8 Carbon Nanotubes for Heavy Metals Removal Shadpour Mallakpour*,†, Elham Khadem* *Organ ic P olymer C hemistry R esearch L aboratory, Dep artment of Chemistry , I sfaha n U ni vers ity o f Te c hnol ogy, Is faha n, Isl ami c Rep ubli c of I ran † Research In stitu te for N a note ch nol ogy a nd A dva nced Mate ria ls, Isf aha n Uni versi ty of Te ch nolo gy, Isfa han , Is lam ic Re publ ic o f Ira n
1 Introduction The ecosystem is continuously being polluted with various toxic agents from both natural and anthropogenic sources. Rapid growth of population, agricultural activity, and industrial development are the main sources of discharge of numerous hazardous elements and materials within the environment [1,2]. Global environmental issue induced by heavy metal ions is usually categorized to metal cations, for example, Ni(II), Hg(II), Cu(II), Pb(II), Cd(II), etc. Generally, the elements with the atomic weight in the range of 63.5–200.6 g/mol and a specific gravity greater than 5.0 are introduced as heavy metal. The toxic metals in wastewater are found either in chemical form or mixed form. Therefore, the removal of toxic metals from wastewater is difficult [2,3]. By discharging of toxic metals into the water, they are converted into hydrated ions with higher toxic than the initial form. Thus, uptake of them is essential to lower the common hazards. To reduce contamination, an admissible discharge level of toxic metal into the ecosystem was determined by Environmental Protection Agency (EPA) and World Health Organization (WHO). Nevertheless, pollution of discharged effluent is more than permissible limits [4,5]. Water contamination and accumulation of toxic metals in human body and environment give rise to serious unwanted health and environmental dangers [6–8]. Generally chemical, physical, and biological approaches have been used to treat the trouble of toxic elements, organic contaminations, and biomaterials from water . In this regard, strategies such as adsorption, precipitation, coagulation, ion exchange, membrane filtration, flotation, and electrochemical processes have been proffered to sequestration the heavy metal contaminants from water [3,10]. A comparison of merits and demerits of various treatment practices accessible for the uptake of heavy metals from wastewater was performed by Carolin et al. . Among these, adsorption/ion exchange was introduced as a noteworthy technique and extensively adopted in industries due to its low cost, ease
Composite Nanoadsorbents. https://doi.org/10.1016/B978-0-12-814132-8.00009-5 © 2019 Elsevier Inc. All rights reserved.
of operation, simplicity of design, insensitivity to toxic pollutants, and less sludge production [10,11]. Advantage and disadvantage of other methods were discussed by researchers [12,13]. The toxicity of heavy metal anions, such as chromate, arsenite, mercury, copper, arsenate, etc., on the human life and health has been studied in other work [6,7]. In the recent study, organic compounds comprising neem leaf , rice husk , conducting polymers , sawdust , orange peel , and bagasse fly ash ; nanoparticles, such as calcium carbonate [20,21] and manganese dioxide ([22–24]); and also nano-sheet such as layered double hydroxide inserted into polymer  were introduced as adsorbent of contaminant materials. Among them, carbonaceous materials like carbon nanotube (CNT) and CNT-based nanocomposites (NCs) have opened fascinating prospects as the most promising remediation materials in minimizing potential environmental threats and trapping of heavy metal cations and other pollutant organic compound from impure water and air [26,27]. The main reasons to this phenomenon can point out high surface area and abundant pore structures, surface hydrophobic π–π interaction, high negative charge density, hydrophilicity, and easily synthesized from the abundant natural graphite, which assists diffusion and adsorption of metal oxides [28,29]. CNTs are allotropic form of carbon. They generally found in two forms with one or several concentric graphitic tubules of same or different chiralities stacked, referred as single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs), respectively [30–32]. Properties and structures of CNT has been completely described in other literatures [33,34]. In all of them pointed out that CNT, due to its unique properties, can be mixed with various polymers to improve physicochemical properties of polymer and used in fields, such as hydrogen storage, drug delivery, electromagnetic interference (EMI) shielding, devices, optical switches, packaging, aerospace and automotive materials, adhesive and coatings, chemical sensor, etc. This chapter introduces recent publication (2016–18), focusing on treated CNT-based adsorption against the removal of types of divalent metal cations from aqueous solution and discussed their adsorption behavior and mechanisms under different conditions.
2 Importance of CNT As known, CNT is an infinite cylindrical graphitic sheets with sp2 hybridization, which is organized into a signal network with hexagonal cells [35,36]. According to reported results in literatures, the presence of polar functional group on the surface of nanotube has an effective role in increasing the performance of adsorbent. In other word, removal efficiency of heavy metals intensely depends upon their surface total acidity (SWCNT or MWCNT) and does not have a positive correlation with kind of CNT (SWCNT and MWCNT), specific volume of pore, mean pore diameter, and specific surface area [8,32]. In CNT, adsorption process is usually controlled by the following four possible active sites: (i) the hollow interior of separate CNTs designated as internal sites; (ii) the interstitial channels between separate CNTs in the stacks; (iii) the grooves existing on
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
the side of a CNT stack and the external surface; and (iv) the external surface of separate CNTs. ([37,38]. During adsorption, structure ends of prepared CNTs are initially closed and sites of interstitial channels and grooves among adjacent nanotubes stacks are accessible for initial adsorption . In next step, the adsorption is carried out on external walls and continued by accumulating of molecules on internal axial sites. From a kinetic viewpoint, internal sites (interstitial channels and inside the tube) reach equilibrium values at more times than the external sites (grooves and outer surface) under the same conditions. This can be due to difficult accessible to adsorbing materials and resistance to diffusion of adsorbents to the internal sites. Consequently, opening the ends of CNTs and increasing number of accessible active sites in CNT can enhance the saturation capacity and kinetic rate . The cost of wastewater treatment technologies by CNT-based adsorbents depends on various factors including the cost/complexity of CNT functionalization, the necessity for solid/liquid segregation, the type of wastewater treatment, and the recycling cost and efficiency . In spite of prohibitive cost CNT and its complex interactions with other compounds, CNT is introduced a promising sorbent in the future and researchers are in quest to further develop its properties. Up to now, hundreds of book chapters and review research papers have been published , which focus on the applications of CNTs-based composite in wastewater treatment, as shown in Fig. 1.
3 Adsorption Affinity of CNT The adsorption affinity of CNT and its polymeric NCs to heavy metal ions depends remarkably on various factors that considered as guidelines to examine of metal ions sorption on the adsorbent. (i) The ionic radius; among of heavy metals, order of ionic radius was Pb(II) (120 pm)>Sr(II)>Ca(II) (100 pm)>Cd(II) (99 pm)>Mn(II) (80 pm)>Cu(II) (77 pm)>Zn(II) (74 pm) and Co(II) (72 pm)>Ni(II) (69 pm). (ii) The metal electronegativity; this rule explains that metals with higher electronegativity have stronger adsorption on the negatively charged CNT surface. Metal electronegativity with Pauling unite followed the order of Pb(II) (2.33)>Ni(II) (1.91)>Cu(II) (1.90)>Co(II) (1.88)>Cd(II) (1.69)>Mn(II) (1.55)>Ca(II) (1.00). (iii) First stability constant of the associated metal hydroxide; the sorption theory suggested that M(II) of less soluble complexes with hydrate have enhanced adsorptive capability. In an adsorption model, oxygenous functional groups on the CNT surface (such as hydroxyl and carboxyl groups) are complexed to heavy metal species, which the speciation of them is determined by the stability constant. For example, log K1 values of Pb(OH)+, Cu(OH)+, Cd(OH)+, Zn(OH)+ in a complexation reaction (M2+ + OH $ M(OH)+) were 7.82, 7.0, 4.17, and 4.4, respectively. (iv) Reduction potential of the heavy metal ions; the theory stated that sorbent affinity reduces by increasing of reduction potential. According to affinity order, the standard reduction potentials of Pb2+/Pb, Cu2+/Cu, Cd2+/Cd, and Zn2+/Zn were 0.1262 V, 0.3419 V, 0.4030 V, and 0.4618 V, respectively [11,28].
2007 2008 2009 2010 2011 2012 2013 2014 2015 2016
Wastewater recovery 38.6% Water desalination 61.4%
Heavy metal ions 25.4%
Emerging pollutants 22.9%
Waste water recovery
FIG. 1 (A) Annual number of research papers for CNTs-based composite membranes reported during 2007–16 (all date updated on January 9, 2017) and (B) existing water treatment methods for CNTs-based composite membranes
4 Mechanism Adsorption on the CNT The mechanism of adsorption of various toxic metal cations on the CNT-based compounds depended on the type of adsorbent and pollutant. The adsorption in CNT and their polymeric NCs took place mainly through ion exchange process, π-π interaction, electrostatic interaction, and surface complexation . In ion exchange process, protons on the adsorption active sites (dCOOH and dOH) are displaced by metal cations presence in solution. This process can also cause a lower equilibrium solution pH than the initial pH value. Different reactions represent to adsorption of M2+ by oxygenous groups through ion exchange; M2 + + COOH ! ðCOO MÞ + + H + M2 + + OH ! ðO MÞ + + H +
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
M2 + + 2 COOH ! COO M OOC + 2H + M2 + + 2 OH ! O M O + 2H + M2 + + COOH + OH ! COO M O + 2H +
The isoelectric point of CNT controls by solution pH and plays a significant role in improving the adsorption ability of CNT through electrostatic interaction. At pH values higher than isoelectric point, attraction between negatively charged CNT surface and positively charged heavy metal affords a driving force for the removal . In this regard, the presence of heteroatoms like S, O, and N with electrons lone pairs has a promising role in improving the adsorption capacity of M2+ on the adsorbent through electrostatic interaction. While, reduction in the removal of cations and desorption of metal ions occur at pH values lower than the isoelectric point. Additionally, chelating sites place at the edges of CNT can simultaneously participate in the complexation with metal ions. During metal adsorption, factors such as small structural defects, inner cavity, and pores of the CNT structure chip are involved [11,43].
5 Effective Parameters in Adsorption In adsorption process different factors such as oxygenous active site, species of metal cations in solution, and experimental conditions can change the uptake of toxic metal cations on the CNT surface. Among them, the experimental conditions, such as ionic strength, adsorbent amount, contact time, foreign ions, solution pH, and temperature, have effective role in removal efficiency and cleanup solution [5,6,44,45]. During adsorption, increase of the sorbent dose leads to enhance percentage removal effectiveness. This associated to increase in number of active site and facilitate metal uptake on adsorbent sites. Fig. 2 shows the effect of adsorbent dose on the kinetic rate constants (k2) (a) and
FIG. 2 Changes of k2 (A) and h (B) with the adsorbent contents. Adapted from W.-L. Sun, J. Xia, Y.-C. Shan, Comparison kinetics studies of Cu (II) adsorption by multi-walled carbon nanotubes in homo and heterogeneous systems: effect of nano-SiO2, Chem. Eng. J. 250 (2014) 119–127, with kind permission of Elsevier.
initial adsorption rate (h) on the various adsorbents (b) . An increase in adsorbent content enhances the values of k2 and h. As seen, at first k2 is high and then started to decrease with future rise in sorbent content because most of Cu ions are adsorbed at higher sorbent dose, so initial rate reduces at 320 mg/L of CNT-COOH and 360 mg/L of CNT-OH. Also, they found that the CNTs-OH reaches equilibrium much quicker than the CNTs-COOH for removal of Cu(II). The solution pH has an effective role in improving the adsorption performance of the CNT and their respective polymeric hybrids for water clean-up application. The pH expressively alerts both the adsorbent chemistry and surface chemistry of the CNTcontaining composites. At pH value > zero charge (pHPZC), negative surface charge of adsorbent adsorbs cations by the electrostatic interactions. While, decrease of pH provides a neutral surface charge and leads to decrease of adsorption of cations. The adsorption capacity of CNTs depends on pH change of solution, which changed competing complexation reactions and metal ion species. The optimum pH solution for reaching high adsorption capacity of Ni(II), Cu(II), Pb(II), Zn(II), and Co(II) can change in range of pH 4–8, due to their variances in the electronegativity of cations, the first stability constant of the metal hydroxide, and the standard reduction potential of the metal ions . At basic pH, the predominant divalent metal ions species in the liquid are M(OH) n+1 and M(OH)2 , which led to electrostatic repulsion between negative species and the negan+1 tively charged functional groups on CNT. Fig. 3 shows the effect of various pH on the adsorption capacity of Cd(II) ion . It confirmed that most of removal of metal ion was obtained at pH 7 due to electrostatic interaction. Other possible interactions were indicated in Fig. 3. Fig. 4 represents the role of pH on the removal of arsenic on magnetic iron oxide-CNT (MIO-CNT) hybrid . It is shown that increasing the pH of the solution increases the
FIG. 3 (Right) Effect of pH on the adsorption of Cd(II) ion by Al2O3/MWCNTs and (left) the schematic presentation of interaction between them. Adapted from J. Liang, J. Liu, X. Yuan, H. Dong, G. Zeng, H. Wu, H. Wang, J. Liu, S. Hua, S. Zhang, Facile synthesis of alumina-decorated multi-walled carbon nanotubes for simultaneous adsorption of cadmium ion and trichloroethylene, Chem. Eng. J. 273 (2015) 101–110, with kind permission of Elsevier.
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
FIG. 4 Effect of pH on arsenic (V) (A) and arsenic (III) (B) adsorption by MIO-CNT hybrid at 298 K. The initial arsenic concentration was 10 mg/L and the dosage of adsorbents was 0.2 g/L. Adapted from B. Chen, Z. Zhu, J. Ma, M. Yang, J. Hong, X. Hu, Y. Qiu, J. Chen, One-pot, solid-phase synthesis of magnetic multiwalled carbon nanotube/iron oxide composites and their application in arsenic removal, J. Colloid Interface Sci. 434 (2014) 9–17, with kind permission of Elsevier.
adsorption efficiency of As(III) and As(V) on the MIO-CNT. The low removal capacity of MIO-CNTs for As(V) in acidic pH mainly owns to the Fe release within the liquid. With increasing of pH value, the Fe release decreased and As(V) removal increased obviously under relatively strong acidic conditions of pH 2–4. While optimum pH value for As(III) occurred at 8. In pH <9.2, As(III) is found in form of H3AsO3 and H2AsO 3 and adsorbed on adsorbent by a surface complexation mechanism.
The other important factor in adsorption rate is temperature. Increasing temperature boosts the mobility of pollutants and diffusion rate of heavy metal ions, thus facilitates available adsorption sites on the CNT surface. This condition accelerates to the adsorption rate. It indicated that the removal process is endothermic, feasible, and spontaneous. Yang et al.  investigated the thermodynamic analysis of Cu(II) onto [email protected]
(diamine functionalized mesoporous silica on MWCNTs) and showed an ascension in adsorption capacity of Cu(II) with the increase of temperature, so that adsorption capacity is 66.577, 70.872, and 74.627 mg/g for temperature of 25°C, 35°C, and 45°C, respectively. In other work, Salehi et al.  investigated effect of various temperatures on equilibrium values and correlation coefficients (R2) of Freundlich and Langmuir isotherms for the removal of Cu(II). Chitosan/poly(vinyl alcohol) (CS/PVA) NCs containing various amounts of MWCNT-NH2 were prepared as adsorbent by casting-evaporation method. Based on higher R2 values, the Freundlich isotherm has better fit to the equilibrium adsorption and incorporation of MWCNTs into polymer improved the adsorption capacity of the CS/PVA membrane almost up to two times (from 9.45 to 18.32 mg/g at T ¼ 20°C). Also, increase of temperature has a positive effect in uptake of metal ion. The MWCNT-NH2 by increasing the number of the reactive adsorption sites, providing nanochannel pathways and introducing additional mass transport network through the membrane matrix could increase number of accessible pathways to transport of Cu(II) toward active sites. De-li Xiao et al.  studied the effects of the coexisting ions on the recoveries of Cu(II) on the carboxylated MWCNT-Fe3O4 magnetic hybrids (c-MWCNTMCs) and indicated that the effect of foreign ions on the removal is slight and the recovery of Cu(II) in solution is all above 90% (Table 1). This negligible influence may own to existence of both cations and anions in the solution that balance the effect of each other.
Table 1 The Maximum of Coexisting Ion Concentrations Under Which Ones’ Interferences Are Insignificant Ions
Concentration (C) (mg/L)
Na2+ K+ ClNO-3 Ca2+ SO24 Al3+ Fe3+
NaCl KCl NaCl NaNO3 CaCl2 Na2SO4 Al2(SO4)3 FeCl3
10,000 10,000 10,000 5000 500 500 50 10
98.9 2.0a 95.3 0.7a 94.1 2.1a 96.1 1.4a 93.7 4.2a 92.5 2.3a 94.1 2.5a 92.9 1.6a
Note: C[Cu(II)]¼0.2 mg/L, V ¼ 250 mL, mc-MWCNTs ¼ 20 mg, t ¼ 25°C, N¼ 4. a Mean standard deviations. Adapted from D.-L. Xiao, H. Li, H. He, R. Lin, P.-L. Zuo, Adsorption performance of carboxylated multi-wall carbon nanotube-Fe3O4 magnetic hybrids for Cu(II) in water, New Carbon Mater. 29 (2014) 15–25, with kind permission of Elsevier.
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
In other work by Zeng et al. , the effect of pH and temperature on the chitosan composites based on CNT was investigated. At first polydopamine (PDA) thin films via self-polymerization of dopamine under alkaline solution was coated on the CNT surface. Then, the carboxymethyl chitosan was linked onto CNT-PDA by Michael addition process between the PDA and the dNH2 groups of carboxymethyl chitosan. The results showed that the adsorption capacity of Cu(II) was enhanced from 19 to 36.0 mg/g as the pH was upped from 3 to 10. Also, adsorption was enhanced and ΔH° became positively with the evaluation of temperature, which proved the Cu(II) removal onto CNT-PDA-CS is endothermic.
6 Adsorption of Heavy Metals by CNT 6.1 Oxide Functionalization of CNT As known, as-produced CNTs are hydrophobic and trend to agglomeration. On the other hand, the presence of functional groups can increase of amount of surface total acidity of CNT. Thus, modification of the external wall of CNT leads to increase of homogeneity and adsorption capacity. There are principally two methods of functionalizations. The first is covalent functionalization using oxidizing agents, such as nitric acid, mixed nitric acid, and sulfuric acid, gaseous oxygen, ozone, and plasma, which attached oxygenous functional groups of dOH, dC]O, and dCOOH and basic of amine and thiol on nanotube wall. Moreover, oxidation process causes the opening up of nanotube ends and forming of defects on the sidewall. The second modification is noncovalent that surfactant adsorbed to nanotube wall to obtain a more homogeneous suspension of CNTs [34,38,53]. From other methods can point out mechanical grinding and adsorption of mediating molecules onto the CNT surfaces to develop the dispersal properties and adsorption ability of CNT . As mentioned above, CNT reactivity and its role in hazardous pollutants removal depended on the structural defects in the sidewalls that are introduced under growth processes and oxidative conditions. These defects are suitable places to attach various functional groups, such as dOH, C]O, and dCOOH, which intentionally formed onto nanotube surfaces by air oxidation or acid oxidation. These functional groups introduce active sites for heavy metal adsorption on CNTs. In an experimental work , the efficiency of pristine MWCNTs (P-MWCNT) and modified MWCNTs (M-MWCNT) was surveyed to adsorb Ni(II) from aqueous solution under various conditions, such as ion concentration, pH, and filter mass and was studied. For this purpose, the surface of nanotubes was purified and functionalized using a mixture of HCl and H2O2 and oxidized by nitric acid (Fig. 5). As seen in Fig. 5, pH has effective role in the removal efficiency, so that the adsorption of metal ion by M-MWCNTs filter rose from 22% at pH 3 to 85% at pH 8. This showed that the functionalized MWCNT is favorable candidate in removal of toxic metals.
FIG. 5 (Top) Schematic diagram for the steps of MWCNTs filter design and filtration mechanism. (Down) Variation in removal efficiency with pH of Ni(II) aqueous solution. Adapted from E. M. Elsehly, N. Chechenin, A. Makunin, H. Motaweh, E. Vorobyeva, K. Bukunov, E. Leksina, A. Priselkova, Characterization of functionalized multiwalled carbon nanotubes and application as an effective filter for heavy metal removal from aqueous solutions, Chin. J. Chem. Eng. 24 (2016) 1695–1702, with kind permission of Elsevier.
On the other work, Yadav et al.  used the UV-visible spectrophotometer to calculate adsorption and deadsorption of Mn+7 ion by MWCNT. Gupta et al.  studied the effect of functionalized MWCNT (F-MWCNT) in removal of Cu(II) ions from the aqueous solution. They found that the optimum values of pH, contact time, sorbent dosage, and initial Cu(II)
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
+ Cu+2 Cu(II)
COOH + Cu
FIG. 6 Schematic diagram of chemical mechanism for adsorption of Cu(II) ions onto f-MWCNTs. Adapted from V. K. Gupta, S. Agarwal, A. K. Bharti, H. Sadegh, Adsorption mechanism of functionalized multi-walled carbon nanotubes for advanced Cu (II) removal, J. Mol. Liq. 230 (2017) 667–673, with kind permission of Elsevier.
concentration are 3, 60 min, 10 mg/L, and 20 mg/L, respectively. For this process, Langmuir adsorption isotherm with 93% removal efficiency and adsorption capacity of 118.41 mg/g was determined. In Fig. 6 electrostatic attraction and complex formation were proposed as adsorption mechanisms between metal ion and the surface active sites of F-MWCNTs.
Also, this process was performed for removal adsorption of Pb(II) and the results displayed a noteworthy potential of MWCNT-COOH as an adsorbent for Pb(II) uptake .
6.2 Organic Functionalization of CNT Researchers by creating functional groups on the CNT surface increased the adsorption performance of nanotube. Poly-amidoamine dendrimer (PAMAM) is a hyperbranched polymer with tertiary and primary amine groups in their inner and surface structure. The presence of these groups on CNT provides great efficiency to adsorb metal ions via coordination and chelation linkages. Hayati et al.  investigated the removal of two heavy metals onto the PAMAM-MWCNT and showed high adsorption capacity of 3333 and 4870 mg/g for Cu(II) and Pb(II) at optimum pH 7, respectively. Abdel Salam et al.  compared the adsorption performance of MWCNT and MWCNT modified with 5,7-dinitro-8-quinolinol under different conditions (shaking time, pH, ionic strength, metal ion concentration, and adsorbent dosage). The experimental data were fitted by the Langmuir isotherm and high adsorption capacities to single ion were computed 142.8 mg/g for Cu(II), 250 mg/g for Zn(II), 111.1 mg/g for Fe(II), and 200 mg/g for Pb(II) using MWCNTs while, modified MWCNTs showed 333.3 mg/g for Cu(II), 500 mg/g for Zn(II), 200 mg/g for Fe(II), and 333.3 mg/g for Pb(II). In other work, it was used from amidoamine-functionalized MWCNT to adsorptive removal of NpO+2 and NpO2+ 2 . The presence of functional group on the MWCNT has effective role in adsorption of heavy metals. For example, Iannazzo et al.  investigated chelating abilities of dendrimer-functionalized MWCNTs containing amino groups (MWCNT-TD2) and α-aminophosphonate (MWCNT-TD2P) for selective removal of the toxic metal ions Pb2+, Hg2+, and Ni2+ and the harmless Ca2+ ion in the two salts concentrations of 1 mg/mL or 1 μg/mL. The results proved remarkable chelating ability of the MWCNT-TD2P dendrimer to metal ion of Hg2+ (98.9%) and the lower affinity of both systems to the harmless Ca2+ ion (60.2% and 58.4%) (Fig. 7). Hadavifar et al.  introduced both amino and thiolated functional groups on the nanotubes surface and utilized them for the adsorption of Cd(II) and Hg(II) ions from aqueous solution (Fig. 8). Experimental data specified that the MWCNTs-SH has lower adsorption capacity of Cd(II) ion than Hg(II) ion in the solutions. At optimum conditions of pH 6 and adsorbent dose of 200 mg/L, the experimental highest adsorption capacity of MWCNTs-SH for Cd(II) and Hg(II) ions in single component systems were calculated 61.1 and 205 mg/g, respectively. While, in binary metal ions system the adsorption capacity reached 14.09 and 35.89 mg/g for Cd(II) and Hg(II) ions, respectively. The results of specific surface area of raw MWCNT are 136.16 m2/g, which raised to 234.89 m2/g after amine functionalization with ethylenediamine (MWCNTs-EDA). It may be due to removal of the amorphous carbon and break the intertube spaces. Additionally, by functionalizing with thiol (MWCNTs-SH), the SBET was decreased to 189.61 m2/g. This can be attributed to
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
FIG. 7 Schematic diagram of MWCNT-TD2 and MWCNT-TD2P. Percentages of metal ions absorbed by p-MWCNT, MWCNT-TD2, and MWCNT-TD2P samples calculated as difference from the concentration values obtained by ICP-MS analyses performed on the water washing solutions at the concentrations , R. Romeo, N. Cicero, G. D. of salts of 1 mg/mL and 1 μg/mL. Adapted from D. Iannazzo, A. Pistone, I. Ziccarelli, C. Espro, S. Galvagno, S. V. Giofre Bua, G. Lanza, Removal of heavy metal ions from wastewaters using dendrimer-functionalized multi-walled carbon nanotubes, Environ. Sci. Pollut. Res. (2017) 1–13, with kind permission of Springer.
FIG. 8 (Right) Schematic diagram of functionalization of MWCNTs. (Left) Effects of adsorbent dose on mercury (A) and cadmium (B) removal (pH 6, initial ion concentration 10 mg/L). Adapted from M. Hadavifar, N. Bahramifar, H. Younesi, M. Rastakhiz, Q. Li, J. Yu, E. Eftekhari, Removal of mercury (II) and cadmium (II) ions from synthetic wastewater by a newly synthesized amino and thiolated multi-walled carbon nanotubes, J. Taiwan Inst. Chem. Eng. 67 (2016) 397–405, with kind permission of Elsevier.
entail of heavy molecular weight functional groups and organic groups linked on and inside the MWCNTs-SH. Gouda and Ghannam  used solid-phase extraction (SPE) as an environmentally friendly green procedure, simplicity, cost effectiveness, and sensitive to separation and determination of the heavy metal ions, Cd(II), Cu(II), Ni(II), Pb(II), and Zn(II) at trace levels in water and food samples. The MWCNTs impregnated with 2-(benzothiazolylazo)orcinol (BTAO) were used as adsorbent. The limits of detection (LODs) for Cd(II), Ni(II), Cu(II), Zn(II), and Pb(II) were found at 0.70, 0.80, 1.2, 2.2, and 2.6 μg/L, respectively, and the capacity of 1.0 g impregnated MWCNTs at pH 7 for Pb(II), Cd(II), Ni(II), Cu(II), and Zn(II) was calculated about 4.0, 4.6, 4.8, 5.4, and 6.4 mg/g, respectively.
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
6.3 Magnetic CNT Combination of CNTs with magnetic nanomaterials comprising Fe, Ni, and Co and their alloys are introduced as one the vital functional materials in removal of toxic metal ions. Magnetic nanoparticles are unstable in reactive system and easily gather due to their small diameter and the active surface. On the other hand, CNTs have imitations in adsorption due to difficulties, such as insignificant distribution and inadequate separation in aquatic phase. In this regard, more efforts were accomplished to prepare of magnetic carbon NCs, which can point to filling process, hydrothermal/solvothermal method, template-based synthesis, self-assembly method, pyrolysis procedure, detonation induced reaction, chemical vapor deposition, and sol-gel process [64,65]. Azimi and Es’haghi  prepared MWCNTs-Fe3O4 (Fig. 9) to uptake metal ions. In this process, the inductively coupled plasma-atomic emission spectroscopy was applied to detect ions. The results showed maximum adsorption capacities of 45, 28, and 20 mg/g for Pb(II), Cd(II), and As(II), respectively. According to R2 values (more than 0.9), the adsorption process obeyed Langmuir and Freundlich isotherm models. Gatabi et al.  reported a high removal capacity of 108.71 mg/g at 35°C and pH 8.0 for Cd(II) adsorption using Fe3O4-functionalized MWCNTs. The magnetic structure and surface charge properties of the sorbent were determined by Fourier transform-infrared spectroscopy, X-ray diffraction, transmission electron microscope (TEM), and pHPZC analyses. Based on pH drift method, value of pHPZC was computed about 3.63. In pH value lower than pHPZC, the Cd(II) adsorption is low due to positive charge on the nano-sorbent surface. While, at values higher than pHPZC, the electrostatic interaction between negative charge of sorbent surface and Cd(II) increased the removal performance.
FIG. 9 Microscopic images of the MWCNTs-Fe3O4 NPs: (A) SEM and (B) TEM. Adapted from S. Azimi, Z. Es’haghi, A magnetized nanoparticle based solid-phase extraction procedure followed by inductively coupled plasma atomic emission spectrometry to determine arsenic, lead and cadmium in water, milk, Indian rice and red tea, Bull. Environ. Contam. Toxicol. 98 (2017) 830–836, with kind permission of Springer.
FIG. 10 (A and B) Representative TEM image of Fe3O4/MWCNTs NCs. Adapted from V. Alimohammadi, M. Sedighi, E. Jabbari, Experimental study on efficient removal of total iron from wastewater using magnetic-modified multi-walled carbon nanotubes, Ecol. Eng. 102 (2017) 90–97, with kind permission of Elsevier.
In other work, from Fe3O4/MWCNTs NC was applied as economical and effective sorbents for total Fe uptake from industrial wastewater . TEM images (Fig. 10) showed that the Fe3O4 NPs completely attached on the MWCNT surface and average particle size of Fe3O4 was about 2–5 nm. In this process, 98.97% removal efficiency and 200 mg/g of high adsorption capacity were obtained under optimum pH 8.2 and (D/C) ¼ 5 (sorbent amount per initial concentration of metal ion). From magnetic MWCNT functionalized with 8-aminoquinoline was used for the fast separation and preconcentration of Cd(II), Ni(II), and Pb(II) ions in different matrixes such as in soil, sediment, fish, and water samples . LODs for Cd(II), Ni(II), and Pb(II) ions were 0.09, 0.72, and 1.0 ng/mL, respectively. In optimum condition, the sorption capacities of Pb(II), Cd(II), and Ni(II) were estimated about 150, 201, and 172 mg/g, respectively. Therefore, the prepared composite can be utilized to the quick extraction of trace quantities of toxic metal ions. For the first time, the adsorption capacity of boron onto the MWCNT functionalized with tartaric acid (TA-MWCNT) was investigated by Zohdi et al. . To further separation, the sorbent was modified by magnetic particles of iron oxide. Results of adsorption showed that the sorption equilibrium fitted by Freundlich isotherm and the maximum adsorption capacities (1.97 mg/g) provided at the pH 6.0. At this pH, the surface of TA-MWCNTs was positive and boron species existed in form of B(OH)3, the B(OH)4 with negative charges. Therefore, electrostatic attraction made the highest affinity for boron adsorption. AlOmar et al.  prepared six deep eutectic solvents (DESs) systems containing choline chloride and six diverse hydrogen bond donors (HBDs) and then were linked with CNTs as adsorbent to uptake of Pb(II) sorbents from aqueous solution. In optimum conditions (pH 5, adsorbent dosage of 5 mg, and contact time of 15 min), the maximum
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
adsorption capacity was achieved for triethylene glycol-K-CNTs with value of 288.4 mg/g. In this work, the K-CNT produced from oxidizing of pristine MWCNTs into KMnO4. Jiang et al.  introduced magnetic MWCNTs composite containing Fe3O4, amino, and thiol groups onto the side walls of MWCNT (designed as N2H4-SH-Fe3O4/o-MWCNTs), which prepared by trimethoxysilylpropanethiol (MPTs), hydrazine, ammonium ferric sulfate, and ammonium ferrous sulfate, in sequence. Steps of preparation and morphology of materials in each step indicated in Fig. 11. Then, adsorption capacity of N2H4SH-Fe3O4/o-MWCNTs as adsorbent under various conditions including effect of pH, contact time, initial concentrations, and temperatures was investigated. Based on the results, the highest equilibrium adsorption capacity of adsorbent for phenol, zinc, and lead was 38.97, 169.89, and 195.81 mg/g at pH 6, respectively. The adsorption isotherm and the adsorption kinetics were consistent with the Freundlich model and pseudo-second order, respectively. This may be due to form bundles and aggregation of MWCNTs by strong van der Waals forces in the solution, which lead to the formation of gap, groove, and inner areas on the MWCNT. Thus, adsorption of phenol, Zn(II), and Pb(II) is proceeded through heterogeneous and multilayer adsorption. Ionic imprinting technique is one of the effective techniques to remove trace heavy metal ions in water due to advantages like good selectivity, high adsorption capacity, and reusability. Recently, nickel ion-imprinted polymers (IIPs) containing MWCNTs were prepared by inverse emulsion system, using acrylic acid and chitosan as the functional monomers, Ni(II) as the template, and N0 ,N-methylene bis-acrylamide as the cross-linker for selective adsorption of nickel ion . In the presence of ions Pb(II) and Cu(II), the removal rate of IIPs toward the template ion Ni(II) reached to 94.83% and selectivity coefficients of Ni(II)/Pb(II) and Ni(II)/Cu(II) estimated 13.09 and 4.42, respectively. The value of selectivity coefficients at the optimum adsorption pH 5–7 was greater than 1. It proved IIPs had stronger selective adsorption capacity toward Ni(II). Oxidation treatment or surface modification can provide plentiful active sites such as COOH and OH on the sidewall of CNT, which could greatly interact with heavy metal ions. Thus, functionalized CNT have better sorption performance than CNT. Wang et al.  compared adsorption performance of magnetic hydroxyapatite-immobilized-oxidized multi-walled carbon nanotubes (mHAP-oMWCNTs) with mHAP, mMWCNTs, and HAP-oMWCNTs. In this experiment, mHAP-oMWCNTs displayed the best adsorption performance (96.2%) with maximum adsorption capacity of 698.4 mg/g for Pb(II) at pH of 4.1. Furthermore, results of interference of coexisting metal ions indicated little decrease (1.5%) in removal efficiency of Pb(II). Thus mHAP-oMWCNTs could be broadly applied in water clean-up with good performance after five recycles (Fig. 12). Incorporation of metal oxide onto CNT surface is other form of modified CNT for removal of heavy metal ions. For example, Liu et al.  prepared ZrO(OH)2/CNT NCs by filtration-steam hydrolysis method for uptake of As(III) and As(V). The ZrO(OH)2/CNTs showed more efficient for As(III) and As(V) adsorption than ZrO(OH)2 nanoparticles. The maximum adsorption capacity of ZrO(OH)2/CNTs was calculated about 78.2 mg/g for As(III) and 124.6 mg/g for As(V) at pH 7, according to the Langmuir fitting.
120mLH2SO4, 40mL HNO3
300r/min, 50 ⬚C, pH = 11
(NH4)2Fe(SO4)2 • 6H2O NH4Fe(SO4)2 • 6H2O
N2H4-SH-MWCNTs/Fe3O4 Fe3O4 JCOOH
NH SH (CH2)3
(A) Adsorption experiments
FIG. 11 (Top) Schematic illustration of the adsorption experiment of N2H4-SH-Fe3O4/o-MWCNTs. (Down) (A) o-MWCNTs with abundant carboxyl groups on the surface. (B) Fe3O4/o-MWCNTs with co-precipitation method. (C) SH-Fe3O4/o-MWCNTs with SH groups on the surface. (D) N2H4-SH-Fe3O4/o-MWCNTs with amino and thiol groups on the surface. (E) Adsorption experiments of N2H4-SH-Fe3O4/o-MWCNTs for Pb(II), Zn(II) and phenol. TEM images of MWCNTs (a), o-MWCNTs (b), N2H4-SH-Fe3O4/o-MWCNTs (c), and a high magnification image of N2H4-SH-Fe3O4/ o-MWCNTs (d). Adapted from L. Jiang, S. Li, H. Yu, Z. Zou, X. Hou, F. Shen, C. Li, X. Yao, Amino and thiol modified magnetic multi-walled carbon nanotubes for the simultaneous removal of lead, zinc, and phenol from aqueous solutions, Appl. Surf. Sci., 369 (2016) 398–413, with kind permission of Elsevier.
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
FIG. 12 (A) Demonstration of magnetic separation, (B) SEM image, (C) recycle experiment for Pb(II) using EDTA of mHAP-oMWCNTs (m ¼10 mg, V ¼ 50 mL, natural pH, temperature at 298 K). Adapted from Y. Wang, L. Hu, G. Zhang, T. Yan, L. Yan, Q. Wei, B. Du, Removal of Pb (II) and methylene blue from aqueous solution by magnetic hydroxyapatite-immobilized oxidized multi-walled carbon nanotubes, J. Colloid Interface Sci. 494 (2017) 380–388, with kind permission of Elsevier.
7 Adsorption of Heavy Metals by Polymer/CNT Nanocomposite From the past till now, various kinds of nanomaterial have been used as filler to improve the physicochemical properties and increase performance of polymer [76–79]. Among them, CNT, due to the unique combination of its properties with polymer, not only enhances strength and modulus but can also leads to improvements in chemical resistance, electrical conductivity, thermal conductivity, and dimensional stability. On the other hand, many studies showed that the incorporation of CNTs into polymers has a positive role in toxic pollutants removal and their reuse from aqueous solution. Generally, two methods, grafting to and grafting from, proposes to interaction between CNT and polymer . Ansari et al.  have prepared ternary composites by in-situ oxidation polymerization of polyaniline (Pani) on mixture of CNT and graphene oxide (GO). In order to make extra functionality, para toluene sulphonic acid (pTSA) was doped on the [email protected]
(Fig. 13). In scanning electron microscope (SEM) and TEM images (Fig. 13), the [email protected]
NC showed a network like interwoven fibrous intermingled structures with good dispersion of MWCNT. These proved that MWCNT coated with Pani in the matrix of GO and Pani. After characterization of NC, it was applied for the adsorptive removal of Cr(IV) and the maximum adsorption capacity (142.85 mg/L) observed in acidic pH 2. Additionally, removal of metal ion is endothermic and removal boosts with rising solution temperature. In these process, electrostatic force and coordination are main interactions in removal of Cr(VI) by [email protected]
NC. Recently, role of MWCNTs as stationary phases in dispersive solid-phase extraction (dSPE), joined with gas chromatography-mass spectrometry and atomic absorption spectrometry was investigated to the simultaneous separation of chromium, cadmium,
FIG. 13 (Top) Schematic demonstration of the synthesis of [email protected]
nanocomposite as well as its functionalization. (Down) (A) SEM, (B and C) TEM and (D) HR-TEM images of the [email protected]
nanocomposite. The inset (D) shows the SAED patterns of the [email protected]
nanocomposite. Adapted from M.O. Ansari, R. Kumar, S. A. Ansari, S. P. Ansari, M. Barakat, A. Alshahrie, M. H. Cho, Anion selective pTSA doped [email protected]
graphene oxide-multiwalled carbon nanotube composite for Cr (VI) and Congo red adsorption, J. Colloid Interface Sci. 496 (2017) 407–415, with kind permission of Elsevier.
and lead in solution . The result showed that the maximum extraction efficiencies were achieved in pH 6–7 by 50 mg OH-MWCNT or COOH-MWCNTs adsorbent per 250 mL of water sample at contact time 30 min. Fig. 14 showed the recovery of metal ions from various type of MWCNT. As seen, each sorbent showed different effects on
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
FIG. 14 Effect of CNTs type on the recovery of cadmium, chromium, and lead ions (conditions: 250 mL of water sample containing 0.1 μg of each PAH and 0.5 μg of each of the metal ions, 3 M HNO3 as eluent, pH 6, 100 mg of CNTs, , P. Stepnowski, Optimization sorption time 60 min). Adapted from M. Paszkiewicz, M. Caban, A. Bielicka-Giełdon of a procedure for the simultaneous extraction of polycyclic aromatic hydrocarbons and metal ions by functionalized and non-functionalized carbon nanotubes as effective sorbents, Talanta 165 (2017) 405–411, with kind permission of Elsevier.
extraction of metal ions. For example, the extraction efficiency of Cr is lower and MWCNT with a diameter <8 nm has higher recovery values than MWCNT with higher diameter > 50 nm. After adsorption process, regeneration of the solid sorbents from aqueous solution was considered as a serious problem in environmental remediation. Centrifugation and filtration methods can be introduced to regenerate and separate of adsorbents from solution, which put on extra stage and costs to the ecosystem treatment. In this regard, the magnetic-separation technology was proposed as one of the most promising ways to exit such a problem. Segregation of the solid sorbents based on their magnetic behaviors has been attracted the attention of many researchers. Salam  used mixing chitin and MWCNT to enhance the adsorption of Cr (VI) then mixed with magnetic nanoparticles to facilitate the separation of sorbent. Adsorption of Cr(VI) by chitin/magnetite/MWCNTs (CMM) NC was studied at 283, 295, 308, and 323 K, and it found that Cr(VI) removal decreased by rising temperature of solution. It can be due to the exothermic nature of the process (Fig. 15A). TEM image of CMM clearly indicated that MWCNT and spherical magnetic NPs uniformly dispersed into the NC (Fig. 15B). Mallakpour and Nezamzadeh Ezhieh  functionalized surface of MWCNTs with both valine and starch for the first time. These two compounds were used to increase dispersion and compatibility of MWCNTs into the CS-PVA matrix. After characterization, the prepared composite was used as adsorbent for removal of Cd(II) from the aqueous solution. Maximum adsorption capacity at pH 6 was estimated about 41.84 mg/g and Freundlich isotherm model was derived to describe the multilayer adsorption of Cd(II) onto a
FIG. 15 (A) Effect of solution temperature on the adsorption kinetics of Cr(VI) by CMM magnetic nanocomposite (experimental conditions: 10.0 mL solution, pH 8.0, 2.5 mg CMM magnetic nanocomposite, and Cr(VI)concentration 5.0 mg/L). (B) Transmission electron microscope image of chitin/magnetite/MWCNTs magnetic nanocomposite. Adapted from E. A. Salam, K. A. El-Nour, A. Awad, A. Orabi, Carbon nanotubes modified with 5, 7-dinitro-8-quinolinol as potentially applicable tool for efficient removal of industrial wastewater pollutants, Arab. J. Chem. 233 (2017) 197–202, with kind permission of Elsevier.
heterogeneous surface of CS-PVA/starch-MWCNT-valine NC. In this process, amide, hydroxyl, and carboxylic acid acted as chelating sites to form coordinate bonds with heavy metal. Beyki and Fazli  prepared polyhydroxyquinoline/MWCNT NCs under hydrothermal route for lead adsorption. 8-Hydroxyquinoline and p-formaldehyde were utilized as monomer and linker for preparation of polymer. Based on the finding, equilibrium adsorption was occurred at pH 4, with a contact time of 15 min and adsorbent amount of 15 mg. The adsorption process for Pb(II) was fitted the Langmuir isotherm model with maximum adsorbent capacity of 250 mg/g. Vegetable tannins were introduced as natural raw material, which have a good potential in metal ion removal. Luzardo et al.  prepared resin based in polycondensation reaction between mimosa tannin and formaldehyde and then, the CNT was fixed into it to improve adsorption capacity of Pb(II) ions in water. They found that the adsorption equilibrium followed by Langmuir model and pseudo-second-order kinetic and maximum adsorption capacity was 13.8 mg/g at pH 3.5–5.5, during 24 h. Yuan et al. 
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
compared the effect of polyethylene terephthalate yarn-CNT as novel cathode materials with Pt/Ti for the electrokinetic (EK) remediation of multi-metals (Cd, Cu, Ni, Pb, Zn) contaminated kaolin and found that the removal efficiencies of Cd, Ni, and Zn were enhanced at least about 30% and Cu and Pb at least 16.6% and 6.9%, respectively, in the PET-CNT treatment. Because, PET-CNT clearly raised electro-osmotic flow and electric current, considerably reduced kaolin pH, and improved toxic metals removal efficiencies. In an experimental work, the adsorption ability of poly(vinyl pyrrolidone)doped MWCNTs/polyrhodanine NCs was evaluated for the removal of Pb(II) from aqueous solution . The most monolayer adsorption capacity at optimum pH (3–6) was estimated about 8118 mg/g. The role of temperature on the adsorption was studied at 25°C, 35°C, 45°C, and 60°C and indicated that the adsorbent has an endothermic and spontaneous nature during the Pb(II) adsorption. In 2016, Hayati et al.  prepared an eco-friendly adsorbent of PAMAM/CNT NC to removal of Ni2+, Zn2+, As3+, and Co2+ under different conditions, such as initial metal ion concentration, dosage of sorbent, temperature, pH, and contact time. The experimental data were fitted well with Langmuir models and pseudo-second-order kinetics. Also, the highest adsorption of Ni2+, Zn2+, As3+, and Co2+ at pH 7, T ¼ 298 K, and CPAMAM/ CNT ¼ 0.03 g/L was estimated about 3900, 3650, 3500, and 3800 mg/g, respectively. This is can be due to the readily accessible to –NH2 groups on the surface sites and mesopores. Albakri et al.  investigated the role of acylchloride-functionalized CNT in improving the adsorption properties of polyamine (TRI), which prepared by in situ polymerization of benzene-1,3,5-triamine, paraformaldehyde, and various alkydiamines. After characterization, the obtained results from lead ion removal showed that the adsorption process was matched Freundlich isotherm model. Also, based on kinetic models, the adsorption mechanism was controlled by intraparticle diffusion and film diffusion mechanism. Recently, hybridization of CNT with other nanomaterials, such as metal oxides and LDH, can provide remarkable properties into polymer matrix. For example, research groups of Mallakpour examined the effect of MWCNT-ZnO quantum dots on the heavy metal adsorption and electrical conductivity of recycled polyethylene terephthalate (r-PET) . The efficiency of rPET/f-MWCNT composite and rPET/MWCNT-ZnO QD 4 wt% was evaluated for the removal of Cd(II) ions from water. The maximum sorption capacity of the rPET/MWCNT-ZnO QD composite was calculated 56 mg/g, which was approximately 4 times more than that of rPET/f-MWCNT composite with value of 13 mg/g. In other work, they studied role of MWCNT/LDH for the Cd(II) sorption by PET and observed that maximum adsorption was obtained about 38.91 mg/g by 20 mg of dosage at T ¼ 25°C (Fig. 16) . In this analysis, adsorption was done by electrostatic interaction between charged sorbent surfaces and metal cation. In TEM images, the outer surface of the MWCNT coated by a LDH layer and size of formed layers on the surface of nanotubes were below 50 nm (Fig. 16).
FIG. 16 (A) Digital pictures of r-PET/MWNT/LDH NC suspensions in DMAc and (B) feasible interactions of r-PET matrix and MWNT/LDH as a filler. TEM images corresponding to r-PET/MWNT/LDH NC 4 wt% at three different magnifications. Adapted from S. Mallakpour, V. Behranvand, Recent progress and perspectives on biofunctionalized CNT hybrid. Hybrid Polymer Composite Materials: Properties and Characterisation, 2017, 311 p, S. Mallakpour, V. Behranvand, Recycled PET/MWCNT-ZnO quantum dot nanocomposites: adsorption of Cd (II) ion, morphology, thermal and electrical conductivity properties, Chem. Eng. J. 313 (2017) 873–881, and S. Mallakpour, V. Behranvand, Water sanitization by the elimination of Cd2 + using recycled PET/MWNT/LDH composite: morphology, thermal, kinetic and isotherm studies, ACS Sustain. Chem. Eng. 5 (2017) 5746–5757, with kind permission of American Chemical Society.
Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
8 Conclusions CNT-based materials as an adsorbent has obtained remarkable attention from research groups because of its unique properties, such as high surface area and abundant pore structures, π-π interaction, high negative charge density, and hydrophilicity. To improve the performance of CNT and better understand of the mechanism reaction as well as separation and reusability of adsorbent, some approaches (such as surface modification and incorporation into polymer) were employed to obtain the CNT-based materials. The adsorption mechanism proposes that the active sites, such as dOH, dCOOH, and dNH2 and dSH on the surface of CNT, are mainly responsible for linking toxic metal cations by electrostatic force or covalent bonds. In this chapter, provided the up to date information about the most important features of CNT-based adsorbents and investigated the elimination of heavy metal cations by the CNT and the polymer/CNT NCs. Also, the importance of factors, such as pH value, adsorbent content, temperature, contact time, and coexisting ions, was considered for better performance of adsorption process. Finally, all investigations confirmed that CNT-based materials are a promising candidate for removal of toxic materials and researchers are in quest to further develop its properties.
Acknowledgments This work was financially supported by 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 Excellency in Sensors and Green Chemistry Research (IUT) Isfahan, I. R. Iran is gratefully acknowledged.
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Chapter 8 • Carbon Nanotubes for Heavy Metals Removal
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