Mobility of selenium oxyanions in clay-rich media: A combined batch and diffusion experiments and synchrotron-based spectroscopic investigation

Abstract

Abstract Selenium-79 is considered as a critical radionuclide in safety assessment of high-level radioactive waste deep geological repository in clay-rich formations, owing to its long half-life and its anionic form in three relevant oxidation states (VI, IV,-II). An in-depth understanding of the diffusive transport properties of Se is therefore essential, especially due to its high sensitivity to oxido-reduction conditions. Here, the mobility of selenite and selenate in claystone samples (Callovo-Oxfordian, East of Paris Basin, France) under anoxic conditions is assessed by combining batch and diffusion experiments, and synchrotron-based spectroscopic mapping of diffusion fronts. Results showed that selenite and selenate behaved distinctly. Selenate diffused similarly to the reference anionic tracer, 36Cl−, without significant sorption or reduction. Selenite showed a sorption behaviour that increased with the decrease of the initial concentration from 10−3 to 10−6 mol L−1, values of distribution ratio (Rd) from batch experiments ranging from 10 to about 250 mL g−1. Selenite in-diffusion experiments showed that a pure diffusion model with reversible sorption was able to roughly reproduce the evolution of dissolved selenite in solution over time. However, additional reactions were required to properly describe the irregular shape of the selenite diffusion profiles in the solid. Mapping of Se oxidation state in diffusion fronts within the claystone coupons showed the presence of reduced Se (Se (0), Se(-I) and/or Se(-II)) along a fringe located at about 2 mm beneath the surface, possibly explaining the irregular shape of diffusion profiles. In the superficial zone (0 to 2 mm), oxidizing perturbations are assumed to account for the preservation of selenite under this oxidation state. These perturbations would also bias batch results in the same manner. Deeper in the claystone sample, diffusing selenite was observed, with a content steadily decreasing up to 7 mm from the inlet. The results suggest that, even after more than 500 days of diffusion, Se reduction would occur progressively along the diffusion pathways.