Engineering Charged Nanofiltration Membranes towards Advanced Ion Separations

Charged polymeric membranes are broadly used for the selective separation of ions in various process industries. The ion separation capability of these membranes is greatly influenced by electrostatic repulsion/attraction forces arising from membrane charges. For excellent ion sieving, particularly in Donnan-dominated separation regimes, appropriately regulated membrane charge polarity and spatial distribution can enhance electrostatic discrimination toward target ions. However, ion selectivity is not governed by charge characteristics alone; pore size, ion hydration, and specific ion–membrane interactions jointly determine the overall sieving performance. In practice, complex material exchanges and chemical reactions during membrane fabrication can produce unsatisfactory membrane charge properties. Therefore, additional charge manipulation is necessary, while this procedure currently lacks systematic guidance from fundamental theories to material choices. This review aims to provide a foundation for manipulating the charge characteristics of polymeric membranes, which starts by introducing representative models for formation, characterization of membrane charges, and their influences on ion transport. Then, the key strategies for manipulating membrane charges are highlighted. To illustrate the economic and environmental values of charge manipulation, the ion sieving applications of charged membranes are also summarized. Finally, the current challenges and potential opportunities for developing charged membranes are identified.