A review on electrochemically modified carbon nanotubes (CNTs

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A review on electrochemically modified carbon nanotubes (CNTs) membrane for desalination and purification of water To cite this article: Zaira Zaman Chowdhury et al 2018 Mater. Res. Express 5 102001

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Mater. Res. Express 5 (2018) 102001

https://doi.org/10.1088/2053-1591/aada65

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A review on electrochemically modified carbon nanotubes (CNTs) membrane for desalination and purification of water Zaira Zaman Chowdhury1,6 , Suresh Sagadevan2,3,6 , Rafie Bin Johan1, Syed Tawab Shah1, Abimola Adebesi1, Sakinul Islam Md4 and Rahman Faizur Rafique5

PUBLISHED

1

24 August 2018

2 3 4 5 6

Nanotechnology and Catalysis Research Center (NANOCAT), University of Malaya, Malaysia Centre for Nanotechnology, Academy of Maritime Education and Training (AMET), Chennai 603112, India Department of Physics, Centre for Defence Foundation Studies, National Defence University of Malaysia, Malaysia Royal Melbourne Institute of Technology (RMIT), Australia Rutgers Cooperative Extension Water Resources Program, Rutgers, The State University of New Jersey, United States of America Authors to whom any correspondence should be addressed.

E-mail: [email protected] and [email protected] Keywords: electrochemical activity, CNT membaranes, water treatment, desalination

Abstract Recently extraordinary breakthroughs have been made towards applying nano-structured materials such as carbon nanotubes (CNTs) and porous graphene membranes in water purification and desalination applications. In this regard, the potential of the electrochemically active carbon nanotube (CNTs) membrane has been highly strengthened for the last few decades. One of the main advantages for such approach is the capability of CNT channel to permit the water flow easily. The perusal of the literature showed that, the performance of CNT based membrane can be three times higher than that of the conventional membrane devices. The unique and excellent characteristics of CNT membrane can outperform the conventional polymer membranes. CNT membrane has been widely used to adsorb chemical and biological contaminants as well as ion separation from sea water due to their high stability, great flexibility, and large specific surface area. Electrochemically active CNT filters deliver further Electro-oxidation of the adsorbed contaminants. Usually polymeric membranes have flexible chains for which it fails to have well-defined pores necessary for filtration. On the contrary CNTs based filters can provide pores with appropriate sizes and configurations by tailoring the growth parameters. The narrow pores of CNTs are capable of filtering water while eliminating ions (Na +/Cl–). Even this type of membranes is capable of removing bacteria from water and heavy hydrocarbon from petroleum. However, the success of desalination entirely depends on the basic design of the CNT-based filter with detailed optimization of the process parameters. Polymer filters cannot be recurrently used through several cycles since elimination of fouling ingredients is difficult. Even though electrochemically active CNT-based membranes have lot of advantages due to their hydrophobic nature, high porosity and specific area; there are numerous traits, which are yet to be considered and optimized. Thus the intrinsic properties of CNT as well as the fabrication of the membrane could be a critical factor for their applicability in various water treatment processes. This chapter provides an explicit and systematic overview of the recent progress of electrochemically active CNT membranes addressing the current prevalent problems associated with water treatment and desalination. The physio-chemical aspect including the working principles of this type of membrane have been discussed. The prevailing challenges and future perceptions are also discussed.

1. Introduction The escalating population growth together with rapid industrialization and upsurge in energy demands has led to serious challenge towards the ever diminishing fresh and purified water resources [1–3]. This has steered to

© 2018 IOP Publishing Ltd


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Figure 1. Graphic representation of (a) Formation of single-walled carbon nanotubes by rolling of a graphene sheet along lattice vectors (b) Structural orientation of armchair, zigzag, and chiral tubes and (b) the three types of carbon nanotubes.

numerous research activities into new water treatment technologies which will ensure and develop innovative water supply strategies [4]. The techniques should be highly energy efficient as well as environmentally friendly [5]. Difficulties with seawater are its’ salinity which makes it unsuitable for consumption. The salinity for sea water is mainly caused by presence of NaCl [6]. The alkali metal Na is 9 times higher than the alkaline earth metal Mg, 12 times higher than the non-metal of S, 17 times higher than K and 180 times higher than Br. Therefore the usage of sea water for domestic purposes needs energy efficient techniques whereby less input energy with minimum amount of chemicals should be used to minimize the adverse effect towards the environment [7, 8]. The membrane-based separation method has been considered ahead of the other traditional methods of wastewater treatment in view of its numerous advantages, including lower energy demand unlike distillation or electrolysis and less space requirement compared to sorption methods. Furthermore, it has higher separation selectivity and can be operated continuously [9, 10]. However, to maintain a robust strength mechanically so as to enjoy maximum permeation, to be selective through its favorable pore size distribution which must be narrow and maintain a high chemical inertness- the separation membrane should be thin [11–14]. To make a preference between the available polymeric and inorganic materials based membrane, such factors such as flux, selectivity, fabrication cost and stability under usage are usually considered. While polymeric membranes enjoy high selectivity and fast flux, they often exhibit high intolerance towards high temperatures, corrosive surroundings and organic solvents. Inorganic membranes on the other hand enjoy high thermal, structural and mechanical strengths coupled with high selectivity but limited by extremely low flux and high cost of fabrication [15–19]. In order to resolve the limitations of these membrane types, the use of welldefined nanostructured materials such as zeolites [20], organic frameworks made of metals [21] and materials that are carbon-based [22, 23] have been considered for development of membranes. Among these materials, CNTs have received highest consideration due to its hollow structure considered to be one dimensional and its mechanical strength considered to be very strong [24, 25]. Membranes or filter bed based on micro or nano structured carbonaceous materials provide a convenient means to overcome these difficulties. Based on geometrical arrangement, CNTs can be of three different types, namely: armchair, zig-zag, and chiral [e.g. zigzag (n, 0); armchair (n, n); and chiral (n, m)] (figure 1). This structural difference is based on the rolling up of graphene sheets during preparation process [26]. The nanoscale cylindrical shaped graphene is considered as carbon nano tubes (CNTs). The outer radius of single walled CNTs (SWNTs) can be 0.5–1.5 nm having inner radius of 0.2–1.2 nm (figure 1). The radius of Multi-walled CNTs (MWNTs) can vary from ∼1 nm (double walled nanotubes) up to ∼50 nm with tens of walls. Three types of geometrical shapes are shown by CNTs (figure 2). Recently electrochemically active CNT membrane filters are extensively used for desalination and purification of aqueous stream [27, 28]. CNT based filters were used earlier for separation of organic compound [29], proteins [30], virus [31], azo dyes [32], pharmaceuticals [33], fluorine based chemicals [34], mono and multi substituted phenolic compound [35]. Electrochemically active CNT membrane can reduce the fouling of filtration medium by biological inactivation [29, 36]. It can ensure optimum permeability of the membrane [37]. The modified ultra-filtration membrane (UF) containing CNT poly-vinylidene fluoride based cathode can reduce organic fouling up to a greater extent [38]. Even it has demonstrated its’ high potential for reducing energy consumption up to two fold than the unmodified ultra-filtration membrane (UUF). CNT based electrodes are efficient than glass electrode due to it’s’ less over potential and can be used effectively for micro2


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Figure 2. Different types of carbon nanotube.

pollutant sensors [39]. However, the carbon nanotubes have their challenges such as high cost of fabrication together with problems associated with vertical alignment of CNT arrays with very high density thereby limiting its usage to theoretical studies only [40]. In spite of using carbonaceous nano materials for desalination and purification techniques, several challenges are still remaining. The major focus of this chapter is to provide a comprehensive idea about the utilization of CNT based separation process used for desalination as well as purification of specific contaminants from waste stream. The chapter highlighted the advantages and disadvantages of different types of membrane technologies. Based on this perspective, the feasibility of incorporating CNT based materials in membrane technology, the existing obstacles and imminent challenges including its toxicological effects are reviewed. A brief summary of existing literature on CNT filters will help to develop an efficient and cost-effective technique for water treatment technologies.

2. Filtration mechanism for desalination and purification 2.1. Mechanism for desalination Desalination process can efficiently separates the salt ions from water using different mechanism and can be classified as follows: 2.1.1. Mechanisms involving phase change To separate a mixture of salt and water would normally involve a phase change from liquid to either a vapor or ice stage. This has been a popular mechanism used in desalination. While multi effect distillation (MED) involves evaporation of brine that may be available on the surfaces of pipes supplying hot air. Multi stage flash distillation (MSF), on the other hand do take place through flashing of salt-containing water in a low pressure chamber thereby leading to evaporation. Distillation processes such as humidification or dehumidification, solar distillation and or membrane distillation all make use of evaporation technique to operate [41]. Another popular technique usually adopted in desalination is freezing method which is considered to be more advantageous than vaporization. Some of its advantages include lower energy consumption and less scaling problems [42]. However, it has its limitation in the plant design which is considered complex. There is possible risk of secondary contamination which may occur during the process of freezing [43, 44]. 2.1.2. Mechanisms relating short-distance (∼1 nm) electrostatic interfaces The use of distant electrostatic fields arises due to availability of positive or negative charges on ions and nonavailability of net charge on water which enables the fields to utilize the ions selectively. Electrical fields have the ability to deplete, accumulate, or even transport ions. The subject that deals with this study is known as electrokinetics [47]. Thus it is possible for the surfaces with charge to attract ions with opposite charges but repel ions with same charges. This phenomenon results in the formation of electric double layer (EDL). The EDL thickness depends on the concentration of the salt which usually ranges from 1 micron to 1 nm for deionized water and sea water respectively. It is therefore possible to be adsorbed preferentially on the surfaces with specific charges or transported selectively through the pores of the membrane due to long-range electrostatic interactions. Overall the classification of desalination process is illustrated by figure 3. 2.2. Mechanism for purification 2.2.1. Electrochemically activated electrode The filter bed containing CNT as purification medium has at least tens of hundreds of different tubes that are entangled with each other through Vander Waals forces [48]. Thus the resulting surface area becomes big and can be varied from 30–500 m2 g−1 that can be utilized for chemical and biological contaminants. The configurationof a representative electrochemical CNT filter is shown in figure 4 [32]. Previously a commercial polycarbonate based filtration medium was prepared by using stainless steel or titanium cathode and a titanium anode ring inside which multi-walled nanotube (MWNT) anode can be used for electro-oxidation [32]. Nevertheless electro- oxidation of organic contaminants using CNT based filter medium is less time consuming than the conventional biological wastewater treatment systems [49]. Even some contaminants which are difficult to be adsorbed can be oxidized electrochemically easily [50]. CNT based electrochemically active filtration medium has following three advantages [32]: 1. The mass transfer is increased hydro-dynamically, 2. Ensure physical or chemical sorption/desorption which is temperature dependent and 3. Transfer of electron which is dependent on overall voltage. Numerous chemical contaminants are removed by CNT based filtration medium and the research showed that the breakthrough point reached within few hours using column based adsorption techniques [51–53]. 4


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Figure 4. (a) Basic Layout of electrochemically active CNT filter where (1) a perforated stainless steel cathode, (2) an insulating silicone rubber separator and seal, (3) a Titanium Anodic ring that is pressed into the carbon nanotubes anodic filter, and (4) the MWNT anodic filter supported by a PTFE membrane. (b) and (c) Modified filter casing. (d) MWCNT Filter Bed before filtration and (e) MWCNT Filter Bed after electrochemical filtration, respectively. Reprinted with permission from [32]. Copyright 2011 American Chemical Society.

Figure 5. Sandwiched electro-Fenton system based on carbon nanotube membrane stacks. (A) Images of the unfolded sandwich membrane stacks including four layers and (B) schematic of main roles of every layer in membrane stacks, and [P] and [P]m are pollutants and their oxidation intermediates, respectively. Reprinted (adapted) with permission from [54]. Copyright 2015 American Chemical Society.

2.2.2. Electrochemically active fenton process Previously CNT was used in electro-Fenton system where H2O2 was used as Fenton agent to improve the treatment efficiency [54]. Overall the system contains two cathodic medium one is purely made up of CNT cathode and a second layer of CNT–COOFen+cathode. A PTFE separator was used after the cathodic layer to separate it from CNT anode. The schematic representation of this process illustrated earlier in literature using figure 5 [54]. In this system, first reduction of oxygen will produce hydrogen per oxide at cathode which will later on react with Fe2+ to produce H2O2 first, and H2O2 further reacts with Fe2+ to produce hydroxyl radical (OH˙). These radicles will further oxidize the contaminants according to the following equations:

H2 O2 + Fe 2 + + H+  Fe3 + + H2 O + OH-1

(1)

Fe3 + + e  Fe 2 +

(2)

The reaction will generate Fe3+ in the cathode and Fe2+ is reused in the subsequent reactions. The residual intermediates or are oxidized in the CNT anode. In this method, H2O2 is produced on spot, the expenditure and 5


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hazard related to handling of H2O2 can be reduced. The system does not require addition of acid or bases as the process is carried out under neutral pH. Due to electro-regeneration of Fe2, there is no need for the addition of Fe in the reaction medium and thus the process is free from sludge formation.

3. Potential application of CNTs in membrane desalination The structure of Carbon nanotubes can be described just as the modified version of a fullerene with single dimension; having a micrometer scale graphene sheet being rolled into a cylinder-the diameter of which is in nanoscale range. CNTs can either be single-walled or multi-walled depending on the methods of synthesis. Synthesis methods and applications of CNTs have been widely reported by researchers [55–59]. The emerging need to further upgrade the desalination technique has pushed this nanostructured material to the versatile edge of uses because of their extraordinary water transport properties. 3.1. Mechanism for water and ion transport through carbon nanotubes CNTs have nano confined inner pores which enable them to use as model framework for producing membrane. The membrane system thus obtained can ensure water ion transportation and water ion interactions also [60, 61]. CNTs structures can closely mimic the biological pores based on their hydrophobicity and size of pores [62]. The fluid transport through CNT channel is almost similar with water transport through biological membranes [63, 64]. There is still a lot to be learned regarding the rapid water transport properties of CNTs. However some propositions have been made that, the smooth nature of their walls with hydrophobic structure contributed to high water flux during the desalination process [64]. This enables frictionless water transport throughout the process [64]. Suggestions have also been made that the shielding effect due to liquid water molecule layer along the walls was the primary reason for the high water transport [65]. To fully utilize CNTs as a structural material, the comprehensive understanding of the flow of fluid in the cylindrical channel is highly desirable. To this end, computer modelling approach has been developed and this has provided engineering approach for using CNTs as transport medium especially in desalination applications [66]. A large number of theoretical approaches to CNTs transportation characteristics have been modeled and reported [63, 67, 68]. Of these, molecular dynamic modeling has revealed that the magnitude of flux through CNTs is greater than the conventional pores [63, 69]. It has been discovered that the electrostatic charge distribution plays significant role in water and ion intake by CNTs and it has been observed that changing the pattern of the alternating charge on the CNTs channel would favor water intake but prevent ion intake into the nano-channels [70, 71]. It has been revealed that the hollow structure of CNTs will allow passage of water molecules rapidly but will hinder the passage of ions [71]. The enhancement of water flux by utilizing CNT based membrane will certainly reduce the cost of desalination process. The surface curvature of CNTs has also been reported to play an important role in determining their friction coefficient [72]. The curvature regulates the energy with which water molecules interact with CNT wall [72, 73]. Earlier membrane desalination was carried out by immobilizing MWCNTs onto the porous hydrophobic PVDF membranes (figure 6) [74]. CNTs served as adsorbent to initiate solute transport and it stimulated vapor transportation white retaining liquid water molecule penetration through the pores of the membrane due to its’ hydrophobic characteristics [74]. 3.2. Modification of carbon nanotubes Even though CNTs have remarkable properties which make them appealing candidates for a numerous form of membrane filtration, their lack of solubility and process ability in most common solvents has imposed barriers on their large scale industrialization for precise applications. CNT bundles typically form large aggregates due to van der waals’ interactions which make them insoluble in common organic solvents and aqueous solutions [75]. CNTs can be dispersed in solvents through ultrasonication, but precipitation straight away happens in most instances. This phenomenon interrupts overall desalination process. Surface modification of CNTs by covalent functionalization or non-covalent wrapping or adsorption of various functionalized molecules onto the surfaces of CNTs, both are well-known strategies for easing their dispersion into solution [76]. Various functional groups such as –COOH, –COH, –NH2, and –OH can be attached onto the surface of CNTs using covalent bonds (figure 7) [77]. The sidewall functionalization of CNTs can change the hybridization of carbon from sp2 to sp3 with concurrent loss of π- conjugation in the graphene layer. Presence of these functional groups allow CNTs to go for additional chemical reactions, such assilanation, polymer grafting, etherification, thiolation, alkylation, arylation and addition of other biomolecules onto their surface [76, 78, 79]. The attachment of these functional groups makes CNTs soluble in many organic solvents. These polar groups alter their normally hydrophobic nature to hydrophilic on e. Chemically functionalized CNTs can produce robust interfacial bonds with many polymers, permitting the practice of CNT-based nanocomposites that demonstrate notable mechanical properties. A prime disadvantage of this technique is the extensive damage to 6


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Figure 6. Typical Process for CNTs based Membrane Desalination. Reprinted (adapted) with permission from [74]. Copyright 2011 American Chemical Society.

Figure 7. Surface functionalization of CNTs by thermal acidic oxidation and esterification or amidation of the carboxyl groups. Reprinted from [77], Copyright 2012, with permission from Elsevier.

the sp2 hybridized carbon structure that takes place because of the advent of different functional groups. Therefore, the widespread effort has been dedicated to developing techniques for solubilizing of CNTs that are convenient to apply and cause less damage to their structure. Non-covalent surface alteration is an alternative scheme for amendment of the interfacial traits of CNTs. This approach is appropriate as it does not compromise the physical characteristics of CNTs, but does mend their solubility and process ability. Non covalent functionalization principally implicates enfolding the external layer of CNTs with polymer, bio-macromolecular or surfactant molecules. The aptitude to disperse CNTs into solution by using polymers such as poly(phenylene vinylene) and polystyrene, was described earlier to be the result of covering the tubes to form supra-molecular complexes [80, 81]. The polymer wrapping process includes van der Waals and π–π interactions between the CNTs and polymer chains having aromatic rings. A great deal of research has revealed that a variety of proteins, comprising bovine serum albumin and lysozyme, are 7


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Figure 8. (a) Schematic Representation of Chitosan Molecule enfolding over the CNT surface (b) TEM Image of individual CNT Tube wrapped by Chitosan. Reprinted from [87], Copyright 2007, with permission from Elsevier.

correspondingly proficient of forming regular aqueous dispersions of CNTs [81, 82]. The use of protein dispersants is of unique importance due to their privation of toxicity related to surfactant and different representative dispersant molecules collectively with their biocompatibility [83] but, proteins incorporate special kinds of reactive functional groups including hydroxyls, carboxylic acids, amines and thiols, which commendably provide sites for additional surface change of CNTs [83]. The dispersion of CNTs -wrapped by using protein molecules encompasses an electrostatic interaction mechanism [82, 83]. The technique is distinctly reliant at the charge distribution present alongside the protein and pH of the solution [82, 83]. Carbohydrates consisting of chitosan and gelatin gum have also been shown to be surprisingly effective at wrapping themselves round CNTs to facilitate formation of aqueous dispersions of the latter [84]. Usually biopolymers are either protonated or deprotonated in aqueous medium. Presence of biopolymers onto the surface of CNTs minimizes aggregation of CNTs due to electrostatic repulsion and steric hindrance mechanisms [85]. Chitosan was observed to be very beneficial for setting apart SWCNTs on the basis of differences in their size. It was reported earlier that only the smaller diameter nanotubes will be non-covalently functionalized through the biopolymer [86]. It has been suggested that chitosan chains wrap alongside the nanotubes axis as shown schematically by following figure 8. Gelatin gum is another polysaccharide which can act as fantastic biomolecules for making both SWNTs and MWNTs disperse in aqueous solution. For instance, it has been observed that solutions containing gelatin gum at concentrations as low as 0.0001% (w/v) have improved the dispersion properties of MWNTs [88]. DNA has also been proven to be able to dispersing CNTs into aqueous solution. This was attributed to the capability of the DNA bases to bind over the nanotubes through π–π interactions. When the polar backbone of DNA molecule is exposed to the solvent, then dispersion of CNTs become easier. Surfactants are amphiphilic in nature and it has proven to be highly effective dispersing agents for CNTs [89, 90], and [91]. The surfactant sodium dodecylsulfate (SDS) has hydrophilic head groups, which can help in dispersion of CNTs by electrostatic repulsion between micellar domains [92]. Another surfactant, polyoxyethylene octylphenylether (Triton X-100), a commonly used non-ionic surfactant attaches itself around the individual nanotubes. Its’ hydrophilic ends form a large solvation shell around the assembly [93]. The type of interaction between surfactant molecules and CNTs depends on the structure and properties of the surfactant, including it’s the alkyl chain length, bulkiness of head group and charge dominates the attachment of surfactant molecule over the surface of CNTs. That’s why Triton X-100 and sodium dodecylbenzene sulfonate (SDBS) display stronger interactions with the surfaces of nanotubes than SDS. This phenomenon is attributed to the presence of the benzene rings in the former surfactants [93]. There is the general need for continuous modification of the CNTs characteristics for multidisciplinary applications and these have remained a great challenge due to certain inconsistencies [94]. Different strategies have been employed to tackle these problems through chemical modification and functionalization of CNTs to improve their selective and controlled transport. Several reports have been given on various purification processes and effective functionalization of CNTs [95–98] but the findings indicated that that there is still room for improvement in the structural modification for the optimization of the process. Technological solutions are also suggested for functionalization of the pore tips to improve the selective characteristics and thereby increase the surface hydrophilicity. Other form of modifications which include oxidation and chemical modifications using hydrophilic functional groups can be done to enhance the binding energy between the water and the nanotubes which will invariably improve water flux [99, 100]. 8


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Figure 9. (a) Schematic Illustration of SWCNTs Bundle (b) TEM Image of SWCNTs (c) TEM images of MWCNTs.

4. Potential application of CNTs water purification Adsorption is an easy and green technique for the elimination of organic and inorganic compounds from aqueous stream. Activated carbons (ACs) are considered as one of the best option for adsorptive removal of pollutants due to its’ chemical inertness and thermal stability. However, regeneration and recycling of ACs is difficult. Furthermore, the sorption kinetics is slow using ACs for removal process. Activated carbon fibers (ACFs) had been developed and it is considered as the second generation carbon based sorbents [101]. The diffusion distance of pollutants to the adsorption sites of ACFs is relatively smaller as the micropores of ACFs located over the external surface of the carbon [102]. Consequently, ACFs characteristically exhibit better sorption kinetics than the powdered or granular ACs. CNTs, can be considered as the contracted version of ACFs [4]. CNTs are hollow tubes having versatile surface chemistry which can be modified based on end application. Hypothetically, CNTs are basically the advanced generation of carbonaceous adsorbents. The outstanding capacity of CNTs to work as adsorptive medium is ascribed to its ‘structural and functional properties. Furthermore it has microbial cytotoxicity that has fractional impact in concentrating biological contaminants for removal process. 4.1. Adsorptive removal of organic contaminants by CNTs During adsorption of organic compound, several chemical forces act concurrently. These include electrostatic interaction, π–π Interaction, hydrophilic-hydrophobic effect, π–π electron-donor-acceptor (EDA) effect and hydrogen bonding. The outer surface of CNTs is hydrophobic which make them bind the non polar organic compounds [103, 104]. The π electrons on the surface of CNTs enable to go for π–π coupling reactions with Benzene ring. Previous studies concluded that adsorption of polar and non-polar compounds onto the surface of CNTs do not take place by hydrophobic effect [105]. Same study showed that the adsorption of C6H5NO2 was stronger than C6H6, C6H5CH3, and C6H5Cl, despite of it’s’ less hydrophobic nature. In those cases, π–π EDA interaction between the nitrobenzene ring (π acceptors) and the graphene sheets (π donors) of CNTs played the vital role for adsorption. The curving and chirality of CNTs is anticipated to have an exquisite impact on the sorption of organic molecules. The nano-scale curvature effect was predominant for differences in sorption percentages of tetracene and phenanthrene onto the surface of CNTs [106]. Due to the variation of chiral angle of CNTs, the sorption capacities of Benzene varied significantly [107]. The aggregation tendency of CNTs reduces the nano-curvature of their wall. Usually, the aggregation of CNTs follows such an order: single-walled CNTs (SWCNTs)>double-walled CNTs (DWCNTs)>multiwalled CNTs (MWCNTs) [101]. SWCNTs typically exist as packs of wires whilst MWCNTs are arbitrarily entwined as isolated tubes as illustrated by figure 9 [108]. Due to aggregation, the external surface is decreased which might lead to the development different sorption sites inside the CNTs channel (figure 9) [108]. Thus adsorption of some synthetic organic compound (SOCs) becomes difficult on aggregated CNTs. Earlier researchers have proposed some methods to calculate the reduction in pore volume and surface area for aggregation. Ultrasonication considerably enhances the dispersion of CNTs resulting better sorption kinetics of SOCs onto the surface of CNTs [109]. Modification of CNTs surface by attaching some specific functional groups cane make it hydrophilic resulting greater removal efficiencies of low molecular weight and polar organic compounds like phenol [102] and 1,2-dichlorobenzene [110]. It was reported earlier that increasing the number of oxygen containing 9


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Figure 10. Degradation Mechanism of TiO2 coated CNTs based Composites (a) Mechanism for Overall Photo-catalytic Degradation Using CNTs (b) Photo-initiated electrons by TiO2 will be stored by CNTs which in turn prevent charge recombination (c) Generating Electron-hole pair in CNTs (d) The role of CNTs as Dopant by formation of Ti–O–C bonds [101, 114, 115]. Reprinted from [115], Copyright 2011, with permission from Elsevier.

functional groups will reduce the adsorption of relatively nonpolar chemicals like naphthalene. Reprinted (adapted) with permission from [111]. Copyright 2009 American Chemical Society. 4.2. Catalytic photo-degradation of organic compound using CNTs Photo-catalysis has been an interesting issue for the degradation of organic contaminants for quite a few years [112]. Conventional photo-catalysts incorporate some semiconductors oxide such as TiO2, CdS, Fe2O3, ZnO etc. Application of these semiconductors has some adverse effects. Due to vast band gap in TiO2, it has to be intensively energized by UV light. Therefore, it can’t be viably use under daylight. CdS and ZnO hold the disadvantage of photo-corrosion. Overall it can decline the photo-activity and stability. The electrons and holes generated during photo-catalysis have tendency to recombine rapidly without initiating the photo-catalytic activities. Attributable to their outstanding physiochemical properties, CNTs can fill in as a perfect building block for composites. It can improve the photocatalytic properties of the resultant composites. CNTs can substantially store electrons. It was observed that 32 carbon molecules in SWCNTs can supply one electron [113]. Once it is incorporated with TiO2, it can incite electron exchange from the conduction band of TiO2 to the surface of CNTs because of their lower Fermi level [113]. Hence, CNTs can collect photo-generated electrons and restrain the recombination of charges. These stored electrons, later on, can be exchanged to another electron acceptor, for example, sub-atomic oxygen. Subsequently this can form different oxygen species (O2, H2O2 and OH) which initiates photo-degradation and additionally minimize the organic pollutants in aqueous stream. CNTs are coated with thin and uniform TiO2 nanoparticles which results in carbon diffusion into oxide phase. The light adsorption capacity under visible region can be enhanced by carbon doping. Presence of carbon can produce a medium band gap adjacent to the valance band of the TiO2 particles (figure 10) [114, 115]. CNTs can be used as photo-sensitizers and introduce the photo-excited electrons to the conducting band of TiO2 due to their semiconductor property [116]. The photo-catalytic oxidation activity to phenol was enhanced by CNT/TiO2 composites. SWCNTs can improve the photo-catalytic properties of TiO2 more than MWCNTs as the surface of SWCNTs can ensure more contact with TiO2 nanoparticles [117]. Organic chemicals like dyes [118], aromatic compounds [119], and carbamazepine [120] can be competently degraded by CNT-TiO2 composites. Incorporation with metal can significantly improve CNTs performance as photo-catalyst. It was revealed that the stacking of Ag to CNTs clearly upgraded the photo-catalytic properties of CNTs [121]. The Ag/CNT composite displayed enhanced photo-catalytic properties for RhB and the mechanism is illustrated by figure 11 [121]. At initial stage, RhB molecules are adsorbed onto the surface of CNTs. After that, Due to presence of visible light, RhB molecules were excited and the electrons thus engendered are transported along CNT surface. 10


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Figure 11. Photo-catalytic Degradation of RhB using Ag/CNTs. Reprinted from [121], Copyright 2011, with permission from Elsevier.

These electrons were confined by Ag particles. Those confined electrons help in reduction of adsorbed oxygen to form superoxide anion radicals. This further initiates the degradation of RhB. 4.3. Microbial decontamination using CNTs Elimination of bacteria and other microbes from drinking water is essential. Both SWCNTs and MWCNTs have shown strong antimicrobial activities. SWCNTs based filter showed high bacterial retention and MWCNTs has removed virus even at low pressure [122, 123]. Recently hybrid filter incorporating both types of SWCNTs and MWCNTs were used for inactivation of bacterial and viral retention using low pressure [124]. The transportation of viral particles was enhanced by applying external electric field [125]. Silver nanoparticles and nanowires were incorporated to achieve enhanced antibacterial activity [125, 126]. Recently membrane with aligned CNTs has been fabricated by spray pyrolysis process which has uniform nanoporous texture [127]. This makes it favorable for filtration process [127]. It has been used for elimination of E. coli bacteria from aqueous effluents successfully [127]. Filters fabricated with CNTs have several advantages than the conventional membrane. Ultrasonication and autoclaving are used for cleaning of CNT based membrane. After wards the regeneration, the filter can be used for subsequent cycles showing full filtering efficiency [127]. Microbial fuel cell (MFC) using CNTs as electrodes uses self-sustained electricity using microorganisms for wastewater treatment. Until now the anode of MFC was produced from carbon cloth, carbon paper, and carbon foam. But these carbonaceous materials have less electro-catalytic properties for microbial reaction at electrode. CNTs can increase the surface area of electrode which consequently can facilitate the charge transfer process. Due to cellular toxicities of CNTs, it is recommended to use it by coated it with conductive polymers. Some commonly used conductive polymers are polyaniline and polypyrrole [128]. Some macroscale porous substrate such as textile or sponge can be coated by CNTs to produce anode for MFCs [129, 130]. The anode constructed by this way can exhibit decreased charge transfer resistance and improves the performance of MFCs as illustrated by figure 12 [129]. The electrons are efficiently accepted by oxygen at the cathode surface of MFCs. However, the oxygen reduction reactions under common operation conditions are hindered sometimes. Presence of some bacteria in the cathode will catalyze oxygen reduction reactions. CNTs can interact with the redox proteins of the bacteria [131]. Accordingly the cathode using CNT will enable the electron transferal easily and increase the reduction of oxygen simultaneously [131]. 4.4. Oil water Careless disposal of oil-water emulsion generated from oil refineries, sewage waste and crude oil production can affect aquatic environment and cause severe harm to human health. To address this problem, microfiltration (MF), ultra filtration (UF), and nanofiltration (NF) have been extensively used with discrete benefits. These processes can ensure high quality purified water with low energy consumption [132–134]. However these techniques are dependent on size exclusion separation of oily droplets. CNTs-based composite membrane was earlier fabricated by inserting CNTs inside the porous channels of an Y2O3–ZrO2 (YSZ) membrane [135]. Presence of CNTs inside the YSZ membranes exhibited 100% rejection rate of oil particles and with water flux of 0.6 L·m−2·min−1 having pressure drop of only 1 bar during 3 days of operation. The lipophilic soft layers produced on CNTs could ensure adsorption as well as size exclusion separation as compared to size exclusion separation for YSZ membranes. Incorporation of CNTs provide improved mechanical properties such as tensile strength, Young’s modulus, and toughness to the membrane and make it durable for oil-water treatment [136]. Moreover, a super hydrophobic CNTs-polystyrene composite membrane was fabricated for emulsified oil11


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Figure 12. Schematic Illustration of the electrode material and electron-transfer mechanisms for the CNT-textileanode and carbon cloth anode. Reprinted with permission from [129]. Copyright 2011 American Chemical Society.

Figure 13. Oil-water separation for super-hydrophobic CNTs-polystyrene composite membranes (a) Composite Membrane (b) Oily Emulsion separation. Adapted from [136] with permission of The Royal Society of Chemistry.

water separation which could effectively separate an inclusive array of surfactant stabilized water-in-oil emulsions as shown in figure 13 [136]. The membrane showed high rejection efficiency (>99.94%) for oil droplets with water flux of 5000 L·m−2·h−1·bar−1 [136]. 4.5. Heavy metal Careless disposal of waste effluents from many industries such as mining operation, metal plating facilities, chemical manufacturing, and battery industries hastoxic cations [137]. Adsorptive removal of these divalent cations using CNTs based filter medium have gained attraction recently [138, 139]. A recent literature has demonstrated almost 100% removal of metallic cations using CNTs based composite membrane [96]. Correspondingly, another type of membrane was fabricated for Ni2+ions removal from water where CNTs were incorporated inside the pore channels of ceramic membrane using CVD technique [140]. The sorption process followed well with Langmuir and Freund lich isotherm models. However, ceramic based CNT membrane can’t be used for continuous operation because the efficiency of the membrane low due to the larger pore diameter of ceramic [140]. Previously phase inversion method was used to fabricate CNTs-PSF composite membranes [141]. The membrane showed excellent performance due to its smaller pore size of 20–30 nm and was successfully used for continuous filtration process [141]. The composite membrane has even efficiently removed 94.2% Cr (VI) and 78.2% Cd2+ whereas the PSF membrane alone could eliminate only 10.2% of Cr(VI) and 9.9% Cd2+ for the bare PSF membranes, respectively [141]. 12


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5. Factors affecting electrochemical purification of water using CNTs 5.1. Electrochemical properties of cathode Different types of cathodic materials were used earlier for electrochemical systems, such as graphite felt [142], carbon felt [143], and MWNT [39]. The oxidation rate was significantly different for different carbonaceous materials. Simply 50% of ferrocyanide was oxidized at 0.3 V anode potential using Ti cathode, whereas more than 80% of the ferrocyanide was oxidized under the identical conditions using the CNT cathode [142]. 5.2. Electrochemical properties of anode The potential of anode plays a vital role for electro-oxidation of organic pollutant using the CNT filters. The potential of Anode should be higher than the redox potential to facilitate the oxidation process. Nevertheless, the phenomenon of water splitting should be taken under consideration by applying high anode potential. In that case, there is an uninterrupted competition between the oxidation of the organic contaminants and evolution of oxygen species as illustrated by following equation:

1/2H2 O  1/4O2 + H+ + E-,

E 0 = +1.03 V versus Ag/AgCl

(3)

Over potential of anode may cause degradation of CNTs [144] and confines the uses of CNTs for electrochemical treatment of waste stream. When the applied potential was 3 V using electrochemical CNT filtration, the MS-2 virus having concentration of 106 ml−1was completely eliminated and it could not be detected in the effluent [39]. Previous literature suggested that higher anode potentials caused higher anoxic oxidation when total applied potential was below 3 V. This trend was further confirmed for electro oxidation of ferroxyanide where rate of anoxic oxidation increased under higher anode potentials [145, 146]. Current densities were detected to increase with anode potentials [147, 148]. CNT reactive sites are affected by anode potential also. The sp-2conjugated sidewall CNT sites are governing the electrochemical filtration process when the anode potentials are low ( 0.2 V) [147, 148]. Even the CNT tips were also electro-active when anode potentials are high ( 0.3 V) [147, 148]. Recently electro oxidative CNT filter has been developed which can effectively define the correlation among target species, adsorption, mass transport and electron transfer with subsequent product desorption [48]. Recently binder free Bismuth doped conductive 3D CNT network was fabricated and it was coated with uniformly coated with tin oxide (BTO) nanoparticles [148]. BTO-coated CNT filters showed better stability of anode and efficiency for organic electro oxidation [148]. BTO coated CNT filters showed over potential of 1.71 V which was 0.44 V higher than uncoated CNT filters [148]. Even electroxidation using BTO coated CNT filter for other compounds like ethanol, methanol, formaldehyde, and formate were also very successful and it demonstrated four to five times less energy consumption than those of the uncoated CNT anode [148]. 5.3. Flow rate The reaction kinetics for electro oxidation using CNT filters are greatly affected by flow rate. The rate of ferroxyanide electro oxidation and flow rate was proportional provided the flow rate was up to 4 ml min−1 and anode potential was 0.3 V [145]. Current efficiency was observed to increase slightly with increasing flow rate from 0.5–4.0 ml min−1 and reached a plateau at the higher flow rates [145]. This was expected as due to increased mass transfer to the surface of anode with increasing flow rates [145].

6. Advantages of using CNTs in membrane The major disadvantage of polymeric membrane is its fouling tendencies which results from the interaction between the surface of the membrane and the foulants. Modification of the membrane surface by enhancing the hydrophilicity of the surface can resist fouling. CNTs, itself is hydrophobic. But its’ chemistry can be altered to hydrophilic one by oxidation using acid. Blending of CNTs with some polymers like polysulfone [149] and polyethersulfone [150] can make it more resistant to fouling. This is owing to presence of –COOH groups onto the surface of CNTs. The fouling can be also resisted using functional groups like hydrophilic isophthaloyl chloride groups [151] and amphilic polymer groups with protein-resistant ability [151] onto the surface of CNTs. Even biofouling that is the growth of biofilm onto the surface of membrane can be prevented by integration of CNTs in membrane [152]. Due to antibacterial properties of CNTs, it can impart biocidal properties to the polymeric membrane [152]. Ag nanoparticles incorporated with CNTs can ensure more direct contact with bacteria which in turn shows more effective antibacterial activity and is illustrated by figure 14 [153]. Incorporation of CNTs provides better mechanical strength to the polymer membrane. Adding 2 wt% of CNTs in polyacrylonitrile membranes [154] and chitosan based membrane [155] can provide increased tensile strength of 97% and 90%, respectively. It can also enhance the permeability of water through the polymer membrane. High flux filtration can be achieved using surface-oxidized MWCNTs [156]. Increasing the amount 13


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Figure 14. Polyacrylonitrile fiber coated by CNTs/Ag showing antibacterial activity. Reprinted with permission from [153]. Copyright 2011 American Chemical Society.

of MWCNT could increase the water flux as well as maintaining the rejection of net organic content. The rate of flux was improved by nano-channels of CNTs for passage of water inside the composite membranes [154]. The presence of ionic groups in sea water cause scaling of membrane during desalination process. This happens due to precipitation of sparingly dissolvable salts. The cytotoxic nature of CNTs can forbid the microorganisms development [157].

7. Challenges of using carbon nano-materials The distinctive and adjustable morphological, chemical, and electrical characteristics of hollow CNTs made them suitable for producing unique and high-performance materials for water desalination and treatment. To obtain synergistic effect, CNT can be incorporated with other nano-materials for water treatment. The limiting factor for using CNT is its high cost. However, contemporary advances have confirmed that it is promising to fabricate high quality CNTs at low prices. Catalytic chemical vapor deposition method using fluidized bed reactor can yield 595 kg h−1 of CNT [158]. The large scale manufacturing may lower the price of CNTs up to a greater scale. It was observed that CNT based filters can ensure high magnitude of water flux while maintaining the low pressure for the process [79]. The tendency to form biofilm can be reduced due to its’ cytotoxicity. CNT based filters are more easily regenerated than granular activated carbon based filters (GAC). Even CNT filters have shown their ability to remove large range of organic, inorganic pollutants as well as bacteria and viruses also. Consequently they are used as disinfectant also. However filters carrying CNTs should be used carefully before its large scale utilizations in term of the leakage of CNTs into the drinking water. CNT Membranes comprising carbonaceous nano-materials represent an emerging part of membrane separation process in case of filtration and seawater desalination. Basically CNT based membranes are easier to regenerate using sonication and autoclaving. Due to their excellent electrical conductivity, the flux regeneration properties can be improved using principles of electrochemistry. The CNT pores can ensure high water flux as well as high permeability and water rejection. The cumulative effects of these advantageous properties are suitable preparation of membrane for desalination process. For electrochemically activated CNTs in filter, when the voltage applied is more that 2 V, and then hydrogen produced at cathode continuously and oxygen at anode. Formation of bubbles blocks the pores of CNTs, Increases back pressure, lessens the mass transfer efficiencies and subsequently the performance of the filter drops down. Agglomeration tendencies of CNTs are other major drawbacks when the applied potential is higher than 2.5 V [159]. The electrochemical process is hindered if the overall filtration process is carried out for 3 h due to accumulation of organic polymer; inorganic sodium per sulfate deposition on anodic plates [159] as these increases the resistance of electron transfer. The specific configuration of CNTs with dead-end system may limit the overall water flux. The filters using CNTs are fragile and can be fractured. After partial oxidation some toxic metabolites might produce which are difficult to treat as their chemical properties are still not well understood. Until recently more effort has been given on for CNT supported catalysts, where by the catalytic activity of those types of catalyst has been monitored using some model pollutants, such as phenol and dyes. Nevertheless greater emphasis should be given for the degradation of more recalcitrant organic pollutants using CNT based composite materials. The catalytic activity to produce degraded chemicals should be carefully analyzed as some intermediate products formed during the process might be more hazardous. In that case additional methods should be carried out for further degradation of those intermediates. 14


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Until now many obstacles still remain and make the overall process challenging. The challenge starts with CNT based membranes’ reproductively, huge cost and requirement of uniform pore size distributions. For synthesis of desalination membrane, the capacity for salt rejection should be >95% and it requires less than 1 pore in 100 having the size greater than 1 nm in diameter [107]. Another obstacle is membrane fouling by algae and other contaminants accumulation during water treatment as well as desalination. The perusal of literature clearly reflects that CNT based nano-materials have lot of advantages over conventional materials for desalination and purification after necessary modification. To overcome the challenges, expansion of costeffective technological methods may show the proper way to integrate CNTs into the composite matrix for membrane fabrication. Surface modification with definite functional groups and manipulation of pore size distribution of CNTs are perhaps the appropriate ways to entirely take the advantages of CNTs’ unique physical, chemical and electrical properties. CNTs can be randomly arranged to form Bucky paper membrane. It is a simple type of membrane containing self-supporting, randomly grown CNTs [160]. This type of membrane can be flexible with excellent chemical, physical, electrical and mechanical properties. It is simple and inexpensive to fabricate compared to align CNTs membrane. Basically CNTs always exhibit strong van der Waals interactions for which they have aggregation tendencies. This is the force which holds the CNTs together inside the consistent Bucky-paper membrane. However, narrow, few walled and more purified form of nanotubes can form stronger Bucky paper based membrane having high tensile strengths [161]. When the diameter of the randomly arranged MWCNTs are increased, then the van der Waals interactions becomes less resulting membrane with reduced tensile strength. In that case, further surface functionalization or polymerization is required for randomly oriented CNTs based membrane to ensure greater desalination and purification performances under controlled environment [161]. Purification of CNTs involves oxidative treatment combined with filtration and centrifugation [162]. Treating with HNO3 acid or heating under air is frequently used to eliminate carbonaceous impurities. Because the impurities have imperfect graphitic structure which will be oxidized earlier than the CNTs itself [163]. Metallic impurities are removed from CNTs by secondary treatment of HCl [164]. However, these extra purification strategies used to fabricate Bucky paper membrane shorten the length of CNTs and damage their original structure but at the same time functionalize them with –COOH and –OH groups resulting hydrophilic membrane [165, 166]. Thus it will be well dispersed in polar solvent like water. Until recently Buckypaper based randomly oriented CNT membrane has been used as fine filters [100]. It was reported that 2 μm thick CNT was used to fabricate Bucky-Paper film over cellulose acetate disc to filter fine particles having diameter of 100–500 nm and the performances are better than the HEPA filters [60]. Even randomly oriented Bucky paper based CNTs membrane can be used to filter powdered organic dyes and condensed lead fumes [100]. It was observed that 2 μm size randomly oriented CNTs membrane is efficient to retain E. coli cells [167]. In membrane distillation, this type of membrane can be widely used [168]. In case of ordered or aligned CNTs based membrane, there are narrow internal gaps between the tubes [40]. Not only desalination, even different types of proteins and biological molecules can be separated using aligned CNTs membrane [2, 40]. The major advantage of aligned CNT membranes is that, its’ pore size can be precisely controlled by changing the catalyst type during their growth. Thus their properties can be adjusted suitably for desalination and purification process [169, 170]. The molecular selectivity can be modified by covalently functionalizing the tubes’ end [169, 170]. Aligned CNT membranes show better rejection rate with improved water flux (3 times higher) and biofouling resistances than the commercial ultrafiltration membrane [171]. Vertically aligned MWNTs were incorporated with PES membrane and it exhibited 3 times more water flux than randomly arranged MWCNTs inside the PES and 10 times more flux than pure PES membrane under identical condition [172]. With the approaching era of nano, CNT based membrane or filtration medium will be widely used in future but the process is still suffering from lot of disadvantages. The electrochemical CNT filters are still expensive to fabricate especially CNT sheets having larger sizes with porous texture. The cost for synthesizing CNTs are still comparatively high than the other commonly used water treatment materials, such as activated carbon.

8. Conclusion Right now, polymeric or inorganic membrane separation procedures have achieved an edge at which it is difficult to enhance the separation performance just by means of further optimization for the fabrication conditions of the membrane itself. The blend of nanotechnology and membrane separation offers new ways to deal with the challenge. Presence of inorganic functional groups onto the surface, side walls or tips of CNTs improves the dispersion and performance of CNT based composite membrane during desalination and purification. CVD method is quite efficient to fabricate inorganic (ceramic)/CNTs-based composite membranes. Moreover the blending method is relatively flexible for polymeric/CNTs-based membranes. CNTs-based composite membranes have better performance compared to the polymeric and inorganic 15


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membranes itself for desalination and purification process. Nevertheless, penetrating research and development on carbon nanotube based membranes endures with substantial industrial involvement. Carbon nanotube membranes can ensure high flux which makes it a promising candidate to compete with the existing commercial nanofiltration and RO polymeric systems, if new economical fabrication methods can be developed. Moreover, the applications of CNTs-based composite membranes should be monitored carefully due to its’ potential toxicity effects. Thus, some studies emphasizing the adherence between CNTs and the matrix should be performed in the near future. The membrane separation process coupled with other techniques such as adsorption, catalysis and electro-chemistry should be studied elaborately for the comprehensive understanding of the mechanisms using CNTs-based composite membranes. Therefore, more investigations are needed in terms of water chemistry, process parameters for membrane separation process, physio-chemical properties of CNTs as well as the function of composite membranes under precise circumstances. There is no hesitation that the incorporation of this nanostructured tube in composite membranes has prominent role for desalination and purification of aqueous effluents. It can help in addressing critical water issues up to a greater extent in future.

Acknowledgments This work has been made possible because of the generous grants RP044C-17AET and RP044D-17AET from the University Malaya, Malaysia.

ORCID iDs Zaira Zaman Chowdhury https://orcid.org/0000-0003-2175-0575 Suresh Sagadevan https://orcid.org/0000-0003-0393-7344

References [1] Shannon M A, Bohn P W, Elimelech M, Georgiadis J G, Marinas B J and Mayes A M 2008 Science and technology for water purification in the coming decades Nature 452 301–10 [2] Elimelech M and Phillip W A 2011 The future of seawater desalination: energy, technology, and the environment Science 333 712–7 [3] Surwade S P, Smirnov S N, Vlassiouk I V, Unocic R R, Veith G M, Dai S and Mahurin S M 2015 Water desalination using nano-porous single-layer graphene Nat. Nanotechnol. 10 459–64 [4] Qu X L, Brame J, Li Q L and Alvarez P J J 2013 Nanotechnology for a safe and sustainable water supply: enabling integrated water treatment and reuse Acc. Chem. Res. 46 834–43 [5] Hu M and Mi B X 2013 Enabling graphene oxide nano-sheets as water separation membranes Environ. Sci. Technol. 47 3715–23 [6] Eugster H P, Harvie C E and Weare J H 1980 Mineral equilibria in a six-component seawater system, Na–K–Mg–Ca–SO4–Cl–H2O, at 25 °C Geochimica Cosmochimica Acta. 44 1335–47 [7] Oren Y 2008 Capacitive deionization (CDI) for desalination and water treatment—past, present and future (a review) Desalination 228 10–29 [8] Duke M C, Mee S and Da Costa J C D 2007 Performance of porous inorganic membranes in non-osmotic desalination Water Res. 41 3998–4004 [9] Gin D L and Noble R D 2011 Designing the next generation of chemical separation membranes Science 332 674–86 [10] Liu G P, Jin W Q and Xu N P 2015 Graphene-based membranes Chem. Soc. Rev. 44 5016–30 [11] Mahmoud K A, Mansoor B, Mansour A and Khraisheh M 2015 Functional graphene nanosheets: the next generation membranes for water desalination Desalination 356 208–25 [12] Lau W J and Ismail A F 2009 Polymeric nanofiltration membranes for textile dye wastewater treatment: preparation, performance evaluation, transport modelling, and fouling controls—a review Desalination 245 321–48 [13] Geise G M, Lee H S, Miller D J, Freeman B D, Mcgrath J E and Paul D R 2010 Water purification by membranes: the role of polymer science J. Polym. Sci. Pol. Phys. 48 1685–718 [14] Marchetti P, Solomon M F J, Szekely G and Livingston A G 2014 Molecular separation with organic solvent nanofiltration: a critical review Chem. Rev. 114 10735–806 [15] Van der Bruggen B, Vandecasteele C, Van Gestel T, Doyen W and Leysen R 2003 A review of pressure-driven membrane processes in wastewater treatment and drinking water production Environ. Prog. 22 46–56 [16] Joshi R, Alwarappan S, Yoshimura M, Sahajwalla V and Nishina Y 2015 Graphene oxide: the new membrane material Appl. Mater. Today 1 1–12 (2015) [17] Tsuru T 2008 Nano/subnano-tuning of porous ceramic membranes for molecular separation J. Sol-Gel Sci. Technol. 46 349–61 [18] Wu P, Xu Y Z, Huang Z X and Zhang J C 2015 A review of preparation techniques of porous ceramic membranes J. Ceram. Process Res. 16 102–6 [19] Ji C, Tian Y, Li Y D and Lin Y S 2014 Thin oriented AFI zeolite membranes for molecular sieving separation Micropor. Mesopor. Mater. 186 80–3 [20] Brown A J, Brunelli N A, Eum K, Rashidi F, Johnson J R, Koros W J, Jones C W and Nair S 2014 Interfacial microfluidic processing of metal-organic framework hollow fiber membranes Science 345 72–5 [21] Baek Y, Seo D K, Choi J H, Lee B, Kim Y H, Park S M, Jung J, Lee S and Yoon J 2016 Improvement of vertically aligned carbon nanotube membranes: desalination potential, flux enhancement and scale-up Desalin. Water Treat 57 28133–40

16


Z Z Chowdhury et al

[22] Joshi R K, Carbone P, Wang F C, Kravets V G, Su Y, Grigorieva I V, Wu H A, Geim A K and Nair R R 2014 Precise and ultrafast molecular sieving through graphene oxide membranes Science 343 752–4 [23] Ismail A F, Goh P S, Sanip S M and Aziz M 2009 Transport and separation properties of carbon nanotube-mixed matrix membrane Sep. Purif. Technol. 70 12–26 [24] Holt J K, Park H G, Wang Y M, Stadermann M, Artyukhin A B, Grigoropoulos C P, Noy A and Bakajin O 2006 Fast mass transport through sub-2-nanometer carbon nanotubes Science 312 1034–7 [25] Konduri S, Tong H M, Chempath S and Nair S 2008 Water in single-walled aluminosilicate nanotubes: diffusion and adsorption properties J. Phys. Chem. C 112 15367–74 [26] Saito R, G. Dresselhaus G and Dresselhaus M S 1998 Physical Properties of Carbon Nanotubes (London and Imperial College Press) [27] Liu Y B, Liu H, Zhou Z, Wang T R, Ong C N and Vecitis C D 2015 Degradation of the common aqueous antibiotic tetracycline using a carbon nanotube electrochemical filter Environ. Sci. Technol. 49 7974–80 [28] Brady-Estévez A S, Schnoor M H, Vecitis C D, Saleh N B and Elimelech M 2010 Multiwalled carbon nanotube filter: improving viral removal at low pressure Langmuir 26 14975–82 [29] De Lannoy C F, Jassby D, Gloe K, Gordon A D and Wiesner M 2013 Aquatic biofouling prevention by electrically charged nanocomposite polymer thin film membranes Environ. Sci. Technol. 47 2760–8 [30] Sun X, Wu J, Chen Z, Su X and Hinds B J 2013 Fouling characteristics and electrochemical recovery of carbon nanotube membranes Adv. Funct. Mater. 23 1500–6 [31] Rahaman M S, Vecitis C D and Elimelech M 2012 Electrochemical carbon-nanotube filter performance toward virus removal and inactivation in the presence of natural organic matter Environ. Sci. Technol. 46 1556–64 [32] Vecitis C D, Gao G and Liu H 2011 Electrochemical carbon nanotube filter for adsorption, desorption, and oxidation of aqueous dyes and anions J. Phys. Chem. C 115 3621–9 [33] Li H, Zhang D, Han X and Xing B 2014 Adsorption of antibiotic ciprofloxacin on carbon nanotubes: pH dependence and thermodynamics Chemosphere 95 150–5 [34] Deng S, Zhang Q, Nie Y, Wei H, Wang B, Huang J, Yu G and Xing B 2012 Sorption mechanisms of perfluorinated compounds on carbon nanotubes Environ. Pollut. 168 138–44 [35] Gao G D and Vecitis C D 2011 Electrochemical carbon nanotube filter oxidative performance as a function of surface chemistry Environ. Sci. Technol. 45 9726–34 [36] Ajmani G S, Goodwin D, Marsh K, Fairbrother D H, Schwab K J, Jacangelo J G and Huang H 2012 Modification of low pressure membranes with carbon nanotube layers for fouling control Water Res. 46 5645–54 [37] Boo C, Elimelech M and Hong S 2013 Fouling control in a forward osmosis process integrating seawater desalination and wastewater reclamation J. Membrane Sci. 444 148–56 [38] Zhang Q and Vecitis C D 2014 Conductive CNT-PVDF membrane for capacitive organic fouling reduction J. Membrane Sci. 459 143–56 [39] Vecitis C D, Schnoor M H, Rahaman M S, Schiffman J D and Elimelech M 2011 Electrochemical multiwalled carbon nanotube filter for viral and bacterial removal and inactivation Environ. Sci. Technol. 45 3672–9 [40] Hinds B J, Chopra N, Rantell T, Andrews R, Gavalas V and Bachas L G 2004 Aligned multiwalled carbon nanotube membranes Science 303 62–5 [41] National Research Council (US) Committee on Advancing Desalination Technology and National Academies Press (US) 2008 Desalination: A National Perspective vol xiv (Washington, DC: National Academies Press) pp 298 [42] Khawaji A D, Kutubkhanah I K and Wie J M 2008 Advances in seawater desalination technologies Desalination 221 47–69 [43] Hahn W J 1986 Measurements and control in freeze desalination plants Desalination 59 321–41 [44] Sonune A and Ghate R 2004 Developments in wastewater treatment methods Desalination 167 55–63 [45] Bryjak K N M, Schlosser S, Kitis M, Avlonitis S, Matejka Z, Al-Mutaz I and Yuksel M 2008 Adsorption-membrane filtration (AMF) hybrid process for boron removal from seawater: an overview Desalination 223 38–48 [46] Kabay N M, Sarper S, Mithat Y, Arar O Z and Marek B 2007 Removal of boron from seawater by selective ion exchange resins React. Funct. Polym. 67 1643–50 [47] Probstein R F 2003 Physicochemical Hydrodynamics: An Introduction 2nd edn vol xv (Hoboken, NJ: Wiley-Interscience) pp 400 [48] Balasubramanian K and Burghard M 2005 Chemically functionalized carbon nanotubes Small 1 180–92 [49] Szpyrkowicz L, Kaul S and Neti R 2005 Tannery wastewater treatment by electro-oxidation coupled with a biological process J. Appl. Electrochem. 35 381–90 [50] Martinez-Huitle C A and Ferro S 2006 Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes Chem. Soc. Rev. 35 1324–40 [51] Liu X, Wang M, Zhang S and Pan B 2013 Application potential of carbon nanotubes in water treatment: a review J. Environ. Sci. 25 1263–80 [52] Pan B and Xing B 2008 Adsorption mechanisms of organic chemicals on carbon nanotubes Environ. Sci. Technol. 42 9005–13 [53] Dichiara A B, Harlander S F and Rogers R E 2015 Fixed bed adsorption of diquat dibromide from aqueous solution using carbon nanotubes RSC Adv. 5 61508–12 [54] Gao G, Zhang Q, Hao Z and Vecitis C D 2015 Carbon nanotube membranestack for flow-through sequential regenerative electrofenton Environ. Sci. Technol. 49 2375–83 [55] Chavan R, Desai U, Mhatre P and Chinchole R 2012 A review: carbon nanotubes Int. J. Pharm. Sci. Rev. Res. 13 124–34 [56] Ying L S, Bin Mohd Salleh M A, Mohamed Yusoff H B, Abdul Rashid S B and Abd. Razak J B 2011 Continuous production of carbon nanotubes—a review J. Ind. Eng. Chem. 17 367–76 [57] Liu Q, Ren W, Liu B, Chen Z-G, Li F, Cong H and Cheng H-M 2009 Synthesis, purification and opening of short cup-stacked carbon nanotubes J. Nanosci. Nanotechnol. 9 4554–60 [58] Pillai S K, Ray S S and Moodley M 2008 Purification of multi-walled carbon nanotubes J. Nanosci. Nanotechnol. 8 6187–207 [59] Ismail A F, Goh P S, Tee J C, Sanip S M and Aziz M 2008 A review of purification techniques for carbon nanotubes Nano 3 127–43 [60] Yu M, Funke H H, Falconer J L and Noble R D 2010 Gated ion transport through dense carbon nanotube membranes J. Am. Chem. Soc. 132 8285–90 [61] Hinds B J, Chopra N, Rantell T, Andrews R, Gavalas V and Bachas L G 2004 Aligned multi-walled carbon nanotube membranes Science 303 62–5 [62] Lu D, Li Y, Ravaioli U and Schulten K 2005 Empirical nanotube model for biological applications J. Phys. Chem. B 109 11461–7 [63] Hummer G, Rasaiah J C and Nowotyta J P 2001 Water conduction through the hydrophobic channel of a carbon nanotube Nature 414 188–90

17


Z Z Chowdhury et al

[64] Noy A, Park H G, Fornasiero F, Holt J K, Grigoropoulos C P and Bakajin O 2007 Nanofluidics in carbon nanotubes Nano Today 2 22–9 [65] Kotsalis E M, Walther J H and Koumoutsakos P 2004 Multiphase water flow inside carbon nanotubes Int. J. Multiphase Flow 30 995–1010 [66] Chan Y and Hill J M 2011 A mechanical model for single-file transport of water through carbon-nanotube membranes J. Membr. Sci. 372 57–65 [67] Whitby M and Quirke N 2007 Fluid flow in carbon nanotubes and nanopipes Nat. Nanotechnol. 12 87–94 [68] Giovambattista N, Rossky P J and Debenedetti P G 2007 Effect of pressure on the phase behavior and structure of water confined between nanoscale hydrophobic and hydrophilic plates J. Phys. Chem. C 111 1323–32 [69] Kolesnikov A I, Zanotti J M, Loong C K, Thivagarajan P, Moravsky A P, Loutfv R O and Burnham C J 2004 Anomalously soft dynamics of water in a nanotube: a revelation of nanoscale confinement Phys. Rev. Lett. 93 035503 [70] Banerjee S, Murad S and Puri I K 2007 Preferential ion and water intake using charged carbon nanotube Chem. Phys. Lett. 434 292–6 [71] Song C and Corry B 2009 Intrinsic ion selectivity of narrow hydrophobic pores J. Phys. Chem. B 113 7642–9 [72] Falk K, Sedlmeier F, Joly L, Netz R R and Bocquet L 2010 Molecular origin of fast water transport in carbon nanotube membranes: superlubricity versus curvature dependent friction Nano Lett. 10 4067–73 [73] Melillo M, Zhu F, Snyder M A and Mittal J 2011 Water transport through nanotubes with varying interaction strength between tube wall and water J. Phys. Chem. Lett. 2 2978–83 [74] Gethard K, Sae-Khow O and Mitra S 2011 Water desalination using carbon nanotube enhanced membrane distillation Appl. Mater. Interfaces 3 110–4 [75] Green M 2010 Analysis and measurement of carbon nanotube dispersions: nanodispersion versus macrodispersion J. Polymer International 59 1319–22 [76] Tasis D, Tagmatarchis N, Bianco A and Prato M 2006 Chemistry of carbon nanotubes Chem. Rev. 106 1105–36 [77] Kim S, Kim W, Kim T, Choi Y S, Lim H S, Yang S J and Park C R 2012 Surface modifications for the effective dispersion of carbon nanotubes in solvents and polymers Carbon 50 3–33 [78] Ma P C, Kim J K and Tang B Z 2006 Functionalization of carbon nanotubes using a silane coupling agent Carbon 44 3232–8 [79] Ma P C, Siddiqui N A, Marom G and Kim J K 2010 Dispersion and functionalization of carbon nanotubes for polymer-based nanocomposites: a review Composites Part A: Applied Science and Manufacturing 41 1345–67 [80] Star A, Stoddart J F, Steuerman D, Diehl M, Boukai A, Wong E W, Yang X, Chung S W, Choi H and Heath J R 2001 Preparation and properties of polymer-wrapped single-walled carbon nanotubes Angewandte Chemie International Edition 40 1721–5 [81] Hill D E, Lin Y, Rao A M, Allard L F and Sun Y P 2002 Functionalization of carbon nanotubes with polystyrene Macromolecules 35 9466 9471 [82] Nepal D and Geckeler K E 2007 Proteins and carbon nanotubes: close encounter in water Small 3 1259–65 [83] Nepal D and Geckeler K E 2006 pH‐sensitive dispersion and debundling of single‐walled carbon nanotubes: lysozyme as a tool Small 2 406–12 [84] Gorityala B K, Ma J, Wang X, Chen P and Liu X W 2010 Carbohydrate functionalized carbon nanotubes and their applications Chem. Soc. Rev. 39 2925–34 [85] Rinaudo M 2006 Chitin and chitosan: properties and applications Prog. Polym. Sci. 31 603–32 [86] Yang H, Wang S C, Mercier P and Akins D L 2006 Diameter-selective dispersion of single-walled carbon nanotubes using a watersoluble, biocompatible polymer Chem. Commun. 0 1425–7 [87] Peng F, Pan F, Sun H, Lu L and Jiang Z 2007 Novel nanocomposite pervaporation membranes composed of poly(vinyl alcohol) and chitosan-wrapped carbon nanotube J of Membrane Sci. 300 13–39 [88] Panhuis M I H, Heurtematte A, Small W R and Paunov V N 2007 Inkjet printed water sensitive transparent films from natural gum– carbon nanotube composites Soft Matter 3 840–3 [89] Geng Y, Liu M Y, Li J, Shi X M and Kim J K 2008 Effects of surfactant treatment on mechanical and electrical properties of CNT/epoxy nanocomposites Composites Part A: Applied Science and Manufacturing 39 1876–83 [90] Yu J, Grossiord N, Koning C E and Loos J 2007 Controlling the dispersion of multi-wall carbon nanotubes in aqueous surfactant solution Carbon 45 618–23 [91] Kim T H, Doe C, Kline S R and Choi S M 2007 Water‐redispersible isolated single‐walled carbon nanotubes fabricated by in situ polymerization of micelles Adv. Mater. 19 929–33 [92] O’Connell M J et al 2002 Band gap fluorescence from individual single-walled carbon nanotubes Science 297 593–6 [93] Moore V C, Strano M S, Haroz E H, Hauge R H, Smalley R E, Schmidt J and Talmon Y 2003 Individually suspended single-walled carbon nanotubes in various surfactants Nano Lett. 3 1379–82 [94] Frizzell C J, Panhuis M, Coutinho D H, Balkus K J, Minett A I, Blau W and Coleman J 2005 Reinforcement of macroscopic carbon nanotube structures by polymer intercalation: the role of polymer molecular weight and chain conformation Physical Review B (Condensed Matter and Materials Physics) 72 245420-1-245420-8 [95] Tanaka T 2010 Formation of microporous membranes of poly (1, 4-butylene succinate) via nonsolvent and thermally induced phase separation Desalination and Water Treatment 17 176–82 [96] Whitby R L D, Fukuda T, Maekawa T, James S L and Mikhalovsky S V 2008 Geometric control and tuneable pore size distribution of buckypaper and buckydiscs Carbon 46 949–56 [97] Gou J 2006 Single-walled nanotube bucky paper and nanocomposite Polymer International 55 1283–8 [98] Coleman J N, Blau W J, Dalton A B, Muñoz E, Collins S, Kim B G, Razal J, Selvidge M, Vieiro G and Baughman R H 2003 Improving the mechanical properties of single-walled carbon nanotube sheets by intercalation of polymeric adhesives Appl. Phys. Lett. 82 1682 [99] Boge J, Sweetman L J, Panhuis M and Ralph S F 2009 The effect of preparation conditions and biopolymer dispersants on the properties of SWNT bucky-papers J. Mater. Chem. 19 9131–40 [100] Viswanathan G, Kane D B and Lipowicz P J 2004 High efficiency fine particulate filtration using carbon nanotube coatings Adv. Mater. 16 2045–9 [101] Xitong L, Shujuan Z and Bingcai P 2012 Potential of carbon nanotubes in water treatment Recent Progress in Carbon Nanotube Research (Rijeka, Croatia: InTech) (https://doi.org/10.5772/51332) [102] Lin D H and Xing B S 2008 Adsorption of phenolic compounds by carbon nano‐tubes: role of aromaticity and substitution of hydroxyl groups Environ. Sci. Technol. 42 7254–9 [103] Gotovac S, Song L, Kanoh H and Kaneko K 2007 Assembly structure control of single wall carbon nanotubes with liquid phase naphthalene adsorption Colloids and Surfaces, A: Physicochemical and Engineering Aspects 300 117–21

18


Z Z Chowdhury et al

[104] Yang K, Zhu L Z and Xing B S 2006 Adsorption of polycyclic aromatic hydro‐carbons by carbon nanomaterials Environ. Sci. Technol. 40 1855–61 [105] Chen W, Duan L and Zhu D Q 2007 Adsorption of polar and nonpolar organic chemicals to carbon nanotubes Environ. Sci. Technol. 41 8295–300 [106] Gotovac S, Honda H, Hattori Y, Takahashi K, Kanoh H and Kaneko K 2007 Effect of Nanoscale curvature of single-walled carbon nanotubes on adsorption of polycyclic aromatic hydrocarbons Nano Lett. 7 583–7 [107] Tournus F and Charlier J C 2005 Ab initio study of benzene adsorption on carbon nanotubes Phys. Rev. B 71 [108] Zhang S J, Shao T, Bekaroglu S S K and Karanfil T 2009 The impacts of aggre‐gation and surface chemistry of carbon nanotubes on the adsorption of synthetic organic compounds Environ. Sci. Technol. 43 5719–25 [109] Zhang S J, Shao T, Kose H S and Karanfil T 2012 Adsorption kinetics of aro‐matic compounds on carbon nanotubes and activated carbons Environmental Toxicol- ogy and Chemistry 31 79–85 [110] Peng X J, Li Y H, Luan Z K, Di Z C, Wang H Y, Tian B H and Jia Z P 2003 Adsorption of 1, 2-dichlorobenzene from water to carbon nanotubes Chemical Physics Letters, 376 154–8 [111] Cho H H, Smith B A, Wnuk J D, Fairbrother D H and Ball W P 2008 Influence of surface oxides on the adsorption of naphthalene onto multiwalled carbon nanotubes Environ. Sci. Technol. 42 2899–905 [112] Hoffmann M R, Martin S T, Choi W Y and Bahnemann D W 1995 Environ‐mental applications of semiconductor photocatalysis Chem. Rev. 95 69–96 [113] Kongkanand A and Kamat P V 2007 Electron storage in single wall carbon nano‐tubes. Fermi level equilibration in semiconductorSWCNT suspensions ACS Nano 1 13–21 [114] Lu S Y, Tang C W, Lin Y H, Kuo H F, Lai Y C, Tsai M Y, Ouyang H and Hsu W K 2010 TiO2-coated carbon nanotubes: a redshift enhanced photocataly‐sis at visible light Appl. Phys. Lett. 96 231915 [115] Cong Y, Li X K, Qin Y, Dong Z J, Yuan G M, Cui Z W and Lai X J 2011 Carbon-doped TiO2 coating on multiwalled carbon nanotubes with higher visible light photocatalytic activity Applied Catalysis, B: Environmental 107 128–34 [116] Wang W D, Serp P, Kalck P and Faria J L 2005 Visible light photo-degradation of phenol on MWNT-TiO2 composite catalysts prepared by a modified sol-gel meth‐od Journal of Molecular Catalysis a-Chemical 235 194–9 [117] Yao Y, Li G, Ciston S, Lueptow R M and Gray K A 2008 Photoreactive TiO2/arbon nanotube composites: synthesis and reactivity Environ. Sci. Technol. 42 4952–7 [118] Yu Y, Yu J C, Chan C Y, Yan K, Zhao J C, Ding L, Ge W K and Wong P K 2005 Enhancement of adsorption and photocatalytic activity of TiO2 by using car‐ bon nanotubes for the treatment of azo dye Applied Catalysis B: Environmental 6 1 [119] Silva C G and Faria J L 2010 Photocatalytic oxidation of benzene derivatives in aqueous suspensions: synergic effect induced by the introduction of carbon nano‐tubes in a TiO2 matrix Applied Catalysis, B: Environmental 101 81–9 [120] Martinez C, Canle M, Fernandez M I, Santaballa J A and Faria J 2011 Kinetics and mechanism of aqueous degradation of carbamazepine by heterogeneous photo‐catalysis using nanocrystalline TiO2, ZnO and multi-walled carbon nanotubes-ana‐tase composites Applied Catalysis, B: Environmental 102 563–71 [121] Yan Y, Sun H P, Yao P P, Kang S Z and Mu J 2011 Effect of multi-walled car‐ bon nanotubes loaded with Ag nanoparticles on the photocatalytic degradation of rhodamine B under visible light irradiation Appl. Surf. Sci. 257 3620–6 [122] Brady-Estevez A S, Kang S and Elimelech M 2008 A single-walled-carbon-nano‐tube filter for removal of viral and bacterial pathogens Small 4 481–4 [123] Brady-Estevez A S, Schnoor M H, Vecitis C D, Saleh N B and Ehmelech M 2008 Multiwalled carbon nanotube filter: improving viral removal at low pres‐sure Langmuir 26 14975–82 [124] Brady-Estevez A S, Schnoor M H, Kang S and Elimelech M 2010 SWNT/MWNT Hybrid filter attains high viral removal and bacterial inactivation Lang-muir 26 19153–8 [125] Rahaman M S, Vecitis C D and Elimelech M 2012 Electrochemical carbonnanotube filter performance toward virus removal and inactivation in the presence of natural organic matter Environ. Sci. Technol. 46 1556–64 [126] Akhavan O, Abdolahad M, Abdi Y and Mohajerzadeh S 2011 Silver nanoparti‐cles within vertically aligned multi-wall carbon nanotubes with open tips for antibacterial purposes J. Mater. Chem. 21 387–93 [127] Srivastava A, Srivastava O N, Talapatra S, Vajtai R and Ajayan P M 2004 Car‐ bon nanotube filters Nat. Mater. 3 610–4 [128] Qiao Y, Li C M, Bao S J and Bao Q L 2007 Carbon nanotube/polyaniline com‐ posite as anode material for microbial fuel cells J. Power Sources 170 79–84 [129] Zou Y J, Xiang C L, Yang L N, Sun L X, Xu F and Cao Z 2008 A mediator‐ less microbial fuel cell using polypyrrole coated carbon nanotubes composite as anode material Int. J. Hydrog. Energy 33 4856–62 [130] Xie X, Hu L B, Pasta M, Wells G F, Kong D S, Criddle C S and Cui Y 2011 Three-dimensional carbon nanotube-textile anode for highperformance microbial fuel cells Nano Lett. 11 291–6 [131] Liu X W, Sun X F, Huang Y X, Sheng G P, Wang S G and Yu H Q 2011 Carbon nanotube/chitosan nano-composite as a biocompatible bio-cathode material to enhance the electricity generation of a microbial fuel cell Energy & Environmental Science 4 1422–7 [132] Padaki M, Murali R S, Abdullah M S, Misdan N, Moslehyani A, Kassim M A, Hilal N and Ismail A F 2015 Membrane technology enhancement in oil–water separation. A review Desalination 357 197–207 [133] Zhu L, Chen M, Dong Y, Tang C Y, Huang A and Li L 2016 A low-cost mullite-titania composite ceramic hollow fiber microfiltration membrane for highly efficient separation of oil-in-water emulsion Water Res. 90 277–85 [134] Chen M, Zhu L, Dong Y, Li L and Liu J 2016 Waste-to-resource strategy to fabricate highly porous whisker-structured mullite ceramic membrane for simulated oil-in-water emulsion wastewater treatment ACS Sustainable Chem. Eng. 4 2098–106 [135] Chen X, Hong L, Xu Y and Ong Z W 2012 Ceramic pore channels with inducted carbon nanotubes for removing oil from water ACS Appl. Mater. Interfaces 4 1909–18 [136] Gu J C, Xiao P, Chen J, Liu F, Huang Y, Li G, Zhang J and Chen T 2014 Robust preparation of superhydrophobic polymer/carbon nanotube hybrid membranes for highly effective removal of oils and separation of water-in-oil emulsions J. Mater. Chem. 2 15268–72 [137] Fu F L and Wang Q 2011 Removal of heavy metal ions from wastewaters: a review J. Environ. Manage. 92 407–18 [138] Rao G P, Lu C and Su F 2007 Sorption of divalent metal ions from aqueous solution by carbon nanotubes: a review Sep. Purif. Technol. 58 224–31 [139] Mubarak N M, Sahu J N, Abdullah E C and Jayakumar N S 2014 Removal of heavy metals from wastewater using carbon nanotubes Sep. Purif. Rev. 43 311–38 [140] Tofighy M A and Mohammadi T 2015 Nickel ions removal from water by two different morphologies of induced CNTs in mullite pore channels as adsorptive membrane Ceram. Int. 41 5464–72

19


Z Z Chowdhury et al

[141] Shah P and Murthy C N 2013 Studies on the porosity control of MWCNT/polysulfone composite membrane and its effect on metal removal J. Membr. Sci. 437 90–8 [142] Abda M, Oren Y and Soffer A 1987 The electrodeposition of trace metallic impurities: dependence on the supporting electrolyte concentration—a comparison between bipolar and monopolar porous electrodes Electrochim. Acta 32 1113–5 [143] Delanghe B, Tellier S and Astruc M 1990 The carbon-felt flow-through electrode in waste water treatment: the case of mercury (II) electrodeposition Environ. Technol 11 999–1006 [144] Ohmori S and Saito T 2012 Electrochemical durability of single-wall carbon nanotube electrode against anodic oxidation in water Carbon 50 4932–8 [145] Schnoor M H and Vecitis C D 2013 Quantitative examination of aqueous ferrocyanide oxidation in a carbon nanotube electrochemical filter: effects of flow rate, ionic strength, and cathode material J. Phys. Chem. C 117 2855–67 [146] Liu H and Vecitis C D 2012 Reactive transport mechanism for organic oxidation during electrochemical filtration: mass-transfer, physical adsorption, and electron-transfer J. Phys. Chem. C 116 374–83 [147] Liu H, Vajpayee A and Vecitis C D 2013 Bismuth-doped tin oxide-coated carbon nanotube network: improved anode stability and efficiency for flow-through organic electrooxidation ACS Appl. Mater. Inter. 5 10054–66 [148] Liu H, Liu J, Liu Y B, Bertoldi K and Vecitis C D 2014 Quantitative 2D electrooxidative carbon nanotube filter model: insight into reactive sites Carbon 80 651–64 [149] Choi J H, Jegal J and Kim W N 2006 Fabrication and characterization of multiwalled carbon nanotubes/polymer blend membranes J. Membr. Sci. 284 406–15 [150] Celik E, Park H, Choi H and Choi H 2011 Carbon nanotube blended polyether‐sulfone membranes for fouling control in water treatment Water Res. 45 274–82 [151] Qiu S, Wu L G, Pan X J, Zhang L, Chen H L and Gao C J 2009 Preparation and properties of functionalized carbon nanotube/PSF blend ultrafiltration membranes J. Membr. Sci. 342 165–72 [152] Liu Y L, Chang Y, Chang Y H and Shih Y J 2010 Preparation of amphiphilic polymer-functionalized carbon nanotubes for lowprotein-adsorption surfaces and protein-resistant membranes ACS Applied Materials & Interfaces 2 3642–7 [153] Tiraferri A, Vecitis C D and Elimelech M 2011 Covalent binding of single-Wal‐ led carbon nanotubes to polyamide membranes for antimicrobial surface proper‐ties ACS Applied Materials & Interfaces 3 2869–77 [154] Gunawan P et al 2011 Hollow fiber membrane decorated with Ag/MWNTs: toward effective water disinfection and Bi‐ofouling control ACS Nano 5 10033–40 [155] Majeed S, Fierro D, Buhr K, Wind J, Du B, Boschetti-Fierro De A and Abetz V 2012 Multi-walled carbon nanotubes (MWCNTs) mixed polyacrylonitrile (PAN) ultrafiltration membranes J. Membr. Sci. 403 101–9 [156] Tang C Y, Zhang Q, Wang K, Fu Q and Zhang C L 2009 Water transport be‐ heavier of chitosan porous membranes containing multiwalled carbon nanotubes (MWNTs) J. Membr. Sci. 337 240–7 [157] Upadhyayula V K K and Gadhamshetty V 2010 Appreciating the role of carbon nanotube composites in preventing biofouling and promoting biofilms on material surfaces in environmental engineering: a review Biotechnol. Adv. 28 802–16 [158] Agboola A E, Pike R W, Hertwig T A and Lou H H 2007 Conceptual design of carbon nanotube processes Clean Technologies and Environmental Policy 9 289–311 [159] Liu Y B, Liu H, Zhou Z, Wang T R, Ong C N and Vecitis C D 2015 Degradation of the common aqueous antibiotic tetracyclineusing a carbon nanotube electrochemical filter Environ. Sci. Technol. 49 7974–80 [160] Hone J, Liaguno M C, Nemes N M, Johnson A T, Fischer J E, Walters D A, Casavant M J, Schmidt J and Smalley R E 2000 Electrical and thermal transport properties of magnetically aligned single wall carbon nanotube films Appl. Phys. Lett. 77 666–8 [161] Xu G, Zhang Q, Zhou W, Huang J and Wei F 2008 The feasibility of producing MWCNT paper and strong MWCNT film from VACNT array Appl. Phys. A: Mater. Sci. Process 92 531–9 [162] Bandow S, Asaka S, Zhao X and Ando Y 1998 Purification and magnetic properties of carbon nanotubes Appl. Phys. A: Mater. Sci. Process 67 23–7 [163] Xu Y Q, Peng H, Hauge R H and Smalley R E 2005 Controlled multistep purification of single walled carbon nanotubes Nano Lett. 5 163–8 [164] Park T J, Banerjee S, Hemraj-Benny T and Wong S S 2006 Purification strategies and purity visualization for single-walled carbon nanotubes J. Mater. Chem. 16 141–54 [165] Ziegler K J, Gu Z, Peng H, Flor E L, Hauge R H and Smalley R E 2005 Controlled oxidative cutting of single-walled carbon nanotubes J. Am. Chem. Soc. 127 1541–7 [166] Hu H, Zhao B, Itkis M E and Haddon R C 2003 Nitric acid purification of single-walled carbon nanotubes J. Phys. Chem. B 107 13838–42 [167] Kang S, Pinault M, Pfefferle L D and Elimelech M 2007 Single-walled carbon nanotubes exhibit strong antimicrobial activity Langmuir 23 8670–3 [168] Lawson K W and Lloyd D R 1997 Membrane distillation J. Membr. Sci. 124 1–25 [169] Majumder M, Chopra N and Hinds B J 2005 Effect of tip functionalization on transport through vertically oriented carbon nanotube membranes J. Am. Chem. Soc. 127 9062–70 [170] Majumder M, Keis K, Zhan X, Meadows C, Cole J and Hinds B J 2008 Enhanced electrostatic modulation of ionic diffusion through carbon nanotube membranes by diazonium grafting chemistry J. Membr. Sci. 316 89–96 [171] Baek Y et al 2014 High performance and antifouling vertically aligned carbon nanotube membrane for water purification J. Membr. Sci. 460 171–7 [172] Li S, Liao G, Liu Z, Pan Y, Wu Q, Weng Y, Zhang X, Yang Z and Tsui O K C 2014 Enhanced water flux in vertically aligned carbon nanotube arrays and polyethersulfone composite membranes J. Mater. Chem. A 2 12171–6 [173] Liu Y, Dustin Lee J H, Xia Q, Ma Y, Yu Y, Lanry Yung L Y, Xie J, Ong C N, Vecitis C D and Zhou Z 2014 A graphene-based electrochemical filter for water purification J. Mater. Chem. A 2 16554–62

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