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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+, CO32-, 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 CO32- 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.