Effect of hydrothermal alteration of Ca-bentonite in K-rich solutions with different pHs on the physicochemical, swelling, and Cs adsorption properties

Understanding the stability of the bentonite buffer when exposed to cement leachates at temperatures exceeding 100°C is crucial for optimizing the design of deep geological repositories (DGRs) for high-level radioactive wastes (HLRWs) and minimizing the required site area. Experiments were conducted under hydrothermal conditions at 150°C for 30–150 days using deionized (DI) water (pH 6) and K-rich solutions (1 mol/L KCl, pH 6 and 1 mol/L KOH, pH 13) on Ca-bentonite. The 1 mol K concentration is comparable to 15 times the cation exchange capacity (CEC) of raw bentonite. Using various analytical techniques, the mineralogical, physicochemical, swelling, and Cs adsorption characteristics of raw and reacted bentonite samples were determined. Almost no changes in the properties of bentonite reacted with DI water were found for a given reaction time. For bentonite reacted with a 1 mol/L KCl solution, the cation exchange of Ca by K was a primary alteration process, resulting in a slight decrease in swelling capacity. However, almost no mineralogical changes were observed and consequently, there was minimal change in the Cs adsorption capacity. In contrast, for the bentonite reacted with a 1 mol/L KOH solution, the dominant alteration process was the transformation of minerals in bentonite into zeolite minerals, which resulted in significant changes in physicochemical properties, in particular, a decrease in the swelling capacity. On the other hand, the Cs adsorption capacity increased up to 2 times compared with the raw bentonite.