Reactive mixed-monomer engineering of polyamide networks enables selective surface-water purification

Achieving high water permeance, sulfate/NOM selectivity, and mineral retention simultaneously remains challenging for polyamide nanofiltration membranes. Here, we introduce a reactive mixed-monomer strategy using piperazine (PIP) and 1,4-bis(3-aminopropyl)piperazine (BAPP) to jointly regulate interfacial polymerization dynamics and polyamide-network chemistry. Molecular simulations reveal that BAPP suppresses PIP mobility and creates a more open network with ionizable tertiary amines, whereas higher PIP fractions enhance network packing and size-sieving. The optimized PIP-BAPP-80 membrane forms a thin, continuous, hydrophilic, negatively charged selective layer, achieving a water permeance of 15.7 L m −2 h −1 bar −1 , approximately 98% Na 2 SO 4 rejection, and a mixed-electrolyte Ca 2+ /SO 4 2− separation factor of 34.4 ± 0.4. For natural surface water, it removes 90.5% of dissolved organic carbon while limiting demineralization. Compared with NF270, it also shows lower flux loss and higher flux recovery under gypsum scaling and humic acid fouling. This work demonstrates mixed-monomer engineering as an effective route to couple reaction-front regulation with network design for selective surface-water purification.