Desalination 326 (2013) 77–95Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal A review on membrane fabrication: Structure, properties and performance relationship Boor Singh Lalia a, Victor Kochkodan b, Raed Hashaikeh a,⁎, Nidal Hilal a,b a Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, United Arab Emirates b Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Swansea SA2 8PP, UK H I G H L I G H T S • Membrane fabrication techniques • Structure–property relationship of membranes • Structure parameters affect the membrane performance. a r t i c l e i n f o a b s t r a c t Article history: In this review, polymeric membrane fabrication techniques for pressure driven membrane processes and Received 18 April 2013 membrane distillation are discussed. The fabrication technique, properties of the fabricated membranes Received in revised form 18 June 2013 and performance in water desalination are related. Important parameters which affect the membrane perfor- Accepted 20 June 2013 mance such as crystallinity of the membrane based polymer, porous structure, hydrophobicity/hydrophilicity, Available online 16 August 2013 membrane charge and surface roughness are analyzed. Despite the fact that extensive knowledge exist Keywords: on how to ‘tailor’ membrane pore structure including its surface properties and cross-section morphology Polymer membranes by selection of appropriate fabrication methods, there is still a challenge to produce reliable membranes Membrane fabrication with anti-fouling properties, chemical resistance, high mechanical strength with high flux and selectivity. Porous structure To ensure progress in membrane performance, further improvements are needed of common membrane fab- Membrane performance rication techniques such as phase inversion and interfacial polymerization. At the same time, the potential of novel fabrication techniques such as electrospinning and track-etching needs to be assessed. A comprehensive understanding between structure-surface properties and performance is a key for further development and progress in membrane technology for water desalination. © 2013 Elsevier B.V. All rights reserved. Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2. Membrane fabrication methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.1. Phase inversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.1.1. Immersion precipitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 2.1.2. Evaporation-induced phase separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 2.2. Interfacial polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 2.3. Stretching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 2.4. Track-etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 2.5. Electrospinning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 3. Structure–property–performance relationship . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3.1. Crystallinity of the polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3.2. Pore structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3.3. Surface properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 ⁎ Corresponding author. Tel.: +971 28109152. E-mail address:
[email protected] (R. Hashaikeh). 0011-9164/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2013.06.016 78 B.S. Lalia et al. / Desalination 326 (2013) 77–95 3.3.1. Hydrophilic–hydrophobic properties of membrane surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88 3.3.2. Surface charge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89 3.3.3. Surface roughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 List of List of abbreviations abbreviations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 1. Introduction manner from a liquid to a solid state [4]. This transformation can be accomplished in several ways [5], namely: According to the world population clock, the population exceeds (a) Immersion precipitation. The polymer solution is immersed in 7 billion and will reach 10 billion by 2050. Pure drinking water would a non-solvent coagulation bath (typically water). Demixing be a major problem for the developing countries in the world. The and precipitation occur due to the exchange of solvent (from improvement in the efficiency and cost of water treatment is a major polymer solution) and non-solvent (from coagulation bath), challenge to overcome the scarcity of portable water. Different membrane that is, the solvent and non-solvent must be miscible. methods have been used for water treatment, including microfiltration (b) Thermally induced phase separation. This method is based on the (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) and phenomenon that the solvent quality usually decreases when the membrane distillation (MD) [1]. UF and MF are well-developed tech- temperature is decreased. After demixing is induced, the solvent niques used for water treatment, whereas RO is widely used for water is removed by extraction, evaporation or freeze drying. desalination and purification. MD is a new developing technique and it (c) Evaporation-induced phase separation. The polymer solution is has potential for desalinating highly saline water [2,3]. The membranes made in a solvent or in a mixture of a volatile non-solvent, and play a key role in membrane-based water treatment processes and deter- the solvent is allowed to evaporate, leading to precipitation or mine the technological and economical efficiency of the aforementioned demixing/precipitation. This technique is also known as a solu- technologies; membrane improvement can greatly affect the perfor- tion casting method. mance of current technology. The material selection and pore size of the (d) Vapor-induced phase separation. The polymer solution is exposed membranes depend on the application for which it would be used. to an atmosphere containing a non-solvent (typically water); Fig. 1 represents the average pore size requirement for membranes for absorption of non-solvent causes demixing/precipitation. different water treatment processes. Different fabrication techniques and polymers used for the prepa- However, among these techniques, immersion precipitation and ration of polymeric membranes are summarized in Table 1. Details of thermally induced phase separation are the most commonly used the fabrication techniques process and the material structural charac- method in the fabrication of polymeric membranes with various mor- teristics will be discussed in the subsequent sections. phologies [6,7]. In this article, the recent development of polymeric membrane ma- terials and membrane preparation methods with focus on structure– 2.1.1. Immersion precipitation property relationships for pressure-driven membrane processes and Immersion precipitation is a process where a polymer solution is cast MD will be discussed. This review article will provide a reference to on a suitable support, then immersed in a coagulation bath containing a the researchers and manufacturers working on fabrication of mem- non-solvent, where an exchange of solvent and non-solvent takes place branes and materials for water treatment. and the membrane is formed [8]. Schematic presentation of processes after polymer solution immersion in a non-solvent bath is shown in Fig. 2. The solvent diffuses into the coagulation bath (at a flux = J2) 2. Membrane fabrication methods whereas the non-solvent will diffuse into the cast film (at a flux = J1). After a certain time the exchange of solvent and non-solvent proceeds The selection of a technique for polymer membrane fabrication until the solution becomes thermodynamically unstable and demixing depends on a choice of polymer and desired structure of the membrane. takes place. A solid polymeric film finally is obtained with an asymmet- The most commonly used techniques for preparation of polymeric mem- ric structure. Usually at J2 ≫ J1 “skin” UF membranes with pore size of branes include phase inversion, interfacial polymerization, stretching, 10–300 Å are obtained, while at J2 = J1 mainly MF membranes with track-etching and electrospinning. pore size of 0.2–0.5 μm are fabricated. For membrane technologies, the development of the first high-flux 2.1. Phase inversion anisotropic acetate cellulose (CA) RO membranes via immersion precip- itation by Loeb and Sourirajan [10] was one of the most critical break- Phase inversion can be described as a demixing process whereby the throughs in desalination. Today, extensive knowledge exists on how initially homogeneous polymer solution is transformed in a controlled to ‘tailor’ the membrane's pore structure including its cross-section Nanofiltration Microfiltration Ultrafiltration Membrane Reverse osmosis distillation 0.1nm 1nm 10nm 100nm µm 1µ 10µm Fig. 1. Average pore size of the membranes used in different membrane processes. B.S. Lalia et al. / Desalination 326 (2013) 77–95 79 Table 1 Summary of commonly used polymers and fabrication techniques for the preparation of polymeric membranes for water treatment processes. Water treatment process Polymers used for membrane fabrication Fabrication techniques Average pore size of the membrane RO Cellulose acetate/triacetate Phase inversion 3–5 Å Aromatic polyamide Solution casting Polypiperzine Polybenziimidazoline NF Polyamides Interfacial polymerization 0.001–0.01 μm Polysulfones Layer-by-layer deposition Polyols Phase inversion Polyphenols UF Polyacrylonitrile (PAN) Phase inversion 0.001 – 0.1 μm Polyethersulfone (PES) Solution wet-spinning Polysulfone (PS) Polyethersulfone (PES) Poly(phthazine ether sulfone ketone) (PPESK) Poly(vinyl butyral) Polyvinylidene fluoride (PVDF) MF PVDF Phase inversion 0.1–10 μm Poly(tetrafluorethylene) (PTFE) Stretching Polypropylene (PP) Track-etching Polyethylene (PE) PES Polyetheretherketone (PEEK) MD PTFE Phase inversion 0.1–1 μm PVDF Stretching Electrospinning morphology by the selection of polymer, solvents and non-solvents, casting. An aprotic polar solvent such as N-methyl-2-pyrrolidone, additives, precipitation time, bath temperature and other parameters dimethyl formamide, dimethyl acetamide or dimethyl sulfoxide is during immersion precipitation [11–17]. For example, different casting preferable for rapid precipitation (instantaneous demixing) upon im- conditions and post-treatments were proposed to improve the water mersion in the non-solvent water and this produces anisotropic mem- flux and salt rejection of the CA membranes [18–24]. Main polymers branes with a high porosity [6]. used in membrane formation and their advantages and disadvantages To improve the membrane morphology and properties, various are presented in Table 2. inorganic (such as LiCl) and high molecular weight organic (such as Apart from the chemical nature of a casting polymer, the concen- polyvinyl pyrrolidone (PVP) or poly(ethylene glycol (PEG))) addi- tration of the polymer is very important in membrane fabrication via tives to casting solution are often used [29,30]. An additive can func- immersion precipitation. Increasing polymer concentration in the tion as a pore former, increase solution viscosity or accelerate the casting solution produces membranes with low porosity and pore phase inversion process. For example, the effect of LiCl addition in size. In this case, the macrovoid formation is suppressed and the ten- the membrane formation was investigated in the studies [31–35]. dency to form sponge-like structures is enhanced. The UF membranes Fontananova et al. [32] found that LiCl addition in the PVDF/ are obtained within a range of polymer concentration of 12–20 wt.%, dimethylacetamide dope increases flux of the casted membranes at whereas RO membranes are typically prepared from casting solutions low LiCl concentration of 2.5 wt.%, but it suppressed macrovoid for- with polymer concentrations ≥ 20 wt.% [1]. mation at a high concentration of 7.5% LiCl and resulted in a decrease Selection of solvent/non-solvent system also strongly affects of the membrane permeation flux. The similar results were obtained morphology and properties of casted membranes. The low miscibility by Lee et al. [33] for poly(amic acid) (PAA) casting solutions in of polymer in the solvent leads to fabrication of a nonporous membrane, N-methyl-2-pyrrolidone. They found that by increasing LiCl concen- while more porous membranes are obtained when the miscibility is tration in the PAA/N-methyl-2-pyrrolidone system, the solution high. Generally aprotic solvents, where there are no hydrogen atoms able to contribute to hydrogen bonding, are preferred for membrane Table 2 Main polymers used in membrane formation via immersion precipitation [25–28]. Polymer Advantages Disadvantages CA • Hydrophilicity •Low thermal resistance • Flexibility in fabrication (b30 °C) • Low cost •Low chemical resistance, pH range (2–8) • Poor resistance to chlorine PS and • High thermal resistance •Low operating pressure limits PES (up to 75 °C) • Hydrophobicity • Wide pH tolerances (1–13) • Good chlorine resistance • Flexibility in membrane fabrication (wide range of pore size) • High mechanical characteristics PVDF •High mechanical strength • Hydrophobicity and chemical resistance Fig. 2. Schematic representation of a film/bath interface: J1 is the non-solvent flux and J2 the •High thermal stability (up to 75 °C) solvent flux. X is the position of the interface between the film and the coagulation bath, x is Polyamide •Wide pH tolerance • Poor chlorine resistance the spatial position coordinate normal to the membrane surface, y = −x − X(t) is the (PA) •High thermal stability position coordinate that moves with the interface. m is the position coordinate in the •High mechanical properties polymer-fixed frame of reference, and M is a support [9]. 80 B.S. Lalia et al. / Desalination 326 (2013) 77–95 viscosity can be raised to the point where macrovoid formation is have with the surrounding polymer matrix [45]. A comprehensive hindered and development of a finely porous structure is favored. review on polymeric membranes incorporated with metal/metal oxide The above observations were believed to be associated with the nanoparticles has been published recently by Ng et al. [46]. change of the thermodynamic and kinetic properties of the polymer Zodrow et al. [47] prepared polysulfone membrane contained dope system before and after LiCl addition. It was shown that LiCl Ag nanoparticles (1–70 nm) via the phase-inversion process by dis- addition increased the dope's thermodynamic instability in reaction persing nanoparticles in the casting solution. It was shown that with water, which facilitated a rapid phase demixing and resulted polysulfone membranes impregnated with 0.9 wt.% Ag nanoparticles in macrovoid formation (thermodynamic effect) [34]. On the other possess similar permeability and surface charges compared with hand, LiCl possesses strong interactions with the polymer and sol- pure polysulfone membranes, however they were significantly more vent, which was supported by the significant increase in viscosity of hydrophilic with 10% reduction in contact angle. It was found that LiCl added casting solutions [31,33]. The strong interactions among the addition of Ag nanoparticles does not visibly alter the membrane the components of the casting solution tended to delay the dope pre- structure. Similar results were obtained by Yan et al. [48], which used cipitation (the kinetic effect), which partially offset the thermodynamic nano-sized Al2O3 particles in dimethylacetamide casting solutions for impact of LiCl addition. As a result, the size of the macrovoids in the preparation of PVDF membranes. It was found that increased Al2O3 fabricated membranes is reduced at high LiCl dosage in the casting concentrations from 0 to 2% in the casting solution had led to in- solutions [33–35]. creased water permeate fluxes due to an increase in the membrane Saljoughi et al. [36] reported that an increase of PVP concentration hydrophilicity. SEM images showed that the addition of nano-sized in the cast film from 0 to 1.5 wt.% resulted in the facilitation of Al2O3 particles did not affect the surface, cross-section, and inner macrovoid formation in the membrane sub-layer, which increased pore membrane structures. Both pure PVDF and PVDF − Al2O3 mem- pure water flux. However, in the same study, it was observed that an branes showed typical asymmetric morphology with finger-like increase in PVP concentration from 1.5 to 3, 6 and 9 wt.% resulted in a pores. decrease in water flux, where the macrovoid had been suppressed grad- On the other hand, Yang et al. [49] showed that the addition of TiO2 ually. Wang et al. [37] showed that the PVP-added PES membrane has a nanoparticles has a large effect on the membrane structure of TiO2/PS higher water flux and lower water contact angle than the neat PES membranes casted from 18 wt.% PS solution in N,N-dimethylacetamide membrane. The contact angle decreased by 16% when the PVP content with N-methyl-2-pyrrolidinone. The cross-section morphologies of in the casting solution was 10 wt.%. Ochoa et al. [38] proved that the membranes are shown in Fig. 3, which illustrates that the macrovoids addition of PVP to the casting solution increases the UF PES membrane grow and become run through at low TiO2 concentrations and then permeability without significant changes in selectivity. are suppressed or disappear at higher additive dosages (≥3 wt.%), the Marchese et al. [39] reported that the reasons behind the increase thickness of skin layer increases with the increase of TiO2 dosage. of membrane permeability when PVP is added are an increment in Fig. 4(a–c) shows a log-normal pore size distributions for the mem- the pore density, a decrease of the effective thickness of the dense branes with TiO2 content of 1–2 wt.% and the number of small pores layer due to macrovoids in the support layer and an increment in the increases compared with the PSF membrane without nanoparticles. hydrophilicity of the surfaces on the membrane and inside the pores. While adding more TiO2 (≥3%) to the casting solution enhances the The formation phenomena of macrovoids, which are large elongated formation of larger pores (50–70 nm) caused by the nanoparticle spaces below the upper surface of the membrane, have been widely aggregate phenomenon, which leads to a bimodal pore distribution discussed by Smolders et al. [12], Wang et al. [40], and McKelvey and (Fig. 4d). The mean pore radius of the membrane with 1–2 wt.% TiO2 Koros [41]. content decreased and then increased at higher TiO2 content due to Arthanareeswaran et al. [42] concluded that the presence of the presence of large pores. These results demonstrate that adding low molecular weight PEG additive in the cast solution film increased appropriate TiO2 nanoparticles to PS matrix can improve its porosity porosity/permeability of the prepared membranes. Saljoughi et al. and increase the number of small pores. As a result, the flux through [43] studied the effects of PEG concentration (0 wt.%, 5 wt.% and such membranes can be increased significantly. It was also shown that 10 wt.%) on morphology, pure water permeation flux of the prepared the addition of TiO2 nanoparticles causes the decrease of contact angle membranes at different coagulation bath temperatures (0 and 25 °C). from 85° for pure PS membrane to 41–52° for TiO2/PS membranes, indi- Increasing PEG concentration in the cast film results in the facilitation cating that TiO2 addition enhances the hydrophilicity of membrane as a of macrovoid formation in the membrane sub-layer, which increases few of hydrophilic TiO2 nanoparticles adsorb and stick on the mem- flux and rejection of human serum albumin. Susanto and Ulbricht brane surface. [44] compared the effect of three different macromolecular additives It should be mentioned however, that one of the limiting factors PVP, PEG and poly(ethylene oxide)-b-poly(propylene oxide)-b-poly for incorporation of nanoparticles into polymeric membranes is high (ethylene oxide) (Pluronic) on the membrane structure and their aggregation of nanoparticles that results in a low dispensability in stability in the polymer membrane matrix of the PES membrane. the casting solution. Also, careful control and monitoring of the nano- They found that the addition of Pluronic as a modifier agent showed particles released from the modified membranes are necessary to the best membrane performance and stability. The authors suggest minimize potential (eco) toxicity effects. that the reason for this phenomenon would be that the hydrophobic part of Pluronic enhances the PES–additive interaction. Ultrafiltration 2.1.2. Evaporation-induced phase separation experiments also demonstrated that more than 70% of the initial Evaporation-induced phase separation is a facile technique to pre- water flux could be recovered after UF of bovine serum albumin pare membranes for various applications. At the first stage, a sufficiently (BSA) solution just by external cleaning with water. It was proposed viscous polymer solution is prepared in a solvent (or in binary/ternary that a highly hydrated and dense poly(ethylene oxide) polymer mixture of solvents) and a non-solvent. Then a prepared polymer solu- layer formed on the membrane surface prevented protein molecule tion is casted on a flat porous substrate using a doctor blade technique from contacting membrane surface directly, and the protein mole- [50]. When the volatile solvent evaporates from the casted solution, a cules deposited on the poly(ethylene oxide) layer can be removed thin polymer film is formed on the porous support. The morphology easily by water washing. of the solution casted films can be controlled by using solvents with Recently the use of inorganic nanoparticles as additives to polymeric different boiling points. Nguyen et al. [51] developed PVDF, PVC, PS membranes has begun to attract wide interest due to the improved and PVAc microporous membranes using different organic solvents membrane properties, including increased strength and modulus, and studied the effect of different solvents on the surface morphology which result from the strong interfacial interactions the nanoparticles and pore size/shape. Kim et al. [52] prepared microporous polystyrene B.S. Lalia et al. / Desalination 326 (2013) 77–95 81 Fig. 3. SEM images of the morphology of PS/TiO2 membranes with (a) 0 wt.% TiO2, (b) 1 wt.% TiO2, (d) 3 wt.% TiO2, and (e) 5 wt.% TiO2 [49]. Fig. 4. Pore size distributions of PS/TiO2 membranes with (a) 0 wt.% TiO2, (b) 1 wt.% TiO2, (c) 2 wt.% TiO2, and (d) 3 wt.% TiO2 [49]. 82 B.S. Lalia et al. / Desalination 326 (2013) 77–95 Fig. 5. SEM images of the surface morphologies of the porous silicon rubber membranes prepared at different liquid paraffin concentrations (a) 10, (b) 15, (c) 20, (d) 25, (e) 30 and (f) 40 wt.% [53]. films using PEG used as pore former. The pore size of the membranes rejection than that of an integrally-skinned asymmetric cellulose was in the 5–12 μm range and controlled by varying polystyrene/PEG acetate membrane and high water flux [55]. concentrations and different molecular weights of PEG. Zhao et al. [53] Due to the significant advantages of IP technique in optimizing prepared silicon rubber microporous membranes using this technique. independently the properties of skin layer and microporous substrate The pore size and the pore structure of the membranes were tuned by layer, a wide variety of TFC membranes have been successfully devel- varying casting temperature and the concentration of liquid paraffin. oped [56,57]. The various factors such as concentration of monomers, Fig. 5 showed the formation of porous silicon membranes with different solvent type, reaction time and post-treatment conditions affect the pore shapes and sizes. structural morphology and composition of the barrier membrane layer [57–60]. Most of NF and RO membranes produced by IP method have PA thin 2.2. Interfacial polymerization layer on top of the membrane support. Among the used active monomers to form functional PA layer in RO/NF membranes, m-phenylenediamine Interfacial polymerization (IP) is the most important method (MPD) and trimesoyl chloride (TMC) are the most common (Fig. 6). for commercial fabrication of thin-film composite (TFC) RO and NF Other amine monomers for production of TFC PA membranes include: membranes. The first interfacially polymerized TFC membranes p-phenylenediamine [61], piperazine [62], triethylenetetramine [63], were developed by Cadotte et al. [54] and represented a break- N-N′-diaminopiperazine [62], N-(2-aminoethyl)-piperazine [62], and through in membrane performance for RO applications [55]. The orig- poly(ethyleneimine) [64]. inal IP protocol involved soaking a microporous polysulfone support Recently, novel monomers have been suggested for the prepara- in an aqueous solution of a polymeric amine and then immersing tion of TFC membranes via IP technique [62,64]. These monomers the amine impregnated membrane into a solution of a di-isocyanate contain more functional or polar groups, so the prepared membrane in hexane. The membrane was then cross-linked by heat-treatment exhibits smoother surface or better hydrophilicity, which is advanta- at 110 °C [54]. The resulting TFC polyurea membrane had better salt geous to the improvement of antifouling property of the membranes. Fig. 6. Preparation of thin film PA membrane from m-phenylenediamine (MPD) and trimesoyl chloride (TMC) via IP [56]. B.S. Lalia et al. / Desalination 326 (2013) 77–95 83 For example, Li et al. [65] synthesized two novel tri- and tetra- [75]. This is a solvent free technique, in which the polymer is heated functional biphenyl acid chloride: 3,4′,5-biphenyl triacyl chloride above the melting point and extruded into thin sheet forms followed and 3,3′,5,5′-biphenyl tetraacyl chloride, which were then used to pre- by stretching to make it porous [76–78]. This technique is suitable for pare TFC RO membranes with MPD. Similarly, Liu et al. [66] presented the highly crystalline polymers where the crystalline regions of the a novel RO composite membrane prepared from 5-isocyanato- polymer provide strength and amorphous regions formed the porous isophthaloyl chloride and MPD. structure. Usually, stretching is carried out in two steps, first cold Besides the exploration of novel monomers for IP, the efforts have stretching followed by the hot stretching. Cold stretching is used to been done on the improvement of IP process via adding of active nucleate the micropores in the precursor film followed by the hot organic modifiers into TMC or MPD solutions. The modifiers can par- stretching to increase/control the final pore structure of membranes. ticipate in the reaction and are introduced into the functional barrier In this process, material's physical properties (like crystallinity, layer, thus improving the surface property and fouling resistance melting point, tensile strength etc.) and the applied processing of resultant RO membranes. For example, Rana et al. [61] added parameters control the final porous structure and properties of the 4,4′-methylene bis(phenyl isocyanate) and PEGs of average molecu- membranes [76]. Carreau's research group reported PP membrane lar weight 200 and 1000 Da into organic phase containing TMC to in- fabrication by stretching and studied various factors affecting the corporate in situ hydrophilic surface modifying macromolecules. The morphology and permeation [79–81]. They found that molecular prepared membranes, which exhibited significantly more hydrophilic weight of PP is a key parameter to control the structure of the mem- surface, were then subjected to long term fouling studies using model branes in addition to stretching (hot and cold) and annealing. In foulants including sodium humate, silica particles and chloroform another study, they developed blend membranes of long chain spiked in the feeding NaCl solution. The results showed that the flux branched PP and linear chain PP and studied the effect of blending decline was reduced significantly after incorporating organic modi- on the orientation of crystalline and amorphous in the precursor fiers into the TFC membranes. A similar approach was also used by film [80]. Addition of long branched PP improved the lamellae thick- An et al. [67], who added polyvinyl alcohol (PVA) into piperazine ness for the blend and increased the porosity of the blend membrane. solution during IP to prepare low fouling NF membrane. It was also They also prepared porous membranes using PVDF of different melt shown that the addition of an acid-acceptor e.g. salt of triethylamine flow indexes and studied the effect of polymer melt rheology on with sulfonic acid in aqueous solution could speed-up IP reaction by the row-nucleated lamellar morphology. The blending of PVDF with removing hydrogen halide by-products formed during amide bond different melt rheologies improved the water vapor permeability of formation [68,69]. It was found that sodium lauryl sulfate or sodium the PVDF membranes [81]. Kurumada et al. discussed the uniaxial dodecyl sulfate could prevent pore collapse in PS substrate during and biaxial stretching operations on the structure and morphology post-heat treatment of TFC membrane [70]. of the PTFE membranes [82]. SEM images of the uniaxially and biaxi- Many efforts have been done to improve chlorine stability of TFC ally stretched membranes (Fig. 7) showed the effect of stretching on PA membranes against oxidative degradation [27,71]. It was noted the fibrils connecting the island like fraction of the film. that the chemical modification of PA layer through the use of diamine Tabatabaei et al. investigated the effect of blending (different mo- moieties could greatly enhance the chlorine resistance of the mem- lecular weights) PP on the crystallinity, pore density, pore uniformity brane and the effectiveness of chlorine resistance is in the order of and tensile properties [83]. The increase in high molecular weight aromatic, cycloaliphatic and aliphatic diamines, respectively [72]. species leads to more uniform pores, better connectivity and distribu- Besides the TFC PA membranes, TFC polyester and polyesteramide tion of pores. The addition of high molecular weight PP does not sig- membranes were also developed via IP technique [73,74]. The incor- nificantly influence the mechanical properties of the membranes. Kim poration of ester linkage increased the oxidation resistance of the et al. studied the influence of annealing on crystallite size and crystal- membrane and this significantly increased the membrane tolerance linity of precursor PP hollow fibers before the stretching process [84]. on chlorine attack. Un-annealed melt spun hollow fibers does not form pores upon hot stretching. Pre-annealing of the precursor PP hollow fibers is found 2.3. Stretching to be essential for the formation of porous structure. Microporous membranes commonly used in MF, UF and MD are 2.4. Track-etching fabricated by extrusion followed by stretching technique. Polymer membrane fabrication using stretching was developed in 1970s and In this technique, a nonporous polymeric film is irradiated with its proprietary was owned the companies. Celgard® is known for pro- energetic heavy ions leading to the formation of linear damaged ducing PE and PP based membranes for use in energy storage devices tracks across the irradiated polymeric film. The schematic of single Fig. 7. SEM image showing the patterned structure of the PTFE porous membranes produced by the stretching operation: (a) uniaxially stretched membrane, and (b) biaxially stretched membrane prepared after the first stretching operation at the condition of (a) [82]. 84 B.S. Lalia et al. / Desalination 326 (2013) 77–95 ion-irradiation setup is shown in Fig. 8. This technique is famous for which is achieved by varying the solution viscosity, environmental its precise control on the pore size distribution of the membrane conditions, applied electric potential and the flow rate of the solution and; pore size and pore density are independent parameters and [98]. Porosity, pore size distribution, hydrophobicity and surface mor- can be controlled in a wide range from a few nanometers to tens of phology of the electrospun mats are controlled by the fiber diameter micrometers and 1–1010 cm−2 respectively. Due to these properties, and its morphology [96]. Due to the precise control on the fiber size, it makes a simple relationship between water transport properties shape and morphology electrospun fibrous membranes have been and membrane structure i.e. pore size/shape. Polycarbonate (PC) track used for filtration and MD processes [97,99–104]. Zong et al. investi- etched membranes were early available in 1970s [85]. The basic infor- gated the effect of polymer solution viscosity, applied potential mation on the formation of particle tracks, track production mecha- strength, solution feed rate and ionic salt addition (to improve the nism, track etching recipes and potential applications can be found in conductivity of solution) on the fiber diameter and nanostructured the book by Fleischer et al. [86]. The membrane porosity is mainly morphology [105]. Minimum potential difference of 20 kV was re- determined by the duration of irradiation, the pore size is determined quired to form a stable jet using 30 wt.% solution of poly(D,L-lactic by the etching time and temperature. The resulting membranes have acid) in dimethyl formamide. The electrospun fibers with few beads a rather low porosity (up to 15%) or pore density (e.g. 6 × 108 cm−2 were obtained at 20 kV (keeping the feed rate and concentration of for 50 nm and 2 × 107 cm−2 for 1 μm) [86,87], in order to reduce the polymer solution constant). Further increase in potential above probability of defects, i.e. double or triple pores. Under those conditions, 20 kV resulted in the formation of more beaded structure of the the pore size distribution can be very sharp [88]. There is some evidence fibers. Viscosity of the polymer solution also plays an important role that the pore geometry for the smaller pore size track-etched mem- to form a beaded free smooth fibers, a minimum viscosity of solution branes may deviate from an ideal cylindrical shape, which can be is needed to form smooth fibers which was 35 wt.% poly(D,L-lactic explained by the chemistry behind the manufacturing process [89]. acid) in dimethyl formamide in the present case. However, the mini- Komaki et al. prepared porous polyethylene naphthalate (PET) mum viscosity requirements vary with the selection of polymers and films irradiated by fission fragments obtained from thermal neutron its molecular weights. Addition of salt (sodium chloride, sodium-/ fission of uranium-235 and studied the effect of etching rate on the potassium phosphate) was found to tune the diameter of the fibers, hole diameter and hole density of the membranes [91]. In another fibers obtained from PDLA solution with 1 wt.% NaCl possessed the study, they first bombarded the polyimide films by several kinds smallest average fiber diameter. This was supposed to be due to the of heavy ions (127I, 107Ag, 79Br, 64Cu) and then treated with gamma increase in charge density on the jet results in a higher stretching rays in the presence of oxygen. The growth of etched tracks increases force on formation of fibers. Liu et al. discussed the formation of with gamma ray exposure in the presence of oxygen. This effect was poly(methyl methacrylate) (PMMA) polymer cup by playing with more prominent on the films bombarded with the lighter ions [92]. the concentration of polymer solution in different solvents viz. ace- Starosta et al. irradiated PET films with heavy ion beams obtained tone, dimethyl formamide, methylene chloride, acrylonitrile, nitro- generated by cyclotron and chemically etched with NaOH solution. methane and formic acid [106]. SEM images of different structures The pore size was obtained in 0.1–0.5 μm [93]. In the above discus- prepared from PMMA solution in methylene chloride, acetone and sion, PET and PC are most commonly used polymers for track etching nitromethane were shown in Fig. 10. due to their stability towards acids, organic solvents and mechanical Pai et al. studied the effect of non-solvent and humidity on the properties. In addition, attempts were also made to use fluorinated morphology and fiber size of the polystyrene (PS) electrospun fibers polymer viz. PVDF and its copolymers [94]. However, PVDF's resis- [107]. They found that fiber diameter of the PS fibers, electrospun tance towards strong oxidizers takes long hours for the formation of from 30 wt.% PS/DMF solution, increased from 0.9 μm to 3.93 μm pores. with an increase in relative humidity from 11 to 43%. The relative humidity affects the solidification rate of electrospun fibers, the fibers 2.5. Electrospinning electrospun above 24 %RH had smooth surface. Below 24 %RH the fibers have wrinkled surface. Due to the formation of wrinkles Electrospinning is a comparatively new technique to fabricate at low RH, the diameter of the fibers becomes smaller. porous membranes for various applications including filtration and Lin et al. studied the formation of micro- and nanoporous struc- desalination [95–97]. A high potential is applied between the polymer tures of the PS fiber by varying the solvent composition and solution solution droplet and the grounded collector. When the electrostatic concentration of PS solution [108]. The fibers electrospun from potential becomes sufficiently high and overcomes the surface 30 wt.% PS solution in THF showed ribbon like structure with densely tension of the droplet, a charged liquid jet is formed as shown in packed nanopores. With the addition of DMF in THF, nanopores dis- Fig. 9. The unique features of these fibrous membranes are con- appear and fiber surface becomes smooth. The decrease in concentra- trollable aspect ratios (aspect ratio = L/d; L—length of the fiber and tion of PS in DMF/THF resulted in the formation of beaded structure of d—diameter of the fiber) and morphology of the nano/microfibers, nanofibers. Fig. 8. Schematic showing single ion-irradiation setup used to fabricate track etched membrane [90]. B.S. Lalia et al. / Desalination 326 (2013) 77–95 85 Fig. 9. Schematic showing electrospinning of polymer solution. Kaur et al. fabricated the PAN nano-fibrous membranes of differ- helped to improve the salt rejection of the membrane at the expense ent fiber sizes by varying the concentration of PAN solution and of water flux. Further, the mean pore size, pore size distribution they found that a decrease in the fiber size diameter decreases the and mechanical properties of the electrospun PAN membranes can pore size of the microfiltration nano-fibrous membranes [109]. This be controlled by hot pressing. Hot pressing helps to fuse the fibers Fig. 10. Electroprocessed structures from (a) 1.5 wt.% PMMA in methylene chloride, (b) 8 wt.% PMMA in acetone, (c) 4 wt.% PMMA in nitromethane, and (d) 16 wt.% PMMA in nitromethane [106]. 86 B.S. Lalia et al. / Desalination 326 (2013) 77–95 together and reduce the thickness of the electrospun membrane size and free volume hole density resulting in improvement in water and improve the mechanical as well as pore size distribution diffusivity related to decrease in crystallinity of the membrane [120]. [96,110]. Recently, Prince et al. studied the effect of incorporation of Yu et al. reported that incorporation of higher concentration of SiO2 nanoclay in the PVDF nanofiber membranes on the hydrophobicity in the PVDF hollow fiber membranes resulted in a transition from and performance in direct contact membrane distillation [95]. Addi- α-phase to β-phase crystal structure and restricted the movement of tion of 8 wt.% nanoclay improves the membranes' contact angle PVDF. This led to deterioration of transport properties of the membrane. from ~ 128° to ~ 154° and salt rejection from 98.2% to 99.9% in a direct However, PVDF–SiO2 membrane with 3 wt.% tetraethyl orthosilicate contact membrane distillation process using 3.5 wt.% NaCl aqueous (SiO2 precursor) improved the UF, antifouling, mechanical and thermal solution. Kim et al. studied the effect of silica addition and thermal properties [121]. Peng et al. studied the effect of degree of cross-linking treatment in PVDF nanofibrous membrane [111]. Improvement in of PVA coating on the PS UF membranes performance. The increase in contact angle 128° to 134° with the addition of 40 wt.% tetramethyl PVA's degree of cross-linking improves the pure water permeability orthosilicate in PVDF nanofibrous membrane was observed. Further, and decreases the crystallinity of the PVA film [122,123]. Minelli et al. thermal treatment increased the surface roughness of the membrane investigated the effect of plasticization (glycerol as plasticizer) of due to formation of silica particles by sol–gel reaction of tetramethyl microfibrillated cellulose on the water sorption, diffusion coefficient orthosilicate. The increase in surface roughness of the fiber further and structure of polymer films. The addition of plasticizers generally increases the contact angle to ~ 145°. increases the mobility of the polymer chains in plasticized material which results in an improvement of water molecule diffusion [124]. 3. Structure–property–performance relationship Tseng et al. studied the effect of blending PVDF in the asymmetric cellu- lose acetate propionate on the porosity, pure water flux and thermal Usually, the membrane performance (flux, rejection and fouling) stability [125]. They found that up to 75 wt.% blending of PVDF, the is strongly influenced by membrane polymer properties, porous struc- pure water flux improved. But, above 75 wt.% crystallization dominates ture and specific membrane surface features [112–114]. The most resulting in the reduction of flux and porosity of the membrane. important parameters, which affect the membrane performance, such as crystallinity of the membrane based polymer, porous structure, hy- 3.2. Pore structure drophobicity/hydrophilicity, membrane charge and surface roughness are discussed in details below. Water flux and solute rejection in NF, UF, MF and MD are primarily controlled by the porosity, pore size distribution and pore tortuosity 3.1. Crystallinity of the polymer of the membranes. The aforementioned properties of the membrane are basically attributed by the pore geometry of the membranes. Crystallinity of polymers is a major property in determining Furthermore, the pore geometry of the membranes can be controlled the mechanical stability and permeability of the polymer, particularly by choosing an appropriate fabrication technique. A relationship be- for nonporous membranes. The glass transition temperature and tween fabrication technique and pore structure of the membrane crystallinity are determined by the chain flexibility, chain interactions helps to design a membrane for particular application. Some charac- and molecular weight of the polymer [115]. In general, most poly- teristic pore geometries of the membranes are illustrated in Fig. 11. mers exist in the semi-crystalline (consist of amorphous and crystal- The simplest geometry of the membrane is characterized by paral- line phases) form. In crystalline phase, the polymer chains are packed lel cylindrical pores perpendicular to the membrane surface. Typical in a regular pattern due to strong intermolecular interaction such example of parallel pore geometry is track etched membranes. The as hydrogen bonding in the case of PVA. The crystallites of polymer pore length is equal to the thickness of the membranes. Hagen– are connected by the randomly oriented molecular chains known as Poiseuille equation defines the permeate flux as: amorphous phase. The transport of liquid in NF (partially) and RO membranes is accompanied by the sorption and diffusion and their εr 2 ΔP J¼ ; product yields the permeability (sorption × diffusion) of the perme- 8ητ Δx able membrane. The crystallites of the polymers have highly packed structure; liquid permeates cannot penetrate inside the packed struc- where J is solvent flux, ɛ is surface porosity, r is pore radius, ΔP ture and the transport of liquid takes place through the amorphous is pressure difference across the membrane of thickness Δx, η is vis- layer. Peterlin et al. studied the effect of crystallinity, spatial distri- cosity of the solution and τ is the pore tortuosity. bution of crystallites and fractional free volume on the sorption and However, few membranes follow Hagen–Poiseuille equation, diffusion in polymers [116]. Diffusion coefficient and crystallinity of because of different pore geometries. Membranes prepared by phase in- the polymers can be described as [117]: version, stretching, solution casting and electrospinning have irregular pore geometry and tortuosity. Kozeny–Carman modeled the relation- n Di ¼ Di;o Ψc =B ; ship between flux and geometrical parameters of the membrane pores: where Di,o diffusion coefficient at zero concentration, Ψc is the frac- ε3 ΔP J¼ ; tion of crystalline material, B is constant and n is an exponential factor KηS ð1−εÞ2 Δx 2 (n b 1). Gliozzi et al. studied the transport behavior of cross-linked colla- where, ɛ is the volume fraction of pores, ΔP is pressure difference across gen membranes and found that filtration coefficient increases when the membrane of thickness Δx, η is viscosity of the solution, K is swelling of membrane increases in the amorphous state. The behavior Kozeny–Carman constant (depends on the shape of the pores and tortu- is opposite for the crystalline phase, the filtration coefficient decreased osity) and S is the internal surface area of the membrane pores. when swelling increased in crystalline phase [118]. Gholap et al. grafted In the phase inversion technique, the composition of the mem- N-tertiary butyl acrylamide on PVA molecular chains to improve the brane forming system and coagulation media control the porosity, hydrophobicity and reduces the swelling of the membrane. The heat pore structure and pore size distribution of the membranes. Matsuura treatment of the membranes induces crystallinity which results in re- and co-workers investigated the effect of hot water temperature duction of permeation flux through the membranes [119]. Lue et al. on the pore structure of CA membranes, effect of PVP additive in discussed the correlation between the water diffusivity and free volume PES membrane, dry jet-wet spinning conditions on the geometry of of the PVA. The addition of silica in PVA increases the free volume hole hollow fiber membranes [30,129–131]. They found that hot water B.S. Lalia et al. / Desalination 326 (2013) 77–95 87 (a) (b) (c) (d) Fig. 11. SEM micrographs of (a) stretched high density PE microporous membranes, cold stretching of 55%, followed by hot stretching of 75% [126], (b) surface modified PES phase inversion membranes [127], (c) track etched PC membrane irradiated with Kr ions [128] and (d) electrospun PVDF-co-hexafluoropropylene membrane [96]. treatment of cellulose, CA and cellulose triacetate membranes lead solution preparation, and studied the formation of highly dense to shrinkage of the surface pore size. Cellulose demonstrated high support membrane using 100% DMF and highly porous structure resistance to shrinkage compared to CA/tri-acetate. This is due to using 100% NMP. Also, the weight percent of the polymer was the highly ordered and close packed structure of cellulose i.e. due to changed and its effect on the porosity and pore shape of the PS mem- high crystallinity. Acetylation of cellulose reduces the crystallinity of brane was studied. They investigated how the transport properties of the cellulose and made it more sensitive to high temperature shrink- the support layer and active layer are important to be optimized for age than native cellulose [129]. In other report, they investigated the the high performance of the membranes. Penky et al. demonstrated effect of addition of PVP in PES UF membranes. The strong interaction the visualization of development of macrovoid pores in dry caste cel- between PES and PVP was observed when their weight ratio is in lulose acetate/acetone/water solution using video-microscopy flow- unity. The interactions were supposed to be due to O_C\N b visualization. They explained the formation of macrovoids in three unctional groups in the PVP and O_S_O functional group in the stages namely fast initial growth, slow growth and collapse (active PES polymer. These chemical interactions between PVP and PES collapse and passive collapse); and conclude that MV growth requires increased the largest pore size of the membranes resulting in an a homogeneous supersaturated solution layer separating the demixed increase in permeation flux through the membrane [30]. Recently, fluid layer from a homogeneous stable solution layer. Supersaturated the effect of polymer concentration and fabrication parameters on solution layer provides the driving force to the growing macrovoid the surface morphology, hydrophobicity, mean pore size and void [135]. Hernandez et al. studied the pore size distribution of track fractions of polyethereimide hollow fiber membranes were reported etched membranes and found that permeability of the membranes by Matsuura and co-workers [132,133]. It was observed that increas- is determined by the bulk pore size distribution [136]. ing PEI concentration in the spinning solution decreases the mean Wu et al. prepared PAN based symmetric membranes using pore size, effective surface porosity and formed the finger like thermally induced phase separation technique and investigated the macrovoids (below the surface layer) of the membranes. The small effect of glycerol content, polymer concentration and cooling rate pores at the surface decrease the wettability of the membrane and on the pore shape, pore size, porosity, water flux and mechanical macrovoids decrease the mass transfer resistance. They modified properties of the membranes [137]. The increase in PAN concentra- the hydrophobicity of the PEI hollow fiber membranes by introducing tion from 10 to 22 wt.% reduces the pore size of the membranes surface modifying macromolecules. Modified-PEI membranes have from 5.3 to 0.8 μm and correlated this with the viscosity of the higher mean pore size, permeation rate, inner and outer surface con- solution. The membranes have cellular like pores for mixed diluent tact angles. Tiraferri et al. relates the structure of PS support layer (glycerol and dimethylsulfone) with 20–30 wt.% glycerol, whereas and performance of TFC membranes [134]. They varied the weight needle like pores for 10–15 wt.% glycerol. The size of pores increases ratio of n-methyl-2-pyrrolidone (NMP) and dimethyl formanide when the temperature of the cooling water bath increases and the (DMF) (kept the polymer concentration constant), solvents used for same trend was observed when dried in air at different temperatures. 88 B.S. Lalia et al. / Desalination 326 (2013) 77–95 Zhao et al. prepared asymmetric PVDF membranes using solution by porous CA membranes decreases when acidity and hydrogen bond- cast and electrospinning methods [138]. The effect of concentration ing ability of the solutes decreased. Taft and Hammett parameters, of PVDF and weight ratio of dimethyl formamide (DMF) and tetrahy- which indicate an effect of the substituent group on polarity, were drofuran (THF) solution on structure formation was studied. Increase found to correlate with the rejection of these compounds [150]. It was in polymer concentration from 5 to 8 wt.% in DMF/THF (5/5 w/w assumed that highly polar compounds can sorb into CA membrane ratio) leads to the formation of entangled polymer network with material via hydrogen bonding, diffuse across the membrane and result small porosity and pore size. The effect of variation of solution con- in negative rejection values due to subsequent flux decline. It was also centration was studied by keeping the PVDF concentration constant shown [149,151] that polar compounds with sizes similar to membrane i.e. 5 wt.%. The increase in THF weight ratio in the solvent mixture pore diameters caused the greatest amount of flux decline through pore of THF/DMF results in the formation of small pores and porosity. blocking or adsorption within the pores. Razmjou et al. studied the effect of addition of TiO2 nanoparticles Apart from membrane rejection, hydrophilic–hydrophobic properties (mechanically and chemically modified) on the membrane perfor- of the polymer membrane have a crucial influence on its anti-fouling mance and its structural and mechanical properties [139]. Addition property and permeate flux [152,153]. It has been well documented of mechanically modified TiO2 increases the number of microvoids in that membranes with hydrophilic surfaces are less susceptible to fouling the membrane whereas chemically and modified TiO2 nanoparticles with organic substances, microorganisms, and charged inorganic parti- lead to elongation of finger like microvoids. These structural modifica- cles [154,155] due to a decrease in the interaction between the foulant tions with incorporation of TiO2 nanoparticles in the membrane im- and the membrane surface. prove the pore size, porosity and hydrophilicity of the membrane Nabe et al. found [156] a direct correlation between an extent which results in an improvement of water flux through the modified of membrane fouling and hydrophilic properties of the membrane membrane. surface. They showed that during filtration of bovine serum albumin Han et al. prepared the ultrafiltration membranes of carboxymethyl solution the relative flux decreases, which correlates with the mem- cellulose acetate (CMCA)/cellulose acetate (CA) using phase inversion brane fouling, with an increase of contact angle of the membrane method and studied the effect of addition of CMCA and polyethylene surface, i.e. with an increase in membrane hydrophobicity. glycol (PEG) on the structure of skin layer and sub-layer of the mem- Due et al. [157] improved hydrophilicity and surface smoothness brane. Incorporation of CMCA and PEG caused the formation of more of commercial UF PVDF membranes by surface coating with a PVA pores and macrovoids beneath the skin layer. Addition of CMCA de- aqueous solution followed by solid-vapor interfacial cross-linking creases the pure water contact angle on the surface of the membrane with glutaraldehyde. It was shown that during UF of surface water and improved the pure water flux through the membrane. with total organic carbon of 7 mg/l, the flux of the PVA/PVDF mem- brane was 14% higher than that of the unmodified PVDF membrane 3.3. Surface properties after 4 h of filtration and 95% higher after 18 h of filtration (Fig. 12). As can be seen in Fig. 12, the flux of the modified membrane reached 3.3.1. Hydrophilic–hydrophobic properties of membrane surface a plateau with increasing operating time, while the flux for the As known, the common-evaluation approach for hydrophilic- unmodified membrane kept decreasing thus indicating that foulants hydrophobic properties of membrane is based on evaluation of a contact continued to accumulate on the surface over time. Additionally, the angle formed between the liquid–gas tangent and membrane–liquid cake-fouling layer could also be more easily removed from the PVA boundary [61,140]. Most commercial membranes for pressure-driven modified membrane by alkaline cleaning. The higher performance of processes are made from hydrophobic polymers with high thermal, the modified membrane was related to the increasing in hydrophilic- chemical and mechanical stability. Usually these materials are charac- ity and smoothness of the membrane surface after coating with the terized by high contact angle values and are prone to adsorption of PVA layer. the various solutes from feed streams [13]. The major reason for hydrophobic membrane fouling with organic The adsorption of hydrophobic compounds onto membranes may compounds is that there are almost no hydrogen bonding interactions be an important factor in solute rejection during membrane applica- in the boundary layer between the membrane interface and water. tions. Recently, it was shown [141,142] that membranes with larger The repulsion of water molecules away from the hydrophobic mem- contact angles could reject and adsorb more mass per unit area of a brane surface is a spontaneous process with increasing entropy and hydrophobic solute than a membrane characterized by a smaller con- therefore foulant molecules have a tendency to adsorb onto mem- tact angle. Chang et al. [143] reported that the steroid estrone was brane surface and dominate the boundary layer. In contrast, the highly rejected with MF hollow fibers due to adsorption mechanism membrane with the hydrophilic layer possesses a high surface ten- despite of a wide pore size of the membrane. Estrone rejection sion and is able to form the hydrogen bonds with surrounding decreases with filtration time due to saturation of the adsorption water molecules to reconstruct a thin water boundary between the capacity of the membrane with solute. Similar studies examining membrane and bulk solutions. It is difficult therefore for hydrophobic adsorption of hydrophobic compounds onto NF and RO membranes solutes to approach the water boundary and break the orderly struc- found that the initial adsorption of hydrophobic compounds could ture because an increase of energy would be required to remove the not be considered a long-term removal mechanism since solute sorp- water boundary and expose the membrane surface [7]. Therefore an tion and rejection decreased with time [114,144–148]. increase in the hydrophilicity of the membrane surface is often a Kiso et al. [148] reported that the rejection of hydrophobic mole- key goal to reducing membrane fouling with colloids, microorgan- cules by NF membranes increased linearly with affinity of the solute isms and organic pollutants. For example, it was found that the im- for the membrane as expressed through the octanol–water distribu- provement on hydrophilicity can favor enhancing the permeate flux tion coefficient (Kow). Thus, hydrophobic interactions between the of RO membrane [152,158] due to increasing interaction between solute and membrane are the dominant rejection mechanism for membrane surface and water molecule via hydrogen bond and/or hydrophobic compounds. On the other hand, Van der Bruggen et al. electrostatic attraction. [149] concluded that although the Kow value was the best parameter It should be mentioned, however, that a contact angle on the mem- to describe the hydrophobic adsorption of compounds to membranes, brane closely related with its surface features, including functional molecular size of solute also played an important role. groups, zeta potential, and surface roughness [159,160]. It was demon- Apart from the hydrophobic compounds, highly polar organic solutes strated that the increase of the density of surface hydrophilic-group, can also interact with membrane surfaces. Matsuura and Sourirajan such as -OH, and -NH2, is favorable for improving the membrane hydro- showed [150] that rejection of alcohols, phenols, and carboxylic acids philicity [161,162]. Tang et al. [163] reported that the contact angles B.S. Lalia et al. / Desalination 326 (2013) 77–95 89 Fig. 12. Variation of the normalized fluxes (Grand River water flux/clean water flux) between base PVDF and modified PVA/PVDF membranes at the beginning (a) and at the end of UF (b). Duration of UF cycle is 2 h, followed by the membranes cleaning with NaOH solution [157]. of PVA coated RO membranes are smaller than those of uncoated that while the rejection of uncharged urea decreased slightly from membranes, due to the presence of abundant -OH groups of PVA mac- 35 to 28%, the rejection of acetic acid increased from 32% in the neu- romolecules on the membrane surface. High zeta potentials and rough tral form at pH 3 to 100% in the negatively charged form at pH 9. The surfaces of RO membranes are also favorable for the decrease of their increase in the rejection of acetic acid at pH values above the pK; is contact angles [152,164]. Generally, the rougher the RO membrane most likely caused by the increasing negative charge of the mem- surface, the smaller its contact angle [163]. It may be because the brane surface repelling the negatively charged ions of acetic acid. rough surface increases the solid–liquid interfacial free energy, and the On the other hand, the effect of membrane surface charge on larger free energy is favorable for the decrease of contact angle [164]. membrane pore structure, permeate flux and the rejection of un- However, the effect of surface roughness on membrane contact angle charged organics at various pH is somewhat contradictory. It has is smaller than that of surface functional groups. been reported that the rejection of uncharged solutes decreased, Additionally, the contact angle of the membrane sample is affected while permeate flux increased at high pH values (8–10) due to an in- by sample pre-treatment, water temperature and water salinity. Li crease in pore size of a membrane caused by the electrostatic repul- et al. [165] showed that contact angles of polyamide RO membrane sion between the functional groups within the membrane [179]. gradually decrease from 69.1° ± 0.8° to 54.0° ± 0.5° with the NaCl Some studies have found small dependence of the rejection of concentration increasing from 0 to 16.000 ppm. It was explained uncharged organics and permeate flux on pH [173,178]. It was also that because a large amount of electronegative carboxyl and hydroxyl found that increasing the pH of a feed solution leads to pore shrinkage groups is present on the membrane surface, these groups can adsorb of UF membranes and subsequently decreased permeability and Na+ ions from the solution. These adsorbed cations can orient sur- increased rejection [177]. In addition, it has been reported that salts rounding water molecules by Coulombic attraction with the water- present in the feed water could reduce the negative charge on a mem- negative dipoles [166]. Thereby, the interactions between the interfacial brane surface by “shielding” the charge [174]. It was found that the water molecules and RO membrane surface are strengthened. Debye length was small at higher ionic strengths, the zeta potential was more positive, electrostatic interaction was minimized within 3.3.2. Surface charge the membrane, and the pore radii could shrink [180,181]. At low Electrostatic interactions between charged solutes and a porous ionic strength when the Debye length is longer and the zeta potential membrane have been frequently reported to be an important rejec- is more negative, pore radii can increase in size to minimize electro- tion mechanism [112,167–170]. These interactions depend on the static repulsion between the negative functional groups in membrane membrane charge which is usually quantified by zeta potential mea- body [180,182]. surements [112]. The membrane surface of RO, NF and UF mem- The electrostatic charge of membranes is a particularly important branes often carries a negative charge to increase the rejection of consideration for the reduction of membrane fouling where foulants dissolved salts and minimize the adsorption of negatively charged or- are charged, which is often the case. When the surface and the ganic foulants and microorganisms [171,172]. Negative charge on the foulant have similar charge, electrostatic repulsion forces between membrane surface is usually caused by sulfonic and/or carboxylic the foulants and the membrane prevent the foulant deposition on acid groups in a skin membrane layer, which may be deprotonated the membrane thereby reducing the fouling [183,184]. For example in feed solution. Yoon et al. [173] showed that rejection of perchlorate a negative surface charge of the membrane will have a beneficial ions by negatively charged NF and UF membranes was greater than effect on separation proteins around neutral pH, because most of pro- expected based on only steric/size exclusions for these membranes teins have also negative charge at such conditions [13]. It was shown due to an electrochemical interaction mechanism. Usually, increasing that most of the colloidal particles such as natural organic matter the pH of feed solutions increases the negative surface charge of the (NOM) that deposit on the membrane surface are negatively charged membranes due to an increase in dissociation of carboxylic/sulphonic [185]. As a result, PVA coated highly negatively charged membranes functional groups [171,174,175]. As a result, electrostatic repulsion may exhibit stable flux due to the strong electrostatic repulsion between a negatively charged solute and membrane increases. between negatively charged NOM and membrane [163]. Although, Besides inorganic salts, the membrane charge also affects rejection as mentioned, to date RO/NF membranes are mainly negatively of charged organics. Hu et al. [176] and Schafer et al. [177] found that charged, however, development of positively charged membranes low molecular weight but charged organic acids had higher rejections with high hydrophilicity and rejection for multivalent cations could by RO and UF membranes than larger neutral organics due to electro- be also beneficial [186]. These membranes may be used for the recov- static repulsion. Ozaki and Li [178] studied the rejection of urea and ery of valuable cationic macromolecules in biotechnology. Similar to acetic acid, both having the same molecular weight, at different pH the negatively charged surface, the positively charged membrane sur- ranges with an RO membrane (ES20, Nitto Denko). It was shown faces exhibited electrochemical repulsion against positively charged 90 B.S. Lalia et al. / Desalination 326 (2013) 77–95 proteins [187], and may be used for the removal of heavy metals and relationship was attributed to surface unevenness of the RO membrane dyes from water [188,189]. skin layer, which resulted in larger effective surface area. Kwak and Ihm [195] did not find a linear relationship between 3.3.3. Surface roughness membrane surface roughness and flux in filtration tests with There is a strong correlation between the membrane fouling thin film polyamide RO membranes. The performance tests were car- and the surface roughness for RO and NF membranes. It was shown ried out with 0.2% NaCl solution at 15 bar and 25 °C. However, the that RO hydrophilic CA membranes with smooth membrane surfaces roughest membrane did have the highest flux. On the other hand, are less prone to colloidal fouling compared to the relatively more Stamatialis et al. [196] showed that the lower surface roughness of hydrophobic and rougher PA membranes [190]. CA and cellulose acetate butyrate membranes the lower the flux and As seen in Fig. 13, fluxes for commercial RO (Hydranautics LFC-1, the higher the rejection during filtration of 3500 ppm NaCl solution Trisep X-20) and NF (Dow-FilmTec NF-70, Osmonics HL) membranes at operating pressure of 40 bar. However, no explanation of these during filtration of NaCl solution with addition of silica particles findings has been provided. Madaeni [197] also found that the rough- decrease when surface roughness of the membrane samples increase. er RO membrane the lower the permeation rate due to the adsorption A greater roughness increases the total surface area to which and trapping of ions on the rough surface membrane. foulants can be attached, and the ridge-valley structure favors accu- Moreover, Al-Jeshi and Neville [198] showed that linear correla- mulation of foulants at the surface. As a result, membranes with tion between surface roughness and flux does not exist for the four rougher surfaces are observed to be more favorable for foulants' commercial Osmonics RO membranes. attachment resulting in faster fouling rates. With AFM technique In attempts to clarify the role that membrane morphology plays in Vrijenhoek et al. [114] clearly showed that particles preferentially transport through composite structures Ramon et al. [199] simulated accumulate in the “valleys” of rough membranes, resulting in “val- the interactions between rough coating films and the underlying ley clogging” which causes more severe flux decline than in smooth porous support membrane. They showed that: i) permeability in- membranes. Abu Seman et al. [191] also reported that irreversible creases with surface roughness if the roughness is created while pro- fouling of PES membranes increases with an increase of surface ducing thin regions in the coating film, essentially reducing the base roughness during filtration of humic acids. film thickness; however, when roughness is formed on top of an Using an AFM adhesion force measurement technique, Bowen unvarying base film thickness, the permeability of the film decreases et al. [192] characterized the interaction force between a colloidal with increased roughness; and ii) when surface roughness comes at silica probe and a rough membrane surface. It was found that mem- the expense of base film thickness, the thinner regions (“valleys”) brane surface roughness significantly reduced electrostatic repulsion present locally higher flux (“hot spots”) than the thicker regions between the colloid and the surface, and the valley regions experi- (“peaks”), and hence, these hot spots may be points of initiation for enced a greater adhesion force. colloidal and organic deposition, as well as mineral scale formation. On the other hand, Boussu et al. [193] suggested that while colloi- Note that membrane surface roughness is not a fixed value and dal fouling was affected by both membrane roughness and hydropho- can change upon varying the conditions the membrane is exposed bicity, membrane hydrophobicity seems to play a more significant to. For example, it was found [198] that water adsorption on TriSep role for promoting fouling. Similarly, Park et al. [194] also observed X20 membrane causes up to 35% change in the membrane surface lower fouling potential for smooth and hydrophilic semiaromatic roughness. Additionally up to 33% difference in surface roughness piperazine based PA membranes compared to the more hydrophobic was observed upon imaging different areas of the membrane. These m-phenylene-diamine based fully aromatic membranes. These au- important factors are often ignored when establishing correlations thors attributed the greater anti-fouling tendency to the large repul- between roughness and membrane performance parameters. sive acid–base interaction for the more hydrophilic poly(piperazine) It should be noted that while analyzing membrane fouling with membranes. complex feed solutions different membrane surface properties should While results on membrane fouling depending on surface rough- be taken into account. Jin et al. [200], suggested when feed contains ness seem fairly consistent (i.e., rougher membranes foul more both positively and negatively charged pollutants, a smooth hydro- quickly and are harder to clean), contradictory findings are reported philic surface with no carboxylic moieties is presumably the best on effect of surface roughness on membrane flux: higher surface rough- membrane option. In their study, the membrane with a hydrophilic ness can mean higher flux, lower flux or have no effect on flux. Hirose PVA coating showed significantly better fouling resistance against et al. [113] suggested an approximately linear relationship between alginate compared to the membrane without PVA coating. membrane surface roughness and flux for cross-linked aromatic poly- On the other hand, Tang et al. [201] found that fouling by humic acid amide RO membrane, where permeability increased with increasing and surfactant was less significant for semi-aromatic PA membranes surface roughness. The tests were conducted using 1500 ppm NaCl (smooth and hydrophilic surfaces) under mild fouling condition. solution (pH 6.5) at 15 bar pressure and 25 °C temperature. The linear However, membrane properties were completely masked by the foulant layer and further fouling tended to be dominated by foulant– deposited-foulant interaction under severe fouling conditions [202]. An overall comparison between the various methods of fabrica- tion of polymer membrane, the membranes properties and perfor- mance are presented in Table 3, which is our interpretation of the available literature data. It should be noted, that here we only attempt to give a general comparison, because it is quite complicated to inter- pret and compare results not only for different fabrication methods but even for the same fabrication technique. The reason for this is that the formation of membranes with each of fabrication methods depends on a large number of material- and process-specific parame- ters. For example, the formation of membrane made by the immer- sion precipitation depends on choice of the polymer, solvents and Fig. 13. Correlation between the surface roughness of commercial RO/NF membranes additives, composition and temperature of the casting solution, and their flux decline for the filtration of 0.05 M NaCl solution, which contained choice of the quench medium and its temperature, evaporation con- 200 mg/L silica particles (0.10 μm). Plotted using the data of Table 1 from [114]. ditions, casting thickness, type of membrane support material, drying B.S. Lalia et al. / Desalination 326 (2013) 77–95 91 Table 3 Membrane fabrication methods: fabrication parameters and membrane performance and properties. Fabrication method Fabrication parameters Membrane performance Membrane properties Simplicity Reproducibility Cost Flux Retention Antifouling properties Width of pore size distribution Surface roughness Surface charge Phase inversion H H L H M M H H H Interfacial polymerization H H L L H H L L H Stretching M M M H L L M L L Track-etching L H H L L M L L L Electrospinning M M M H L M H M L H — High; M — Medium, L — Low. conditions, etc. Additionally many membrane properties may be solute rejection, the role of membrane structure and surface properties influenced simultaneously with the same fabrication process. in membrane performance is still not thoroughly understood. A system- It can be seen in Table 3 that membrane properties and per- atic work is still needed to clarify for example the role that membrane formance can be tuned to larger extend through the discussed morphology plays in trans-membrane transport, the effect of surface membrane fabrication methods. Phase inversion and interfacial poly- roughness on membrane flux during membrane treatment of complex merization are probably the most versatile and cost effective mem- feed solutions with organic and inorganic species, the influence of brane fabrication techniques for preparation of polymer membranes membrane surface charge on membrane pore structure at various pH, in a wide range of pore sizes (from RO to MF membranes) with vari- or for studying foulant-membrane interactions under severe fouling ous hydrophilic/hydrophobic and charged properties at high values conditions. Identification of these parameters and others which can of average porosity. However in many cases the width of pore size effectively be used to predict relationships between membrane fabrica- distribution for phase inversion membranes should be improved to tion, structure, surface properties and performance could be very mean- provide better retention properties. The additional advantages of ingful for further development of membrane based technologies for the interfacial polymerization include the possibility of tuning inde- water treatment. pendently the properties of skin/support layers and a low surface roughness of the prepared membranes, which are of great impor- List of abbreviations tance for development of low fouling TFC polymer membranes for AFM atomic force microscopy pressure-driven membrane processes. Stretching is a simple and BSA bovine serum albumin low cost method for fabrication of highly porous hydrophobic mem- CA cellulose acetate branes for MF and MD, while these membranes are still prone to DMF dimethyl formamide organic and (bio) colloidal fouling. The track-etched polymer mem- HFP hexafluoropropylene branes are usually in a MF range of pore sizes with low surface rough- IP interfacial polymerisation ness and their unique futures are practically ideal cylindrical shaped Kow octanol–water distribution coefficient pores and a very sharp pore size distribution. However because of MD membrane distillation low surface porosity, the membrane fluxes are rather low. The other MF microfiltration drawback of these membranes is high cost, obviously, due to techni- MPD m-phenylenediamine cal complexity of their fabrication. With respect to electrospinning, NF nanofiltration it is a very promising method for preparation of highly productive NMP n-methyl-2-pyrrolidone hydrophobic membranes for MF and MD but is in need of further de- NOM natural organic matter velopment in terms of pore size distribution, antifouling properties, PA polyamide cost reduction and fabrication on a large scale. PAA poly(amic acid) PAN polyacrylonitrile 4. Conclusions PC polycarbonate PE polyethylene In this review, relationships between the synthesis of polymer PEEK polyetheretherketone membranes, their structure, surface properties and performance PEG poly(ethylene glycol) were discussed. It was shown that to date remarkable progress has PES polyethersulfone been made in the fabrications of membranes for water treatment. PET polyethylene terephthalate However, there is still a challenge to produce reliable membranes Pluronic poly(ethylene oxide)-b-poly(propylene oxide)-b-poly with anti-fouling properties, high mechanical strength, high tolerance (ethylene oxide) on chlorine attack and minimal thickness of the membrane barrier PP polypropylene layer to provide a high flux. To ensure progress in these fields, PPESK poly(phthazine ether sulfone ketone) more efforts are needed for further improvement of common mem- PS polysulfone brane fabrication methods as well as the development of new fabri- PTFE ` poly(tetrafluorethylene) cation techniques. A comprehensive understanding between porous PVA polyvinyl alcohol structure-surface properties and the performance of membranes in PVAc polyvinyl acetate water treatment processes is crucial for further development of poly- PVDF polyvillidene fluoride mer membranes and optimization of fabrication processes. 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