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PI-Submitted Research Highlights for
Subsurface Biogeochemical Research Program

New Molecular Insights into the Structural Mechanism of Uraninite (UO2) Oxidation

John Bargar
SSRL, SLAC National Accelerator Laboratory

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Molecular-scale information reveals non-classical diffusion behavior during the initial stages of uranium dioxide corrosion

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, suggesting a mixed termination of the oxidized surface with hemi-uranyl, hydroxyl and/or molecular water. The ordered oscillatory oxidation front with a three-layer periodicity observed here is distinct from previously proposed models of oxidative corrosion under vacuum and offers important molecular-scale insights into UO2 oxidation under ambient conditions.

Contacts (BER PM)
Roland F. Hirsch, SC-23.2, roland.hirsch@science.doe.gov, (301) 903-9009

(PI Contact)
John Bargar
SSRL, A directorate of the SLAC National Accelerator Laboratory
bargar@slac.stanford.edu

Funding
Support was provided by U.S. DOE Office of Biological and Environmental Research (BER), Subsurface Biogeochemical Research activity, through the SLAC SFA program (Contract No. DE-AC02-76SF00515) and by the Geosciences Research Program at PNNL, funded by the U.S. DOE Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences, and Biosciences. XPS data were collected in the Radiochemistry Annex at EMSL, a national scientific user facility located at PNNL. A portion of the DFT study was also performed using the computational resources of EMSL. GeoSoilEnviroCARS is supported by the National Science Foundation-Earth Sciences (EAR-1128799) and U.S. DOE-BES GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility.

Publications
Stubbs, J.E. et al. (2015). UO2 Oxidative Corrosion by Nonclassical Diffusion. Physical Review Letters 114, 246103. DOI: 0031-9007/15/114(24)/246103(5)

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Article (PDF):
Stubbs_oxidized_UO2_surface_structure_PhysRevLett.114.246103_2015.pdf

Performer is an SFA, funded by DOE-BER, Climate and Environmental Sciences Division, Subsurface Biogeochemical Research Activity.  Additional support was provided by: (i) GeoSoilEnviroCARS which is funded by the National Science Foundation-Earth Sciences (EAR-1128799) and U.S. DOE-BES GeoSciences (DE-FG02-94ER14466); and (ii) by the Geosciences Research Program at PNNL, funded by the U.S. DOE Office of Basic Energy Sciences (BES), Division of Chemical Sciences, Geosciences, and Biosciences. Data were fit using the GenX program. The Environmental Molecular Sciences Laboratory (EMSL) user facility at the Pacific Northwest National Laboratory (PNNL) was used to perform XPS analyses.  The Advanced Photon Source (APS) user facility at Argonne National Laboratory (ANL) was used to acquire CTR data.


Oxygen penetrates the mineral lattice of UO2 and occupies interstitial sites in every third layer. The sphere of influence for each interstitial oxygen defect extends over several shells of neighboring cations and influences the positions of subsequent interstitials.

 

Image courtesy of SLAC National Laboratory, based on a modification of Figure 2c in Stubbs et al. (2015) Phys. Rev. Lett.

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