One promising approach to stabilize uranium contamination in soils is to envelop the radioactive uranium into iron-bearing minerals like hematite. But how well does uranium bind with hematite and for how long? Scientists have disagreed on the answer to that question. A team has discovered the answer. And it's not what anyone expected. Vacancies created in hematite's atomic structure during its formation accommodate uranium, which is surprisingly flexible.
Uranium contamination lurks in groundwater and soils at certain Department of Energy (DOE) sites and under many industrial areas around the world. Some forms of uranium can be readily transported in the soil or water. One approach for limiting the mobility of uranium is to enhance its binding with iron oxides or other minerals. Doing so could also enable scientists to better predict its long-term behavior to ensure the uranium stays put for thousands of years.
While scientists have been studying the binding of uranium to iron-bearing minerals for some time using X-ray spectroscopy, different researchers have interpreted similar data in drastically different ways. This has been a tough problem because uranium, like a square peg in a round hole, should not fit into the crystal structure of hematite, one of the most abundant iron minerals found in soils. The solution, developed by researchers at the Pacific Northwest National Laboratory and the University of Manchester, turns previous work on its head. The team calculated many possible atomic structures of uranium incorporated into the structure of this mineral. They discovered that vacancies created in the atomic structure of hematite during its formation accommodate the uranium. Neither this accommodation nor the flexibility shown by the uranium was expected. This binding process had never before been identified, but the methods used to make this finding could explain a number of mysteries previously reported in the scientific literature. The work opens the door to new studies on how other radioactive contaminants bind to soil minerals and will lead to more accurate predictions of how these contaminants behave in the environment.
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This work was supported by the Department of Energy's Office of Science, Office of Basic Energy Sciences (BES), Geosciences Program at Pacific Northwest National Laboratory. A portion of the work was performed at the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility supported by the Office of Biological and Environmental Research. Extended X-ray absorption fine structure data for a sample (EXAFS) were collected as part of the Natural Environment Research Council Biogeochemical Gradient and Radionuclide Transport Consortium under beamtime grants. Support was also provided by the Science & Technology Facilities Council Environmental Radioactivity Network. EXAFS data for another sample were collected at the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, which is supported by DOE's Office of Science, BES. EXAFS spectra for a uranyl-sorbed ferrihydrite standard sample were collected at GeoSoilEnviroCARS, Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences and DOE – Geosciences. The APS is a DOE Office of Science user facility operated by Argonne National Laboratory. The authors acknowledge the use of VESTA structural analysis software.
M.E. McBriarty, S. Kerisit, E.J. Bylaska, S. Shaw, K. Morris, and E.S. Ilton, "Iron vacancies accommodate uranyl incorporation into hematite." Environmental Science and Technology 52, 6282 (2018). [DOI: 10.1021/acs.est.8b00297]
Environmental Molecular Sciences Laboratory (EMSL) science highlight: Fitting a Square Peg in a Round Hole: The Surprising Structure of Uranium Bound in Hematite
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