Confirming the potentials of an Italian, chabasitic tuff in the management of radioactive Cs-bearing waste streams: a physical-chemistry investigation

Caesium plays a critical role in radioactive waste management due to the high mobility of Cs⁺ ions: Cs forms volatile compounds and is easily leached from traditional encapsulation matrices, while the decay of Cs-137 generates heat, further challenging its confinement (Mukiza et al., 2024). Conventional coagulation and filtration are ineffective for Cs removal from effluents, while zeolites have proven highly effective, alongside materials like Prussian blue and metal-organic frameworks. Similarly, immobilization methods using cementitious alkalinated and geopolymeric binders, combined with zeolitic fillers, have shown promising results through chemical binding and structural incorporation of Cs (Misaelides et al., 2018). In this work, a physico-chemical study on the interactions of Cs+ ions with an Italian, chabasitic tuff was carried out. Owing to its natural origin, ready availability, low costs, and minor environmental footprint, this tuff has been the subject of previous investigations in the field of radioactive waste management, which had demonstrated its compatibility with alkali-activated and geopolymeric binders (Santi et al., 2022), and confirmed its positive impact on the lixiviation behaviour of Cs-bearing waste forms (Galluccio et al., 2024). For the purposes of this work, the tuff was preliminarily characterized by X-ray powder diffraction, scanning electron microscopy and elemental chemical analysis, confirming a 65 wt.% chabazite content. Kinetics tests showed that the tuff rapidly interacts with the simulated waste: under the tested conditions, 80% of the Cs+ ions is removed after only 4 h of contact, while thermodynamic equilibrium is reached at 100 h. The sorption kinetics of Cs+ ions follows a fourth-order kinetics, consistently with the presence of multiple competitive interaction mechanisms attributable to the tuff mineralogy. In agreement with the literature on nearly-pure chabazite, the sorption isotherm curves confirm the selectivity of the tuff toward Cs+ ions and follow the Langmuir adsorption equation. Modelling the sorption as ion exchange reactions, it was possible to calculate the values of the equilibrium constant values, from which it was possible to derive the values of ΔH, ΔS and ΔG via the van’t Hoff and Gibbs-Helmholtz equations. For temperatures between 10°C and 90°C, typical of many Cs-bearing industrial effluents, ΔG values are negative, indicating spontaneous fixation of the Cs+ ions onto the tuff. Eventually, the selectivity of the tuff for the removal of Cs was confirmed in competitive conditions with a more representative simulant solution containing activation and fission products commonly found in low- and intermediate-level waste streams. Thanks to the results of this work and the previous proof of the compatibility of the tuff with the aluminosilicate binders, the potentials of the application of the chabasitic tuff to more straightforward management of Cs-bearing waste streams are confirmed.