Regulating Ionic Composition to Tune Monovalent–Divalent Ion Selectivity in Nanofiltration Membranes

High Resolution Image Download MS PowerPoint Slide Monovalent–divalent ion selectivity in nanofiltration (NF) is central to many separation processes, such as water softening and lithium recovery, yet the role of feed ionic composition in regulating selectivity remains insufficiently understood. Here, we systematically show how ion concentration and composition can modulate monovalent–divalent ion selectivity in a polyamide NF membrane by coupling zeta potential measurements, single- and mixed-salt filtration, concentration polarization analysis, and temperature-dependent permeation experiments. Increasing salinity enhanced charge screening, reducing the rejection of monovalent salts (e.g., NaCl and LiCl) while increasing the rejection of divalent salts (e.g., MgCl 2 and CaCl 2 ). In mixed-salt systems, high flux of chloride associated with strongly rejected divalent cations promoted electromigration effects; that is, cotransport of monovalent cations to maintain electroneutrality, dramatically lowering monovalent salt rejection and, under highly screened conditions, inducing negative LiCl rejection. Transition-state theory analysis revealed that electromigration reduces the effective free-energy barrier for Na + transport primarily through favorable entropic contributions. Case studies simulating groundwater softening and filtration of lithium-rich brines demonstrate that tuning ionic composition can strategically enhance monovalent–divalent selectivity without altering membrane material properties. These findings establish charge screening and electroneutrality-driven electromigration as complementary and controllable mechanisms for optimizing NF separations under realistic multicomponent conditions.