Effect of Salt on the Uranyl Binding with Carbonate and Calcium Ions in Aqueous Solutions

The Ca₂(UO₂)(CO₃)₃ complex has been shown to be the dominant species of uranyl in different aqueous environments, and thermodynamic data of the complexation have been measured accurately recently. However, a detailed understanding of the binding processes with explicit consideration of the water molecules in the presence of common salt ions such as Na⁺ and Cl^- has been lacking. Here we use classical molecular dynamics combined with umbrella sampling to map the complete binding processes and their free-energy profiles leading to formation of the Ca₂(UO₂)(CO₃)₃ complex from UO₂²⁺, CO₃^2-, and Ca²⁺ in an aqueous NaCl solution to simulate the seawater conditions. We find that the presence of Na⁺ ions affects the binding between UO₂²⁺ and CO₃^2- as well as between [(UO₂)(CO₃)₃]^4- and Ca²⁺ by changing the coordination mode of carbonate to UO₂²⁺. The free energies of binding from our simulations are in good agreement with the experimental data for both pure water and the NaCl solution. Our work shows that free-energy simulations based on classical molecular dynamics simulations can be a useful tool to examine the atomistic process of the ligand binding to form the Ca₂(UO₂)(CO₃)₃ complex under different aqueous environments and that the presence of common ions can impact the complexation chemistry of uranyl.