Contamination of soil and water by uranium is especially problematic because uranium takes hundreds of millions of years to decay radioactively. Furthermore, in the United States alone, a multitude of facilities are contaminated with radioactive waste as a legacy of nuclear weapons production. Within the Department of Energy (DOE), over half of its 3,000 contaminated disposal sites have radioactive groundwater or soil contamination.
The fate and transport of uranium is governed by its oxidation state, which is either hexavalent (U(VI)) or tetravalent (U(IV)). In its hexavalent state, uranium is very soluble and travels with water, but uranium in the tetravalent state is insoluble and essentially immobile. Certain bacteria are able to reduce uranium from its hexavalent state to its tetravalent state, thus decreasing its solubility and trapping it in the soil underground. We have carried out a series of laboratory- and field-scale experiments in which we have added a carbon and energy source to stimulate microbial activity that removes uranium from groundwater. The immobilization of uranium underground provides for its long-term immobilization.