Counter Ion Dependent Adsorption and Diffusion of Aqueous Uranyl Ions onto Lizardite and Kaolinite Clays: 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 the diffusion behaviour is crucial for environmental 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-) on complexation, adsorption and diffusion 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 complex is formed, counter ions remain in that state throughout the simulation. Uranyl ions get adsorbed onto siloxane surfaces of clay primarily through outer-sphere complexation, mediated by water molecules. By calculating standard free energy change and distribution coefficients, it was observed that, for lizardite, extent of uranyl adsorption follows the order NO3- > CO32- > SO42-; whereas a different order as SO42- > NO3- > CO32- is followed in kaolinite system. Highest adsorption of uranyl sulfate in kaolinite may be linked to adsorption of uranyl ions on both siloxane and gibbsite surfaces. Calculated distribution coefficients (Kd) and diffusion coefficients are compared with those from experiments wherever possible. It was observed that the diffusion in both lizardite and kaolinite pores follows the same order: NO3- > SO42- > CO32-. Nature of diffusion in terms of sub-, normal or super-diffusion has also been analysed.