Correlation between Na–Cs Ion Exchange Properties in the Alkaline Form and Acid Strength in the Proton Form of Zeolite

The ion exchange function of zeolite is useful for such a purpose as the removal of radioactive Cs species from water. In the very early days of zeolite science, the affinity of zeolites for metal cations was explained based on geometry. After the explanation presented above was proposed, many new zeolites and related materials were discovered or synthesized. Furthermore, it has become clear that the chemical nature of the ion exchange sites is strongly dependent on the framework topology. In this study, the ion exchange behavior between Na-form zeolites with different framework topologies and Cs-containing aqueous solutions was analyzed, and the equilibrium constant was calculated based on the Langmuir type equation to investigate the influence of the chemical nature of the zeolite framework on ion selectivity. The equilibrium constants at room temperature were in the order <b>FAU</b> < <b>LTA</b> < <b>MFI</b> < <b>YFI</b> < <b>MOR</b>. This order is the same as the order of Bro̷nsted acid strengths in the corresponding proton form zeolites. It is suggested that the delocalization of charge around the ion exchange site stabilizes a large cation whose charge is delocalized. In contrast, the equilibrium constant was not related to the pore and cavity size. This opens new insight into the influence of the chemical nature of zeolite on the affinity to cation.