Polyvinylidene fluoride nanofiber-supported polyvinyl alcohol thin film nanofibrous composite membrane for forward osmosis process

Abstract

Development of alternative water sources through seawater desalination by applying osmotic pressure-driven forward osmosis (FO) technologies has become one of the most reliable approaches to overcome the global water scarcity. In this study, as to achieve lower internal concentration polarization (ICP), nanofiber membrane with highly porous, thin and has low tortuosity was used as the substrate for FO membrane. Additionally, to minimize the penetration issue arising from conventional method of fabricating selective layer in which the casting solutions penetrated into the scaffold structure of nanofibrous substrates, this current work utilized the dual-layered nanofibrous mats and cross-link technique to fabricate polyvinyl alcohol (PVA) selective layer. PVA was chosen because it is highly hydrophilic and possesses good film-forming properties. The main objective of this research is to fabricate polyvinylidene fluoride (PVDF) nanofiber-supported PVA thin film nanofibrous composite (TFNC) membrane for FO process. Meanwhile, the sub-objectives are to investigate the effects of PVA loadings and to evaluate the performances of the resultant composite membranes in terms of intrinsic separation properties, FO testing and ICP comparison with asymmetric structure-supported composite membrane prepared via phase inversion technique. The experiment began with the fabrication of PVDF nanofibrous substrate via electrospinning. Subsequently, the selective layer of PVA nanofibers was prepared via electrospinning at different PVA loadings (2, 4, 6, 8, 9 and 10% w/v), prior to PVA cross-linking process. The characterizations of the substrate and PVA/PVDF composite membranes were done using scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, atomic force microscopy (AFM), Fourier transform infrared (FTIR) spectroscopy, contact angle measurement and mercury intrusion porosimetry (MIP). Finally, the resulting TFNC membranes were evaluated in terms of intrinsic separation properties and FO performances, followed by the ICP evaluation comparison between the chosen nanofiber-supported TFNC membranes and asymmetric structure-supported TFC membranes. It was found that the best loading of PVA was 8% w/v (denoted as PVA 8/PVDF) as it possessed the highest water permeability (1.33 L/m².h.bar) with 43.35% salt rejection and a ratio of 6.20 kPA between water and salt permeabilities. Also, PVA 8/PVDF composite membranes showed considerable performances in FO process with water flux of 4.83 LMH, reverse solute flux of 0.0242 gMH and significantly low specific reverse solute flux of 0.005 g/L. Besides, ICP effect in nanofiber-supported TFNC membrane was reduced by 30% as compared with asymmetric structure-supported TFC membrane. Therefore, this study may lead to a new strategy of developing dual-layered TFNC membrane via PVA cross-linking technique which could provide a good solution to solve the penetration issue, apart from mitigating the ICP bottleneck.