Abstract
Uranium is an important carbon-free fuel source and environmental contaminant that accumulates in the tetravalent state, U(IV), in anoxic sediments, such as ore deposits, marine basins, and contaminated aquifers. However, little is known about the speciation of U(IV) in low-temperature geochemical environments, inhibiting the development of a conceptual model of U behavior. Until recently, U(IV) was assumed to exist predominantly as the sparingly soluble mineral uraninite (UO2+x) in anoxic sediments; however, studies now show that this is not often the case. Yet a model of U(IV) speciation in the absence of mineral formation under field-relevant conditions has not yet been developed. Uranium(IV) speciation controls its reactivity, particularly its susceptibility to oxidative mobilization, impacting its distribution and toxicity. Here we show adsorption to organic carbon and organic carbon-coated clays dominate U(IV) speciation in an organic-rich natural substrate under field-relevant conditions. Whereas previous research assumed that U(IV) speciation is dictated by the mode of reduction (i.e., whether reduction is mediated by microbes or by inorganic reductants), our results demonstrate that mineral formation can be diminished in favor of adsorption, regardless of reduction pathway. Projections of U transport and bioavailability, and thus its threat to human and ecosystem health, must consider U(IV) adsorption to organic matter within the sediment environment.
| Original language | English |
|---|---|
| Pages (from-to) | 711-716 |
| Number of pages | 6 |
| Journal | Proceedings of the National Academy of Sciences of the United States of America |
| Volume | 114 |
| Issue number | 4 |
| DOIs | |
| State | Published - Jan 24 2017 |
| Externally published | Yes |
Funding
The authors thank Ann Marshall for assistance in collecting TEM images at the Stanford Nano Shared Facilities. Funding was provided by the DOE Office of Biological and Environmental Research (BER), Subsurface Biogeochemistry Research activity to the SLAC Science Focus Area program under contract DE-AC02-76SF00515 to SLAC (S.E.B. and J.R.B.). Use of the Stanford Synchrotron Radiation Lightsource (SSRL) is supported by the US DOE, Office of Basic Energy Sciences. A portion of the research was performed using the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility sponsored by the Office of BER (located at Pacific Northwest National Laboratory). Research described in this paper was performed at beamline 10ID-1 at the Canadian Light Source (CLS), which is supported by Natural Sciences and Engineering Research Council, Canadian Institutes of Health Research, National Research Council, Western Economic Diversification Canada, the University of Saskatchewan, and the Province of Saskatchewan.
Keywords
- EXAFS
- NanoSIMS
- Organic matter
- STXM
- Uranium