Unexpected Uranium Chemistry Found In Contaminated Aquifer

Cleaning up water in aquifers contaminated with uranium from processing ore for nuclear fuels could be carried out more effectively if the underground chemistry—how uranium converts between water-soluble U⁶⁺ and insoluble U⁴⁺—was well understood. Although scientists had believed that all U⁴⁺ in aquifers occurred as UO₂, researchers report in a recent paper and at the American Chemical Society national meeting in New Orleans that a combination of microbial and abiotic reactions also produces other U⁴⁺ complexes (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1219198110).

More than a dozen former ore-processing sites in the western U.S. are contaminated with uranium. Cleaning up the sites is necessary to protect people, wildlife, and livestock from being exposed to uranium-contaminated groundwater.

One approach to treating uranium-contaminated aquifers is to add a microbial nutrient to create a bacterial bloom that consumes all of the oxygen and creates reducing conditions, say research team leaders John R. Bargar and Noémi Janot of SLAC National Accelerator Laboratory. After using this method on a uranium-contaminated aquifer in Rifle, Colo., the researchers found that the mechanism of U⁶⁺-to-U⁴⁺ reduction is complex. Iron sulfide minerals that deposit on biomass appear to reduce U⁶⁺ to U⁴⁺. Then, U⁴⁺ is not only converted to UO_2, it also forms complexes with phosphate or other ligands from nearby biological material. The microbes themselves may also reduce U⁶⁺ to U⁴⁺.

The results will change how aquifer behavior is modeled, Bargar says. Right now, models assume that all U⁴⁺ turns into UO₂.

Seeing the unexpected products indicates that scientists still have much to learn about uranium chemistry in natural systems, says Argonne National Laboratory’s Max Boyanov, who also studies subsurface mineral transformation.