Advances in Nanostructured Membranes for Water Desalination

Advances in Nanostructured Membranes for Water Desalination

CHAPTER Advances in Nanostructured Membranes for Water Desalination 7 Madhuleena Bhadra and Somenath Mitra Department of Chemistry and Environmenta...

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CHAPTER

Advances in Nanostructured Membranes for Water Desalination

7

Madhuleena Bhadra and Somenath Mitra Department of Chemistry and Environmental Science, New Jersey Institute of Technology, University Heights, Newark, NJ, USA

7.1 Introduction .....................................................................................................109 7.2 Desalination technologies ................................................................................110 7.2.1 State of the art in RO......................................................................110 7.2.2 State of the art in MD .....................................................................111 7.3 Nanostructured membranes ..............................................................................112 7.3.1 Nanozeolite membranes ..................................................................112 7.3.2 Clay nanocomposite membranes ......................................................113 7.3.3 CNT membranes.............................................................................114 7.4 Application of nanostructured membranes .........................................................116 7.4.1 CNT membranes in RO ...................................................................117 7.4.2 CNT membranes in MD ...................................................................117 7.5 Commercial efforts to date................................................................................119 7.6 Future challenge of energy-efficient CNT membranes for desalination .................120 Acknowledgments ...................................................................................................120 References .............................................................................................................120

7.1 Introduction According to a report provided by the World Health Organization, 1.1 billion people lack access to adequate drinking water facilities. Furthermore, the anticipated 40 50% growth in the human population over the next 50 years, coupled with global industrialization and urbanization, will lead to freshwater shortage as a real threat. Since only 0.5% of the 1.4 billion cubic kilometers of water in the world is accessible as freshwater, the water scarcity will force us to use inferior quality and unconventional water sources such as seawater. Street, Sustich, Duncan and Savage. Nanotechnology Applications for Clean Water, 2nd Edition. © 2014 Elsevier Inc. All rights reserved. DOI: http://dx.doi.org/10.1016/B978-1-4557-3116-9.00007-X

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Membrane separation technology can address and meet some of the challenges in water shortage. Since their inception in the early 1960s, membranes have revolutionized many separation processes [1]. Membranes can proficiently remove specific particles and molecules from liquids, and this process has been used to develop several water treatment technologies. In the areas of seawater desalination and saline water use, the two membrane-based techniques are reverse osmosis (RO) and membrane distillation (MD). The main barriers to expanding these technologies are high-energy requirements and infrastructure costs. The key to economical development of these technologies is the creation of novel membrane materials with high water permeability, superior selectivity, flux, fouling resistance, and stability. In an effort to develop next-generation membranes with enhanced permeability, salt rejections, and low energy consumption, much effort has gone into both the synthesis of new membrane materials and new membrane architecture [2]. Of particular interest are the recent innovations in nanotechnology, where nanomaterials have been incorporated within membranes [2]. These could be successfully engineered for specific pore size, functionalities, physical, and chemical properties, thereby leading to the development of the next generation of desalination membranes. A variety of materials including carbon nanotubes (CNTs), zeolite, and nanoclay have been implemented in membrane structures for successful fabrication of nanostructured membranes. These nanomaterials have attracted significant attention due to their ability to display superior durability and separation characteristics. This chapter documents a review that intends to provide an in-depth insight into nanostructured membranes and their desalination applications by both RO and MD.

7.2 Desalination technologies 7.2.1 State of the art in RO In RO, desalination is achieved by applying high pressure to feed water, forcing it through a semipermeable membrane that permits the flow of water molecules, while restricting the flow of salt ions. The RO process requires pressure higher than the osmotic pressure, which increases with salt concentration [3]. RO is a growing seawater desalination technology [4,5] because the energy cost is lower than thermal distillation processes such as multistage flash distillation. Currently, RO plants account for 41% of the world’s desalination capacity and 80% of the number of operating plants. RO processes were initially developed using cellulose acetate membranes and commercialized in the 1960; however, the flux was low and they were subject to biological degradation. Current state-of-the-art RO membranes are asymmetric polyamide (PA) and thin film composite (TFC) membranes fabricated using interfacial polymerization. These membranes consist of a hierarchical structure with a thin (100 1000 nm) PA selective layer fabricated on a porous polysulfone layer that offers mechanical support and minimizes pressure drop.

7.2 Desalination technologies

While RO systems have already been commercialized, improving flux, salt rejection, and fouling resistance in a cost-effective way is required to advance this technology to meet future needs. It is anticipated that higher permeability membranes may enable operation closer to the ideal osmotic pressure, thereby decreasing the energy cost; alternatively, such membranes may reduce capital cost by requiring less membrane area for a given desalination capacity. However, optimizing flux, salt rejection along with other properties such as mechanical stability and fouling resistance requires careful membrane design. This is a major challenge because improving one parameter tends to adversely affect the others [6]. Advances in nanotechnology have enabled unparalleled control over the fabrication of size-selective membranes with pore sizes in the sub-nanometer region which allow water molecules to pass through, while impeding the passage of ions that have a larger effective diameter due to their hydration shells. For example, ˚ ; theoretically, if the pore diathe diameter of a hydrated sodium ion is B7.6 A meter is smaller than that of a solvated ion but larger than a water molecule, the pore could act as a molecular sieve. It is anticipated that nanomaterials such as CNTs can serve as size exclusion membrane for small ionic species such as Na 1 or Cl 2 . Additionally, simulation studies have also predicted that CNTs with diameter 0.34 nm with a negative charge will conduct K 1 ions and exclude Cl 2 , whereas positively charged CNTs with diameter of 0.47 nm will conduct Cl 2 and help in the possible exclusion of K 1 ions [7,8]. In summary, while significant advances have been made in polymeric RO membrane technology, new approaches need to be exploited to develop the next-generation membranes. In the following sections, we review the progress in the fabrication and understanding of the transport characteristics of these nanomaterials to achieve improved desalination by RO.

7.2.2 State of the art in MD MD is a thermally driven, membrane-based separation process which was introduced in late 1960s. In MD, hot aqueous solution is passed through the lumen of a porous hydrophobic hollow fiber [9]. While preventing the transport of the liquid phase, MD relies on the net flux of water vapor from the warm to the cool side of the membrane. Typically, MD is carried out at 60 90 C, which is notably lower than conventional distillation. Therefore, it has the potential to generate high-quality drinking water using only low-temperature heat sources such as waste heat from industrial processes and solar energy. In general, MD has received much attention as an alternative to thermal distillation and RO desalination of sea and brackish waters. Various types of MD have been known for the past several years: direct contact MD (DCMD), air gap MD (AGMD), sweeping gas MD (SGMD), and vacuum MD (VMD). These terms refer to the permeate side of the membrane. In all cases, the feed is in direct contact with the membrane. In DCMD, both sides of the membrane contact a liquid phase. The liquid on the permeate side is used as the condensing medium for the vapors. In AGMD

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the condensed permeate is not in direct contact with the membrane, in SGMD a sweep gas is used to remove the water vapor, and in VMD the permeate side has a vacuum. A key component in MD is the membrane itself because it determines both flux and selectivity. As of now the throughput of MD processes is relatively low, since two major factors were impeding its development, namely membranes with adequate characteristics and unfavorable economics of the process compared to RO. Recent developments convey the coupling of MD with waste heat and renewable energy-driven systems such as geothermal and solar energy to provide greater efficiency. Some advantages of MD include the ability to operate at lower temperature, lower pressure, and the capacity to handle higher brine concentrations. In conclusion, the structural design and chemistry of membranes engineered for MD are critical to achieve high performance. In an effort to address such issues, recent times have witnessed the development of novel architectured nanostructured membranes which have enabled unprecedented control for membrane fabrication that is of great importance to enhance the desalination performances.

7.3 Nanostructured membranes As previously mentioned, the two important membrane characteristics are flux and selectivity, which are typically controlled by physical and chemical characteristics, morphology, and presence/absence of pores. Typically, the assessment of permeability and selectivity has shown asymptotic limitations on the separation capability of conventional desalination membranes. Efforts to improve these have looked upon the development and incorporation of novel nanomaterials within the membrane structures in order to alter their structure and morphologies [2]. Recent advances have focused on incorporating nanomaterials such as nanozeolite, clay, and CNTs on the bulk membrane matrix or the surface, to enhance flux, selectivity, and fouling resistance for desalination applications.

7.3.1 Nanozeolite membranes In 2001, molecular dynamics simulations showed that zeolite membranes previously applied for gas separations may be applicable for aqueous osmotic separations as well. Since then, nanozeolite membranes have been studied extensively for RO desalination [10 12]. Typically, the incorporation of zeolite can improve the separation performance due to the combined effect of molecular sieving, selective adsorption, or ion exchange. In this context, Li et al. [13] used mordenite framework inverted (MFI) type zeolites depicted in Figure 7.1, on a porous α-alumina support to develop 0.3-µm-thick membranes for RO desalination applications. Under an applied pressure of 2.07 MPa (20.7 bar) and with 0.1 M NaCl as feed water concentration, the modified membranes rejected 76.7% of Na 1 ions while permitting a water flux of 0.112 kg/m2/h. The membrane was

7.3 Nanostructured membranes

0.54 nm × 0.56 nm b 0.51 nm × 0.55 nm

a

[h0h] c

FIGURE 7.1 Structure of the MFI zeolite [13].

also tested with real RO feed water and the resulting permeate flux and rejection was comparatively lower. The cause for this lower rejection was recognized as ion transport across nanometer-sized interstitial defects, which were created during the membrane synthesis process. At this point, it is somewhat difficult to establish the role that zeolites might have in water transport and salt rejection. Additionally, if salt can transport around the zeolite crystal, perfect salt rejection cannot be achieved. Hence, further research is desirable to determine the specific transport mechanisms within the sub-nanometer zeolite pores and in the fabrication of such membranes.

7.3.2 Clay nanocomposite membranes In addition to nanozeolites, nano clay offers interesting options for potential fillers for fabrication of nanostructured membranes. Prince et al. [14] carried out amalgamation of clay nanoparticles in polyvinylidene fluoride (PVDF) polymer solution, followed by electro spinning of the solution to produce a PVDF-clay nanocomposite membrane. Various concentrations (2, 4, and 8 wt%) of clay nanoparticles were incorporated into the nanocomposite membranes for DCMD applications. In their work, the authors observed that when the concentrations of the nanoparticles were increased, the hydrophobicity and melting point of the nanocomposite also increased. Highest water flux was observed for membranes containing 8 wt% clay. Further, the same group demonstrated that salt rejection of higher than 99.95% in DCMD process can be achieved by blending of clay in the nanocomposite nanomembranes when compared to control membrane of only 98.27%. This was due to prevention of pore wetting in clay composite membrane.

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In conclusion, their study confirmed that one of the serious issues of membrane pore wetting in MD can be reduced by blending of nanoparticles and further surface properties of the membrane can also be altered by the incorporation of nanoparticles within the membranes.

7.3.3 CNT membranes CNTs are an allotrope of carbon consisting of rolled up graphene sheets. There has been much interest in these materials because of their excellent adsorption, hydrophobicity, transport, separation characteristics, ease of functionalization, and fouling resistance. At present, few methods have been adopted to fabricate CNT membranes that could enable the development of a new generation of membranes whose permeation abilities, rejection properties, and robustness dramatically exceed those of conventional membranes for desalination applications. These fabrication approaches are documented in the following section.

7.3.3.1 CNT composite membrane The first CNT nanocomposite membrane to reject NaCl was designed by Shawky et al. [15]. A polymer grafting process was used for the synthesis of MWCNT PA composite membrane. In their study, the MWCNTs were initially dispersed in a solvent via ultrasonication prior to polymer grafting and membrane casting. Their method proposed that the addition of MWCNTs improved the rejection of both salt and organic matter relative to the 10% PA membrane base case. The nanocomposite membrane synthesized with 15 mg/g MWCNT in a 10% PA casting solution rejected NaCl and humic acid by factors of 3.17 and 1.67, respectively, relative to the unmodified PA membrane without MWCNTs, while membrane permeability decreased by 6.5%. More recently, Dumee et al. [16] established CNT buckypaper (BP) membranes by vacuum filtration of CNTs in a 99.8% pure propan-2-ol solution. Well-dispersed CNT solutions were obtained by repeated sonication at 15 min intervals. Following this procedure, they performed vacuum filtration with a 47-mm-diameter Millipore filtration unit with house line vacuum (95 kPa). The CNTs were filtered onto a poly (ethersulfone) (PES) 0.2 nm pore size Millipore membrane and then peeled off to form a self-supporting membrane for DCMD applications.

7.3.3.2 Aligned CNT membrane Recent novel fabrication techniques that permit the assembly of vertically aligned (VA) CNTs in membrane platforms are enabling the testing of their transport performance. The first prototype for a VA CNT membrane was introduced by Hinds’s research group [17]. After being grown on an iron catalyst using the CVD process, multiwalled carbon nanotubes (MWNTs) were embedded in polymeric filler composed of polystyrene (PS). Hinds and coworkers performed a series of supplementary pressure-driven flow experiments with the MWNT/PS membrane. They found that water flow rates increased four- to fivefold over those

7.3 Nanostructured membranes

of conventional fluid flow, which was estimated from the Hagen Poiseuille equation. Originally, the Hinds’s research group developed the CNT membrane as a chemically selective gatekeeper, which could separate different sized enzymes. Thus, they did not report ion selectivity, which is strongly related to desalination potential during the desalination process.

7.3.3.3 Interfacial polymerization synthesis of thin film nanocomposite membrane Kim et al. [18] developed a new type of thin film nanocomposite (TFN) membrane, where MWNTs were entrapped during the formation by phase separation (Figure 7.2) of the support layer, while nanosilver (nAg) particles were incorporated as a thin film layer via interfacial polymerization. Microscopic characterization confirmed that MWNTs and nAg particles were distributed in the support layer and the thin film layer, respectively. MWNTs at a concentration of 5.0 wt% in the support layer and nAg particles at 10 wt% in the thin film layer enhanced the pure water permeability of the n-TFN membrane by 20 25%. Increase in pure water permeability and hydrophilicity of the n-TFN membrane were attributed to the diffusive effect of nanopores in the MWNTs.

7.3.3.4 Carbon nanotube immobilized membrane Mitra and his group [19] have pioneered a novel architecture in producing CNTbased membranes for MD desalination. In their work, CNTs were immobilized within the pores of a commercially available membrane leading to the development of a unique membrane structure referred to as the carbon nanotube immobilized membrane (CNIM) (Figure 7.3). This was achieved by immobilizing CNT using dispersion in a polymer solution. Typically, the dispersion was injected into the lumen of a conventional hollow fiber under pressure. This served as the immobilization step, and the polymer served as the glue that typically held the Phase inversion casting

Acid-modified MWNTs

PSU polymer

MWNTs support layer

nAg Interfacial polymerization MWNTs support layer

FIGURE 7.2 Schematic representation of n-TFN membrane synthesis [18].

nAg thin film layer

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FIGURE 7.3 (A) Photograph of CNIM; (B) photograph of pure polypropylene [19]; (C) SEM image of unmodified polypropylene membrane; and (D) CNIM [19].

CNTs in place. Such membranes were robust, thermally stable, and possessed high selectivity. The goal here was to immobilize CNTs without covering its active surface with the polymer, or having a thick polymeric layer over it, so that their surface is free to interact directly with the water vapor. The membrane produced by this method has been used for other separations including extraction and pervaporation [20,21].

7.4 Application of nanostructured membranes The nanostructured membranes are fairly innovative developments when it comes to desalination technologies. Some applications that show a great deal of promise are presented here. In the membrane desalination field, the largest application has been with the application of CNT membranes in RO and MD processes. This is attractive because the CNTs are exceedingly hydrophobic with high aspect ratios, smooth surface structure, excellent adsorption, and separation properties, which enhance permeability and selectivity. Additionally, the functionalization of CNT tip can introduce the required physicochemical characteristics into the membrane

7.4 Application of nanostructured membranes

surface, which could lead to selectivity based upon physicochemical interaction of species with the functional group present over the CNT tip.

7.4.1 CNT membranes in RO Recently developed novel fabrication techniques that permit the assembly of VA CNTs in membrane platforms are enabling the testing of their transport performance for RO applications. The size and uniformity of tubes which is required to achieve a desired salt rejection for water desalination using RO have been studied in depth by Corry et al. [22]. They studied the transport of water and ions through ˚ using molecumembranes formed from CNTs ranging in diameter from 6 to 11 A lar dynamics simulations under hydrostatic pressure and equilibrium conditions. Ions face a large energy barrier and will not pass through the narrower CNTs studied ((5, 5) and (6, 6) “armchair” type tubes) but can pass through the wider (7, 7) and (8, 8) nanotubes. By measuring this conduction rate under a hydrostatic pressure difference, it was determined that membranes incorporating CNTs can, in principle, achieve a high degree of desalination far in excess of existing RO membranes. Molecular dynamics simulations to examine water and ion transport showed that CNT functionalization [23] can lead to improved membrane performance. A range of differently charged polar functional groups were added to a 1.1-nmdiameter (8, 8) CNT that was previously found to be only moderately effective at rejecting salt ions. These CNTs were incorporated into membranes and simulations and conducted with a hydrostatic pressure difference to determine the ion rejection and flux of water passing through each as well as the energy barriers presented to ions and water molecules. It was observed that the performance of these membranes in the simulations is still many times more efficient than existing technologies and thus the inclusion of functionalized CNTs in desalination membranes may prove to be useful in achieving enhanced salt rejection and rapid water flow.

7.4.2 CNT membranes in MD A novel desalination membrane process that also uses CNT-based membranes in MD is the CNIM [19]. It was postulated that the presence of CNTs serves as a sorbent to provide an additional pathway for solute transport. The CNTs favorably altered the water membrane interactions to promote vapor permeability while preventing liquid penetration into the membrane pores due to their high hydrophobicity. A significant increase in the vapor permeation was observed. Figure 7.4A schematically depicts the transport and separation mechanism of the MD process with the presence of CNTs. Compared to conventionally used MD membranes, the CNIM can facilitate the MD process at relatively lower temperature. It also exhibits greater acceptability for high saline concentration up to the

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(A)

Membrane Fast transport along CNT surface Activated diffusion via adsorption on CNT surface

Sample

Sweep air

Direct permeation through membrane pores Hydrophobic effect

Liquid water molecule

CNT

Water vapor molecule

(B)

2 µm

FIGURE 7.4 (A) Mechanisms of MD by CNIM made by immobilizing CNTs on polymeric membranes [19] and (B) SEM image of BP membrane surface made purely of CNTs [16].

equivalent of seawater. For a salt concentration of 34,000 mg/L and at 80 C, the nanotube incorporation led to a dramatic 1.85 times enhancement in flux. A parallel development is the use of BP CNTs as a membrane for desalination by MD. Dumee et al. [16] demonstrated that BPs made by self-supporting CNT BP as observed from an SEM image (Figure 7.4B) can be used as potential high-performance membranes for desalination in a DCMD setup with excellent salt rejection (99%) and a flux rate of up to 12 kg/m2/h at a water vapor partial pressure difference of 22.7 kPa. Additionally, they observed that increasing the hydrophobicity of CNTs may slow down the rate of crack formation and may reduce surface pore wetting and thereby lead to increased water permeability. Hence, they carried out a stepwise CNT modification, where the CNTs, after

7.5 Commercial efforts to date

treatment with ozone created hydroxyl and carboxyl functional groups, followed by reduction with LiAlH4 (to transform carboxyl groups into hydroxyl functional groups) and subsequent reaction of hydroxyl groups with silage groups. Following this, the CNT BP membranes fabricated with these modified CNTs showed increased hydrophobicity (contact angle of 140 compared to 125 for unmodified), thermal stability, and membrane limit entry pressure. The flux of the modified CNT BP membranes nearly doubled when compared to unmodified CNT BP. Additionally, the salt rejection of modified CNT BP membranes by DCMD was higher (97%) than that of unmodified CNT BP (95 97%). Overall, this study has suggested that life span, salt rejection and permeability of CNT BP membranes can be enhanced by silane functionalization, which may be further improved by fluoro silane modification in the future.

7.5 Commercial efforts to date Recently, a start-up company, NanoH2O Inc. (El Segundo, CA), has commercialized the next-generation RO membrane [24]. Based on nanostructured materials and industry-proven polymer technology, NanoH2O’s Quantum Flux membranes appear to significantly improve desalination efficiency and productivity. Additionally, they proposed a method to protect against chlorine damage with a protective layer including reactive nitrogen which forms chloramines on the surface of the membranes that reduce chlorine penetration. The protective layer additionally provides substantial antifouling capabilities, whether used with a chlorinated or unclorinated feed stream, due to the antibacterial properties of chloramines. It is anticipated that more such membranes will be developed that will move forward the field of nanostructured membranes for seawater desalination. On a similar note, recently, the excellent water transport properties of biological membranes have led to the innovative study of biomimetic nanostructured membranes incorporating aquaporins, which are proteins functioning as waterselective channels in biological cell membranes. Membranes incorporating bacterial aquaporin Z proteins have been reported [25] to show superior water transport efficiency and enhancements relative to conventional RO membranes. Aquaporins were incorporated into the walls of self-assembled polymer vesicles constituted of triblock copolymer, poly(2-methyl-2-oxazoline)-blockpoly (dimethylsiloxane)block-poly(2-methyl-2-oxazoline). An initial permeability test was carried out on the aquaporin-triblock polymer vesicles by stopped-flow light-scattering experiments. The results show at least an order of magnitude improvement in permeability compared to commercially available TFC RO membranes. Although a salt separation test has yet to be reported, extremely high salt rejection is expected from aquaporins since their functional biological performance is to only allow the passage of water molecules. Hence, they represent an ideal opportunity for the future production of ultrapure water.

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7.6 Future challenge of energy-efficient CNT membranes for desalination There is a growing need to improve desalination technologies as the demand for clean water mounts, energy costs rise, and natural resources diminish. As current desalination technologies reach maturity, nanostructured membranes will play an increasingly important role in realizing next-generation desalination technologies. However, this offers many challenges since nanostructured membrane applications require the balancing of many parameters such as composite formation, nanomaterial functionalization, and adequate mechanical strength. However, the incorporation of nanomaterials with various functionalities that can positively alter both selectivity and permeability is probably one of the most promising approaches to the development of the next-generation desalination membranes. The nanomaterials offer major advantages by providing uniform pore size, high permeability, higher thermal stability, targeted functionalization, and they can be the nucleus of additional physical chemical interactions such as enhancing the overall hydrophobicity, thus providing enhanced selectivity as well as flux. Considering all the above factors, much is to be done to move this technology forward to penetrate into the desalination market. For example, CNTs with a narrower distribution in pore sizes with an idyllic pore that might be less than 1 nm may be of great benefit. Regardless of all challenges, potential of nanostructured membranes is tremendous when it comes to generating higher flux than conventional membranes. Finally, there is always a question of economies of scale in real-world applications and process economics. In addition to this, the toxicity of the nanomaterials may also be an important consideration.

Acknowledgments All authors acknowledge the support from the Electric Power Research Institute.

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