Reduction of Uranium by Bacterial Products

The Old Rifle Mill Processing site at Rifle, CO, contains uranium contaminated groundwater. The presence of uranium is one of the major problems at Department of Energy legacy sites. There is an initiative for attenuation of uranium by the Department of Energy. Uranium undergoes oxidation/reduction reactions with the substances at the site. Uranium’s oxidation state determines its solubility and mobility in the aquifer. The oxidation reduction pathways at this site have mineralogical, microbial and geochemical components. Understanding the oxidation/reduction pathways of these components will allow for better predictions of the changes and movement of uranium. Sulfide [S2- ] and Ferrous [Fe2] ions are products of microbial activity. These ions can reduce uranium [U(VI)], but bicarbonate ions [HCO3-] in the aquifers slows down the reaction. However, we believe that organic matter in the environment enhances U(VI) reduction by Fe2+and S2- in the absence of microbes. To address this, U(VI) was mixed with Fe2+ or S2- in autoclaved biomass from Rifle and artificial groundwater for seven to eleven days. Aqueous samples from the vials will be analyzed for Uranium presence using ICP-MS [Inductively Coupled Plasma- Mass Spectrometry]. The biomass from each of the samples will be analyzed using XAS [X-Ray Absorption Spectroscopy] to determine the ionization state of Uranium. Based on the data, we can conclude that there is a significant decrease of the concentration of uranium from the reaction when the biomass was not sterilized. There were no differences within in the vials that had Fe2+ or S2-. The XAS data shows a mixture of U(IV) and U(VI) in the biomasss and more U(IV) in the biomass that was not sterilized. This suggests that the bacterial products alone is not completely responsible for the reduction of uranium

Understanding the Behavior of Sulfidic Colloids in the Presence of Metals

Riverton, Wyoming was host to a former uranium and vanadium ore processing plant, which operated from 1958 to 1963. The milling operations at the site contaminated the surface and shallow groundwater. The area became a Department of Energy (DOE) legacy site, where the U.S. Nuclear Regulatory Commission approved the DOE’s natural flushing compliance strategy. Up until the flooding in 2010, the natural flushing compliance strategy was going underway as expected. Sampling after the flood revealed a significant increase in contaminant concentration. New updated models need to be developed to help understand the situation at Riverton, for which this laboratory experiment is conducted. We want to understand the behavior of sulfidic colloids in the groundwater to the presence of metals. At the lab of Stanford’s Green Earth Sciences Building, samples of Riverton groundwater solution and 0.1 M NaCl water solution were place under different parameters. These parameters were: different Ferrihydrite and Sulfur ratios (0.05, 0.1, 0.5, and 2), the duration of agitation (3h, 9h, 24h, 48h, 3d, 5d, 10d, and 14d), and the metals used (Uranium, Zinc, Copper, Nickel, and Molybdenum). The generation of sulfidic colloids is closely monitored throughout the progression of the experiment, which will give insight to their behavior

Coupled Biogeochemical Processes Governing the Stability of Bacteriogenic Uraninite and Release of U(VI) in Heterogeneous Media: Molecular to Meter Scales

In-situ reductive biotransformation of subsurface U(VI) to U(IV) (as ?UO2?) has been proposed as a bioremediation method to immobilize uranium at contaminated DOE sites. The chemical stability of bacteriogenic ?UO2? is the seminal issue governing its success as an in-situ waste form in the subsurface. The structure and properties of chemically synthesized UO2+x have been investigated in great detail. It has been found to exhibit complex structural disorder, with nonstoichiometry being common, hence the designation ?UO2+x?, where 0 < x < 0.25. Little is known about the structures and properties of the important bacteriogenic analogs, which are believed to occur as nanoparticles in the environment. Chemically synthesized UO2+x exhibits an open fluorite structure and is known to accommodate significant doping of divalent cations. The extent to which bacteriogenic UO2+x incorporates common ground water cations (e.g., Ca2+) has not been investigated, and little is known about nonstoichiometry and structure defects in the bacteriogenic material. Particle size, nonstoichiometry, and doping may significantly alter the reactivity, and hence stability, of bacteriogenic UO2+x in the subsurface. The presence of associated sulfide minerals, and solid phase oxidants such as bacteriogenic Mn oxides may also affect the longevity of bacteriogenic UO2 in the subsurface

Environmental Remediation Sciences Program at the Stanford Synchrotron Radiation Laboratory

Synchrotron radiation (SR)-based techniques provide unique capabilities to address scientific issues underpinning environmental remediation science and have emerged as major research tools in this field. The high intensity of SR sources and x-ray photon-in/photon-out detection allow noninvasive in-situ analysis of dilute, hydrated, and chemically/structurally complex natural samples. SR x-rays can be focused to beams of micron and sub-micron dimension, which allows the study of microstructures, chemical microgradients, and microenvironments such as in biofilms, pore spaces, and around plant roots, that may control the transformation of contaminants in the environment. The utilization of SR techniques in environmental remediation sciences is often frustrated, however, by an ''activation energy barrier'', which is associated with the need to become familiar with an array of data acquisition and analysis techniques, a new technical vocabulary, beam lines, experimental instrumentation, and user facility administrative procedures. Many investigators find it challenging to become sufficiently expert in all of these areas or to maintain their training as techniques evolve. Another challenge is the dearth of facilities for hard x-ray micro-spectroscopy, particularly in the 15 to 23 KeV range, which includes x-ray absorption edges of the priority DOE contaminants Sr, U, Np, Pu, and Tc. Prior to the current program, there were only two (heavily oversubscribed) microprobe facilities in the U.S. that could fully address this energy range (one at each of APS and NSLS); none existed in the Western U.S., in spite of the relatively large number of DOE laboratories in this region

Correlation Between Luminescent Properties and Local Coordination Environment for Erbium Dopant in Yttrium Oxide Nanotubes

The local dopant coordination environment and its effect on the photoluminescent (PL) spectral features of erbium-doped yttrium oxide nanotubes (NTs) were probed by synchrotron-based x-ray diffraction (XRD), x-ray absorption near-edge spectroscopy (XANES), and extended x-ray absorption fine structure (EXAFS). XRD, XANES, and EXAFS data demonstrate that single phase solid solutions of Y (2-x) Erx O3 were formed at 0≤

Submitted to Geochimica et Cosmochimica Acta

FTIR and EXAFS spectroscopic measurements were performed on Pb(II)EDTA adsorbed on goethite as functions of pH (4-6), Pb(II)EDTA concentration (0.11 µM- 72 µM), and ionic strength (16 µM- 0.5M). FTIR measurements show no evidence for carboxylate-Fe(III) bonding or protonation of EDTA at Pb:EDTA = 1:1. Both FTIR and EXAFS measurements suggest that EDTA acts as a hexadentate ligand, with all four of its carboxylate and both amine groups bonded to Pb(II). No evidence was observed for inner-sphere Pb(II)-goethite bonding at Pb:EDTA = 1:1. Hence, the adsorbed complexes should have composition Pb(II)EDTA 2-. Since substantial uptake of PbEDTA(II) 2- occurred in the samples, we infer that Pb(II)EDTA 2- adsorbed as outer-sphere complexes and/or as complexes that lose part of their solvation shells and hydrogen bond directly to goethite surface sites. We propose the term “hydration-sphere ” for the latter type of complexes because they should occupy space in the primary hydration spheres of goethite surface functional groups, and to distinguish this mode of sorption from common structural definitions of inner- and outer-sphere complexes. The similarity of Pb(II) uptake isotherms to those of other divalent metal ions complexed by EDTA suggests that they too adsorb by these mechanisms. The lack of evidence for inner-sphere EDTA-Fe(III) bonding suggests that previously proposed metal-ligand- promoted dissolution mechanisms should be modified, specifically to account for the presence of outer-sphere precursor species

Characterizing Sediment from Riverton, WY

Riverton, Wyoming is home to the seventh-largest Native American Reservation by area and a former uranium processing facility. Milling activities at this site have left the sediments and groundwater with elevated concentrations of uranium that occasionally disrupt water quality to the 12,000 residents of the reservation. The floodplain becomes seasonally wet and sometimes flooded from snowmelt that can be amplified by El Niño events. As a result, the sediments of this area experience periodic droughts and floods. There is concern that a decreased water stage can have major impacts on the geochemical makeup of this ecosystem as 1) a decreased water volume may result in an increase of pollutant concentrations and 2) a decreased water stage can have a significant impact the redox cycling within the subsurface and affect major biogeochemical cycles. This study addresses the need to understand current subsurface elemental distributions in Riverton, Wyoming. Overall, the project explores the sediment characteristics of several Riverton cores in response to drought and flood conditions. Samples for this study were taken over the course of twelve months - at three different time points. This summer, 105 Riverton sediment samples from August 2016 (the final time point) were geochemically characterized and compared to samples from dry conditions (August 2015) prior to the second largest flood on record at this location. We expect the sediments collected prior to the flood to vary greatly from the sediments collected after the flood because the activity of microbial communities is affected by differences in the physical structure of the soil. This data, along with an analysis of the microbial communities present in these sediments, will help the Francis Lab understand what factors shape the distribution and diversity of microbial communities present in order to better understand the subsurface biogeochemistry at Riverton

Characterization of Organic Carbon in Sediments from Old Rifle, CO, a Former Uranium Mill

Characterization of sediments from Old Rifle, CO, a former uranium mill More than 34 million gallons (~129 million liters) of groundwater are contaminated with uranium at Old Rifle, Colorado – a former uranium-processing site that operated until 1958. The original Department of Energy strategy for remediation, involving natural flushing of U from the groundwater through mixing with surface water, has not been as successful as predicted. The uranium plume is replenished when insoluble U(IV) is oxidized to the more mobile U(VI). Relatively thin pockets of silt-, clay-, and organic-rich sediments contain reduced uranium, iron and sulfur and are referred to as naturally reduced zones (NRZs). There is a correlation between organic carbon (OC) and U concentrations; thus it can be inferred that OC is controlling U distribution and speciation. Sediment samples representing five different depths from the JB-02 well at Old Rifle were collected; two depths are above the NRZ, two are within the NRZ and one is below the NRZ. Sub-samples were then extracted using deionized water, NaCl and NaOH. The extractions were analyzed for non-purgeable organic carbon (NPOC) concentrations. Base extractions produced the highest concentrations of dissolved organic carbon (DOC) at all depths. Sediments within the NRZ produced more DOC than sediments above or below the NRZ. Further analysis by X-ray absorption spectroscopy (XAS) is expected to give key information on which organic functional groups are present within the sediments and their extractable carbon fractions, which will inform uranium management strategies. Additionally the amount of permanganate oxidizable carbon will be determined to further compare the carbon pools in and out of the NRZ

Using bromide tracer to measure uranium diffusivity in ground water sediments

More than 129 million liters of groundwater are contaminated with uranium at Old Rifle, Colorado – a former uranium-processing site that operated until 1958. The original Department of Energy (DOE) strategy for remediation, involving natural flushing of U from the groundwater through mixing with surface water, has not proven successful. Thin pockets of silt-, clay-, and organic-rich sediments referred to as naturally reduced zones (NRZs) act both as sinks and sources of U to the aquifer, contribute to plume persistence, and appear to be diffusion limited controlled. To better understand how the NRZs are diffusion limited controlled, a bromide tracer was used to measure uranium diffusivity at two depths from the JB-02 well at Old Rifle: one depth in the middle of the NRZ and one depth at the bottom edge of the NRZ. A NaBr reservoir was allowed to diffuse into the sediments for several days with reservoir samples collected twice a day and analyzed using inductively coupled plasma mass spectrometry (ICP-MS) for bromide concentrations. This data was then used to calculate net flux, effective diffusivity, and the tortuosity effect within the sediments, which will inform uranium management strategies not only at the Old Rifle site but potentially other DOE legacy sites including Riverton, Wyoming and Shiprock, New Mexico