Biotic-Abiotic Pathways: A New Paradigm for Uranium Reduction in Sediments

March 2013 SSRL Science Summary by Lori Ann White, SLAC Office of Communications

As part of a larger, DOE-funded investigation into bioremediation of uranium in contaminated aquifers, a group of SSRL scientists made a surprising discovery about how uranium ions behave in the environment. In addition to overturning current scientific models, this research will lead to more efficient, less costly methods for uranium cleanup and mining. Their research hinged on the fundamental subject of electron transfer– redox reactions – in this case, what atoms gave up electrons to uranium. Prior to this study it was generally thought that enzymes on bacterial cells could donate electrons to highly soluble, highly mobile, and thus highly undesirable U⁶⁺ (uranyl), reducing it to the more stable U⁴⁺ oxidation state in the form of the mineral uraninite (UO₂). Another possible path: iron sulfide (FeS) is also capable of donating electrons to uranyl, reducing it to UO₂.

In the field, the SSRL team led by Senior Staff Scientist John Bargar placed fresh sediments into wells in a uranium contaminated aquifer in Rifle, CO during a bioremediation experiment conducted by researchers from Lawrence Berkeley National Laboratory. At the end of the experiment, the sediments were harvested and brought back to SSRL. The research team used x-ray imaging at SSRL Beam Lines 10-2 and 2-3 to measure the micron-scale distribution of uranium in the aquifer sediments and to determine if it was closely associated with iron oxides or iron sulfides. Most of the U⁴⁺ was found to be associated with FeS, suggesting that FeS was supplying electrons to reduce U⁶⁺ to U⁴⁺. The team also used SSRL Beam Line 11-2 to perform x-ray absorption spectroscopy measurements to characterize the local molecular structure around U⁴⁺ at very low concentrations. These measurements showed that the dominant U⁴⁺ products were bound to biopolymers; UO₂ was present but only at relatively low concentrations. Electron microscopy and chemical extraction measurements confirmed and enhanced these conclusions.

These results lead to the surprising conclusion that both bacterial biomass and FeS are required to explain reduction of uranyl to U⁴⁺ in the aquifer. At least some of the electrons required to reduce uranyl are supplied by FeS. However, biomass is required to first form the FeS, and then to chemically bind U⁴⁺. The findings provide important new clues about how to improve bioremediation strategies and uranium extraction from ore bodies.

This research was funded by was funded by the U.S. DOE Office of Science, Office of Biological and Environmental Research (FWP 10094) and Office of Basic Energy Sciences.

 

Primary Citation

J. R. Bargar, K. H. Williams, K. M. Campbell, P. E. Long, J. E. Stubbs, E. I. Suvorova, J. S. Lezama-Pacheco, D. S. Alessi, M. Stylo, S. M. Webb, J. A. Davis, D. E. Giammar, L. Y. Blue and R. Bernier-Latmani, "Uranium Redox Transition Pathways in Acetate-amended Sediments", Proc. Natl. Acad. Sci. USA 110, 4506 (2013)

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John R. Bargar, SSRL