Mechanism of Eu(Ⅲ) adsorption and mineral alteration response of Tamusu claystones in Inner Mongolia

Elucidating the immobilization mechanisms of radionuclides within heterogeneous host rocks is significant for the long-term safety assessment of high-level radioactive waste (HLW) geological repositories. This work evaluates the immobilization of Eu(III)—as a surrogate for trivalent actinide ions—on heterogeneous Tamusu claystones, focusing on the coupling effects of mineralogy and temperature (25–80 °C). At 25 °C, Eu(III) adsorption is governed primarily by the analcime content. Under elevated temperatures (50 °C and 80 °C), however, a contrast in retention behavior emerges between lithologies. Analcime-rich samples maintain high sorption stability (with the mineralogical ratio of (dolomite + ankerite)/analcime serving as a proxy for retention performance), whereas carbonate-rich samples exhibit complex behavior driven by the thermally induced dissolution of dolomite and ankerite. Thermodynamic modeling indicates that temperature elevation facilitates carbonate dissolution and shifts the dominant aqueous species at pH ∼9.3 from EuOH(CO 3 ) 2 - to EuOHCO 3 (s), thereby strengthening the retention capacity of Tamusu claystone to Eu(III)/Am(III). X-ray diffractometer (XRD) and X-ray photoelectron spectroscopy (XPS) evidence reveal that Eu(III) retention is driven by a dual mechanism: ion exchange on analcime lattices and inner-sphere surface complexation on carbonate minerals. Therefore, dolomite/ankerite-rich samples may enhance Eu(III)/Am(III) immobilization through carbonate dissolution, and analcime-rich samples offer greater long-term predictability and stability. These trends are dependent on both temperature and pH changes. The findings highlight the synergistic role of mineral phases and radionuclide immobilization under near-field thermal conditions and provide essential thermodynamic data for radionuclide migration modeling and assessing the repository performance.