Adsorption behaviors of Eu(III) on granite: batch, electron probe micro-analysis and modeling studies

Although granite has been widely considered as the host rock of the high-level radioactive waste (HLRW) repository in the world, the adsorption behaviors of radionuclides on granite are quite complicated and still unclear, especially at molecular scales. In this study, the adsorption behaviors of europium(III) on Beishan granite (BSG), a preliminary selection of host rock for the HLRW repository in China, were explored under environmental conditions combining batch, electron probe micro-analyzer (EPMA), and modeling approaches. X-ray diffraction (XRD) pattern confirmed that albite, quartz and biotite were the main mineralogical components for the BSG grains. Eu(III) species on BSG grains are predominant as the ion exchange (≡X₃Eu⁰) and the inner sphere complexes of ≡S^wOEu(OH)₂⁰, ≡S^sOEu(OH)₂,⁰ ≡S^sOEu²⁺, and ≡S^wOEu²⁺. EPMA showed that the distribution of Eu(III) on BSG grains was strongly correlated to the biotite, which suggested that biotite is the host phase for retarding trivalent actinides in the BSG grains. The presence of soil humic and fulvic acids could enhance Eu(III) adsorption on BSG grains under low-pH conditions, and inhibit the adsorption under higher pH range.