Molecular dynamics study of uranyl adsorption from aqueous solution to smectite

Abstract

Adsorption of aqueous uranyl species in pore spaces of clay minerals plays a key role in post-closure safety assessments for geological disposal of radioactive waste. Molecular Dynamics Simulations (MDS) were performed to study the adsorption of uranyl (UO22+) to the clay mineral smectite from its aqueous solution in the presence of carbonate (CO32−) and Na+ counter ions. The modelled system consisted of water-saturated clay layers and a 0.162 M aqueous uranyl carbonate solution. The large system size (up to 25,560 atoms) and long simulation times (over 200 ns) allowed investigating the effect of the electrical double layer on uranyl adsorption to smectite for different pore sizes (8.37, 25.1, and 33.5 nm). This study identified various polynuclear uranyl carbonate complexes on clay surfaces (e.g., [Na(UO2)3(CO3)3]+) and in aqueous solution (e.g., [Na2(UO2)5(CO3)6]0) which were not seen in other MDS studies due to statistical limitations caused by the small number of uranyl and carbonate ions in their super cell and/or insufficient simulation time. Uranyl complexes represented the majority of adsorbed species relative to uncomplexed uranyl, with different complexes for different pore sizes. The sorption parameter KD for uranyl ranged from 59 to 151 mL/g and was dependent on pore size (smaller KD for larger pore size). Electrostatic factors controlled the formation and locations of uranyl complexes. MDS-based KD values were in agreement with experimentally derived values for similar experimental conditions. MDS provides an efficient tool to derive sorption parameters for safety assessments, especially in the early stages of site selection and characterisation when access to cores from deep rocks may be limited.