Fabrication Of High-performance Thin-film Polyamide Composite Membrane For Forward Osmosis Process

As a new membrane separation technology, forward osmosis technology has attracted growing attention in many potential applications, such as water purification, seawater desalination, power generation, and liquid food concentrates, due to the advantages of lower fouling tendency, easier fouling removal, lower energy-consumption and higher water-recovery. One of the major challenges to current FO technology is the lack of high performance membranes which should exhibit high water permeability, low reverse solute flux, and sufficient high mechanical strength. This paper developed three types of high-performance thin-film composite membranes for forward osmosis (TFC-FO) with polysulfone (PSf) substrate, polyacrylonitrile (PAN) nanofiber film and PAN/carbon nanotubes (MWNTs) hybrid nanofibers (HNF) film as support, respectively, and characterized their surface, structure properties and FO performance.TFC-FO membranes were synthesized by interfacial polymerization on top of the porous PSf supporting layer films, which were prepared via phase inversion with addition of polyvinyl pyrrolidone (PVP) and lithium chloride (LiC1). This part focused on the effects of additives and the concentration of polysulfone membrane on the structure and performance of the membrane. The results indicate that the addition of LiCl makes the finger-like pore more uniform, reduces the thickness of sponge-like pore layer, and also promotes the porosity of supporting layer, resulting in high water flux in forward osmosis process. PVP increases the hydrophilicity of TFC-FO membrane and improves the property of membrane-formation. TFC-FO membranes with addition of PVP exhibit high water flux and low reverse salt flux. With PSf concentration increasing, the porosity of supporting layer decreases, the sponge-like pore layer thickens and polyamide active layer becomes dense, which can lead to more serious internal concentration polarization and lower water flux in FO process. The TFC-FO membrane with 9% PSf concentration and addition of PVP and LiCl exhibits more uniform, porous and hydrophilic characteristics (porosity 68.0%, contact angle 48.5°) than two commercial FO membranes (porosity 32.6% and 25.4%, contact angle 76.5° and 73.5°). The fabricated membrane shows a high performance in FO process with low reverse salt-flux of 19.9 g/(m2-h) as well as high water-flux of 17.0 L/(m2-h), which is better than that of two commercial FO membranes (CTA-NW 9.5 L/(m2-h) and CTA-ES 14.4 L/(m2-h)) and some reported TFC-FO membranes at similar testing conditions.TFC-FO membranes with PAN nanofibers as a support was developed via interfacial polymerization on top of electrospun PAN nanofibers mat. The effects of the polymer concentration, flow rate of polymer solution and electrospinning-voltage on the structure properties and mechanical property of PAN-nanofiber film were investigated, and FO performance of developed TFC-FO membrane and conventional TFC-FO membrane was compared. The results show that with the increase of PAN concentration from 8% to 16%, the diameter of nanofiber goes up to 750 nm from 100 nm, the tensile strength reaches 36.99 MPa from 4.9 MPa, and porosity increases initially and then reduces. A good nanofiber film for FO membrane with high porosity of 91.33%, high tensile-strength of 27.84 MPa, uniform fiber of 550 nm diameter, can be prepared at the optimum conditions (14% PAN in DMF, eletrospinning voltage 15 kV, flow rate 0.5 ml/h, collection distance 8 cm, temperature 35 ℃ and collector-rotate speed 2000 r/min). The developed FO membrane with PAN-nanofiber film as support exhibits hydrophilic characteristics (contact angle 45.4°), and thus shows a high performance in FO process with high water-flux of 17.0 L/(m2-h) as well as low reverse salt-flux of 19.9 g/(m2-h), which is much better than that of present reported TFC-FO membranes with water flux of 10-50 L/(m2-h) and reverse salt flux of 10-125 g/(m2-h).A novel TFC membrane with PAN/carbon nanotubes (MWNTs) hybrid nanofibers (HNF) as a support was developed for promoting the mechanical properties. HNF film was prepared via electrospinning of uniform mix of oxidized MWNTs and PAN solution and the developed TFC membrane was fabricated by using interfacial polymerization of polyamide on top of HNF film. The surface, structure and mechanical properties of HNF film and the developed TFC membrane were investigated and also FO performance of the THC membrane was tested. The results indicate that the addition of MWNTs, the diameter of PAN-fiber increases by 35% and the tensile-strength improves by 106%. The developed TFC membrane, which is synthesized at the optimum conditions (12% MWNTs and 14% PAN in DMF), exhibits thin thickness of 81 μm, hydrophilic characteristics (contact angle 36.6°), and high tensile-strength of 27.84 MPa as well as high FO performance (water flux of 163.5 L/(m2-h) and reverse salt flux of 41.7 g/(m2·h)), indicating that HNF overcomes the trade-off of FO performance and mechanical properties.