The Science

A new multitechnique study using X-ray absorption spectroscopy at the Stanford Synchrotron Radiation Laboratory (SSRL) and Nano-Secondary Ion Mass Spectroscopy (NanoSIMS) at the Environmental Molecular Sciences Laboratory (EMSL), a Department of Energy (DOE) Office of Science user facility has revealed crisp new details about the mechanisms of uranium binding in sediments. Surfaces of natural organic matter bind uranium more strongly than minerals under field-relevant conditions.

The Impact

Uranium is less stable and more easily remobilized when bound to surfaces of organic matter and mineral as compared to being incorporated with mineral precipitates. This new finding implies that reduced uranium is much more reactive and able to participate in repeated biogeochemical cycling than previously thought to be the case.

Summary

Uranium is an important carbon-neutral energy source and major subsurface contaminant at DOE legacy sites. Anoxic sediments, which are common in alluvial aquifers, are important concentrators of uranium, where it accumulates in the tetravalent state, U(IV). Uranium-laden sediments pose risks as “secondary sources” from which uranium can be re-released to aquifers, prolonging its impact on local water supplies. In spite of its importance, little is known about the speciation of U(IV) in these geochemical environments. Uranium analysis is challenged by its low concentrations and the tremendous chemical and physical complexity of natural sediments. U(IV) binds to both organic matter and minerals, which can be co-associated at the scale of 10s to 100s of nanometers. Because of the multiplicity and similarity of binding sites present in samples, “standby” analytical techniques such as X-ray absorption spectroscopy (XAS) are challenged to distinguish the molecular structure of U(IV) in these natural sediments. The molecular nature of accumulated U(IV) is, however, a first-order question, because the susceptibility of uranium to oxidative mobilization is mediated by its structure.

In an SSRL-based study, Bone et al (2017) overcame these challenges by combining XAS, NanoSIMS, and scanning transmission X-ray microscopy (STXM) measurements to characterize the local structure and nanoscale localization of uranium and the character of organic functional groups. This work showed that complexes of U(IV) adsorb on organic carbon and organic carbon–coated clays in an organic-rich natural substrate under field-relevant conditions. Furthermore, 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), this work demonstrated that precipitation of U(IV) minerals, such as uraninite (UO₂), can be inhibited simply by decreasing the total concentration of uranium, while maintaining the same concentration of sorbent. This conclusion is significant because UO₂ and other minerals are much more stable and more readily remobilized than surface-complexed forms of U(IV). Thus, the number and type of organic and mineral surface binding sites that are available have a profound influence on U(IV) behavior. Projections of uranium transport and bioavailability, and thus its threat to human and ecosystem health, must consider U(IV) adsorption to organic matter within the local sediment environment.

Principal Investigator

John Bargar
SLAC National Accelerator Laboratory
bargar@slac.stanford.edu

Program Manager

Paul Bayer
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
paul.bayer@science.doe.gov

Funding

Funding was provided by the Subsurface Biogeochemistry Research (SBR) program of the Office of Biological and Environmental Research (BER), within the U.S. Department of Energy Office of Science, to the SLAC Scientific Focus Area (SFA) under contract DE-AC02-76SF00515 to SLAC. Use of the Stanford Synchrotron Radiation Laboratory (SSRL) is supported by the Office of Basic Energy Sciences within the DOE Office of Science. A portion of the research was performed using the Environmental Molecular Sciences Laboratory (EMSL), a DOE Office of Science user facility sponsored by BER (located at Pacific Northwest National Laboratory). Research described in this paper was performed at beamline 10ID-1 the CLS, which is supported by the following Canadian entities: Natural Sciences and Engineering Research Council {NSERC), Canadian Institutes of Health Research (CIHR), National Research Council of Canada (NRC), Western Economic Diversification Canada (WEDC), the University of Saskatchewan, and the Province of Saskatchewan. The authors thank Ann Marshall for assistance in collecting transmission electron microscopy (TEM) images at the Stanford Nano Shared Facilities (SNSF).

References