Uranium Reduction

 


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. As a consequence, we proposed the stimulation of microbial activity to remove uranium from groundwater in-situ. The immobilization of uranium underground provides for its long-term immobilization during the radioactive decay process.

Schematic of Microbial Uranium Reduction

At the DOE's Natural and Accelerated Bioremediation Research (NABIR) program Field Research Center (FRC) in Oak Ridge, TN, our group is working as part of a larger team at Stanford and Oak Ridge National Laboratory to investigate the feasibility of stimulating microbial reduction of mobile and soluble U(VI) to immobile and insoluble U(IV). Our field site at Oak Ridge is currently set up with recirculation wells to enable delivery of electron donor (food) and nutrients for subsurface reduction of U(VI). It also has capabilities for ex-situ removal of groundwater contaminants.

To understand subsurface U(VI) transformations, we tested sediment from our field site in a series of microcosm experiments. We identified electron donors that can stimulate microbial uranium reduction and previewed the response of the field system to the addition of electron donor and nutrients. We are now using bacterial enrichments to develop and test kinetic models of microbial growth and U(VI) reduction, as well as to investigate the community structure of U(VI)-reducing communities. Our plan is to incorporate microbial kinetic models into a larger model capable of simulating and predicting the response of the field system. This larger model is being developed by Prof. Kitanidis' group in collaboration with Dr. Olaf Cirpka. Our group in the next two years will be evaluating the long-term feasibility of the bioremediation of uranium and further developing models to help design the treatment of other sites.

This page was written by Dr. Jenny Nyman, former graduate student.