Bridging mechanism and fabrication: New horizons in nanofiltration membranes for ion separation

The growing demand for water sustainability, resource recovery, and emerging energy technologies has intensified the need for efficient ion separation processes. Nanofiltration (NF) membranes, featuring sub-2 nm pores and intrinsic surface charges, enable selective ion transport governed by multiple mechanisms, including size exclusion, Donnan exclusion, and dielectric effects. However, the complex coupling among these mechanisms, together with interfacial interactions such as ion dehydration and specific adsorption, leads to highly nontrivial separation behaviors that are not yet systematically understood. This review presents a unified framework linking separation mechanisms, membrane structure design, and performance regulation. First, the fundamental roles and coupling effects of size exclusion, Donnan effect, and dielectric exclusion are analyzed to elucidate their intrinsic contributions to ion selectivity. Second, mechanism-driven membrane design strategies are restructured, focusing on the regulation of pore size and distribution, charge density and spatial heterogeneity, and synergistic multi-mechanism effects. Third, emerging approaches based on interfacial interactions, including selective ion dehydration and adsorption, are integrated with biomimetic ion channel design to provide additional dimensions for tuning ion transport. The incorporation of biomimetic channels with angstrom-scale confinement and specific coordination environments demonstrates promising pathways toward achieving high selectivity and permeability. Finally, key challenges and opportunities are discussed, including the need for multiscale modeling, advanced characterization of membrane microstructure, scalable fabrication, and performance stability in complex systems. This review provides a comprehensive perspective for the rational design of next-generation NF membranes for high-selectivity ion separation.