08/01/2012

How Iron in Minerals Affects Subsurface Uranium

Summary

Subsurface minerals help control the chemical form of contaminants such as uranium (U). The redox (reduction and oxidation) state of soils and sediments exists on a continuum from oxidized to reduced and can affect the mobility of uranium plumes. Under oxidized conditions, U is rather soluble as a uranyl ion in the U⁶⁺ valence state, whereas under reducing conditions U can become immobilized in the less-soluble U⁴⁺ valence state. Researchers at the University of Iowa and Argonne National Laboratory have found that a complex mixture of ferrous iron (Fe²⁺)-bearing minerals in a naturally reduced soil is capable of reducing and immobilizing uranium. Using Mössbauer spectroscopy at the University of Iowa and synchrotron x-ray absorption spectroscopy at the Advanced Photon Source at Argonne, the researchers found that uranium was reduced by Fe²⁺ in clay minerals and by a less-common, transient, and highly reactive Fe²⁺-mineral called green rust. The researchers also observed that the reduced U⁴⁺ atoms formed a product different from the uraninite mineral (UO₂) commonly observed in laboratory studies, providing evidence for the diversity in chemical speciation of reduced U in natural systems. This study provides detailed information necessary for understanding toxic and radioactive contaminant mobility which will contribute to the long-term stewardship of U.S. Department of Energy legacy sites.

References

Latta, D. E., M. I. Boyanov, K. M. Kemner, E. J. O’Loughlin, and M. M. Scherer. 2012. “Abiotic Reduction of Uranium by Fe(II) in Soil,” Applied Geochemistry 27, 1512–24. DOI: 10.1016/j.apgeochem.2012.03.003