Positively charged nanofiltration membrane with superior chemical stability for separation of dyes from salts

Membranes constructed via C-C bond formation exhibit superior chemical resistance compared to conventional polyamide (PA) membranes of which the performance often deteriorates under extreme pH conditions, making them promising candidates for dye wastewater treatment. In this study, we developed a novel positively charged nanofiltration (NF) membrane, denoted as PCNM, through redox-initiated interfacial free-radical polymerization. Quaternary ammonium groups were introduced to impart a stable positive charge, significantly enhancing dye separation performance. Membrane morphology and surface roughness were characterized using SEM and AFM. The PCNM membrane demonstrated exceptional rejection (>99%) for all tested cationic dyes, including Crystal violet (CV), Brilliant green (BG), Methylene blue (MEB), and Safranine O (SO) with an outstanding separation factor of 736.6 (CV over Na − SO 4 ). Notably, it retained 99% rejection against CV after 48-hour exposure to 2000 ppm NaClO (96,000 ppm·h), highlighting its outstanding chlorine resistance. Furthermore, the membrane exhibited remarkable pH stability, maintaining 99% rejection against CV after 24-hour immersion in 1 mol/L H 2 SO 4 and 1 mol/L NaOH, respectively. Consequently, this work presents a new strategy for fabricating high-performance positively charged NF membranes via interfacial free-radical polymerization, with significant potential for industrial dye wastewater treatment.