Physcial Sciences Division
Research Highlights

November 2009

Class-ing Up Uranium

Geochemist shares insights on troublesome radionuclide with Pacific Northwest National Laboratory

John Bargar

Primitive: that's how Dr. John Bargar describes the state of knowledge of the reactivity of biogenic uranite in ground water. Biogenic uranite is a form of uranium, produced by bacteria, that is fairly immobile in groundwater. Bargar discussed his work to turn this primitive knowledge into a sophisticated understanding at Pacific Northwest National Laboratory's Frontiers in Biogeochemistry Seminar Series. The series features scientists who discuss novel ideas and advancements in geochemical research and development.

Primitive state: Biogenic uranite is produced by microbes. The tiny organisms in the soil convert a mobile form of uranium, which moves easily through the soil, into a less mobile form. The movement of the uranium is a concern because of its potential to flow from former nuclear sites into nearby lakes and waterways.

One option to halt the uranium's progress is to help microbes turn it into an immobile form (uraninite) that stays trapped in the soil, never reaching the waterways.

Before such a remediation system can be designed, researchers need a first-order understanding of biogenic uraninite in ground water. This knowledge has proven difficult to acquire for two reasons, according to Bargar:

• The geochemistry of uraninite particles is deceptively complex, making simple assumptions far from useful.
• Microbes work in a soup of chemicals, making it hard to isolate reactions.

Learning more: Bargar and his team studied how microbes create the immobile form. This work began at the SLAC National Accelerator Laboratory, Ecole Polytechnique Federale Lausanne, Washington University, and Pacific Northwest National Laboratory. Based on the experiments, the team created a hypothesis regarding the type of microbes involved.

Then, they took their hypothesis to the field, specifically a test site at Rifle, Colorado managed by Phil Long at PNNL. Here, they installed biogenic nano-uraninite into wells to monitor dissolution rates and mechanisms. Several months later, they removed the samples and found that uraninite is dissolving more slowly than originally projected.  These results suggest that biogenic uraninite will have long-term stability in ground water.

What's next: Bargar and his team are continuing to work with Pacific Northwest National Laboratory to improve the understanding of immobile form of uranium that these bacteria produce.

About Dr. John Bargar: Dr. Bargar is an environmental geochemist at the SLAC National Accelerator Laboratory. His expertise is in low-temperature aquatic geochemistry, with an emphasis on synchrotron-based x-ray spectroscopy and scattering techniques. His research interests include environmental biogenic oxides and heavy metal contaminants in natural waters.