Polyamide thin-film composite membranes based on carboxylated polysulfone microporous support membranes for forward osmosis

• Carboxylated polysulfone membranes were prepared as promising support layers for FO membranes.
• CPSF membranes showed moderate hydrophilicity and stable membrane structures.
• PA-CPSF TFC membranes showed higher FO water flux than pristine PA-PSF TFC membranes.

Due to its simple process and low energy consumption, forward osmosis (FO) has gained significant attention in the fields of portable hydration bags, desalination, landfill leachate treatment, and brine concentration. However, current state-of-the-art reverse osmosis (RO) membranes show relatively low water fluxes in FO processes due to high internal concentration polarization (ICP) and high mass transfer resistance in commercially available microporous support membranes. In this study, carboxylated polysulfones (CPSFs) were synthesized via direct polysulfone (PSF) functionalization and considered as hydrophilic, mechanically stable microporous support membranes. The incorporation of hydrophilic groups into hydrophobic polymer backbones often reduces mechanical strength due to excessive water swelling. However, the mechanical properties of CPSFs (degree of substitution, DS=0.49–0.85) were similar to those of pristine PSF, and they retained their hydrophilic nature. Microporous CPSF membranes were prepared in various conditions, and FO water fluxes and salt passages of polyamide thin-film/CPSF composite membranes were measured and compared with each other. CPSF-based FO membranes showed significantly higher water fluxes (water flux in FO mode: 18 L/m2 h, salt passage: 2.2  g/m2 h under 1 M MgCl2 as a draw solution, active layer facing DI water) than PSF-based FO membranes (10.5 L/m2 h, 1.5  g/m2 h at the same conditions), which might be due to enhanced hydrophilicity and reduced ICP.