A combined DFT and molecular dynamics study of U(VI)/calcite interaction in aqueous solution

Here we present a combined DFT and molecular dynamics study of uranyl (U(VI)) interaction mechanisms with the calcite (104) surface in aqueous solution. The roles of three anion ligands (CO₃^2-, HCO₃^-, OH^-) and solvation effect in U(VI) interaction with calcite have been evaluated. According to our calculations, water adsorbed on the calcite (104) surface prefers to exist in molecular state rather than dissociative state. Energy analysis indicate that the positively charged uranyl species prefers to form surface complexes on the surface, while neutral uranyl species may bind with the surface via both surface complexing and ion exchange reactions of U(VI)→Ca(II). In contrast, the negatively charged uranyl species prefer to interact with the surface via ion exchange reactions of U(VI)→Ca(II), and the one with UO₂(CO₃)₂(H₂O)^2- as the reactant becomes the most favorable one in energy. We also found that uranyl adsorption increases the hydrophilicability of the (104) surface to different extents, where the UO₂(CO₃)₃Ca₂ species contributes to the largest degree of energy changes (-53kcal/mol). Our calculations proved that the (104) surface also has the ability to immobilize U(VI) via either surface complexing or ion exchange mechanisms under different pH values.