Microstructure, adsorption site energetics, and formation enthalpy control for FAU-Zeolite Cs+ exchange

Cs-137 is a radionuclide fission product that poses a significant risk to life, making it crucial to develop effective methods for its separation and sequestration from nuclear waste streams. Zeolitic structures have emerged as promising materials. This work examines the influence of structure, exchange site energetics, and formation enthalpies of nascent and cation-exchanged Faujasite-X, -Y, and -HY zeolites in terms of their Cs-exchange selectivity. Their interplay was quantified with the application of high-temperature calorimetry, adsorption isotherms, X-ray diffraction and density functional theory (DFT) calculations. Greater efficacy of Cs+ exchange was demonstrated for the Na+-substituted Fau-Y (NaY) zeolite than that of the Fau-X (NaX) and Fau-HY (Na-HY) zeolites. This is explained by a higher amount of Na+ in un-exchangeable sites in the case of NaX and a lower stability in NaY that favors the ionic exchange with Cs+. Moreover, Cs+ incorporation in the structure increases the stability of each kind of zeolite. Correspondingly, structure and DFT analyses demonstrated site-exchange thermodynamic favorability as well as the contribution from cage cell, which resulted in an energy landscape far more conducive to Cs+ incorporation for NaY than either NaX or Na-HY.