Nano Today (2012) 7, 385—389
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NEWS AND OPINIONS
Carbon nanotube membranes for desalination and water puriﬁcation: Challenges and opportunities Soumitra Kar ∗, R.C. Bindal, P.K. Tewari Desalination Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India Received 16 March 2012; received in revised form 30 August 2012; accepted 7 September 2012
KEYWORDS Water; Carbon nanotube membrane; Nanocomposite
Summary The importance as well as impact of application of carbon nanotube (CNT) membranes in the area of water technology development is tremendous. A substantial amount of work have been carried out with impregnation of CNTs (simply as one of the reinforcements for incorporation of better properties like antibiofouling and/or better strength, etc.) in polymer host matrix. However, the volume of work enabling CNTs as ﬂow channels (in a membranous structure) is not signiﬁcant and that is where the potential beneﬁts of CNTs lie. Moreover, from impressive works carried out in this direction, it is quite obvious that still signiﬁcant challenges have to be addressed to align the CNTs, to reinforce it in a suitable host matrix without disturbing the alignment and inhibiting the agglomeration (adoption of suitable nanocomposite fabrication route), to open the tips preferentially and to scale up favorably. It is believed that the innovative attributes put forth by CNTs and the application areas identiﬁed with CNTs are getting matured, while the area of development of CNT (based) membranes is far from being matured and it still needs to be looked into in the light of involvement of materials as well as process challenges. © 2012 Elsevier Ltd. All rights reserved.
Water, a nonsubstitutional natural resource, was already scarce, and is becoming increasingly scarce day by day. Water scarcity is among the main problems to be faced by many societies and the World in the XXIst century. Water use has been growing at more than twice the rate of population increase in the last century. As per a report from United Nations (UN), by 2025, 1800 million people will be living in countries or regions with absolute water scarcity, and two-thirds of the world population could be under stress conditions . The situation will be exacerbated as rapidly
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growing urban areas place heavy pressure on neighboring water resources. Most countries in the Near East and North Africa suffer from acute water scarcity, as do countries such as Mexico, Pakistan, South Africa, and large parts of China and India. Growing scarcity and competition for water stand as a major threat to future advances in poverty alleviation, especially in rural areas. As emphasized in one of the millennium development of goals (MDGs) by UN, water scarcity calls for strengthened international cooperation in the ﬁelds of technologies for enhanced water productivity. Recent years have witnessed impressive breakthroughs toward application of nanostructured materials like, carbon nanotubes (CNTs), metal/metal-oxide nanoparticles, zeolites, dendrimers in the ﬁeld of water puriﬁcation. However,
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386 the credentials put forth by CNTs are remarkable and amazingly important to a membranologist to come out with next generation membranes with high ﬂux, high selectivity, low fouling (hydrophilic-, hydrophobic- and bio-fouling). The CNTs have got the potential to overcome the inherent limitation of trade-off between ﬂux and selectivity that any membrane process carries along with. The inner hollow cavity of CNT offers tremendous Opportunities to material scientists, chemists and engineers to do excellent R&D in the nanoscale test tube. The extremely high aspect ratios, molecularly smooth hydrophobic graphitic walls, and nanoscale inner diameters of CNTs give rise to the unique phenomenon of ultra-efﬁcient transport of water molecules through these ultra-narrow molecular pipes. Water molecules move through nanotube pores orders of magnitude faster than through other pores of comparable size. Because of the reduced diameter of CNTs (in the order of nanometers), thermodynamic and transport properties of conﬁned water differ substantially those observed in the bulk. Concerning water, the scientiﬁc interest was centered initially on its behavior within short, narrow tubes, where one single walled nanotube (SWNT) with diameter of 0.8 nm and length of 1.35 nm immersed in bulk water . It was observed that water molecules move occasionally along the nanotube axis via bursts of hydrogen-bonded clusters of molecules. In CNTs with smaller diameters, however, water molecules have been shown to assemble into diameter-dependent one-dimensional structures. The functionalization of CNTs opens up excellent avenues to gain the potential beneﬁts of CNTs and achieve improved separation characteristics (termed as ‘‘gate-keeper controlled’’ separation) without compromise in throughput. In addition, attaching organic moieties leads to better anchoring of nanotubes in host materials and thus, yields better reinforcement of composites. The few studies available credited SWNTs with antimicrobial activity toward Gram-positive and Gram-negative bacteria, and the damages inﬂicted were attributed to either a physical interaction or oxidative stress that compromise cell membrane integrity [3,4]. Carbon nanotubes may therefore be useful for inhibiting microbial attachment and biofouling formation on surfaces. In addition to the inhibition of biofouling, the surface and tip of CNTs can be functionalized appropriately to take care of hydrophilic as well as hydrophobic fouling. Though there have been a lot of studies and application of as-grown/functionalized CNTs in water decontamination with respect to heavy metal, ﬂuoride, arsenic and toxic organics, it is note-worthy that the beneﬁcial effects of CNTs can be truly exploited only in impregnating CNTs in a host matrix (through synthesizing a membranous structure, without disturbing the alignment of nanotubes) and subsequently functionalizing the tubes for incorporation of the capability for speciﬁc separation. For the ﬁrst time in 2004 , an array of aligned CNTs was incorporated across a polymer ﬁlm to form a well-ordered nanoporous membrane structure. Subsequently, a group of researchers  have spent years studying how ﬂuid moves through nano-sized devices. They found that they were transporting 1000 times more water than expected. Membranes with CNT tips that were
S. Kar et al. functionalized  demonstrated the ability to gate molecular transport through CNT cores for potential applications in chemical separations and sensing. A macrogeometry of aligned CNTs was developed for water puriﬁcation applications by the researchers  and an approach toward development of an integrated membrane system was provided . The functionalization of CNT tip can introduce the required physico-chemical characteristics into the membrane surface, which could lead to very selective removal of contaminants based upon physico-chemical interaction of species with the functional group present over the CNT tip. The most important advantage of immobilizing the CNTs in a host matrix for separation applications (other than high ﬂux and high selectivity), lies in minimizing the probable health risk associated with the toxicity of as-grown CNTs when they are being applied as sorbent in water puriﬁcation applications, where there are fair chances of product water getting contaminated with CNTs. There are four approaches  being practiced for synthesis of CNT based membranes: 1. Deposition of carbonaceous materials inside preexisting ordered porous membranes, such as anodized alumina, also known as the template synthesized CNT membranes ; 2. Membranes based on the interstice between nanotubes in a vertical array of CNTs, referred to as the dense-array outer-wall CNT membrane ; 3. Encapsulation of as-grown vertically aligned CNTs by a space-ﬁlling inert polymer or ceramic matrix followed by opening up the CNT tips using plasma chemistry, or the open-ended CNT membrane [5,6]; and 4. Membranes composed of nanotubes as ﬁllers in a polymer matrix, also known as mixed-matrix membranes. Keeping in view the application of CNT-based composites in the area of water puriﬁcation, the ex situ alignment method (Approach 3) is preferred where the CNTs are aligned in advance using chemical vapor deposition (CVD) method and then they are compounded with the polymeric matrix by either in situ polymerization of some monomers or by spin coating/dip coating of polymer solution onto the aligned CNT matrix. The scheme for ex situ alignment has been shown in Fig. 1. It is believed that a cylindrical macrogeometry of aligned CNTs should be a better option than growing the aligned CNTs on a ﬂat support, while the method of CVD is being considered for aligned growth of CNTs. The tubular macrogeometry would essentially enable a technically smoother process of subsequent composite development using polymer/ceramic ﬁller (using dip-coating followed by necessary heat treatment) and also in preparation of viable and technically feasible membrane modules either in single channel or multichannel conﬁguration. The multichannel conﬁguration of CNT based membrane module can prove to be a well-engineered scaled-up product, which is the indispensable requisite that any water treatment system should fulﬁll to adhere to wider and simpler applicability standards. On the other hand, the scaled-up CNT based membrane can also be targeted in a ﬂat sheet conﬁguration by using a combination of self assembly and ﬁltration methods . To conﬁrm to the requirements of desalination and water puriﬁcation, the membrane should have the desirable
Carbon nanotube membranes for desalination and water puriﬁcation
A schematic of steps involved for development of CNT based membrane.
porosity and pore size distribution. For water puriﬁcation and waste water treatment ultraﬁltration (UF) type membranes are widely used world wide where the pore size varies from 5 nm to 100 nm and the separation is governed by the mechanism of size exclusion. A self-standing network of as-grown aligned multi walled CNTs can suitably act as an ultraﬁltration media. On the other hand, there are innumerable opportunities to functionalize CNTs [14,15] mainly grouped under (a) the covalent attachment of chemical groups through reactions onto the -conjugated skeleton of CNT; (b) the noncovalent adsorption or wrapping of various functional molecules; and (c) the endohedral ﬁlling of their inner empty cavity. The tips of CNTs can be appropriately functionalized to result in positively charged and negatively charged membranes to enable desalination of water. Figs. 2 and 3 show proposed pictorial representations of single walled CNT functionalized with positively charged (quaternary ammonium group) and negatively charged (sulfonic acid group) moieties respectively, which can serve as a building block toward development of charged membranes for size/charge based nanoﬁltration (NF) applications to enable desalination. The important, however unavoidable, problem encountered by any membrane applied in ﬁeld conditions for desalination/water puriﬁcation is membrane
fouling, where the extent of fouling depends upon the feed water conditions, membrane material, type of membrane, membrane module conﬁguration and of course process operation conditions. While the impregnation of as-grown CNTs in a polymeric matrix helps in reduction of fouling tendency of a membrane , the tip and sidewall of the CNTs can be functionalized with the required chemical groups to further alleviate the problem of membrane fouling. The severe problem of biofouling of membranes can also be minimized by impregnation of CNTs [3,4] which must make the membrane restore its inherent performance (desirable throughput and selectivity) for quite a longer period of time and in turn make the membrane based separation more economic. Though CNT based membranes have shown tremendous promises to serve as an wonderful aid in desalination and water puriﬁcation applications, however, there are numerous challenges involved with each step of membrane making starting from growth of CNTs to membrane performance evaluation and scale up. To grow 12—13 orders of magnitude of aligned CNTs per sq.cm is a real technological challenge.
Figure 2 Pictorial representation of functionalization of CNT tip with quaternary ammonium group for development of positively charged nanoﬁltration membrane.
Figure 3 Pictorial representation of functionalization of CNT tip with sulfonic acid group for development of negatively charged nanoﬁltration membrane.
388 Nanocomposite membrane fabrication route has to conﬁrm that CNTs are well dispersed and well aligned, which may require functionalization of CNTs to have better dispersion in the host matrix. Functionalization of CNTs with desired functional groups is difﬁcult to achieve, for CNT is hardly soluble in any solvent. The most critical step, that is, opening of the CNT tips with either acid treatment or plasma based oxidation may cause thinning of CNT wall and disruption of tube integrity and subsequent failure of membrane channels. Finally, scale-up with respect to CNT growth, CNT alignment, nanocomposite formation (ﬂat sheet or tubular), CNT tip or side-wall functionalization (with positively/negatively charged functional groups), all are highly complex and lot of material as well as process challenges are involved. A number of works has been carried out in literatures on theoretical nanoﬂuidics [17—20] explaining the nature of fast transport of water through CNT channels by consideration of slip length, strong hydrogen-bonding network between the water molecules, interfacial friction of water at graphitic interfaces and inherent smoothness of the interior of carbon nanotubes. Liquid slip at the solid—liquid boundary, conﬁnement induced reductions in the liquid viscosity, and subcontinuum changes to the liquid structure can all cause the actual hydraulic conductivity to exceed that calculated from the Poiseuille relation . The studies on theoretical nanoﬂuidics dictate the need for generation of a database on real experimental nanoﬂuidics of CNTs, which is seriously lacking, to ensure fruitful practical application of CNT based systems. Though the ion transport through the CNT channels has been investigated both experimentally and computationally , it was observed that ion rejection is not sufﬁciently high for desalination, and in addition, the monovalent salt rejection has not been tested yet to the best of our knowledge. The transport of water and ions through membranes formed from carbon nanotubes ranging in diam˚ were studied using molecular dynamics eter from 6 to 11 A simulations  and membranes incorporating CNTs were found to be promising candidates for water desalination using reverse osmosis (RO). Also researchers [24,25] are of the opinion that the CNT based membranes are going to be the next generation sea water reverse osmosis (SWRO) membranes. However, it was highlighted  that amount of energy that can be saved by usage of CNT based membrane is likely to very small unless the approach toward design of membrane module conﬁguration is redeﬁned to minimize concentration polarization, and membrane fouling issues are addressed appropriately. The possible usage of carbon nanotube based membrane in desalination and water puriﬁcation is proposed to be a sincere augmentation to the water technologies. In this regard, further research is proposed to analyze aligned-growth of CNTs, effect of CNT-tip functionalization, water permeation behavior with CNT membranes under real ﬁeld conditions (with multi component mixtures), and more importantly, assess the toxic effects of CNTs on living beings as well as environment so as to generate robust composite membranes, which can serve as a universal water ﬁlter.
S. Kar et al.
Acknowledgements We thank Mr. R.S. Tidke, Draftman, Desalination Division for preparing the schematic of CNT based nanocomposite membrane fabrication steps and Mr. Avishek Pal, Senior Research Fellow, Desalination Division for providing sincere technical suggestions on potential application of CNT based membranes in nanoﬁltration domain.
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Carbon nanotube membranes for desalination and water puriﬁcation Soumitra Kar is a Young Scientist working in the Desalination Division of BARC. He is associated with the development of inorganic and organic membranes and membrane-assisted physicochemical processes for gas separation, water puriﬁcation and efﬂuent treatment. He is also actively involved in the development of carbon nanotube-based separation systems. He is a Life Member of American Nano Society and the Indian Desalination Association. R.C. Bindal is a Senior Scientist in the Desalination Division of BARC. He has been associated with the development of different types of membranes and membranebased water puriﬁcation devices. He is instrumental in developing ﬁve different water puriﬁcation/industrial efﬂuent treatment technologies including the technologies for microorganism, ﬂuoride and arsenic decontamination. Currently his area of interest includes development of inorganic and mixed matrix nanocomposite membranes for radioactive waste treatment and gas permeation studies. He is member of several professional bodies associated with water technologies.
P.K. Tewari is a Outstanding Scientist and working as Head, Desalination Division, BARC and Professor in Homi Bhabha National Institute. He is the President of the Indian Desalination Association and sits on the Board of Directors of the International Desalination Association. He is the Chairman of the International Nuclear Desalination Advisory Group (INDAG) of the International Atomic Energy Agency (IAEA). He has been involved in providing consultancy services to several organisations on desalination and water puriﬁcation. He is the Co-Chairman of the Editorial and Scientiﬁc Committee of the International Journal of Nuclear Desalination, member of the Editorial Board of Desalination and water Treatment Science and Engineering and Editor in-chief of International Journal of Nuclear Hydrogen Production and Applications.