Salt-regulated interfacial polymerization enables selective and antifouling poly(homopiperazine-amide) nanofiltration membranes for efficient surface water purification

Low-concentration sodium chloride (NaCl) was adopted as a kinetic regulator of homopiperazine/trimesoyl chloride interfacial polymerization to fabricate polyamide nanofiltration membranes for surface water treatment. Characterization showed that NaCl simultaneously slowed amine transport toward the aqueous-organic interface, promoted hydrolysis of residual acyl chloride groups to form carboxyl/carboxylate functionalities, and facilitated the retention of sodium-associated species within the polyamide network. These coupled effects yielded a smoother, more hydrophilic, and more negatively charged selective layer with a refined pore structure. The optimized membrane, NFM-Na2, achieved removals of 96.42% for sulfate, 87.53% for dissolved organic carbon, and 97.58% for ultraviolet absorbance at 254 nm from real surface water, while substantially reducing fluorescent dissolved organic matter. NFM-Na2 also exhibited improved resistance to combined humic acid and calcium fouling and retained a flux recovery ratio of 82.47% after five fouling-cleaning cycles, markedly exceeding that of the NaCl-free membrane. These findings demonstrate that low-concentration salt regulation couples monomer-transport control with selective-layer chemistry and hydration tuning, providing a simple and transferable approach for balancing ion selectivity, natural organic matter removal, and fouling resistance under environmentally relevant conditions.