Polymeric membranes incorporated with metal/metal ...

Desalination 308 (2013) 15–33

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Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l

Polymeric membranes incorporated with metal/metal oxide nanoparticles: A comprehensive review Law Yong Ng a, Abdul Wahab Mohammad a,⁎, Choe Peng Leo b, Nidal Hilal c,⁎ a

Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Malaysia School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai Selatan, Pulau Pinang, Malaysia Centre for Water Advanced Technologies and Environmental Research (C WATER), Multidisciplinary Nanotechnology Centre, College of Engineering, Swansea University, Swansea, SA2 8PP, UK

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a r t i c l e

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Article history: Received 3 October 2010 Received in revised form 16 November 2010 Accepted 18 November 2010 Available online 23 December 2010 Keywords: Membrane application Membrane fouling Metal nanoparticles Metal oxide nanoparticles Permeability Polymeric materials Polymeric membranes

a b s t r a c t Synthetic membranes have become the focus of separation processes in different industries. Synthetic membranes may be composed of inorganic materials (such as ceramics) and organic materials (such as polymers). Current research on membranes focus more on polymeric membranes due to better control of the pore forming mechanism, higher flexibility, smaller spaces required for installation and lower costs compared to inorganic membranes. Though polymeric membranes have these properties which make them better materials in membrane fabrication, they also have some disadvantages which need to be overcome. Common problems faced by polymeric membranes, such as high hydrophobicity, exposure to biofouling, low fluxes and low mechanical strength have become the focus of researchers in order to improve these disadvantages. The incorporation of nanoparticles into polymeric membranes has been the trend in the field of membrane research recently. Incorporation of nano-sized materials could produce synergistic effects when incorporated with different types of materials. This paper discusses a few types of nanoparticles incorporated into various types of polymeric membranes. Nanoparticles that will be discussed include silver, iron, zirconium, silica, aluminium, titanium, and magnesium based nanoparticles. Nanoparticles affect the permeability, selectivity, hydrophilicity, conductivity, mechanical strength, thermal stability, and the antiviral and antibacterial properties of the polymeric membranes. Though nanoparticles usually improve the performances of the membranes, they also might change or even deteriorate the performances of the membranes. Thus, careful study needs to be done in order to choose the most appropriate types and composition of nanoparticles to be incorporated into polymeric membranes. © 2010 Elsevier B.V. All rights reserved.

Contents 1.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1. Introduction to membranes . . . . . . . . . . . . . . . . . 1.2. Introduction to metal and metal oxide nanoparticles . . . . . 2. Polymeric membranes impregnated with a variety of nanoparticles . . 2.1. Membranes impregnated with silver-based nanoparticles . . . 2.2. Membranes impregnated with iron-based nanoparticles . . . . 2.3. Membranes impregnated with zirconium-based nanoparticles . 2.4. Membranes impregnated with silica-based nanoparticles . . . 2.5. Membranes impregnated with aluminium-based nanoparticles 2.6. Membranes impregnated with titanium-based nanoparticles . 2.7. Membranes impregnated with magnesium-based nanoparticles 3. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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⁎ Corresponding authors. N. Hilal is to be contacted at tel.: +441792606675, fax: +441792295676. Mohammad, tel./fax: +603 89216410/603 89216148. E-mail addresses: [email protected] (A.W. Mohammad), [email protected] (N. Hilal). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.11.033

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L.Y. Ng et al. / Desalination 308 (2013) 15–33

1. Introduction 1.1. Introduction to membranes A membrane can be defined as an interphase between two phases acting as a selective barrier. Low operation cost, relatively small footprint, and complicity with environmental regulations are the major benefits which polymeric membranes have over conventional counterpart technologies. Separation using membranes does not require additives, thus, it can be performed isothermally at lower temperatures with lower energy consumption as compared to other thermal separation processes. Besides, up-scaling and down-scaling of polymeric membrane processes as well as their integration into other separation or reaction processes are easier [1]. Membranes have been used in discontinuous mode such as dialysis, which aims to remove salts and low molecular weight solutes from solution [2,3]. However, there are some important criteria which are required in order for a membrane to be considered to be of high quality. A good membrane should be designed to have a higher filtration flux with stable flux and lower filtration pressure, to be space saving (higher filtration flux needs less of a footprint), and have a simple and highly reliable process based on the use of membrane, high quality of the water produced, and with thorough pretreatment unnecessary. Considering the large diversity of membranes suited for technical applications [1], it will be useful to introduce the membrane classifications as membrane materials, membrane cross-section, preparation method, and the membrane shape. Table 1 displayed the classification of the membranes in four different categories with their respective description. Researchers are focusing more on polymeric materials because of better pore-forming control and lower cost compared with inorganic materials. Polymeric materials have been used to fabricate membranes with different characteristics. Each of the polymeric materials have specific characteristic which make them suitable in different separation process. Polymeric materials with their respective characteristics were discussed further in another paper [1]. Polymeric membranes have many advantages, such as easy-forming properties, selectively transfer of chemical species, and the materials used are usually inexpensive. The use of polymeric membranes has increased in popularity recently due to their wide range of applications. For instance, polymeric membranes have been investigated for their performances in drug delivery development [4–6]. Besides, various polymeric membranes of different pore sizes have been studied for whey protein fractionation, including cellulose [7,8], polysulfone [9,10] and polyether sulfone (PES) [10,11] membranes on a laboratory scale. However, inorganic membranes are currently in competition with organic membranes for commercial use. Inorganic membranes are much more resistant to chemical attack, have good mechanical strength and a high tolerance of pH extremes and oxidation [12]. Inorganic membranes composed of metal oxides are gaining a presence in the marketplace due to their higher durability in many water purification applications [13]. Besides, increased conversions, better selectivity, milder operating conditions and decreased separation load are some of the attractive features which promote inorganic membranes as

Table 1 Classifications and description for membrane. Adapted from: [2]. Classifications Membrane materials

Description

Organic polymers, inorganic materials (oxides, ceramics, metals), mixed matrix or composite materials. Membrane cross-section Isotropic (symmetric), integrally anisotropic (asymmetric), bi- or multilayer, thin-layer or mixed matrix composite. Preparation method Phase separation (phase inversion) of polymers, sol–gel process, interface reaction, stretching, extrusion, track-etching, micro-fabrication Membrane module configuration Flat-sheet, hollow fiber, hollow capsule

chemical reactors in many established and novel reaction systems [14]. In many of the harsh operational environments, only inorganic membranes offer needed solutions. However, inorganic membranes are not used extensively because of the high costs and relatively poor control in pore size distribution. Besides, the effective membrane layer is very thick in comparison to the mean pore size, which results in a reduced flow rate. So, organic–inorganic polymer hybrids [15,16] constitute an emerging research field which has opened the possibility of tailoring new materials combining properties of inorganic glasses and organic polymers. Thus, incorporation of inorganic nanoparticles into the polymeric membranes has been considered as a way to make polymeric membranes more attractive to be commercialized. Regardless of the membrane materials used, the general aim of researchers and technology is to produce membranes with a high permeability, stable flux and excellent rejection of foulant materials. In order to obtain higher and more stable fluxes, one of the factors that needs to be examined is the pore sized formed by the polymeric materials. Filtration membranes having a screening pore size ranging from several angstroms to several micrometers (Fig. 1) can be obtained by forming a polymeric material into a porous body using the proper technique [17]. In an industrial use, the following two characteristics are important. First, pore size must be appropriate for the purpose of the separation. Smaller pore size is not necessarily better because it may be necessary to concentrate a valuable component (such as an enzyme) and also necessary to purify (separation of impurities that are larger than a valuable component) by permeating a valuable component. Furthermore, a narrow pore size distribution is also important. In addition, economic efficiency must be adequate to allow for the variations in pore sizes needed. Smaller pore size might require higher pressure to drive the permeation of the component; thus a higher energy and cost required. Membrane technologies are getting more and more attention nowadays due to their reliable contaminant removal without production of any harmful by-products, especially in water and wastewater treatment processes. In spite of that, the most common disadvantage associated with the application of the membrane process in water and wastewater treatment is membrane fouling, which results in flux decline during the operation [18]. There are several kinds of fouling which may occur in membrane systems, such as crystalline fouling, organic fouling, particulate and colloidal fouling, and microbial fouling [19]. Though after decades of development, particulate and colloidal fouling still remain the main reason for flux decline in the process of industrial wastewater treatment [20]. The consequence of fouling is a reduction of membrane performance, either due to the build-up of an additional barrier layer or due to a failure of the barrier, e.g. because the wetting-ability of a porous membrane in a membrane contactor had been increased. Other process conditions also have influence on the extent of fouling. Severe membrane fouling may require intense chemical cleaning or membrane replacement. As a result, operating costs of a treatment plant are therefore increased. Membrane fouling originates from the attachment of solutes onto the membrane surface or into the internal structure of the membrane. The fouling materials set up an additional barrier or block the membrane pores preventing the solvent from transporting through the membrane, hence raising the trans-membrane pressure and lowering the permeate productivity. Some fouling materials even destroy the membrane and shorten its service life. The main approach towards minimizing polymeric membrane fouling is the prevention of the undesired adsorption or adhesion processes on the surface of the membrane, because this will prevent or, at least, slow down the subsequent accumulation of colloids. There are various solutions available to overcome this disadvantage, such as pretreatment process installation, membrane surface modifications, ultrasonic entrenchment, chemical and physical membrane cleaning, and so forth. Physical cleaning techniques like relaxation (when no filtration takes place) and backwashing (when permeate is used to flush the membrane backwards) have been incorporated as standard operating strategies to limit the fouling [21]. However, membrane surface modification has become one of the most important fields in the


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Fig. 1. Pore size range of various membranes. Adapted from: [27].

research. It has been believed that membrane fouling could be reduced by developing more hydrophilic membranes [22]. In membrane surface modification, the techniques commonly used are blending [23], grafting [24], surface chemical reaction [25,26], and nanoparticle incorporation [27]. The presence of finely dispersed inorganic nanoparticles in the polymeric matrix has been proven to be very useful in the improvement of membrane performance for a wide spectrum of processes, ranging from gas separation and pervaporation [28,29], to nanofiltration and ultrafiltration [30–33]. 1.2. Introduction to metal and metal oxide nanoparticles As the size of a solid particle decreases in the order of one millionth of a millimeter, the number of atoms constructing the particle becomes small and in the order of several hundreds or thousands. At this state, the fundamental physical properties, such as the melting point, can change drastically and ceramic materials may be sintered at a lower temperature. Also, as particles get smaller than the wavelength of visible light, they not only become transparent but also emit special light by plasma absorption. They show completely different electromagnetic or physicochemical properties from their bulk counterparts, although they are made of the same materials. The definition of nanoparticles differs depending upon the materials, fields and applications concerned. In the narrower sense, they are regarded as the particles smaller than 10–20 nm, where the physical properties of solid materials themselves would drastically change. On the other hand, the particles in the three digit range of nanometer from 1 nm to 1 μm may also be called nanoparticles [34]. Particle size is the most important information in practical applications of powder-particles. Usually, powder is constituted by particles of various sizes and, therefore, it is necessary to obtain not only the mean particle size but also the size distribution. Recently, the methods for particle size analysis have been greatly developed, especially analytical techniques with rapid response, high repeatability and covering a wide range of particle sizes have been developed, as in the case of laser scattering and diffraction methods. Various kinds of nanoparticles, produced by different methods, are applied as raw materials in different fields. For example, for production of cosmetics, medical supplies, catalysts, pigments, toner and ink. Nanoparticle incorporation in the design and synthesis of new materials has been an area of intense research in recent years. Such materials have gained a widespread interest in many areas of science and technology due to their remarkable changes in properties such as mechanical [35], thermal [36], and magnetic [37] as compared to virgin organic polymers. Thus, one of the latest applications includes the incorporation of nanoparticles into polymeric membranes in order to increase the performances of the membranes such as permeability, selectivity, strength, and hydrophilicity. For instances, poly(vinylidene fluoride) membranes incorporated with silica nanoparticles can withstand higher temperature, higher selectivity and higher diffusivity [27]; chitosan/zinc

oxide nanoparticles membrane exhibited good mechanical properties and high antibacterial activities [38]; polysulfone membranes incorporated with silica nanoparticles exhibited enhanced gas permeability [39]; polyethersulfone/aluminium oxide membranes exhibited lower flux decline, higher porosity and pseudo steady-state permeability [40]; and polybenzimidazole/silica nanoparticles membranes showed increased permeability and selectivity in gas separation [41]. Some studies also indicated that the particles added into the polymer matrix might have a hope of stabilizing the polymeric membrane against change in perm-selectivity with temperature [42]. Incorporation of nanoparticles into polymeric membranes has some drawbacks. One of the limiting factors is the dispersion of the nanoparticles in the polymers. The aggregation/dispersion behavior control, which is the first process for the preparation of new functional materials incorporating nanoparticles, is very difficult for nanoparticles with less than 100 nm in diameter due to surface interactions. Examples of surface interaction between particles in liquid phase are shown in the Table 2. The surface interaction would take place when the nanoparticles in polymeric solution fulfilled the conditions as stated. Though researchers understand the surface interaction theories, but the factors that would contribute to enhance or further induce the agglomerations remains unclear. This causes difficulty in dispersing the nanoparticles during membrane fabrication. However, Yu et al. [43] suggested that the increment in concentration of nanoparticles could lead to an increase in nanoparticles agglomeration. Besides, Benjamin et al. [44] suggested that ionic strength and pH of the solution also would induce agglomeration between nanoparticles. 2. Polymeric membranes impregnated with a variety of nanoparticles Membrane fouling has usually been explained by pore blocking, cake formation, ligand exchange reaction, charge interaction, or

Table 2 Examples of surface interaction between particles in liquid phase. Adapted from: [31]. Surface interaction

Generation mechanism

Van der Waals interaction

Short-ranged electromagnetic force between molecule and/or atoms Electrical interaction by the overlap of electric double layer around particle in solution Short-ranged interaction by the overlap of adsorbed polymer layer on particles Formation of the bridge of polymer binder and/or surfactant between particles Overlap of hydrogen-bonded water molecule on hydrophilic surface on particle Negative adsorption of solute and polymer by having less affinity for the surface than the solvent

Overlap of electric double layer Steric interaction of adsorbed polymer Bridge force Hydration force Depletion

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hydrophobic interaction [45]. However, membrane material is frequently accepted as one of the predominant fouling modulators, with membrane fouling expected to be more severe with hydrophobic than hydrophilic membranes [46,47]. Thus, several strategies to alleviate the membrane fouling have been investigated. One of these methods is hydrophilic modification of membrane surface [48]. There are various methods available for hydrophilic modification of membrane surface [24,49–54]. Among these methods, incorporation of various kinds of nanoparticles into the polymeric membranes has been the focus of numerous investigations in recent years. Common preparation methods for the incorporation of nanoparticles into polymeric membranes can be simplified as follows. Firstly, casting solutions were prepared by dissolving a certain ratio of polymers and nanoparticles in the solvent [27,40,55–58]. Dispersant might be required in order to make the nanoparticles well-dispersed in the casting solutions [59–62]. Immersion of the glass plate into a coagulation bath of water at room temperature is required for polymeric membranes prepared by wet phase inversion method [63–70]. The unique chemical and physical properties of nano-sized metals as compared to their bulk particles have increased the studies of nanoparticle synthesis for specific optical, magnetic, electronic, and catalytic purposes. Polymeric systems have been used in the preparation of nano-scaled particles due to the presence of specific functional groups on the backbone of the polymer chain. These groups are often ionic in nature or have lone-pair electrons that can be served as a chelating agent as well as imposing a stabilizing effect on the synthesized nanoparticles. A typical nanoparticle containing atoms or molecules numbering from tens to tens of thousands has a length between a few and a few tens of nanometers. The introduced nanoparticles to polymer membranes might be silica [71], Fe3O4 [72], ZrO2 [73], TiO2 [74–77], CdS [78], and polymeric nanoparticles [79]. Each of these nanoparticles can be incorporated with most of the polymeric materials available in order to produce membranes with specific characteristics, as a result of the synergism properties between the polymeric materials and nanoparticles. Some of the specific properties of the nanoparticles listed above which incorporated into the polymeric membranes, would be discussed further in the following discussion. 2.1. Membranes impregnated with silver-based nanoparticles Biofouling of membranes including microbial release of extracellular polymeric substances (EPS) and the associated decrease in membrane flux increases energy costs and shortens membrane life [80]. Membrane fouling is usually controlled by pretreatment or chemical cleaning during backwash. Although pretreatment can be an effective form of biofouling control, many polymeric membranes cannot withstand the corrosiveness of chemical cleaners [81]. Incorporation of antimicrobial nano-materials into membranes offer an innovative potential solution to biofouling control [82–84]. Silver nanoparticles are well known as antibacterial materials to be incorporated into the polymer matrix of membrane. The antibacterial mechanism of silver, as reported, is related to its interaction with sulfur and phosphorous, most notably thiol groups (S–H) present in cysteine and other compounds [85]. Interaction of ionic silver (which can be released from nAg) with thiol groups and formation of S–Ag or disulfide bonds can damage bacterial proteins, interrupt the electron transport chain, and dimerize deoxyribonucleic acid (DNA) [86–88]. Similarly, the antiviral properties of silver ions involve the interaction with viral DNA and thiol groups in proteins [89]. Besides that, silver nanoparticles are believed to be able to perform well as selective barrier and facilitate transportation of certain components. This special property allowed the membranes incorporated with silver nanoparticles to be used in different applications to transport high value products. Silver ions and silver nanoparticles (nAg) have been studied for a wide variety of water treatment processes, including water filtration membranes. Nano-silver has been incorporated into cellulose acetate [90], polyimide [91], polyamide [92], and poly (2-ethyl-2-oxazoline)

[93] membranes. However, the long-term effectiveness in preventing biofouling by the incorporated nAg during continuous filtration has not been clearly addressed. Little research was conducted to determine properties of polysulfone membranes incorporated with nAg, which is notable for their widespread application in water filtration (microfiltration, ultrafiltration, or nanofiltration membranes). Thus, detail on the characterization and application of polysulfone membranes impregnated with silver-based nanoparticles reported by Zodrow et al. [81] could provide us a better understanding. The composite ultrafiltration membrane was fabricated using the wet phase-inversion process as reported by Mulder [94]. Silver nanoparticles (1–70 nm) were blended into polysulfone membrane by dispersing nanoparticles in the casting solution before dissolving polysulfone resin. 10% polyvinyl pyrrolidone (PVP) were utilized as pore former as a common practice in the phase-inversion process. Zodrow et al. [81] found that polysulfone membranes impregnated with 0.9 wt.% nAg possess similar permeability and surface charges compared with pure polysulfone membranes without nAg. However, the composite membranes were significantly more hydrophilic than pure polysulfone membranes, with 10% reduction in contact angle. The asymmetric structure of silver impregnated membranes was apparent. The addition of nAg does not visibly alter the membrane structure. Impregnation of nAg (0.9% by weight) significantly decreased the number of Escherichia coli grown on the membrane surface after filtration of a dilute bacteria suspension, as indicated by the number of colony forming units (CFU) per 9.35 cm2 membrane coupons. Though some of the properties of the polysulfone membranes were improved by the addition of silver-based nanoparticles, there were also some weaknesses which need to be overcome. By performing inductively-coupled plasma (ICP) and transmission electron microscopy (TEM) tests, Ag+ was determined to be leached out of the membrane. However, no silver was detected in filtrate with ICP after 0.31 L/cm2 of water was filtered. The membrane lost about 10% of total silver. TEM analysis of a concentrated solution of the filtrate revealed that the silver leached from the membrane predominantly in ionic form. Besides, it was determined that the Ag loss was mostly from the surface, the most likely location for membrane-bacteria and membrane-virus interactions [81]. Leaching of silver from the membranes caused the performances of the membranes to drop over the time. Its anti-biofouling properties would decreased and make it not suitable to be used over a long period of time. Besides, leaching of silver nanoparticles could pose the danger of water poisoning if the membranes incorporated with silver nanoparticles are used in drinking water purification processes. Silver nanoparticles are also incorporated into the membranes to facilitate the separation of olefin. Olefin separations are conventionally achieved by fractional distillation, which is highly energy intensive and moving bed adsorption. Sungpet et al., [95] described that oxidized poly(pyrrole), an electrically conducting polymer was found to be capable of changing the local electronic environment between the silver (I) ion and the counter ion as well as allowing silver (I) ions to form a complex with ethylene in the absence of water. Nafion-poly (pyrrole) composite membranes can be prepared by polymerizing pyrrole into a Nafion film using a solution containing pyrrole and hydrogen peroxide was prepared. To obtain sodium (I)-form or silver (I)-form membrane by ion exchange, the proton-form membrane was immersed in sodium hydroxide solution or silver nitrate solution. Liquid-phase olefin separations were performed in the perstraction mode at 25 °C [96]. Simultaneous complexation of olefin-silver (I) ionpoly (pyrrole) is postulated to be a necessary condition for facilitation. Although silver is postulated to be able to facilitate the transportation of certain components in this research, the researchers did not explained in details the condition or limitation in order for this facilitated transportation to occur. The researchers did not clearly state whether this facilitated transportation can only be done by silver (I) ions or any other metal ion with the same charges. Thus, there would be some room


for future investigation on the properties of different types of metal ions with the same charges, to facilitate in the separation processes. Currently, there are various silver compounds which are applicable to the preparation of composite membrane, such as silver tetrafluoroborate (AgBF4), silver triflate (AgCF3SO3), silver nitrate (AgNO3), and so on [97]. Types of silver compound nanoparticles preferred in the preparation of composite membranes are dependent on reversible complexation of silver ions with the chemical compounds (such as olefin) to be transported, which directly depends on the interactions of the silver with its counter anion and with polymer [98]. Research has been done to determine the performance of silver bromide nanoparticles as olefin carrier in membranes [99]. Materials used to fabricate the polymer membranes impregnated with silver bromide nanoparticles included: PVC, 4-vinyl pyridine (4VP), copper (I) chloride (CuCl), 1,1,4,7,10,10-hexamethyltriethylene tetramine (HMTETA), 1-bromohexane, silver p-toluenesulfonate (AgPTS), and 1-methyl-2-pyrrolidinone (NMP). Polymer membrane preparation was divided into three steps: synthesis of PVC-g-P4VP graft copolymer, synthesis of N-PVC-gP4VP, and followed by preparation of N-PVC-g-P4VP/AgBr nanocomposites. Since the precipitation takes place in the vicinity of the polymer chains, the growing AgBr nanoparticles are stabilized and prevented from aggregating by the capping action of the coordinating pyridine groups [100]. The membranes containing AgBr nanocomposites were also tested for the separation of olefin/paraffin mixtures. The incorporation of AgBr nanocomposites in the membranes resulted in an increment of mixed gas total permeance and mixed gas selectivity. In particular, both the permeance and the selectivity initially increased markedly with the increasing weight fraction of AgBr nanocomposites up to 0.1. The enhanced performance probably resulted from facilitated olefin transport due to the interactions of propylene with the partially positively charged surface of AgBr nanoparticles [101]. Furthermore, AgBr nanoparticles in the nanocomposite membranes have a great resistivity to acetylene, which is known to be a highly

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reactive compound with respect to metallic silver or silver ions, as reported [99]. Despite the extremely high separation performance of silver polymer electrolyte membranes, the long-term stability of their separation performance is in doubt because silver ions can be reduced to form silver metal nanoparticles and thus lose their olefin carrier activity. To improve the stability of the facilitated transport membrane, investigation has been carried out by Kim et al. [102]. It is learned that the electrolyte membranes consisting of poly(ethylene phthalate) and AgBr4 exhibit a much long-term stability in the separation of olefin/ paraffin mixtures than poly(2-ethyl-2-oxazoline)/silver bromide (POZ/ AgBr4) membrane. The stability is attributable to the suppression of silver ion reduction to silver metal nanoparticles. Besides silver-based nanoparticles, CuCl, which is insoluble in water, has been considered in the power form for olefin/paraffin separation [103,104] AgCl/poly(methyl methacrylate) (PMMA) membranes were prepared via microemulsion in situ polymerization using MMA as the oil phase and aerosol diisooctyl succinate (AOT) as the emulsion solvent. The reverse micelle structure, hybrid membranes morphology and the sorption behavior of cyclohexene and cyclohexene in hybrid membranes were investigated [105]. MMA micro-emulsion was prepared according to the reported methods [106]. Fig. 2 showed the TEM image of AgCl particle (the size is less than 50 nm) in MMA and scanning electron microscope (SEM) images of surface and crosssection of the AgCl/PMMA membrane. Cyclohexene has π electrons, which show stronger affinity to polar molecules and reversible coordination interaction between cyclohexene and carrier. Therefore, a polymer possessing polar or carrier groups facilitates the permeation of cyclohexene through membrane [107,108]. Polar membrane is suggested to be suitable for benzene/ cyclohexene separation since benzene π electrons have a strong affinity for polar molecules, which gives it a higher adsorption selectivity than cyclohexane [109]. Cyclohexene uptake in the hybrid membrane was higher than in the pure PMMA membrane. This demonstrates that in addition to the interaction between cyclohexene

Fig. 2. (a) AgCl/PMMA hybrid membrane surface; (b) AgCl particle in PMMA; (c) cross-section; (d) AgCl particle in MMA. Adapted from: [111].

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and polar groups in the membranes described above, specific interaction between cyclohexene and Ag+ ions must play a crucial role in cyclohexene sorption [110]. It has been shown that silver ions form coordination complexes with olefin double bond [93]. Incorporation of silver compound in PMMA showed its suitability to form a membrane with higher selectivity by forming complexes with olefins double-bonds. However, silver compounds are not only limited to those polymeric membranes discussed above. Silver compounds will have to be further investigated for their unique properties in different applications with different polymeric membranes. Thus, in other research works, silver compounds are incorporated into the polyimide membranes in order to investigate the properties of the membranes fabricated. Polyimides are popular membrane materials due to their good resistance to temperature and chemicals. Polyimides containing bulky CF3-groups show excellent film formation and separation properties [111,112]. In order to avoid any depletion of silver ions as reported by others, dry copolyimide membranes were used in which the metal ions are fixed in the cavity of crown ether. Examples of the copolyimide membranes are 4,4′-hexafluoro-isopropylidene diphthalic anhydride (6FDA) based copolyimides [113] and 3,5-diamino benzoic acid (DABA). Crown ether units of the 4′,4″(5″)-diamino-dibenzo-15-crown-5 (diamines 15-crown-5) were incorporated into the main chain of the polymer and/or were used as cross-linkers. Further incorporation of silver ions into the polymer structure leads to a considerable improvement in selectivity at the expense of permeability due to a loss in free volume. Higher selectivities were found for AgBF4-impregnated membranes compared to the AgNO3containing membranes, because AgBF4 shows a better solubility in the membrane materials. High selectivities were found in mixed gas permeation experiments, but they decreased rapidly with increasing feed pressure. At higher pressures, the selectivity of the membranes without silver ions was nearly reached. In long term studies at constant feed pressure a reduction of the separation factor was observed. Such a loss in selectivity is caused by chemical reduction of silver ions making facilitated olefin transport impossible. In membranes not containing crown ether units in their main chain, silver ions seem to penetrate deeper into the membrane material due to the larger free volume of the polymer. In this case the membranes are relatively stable. In nearly all membranes with crown ether units in their main chain, silver ions seem to penetrate only into layers near the membrane surface where they are reduced quickly [113]. Although there are a large number of advantages to incorporating silver nanoparticles into polymeric materials, researchers still doubt their safety in commercial use. Increasing use of silver nanoparticles necessitates a health and environmental risk assessment of nanoparticles [114]. Silver nanoparticles are highly reactive, which is one of the common characteristics that can be observed for any kind of nanoparticles. Application of polymeric materials incorporated with silver nanoparticles in drinking water filtration system may cause the leaching of silver nanoparticles into the drinking water. The leaching process may be caused by physical damage or improper nanoparticle incorporation techniques. Exposure to high levels of silver over a long period of time can result in a condition called argyria, a blue–gray discoloration of the skin and other organs. Lower-level exposure to silver is also known to cause silver to be deposited in the skin and other parts of the body [115]. Besides, in vitro investigation indicated that the silver nanoparticles caused DNA damage and apoptosis in mouse embryonic stem cells and fibroblasts [116]. 2.2. Membranes impregnated with iron-based nanoparticles Iron, the most ubiquitous of the transition metals and the fourth most plentiful element in the Earth's crust, is the backbone of our modern infrastructure. It is therefore ironic that as nanoparticles, iron has been somewhat neglected in favor of its own oxides, as well as other metals

such as cobalt, nickel, gold, and platinum. This is unfortunate, but understandable. Iron's reactivity is important in macroscopic applications (particularly rusting), but is a dominant concern at the nano-scale. Finely divided iron has long been known to be pyrophoric, which is a major reason that iron nanoparticles have not been more fully studied to date. This extreme reactivity has traditionally made iron nanoparticles difficult to study and inconvenient for practical applications. Iron however has a great deal to offer at the nano-scale, including very potent magnetic and catalytic properties. Recent work has begun to take advantage of iron's potential, and to be applied in membrane separation processes which will be discussed here. As described above, high reactivity of the iron metal causes it to be unsuitable for use as pure metal nanoparticles. Thus, in most cases, iron compounds instead of pure iron nanoparticles were incorporated into the polymeric membranes. Basically, the addition of the iron compounds nanoparticles leads to better performance of the membranes in their selected applications. One of the polymeric membranes incorporated with iron compounds is Nafion. Nafion is a sulfonated tetrafluoroethylene based fluoropolymer-copolymer. It is the first of a class of synthetic polymers with ionic properties which are called ionomers. Nafion has received a considerable amount of attention as a proton conductor for proton exchange membrane (PEM) fuel cells because of its excellent thermal and mechanical stability. Nafion based films are of interest as proton-conducting membranes in direct methanol fuel cells (DMFC). However, unmodified Nafion membranes generally possess high methanol permeability, which makes them not suitable to be applied in the current generation of commercial fuel cells [117]. Thus, incorporation of nanoparticles of highly acidic inorganic materials into Nafion membranes has been proved to be one of the effective approaches to reduce methanol permeability [118–124]. High surface acidity of incorporated nanomaterials would allow high proton conductivity to be achieved in the composite membranes, while blockage of the pores by the particles, reduces methanol transport. The sol–gel syntheses of inorganic phases (SiO2, TiO2, ZrO2) inside the pores of nafion membranes had been reported as an effective modification route to achieve high selectivities [124–127]. One of the modifiers in nafion membranes is zeolites. This is due to their inherent narrow pore size distributions, high surface acidity and high water intake [128]. In one of the current works, results on methanol transport properties of several nafion-composite membranes incorporating micro- and nano-sized particles of zeolites (Fe-silicate-1), amorphous silica and in situ crystallized Fe-silicalite-1 have been reported in comparison with an unmodified commercial nafion-115 membrane [117]. In preparation of the composites, supercritical carbon dioxide treatment of some of the membranes was used prior to incorporation of the inorganic phase. Two methods of deposition of zeolites such as deposition from colloid or suspension solution and direct in situ synthesis inside the pores of nafion membrane were used. Very low methanol permeability was achieved for composite membranes that were prepared using the colloidal intercalation route (from colloidal Fe-sillicate-1 as well as silica solution) and from in situ synthesis of Fe-silicate-1 inside the pores of nafion membrane. Supercritical CO2 activation of a nafion membrane prior to zeolites deposition was used to modify its structure. Thus, prepared nafionzeolite composite membranes showed a dramatic decrease in methanol permeability (if colloidal rather than suspended Fe-silicalite-1 particles were used for deposition), and 19-fold higher selectivity compared to either composite membranes prepared without previous supercritical treatment or pure commercial nafion-115 membrane. The method of in situ synthesis of zeolite inside the membrane pores was found to be well effective for preparation of composites, giving a six-fold higher selectivity for the composite membrane compared to pure nafion [117]. Besides its suitability for use with Nafion membranes to reduce methanol permeability, iron compounds are also incorporated into other polymeric membranes to produce proton exchange membranes.


Research work on fabrication of ion-conducting membranes by selfassembly of surface-charged nanoparticles was done by Gao et al. [129]. In that respective study, the researchers found that the membranes made from closely packed nanoparticles had a significantly higher proton conductivity compared to solution-cast films of similar composition and ion-exchange capacity (IEC). However, there was a limitation on maximum IEC, as high-IEC membranes exhibited excessive swelling in water which hindered testing for proton conductivity. Membranes with proton-conducting particles assembled in a suitable matrix can be designed to avoid such problems. The particles can be aligned to obtain the percolation needed for proton conduction, and the swelling in water or methanol can be controlled by choosing a water-resistant matrix. In a similar work, the synthesis of composite particles with sulfonated cross-linked polystyrene (SXLPS) for application in proton

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exchange membranes (PEM) for fuel cells was described [130]. The technique used for polymerization was similar to the mini-emulsion polymerization reported [131]. However, some modification to the procedure was involved to make crosslinked and functional polymeriron oxide composites. Also reported is the membrane fabrication process, which includes the alignment of synthesized particles in a high-performance sulfonated poly(etherketoneketone) (SPEKK) matrix [132], and properties of such PEMs for fuel cell applications. The final properties of the membrane depend on various factors, such as the IEC of the particles and the matrix and the size of particles. However, the main emphasis of that research was to demonstrate a useful membrane-fabrication technique which can be utilized to enhance the conductivity of the PEMs. As reported [130], the composite ion-conducting nanoparticles were synthesized by emulsion polymerization. The polymeric component of

Fig. 3. TEM images of the starting material γ-Fe2O3 and synthesized crosslinked polystyrene γ-Fe2O3 particles. Adapted from: [136].

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the particles comprised sulfonated crosslinked polystyrene and the inorganic component was γ-Fe2O3. A particle synthesis mechanism was proposed to explain the unusual morphologies of composite particles. The average diameters of the synthesized particles with different feed compositions ranged from 230 to 340 nm; and the distribution breadth increased with sulfonated content (Fig. 3). Using thermo gravimetric methods, it was determined that the particles contained about 75–80 wt.% of polymer. This was also confirmed by measuring the magnetic susceptibility. The synthesized particles could be easily aligned in SPEKK for PEM application. The proton conductivity of membranes with particles was less than plain SPEKK due to low IEC of the particles. However, the conductivity appeared to be improved with alignment. 2.3. Membranes impregnated with zirconium-based nanoparticles ZrO2 has been used to prepare PVDF ultrafiltration membrane [73]. The membrane performance can be properly varied by changing the PVDF solvent or the ZrO2 concentration ratio in the ternary suspension. However, addition of ZrO2 should be well controlled as the increment of ZrO2 concentration in PVDF membrane could lead to an increase in permeate flux at the expense of retention (Fig. 4). ZrO2 was also used in sulfonated polyetherketone to reduce water and methanol permeability. This property makes the zirconium dioxide and polyetherketone combination as one of the candidates to be used in fuel cell applications. The fuel cell has been developed as a promising alternative for energy conversion. The polyelectrolyte membrane fuel cell (PEFC) is particularly interesting for mobile applications and is currently an important research topic in all leading automobile industries. However, storage and delivery still provide complex problems. An alternative is the use of reformers to generate hydrogen from liquid fuels, such as methanol or gasoline. The reformer could be eliminated, gaining in technical simplicity and saving space, if methanol could directly feed the fuel cell. For the breakthrough of direct methanol fuel cell (DMFC) technology, suitable membranes with high proton conductivity and low water and methanol permeability are required [133]. Nafion membranes have been intensively used for fuel cells because they show high proton conductivity and chemical stability, but methanol permeability is too high. However, the critical aspect of Nafion is still its high-cost. Several non-fluorinated membranes, with potentially lower costs, have been tested for fuel cells. Sulfonated polysulfone, sulfonated polyether ether ketone, sulfonated polyphosphazane and sulfonated polyamides with good performance for hydrogen fuel cells are described in several reports [134–137]. However, the methanol permeability in many cases still relatively high. In one of the research works reported [133], the polymers chosen for the membrane preparation were sulfonated polyetherketone (SPEK) and sulfonated poly(ether ether ketone) (SPEEK). Inorganic

Fig. 4. Effect of additional amounts (g) of ZrO2 added to PVDF binary solutions (100 g) on the permeate flux and dextran 40 k retention of the unsupported membranes. PVDF solvent: NMP, ■ Flux, □ Retention; TEP, ● Flux, ○ Retention. Adapted from: [79].

Fig. 5. Methanol and water flux in pervaporation experiments at 55 °C by modification of an SPEK membrane with hydrolyzed TEOS. Adapted from: [140].

networks were generated in the organic polymer matrix by hydrolysis of tetraethoxy silane (TEOS) and of 1-(3-triethoxysilyl propyl)-4,5dihydroimidazole (I-silane). The inorganic modification with SiO2 led to a considerable decrease of permeability as shown in Fig. 5. It was also reported that there was a reduction in water and methanol permeability after modification with titanium oxide (TiO2) and zirconium oxide (ZrO2). Fig. 6(a) illustrates the considerable reduction of water and methanol permeability after the modification with TiO2. With this oxide, only little improvement was reached by

Fig. 6. Methanol and water flux in pervaporation experiments at 55 °C after modification of an SPEK membrane with (a) TiO2 and (b) ZrO2. Adapted from: [140].


the pre-treatment with 1,1′-carbonyl-diimidazole (CDI) and aminopropylsilane (AS). Distribution of ZrO2 into SPEK membranes strongly reduced the permeability of methanol and water as well (Fig. 6(b)). No additional improvement was observed using the pre-treatment with CDI and AS [133]. Besides, the distribution of the inorganic phase in the membrane was very homogeneous. The SEM of an SPEK membrane modified with 22 wt.% TiO2 is shown in Fig. 7. The well distribution of the nanoparticles reported were contributed by the method itself where the inorganic networks were generated in the polymeric matrix by the hydrolysis of TEOS and I-silane. It has been demonstrated that Nafion membrane modified by incorporating hygroscopic inorganic nanoparticles such as SiO2 and ZrO2 can improve water retention and enhance proton conductivity [126,138–146]. More recently, hybrid membranes of Nafion and mesoporous silica containing sulfuric acid groups using sol–gel process was synthesized [147]. Compared to standard Nafion 112 membrane, the hybrid membranes show improved proton conductivity at 95 °C and 120 °C and over the whole range of relative humidity. In another report [148], zirconia nanoparticles with the diameter of 6.3 ± 0.5 nm were in situ synthesized in Nafion solution by hydrolysis and condensation of tetrabutylzirconate (TBZ) precursor. Nafion solutions used in the study were prepared by dissolving Nafion resin in NMP, which was obtained by solvent evaporation of the purchased Nafion solution under vacuum at 60 °C. The desired quantity of TBZ solution in n-butanol was added drop wise to the Nafion solution under vigorous stirring in an inert nitrogen atmosphere at 80 °C. After the desired amount of 2 M HCl solution was added, the mixture was continuously stirred for 1 h at 80 °C and allowed to cool down to room temperature. After addition of desired amount of deionized water, the mixture was then continuously stirred for another 8 h to complete condensation of TBZ and a clear sol containing hybrid Nafion–zirconia nanoparticles was obtained. Unless otherwise stated, the final concentration of Nafion is about 2% in weight and the content of zirconia is about 5% in weight regarding to Nafion. TEM micrograph (Fig. 8) shows transmission electron microscopy image for Nafion–zirconia hybrid dispersion with initial Nafion content of 2% in weight in NMP. Samples for TEM measurement were prepared by directly placing a drop of the solution on a thin carbon film supported by a copper grid. It is evident that the in situ formed zirconia nanoparticles were uniformly distributed in the dispersion and the average diameter of the formed nanoparticles was about 6.3 ± 0.5 nm [148]. Fig. 9 adapted from past report [148] shows the FTIR spectrum of Nafion–ZrO2 hybrid membrane. For comparison, the recorded FTIR spectrum of a recast Nafion membrane was plotted. The appearance of a peak at 1016 cm− 1 in the spectrum of Nafion–ZrO2 hybrid membrane could be attributed to the vibrational mode of Zr–O, indicating the existence of zirconia nanoparticles. Other characteristic

Fig. 7. SEM of an SPEK membrane modified with 22 wt.% TiO2. Adapted from: [140].

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Fig. 8. TEM micrograph of hybrid dispersion of in situ formed zirconia nanoparticles in 2 wt.% Nafion solution. Adapted from: [146].

absorption peaks for zirconia in the spectrum of hybrid membrane cannot be clearly identified because of the overlap with absorption peaks of Nafion molecules. As all characteristic absorption peaks for Nafion were found without shift of wave numbers in the hybrid membrane, the addition of zirconia nanoparticles does not affect the crystallinity and the structure of Nafion molecules in the membrane. The existing Nafion molecules can be self-assembled onto zirconia particles through electrostatic interactions and prevent the further growth of the initial formed zirconia nanoparticles. The formed Nafion–zirconia hybrid membrane shows enhanced water retention ability compared to recasted pure Nafion membrane at elevated temperature, especially at high relative humidity. 2.4. Membranes impregnated with silica-based nanoparticles Silica nanoparticles have been investigated intensively and proven to be an ideal protein host, since they are chemically and thermally stable, they have large surface area, fine suspendability in aqueous solution, and are relatively environmentally inert [149]. In aqueous solution, the silica surface because of electrostatic stabilization promotes the dispersion of the nanoparticles, and thus it is considered as highly miscible. There are many biomolecules that can be

Fig. 9. FTIR–ATR spectra of Nafion–zirconia nanocomposite membrane (solid line) and recasted Nafion membrane (dash line). Adapted from: [146].

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conjugated to silica nanoparticles, including biotin–avidin, antigen– antibodies, peptides, proteins, and DNA [150]. Currently, a large number of conventional methods including gravity settling, dewatering, and incineration cannot efficiently treat emulsified and soluble oil [151]. Thus, membrane applications become an option for the concentration and clarification of wastewater containing oil. However, large-scale applications of the membrane separation process for wastewater containing oil are limited because polymer membranes are easily contaminated by oil. As a result, researchers are trying to modify the polymeric membranes such as polysulfone membranes, in order to make it more applicable to treat wastewater containing oil. Polysulfone (PSF) membranes are broadly applied in different fields at present due to their good physicochemical stability, resistance to oxidation and chlorine. But their surface energy is lower and their hydrophobicity is stronger, so their anti-fouling ability is weaker. Lots of methods to enhance their hydrophilic property are reported [33,152–154].

One of the most common methods is the doping of inorganic oxide particles to polymer to prepare organic–inorganic composite membranes. This method is intensively studied for its simple operating process and preparation technology. Some of the latest research [155] has been focused on Ce-doped non-stoichiometric nano-silica (modified nano-silica) which was prepared first, and then added to PSF to prepare novel composite membranes. However, it is important to note that the composition of silica nanoparticles in polysulfone is not varied for different compositions during this study. The compositions of silica nanoparticles were only fixed at 10 wt.% because the study was done purposely to differentiate between the effects of modified silica nanoparticles, unmodified silica nanoparticles, and without any addition of nanoparticles on the polysulfone membranes. Nanoparticles can increase the performances of membranes such as permeability, mechanical strength, and so on. However, higher weight percentage of nanoparticles in membranes might deteriorate the performances of membranes and cause losses. From the report [155], TEM examination confirmed that all particles with mean diameter of 30 nm are essentially spherical and the particle diameter distribution range from 20 nm to 50 nm. ESEM micrographs of the composite membrane sample adapted from other report [155] were shown in Fig. 10. The asymmetric structure of the cross-section in Fig. 10 indicates that the finger-like

Fig. 10. ESEM micrographs of the cross-section and surfaces of the composite membrane sample: (a) cross-section; (b) upper surface; (c) bottom surface. Adapted from: [163].

Fig. 11. (a) Pure water fluxes of membranes cast from different casting solutions & (b) Fluxes of membranes cast from different casting solutions in oil–water separation process. Adapted from: [163].


cavities start from the bottom surface towards the upper surface and that the upper compact skin layer which can hold back macromolecular matter is supported by a porous substrate. It may be concluded that the upper skin dominates the transport resistance of the composite membrane. In Fig. 11 which was adapted from other study done [155], the separation performances of membranes prepared from different casting solutions were compared. The average trans-membrane pressure (TMP) is 0.05 MPa and permeate should be collected for analysis within 90 min operating time. For modified nanosilica/PSF composite membrane, the pure water flux leveled off after 90 min is 120 L/(m2h) and the permeate flux leveled off after 90 min is 80 L/(m2h) in the process of oil–water separation. The addition of inorganic oxide nanoparticles causes an increase in tensile strength to some extent. The reason reported was that the free motion of polymeric chains is partly restricted by the intermolecular forces between the polymeric chains and the inorganic oxide nanoparticles dispersed uniformly in PSF and the tensile strength of membranes is sequentially enhanced. At the same time inorganic oxide nanoparticles are packed by polymeric chains twisting mutually, so the tensile strength of membranes is also improved [156]. Moreover, the membrane hydrophilic property can be enhanced by doping inorganic oxide nanoparticles. This is because there are hydrophilic hydroxide radicals on the surface of inorganic oxide nanoparticles [157]. Porosity analysis showed an improvement in membrane porosity by adding inorganic oxide nanoparticles. The improvement of the membrane porosity comes from the PSF-inorganic oxide nanoparticles boundary layer. In inorganic oxide nanoparticles, there are many defects which can act as tunnels of mass transfer. Meanwhile, the presence of inorganic oxide nanoparticles does reduce the crystallinity of PSF and increases the amorphous portion, thus the membrane porosity is augmented. Last but not least, it can be concluded that modified nanosilica/PSF membrane is not only resistant to fouling but also tolerant to tensile force and treatment of wastewater containing oil using the composite membrane developed is feasible and desirable. Besides the incorporation of silica nanoparticles into the polysulfone membranes, other polymeric membranes in other study also had been modified and some of them have been improved by the addition of silica nanoparticles. Another common polymeric membrane modified by silica nanoparticles is poly(vinyl alcohol) membrane. Poly(vinyl alcohol) (PVA) is a glassy, semi-crystalline polymer, and is used in many areas of science and technology, including membrane separation [158], drug delivery systems [159], artificial biomedical devices [160], and fuel cell electrolytes [161,162]. Unfortunately, the hydrophilic nature of PVA in water systems leads to instability of its chemical and mechanical properties, limiting its use in aqueous environments [163]. Many researchers have introduced cross-linking, blending, or grafting into PVA to enhance membrane stability and mechanical strength [160,164]. An alternative means to improve membrane stability and mechanical strength involves the incorporation of filler materials into the polymer matrix [165–167]. PVA composites containing inorganic fillers have been shown to withstand higher water concentrations and improved pervaporation selectivity and membrane thermal stability [165,166]. In addition, many studies have shown that polymericinorganic filler composites exhibit improved selectivity and high permeability in gas separation applications [168,169]. Usually chemical cross-linkers enhance mechanical and thermal stability but decrease gas and liquid permeation. The inorganic fillers, however, seem to increase membrane stability [170] while facilitating gas transfer. A further examination of the composite microstructure and crystallinity are therefore needed to elucidate the effect of nanoparticles incorporation on permeant transport properties. In one study it was reported [171], the free-volume increased due to the interruption of fumed-silica (FS) on PVA crystallization during film formation. The images of the composite with different magnification are shown in Fig. 12.

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Fig. 12. Field emission scanning electron micrographs of cross-sectional views on pristine PVA (a), PVA–FS nano-composites at 30,000× (b) and 100,000× (c) magnifications. Adapted from: [179].

Fig. 13 adapted from other report [171] showed the water vapor diffusion coefficient as a function of fumed silica content in PVA and PVA–FS composites. The increase of the relative fractional freevolume (FFV) was due to the increased number and the enlarged sizes of free-volume holes as FS concentration increased from 0 to 30%. This increased FFV facilitated water vapor diffusion into the composites. The pristine PVA had a fractional free-volume of 1.7%, which was typical for a glassy, semi-crystalline polymer. The FFV increased to 2.8% as the FS content was increased to 30% as shown by Fig. 14. 2.5. Membranes impregnated with aluminium-based nanoparticles Like other metal oxide nanoparticles, aluminium-based nanoparticles have properties which could increase the performance of

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Fig. 13. Water-vapor diffusion coefficients as a function of fumed silica content in PVA and PVA–FS composites. The dotted line is the predicted diffusivity using Maxwell's equation. Adapted from: [179].

membranes fabricated using them. The discussion below introduces the advantages of adding the nanoparticles into the polyvinylidene fluoride membranes. From this particular research work, it was also shown that the addition of aluminium-based nanoparticles into the polymeric membranes should be carried out carefully since excessive addition could lead to a decline in membrane strength. Polyvinylidene fluoride (PVDF) is a material which can form asymmetric membranes. This material can produce membranes with high permeability, high surface porosity, and good pore structure. PVDF is thermally stable and resistant to corrosion by most chemicals and organic compounds. PVDF membranes show outstanding anti-oxidation activities, strong thermal and hydrolytic stabilities, and good mechanical and film-forming properties. PVDF membranes can be used in ultrafiltration processes through various modifications [172]. However, there are still rooms for improvement to increase the performance of PVDF membranes. Improvements of the PVDF membrane can be made through physical blending, chemical grafting, and surface modifications. The blending of polymers has the advantage of easy preparation using phase inversion. Addition of hydrophilic materials to the dope solution increases the water permeability of a membrane with

Fig. 14. Effect of the FS mass fraction on FFV and the amorphous fraction (1-crystallinity) of the composites. Adapted from: [179].

similar pore size and pore distribution, due to an increase in pore density as well as in the hydrophilicity of the membrane surface and inside the pores [173,174]. Polymethyl methacrylate (PMMA) is an organic material that usually blended with PVDF to improve the membrane pore size distribution, pore structure, and enhances its permeation performance without a loss of retention [52,175]. In order to increase the performance of PVDF membranes, alternative methods have been used, such as using the addition of inorganic fillers. The addition of inorganic fillers has led to increased membrane permeability and improved control of membrane-surface properties [30,32,33]. Inorganic materials that had been used to be blended with PVDF included lithium salts [176], silica [32], and zirconium dioxide (ZrO2) [73]. In one study [172], PVDF UF membranes were modified by inorganic nano-sized Al2O3 particles. This research was intended to prepare Al2O3–PVDF composite membranes by using phase inversion method. This method included a small proportion of Al2O3 particles which effectively improved the membrane performance. The effects of the Al2O3-particle concentration in the casting solution on the membrane hydrophilicity, permeation flux, morphology, mechanical properties, and anti-fouling performance were examined. Al2O3– PVDF composite membranes were prepared by the phase-inversion method. Different Al2O3-particle concentrations (0–4%, by weight of PVDF) of polymer dopes consisting of PVDF (19%, by weight of the solution), dimethylacetamide (DMAC) (78%, by weight of the solution), and additives (1% sodium hexad-phosphate, by weight of PVDF; 3% polyvinyl pyrrolidone (PVP), by weight of the solution) were prepared. There were some important findings from this particular paper [172]. Membrane fluxes were affected by the addition of nano-sized Al2O3 particles. It was showed that increased Al2O3 concentrations had led to increased water permeate fluxes (as shown in Fig. 15), although this trend ceased when the quantity of the nano-sized Al2O3 particles reached a certain level. This finding can be interpreted as follows. PVDF is a hydrophobic polymer. Its hydrophilicity can be improved significantly by the addition of nano-sized Al2O3 particles, which have some favorable characteristics, such as hydrophilicity and higher ratio surface areas. Thus, the water fluxes are increased. However, with casting drops containing more than a certain concentration of nano-sized Al2O3 particles, the flux cannot be improved further as the nano-sized Al2O3 particles coalesce. In terms of both economy and practicality, 2% (weight) of nano-sized particles in the casting solution were found to be ideal. Fig. 16 shows SEM micrographs of the surface, cross-section, and inner porous-surface structures of the membranes PVDF-0 and PVDF2. Micropores were distributed on both membrane surfaces and there

Fig. 15. Distilled water fluxes of membranes prepared using dopes with different nanosized Al2O3 particles. Adapted from: [180].


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Fig. 16. SEM micrographs of the membrane surface, cross-section, and inner porous structures: (a, c, and e) PVDF-0 and (b, d, and f) PVDF-2. Adapted from: [180].

were no apparent differences between the modified and unmodified membranes. Both membranes showed typical asymmetric morphology with finger-like pores linked by sponge walls; the latter contained large numbers of micropores that allowed the finger-like pores to communicate with each other. These findings indicate that the addition of nano-sized Al2O3 particles did not affect the structures of the surface, cross-section, and inner pores. Therefore, the mechanism of PVDF membrane-structure formation was not altered by the addition of inorganic nano-sized Al2O3. Contact angle is another important parameter for measuring surface hydrophilicity [177]. As observed from the research, increasing the Al2O3-particle concentration caused a decrease of the contact

angle. However, the porosity, rejection, and molecular-weight cut-off values were not affected by the Al2O3-particle concentration. These results demonstrated that adding Al2O3 particles to PVDF polymer could improve its hydrophilicity, but did not affect the pores size or the number of pores. As a result, the flux through the composite membranes increased significantly. Fig. 17 illustrates that most Al2O3 particles were dispersed uniformly in the membrane, with the exception of a few large clusters that might have resulted from particles overlapping or from nanosized particles coalescing in the membrane. Fig. 18 shows that the permeation fluxes of alpha-amylase solution for the membranes declined as the filtration time increased, followed

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Fig. 17. Differential coefficient interference pattern of Al2O3-particle distribution in the modified membrane. Adapted from: [180].

by a long period at a steady value. The rate of flux decline, which was defined as the flux of the membrane at the time of consistent flux divided by the initial flux of the clear membrane, was no more than 27.4% for the modified membranes; the lowest value was 18.2% compared with 38.5% for the unmodified membrane. This shows that the composite membranes modified by Al2O3 particles had a more favorable anti-fouling performance. Moreover, adding an appropriate amount of nano-sized Al2O3 particles to a PVDF solution can improve the membrane's mechanical properties. However, an excessive amount of inorganic Al2O3 particles in the cast solution can cause the membrane elasticity to decline, thereby leading to a decrease in the membrane's elongation-at-break value. 2.6. Membranes impregnated with titanium-based nanoparticles Titanium dioxide (TiO2) has been the focus of numerous studies in recent years, because of its photocatalytic effects which decompose organic chemicals and kill bacteria [178]. Most of the work carried out focused on the use of TiO2 powders suspended in the water as a catalyst [179]. It has been applied to a variety of environmental problems in addition to water and air purification [180]. As discussed, PVDF membrane is a common material used in the fabricate membranes. However, it is readily contaminated by proteins and other impurities during water and wastewater treatment, which leads to a sharp drop in the membrane flux [181]. Chemical modification methods could be employed to improve the hydrophilicity of the membrane, but the main chain of PVDF molecule would be changed and the advantages of the PVDF membrane may be decreased [182,183].

Fig. 18. Permeate-flux decline with increasing time for the different membranes measured at 0.1 MPa trans-membrane pressure. Adapted from: [180].

Usually, physical modification method such as mixing was used, and the mixture materials were macromolecules [32,184,185]. When TiO2 nanoparticles are dispersed in PVDF membranes, the addition of nanoparticles not only improves the hydrophilicity of PVDF membranes but mitigates the biofouling problem of PVDF membrane and membrane bioreactor (MBR) systems [74,75] and the microbial biofouling of RO membranes [76]. In addition, the TiO2/ PVDF membrane could significantly increase the degradation rate of the phenylurea herbicide known as isoproturon [77]. In one of the recent studies [64], TiO2/PVDF ultrafiltration membranes were made using phase inversion method and characterized by the pure water flux and retention efficiency of blood serum albumin (BSA). The main purpose of this work was to make use of various methods to investigate and compare the effect of TiO2 nanoparticle dimensions on the performance and structure of the PVDF membrane. Smaller nano-sized TiO2 particles have better antifouling effects in the PVDF composite membrane. According to measurements using AFM and SEM, the TiO2/PVDF membrane with smaller nanoparticles have a smaller mean pore size and lower roughness on its surface and more apertures inside the membrane. X-ray diffraction (XRD) experiments also suggested that the smaller TiO2 nanoparticles had stronger effect on the crystallization of PVDF molecules. Due to vastly expanding populations, increasing water demand, and the deterioration of water resource quality and quantity, water is going to be the most precious resource in the world. Membrane technologies have made great progress, and commercial markets have been spreading very rapidly throughout the world. Among the desalination technologies available today, reverse osmosis (RO) is regarded as the most economical desalination process. Reverse osmosis (RO) thin-film-composite (TFC) membranes are receiving the increased attention for a variety of applications in water desalination, ultrapure water production, waste water treatment, and so on. It has been sa tandard practice to control biological growth in the feed water by the use of chlorine. Biofilm was observed not to form from water treated with disinfectant, such as chlorinated water [186]. However, chlorination generates harmful byproducts such as trihalomethanes and other carcinogens. The method of self-assembly of TiO2 on the surfaces with the terminal functional groups (for example, single-crystal silicon, quartz, and glass substrates) has been used to fabricate multilayer ultrathin films without requirements such as high temperature, solvent involvement, costly fabrication, and complex process control [187– 189]. The self-assembly behavior of TiO2 on polymer with COOH group is explained by two different adsorption schemes. One scheme was that TiO2 was bound with two oxygen atoms of carboxylate group via a bidentate coordination to Ti4+ cations. The other scheme was to form H-bond between carbonyl group and the surface hydroxyl group of TiO2 [190]. Thus, it is probable to self-assemble the TiO2 nanoparticles on TFC membrane surface. Fouling of RO membranes is markedly influenced by membrane surface morphology. The rougher surface and the larger surface area of TFC membranes make it possible to have contact with more water, which cause higher permeability [191]. However, surface roughness increases membrane fouling by increasing the rate of foulant attachment onto membrane surface [192]. Anti-fouling and fouling mitigation is essential to the flux-enhanced RO membrane. Besides TFC membrane, poly(4-methyl-2-pentyne) (PMP) membranes incorporated with titanium dioxide nanoparticles were also studied. PMP is an amorphous, di-substituted acetylene-based high free volume glassy polymer. It is one of the most permeable purely hydrocarbon-based polymers known [193]. The unique permeation property has been reported by several authors [168,194–198]. However, it has also been documented that the gas permeability is not stable over time, and that seems to be sensitive to processing history. PMP undergoes significant physical aging over time caused by the gradual relaxation of non-equilibrium excess free volume in glassy


polymers [168,196]. For example, nitrogen permeability coefficient in PMP has been reported to decrease by 25% over a period of 29 days [168]. Recently it was reported that the addition of nonporous fumed silica nanoparticles to PMP and poly(1-trimethysilyl-1-propyne) (PTMSP) increased their permeabilities with increasing filler content up to a level as high as 30 vol.% [168,194,199]. This behavior is contrary to the observed fact that permeability typically decreases with increasing filler loading in traditionally filled polymer systems. The increased permeability has been ascribed to increased free volume sizes in the polymer caused by the nano-sized fumed silica particles disrupting the packing of the polymer chains. In another study [200], cross-linking PMP with bis(aryl azide) had been shown to increase the chemical and physical stability. Crosslinked PMP membrane is insoluble in good solvents of PMP. Compared to the pure PMP membrane, the permeability of the crosslinked membrane is initially reduced for all gases tested due to the cross-linking (Fig. 19). By adding nanoparticles (FS, TiO2), the permeability is again increased. Permeability coefficients in PMP increase systematically with the addition of nanoscale nonporous inorganic filler particles,

29

such as fumed silica (FS) and TiO2. The increased permeability has been ascribed to increased free volume in the polymer matrix caused by the nanoparticles disrupting the packing of the polymer chains. The enhancement in permeability obtained for a given filler loading increases as the primary particle size of the filler decreases. This result is likely related to smaller particles yielding larger polymer/particle interfacial area, since at a fixed volume fraction of particles there are a large number of small particles per unit volume of nanocomposites, which gives them a greater capacity disrupt chain packing, thereby affecting transport property. In another study [201], the polysulfone membranes are modified with TiO2 nanoparticles and their properties are tested. Surface modification of TiO2 nanoparticles was performed to overcome their aggregation and to increase their dispersibility in the casting solution. The viscosities of casting solutions increase with the increase of TiO2 content, in particular, the viscosity is significantly increased over 2 wt.% TiO2 content. This fact can be interpreted in terms of the adsorption between the exposed hydroxyl groups at the surface of TiO2 nanoparticles with high specific surface area and surface energy and the polymeric chains [202].

Fig. 19. (a–e) The effect of nanoparticles (FS, TiO2) content on gas permeabilities and selectivities of crosslinked PMP membranes containing 2 wt.% HFBAA crosslinker; temperature: 35 °C, feed pressure: 2.0 bar. Adapted from: [207].

30


Fig. 20. SEM pictures of the morphology of PSF/TiO2 membranes with (a) 0 wt.% TiO2, (b) 1 wt.% TiO2, (c) 2 wt.% TiO2, (d) 3 wt.% TiO2, (e) 5 wt.% TiO2, and (f) e's local magnifying figure. Adapted from: [208].

The cross-section morphologies of membranes are shown in Fig. 20 (adapted from [201]), which illustrates that the macrovoids grow and become run-through at low filler concentration and then are suppressed or disappear at higher filler concentration (≥3 wt.%), the thickness of skin layer increases with the increase of filler concentration. These findings indicate that the addition of TiO2 nanoparticles has a large effect on membrane structure. So we can conclude that the mechanism of composite membrane is altered by the addition of inorganic fillers. In UF experiments, the influence of the addition of TiO2 on the permeability and the retention of membrane was examined. As shown in Fig. 21 (adapted from [201]), the permeability of the membrane initially increases and then decreases with the increase of TiO2 content and has a maximum of 488 L/h m2 at 20 °C when TiO2 content is 2 wt.%. The facts can be interpreted that the increase of the

membrane's porosity and hydrophilicity as well as the asymmetric and opened structure of membrane with 1–2 wt.% TiO2 content can attract water molecules inside the membrane matrix and promote them to pass through the membrane and accordingly enhance the permeability. However high filler concentration (≥ 3 wt.%) forms a highly viscous casting suspension, which slows down the formation process of PSF/TiO2 composite membrane and causes to form a thicker skinlayer and a symmetrical structure, resulting in a negative effect on the permeability. The presence of TiO2 in polymeric membranes resulted in decreased porosity of the membranes. Nanofiltration experiments showed that TiO2 nanoparticles are helpful in preventing the porous structure from collapsing and therefore, reduce flux decline. Incorporation of TiO2 nanoparticles into the membranes enhanced the hydrophilicity and mechanical strength of the membranes [55]. However, researchers concluded that TiO2 nanoparticles have serious deleterious effect on erythrocyte in a dose-dependent way in vitro [203]. 2.7. Membranes impregnated with magnesium-based nanoparticles

Fig. 21. Pure water flux and retention of PSF/TiO2 composite membranes as a function of TiO2 concentration. Adapted from: [208].

Membranes have been successfully developed for separation of gas molecules and they are widely used in chemical and petrochemical plants. Separation of hydrogen from its mixtures with nitrogen or hydrocarbons, nitrogen purification and CO2 removal from natural gas are recognized as major established and high value industrial applications of membranes. However, conventional polymeric membranes generally suffer from the trade-off limitation between productivity and selectivity which yet remains as one of the most important challenges for further developments in this field [204]. Nanocomposite membranes (also known as mixed matrix membranes) and facilitated transport membranes (FTM) are two distinguished and promising generations of gas separation membranes which have attracted considerable amount of attention for their potential use in gas separation and purification. The presence of affinity and interaction between magnesium oxide (MgO) surfaces and some gas species such as CO2 [205] provides a great motivation to investigate the potential application of this


31

Fig. 22. The comparison of gas permeability for pure Matrimid and nanocomposite membranes with different loadings of MgO nanoparticles (left: permeability of helium, hydrogen and carbon dioxide; right: permeability of oxygen, nitrogen and methane). Adapted from: [213].

material as carriers in FTM structure for separation of gas molecules. Flat nano-composite membranes were fabricated in another study by the introduction of magnesium oxide nanoparticles into the Matrimid polymer matrix [206]. The increment in the gas permeability as a function of particle content and the drop in gas selectivity reflected the fact that the average pore size of MgO is much larger than the kinetic diameters of gas molecules. Fig. 22 (adapted from [206]) exhibits the gas permeability of neat Matrimid and nanocomposite membranes with different MgO loadings. It can be seen that the permeability of nanocomposite membranes is higher than that of neat Matrimid membrane. Fig. 22 also shows a monotonous increase in permeability with increasing the MgO content for all the gases. The average increment in gas permeability is about 75% which is obtained for the membrane with 40 wt.% MgO loading. Clearly, the enhancement in gas permeability is mainly due to the incorporation of highly porous MgO nanoparticles which have substituted some portions of the dense structure of polymer chains in the membrane structure. In principle, the increase in permeability of nanocomposite membranes may be attributed to the presence of microvoids at the polymer–particle interface and/or the inherent transport properties of porous particles if encompassing pores larger than the kinetic diameter of gases. 3. Conclusion For applications where low-cost filtration would be highly desirable, polymeric materials are the ideal choice for membrane fabrication. Thus, it is worthwhile to investigate further the performance of polymeric membrane with different ways of modification. This paper discussed various combinations of polymeric materials and nanoparticles which were fabricated by other researchers and their performances were tested and reported. As discussed, there are wide varieties of polymeric materials which are suitable to be used in membrane separation processes either liquid or gas separation. However, the performance of pure polymeric materials as membranes may not be ideal for industrial use. Thus, modification by adding nanoparticles to the polymeric materials is reported to have more advantages over pure polymeric materials alone in most cases. Polymeric membranes modified by adding nanoparticles might increase in permeability, lowering the fouling, higher tensile strength, higher selectivity of certain components, better performance in wider temperature and pH range, and higher diffusion rate. Silver and titanium-based nanoparticles are ideal for incorporation into the membranes in order to reduce the biofouling of the polymeric membranes. Silver nanoparticles are also involved in facilitated olefin transportation which has been discussed above. However, incorporation of silver nanoparticles has been said to be able to reduce the void

volume thus permeability of the membranes fabricated. For iron-based nanoparticles, they have been proved to be useful in proton exchange membranes which could be used in fuel cell applications. Iron-based nanoparticles have the same properties as exhibited by zirconiumbased nanoparticles. Both of these kinds of nanoparticles could be useful in proton exchange membranes in fuel cell application. Besides conductivity, it was also reported that an increase in ZrO2 concentration leads to an increase of the permeate flux at the expense of retention, which property is similar to that exhibited by magnesium and aluminium-based nanoparticles. For silica nanoparticles, it has been proved that the silica nanoparticles embedded in membranes may help to produce gases free of impurities. Because of the ability of nanocomposites to trap molecular-sized impurities, silica nanoparticles could be further used in processes such as environmental remediation, sea water desalination, and petroleum chemicals and fuel production. Further investigation of the properties of various polymeric membranes incorporated with nanoparticles should be carried out in order to find the most appropriate combinations and applications of the membranes fabricated. In drinking water application, nanoparticles used should be handled carefully due to the potentially toxic properties exhibited by the nanoparticles. For instance, silica nanoparticles are more suitable to be incorporated into the membranes which will be used in drinking water application because silica exhibits lower toxicity and is environmentally inert. Besides, the amount of nanoparticles and polymeric materials should be optimized in order to produce more costcompetitive and higher performance membranes in the future. Acknowledgment The authors wish to express their gratitude for the financial support from the UKM-GUP-KPB-08-32-129 and 02-01-02-SF0529 Grants. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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