Selective Ion Separation in Nanoporous Materials: Confinement Sieving, Chemical Recognition, and Dynamic Gating
Selective ion separation is critical for resource recovery, water treatment, lithium extraction from salt lakes, and nuclear waste management, yet the differences in size, solvation structure, and coordination behavior among ions are often minimal, and separation is further complicated by valence, interfacial charge, and competing ions. Nanoporous materials with tunable sub-nanometer channels and chemically active interfaces can regulate ion entry, solvation reorganization, interfacial partitioning, intrapore migration, and release. This review examines three mechanistic categories—size and solvation sieving, chemical recognition, and dynamic gating—from the perspective of confined transport and ion–pore interactions, and compares their roles and coupling in systems of monovalent–monovalent, divalent–divalent, heterovalent, and chemically similar multivalent ions. We further distinguish selective adsorption, membrane enrichment, and transmembrane transport, and discuss how selectivity definitions, ion flux, feed composition, driving force, and operating time affect performance evaluation. Current research faces three major challenges: lack of comparability of performance data across different test conditions, insufficient direct evidence of ion solvation, site occupancy, and migration under operating conditions, and the complexity of feed streams. By adopting a sequential ion transport process as a unified conceptual framework, this review systematically compares separation mechanisms across diverse nanoporous materials, including MOFs, COFs, zeolites, 2D materials, microporous polymer membranes, ion-exchange membranes, biomimetic nanochannels, organic–inorganic composites, functionalized porous carbons, and biochars. This transport-process-oriented framework provides a general and mechanistic perspective for understanding and comparing selective ion separation across diverse nanoporous platforms.