Enhanced cesium retention through zeolitization of bentonite under elevated temperature and potassium-rich alkaline environment

This study investigated the hydrothermal transformation of montmorillonite in bentonite into zeolite under alkaline conditions using KOH solutions (0.02–1 M) at 150 °C, simulating the alkaline and thermal environments of high-level radioactive waste geological repositories. The Cs retention increased with increasing KOH concentrations. In the samples reacted with KOH solutions, the Cs retention capacity was approximately 1.3–1.8 times higher than that in the raw bentonite sample. At KOH solutions, the montmorillonite in bentonite underwent substantial structural and chemical alterations, forming K-zeolite (merlinoite) with a stable aluminosilicate framework and enhanced cation exchange capacity (CEC). The eight-membered ring (8-MR) channels (3.3–4.4 and 5.1 Å) in merlinoite provide an ideal environment for Cs retention due to ionic size compatibility and the low hydration energy of Cs⁺. Thermal and chemical analyses further confirmed the improved thermal and structural stability of merlinoite under high pH conditions. The results indicate that merlinoite formation in bentonite effectively immobilizes Cs⁺ ions under high temperatures and K-rich alkaline conditions, relevant to radioactive waste disposal.