Molecular simulation of the diffusion of uranyl carbonate species in aqueous solution

Potential-based molecular dynamics simulations of aqueous uranyl carbonate species (MxUO2(CO3)(y)(2+2x-2y) with M = Mg, Ca, or Sr) were carried out to gain molecular-level insight into the hydration properties of these species. The simulation results were used to estimate the self-diffusion coefficients of these uranyl carbonate species, which often dominate uranyl speciation in groundwater systems. The diffusion coefficients obtained for the monoatomic alkaline-earth cations and polyatomic ions (uranyl, carbonate, and uranyl tri-carbonate) were compared with those calculated from the Stokes-Einstein (SE) equation and its variant formulation by Impey et al. (1983). Our results show that the equation of Impey et al. (1983), originally formulated for monovalent monoatomic ions, can be extended to divalent monoatomic ions, with some success in reproducing the absolute values and the overall trend determined from the molecular dynamics simulations, but not to polyatomic ions, for which the hydration shell is not spherically symmetrical. Despite the quantitative failure of both SE formulations, a plot of the diffusion coefficients of the uranyl carbonate complexes as a function of the inverse of the equivalent spherical radius showed that a general linear dependence is observed for these complexes as expected from the SE equation. The nature of the alkaline-earth cation in the uranyl carbonate complexes was not found to have a significant effect on the ion's diffusion coefficient, which suggests that the use of a single diffusion coefficient for different alkaline-earth uranyl carbonate complexes in microscopic diffusion models is appropriate. The potential model reproduced well published quantum mechanical and experimental data of CaxUO2(CO3)(3)(2x-4) and of the individual constituent ions, and therefore is expected to offer reliable predictions of the structure of magnesium and strontium uranyl carbonate aqueous species, for which there is no structural data available to date. In addition, the interatomic distances reported for CaxUO2(CO3)(3)(2x-4) could help with the refinement of the interpretation of EXAFS data of these species, which is made difficult by the similar uranium-distant carbonate oxygen and uranium-calcium distances. An analysis of the dynamics of water exchange around the alkaline-earth cations revealed that the presence of the uranyl tri-carbonate molecule has a strong influence on the geometry of the cation's first hydration shell, which, in turn, can considerably affect the water exchange kinetics depending on whether the imposed geometry matches that around the isolated alkaline-earth cation. This result shows that the alkaline-earth uranyl carbonate complexes have distinct water exchange dynamics, which may lead to different reactivities. Finally, significant changes in water residence time were also predicted when replacing carbonate for water ligands in the uranyl coordination shell. (C) 2010 Elsevier Ltd. All rights reserved.