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The forward osmosis process has obtained renewed interest nowadays and it might become an alternative so- lution for many industrial applications to meet the current and future requirements for potable water. The FO process depends on the osmotic pressure gradient between a high salinity draw solute and low salinity feed solution across a semi-permeable membrane to extract pure water. Despite the potential advantages of FO, there are some technical drawbacks that hinder FO application for water desalination. One of the most significant critical challenges is the need for membrane compatible with the FO process. To improve FO desalination feasibility, membrane development is required to obtain maximum water permeability and minimum reverse solute flux over long-term operations. Therefore, this review starts by demonstrating the fundamentals and membrane development over the years. Fabrication modifications for the support layer of FO membranes and the crucial challenges of the FO process are summarized. Recent trends of the chemical modifications of the bulk and substrate are discussed. The advantages and disadvantages of the modifications on the FO membrane productivity are also addressed. Finally, concluding with future perspectives. Keywords: Forward osmosis Desalination Chemical modifications Membrane fabrication Support layer Synthetic polymer Nanotechnology * Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Fabian Way, Swansea SA1 8EN, UK > College of Engineering, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates Wafa Ali Suwaileh*, Daniel James Johnson“, Sarper Sarp*, Nidal Hilal**

Table 1 The forward osmosis process has obtained renewed interest nowadays and it might become an alternative so- lution for many industrial applications to meet the current and future requirements for potable water. The FO process depends on the osmotic pressure gradient between a high salinity draw solute and low salinity feed solution across a semi-permeable membrane to extract pure water. Despite the potential advantages of FO, there are some technical drawbacks that hinder FO application for water desalination. One of the most significant critical challenges is the need for membrane compatible with the FO process. To improve FO desalination feasibility, membrane development is required to obtain maximum water permeability and minimum reverse solute flux over long-term operations. Therefore, this review starts by demonstrating the fundamentals and membrane development over the years. Fabrication modifications for the support layer of FO membranes and the crucial challenges of the FO process are summarized. Recent trends of the chemical modifications of the bulk and substrate are discussed. The advantages and disadvantages of the modifications on the FO membrane productivity are also addressed. Finally, concluding with future perspectives. Keywords: Forward osmosis Desalination Chemical modifications Membrane fabrication Support layer Synthetic polymer Nanotechnology * Centre for Water Advanced Technologies and Environmental Research (CWATER), College of Engineering, Swansea University, Fabian Way, Swansea SA1 8EN, UK > College of Engineering, University of Sharjah, P.O. Box 27272, Sharjah, United Arab Emirates Wafa Ali Suwaileh*, Daniel James Johnson“, Sarper Sarp*, Nidal Hilal**

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Fig. 1. A schematic diagram of a commercial FO-TFC membrane. Reproduced with permission from [116]. Copyright 2017 Elsevier. Considerations, such as low water quality in a single stage [3], draw solution properties, reverse salt flux [5] and the choice of an ideal FO membrane are critical for the development of the FO desalination system [11]. A wide range of membranes have been used for the FO process in both flat sheet and hollow fibre configurations including cellulose acetate (CA), cellulose triacetate (CTA), thin film composite (TFC), and bio-mimetic membranes [13]. Amongst FO membranes, the cellulosic membrane has been employed widely for the FO process; however, it suffers low selectivity, is prone to biological attack and chemical hydrolysis [14]. Thereafter, desalination researchers focused on unique TFC-FO membranes for the FO system because of excellent Phase inversion is the most important and successful technique used for the preparation of flat sheet and hollow fibre FO membranes. It involves mixing a polymer with solvent and casting of the polymer suspension on a support layer. The next step is the immersion pre- cipitation of this support layer which could be described as soaking a polymer solution in a non-solvent coagulant bath [21]. In addition to this process, a dry-jet wet spinning process was also used to fabricate a hollow fibre substrate [22]. It consists of preparing a polymer dope which was extruded by a spinneret at a volume rate that was fixed by a gear pump. Subsequently, it was subjected to an air gap followed by a soaking in a coagulation bath. Electro-spinning is another technique used for the formation of fibrous polymer in various configurations and functions. It has been employed to fabricate flat sheet and hollow fibre membranes. It involves applying a high electrical field to a polymer
Fig. 1. A schematic diagram of a commercial FO-TFC membrane. Reproduced with permission from [116]. Copyright 2017 Elsevier. Considerations, such as low water quality in a single stage [3], draw solution properties, reverse salt flux [5] and the choice of an ideal FO membrane are critical for the development of the FO desalination system [11]. A wide range of membranes have been used for the FO process in both flat sheet and hollow fibre configurations including cellulose acetate (CA), cellulose triacetate (CTA), thin film composite (TFC), and bio-mimetic membranes [13]. Amongst FO membranes, the cellulosic membrane has been employed widely for the FO process; however, it suffers low selectivity, is prone to biological attack and chemical hydrolysis [14]. Thereafter, desalination researchers focused on unique TFC-FO membranes for the FO system because of excellent Phase inversion is the most important and successful technique used for the preparation of flat sheet and hollow fibre FO membranes. It involves mixing a polymer with solvent and casting of the polymer suspension on a support layer. The next step is the immersion pre- cipitation of this support layer which could be described as soaking a polymer solution in a non-solvent coagulant bath [21]. In addition to this process, a dry-jet wet spinning process was also used to fabricate a hollow fibre substrate [22]. It consists of preparing a polymer dope which was extruded by a spinneret at a volume rate that was fixed by a gear pump. Subsequently, it was subjected to an air gap followed by a soaking in a coagulation bath. Electro-spinning is another technique used for the formation of fibrous polymer in various configurations and functions. It has been employed to fabricate flat sheet and hollow fibre membranes. It involves applying a high electrical field to a polymer
Fig. 2. Showing the typical FO system which operates under osmotic pressure difference using seawater feed, NaCl draw solute and FO membrane. Adapted with permission from [127]. Copyright 2012 Elsevier. This osmotic pressure (st) is influenced by the draw solute con- centration and its temperature [46]. Accordingly, the osmotic pressure
Fig. 2. Showing the typical FO system which operates under osmotic pressure difference using seawater feed, NaCl draw solute and FO membrane. Adapted with permission from [127]. Copyright 2012 Elsevier. This osmotic pressure (st) is influenced by the draw solute con- centration and its temperature [46]. Accordingly, the osmotic pressure
which can be re-expressed as follows [10]: Fig. 3. The most important properties for selecting the ideal membrane for osmosis process.
which can be re-expressed as follows [10]: Fig. 3. The most important properties for selecting the ideal membrane for osmosis process.
Table 1 (continued) synthesized with double active layers on PES a sub-layer [76]. The sub- layer contained a dense and porous finger-like macrovoid structure, and S value was of 996 um. In this case, the outer selective layer successfully minimized the ICP and fouling into the sub-layer against real waste- water brine. This new membrane had acceptable mechanical strength for hydraulic pressure over 20 bar and superior fouling resistance. An excellent water permeability of 1.5 LMH and a very low salt perme- ability of 0.02 LMH and a salt rejection of 94.2% were recorded using DI water and 1000 ppm NaCl FS respectively.
Table 1 (continued) synthesized with double active layers on PES a sub-layer [76]. The sub- layer contained a dense and porous finger-like macrovoid structure, and S value was of 996 um. In this case, the outer selective layer successfully minimized the ICP and fouling into the sub-layer against real waste- water brine. This new membrane had acceptable mechanical strength for hydraulic pressure over 20 bar and superior fouling resistance. An excellent water permeability of 1.5 LMH and a very low salt perme- ability of 0.02 LMH and a salt rejection of 94.2% were recorded using DI water and 1000 ppm NaCl FS respectively.
In summary, other players in the market include Porifera, Toray and Aquaporin. The TFC membranes performed better than cellulosic membranes because of higher tolerance to pH ranges from 2 to 11 and were stable at a temperature over 60°C. As mentioned previously, it exhibited higher water permeability and can tolerate the process con- ditions of pressure retarded osmosis (PRO) [77]. In contrast, the two stages of preparation result in higher cost and there is a challenge in performance outperformed the commercial HTI-CTA mem- branes.1.16LMH using DI water and salt rejection of 94.1% using 50 mM NaCl feed in RO mode [11]. However, the sub-layer is thick and the structural parameter (492 um) needs optimization. Moreover, re- cent TFC-HTI membranes included a mesh-embedded sub-layer was synthesized. It exhibited 2 folds higher water flux (1.4LMH) than commercial CA membrane but lower salt rejection of 92% [16]. The sub-layer thickness is double CA and it had a high S value of 533 um. In addition, a flat sheet TFC made of both spongy skin and a finger-like structure produced a structural parameter of 670-710 um. This may decrease the tortuosity, yielding to quite low ICP impact [29]. The water flux was higher than 12 LMH as compared to commercial CTA membrane (5LMH) and a brackish water RO membrane (0.78 LMH) using 10mM NaCl FS and 0.5M NaCl DS. It appears that the straight finger-like pore structure is more significant than the sponge-like structure to mitigate the ICP. Presently, a hollow fibre membrane Fig. 4. (a) a schematic illustration of Layer by Layer process. Adapted with permission from [87]. Copyright 2011 American Chemical Society
In summary, other players in the market include Porifera, Toray and Aquaporin. The TFC membranes performed better than cellulosic membranes because of higher tolerance to pH ranges from 2 to 11 and were stable at a temperature over 60°C. As mentioned previously, it exhibited higher water permeability and can tolerate the process con- ditions of pressure retarded osmosis (PRO) [77]. In contrast, the two stages of preparation result in higher cost and there is a challenge in performance outperformed the commercial HTI-CTA mem- branes.1.16LMH using DI water and salt rejection of 94.1% using 50 mM NaCl feed in RO mode [11]. However, the sub-layer is thick and the structural parameter (492 um) needs optimization. Moreover, re- cent TFC-HTI membranes included a mesh-embedded sub-layer was synthesized. It exhibited 2 folds higher water flux (1.4LMH) than commercial CA membrane but lower salt rejection of 92% [16]. The sub-layer thickness is double CA and it had a high S value of 533 um. In addition, a flat sheet TFC made of both spongy skin and a finger-like structure produced a structural parameter of 670-710 um. This may decrease the tortuosity, yielding to quite low ICP impact [29]. The water flux was higher than 12 LMH as compared to commercial CTA membrane (5LMH) and a brackish water RO membrane (0.78 LMH) using 10mM NaCl FS and 0.5M NaCl DS. It appears that the straight finger-like pore structure is more significant than the sponge-like structure to mitigate the ICP. Presently, a hollow fibre membrane Fig. 4. (a) a schematic illustration of Layer by Layer process. Adapted with permission from [87]. Copyright 2011 American Chemical Society
Fig. 5. (a) schematic diagram of the preparation to make a membrane with vertical pores. (b, c, d) SEM micrographs of TFC membrane showing vertical pores having open structure in both sides of sub-layer creating spongy structure. Adapted with permission from Liang et al. [105]. Copyright 2017 Elsevier.
Fig. 5. (a) schematic diagram of the preparation to make a membrane with vertical pores. (b, c, d) SEM micrographs of TFC membrane showing vertical pores having open structure in both sides of sub-layer creating spongy structure. Adapted with permission from Liang et al. [105]. Copyright 2017 Elsevier.
The electro-spinning process produces polymeric fibres with a dia- meter between 5 and 500 nm and less than several micrometers, a small pore size and large surface area [107,108]. The scaffold-like nanofibres can be transformed to more porous with interconnected pores to obtain low tortuosity and S value [109]. Compared to PI technique, this structure can be obtained easily by the electro-spinning method [110,111]. The morphologies, orientation of prepared nanofibre, dif- ferent diameters and aspect ratio can be fulfilled by controlling the environment conditions, the solution viscosity, the flow rate of the Fig. 6. a, b, c, d represents TFC membrane embedded polyacrylonitrile (PAN)/cellulose acetate (CA) blended nanofibres mat that synthesized in DMF at various weight ratios: (a) PAN, CA 2/8, (b) PAN/CA 5/5, (c) PAN/CA 8/2, (d) pure PAN. (e-f) Cross-sectional SEM image of TFC membranes embedded nanofibrous PAN layer. Adapted with permission from Bui and McCutcheon [113]. Copyright 2012 American Chemical Society. Liang et al. [105] proposed using the PI combined bidirectional freezing process to tailor the PVDF substrate of a TFC membrane. It was chosen to make a substrate with vertically oriented pores in the sub- layer. The preparation process is illustrated in Fig. 5(a) so the polymer solution was added into a mould made of two stainless steel and glass plates [106]. Then the thickness was tailored perfectly and the mould was located in a water bath with various temperatures. Next, it was soaked vertically in an extraction solvent to obtain the membrane containing vertical pores. The newly developed FO membrane showed a nodular structure on the top and its porosity was estimated at 74%. It can be seen from Fig. 5(b) that the pores were formed vertically with a pore size of about 1.8 um. It can be observed that pores have an open shape in the bottom and top sides, as well as a spongy structure in the
The electro-spinning process produces polymeric fibres with a dia- meter between 5 and 500 nm and less than several micrometers, a small pore size and large surface area [107,108]. The scaffold-like nanofibres can be transformed to more porous with interconnected pores to obtain low tortuosity and S value [109]. Compared to PI technique, this structure can be obtained easily by the electro-spinning method [110,111]. The morphologies, orientation of prepared nanofibre, dif- ferent diameters and aspect ratio can be fulfilled by controlling the environment conditions, the solution viscosity, the flow rate of the Fig. 6. a, b, c, d represents TFC membrane embedded polyacrylonitrile (PAN)/cellulose acetate (CA) blended nanofibres mat that synthesized in DMF at various weight ratios: (a) PAN, CA 2/8, (b) PAN/CA 5/5, (c) PAN/CA 8/2, (d) pure PAN. (e-f) Cross-sectional SEM image of TFC membranes embedded nanofibrous PAN layer. Adapted with permission from Bui and McCutcheon [113]. Copyright 2012 American Chemical Society. Liang et al. [105] proposed using the PI combined bidirectional freezing process to tailor the PVDF substrate of a TFC membrane. It was chosen to make a substrate with vertically oriented pores in the sub- layer. The preparation process is illustrated in Fig. 5(a) so the polymer solution was added into a mould made of two stainless steel and glass plates [106]. Then the thickness was tailored perfectly and the mould was located in a water bath with various temperatures. Next, it was soaked vertically in an extraction solvent to obtain the membrane containing vertical pores. The newly developed FO membrane showed a nodular structure on the top and its porosity was estimated at 74%. It can be seen from Fig. 5(b) that the pores were formed vertically with a pore size of about 1.8 um. It can be observed that pores have an open shape in the bottom and top sides, as well as a spongy structure in the
Fig. 7. (a) The synthesis process of carboxylated polysulfone (CPSF). (b) A comparison between the pristine PSf and CPSf in terms of microstructure. It is shown a spongy-like structure in the CPSf support layer. Adapted with permission from [149]. Copyright 2013 Elsevier. In Zhong et al.'s [145] study, interfacial polymerization was adopted to prepare PPSU-TFC based hollow fibre membrane. It was found that the sulfonation preparation greatly influenced the membrane micro- structure. This is because nonsulfonated membrane formed a macro- void-free structure caused by quick de-mixing while the modified sub- strate (1.5mol% sPPSU) displayed a sponge-like structure. The structural parameter was dropped to 1.63x10°*m from 7.46 x 107*m for a pristine PPSU membrane. This allows more ef- fective water diffusion within the membrane resulting in higher water permeability of 1.99LMH, reduced salt passage of 0.0399LMH and acceptable salt rejection of 90.9% against DI water and 1000 ppm NaCl in RO mode. When Widjojo et al. [144] used sPPSU mixed with different past, the sulfonation process was used to functionalize polysulfone polymer to obtain a high hydrophilicity, produce the desired macro- voids, open pore structures and to enhance rejection of organics [141,142], leading to improved water fluxes in forward osmosis sys- tems. In one study exploring the impacts of a sulfonated copolymer composed of PES and PPSU when blended with polyethersulfone (PESU E6020P) in a TFC membrane, it was found that the higher concentra- tion of the sulfonated polymer in the substrate lead to lower surface roughness [143]. When 50 wt% sulfonated material was embedded in the support layer, a more favourable sponge-like structure free of macrovoids was formed. The porosity and pore size increased due to slow de-mixing in the phase inversion compared to pure PESU. The structural parameter exhibited a low value of 3.24 x 107~*m when increasing the amount of sulfonated polymer and therefore reduced ICP effects. Another advantage of this modification was the formation of a high permeable active layer on the substrate. In addition to this, the effect of directly sulfonated polymer with various sulfonation degrees on the morphology, mechanical properties and performance were in- vestigated. It is important to note that the greatest water permeability approached 0.73LMH for 50wt% sulfonated polymer embedded membrane as compared to pristine PESU membrane because of im- proved hydrophilicity. The salt rejection and solute permeability almost remained the same when using 400 ppm NaCl solution in RO experi- ment. monomers to fabricate the support layer, the thickness of both the non- sulfonated and sulfonated polymers was in the range of 35-50 um. The sulfonated support layer was hydrophilic and had a sponge-like struc- ture compared to the non-sulfonated sample with macrovoids only. A reduction in the ICP and a good wettability of the sub-layer were ob- servable. When the polymer contained 2.5mol% of 4,4’-Di- chlorodiphenyl sulfone, the membrane displayed very low water per- meability of 3.23 LMH and salt passage of 1.05 LMH against DI water and 400 ppm NaCl feeds in RO filtration. In contrast, the pristine membrane showed water permeability of 12.53 and higher salt passage of 5.78 LMH. Interestingly, the salt rejection of the developed mem- brane was improved of about 84.1% versus the pristine membrane (81.7%).
Fig. 7. (a) The synthesis process of carboxylated polysulfone (CPSF). (b) A comparison between the pristine PSf and CPSf in terms of microstructure. It is shown a spongy-like structure in the CPSf support layer. Adapted with permission from [149]. Copyright 2013 Elsevier. In Zhong et al.'s [145] study, interfacial polymerization was adopted to prepare PPSU-TFC based hollow fibre membrane. It was found that the sulfonation preparation greatly influenced the membrane micro- structure. This is because nonsulfonated membrane formed a macro- void-free structure caused by quick de-mixing while the modified sub- strate (1.5mol% sPPSU) displayed a sponge-like structure. The structural parameter was dropped to 1.63x10°*m from 7.46 x 107*m for a pristine PPSU membrane. This allows more ef- fective water diffusion within the membrane resulting in higher water permeability of 1.99LMH, reduced salt passage of 0.0399LMH and acceptable salt rejection of 90.9% against DI water and 1000 ppm NaCl in RO mode. When Widjojo et al. [144] used sPPSU mixed with different past, the sulfonation process was used to functionalize polysulfone polymer to obtain a high hydrophilicity, produce the desired macro- voids, open pore structures and to enhance rejection of organics [141,142], leading to improved water fluxes in forward osmosis sys- tems. In one study exploring the impacts of a sulfonated copolymer composed of PES and PPSU when blended with polyethersulfone (PESU E6020P) in a TFC membrane, it was found that the higher concentra- tion of the sulfonated polymer in the substrate lead to lower surface roughness [143]. When 50 wt% sulfonated material was embedded in the support layer, a more favourable sponge-like structure free of macrovoids was formed. The porosity and pore size increased due to slow de-mixing in the phase inversion compared to pure PESU. The structural parameter exhibited a low value of 3.24 x 107~*m when increasing the amount of sulfonated polymer and therefore reduced ICP effects. Another advantage of this modification was the formation of a high permeable active layer on the substrate. In addition to this, the effect of directly sulfonated polymer with various sulfonation degrees on the morphology, mechanical properties and performance were in- vestigated. It is important to note that the greatest water permeability approached 0.73LMH for 50wt% sulfonated polymer embedded membrane as compared to pristine PESU membrane because of im- proved hydrophilicity. The salt rejection and solute permeability almost remained the same when using 400 ppm NaCl solution in RO experi- ment. monomers to fabricate the support layer, the thickness of both the non- sulfonated and sulfonated polymers was in the range of 35-50 um. The sulfonated support layer was hydrophilic and had a sponge-like struc- ture compared to the non-sulfonated sample with macrovoids only. A reduction in the ICP and a good wettability of the sub-layer were ob- servable. When the polymer contained 2.5mol% of 4,4’-Di- chlorodiphenyl sulfone, the membrane displayed very low water per- meability of 3.23 LMH and salt passage of 1.05 LMH against DI water and 400 ppm NaCl feeds in RO filtration. In contrast, the pristine membrane showed water permeability of 12.53 and higher salt passage of 5.78 LMH. Interestingly, the salt rejection of the developed mem- brane was improved of about 84.1% versus the pristine membrane (81.7%).
Several modification procedures have also been used to improve the FO membrane properties. The chemical modification involves the ad- dition of a chemical modifier which adheres strongly to the surface by the covalent bonding providing chemical stability [173]. One of its advantages is providing low severe effects of ICP in the support layer and improving the water permeation [16]. For example, the separation
Several modification procedures have also been used to improve the FO membrane properties. The chemical modification involves the ad- dition of a chemical modifier which adheres strongly to the surface by the covalent bonding providing chemical stability [173]. One of its advantages is providing low severe effects of ICP in the support layer and improving the water permeation [16]. For example, the separation
Fig. 9. (a) A schematic diagram demonstrating the synthesis procedure of CaCO3-coated PES support layer of TFC-FO membrane. (b) Micrograph images showing the PES top surfa (a—c). Adapted from Liu et al. [200]. Copyright 2016 Nature/Scientific reports. to provide the negative charges. Then, PI was formed on the top surface followed by the mineral coating on the inner surface substrate. The PES/PAAS5 appeared rough after the alternative immersion procedure which indicated effective CaCO3 coating as can be seen in Fig. 9(b). This coating enabled an effective hydrophilicity via the electrostatic interaction. This means the interaction between Ca?* and carboxylate groups of PAA accelerated the nucleation of Ca?* on the pore surface followed by an interaction with CO; ? to form the coating on the substrate [200,14]. Additionally, the mineral coating contributed to a little decline in the water permeability when using DI water in RO experiment. This decrease caused by the swelling issue of PAA leading to contraction and pore blocking. However, it was useful in removing An alternative hydrophilization procedure is the bio-mineral coating of CaCO; on the PES substrate to improve wettability, which decreases the S value and ICP effects in TFC-FO membrane [200]. This coating was used for other polymeric membranes in previous studies [14,201,202]. It can provide the substrate with the required hydro- philicity because it can be connected to water through strong ionic hydrogen bonding. A PES sub-layer was exposed to bio-mineral (CaCO3) coating to cover the inner surface of this layer [200]. In Fig. 9(a), the three stages for the preparation of the mineral coating are described. First, the polyacrylic acid (PAA) deposited into PES polymer
Fig. 9. (a) A schematic diagram demonstrating the synthesis procedure of CaCO3-coated PES support layer of TFC-FO membrane. (b) Micrograph images showing the PES top surfa (a—c). Adapted from Liu et al. [200]. Copyright 2016 Nature/Scientific reports. to provide the negative charges. Then, PI was formed on the top surface followed by the mineral coating on the inner surface substrate. The PES/PAAS5 appeared rough after the alternative immersion procedure which indicated effective CaCO3 coating as can be seen in Fig. 9(b). This coating enabled an effective hydrophilicity via the electrostatic interaction. This means the interaction between Ca?* and carboxylate groups of PAA accelerated the nucleation of Ca?* on the pore surface followed by an interaction with CO; ? to form the coating on the substrate [200,14]. Additionally, the mineral coating contributed to a little decline in the water permeability when using DI water in RO experiment. This decrease caused by the swelling issue of PAA leading to contraction and pore blocking. However, it was useful in removing An alternative hydrophilization procedure is the bio-mineral coating of CaCO; on the PES substrate to improve wettability, which decreases the S value and ICP effects in TFC-FO membrane [200]. This coating was used for other polymeric membranes in previous studies [14,201,202]. It can provide the substrate with the required hydro- philicity because it can be connected to water through strong ionic hydrogen bonding. A PES sub-layer was exposed to bio-mineral (CaCO3) coating to cover the inner surface of this layer [200]. In Fig. 9(a), the three stages for the preparation of the mineral coating are described. First, the polyacrylic acid (PAA) deposited into PES polymer
Fig. 10. (a) A diagram presenting the modification procedure to form AqpZ-incorporated SLB forward osmosis (FO) membrane. The PSf substrate (gray) was treated with PDA an followed by the deposition of the amide bonded DOPE SLB (orange). (b) SEM micrographs show the asymmetrical structure with flat surface of the modified support layer. The SLB fille the gaps on the PDA-coated PSf membrane successfully. Adapted with permission from [206]. Copyright 2015 The Royal Society of Chemistry. Currently, the water channel protein aquaporin is used in fabri- cating FO membrane to facilitate water transport through the mem- brane [80]. A wide range of research has been carried out using dif- ferent procedures to form aquaporin incorporated membranes [80,90,203-205]. Scientists are interested in aquaporin due to its ef- fective performance. This was attained with the preferential water Additionally, a novel strategy was developed to form stable aqua- porin based biomimetic membranes with excellent FO performance [206]. This strategy is described in Fig. 10, it involved applying PDA coating on the PSf substrate and the deposition of proteoliposomes and
Fig. 10. (a) A diagram presenting the modification procedure to form AqpZ-incorporated SLB forward osmosis (FO) membrane. The PSf substrate (gray) was treated with PDA an followed by the deposition of the amide bonded DOPE SLB (orange). (b) SEM micrographs show the asymmetrical structure with flat surface of the modified support layer. The SLB fille the gaps on the PDA-coated PSf membrane successfully. Adapted with permission from [206]. Copyright 2015 The Royal Society of Chemistry. Currently, the water channel protein aquaporin is used in fabri- cating FO membrane to facilitate water transport through the mem- brane [80]. A wide range of research has been carried out using dif- ferent procedures to form aquaporin incorporated membranes [80,90,203-205]. Scientists are interested in aquaporin due to its ef- fective performance. This was attained with the preferential water Additionally, a novel strategy was developed to form stable aqua- porin based biomimetic membranes with excellent FO performance [206]. This strategy is described in Fig. 10, it involved applying PDA coating on the PSf substrate and the deposition of proteoliposomes and
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