Reactive diffusion of selenium anions into clay-rocks: A sorption in batch and through-diffusion investigation

Selenium-79 is a critical radionuclide in the safety assessment of high-level radioactive waste in deep geological repositories in clay-rich formations, due to its long half-life and speciation. It exhibits three relevant oxidation states in anionic forms: selenate(+VI), selenite(+IV), and selenide(-II) whose mobility through clay-rocks needs to be evaluated. To this end, batch and diffusion experiments were performed in the French Callovo-oxfordian clay-rocks under anoxic conditions and at ultra-trace concentrations, using the ⁷⁵Se isotope. Our findings reveal that selenate is weakly retained and diffuses similarly to the inert anionic tracer ³⁶Cl^-. In contrast, selenite exhibits strong retention in diffusion experiments conducted with and without the addition of hydrazine. The steep ⁷⁵Se rock profile and the ⁷⁵Se(+IV) activity evolution in solution were modeled using high distribution ratio values (1000 -3300 L kg^-1). The oxidation of selenide, HSe^- species, into the precipitate Se(0)_(s) was strongly delayed by its polymerization into polyselenides (Se_n^2-), with no ⁷⁵Se(0)_(s) observed after 100 days of diffusion. Adding hydrazine to one of the diffusion cells slowed the polymerization rate of selenide, thus allowing more selenide to diffuse into the solid. Based on a simplified model, we proposed that the selenide diffusion would be similar to that of selenate and ³⁶Cl^-.