Molecular design of ion-selective polymer membranes for electrochemical water purification and resource recovery

Ion-selective membranes enable electrochemical water purification and resource recovery. Effective molecular design must balance preferential target-ion uptake, rapid transport, and limited water crossover under changing feed and electrical conditions. This review examines how fixed charge, hydration, pathway connectivity, and ion-specific interactions govern transport in charged polymers, microporous materials, recognition membranes, selective surface layers, and covalent organic frameworks. Hydration reshapes transport pathways, making their hydrated dimensions and connectivity more informative than dry pore size alone. Strong binding can favor uptake while slowing release and passage, so selective uptake need not yield selective flux. Designs that partly decouple charge density from swelling or binding affinity from mobility offer ways to balance discrimination and throughput. Practical deployment requires reproducible selectivity, resistance, and water-transport properties across large-scale membrane production. Transport-state retention then requires maintaining these properties as polymer chemistry, hydration, and interfaces evolve during operation. Evaluating them together connects molecular design, manufacturing quality, and durability to current efficiency, product purity, concentration, recovery, and energy consumption in complex feeds.