New Molecular Insights into the Structural Mechanism of Uraninite Oxidation

The Science

Density-functional theory and X-ray based methods sensitive to surface atomic structure and oxidation state [crystal truncation rod (CTR), X-ray diffraction, and X-ray photoelectron spectroscopy (XPS)] were used to determine the behavior of the natural cleavage surface of uraninite (UO₂) in water at ambient conditions. Oxygen was found to react strongly with UO₂. However, rather than following classical diffusion patterns, oxygen self-organized as interstitial atoms within the mineral lattice of every third atomic layer.

The Impact

Uranium dioxide occurs naturally in anoxic sediments, is the desired product of in-situ bioremediation of uranium-contaminated aquifers, and is likely to control uranium release from such sediments over the long-term. These surprising insights indicate that UO₂ oxidation is far more complicated that previously known and offer a new conceptual molecular-scale framework for understanding UO₂ fate in the environment.

Summary

CTR X-ray diffraction measurements of a polished UO₂ (111) surface exposed to atmospheric oxygen revealed a periodic, oscillatory structure of the oxidation front perpendicular to the mineral-water interface. This behavior could be explained by quantum mechanic considerations of the electron transfer from U 5f orbitals to O 2p orbitals, assuming at least partial contribution from hemi-uranyl (resembling half of the UO₂²⁺ uranyl cation (i.e., with only a single short U-O bond) termination groups at the mineral surface, which favor the incorporation of interstitial oxygens into slab 3 of the UO₂ lattice. The presence of hemi-uranyl termination groups was supported by XPS analyses, revealing that both U(V) and U(VI) were present at the mineral surface and suggesting a mixed termination of the oxidized surface with hemi-uranyl, hydroxyl, and molecular water. The ordered oscillatory oxidation front with a three-layer periodicity observed is distinct from previously proposed models of oxidative corrosion under vacuum and offers important molecular-scale insights into UO₂ oxidation under ambient conditions.

Principal Investigator

John Bargar
SLAC National Accelerator Laboratory
[email protected]

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

Support was provided by the Subsurface Biogeochemical Research program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy (DOE) Office of Science, through the SLAC Scientific Focus Area (SFA) program (Contract No. DE-AC02-76SF00515); and by the Geosciences Research Program at Pacific Northwest National Laboratory (PNNL), funded by the Office of Basic Energy Sciences (BES) within DOE’s Office of Science. X-ray photoelectron spectroscopy (XPS) data were collected in the Radiochemistry Annex at the Environmental Molecular Sciences Laboratory (EMSL), a DOE user facility located at PNNL. A portion of the  density functional theory (DFT) study was also performed using the computational resources of EMSL. GeoSoilEnviroCARS is supported by the National Science Foundation-Earth Sciences (EAR-1128799) and BES GeoSciences (DE-FG02-94ER14466). This research used resources at the Advanced Photon Source, a DOE user facility at Argonne National Laboratory.

References