RSC Advances REVIEW. A critical review on recent polymeric and nanoenhanced membranes for reverse osmosis. 1. Introduction

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1 RSC Advances REVIEW Cite this: RSC Adv., 2016,6, 8134 Received 25th August 2015 Accepted 9th December 2015 DOI: /c5ra17221g A critical review on recent polymeric and nanoenhanced membranes for reverse osmosis Adewale Giwa, Nawshad Akther, Virginie Dufour and Shadi Wajih Hasan* In this paper, current and recent advances in polymeric and nano-enhanced membrane development for reverse osmosis have been reported in terms of membrane performance and fouling. Graphene, zeolites, carbon nanotubes, silica, silver, and titanium dioxide are the predominantly tested nanoparticles in current and recent investigations. Membranes from graphene, zeolites, and carbon nanotubes have all been shown to enhance membrane water permeability. Silica has been observed to exhibit high affinity for water and improve the hydrophilicity of RO membranes. Silver and titanium dioxide have strong antimicrobial properties and can be included in RO membranes to reduce biofouling. However, the use of nanomembranes for commercial and industrial RO applications is still under development as their scalability is still a challenge. Polymeric membranes, such as cellulose acetate and polyamide, and their integration with other polymers or nanoparticles have also been presented in this paper. Overall, the choice of membrane materials for future RO processes would depend largely on the required permselectivity and the targeted foulants. However, membrane performance and antifouling features would have to be taken into consideration for sustainability of the type of RO membrane desired for a specific application. 1. Introduction Over the last few years, the market share of reverse osmosis (RO) has witnessed continued domination in desalination when compared with other technologies. 1 RO tends to be the most preferred choice because it is simple and requires relatively low speci c energy consumption; therefore, more than half of the total installed desalination plants in the world today use RO technology. 2 Progress in RO desalination has also been directed towards RO membrane synthesis in recent times. It seems RO is indispensable for today s pure water production and many membrane production rms have sprung up lately. Although commercial RO membranes are known for their high salt rejection rates currently, 3 it is worth noting here that fresh water scarcity is still prevalent and many people still do not have access to usable water as a result of natural and anthropogenic factors. 4,5 The RO market is currently a multibillion dollar market and a huge amount of money is being spent yearly on the research and development of new membrane materials and technology in order to reduce the fresh water supply shortfall. 2 Advancements in RO membrane materials in terms of performance and fouling resistance are necessary to further improve the contribution of RO to the world s pure water production. Department of Chemical and Environmental Engineering, Masdar Institute of Science and Technology, P.O. Box 54224, Abu Dhabi, United Arab Emirates. agiwa@ masdar.ac.ae; nakther@masdar.ac.ae; vmdufour@masdar.ac.ae; swajih@masdar. ac.ae The currently available RO membrane materials still suffer from fouling and regular replacement, leading to a high lifecycle cost of the RO process. 6,7 Therefore, materials that can maintain high salt rejection, high water ux and productivity, and low vulnerability to fouling are needed to produce RO membranes at low cost so that fresh water can be easily made available. Improved technologies are needed for the fabrication, testing, and commercial applications of these RO membranes so that enhanced performance can be guaranteed. RO is a pressure driven process that permits the ow of water through a semipermeable membrane by a solution-diffusion mechanism. 1,8,9 Because high pressure is needed to stimulate water diffusion and subdue the osmotic pressure of the saline feed, the properties of RO membranes are quite important as factors that signi cantly determine the process performance and cost. Recently, some state-of-the-art RO membranes have been prepared from nanomaterials, polymers and their combinations in order to overcome the challenges of limited water ux, salt rejection and the propensity for membrane fouling. The most recent articles on RO membrane materials have presented novel fabrication methods, improved membrane development conditions, newer composites or combinations of materials to integrate their competitive advantages, advanced membrane structural modi cation methods, and challenges and/or drawbacks of the latest RO membranes. Nanomaterials are dominating the current wave of novel membrane material development because of their inherent speci c physicochemical features that make them 8134 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

2 Review suitable for the RO process. 10,11 Buonomenna 12 has indicated that the use of nano-enhanced RO membranes for desalination has become attractive recently as a result of the tendency of nanomaterials to improve the ux of fresh water through the membranes and reduce the energy required for transmembrane transport. Buonomenna 12 emphasized that cost savings might be achieved from nano-enhanced RO membranes as a result of reduced energy consumption, the antifouling characteristics of some of these membranes, and the requirement for lower quantities of cleaning chemicals for fouling control. These bene ts would translate to the reduction of the environmental impact associated with RO desalination. Although the commercially feasible use of nanomembranes for practical large-scale RO desalination is yet to be achieved, the commercial scale-up of effective small-scale nanomembrane systems would be attained if the performance improvements that have been recorded recently are sustained and can be reproduced in real-life desalination. Graphene, zeolites, carbon nanotubes, silica, silver, and titanium dioxide are the predominantly tested nanoparticles in current and recent investigations. Membranes from graphene, zeolites, and carbon nanotubes have all been shown to enhance membrane water permeability Silica has been shown to exhibit high affinity for water and improve the hydrophilicity of RO membranes. 16 Silver and TiO 2 have strong antimicrobial properties and can be used to develop RO membranes that are less susceptible to biofouling. 17,18 However, the use of nanomembranes for commercial and industrial RO applications is still under development and scaling membranes from nanoparticles is still a challenge. 10,19 Lately, some advances in polymeric membrane development have been reported in the literature. The integration of commonly used polymeric materials such as cellulose acetate and polyamide with other polymers or nanoparticles has ensured an improvement in the membrane performance. Therefore, this review presents the most recent RO membranes, their fabrication, properties, applications, and performance characteristics. The bene ts and drawbacks of these membranes are also explained. The modi cation of the membrane surface features through material selection has also been presented. The impact of the operating conditions on the performance of recently developed RO membranes and state-ofthe-art methods that have been devised to curtail membrane fouling are also discussed in this review. 2. Nano-enhanced membranes and polymer nanocomposites 2.1 Graphene and graphene oxide Today, graphene could be an interesting ultra-permeable membrane for RO due to its atomic thickness and its precise sieving properties. 13,20 26 However, there is limited information regarding the ideal conditions for the use of graphene in terms of the operating pressure in the literature. 27 Meanwhile, the water permeability of nanoporous graphene (NPG) under different pressures has been studied for the RO process. 27 A RSC Advances molecular simulation through visual molecular dynamics (VMD) was applied to see if real-world RO pressures ( bars) could have an impact on the ultrahigh water permeability of NPG. The data showed that NPG maintained its ultrahigh permeability at high pressures. A permeability of L m 2 h 1 bar 1 was achieved for NPG, assuming a nanopore density of cm 2. This work proved that NPG could be a high-performing membrane for RO desalination. However, this is only a theoretical study; an experimental investigation would be required to con rm the simulation results obtained. On the other hand, the successes recorded from the incorporation of graphene oxide (GO) into polymeric membranes for water permeability improvements have continued lately. Some recent research has been directed towards the incorporation of GO and poly(allylamine hydrochloride) (PAH) via electrostatic interactions on a porous poly(acrylonitrile) support in a layerby-layer assembly. 28 Characterization techniques revealed that the mass of GO was 2 5 times higher than that of PAH in each GO PAH bilayer of 16.5 nm thickness. The transport of water and selected solutes were studied for this membrane. It was shown that the water permeability of the GO membrane was about one order of magnitude higher than that of the commercial polymeric membrane. The GO membrane retained a tight structure in solutions of low ionic strength and exhibited a high rejection of sucrose (99%) with a membrane weight cutoff of around 342 or an equivalent pore size of around 1 nm. However, this membrane was only well suited for applications in the pressure retarded osmosis (PRO) or forward osmosis (FO) modes. The performance of different kinds of RO membranes for FO desalination has been discussed elsewhere. 29 Safarpour et al. has also prepared a novel thin lm nanocomposite (TFN) RO membrane via interfacial polymerization (IP) of m-phenylenediamine (MPD) and trimesoyl chloride (TMC) monomers, and introduced a reduced hydrophilic graphene oxide titanium dioxide (rgo TiO 2 ) nanocomposite in the membrane s active PA layer to enhance the membrane water ux and salt rejection. 20 The rgo TiO 2 nanocomposite was prepared by employing a hydrothermal method, and was then distributed into the MPD solution before being embedded into the PA layer. The three-dimensional atomic force microscopy (AFM) images showed that the modi ed membranes had a rough surface (Fig. 1). It was observed from contact angle measurements that the hydrophilicity of the membranes could be increased by increasing the concentration of rgo TiO 2 because of the existence of several negatively charged and hydrophilic functional groups on the rgo TiO 2 nanocomposite material. The best performance for the modi ed membranes was observed using 0.02 wt% rgo TiO 2 to obtain a water ux of 51.3 L m 2 h 1 (LMH) and salt rejection of 99.45%. A GO thin lm composite (TFC) membrane has also been prepared through fractionation and dispersing in a solution of MPD. 30 The fabricated membrane was then characterized and changes in the hydrophilicity, surface charge, surface roughness and thickness were noticed. It was observed that the GO embedding enhanced the water permeability and anti-biofouling property by 80 and 98%, respectively. Furthermore, with a chlorine concentration at ppm, a high salt rejection was still This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

3 RSC Advances Review Fig. 1 Three-dimensional images of rgo TiO 2 enhanced TFN membranes. 20 retained. Overall, the GO layer enhanced the performance of the TFC membrane. A rgo TiO 2 nanocomposite has also been integrated into the active polyamide (PA) layer of PA TFC RO membranes in order to impart antifouling properties to the membranes. Filtration of bovine serum albumin (BSA) solution was used to test the antifouling and chlorine-resistant properties of the developed membranes. 20 The best antifouling ability was demonstrated by the RO membrane with 0.02 wt% rgo TiO 2. Moreover, the chlorine tests revealed that the salt rejection declined by 30% for the pristine TFC RO membranes, and declined by only 3% for a TFC RO membrane integrated with 0.02 wt% rgo TiO 2.It was concluded that the developed nanocomposite has better antifouling and chlorine-resistant efficiency when compared with the pristine PA TFC membranes. 2.2 Zeolites Zeolites as nanomaterials have also been used for RO membrane development. Zeolites are chemically stable and have the necessary characteristics required for rejecting ions. 10,31 34 In order to prevent the expensive pretreatment required for current polymeric RO membranes, Zhu et al. have recently proposed the implementation of mordenite framework inverted (MFI)-type zeolite membranes for recycled saline wastewater desalination. 32 A collective method of rubbing and secondary hydrothermal growth was employed to develop the MFI-type zeolite membrane on a tubular a-alumina substrate. The zeolite membrane demonstrated a water ux of 4 LMH and a salt rejection of 80% when tested at a pressure of 7 MPa, utilizing recycled wastewater from a local treatment plant situated in Melbourne, Australia as the feed. The zeolite membranes also attained a rejection of 90% or higher for divalent calcium and magnesium ions, and a rejection greater than 70% for monovalent potassium and sodium ions when tested at 21 C with applied pressures of 3 MPa and above. The modi ed membranes also eliminated more than 90% of the organic compounds comprising protein-like and aromatic substances at 21 C with applied pressures of 3 MPa and above. Batch concentration runs with saline wastewater were conducted for a test period of 48 hours, and the results demonstrated a water recovery of 43%. Also, a marginally lower salt rejection was observed when the zeolite membrane was tested for RO performance using saline wastewater as the feed. Additionally, the water ux remained approximately at 2 LMH during the test period of 48 hours, which indicated the resistance of the zeolite membrane to organic fouling. Moreover, the long-term chlorine stability tests conducted for 7 days using strong hypochlorite cleaning (chlorine exposure of mg L 1 h) revealed that the zeolite membranes did not signi cantly retard the water ux or salt rejection, thereby proving the outstanding chemical resilience of the zeolite membranes. Hence, easy biofouling control and cleaning techniques were also observed. However, there is a need to enhance the water ux and salt rejection so that the proposed zeolite membranes can compete with the current polymer RO membranes to achieve water of higher purity. Zeolites may be affected by the ph of treatment. The chemical resistance and acid stability of Linde Type A (LTA) zeolites during the treatment of synthetic low ph wastewater brine prepared by the United States National Aeronautics and Space Administration (NASA) has also been investigated in order to evaluate the effectiveness of LTA zeolites in producing potable water. 35 It was observed that the LTA zeolites disintegrated the synthetic wastewater brine quickly. The effects of monobasic potassium phosphate, sulfuric acid, chromium(vi) oxide and monobasic sodium phosphate dihydrate, which are acid producing components present in wastewater brine, were studied precisely to identify the acidic components that caused degradation. The impact of the acids on the surface morphology, bonding, solid phase composition and crystal structure of the LTA zeolites was examined. It was observed that the zeolites only incurred some damage a er 24 hours when exposed to the synthetic wastewater brine at a ph of However, the LTA zeolites were degraded extensively a er 128 hours of exposure to synthetic wastewater brine at a ph of The LTA particles crystal structure was damaged completely by all acidic solutions and phosphate solutions with ph values below 1 and 5, respectively. It was observed that the elimination of aluminum from the framework of the zeolite caused the structural degradation. Moreover, phosphate removed the silicon from zeolite framework and formed precipitates of aluminum phosphate, thereby altering the zeolite bonding. Furthermore, it was revealed that the acidic anions, rather than the ph, played the most important role in damaging the LTA structures present in the solution containing dihydrogen phosphate. In conclusion, the experimental results indicated that the LTA zeolites were not suitable for applications in low ph environments, and especially in the presence of phosphate. The integration of zeolites into a PA membrane has also resulted in an impressive membrane performance. For example, TFN membranes incorporated with NaY zeolite nanoparticles have been fabricated on polysulfone supports by IP of MPD and TMC. 31 These zeolite nanoparticles were 8136 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

4 Review characterized as having cubical shapes with a particle size range of nm (Fig. 2). It was observed from the experimental results that a higher salt rejection could be obtained by increasing the IP reaction time in order to obtain a denser zeolite PA layer. A zeolite loading of 0.15 wt% was found to be the optimal loading. The inclusion of NaY zeolite nanoparticles in the TFN membranes enhanced the water ux from to LMH with a salt rejection of 98.8% under the optimal conditions of 25 C, 1.55 MPa, and 2000 ppm NaCl solution. The water ux was further enhanced to LMH, but with a salt rejection of 98.4% a er the post-treatment of the TFN membranes using the optimal solution composition of sodium lauryl sulfate, camphorsulfonic acid triethylamine salt and glycerol. The water ux of post-treated TFN membranes with NaY zeolite nanoparticles was two times higher than that obtained using a TFC membrane devoid of zeolite nanoparticles. The feasibility of employing zeolite incorporated membranes for seawater desalination using RO has been investigated in another study. 36 Two kinds of zeolite hydrophilic faujasite (FAU) and hydrophobic MFI with distinct wetting properties were used for RO membrane fabrication. A salt rejection of almost 100% was achieved using both membranes. Additionally, the membrane permeability was signi cantly improved to 720 L m 2 h 1 bar 1 for a membrane thickness of less than 3.5 nm, which is 100 times higher than the permeability of commercial RO membranes. Moreover, the pressure drop in the FAU membrane was found to be less due to its hydrophilicity, whereas a larger pressure drop was observed in the hydrophobic MFI zeolite membrane due to capillary resistance. It was concluded that the pressure drop during desalination could be sustained by placing the nanoscale zeolite membrane on a porous substrate and optimizing the ratio of the nanomembrane thickness to the pore radius of the supporting substrate. NaX zeolite nanoparticles have also been used in TFN membranes for an improved membrane performance. 37 The TFN membranes comprising NaX zeolite nanoparticles were synthesized on porous polyethersulfone (PES) ultra ltration (UF) supports by IP of MPD and TMC. The results obtained demonstrated that the NaX zeolite TFN membranes have higher water permeability and greater thermal stability compared to the pure PA membranes. In addition, increasing the content of NaX zeolite nanoparticles reduced the lm thickness of the PA RSC Advances membrane and enhanced its surface properties, water ux, and pore size. Increasing the concentration of MPD and TMC in the IP of the TFN membranes resulted in a higher water ux but reduced solute rejection. The optimum membrane performance was achieved for TFN membranes comprising approximately 0.2% (w/v) NaX zeolite nanoparticles, 2% (w/v) MPD and 0.1% (w/v) TMC. The optimum water ux through the hybrid membrane was 1.8 times greater than that of the pure PA membrane with no change in the salt rejection. Interesting results have also been reported for a hybrid membrane integrated with zeolitic imidazolate framework-8 (ZIF-8). The effect of introducing ZIF-8, which is a microporous and hydrophobic hybrid material, as ller into the PA selective layer of TFN RO membranes has been investigated experimentally. 14 The incorporation of ZIF-8 nanoparticles enhanced the water permeability by up to 162% at 0.4% (w/v) loading compared to pristine PA membranes. Salt rejection, however, remained at 98% for simulated brackish water (2000 ppm sodium chloride solution) desalination with a hydraulic pressure of 15.5 bars. Additionally, the inclusion of ZIF-8 caused the TFN selective layer surface to become more hydrophilic and less cross-linked compared to that of the pristine PA membrane. In general, a higher water permeability was obtained using microporous hydrophobic llers (silicalite-1, carbon nanotubes and ZIF-8) compared to hydrophilic llers (zeolite 4A). This is because of the greater compatibility of ZIF-8 with the PA matrix, and faster transport of water within the hydrophobic framework due to less attraction of water to the pore wall compared to the classic zeolite hydrophilic framework. In addition, the performance of the TFN RO membranes can be improved further by reducing the particle sizes of ZIF-8 and including a metal organic framework with an identical surface hydrophobicity but marginally bigger pores. Another study on a nanocomposite zeolite membrane with aminated template free zeolite nanoparticles (atma) has given impressive water uxes. 38 Kim et al. fabricated nanocomposite TFC RO membranes from a sulfonated poly(arylene ether sulfone) material comprising aminated template free zeolite nanoparticles (atma) and amino groups (apes) to improve the chlorination resistance and membrane performance. 38 The proposed membranes were synthesized via IP. Based on the performance test, the nanocomposite membranes had a water ux and salt rejection of 37.8 LMH and 98.8%, respectively. A er chlorination, the water ux increased by 2.5 LMH but the salt rejection was reduced by 12.7% compared to that of pristine PA. An improved performance of the hybrid RO membrane resulted from the nanoparticle loading because of the change in the three-dimensional structure of the PA network. As a result of this structural modi cation, a high degree of cross-linking leading to stiffness of the copolymer chain in the RO membranes was ensured. The aminated nanoparticle composition protected the underlying active layer from chlorine attack. Consequently, apes and atma facilitated an increase in water permeability and improved the chlorine resistance. The active layer of the RO membrane was also protected from degradation. Fig. 2 SEM image of NaY zeolite nanoparticles. 31 This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

5 RSC Advances 2.3 Silica Silica nanoparticles belong to another class of nanomaterials that have been tested recently for improved RO membrane performance. Recent studies have recommended the incorporation of silica with composite membranes for improved water permeation Pure silica membranes can exhibit microporous structures but would lose stability during desalination because the water permeating though the micropores would increase the pore sizes of the silica lm by interacting with the silane groups. 42 Therefore, the incorporation of silica in polymeric nanocomposites may be necessary for chemical and mechanical stability. A modi ed acetate polyethylene glycol (PEG) polymer has been combined with silica to produce different nanocomposite RO membranes with different levels of performance. 40 This work entailed the optimization of the salt rejection and the permeation properties of the modi ed acetate PEG membranes with varying amounts of silica for RO. A 2-stage phase inversion protocol with varying cellulose acetate (CA)/PEG ratios was followed in order to optimize the membranes. The incorporation of silica into the membranes increased the permeation properties of the fabricated membranes in terms of: water ux (from 0.35 to 2.46 LMH), salt rejection (increase of 11.41%), and mechanical stabilization (from 1 to 4%). The improved trend observed for all the measured membrane performance properties resulting from the addition of silica showed the need to optimize the silica particle loading because the silica had a negative effect on the hybrid RO membrane properties beyond a critical load. Peyki et al. has modi ed the surfaces of PA TFC RO membranes by incorporating silica nanoparticles on the PA active layer. 16 Different amounts of silica nanoparticles were added to the amine solution. The hydrophilicity of the modi ed membrane was higher than the pristine membranes as observed from contact angle measurements. The hydrophilicity of the membrane surface was found to increase with increasing silica content in the aqueous solution. Additionally, a signi cant alteration in the membrane surface structure occurred due to the incorporation of silica nanoparticles. The roughness of the membrane surfaces increased with an increase in the silica concentration in the amine solution. Moreover, cross- ow permeation tests were used to test the performance of the modi ed membranes at an operating pressure of 44 bars with aqueous sodium chloride solution. The short-term experimental results indicated that the water ux of the modi ed membranes progressively improved at lower nanoparticle levels, up to 0.1 wt%, and then reduced steadily. Furthermore, an increase in sodium chloride rejection was observed for lower silica concentrations (0.005 and 0.01 wt%) followed by a reduction in salt rejection with increasing silica concentration in amine solution. Long-term experiments revealed a lower water ux decline for the modi ed RO membranes compared to pristine TFC RO membranes. Lastly, the modi ed membranes also had better anti-fouling properties than the unmodi ed membranes. Silica nanoparticles have also been synthesized on a CA TFC RO membrane to impart antibacterial properties to the Review membrane. This work studied the effect of the development of dry-cast CA TFC RO membranes by swelling in roomtemperature water baths before the annealing treatment. 45 A er the swelling in water, the performance of the membranes was also studied, as well as the membrane s physical properties, by ellipsometry, contact angles, and SEM. It was found that the silica CA TFC membrane could reduce the bacterial density in both the feed solution and on the surface of the membrane for biofouling control. The CA TFC membrane seemed to be a good platform to study the CA selective layer in the RO membrane and the results obtained also showed that a combined swelling and annealing treatment improved the desalination performance with a salt rejection of 94% for CA TFC membranes compared to 55% obtained for asymmetric CA RO membranes without deterioration of ux. The water swelling time improved the permeability and the selectivity was slightly increased. Hydrophilic water-swollen TFC membranes can also be used for other applications such as the removal of water vapor, carbon dioxide, and hydrogen sul de simultaneously from agro biogas. 46 Swelling of RO membranes has also been used lately to detect a new fouling mechanism in the membranes Other nano-enhanced membranes Recent insight on the integration of nanoparticles with polymer-based membranes has revealed the antifouling properties of these nanoparticles. Today, silver nanoparticles are largely considered to be biocidal but the loading of silver nanoparticles onto membranes is still a real challenge. 11,19,48 51 Meanwhile, a new procedure aimed at loading silver nanoparticles on TFC RO membranes has been developed. 19 The method involved the reaction of the silver salt with a reducing agent on the membrane to form a properly-bound nanocomposite. The results showed that water permeability through the nanocomposite membrane was affected by silver nanoparticle functionalization. 19 Water permeability decreased by up to 17%, whereas improvements in the salt selectivity, hydrophilicity, zeta potential, and surface roughness were achieved. Interestingly, a strong antibacterial activity was reported with a reduction of more than 75% in the number of live bacteria on the membrane and with a 41% reduction in the total biological volume. The simplicity of the method, short reaction time, ability to load the silver nanoparticles on-site, and strong imparted antibacterial activity highlight the potential of this method for real-world RO membrane applications. Novel surface coatings have also been developed on PA TFC membranes using biocidal silver nanoparticles and antifouling polymer brushes via polyelectrolyte layer-by-layer (LBL) selfassembly. 17 The polyelectrolyte LBL lms contained poly(acrylic acid) (PAA) and poly(ethylene imine) (PEI). The PEI was used either as pure PEI or in the form of silver nanoparticles coated with PEI (Ag PEI). Because PA TFC membranes are prone to biofouling due to their characteristic physicochemical surface features, the membrane surfaces were modi ed by coating in order to enhance their antifouling properties for this study. Then the surface energy and hydrophilic properties were studied to show the microbial adhesion to the surfaces. The 8138 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

6 Review results showed that in general, modi ed surfaces containing silver nanoparticles demonstrated a low adhesion in a microbial adhesion test with E. coli and the membranes coated with PAA Ag PEI could remove 95% of the bacteria attached to the surface with 1 hour of contact time. Both antifouling and antimicrobial studies proved that these novel coated membranes could be a potential solution to prevent and control biofouling in PA TFC RO membranes. The in uence of integrating a polyhedral oligomeric silsesquioxane (POSS) nano ller and PA on water ux and salt rejection improvements has also been investigated. 52 This study involved the investigation of the in uence of adding a POSS nano ller in a PA selective layer. POSS is a hybrid intermediate silicone-based compound which behaves like a nanostructured ber when mixed with a polymer. 53 Four different POSS materials (P-8NH 2, P-8phenyl, P-1NH 2 and P-8NH 3 Cl) were incorporated into the PA selective layer during IP by either chemical cross-linking or physical blending. The fabricated membranes were analyzed at a pressure of 15.5 bar using sodium chloride solution with a concentration of 2000 ppm. Compared to the pristine PA membrane, the water ux increased by 65% and the salt rejection remained at 98% when 0.4% (w/v) of P-8phenyl was added to the organic phase and physically blended into the selective layer. Similar results were obtained when P-8NH 2 and P-8NH 3 Cl were added to the organic phase. An increased water ux was also obtained from the chemical xation of P- 8NH 2 and P-8NH 3 Cl into the selective layer. However, the water ux was unsatisfactory for chemical cross-linking of P-1NH 2 in the organic phase due to the hydrophobicity of the membrane produced. The recent integration of carbon nanotubes into polymeric membranes for the RO process has also yielded positive results. 2,10,54 58 Zhao et al. has enhanced the performance of nanocomposite PA RO membranes incorporated with carboxyfunctionalized multi-walled carbon nanotubes (MWCNTs). 15 Mixed acids were used to pretreat the virgin MWCNTs prior to modifying them with diisobutyryl peroxide to improve their chemical activity and dispersity. The synthesized membranes had a skin layer of thickness nm within which the modi ed MWCNTs were inserted (Fig. 3). It was observed that the nanocomposite membrane surface was more negatively charged compared to the pristine PA membranes. Additionally, the morphology of the membrane was noticeably altered when the carbon nanotube loading was increased, thereby causing the permeate ux to increase signi cantly from to LMH. However, only a marginal decrease in salt rejection was observed. Moreover, the modi ed nanocomposite membranes demonstrated better anti-oxidative and anti-fouling properties than the PA membranes free of MWCNTs under similar testing conditions. Nanotubes have the ability to facilitate fast water transport through membranes 59 and the incorporation of functionalized MWCNTs into TFN RO membranes is capable of enhancing the water ux obtained from these membranes. 60 Thin lm bipolar RO desalination membranes have also been synthesized from PAA and PAH through a spin assisted layer by layer (SA-LbL) coating technique. 61 Polysulfone UF membranes and silicon wafer substrates were used for RSC Advances depositing these polyionic coatings. The thickness of the lm could be regulated by altering the number of layers, varying the ph or changing the spin speed. The thickest lm was made when the low ph of PAA was combined with the high ph of PAH, and vice versa for the formation of the thinnest lm. The membrane performance test conducted with feed water at a pressure of 700 psig and a sodium chloride concentration of ppm showed that 35 bi-layers of PAH PAA on polysulfone UF membranes demonstrated a water permeability of 0.22 L m 2 h 1 bar 1 and a salt rejection of 88%. The morphology of the membrane lm was observed to change a er the permeation/performance test due to the application of high pressure that enabled salts to diffuse further into the lm and change the polymer chain conformation. The SA-LbL method was found to be very effective for fabricating RO membranes for membrane thickness regulation. 3. Recently developed polymeric and hybrid polymer membranes 3.1 Fabrication and performance studies A commonly used polymeric membrane material for the RO process in recent times is PA. 39,62 65 This has prompted the molecular study of a PA RO membrane to give a better comprehension of the separation process that can be achieved through the membrane at the molecular level. 66 A computational molecular simulation was used to build a fully aromatic PA (FAPA) RO membrane made from identical monomers, such as those found in commercial membranes. 66 The cross-linking degree and solid liquid interface of the membrane were controlled by this simulation. This study con rmed the previous data available regarding the water structure, water dielectric properties, and water dynamics through the membrane. Indeed, the con nement inside the membrane cavities would not disturb the overall structure of the water and the relaxation of water dipoles was much slower inside the membrane than in the bulk phase. In addition, the dielectric properties of con ned water were severely modi ed with respect to the bulk phase. This work showed that a FAPA RO membrane could play a signi cant role in salt rejection and proposals for future research to establish the relationship between the investigated molecular dynamics and salt rejection were suggested. Another study was performed to attempt to develop a model to simulate the transport of water molecules across highly crosslinked PA RO membranes. 67 The mechanism behind ion transport through highly cross-linked PA membranes at the molecular level was also investigated in this study. Fujioka et al. studied the internal structure of the PA RO membranes selective skin layer using positron annihilation spectroscopy (PAS) to obtain a better understanding of the water and solute transport, which will help in introducing further enhancement to RO membranes. 3 PAS showed that the RO membranes contain a very thin selective skin layer with free volume as subnanometer sized holes. Previous studies using data from positron annihilation lifetime spectroscopy (PALS) reported that the This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

7 RSC Advances Review Fig. 3 (A and B) surface morphology of the unmodified PA composite/pa MWCNT nanocomposite from SEM; (C and D) cross-section of the unmodified PA composite/pa MWCNT nanocomposite from transmission electron microscopy (TEM). 15 radius of average free volume hole for commercial RO membranes was between nm in the selective skin layer with a thickness of around 100 nm. The experimental results obtained using PALS showed that the size of the free volume hole is the most signi cant parameter in governing rejection (of boron). Additionally, the selective skin layer s free volume fraction was found to affect the solute rejection. The solute transport could also be in uenced by the thickness of the selective layer. The measurements of the shape of the free volume hole, the free volume fraction and the distribution of the hole size limit the possibility of further investigating the solute transport. Using molecular dynamics simulations in addition to the free volume hole size measured by PALS can help to achieve a better understanding of solute rejection mechanisms through membrane polymer chains. Cellulose triacetate (CTA) is another polymeric membrane material whose continued use for RO has been sustained in recent times. 62,68 Although CTA membranes are in a distant second position to PA TFC membranes in terms of their market share in recent applications, they are preferred most especially in the Middle East because of their superior ability to withstand chlorine attack. 62,68 Fujioka et al. evaluated the performance of CTA hollow ber RO membranes for reusing potable water by observing its rejection behavior for forty one trace organic chemicals (TrOCs). The results obtained were compared with those achieved using PA membranes. 69 The results obtained for the CTA membranes were largely similar to those obtained for the TrOC rejection behavior of PA membranes. However, compared to PA membranes, TrOC rejection by CTA membranes was found to be greatly in uenced by hydrophobic interactions and less dependent on electrostatic interactions. No variation was observed in the rejection of positively and negatively charged TrOCs when CTA membranes were used. Nonetheless, there was great variation in neutral TrOC rejection by CTA membranes (unlike PA membranes) because the neutral TrOC rejection was observed to depend on their minimum projection area (molecular size) which strongly correlated with the PA membrane structure. Additionally, hydrophobicity was found to greatly in uence the rejection of N-nitrosamines by CTA membranes, whereas the molecular size in uenced the rejection of N-nitrosamines by PA membranes. Finally, the charged TrOCs exhibited much greater rejection by CTA membranes than the neutral TrOCs irrespective of the molecular size. N-Nitrosamine rejection has also been evaluated using a spiral wound PA nanocomposite RO membrane from Hydranautics (ESPA2-4040). In this study, a mathematical model was developed using hydrodynamic calculations and the principle of irreversible thermodynamics to precisely describe N-nitrosamine rejection by the spiral wound RO membrane systems for a set of operating conditions. 70 The N-nitrosamine rejections obtained from the mathematical model were validated with experimental results from a pilot RO system. The modeled results showed that the rejection of N-nitrosamines with low molecular weight could be increased by increasing the permeate ux, and decreased by increasing the system recovery. Also, a reduction in localized rejection was observed along the ow path in the membrane vessel. The developed model can be bene cial for use in regulatory monitoring and system design. In general, these studies con rm the appropriateness of RO 8140 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

8 Review RSC Advances membranes for rejecting micropollutants or carcinogenic disinfection products and trace organics from water. 71 The incorporation of hyperbranched polyesters (HBPE) into CA for improved membrane performance has been tested. CA membranes with different compositions of HBPE blended into the membrane structure were prepared via phase inversion. 72 HBPEs were investigated because of the hydrophilic nature of the polymers. The addition of HBPE improved certain morphological features of the membrane, such as the ltration properties, wettability, thermal behavior and hydrophilicity. The membranes were tested for 30 minutes at a pressure of 500 kpa and pure water uxes were measured at 400 kpa. The results obtained showed that a membrane prepared from pure CA has the lowest pure water ux because of its dense lowhydrophilicity surface structure. The main reasons for the formation of such a structure include the slow phase separation, time required for uniform dispersion, and nucleus growth on the surface. Hybrid CA HBPE membranes (CA HBPEG3) with small dimensions showed higher uxes. The pure water ux of these membranes increased from 24.6 to 37.6 LMH when the HBPEG3 content was increased from 2.5 to 10 wt%. This was due to the formation of a greater number of pores on the membrane surface at higher HBPEG3 contents. Another set of hybrid CA HBPE membranes (CA HBPEG4) was developed with larger pores formed by partial phase separation. However, these membranes had relatively small water uxes despite having larger pores. This effect was possibly related to the high permeation resistance caused by the honeycomb-like voids in the membrane matrix. This study also indicated that the molecular weight cutoff (MWCO) values for CA HBPEs are low but can be slightly increased through the controlled increase of surface porosity. The incorporation of CA with poly(vinylidene uoride) (PVDF) with TiO 2 loading has also been investigated by Liu et al. 73 TiO 2 nanoparticles are antimicrobial agents and they can curtail organic fouling. 18 A novel dual-layer CA PVDF hollow ber membrane was fabricated via a two-step thermally induced phase separation (TIPS) and non-solvent induced phase separation (NIPS) process. 73 The morphology and porosity of the synthesized membranes were found to be in uenced by the outer layer s dope solution composition: cosolvent (acetone) ratio, polymer concentration, and TiO 2 nanoparticle loading. Increasing the acetone ratio and TiO 2 nanoparticle loading resulted in a narrower pore size distribution (Fig. 4) and a thinner outer layer that signi cantly improved the rejection of electrolytes and dextran, and maintained the pure water permeability of the hollow ber membrane. Increasing the loading of TiO 2 nanoparticles also helped to improve the hydrophilicity, anti-bacterial and antibiofouling properties of the dual-layer hollow ber membrane. The mechanical strength of the dual-layer membrane was observed to be greater than 8 MPa, in addition to the sodium sulfate rejection of 90 95% and pure water permeability of 1 to 4 L m 2 h 1 bar 1. The possibility of using the resultant membrane for treating RO concentrates was veri- ed from its performance test that yielded a TOC removal greater than 90% and a rejection of total dissolved salts of less than 60%. Permeate from the fabricated hybrid membrane was Fig. 4 Pore size distribution of CA PVDF and pure CA flat sheet membranes. 73 safe for discharge to underground and surface waters as a result of the antibacterial effect of TiO 2. It has also been demonstrated that the integration of polyvinyl alcohol (PVA) with CA could lead to an improvement in the CA RO membrane performance for desalination of brackish water and seawater. Muhammed et al. have fabricated and characterized a PVA CA composite membrane by using different concentrations of maleic acid and varying the reaction times to cross-link the PVA layer. 74 Thin lms comprising PEG and CA were interfacially polymerized over PVA support membranes having different chemistry and pore structures. Large pores were added onto the hydrophobic skin layer resulting in membranes with higher water permeability. A more rough composite membrane surface was achieved from a smaller percentage of PVA formation with the membrane pores. As a result of this approach, the overall path length became shorter for solute and water transport from the feed to the permeate side of the PVA layer. Most RO membranes are composites because they combine the properties of the different components. 75,76 PA TFC membranes have found the most important niche in the current RO industry. 43,77 81 They provide the bene ts of high ux and high salt rejection when compared with asymmetric CTA membranes. 77,78,80 However, in contrast to their impressive water permeation and salt rejection features and greater resistance to biodegradability, PA TFCs are more susceptible to chlorine attack because they contain chlorine-sensitive nitrogen sites More developments in the fabrication and evaluation of the performance of PA TFC RO membranes for a hightemperature selection process have also been reported lately. 83 Apart from susceptibility to chlorine deformation, the in uence of temperature changes on the performance of a PA TFC membrane has been tested in a recent work. 84 In this study, gas separation tests were used to evaluate the impact of increased temperature on TFC RO membranes a er different pretreatment procedures. It was shown that high permeance results can be obtained for a maximum He/N 2 selectivity at temperatures This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

9 RSC Advances Review up to 150 C. The temperature-induced changes in the polymer structure and in the transport of compounds suggested the appropriateness of the use of PA membranes for hightemperature separation processes. The use of a PA TFC RO membrane to remove substances like uorinated surfactants, which are resistant to chemical or thermal attack, has also been studied. 85 Fluorinated surfactants can reduce the surface tension of re- ghting waters. Therefore, a PA thin lm membrane has been used to signi cantly reduce the concentration of uorinated surfactants in re- ghting water from 30 to 0.1 mg L 1. The results obtained also showed consistent permeability over several days (0.5 L m 2 h 1 bar 1 ) and the retention rate achieved was around 99.4 to 99.9%. A recent study of PA TFC RO membranes has shown that the performance of these membranes in terms of the water ux and salt rejection can be further improved through the post-treatment of the membrane with dimethylformamide (DMF). 86 The group fabricated and investigated the performance of PA TFC RO membranes that were prepared by IP of MPD and TMC, and casted on a cross-linked polyimide support. The effect of using the activating solvent, DMF, on the performance of the membrane was studied. The results demonstrated an increase in water ux by eight times a er post-treating the membrane with DMF. An improvement in salt (sodium chloride) rejection was also observed upon post-treatment with DMF, especially for membranes with initial salt rejections of less than 50%, resulting in a peak rejection of 94%. The behavior of a commercial PA composite RO membrane has also been tested under gamma rays in the presence of water and oxygen in order to detect the stability of the membrane under irradiation. 87 The PA composite membrane used was tested before and a er two doses of gamma rays (0.1 and 1 MGy). The membrane s performance in terms of its NaCl rejection and water permeability was obtained. Commercial PA TFC membranes are usually composed of an aromatic PA active layer with a polysulfone microporous support layer and a bottom structure made up of non-woven polyester. Membrane texture analysis and chemical composition were carried out to characterize the membrane by advanced analysis techniques. It was observed from AFM images that the membrane degraded a er irradiation at 1 MGy by structural modi cations resulting from increased roughness and chain breakage at different layers (Fig. 5). The drop of the permselectivity properties of the membrane was a direct consequence of this structural modi cation. Irradiation at 0.1 MGy seemed to have only a minor effect. It was concluded that PA composite RO membranes can be resistant to gamma rays until a dose level of 0.1 MGy is reached. Structural modi cations of the membrane surface would occur around 1 MGy. Further research is ongoing to show the performance of RO membranes when exposed to different levels of irradiation and radioactive materials in water Operating conditions have also been shown to play a significant role on the efficiency of PA TFC membranes for the RO process. 90 For example, the behavior of Dow s BW30LE PA TFC RO membranes for brackish water desalination has been examined using varying salinity and ph conditions in both experimental and numerical modeling studies. 91 It was found Fig. 5 Atomic force microscopy (AFM) images of (a) a virgin PA composite RO membrane, and membranes irradiated at (b) 0.1 and (c) 1 MGy gamma irradiation. 87 experimentally that the membrane charge, thickness and pore size were dependent on both the salinity and ph levels. In particular, the membrane thickness increased with increasing feed salinity and ph. Additionally, the ux of hydroxide and hydronium ions were inspected using similar experimental conditions to examine the membrane performance. An increase in the rejection of hydroxide and hydronium ions was observed with rising surface charge density. The ux was not affected by the applied pressure at zero salinity. However, negative rejection of both ions was observed a er adding salt in order to sustain the electroneutrality in the permeate solution. The ux of hydronium ions increased signi cantly when the permeate 8142 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

10 Review ux was increased. In order to predict hydroxide and hydronium ion transport through the RO membrane, modeling was carried out using the extended Nernst Planck equation and the interface partitioning was evaluated based on steric hindrance and Donnan equilibrium. A good agreement was obtained with experimental results without adjusting any parameters. Richards et al. also used Dow s PA TFC membrane to demonstrate a safe operating window (SOW) for a RO desalination system powered by renewable energy. 92 This study was the rst to develop the methodology to experimentally determine the SOW for a renewable energy powered membrane ltration system used for brackish water desalination. The SOW for the wind-powered membrane ltration system was dependent on the following factors: available power of the pump (pressure and ow rate), highest suggested recovery and the osmotic pressure of feed water. The RO membrane modules (and brackish feed water salinity) used in this study were: BW (5500 and mg L 1 ) and aged BW (5500 mg L 1 ). The maximum recovery of 30% was found to be the main limiting factor at lower salinity values, whereas the osmotic pressure was the chief constraint at higher salinity values. Constant recovery was observed to be the best operating approach as it yielded the maximum water ux and sustained good retention at a speci c power consumption, which resulted in the lowest speci c energy consumption (SEC). Nonetheless, a constant set-point mode was the preferred the operating strategy over the constant recovery approach as this is challenging to apply in practice. The mode of constant set-point offered an effective and robust solution for water provision in remote areas with a slight decrease in the system performance. Overall, it was possible to power the suggested approach by solar (photovoltaics) or wind energy due to its low SEC (approximately 3 kw h m 3 ). This allowed the system to function over a wide power range ( W) in order to attain the anticipated pressure range ( bar). Generally, this SOW methodology allows the performance assessment of a large range of factors for PA TFC RO membrane ltration systems. This is against the backdrop that energy efficiency is a crucial factor for RO desalination Cross- ow velocity has also been studied as an operating parameter that in uences the concentration layer of PA TFC membranes during RO desalination. An experimental investigation and numerical modelling has been carried out recently to investigate the development of a concentration polarization (CP) layer during cross- ow in a RO membrane slit channel. 97 The development of a CP layer in the slit was directly visualized as an interference fringe pattern using a digital holographic interferometry (Fig. 6). It was observed that the polarization layer along the slit-type channel continuously increased irrespective of the cross- ow velocity, except near the cell outlet because of the edge effect. However, the growth of the polarization layer was found to be bigger at a lower Reynold s number but the permeate ux was not in uenced noticeably. Nonetheless, the permeate ux was signi cantly affected by the concentration of the feed. Overall, the polarization layer development was found to be dependent on the driving force, ux resistance and hydrodynamics of the process. The simulation RSC Advances results obtained from commercial CFD so ware were observed to be in good agreement with the experimental results. Novel TFC RO membranes with high permeate uxes have also been fabricated for seawater desalination by adding the hydrophilic o-aminobenzoic acid triethylamine (o-aba TEA) salt onto the PA TFC RO membrane during IP of MPD and TMC on a polysulfone support. 98 Several membrane fabrication conditions, such as the o-aba TEA salt and isopropanol concentration, extra drying time of the amine and the hydrocarbon removal time, were enhanced based on membrane performance tests. The optimal additive concentration was found to be 1.0 wt%. The modi ed membranes were more hydrophilic than the pristine membranes due to the incorporation of hydrophilic materials. Performance tests were conducted using a synthetic sodium chloride solution of 3.28 wt% at 5.52 MPa and 25 C. The modi ed membranes had a salt rejection of 99.41% and a water ux of 83.5% (75.42 LMH), higher than the pristine membranes. The hydrophilic additive produced an added route for water transport enhancement and delivered a charge repulsion that improved the salt rejection. The synthesized membranes were further tested for their performance under the same conditions using seawater from Port Hueneme in California, United States. A lower water ux was attained using the seawater due to the presence of a higher solid content (3.45%) in the tested seawater. Moreover, the stability test conducted using this seawater as the feed solution demonstrated a stable desalination performance for a period of 30 days. The surface and structural features of PA TFC RO membranes also play a crucial role in the membrane performance in terms of water permeability and salt rejection Liu et al. investigated the in uence of surface properties and the structure of several TFC PA RO membranes on the performance of these membranes. 99 These membranes, with slight structural variances, were synthesized via the reaction of 5- chloroformyloxy-isophthaloyl chloride (CFIC), 5-isocyanatoisophthaloyl chloride (ICIC) and TMC with MPD separately through the IP technique on a polysulfone supporting mesh. The performance of the fabricated RO membranes was evaluated based on their water permeability and salt rejection. The properties of the membrane surfaces were characterized using XPS, SEM, ATR-IR, AFM, streaming potential and contact angle measurements. The stability of chlorine was examined by evaluating the performance of the membrane before and a er exposing it to hypochlorite. The results indicated that the performance, surface properties and chlorine stability of the RO composite membranes were strongly in uenced by the structure of polyacyl chloride. Incorporation of the isocyanato group into polyacyl chloride was found to improve the water permeability, surface smoothness and hydrophilicity but reduce the chlorine stability of the TFC membrane. On the other hand, integration of a chloroformyloxy group into polyacyl chloride was found to increase the salt rejection and increase the surface roughness but reduce the water permeability of the TFC membrane. The most hydrophilic PA TFC membrane was MPD ICIC and the least hydrophilic was MPD CFIC. Among the three membranes, the surface of MPD CFIC was found to be the This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

11 RSC Advances Review Fig. 6 Concentration profile from digital holographic interferometry. 97 roughest followed by MPD TMC, and MPD ICIC had the smoothest membrane surface. The coefficient of pure water permeability was 2.87, 3.53, and 4.05 L m 2 h 1 bar 1 for MPD CFIC, MPD TMC, and MPD ICIC, respectively. The salt rejection of the three TFC PA membranes was greater than 98% when tested at 1.6 MPa using a sodium chloride feed solution with a concentration of 2000 mg L 1. The rate of salt rejection was highest for MPD CFIC and lowest for MPD ICIC. Moreover, the MPD CFIC membrane was veri ed to be the most suitable for processing highly concentrated aqueous salt solutions. The MPD ICIC membrane was the least chlorine resistant among the tested membranes. The functionalization of PA TFC RO membranes with acyl chloride monomers has also been tested in another recent investigation as a method of improving the membrane performance. Wang et al. has synthesized three novel monomers of polyacyl chloride 2,3 0,4,5 0,6-biphenyl pentaacyl chloride (BPAC), 2,4,4 0,6-biphenyl tetraacyl chloride (BTAC) and 2,2 0,4,4 0,6,6 0 -biphenyl hexaacyl chloride (BHAC) to investigate the impact of their functionality on the PA TFC RO membrane properties. 103 The TFC RO membranes were fabricated on a polysulfone support via IP of TMC, BPAC, BTAC and BHAC with MPD. The characterization and separation performances of the synthesized membranes were evaluated using SEM, ATR FT-IR spectroscopy, AFM, XPS, contact angle, streaming potential measurements and cross- ow RO performance tests. The results indicated that the RO membrane properties were signi cantly in uenced by the acyl chloride monomer functionality. It was observed that the skin layer of the prepared membrane became thinner, more negatively charged and smoother as the acyl chloride monomer functionality increased. This occurred because of the higher degree of cross-linking of the nascent PA lm with severe resistance to amine diffusion into the organic phase throughout the process of IP. Nonetheless, no trend was observed for the hydrophilicity change due to the combined in uences of the carboxylic acid group content and surface roughness. Moreover, the RO performance tests yielded salt rejection rates that were close for all the four membranes. However, it was suggested that the quantity of the acyl chloride functionality needs to be optimized for an improved membrane performance. An uncontrolled increase in acyl chloride functionality was observed to decrease the water ux as a result of reduced surface roughness, greater number of carboxylic acid groups on the membrane surface, and reduced exibility of the cross-linked PA chains. The modi cation of the PA active layer of a RO membrane by coating it with a hyperbranched poly(amido amine) (PAMAM) hydrophilic polymer has also been studied for membrane performance improvement. 100 PAMAM was coated on the PA active layer of the RO membrane via chemical coupling. The RO membrane was coated by spraying a solution containing 10 wt% PAMAM on the membrane surface using either water or methanol as solvent. Membrane modi cation using water and methanol as solvents improved the water ux by 19.6 and 24.8%, respectively. The increase in water ux was due to the suppression of cross-linking following the nal step of curing. A stable membrane surface modi cation occurred under ltration conditions. It was observed that a smooth thin PAMAM lm was produced with almost complete surface coverage when water was used as a solvent (Fig. 7). However, using methanol as solvent caused PAMAM to deposit as clusters on the membrane surface. Surface modi cation with PAMAM increased the water permeability by 20 25% compared to the uncoated membrane, but did not in uence the salt rejection. However, a lower salt rejection resulted for the modi ed membrane when methanol was used as a solvent. It was also observed that the modi ed membranes coated with aqueous PAMAM solution exhibited low protein fouling when compared with the pristine membranes and those that were coated with methanolic PAMAM solution. The enhanced performance of the modi ed membranes with aqueous PAMAM solution was due to the development of a second homogenous hydrophilic hydrogel layer. The pore structure of the surfaces of PA TFC RO membranes could also contribute to the performance of these membranes. Therefore, it is necessary to understand the mechanism 8144 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

12 Review RSC Advances Fig. 7 AFM pictures of (a) unmodified TFC PAMAM, (b) TFC PAMAM with methanol as solvent and (c) TFC PAMAM with water as solvent. 100 involved during pore formation on the lm surfaces of PA TFC RO membranes. Yan et al. characterized and studied the complex porous structures of commercial FAPA RO membranes using TEM and SEM. 101 Experimental results showed that the formation of pores on the FAPA lm surface always took place on the side facing the aqueous phase. A novel FAPA lm structure model was proposed for the RO membranes using the observations obtained from the TEM and SEM images of the FAPA lm cross-section. The RO membrane s main barrier layer was the dense selective layer that formed on the top surface of the PA. The pores and the dense layer on the FAPA lm s back surface were linked through the asymmetrical interconnecting tunnels and voids of the ridges and protrusions. These tunnels and voids offered passages to permit MPD to move through the FAPA lm during the process of IP formation and allowed water to penetrate through during RO ltration. The partial replacement of the conventional MPD monomer in PA TFC RO membranes with the linear monomer, 1,3-diamino-2- hydroxypropane (DAHP) has also been studied to evaluate the effect of this substitution on the membrane performance. TFC PA RO membranes were fabricated via IP in the aqueous phase to increase the water ux. 102 The linear DAHP monomer was used to partially replace the conventional MPD monomer and the permeability increased by approximately 22% ( L m 2 h 1 bar 1 ) for the synthesized membrane at an optimal DAHP/MPD ratio of 12.8%. However, no signi cant change in salt rejection was observed, which was maintained from 96 to 98%. The permeability increased to L m 2 h 1 bar 1 when the support surface was washed to remove protective surface coatings before the membrane development. In general, an enhanced water ux occurred for both washed and unwashed supports when DAHP units were incorporated into the PA network. This achievement was reported because the integrated DAHP changed the membrane surface structure of the PA lm from large ridge and valley to enlarged nodular structures. The introduction of DAHP to the PA TFC membrane also reduced the thickness of the selective layer, reduced the surface roughness and increased the hydrophilicity of the membrane that caused an increase in the water ux. To improve the performance of asymmetric RO membranes, Stevens et al. used 20% disulfonated biphenol and an aryl sulfone (BPS) copolymer to develop an optimal recipe. 81 The membranes were fabricated via the phase inversion of BPS solutions. In addition to optimizing the recipe, a number of post-treatment steps were included for the formation of a dense rejection layer and healing of the membrane defects in order to optimize the salt passage. The post-treatment involved boiling the prepared membrane in concentrated electrolytes to break down the pores and dehydrate the membranes so as to produce almost defect-free skins. This step was followed by in situ poly(vinyl alcohol) coating to repair any le over holes or defects so that a highly salt rejecting membrane surface could be produced. An enhanced water ux and salt rejection resulted from the post-treatment of the membranes. The nal salt passage was observed to be strongly dependent on the temperature and ionic strength of the annealing solution. The salt passage ranged from 0.34 to 1.81% across a water ux range of 2.5 to 20 LMH when tested at a pressure of 15.5 bar with 2000 This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

13 RSC Advances ppm sodium chloride solution. Although the performance of the membrane was not good enough to compete with commercial PA RO membranes, the obtained results demonstrated that the BPS membranes have the potential to be used for RO desalination. RO membrane performance is also strongly dependent on the properties of the membrane support. Several studies have estimated the dependence of the structural parameters of the membrane on its performance. 104,105 Conventional methods, such as SEM, have been used to characterize the tortuosity and porosity of the membrane support layer. However, these methods are difficult to implement and cannot be used to extensively visualize the pore structure. Manickam et al. used X- ray computed tomography for imaging the pore structure and calculating the tortuosity and porosity. 106 The structural parameters of two commercial TFC RO membrane support layers were measured using imaging (X-ray microscopy (XRM)) and analytical (mercury intrusion porosimetry (MIP)) methods. The structural parameters found using XRM images and MIP were compared with the ones obtained from the conventional model using empirical data. The structural parameters obtained using the different techniques were found to differ signi cantly, which indicates that revisions are required to the customary methods of characterizing membrane structures. The advantage of XRM over other techniques was its ability to calculate the porosity of the membrane as a function of depth. The tests which have been carried out to evaluate the performance of recently developed RO membrane materials and the achievements reported from these tests are generally summarized in Table Fouling studies, mitigation, and detection Detection and monitoring of membrane fouling. Fouling remains a serious drawback for membrane application to RO desalination. 6, Fouling increases the operating cost of the RO process and reduces the lifetime of RO membranes. 116,118,121 Recently, biofouling has been proven to cause some of the most serious limitations of membrane performance, most especially during commercial water desalination. 117,119,120,122 The inability to detect biofouling early enough for quick control may lead to even worse deleterious effects on membranes. A recent investigation on biofouling detection through the use of Earth s Field Nuclear Magnetic Resonance (EF NMR) suggested that early biofouling detection is possible. This work involved the analysis of the feasibility of using EF NMR for non-destructive biofouling detection at its onset within a commercial RO spiral-wound membrane module. 123 EF NMR is an appropriate technique for early biofouling detection as it can assess the existence of biofouling very quickly by evaluating the change in total EF NMR signals caused by variations in the uid ow eld due to biofouling within the membrane module. Likewise, the EF NMR measurement technique can be used for high operational ow rates and pressures, and can be located anywhere along the RO membrane module. In a related study, an online feed fouling monitor (FFM) with a salt tracer response technique (STRT) has been investigated for the prediction and Review monitoring of RO membrane fouling under steady ux ltration. 124 The foulant loads were determined using the FFM, whereas the CP development was measured using the STRT to evaluate the contribution of cake-enhanced osmotic pressure (CEOP). The RO fouling trends were predicted using a developed model that included both the CEOP effect and cake resistance because of cake formation. The model foulants that were used to represent inorganic and organic fouling were colloidal silica and humic acid, respectively, in sodium chloride solutions. The FFM fouling and RO experiments were run at varying constant uxes at a cross- ow velocity of 0.1 m s 1 and with the same feed solutions. It was observed that higher uxes augmented the fouling rate. Moreover, the higher ux signi cantly increased the cake resistance contribution to RO fouling, which was more apparent for the experiments using humic acid as the model foulant. Dead end ltration was used to study the impact of salt concentration on the speci c cake resistance. It was found that the cake layer s speci c cake resistance increased for humic acid over a wide range of sodium chloride concentrations due to a decline in the particle particle repulsion and an alteration in the packing of the foulant layer when the salt concentration was increased. The results demonstrated that FFM can be used together with STRT to reasonably assess the trends of RO fouling for both humic acid and colloidal silica. Furthermore, the substantial in uence of CEOP on RO fouling can be observed from the predicted pro les of transmembrane pressure. In a recent study, diffusiophoresis has also been shown to contribute considerably to fouling mechanisms in low salinity RO systems. 125 This study showed through experimentation and modeling that diffusiophoresis is a colloidal fouling mechanism in RO which signi cantly adds to cake formation and particle formation on RO membranes. The particle ux decline varied for the different salts tested in this study. It was also observed that cake-enhanced CP increased particulate fouling over time in a positive feedback loop resulting from diffusiophoresis. Improved fouling control has also been ensured through the redesign of the spacer in the RO membrane module con guration. Koutsou and Karabelas studied a new retentatespacer with a net-type structure that has parallelogram unit cells formed from symmetrically joining spherical nodes with cylindrical laments. 126 The most important characteristic of the proposed con guration was the presence of small contact regions (contact points) between the nodes and the bounding spiral wound membranes that prevented dead ow zones. Numerical simulations of mass transfer and ow eld were conducted directly in detail within narrow channels. It was observed from the results of numerical simulation that the newly developed spacers helped to alleviate the fouling phenomena and reduce CP in the membrane process. The possibility of early detection of fouling in hybrid polymer membranes would reduce the vulnerability of the membranes to a short life span. Electrical impedance spectroscopy (EIS) is a recent method that has been tested for real time monitoring of foulants in RO systems. 127,128 EIS has been employed for real time monitoring of electrical properties and early detection of calcium sulfate scale formation in RO systems 8146 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

14 Review RSC Advances Table 1 Performance tests of recently developed RO membrane materials and reported achievements Performance test Notable achievements Reference CA hollow ber Removal of micropollutants or TrOCs such as 69 charged N-nitrosamines CA with HBPE Enhancement of membrane ltration 72 properties, wettability, thermal behavior and hydrophilicity CA with PVDF and TiO 2 Enhancement of membrane hydrophilicity, 73 sodium sulfate and TOC rejection, antibacterial, and anti-biofouling properties CA with HBPE, PEG, PVDF, or PVA Better productivity and higher fouling 40, 54 and resistance CA with TiO 2 nanoparticles Mechanical strength improvement 73 CA with PVA Addition of large pores on the skin layer and 74 increase in water permeability of membrane Spiral wound PA nanocomposite Rejection of low molecular weight TrOCs 70 TFC for uorinated surfactant rejection from More than % rejection with consistent 85 re- ghting water ux of 0.5 L m 2 h 1 bar 1 TFC with o-aba TEA salt High salt rejection of 99.41% and water ux of m 3 m 2 d 1 for seawater desalination Incorporation of isocyanato group into polyacyl Improvement in the water permeability, surface 99 chloride during TFC fabrication smoothness and hydrophilicity of membrane but reduction in chlorine stability Incorporation of chloroformyloxy group into Increase in membrane salt rejection and surface 99 polyacyl chloride during TFC fabrication roughness but decrease in water permeability TFC with acyl chloride monomers such as BPAC, Enhancement of water ux 103 BTAC, and BHAC TFC with PAMAM Enhancement of water ux 100 Partial replacement of conventional MPD Increase in membrane hydrophilicity and water 102 monomer DAHP in TFC ux TFC with NaY zeolite High ux of 50.6 gfd 31 TFC with silica nanoparticles Biocidal properties imparted on the membranes 19 TFC with amine groups (PAMAM, L-DOPA amino Enhancement of water ux, salt rejection, and 100 and acid, urethane, DMMPD, PNIPAm) anti-biofouling property TFC with acrylates (PFDA, HEMA) Antifouling enhancement 112 and 113 TFC with barium sulfate Antifouling enhancement 114 TFC with SA and QA post-treatment Antifouling enhancement 115 Phase inversion of BPS solutions Low salt passage ( %) and high water ux ( LMH) during brackish water desalination 81 on a lab scale. 128 The laboratory experiments were performed using commercial PA TFC membranes from Dow (BW30-FR Filmtec). The EIS signals and conductance obtained at a frequency range of 10 1 to 10 5 Hz were associated with a permeate ux decline for batch and recirculation modes. It was observed that the rate of change of conductance was greater than the permeate ux decline. For the experiments conducted in recirculation mode, a signi cant change in the value of capacitance was observed at a frequency of 73 Hz. For the batch mode experiments, signi cant changes were observed at all frequencies but the change in conductance at a frequency of 38 Hz was found to be the most effective and it corresponded to the coating layer on the membrane surface. Within the speci ed range of frequency, the proposed scale monitoring method was found to be effective in detecting scale formation at its initial stage before observing any considerable decline in permeate ux. Hence, EIS could be attached to a full scale RO module as a side stream for detecting scale formation before any noticeable decline in permeate ux can occur. It is also possible to comprehend the separation performance of membranes by characterizing the membranes electrical surface properties. 129 In one study, experimental measurements of streaming current were performed in the tangential mode for several RO membranes in an inert nitrogen environment, to provide a controlled environment, and with degassed background solutions. 130 This was done to obtain precise electrokinetic measurements even within the basic ph range. It was rst demonstrated in this work that the streaming current technique is an effective method to reveal the existence of additional coating layers on commercial PA TFC membrane surfaces. It was further observed that conducting the experiments under a controlled atmosphere was much better than the alternative attenuated total re ectance Fourier transform infrared spectroscopy (ATR-FTIR) technique, as it allowed clear differentiation between a coated and uncoated membrane surface. This is due to the sensitivity of the streaming current measurements to the ionization of the functional groups that are found on membrane s coating layer. Moreover, the experiments that were This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

15 RSC Advances conducted in the presence and absence of a nitrogen environment showed speci c adsorption of carbonate and bicarbonate ions on the RO PA membrane surfaces. Lutskiy et al. described a robust assay for quantifying bio lm growth on the surface of different types of PA TFC RO membranes in the initial stage. 131 The procedure involved the attachment of bacteria on the PA TFC membrane, bio lm development on the membrane surface, quantitative estimation of bacteria, and detachment of bacteria from the membrane surface through seconds of sonication. Bacterial growth and adhesion on the membrane surfaces were investigated with respect to time under static conditions, along with continual elimination of planktonic bacteria. The proposed assay gave a more quantitative result compared to using uorescence microscopy for observing bacterial growth. The extent of bio lm growth on these membranes was found to be similar. The proposed assay can be used to predict the vulnerability of RO membranes to bacterial adhesion and bio lm growth. This assay can also be used to screen microbial and bio lm resistant membrane coatings. Additionally, a deeper understanding of the procedure of surface tethered antimicrobial agents can be obtained from this assay Dependence of membrane fouling on the RO operating conditions. The RO operating conditions are known to in uence membrane fouling The dependence of ltration resistance on the RO operating conditions using a wide range of polymeric membranes has also been investigated. This work was a study of the performance of RO membranes for inhibitor separation from sugars, and for concentrating sugars from a biomass hydrolysate to increase the performance efficiency. 132 A variety of commercial RO membranes with varying pore sizes, porosity and membrane chemistrya were tested for their performance based on inhibitor separation, permeate ux, sugar yield and ease of cleaning. The materials of these members include PA (from Dow and Toray), thin lm (from GE Osmonics), and PA TFC (from Koch). The performance of the different membranes tested varied and the yield of sugar in the retentate ranged from under 20% to approximately 100%. In general, lower sugar yields resulted from greater inhibitor separation factors. Moreover, a reduction in permeate ux mainly occurred due to reversible fouling, and from increased osmotic pressure caused by the retention of soluble compounds. It was observed that greater porosity, higher hydrophilicity and lower surface roughness of membranes resulted in reduced membrane surface fouling and a sustained elevated permeate ux for the RO membranes during the processing of enzymatic hydrolysate. In a related study, Nguyen et al. screened some commercial FAPA RO membranes on a at sheet to remove toxic compounds from lignocellulosic hydrolysates and prevent inhibition of the fermentation process. 133 These membranes included CPA2, CPA3, and ESPA2 (from Hydranautics), XLE Filmtec (from Dow), and SG (from GE Osmonics). These membranes were tested for their capacity to isolate C5 and C6 sugars from a model solution containing furfural, acetic acid, vanillin and 5-hydroxymethyl furfural. The highest sugar rejection obtained was greater than 97% but the transmission of inhibitors was found to be low. Review The fouling behavior of gypsum (CaSO 4 $2H 2 O) scalants and sodium alginate on PES TFC membranes under different operating conditions has been investigated in several studies. 134 Gypsum and sodium alginate are two of the most critical foulants hindering membrane performance and causing a membrane ux decline. The effect of the applied hydraulic pressure (up to 18 bars) on the extent of membrane fouling was analyzed using SEM. It was observed that the alginate fouling was minimal in the absence of applied pressure but increased with higher hydraulic pressures. However, gypsum scaling was observed in the absence of applied pressure and was negligible between 8 bars and 18 bars. Combined fouling was severe with the co-existence of gypsum crystals and alginate in the absence of applied pressure. The study concluded that the removal of alginate-type foulants from the feed water stream may become essential for the success of PES TFC membranes at high pressures. Khan et al. conducted a comparative investigation on fouled PA TFC RO membranes that were collected from seawater RO pilot plants to study the in uence of the operating parameters and water quality on membrane fouling. 135 The pilot plants were situated in Vilaseca (East Spain) and used UF as a pretreatment method. From the results obtained using chemical, ultrastructural and microbiological analyses on fouling layers, it was observed that the seawater RO train was greatly affected by bio/organic fouling both at the lead and terminal position elements. It was discovered that the microbial population causing bio/organic fouling largely comprised alpha and gamma proteobacteria, whereas the inorganic fouling layer mostly comprised calcium, sulfur and iron elements. It was veri ed from these results that the composition of the RO fouling layer is strongly dependent on the water quality of the source Dependence of membrane fouling on the membrane surface properties. For PA membranes, the chemical structure of the membrane surface might play a key role in determining the fouling propensity of these membranes. 136 The chemical heterogeneity of the surface of PA RO membranes and its effect on membrane fouling has been studied using AFM. 137 The AFM colloidal probes were made using positively charged aliphatic amine latex (AAL) and negatively charged carboxyl modi ed latex (CML). As a result of the negatively charged membrane surfaces, the adhesive forces were observed to be considerably stronger with the AAL probe compared to the CML probe. It was further observed that for the RO membranes having similar average surface properties, it was possible for them to have surfaces that are chemically heterogeneous. The results obtained from fouling experiments conducted using two commercial RO membranes with different chemical heterogeneities showed that fouling was more prominent for membranes with higher chemical heterogeneity. This veri ed that the fouling of PA RO membranes is dependent on the surface charge distribution. Hence, the surface chemical heterogeneity can be used as an important parameter for synthesizing RO membranes that are resistant to fouling. Wu et al. studied the impact of speci c surface functional groups found on a common PA RO membrane surface on alginate membrane fouling under solution conditions relevant to 8148 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

16 Review seawater desalination. 138 Self-assembled monolayers (SAM) with desired terminal functional groups such as COOH, NH 2, CONH 2 and OH were prepared and alginate adsorption and removal was studied using a quartz crystal microbalance with dissipation monitoring. The initial adsorption rate and the reversibility of the adsorbed layer were studied. It was discovered that the NH 2 group has a strong affinity for alginate; however, the COOH group showed the highest adsorption rate for simulated seawater. Furthermore, the reversibility was shown at high ionic strength and calcium ion concentration where alginate alginate interactions and alginate aggregation played an important role for its adsorption behavior. Speci c functional groups seemed to have no signi cant in uence on the equilibrium adsorption of alginate; however, it would be interesting to further investigate the role of surface heterogeneity in membrane fouling. Biofouling in PA RO membranes has also been controlled by immobilizing the membranes with lysozyme, which is an antibacterial enzyme. 139 The antibacterial enzyme was immobilized on the PA layer of RO membranes through 6-amino caproic acid molecules by using the method of IP and amine coupling. An increase in the surface density of the immobilized lysozyme was observed (Fig. 8) as the concentration of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide and N- hydroxysuccinimide was increased in the amine coupling reaction. The highest surface density of the immobilized lysozyme was found to be around 0.1 mg cm 2. The modi ed membranes caused a reduction in water permeability but sustained the same salt rejection ratio as the original PA RO membrane. Moreover, it was found from biofouling experiments that when the fabricated antibacterial PA RO membrane was exposed to the feed solution containing bacteria, it efficiently inhibited bio lm formation which could have resulted in adverse biofouling. PA RO membranes with high chlorine tolerance properties and good permeation have also been prepared on microporous polysulfone membrane supports via conventional IP of different diamine compounds with TMC or isophthaloyl chloride. 140 The diamine compounds used for polymer preparation were metaphenylene diamine, 2,4-diaminotoluene, 2,6-diaminotoluene, 3,4-diaminotoluene, N,N 0 -diphenylethyldiamine, 4-chlorometaphenylene diamine and 2,4-diaminoanisol. All the polymers prepared using the different diamine compounds were Fig. 8 Analyzed images of unmodified and lysozyme-immobilized membranes from COMSTAT software. 139 RSC Advances tested for their hydrophilicity and chlorine tolerance. Of the diamines, only metaphenylene diamine and 2,6-diaminotoluene were used for the preparation of TFC RO membranes via IP with trimesoyl chloride. It was observed from the chlorine tolerance that the TFC membranes synthesized from 2,6-diaminotoluene demonstrated better chlorine tolerance compared to those synthesized from metaphenylene diamine as an amine monomer. However, performance tests showed that the water permeability and salt rejection of both the membranes were similar. The surface composition and properties of hybrid polymer membranes are crucial determining factors of the fouling propensities of the membranes Initial biofouling in polymeric composites could be attributed to the presence of defects on the membrane surface. 146 Up to 30% of the membrane surface could be covered by bacteria due to defects in the membrane surface. 146 The areas of heterogeneity on the membrane surface can provide niches for promoting bio lm growth. Therefore, the application of a conditioning lm can be adopted in order to reduce the impact of the membrane heterogeneity on the RO membrane performance. Although there are some reports that membrane surface properties such as hydrophilicity, surface charge, and roughness could affect bio lm formation, Baek et al. argued that bio lm formation and ux reduction are experienced despite differences in the surface properties. 147 This observation suggests that the current research interest directed towards the manipulation of membrane surface conditions to reduce biofouling needs to be reconsidered. The mitigation of bio lm formation through the addition of anti-biofouling membrane surfaces or effective pretreatment might prove to be more worthwhile. The in uence of the surface features of four commercial PA composite RO membranes on membrane fouling has been investigated in a recent study. 148 These membranes included SW22 (from Vontron Technology Inc.), SW30 (from Film Tec Inc.), SWC1 (from Hydranautics), and TM810 (from Toray Bluestar Membrane Inc.). The surface features and microstructures of these commercial RO membranes were characterized and investigated for seawater desalination. It was found from experimental results that only SW22 and SW30 had hydrophilic coatings. It was further observed that the main role in the desalting process was played by the mid-section of the separation layer, which differed for each membrane because of the different approaches for the membrane synthesis. The RO membranes were also tested for their resistance to fouling using aqueous solutions containing different foulants with a sodium chloride concentration of ppm. It was discovered that SW22 had the highest resistance to fouling from humic acid due to the existence of substantial hydrophilic groups on the surface of the membranes. SWC1 strongly resisted fouling from silica colloids and BSA because the membrane was negatively charged. The antifouling properties of the RO membranes greatly depended on their surface features like the surface charge properties, functional groups and surface roughness. A smooth membrane surface would improve the anti-fouling properties of the membranes by hindering the foulants deposition on membranes under hydraulic shear forces during the This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

17 RSC Advances desalting process. 149 Furthermore, the fouling mechanism of BSA and humic acid was observed to vary signi cantly from that of silica colloids. Another work was an investigation of the modi cation of a PA RO membrane with L-3,4-dihydroxyphenylalanine (L-DOPA) amino acid for minimizing membrane fouling. 107 This study presented a comparison between a virgin PA membrane and an L-DOPA modi ed PA membrane. The effects of this modi cation on these membranes and fouling agents in terms of physicochemical interactions were examined. The extended Derjaguin Landau Verwey Overbeek (XDLVO) theory and the calculation of adhesion free energies were used to show the impact of the fouling agents (BSA and alginate) on the membranes. The results obtained showed that the adhesion of both BSA and alginate on the membranes was mainly due to acid base interactions. L-DOPA seemed to affect the acid base component of the adhesion free energy for the modi ed membrane. The results indicated that the modi cation of the virgin PA membrane with L-DOPA could restrict the membrane fouling propensity. However, L-DOPA needs to be further explored for industrial and commercial anti-fouling processes. PA-urethane composites form another class of RO membranes that have been investigated lately because of their improved mechanical properties. 108,109 An attempt has also been made to endow antifouling and chlorine resistance properties on PA-urethane TFC RO membranes by modifying the membranes with a functional monomer. Liu et al. has fabricated a PA-urethane RO composite membrane by a two-step IP technique using dimethyl-m-phenylenediamine (DMMPD) as the main functional monomer on a polysulfone support layer. 108 The modi ed RO TFC membranes, poly(amideurethane@imide), were more chlorine resistant due to the inclusion of an ultrathin polyimide lm on the outermost surface of the synthesized TFC RO membrane. The chlorine tolerant properties of the poly(amide-urethane@imide) membranes were veri ed by a combination of XPS and infrared (IR) analyses. In addition, the modi ed membranes prepared via the two-step IP technique demonstrated enhanced antifouling properties, as they possessed much smoother and more hydrophilic surfaces than the conventional poly(amide/ imide-urethane) membranes that are prepared by a one-step IP technique. However, the water ux and salt rejection of the poly(amide-urethane@imide) were marginally lower than the conventional poly(amide/imide-urethane) membrane and the commercial PA membrane. A PA-urethane TFC RO membrane has been fabricated in another study for seawater desalination via IP of MPD and CFIC. 109 However, in this study, only the membrane performance enhancement was investigated. The impacts of utilizing mixed cross-linking agents, choosing organic solvents of varying diffusivity and/or solubility for MPD, changing the temperature of the organic solvent, implementing a modi ed curing process, and post-treatment on the performance of the modi ed TFC RO membrane were investigated. The experimental results demonstrated that water ux through the membrane improved effectively. The salt rejection remained unchanged or increased under the following conditions: addition of diacyl chloride, selection of an organic solvent with higher amine diffusivity, synthesis of a membrane at Review a reasonably higher temperature, curing the nascent membrane via a two-stage curing process, soaking the membrane in aqueous hypochlorite solution. The modi ed membrane s water ux increased from around 35 LMH to approximately 42 LMH for seawater desalination and the salt rejection was always observed to be greater than 99.4% for a feed solution containing 3.5 wt% sodium chloride at a pressure of 5.5 MPa. Moreover, the PA urethane TFC RO membrane with the most efficient performance comprised a distinct cross-linked selective skin layer and a comparatively hydrophobic surface. This study proved that the modi cation of the fabrication and process conditions for the PA-urethane TFC RO membrane could improve the membrane performance in addition to its antifouling bene ts. 109 Zwitterionic polymers have also been reported and con rmed to exhibit antifouling properties. 110,150,151 Choi et al. prepared and applied amphiphilic zwitterionic carboxylated polyethyeleneimine (carboxylated PEI) and comb-like hydroxyl poly(oxyethylene) methacrylate homopolymer (HPOEM) antifouling layers on the surface of commercial seawater RO membranes to enhance the fouling resistance of these membranes. 151 No signi cant changes occurred to the morphologies of the modi ed membranes because of the preparation of the ultrathin coating layer under mild conditions. The performances of the modi ed membranes were tested in both seawater and brackish water conditions. Fouling tests were conducted on the modi ed membranes using BSA and alginate as model foulants. Fouling tests at high salt concentrations revealed a salting-in effect for the membranes coated by zwitterionic carboxylated PEI, and a salting-out effect for the membranes coated by HPOEM. The zwitterionic carboxylated PEI coated membrane demonstrated a lower affinity for deionized water than for sodium chloride solution, and showed better fouling resistance in seawater conditions. Hence, an efficient fouling resistant layer made from zwitterionic materials can be proposed for seawater desalination. The HPOEM coated membranes also showed improved fouling resistance under the conditions of brackish water. Meng et al. attempted to minimize fouling on a commercial PA TFC RO membrane by gra ing a salt-responsive zwitterionic polymer poly(4-(2-sulfoethyl)-1-(4-vinylbenzyl)pyridinium betaine) (PSVBP) on the membrane. 110 The investigation of the synthesized membrane revealed that the PSVBP gra ing enhanced the membrane hydrophilicity and caused the membrane surface to be negatively charged. It was observed from RO performance tests that the PSVBP gra ing resulted in an increase in salt rejection from 98.0 to 99.7%; however, the permeation ux decreased by 20%. Additionally, the fabricated membrane was found to have good antifouling properties over the short term, and was able to recover 90% of the initial permeate ux a er being rinsed with brine. The enhanced antifouling properties of the membrane resulted from the suppression of hydrophobic hydrophobic interactions and negatively charged organic foulants in the surface water. Moreover, the PSVBP membrane s salt-responsive properties provided the driving force for easy elimination of protein foulants from the membrane surface to improve its cleaning efficiency, as shown in Fig RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016

18 Review RSC Advances Fig. 9 Synthesis and easy cleaning of a salt-responsive PA TFC membrane. 110 The imparting of antifouling properties on a commercial PA TFC RO membrane (LE-400 Film Tec from Dow) through membrane surface modi cation has also been investigated by Nikkola et al. 152 In this study, the surface properties of the membrane were modi ed by coating the membrane surface via an inorganic trimethylaluminium-based atomic layer deposition (ALD) process. The processing temperatures and number of ALD cycles were varied systematically and observed to in uence membrane surface properties such as surface polarity, hydrophilicity and surface roughness. The ALD coatings were found to enhance the antibacterial and antifouling properties of the RO membrane. Among all the ALD-coated membranes produced, the colony forming unit (CFU) test results showed that the lowest bacterial adhesion took place on the membrane with the most polar and hydrophilic surface. In addition, the RO membrane performance in terms of water permeability, salt permeability and salt rejection was observed to be affected by the processing temperatures and the number of ALD cycles. Generally, less ALD cycles and low temperature contributed to the improved RO performance of the ALD-coated membranes in terms of the permeability and antifouling properties. The gra ing of imidazolidinyl urea (IU) on a commercial aromatic PA RO membrane (RE4021-TL from Woongjin Chemical Co., Korea) using the carbodiimide induced method has also been shown to be a way to improve the anti-biofouling and chlorine resistant properties of the membrane. 111 The gra ed IU worked like an anti-microbial agent to enhance the anti-biofouling properties of the membrane surfaces, and worked as the precursor of N-halamine to provide lasting and regenerable anti-biofouling and chlorine resistant properties to the membrane. An improved hydrophilicity was observed for the modi ed membrane and the membrane surface s isoelectric point was found to shi towards a higher value. No changes were observed in the morphology and roughness of the membrane surface. Compared to pristine PA RO membranes, which do not have anti-biofouling properties, the IU modi ed membrane demonstrated very high anti-biofouling properties and eliminated all the possible E. coli cells from the IU-modi ed membrane surface. The water ux of the pristine membrane was almost negligible in the case of severe biofouling, whereas the salt rejection and water ux of the IU-modi ed membrane decreased by 2.7 and 16.0%, respectively. Additionally, the IUmodi ed membrane s anti-biofouling properties were observed to improve considerably a er chlorination because the gra ed IU on the surface of the membrane worked as sacri cial pendant groups to avoid the chlorination of PA. Moreover, the IU-modi ed membrane s anti-biofouling and the chlorine resistant properties retained a high regeneration capacity due to reversible transition between the IU and chlorinated IU. Furthermore, results from the long term investigation of this method demonstrated that the lifetime of the commercial RO membrane can be increased for practical applications with the inclusion of IU modi cation along with occasional free chlorine pretreatment in the RO systems. The deposition of N-isopropylacrylamide-co-acrylic acid copolymers (P(NIPAm-co-AAc)) on the surface of a PA TFC RO membrane is another way to improve the antifouling properties of the membrane. Yu et al. has modi ed the surface of interfacially prepared commercial aromatic PA TFC RO membranes by depositing P(NIPAm-co-AAc) on the membrane surface. 153 Free radical copolymerization was used to prepare P(NIPAm-co- AAc) copolymers and varying concentrations of their aqueous solutions were used for PA membrane modi cation through sedentary surface coating technique. Fouling experiments using an aqueous solution of BSA demonstrated that the modi ed membrane had signi cantly higher fouling resistance to BSA compared to unmodi ed membranes due to the increased hydrophilicity and negative charge of the membrane surface at neutral ph. It was revealed from the washing experiments using deionized water that the P(NIPAm-co-AAc) coating layer enhanced the cleaning efficiency of the membrane. Furthermore, the elimination of foulants situated on the membrane surface would be possible when the coating layer s phase transition occurs at temperatures higher than its low critical solution temperature. The membrane surface became more hydrophilic and the surface charge increased at neutral ph due to hydrophilic P(NIPAm-co-AAc) layer deposition; but the water permeation resistance increased. It was observed from permeation tests that the sodium chloride and sodium sulfate permeability decreased under neutral and alkaline conditions due to membrane modi cation. The decline in permeability This journal is The Royal Society of Chemistry 2016 RSC Adv., 2016,6,

19 RSC Advances was higher for sodium sulfate (divalent anion) compared to sodium chloride (monovalent anions). The surface modi cation of commercial RO membranes by the deposition of ultrathin copolymer lms on the membrane surfaces could also be used to promote biofouling resistance. In a recent study, four commercial seawater RO membranes made from PA were tested: the defunct 80B (from Toray), AD (from GE Osmonics), and SW30HR (from Dow Film Tec). 112 The initiated Chemical Vapor Deposition (icvd) technique was used for the direct deposition of the two copolymerized monomers (per- uorodecyl acrylate (PFDA) and hydroxyethyl methacrylate (HEMA)) with tert-butyl-peroxide as the initiator, forming an amphiphilic surface measured by the contact angles. Several analyses were used to con rm the presence and smoothness of the lm on the membrane surface, such as FTIR, XPS and AFM. Short-term permeation tests were conducted to show the impact of this coating on the salt rejection. Analysis of the membranes showed that the surface was smooth and that the thin lm was well-deposited on the membrane surface. The permeation tests revealed that the lm was as permeable as the PA layer and that it would not cause a signi cant decline in water ux. Moreover, this deposition technique could be used to prevent biofouling because the bacterial composition around the membrane surfaces would be hampered by their amphiphilic chemistry, rather than hydrophobic chemistry. However, further studies are needed to evaluate the long-term viability of this approach. In a related study, commercial PA TFC RO membranes were modi ed by depositing copolymer lms on the membrane surfaces to decrease bacterial adhesion and control biofouling. 113 The icvd technique was used to deposit the copolymerized hydrophobic PFDA and hydrophilic HEMA monomers on several substrates. The thickness and chemistry of the deposited lms were controlled by regulating the comparative gas ow rates of the two monomers. It was observed from Quartz Crystal Microscopy (QCM) that the amphiphilic chemistry copolymer lm exhibited relatively less foulant adsorption than the pure homopolymers. E. coli cells were used for short-term batch studies to test for the coated and virgin membranes resistance to microbial adhesion. The minimum bacterial adhesion was observed for the copolymer Fig. 10 Number of E. coli cells attachments to amphiphilic and virgin PA TFC membranes for images across the membrane surfaces. 113 Review lm membrane with amphiphilic chemistry (Fig. 10). Moreover, it was discovered from the quantitative analysis of the optimized modi ed membrane and virgin membrane that the existence of the amphiphilic copolymer lm on the membrane surface reduces the bacterial adhesion by around 100 times. The copolymer lms can therefore be used as antifouling coatings on RO membranes as they can effectively prevent bacterial adhesion and foulant adsorption. McVerry et al. proposed the modi cation of commercial PA TFC RO membranes using the per uorophenyl azide (PFPA) photochemistry method to enhance their anti-fouling properties in addition to retaining the scalability and ensuring a rollto-roll manufacturing process for RO membranes. 154 The commercial RO membranes were rst immersed into an aqueous solution containing PFPA-terminated poly(- ethyleneglycol) (PEG) species, followed by UV light exposure under ambient conditions. Contact angle measurements and ATR-IR spectroscopy con rmed the effective covalent modi cation of the RO membranes. Moreover, XPS demonstrated that the PFPAs went through nitrene addition generated by UV and were attached to the RO membrane via aziridine linkage. Furthermore, the results from fouling experiments revealed that the RO membranes modi ed with PFPA-PEG derivatives had high resistance to fouling. However, RO performance tests demonstrated that hydrophilic polymers on the modi ed membrane surfaces reduced the water permeability and increased the salt (sodium chloride) rejection compared to the original commercial RO membranes because of steric hindrance. When the molecular weight of hydrophilic PEG brush polymers was systematically increased, the water ux and salt rejection of the modi ed membranes were also signi cantly affected because of bigger exible polymer chains. Despite a reduction in the initial water ux of PFPA-PEG modi ed membranes, the modi ed membranes provided higher water uxes than most commercial RO membranes with a similar sodium chloride salt rejection. The anti-fouling experiments on the modi ed and unmodi ed membranes were conducted by uorescent microscopy using E. coli as the model bacteria. It was revealed that the bacterial adhesion on the modi ed membranes was signi cantly less than for the unmodi ed membranes. Moreover, the anti-fouling resistance of the modi ed RO membranes improved with increasing PEG molecular weight. Overall, the production of anti-fouling RO membranes can reduce the energy and high maintenance costs associated with RO desalination by eliminating biofouling on the membrane surface. The enhancement of the antifouling properties of commercial PA TFC RO membranes has also been investigated in a recent study by dip-coating a synthesized terpolymer on the membrane surface. 155 In this work, the novel terpolymer poly(- methylacryloxyethyldimethyl benzyl ammonium chloride-racrylamide-r-2-hydroxyethyl methacrylate) [P(MDBAC-r-Am-r- HEMA)] was prepared by free radical polymerization. The synthesized terpolymer was dip-coated and cross-linked on the surface of commercial TFC PA RO membranes (LCLE and BW30 Film Tec from Dow) to enhance their antimicrobial properties and chlorine resistance (Fig. 11). The chlorine resistance of the 8152 RSC Adv., 2016,6, This journal is The Royal Society of Chemistry 2016