11/17/2015

Calcium and Phosphate Can Affect How Uranium Contamination Travels Through the Environment

The molecular form and the oxidation of subsurface uranium are affected by common groundwater constituents.

The Science

Calcium and phosphate are environmental components found in animal bone, various minerals, and groundwater. A recent study has demonstrated that these ions slow down the oxidation of subsurface-relevant uranium species and change the oxidized form of uranium into a more stable mineral.

The Impact

The effect of environmental factors like calcium and phosphate on transformations of some newly discovered forms of uranium found in groundwater are currently unknown. This study determined uranium valence and molecular structure using state-of-the-art spectroscopy techniques to provide information necessary for accurately predicting uranium transport.

Summary

The mobility of uranium in subsurface environments depends strongly on its oxidation state, with chemically reduced U^IV phases being significantly less soluble than U^IV minerals. A team of scientists from Argonne National Laboratory, Illinois Institute of Technology, and Bulgarian Academy of Sciences compared the oxidation kinetics and mechanisms of two potential products of U^IV reduction in natural systems: a nanoparticulate UO₂ phase and an amorphous U^IV-Ca-PO₄ phase. The valence and molecular structure of uranium was tracked by synchrotron x-ray absorption spectroscopy. Similar oxidation rates for the two phases were observed in solutions equilibrated with atmospheric O₂ and CO₂. Addition of up to 400 µM Ca and PO₄ decreased the oxidation rate by an order of magnitude for both UO₂ and U^IV-phosphate. In the absence of Ca or PO₄, the product of UO₂ oxidation was Na-uranyl oxyhydroxide, whereas the product of U^IV-Ca-PO₄ oxidation was a U^IV-phosphate phase (autunite). In the presence of Ca or PO₄, the oxidation proceeded to U^IV-phosphate for both pre-oxidation forms of U^IV. Addition of Ca or PO₄ changed the mechanism of oxidation by causing the formation of a passivation layer on the particle surfaces.

Principal Investigator(s)

Kenneth Kemner
Argonne National Laboratory
[email protected]

Funding

This research is part of the Subsurface Science Scientific Focus Area at Argonne National Laboratory (ANL), which is supported by the U.S. Department of Energy’s (DOE) Subsurface Biogeochemical Research Program, Office of Biological and Environmental Research, Office of Science. Use of the Electron Microscopy Center and Advanced Photon Source at ANL is supported by DOE’s Office of Science, Office of Basic Energy Sciences. MRCAT/EnviroCAT operations are supported by DOE and the MRCAT/EnviroCAT member institutions. All work at ANL was under Contract DE-AC02-06CH11357.

References

Latta, D. E., K. M. Kemner, B. Mishra, and M. I. Boyanov. 2016. “Effects of Calcium and Phosphate on Uranium(IV) Oxidation: Comparison Between Nanoparticulate Uraninite and Amorphous UIV–Phosphate,” Geochimica et Cosmochimica Acta 174, 122–42. DOI: 10.1016/j.gca.2015.11.010.