Ultra permeable positively-charged TMA/VBC-g-PSf loose nanofiltration membranes for effective salt/dye separation

The high-salinity wastewater generated by the textile industry presents a substantial challenge in the effective separation of dyes and inorganic salts, particularly concerning cationic dyes, which pose severe environmental pollution risks. Leveraging the combined effects of size exclusion and electrostatic repulsion, positively charged and loose nanofiltration (LNF) membranes have demonstrated the capability to efficiently reject cationic dyes while ensuring high permeance of inorganic salts. Herein, a new type of positively charged LNF membrane was prepared with trimethylamine (TMA) nucleophilic substitution reaction on the 4-Vinylbenzyl chlorostyrene (VBC)/polysulfone (PSf) chloride-grafted (TMA/VBC-g-PSf) membranes. VBC, a highly reactive small molecule featuring two functional groups (active vinyl (CH₂=CH–) and benzyl chloride (C-Cl)), was stably anchored onto the surface of PSf ultrafiltration (UF) membranes via vinyl groups. Furthermore, the C-Cl was nucleophilically replaced by the tertammonium group of TMA, forming a quaternary ammonium layer. Thereby, an ultra-thin positively charged selective layer with micro-convex structure was formed onto the PSf membrane surface, endowing the LNF membrane with ultra permeance and high Na₂SO₄/Victoria blue B (VB) selectivity. The optimized M-2 membrane, with 0.5 wt% VBC and 30 wt% TMA grafting concentration, yielded both high levels of pure water permeance (186.42 L·m^-2·h^-1·bar^-1) and an excellent Na₂SO₄/VB selectivity of 497.5 (high VB rejection of 99.8 % compared to low Na₂SO₄ rejection of 0.5 %). After 16 h of continuous operation, the separation efficiency of the M-2 membrane for both mixed solution and single-component solution remained consistent and stable, demonstrating its feasibility and high potential in the field of inorganic salts/cationic dyes separation. This study offers valuable insights into innovative methodologies for the production of LNF membranes tailored for dye wastewater treatment.