Molecular dynamics simulations of exchange behavior of radionuclides into montmorillonite: Unraveling the dynamic processes and microscopic structures

The reliability of long-term safety assessments of radioactive waste repositories can be improved by a molecular-level knowledge of the geochemical processes onto clay minerals, such as montmorillonite. However, a mechanistic understanding of the dynamic exchange processes of radionuclides into the interlayer region of montmorillonite is still unclear. Using molecular modeling technique, we elucidate the dynamic exchange characteristics, microscopic structures, aqueous species distributions, and diffusion properties of the exchange of radioactive nuclides. The exchange ratios are in the order of Cs ⁺ ≈ Rb ⁺ ≈ UO ₂ ²⁺ > > Ba ²⁺ ≈ Sr ²⁺ > > Eu ³⁺, and Cl ⁻ can enter the interlayer space by associating with nuclides. Formation of aqueous CO ₃ ²⁻-UO ₂ ²⁺ clusters largely reduces exchange amount. Unlike CO ₃ ²⁻, other anions (Cl ⁻, I ⁻, SO ₄ ²⁻, and TcO ₄ ⁻) show relatively weaker binding ability with UO ₂ ²⁺. We find that multivalent cations prefer to stay in the mid-plane of interlayer region and Cs ⁺ and Rb ⁺ are located closer to the basal plane via inner-sphere complexation. Furthermore, the orientations of interlayer UO ₂ ²⁺ are nearly perpendicular to basal surface. The effects of temperature, montmorillonite type, interlayer water content, and concentration of nuclide on the exchange processes are further clarified. This study provides fundamental data for a profound understanding of environmental behavior of radionuclides.