Epoxy-Based Ion-Exchange Materials for Water Treatment and Resource Recovery: A Critical Review of Structure–Performance Relationships and Practical Limitations

Epoxy- and glycidyl methacrylate (GMA)-based polymers offer versatile platforms for ion exchange, contaminant removal, and resource recovery. However, their reported adsorption capacities do not always reflect the number of functional groups that remain accessible and effective under practical treatment conditions. This review critically evaluates the relationships among polymer architecture, functionalization chemistry, binding mechanisms, mass transfer, and operational performance. Particular attention is given to the distinction between theoretical, total, accessible, and operational capacities. The literature indicates that increasing functional-group density does not necessarily improve adsorption performance because crosslinking, limited swelling, pore accessibility, and steric hindrance can restrict ion transport and site utilization. Mechanistic interpretations also remain uncertain in many studies, as ion exchange, coordination, and chemisorption are frequently inferred from equilibrium or kinetic model fitting without sufficient counter-ion balance or direct spectroscopic evidence. Furthermore, adsorption performance is predominantly evaluated in single-solute laboratory systems, while systematic comparisons under competitive-ion conditions, authentic water matrices, and continuous-flow operation remain limited. Regeneration studies often demonstrate short-term capacity retention but provide insufficient information on ligand leaching, chemical consumption, recovered-product purity, and long-term durability. Direct benchmarking against commercial ion-exchange resins is also uncommon. This review therefore identifies accessible binding-site density, experimentally verified mechanisms, dynamic separation performance, and regeneration stability as essential criteria for evaluating epoxy-based ion-exchange materials. Future research should prioritize standardized testing, realistic feedwaters, extended regeneration cycles, and process-level comparisons to establish their practical feasibility and sustainability.