Carbon Nanotubes for Dyes Removal

9 Carbon Nanotubes for Dyes Removal Shadpour Mallakpour*,†, Shima Rashidimoghadam* *Organ ic P olymer Ch emistry R esearch Labor atory, Department of Chemistry, I sfahan Unive rsi ty of T echn olog y, I sfa han, I sla mic Re publ ic o f Ira n † R esearch Institute for Nanote c hnol ogy and A dva nced Mate ria ls, I sfah an Unive rsit y of Te chnol ogy, I sfah an, I sla mic Re publ ic o f Ira n

1 Introduction 1.1 Water Pollution Today, due to the fast growth of the world population, agricultural activities, intensive industrialization, and unplanned urbanization, the various toxic organic and inorganic compounds are discharged into the environment, which influence soil and water quality [1,2]. The contaminated water originates from the industries and untreated sewage sludge from the domestics, which release many pollutants such as phosphor, heavy metal, dye, nitrogen, cyanide, toxic organics, suspended solids, phenols, and turbidity into water [3]. Freshwater is essential for life and plants, animals, and humans all need freshwater to survive. Therefore, water pollution is a danger to human being and aquatic life and is one of the major environmental problems [4–8].

1.2 Dyes Dyes, which recognized as aromatic, colored, and ionizing substances, are one of the most important water contamination sources. These compounds which originate from coal tarbased hydrocarbons, such as toluene, benzene, naphthalene, anthracene, xylene are employed to color different substrates, such as drugs, paper, fur, greases, leather, cosmetics, waxes, hair, plastics, and textile materials. They have color due to (1) existence of a conjugated system which is a system of connected p-orbitals with delocalized electrons in molecules with alternating single and multiple bonds, (2) absorption of light in the visible spectrum (400–700 nm) by these compounds, (3) the possibility of the resonance of electrons in their structure, and (4) existence of a chromophore which is a region of the molecule where the energy difference between two separate molecular orbitals falls within the range of the visible spectrum. Also, the existence of auxochromes, such as hydroxyl, amino, sulfonic acid, and carboxylic acid groups in several dyes imparts the color of the colorant and affects the solubility of dye. However, these functional groups are not responsible for producing the color [9–12].

Composite Nanoadsorbents. https://doi.org/10.1016/B978-0-12-814132-8.00010-1 © 2019 Elsevier Inc. All rights reserved.

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Dyes are categorized into two general groups: 1) Anionic dyes, such as azine, triphenylmethane, anthraquinone, nitroso, and xanthene, are utilized for silk, modified acrylics, nylon, wool, etc. 2) Cationic dyes, such as crystal violet, methylene blue, and amaranth, are utilized for modified nylons, polyesters, paper, polyacrylonitrile, and in medicines [13]. Also, dyes can be categorized according to the chemical composition (chromophore) or based on their applications. According to the nature of their chromophore, dyes are divided into: 1) Nitro dyes have a nitro (NO2) group which is conjugated with an electron donating group (usually hydroxyl and amino groups) in an aromatic system. 2) Nitroso dyes, which contain a nitroso (dN]O) group as a chromophore in the orthoposition to a hydroxyl group, are prepared via reaction between nitrous acid and phenols or naphthols. They have not themselves dyeing properties but they can produce metal complexes that are either pigments or, if the starting compound bears hydrophilic groups, are acid dyes. These dyes are employed for dyeing rubbers, in the fabrication of wallpaper and pencils, and in the paint and varnish industry. 3) Acridine dyes are heterocyclic compounds including acridine and its derivatives in their structure. These molecules mainly utilized for imparting color to mordant cotton and leather. 4) Cyanine dyes are molecules including polymethine bridge between two nitrogen atoms with a delocalized charge. These dyes are employed in industry, and more recently in biotechnology (labeling, analysis) because they are useful labels for proteins and nucleic acids and remain some of the most extensively utilized fluorophores in biochemical and biological researches. 5) Oxazine dyes are characterized by oxazine ring as a chromophore in which the O atom is in para-positions to N atom. 6) Thiazole dyes which contain the thiazole ring system are used mainly for cotton. 7) Anthraquinone dyes and their structure are based on anthraquinone. The resonance structure of the aromatic rings and the carbonyl groups provide the chromophore. These dyes are employed for dyeing textiles, especially cotton, rayon, and silk. 8) Arylmethane dyes are so-called because they are derived from methane, but in which some of the H atoms are replaced with aryl rings. They include two subgroups: (a) diarylmethane dyes, which have two aryl rings and (b) triarylmethane dyes, which have three aryl rings. 9) Phthalocyanine dyes contain the derivatives of tetrabenzotetraazoporphyrin (phthalocyanine) and its complex compounds with metals. The water-soluble phthalocyanine dyes are employed for color fabrics and the water-insoluble phthalocyanine dyes are extensively utilized as pigments in the fabrication of printing colors, plastics, resins, and other products.

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10) Indigoid dyes which contain indigo as chromophore are used to dye cellulose and protein fibers as well as for cotton printing [10,14–18]. Dye classification on the basis of the use or application is as follows: 1) Azo dyes which are aromatic compounds with at least one azo groups (dN]Nd) are employed particularly for dyeing of polyester, rayon, and cotton. 2) Acid dyes are sodium salts of sulfonic acid or other acids, such as carboxylic or phenolic organic acids. These water-soluble dyes are usually employed for dyeing polyamide, silk, wool and modified acrylics, and polypropylene fibers. 3) Basic dyes are recognized as water-soluble cationic dyes with bright color. These dyes carry a positive charge in their molecule, which is generally localized at an ammonium group. They are utilized for acrylic, modacrylic, cationic dyeable nylon, cationic dyeable polyester, protein, and cellulosic fibers. 4) Disperse dyes are water-insoluble dyes which are free of the ionizing group. These dyes are used in the form of an aqueous dispersion utilized for dyeing cellulose, cellulose acetate, nylon, and acrylic fibers. 5) Reactive dyes have at least one reactive group capable of forming a covalent bond with a compatible fiber group. These compounds show good fastness properties because of the bonding that occur during dyeing. They are employed for dyeing cotton, rayon, and some nylons via the easy dyeing procedure. 6) Direct dyes are soluble in water and ionizable to produce colored anions, which can directly color the paper, leather, cotton, and regenerated cellulose. These dye are very easy to apply and available in the market. 7) Sulfur dyes are so-called because they have sulfur-containing heterocyclic rings in their chemical structure. These water-insoluble dyes are utilized in dyeing cotton. 8) Mordant dyes, which require auxiliary substances, are called mordants in their application and which upon combination with the mordant imparts insoluble color on the substrate, for example, dyes with metal chelating groups. These dyes are employed for dyeing cotton, wool, or other protein fibers. 9) Vat dyes are water-insoluble polycyclic compounds based on quinine structure, which gets impregnated into the fiber under reducing conditions and get oxidized to an insoluble color. They are utilized most often in dyeing and printing of cotton and cellulose fibers and also for color polyamide and polyester blends with cellulose fibers. 10) Solvent dyes are characterized by their solubility in organic solvents. These compounds, which are soluble in organic solvents, such as chlorinated hydrocarbons, liquid ammonia, and alcohols and insoluble in water, are employed for dyeing organic solvents, plastics, oil, gasoline, lubricants, and waxes. As a general tendency, these dyes do not ionize. 11) Drug, food, and cosmetic dyes are synthetic and natural dyes which are generally utilized in drugs, foods, and cosmetics, of which synthetic dyes belonging to triarylmethane, azo, carotenoid, and anthraquinone groups are the most frequently employed [10,15,16,19].

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Nowadays, more than 100,000 different types of dyes are fabricated with a rate of 9
106 tons/year. These dyes are extensively utilized in many industries, such as wall colors, printing, textiles, paper, and foodstuff industries. However, the main source of release of color into the water is associated with the textiles and dye manufacturing industries. The estimation of the amount of discharged dyes into water is not easy. Recent estimates suggest that the total dye consumption in textile industry worldwide is >10,000 tons/year and about 100 tons/year of dyes are released into waste streams. Because of the increasing utilization of dyes, the dye-polluted water is becoming a serious environmental problem. Even the existence of very small amounts of dyes (<1 ppm) is very visible and changed the water quality. Therefore, the elimination of these pollutants from wastewater is necessary for supplying disease-free health to our society [20].

1.3 Dyes Removal Techniques There are several methods to remove the dye molecules from dye-polluted water. These technologies can be divided into three categories: physical, chemical, and biological treatments [21].

1.3.1 Biological Treatments Biological techniques decompose organic matters in wastewater using microorganisms, such as bacteria, fungi, and algae (mostly bacteria). These techniques are relatively economical, having low running costs, and the end products of complete mineralization are stable and harmless. Biological methods require fewer chemicals and energy than physical and chemical methods. Biological treatments contain some methods, such as decolonization by white-rot fungi, adsorption by living/dead microbial biomass, and anaerobic textile-dye bioremediation systems.

1.3.2 Chemical Methods Chemical methods are referred to as additive processes, as they require the addition of chemicals to react with the pollutant by means of chemical reactions to eliminate them. These techniques are not much utilized because of some disadvantages, such as large consumption of the chemical reagents, high cost, high electrical energy demand, and disposal problem due to the accumulation of the concentrated sludge. Chemical methods include a number of processes such as oxidative processes, H2O2-Fe (II) salts (Fenton’s reagent), and ozonation, photochemical, sodium hypo chloride (NaOCl), cucurbituril, and electrochemical destruction.

1.3.3 Physical Methods Physical treatment involves physical separation of contaminants from wastewater without causing a considerable change in the chemical or biological characteristics of the treated water. The major drawbacks of these techniques are that they simply transfer the dye

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molecules to another phase rather than destroying them and they are effective only when the effluent volume is small. Physical treatment methods for the removal of dyes are adsorption, membrane filtration, ion exchange, irradiation, and electro kinetic coagulation. Among all the physical methods, adsorption is an effective and best method for the removal of different kinds of dissolved organic dyes from the wastewater. It is a surface phenomenon, which involves the transfer of soluble organic dyes from wastewater to the surface of the adsorbent, which is solid and highly porous material. The main advantages of this method are low-cost, the simplicity of design, use of operation and insensitivity to toxic substances. This method is categorized into chemical and physical adsorption. Chemical adsorption, or chemisorptions, is adsorption in which the adsorbate is chemically bound to the adsorbent’s surface because of the exchange of electrons. Physical adsorption, or physisorption, is a process in which the adsorbate is attached to the adsorbent’s surface via physical forces, such as hydrogen bond, polarity, Van der Waals forces, hydrophobicity, π–π interaction, static interaction, dipole-dipole interaction, etc. [22–27].

1.4 Adsorbents Basically, adsorption is the accumulation of a substance on the surface or interface. In case of water treatment, the process happens at an interface between the solid adsorbent and polluted water. The adsorbed contaminant is called as adsorbate and the adsorbing phase as adsorbent [21]. Adsorbent has a very important role in the efficiency of the adsorption process. The major cost of the adsorption process is incurred in the adsorbent. A good adsorbent must have low cost, great selectivity, wide availability, great adsorption efficiency, and long life. Numerous substances, such as activated carbon, many low-cost adsorbents, and nanomaterials, have been studied as adsorbents for removal of pollutant from contaminated water [16].

1.4.1 Activated Carbon Activated carbon is an internally porous microcrystalline, non-graphitic form of carbon which is the most commonly utilized adsorbent for removal of a wide range of dyes from wastewater due to its great capacity of adsorption, large surface area, great degree of surface reactivity, and micropore structures, but the price of production and regeneration of this adsorbent is too high. Also, generation by solution generates a small additional effluent while regeneration via refractory method causes a 10%–15% loss of adsorbent and its removal efficiency [28–30]. Thus, there is a need to search for cheaper and effective adsorbents.

1.4.2 Low-Cost Adsorbents Due to high cost and low regeneration of activated carbon, search for low-cost adsorbents is necessary. These low-cost adsorbents should be abundant in nature, effective, and require minimal processing. These adsorbents can be divided into three different categories: (1) agricultural and industrial by-products [such as teak wood bark, papaya seeds,

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sugar industry mud, grass waste, peels (pomelo, jackfruit, banana, garlic), rubber seed shell, fly ash, and coconut tree flower] (2) bioadsorbents [such as biomass (algae, activated sludge), fungi, and microbial] (3) natural materials [such as clay (montmorillonite, bentonite, fibrous clay, palygorskite, kaolin), and glass wool]. These adsorbents demonstrated mixed results and in very few cases their adsorption efficiency is greater than activated carbons. Thus, the explore to develop effective adsorbents which possess the greater adsorption ability and better regeneration capacity is still going on [26,31,32].

1.4.3 Nanomaterials Nanotechnology is the study and use of structures on the scale of less than 100 nm and the nanomaterials, also referrers to as nanoparticles, are particles that fall within the size range of 1–100 nm. Nanotechnology has proved to be one of the finest and advance ways for the removal of toxic pollutants, such as the pharmaceutical and personal care products, polychlorinated biphenyls, polycyclic aromatic hydrocarbons, phthalates, agrochemicals and pesticides, furans and dioxins, volatile organic compounds, dyes, inorganic pollutants, viruses and bacteria, etc. from wastewater using various efficient, cost-effective, and co-friendly nanomaterials, which have exceptional properties for potential decontamination of industrial effluents, surface water, ground water, and drinking water [33–49]. Compared with conventional adsorbents, nanomaterials (nano adsorbent) have greater efficiencies and rapid adsorption rates over a broad pH range due to the extremely high specific surface area (SSA) and associated sorption sites, short intraparticle diffusion distance, and tunable pore size and surface chemistry. Also, they can be simply fabricated at a lower price and smaller amounts are needed for efficient elimination of dyes. Therefore, these materials are cheaper than activated carbon for adsorption applications. One important class of nanomaterials is carbonaceous nanomaterials, which include fullerenes, carbon nanotubes (CNTs), graphene, and carbon nanofibers and have strong covalently bonded carbon molecules. In comparison with conventional activated carbon, carbonaceous nanomaterials demonstrate great potential for the removal of dye from wastewater due to their high adsorption kinetic and capacity and higher affinity. They can remove dyes through electrostatic and covalent interaction, H-bonding, hydrophobic effect, and π–π stacking. Recently, adsorption of dyes onto CNTs and CNTs-based nanocomposites (NCs) have been receiving great attention for environmental application of CNTs in water treatment because of surface characteristics and unique structure of CNTs [50–52].

1.5 Application of CNTs in the Removal of Dyes CNTs are allotropes of carbon, made of graphite, and constructed in cylindrical tubes with nanometer in diameter and several millimeters in length. CNTs are categorized as singlewalled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs) (Fig. 1). SWCNTs and DWCNTs contain one or two (concentric) graphene cylinder, respectively, whereas MWCNTs include several

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FIG. 1 (A) Single-walled carbon nanotubes (SWCNTs), (B) double-walled carbon nanotubes (DWCNTs), and (C) multi-walled carbon nanotubes (MWCNTs).

concentric cylindrical shells of graphene sheets. Due to their large surface area, extraordinary strength, current capacity, large flexibility, rapid electron transfer, low weight, high aspect ratio, and metallic/semiconducting characteristics, CNTs have been widely used for several applications [53–57]; Mallakpour et al. [4,58–60]. Recently, CNTs have attracted increasing research interest as a new adsorbent owing to their hollow and layered structures, high chemical and thermal stability, large SSA, π-conjugative structures, high porosity, active sites, and curvature of sidewalls. CNTs can strongly adsorb various dyes because of the various chemical functional groups of dyes-CNT interactions including covalent and electrostatic interactions, π–π bonds, hydrophobic effects, and hydrogen bonds. Although molecular structure which is considerably affected by the charge load of the dyes and CNTs and also the π–π bonds is the main driving force of interaction between CNTs and dyes [61]. The adsorption ability of the CNTs can be tuned via surface modification, which introduces various functional groups onto the surface of and then provides new adsorption sites for organic dyes. Different types of surface modification methods such as functional molecules/group grafting, metals impregnation, and acid treatment were used to change the features of CNTs surface, for example, surface charge, surface area, hydrophobicity, and dispersion [35,62,63]. The dyes adsorption onto CNTs depends on various parameters, such as physical properties of CNTs, nature of adsorbate, chemical treatment of CNTs, process variables (including CNT dosage, temperature, solution pH, contact time, and initial concentration of dye) [9,64]. Although, pure CNTs can be utilized for dye removal due to their high adsorption efficiency, their industrial use is limited due to their high cost. In addition, separation of CNTs from aqueous solution is very complicated due

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to its smaller size and high aggregation property. Thus, composites of CNTs with a metal oxide, polymers, carbon, etc., which act as a stable matrix to the CNTs, can solve these problems [9,61]. There are numerous published researches addressing the removal of dyes by CNTs-based materials. To the best of our knowledge, very few chapters have been written about the dye adsorption using CNTs and their NCs. So we were encouraged to write a chapter about CNTs-based adsorbent for dyes removal. The main aim of this chapter is to provide an overview of recent advances in the application of CNTs (SWCNTs and MWCNTs) and CNTs-based NCs in the elimination of dyes reported in the years of 2012–18. The emphasis is on the application of CNTs and polymer-CNTs NCs in dye removal.

2 Dye Adsorption on CNTs and CNTs-Based NCs 2.1 Dye Adsorption on CNTs As was mentioned above, CNTs is one of the most effective materials for removal of dye from wastewater due to its fantastic properties. It was reported that pure CNT (without any need to graft or composite with other materials) effectively remove the various kinds of dyes from wastewater. Ma et al. [65] showed that activated CNTs (CNTs-A) which prepared by an alkaliactivated technique can be used as an efficient adsorbent for adsorption of methyl orange and methylene blue dyes. Alkali-activation treatment of CNTs introduced oxygencontaining functional groups on the surface of CNTs-A and enhanced the pore volume and SSA which leads to higher adsorption ability of CNTs-A for methyl orange and methylene blue dyes. It was found that CNTs-A effectively adsorbed a large amount of methyl orange (149 mg/g) and methylene blue (400 mg/g) by multiple adsorption interaction mechanisms, such as π–π EDA interactions, H-bonding, electrostatic interactions, mesoporefilling between CNTs-A and dyes (Fig. 2). The surface characteristic changes CNTs-A before and after adsorption of methyl orange and methylene blue dyes were investigated via scanning electron microscopy (SEM) analysis (Fig. 3). As can be seen, the adsorption of dyes onto the surface of CNTs-A leads to the formation of a white layer on the surface of adsorbent and formation of the molecular cloud of the dye over the surface (Fig. 3B and C). Also, CNTs-A-methylene blue has a greater diameter than CNTs-A (Fig. 3C) [65]. Machado et al. [66] reported the utilization of MWCNTs and SWCNTs as adsorbents for the elimination of reactive blue 4 textile dye from aqueous solutions. They showed that the highest amount of adsorbed dye for MWCNT and SWCNT is 502.5 and 567.7 mg/g, respectively, due to the greater SSA and total pore volume of SWCNTs than the respective values of MWCNTs. It was found that pH of 2.0, and contact time of 4.0 h was found as the optimum condition for both adsorbents and Liu isotherm model fitted the experimental results well [66]. Kumar et al. [67] investigated the adsorption of rhodamine B dye onto MWCNTs which fabricated via chemical vapor deposition (CVD) technique. They showed that the dye uptake was increased as the pH increased due to electrostatic attraction between the negatively charged CNT and the cationic dye.

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FIG. 2 Adsorption schematic diagrams of methyl orange or methylene blue on CNTs-A though (A) hydrogen bonding, (B) π–π EDA interactions, (C) electrostatic interactions, and (D) mesopore filling (MO ¼ methyl orange, MB ¼ methylene blue). Adapted from J. Ma, F. Yu, L. Zhou, L. Jin, M. Yang, J. Luan, Y. Tang, H. Fan, Z. Yuan, J. Chen, Enhanced adsorptive removal of methyl orange and methylene blue from aqueous solution by alkali-activated multiwalled carbon nanotubes, ACS Appl. Mater. Interfaces 4 (2012) 5749–5760. With kind permission of American Chemical Society.

Also, it was found that as the initial concentration of dye increase, more the removal of dye was observed because of the increase in the driving force of the concentration gradient for mass transfer with the increase in initial dye concentration. The Langmuir and Temkin isotherms fitted the data well and the pseudo-second-order kinetic model provided the best fit to the experimental data [67]. The MWCNTs decorated calcined eggshell (CES) was fabricated via a simple hydrothermal technique and employed for the first time for removal of congo red from aqueous solution by Seyahmazegi et al. [68]. This adsorbent effectively adsorbed congo red via ionic interaction and H-bonding between -OH group of MWCNTs/CES and aromatic rings of dye, π–π interactions, and H-bonding between electronegative groups of dye and dOH group of adsorbent (Fig. 4). It was found that increase in MWCNTs/CES dosage results in a greater percentage of dye removal, which is related to an enhancement in adsorption surface area and the number of active sites for adsorption. Almost 100% congo red adsorption was observed by 0.5 g non-treated eggshell, 0.05 g CES, and 0.02 g MWCNTs/CES which demonstrated that MWCNTs/CES shows increased

FIG. 3 SEM of (A) CNTs-A, (B) MO adsorbed CNTs-A, and (C) MB adsorbed CNTs-A (MO ¼ methyl orange, MB ¼ methylene blue). Adapted from J. Ma, F. Yu, L. Zhou, L. Jin, M. Yang, J. Luan, Y. Tang, H. Fan, Z. Yuan, J. Chen, Enhanced adsorptive removal of methyl orange and methylene blue from aqueous solution by alkali-activated multiwalled carbon nanotubes, ACS Appl. Mater. Interfaces 4 (2012) 5749–5760. With kind permission of American Chemical Society.

FIG. 4 Possible interactions between congo red and MWCNTs/CES: (i) π–π interaction, (ii) hydrogen bonding between hydroxyl group of MWCNTs/CES and electronegative residues in the congo red, (iii) H-bonding between hydroxyl group of MWCNTs/CES and aromatic residue in the congo red, and (iv) ionic interaction between. Adapted from E.N. Seyahmazegi, R. Mohammad-Rezaei, H. Razmi, Multiwall carbon nanotubes decorated on calcined eggshell waste as a novel nano-sorbent: application for anionic dye Congo red removal, Chem. Eng. Res. Des. 109 (2016) 824–834. With kind permission of Elsevier.

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FIG. 5 Effects of different experimental parameters on congo red adsorption: adsorbent dosage (A), contact time (B), and initial dye concentrations (C) obtained for the NES, CES, and MWCNTs/CES. Adapted from E.N. Seyahmazegi, R. Mohammad-Rezaei, H. Razmi, Multiwall carbon nanotubes decorated on calcined eggshell waste as a novel nano-sorbent: application for anionic dye Congo red removal, Chem. Eng. Res. Des. 109 (2016) 824–834. With kind permission of Elsevier.

adsorption capacity with regard to non-treated eggshell and CES. Also increasing the contact time leads to a considerable enhancement and the equilibrium time is 50 min for non-treated eggshell, 40 min for CES and MWCNTs/CES. Because of great driving force for mass transfer in high dye concentration, the percentage of dye removal enhances with increasing initial congo red concentration (Fig. 5) [68]. Amorphous CNT was fabricated via solid-state reaction and used for the first time, for the elimination of methyl orange (anionic dye) and rhodamine B (cationic dye) from aqueous solution by Banerjee et al. [69]. It was found that acidic pH is favorable for the adsorption of both dyes because of inter-molecular H-bonding between dCOOH groups of rhodamine B and the dOH groups of a-CNTs and electrostatic interaction between positively charged N(CH3)2 groups of methyl orange and negatively charged dOH group of a-CNTs (Fig. 6A). Furthermore, the amount of adsorbed dye is increased with the increase in adsorbent dosage due to increase in the adsorbent SSA and availability of more

FIG. 6 (A) Effect of solution pH on adsorption of (a) RhB and (b) MO by a-CNTs, (B) (a) effect of adsorbent dosage on adsorption of (a) RhB and (b) MO by a-CNTs and with corresponding color change shown inset, (C) (a) effect of contact time on adsorption of (a) RhB and (b) MO by a-CNTs with corresponding color change of the solution shown inset, and (D) variation of (a) Qe and (b) dye removal efficiency () with initial dye concentration for both the dyes (RhB ¼ rhodamine B, MO ¼ methyl orange, Qe ¼ amounts of dye adsorbed at equilibrium). Adapted from D. Banerjee, P. Bhowmick, D. Pahari, S. Santra, S. Sarkar, B. Das, K.K. Chattopadhyay, Pseudo first ordered adsorption of noxious textile dyes by low-temperature synthesized amorphous carbon nanotubes, Physica E Low Dimens. Syst. Nanostruct. 87 (2017) 68–76. With kind permission of Elsevier.

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number of adsorption sites (Fig. 6B). Also, higher contact time and lower initial concentration cause an increase in the adsorption of rhodamine B and methyl orange dye onto aCNTs (Fig. 6C and D). Adsorption of these dyes is followed by classical Langmuir-Freundlich and Temkin adsorption isotherm. Also, adsorption of both the dyes followed the pseudo-first-order model [69]. Samadi et al. [70] investigated the influence of solution pH, initial dye concentration, and adsorbent content on the removal of reactive black 5 by MWCNT. They showed that the high adsorption capacity of MWCNT for this dye was obtained at pH 7, which is due to the effect of solution pH on the surface of MWCNT. Furthermore, the amount of dye adsorbed is decreased with the increase in initial reactive black 5 concentrations because at higher initial concentrations of the dye free places available on adsorbent for dye removal is limited. The amount of adsorbed reactive black 5 increased with increasing adsorbent content due to the availability of more surface area of the MWCNTs [70]. Sobhanardakani and Zandipak [71] reported that oxidized MWCNTs can act as an effective adsorbent for the elimination of anionic dyes (direct blue 106 and acid green 25) from wastewater. The results of Brunauer, Emmett, and Teller (BET) analysis demonstrated that the oxidized MWCNTs have greater pore volume and average pore diameter than MWCNTs. Because during oxidization of MWCNTs with nitric acid, MWCNTs divided into smaller pieces with a large number of defects on their surface and open the tips, probe the holes through the MWCNTs. Thus the adsorption ability of oxidized MWCNTs for dyes is higher than MWCNTs. It was found that optimized conditions for dye removal are solution pH 2, an adsorbent dose of 0.025 g, and contact time of 15 min. Also, the amount of adsorbed dye is decreases with temperature. The equilibrium data and the adsorption kinetics were best fitted to the Langmuir isotherm model and pseudo-second-order model, respectively [71]. Moussavi and MohammadianFazli [72] demonstrated that the adsorption efficiency of MWCNTs toward acid violet 17 dye is depended on variable parameters, such as solution pH, initial dye concentration adsorbent content, and contact time. The optimum conditions for acid violet 17 removals were found to be the MWCNTs content of 0.4 g/L, contact time of 3 h, and solution pH of 4 and Freundlich isotherm and Pseudo-second order kinetic models fitted well for the experimental data. It was concluded that MWCNTs can be effective in removal of acid violet 17 dye because of a unique network arrangement, small size, crystal form, large section area, extremely high reactivity, and the high ability for the elimination of organic contaminants from wastewater [72]. Vuono et al. [73] showed that due to its exceptional structural features, MWCNTs have high adsorption capacity toward reactive yellow 81, reactive blue 116, and reactive red 159 on comparing with active carbon. They investigated the influence of surface functionalization of MWCNTs on the adsorption capacity of MWCNTs and the results showed that neither the presence of residual catalyst nor common surface treatment (oxidation) influences the MWCNTs performances [73]. Maleki et al. [74] examine the adsorption of anionic dyes (acid blue 45 and acid black 1) in single and binary (mixture of dyes) systems onto amine-functionalized MWCNTs (CNT-NH2) which prepared by a feasible and inexpensive technique. In both single and binary dye systems,

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the adsorption of acid blue 45 onto CNT-NH2 is higher than acid black 1 which is due to the stronger interactions between CNT-NH2 and acid black 1. The adsorption capacity was higher at higher dye concentration and the best CNT-NH2 content for a single and binary system of acid blue 45 and acid black 1 was 0.1 g/L. The adsorption capacity was reduced by enhancing CNT-NH2 content because of an excess of adsorption sites. Also, the optimum pH for dye removal is 2 in single and binary systems. This is due to effective electrostatic interactions which occurred between dNH2 groups of CNT-NH2 and negative groups of acid blue 45 and acid black 1 in acidic medium. Also, the adsorption isotherm and kinetics are consistent with the Langmuir isotherm and pseudo-second-order kinetics, respectively [74]. Dutta et al. [75] analyzed the adsorption behavior of a mixture of cationic and anionic dyes onto heat-treated MWCNT (HCNT), acid-treated MWCNT (ACNT), and amine treated MWCNT (NH2CNT). They showed that MWCNT, HCNT, and ACNT have high adsorption capacity toward cationic dyes (rhodamine B, methylene blue, and malachite green) because of the electrostatic attraction of negatively charged groups on their surface which has been ascertained through zeta potential measurements and NH2CNT demonstrates great adsorption ability toward anionic dye (methyl orange) due to positively charged amine groups on its surface. Also, it was found that the composites of NH2CNT with MWCNT and ACNT can be used as an excellent adsorbent for the elimination of mixture of anionic and cationic dyes from wastewater. Also, the simple and successful regeneration of adsorbed malachite green onto ACNT shows excellent multicyclic efficiency [75]. Dutta et al. [76] investigated the removal of methyl orange and rhodamine B dyes from aqueous solution using an amorphous analogue of CNT which prepared via lowtemperature solid-state reaction. The results showed that with decreasing pH the removal efficiency was increased. Classical Langmuir-Freundlich, Temkin, and Freundlich adsorption models and also pseudo-first-order kinetic models fitted well for the experimental data. Also, they reported that exposure of UV light has no considerable influence on the adsorption capability [76].

2.2 Dye adsorption on CNTs-Based NCs As mentioned above, because of its small size and a great tendency to aggregation, the separation of CNTs from aqueous solution is very difficult. The incorporation of CNTs into a stable matrix, such as polymers, metal oxide, and carbon can solve these problems and these materials can be effectively utilized for the elimination of dyes. Here, the emphasis is on the application of polymer-CNTs NCs in dye removal. Zhu et al. [77] used a suspension cross-linking technique for fabrication of novel chitosan-modified magnetic-graphitized MWCNTs (CS-m-GMCNTs). Then, the obtained CS-m-GMCNTs were employed as an efficient adsorbent for the elimination of congo red from wastewater. The BET surface area measurements showed that CS-m-GMCNTs have a high SSA (39.20 m2/g) which is essential for an excellent adsorbent. With the increasing initial concentration of congo red, removal efficiency was enhanced due to an

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enhancement in the driving force of a concentration gradient with the increase in the initial concentration. The highest removal was obtained at pH 6.3 because of the enhanced electrostatic interactions between cationic groups of adsorbent and anionic groups of dye at lower pH values. The equilibrium data and the adsorption kinetics were best fitted to the Langmuir isotherm model and pseudo-second-order model, respectively [77]. Natarajan et al. [78] reported that MWCNT-TiO2 nanotube (TNT) NCs are stable and reusable photocatalysts with great photocatalytic decomposition efficiency for the degradation of rhodamine 6G dye. The x% MWCNT/TNT NCs (x ¼ 1, 3, 5, 7, and 10) were fabricated through hydrothermal treatment of anatase TiO2 by in situ addition of MWCNT. It was found that the NC containing 10% amount of MWCNT has greater photocatalytic decomposition efficiency in comparison to other NCs, pure TNT, anatase TiO2, and Degussa P25 TiO2. This enhancement is owing to the addition of MWCNT which reduced the electronhole recombination on the TNT (Fig. 7) [78]. Esfandiyari et al. [79] prepared the MWCNTcarbon ceramic NC via sol-gel method and studied its adsorption capacity for basic yellow 28 dye. The optimum condition of basic yellow 28 dye removal was found at pH of 8, adsorbent amount of 1.5 g/L, and initial dye concentration 15 mg/L for 80 min contact time. The obtained results are well fitted with the Freundlich adsorption isotherm model and pseudo-second-order kinetic model which demonstrate the great affinity of MWCNTcarbon ceramic NC for basic yellow 28 dye [79]. The removal of methylene blue, malachite green, and methyl violet dyes from wastewater using water-soluble hyperbranched polyamine functionalized MWCNTs NC (WHPA-OMCNT) was studied by Hu et al. [80,81]. They suggest that these dyes can be adsorbed onto surface of adsorbent by H-bonding and electrostatic interaction between the hydroxyls groups and amine groups of WHPA-OMCNT NCs and the active sites of cationic methylene blue, malachite green, and methyl violet dyes and also π–π stacking interactions between aromatic rings of dyes and the hexagonal skeleton of WHPA-OMCNT (Fig. 8). The optimized conditions for dye removal are solution pH 6, an adsorbent dose of 5 mg, and contact time of 10 min (for methylene blue) and 120 min (for malachite green and methyl violet).

FIG. 7 Schematic representation of photogenerated electron-hole separation mechanism (CB¼ conduction band, VB ¼ valence band). Adapted from T.S. Natarajan, J.Y. Lee, H.C. Bajaj, W.-K. Jo, R.J. Tayade, Synthesis of multiwall carbon nanotubes/TiO2 nanotube composites with enhanced photocatalytic decomposition efficiency, Catal. Today 282 (2017) 13–23. With kind permission of Elsevier.

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N

= O–

= OH

= NHR1, NH2 N

N

N

N

H-bonding

H-bonding

H-bonding

MB

MG

MV

S +

+

N

N

+

N

Electrostatic attraction – stacking

WHPA-OMCNT

WHPA-OMCNT

Electrostatic attraction

Electrostatic attraction

– stacking

– stacking

WHPA-OMCNT

FIG. 8 Proposed adsorption mechanism of methylene blue, malachite green, and methyl violet adsorption onto WHPA-OMCNT. Adapted from D. Hu, X. Wan, X. Li, J. Liu, C. Zhou, (Synthesis of water-dispersible poly-l-lysinefunctionalized magnetic Fe3O4-(GO-MWCNTs) nanocomposite hybrid with a large surface area for high-efficiency removal of tartrazine and Pb(II), Int. J. Biol. Macromol. 105 (2017) 1611–1621 and L. Hu, Z. Yang, Y. Wang, Y. Li, D. Fan, D. Wu, Q. Wei, B. Du, Facile preparation of water-soluble hyperbranched polyamine functionalized multiwalled carbon nanotubes for high-efficiency organic dye removal from aqueous solution, Sci. Rep. 7 (2017) 3611. Open access journal.

The adsorption process of all three dyes was well represented by Langmuir isotherm and a pseudo-second-order kinetic model [80]. Li et al. [82] investigated the adsorption behavior of anionic (methyl blue, methyl orange, and acid fuchsin) and cationic (methylene blue) dyes onto polypyrrole/CNTs-CoFe2O4 magnetic (CNTs-CoFe2O4@PPy)NCs which were fabricated via a chemical oxidative polymerization of pyrrole in the presence of CNTs-CoFe2O4.CNTs-CoFe2O4@PPyNC can be easily separated from the aqueous solution by placing a permanent magnet near the glass bottle. Also, Co+2 can activate peroxymonosulfate (PMS) to produce reactive radicals, such as hydroxyl radicals (●OH) and sulfate radicals (SO 4 •) for efficient degradation of these dyes (Fig. 9). Therefore this NC can be utilized as an effective adsorbent for dye removal due to their great adsorption ability, effective separation, exceptional catalytic activity, and good reusability [82]. Mallakpour et al. [4] reported the preparation of starch/ascorbic acid (AA)-MWCNTs NC by ultrasonication technique (Fig. 10). Then they studied the removal of methyl orange from aqueous solution using the fabricated NC as an adsorbent. It was found that starch/ AA-MWCNTs NC can effectively adsorb a great amount of methyl orange from wastewater by electrostatic attraction between the methyl orange dye and NC adsorption site. The high adsorption capacity of starch/AA-MWCNTs NC was obtained at 0.04 g adsorbent, pH 2, and initial dye concentration 20 ppm and Langmuir isotherm model was in good agreement with experimental data (Fig. 11) [83]. Wu et al. [84] investigated the adsorption ability of oxide-MWCNTs/polyaniline NC for the removal of alizarin yellow R from aqueous solutions. This NC was fabricated by in situ chemical oxidation polymerization. The results of adsorption experiments showed that the optimized conditions for dye removal are solution pH 8.45, contact time of

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FIG. 9 Synthesis route of CNTs-CoFe2O4@PPy magnetic composites. Adapted from X. Li, H. Lu, Y. Zhang, F. He, Efficient removal of organic pollutants from aqueous media using newly synthesized polypyrrole/CNTs-CoFe2O4 magnetic nanocomposites, Chem. Eng. J. 316 (2017) 893–902. With kind permission of Elsevier.

120 min, and initial concentration of 300 mg/L. The pseudo-second-order kinetic model and Langmuir isotherm model fitted well the adsorption data [84]. Wasim et al. [85] prepared novel thin film composite (TFC) membranes via decorating chitosan in the porous structure of polyvinylidene fluoride substrate doped with MWCNTs for the elimination of reactive orange 16 from wastewater. The decoration of chitosan was proved via the elimination of pores from polyvinylidene fluoride, which enhances the active sites and adsorption capacity for removal of reactive orange 16. The utilization of chitosan as a pore decorating material produced the positive charge on the surface of TFC membrane, and the adsorption of dye occurred via electrostatic attractions between the anionic group of reactive orange 16 and the cationic group of chitosan surface. The changes in TFC membranes before and after adsorption of reactive orange 16 were studied via SEM analysis (Fig. 12). As can be seen, both the substrate layer and chitosan demonstrated a change in their SEM micrographs. The rough surface of chitosan layer is due to the agglomeration of reactive orange 16 on chitosan layer which confirms the adsorption of dye by chitosan layer [85]. Abbasi [86] showed that reactive blue 19 and direct blue 71 can be effectively eliminated from wastewater by novel magnetic NC of Chitosan/SiO2/CNTs (MNCSC) which was prepared via gelating technique with utilized glutaraldehyde as a crosslinker. It

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FIG. 10 Schematic illustration for the preparation of the starch/modified MWCNTs NCs. Adapted from S. Mallakpour, S. Rashidimoghadam, Starch/MWCNT-vitamin C nanocomposites: electrical, thermal properties and their utilization for removal of methyl orange, Carbohydr. Polym. 169 (2017) 23–32. With kind permission of Elsevier.

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FIG. 11 (A) Effect of pH on the adsorption of MO on starch/AA-MWCNTs NCs 6 wt%, (B) the influence of adsorbent dose on the adsorption of MO by starch/AA-MWCNTs NC 6 wt%, (C) the influence of initial dye concentration on the uptake of MO dye by starch/AA-MWCNTs NC 6 wt%, (D) Langmuir isotherm plot of MO dye uptake by adsorption onto starch/AA-MWCNTs NC 6 wt%, (E) Freundlich isotherm of dye adsorption onto starch/AA-MWCNTs NC6 wt% (MO ¼ methyl orange). Adapted from S. Mallakpour, S. Rashidimoghadam, Starch/MWCNT-vitamin C nanocomposites: electrical, thermal properties and their utilization for removal of methyl orange, Carbohydr. Polym. 169 (2017) 23–32. With kind permission of Elsevier.

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FIG. 12 Surface images of fabricated membranes before and after testing with RO-16 (RO-16 ¼reactive orange 16, PVDF ¼ polyvinylidene fluoride). Adapted from M. Wasim, S. Sagar, A. Sabir, M. Shafiq, T. Jamil, Decoration of open pore network in Polyvinylidene fluoride/MWCNTs with chitosan for the removal of reactive orange 16 dye, Carbohydr. Polym. 174 (2017) 474–483. With kind permission of Elsevier.

was found that the highest adsorption ability happened at pH 2.0 for reactive blue 19 and pH 6.8 for direct blue 71. For both reactive blue 19 and direct blue 71 dye, the adsorption capacity was enhanced with reducing of initial dye concentration and increasing the MNCSC amount due to enhanced available active sites. Also, the equilibrium data and the adsorption kinetics were best fitted to the Langmuir isotherm model and pseudosecond-order model, respectively [86]. Wang et al. [87] demonstrated that magnetic hydroxyapatite (mHAP)-immobilized oxidized MWCNT (oMWCNTs) is an effective adsorbent for the elimination of Pb(II) and methylene blue from wastewater. This adsorbent was fabricated by loading mHAP beads on oMWCNTs. The results of adsorption studies

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demonstrated that the mHAP-oMWCNTs are excellent adsorbent due to the great SSA and plentiful oxygenic functional groups of oMWCNTs which act as available adsorption sites, and also strong ion exchange properties of HAP and the easy solid-liquid separation of Fe3O4. The highest adsorption capacity was achieved at pH of 8.1 for methylene blue and 4.1 for Pb(II). The adsorption process was well represented by Freundlich and Langmuir isotherm and a pseudo-second-order kinetic model [87]. The MWCNTs functionalized with 3-aminopropyltriethoxysilane (functionalized-CNTs) and loaded with TiO2 to study its adsorption capacity for methyl orange by Ahmad et al. [88]. The TiO2 clusters were connected to sidewalls of functionalized-CNTs because of interactions between the amine groups of 3-aminopropyltriethoxysilane and the -OH groups of TiO2. The mechanism of dye adsorption process is as follow: the dye was distributed on the surface of the functionalized-CNTs-loaded TiO2 NC and then an effective interaction happened between the surfaces of NC and methyl orange molecules. The pseudo-secondorder kinetics model fitted well the adsorption data [88]. Kheirandish et al. [89] reported the preparation of novel material based on chitosan and MWCNTs for ultrasound-assisted elimination of eriochrome cyanine R dye from wastewater. Chitosan was extracted from lobster shells, and grafted with amino functionalized MWCNT. The ultrasound waves increase the mass transfer between the dye and adsorbent, enhance the chemical reaction rate, and reduce the adsorption time as well as energy consumption. The optimum condition of eriochrome cyanine R dye adsorption was found at sonication time of 9.35 min, adsorbent amount of 0.018 g, and initial dye concentration 18.20 mg/L [89]. Shouman et al. [90] investigated the adsorption ability of MWCNTs and MWCNTs-poly conducting polymers (o- and m-Toluidine) NCs for removal of methylene blue and Pb(II) ions from wastewater. The NCs were fabricated via in situ oxidative chemical polymerization of two monomers [2-methylaniline (o-Toluidine) and 3-methylaniline (m-Toluidine)] on the MWCNTs. The results of BET surface area demonstrated that after treatment of MWCNT with o- and m-Toluidine, its SSA was decreased because the polymer occupies the inner pores of the MWCNTs. It was found that MWCNTs and NCs have great adsorption ability toward methylene blue in comparison with NCs because of the presence of carbonyl groups on the surface of MWCNTs which react with methylene blue and also the high SSA of MWCNTs which considerably improved the removal efficiency. On the other hand, the modification of MWCNT by polymer reduces its surface area thus the NCs can’t be completely accessed by the methylene blue molecule because of the pore blockage. The fabricated NCs have great adsorption ability for Pb (II) ions at pH 5.5. At this pH, the effective reaction occurred between metal ion with the methyl group and the amine functional group of o- and m-Toluidine resulting in the generation of the metal complex. During this reaction, the polymer is changed into an undoped form, then, the free amine or imine will be available to metal chelating. This phenomenon, considerably improved the removal efficiency of NCs toward Pb+2 [90]. The preparation of MnFe2O4/MWCNT NC via the hydrothermal method and its application for elimination of direct red 16 and yellow 40 dyes from wastewater was reported by Kafshgari et al. [91]. The removal of direct red 16 by MnFe2O4/MWCNT reached a maximum at pH 2 and decreased by enhancing the

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pH value. At this pH, the surface of adsorbent was positively charged due to the high concentration of H+ ions and electrostatic attractions between the cationic groups of adsorbent and the anionic groups of direct red 16 was occurred. Also, the highest removal efficiency was achieved at about pH 6 for yellow 40 due to the electrostatic attractions between yellow 40 zwitterions and the negatively charged surface groups on the MnFe2O4/MWCNT. The obtained results are well fitted in the Sips adsorption isotherm model and pseudo-second-order kinetic model [91]. Azqhandi et al. [92] fabricated the Cd-doped ZnO/CNT NCs (Cd@ZnO/CNT-NCs) via microwave-assisted hydrothermal technique. Then the photocatalytic performance of the NCs was examined through the measurements of methyl orange degradation under UV-visible light. It was found that the photocatalytic performance of Cd@ZnO/CNT-NCs is higher than that of undoped ZnO-CNT NCs. The incorporation of cadmium ions into the ZnO lattice reduces the SSA and enhances the crystal size connected to the CNTs surface but doesn’t affect the lattice constants of the ZnO crystals. The incorporation of cadmium ions was confirmed via the reduction of band-gap energy value, which improves the photocatalytic property of the Cd@ZnO/CNT-NCs because it succeeds in adsorbing wider edge of irradiations (Fig. 13) [92]. Abdi et al. [93] reported the preparation of zeolitic imidazolate framework (ZIF-8) and its hybrid NC based on CNTs and graphene oxide (GO) through simple and ambient temperature technique. This NC was utilized as an adsorbent for the elimination of cationic malachite green dye from real wastewater. The preparation of ZIF-8@CNT and ZIF-8@GO NCs have several benefits like high thermal stability, proper pore size, impeding aggregate formation, improved dispersive forces, and great surface area. Also, the ZIF-8@GO showed greater adsorption ability in comparison with ZIF-8 and ZIF-8@CNT which shows that the presence of other salts and ions in real wastewater did not considerably affect the removal of malachite green dye by ZIF-8@GO [93]. Hu et al. [80,81] showed that poly-L-lysine (PLL)-functionalized magnetic Fe3O4-(GO-MWCNTs) hybrid NC is an effective adsorbent for the elimination of tartrazine dye and Pb(II) from wastewater due to its great surface area and plentiful hydroxyl and amino groups. The GO-MWCNT hybrid NC was prepared through

FIG. 13 Schematic illustration of band-gap structure of Cd-doped CNT/ZnO NCs. Adapted from M.H.A. Azqhandi, F.B. Vasheghani, F.H. Rajabi, M. Keramati, Synthesis of Cd doped ZnO/CNT nanocomposite by using microwave method: photocatalytic behavior, adsorption and kinetic study, Results Phys. 7 (2017) 1106–1114. With kind permission of Elsevier.

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chemical-free technique. Then, Fe3O4-(GO-MWCNTs) was fabricated by a combination of GO-MWCNTs with Fe3O4 through chemical co-precipitation technique. After that, PLL was grafted onto Fe3O4-(GO-MWCNTs) via a nucleophilic reaction between amine groups of PLL and the carboxyl groups of (GO MWCNTs) to the synthesis of PLL-Fe3O4-(GO-MWCNTs) NC hybrid. This adsorbent effectively adsorbed the dye pH 2 due to electrostatic attractions between the positively charged surface adsorbent and anionic groups of tartrazine dye and also the H-bonding, van der Waals forces, and covalent bonds between dye and C]C, dOH, dCOOH, and dNH2 functional groups of adsorbent. The sorption data fitted into Langmuir isotherms model and pseudosecond-order kinetics model [81]. Mallakpour et al. [94] reported that glycerol plasticized-starch (GPS)/ascorbic acid (AA)MWCNTs NCs are excellent adsorbents for the elimination of methylene blue dye from wastewater. This adsorbent was synthesized by solution casting and ultrasonic dispersion technique, which facilitates the H-bonding between GPS and carbonyl, carboxyl, and hydroxyl groups of AA-MWCNTs (Fig. 14). The transmission electron microscopy (TEM) images of GPS/AA-MWCNTs NC demonstrated the homogeneous distribution of the AA-MWCNTs in the GPS matrix (Fig. 15). The highest adsorption capacity was achieved at pH 4, initial dye concentration ¼ 20 ppm, and adsorbent dosage ¼ 0.04 g (Fig. 16). At pH <4, the competition of great amount of H+ ions with the cationic groups of methylene blue for adsorption sites occurred. The electrostatic repulsion between cationic methylene blue dye and cationic groups of GPS/AA-MWCNTs NCs which produced by protonation of carbonyl, hydroxyl, and carboxyl groups of GPS is another reason for low adsorption ability at lower pH. The reduction of adsorption capacity at pH >4 showed that the electrostatic interactions are not enough for elucidation the mechanism of dye removal and other interactions, such as hydrogen bonds between methylene blue and adsorbent, π–π interactions between organic molecules with benzene rings or C]C of methylene blue and bulk π systems on surfaces of NC can affect the adsorption process. It was found that Langmuir isotherms model was in good agreement with experimental data [95]. Jin et al. [96] fabricated the Nickel nanoparticles encapsulated in porous carbon/CNT (Ni/PC-CNT) NCs via facile and economical synthetic technique. The prepared NC demonstrates good removal efficiency for malachite green, methylene blue, rhodamine B, congo red, and methyl orange from wastewater due to great pore volume and great surface area. These dyes can absorb onto carbon surface, because of the π–π interaction between the sp2 graphitic carbon in the adsorbent and the dye molecule. The adsorbent is negatively charged in aqueous solution, and therefore it has great adsorption ability toward malachite green, rhodamine B, and methylene blue in comparison with methyl orange. The congo red dye can be adsorbed via hydrogen bond between the amine groups in congo red and the hydroxyl groups of the adsorbent. It was found that the highest removal efficiency of NCs are 271, 312, 395, 818, and 898 mg/g for methyl orange, methylene blue, rhodamine B, congo red, and malachite green dyes, respectively, which were considerably greater than most of other adsorbents [96]. Kang et al. [97] prepared sandwich-like NC membrane by layer-by-layer self-assembly technique by GO and oxidized CNTs (OCNTs) alternately

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FIG. 14 Schematic illustration for the fabrication of the GPS/AA-MWCNTs NCs. Adapted from S. Mallakpour, S. Rashidimoghadam, Application of ultrasonic irradiation as a benign method for production of glycerol plasticized-starch/ascorbic acid functionalized MWCNTs nanocomposites: investigation of methylene blue adsorption and electrical properties, Ultrason. Sonochem. 40 (2018) 419–432. With kind permission of Elsevier.

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FIG. 15 TEM micrographs of GPS/AA-MWCNTs NC 6 wt%. Adapted from S. Mallakpour, S. Rashidimoghadam, Application of ultrasonic irradiation as a benign method for production of glycerol plasticized-starch/ascorbic acid functionalized MWCNTs nanocomposites: investigation of methylene blue adsorption and electrical properties, Ultrason. Sonochem. 40 (2018) 419–432. With kind permission of Elsevier.

on the hydrolyzed polyacrylonitrile under the cross-linking effect of polyelectrolyte. The rate of OCNTs/GO was 1/3, 1/5, 1/10, and 1/15 and the membranes were named after (GO3/OCNTs1)M, (GO5/OCNTs1)M, (GO10/OCNTs1)M, and (GO15/OCNTs1)M, respectively. The fabricated NCs were utilized for removal of dye from aqueous solution. It was found that (GO10/OCNTs1)M had water flux of 21.71 L/(m2h) and rejection of 99.3% for methyl blue which indicates that (GO10/OCNTs1)M had greater flux in comparison with the reported literature in which rejection also reached up to 99%. After operating time of 50 h, dye rejection for methyl blue of (GO10/OCNTs1)M and (GO15/OCNTs1)M remained 99.3% and 99.6%, respectively [97]. Pentaerythritol modified MWCNT (ox-MWCNT-PER) NCs were used for removal of alizarin red S and alizarin yellow R dyes from aqueous solutions by Yang et al. [98]. These

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FIG. 16 (A) Influence of pH on the adsorption of MB on GPS/AA-MWCNTs NC 6 wt%, (B) the influence of adsorbent content on the removal of MB using GPS/AA-MWCNTs NC 6 wt%, and (C) the influence of initial MB concentration on the uptake of MO dye by GPS/AA-MWCNTs NC 6 wt% (MB ¼ methylene blue, MO ¼ methyl orange). Adapted from S. Mallakpour, S. Rashidimoghadam, Application of ultrasonic irradiation as a benign method for production of glycerol plasticized-starch/ascorbic acid functionalized MWCNTs nanocomposites: investigation of methylene blue adsorption and electrical properties, Ultrason. Sonochem. 40 (2018) 419–432. With kind permission of Elsevier.

dyes were successfully removed due to H-bonding and π–π stacking interactions between adsorbent and dyes. The results of adsorption experiments showed that the removal efficiency was reduced by enhancing the temperature thus the adsorption process is spontaneous and exothermic. Also, it was found that the Langmuir isotherm and pseudo-second-order kinetic model showed a better fit to the adsorption of both the dyes [98]. Aliabadi and Mahmoodi [99] reported the elimination of sunset yellow and congo red dyes from aqueous solutions by polypyrrole, polyaniline nanoparticles alone, and their NCs (polyaniline/MWCNTs and polypyrrole/MWCNTs NCs) using ultrasonic irradiation method at room temperature. It was found that the high removal efficiency of polypyrrole/MWCNTs and the polyaniline/MWCNTs NCs which synthesized with FeCl3 and (NH4)2S2O8 as initiators are because of the electrostatic attraction and the π-π electron donor-acceptor interaction. The results showed that the greatest adsorption capacity for sunset yellow dye (99%) was obtained at room temperature, ultrasonic irradiation power 500 W, 0.007 g of polypyrrole/MWCNTs NC, and pH 2 and for congo red dye the

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greatest adsorption capacity (98%) was obtained at room temperature, ultrasonic irradiation power 500 W, 0.01 g of polypyrrole/MWCNTs NC, and pH 2 [99].

3 Conclusions The potential of CNTs and polymer/CNTs NCs, for removal of dyes from wastewater, has been discussed in this chapter. CNTs can effectively adsorb various dyes owing to the different chemical functional groups of dyes-CNT interactions including covalent and electrostatic interactions, π–π bonds, hydrophobic effects, and hydrogen bonds. Although molecular structure, which is considerably affected by the charge, loads of the dyes and CNTs and also π–π bonds is the main driving force of interaction between CNTs and dyes. Although, pure CNTs can be utilized for dye removal due to their high adsorption efficiency, their high price limits their industrial application. In addition, separation of CNTs from aqueous solution is very complicated due to its smaller size and high aggregation property. To overcome these problems researchers have focused their attention on the preparation of NCs of CNTs with polymers, metal oxide, carbon, etc. which act as a stable matrix to the CNTs. Effect of various parameters including physical properties of CNTs, nature of adsorbate, chemical treatment of CNTs, and process variables (including CNT dosage, temperature, solution pH, contact time, and initial concentration of dye) on the removal of dyes by CNT-based materials has been reported in this study. The high adsorption capacity of these materials toward various classes of dyes suggests that they can act as a potential adsorbent for the removal of dyes from wastewater.

Acknowledgments “The authors are thankful to the Research Affairs Division Isfahan University of Technology (IUT), Isfahan, I. R. Iran, for partial financial support. Further financial support from National Elite Foundation (NEF), Tehran, I. R. Iran, Iran Nanotechnology Initiative Council (INIC), Tehran, I.R. Iran, and Center of Excellence in Sensors and Green Chemistry Research (IUT), Isfahan, I. R. Iran is gratefully acknowledged.”

References [1] V.K. Gupta, Suhas, Application of low-cost adsorbents for dye removal—a review, J. Environ. Manag. 90 (2009) 2313–2342. [2] H. Sadegh, R. ShahryariGhoshekandi, A. Masjedi, Z. Mahmoodi, M. Kazemi, A review on Carbon nanotubes adsorbents for the removal of pollutants from aqueous solutions, Int. J. Nano Dimens. 7 (2016) 109–120. [3] Y.B. Che Ani, Adsorption studies of dyes using clay-based and activated carbon adsorbents, Universiti Sains Malaysia, Malaysia, 2004. [4] S. Mallakpour, A. Abdolmaleki, F. Azimi, Ultrasonic-assisted biosurface modification of multi-walled carbon nanotubes with Thiamine and its influence on the properties of PVC/Tm-MWCNTs nanocomposite films, Ultrason. Sonochem. 39 (2017) 589–596.

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