Nanofiltration membranes with ultra-high negative charge density for enhanced anion sieving and removal of organic micropollutants

Nanofiltration membranes with high charge density are highly attractive for selectively removing organic micropollutants and divalent anions from water environments. Here we constructed polyamide (PA) membranes with ultra-high negative charge density via a sea-squirt nanofibrillated cellulose restricted interfacial polymerization process. Sea-squirt nanofibrillated cellulose, which contains a high content of 7.0% carboxyl groups and 29.8% hydroxyl groups, effectively fettered piperazine and regulated the interfacial polymerization reaction kinetics. As a result, the optimized membrane had an ultra-high zeta potential of −148 mV at pH 7 and a charge density of −32.6 mC m⁻². This membrane achieved outstanding performance metrics, including a water permeance of 41.5 l m⁻² h⁻¹ bar⁻¹, exceptional SO₄²⁻/Cl⁻ selectivity of 144.5 and greatly increased water/organic micropollutant selectivity. Molecular dynamics simulations revealed a 73.1% reduction in the diffusion rate of piperazine due to competitive forces, leading to a PA surface enriched with -COOH groups. This work provides an effective strategy for tuning the PA membrane charge density to increase water purification and wastewater treatment efficiency.