A molecular dynamics study of uranyl-carbonate complexes adsorbed on basal surfaces of clay minerals

Abstract

We use molecular dynamics simulation to study the mechanisms involved in the adsorption of aqueous uranyl species (\( {\text{UO}}_{2}^{2 + } \)) to the basal surfaces of clay minerals, including kaolinite, pyrophyllite and montmorillonite. Uranyl ion can form various complexes with carbonates, namely, [UO2(H2O)5]2+, [UO2(H2O)3(CO3)], [UO2(H2O)2(CO3)2]2−, [UO2(CO3)3]4−. The simulations show that at aqueous clay interfaces, both uranyl species and surface type control the adsorption pattern. The noncarbonato and monocarbonato uranyl species can form outer-sphere complexes on siloxane surfaces through electrostatic interaction, but the dicarbonato and tricarbonato uranyl complexes rarely adsorb on the siloxane surfaces. Strong outer-sphere adsorptions of the uranyl-carbonate complexes on gibbsite surfaces are observed, which are fixed by hydrogen bonds between the ligands (carbonate and/or H2O) and surface hydroxyls. The sorption behaviors derived in this study provide new insights into understanding the migration and enrichment of uranium and other radionuclides.