Behaviors and Effects of Differing Dimensional Nanomaterials in

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Behaviors and Effects of Differing Dimensional Nanomaterials in Water Filtration Membranes through the Classical Phase Inversion Process: A Review Panpan Wang,† Jun Ma,*,†,‡ Fengmei Shi,† Yuxin Ma,‡ Zhenghui Wang,† and Xiaoyu Zhao† †

School of Municipal and Environmental Engineering, Harbin Institute of Technology, Harbin 150090, P. R. China National Engineering Research Center of Urban Water Resources, Harbin 150090, P. R. China

‡

ABSTRACT: A polymer membrane has been developed as a competitive technology in the field of water purification and treatment. The coupling of nanomaterials and membrane technology might inspire some interesting and extraordinary harvests in the water-treatment application of polymer−nanomaterials composite membranes. We discuss the behaviors of different types of nanomaterials in the process of classical phase inversion and the resultant effects on the structure−performance relationship of an as-prepared polymer−nanocomposite membrane in water treatment. It is very necessary to sort and compare the current complexities and controversies in this field. The further development of polymer−nanomaterial composite membranes in water treatment is also pointed out and discussed. coating10,11) inevitably influences membrane chemistry and pore morphology as a post-treatment, while blending with macromolecular/polymeric additives does not improve the mechanical performance of polymer alloy membranes.12,13 An emerging technology, polymer-nanomaterial composite membranes (i.e., mixed matrix membrane, MMM), dispersing nanoscale fillers into the large polymer matrix, has been attempted to solve some of the issues of permeability, selectivity, fouling, and mechanical strength in the application of water treatment. The present review discusses the behaviors and effects of differing dimensional nanomaterials in water filtration membrane through a classical phase inversion process, except for the interfacial polymerization process of dense skin layers in nanofiltration, reverse/forward osmosis membrane.

1. INTRODUCTION Pollution and scarcity of fresh water around the world, accompanying increasing population and stringent standards of drinking water or wastewater discharge, make membrane filtration emerge as a competitive alternative in future water treatment processes.1,2 Membrane engineering has been already applied to solve some major industrial problems. Especially in water engineering, membrane reactors (MBR), various kinds of membrane operations (MF, UF, NF, RO, FO, MD etc.), and integrated membrane systems play a dominant role in the field of municipal water treatment and water desalination.3,4 As we know, membrane materials and modules are undoubtedly the core components of prospective green membrane engineering characteristics of low footprint, high efficiency, and modularity. Polymeric membranes, mostly adopted in membrane engineering, have been already dominating the academic field and commercial market since the invention and development of asymmetric cellulose acetate RO membranes by Loeb and Sourirajan.5,6 Good membrane-forming (flexibility) and excellent physicochemical properties contribute to the great popularity of polymeric membranes in water treatment. Unfortunately, inherent hydrophobic surfaces of polymeric membranes are apt to adsorb or block organic foulants (NOMs, pathogens, polysacchrides, and proteins, etc.) from contaminated water, which in turn decreases membrane permeation flux and utility efficiency. Meanwhile, the breakdown of membrane fibers caused by high pressure or long-term operation deteriorates the water quality of permeates in engineering applications. Therefore, the organic/biofouling resistance and mechanical strength of polymeric membranes, depending on surface chemistry and structures, remain a significant obstacle for the further development of membrane applications.1 Hydrophilic modification of polymeric membranes has become an accepted solution to improve membrane fouling resistance in water treatment.1,7 Among a tremendous amount of effort, surface modification (physical treatment,8 grafting,9 © XXXX American Chemical Society

2. SELECTIVITY AND PROPERTIES OF TYPICAL NANOMATERIALS Polymer−nanomaterial composites, consisting of different couplings of polymers and inorganic nanoparticles, are promising systems for varieties of applications.14 A lot of combination moieties (polymers and nanomaterials) and methods (in situ and ex situ) contribute to the extraordinary properties and considerable applications of these polymer− nanomaterial composites. Polymer−nanomaterial composite membranes (MMMs) have been widely used in gas separation.15,16 Therein, different inorganic nanoparticles (TiO2,17 SiO2,18 MgO,19 CNTs,20 clays,21 zeolites22) could create void spaces or gas channels within the dense nanocomposites, which modulate gas permeability and selectivity through the MMMs. The gas Special Issue: Enrico Drioli Festschrift Received: November 29, 2012 Revised: February 4, 2013 Accepted: February 6, 2013

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dx.doi.org/10.1021/ie303289k | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 1. Effects of TiO2 dosage on the PVDF membrane morphologies, permeability and mechanical strength (SEM photographs of PVDF/Ti0.3). Reprinted with permission from ref 27. Copyright 2012 Elsevier.

resistance of polymeric membranes. On the other hand, carbon nanotubes could improve the mechanical strength of polymeric membranes in the application of water treatment. 2.3. Exfoliated 2D Nanomaterials. 2D silicate sheets (clays) are also adopted to improve the hydrophilicity/ hydrophobicity and mechanical strength of water filtration membranes, which mostly were exfoliated in gas separation and proton exchange membranes. 2.4. Porous 3D Nanomaterials. 3D zeolites are microporous, crystalline alluminosillicates with well-defined nanoscale pore structures in their regular frameworks. The hydrophilicity and permeability of 3D zeolites facilitate the permeability/selectivity and fouling resistance of ultrafiltration and reverse osmosis membranes.

transport and composite density (free volume) exhibit a strong dependence on nanoparticle impermeability, loading, dispersion, and interaction with the polymer matrix. Similarly, the inorganic nanoparticles may also give some changes to liquid separation of membranes. Recently, the polymer−nanomaterial composite membrane (MMM) technology has introduced a new concept to prepare excellent water-treatment membranes in pursuit of high water production, proper solute rejection, and low fouling propensity.1 Obviously, the selectivity and properties of typical nanomaterials are the first step of importance to fabricate polymer−nanomaterial composite membranes. Like a “blackbox”, there are varieties of nanomaterials adopted in the published works. What are the standards of nanomaterials incorporated into polymeric membranes? These inorganic nanoparticle fillers that are commonly adopted refer to varieties of dimensional nanoscale (0D, 1D, 2D, 3D) materials. Whether nanocomposite membranes could achieve one or two of the performance enhancements, such as balance of permeability and selectivity, fouling, and mechanical strength, becomes the significant standard of nanomaterials incorporated into polymeric membranes for water treatment. 2.1. Hydrophilic 0D Nanomaterials. 0D nanoparticles (TiO2, Al2O3, ZrO2, SiO2, ZnO, Ag0, etc.), consisting of metal oxides and metals etc., are usually introduced into a polymer matrix to form microfiltration/ultrafiltration membranes for organic/biofouling reduction, due to the characters of surface hydrophilicity, ion release, or catalytic effects. 2.2. Channel-Possessing 1D Nanomaterials. In addition to the hydrophilicity of nanoparticles, water transport channels of 1D nanotubes (single/multiwall carbon nanotubes, titania nanotubes, etc.) also facilitate the permeability and fouling

3. STRUCTURE AND PERFORMANCE OF NANOCOMPOSITE MEMBRANE In general, different incorporation routes (in situ and ex situ) and positions (surface and all-through) of nanomaterials dispersed in a polymeric matrix have been adopted to fabricate polymer−nanomaterial composite membranes in the form of microfiltration, ultrafiltration, nanofiltration, reverse osmosis, etc. Commonly, polymer−nanomaterial composite membranes are always prepared through either a phase inversion process or interfacial polymerization process, in which inorganic nanomaterials or other additives are involved. Our group has already made some attempts and efforts to disperse different dimensional nanomaterials (0D, 1D, 2D, etc.) into engineering polymers to form water-treatment membranes.1,23−29 Owing to unique properties, the coupling of nanomaterials and membrane technology is gaining more and more attention in the academic and industrial fields. Therefore, B



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these catalytic PVDF/TiO2 hybrid membranes were badly needed performances to further the highly efficient application of membranes in water treatment. Compared to the high-performance PVDF membranes through the TIPS method, the conventional phase inversion via immersion precipitation method still remains a convenient and promising way to produce high-efficient polymeric membranes in water treatment. Considering the susceptibility of PVDF membranes in alkaline solution, Li et al. demonstrated that some basic Al2O3 nanoparticles could directly bond PVDF chains through acid-catalyzed grafting reactions between them, due to the formation of PVDF conjugated double bonds in the process.30 As a result, the hydrophilic PVDF/Al2O3 complex could be segregated onto the surfaces of as-prepared PVDF membranes in the process of nonsolvent induced phase inversion (NIPS), which in turn improved the surface hydrophilicity and antifouling performance of PVDF/Al2O3 nanocomposite membranes. Interestingly, Li et al. also prepared PVDF hollow fiber membranes with the additives of SiO2 nanoparticles through a conventional immersion precipitation method, after which the SiO2 nanoparticles dispersed in the membrane matrix were completely removed by either the basic washing (NaOH) or acid washing (HF) of the post-treatment process.31 As a result, they could gain high-flux PVDF hollow fiber membranes with unaffected mechanical strength and higher ductility through a 10 min washing post-treatment of HF acid solution. However, we wonder whether it is enough to incorporate nanomaterials only into the casting system of the polymer solution. Nanomaterials and Dispersants/Porogens. On the basis of plenty of experimental results, our group found that nanomaterials were not hydrophilic enough to induce or develop pores during the wet phase inversion process, in which the nanoparticles were apt to aggregate to each other. Therefore, varieties of dispersants or porogens (pore forming agents) were involved in the casting system of the polymer solution. Ma et al. introduced TiO2 nanoparticles of different sizes into a PVDF casting solution to fabricate ultrafiltration membranes through a nonsolvent phase separation (NIPS) method.24 In the presence of poly(ethylene glycol) (PEG) as a pore-forming agent, smaller TiO2 nanoparticles (10 nm) could enhance the antifouling performances of PVDF membranes more obviously, due to the crystallization changes of PVDF and smaller mean pore sizes on membrane surfaces. To avoid agglomeration of nanoparticles, Razmajou et al. prepared PES/TiO2 ultrafiltration membranes in the presence of polyvinylpyrrolidone (PVP K40) as the porogen through a phase inversion process, after the mechanical and chemical modification of TiO2 nanoparticles.32 A high flux recovery ratio and much lower TMP during filtration with bovine serum albumin (BSA) demonstrated the enhanced antifouling properties of a PES membrane with low TiO2 loading. Yan et al. prepared polyvinylidene fluoride (PVDF) ultrafiltration membrane incorporated with Al2O3 nanoparticles in the precence of hexadisodium phosphate as the dispersant and PVP as the porogen.33,34 The addition of Al2O3 nanoparticles did not affect the membrane pore structures, but only improved the hydrophilicity of the resultant membranes and consequently the antifouling performance of membranes was enhanced. Interestingly, Ma et al. pointed out that a novel pure spongelike ultrafiltration membrane bearing both high water permeability and high retention capacity could be fabricated

it is greatly necessary and significant to sort and compare different coupling methods adopted in the published works, through which we attempt to primarily reveal the “black box” of polymer−nanomaterial composite membranes in water treatment. On the basis of the unique properties above-mentioned, how do nanomaterials change the structures, morphologies, and performances of polymeric membranes that are prepared from these polymer−nanomaterial composite casting solutions? Whether these systems of polymer−nanomaterial composite casting solutions need other additives has been always controversial in recent research. 3.1. Polymer-0D Nanomaterial Composite Membrane. Nanomaterials Only. Focusing on 0D nanomaterials, Ma and co-workers introduced TiO2 nanoparticles into the diluted solution of PVDF/dimethal phthalate (DMP) to form isotropic microfiltration membranes through a thermally induced phase separation (TIPS) method.27 They found that TiO2 nanoparticles, as heterogeneous nuclei, could disturb the formation of PVDF spherulitic crystals in the quenching process, which facilitates the fabrication of microfiltration membranes with high porosity and mechanical strength. Figure 1 showed the structures, pure water flux, and mechanical strength of PVDF membranes with different additions of TiO2 nanoparticles. All of the membranes possess isotropic cross-sectional structures of PVDF spherulites and dispersed TiO2 nanoparticles. Proper dosage of TiO2 nanoparticles could effectively enhance membrane permeation, tensile strength, and elongation at break, which is necessary to prevent a breakdown of water filtration membranes in the engineering application. To improve the compatibility between PVDF matrix and TiO2 nanoparticles, Ma et al. reported that the novel PVDF microfiltration membranes, hybridized with ionic liquid modified TiO2 (IL-TiO2) nanoparticles through a sol−gel process, had higher pure water flux and antifouling properties at lower contents of addition.29 Of great importance, they found that the antifouling properties of PVDF membranes were achieved by the high-efficient cleaning of hydroxyl radicals from TiO2-catalyzed H2O2 process. As depicted from Figure 2, the permeation flux of the PI-0.45 membrane (IL-TiO2 content in PVDF membrane is 0.45 wt %) could almost completely recovered after a 30 min cleaning with H2O2 aqueous solution. The low fouling propensity and rapid cleaning efficiency of

Figure 2. Comparison of the permeation flux recovery of PVDF and catalytic PVDF/TiO2 hybrid membrane (PI-0.45, i.e., 0.45 wt % ILTiO2 nanoparticles in PVDF membrane) based on the cleaning process of H2O2 aqueous solution. Reprinted with permission from ref 29 . Copyright 2013 Elsevier. C



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from a composite casting solution of PVDF and SiO2 nanoparticles through a nonsolvent-induced phase inversion (NIPS) process.35 In the presence of FeCl2 or CaCl2 as the porogen, the addition of SiO2 nanoparticles could increase the viscosity and thermodynamic stability of a casting solution and in turn lower the mutual diffusion/exchange between solvent and nonsolvent in the NIPS process. As a result, the pure sponge-like structures through the whole membrane (shown in Figure 3) were developed dependent on controlling the phase

the fouling propensity through well-dispersed hyperbranched poly (amine-ester) functionalized multiwalled carbon nanotubes (MWCNTHPAE).28 As shown in Figure 4, the surface

Figure 3. Cross-sectional morphologies of PVDF/SiO2 nanocomposite ultrafiltration membranes and their counterpart membranes in the presence of different porogens (FeCl2 or CaCl2) through nonsolvent induced phase inversion (NIPS) process. Reprinted with permission from ref 35. Copyright 2011 Elsevier.

Figure 4. Dependence of water permeability and fouling propensity (BSA solution, 1 mg/mL, pH 7.0) of PVDF/MWCNTHAPE nanocomposite ultrafiltration membranes on the content and dispersed state of MWCNTHAPE in the casting solvent (1.5 mg/mL), dimethyl formamide (DMF). Reprinted with permission from ref 28. Copyright 2012 Elsevier.

inversion process, which possessed higher water permeability and selectivity over their macroporous counterpart membranes without SiO2 nanoparticles. 3.2. Polymer-1D Nanomaterial Composite Membrane. Nanomaterials Only. Carbon nanotubes are the typical representative of 1D nanomaterials, which used to being filled in the gas separation membrane 36 and ion exchange membrane37 etc. In the field of water filtration membrane, Choi et al. introduced multiwalled carbon nanotubes into a polyethersulfone (PES) ultrafitration membrane in pursuit of alleviated natural water or protein fouling, after strong acid treatment of multiwalled carbon nanotubes (MWCNTs).38,39 They pointed out that the well dispersion of acid-MWCNTs in the PES membrane was ascribed to the hydrogen bonding interactions between sulfonic groups of PES chains and carboxylic groups on the acid activated MWCNTs. As a result, the hybridized MWCNTs changed membrane morphologies and structures, which improved membrane roughness, hydrophilicity, porosity, and water flux. During the ultrafiltration processes of natural water or protein solution, the antifouling performances of these PES/MWCNTs composite membranes were enhanced to some extent, depending on the content of MWCNTs addition. Although the membranes had better separation properties, unselected voids were formed in the membranes due to the poor interfacial compatibility of carbon nanotubes with polymers. Recently, Ma et al. synthesized the PVDF/MWCNTs nanocomposite ultrafiltration membranes in order to reduce

coverage of hydrophilic hyperbranched poly(amine-ester) (HPAE) groups facilitated the good dispersion of MWCNTs in the casting solvent, dimethyl formamide (DMF). They found that the water permeability/recovery and fouling propensity of PVDF/MWCNTs nanocomposite membranes depended on the content and dispersed state of MWCNTs in the polymer matrix, in which 1.5 wt % of MWCNTHPAE addition was the best. However, they also claimed that the modified MWCNTHPAE was not hydrophilic enough to induce the macropore formation in the phase inversion process, and in turn relative low water permeability flux could be achieved as shown in Figure 4. Nanomaterials and Dispersants/Porogens. As the abovementioned claims, poor pore-forming potential and dispersion of carbon nanotubes in a polymer matrix made highly efficient pore-forming agents necessary in the preparation of polymer1D nanomaterial composite membranes via the classical phase inversion method. Similar to the authors in refs 38 and 39, Madaeni et al. reported that a novel antifouling nanofiltration membrane, consisting of acid-oxidized multiwalled carbon nanotubes and polyethersulfone (PES), could be fabricated in the presence of polyvinylpyrrolidone (PVP, porogen) through nonsolvent-induced phase inversion technique.40 Through proper addition of these acid-oxidized multiwalled carbon nanotubes, water permeability and salt rejection of PES/ MWCNTs nanofiltration membrane could be both possibly increased due to the formation of very large macrovoids in the membrane support layers. Owing to the surface segregation D



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while membrane surfaces became more even, porous and smooth. As a result, water flux permeation and fouling tendency of these membranes declined and meanwhile separation property of Cd ions increased remarkably compared to the unmodified PES membranes. The interaction between carboxylate functional groups of PCL-MWCNTs nanocomposites and Cd ions leads to more deposition of ions on the membrane surface, resulting in a higher Cd ions retention. 3.3. Polymer-2D Nanomaterial Composite Membrane. Nanomaterials Only. Recently, silicate clays representative of 2D nanomaterials have been getting more and more attention in the preparation of water filtration membranes, which used to be filled in the gas separation and proton exchange membranes. Monticelli et al. reported that different types and amounts of clays in casting solution had great effects on the dispersion state of clay particles in resultant polysulfone membranes and in turn dominated the separation performance and mechanical characters of these nanocomposite membranes.42 They demonstrated that Cloisite 30B, modified by a methyl tallow bis(2-hydroxyethyl) quaternary ammonium salt, could be intercalated or exfoliated in a polymer matrix, and the asprepared membrane had the best separation properties and mechanical strength at 2 wt % loading. On the basis of the hydrophilic/hydrophobic clays, Chung et al. demonstrated that these mixed matrix hollow fiber membranes possessed enhanced separation performance and mechanical strength in the long term distillation process.43 Interestingly, Lin et al. took advantage of the cationic ion exchange capacity of Na+Montmorillonites to prepare a novel porous poly(methyl methacrylate) (PMMA) cation-exchange membrane through emulsion polymerization of the MMA monomer and wet phase inversion process. For the cationic dye adsorption, their results showed that about 95% methyl violet adsorption could be attained in 2 h and more than 92% of them were easily desorbed from these nanocomposite membranes through consecutive cleaning of optimal desorption solution (1 M KSCN in 80% methanol), which demonstrated that a cationexchange polymeric membrane, high-efficiency of adsorption− desorption, could be further developed in the application of water treatment. Emphasizing the dispersion state of nanomaterials, Ma et al. prepared the PVDF/PVP-g-MMT nanocomposite ultrafiltration membranes based on the NVP-grafted polymerization modification of montmorillonites (MMTs).1 The novel hydrophilic nanocomposite additive (PVP-g-MMT) played a complex role of hydrophilic modifier, self-dispersant, and pore-forming agent (porogen) in the sequential processes of dispersed casting solution and surface-segregation through nonsolvent-induced phase inversion (NIPS), shown in Scheme 1 and Figure 7. These multifunctionalities of PVP-g-MMT in PVDF membranes were ascribed to “grafting-from” polymerization of N-vinyl-2-pyrrolidone monomer, as an active chain block, on the MMT surface and in turn the structures, morphologies, surface composition, and chemistry of as-prepared nanocomposite membranes were also changed, owing to the existence and dispersion of PVP-g-MMT in the membrane matrix. As a result, all of the PVDF/PVP-g-MMT nanocomposite membranes showed significant improvement of water permeability and proper BSA retention, compared to the control PVDF membrane. Meanwhile, dynamic BSA fouling (especially irreversible fouling) resistance and flux recovery properties were also greatly enhanced due to the changes of

behavior of hydrophilic MWCNTs, PES membranes resulting from the phase inversion process possessed minimized fouling during filtration experiments of protein (bovine serum albumin) aqueous solutions. They pointed out that surface roughness played an important role in antifouling performance of these membranes. Finally, they also took the advantage of negative charges from MWCNTs to improve the salt rejection through the Donnan exclusion mechanism. To enhance the compatibility of MWCNTs and PES matrix, the chemical modification of the outside walls of the MWCNTs has been a well-known approach to fabricate composite membranes, although some pore-forming agents might be also needed in the phase inversion process.28 Mansourpanah et al. investigated the effects of polycaprolactone-modified MWCNTs on the structures and performances of PES-based mixed matrix membranes through a classical phase inversion process, during which the polyvinylpyrrolidone (PVP) was involved as the pore-forming agents (porogens).41 The polycaprolactone-modified MWCNTs (PCL-MWCNTs, shown in Figure 5) with hydrophilic groups facilitated the

Figure 5. TEM image of PCL-MWCNTs nanocomposite based on the out-wall modification of MWCNTs through caprolactone monopolymerization. Reprinted with permission from ref 41. Copyright 2011 Elsevier.

transfer of sponge-like pores into finger-like pores in the sublayers of PES/PCL-MWCNTs blend membranes and in turn membrane porosity was increased (as shown in Figure 6),

Figure 6. SEM images from the cross-sectional structures of PES/ PCL-MWCNTs nanocomposite membranes based on the different weight percents (w/v %) of PCL-MWCNTs addition. Reprinted with permission from ref 41. Copyright 2011 Elsevier. E



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coagulation bath. As a result, water permeability of resultant nanocomposite membranes were obviously improved with the increase of clay content, which was ascribed to the changes of surface hydrophilicity and pore size distribution. The most fascinating advantage in these researches was the enhanced mechanical performance of polysulfone membranes after content-dependent dispersion of clay nanosheets therein, while the mechanical strength was also reduced by the membrane porosity depending on pore-forming potential of different porogens (PEG or LiCl). 3.4. Polymer-3D Nanomaterial Composite Membrane. Based on the three-dimensional channels, varieties of zeolites have been dispersed into the thin polyamide film of nanofiltration and reverse/forward osmosis membranes since the new concept was introduced in 2007.44 The study of these nanocomposite membranes concerned the behaviors and effects of zeolites in the processes of interfacial polymerization on support layers, which was not discussed in this review. However, the behaviors and effects of zeolites in the processes of classical phase inversion are also of importance in the fabrication of asymmetric polymer membranes. Jian et al. focused on the effects of NaA zeolite particles on the structures and performances of poly(phthalazinone ether sulfone ketone) composite ultrafiltration membranes.45 They found that hydrophilic zeolites could aid these composite membranes to maintain excellent water permeability, hydrophilicity, and antifouling properties with enhanced solute rejections at a low content (3 wt %) of zeolites, while these additives were apt to agglomerate to each other at a high content and in turn affect resultant performances. The strong hydrophilicity of these zeolites could facilitate the in-flow velocity of nonsolvent (water) in the phase inversion process that results in the formation of finger-like pores in the porous support layers. On the other hand, hydrophilic zeolites could absorb water vapor in the evaporation process, which caused the formation of dense skin layers on the membrane surface and in turn improved the selectivity of these nanocomposite membranes. To alleviate the agglomeration of zeolite nanoparticles, external dispersants or pore-forming agents should be involved in the process of membrane formation. 3.5. Challenges: Be Better on More Than One Side. As aforementioned, differing dimensional nanomaterials (TiO2, Al2O3, SiO2, MWCNTs, clays, and zeolite) were introduced into polymeric membranes through the classical phase inversion process. Various characters of these nanomaterials resulted in different structures and performances of the prepared nanocomposite membranes, which may possess advantages or disadvantages in the field of water treatment. Focusing on the nanomaterial-causing strutures, “Threshold” content, good dispersion, and pore-forming agent (porogen) have become three dominating factors during the preparation process of polymer−nanoparticle mixed matrix membranes by the classical phase inversion method. Good dispersion of all the nanomaterials, interlinking with “threshold” content, is greatly important to the development of membrane structures. Hydrophilic 0D nanoparticles (TiO2, Al2O3, SiO2) could be well dispersed through an in situ incorporation method, although these 0D nanoparticles may be apt to leak out of the membrane matrix during the processes of preparation and filtration. 1D/2D/3D nanomaterials have relative stablility in the membrane through the wrapping/intercalating/exfoliating of polymer chains, which could prevent the involved nanomaterials from leaking out of membrane matrix. “Threshold”

Scheme 1. Schematic Representative of PVP-g-MMT Dispersion in Casting Solution and Segregation onto the Membrane or Pore Surfaces via Immersion Precipitation of as-Spread PVDF Composite Films during the Nonsolvent (H2O) Induced Phase Inversion (NIPS) Processa

a

Reprinted with permission from ref 1. Copyright 2012 American Chemical Society.

Figure 7. SEM images of cross-section morphologies for prepared PVDF/PVP-g-MMT nanocomposite membrane M0, M1, M2, M4, and M6 (the numbers represent the weight percents of PVP-g-MMT addition). Reprinted with permission from ref 1. Copyright 2012 American Chemical Society.

surface hydrophilicity and morphologies, shown in Figure 8. The enhanced performances could be explained by the poreforming and segregation behavior of hydrophilic nanocomposite PVP-g-MMT: further-developed pore structures favored permeation, hydration layer caused by segregation behavior of hydrophilic segments endowed the surface with outstanding antifouling resistance potential. Simultaneously, the synergy of hydration layer and “water corona” around protein molecules avoided the conformational transformations of proteins to some extent, which also enhanced the fouling resistances of nanocomposite membranes. Hopefully, the demonstrated modification method of hydroxyl group-containing inorganic nanoparticles was favorable to fabricate hydrophilic nanoparticle-enhanced polymer membranes for water treatment. Nanomaterials and Dispersants/Porogens. Ma et al. prepared polysulfone(PSf)/clay nanocomposite ultrafiltration membranes with PEG and LiCl as pore-forming agents, respectively.25,26 The exfoliated clay nanosheets in casting solution could facilitate the exchange of solvent and water in the phase inversion process, although the PEG/LiCl played the role of a strong pore-forming agent. In the other hand, the viscosity of polysulfone casting solution incorporated with clay nanoparticles could be rapidly increased with the increase of clay addition, which dominated the phase separation and lowered the exchange velocity of solvent and water in F



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Figure 8. Ultrafiltration performances of PVDF/PVP-g-MMT nanocomposite membranes (M0−M6) during BSA−PBS solution (0.2 g L−1, pH 7.4) filtration: (a) pure water permeability and protein rejection, (b) time-dependent water permeation flux, (c) water flux recovery ratio after hydraulic washing, and (d) calculated resistances of membrane fouling based on resistance-in-series model. Reprinted with permission from ref 1. Copyright 2012 American Chemical Society.

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CONCLUSIONS AND FUTURE TREND Polymer−nanomaterial composite membranes prepared from the classical phase inversion method are of great interest and have been researched extensively in the fields of membrane and water treatment, although the varieties of complexities and controversies are still to be studied. At present, some conclusions can be drawn from the works of publication as follows: 1. Nanomaterials incorporated into polymer membranes were various and the standards of selectivity were not identified. 2. Fabrication methods of polymer−nanomaterial composite membranes were various when the classical phase inversion process was used, including nonsolvent induced phase inversion (NIPS) and thermally induced phase inversion (TIPS). 3. Incorporation routes of nanomaterials into polymer membranes include in situ, ex situ, and biogeneration, etc. 4. Dispersion methods of nanomaterials in polymer membranes include differing additions of original nanomaterials, modified nanomaterials, dispersants, and porogens, etc. 5. Performance and functionality of polymer−nanomaterial composite membranes were multifarious and controversial in the application of water treatment. In the future, further developments of polymer−nanomaterial composite membrane might emphasize on the exploration of novel functional nanomaterials (water-channel possessing, biodegradability, high-strength, heat-insulation etc.), new

contents of these nanomaterials are different, based on their characters, in which 1D CNTs have the smallest “threshold” content. Beyond the “threshold” content, all nanomaterials could increase the viscosities of the casting solution and in turn inhibit the development of pore-structures and active layers. Pore-forming agents (porogens) could facilitate the dispersion of these nanomaterials in a membrane matrix, while overlapping the pore-forming effects of hydrophilic nanomaterials themselves. Regarding the properties that result from the use of nanomaterials, a balance of flux-rejection, antifouling, and mechanical strength have become the three dominating performances of polymer−nanoparticle mixed matrix membranes, which depend on the character and dispersion states of differing nanomaterials. Hydrophilic 0D nanoparticles (TiO2, Al2O3, SiO2) and exfoliated 2D nanosheets (MMT) could effectively enhance the wettability of membrane surfaces and improve the water flux. In addition to the hydrophilicity of nanoparticles, channel-possessing 1D MWCNTs and porous 3D zeolites could modulate the trade-off of water flux and selectivity. As to antifouling properties, the fouling layers could be rapidly cleaned throuth the TiO2 catalytic process. At the “threshold” content, all the nanomaterials could improve the mechanical strength of polymeric membranes. However, the effect of differing dimensional nanomaterials on the structures and properties of mixed matrix membranes is monofunctionality at the expense of other advantages, which often greatly reduces the membrane life or deteriorates the permeate water. G



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coupling methods of nanomaterials and membranes (bioinspiration, biogeneration, biodispersion), and multifunctionality of composite membranes (biocidal ability), although some attempts have been conducted in the fields of thin film composite (TFC) membranes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank financial support from the National Natural Science Foundation of China (Grants 50978067, 51178134). It is also supported by the Ministry of Science and Technology of China (2009ZX07424-006, 2012BAC05B02).

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