Counter ion dependent adsorption and diffusion of aqueous uranyl ions in slit pores of lizardite and kaolinite clays: a molecular dynamics simulation study

Abstract

Leaching and migration of radioactive actinide ions into groundwater pose severe environmental challenges. Therefore, understanding the interactions of actinyl ions with soil clay minerals and their diffusion behaviour is crucial for the ecological safety of groundwater, particularly near geological repositories for nuclear waste disposal. Extensive molecular dynamics simulations were used to probe the role of different counter ions, namely, nitrate (NO₃⁻), carbonate (CO₃²⁻) and sulfate (SO₄²⁻), in the complexation, adsorption and diffusion of uranyl ions in the slit pores of two different 1 : 1 layer-type clay minerals viz., lizardite and kaolinite. Uranyl ions form various mono- and poly-nuclear complexes involving other uranyl ions, water and counter ions. For carbonate and sulfate ions, once a complex is formed, counter ions remain in that state throughout the simulation. Uranyl ions get adsorbed onto the siloxane surfaces of clay primarily through outer-sphere complexation, mediated by water molecules. The calculated distribution coefficients (K_d) and diffusion coefficients were compared with those from experiments wherever possible. It was observed that the diffusion of uranyl ions in both lizardite and kaolinite pores follows the same order: NO₃⁻ > SO₄²⁻ > CO₃²⁻. The nature of diffusion in terms of sub-, normal or super-diffusion was also analysed.