Improving the Structural Parameter of the Membrane Sublayer for Enhanced Forward Osmosis

The structural (S) parameter of a medium is used to represent the mass transport resistance of an asymmetric membrane. In this study, we aimed to fabricate a membrane sublayer using a novel composition to improve the S parameter for enhanced forward osmosis (FO). Thin film composite (TFC) membranes using polyamide (PA) as an active layer and different polysulfone:polyethersulfone (PSf:PES) supports as sublayers were prepared via the phase inversion technique, followed by interfacial polymerization. The membrane made with a PSf:PES ratio of 2:3 was observed to have the lowest contact angle (CA) with the highest overall porosity. It also had the highest water permeability (A; 3.79 ± 1.06 L m^-2 h^-1 bar^-1) and salt permeability (B; 8.42 ± 2.34 g m^-2 h^-1), as well as a good NaCl rejection rate of 74%. An increase in porosity at elevated temperatures from 30 to 40 °C decreased S from 184 ± 4 to 159 ± 2 μm. At elevated temperatures, significant increases in the water flux from 13.81 to 42.86 L m^-2 h^-1 and reverse salt flux (RSF) from 12.74 to 460 g m^-2 h^-1 occur, reducing S from 152 ± 26 to 120 ± 14 μm. S is a temperature-dependent parameter, whereas S can only be reduced in a high-water- permeability membrane at elevated temperatures.