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.
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 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.
The mobility of uranium in subsurface environments depends strongly on its oxidation state, with chemically reduced UIV phases being significantly less soluble than UIV 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 UIV reduction in natural systems: a nanoparticulate UO2 phase and an amorphous UIV-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 UIV-phosphate. In the absence of Ca or PO4, the product of UO2 oxidation was Na-uranyl oxyhydroxide, whereas the product of UIV-Ca-PO4 oxidation was a UIV-phosphate phase (autunite). In the presence of Ca or PO4, the oxidation proceeded to UIV-phosphate for both pre-oxidation forms of UIV. Addition of Ca or PO4 changed the mechanism of oxidation by causing the formation of a passivation layer on the particle surfaces.
Argonne National Laboratory
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.
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.