Geopolymer-Assisted Conversion of Cs-Exchanged 13X Zeolite into Stable Cs-Aluminosilicates for Long-Term Cesium Immobilization

A promising approach for the removal of Cs+ ions from aqueous solutions and their long-term immobilization was investigated, as the safe containment of radioactive cesium is essential due to its long half-life and environmental hazards. The approach involves ion exchange using zeolite, geopolymerization, and subsequent heat treatment of the geopolymer/zeolite composite. Cs+ ions were removed from water by ion exchange using Na-13X zeolite, in which Cs+ ions replaced Na+ ions within the zeolite framework. The resulting Cs-13X zeolite was subsequently combined with kaolinite to prepare geopolymer/Cs-13X zeolite composites containing 10–50 wt% Cs-13X zeolite. During geopolymerization, a portion of the Cs was released from Cs-13X zeolite and incorporated into the geopolymer matrix, while the remaining Cs remained immobilized within the zeolite structure. Heat treatment of the geopolymer/Cs-13X zeolite composites at 950 °C induced crystallization of two stable Cs-bearing phases. The Cs incorporated into the geopolymer matrix crystallized as the CAS phase, whereas the Cs retained in the Cs-13X zeolite crystallized as pollucite. The crystalline phase content in the heat-treated composite samples increased with Cs-13X zeolite loading, resulting in improved density and compressive strength. The highest values of crystallinity (72%), bulk density (2.42 g/cm3), and compressive strength (~53 MPa) were measured for samples containing 50 wt% Cs-13X zeolite. This level of compressive strength is sufficient to ensure safe handling and long-term containment of the immobilized Cs+ ions.