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Improving Subsurface Uranium Cleanup Methods

View of a field that was a tailings site. Colorado River is on the upper right.
A former uranium mill tailings site near Rifle, Colorado, is next to the Colorado River. Photo credit: John Bargar, SLAC National Accelerator Laboratory.

A team of scientists has documented multiple chemical reactions that transform hexavalent uranium [U(VI)] to more insoluble forms of tetravalent uranium [U(IV)] in subsurface sediments at a uranium contamination site. This information could lead to improved cleanup methods by enabling remediation designers to enhance the transformation of soluble U(VI) to more insoluble forms of U(IV), such as the mineral uraninite (UO2). The objective of such remediation is to stabilize (stop the downgradient movement of) uranium plumes in groundwater or to immobilize the uranium in the aquifer material. Before scientists documented the current study, the chemical reactions and mineralogic endpoints of U(VI) in natural sediments had not been well defined.

Working at the Integrated Field Research Challenge (IFRC) site—a former uranium mill tailings site near Rifle, Colorado—the team of scientists from SLAC National Accelerator Laboratory; Lawrence Berkeley National Laboratory; U.S. Geological Survey (USGS); Ecole Polytechnique Fédérale de Lausanne, Switzerland; and Washington University in St. Louis, Missouri, showed that in subsurface sediments, both nonbiological and biologically mediated transformations ofU(VI) to U(IV) can take place simultaneously in contaminated sediments, resulting in more than one form of U(IV) in the same location. The most desired from of U(IV) is the mineral uraninite, because it is insoluble and is less likely to move in groundwater. Of special interest to cleanup professionals is that some forms of U(IV) more soluble than uraninite could conceivably be redissolved if the chemistry of groundwater changes in the future.

This study was funded by the U.S. Department of Energy, U.S. Geological Survey National Research Council Postdoctoral Fellowship Program, Swiss National Science Foundation, Marie Curie Grant Program, and National Institutes of Health.


Bargar, J.R., Williams, K.H., Campbell, K.M., Long, P.E., Stubbs, J.E., Suvorova, E.I., Lezama-Pacheco, J.S., Alessi, D.S., Stylo, M., Webb, S.M., Davis, J.A., Giammar, D.E., Blue, L.Y., and Bernier-Latmani, R., 2013, Uranium redox transition pathways in acetate-amended sediments: Proceedings of the National Academy of Sciences, v. 110, no. 12, p. 4506-4511, doi:10.1073/pnas.1219198110.

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