01/09/2014

Chemical View of Single Uranium Atoms Attached to Mineral Surfaces

Summary

Uranium fission currently provides a significant portion of the world’s low-carbon energy. Mining of uranium ores, fuel processing, nuclear accidents, and spent fuel disposal all have resulted in the release of uranium to the environment. Furthermore, because of its chemistry, uranium is often enriched in fossil fuel ores (e.g., kerogens and black shales). Accurate modelling of uranium transport is paramount to understanding the risks from uranium release in the environment. Current models assume that reduction of U⁶⁺ to U⁴⁺ in bioreduced sediments results in the precipitation of the insoluble mineral uraninite (UO₂). However, researchers at Argonne National Laboratory have shown that mineral surfaces have a significant role in stabilizing U⁴⁺ as isolated, adsorbed U⁴⁺ atoms at low U:surface ratios that are more typical of contaminated sites. Using two model minerals, rutile (TiO₂) and magnetite (Fe₃O₄), the researchers found that the surface area threshold at which U⁴⁺ forms single-atom surface complexes and the stability of these complexes over time is dependent on the mineral chemistry. Adsorbed U⁴⁺ was produced by reactions representative of both biotic and abiotic U⁶⁺ reduction pathways, suggesting that non-uraninite U⁴⁺ associated with minerals may account for a significant portion of the U⁴⁺ balance in sediments. The results indicate the need to include such forms of U⁴⁺ in reactive transport models to better predict uranium mobility in the environment.

References

Latta, D. E., B. Mishra, R. E. Cook, K. M. Kemner, and M. I. Boyanov. 2014. “Stable U(IV) Complexes Form at High-Affinity Mineral Surface Sites,” Environmental Science and Technology 48(3), 1683-91. DOI: 10.1021/es4047389.