Nanomaterials for Air and Water Purification E-Book

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Wissenschaft und neue Technologien
• Sprache: Englisch
• Veröffentlichungsjahr: 2024

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Beschreibung

The primary resource for researches to gain insights into the fundamentals, attributes and characteristics of a wide range of environment-friendly polymers and their nanocomposites, including analytical approaches as well as fabrication of materials and composites.

Leseprobe

Nanomaterials for Air and Water Purification

Edited by Deepu A. Gopakumar,

Jyotishkumar Parameswaranpillai,

Jinu Jacob George, and Midhun Dominic C.D.

The Editors

Dr. Deepu A. GopakumarCochin University of Science and TechnologyDepartment of Polymer Science and Rubber TechnologyKochi 682022, KeralaIndia

Dr. Jyotishkumar ParameswaranpillaiAlliance UniversityBengaluru 562106, KarnatakaIndia

Dr. Jinu Jacob GeorgeCochin University of Science and TechnologyDepartment of Polymer Science and Rubber TechnologyKochi 682022, KeralaIndia

Dr. Midhun Dominic C.D.Sacred Heart CollegeDepartment of Applied ChemistryThevara 682013, Kochi, KeralaIndia

Cover Image: © SkillUp/Shutterstock

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Preface

Air and water are essential elements for the occurrence and continuity of life, including humans, plants, and animals. With the increasing needs of modern society and the advancement of technical knowledge from the beginnings to the present day, air and water are increasingly demanded. Water is one of the planet’s most important natural resources and is required for all life forms to live. Drinking, farming, and other industrial processes make the most use of water. Currently, industrial sectors use a significant amount of water for their activities and discharge that water as wastewater into the ground or waterways. This leads to a shortage of water and environmental contamination. Wastewater contains many contaminants that are harmful to living things. The contaminated water must therefore be treated before it may be used for other uses. With the growing demands of modern society, water and air are compromised, as the concentration of contaminants increases. Due to their high surface area and chemical reactivity, nanomaterials have gained considerable attention in the fabrication of various functional constructs for air and water purification. Recent breakthroughs in nanotechnology and nanomaterials have opened up new possibilities for the development of processes and devices for air and water purification and lowering associated health risks. Several techniques are now being used to cleanse contaminated waters. Due to their numerous benefits, including their straightforward synthesis, unique chemical and physical properties, distinctive photocatalytic activity, advantageous antimicrobial properties, and high efficiency, use of nanoparticles/composites has attracted lot of attention as an effective purification technology among other purification methods. All nanoparticles, composites, and cutting‐edge methods used in water and air purification are thoroughly covered in the book Nanomaterials for Air and Water purification, and new insights are provided. Academicians and researchers from prestigious institutions and universities in India and overseas have each contributed a chapter to this textbook with their research or review work.

The sections of this book focus on various aspects dealing with nanomaterials for air and water purification. This book comprises of nanocomposite for effective air and water filtration systems, water and air filter membranes, photocatalytic materials and technologies for air and water purification, carbon‐based nanomaterials for air purification, etc. This book is mainly focused on the nanotechnology approach, green chemistry, and sustainable approaches of advanced nanomaterials in the area of air and water purification. This book also focuses comprehensively on nanomaterials and the methodologies already developed for air and water purification. Academicians, researchers, textile and environmental engineers, the pollution control board, policymakers, stakeholders, and governmental and non‐profit groups will benefit from this book. This will be an invaluable resource for environmental engineers and materials scientists who wish to study how nanotechnology is used to enhance air quality. This book critically examines and accesses the recent progress in the development of nanomaterials for air and water purification and the present techniques of nanomaterials for air and water purification, together with their negative impacts. This book will be a valuable reference source for university professors, researchers, undergraduate and postgraduate students, and scientists primarily working in the field of materials science, applied chemistry, applied physics, and nanotechnology.

04-12-2023Kochi, India

Dr. Deepu A. GopakumarCochin University of Science and TechnologyIndia

Dr. Jyotishkumar ParameswaranpillaiAlliance UniversityIndia

Dr. Jinu Jacob GeorgeCochin University of Science and TechnologyIndia

Dr. Midhun Dominic C.D.Sacred Heart CollegeIndia

1New‐Age Nano Adsorbents for Water Purification

Subhadeep Biswas, Abhishek Johri, and Anjali Pal

Civil Engineering Department, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India

1.1 Introduction

Rapid urbanization and huge industrialization often lead to the contamination of different water bodies. Irresponsible discharge of dyes, heavy metals, and emerging contaminants into the water often causes enormous pollution. A huge and uncontrolled release of unwanted materials into the water bodies deteriorates the water quality and poses a potential threat to aquatic life. The presence of colored substances, even in a trace quantity, in water is objectionable from an aesthetic point of view. Apart from that, many dye molecules show carcinogenicity and adverse toxic effects. In addition to this, illegal disposal of heavy metal ions present in industrial effluents has worsened the situation. In recent times, many harmful compounds, such as surfactants, phenol derivatives, and pharmaceutical waste, have also been detected in surface and subsurface water. These compounds are termed as emerging pollutants.

Considering all these facts, it is obvious that water purification is one of the most urgent needs to maintain the overall ecological balance. Innovative research activities are continuously carried out all over the globe on various wastewater treatment processes like adsorption, advanced oxidation process, electrocoagulation, electrooxidation, and membrane separation. Adsorption is one of the simplest and most efficient water and wastewater treatment techniques. However, the development of environmentally congenial adsorbents with excellent removal capacity and reusability is still a challenge. For a long time, activated carbon materials have been widely used for adsorption purposes. However, regeneration incapability and high cost associated with activated carbon materials are some of the major drawbacks in their use. Pristine materials like alumina, silica, and bentonite have also been explored in the recent past for adsorption purpose. Ma et al. [1] utilized bentonite‐based adsorbents for removal of anionic pollutants from water bodies. Zhu et al. [2] in their recent review article highlighted the superiority of montmorillonite‐based adsorbents for pollutant removal purposes. In this regard, surface modifications, like acid modification, alkali modification, and surfactant modification, were also tried out for increasing adsorption capacity of different materials. Surfactant‐modified alumina and silica have been found to be successful in removal of phenol [3], dyes [4, 5], and heavy metals [6].

In this field, nanomaterials have opened a new dimension for water treatment purpose. Nanoparticles are materials whose dimensions are less than 100 nm. In recent times, there has been a huge interest in the application of nanomaterials as adsorbents due to their excellent surface properties. Recently discovered materials like graphene and MXene have enhanced the scope of nanomaterials by forming compatible composites with them. Neha et al. [7] in their recent review article described the potential of different nanoadsorbents for the eradication of several toxic pharmaceutical compounds from water bodies. Kyzas and Matis documented in their review article the potential of various types of nanoadsorbents for wastewater treatment [8]. They highlighted the fact that the nanoadsorbents show unique physical and chemical characteristics, and the atoms that have high adsorption capacity lie on the surface of the nanomaterials. The current chapter describes the potential of different categories of nanoadsorbents such as metal–organic framework (MOF)‐based nanoadsorbents, MXene‐based nanoadsorbent, ZVI‐based nanoadsorbent, and biochar‐based nanoadsorbents for water purification purposes. Different mechanisms for pollutant removal by these nanoadsorbents have been elaborated. The next section deals with the characterization techniques commonly practiced in order to get insight into the adsorption process. Lastly, current challenges are often faced in this domain and the scope for further research has been presented before conclusion.

1.2 Different New‐Age Nanoadsorbents

1.2.1 Metal–Organic Framework (MOF)‐Based Nanoadsorbents

MOF‐based compounds can be described as three‐dimensional organic–inorganic complexes having porous structures, high specific surface area, and abundant sites exposed for adsorption [9]. In recent years, MOF‐based nanocomposites have been explored by engineers and scientists worldwide for adsorption of different types of pollutants. A novel surfactant‐functionalized MOF@MOF nanoadsorbent was reported by Li et al. [10] for Cr(VI) removal purposes. Thus, synthesized nanocomposite showed promising adsorption capacity and also reusability due to the presence of tunable pores along with abundant active sites on the surface. It showed good performance throughout a large pH range of 1–11, and the highest removal occurred at pH 2, with the maximum adsorption capacity being 932.1 mg g−1. He et al. [11] carried out thermal decomposition of Ce‐MOF micro and nanocomposites under N2 atmosphere at a relatively lower temperature (400–500 °C). The composites thus produced after thermal treatment showed high phosphate removal efficiency. In comparison to ceria, the removal capacity was 2–4 times higher, and the maximum uptake was obtained at 189.4 mg g−1. Yan et al. [12] reported the preparation and application of various polyoxometalates‐MOF nanocomposite for the removal of cationic dye methylene blue (MB). The removal occurred at 98% within five minutes for an initial MB concentration of 100 mg l−1, and the maximum adsorption capacity obtained was 371 mg g−1.

Soltani et al. [13] synthesized layered double hydroxide (LDH)‐MOF nanocomposite without involvement of any toxic solvent and utilized the same for the remediation of heavy metals such as Cd(II) and Pb(II) from water. The LDH/MOF nanocomposite thus prepared showed promising performance toward heavy metal remediation, and the experimental data fitted well with the Langmuir isotherm model and pseudo‐first‐order kinetic model. The calculated maximum adsorption capacity of the nanocomposite was found to be 415.3 and 301.4 mg g−1 toward the removal of Cd(II) and Pb(II), respectively. Yuan et al. [14] in their recent work designed a UiO‐66‐F‐based nanocomposite for trapping carbamazepine from aqueous media. Experimental data fit well with the Langmuir isotherm model and pseudo‐second‐order kinetic model.

Nguyen et al. [15] reported the feasibility of application of zirconium‐organic framework‐based nanocomposites for the efficient adsorption of model cationic and anionic dyes MB and methyl orange (MO). The MOF‐based nanoadsorbent showed excellent reusability for up to five cycles without any significant loss in uptake capacity.

Zr‐based MOF nanocomposites have been explored by different research groups for wastewater treatment purpose. Huang et al. [16] used magnetic Zr‐based nanocomposite Fe3O4@SiO2@UiO‐66 and its amino derivatives for the abatement of heavy metal ions and dyes from water bodies. It showed a high removal capacity for Pb2+ (102 mg g−1), MB (128 mg g−1), and MO (219 mg g−1). The nanoadsorbent has been reported to possess excellent selectivity toward anionic and cationic dyes by adjusting the solution pH. Moreover, it can completely remove both MO and MB at a neutral pH (∼7). With respect to regeneration, it can be reused six times without losing efficiency. Some other MOF‐based nanoadsorbents include Zr‐cluster‐based MOF for tetracycline removal [17], cobalt‐MOF with morpholine for dye removal [18], and LDH/MOF nanocomposite for Orange II and Cr(VI) removal [19].

1.2.2 Graphene, Graphene Oxide (GO), and Reduced Graphene Oxide (rGO)‐Based Nanoadsorbents

Carbon‐based materials are often explored for adsorption purpose. In this regard, graphene is the latest member of the family of carbon allotropes. It is nearly one atom thick, has an excessively large surface‐to‐ volume ratio, and is considered to be one of the most attractive new‐age adsorbents. Besides graphene, graphene oxide (GO), reduced graphene oxide (rGO), and their composites also emerged as viable options of adsorbents. In one of the recent review articles, Kim et al. [20] investigated the potential of graphene‐based nanocomposites for wastewater remediation. Possessing variable structure, chemical strength, and being light in density, the graphene family offers a numerous variety of nanocomposite materials for adsorption purpose. Shoushtarian et al. [21] explored the GO‐based nanoadsorbent for efficient removal of Basic Red 46, a cationic dye. Under the optimized condition, the maximum removal capacity reached up to 360 mg g−1. The prepared GO was reused for four more cycles. However, the adsorption efficiency dropped with consecutive cycles. During four cycles of regeneration, the adsorption capacity dropped from 360 to 209 mg g−1. Calimli et al. [22] reported the application of reduced GO‐supported nano‐Ni composite for the removal of MB. The maximum removal capacity was obtained at 946.12 mg g−1.

ZnO nanoparticles coated with rGO were synthesized by Wang et al. [23] by means of self‐assembly and in situ photoreduction technique. Thus synthesized rGO‐based nanocomposite was utilized for the removal of rhodamine B (RhB) dye. rGO‐supported ZnO nanocomposite showed excellent adsorption performance toward RhB removal. After adsorption, it was further subjected to dye photodegradation. In comparison to bare ZnO nanoparticles, rGO‐supported nanoparticles showed much higher efficiency. Lujaniene et al. [24] explored the Prussian blue GO nanocomposite for the remediation of heavy metals and radionuclides from wastewater.

Verma and Dutta [25] synthesized nitrogen‐enriched rGO‐based nanocomposite for the removal of uranyl ions from water. A detailed experimental investigation showed that the adsorption process was kinetically and thermodynamically feasible. Removal occurred by chemisorption phenomenon, and the maximum uptake capacity reached up to 337.93 mg g−1. High removal has been attributed to the large specific surface area of the nanocomposite (57.086 m2 g−1). El‐Maghrabi et al. [26] reported the preparation and application of magnetic graphene‐based nanocomposite for uranium removal. Excellent removal uptake of 455 mg g−1 was exhibited by this nanocomposite within 60‐minute contact time. Experimental data showed good correlation with pseudo‐second‐order kinetic model. Moreover, a high degree of sustainability has been shown by the nanocomposites (>5 times profiteering).

1.2.3 ZVI‐Based Nanoadsorbents

nZVI is one of the most effective new‐age adsorbents possessing several distinguished properties that immensely help in wastewater treatment. However, despite possessing several advantageous characteristics, it often gets agglomerated. Hence, various modifications have been reported in the literature to overcome this problem. Eljamal et al. [27] used four polymers, namely polyacrylamide, carboxymethyl cellulose, polyethylene sorbitan monolaurate, and polyvinylpyrrolidone, to increase the reactivity of nZVI and also to prevent their aggregation. Thus, different nanocomposites were finally deployed for nitrate and phosphorus removal from water. It was found that the combination of polyacrylamide and nZVI was the best for the adsorption of phosphorus.

Zhu et al. [28] applied activated carbon‐loaded nZVI for the adsorptive removal of As(III) and As(V) from water. Activated carbon was loaded with ferrous sulfate, followed by reduction with NaBH4. Arsenic removal was fast in the first 12 hours. However, equilibrium was reached at nearly 72 hours. It was seen during the interference study that the presence of different cations enhanced the removal of arsenate, but the presence of Fe2+ diminished the uptake of arsenite. In order to prevent the loss of reactivity of nZVI, Bui et al. [29] synthesized core/shell type structure of nZVI/manganese oxide nanocomposite and applied the same for arsenic remediation purpose. It was quite fascinating to observe that the nanocomposite thus produced was superior in adsorption capacity in comparison to the bare nZVI by two to three times. Jiang et al. [30] deployed biochar‐supported nZVI for the removal of glyphosphate from water medium. Langmuir isotherm model fitted well with the experimental values, and the kinetic data fitted with the pseudo‐second‐order kinetic model. Hence, it was concluded that the adsorptive removal process involved both chemical and surface phenomena. Maximum adsorptive uptake capacity was noticed to be 80 mg g−1, which is nearly twice that of biochar only. Thus loaded, nZVI improved adsorption capacity significantly.

For As(V) removal from wastewater in single‐component system as well as in presence of Se(VI), Suazo‐Hernandez et al. [31] deployed nZVI and nZVI‐ functionalized zeolite‐based nanocomposites. Studies revealed that the zeolite‐based nZVI composite was a much better option for arsenic remediation due to the presence of more specific sites on its surface.

Kadu and Chikate [32] prepared different clay‐based nZVI nanocomposites for Cr(VI) removal purpose. The pore diffusion process has been found to be the governing mechanism for chromium abatement. Besides that, the presence of nZVI further facilitated the reduction of adsorbed hexavalent chromium.

1.2.4 Polysaccharide‐Based Nanoadsorbents

Polysaccharides are biopolymers where complex carbohydrate molecules, i.e. long chain of monosaccharides, are linked by glycosidic bonds. They are biodegradable, nontoxic, biocompatible, low‐cost, renewable, and sustainable, and they contain different functional groups. These polysaccharides have huge potential to be used as adsorbents in the field of wastewater treatment. These materials are popular as adsorbents due to their selectivity for pollutants and the capability of forming chelating compounds for trapping the pollutants using the –OH and –NH2 groups present in their structure. In recent times, nanobiopolymers or nanocomposites with biopolymers have emerged as one of the most interesting materials for wastewater treatment. Different types of biopolymer‐based nanocomposites that have been explored for wastewater treatment purpose have been described in the following section.

1.2.4.1 Chitin and Chitosan‐Based Adsorbents

Among the existing polysaccharides, chitin is the second most abundant polysaccharide after cellulose. It is an animal polysaccharide. It is obtained from crustacean (shells) as a byproduct. It is a homopolymer of β‐(1‐4) linked N‐acetyl‐D‐glucosamine, and found in three forms: chitin‐α, β, and γ. Chitosan (poliglusam) is obtained by deacetylation of chitin. It is made up of monomers of N‐acetyl‐D‐glucosamine and D‐acetyl‐glucosamine linked by glycosidic β‐(1‐4) linkages. It is a cationic biopolymer, semicrystalline in nature and exists in solid state. It is insoluble in water and soluble in dilute acid. Solubility of chitosan depends on many factors, such as degree of acetylation, concentration of acid, distribution of acetyl group on chitosan, molecular weight, and pH of solution.

Karimi et al. [33] deployed the chitosan/Al2O3/Fe3O4 nanocomposite for the removal of heavy metal ions such as Cd(II), Cu(II), Zn(II) from wastewater. In the case of Cd(II), the removal efficiency was highest, followed by Cu(II) and Zn(II). Hassan et al. [34] reported the application of chitosan/silica/ZnO nanocomposite for the removal of MB dye. Maximum removal capacity reached up to 293.3 mg g−1 and the experimental data followed a pseudo‐second‐order kinetic model. Khan et al. [35] reported the application of chitosan/Co‐silica nanocomposites for the removal of dyes such as MO, acridine orange, indigo carmine, and congo red (CR).

1.2.4.2 Cellulose‐Based Adsorbents

Cellulose is the most abundant naturally occurring biodegradable polysaccharide. It is a homopolymer made up of β‐D‐glucopyranose units linked by β‐1,4 glycosidic linkages. It is semicrystalline in nature, showing both a crystalline and an amorphous nature depending upon the source of its extraction. Although it is nontoxic, renewable, biocompatible, sustainable, and cost‐effective, it also has some demerits like poor hydrophilicity and low physical and chemical stability. These demerits can be overcome by reducing the size of polymers up to the nanometer. Reduction of polymer size results in availability of larger surface area, high strength, increased hydroxyl functional groups, and enhanced stability and stiffness of the polymer. Nanocellulose can be categorized into three groups on the basis of its extraction: as nanofibrillated cellulose (NFC), cellulose nanocrystal (CNC), and bacterial cellulose.

In order to remove Cr(VI) from aqueous medium, Dong et al. [36] explored CNC‐based adsorbents. Polydopamine nanoparticles of size less than 30 nm were loaded on cellulose surface. Maximum uptake capacity toward Cr(VI) reached up to 205 mg g−1, which is much higher in comparison to the cellulosic materials. Shoukat et al. [37] synthesized nanocomposites made up of bacterial cellulose and TiO2. Thus prepared cellulose‐based nanocomposite was explored for Pb(II) removal from wastewater. With initial concentration of lead at 100 mg l−1, the removal efficiency reached 90% within 120 minutes at pH 7. The nanoadsorbent showed good reusability for three cycles.

1.2.4.3 Starch‐Based Adsorbents

Starch is a homopolysaccharide made up of glucose monomers linked through glycosidic linkages. It is found in plants, staple crops, crop seeds, and food items such as potatoes, rice, wheat, corn, and maize. It forms straight‐chain polymer known as amylose as well as branched‐chain polymer called as amylopectin. It is nontoxic, renewable, easily affordable, and biodegradable in nature. Different physical, chemical, genetic, and enzymatic methods are used for modification of starch. Chemical methods used for modification of starch are acid hydrolysis, cross linking, acetylation and esterification, oxidation, grafting, etc. Lawchoochaisakul et al. [38] reported the synthesis and application of cationic starch intercalated montmorillonite nanocomposite for the treatment of dye wastewater. The presence of cationic starch in the nanocomposite enhanced the interlayer spacing, leading to an improvement in adsorption capacity. Siddiqui et al. [39] developed starch‐ functionalized maghemite nanocomposites for abatement of As(III) from dilute solutions in order to achieve potable water. Starch functionalized nanocomposite was designated as C1 and nonfunctionalized material was denoted as C2. Incorporation of starch in the maghemite enhanced adsorption capacity. Adsorption data fitted well with the Freundlich isotherm model as well as with the pseudo‐second‐order and film diffusion model. A graphical presentation of experimental data has been shown in Figure 1.1.

Figure 1.1 Langmuir {(a) and (b)} and Freundlich {(c) and (d)} plots for As(III) uptake by C1 and C2, respectively.

Source: Siddiqui et al. [39]/with permission of Elsevier.

1.2.4.4 Pectin‐Based Adsorbents

Pectin is a plant‐based linear polysaccharide found in cell walls of plants. It comprises monomers of galacturonic acid linked by α‐(1‐4) glycosidic linkage. The composition of pectin depends on the method of extraction from its sources. It is an anionic polymer containing hydroxyl and carboxyl groups present in its structure, which can be used as adsorption sites for removal of heavy metals and dyes. It shows gel‐forming ability.

Babaladimath and Badalamoole [40] incorporated Ag nanoparticles inside the pectin hydrogel matrix to prepare a novel adsorbent for MB dye. Thus prepared pectin‐based nanocomposite showed much swelling due to the presence of Ag nanoparticles. After three cycles of usage, it showed promising performance, reaching 80% of its initial efficiency. Kodoth and Badalamoole [41] tested Ag nanoparticles embedded in pectin biopolymer nanocomposite for the adsorption of dyes and metal ions. The maximum adsorption capacity obtained was 1950 mg g−1 for crystal violet (CV) dye, 111 mg g−1 for Cu(II), and 130 mg g−1 for Pb(II).

1.2.4.5 Alginate‐Based Adsorbents

Alginate is a natural copolymer made up of two epimer uronic acids, i.e. 1,4‐(β‐D) mannuronic acid (M) and 1,4‐ (α‐L) guluronic acid (G). It is obtained from alginic acid, which is extracted from brown algae. It is biocompatible, nontoxic, highly water permeable, and used as a stabilizing agent for nanoparticles.

Alginate‐based nanocomposites (alginate‐clay, alginate‐phosphate, and alginate‐activated charcoal) were synthesized by Aziz et al. [42] for the removal of Cd(II) from wastewater. The highest adsorption efficiency was obtained by alginate‐activated charcoal nanocomposites. For an initial concentration of Cd(II) equal to 1000 mg l−1, the maximum adsorption capacity reached up to 137 mg g−1. Moreover, due to its excellent stability and high removal capacity, it has been recommended for usage in continuous‐flow real wastewater treatment systems. Majhi and Patra [43] described the applicability of nanocomposites synthesized from polyaniline and sodium alginate toward organic dye removal. Thus prepared alginate‐based nanoadsorbents showed promising efficiency toward both cationic and anionic dyes. Mozaffari et al. [44], in their recent work, incorporated Cu(II) in an alginate‐based hydrogel polymer matrix to prepare a novel nanocomposite useful for dye removal from wastewater. Malachite green (MG) and CV were tried out as the model dyes. Experimental data showed good fitting to the Freundlich isotherm model as well as to pseudo‐second‐order kinetic model. Moreover, one of the fascinating observations regarding this nanoadsorbent is that it showed good performance even after eight cycles.

1.2.5 MXene‐Based Nanoadsorbents

MXenes are undoubtedly one of the most wonderful materials of recent times, possessing several promising qualities required for water treatment purpose. They are 2D carbides of transition metals with unique physicochemical properties, high hydrophilicity, and oxidation resistance. Moreover, cations can be intercalated through its layers. Jeon et al. [45] in their recent review highlighted the importance of MXene‐based nanomaterials as one of the promising new‐age adsorbents for pollution abatement purpose. Kim et al. [46] explored the adsorption capacity of Ti3C2TxMXene for the removal of pharmaceutical compounds like amitriptyline, verapamil, carbamazepine, 17 α‐ethinyl estradiol, ibuprofen, and diclofenac. It was observed that sonicated MXene nanocomposite exhibited better performance in comparison to the pristine MXene. Fard et al. [47] used 2D Ti3C2Tx nanosheets for the adsorptive removal of barium from water bodies. Under optimized conditions, removal efficiency was achieved up to 90% within first ten minutes.

Sodium‐intercalated Ti3C2Tx‐based nanoadsorbent was explored by Ghani et al. [48] for the adsorptive removal of ciprofloxacin from wastewater. Sodium intercalated nanoadsorbent showed better removal efficiency in comparison to the pristine Mxene.

Jun et al. [49] explored the potency of the MXene‐based adsorbent for elimination of radioactive Cs+ from low‐level radioactive wastewater. It showed an excellent adsorption capacity of 148 mg g−1 within one hour. Moreover, the MXene‐based adsorbent could be reused for at least four cycles, which undoubtedly makes it suitable for practical purpose.

Cai et al. [50] explored the potency of phytic acid‐based MXene‐based nanocomposites for the dye removal from sewage. MB and RhB were taken as the model dyes for the adsorption purpose. The nanocomposites showed excellent adsorptive removal capacity, and even after 12 cycles, MB removal efficiency remained at 85%. Shahzad et al. [51] deployed MoS2‐MXene nanocomposite for the adsorption of mercury. The maximum adsorption capacity reached 7.21 mmol g−1.

1.2.6 Biochar‐Based Nanoadsorbents

In the current decade, special attention has been paid to biochar‐based composite materials for wastewater treatment purposes. Biochar refers to the class of organic substances that are produced by heating organic waste mass in presence of low oxygen or totally in absence of oxygen, which is termed as pyrolysis. The main purpose of producing biochar is to reduce the volume of waste and also produce energy. As they are resistant to degradation under ambient conditions, have large surface area, and possess negative surface charge, they are one of the most interesting classes of adsorbents for cationic pollutant removal from water bodies. Sizmur et al. [52] and Tang et al. [53] reviewed the application of biochar‐based composites for wastewater treatment.

Liu et al. [54] reported the utility of biochar‐supported carbon nanotubes and GO for the adsorption of Pb(II) and Cd(II) from water media. The adsorption capacity of the nanocomposite was higher than that of the bare biochar.

By means of ball milling, MgO/biochar nanocomposites were synthesized by Zheng et al. [55] and have been applied for the adsorptive removal of cationic dye and anionic phosphate from water medium. Li et al. [56] used agroforestry waste materials for the huge production of nano‐ and micro‐sized biochar composites for several applications. Thus synthesized nanobiochar has been applied for Pb2+ removal with varying initial concentrations (20–180 mg l−1) at pH 7. The maximum adsorption capacities for upper and lower layer biochar have been reported as 55.56 and 41.67 mg g−1, respectively. Shaikh et al. [57] prepared biochar‐based nanocomposite consisting of doped AgNP and utilized it for the removal of toxic dyes from water bodies. Silver‐doped biochar‐based nanocomposite showed promising performance toward the removal of dyes like CR and RhB (>90%). Dye adsorption was found to be a spontaneous and exothermic process, and the experimental data fitted well with the Freundlich isotherm and pseudo‐second‐order model. Biochar‐based nanocomposite was prepared by the same research group [58] from waste tea leaves and explored for toxic dye removal from wastewater.

1.2.7 Hybrid Nanoadsorbents

In many cases, MOF, GO, Mxenes, biochars, and polysaccharides are often blended to produce nanocomposites of desired properties. Faradonbeh et al. [59] explored MOF‐graphene hybrid nanocomposite for the purpose of Ni(II) removal from water media. For 300 ppm nickel solution, maximum removal efficiency reached up to 96% and the experimental data fitted well with the pseudo‐second‐order kinetic model. Among different hybrids prepared, maximum removal was observed for the nanohybrid synthesized by incorporating 30 wt% graphene into Cu‐organic framework, and the highest capacity reached 1152 mg g−1.

Hou et al. [60] utilized MOF (MIL‐101 (Cr)) loaded nZVI composite for the removal of tetracycline antibiotic from water bodies. Maximum adsorption capacity of the nanocomposite was reported as 625 mg g−1 and the experimental data fitted well with the pseudo‐second‐order kinetic model. Ventura et al. [61] synthesized novel hybrid nanoadsorbent composed of MOFs, Fe3O4, and GO. Hybrid nanoadsorbent thus produced was further applied for the removal of MB dye, and also tested for the adsorption of various metal ions such as Na+, Ca2+, Mg2+, SO42−, and SiO32− from a model brackish water. In comparison to the parent MOF material, the adsorption capacity toward MB increased by an amount of 30.52 and 13.75 mg g−1 for the Co and Ni nanocomposites.

Mittal et al. [62] reported the application of hybrid nanocomposite made up of GO and xanthan gum toward CV removal. Thus prepared nanocomposite showed excellent performance toward CV removal with a maximum adsorption capacity of 1566.97 mg g−1. Experimental data showed good correlation with Langmuir isotherm model and pseudo‐second‐order kinetic model.

To remove heavy metal ions such as Cu(II), Pb(II), and Cr(VI) from the water medium, Najafabadi et al. [63] applied chitosan/GO nanocomposite. It was revealed that the double exponential kinetic model and Redlich–Petersen isotherm best fit the experimental data. For Pb(II), Cu(II), and Cr(VI) the maximum adsorption capacity obtained was 461.3, 423.8, and 310.4 mg g−1, respectively. Moreover, the nanocomposites could be reused without any loss of efficiency.

1.2.8 Other Nanoadsorbents

Apart from the above‐mentioned nanocomposite materials, there are also other nanomaterials reported in the literature for pollutant removal. Abraham et al. [64] used the nanosize thorium(IV) metavandate for the removal of Ni(II) from water media. Kaolin and kaolin/ZnO nanocomposites have been explored by Mustapha et al. [65] for tannery wastewater treatment purpose. Nanocomposites were prepared by sol–gel method followed by wet impregnation technique. In comparison to kaolin, kaolin/ZnO nanocomposites showed better efficiency toward wastewater treatment. Under the same experimental conditions, Cr(VI) removal by the nanocomposite was nearly 100% whereas it was around 78% with kaolin. The increase in treatment efficiency was due to the higher surface area of the nanocomposite.

Table 1.1 List of some nanoadsorbents with their adsorption capacities.

Adsorbent

Pollutant

Adsorption Capacity (mg g

−1

)

Adsorption Isotherm Model

Reference

CTAB-surface-functionalized magnetic MOF@MOF

Cr (VI)

932.1

—

[10]

Ce – MOF- 500(S)

Phosphate

189.4

—

[11]

LDH/MOF nanocomposite

Cd (II)Pb (II)

415.3301.4

Langmuir

[13]

Zr–MFC–N

Pb (II)MB

102128

Langmuir

[16]

Zr–MFC–O

MO

219

GO nano adsorbent

BR 46

370.4

Langmuir

[21]

Ni–rGO

MB

946.12

—

[22]

Amine- reduced graphene oxide nanocomposite (CARGO)

Uranyl ions

337.93

Langmuir

[25]

Magnetic ferberite graphene nanocomposite

U (VI)

455

Langmuir

[26]

nZVI/activated carbon

As (III)As (V)

18.212

Langmuir,Freundlich

[28]

nZVI/Mn oxide

As (III)As (V)

29.435.7

Langmuir

[29]

Biochar-nZVI

Glyphosate

80

Langmuir

[30]

Magnetic chitosan Al

2

O

3

/Fe

3

O

4

nanocomposite

Cu (II)Cd (II)Zn (II)

35.9785.3932.89

Langmuir

[33]

Chitosan silica zinc oxide nanocomposite

MB

293.3

Langmuir

[34]

Polydopamine‐ coated cellulose nanocrystals

Cr (VI)

205

Langmuir

[36]

Cationic starch‐ intercalated montmorillonite

BB66 BY1

49.7 49.9

Langmuir

[38]

Functionalized maghemite

As (III)

8.6

Freundlich

[39]

Pectin‐based nanocomposite (Pec‐

g

‐PAMPS) Pec‐

g

‐PAMPS‐SN

MB

62 89

Freundlich

[40]

Pec‐

g

‐poly (AMPS‐

co

‐Aam)/Ag

CV Cu (II) Pb (II)

1950 111 130

Langmuir

[41]

Sodium alginate‐based nanocomposite (SA‐C Beads) SA‐P Beads SA‐Ch beads

Cd (II)

112 105 137

Langmuir

[42]

Polyaniline sodium nanocomposite

MB RhB O‐II MO

555.5 434.78 476.19 416.66

Langmuir

[43]

Nanocomposite hydrogel

MG CV

—

Freundlich

[44]

Mxene

Ba

9.3

Freundlich

[47]

Mxene (

Ti

3

C

2

T

x

)

Cs

148

Freundlich

[49]

PA‐Mxene‐12

MB RhB

42 22

Langmuir

[50]

Biochar‐supported carbon nanotubes and GO nanocomposites

Pb (II) Cd (II)

38.6 10.8

Dubinnin Radushkevich

[54]

Biochar‐based silver nanocomposite

RhB CR

23.39 23.55

Freundlich

[57]

MOF–5Cu–Graphene hybrid 30%

Ni (II)

1152

Freundlich

[59]

nZVI/MIL–101(Cr)

TC

625

Langmuir

[60]

Ni

3

(BTC)

2

–MOF

Co

3

(BTC)

2

–MOF

MB

67 54

Langmuir

[61]

XG–HNC

CV

1566.97

Langmuir

[62]

Chitosan/graphene oxide nanofibrous adsorbent

Pb (II) Cu (II) Cr (VI)

461.3 423.8 310.4

Redlich ‐Peterson

[63]

Core shell bimagnetic NPs

CoFe

2

O

4

@ γ Fe

2

O

3

P

71.2

Freundlich

[66]

Magnetite – maghemite nanoparticles

As (III) As (V)

3.69 3.71

Freundlich

[68]

Guerra et al. [66] utilized core@shell bimagnetic nanoparticles (CoFe2O4@γ‐Fe2O3) for adsorptive removal of phosphorus from water. Two different‐sized nanocomposites were prepared. Maximum adsorption capacity for the smaller‐sized nanocomposite reached up to 71.2 mg g−1 while it was 46.9 mg g−1 for the bigger‐sized nanocomposite at pH = 2.

Alias et al. [67] immobilized commercial TiO2 and hydrothermally synthesized nano‐flower like TiO2 on basil seed support for dye adsorption purpose. Hydrothermally prepared nanoTiO2 composites exhibited better performance in comparison to the commercial ones. Moreover, after one cycle of use, when the nanocomposite was further subjected to reuse, its efficiency got enhanced significantly.

Chowdhury and Yanful [68] developed a useful and practical method for arsenic and chromium removal from water by using a mixture of maghemite and magnetite nanoparticles. In pH‐controlled environment, almost all the arsenic and chromium species in the water were adsorbed. Maximum adsorption capacity was reported as 3.69 mg g−1 for As(III), 3.71 mg g−1 for As(V), and 2.4 mg g−1 for Cr(VI) at pH 2, while their initial concentration in the solution was 1.5 mg l−1 for As(III) and As(V) and 1 mg l−1 for Cr(VI).

A list of recently reported nano adsorbents has been compiled in Table 1.1, along with their maximum adsorption capacities toward certain pollutants.

1.3 Different Mechanisms

Different interesting mechanisms have been put forward by different research groups regarding adsorption by nanocomposites. Adsorption of pollutants by the carbon material occurs through electrostatic and non‐electrostatic interactions. Fard et al. [47] explained the mechanism of Ba2+ removal by MXene‐based nanoadsorbent. The removal occurred through physisorption as well as chemisorption. After the etching process, the surface of the MXene got covered with OH, F, and O groups. These groups enhanced the removal efficiency of barium. Furthermore, from D‐R model it was clear that physisorption as well as chemisorption are involved in the removal process. Mechanisms can be well understood from Figure 1.2. Barium removal by MXene‐based nanocomposite was proposed by the following two equations:

Shaikh et al. [57] explained the CR uptake as surface complexation accompanied by electrostatic attraction. On the other hand, removal of RhB occurred only through surface complexation. The mechanism has been elucidated in Figure 1.3.

Figure 1.2 Adsorption mechanism of barium ion on surface of MXene layers.

Source: Fard et al. [47]/with permission of Elsevier.

Figure 1.3 Schematic illustration of probable RhB and CR adsorption mechanism using Ag‐nBC.

Source: Shaikh et al. [57]/with permission of Elsevier.

Mozaffari et al. [44] provided a detailed mechanism behind the dye uptake by their newly synthesized tetraamine Cu(II) nanoparticles embedded in a hydrogel matrix adsorbent termed as novel nanocomposite hydrogel (NCH). The nanoadsorbent worked well for CV and MG dye. Enhanced dye removal capacity was attributed due to the increase in surface area of the hydrogel composite owing to the incorporation of the nanoparticles. Considering pKa values of all the dyes taken for experimental purpose (CV, MG, RhB, CR, and MO), it was found that at pH 6, CV and MG existed as cationic form, RhB as neutral, and MO and CR as anionic form. On the other hand, hydrogel‐based nanocomposite matrix existed as anion at pH 6. Therefore, cationic dyes got removed in greater percentages in comparison to the anionic dyes. For anionic dyes, only hydrogen bonding between the adsorbate and adsorbent was responsible for small removal percentage as shown in Figure 1.4.

Figure 1.4 Proposed electrostatic interaction and hydrogen bonding for dye adsorption by the NCH.

Source: Mozaffari et al. [44]/with permission of Elsevier.

1.4 Different Characterization Techniques

1.4.1 FTIR Analysis

FTIR analyses are often done in order to get an idea of the functional groups involved in the adsorption phenomenon. Shaikh et al. [58] explained the dye removal by nanosilver modified biochar composites by FTIR analysis. From the FTIR analysis, it was clear that –OH, C ≡ N, and S=O groups were involved in the adsorption phenomenon. Furthermore, it was also confirmed that the C=C group on the surface of the nanocomposite contains a π‐bond, which creates a π–π stacking with the delocalized π‐aromatic bonds of RhB and CR molecules resulting in enhanced adsorption of these two dyes.

Das et al. [69] conducted FTIR analysis of GO/chitosan‐PVA hydrogel nanocomposites. Strong peak at ∼3270 cm−1 appeared due to the –OH functional group and –NH2 group of chitosan. Peaks at ∼2939 and 1377 cm−1 have been assigned corresponding to the C–H stretching vibration. Due to the reaction that occurred between hydroxyl groups of PVA and the NH2 group of chitosan, C=N has been formed, and a peak has been obtained at ∼1647 cm−1.

1.4.2 Electron Microscopic Analysis

In order to investigate the interesting morphological features of the nanoadsorbents, scanning electron microscopic (SEM) and transmission electron microscopic (TEM) analysis is often done. Jiang et al. [30] carried out the SEM and TEM analysis of biochar, bare nZVI and biochar‐supported nZVI. It was observed that the surface of the original biochar was smooth consisting of lamellar multilayer structure of size around 0.5 μm. Bare nZVI was spherical in appearance with a size of 20–100 nm. After loading on the biochar surface, nZVI particles were uniformly dispersed and showed less sign of reunion. Moreover, the outer layer of the biochar‐nZVI composite looked brown, possibly due to the oxidation. It prevented the interior nZVI from being oxidized and thus retained its reactivity.

From the SEM analysis of tea leaf‐supported Ag nanoparticles [58], it was visualized that the surface of the nanocomposite exhibited porous, irregular surface with the presence of numerous cavities on it. It has been proposed that volatilization of lignin and holocellulose contributed to the pore formation, which ultimately aided in enhanced adsorption capacity of the nanoadsorbent.

In the work of Mozaffari et al. [44], TEM analysis showed that the size of the nanoparticles is within the range of 5–30 nm. From the TEM images of different weight percentages (0.5% and 0.7%) of chitosan/GO nanocomposite [63], it was clear that in 0.7% composite, GO sheets were embedded in the nanofibers, while in 0.5% composite, no such evidence was there.

1.4.3 X‐Ray Diffraction (XRD) Analysis

Powder X‐ray diffraction (XRD) is an analytical technique commonly practiced in order to determine the phase of a crystalline material and also to get an idea about the oxidation state of the metal ions involved in the adsorption process. From the XRD spectrum of Co‐MOF, it was found that the nanocomposites were crystalline in nature possessing lattice structure and unvarying morphology [18]. In the XRD spectrum of pristine GO [21], the diffraction peaks were obtained at 2θ = 11.2° and 42.6° corresponding to (001) and (100) reflections, respectively.

In the work of Verma and Dutta [25], the XRD analysis confirmed the reduction of GO by L‐cysteine. First, formation of GO from graphite was clear from the diffraction peak at 2θ