Improving water flux and salt rejection by a tradeoff between hydrophilicity and hydrophobicity of sublayer in TFC FO membrane

Abstract

This paper presents a novel approach using sulfonated polyethersulfone/zeolitic imidazolate framework (SPES/ZIF-8) porous nanocomposite substrate to enhance the salt rejection and water flux of forward osmosis (FO) membrane. The effect of SPES/ZIF-8 substrate on FO membrane properties containing morphologies, hydrophilicity, water permeability and salt rejection were investigated and completely discussed. Based on the obtained results, the selectivity and permeability of thin-film nanocomposite (TFN) FO membranes can be improved simultaneously through adjusting the both hydrophilic and hydrophobic characteristics. But these two results, is achieved in two different processes and with reverse impacts. After the sulfonation process and using the more hydrophilic polymer (SPES) instead of the neat PES, the porosity and consequently the water flux was increased. Because of more hydrophilicity of SPES which reduces the cross-linking degree of the corresponding polyamide (PA) layer, the salt rejection was reduced. In contrast, after the incorporation of ZIF-8 into the SPES substrate, the salt rejection was increased due to the hydrophobic nature of these nanoparticles (NPs). In terms of both salt rejection and water flux, the sulfonated TFN membrane with 0.5% ZIF-8 NPs (TFN-S0.5 membrane) showed the improved results comparing with the bare Thin-film composite (TFC) membrane. Furthermore, the best result for the ratio of reverse salt flux to water flux (Js/Jw) as the selectivity parameter, was about 0.16 g/L using 10 mM and 2 M NaCl solutions as the feed solution (FS) and the draw solution (DS), respectively, in the FO experiment. Also, the structural parameter (S) which is believed to have a strong relationship with the internal concentration polarization (ICP), was decreased to 0.39 nm as the best result, after modification of FO membrane.