Comparative study of Se oxyanions retention on three argillaceous rocks: Upper Toarcian (Tournemire, France), Black Shales (Tournemire, France) and Opalinus Clay (Mont Terri, Switzerland)

Ex and In Situ Reactivity and Sorption of Selenium in Opalinus Clay in the Presence of a Selenium Reducing Microbial Community

79Se is a critical radionuclide concerning the safety of deep geological disposal of certain radioactive wastes in clay-rich formations. To study the fate of selenium oxyanions in clayey rocks in the presence of a selenium reducing microbial community, in situ tests were performed in the Opalinus Clay at the Mont Terri Rock Laboratory (Switzerland). Furthermore, biotic and abiotic batch tests were performed to assess Se(VI) and Se(IV) reactivity in the presence of Opalinus Clay and/or stainless steel, in order to support the interpretation of the in situ tests. Geochemical modeling was applied to simulate Se(VI) reduction, Se(IV) sorption and solubility, and diffusion processes. This study shows that microbial activity is required to transform Se(VI) into more reduced and sorbing Se species in the Opalinus Clay, while in abiotic conditions, Se(VI) remains unreactive. On the other hand, Se(IV) can be reduced by microorganisms but can also sorb in the presence of clay without microorganisms. In situ microbial reduction of Se oxyanions can occur with electron donors provided by the clay itself. If microorganisms would be active in the clay surrounding a disposal facility, microbial reduction of leached Se could thus contribute to the overall retention of Se in clayey host rocks.

Sorption of selenite on Tamusu clay in simulated groundwater with high salinity under aerobic/anaerobic conditions

The sorption behavior of selenite onto the Tamusu clay from a preselected high-level radioactive waste disposal site in Inner Mongolia, China, was first investigated in simulated groundwater with high salinity by batch sorption experiments under aerobic/anaerobic conditions. The results demonstrated that the K_d values rapidly decreased and then remained steady in the pH range of 2.0-8.0. However, selenite sorption was promoted when pH exceeded 8.0, which might be attributed to the coprecipitation between Ca²⁺ and SeO₃^2-. Besides, the change trend of the K_d values as functions of various parameters was not affected by oxygen. The sorption kinetics and isotherms could be well fitted by the pseudo-second-order kinetic model and the Freundlich model for both aerobic and anaerobic conditions, and the calculated thermodynamic parameters (△G and △H) suggested that the selenite sorption process was a spontaneous and endothermic process. Additionally, the XPS results revealed that Se(IV) could be reduced to Se (0) only in anaerobic conditions and that the different amounts of Fe on the clay surface led to the discrepancy of the Se(IV) K_d values under aerobic and anaerobic conditions even in high-salt simulated groundwater. Overall, our findings in this study are significant in regards to the retardation of selenite on the host rock under high salinity conditions.

Diffusion of organic anions in clay-rich media: Retardation and effect of anion exclusion

The transport of emerging organic contaminants through the geosphere is often an environmental issue. The sorption of organic compounds slows their transport in soils and porous rocks and retardation is often assessed by extrapolation of batch experiments. However, transport experiments are preferable to strengthen migration data and modelling. In this context, we evaluated the adsorption of various organic acids by means of through-diffusion experiments in a sedimentary clay-rich rock (Callovo-Oxfordian, East of Paris Basin, France). A low diffusivity of organic anions was quantified with effective diffusion coefficients, D_e, ranged between 0.5 and 7 10^-12 m² s^-1. These values indicated an organic anion exclusion. As for chloride, the porosity accessible to organic anions was lower than that of water: ε_a(organic anions) < ε(water). The partial exclusion of organic anions from rock porosity was linked to both charge and size effects. A significant retardation was observed for organic anions such as oxalate, citrate or α-isosaccharinate. Yet, retardation measured by diffusion experiments was significantly lower than expected from batch experiments on crushed samples. An empirical correction factor is proposed to account for a possible decrease of retardation with accessible porosity of diffusing solute. This feature has significant implications for the estimation of migration parameters of organic compounds in the environment.