Organic matter controls uranium mobility.
A new multi-technique study using X-ray absorption spectroscopy at the Stanford Synchrotron Radiation Laboratory (SSRL) and Nano-Secondary Ion Mass Spectroscopy (NanoSIMS) at the Environmental Molecular Science Laboratory (EMSL), an 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.
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.
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 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, as 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 X-ray absorption spectroscopy, NanoSIMS, and 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 UO2, can be inhibited simply by decreasing the total concentration of U, while maintaining the same concentration of sorbent. This conclusion is significant because UO2 (uraninite) 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 U 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.
Contacts (BER PM)
DOE Office of Biological and Environmental Research, Climate and Environmental Sciences Division
SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Laboratory
Funding was provided by the DOE Office of Biological and Environmental Research, Subsurface Biogeochemistry Research (SBR) activity to the SLAC Science Focus Area under contract DE-AC02-76SF00515 to SLAC. Use of SSRL is supported by the U.S. DOE, Office of Basic Energy Sciences. A portion of the research was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research (located at PNNL). Research described in this paper was performed at beamline 10ID-1 the CLS, which is supported by NSERC, CIHR, NRC, WEDC, the University of Saskatchewan, and the Province of Saskatchewan. The authors thank Ann Marshall for assistance in collecting TEM images at the Stanford Nano Shared Facilities.
Bone SE, Dynes JJ, Cliff J, & Bargar JR “Uranium(IV) adsorption by natural organic matter in sediments.” Proceedings of the National Academy of Sciences of the United States of America 114(4), 711-716. [10.1073/pnas.1611918114] (Reference link)
SC-23.1 Climate and Environmental Sciences Division, BER
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