Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction By Fe(II) And Result In U(V)-Bearing Ferrihydrite
By Christian Dewey
The Science
Uranium (U) solubility determines the dissolved concentrations and thus its threat to water quality. It is therefore essential to understand the controls on U solubility to predict and mitigate the impacts of U contamination. In freshwater systems (including groundwater), the U oxidation state largely controls U solubility. In the +VI oxidation state, U is generally soluble, whereas in the +IV state, U is sparingly soluble. Reactions that change the U oxidation state from +VI to +IV (i.e., reduce U(VI)) thus transform U from a more soluble (and mobile) phase to a less so (immobile) phase, and thereby lessen the risk of U transport in groundwater and human exposure. We find that the dissolved calcium-carbonato-U(VI) species, which forms in the presence of dissolved calcium and carbonate, cannot be reduced by dissolved ferrous iron (Fe(II)), a common reductant in freshwater. In sharp contrast, non-calcium U(VI) species are quickly reduced by dissolved Fe(II), resulting in a mixed Fe-U solid in which U unexpectedly occurs in the +V valence state.
The Impact
Our work shows that dissolved calcium and carbonate in U-contaminated systems limit the transformation of U from a more soluble (and mobile) to a less soluble (and less mobile) phase, increasing the threat imposed by uranium to water quality. Furthermore, we reveal a hitherto unidentified solid U(V) phase. Through elucidation of the mechanisms that inhibit reduction of the calcium-carbonato U(VI) species and promote stabilization of U(V), we advance our understanding on the controls on U solubility, and thus improve our ability to predict and mitigate the risks associated with U contamination.
Summary
To determine the impact of calcium-uranyl-carbonato species on uranium fate, we varied and tracked common aqueous U species upon reaction with Fe(II). We found that reduction of the calcium-uranyl-carbonato species is inhibited despite favorable thermodynamics. This indicates that a kinetic constrain limits reduction of these species, leading to their persistence or even accumulation in soil and groundwater – even under reducing conditions. Such an effect is especially important to consider in carbonate-rich Karst formations or arid to semi-arid environments, such as the western U.S., where dissolved calcium and carbonate are commonly abundant in freshwater systems. In addition, we found that the dominant product of homogeneous U reduction was U(V) incorporated into ferrihydrite. Such a phase has not previously been documented, though U(V) has been shown to incorporate into other Fe phases. Finally, we show that surface-catalyzed U reduction, which becomes the predominant U reduction pathway as Fe(III) solids accumulate, results in both reduction of the calcium-uranyl-carbonato species and the production of uraninite (UO2). To characterize the U and Fe solids in our reactors, we employed synchrotron-based spectroscopic techniques (U L3 EXAFS and HERFD-XANES) with transmission electron microscopy. Our work furthers our understanding of U solubility, thereby improving our ability to assess and address the threats posed by this common groundwater contaminant.
Contact
Christian Dewey
Stanford University
Funding
This research was supported by the SLAC Groundwater Quality SFA program of the US Department of Energy, Office of Biological and Environmental Research, Subsurface Biogeochemistry Program (SBR), and by the SBR Project Award Number DE-SC0016544. Use of the Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC0276SF00515. Part of this work was performed at the Stanford Nano Shared Facilities (SNSF), supported by the National Science Foundation under award ECCS-1542152.
Publications
Boye, K., Bobb, C., Maher, K., Bargar, J. R., & Francis, C. A. (2020) Stability of floodplain subsurface microbial communities through seasonal hydrological and geochemical cycles. Frontiers in Earth Science, 8(338).
DOI:10.3389/feart.2020.00338
https://www.frontiersin.org/articles/10.3389/feart.2020.00338/full
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