Interactions of radionuclides and CO₂ with clays: elucidating mechanisms at nano-scale level

In order to predict and regulate the environmental impact of human activities such as uranium mining and radioactive waste disposal, it is necessary to understand the behavior of actinides in the environment because their interaction with clay mineral is an important factor to control the migration of radionuclide in the environment. The behavior of actinides in the soil is mainly the surface adsorption interactions, which change the forms of radioactive elements and reduces the mobility of actinides in the natural systems. Therefore, it is important to search how the actinides interact with clay mineral such as the fundamental process of surface precipitation. Uranium is the predominant heavy metal content of the final waste in the nuclear fuel cycle (≥95% UO₂). In addition, uranium is a major contaminant in the soil, subsurface and groundwater as a result of human activity. Under standard environmental conditions, the most stable chemical form of U(VI) is the uranyl ion UO₂²⁺, which is potentially very mobile and readily complexes with organic and inorganic matter. On the other hand, carbon dioxide is an important greenhouse gas, warming the earth's surface to a higher temperature by reducing outward radiation. However, problems may occur when the atmospheric concentration of greenhouse gases increases. Amounts of carbon dioxide were produced since the industrial revolution, which is behind the significant global warming and rising sea level. Clay minerals are of great practical importance here, in storage of carbon dioxide due to its hydraulic permeability and ability to retain mobile species. We have chosen kaolinite and montmorillonite as prototypes of clay minerals of 1:1 and 2:1. Classical Monte Carlo (MC) and molecular dynamics (MD) methods have been used in this work in order to understand the adsorption behaviour of radionuclide and carbon dioxide in clays surface. In this thesis, we will investigate first the adsorption of uranyl on kaolinite surface by means of Monte Carlo and Molecular Dynamics simulation methods. Several adsorption sites have been modeled by considering surface defects in order to have inner or outer-sphere complexes. Then, the adsorptions of uranyl species onto Montmorillonite surfaces in the presence of different counterions will be performed. Interaction energy between Montmorillonite sheets and work of adhesion between the radionuclide and MMT surface will be discussed as well. Finally, we will study the adsorption behavior of carbon dioxide in MMT, and investigate at the same time thermodynamics, structural and dynamical properties. (author)