Highly efficient ion-exchange for Sr2+ capture in zeolites governed by framework Al density and charge-compensating cations

Eight representative zeolites (K-A, Na-A, Ca-A, Na-X, Na-Y, Na-mordenite, Na-ZSM-5, and Na-SSZ-13) with different framework structures and SiO 2 /Al 2 O 3 ratios were investigated to clarify their Sr 2+ ion-exchange mechanisms. The results show that the Sr 2+ ion-exchange capacity is primarily governed by the SiO 2 /Al 2 O 3 ratio rather than the framework type. A higher density of Al sites provides more ion-exchange sites, enabling divalent Sr 2+ to undergo 1:2 exchange with Na + and ensuring charge-compensation within the zeolite framework. In addition, comparative studies of LTA-type zeolites revealed that K-type and Na-type LTA exhibit comparable Sr 2+ ion-exchange capacities, whereas Ca-type LTA shows a significantly lower Sr 2+ ion-exchange capacity. This difference can be attributed to the hydration energies of cations, combined with their strong adsorption energy within the zeolite framework. These findings provide insights into the key factors governing Sr 2+ ion-exchange in zeolites and offer guidelines to design efficient adsorbents for radioactive Sr 2+ removal.