Unusual Uranium Species Found In Contaminated Aquifer

Cleaning up water in aquifers contaminated with uranium from processing ore for nuclear fuels requires understanding the underground chemistry of how uranium is interconverted between water-soluble U⁶⁺ and insoluble U⁴⁺. Although scientists had believed that all U⁴⁺ in aquifer sediments occurred as UO₂, new research shows that a combination of microbial and abiotic reactions also produces other U⁴⁺ complexes (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1219198110). 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, said research team leaders John R. Bargar and Noémie Janot of SLAC National Accelerator Laboratory. After using this method on an aquifer in Rifle, Colo., the researchers found that iron sulfide minerals appear to seed on the outside of biomass and reduce U⁶⁺ to U⁴⁺. The U⁴⁺ then may complex with phosphate or other ligands available from nearby biological material. If such ligands are absent, then UO₂ may form. The microbes themselves may also reduce U⁶⁺ to U⁴⁺. The results will change how aquifer behavior is modeled, Bargar said.