The proposed research will elucidate the principal biogeochemical reactions that govern the concentration, chemical speciation, and reactivity of the redox-sensitive contaminant uranium. The results will provide an improved understanding and predictive capability of the mechanisms that govern the biogeochemical reduction of uranium in subsurface environments. In addition, the work plan is designed to: (1) Generate fundamental scientific understanding on the relationship between U(VI) chemical speciation and its susceptibility to biogeochemical reduction reactions. ? Elucidate the controls on the rate and extent of contaminant reactivity. (2) Provide new insights into the aqueous and solid speciation of U(VI)/U(IV) under representative groundwater conditions.
Lanthanide-ligand complexes are key components of technological applications, and their properties depend on their structures in the solution phase, which are challenging to resolve experimentally or computationally. The coordination structure of the Eu3+ ion in different coordination environments in acetonitrile is examined using ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy. AIMD simulations are conducted for the solvated Eu3+ ion in acetonitrile, both with or without a terpyridyl ligand, and in the presence of either triflate or nitrate counterions. EXAFS spectra are calculated directly from AIMD simulations and then compared to experimentally measured EXAFS spectra. In acetonitrile solution, both nitrate and triflate anions are shown to coordinate directly to the Eu3+ ion forming either ten- or eight-coordinate solvent complexes where the counterions are binding as bidentate or monodentate structures, respectively. Coordination of a terpyridyl ligand to the Eu3+ ion limits the available binding sites for the solvent and anions. In certain cases, the terpyridyl ligand excludes any solvent binding and limits the number of coordinated anions. The solution structure of the Eu-terpyridyl complex with nitrate counterions is shown to have a similar arrangement of Eu3+ coordinating molecules as the crystal structure. This study illustrates how a combination of AIMD and EXAFS can be used to determine how ligands, solvent, and counterions coordinate with the lanthanide ions in solution.
A quantitative comparison of the Cd binding mechanism to Gram‐positive (Bacillus subtilis) and Gram‐negative bacteria (Shewanella oneidensis) is presented. At pH 6.0, EXAFS data for the Gram‐positive bacteria were modeled using carboxyl and phosphoryl sites only. However, additional sulfide sites were required to model the spectrum from the Gram‐negative bacteria under similar experimental conditions. Cd binding to a bacterial consortium at the same pH value, sampled from natural river water, was modeled using the models developed for the individual Gram‐positive and Gram‐negative bacterial strains.
Theoretical EXAFS spectra generated by FEFF6 and FEFF8 are compared. As a test of the effect of charge transfer on EXAFS analysis, we examine the aqueous uranyl (UO22+) ion. We find that the major difference between FEFF8 and FEFF6 is the edge energy position of approximately 5 eV. Modest changes in the forward focusing multiple scattering path of the uranyl resulting with FEFF8 produce a better model of the measured hydrated uranyl spectrum.
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