Charge-Sign-Independent Separation of Mono- and Divalent Ions With Nanofiltration Membranes

Achieving precise selective separation of monovalent and divalent cations, as well as anions, is vital yet challenging in practical applications involving complex component treatments. Current membranes are typically effective for separating either cation pairs or anion pairs. To address this issue, a straightforward strategy for fabricating a nanofiltration (NF) membrane is developed that selectively permeates monovalent ions. This study focused on neutralizing the surface charge and tuning the pore size distribution of the polyamide membranes through a secondary interfacial polymerization using a zwitterionic copolymer consisting of 2-methacryloyloxyethyl phosphorylcholine and 2-aminoethyl methacrylate hydrochloride. The optimized NF membrane prepared in this study, with a near-neutrally charged membrane surface and appropriate pore size distribution, demonstrates favorable performance in precisely separating monovalent and divalent ions, irrespective of the ion charge sign. The optimum NF membrane features high selectivity for both Cl⁻/SO₄²⁻ (93) and Li⁺/Mg²⁺ (67) ion pairs, along with high water permeance of 8.5 L m⁻² h⁻¹ bar⁻¹, making it competitive with many reported membranes. This study offers new insights into the ion-selective mechanisms of polyamide membranes for monovalent/divalent ions and may guide the development of advanced membranes with single-solute selectivity.