Metal Hexacyanoferrates as Selective Ion Exchangers: Mechanism, Performance and Applications

Metal hexacyanoferrates (Me-HCFs) constitute a distinctive class of inorganic ion-exchange materials, characterized by open-framework crystal structures, high chemical stability, and exceptional selectivity for monovalent cations. This review critically evaluates the current state of Me-HCFs as selective ion exchangers, with particular emphasis on their application in the adsorption of NH4+, Cs+, and Rb+ for both water decontamination and resource-recovery purposes. First, the crystallographic structure and ion-exchange mechanisms of Me-HCFs are examined, highlighting the roles of framework vacancies, transition-metal composition, and temperature in ion transport and selectivity. Strategies for material synthesis and immobilization are then discussed with respect to mechanical stability, accessibility of exchange sites, and scalability. The adsorption performance of Me-HCFs is reviewed in terms of kinetics, capacity, and selectivity under environmentally relevant conditions, including competitive ionic matrices. Regeneration strategies and long-term material stability are critically assessed, with particular attention to framework degradation and metal leaching. Finally, the potential of Me-HCF-based systems for circular-economy applications is explored, emphasizing their dual functionality in contaminant removal and selective recovery of valuable ions. This review aims to provide both a mechanistic framework and a practical perspective to guide the rational design and deployment of Me-HCF-based ion exchangers in environmental and resource-recovery applications.