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

The ion exchange\nfunction of zeolite is useful for such\na purpose\nas the removal of radioactive Cs species from water. In the very early\ndays of zeolite science, the affinity of zeolites for metal cations\nwas explained based on geometry. After the explanation presented above\nwas proposed, many new zeolites and related materials were discovered\nor synthesized. Furthermore, it has become clear that the chemical\nnature of the ion exchange sites is strongly dependent on the framework\ntopology. In this study, the ion exchange behavior between Na-form\nzeolites with different framework topologies and Cs-containing aqueous\nsolutions was analyzed, and the equilibrium constant was calculated\nbased on the Langmuir type equation to investigate the influence of\nthe chemical nature of the zeolite framework on ion selectivity. The\nequilibrium 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\nof Bro̷nsted acid strengths in the corresponding proton form\nzeolites. It is suggested that the delocalization of charge around\nthe ion exchange site stabilizes a large cation whose charge is delocalized.\nIn contrast, the equilibrium constant was not related to the pore\nand cavity size. This opens new insight into the influence of the\nchemical nature of zeolite on the affinity to cation.