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Staff Accomplishments

Happy hour for microbes

August 2006
Fueled by ethanol, microorganisms immobilize uranium at former nuclear site By serving microbes a little bit of alcohol for an hour every day, Pacific Northwest National Laboratory researchers and their collaborators found that certain micro-organisms can tie up uranium in the soil. The microbes could be quite valuable at former nuclear weapons sites. The microbes could immobilize uranium in the soil and keep it from spreading into nearby creeks and aquifers. Nuclear weapons development has left uranium in the soil around Bear Creek at the Oak Ridge Reservation in eastern Tennessee. Traditional cleanup methods, including digging up contaminated soil and replacing it with clean gravel and native soil, leave small amounts of uranium in the soil. The PNNL-led research team is working at the Oak Ridge Field Research Center, an outdoor site, to test an alternative cleanup method: biotransformation. In this method, microorganisms turn uranium into an insoluble form, locking it in the soil, which keeps the hazardous radionuclide away from the creek and ultimately away from possible exposure to people. The research team consists of experts from the Pacific Northwest and Oak Ridge national laboratories and the University of Wisconsin. Imbibing microbes swap electrons Studies on biotransformation began in the laboratory. Scientists found that metal-reducing microbes, a broad class of microorganisms, could change the electronic or valence state of uranium under the right conditions. Microorganisms in the laboratory environment, as part of their normal growth and metabolism, add two electrons to soluble uranium, making it relatively insoluble. The site provides the research team with the opportunity to test this process in the field. "Just because you can do it in a lab doesn't mean it will work in the field," said PNNL staff scientist and principal investigator Tim Scheibe. "By working at this test site, we can determine how effective this procedure would be in the real world." At the test site, the researchers pump ethanol or potable grain alcohol into the soil for an hour every day. The ethanol stimulates the native microorganisms, which do not have sufficient natural carbon and nutrients otherwise. Then, the microbes perform a series of reduction-oxidation reactions; that is, transferring electrons from one material to another. These microorganisms use iron and dissolved uranium as electron acceptors, providing the minerals with electrons. For uranium, the added electrons change the valence state from 6 to 4. The reduced valence state allows the uranium to precipitate into a relatively insoluble and immobile mineral that, in effect, renders the uranium much less likely to enter the groundwater. Modeling the behavior By taking soil samples at various depths around the creek, the researchers gathered extensive data on the microbes' activity and the uranium's behavior. To analyze the large volume of data, the researchers built two- and three-dimensional computer models that include subsurface reactions. These models contain diverse and complex biogeochemical, hydrogeologic, and geophysical data. The team found that the microbes effectively immobilize nearly 100 percent of the uranium that was previously moving with the groundwater. Uranium concentrations in the groundwater (moving toward Bear Creek) have been reduced to below drinking water standard limits. However, the success of any cleanup effort depends on the subsurface chemistry, such as iron and sulfate levels, and on the subsurface structure, both of which can enhance or inhibit the microbes. Future studies look at longevity In proposed studies at Oak Ridge, the researchers will look at the longevity of the uranium immobilization. The questions in their minds are, how long will the uranium remain immobilized? Will the groundwater have to be retreated on a regular basis? If so, how often and at what cost? The researchers will also study microbes and phosphate. The microbes release phosphate from organic compounds, initiating reactions that lock away uranium. What happens to the released phosphate and how does it work to lock up uranium? While phosphate minerals are stable in the laboratory, researchers want to study this process in the field. The Environmental Remediation Sciences Program in the U.S. Department of Energy's Office of Biological and Environmental Research funds the work. For additional information, contact Mike Fayer or Tim Scheibe.

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