U.S. Department of Energy Office of Biological and Environmental Research

BER Research Highlights


New Molecular Insights into the Structural Mechanism of Uraninite Oxidation
Published: June 19, 2015
Posted: October 27, 2016

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 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.

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

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

Funding
Support was provided by the U.S. Department of Energy (DOE), Office of Science, Biological and Environmental Research (BER), Subsurface Biogeochemical Research activity, through the SLAC Scientific Focus Area program (Contract No. DE-AC02-76SF00515); and by the Geosciences Research Program at Pacific Northwest National Laboratory (PNNL), funded by DOE’s Office of Basic Energy Sciences (BES). XPS data were collected in the Radiochemistry Annex at the Environmental Molecular Sciences Laboratory, a DOE 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 BES GeoSciences (DE-FG02-94ER14466). This research used resources at the Advanced Photon Source, a DOE user facility.

Publication
Stubbs, J. E.,et al. 2015. “UO2 Oxidative Corrosion by Nonclassical Diffusion,” Physical Review Letters 114, 246103. DOI: 10.1103/PhysRevLett.114.246103. (Reference link)

Related Links
ANL Highlight

Topic Areas:

  • Research Area: Subsurface Biogeochemical Research
  • Research Area: DOE Environmental Molecular Sciences Laboratory (EMSL)

Division: SC-23.1 Climate and Environmental Sciences Division, BER

 

BER supports basic research and scientific user facilities to advance DOE missions in energy and environment. More about BER

Recent Highlights

May 10, 2019
Quantifying Decision Uncertainty in Water Management via a Coupled Agent-Based Model
Considering risk perception can improve the representation of human decision-making processes in age [more...]

May 09, 2019
Projecting Global Urban Area Growth Through 2100 Based on Historical Time Series Data and Future Scenarios
Study provides country-specific urban area growth models and the first dataset on country-level urba [more...]

May 05, 2019
Calibrating Building Energy Demand Models to Refine Long-Term Energy Planning
A new, flexible calibration approach improved model accuracy in capturing year-to-year changes in bu [more...]

May 03, 2019
Calibration and Uncertainty Analysis of Demeter for Better Downscaling of Global Land Use and Land Cover Projections
Researchers improved the Demeter model’s performance by calibrating key parameters and establi [more...]

Apr 22, 2019
Representation of U.S. Warm Temperature Extremes in Global Climate Model Ensembles
Representation of warm temperature events varies considerably among global climate models, which has [more...]

List all highlights (possible long download time)