Predictions about the mobility and fate of actinides in the environment begin with knowledge of their speciation, which is controlled by factors such as pH, concentration, and the presence of complexing ligands. However, the speciation relevant to subsurface conditions often occur at concentrations that are too low for accurate spectroscopic characterization. Alternatively, atomic-scale simulations offer a way to carry out detailed studies under well-defined, model conditions. In this work, we demonstrate how first principles molecular dynamics (FPMD) can complement spectroscopy to reveal the speciation of aqueous actinides. Two cases are presented here. The first application involves the uranyl cation (UO22+), which commonly forms uranyl-hydroxy, uranyl-carbonato, and uranyl-calcium-carbonato complexes in natural waters. In silicate-rich waters, uranyl-silicate complexes can also be important but remain poorly characterized at low concentrations. Extended X-Ray Absorption Fine Structure (EXAFS) spectra were recorded for a series of aqueous solutions of 0.1 mM uranyl and 2 mM silicate with pH ranging from 4 to 7. Of particular note is the uranyl and silicate concentrations in our experiments, which are lower than conditions in previously-reported EXAFS studies and approach conditions in natural groundwater systems. To guide the interpretation of the EXAFS data, we carried out FPMD simulations of 3D-periodic models involving uranyl and silicate in water. These calculations reveal that the uranyl is part of a [UO2(H2O)4H3SiO4]+ complex formed by the substitution of an equatorial uranyl water with a monodentate silicate ligand. The second application involves the tetravalent actinide hydroxides, An(OH)4(aq) (An = Th, Pu). Their monomeric forms, which are expected to be predominant at environmental concentrations, have not been characterized to date due to the low solubility of An(IV). We report FPMD simulation results for the structure, energetics, and dynamical behavior of An(OH)4(aq). The Pu case is particularly challenging for first principles methods due to the presence of strong 5f-electron correlation effects, which we take into account using a DFT+U model derived from previous ab initio calculations on hydrated An(OH)4(H2O)n clusters. In general, we find a set of coordination structures that are close in energy; the metadynamics technique is applied to obtain quantitative estimates for the free energy differences between these structures.

Characterization of aqueous actinide complexes from first-principles molecular dynamics simulations

251st National Meeting (2016)

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