Diffusion and adsorption of uranyl ion in clays: Molecular dynamics study

Molecular dynamics (MD) simulations were performed to study the diffusion and adsorption behaviors of aqueous uranyl species in interlayer space of clay minerals, which are key processes in the safety assessment for geological disposal of radioactive waste. The diffusion behaviors of UO₂²⁺, K⁺, CO₃²⁻ and Cl⁻ and H₂O were evaluated for the aqueous solution system. The diffusion coefficients of these species decreased with increasing solutes concentration, and that of UO₂²⁺ was much smaller than others. In the aqueous solution with higher concentration of carbonate ions, uranyl carbonate complexes such as UO₂CO₃ and UO₂(CO₃)₂²⁻ could be observed. For the clay-aqueous solution system (clay: montmorillonite or illite), the adsorption and diffusion behaviors of UO₂²⁺ and K⁺ were evaluated by MD calculations. The distribution coefficients (K_D) of these solutes increased with the layer charge of clay, and the K_D value of UO₂²⁺ might be smaller than that of K⁺. Further, the diffusion coefficients in the adsorption layer on the basal plane were relatively small and decreased with increasing the layer charge of clay.