The growth of the global population and the rise in water consumption has increased the already great pressure on water and energy systems 1. Two alternatives for increasing water supplies involve desalination of sea or brackish water or the reclamation of wastewater 2. A promising emerging membrane technology for purification of water is forward osmosis process which has gained immense interest since the mid 1970s 3. Osmosis is a natural process which involves the transfer of water molecules throughout a porous membrane 4. In order to induce the flow of water from one to another side, an osmotic pressure differential is required. The FO system operates using a permeable membrane and two solutions with different concentrations, the feed solution (FS) and draw solution (DS). The osmotic pressure gradient between low concentration FS (low osmotic pressure) and high concentration DS (high osmotic pressure) is the driving force. Accordingly, the difference in osmotic pressure drives the water through a semi-permeable membrane to the higher osmotic pressure side while hindering the transport of ions 5, 6, 7, 8. In a typicalFO system, the selective layer is in contact with the feed solution and the support layer is in contact with draw solute. This, along with the fact that FO operates without the need of applied hydraulic pressure, causes low fouling tendency. It is highly efficient for various contaminants, has low energy consumption and potentially achieves high recovery 9.Therefore, FO can be of strategic importance in food and pharmaceutical manufacturing, treating landfill leachate, treatment of highly saline streams, seawater and industrial wastewater purification4, 5, 10, 11, 12.
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 water permeability and selectivity arising from a porous sub-layer and a thin active layer 15. The design of the TFC asymmetric membrane is shown in Fig.1. It has a thin support layer embedded with a mesh for mechanical strength. A thin active layer was formed on the top of the support layer 16. Subsequently,mixed matrix membranes (MMM) have been used for the FO process. They consist of the deposition of filler into the polymer matrix presenting a good surface area, water permeation and separation properties 17. This new domain was considered for two reasons: firstly, the hydrophobic nature of the most membrane polymers and secondly, a hydrophilic sub-layer can be affected by water and got plasticized while the selective layer was more rigid 18. A new attractive protein material is Aquaporin which has been incorporated into either a substrate or selective layer to form a flat sheet or hollow fibre membranes 13, 19.The FO membranes can be fabricated via phase inversion methods that can be further classified into thermally induced phase separation, precipitation by controlled evaporation, vapour phase precipitation, and non-solvent induced phase separation (NIPS) 20.
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 precipitation 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 beenemployed to fabricate flat sheet and hollow fibre membranes. It involves applying a high electrical field to a polymer suspension in a syringe. This will result in ejecting and depositing fine fibres on a collector 23, 24, 25, 26. The layer by layer technique (L-b-L) is used to synthesize FO membranes which involve exposing the prefabricated sub-layer to a polyelectrolyte with an opposite charge for a limited time. This creates a uniform and homogenous ultra-thin selective film on the surface 27, 28. By adding alternately charged polyelectrolytes, multi-layers can be formed. The selective layer was synthesized byinterfacial polymerization that is based on the interaction between m-phenylenediamine (MPD) and trimesoyl chloride (TMC) monomers to form a very thin film 29. This membrane had an asymmetric structure and high permeability and was used for FO processes. The major objective is to synthesize a FO membrane having excellent water permeability, the good rejection rate for salt and foulants, and stable water flux during long-term operations. Nevertheless, the practicality of the sub-layer design is restricted due to some serious problems. For example, the thickness of the porous sub-layer is of crucial importance on the water flux due to an increment of mass transfer resistance and the effects of internal concentration polarization (ICP). The ICP occurs due to the dilution of the concentrated draw solute across the sub-layer while the precipitation of the feed molecules is across the selective layer, causing a decrease in the water flux of up to >80% 9, 30. In order to alleviate the ICP, optimizing the sub-layer structure such as thickness, tortuosity, and porosity via the fabrication methods is essential. Up to date, extensive research efforts have been conducted to manufacture an appropriate support layer for the FO membrane through adjusting the phase inversion procedure. For example, a sub-layer of CTA membrane was modified via different conditions of phase inversion. The solvent and non-solvent were replaced by a mixture of dioxane and acetone and a mixture of lactate and methanol respectively. The concentration of the polymer and additives were also varied 31. Replacing the solvent and non-solvent and changing the concentration on both the polymer and additives resulted in a very open sub-layer. Thus, the water flux enhanced but low salt rejection is still a critical problem. Another attempt proposed modifying the polysulfone (PSf) sub-layer structure of a flat sheet TFC-FO membrane by phase inversion method. It was speculated that the thickness and chemistry of the sub-layer may influence the solute mass transfer resistance 11. Hence, when the PSf sub-layer was made of a dense sponge-like structure, the mass transport was hindered, leading to lower water permeation 32. The performance of FO membranes could be affected by ICP arising from the spongy and tortuous structure 33. In addition, there are several chemical procedures for polymeric membranes such as physical, surface chemical treatments and chemical modification of the base polymer 34, 35. The most common procedures studied for FO membranes are the chemical modification of the base polymer and the substrate. Here, the chemical treatment was accomplished by depositing chemical agents using different methods to improve the membrane performance 36. Chemical modification can be achieved during synthesis by embedding hydrophilic additives or nanomaterials into the polymer matrix. There are two important elements to develop ideal TFC-FO membrane, which involves good wetting and hydrophilicity of the sub-layer. Therefore, the former will facilitate the diffusion of the solution in wetted pores while the latter will help in reducing the ICP impact 37.Some other potential methods are the deposition of inorganic nanoparticles and metal oxide nanoparticles into the sub-layer of the FO membrane 38, 39. When nanosilica particles were incorporated into the polysulfone substrate, more pathways for water transfer were created in the porous sub-layer 40. This was attributed to an increase in the hydrophilicity aiding to enhanced water flux. However, high content of nanoparticles caused a defect in the active layer, arising from an aggregation of the nanoparticles. This possessed lower water flux and higher salt permeability. This can increase the ICP effects, resulting in reducing the water transport through the membrane. Other recent work describes improving the substrate hydrophilicity by Norepinephrine (pNE) coating on a double-structured sub-layer 41. A moderate enhancement in water flux was observable. However, it has some drawbacks; for example, it is expensive and should be prepared with the addition of oxidant under corrosive preparation conditions.