Dual-salt synergistic interfacial polymerization toward loose, negatively charged nanofiltration membranes for safe...

Nanofiltration membranes for drinking-water treatment ideally reconcile three intrinsically conflicting objectives: high water permeance, effective rejection of organic micropollutants, and selective permeation of beneficial mineral ions. Herein, a cost-effective dual-salt synergistic strategy is proposed, where sodium chloride (NaCl) and sodium bicarbonate (NaHCO₃) are introduced into the piperazine (PIP) aqueous phase to fabricate high-performance NF membranes via subsequent interfacial polymerization (IP) with trimesoyl chloride (TMC). Molecular dynamics (MD) simulations revealed that NaCl and NaHCO₃ reduced PIP diffusion through complementary kinetic pathways-NaCl via salting-out-induced elevation of aqueous-organic interfacial tension, and NaHCO₃ via N-H⋯O[double bond, length as m-dash]C hydrogen bonding between the secondary amine groups of PIP and the carbonyl oxygen of HCO₃ ^--collectively minimizing monomer supply at the reaction front and maximizing the fractional free volume (FFV) of the PA network. Consistently, experimental characterization confirmed that the fabricated membranes exhibited a progressively reduced crosslinking degree (minimum 38.10%) and an enhanced negative surface charge, accompanied by increased surface roughness and reduced PA layer thickness-structural and physicochemical features that synergistically govern both water transport and solute selectivity. Compared with the control membrane, the optimized membrane achieved a 147.7% higher pure-water permeance (23.09 L m^-2 h^-1 bar^-1) while maintaining a comparable Na₂SO₄ rejection of 98.27%. It also exhibited a markedly higher CaCl₂/Na₂SO₄ selectivity (S = 51.66) that enabled selective Ca²⁺/Mg²⁺ permeation for mineral retention and antiscaling capability, together with effective rejection of charged antibiotic micropollutants and robust operational stability. This work establishes a rational and scalable design strategy for high-performance NF membranes that reconcile micropollutant removal, mineral retention, and energy-efficient operation in drinking-water treatment.