Effects of thermal treatment on bentonite stability and europium(III) leachability

Abstract This study investigates the thermal effects on europium(III)-sorbed bentonite, a montmorillonite-rich clay, with focuses on the structural changes in montmorillonite and the leachability of Eu(III). Thermogravimetric analysis shows greater water-related weight loss in Eu(III)-sorbed bentonite compared to Na-bentonite, likely due to the expanded c -spacing of montmorillonite and the strong hydration shells of Eu(III). In surface complexation-dominant samples, the abrupt weight loss at ~ 500 to 600 °C suggests the formation of a glassy coating, resulting from the dehydration of surface-complexed Eu(III) and the dehydroxylation of surface aluminol groups. This transformation reduces the leachability of surface-complexed Eu(III), thereby enhancing its retention within the bentonite matrix. X-ray diffraction analysis reveals that the c -spacing of montmorillonite, depending on the type of interlayer cations, decreases with thermal treatment temperature. The reduced c -spacing limits the exchangeability of interlayer Eu(III) upon heat exposure. While Eu(III) sorption initially stabilizes bentonite, thermal treatment alters this effect. Regardless of the dominant sorption mechanism (cation exchange or surface complexation), the leachability of Eu(III) from bentonite decreases significantly at elevated temperatures. These findings underline the critical role of thermal history in bentonite’s performance as a radioactive waste barrier.