Effect of organic compounds on the retention of radionuclides in clay rocks: Mechanisms and specificities of Eu(III), Th(IV), and U(VI)

Abstract The use of geological barriers is widely studied in the context of hazardous waste management and pollution events. In the near field of pollution sources, geological barriers endure disturbances with the concomitant release of hazardous chemicals and organic or saline plumes. In this study, we investigate the effect of organic compounds on the retention of radionuclides in contact with a sedimentary rock, namely Callovo-Oxfordian clay rock (East of Paris Basin). The retention of Eu(III), Th(IV), U(VI), was quantified at near-neutral pH, in presence of model organic compounds released from wastes and engineered barriers, e.g. ortho-phthalate, α-isosaccharinate (α-ISA), succinate, as well as high complexing compounds (oxalate, citrate, EDTA). The studied concentrations of organic compounds, eventually higher than disposal conditions, aimed at quantifying concentration ranges above which the retention of radionuclides decreases, and the underlying mechanisms occurring. A simplified surface complexation model, assuming illite as the main sorbing phase, was used for predictive modelling. The comparison with experimental results allowed the evaluation of the robustness of the surface complexation constants and of the thermodynamic database “ThermoChimie” of the Andra-RWM-Ondraf/Niras consortium, to account for speciation in solution (ThermoChimie V10a, https://www.thermochimie-tdb.com). Specific mechanisms are discussed, such as complexation of Eu(III) by α-ISA, or retention of Th(IV) / citrate complexes. U(VI) displayed a unique behaviour because of uranyl complexation by natural species under geological conditions. The low solid to liquid distribution ratio, Rd(U(VI)/COx) ∼ 7.5 ± 1.6 L kg-1 decreases in presence of organic compounds, but without direct complexation by organic molecules. Indeed, the organic plume induces a disturbance of chemical equilibria in pore water of the clay rock, leading to additional complexation of uranyl by calcium and carbonates. Such effect highlights the crucial role of clay surfaces and carbonate minerals. More generally, these results provide insights on the geochemical buffers and key parameters involved during near-field disturbance of environmental media.