PNNL

Abstract

Ab initio molecular dynamics simulations at 300 K based on density functional theory have been used to study the hydration shell geometries, solvent dipole, and first hydrolysis of the Uranium(IV) (U4+) and Uranyl(V) (UO2+) ions in aqueous solution. The solvent dipole and first of hydrolysis of aqueous Uranium(VI) (UO22+) has also been probed. The first shell of U4+ is coordinated by 8-9 water ligands with an average U-O distance of 2.42 Å. The average first shell coordination number and distance are in agreement with experimental estimates of 8-11 and 2.40-2.44 Å respectively. The simulated EXAFS spectra of U4+ matched well with recent experimental data. The first shell of UO2+ is coordinated by 5 water ligands in the equatorial plane, with the average U=Oax and U-O distances being 1.85 Å and 2.54 Å respectively. Overall, the hydration shell structure of UO2+ matches closely with that of UO22+ except for small expansions in the average U=Oax and U-O distances. Each ion strongly polarizes their respective first shell water ligands. The computed acidity constant (pKa) of U4+ and UO22+ are 0.93 and 4.95, in good agreement with the experimental values of 0.54 and 5.24 respectively. The predicted pKa of UO2+ is 8.5.