PDACs PFB PL PP PWAC RAC RBV5R RDS RHAC RhB RoDS RSM SAC TDS TRD USEPA

Abbreviations

Dragon fruit peel activated carbon loaded with methylene blue Malachite green dye Methyl orange Regenerated activated carbon Microwave-steam activation Microwave Microwave absorbers Natural Olive Stone Olive stone-activated carbon Olive Stone Activated carbon The composite adsorbent Popcorn-Derived Activated Carbons Palm Frond Base Palm Leaflets Pomegranate Peels Parthenium weed activated carbon Regenerated Activated Carbon Remazol brilliant violet 5 R Raw Date Seeds Rice husk-based AC Rhodamine B Roasted Date Seeds Response surface methodology Spent activated carbon Total Dissolved Solids Taguchi robust designs The US Environmental Protection Agency

Chapter 1

Water Pollution

Abstract This chapter provides an introduction to water pollution, polluting sources, especially wastewater during textile processing, and characteristics of dyes, and then outlines technologies for the removal of dyes including biological, chemical, physicochemical, and electrochemical. An overview of commercial activated carbon used, disadvantages, the activation methodology, and finally the activated carbon from natural adsorbents and types of adsorbents with examples. Chapter 2 covers the disadvantages of conventional heating methods for the preparation of AC, the characteristics of microwave heating, the advantages and disadvantages of microwave techniques, a comparison between conventional and MW heating, and also the important variables for the preparation using microwave heating. Keywords Textile effluent · Adsorption · Removal of dyes · Activated carbon · Conventional heating

1.1 Pollution and Environmental Implications The most important objective of the principles of green chemistry is the rejection or diminishment of waste. The green reactions are sustainable, greatly efficacious, simple to use, and ecologically benign. Environmentally, the textile industry is regarded as one of the most polluting in the world. A lot of initiatives have recently been taken to make textile processes more environmentally friendly. Greener dyes and auxiliaries are used, as well as eco-friendly, optimal, and efficient processing, bioprocessing, and reuse of textiles, water, and chemicals in addition to the removal of dangerous compounds (Choudhury, 2017). The challenge of establishing suitable treatment facilities for all polluting sources is tough and expensive (Dassanayake et al., 2021), and thus there is a great demand for novel technology that is of low cost, low maintenance, and energy efficient (de Oliveira et al., 2022; Kumar et al., 2021). As the problem of solid waste disposal has grown in complexity, it is now necessary to either identify appropriate methods

© The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. Abdelghaffar, Effluent Dye Removal by Microwave-Assisted Activated Carbon, SpringerBriefs in Molecular Science, https://doi.org/10.1007/978-3-031-41145-8_1

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for their safe disposal or offer creative uses for them as by-products (Kumar et al., 2021).

1.2 Dyes wastewater—A Growing Concern Water contamination by certain industrial chemicals and dyes from various industries, including textile, cosmetics, rubber, plastics, paper (Hu et al., 2014) paints, varnishes, and leather (Fig. 1.1) (Dassanayake et al., 2021), is a major source of environmental deterioration (Moghazy, 2017; Yanne et al., 2021). Color removal is a significant issue for all types of textile effluents due to the wide range of chemicals used in the dyeing and printing of fiber, yarn, or fabric (Balaji et al., 2015). Colored wastewater from the textile dyeing industry is a major source of worry in developing countries because it has the potential to pollute the environment. Many complex wet processes in the textile industry, including washing, dyeing, printing, finishing, and so on, use a variety of dyes, which result in colorful effluent. Colors and dyes are resistant and non-biodegradable, and they are difficult to remove effectively using traditional treatment methods. If discharged into the environment, they tend to remain over extended distances in flowing water, which can impact sunlight penetration through the water column, inhibit photosynthesis, and diminish the availability of dissolved oxygen in the water. Because of the lower penetration of

Fig. 1.1 Sources and routes for dyes in the environment

1.3 Characteristics of Dyes

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Fig. 1.2 Textile effluent used in wet processes in textile manufacturing

sunlight and the increased biological oxygen demand (BOD), (Elsayed et al., 2020; Ishak et al., 2020) an increase in oxygen consumption is expected. As a result, the amount of life in the water body would plummet. Composite textile effluent has a high total dissolved solids (TDS) content, as well as a high concentration of organic dyes (Regunton et al., 2018) and other chemicals employed in the manufacturing process (Fig. 1.2). Large volumes of dyestuff can also be lost directly to wastewater during textile processing due to inefficiencies in dyeing, which then ends up in the environment (Dassanayake et al., 2021). Furthermore, the problem is not solved by changing the hue! In reality, some of the dyes will break down over time and become colorless. The resultant chemicals are even more toxic (Al-Balushi et al., 2017) and, in some circumstances, carcinogenic than the original colors (Alorabi et al., 2020; Majedi et al., 2014). As a result, new and effective procedures for dye removal from the water must be devised to avoid hazardous by-products.

1.3 Characteristics of Dyes Synthetic colors can be harmful to people, causing allergies and sensitization of the skin and lungs. Furthermore, several dyes and their derivatives are mutagenic and have been indexed as a result. Some carcinogenic dyes (Kumar et al., 2021; Yanne et al., 2021), however, may still be in use, and products colored with them may be sold in areas where the compounds are prohibited. Dyes are categorized according to their chemical type (azo, anthraquinone, indigo, etc.) (Dassanayake et al., 2021) and application method (acid, direct, reactive, etc.)

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Table 1.1 Fixation rates of various dye classes on various substrates and the proportion of dye loss in textile effluent (Carmen & Daniela, 2012) The class of dye

The type of fabric

The degree of fixation (%)

The loss in wastewater (%)

Direct dye

Cellulose

70–95

5–30

Reactive dye

Cellulose

50–90

10–50

Basic dye

Acrylic

95–100

0–5

Disperse dye

Polyester

90–100

0–10

Acid dye

Polyamide

80–95

5–20

(Bharathi & Ramesh, 2013). Azo dyes are the most widely used dyes, they make the largest group of synthetic dyes, and they are also the most dangerous in terms of mutagenicity and carcinogenicity (Dassanayake et al., 2021; O’Neill et al., 1999). The presence of one or more azo bonds (–N=N–) joining aromatic structures is their defining property. Aromatic amines are formed when the azo link is severed chemically or biologically, and they are often more poisonous and mutagenic than the mother molecule. Azo dyes have a stronger color than anthraquinone dyes, and this advantage, combined with the fact that they are relatively inexpensive to manufacture, has led to their market dominance (Koh, 2011). A percentage of dye, 2–50%, does not bond to the fabric and is rinsed away throughout the dyeing process. Table 1.1 shows the reported percentage fixing of different dyes on different substrates and effluent discharge (Carmen & Daniela, 2012; Gopalakrishnan et al., 2020; Kausar et al., 2018).

1.4 Treatment Techniques for Removal of Dyes Several methods for removing color from industrial effluents have been proposed, including aerobic and anaerobic microbial degradation, coagulation (Elsayed et al., 2020; Ishak et al., 2020), flotation, membrane filtration, and chemical oxidation as shown in Fig. 1.3 (Hu et al., 2014), but none of these methods are effective enough for industrialists to use (Kausar et al., 2018). Most current practices for wastewater decolorization treatment fall into the following four main classes:

1.4.1 Physicochemical Techniques Precipitation, coagulation or flocculation, ion exchange, adsorption, and membrane separation (Dassanayake et al., 2021) are all methods used to remove materials (Yanne et al., 2021). These physically remove or separate the color, demonstrating advantages such as low energy consumption and simple operational management;

1.4 Treatment Techniques for Removal of Dyes

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Fig. 1.3 Color removal from textile effluents using various techniques

nevertheless, the main drawbacks are sludge creation and shorter lifetime due to excessive residue accumulation (Mohammed et al., 2014; Yagub et al., 2014). Therefore, these methods may need solid waste disposal (Lima & Asencios, 2021).

1.4.2 Chemical Techniques Chemical oxidation/reduction, ozonolysis, and so on technologies break down the effluent into smaller particles and disrupt the chromophore, which is responsible for color. However, the high expense of chemical reagents and, in some cases, the generation of harmful by-products are two of the most significant barriers to their utilization (Crini, 2006; Lima & Asencios, 2021; Yagub et al., 2014).

1.4.3 Biological Techniques Biological methods involving microorganisms in aerobic and anaerobic digestion, in which decolorization occurs either by dye adsorption on activated sludge or biological destruction of dye molecules, are more cost-effective and easier to implement than physicochemical methods (Zhou et al., 2019a, 2019b). Biological treatments can degrade a wide range of organic molecules; however, some dyes, such as azo, have a complicated structure that is not entirely decomposed under aerobic circumstances using microbiological procedures (Lima & Asencios, 2021).

1.4.4 Electrochemical Techniques Ion oxidation/electrodialysis. It combines the electrolytic oxidation of the dye and other environmental pollutants with the physicochemical precipitation of the sludge.

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Its biggest downside, however, is the enormous need for electrical energy (Balaji et al., 2015; Lima & Asencios, 2021). The adsorption technique has been recognized as the most cost-effective and potent approach for the last few decades (Alorabi et al., 2020; Yanne et al., 2021). Various data on the utilization of diverse adsorbents from various sources have been identified and carried out in recent years for wastewater purification (Singh et al., 2020).

1.5 The Adsorption Treatment Strategy Adsorption is one of the most essential industrial waste treatment separation techniques (Stawi´nski et al., 2018). It is a mass transfer process in which a solid substance (adsorbent) attracts dissolved constituents toward its surface to selectively remove them from an aqueous solution. This process involves the accumulation of concentrated materials at the surface or in the interphase. The adsorbent can be in the form of a liquid, a solid, a gas, or a dissolved solute. Adsorption can be classed as chemical or physical; chemical adsorption involves the exchange of electrons; the adsorbate being chemically attached to the surface. While physical adsorption, waste matter is bonded to the adsorbent surface using physical forces such as hydrogen bonding, polarity, van der Waals forces, and dipole– dipole interactions. The extent of adsorption, on the other hand, is determined by adsorbent properties such as molecular size, molecular structure, molecular weight, solution concentration, polarity, and adsorbent surface parameters such as surface area and particle size (Kausar et al., 2018; Salleh et al., 2011). Adsorption, on the other hand, has been proven to be particularly effective in treating wastewater containing chemically stable contaminants since it is a sludge-free method that can entirely remove even trace amounts of colors from wastewater. A low-cost color removal solution can encourage these industrialists to use such methods and reduce the environmental impact of their characterization techniques, which can be used for adsorbents effluent wastewater. The effectiveness of this method is determined by the chemical and physical properties of the adsorbent and adsorbate as shown in Fig. 1.4, as well as their cost (Barjasteh-Askari et al., 2021) availability, ease of operation, surface area, and lack of toxicity (Anirudhan & Ramachandran, 2015; Kausar et al., 2018). As a result, concerns about high operating and capital costs, efficiency as well as the need for secondary treatment, are lessened (Robinson et al., 2001; Saleem et al., 2019).

1.6 The Commercial Activated Carbon (CAC)

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Fig. 1.4 Adsorption techniques and dye removal from wastewater mechanisms

1.6 The Commercial Activated Carbon (CAC) Activated carbon, or AC, refers to a class of amorphous carbonaceous materials that have a large porosity and internal surface area (Bharathi & Ramesh, 2013; Sultana et al., 2022). Any carbon-rich material can be used as a precursor to activated carbon production (Barjasteh-Askari et al., 2021; Saleem et al., 2019). Activated carbon is the most extensively used sorbent material for the adsorption of contaminants from wastewater (Ma et al., 2021; Singh et al., 2020). The first commercial manufacture of active carbon was from wood which was recorded in Europe in the early 1800s (Masoumi & Dalai, 2020; Rangari & Chavan, 2017). Coal, asphalt, and petroleum coke are still the most common materials used to make commercial ACs. There have been numerous reports of the use of coal-based adsorbents (Singh et al., 2020). Coal (Fig. 1.5) is primarily used in the production of activated carbon. However, it is frequently non-selective and ineffectual, and the higher the quality, the higher the cost (Bharathi & Ramesh, 2013). One of the most significant obstacles to the commercialization of activated carbon (AC) is the high cost of manufacturing utilizing nonrenewable precursors (Singh et al., 2020). Meanwhile, the global demand for activated carbon produced from alternative environmentally acceptable sources has increased year after year (Masoumi & Dalai, 2020). Environmental concern has also influenced the production of activated carbons by introducing the concept that ordinary waste materials can be used to make activated carbons. They can be made from a variety of agricultural waste and industrial waste sources (Saleem et al., 2019).

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Fig. 1.5 Commercially activated carbon

1.7 The Disadvantages of Commercially Activated Carbon Commercially available activated carbon, a common adsorbent, has a high capacity for pollution removal. However, the high cost of treatment and difficult regeneration, both of which drive up the cost of wastewater treatment. As a result, there is a desire for alternative adsorbents that are made of low-cost materials and do not require any further expensive processing. Adsorption onto activated carbon made from a variety of low-cost, readily available, and renewable basic materials has recently received a lot of interest (Banat et al., 2007; Ioannou and Simitzis, 2013; Masoumi & Dalai, 2020). All of the research indicated that the carbons produced have equivalent or higher adsorption capabilities than commercially available non-renewable fossil sources (Masoumi & Dalai, 2020; Saleem et al., 2019). As a result, the majority of adsorption research has centered on untreated industrial, agricultural, fruit, and plant wastes. Adsorption with activated carbon is effective in removing both organic and inorganic pollutants in wastewater effluents. Activated carbon, on the other hand, has been identified by the US Environmental Protection Agency (USEPA) as one of the best-controlled remediation systems in the world (Singh et al., 2020).

1.8 The Shapes, Sizes, and Functional Groups of Activated Carbons Based on their shape and size, they can be classed as granules, powders, or pellets. As illustrated in Fig. 1.6, activated carbons have well-developed micro (< 2 nm), meso (2–50 nm), and macropores (> 50 nm) with a variety of surface oxygenated functional groups which are responsible for adsorption, such as hydroxyl, carboxylic, and epoxy; depending on the source of precursor materials (Fig. 1.7), also they have a large surface area (up to 3000 m2 g1 ), (Ahmad et al., 2015), which help promote adsorption capabilities.

1.9 The Activation Methodology

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Fig. 1.6 Shapes and sizes of activated carbon Fig. 1.7 Schematic presentation of pore structure (Moosavi et al., 2020)

1.9 The Activation Methodology The carbon-containing precursors are dehydrated first, then carbonized by slowly heating in the absence of air (pyrolysis). Because of its large specific surface area, porous structure, and good adsorption characteristics, activated carbon (AC) has been widely employed as a flexible adsorbent for the separation of gases, removal of organic contaminants, and so on (Fukuyama & Terai, 2008). There are three methods for the production of activated carbon as presented in Table 1.2 and Fig. 1.8.

1.9.1 Physical Activation Physical activation is a traditional way of producing activated carbon (Yasin & Pravinkumar, 2020). Carbonization of the raw material in an inert atmosphere is followed by mild oxidation as in Fig. 1.9. To convert this organic precursor to primary

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Table 1.2 Common activation method and activating agents for preparation of activated carbon (Alslaibi et al., 2013) Activation method

Activation agent

Physical activation

Steam only Steam, CO2 CO2 CO2 /N2

Chemical activation with alkali treatment

Potassium hydroxide (KOH) Potassium carbonate (K2 CO3 ) Sodium hydroxide (NaOH) Zinc chloride (Zncl2 )

Chemical activation with acid treatment

Phosphoric acid (H3 PO4 ) Sulfuric acid (H2 SO4 )

Physiochemical activation

Potassium hydroxide (KOH)/CO2

Fig. 1.8 Three methods for the preparation of activated carbon

carbon, it was heated in a nitrogen environment, then activated with CO2 , steam, or a combination of steam and carbon dioxide at temperatures (Masoumi & Dalai, 2020) between 700 to 1000 °C. Physical activation starts with the char coming into touch with the activating agent, then the surface carbons on the pores react with the activating agent. These substances take carbon atoms out of the porous carbon’s structure. Surface carbons react with steam and CO2 in accordance with reaction: C + H2 O → CO + H2 C + 2 H2 O → CO2 + 2 H2

1.9 The Activation Methodology

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Fig. 1.9 Physical process of activated carbon

The carbon is partially gasified, and a porous, highly activated carbon skeleton is produced. Physical activation has the disadvantage of low yield and requires higher temperatures for the activation process.

1.9.2 Chemical Activation Prior to carbonization (Yasin & Pravinkumar, 2020), the carbon-containing precursors are impregnated with certain chemicals, which are typically acids such as phosphoric acid (H3 PO4 ), sulfuric acid (H2 SO4 ), a strong base such as potassium hydroxide (KOH), sodium hydroxide (NaOH), or a salt such as zinc chloride (ZnCl2 ). To obtain a carbonized product with a well-developed porosity in a single operation. They are then carbonized at moderate temperatures (400–800 °C) (Masoumi & Dalai, 2020; Moralı et al., 2018; Sulaiman et al., 2018). The carbonization/activation process is thought to happen at the same time during the chemical activation. It’s also thought that the chemical incorporated into the interior of the precursor particles reacts with the products of the precursor’s thermal decomposition, reducing volatile matter evolution and inhibiting particle shrinkage; as a result, the precursor’s conversion to carbon is high, and once the chemical is removed after heat treatment, a large amount of porosity is formed (Fig. 1.10). Chemical activation has several advantages over physical activation, including a lower activation temperature (Barjasteh-Askari et al., 2021), a one-step process, and a faster activation time (Sureshkumar & Susmita, 2018). Furthermore, compared to physical activation, chemical activation is accomplished to promote the porosity of AC through impregnation with chemicals succeeded by pyrolysis at an elevated temperature. This method has been stated to generate AC with enhanced surface porosity and less fractured surface (Al-Qodah & Shawabkah, 2009) because it may be carried out at a dropped process temperature (> 450 °C) compared to physical activation (> 700 °C) (Mahapatra et al., 2012). Chemicals including H3 PO4 , ZnCl2 ,

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Fig. 1.10 Chemical process of activated carbon

NaOH, and KOH have been studied to see if they may weaken or dissolve the chemical bonds between the lignocellulosic ingredients contained in the biomass utilized as the feedstock to create AC (Jin et al., 2013). That makes it possible for the lignocellulosic ingredients to be effortlessly removed and converted into vaporous gases (such as CO2 and CH4 ) at the time of the following pyrolysis approach, that in turn augments the formalization of pores on the surface of the arising AC produced as a solid output (Say˘gılı et al., 2015). Alkali metal hydroxides, such as KOH and NaOH, in particular, have been found to have the capacity to increase pore formation with high surface area through an intercalation effect brought on by the interaction of K and Na atoms with the carbon structure of the AC, where a greater number of oxygen-carrier functional groups formed and is environmentally friendly (Musa et al., 2015). Consequently, it was proposed that chemical activation using alkali metal hydroxides might result in the creation of AC with superior characteristics in terms of increased porosity and the consequent surface area (Azlan Zahari et al., 2022; Lam et al., 2017). Activated carbons are employed in a variety of industries, including industrial wastewater treatment, petrochemical industries, solvent recovery, catalyst supports, and the pharmaceutical industry. Due to its porous structure, it also has a strong demand for energy storage device production, particularly supercapacitors (Sultana et al., 2022). The two processes of activated carbon manufacturing are shown in Fig. 1.11 (Sulyman et al., 2017; Yasin & Pravinkumar, 2020; Zhou et al., 2019a, 2019b).

1.9.3 Physicochemical Activation The typical physicochemical activation sequence is carbonization, chemical activation, and physical activation. When compared to physical and chemical activations alone, this hybrid kind of activation often results in a greater surface area in AC. Theoretically, upon chemical activation, metallic ions pierce the char matrix’s skeleton

1.10 The Pivotal Determinants for Adsorption of Effective Adsorption …

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Fig. 1.11 Two processes (physicochemical) of activated carbon manufacturing

deeply and form a porous network, largely made up of micropores. The next physical activation step allowed the molecules of the activating gases to infiltrate within the micropore type of pores and bombard them aggressively, increasing the size of the pores from micropores to mesopores and, in some circumstances, macropores. As a result, scientists choose physicochemical activation because the resulting AC is ideal to adsorb water pollutants like dyes, heavy metals, and other contaminants with comparatively bigger molecules than gas pollutants (Alharbi et al., 2022). Physical (carbonization) and chemical activation are combined in this method (Fig. 1.11). It begins with a chemical-activating agent impregnating char, followed by oxidizing gas at a reasonably high temperature for gasification treatment (Yusop et al., 2021).

1.10 The Pivotal Determinants for Adsorption of Effective Adsorption by High-Quality Activated Carbon 1. Starting materials and preparation process with a high adsorbate removal capacity. 2. Adsorbent nature: high carbon content, low ash content, low impurity content, surface area, surface charge, pore distribution, pore volume, particle size (granular, powered), functional groups, polarity, availability, affordability, and renewability. 3. Adsorbate nature: including molecular size, functional groups, and polarity (Moosavi et al., 2020). 4. Adsorption conditions: pH, temperature, ionic strength, adsorbent dosage, contact time, and the initial adsorbate concentration (Dutta et al., 2021; Hamad & Idrus, 2022; Moosavi et al., 2020)

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Fig. 1.12 Types of adsorbents from different sources

1.11 Activated Carbon from Natural Adsorbents Several non-traditional adsorbents have received a lot of interest in recent years as potential cost-effective replacements to expensive adsorbent materials for removing harmful contaminants. This covers the use of waste materials such as agricultural, industrial, natural, and biowastes, which are all readily available. These waste materials go through a number of procedures before being used as adsorbents, including chemical processing, powder conversion, and breakdown.

1.11.1 Types of Adsorbents A variety of agricultural residues are considered and classified into two broad categories. One of the remnants left in the field following harvest or during the falling-leaf season, such as tree fronds. (Al-Swaidan & Ahmad, 2011), oak tree, and dead leaves (Sulyman et al., 2014). The second category includes by-products from industrial processing, such as date stones (Aremu et al., 2020), olive stones (Amar et al., 2020), and tea leaves (Mohammed, 2012; Sulyman, 2014; Sulyman et al., 2017). Agricultural waste and industrial waste (Fig. 1.12) are both good sources of lowcost adsorbents. Producing AC from biomass has two benefits: First, it can stop the generation of CO2 by fixing the carbon, and second, it can naturally leach into the soil (Danish & Ahmad, 2018).

1.11.2 Agricultural Waste Agricultural solid waste adsorbents are inexpensive abundant in nature, have little or no economic value, and are easily available (Sulyman et al., 2017). Activated carbon has been reported to be derived from a variety of agricultural residues (Masengo &

1.11 Activated Carbon from Natural Adsorbents

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Mulopo, 2021), including stalks, leaves, seeds, shells, peels (Singh et al., 2020), husks, and straws (Sulyman et al., 2017).

1.11.3 Industrial Waste Fly ash, bagasse (Abdelghaffar et al., 2019), slurry, and ceramic wastes have all been employed as adsorbents to remove dyes from contaminated water (Dutta et al., 2021; Zhou et al., 2018). Sawdust from the wood sector, for example, is commonly utilized as an adsorbent (Singh et al., 2020). Several research projects have been carried out to transform carbonaceous and other wastes into activated carbons or other adsorbing materials (Saleem et al., 2019). As previously stated, producing high-quality AC from abundant, low-cost, renewable, and sustainable antecedents such as biomassbased agro-industrial wastes is cost-effective (Ioannou & Simitzis, 2013; Húmpola et al., 2016; Mansour et al., 2020). Vehicle tires (Chan et al., 2012), plastic wastes (Bazargan et al., 2013), and textiles, such as silk waste, are all carbon-containing wastes (Saleem et al., 2019). Stone fruit markets have sprung up all over the world, particularly for olives, dates, and fig seeds. These items’ stones are hard and contain considerable amounts of lignin, making them ideal as activated carbon precursors.

1.11.4 Bioadsorbents (Microorganisms) Bioadsorbents are materials generated from various biological sources that use physical–chemical principles in their structure to remove contaminants from wastewater. Microorganisms, such as peat, seaweed, chitosan, lignin, fungi, algae, yeasts, and bacteria, are the most frequent. However, dead biomass has an adsorption capacity equivalent to or greater than living biomass (Kumar et al., 2021), does not require nutrients, is easy to regenerate, and may be stored at room temperature for long periods without deterioration. Biosorbents have gotten a lot of concern because of their good performance in removing contaminants, relative abundance in nature, and efficiency as low-cost biosorbents. Also, they have a large pore volume, a wide surface area, and a variety of functional groups and ligands. In general, biosorbents made from biomass feedstock that have not been treated have a lower sorption capability. Various methods for improving adsorption capacity by increasing the number of functional groups as well as sorption sites on the surface of biosorbents have been documented, including surface oxidation, surface amendment, functionalization, and so on (Singh et al., 2020). As a result, numerous microorganisms, such as fungi, bacteria, yeasts, algae (Abdelghaffar et al., 2021; El-Mekkawi et al., 2019), and others, have been used as adsorbents in conjunction with physicochemical processes.

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1.12 Applications of Conventional Heating of Activated Carbon for Dye Removal It is possible to manufacture AC via chemical activation (ChA), a single-step process, using various chemical-activating chemicals. Due to their availability and high carbon content (approximately 50%), stones are one of the most common agricultural waste materials used to make AC (Daoud et al., 2019). The preparation of AC has included the use of stones from a variety of fruits, including jujube, peach, olive, date, etc. (Srivastava et al., 2021). Olive stone, a necessary by-product of the olive oil extraction industry, is one of the many stone fruits which is regarded as having one of the greatest carbon contents among stone fruits, and the sale of stoneless olive products is also a huge business (Saleem et al., 2019). Using ZnCl2 as an activating agent, Mahmoudi et al. synthesized four activated carbons at different heating temperatures (300, 400, 450 and 500 °C) from natural olive stone (NOS) wastes. The adsorption of a cationic (methylene blue (MB)) and anionic (methyl orange (MO) dye on these activated carbons under various experimental settings revealed that (OSAC 400 °C) might be deemed an effective, appealing, and promising adsorbent for both dyes. This adsorbent’s Langmuir adsorption capabilities were determined to be 303.0 and 277.8 mg g−1 for MB and MO, respectively, which are much higher than other activated carbons (Mahmoudi et al., 2020). Elkholy et al. studied the manufacture of a new adsorbent material, OSMG, by impregnating activated carbon (OS400) from olive stones (agro-waste) with Egyptian black sand (GA). The activation of the raw material with favorable structural and surface chemical properties was found to occur at a temperature of 400 °C. The new composite (OSMG) was able to remove 98% of the MB dye because of its better characteristics. The garnet’s specific surface areas were determined to be 5.157 mg/ g, 1489.598 mg/g, and 546.392 mg/g, respectively (GA), (OS400), and (OSMG). It could be used at least for five consecutive adsorption/desorption cycles while retaining its adsorption capacity. The results indicated that, the adsorption fit best with the Freundlich model and that the adsorption process followed a pseudo-second-order kinetic mechanism (Elkholy et al., 2023). Al-Ghouti et al. effectively prepared AC as an adsorbent for the elimination of MB from an aqueous milieu using black and green olive stones. Freundlich isotherm was found to have the best fit for the equilibrium data, with an R2 value of 98% (Al-Ghouti & Sweleh, 2019). Dates are the fruit of the date palm; as the demand for dates grows, the packing and processing activities for this product produce a variety of by-products, including seed. Date seeds are a hot topic in science because of their high nutritional value and functional component content, which includes dietary fiber and phenolic compounds. Exploring valorization is a must if we are to find viable applications for this waste’s potential recycling (El Marouani et al., 2018). Al-Balushi et al. discovered that date seeds activated carbon (DSAC) produced by chemical activation with phosphoric acid (H3 PO4 ) was superior to CAC in removing

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MB from aqueous solution. The proportion of MB removed by DSAC was higher than that of CAC (Al-Balushi et al., 2017). Zeghoud et al. used palm frond base and palm leaflets as a low-cost adsorbent for removing methylene blue from aqueous solution. The adsorption findings fit the Langmuir model quite well. The MB dye adsorption process was followed a pseudo-second-order paradigm, according to the kinetic studies (Zeghoud et al., 2019). To remove methylene blue (MB) and methyl orange (MO) from aqueous solution, Marouani et al. employed date seeds in raw (RDS), roasted (RoDS), and carbonized (CDS) stages. The response surface method (RSM) determined that 75 mg RDS, 5 mg/L dye, and a solution pH of 5 were the best settings for these three parameters. The retention rate obtained by the roasted date seeds was higher than that of the raw date seeds, which was higher than that of the carbonized ones, and was comparable to that of commercial activated carbon (El Marouani et al., 2018). Another common agricultural waste that comes from the milling of rice is rice husk. Because it has a low rate of combustion and the potential to harm the environment, it requires appropriate management and usage (Satayeva et al., 2018). Due to its high lignocellulose and organic content, it is frequently employed as an AC precursor for the adsorption of colors, heavy metals, emerging pollutants, and other contaminants (Srivastava et al., 2021). Nasehir et al. used the ZnCl2 treatment step of the ChA process to create rice husk-based AC (RHAC). We investigated the impacts of the ZnCl2 : char, impregnation ratio (IR), activation temperature, and activation duration. Two quadratic models were created using the central composite design (CCD) to link the preparation factors with both answers. The important components in each experimental design response were determined using ANOVA testing. The findings showed that activation temperature significantly influenced both responses. The best RHAC was produced with an activation temperature of 500 °C, an activation period of 1.71 h, and an IR of 1.04, which showed a large surface area of 604.34 m2 g−1 and well-developed porosity. In contrast, the IR was minor in both responses (Yahaya et al., 2010). Some fruits and vegetables have protective shells, which are often thrown as garbage. It is economical and beneficial for waste reduction to use this trash as a raw material to create AC. A lot of shells from plants including the palm, walnut, coconut, and peanut are used to make AC (Srivastava et al., 2021). Yanne et al. investigated the removal of methyl orange (MO) from wastewater using four different adsorbents made from peanut shells. Chemical activation with ortho-phosphoric acid (H3 PO4 ) yielded the native biosorbent (BN) and activated biosorbent (BA). Activated carbons (CA1 and CA2) were made by pyrolyzing BN and BA at 650 °C, respectively. The adsorption kinetics of MO on the four adsorbents are described by the pseudo-secondorder model (Yanne et al., 2021). Another source of low-cost lignocellulose wastes produced by trees and plants that are useful as raw materials for the manufacture of AC is leaves. Mango trees, pineapple plants, coconut trees, and even used tea leaves have been used for AC (Srivastava et al., 2021). With potassium acetate acting as the activating agent, Auta and Hameed created AC out of used tea. The IR of the activating agent to waste tea

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(0.3–2.5), the activation temperature (500–800 °C), and the activation period (60– 150 min) were taken into consideration. The activation temperature, activation time, and impregnation ratio (IR) were optimized using the response surface methodology statistical approach; the desired responses were the percentage yield and elimination of the dyes methylene blue (MB) and Acid Blue 29 (AB29). The findings showed that 800 °C, IR 1.4, and 120 min gave a large surface area of 854.30 m2 g1 and were the ideal conditions for good percentage yield and the removal of the two dyes. For AB29 and MB, respectively, the produced mesoporous adsorbent had excellent adsorption capabilities of 453.12 and 554.30 mg g−1 . The production of waste tea AC and its utilization for the absorption of MB/AB29 were shown to be most influenced by the activation temperature and chemical IR among the three factors investigated (Auta & Hameed, 2011). Gurten et al. explored the utilization of the same precursor to manufacture AC using both the standard approach and K2 CO3 . On the properties of the finished product, the impacts of several process parameters, such as carbonization temperature and period, impregnation ratio and period were determined. The largest surface area created at 900 °C with an impregnation ratio of 1.0 was 1722 m2 g. Also, 800 °C is the most practical temperature for the studies carried out at impregnation ratios of 1.0, 1.5 to generate the AC with a high micropore volume fraction. The outcomes showed that the micro- and mesopore volumes, as well as the BET surface area, were significantly influenced by the carbonization temperature and the impregnation ratio (Alslaibi et al., 2013; Gurten et al., 2012). By impregnating Moringa oleifera leaves in H2 SO4 and NaOH, Bello et al. found that acid-activated M. oleifera leaf is a good alternative adsorbent for recovering colors and heavy metals from aqueous solutions and other separation procedures and that it could be used instead of CAC (Bello et al., 2017). Ahmad et al. used lemongrass leaf-based activated carbon to increase the adsorption rate of methyl red dye depending on temperature, dye concentration, and contact duration where their thermodynamic analysis revealed that methyl red dye adsorption is endothermic and follows a physisorption process (Ahmad et al., 2019). In recent research, Ahmad et al. are, also, used a physicochemical technique to manufacture lemongrass leaf-activated carbon (LLAC) for the removal of Remazol brilliant violet 5 R (RBV5R) dye from aqueous solutions (Ahmad et al., 2020). Ding et al. used activated carbon derived from treated rice husk to treat rhodamine B and reached equilibrium elimination (478.5 mg g−1 ) in less than 5 h. The adsorption of rhodamine B on activated carbon was shown to be unaffected by the pH of the original solution (Ding et al., 2014). In another way, Jawad et al. developed sulfuric acid-treated activated carbon obtained from coconut leaves and proved that it is a very effective MB dye adsorbent (Jawad et al., 2016). Ojedokun et al. discovered the potential of activated guava leaf as an efficient precursor for the manufacture of activated carbon and could remove CR dye (Dutta et al., 2021; Ojedokun & Bello, 2017). The potential of Eichhornia crassipes to remove synthetic colors from water has been examined by scientists in recent years. El-Wakil et al. (El-Wakil et al., 2013; Yagub et al., 2014) investigated and compared the adsorption of methylene blue and Rhodamine B by the carbon of E. crassipes stems and leaves, using H3 PO4 to activate the carbon materials. The adsorption of methylene blue and Rhodamine B

1.12 Applications of Conventional Heating of Activated Carbon for Dye …

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was impacted by operational parameters. The number of negatively charged binding sites increased at high pH, facilitating the removal of methylene blue and Rhodamine B by E. crassipes activated carbon (Lima & Asencios, 2021). Peels are mostly a result of fruit and vegetable material, and using them as AC precursors reduces waste and its environmental effects (Srivastava et al., 2021). Pomegranate peel is thrown out as a leftover; however, it may be used as an inexpensive and sustainable biomass origin. In the study by Asmaa E. Elsayed et al., the pomegranate peel-based activated carbon (ACPP) which was activated by HCl acid might be recycled three times for adsorption–desorption cycles until the sorption capacity is less than half of the initial one (Elsayed et al., 2020). Since corn is one of the most widely grown crops in the world, with a huge yield and inexpensive price, many researchers used popcorn in the preparation of activated carbon due to its unique fluffy structure. Yu et al. created an extremely fluffy popcorn structure that is advantageous in forming a developed porous structure with a high specific surface area of carbon. A basic NaOH activation is utilized to make fluffy honeycomb-like activated carbon from popcorn allowing PDAC-4 to have a maximum RhB adsorption capacity and good adsorption properties on other dyes as CR, MO, and MB (Yu et al., 2019). Because maize cob is a common agricultural waste, it was chosen. It is also an environmentally friendly agro-waste produced in large amounts during various maize processing operations. Ojediran et al. looked into the adsorption of malachite green (MG) dyes onto FZMC, a low-cost agro-waste. The sorption appropriateness of the adsorption technique was demonstrated. At pH 8, 120 min contact time, and concentrations of 100–600 ppm, excellent MG dye adsorption in a batch approach was achieved (Ojediran et al., 2020). Wood and sawdust, which is produced as a by-product from the mechanical milling or processing of wood with varying usable sizes, is a known and plentiful preparatory lignocellulosic biomass source that is accessible for the creation of AC. While most of this wood waste is burned directly for energy recovery, which frequently results in air pollution, a minor amount is also placed in landfills. As a result, waste wood use has received considerable attention (Srivastava et al., 2021). Due to its availability and high cellulose, lignin, and hemicellulose content, wood is the most often utilized material in the synthesis of activated carbon. Consequently, several researches have examined the use of wood in various sectors, including petrochemicals, fuel, and activated carbon (Al Jebur & Alwan, 2022). The wood of the guava tree is a good source of activated carbon and is commonly used in turnery and carpentry. Furthermore, this wood is a great fuel source and a valuable charcoal resource. Mansour et al. tested the rejection of the brilliant green dye from wastewater using activated carbon produced from guava tree wood. In an aqueous solution, the adsorbent (AC) may totally remove the BG dye. The adsorption efficiency was improved, and this was also supported by the results of the kinetic study for BG dye removal (Mansour et al., 2020). Microalgae have high sorption capacity and high binding affinity due to their functional groups. El Mekkawi et al. used activated de-oiled microalgae dried biomass to study the biosorption of Acid red1 (AR1) dye from aqueous solutions as shown

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Fig. 1.13 Carbonization and activation process routes for C. Vulgaris algae

in Fig. 1.13. The activation process reduces the size of the biomass surface to a regular and small uniform size, which increases the surface area and, consequently, the adsorption capacity (El-Mekkawi et al., 2021). Furthermore, Moghazy examined the biosorption of methylene blue dye from aqueous solution using dried microalgal biomass (Nile water algae, as well as isolated species (Chlamydomonas variabilis, Anabaena constricta) and activated Nile water algae with (CaCl2 , Na2 CO3 , Na2 SO4 , H2 SO4, and H3 PO4 ) (Moghazy, 2017). Abdelghaffar et al. extracted fatty acids from C. Vulgaris that are appropriate for high-quality biodiesel synthesis. The percentage of acid dye removed is more impacted by acidic solutions at various dye concentrations. The Langmuir isotherm model fits the equilibrium data better than the Freundlich isotherm model (Abdelghaffar et al., 2021). By using oxygen-insensitive or aerobic azoreductases, some aerobic bacterial strains may break the azo link (Varjani et al., 2020). To remove CR, Wang et al. used a nitric acid acidified activated carbon entrapped bacteria as an absorbent. Langmuir’s model fits well with the adsorption of Congo red by bacteria surfacemodified activated carbon (Wang et al., 2021).

References

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References Abdelghaffar, F., Abdelghaffar, R. A., Mahmoud, S. A., & Youssef, B. M. (2019). Modified sugarcane bagasse for the removal of anionic dyes from aqueous solution. Pigment & Resin Technolog, 48(5), 464–471. https://doi.org/10.1108/PRT-01-2019-0003 Abdelghaffar, R. A., El-Mekkawi, S. A., Abdelghaffar, F., & Abo El-Enin, S. A. (2021). Integrated utilization of Chlorella vulgaris as biofuel and dye biosorbent. Desalination and Water Treatment, 235, 241–250. https://doi.org/10.5004/dwt.2021.27665 Ahmad, A., Mohd-Setapar, S. H., Chuong, C. S., Khatoon, A., Wani, W. A., Kumar, R., & Rafatullah, M. (2015). Recent advances in new generation dye removal technologies: Novel search for approaches to reprocess wastewater. Rsc Advances, 5, 30801–30818. https://doi.org/10.1039/ c4ra16959j Ahmad, M. A., Ahmed, N. A., Adegoke, K. A., & Bello, O. S. (2020). Trapping synthetic dye molecules using modified lemon grass adsorbent. Journal of Dispersion Science and Technology, 43(4), 1–15. https://doi.org/10.1080/01932691.2020.1844016 Ahmad, M. A., Ahmed, N.‘Adilah, B., Adegoke, K. A., & Bello, O. S. (2019). Sorption studies of methyl red dye removal using lemon grass (Cymbopogon citratus). Chemical Data Collections, 22, 100249. https://doi.org/10.1016/j.cdc.2019.100249 Al-Balushi, K., Revanuru, S., & Sajjala, S. R. (2017). Preparation of activated carbon from date seeds and evaluation of its applications. In International Conference on Civil, Disaster Management and Environmental Sciences (CDMES-17) Feb, 2–3,113–117. https://doi.org/10.17758/EIRAI. H0217302 Al-Ghouti, M. A., & Sweleh, A. O. (2019). Optimizing textile dye removal by activated carbon prepared from olive stones. Environmental Technology & Innovation, 16, 100488. https://doi. org/10.1016/j.eti.2019.100488 Al-Qodah, Z., & Shawabkah, R. (2009). Production and characterization of granular activated carbon from activated sludge. Brazilian Journal of Chemical Engineering, 26(1), 127–136. https://doi. org/10.1590/S0104-66322009000100012 Al-Swaidan, H. M., & Ahmad, A. (2011). Synthesis and characterization of activated carbon from Saudi Arabian dates tree’s fronds wastes. In 3rd International conference on chemical, biological and environmental engineering, 25–31. Citeseer. Al Jebur, L. A., & Alwan, L. H. (2022). Development of nano-activated carbon and apply it for dyes removal from water. Water Practice and Technology, 17(1), 297–310. https://doi.org/10.2166/ wpt.2021.105 Alharbi, H. A., Hameed, B. H., Alotaibi, K. D., Al-Oud, S. S., & Al-Modaihsh, A. S. (2022). Recent methods in the production of activated carbon from date palm residues for the adsorption of textile dyes: A review, 10, 1–24. https://doi.org/10.3389/fenvs.2022.996953 Alorabi, A. Q., Hassan, M. S., & Azizi, M. (2020). Fe3 O4 -CuO-activated carbon composite as an efficient adsorbent for bromophenol blue dye removal from aqueous solutions. Arabian Journal of Chemistry, 13, 8080–8091. https://doi.org/10.1016/j.arabjc.2020.09.039 Alslaibi, T. M., Abustan, I., Ahmad, M. A., & Abu Foul, A. (2013). A review: Production of activated carbon from agricultural byproducts via conventional and microwave heating. Journal of Chemical Technology & Biotechnology, 88, 1183–1190. https://doi.org/10.1002/jctb.4028 Amar, M. B., Walha, K., & Salvadó, V. (2020). Evaluation of olive stones for Cd (II), Cu (II), Pb (II) and Cr (VI) biosorption from aqueous solution: Equilibrium and kinetics. International Journal of Environmental Research, 14, 193–204. https://doi.org/10.1007/s41742-020-00246-5 Anirudhan, T.S., & Ramachandran, M. (2015). Adsorptive removal of basic dyes from aqueous solutions by surfactant modified bentonite clay (organoclay): kinetic and competitive adsorption isotherm. Process Safety and Environmental Protection, 95, 215–225. https://doi.org/10.1016/ j.psep.2015.03.003 Aremu, M. O., Arinkoola, A. O., Olowonyo, I. A., & Salam, K. K. (2020). Improved phenol sequestration from aqueous solution using silver nanoparticle modified Palm Kernel Shell activated carbon. Heliyon, 6, e04492. https://doi.org/10.1016/j.heliyon.2020.e04492

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Chapter 2

New Trends Using Microwave Techniques

Abstract This chapter provides an examination of microwave technology, including its historical studies, basic chemistry, and practical applications. There has been a gradually rising interest in this area of study due to the revival of activated carbon from biowastes. Microwave technology has been shown to drastically lower costs, accelerate reaction times, improve yields, and selectively activate. Current advancements in the preparation of activated carbons are summarized. In comparison to traditional approaches, the introduction of microwave radiation has been recognized as a major advancement. Also, the main disadvantages and difficulties with its process are highlighted and explored. In conclusion, it is anticipated that microwave energy will be a potentially powerful and practical substitute for fuel technology in a variety of fields, and its advancement represents an increasing topic in the field of adsorption science. Keywords Disadvantages · Microwave heating system · Physical activation · Chemical activation

2.1 Microwave-Assisted Heating Technique Carbon materials are among the many different types of materials that work well as microwave absorbents. Due to this property, these carbonaceous compounds may be microwave heated to create new materials with altered characteristics, which involve the synthesis of an extensive range of carbon substances, including nanostructures, graphite, active carbons, and polymers (Ahmed, 2016; Menéndez et al., 2010). However, compared to other typical techniques based on traditional heating, the number of procedures combining carbons with microwave heating has greatly expanded. Prior to the late 1990s, there were very few scientific publications on these subjects, but during the past decade, the number of publications has increased dramatically (Alslaibi et al., 2013; Ewis & Hameed, 2021). Researchers are increasingly interested in adopting microwave heating techniques as part of the activation process for active carbons (Li et al., 2021; Sousa et al., 2021; Zhou et al., 2021). © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. Abdelghaffar, Effluent Dye Removal by Microwave-Assisted Activated Carbon, SpringerBriefs in Molecular Science, https://doi.org/10.1007/978-3-031-41145-8_2

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2.2 The Disadvantages of Conventional Heating Methods Traditional heating is uneven and sluggish, and results in surfaces, margins, and corners that are significantly hotter than the material’s interior. As a result, the product’s goodness is inconsistent and usually lower than that is expected (Ahmed, 2016). Whereas the traditional technique of heating creates a temperature gradient from the hot surface to the inside of every particle and varied temperature distributions for particles of different shapes and sizes, a decelerated rate of heating with isothermal holding is employed to prevent this thermal gradient at high synthesis temperatures inside the material. This sluggish heating rate at intermediate temperatures lengthens the development procedure in the traditional heating technique, leading to more power usage (Hesas et al., 2013). Gases produced during pyrolysis cannot escape into the environment because of the temperature gradient. As a result, certain volatile components can still be present inside the particles, leading to issues with carbon deposition. The microporous network may become obstructed by the carbon that has been deposited, causing deformation, a heterogeneous microstructure, decreased total pore volume, and poor BET surface area values (Yuen & Hameed, 2009). Therefore, the product’s quality varies and is typically lower than what is intended to happen. Another drawback of the thermal heating approach is conventional quickfiring, which is dependent on the kind of reactor, operating circumstances, process configurations, and AC precursors used (Ao et al., 2018). The processes of carbonization and activation are frequently carried out by pyrolysis utilizing traditional heating sources such as electric ovens or furnaces. This typically necessitates a lengthy process time (1–7 h) (Dural et al., 2011). and elevated procedure temperature (up to 700 °C) (Ahmad & Alrozi, 2010; Garba & Rahim, 2014), leading to significant power consumption and elevated development costs (Lam et al., 2017a, 2017b). As a result, the disadvantages of using a typical furnace to activate a process include substantial energy consumption and a lengthy activation period, which are overcome by employing a microwave heating approach (Hesas et al., 2013). It is well known that utilizing a microwave as a source of heat enables speedy and accurate heating (Lam et al., 2015, 2017a, 2017b; Wan Mahari et al., 2016). MW-assisted activation, that can create better quality activated carbon through onestep (Deng et al., 2010) or two-step activation methods, combines physical and/or chemical activation (Reza et al., 2020).

2.3 Characteristics and Mechanism of Microwave Heating Microwaves are electromagnetic radiations that have wavelengths between 300 MHz and 300 GHz and lengths between 1 mm and 1 m (Ao et al., 2018; Brazil et al., 2022). Domestic microwave ovens operate at 2.45 GHz, whereas commercial ones operate at 915 MHz (Ao et al., 2018; Brazil et al., 2022). These wavelengths are controlled

2.3 Characteristics and Mechanism of Microwave Heating

31

Fig. 2.1 Microwave heating system (Ewis & Hameed, 2021)

for use in industry, science, and medicine to prevent interference (Ao et al., 2018). According to Fig. 2.1, the MW heating system is made up of the power supply, magnetron, applicator, and waveguide. This technology uses MW energy that is guided by a waveguide to heat the material inside a metal applicator that changes according to the process requirements. An insulated conveyor belt is used to transport the material through the oven as it is being heated up to improve energy distribution (Ewis & Hameed, 2021). Microwave heating exhibits various features while acting on various materials. The dielectric material, which makes up the majority of natural substances (like water and food), is composed of molecules (or dipoles) with opposing electrical charges which can absorb microwaves and provide the effect of heating. A specific frequency range of dielectric heating includes microwave heating. Dipole rotation and ionic conduction provide energy to the materials while interfacial and dipolar polarization effects serve as the basis for the dielectric heating process. The dipoles in the medium exhibit chaotic spatial distribution and random mobility in their native form. Molecules that are both polar and non-polar make up the dielectric substance. The dipoles in the dielectric substance will reallocate themselves in the electromagnetic field E according to the principle of opposite-pole attraction when the dielectric medium is subject to the action of electromagnetic field E, such as microwave radiation (Palma et al., 2020). The ion shifts rapidly in a manner akin to the rotation when the dielectric material is subject to electromagnetic field E. The chaotic dipole will then transform into a rearranged array of polarized molecules as a result. Figure 2.2 depicts the impact of an electromagnetic field E on a polar molecule’s dipoles (Yang & Du, 2022). A process known as dipolar polarization which occurs by the electric field component of a microwave at a certain frequency of more than a million times per minute in the atomic level of the staff, makes polar molecules (such as water) to spin and attempt to line up in both persistent and induced dipoles. Because of the increased molecular rotation and mobility, friction, and collision occur more often, which causes heat to be created and swiftly dissipated throughout the material (Ao et al., 2018). In pressed compact materials, microwave radiation directly interacts with the particles, converting electromagnetic energy into thermal energy transfer inside the dielectric substances. Quick volumetric heating is achieved because no outside heat is transferred into the sample (Ahmed, 2016). The impact of the microwave is often

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Fig. 2.2 Effect of an electric field E on the dipoles of a polar molecule (Yang & Du, 2022)

inversely correlated with the induced polarity, indicating that microwave radiation has the properties of selective, uniform, and volumetric heating (Ao et al., 2018).

2.4 Microwave-Assisted Thermal Process Since carbon materials often make excellent microwave absorbers, there has been an increased interest in utilizing them in a range of microwave-assisted thermal procedures (Fig. 2.3). Due to their potential use in commercial applications, some of which have already been demonstrated at pilot or even manufactural scales (Alslaibi et al., 2013; Ewis & Hameed, 2021). When a microwave interacts with a dielectric material, the microwave’s high-frequency vibration directly interacts with the material’s molecules, causing them to move erratically. This causes the dielectric material to heat up overall and drastically reduces the time needed for heat to transfer (Yang & Du, 2022). Thus, it was demonstrated that it was possible to provide a quick and “volumetric” heating to heat the material in large quantities (Lam et al., 2017a, 2017b; Lei et al., 2009).

2.5 The Advantages of Microwave Techniques for the Activation Process Microwave heating provides a lot of benefits over traditional heating, including energy transfer, selective heating, increased efficiency, fewer steps, fewer activation temperatures, greater safety, easiness, littler equipment size, and reduced automation (Ahmed, 2016; Ji et al., 2007). In comparison to traditional heating, microwave heating has several advantages:

2.5 The Advantages of Microwave Techniques for the Activation Process

33

Fig. 2.3 Components of a microwave activation system

Firstly, the microwave is distinct from the traditional heating process, which creates a temperature gradient of elevated outside and low within by radiating heat from the material’s surface to its inside. However, the material’s exterior temperature in the microwave is lower than its internal temperature as a result of the material’s surface dissipating heat, which promotes the material’s outer diffusion and has a positive impact (Yang & Du, 2022). Also, there is not any direct contact between the MW heating supplier and the heated substance while it heats the substance from the interior out using selective heating that relies on the nature of the thing (Dehdashti et al., 2011) as shown in Fig. 2.4.

a

b

Fig. 2.4 Comparison of heating patterns of the conventional furnace (a) and microwave (b) (Ao et al., 2018)

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Fig. 2.5 Schematic of the manufacture of AC from biomass using microwave radiation (PhA = physical activation, ChA = chemical activation, MW = microwave radiation, ChR = chemical reagents, CH = conventional heating) (Ao et al., 2018)

Carbonization and activation are the two primary processes in a reactor-based one-step microwave activation (Li et al., 2016). The advantage of this activation is that it allows for the easy acquisition of an AC with a steady configuration. According to Fig. 2.5, two-step microwave activation includes carbonizing and igniting biochar when it is exposed to microwave radiation (Li et al., 2008). The biomass sources, pyrolysis temperature, radiation power, operating duration, and additives all affect the BET surface area in this process (Ao et al., 2018). In addition to creating ACs with an improvement in pore distribution and textural qualities, microwave activation is a flexible process with incredibly controllable variables (Angın et al., 2013; Reza et al., 2020). As a result of the material activation process taking less time, less activating gas may also be used. The amount of activating gas required might be decreased as a result of the material’s quicker activation time (Yusop et al., 2021). Other benefits of microwave-assisted pyrolysis over traditional electrical-heated pyrolysis methods involve improved procedure control for rapid start-up and shutdown, more evener heat distribution, augmented production speeds, and less expensive production (Kundu et al., 2015; Lam et al., 2017a, 2017b). Finally, it is believed that the use of microwave-assisted pyrolysis in the manufacture of AC may demonstrate advantageous properties that might result in a heating process that uses less energy, takes less time, and produces more desired products (Lam et al., 2017a, 2017b).

2.7 The Disadvantages of Microwave-Assisted Activation Process

35

2.6 Activation Procedures with Microwave-Assisted Pyrolysis There are two primary impacts of employing microwave reactors, the first is the thermal action that is related to the heating of the dielectric and is brought on by molecular dipoles trying to adapt to the shifting electric field of the microwave radiation. A more uniform thermal zone is produced as a result of the friction and collisions that are caused. The heat is then dispersed due to molecular friction and dielectric loss of the molecules themselves. Secondly, thermal processes like conduction, convection, or radiation then occur after this. The interaction of molecules with a dipole moment and the charges of the electric field is connected to a unique non-thermal microwave effect. After stabilization, electrostatic energy is produced, and it may be compared to the alignment of charges when an electric current is passed (Priecel & Lopez-Sanchez, 2018). In this concern, carbon substances are fine microwave absorbers as a result of the interaction of the delocalized π electrons with the microwaves, consequently converting microwave energy into heat.

2.7 The Disadvantages of Microwave-Assisted Activation Process Despite the benefits that microwave pyrolysis has demonstrated, the method has the potential to produce hot patches (due to mineral impurities) to the inner of the carbon molecules where the temperature is significantly greater than the sample’s average temperature (Ahmed, 2016). The amount and severity of the small spots produced during MW heating depend on the chemical makeup, form, and size of the carbon (Ao et al., 2018). As a result, it is practically hard to determine the staff temperature properly; instead, an infrared pyrometer can only estimate the sample’s surface temperature. Due to the internal and volumetric property of microwave heating, the sample’s interior temperature may be tens or even hundreds of degrees greater than the sample surface temperature (Ahmed, 2016). Thus, MW heating can produce hot spots both inside the bulk materials in addition to the carbon particles (Ao et al., 2018). The development of manufactural microwave pyrolysis employments is also constrained by what appears to be a lack of knowledge of microwave systems, the impact of important process variables on the required product, and the technical data needed to design commercial systems for those pyrolysis employments. Additionally, the materials that may be utilized in a reactor’s construction and design are constrained by the usage of microwaves. So, the scaling up and optimization of microwave pyrolysis processes is complicated by these constraints (Lam et al., 2017a, 2017b). The comparison between the advantages and disadvantages of traditional and microwave heating for the production of AC are described in Table 2.1.

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Table 2.1 Advantages and disadvantages of microwave and traditional heating for activated carbon production (Ao et al., 2018) Heating methods

Advantages

Disadvantages

Microwave heating

Quicker processing times and energy savings; volumetric heating that is quick and even; non-contact approach; reduce the activation temperature; gradient in temperature from the center to the surface; impacts of thermal and catalysis in microplasmas; easy pore expansion occurs when light components are liberated; prompt start-up and shutdown, increased safeness, in addition to improved efficiency; reduced feedstock pre-treatment requirements and less equipment size

Improper measuring of the temperature and management; thermal runaway; hot patches brought on by microplasma and unevenness of mediums; MW may possibly leak; the economy must be upgraded; process scale-up is still in its infancy

Conventional heating

Easy scaling up and a relatively developed process

Low efficacy of gaseous product removal; carbon deposition and obstruction of the microporous network; rather a large temperature gradient within the particle; minimal BET surface area and pore volume; uneven microstructure and distortion; more expensive and time-consuming treatment; perhaps quick firing

2.8 Comparison of Traditional and Microwave Heating in the Preparation of AC Table 2.2 compares the preparation of AC using traditional and microwave heating from many angles, including treatment time, heating process, gas consumption, equipment size, AC preparation parameters, and BET surface area of generated AC.

2.9 Different Activated Carbon Factors by Microwave Treatment The characteristics of AC with MW heating, including carbon’s capacity for adsorption, carbon yield, and BET surface area, are influenced by factors including feedstock kinds and sources, impregnation ratio (IR), pyrolysis temperature and radiation period and power, holding duration, and additional activation agents (Du et al., 2017). The extremely adaptable nature of MW heating is due to these highly controlled

2.9 Different Activated Carbon Factors by Microwave Treatment

37

Table 2.2 Comparison of traditional and microwave heating techniques for producing AC (Alslaibi et al., 2013) Parameter

Traditional heating

Microwave heating

Treatment time

To attain the necessary degree of activation, the heat procedure may need many hours or even up to a week, adding to the processing expenses

It is possible to significantly shorten the treatment period, which frequently translates into less energy being consumed

Heating process

Surface heating • The heated surface of the char particle creates a thermal gradient that causes problems with the caloric transfer from the hot surface to the inside • Different shapes and sizes of samples cannot all be heated to the same temperature by the hearth wall of the furnace. This has a negative impact on the quality of the ACs that are created because it creates a temperature gradient from the sample particle’s heated surface to its inside and hinders the efficient evacuation of gaseous outputs to its atmosphere

Both internal and volumetric heating • The microwave-induced reaction may continue fast and efficiently at a decreased bulk temperature due to a large thermal gradient from the interior of the char particle to its cold surface • By using ionic conduction and dipole rotation, the absorbed microwave easily converts to heat inside the particles, offering the benefits of energy efficiency, quick temperature rise, and uniform heating

Gas consumption

Large gas usage throughout the treatment procedure

Because microwave treatment takes place quickly, gas consumption may be decreased

Equipment size

The huge size of the traditional furnaces

In comparison to traditional electric tube heating, microwave furnaces are often littler than traditional furnaces

AC preparation conditions

The ideal experimental parameters for producing AC from agricultural outgrowths were activation temperatures of 400–800 °C, activation times of 1–3 h, and IR ranges of 0.5–2

350–700 W of microwave radiation energy, 5–15 min of microwave radiation duration, and 0.5–2 of IR were found to be the scopes of ideal experimental conditions for creating AC out of agricultural waste

AC surface area

The generated AC has a large surface area

For the same precursor, microwave heating results in the creation of a substantially greater AC surface area than traditional heating

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factors; furthermore, it is feasible to promote those elements to produce ACs with favorable pore formation and surface properties (Du et al., 2017; Yusop et al., 2021).

2.9.1 Microwave Radiation Power Although it is challenging to precisely determine the sample temperature, MW power and temperature are strongly connected. Previous researches (Foo & Hameed, 2012b, 2012c; Hesas et al., 2013) attempted to examine the effects of MW-aided activation and traditional activation techniques on the surface and pore properties of ACs. Nevertheless, it is hard to compare those two methods because of a variety of factors, including the sample’s unknown temperature and various heat transmission pathways. Microwave absorbers (MWAs) may be rapidly heated by MW; on the other hand, it is difficult to heat materials with weak MW absorption ability to the appropriate temperature. To improve the treated materials’ ability to absorb MW, additives are occasionally added to sample preparations. The sample’s overall mass impacts the heating rate and ultimate temperature of the AC precursor, making it another crucial factor in MW radiation power for activation.

2.9.2 Microwave Activation Time Another crucial aspect that affects in the activation of ACs is the MW radiation time, which has a crucial impact on the degree of activation (Foo & Hameed, 2011, 2012a). If the MW power is constant, the longer the MW heating period, the more power is transmitted to the AC precursor and the greater the sample temperature. The number of active sites and pores that initially emerge upon the surface of materials increases as the MW radiation period is extended. As a result, ACs saw an increase in adsorption capacity. The pores in the carbon, however, would be burned out by MW heating when MW radiation duration reached a particular amount, which would decrease the ACs’ capacity to adsorb and lessen their yield. Compared to traditional activation, MW irradiation greatly reduces activation time and energy consumption, primarily because of the special properties of MW heating (Ao et al., 2018).

2.9.3 The Characteristics of the AC Precursor According to the MW absorption capability of the sample, the MW radiation energy and radiation period should correspond to the entire bulk of the AC precursor to prevent excess heating or inadequate one. The distinctive physical characteristics of biomass, such as its irregular form, broad particle size distribution, high moisture enclosure, and decreased particle density, as well as its varied chemical compositions

2.10 The Most Modeling and Optimization Approaches for Adsorption …

39

and structures (Dai et al., 2012) the size and form of the particle, as well as its relationship to its temperature gradient, are crucial for heat transmission. The temperature differential within a particle increases with particle size. Due to MW heating’s selective and volumetric features, the AC precursor’s MW absorption capacity is typically more significant than particle size and form. Moisture in the sample increases energy consumption and slows down the heating rate during traditional activation. However, because water has a strong capacity for MW absorption, it accelerates the pace of heating for MW activation.

2.9.4 Microwave and Chemical Reagent Interactions The activation time and temperature are shortened during chemical activation, due to interactions between the chemical activation chemicals and the impregnation of the AC precursor. Until the ideal impregnation ratio was attained, chemical activation might be first strengthened by raising the mass fraction of the chemical reagent. Augmenting the reagent ratio further might have negative impacts, such as burning and blocking the pores with too much reagent. Thus, the region that is accessible could be less. As previously indicated, depending on the individual AC precursors, secondary pyrolysis, interactions between the AC precursors and additives, process circumstances, MW energy, and MW activation period, the impacts on the AC product are complicated (Ao et al., 2018). The major use is highly reliant on the porosity of the ACs, including the surface area, pore volume, and internal porous structure. The preparation and activation process as well as the precursor have an impact on these adsorptive qualities (Norashiddin et al., 2020).

2.10 The Most Modeling and Optimization Approaches for Adsorption Activated Carbon Production optimization is crucial for AC since it may increase effectiveness without raising prices (Yusop et al., 2021). Whereas, the variables affecting the experimental procedure can also be evaluated simultaneously using a suitable selection of design and optimization models.

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2.10.1 Response Surface Analysis A popular statistical tool for designing the experiment is called response surface methodology (RSM). It is utilized to calculate the influence of the variables (independent factors) on the outcomes (dependent variable) (Amalina et al., 2022) and to improve the operational circumstances. This method involves a variety of experimental data and is a well-liked statistical tool for modeling, examining intricate relationships, and determining the relative importance of various process variables. As a result, RSM looked into and optimized the effects of potential key factors for the preparation employing microwave heating (Du et al., 2017). The use of RSM as an optimization tool is very common because of its many benefits, including the need for a small number of experiments, the interactive action between variables, in addition to the creation of a mathematical relation that connects results to variables (Das & Mishra, 2017). Additionally, the use of three-dimensional graphs allows for the analysis of parameter change (Ghaedi et al., 2016; Yusop et al., 2021). The experimental designs used in RSM include Taguchi robust designs (TRD), Box–Behnken designs (BBD), and centric composite designs (CCD) (Amalina et al., 2022; Gomaa et al., 2021).

2.10.2 Adsorption Isotherm Models The equilibrium performance of adsorbents at constant temperature is described by the adsorption isotherm. It relies on the species that are adsorbed, the adsorbent, the adsorbate, and other solution physical characteristics including pH, ionic strength, and temperature. Adsorption isotherms are often created when an adsorbate comes into contact with an adsorbent for a long enough period of time. During this time, the interface concentration should be in dynamic equilibrium with the adsorbate concentration present in the bulk solution. Isotherm models are used to analyze the adsorption procedure to determine if it is chemisorption or physisorption. The twoparameter isotherm models that were employed for the activated carbon adsorption process are summarized below: Langmuir Model This isotherm model for adsorption was primarily created to represent the adsorption of gases onto solid-phase adsorbates like activated carbon. The Langmuir model is an experimental model that implies monolayer adsorption, where the adsorption procedure takes place at comparable locations. Even on the nearby sites, the adsorbed molecules don’t interact in any other way. According to that model (AlGhouti & Da’ana, 2020), the adsorption is homogeneous. Because the Langmuir model explains chemisorption, the adsorbate molecules interact with the adsorbent surface.

2.10 The Most Modeling and Optimization Approaches for Adsorption …

41

Freundlich Model The reversible and non-ideal adsorption procedure is described by the Freundlich adsorption isotherm model. Unlike the Langmuir isotherm model, the Freundlich model is applicable to multilayer adsorption and is not limited to monolayer formation. According to this isotherm model, the adsorption affinities and heat do not have to be spread equally throughout the heterogeneous surface. The formulation of the Freundlich isotherm model represents the surface’s heterogeneousness as well as the exponential distribution of the active sites and their energy. The Freundlich isotherm model is now broadly used in heterogeneous systems, such as the adsorption of highly interacting organic chemicals or species on molecular sieves or activated carbon (Husien et al., 2022).

2.10.3 Kinetic Model Adsorption kinetics provides the rate of adsorption in terms of the order of the rate constant. Adsorbents have two major qualities that are influenced by the adsorption rate when determining the kind or optimum adsorbent for a given procedure. The first is the capacity for adsorption, while the second is the rate of adsorption. Most adsorption kinetics fit into one of two categories (Corda & Kini, 2018): . Pseudo-first order . Pseudo-second order. Pseudo-First-Order Model Higher-order reactions are seen as first-order reactions in pseudo-first-order kinetic theory. The second-order equation for a reaction rate is K[A][B], where [A] and [B] represent the concentrations of the adsorbent and the adsorbate, respectively. Although B is thought to remain constant when it is tiny in comparison with A, the rate will be k’[A]. As a function of the quantity adsorbed, the pseudo-first-order equation is provided. Pseudo-Second-Order Model This model makes the assumption that the sorption follows second-order chemisorption (Huang et al., 2021; Husien et al., 2022). Intra-Particle Diffusion Model Adsorption kinetics are depicted by the intra-particle diffusion model where the adsorption rate is dependent on the rate at which the adsorbate diffuses through the pores of the adsorbent particles (Husien et al., 2022).

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2.11 Different Techniques for Analyzing the Surface Chemistry of ACs ACs are superior adsorbents because of their porous nanostructure. The weak physical contacts (van der Waals forces) within the adsorbate particles trapped in these nanospaces and the surface (pores) of the carbon intensify. Additionally, at some locations, significant (chemical) interactions between the adsorbate molecules and the activated carbon surface may be seen, resulting in highly reserved (chemisorbed) molecules. In some circumstances, the chemisorbed molecules even break down, and instantly, carbon acts as a catalyzer. Typically, active sites are surface groups and unsaturated atoms on the carbon surface. An extensive range of experimental methodologies and parameters may be used to explore the surface chemistry of activated carbons (Bandosz & Ania, 2006): Boehm and potentiometric chemical titration techniques measurement of carbon pH, point of zero charges, and isoelectric point, spectroscopic methods (infrared spectroscopy— FTIR/DRIFTS, X-ray photoelectron spectroscopy—XPS/ESCA, K-edge X-ray near edge structure—XANES spectroscopy, electron spin resonance—ESR, and electron paramagnetic resonance—EPR), temperature-programmed desorption—TPD or calorimetric techniques. Those methods are helpful for identifying the kind and number of active sites in an AC (Alcañiz-Monge et al., 2022).

2.12 Surface Functional Groups of Different Activation Methods of AC Carbons with comparable physical qualities can have varied adsorption capabilities for comparable adsorbates due to the various surface functional groups that are located on the surface of the AC. Surface groups are significant because their presence or absence may significantly affect how carbon interacts with different adsorbates. The hydrophobic and hydrophilic characteristics of AC are controlled by these surface groups, and they also have an impact on the acidic and basic nature of AC. According to the majority of research, AC frequently has surfaces that contain oxygen. In addition, specialized treatments using oxidizing solutions like HNO3 , H3 PO4 , H2 SO4 , and H2 O2 might lead to the discovery of hydrogen linked to carbon at the margins as well as other functional groups comprising nitrogen, sulfur, and phosphorus atoms (Yunus et al., 2020). The coexistence of acidic and basic groups on the surface is made possible by the amphoteric nature of carbon (Srivastava et al., 2021).

2.13 Applications of Microwave-Assisted Activated Carbon for Dye Removal

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2.13 Applications of Microwave-Assisted Activated Carbon for Dye Removal Pore volume, porosity, and surface area may all be used to assess activated carbon’s (AC) higher adsorption capacity. In order to improve the characteristics of AC supercapacitors, AC with increased pore volume, porosity, and surface area is preferred (Zawawi et al., 2017). Thus, the following applications concentrate on the physical and chemical activation processes, in order to investigate the characterization of the AC.

2.13.1 Physical Activation Physical adaptation enhances the physical properties of the activated carbon by raising the BET surface area and pore volume. Yek et al. used steam activation and microwave heating as part of a novel pyrolysis technique to create ACs using palm shell waste for methylene blue elimination, at 500 W for 10 min, and heating rates up to 150 °C/min. The AC displayed high, homogeneous surface porosity with a high fixed carbon content (73 wt%), the highest AC yield of 80%, and a high micropore of 570.8 m2 g on the BET surface. As a result, it was suggested that microwave-steam activation (MSA) is a wonderful option for creating high-quality adsorbent for color removal (Norashiddin et al., 2020; Yek et al., 2019). Xin-hui et al. compared the output and porousness of the activated carbon made employing traditional heating methods and microwave assistance. To evaluate the relative worth of activating agents and heating techniques, Jatropha hull biomass precursor was activated employing the well-known activating agents, steam, and CO2 . Regardless of the heating technique, the yield of activated carbon was observed to be rather constant for steam activation; however, the AC doubled with microwave heating compared to CO2 activation with traditional heating. By using microwave heating with steam, it was discovered that the pore volume and surface area are double; however employing CO2 activation, it was discovered to be of the same order of magnitude (Xin-Hui et al., 2011). For the first time, Salgado et al. manufactured activated carbons from Babassu endocarp by physically activating them with microwave radiation. The biochar that resulted from the pyrolysis process at 600 °C was heated to 700, 750, and 800 °C for 30 min in a CO2 environment. Nitrogen adsorption isotherms at 77.32 K were used to assess the activated carbons’ porous characteristics, such as their Brunauer– Emmett–Teller (BET) surface area, pore volume, and average pore diameter. Due to its inexpensive cost of manufacture, activated carbon may be used in applications for the adsorption of gaseous pollutants, including iodine, methylene blue, and residual chlorine (Salgado et al., 2018).

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2.13.2 Chemical Activation Certain chemical substances were utilized as activation agents while creating activated carbon. The precursor passes through a number of processes depending on the activating agent that changes the adsorption behavior. The most often utilized compounds as potential activator agents include chemical groups that are alkaline or acidic.

2.13.2.1

Chemical Activation with Alkali Treatment

In the synthesis of activated carbon, potassium carbonate is a well-known activating agent (Abbas & Ahmed, 2016). As it brought about more activated carbon with a greater yield, surface area, and pore volume, potassium carbonate was shown to be a superior activating agent to potassium hydroxide. Additionally, activated carbon made from potassium carbonate has reduced ash and sulfur content and a better ability for adsorbing big molecules (Husien et al., 2022). In a study by Li et al., K2 CO3 activation by microwave radiation was used to study ACs with large surface areas from carbonized tobacco stems. They looked at a variety of microwave radiation periods, from 5 to 60 min, and a microwave energy range from 80 to 700 W. On the yield and adsorption capabilities of the ACs, the impacts of microwave radiation duration and the K2 CO3 /C ratio were assessed. According to the experimental findings, the ideal microwave radiation parameters were 700 W, 30 min, and 1.5:1 for the K2 CO3 /C ratio. Under ideal circumstances, the iodine number, methylene blue adsorption, and yield of AC were each 1834 mg g−1 , 517.5 mg g−1 , and 16.65%, respectively. Additionally, the carbons’ surface area, micropore volume, and pore size dispersion were measured. The findings revealed that the micropore content of the ACs was around 59.98%, with just a few mesopores and macropores; the specific surface area and total pore volume were 2557 m2 g−1 and 1.647 cm3 g−1 , respectively (Alslaibi et al., 2013). Bamboo waste was activated using K2 CO3 as the activating agent, and BWAC was successfully synthesized by Azlan Zahari et al. using a microwave heating technique. The surface area of the mesoporous pores in BWAC was 107.15 m2 /g. The optimal circumstances were 0.08 g/100 mL of adsorbent dosage, pH of 7.62, and contact duration of 8 min for the greatest MB removal (87.36%) of 50 mg/L of MB utilizing BWAC as determined by the BBD model. According to ANOVA experiments, contact duration and pH had the next-highest effects on MB rejection, with an F-value of 121.70, followed by adsorbent dosage. The experimental findings could be explained by the Langmuir model, and BWAC had an adsorption capacity of 85.6 mg/g (Azlan Zahari et al., 2022). In addition to other activating agents, zinc chloride was frequently employed to create activated carbon, particularly from cellulosic and lignocellulosic precursors (Yousefi et al., 2019). For the samples that have been impregnated with zinc chloride during activation, it serves as a dampening agent. Volatile compounds continue to

2.13 Applications of Microwave-Assisted Activated Carbon for Dye Removal

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travel freely through the saturated pores with zinc chloride, and after that, during the activation phase, they are released from the surface of the activated carbon (Husien et al., 2022). Deng et al. studied the AC made from a cotton stalk using ZnCl2 as the activation staff when it was exposed to microwave radiation. The effects on the yield and adsorption capabilities of AC were assessed, and the findings showed that 560 W, 9 min, and 1.6 g g−1 of ZnCl2 were the ideal microwave energy, microwave radiation duration, and ZnCl2 IR values. The yield of the AC synthesized under ideal circumstances was 37.92%, and the iodine number, maximum BET surface area, and methylene blue (MB) adsorption quantity, were each 972.92 mg g−1 , 795 m2 g, and 193.50 mg g−1 , respectively. In order to maximize the manufacture of AC from cotton stalks using microwave heating (Deng et al., 2009). Deng et al. used phosphoric acid. The ideal phosphoric acid content was 50% by volume, the radiation power was 400 W, the radiation period was 8 min, and the impregnation time was 20 h. The MB of the AC synthesized under optimal circumstances had a maximum adsorption capacity of 245.70 mg g−1 and a maximum BET surface area of 653 m2 g (Deng et al., 2010). Microspores are produced in significant quantities on the surface of the activated carbon by chemical activation employing alkaline substances like potassium and sodium hydroxide (Martins et al., 2015). It has been extensively established that sodium and potassium hydroxide are powerful activating agents for the preparation of activated carbon (Husien et al., 2022). Liew et al. researched the production of activated carbon from banana peel utilizing a microwave-activation method to eliminate the malachite green color from wastewater. Two methods were used to create activated carbon; a washed and dried banana peel was carbonized using a 700 W microwave for 20 min in N2 environment. The acquired char had a 35% weight yield. Secondly, microwave power pyrolysis of char impregnated with a KOH/NaOH combination was carried out under N2 environment for chemical activation. To assess the chemical makeup and porosity qualities of the obtained activated carbon, several activating agents, microwave power, radiation period, and impregnation ratios were used. The activation agents were NaOH, KOH, and a KOH/NaOH combination. The produced activated carbons had a micromesoporous structure and had a BET surface area and pore volume of 1038 m2 g−1 and 0.80 cm3 g−1 , respectively. By microwave pyrolysis, a maximum 29 wt% yield of AC was produced from banana peel char. The yield of activation carbon was reduced by 19 wt% by chemical impregnation at a higher ratio (1.5). KOH, NaOH, and Na/ KOH-activation produced activated carbon yields of 22 wt%, 25 wt%, and 24 wt%, respectively. Activated carbon was generated in powder form by KOH activation and in granular form by NaOH activation (Liew et al., 2018; Sultana et al., 2022). Lam et al. conducted an investigation that focused on the employment of orange peels. They successfully created ACs from orange peel under ideal conditions of 500 W microwave power, 5 min of radiation duration, quick heating (up to 112 °C/ min), and low process temperature (460–490 °C). Malachite green dye was then treated using an IR of 1.0 that produced an 87% yield. The AC has a 56.9% removal efficiency. High surface area (1350 m2 /g), extremely porous, and carbon-dense AC were produced by adding chemical activation using alkali metal hydroxides (Lam et al., 2017a, 2017b; Norashiddin et al., 2020).

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2.13.2.2

2 New Trends Using Microwave Techniques

Chemical Activation with Acid Treatment

Phosphoric acid was often used in the activation of several lignocellulosic materials. It is the chemical that is used most frequently in the activation of activated carbon because it can turn raw materials into highly porous activated carbon. Furthermore, H3 PO4 is not volatile and has a low activation temperature (Husien et al., 2022). By turning date sphate into AC, Emami and Azizian used a 40% phosphoric acid concentration. The impregnation ratio, microwave energy level, and radiation duration that generated the best quality AC were 3.0, 700 W, and 2 min, respectively. The AC was next examined for its ability to adsorb methyl orange and for the dispersion of its pores. It was discovered that the AC could remove 98.9% of the targeted dye and that it had type I and II isotherms, indicating that the AC had a mix of microporous and mesoporous structures (Emami & Azizian, 2014; Norashiddin et al., 2020). Liu et al. employed phosphoric acid as the activating agent to manufacture AC from bamboo utilizing a microwave-induced activation technique (Liu et al., 2010). On the activation, the impacts of a number of variables, including microwave radiation strength, microwave radiation duration, and H3 PO4 /carbon IR ratio, were investigated. The outcomes showed that 350 W microwave power, a 20 min microwave radiation period, and a 1:1 H3 PO4 /carbon IR ratio were the best activation conditions, resulting in a surface area of 1432 m2 g−1 and a carbon output of 48%. When compared to the traditional thermal procedure, microwave-induced activation showed a quicker activation rate and a larger carbon yield (Alslaibi et al., 2013). Ali H. Jawad et al. produced mesoporous corn cob activated carbon (CC-AC) from corn (Zea mays) cob by-product using microwave-assisted chemical activation with H3 PO4 at a power of 600 W for 20 min as a renewable and advantageous adsorbent for cationic dye (methylene blue; MB) elimination. The optimal operating parameters for MB dye removal (99.7%) were CC-AC dose of 0.1g, a solution pH of 9.4, a temperature of 39.9 °C, and a contact duration of 34.1 min. The adsorption findings under these ideal circumstances showed that CC-AC was capable of absorbing 183.3 mg/g of MB dye at equilibrium as defined by the Langmuir isotherm (Jawad et al., 2020). Brazil et al. effectively created activated carbons from coffee grinds, olive stones, and Kraft lignin that had been microwave-susceptible after being treated with phosphoric acid. There were yields of 19, 21, and 23%, and surface areas of 550, 1125, and 1170 m2 g−1 , respectively. For the activated carbons based on coffee grounds, olive stones, and Kraft lignin precursors, respectively, excellent values of 80, 90, and 85 mg g−1 , or 82, 95, and 92%, were found for the methylene blue dye adsorption capacity of the generated AC samples. The results allow for the conclusion that microwave-assisted pyrolysis is a technique with significant promise for producing AC quickly and inexpensively (Brazil et al., 2022).

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2.13.3 Physiochemical Activation Yagmur et al. described the creation of AC from used tea leaves (Yagmur et al., 2008). The ACs were made via microwave and H3 PO4 activation. Temperatures within 250 and 700 °C were used to carbonize the waste tea under a nitrogen environment, with various H3 PO4 /precursor IRs ranging from 1:1 to 3:1. Prior to being heated in a furnace, the waste tea and phosphoric acid combination was microwave heated. With the identical precursor and operating circumstances, the maximal BET surface areas for the samples prepared by microwave radiation and traditional technique were 1157 m2 g−1 and 928.8 m2 g−1 , respectively. The findings show that microwave heating had a significant impact on both the BET surface area and the samples’ micropore surface area. The sample acquired using the traditional procedure had a lower micropore surface area than the sample treated with microwave radiation (Alslaibi et al., 2013). Yusop et al. created effectively acacia wood chip-based AC (AWAC) through a physicochemical activation procedure which involved potassium hydroxide (KOH) treatment, followed by the gasification of carbon dioxide (CO2 ) for methylene blue (MB) dye removal while being heated in a microwave. The resultant AWAC was discovered to have a high BET surface area of 1045.56 m2 /g and a mesopores surface area of 689.77 m2 /g utilizing response surface methodology (RSM). According to RSM, the ideal AWAC preparation parameters for radiation power, radiation duration, and IR were 360 W, 4.50 min, and 0.90 g/g, respectively. This led to 81.20 mg/g of MB dye removal and a yield of 27.96% for AWAC. For isotherm and kinetic experiments, the adsorption of MB onto AWAC followed Langmuir and pseudo-second-order, with a Langmuir monolayer adsorption capacity of 338.29 mg/g (Yusop et al., 2021). Activated carbon (AC) was made from dragon fruit peel (DFP) by Ahmad et al. to eliminate the methylene blue (MB) dye from an aqueous solution. The physicochemical activation method entails “CO2 gasification,” chemical treatment with potassium hydroxide, and microwave heating procedures. These ACs were discovered to be with a maximal monolayer adsorption capacity of 232.56 mg/g and were effective for removing cationic MB dye. For DFPAC (725.80 m2 /g, 368.22 m2 /g, and 70.11%), high BET surface area, mesopores surface area, and fixed carbon content were discovered. The Freundlich isotherm, with an estimated sorption capacity of 233 mg/g, provided the most comprehensive explanation of the MB-DFPAC sorption system. The “pseudo-second order” model best characterized the sorption of MB-DFPAC complexes, according to the kinetic analysis (Ahmad et al., 2021).

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Chapter 3

Recycling/Regeneration of AC Using Microwave Technique

Abstract This chapter presents an investigation for comparing the two regeneration processes of spent activated carbons, conventional and microwave heating, to assess the reuse of the regenerated activated carbons. Adsorbents loaded with organic dyes were able to be regenerated using both conventional thermal regeneration and microwave regeneration, however with varying degrees of regeneration efficacy favoring the microwave. Furthermore, microwave-assisted regeneration has a number of benefits, such as quick and accurate temperature control, small space requirements, quick heating, cost savings from fewer exposure times, higher regeneration efficacy over a greater number of cycles, energy savings, and higher efficiency during intermittent use. Keywords Regeneration · Miscellaneous technologies · Spent activated carbon · Microwave technique

3.1 Regeneration of Spent Activated Carbon (SAC) The regeneration choice is typically less costly than a replacement. Therefore, it is crucial to renew adsorbents after using the dye adsorption process as adsorbents may be safely discharged into the environment. So, the regeneration of AC is primarily used to remove pollutants that have been adsorbed in order to restore AC’s adsorption capability (Barjasteh-Askari et al., 2021). Due to its developed pore structure, substantial characteristic surface area, and abundance of surface functional groups, activated carbon (AC) is a type of recyclable and cost-effective carbon adsorption substance with a powerful adsorption capability. During the treatment process, the surface functional groups and the interior pore structure of AC have altered, its characteristic surface area has reduced, and pollutants proceed to assemble upon its surface throughout the adsorption process. As a result, AC’s activity has decreased, it has lost some of its ability to adsorb, and eventually, it loses this ability and turns into waste AC (i.e., the stuff is “spent”). These substances turn into dangerous wastes that are either burned or dumped in landfills. © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 R. Abdelghaffar, Effluent Dye Removal by Microwave-Assisted Activated Carbon, SpringerBriefs in Molecular Science, https://doi.org/10.1007/978-3-031-41145-8_3

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A lot of useable resources will be wasted if the waste AC is not adequately processed, in addition to causing further environmental contamination (Ao et al., 2018). The regeneration of spent adsorbents is a greater appealing option from an environmental and sustainability standpoint to reduce the resource waste and ecological contamination brought out by the waste AC (Genç et al., 2022) to regain its adsorption efficiency and reuse (Yang & Du, 2022). The processes used to regenerate used activated carbons fall into several categories, including thermal and gas treatments (Shah et al., 2014), chemical and extraction processes (Li et al., 2015; Nahm et al., 2012), microbiological processes (Salvador et al., 2015), vacuum processes, electric and electrochemical processes (Narbaitz & McEwen, 2012), supercritical fluid and electrochemical processes, wet oxidation processes, and additional processes involving a microwave (Xia et al., 2015), ultrasound, X-ray irradiation, photochemical processes, and superficial water oxidation. Every technique has advantages and drawbacks of its own (Ma et al., 2017). The thermal regeneration technique is the one that is most frequently utilized. The intensity of the AC structure is diminished as a result of the standard heating and regeneration treatment for AC, which also results in a high rate of AC loss. It is important to note that the pore structure of AC degenerates more visibly with increased regeneration duration and temperature, reducing its ability to absorb substances and its ability to regenerate effectively. Furthermore, while the spent AC is at the lowtemperature desorption phase, the release of organic contaminants will pollute the environment (Yang & Du, 2022). Additionally, the chemical processes used for the regeneration of activated carbons produce organic impurities, and it’s vital to note that these compounds use up a lot of chemical reagents, and must be disposed of. Chemical regeneration is therefore useless for removing the complicated chemicals left over from used catalysts (Ma et al., 2017). The solubility of the chemicals that are adsorbed is a key factor in this process’s efficiency, and following the desorption process, an additional step is often required to recover the extraction agent (Foo & Hameed, 2012a). In contrast, electrochemical, microbiological, and extractive regeneration techniques essentially have no economics (Ma et al., 2017). Despite the fact that bioregeneration is often thought of as a cost-effective method, it is often sluggish and can only be used on materials that may degrade (Foo & Hameed, 2012a).

3.2 Regeneration of Spent Activated Carbon (SAC) Using Microwave Technique Regeneration of spent AC is accomplished using microwave technology, which involves quickly heating the spent AC under microwave heating so as to upset its adsorption equilibrium and cause bulk desorption or disintegration (Yang & Du, 2022). The number, location, and characteristics of the species that have been adsorbed (adsorbates) also have an impact on the microwave regeneration process

3.2 Regeneration of Spent Activated Carbon (SAC) Using Microwave …

55

(Durán-Jiménez et al., 2019). Organic contaminants are able to overpower Van der Waals attraction and accomplish desorption when exposed to microwaves. Some of organic contaminants are pyrolyzed into tiny molecules as a result of the buildup of microwave energy, which occurs as a result of both thermal and non-thermal causes, these small molecules are then eliminated as CO2 , CO, and H2 O. High temperatures cause the carbonization of charred waste and refractory organic contaminants. In doing so, toxins that have been desorbed lessen environmental pollution (Yang & Du, 2022). The quick heating of the activated carbon by microwave radiation has shown extremely positive results. In addition, carbon may be recycled and utilized a lot more than once thanks to microwave technology. This method increases the surface area of the carbon without really damaging it, allowing more contaminants to adhere and raising the value. Microwave regeneration may be superior to the traditional method since it is an effective and clean energy source and can heat objects selectively, more quickly, and with ease using an autonomous control system (Ma et al., 2014; Xin-hui et al., 2012; Zhang et al., 2014). In utilizing microwave regeneration technique for regeneration, the used activated carbon has shown encouraging results (Ma et al., 2017; Xin-hui et al., 2012). Microwave regeneration technique may compensate for the lack of traditional heating regeneration, accomplish a short period of quick regeneration, and comparatively cheap power consumption, Since the qualities of AC regeneration restored well after regeneration, this technology has garnered a lot of attention in recent years (Yang & Du, 2022).

3.2.1 Regeneration Procedures with Microwave-Assisted Pyrolysis The characteristics of AC regeneration in a microwave field might be considered roughly: While the polar material is adsorbed by AC, the dipole will rapidly rotate due to the effect of the microwave, producing heat energy. The phenomenon of desorption of the materials absorbed by AC happens when the heat energy accumulates to a particular level. The adsorbent, which cannot be readily desorbed, will disintegrate or carbonize with further heating. In addition, the microwave field’s impact will cause the space charge polarization of AC, which will give it polarity and enable it to absorb microwave energy. To encourage the decomposition of organic materials, the active center is generated at the adsorption stage (Foo, 2018). Additionally, various activators are often introduced throughout the regeneration process, such as CO2 and water vapor. Both the carbon and the activator are at a greater temperature as a result of the impact of microwaves. At that point, the activator will serve as an etching agent on the carbon wall surface and help rebuild some of the pore structure that has been lost due to the breakdown and high-temperature removal of the carbon wall surface (Yang & Du, 2022).

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3.2.2 Comparison of Conventional and Microwave Heating Regeneration Compared to the material treated in the electric furnace as shown in Table 3.1, it was discovered that microwave radiation significantly preserved the porosity structure of the initial material. Microwave heating has additional benefits over conventional heating methods, including internal heating, quicker heating rates, selective heating, superior control to the heating technique, no direct touch between heated materials and the heating supplier, smaller equipment, and less waste (Zanella et al., 2017). Even while microwave heating is seen to be a good replacement for traditional heating methods in waste management and conversion operations, it is important to note that there are certain restrictions on how microwave heating works. These include a lack of information about the dielectric characteristics of materials like biomass, the requirement for more complex design and execution of the technology, and ambiguity over the true cost of these technologies (Fernández Díez et al., 2011). Table 3.1 Comparison of traditional and microwave heating regeneration (Yang & Du, 2022) Parameter

Conventional heating regeneration

Microwave regeneration

Heating speed and power usage

AC heating has a slow reaction time and uses a lot of energy

High efficiency and energy conservation; regeneration time is only 1/100–1/10 that of conventional heating

Strength of AC following regeneration

The potency of the AC mechanism declines past regeneration; the AC loss rate is higher, often 5–10%

To ensure the potency of regenerated carbon, the carbon loss rate may be significantly decreased by adjusting the microwave irradiation period and heating rate

Effectiveness of regeneration

The pores of AC were closed by the residue left behind after organic pollutants were pyrolyzed, which reduced the effectiveness of AC’s regeneration

The micropores of AC that have been microwave-regenerated are more promoted; also, AC’s adsorption ability is superior to that of the original carbon

Environmental preservation

The environment will experience secondary pollution as a result of the desorbed contaminants

Pollutants that have been adsorbed are broken down into little molecules and released without harming the environment

Cost

High running costs, complex machinery, and expensive energy use

Reduce energy use; minimization of equipment The costs associated with investing are minimal

3.3 Applications of Microwave-Assisted Activated Carbon Regeneration

57

3.3 Applications of Microwave-Assisted Activated Carbon Regeneration It has been demonstrated that in the MW-oxidant system, oxidants such as hydrogen peroxide (H2 O2 ) and persulfate may produce greater oxidative species when stimulated by MW radiation. As opposed to conventional heating, the polarization effect of the water molecules causes the wastewater temperature under MW irradiation to rise more quickly. Due to the Arrhenius equation’s prediction of a rise in the reaction rate constant, this is advantageous for the rapid and successful treatment of wastewater (Wang & Wang, 2016). For the treatment of paracetamol-discoloring spent activated carbon (SAC), Zhang et al. employed a microwave-assisted heating technique. The modified activated carbon (MRAC), which was utilized to remove Congo red (CR) and methylene blue (MB) from an aqueous solution, was created by modifying the regenerated activated carbon (RAC) using hydrogen peroxide (H2 O2 ). The findings of the RAC BET, which had a surface area of 1254.46 m2 /g, demonstrated that the wasted carbon could be successfully regenerated by microwave treatment. The maximal sorption capacities for CR and MB on MRAC (7% H2 O2 ) were 254 and 318 mg/g, respectively, and the Redlich-Peterson model provided the best match to the experimental data from the isotherm. The pseudo-second-order model effectively captured the adsorption kinetics (Zhang et al., 2019). Genc et al. investigated the regeneration of used granular activated carbon (GAC) with reactive dye using sophisticated oxidation techniques based on hydroxyl and sulfate radicals (MW + persulfate (PS), Fe(II) + persulfate, and O3 + H2 O2 ). Under ideal circumstances, the adsorptive capacity was determined to be 4.36, 8.89, and 8.12 mg dye/g GAC, respectively. These values are about equivalent to the adsorptive capacity of raw GAC (8.01 mg dye/g GAC) for the procedures (Fe(II) + PS) and (O3 + H2 O2 ). Over the course of around eight regeneration cycles, the adsorptive ability of GAC produced by the (Fe(II) + PS) regeneration process was increased. Taking into account all factors, the preferred sequence for regeneration processes was (Fe(II) + PS) followed by (MW + PS) and then (O3 + H2 O2 )(Genç et al., 2022). Igbokwe et al. successfully investigated microwave regeneration of Swiss blue dye that had been loaded conventionally with activated carbon from brewer’s leftover grain. It was demonstrated using an oxidant (30% H2 O2 ) that the regeneration efficiency and adsorption capacity of regenerated carbon decreased as the regeneration cycle and starting solid phase concentration increased. The carbon’s adsorptive capacity and regeneration efficiency were investigated after six adsorption–regeneration cycles. The microwave heating technique was found to be effective in regenerating spent activated carbon (Igbokwe, 2016). G. Durán-Jiménez et al. compared the regeneration processes of two activated carbons made from coconut shells (CC) and bituminous charcoal (Ch) that had been saturated with Basic Blue 9 (BB9) and Acid Blue 93 (AB93). The microwave heating procedure was able to renew the wasted adsorbents almost two orders of magnitude faster than the conventional heating process. Microwaves required only 3 min

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3 Recycling/Regeneration of AC Using Microwave Technique

Saturated AC

RegeneratedAC

MW 3 min

Fig. 3.1 Microwave regeneration heating process (Durán-Jiménez et al., 2019)

(Fig. 3.1) to attain equivalent textural qualities as those obtained by conventional heating, which took 190 min (Durán-Jiménez et al., 2019). Foo et al. studied the effects of microwave irradiation on the regeneration of activated carbons made from orange and mangosteen peels and loaded with methylene blue dye. A modified ordinary microwave oven was used to heat the used activated carbons for 2 and 3 min of irradiation. According to the research, microwave irradiation may be used to renew activated carbons made from orange and mangosteen peels. Even after five adsorption–regeneration cycles, the regenerated activated carbons could retain their adsorption uptakes and carbon yield at 238.89–267.91 mg/ g and 75.41–76.69%, respectively (Foo & Hameed, 2012b). In addition, Foo et al. employed microwave heating to regenerate oil palm fiber, empty fruit bunch-, and shell-derived activated carbons that were loaded with methylene blue. A modified regular microwave oven was used to treat the dye-loaded carbons, and it was set to run at 2450 MHz for 2, 3, and 5 min of exposure. The original active sites, pore structure, and adsorption capability of the regenerated activated carbons were all retained by microwave irradiation. Even after five iterations of adsorption–regeneration, the MB’s monolayer adsorption capabilities and carbon production were kept at 154.65–195.22 mg/g and 68.35–82.84%, respectively. The possibility of using a microwave to heat used activated carbon was found by the researchers (Foo & Hameed, 2012a). Arabkhani et al. effectively created Meso-CaAl2 O4 utilizing a citric acid-assisted sol–gel auto-combustion technique as a possible adsorbent to remove the malachite green (MG) from synthetic effluent (Fig. 3.2). Meso-CaAl2 O4 has a nonhomogeneous surface with a very porous nanometric surface shape. Meso-CaAl2 O4 has a specific surface area of 148.5 m2 g and a total pore volume of 1.39 cm3 respectively. The maximum adsorption capacity of the dye, which was determined by fitting the adsorption values using the Langmuir isotherm model, was 587.5 mg g−1 . It was discovered that the results from the adsorption kinetics model matched the pseudosecond-order model. The microwave-assisted heating approach proved successful in regenerating the dye-loaded meso-CaAl2 O4 , and after five reuses, the MG dye adsorption percentage remained over 90%. In conclusion, meso-CaAl2 O4 can be

3.4 Miscellaneous Technologies with Microwave Heating for Producing …

59

Fig. 3.2 MG dye adsorption by meso-CaAl2 O4 and microwave-assisted regeneration (Arabkhani et al., 2021)

recommended for the effective removal of potentially harmful dyes from wastewater, particularly MG dye (Arabkhani et al., 2021).

3.4 Miscellaneous Technologies with Microwave Heating for Producing Activated Carbon (AC) and Regeneration of Spent Activated Carbon (SAC) for Dye Adsorption There have been several findings in recent years that support the treatment of SAC using the electrochemical method (Huang et al., 2015), thermal regeneration (Marques et al., 2017), solvent extraction (Zanella et al., 2017), microwave heating (Wu et al., 2017), and ultrasonic regeneration (Liu et al., 2017). An integration of ultrasound and microwave heating to regenerate SAC has however received comparatively little research. The two specific effects of warmth and mechanical vibration are mostly used in ultrasound regeneration. As a result, the attraction between the adsorbate and adsorbent may be destroyed. It may improve the mass transfer process in addition to speed up mass transfer on the surface of materials (Jiang et al., 2018).

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3 Recycling/Regeneration of AC Using Microwave Technique

Numerous heating and impregnation techniques are employed to alter activated carbon and create the composite adsorbent. As a potential crystallization technique, the ultrasonic-aided impregnation approach has lately attracted a lot of interest (Dou et al., 2015). The development, growth, and breakdown of micrometer-scale bubbles were caused by the passage of a pressure wave through a liquid during the ultrasonicassisted impregnation method, which increased the mass transfer process (Asfaram et al., 2015). Microturbulence within interfacial layers surrounding neighboring solid particles might be produced by shock waves (Asfaram et al., 2015; Dashamiri et al., 2016; Jamshidi et al., 2016). Acoustic streaming, which is the movement of the liquid caused by the acoustic wave, can be thought of as the transformation of sound into kinetic energy (Asfaram et al., 2015; Cheng et al., 2016; Li & Kobayashi, 2016). Cheng et al. (2016) created a composite adsorbent coated with Fe-activated carbon that was microwave-heated and ultrasonically assisted to eliminate methylene blue (MB) from dye waste. Adsorbents were created by immersing carbon that had been ultrasonically activated in a Fe(NO3 )2 solution for 30 min. Fe was spread out into the pores of the adsorbent with the use of ultrasound. While Fe-activated carbon has a larger surface area than raw activated carbon, it was less than ultrasoundactivated carbon. Fe-activated carbon produced under ideal circumstances yielded 83.31% and displayed MB elimination capacity of 257 mg g−1 . Due to the magnetic characteristics of Fe-activated carbon, it is possible to remove it from the dye solution utilizing an external magnetic field (Sultana et al., 2022). According to Zawawi et al., the study used bamboo waste as an initial material, and the activation procedure was carried out utilizing a microwave ultrasonic system. Chemicals utilized in the procedure were KOH and H2 SO4 . For 30 min, microwave activation was done at 100 W and 300 W, and for 60 min, sonication was done at 200 Hz continuously. In a furnace, the material was carbonized for two hours between 400 and 800 °C. The AC supercapacitor’s active surface area and total pore volume were 1167 m2 /g, 0.724 cm3 /g, 740.10 m2 /g, and 0.462 cm3 /g, respectively. The findings showed that MW-ultrasonic activation had the potential to be a rapid, effective, and practical activation technique (Zawawi et al., 2017). Under MW radiation, a system that uses sonolysis can produce more oxidative free radicals. Sequential operations, such as MW irradiation followed by sonolysis, might improve the efficacy of the wastewater treatment (Wang & Wang, 2016). When handling low-concentration organic wastewater, however, the MW-AC system consistently works badly because of the low mass transfer rate of the pollutants (i.e., the pollutant cannot approach the “hot spots” in the solution in a timely manner due to the short lifespan of the “hot spots”). As a result, several kinds of nanomaterials (nano-Fe3 O4 , TiO2 , ZnO, etc.) were put in the MW-AC system in order to efficiently use the MW energy for the treatment of low-concentration organic waste. In the study by Xu et al. used the microwave technique to create the AC-loaded Ti3+ self-doped TiO2 composite substance. Under UV light, rhodamine B dye is employed as a breakdown by-product. The removal rate of Rhodamine B by the composite material approaches 96% by dark adsorption and photocatalysis, and the efficiency is only decreased by 6% when it is recycled three times. At a temperature of 323 K, the modified activated carbon has a maximum capacity of 250.93 mg/

3.5 Environmental Issues

61

g for rhodamine B. As a result, the microwave-prepared composite material may be considered an effective and cost-effective material for wastewater treatment (Xu et al., 2021). Jiang et al. created the Cu-AC by utilizing SAC as a precursor with ultrasound and microwave heating. A heating temperature of 800 °C and a heating period of 30 min were found to be the ideal experimental conditions for the manufacture of Cu-AC. Due to the presence of Cu, CuO, and Cu2 O as well as the established pore structure, Cu-AC performed superior to R-AC in terms of adsorption under these circumstances. Additionally, the Cu-AC showed high reusability. With an elimination effectiveness of 94.7% when exposed to UV light, the produced Cu-AC demonstrated strong photocatalytic activity toward Congo red (CR) solution. The adsorption of Cu-AC and RAC put up with pseudo-second order, according to the kinetic findings. Adsorption isotherm tests showed that Langmuir isotherm was followed. The Cu-AC showed high promise in the analysis of the experimental findings and the effective removal of organic dye (Jiang et al., 2018).

3.5 Environmental Issues Environmental issues in chemical activation are significant, especially in light of the toxic nature of the chemical-activating agents, H3 PO4 has less toxicological and environmental pollutants compared to zinc chloride, and hydroxide (Husien et al., 2022) (ZnCl2 is undesirable or not favored activating agent for these issues), the potential for recycling the chemical-activating agent, and a stream of either liquid or gas from the production process, are considered, as it is clear that ecological considerations in chemical activation are significant. In this field, there is a trend toward investigating and employing moderate activation conditions, with intriguing ways utilizing various residues as carbon precursors (González-García, 2018; Panwar & Pawar, 2020). The prospect of recycling chemical-activating agents has been thoroughly researched in the literature, and it has been determined that the majority of studies in this area focus on KOH activation. This is most likely due to the potassium contained in the activated product may be eliminated by water rinsing, and the bulk of the potassium drawn out water is in the form of carbonate (Yuan et al., 2012). Because of the fact that potassium hydroxide is one of the most popular activating agents due to its superior porosity, extremely small micropore size distribution, low environmental contamination, reduced corrosiveness, cheaper cost (Husien et al., 2022), and high final activated carbon output (Alcañiz-Monge et al., 2022), it should draw attention to the desire for environmentally friendly and sustainable practices. However, maximizing the synthesis of “eco-friendly” and sustainable activated carbons via chemical activation requires a lot more work.

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3.6 Conclusions and Future Outlook Recent researches have focused on the use of available, plentiful, and environmentally friendly sorbents with microwave technology to replace conventional heating process. The microwave technology has a highly promising and beneficial AC regeneration technology, which offers heating, high efficacy, and power savings, as well as decreasing contaminating emissions and the running investment cost is cheap. However, as of right now, most microwave reactors are self-modified, and microwave regeneration technology is still in the laboratory. For use in actual engineering, neither the scale nor the equipment characteristics are sufficient. The study of a convenient microwave heating rate and reaction temperature, as well as the amplifying of the microwave reactor, should be included in future research and development in order to accomplish the effective commercial use of microwave regeneration technology. Second, more research into the process of microwave regeneration is required in order to fully understand the fundamental idea, create a mathematical pattern, and enhance and complete the theoretic database. To boost regeneration effectiveness and eventually encourage the full growth of the waste AC regeneration area, multi-effect regeneration technology might be investigated where a few studies have concentrated on batch kinetic analyses, while most have concentrated on equilibrium isotherm analysis and adsorption capabilities. Response surface methodology (RSM) is extensively used as an optimization method because of the various advantages it provides, such as the need for fewer tests, consideration of the interactive effect of factors, and the production of a mathematical equation that links responses to variables. More kinetic studies and statistical investigations are required to advance the development of activated carbons to the next level of development. In addition, the combination of multiple technologies and microwaves for the treatment of dye industrial waste with resource recovery and sustainability would be made possible by scaling up the processes.

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