U Release from NRZ sediments is inhibited by Transport and Geochemistry | Stanford Synchrotron Radiation Lightsource

Oxidative release of U(IV) from NRZs is slow because of low diffusivity, rapid consumption of oxidants by the sediments, and rapid U(VI) re-reduction

The Science

Floodplain aquifers undergo seasonal water table fluctuations that drive redox cycling of subsurface species, including contaminants such as uranium. We posited that seasonal intrusion of O₂ into anoxic sediments will stimulate production of NO₃^- followed by denitrification. This process has been linked to U(IV) oxidation, leading to release of U(VI) to the aquifer. Floodplains at legacy uranium contaminated DOE ore processing site contain abundant naturally reduced zones (NRZs) that have accumulated large masses of uranium as U(IV). If this model for linked nitrification-denitrification-uranium oxidation were found to be applicable, it would have major implications for uranium mobilization and site management. On the other hand, NRZs often exhibit fine-grained sediment texture and low permeability, which impose diffusion limitations on the exchange of solutes with the surrounding hydraulically conductive aquifer sediments. The net effect of this diffusion limitation is to retard the transport of oxidants into NRZs as well as the subsequent diffusion of U(VI) solutes out of NRZs. Nitrate and denitrification products exhibit relatively high solubility, permitting their diffusion into NRZs, whereas molecular oxygen is less soluble and is readily consumed through a multitude of abiotic (e.g. sulfide oxidation) and biotic reactions (respiration). Given the likely importance of diffusion and the contrasting reactivity of these two oxidants, it is unclear whether or not they can effectively oxidize and release uranium from NRZ sediments. To better understand the importance of, and mechanisms behind, oxidative uranium mobilization from NRZs, we examined the release of uranium from NRZ sediments through nitrate or oxygen exposure.

The Impact

In spite of seasonal increases in O₂ and NO₃^- within subsurface sediments, the oxidative release of U is negligible. These oxidants are rapidly consumed by other reduced species in anoxic the sediments before they reach U(IV). Moreover, bioavailable organic C supports re-reduction of U(VI) to U(IV) at rates faster than diffusion can transport U(VI) out of the reduced zone. These results emphasize the intimate linkage between transport and biogeochemical reactivity of anoxic sediments and indicate there is an ongoing need for robust reactive transport process representations of these coupled processes.

Summary

To test the relevance of this mobilization mechanism, we constructed model, diffusion-limited reactors containing NRZ sediments from Riverton, WY, that were exposed either to oxidant or to anoxic artificial groundwater for 85 days. Uranium (and other solute) efflux from the sediments were monitored in addition to U porewater concentrations. The sediment reaction cells were submerged in circulating 1 mM NO₃^-/artificial groundwater, O₂-saturated/artificial groundwater, or anoxic artificial groundwater solution for 85 days. Aqueous uranium was monitored daily in the circulating (“advection”) solution. Additionally, porewater concentrations of U, O₂, S(-II), NO₃^-, NO₂^- and NO were measured as a function of depth and time, along with the solid-phase concentration and oxidation state of U. Diffusion of O₂ and NO₃^- into the sediments caused short-term oxidative release of uranium out of the sediments relative to the anoxic control: an efflux of uranium (50 – 120 ng/cm² d) was sustained only for the first few days of the experiment, after which, the efflux dropped to approximately zero. In contrast, porewater concentrations of uranium remained elevated for the first month of the experiment, suggesting that uranium oxidation continued to occur, even after the uranium efflux dropped to zero. This suggests that uranium redox cycling within the sediments was faster than diffusional exchange with the advection solution. Nitrate was almost entirely consumed within the sediments and sulfate reduction occurred in both the anoxic control and 1 mM NO₃^- reactors. Additionally, O₂ was entirely consumed within the first 0.5 cm of the sediment tube. These results suggest that the oxidative release of uranium may be limited by the high reducing capacity of the sediments, which leads to rapid O₂ and NO₃^- consumption, especially in combination with relatively high abundance of organic carbon that stimulates microbial respiration.

Contacts (BER PM)

Amy Swain
DOE Office of Biological and Environmental Research, Climate and Environmental Sciences Division
Amy.Swain@science.doe.gov

(PI contact)

John Bargar
SLAC National Accelerator Laboratory, Stanford Synchrotron Radiation Lightsource
Bargar@slac.stanford.edu

Funding

Funding was provided by the DOE Office of Biological and Environmental Research, Subsurface Biogeochemistry Research (SBR) activity to the SLAC SFA program under contract DE-AC02-76SF00515 to SLAC. Use of SSRL is supported by the U.S. DOE, Office of Basic Energy Sciences.

Publications

Bone, S.E.; Cahill, M.R.; Jones, M.E.; Fendorf, S.; Davis, J.A.; Williams, K.H.; Bargar, J.R. Oxidative Uranium Release from Anoxic Sediments under Diffusion-Limited Conditions. Environmental Science and Technology 2017, 51, 11039−11047.