Interfacial alkalinity controls polyamide network evolution for broad-spectrum PFAS rejection

An alkaline interfacial environment alleviates the self-limiting effect of IP • Moderate NaOH suppresses amine protonation and promotes PA cross-linking. • The optimized membrane forms a dense and uniform polyamide selective layer. • The optimized membrane achieves excellent rejection of PFAS. • The selectivity between organic matter and mineral ions exceeds 200. Nanofiltration (NF) is an important membrane technology for industrial water treatment and advanced drinking water purification. Interfacial polymerization (IP) is widely used to fabricate polyamide (PA) NF membranes because of its tunability and scalability. However, the self-limiting effect in conventional IP, caused by the accumulation of acidic byproducts, constrains PA cross-linking and structural uniformity, limiting the rejection of emerging micropollutants such as per - and polyfluoroalkyl substances (PFAS). Here, the effects of aqueous NaOH concentration on interfacial reaction kinetics, PA microstructure, and membrane separation performance were systematically investigated. Moderate NaOH addition neutralized the HCl byproduct and suppressed piperazine protonation, promoting the formation of a dense and uniform PA network. By incorporating 1 wt% NaOH, the PA-OH-1 membrane achieved an optimized cross-linking density that facilitated 99.2% Na 2 SO 4 rejection and 99.4% natural organic matter removal, yielding an ion/organic selectivity coefficient above 200. Consistent with its dense structure, the membrane maintained high rejection (≥ 96%) for all nine PFAS species tested, with particular efficacy against short-chain perfluorobutanoic acid (PFBA) (99.7% rejection). In contrast, excessive NaOH induced alkaline hydrolysis, disrupted the PA network, altered surface morphology, and significantly deteriorated membrane performance. This work provides mechanistic insight into the structure-performance relationship of PA NF membranes under alkaline interfacial conditions and offers guidance for designing high-performance NF membranes for emerging contaminant removal. Download: Download high-res image (232KB) Download: Download full-size image