Hollow fiber membranes comprising of polyvinylamine/polydopamine active layers and a polyvinylidene fluoride substrate for pervaporative concentration of KAc solutions

Abstract

We report on a thin-film-composite hollow fiber membrane comprising a hydrophilic active layer and a hydrophobic substrate for pervaporative concentration of KAc solutions. A hydrophobic substrate was chosen in order to overcome the potential problems of liquid water crossover through the membrane that typically occurred with a hydrophilic substrate over a prolonged period as a result of capillary condensation and gradual pore wetting even after system shutdown. Attempts were made to co-deposit PVAm/dopamine onto a PVDF hollow fiber substrate to form thin-film-composite hollow fiber membranes. However, it was challenging to deposit hydrophilic macromolecules on a hydrophobic surface. To facilitate the deposition of dopamine/PVAm onto a hydrophobic substrate, where PVAm macromolecules were securely attached to the substrate with the assistance of adhesive dopamine, methanol was used as a co-solvent to improve surface wetting and pre-coating of dopamine was used to form a hydrophilic binding layer that was better prepared for subsequent dopamine/PVAm deposition. It was shown that the use of co-solvent water/methanol (90/10 in vol) in the depositing solution and precoating dopamine for 3 h prior to co-deposition of dopamine/PVAm were appropriate for membrane formation, and the resulting membrane showed stable performance in concentrating KAc solutions at a high feed concentration (70 wt%) over a period of 200 h at an operating temperature of 70℃. A stable water flux of 1.7–1.9 kg/m2h was achieved, with an essentially complete salt retention.