Reductive Precipitation Of Uranium Research Articles

Characterizing organic carbon dynamics during biostimulation of a uranium contaminated field site

Uranium contamination of groundwater remains a pressing problem at many former uranium mining and milling operations, such as the Rifle, Integrated Field Research Challenge (IFRC) site. Biostimulation of the subsurface with an organic carbon source such as acetate, followed by the microbially-induced reductive precipitation of uranium has been proposed as an effective remediation strategy. While uranium bioreduction has been studied in several field experiments, the transformation and fate of injected carbon remains poorly understood. This study evaluated the impact of added organic carbon on the long-term biogeochemical attenuation of uranium in the subsurface of a former mill tailings site. Fluorescence and ultraviolet–visible absorbance analyses were used together with dissolved organic carbon (DOC) measurements to track organic carbon dynamics during and post-biostimulation of the 2011 Rifle IFRC experiment. An electron mass balance was performed on well CD01 to account for any unidentified carbon sinks. Measured DOC values increased to 1.76 mM-C during biostimulation, and to 3.18 mM-C post-biostimulation over background DOC values of 0.3–0.4 mM-C. Elevated DOC levels persisted for 90 days after acetate injections ceased. The electron mass balance revealed that assumed electron acceptors would not account for the total amount of acetate consumed. Excitation–emission matrices showed an increase in signals associated with soluble microbial products, during biostimulation, which disappeared post-biostimulation despite an increase in total DOC. Specific ultraviolet absorbance analyses, indicated that DOC present post-biostimulation is less aromatic in nature, compared to background DOC. Our results suggest that microbes convert injected acetate into a form of solid phase organic matter that may be available to sustain iron reduction post-stimulation.

Uranium Biominerals Precipitated by an Environmental Isolate of Serratia under Anaerobic Conditions.

Stimulating the microbially-mediated precipitation of uranium biominerals may be used to treat groundwater contamination at nuclear sites. The majority of studies to date have focussed on the reductive precipitation of uranium as U(IV) by U(VI)- and Fe(III)-reducing bacteria such as Geobacter and Shewanella species, although other mechanisms of uranium removal from solution can occur, including the precipitation of uranyl phosphates via bacterial phosphatase activity. Here we present the results of uranium biomineralisation experiments using an isolate of Serratia obtained from a sediment sample representative of the Sellafield nuclear site, UK. When supplied with glycerol phosphate, this Serratia strain was able to precipitate 1 mM of soluble U(VI) as uranyl phosphate minerals from the autunite group, under anaerobic and fermentative conditions. Under phosphate-limited anaerobic conditions and with glycerol as the electron donor, non-growing Serratia cells could precipitate 0.5 mM of uranium supplied as soluble U(VI), via reduction to nano-crystalline U(IV) uraninite. Some evidence for the reduction of solid phase uranyl(VI) phosphate was also observed. This study highlights the potential for Serratia and related species to play a role in the bioremediation of uranium contamination, via a range of different metabolic pathways, dependent on culturing or in situ conditions.

Uranium isotopes fingerprint biotic reduction

Knowledge of paleo-redox conditions in the Earth's history provides a window into events that shaped the evolution of life on our planet. The role of microbial activity in paleo-redox processes remains unexplored due to the inability to discriminate biotic from abiotic redox transformations in the rock record. The ability to deconvolute these two processes would provide a means to identify environmental niches in which microbial activity was prevalent at a specific time in paleo-history and to correlate specific biogeochemical events with the corresponding microbial metabolism. Here, we demonstrate that the isotopic signature associated with microbial reduction of hexavalent uranium (U), i.e., the accumulation of the heavy isotope in the U(IV) phase, is readily distinguishable from that generated by abiotic uranium reduction in laboratory experiments. Thus, isotope signatures preserved in the geologic record through the reductive precipitation of uranium may provide the sought-after tool to probe for biotic processes. Because uranium is a common element in the Earth's crust and a wide variety of metabolic groups of microorganisms catalyze the biological reduction of U(VI), this tool is applicable to a multiplicity of geological epochs and terrestrial environments. The findings of this study indicate that biological activity contributed to the formation of many authigenic U deposits, including sandstone U deposits of various ages, as well as modern, Cretaceous, and Archean black shales. Additionally, engineered bioremediation activities also exhibit a biotic signature, suggesting that, although multiple pathways may be involved in the reduction, direct enzymatic reduction contributes substantially to the immobilization of uranium.

Influence of some essential environmental factors on the reductive precipitation of uranium by sulfate reducing bacteria

Influence of some essential environmental factors on the reductive precipitation of uranium by sulfate reducing bacteria

Influence of some essential environmental factors on the reductive precipitation of uranium by sulfate reducing bacteria

Influence of some essential environmental factors on the reductive precipitation of uranium by sulfate reducing bacteria

Reductive precipitation of uranium by Desulfovibrio desulfuricans: evaluation of cocontaminant effects and selective removal

The sulfate reducing bacterium, Desulfovibrio desulfuricans can initiate uranium precipitation from solution via direct enzymatic reduction. Additionally, separation of heavy metals from solution can occur via indirect sulfide-mediated precipitation. This study was conducted to evaluate the influence of anions (sulfate, nitrate), heavy metals (zinc, nickel and copper) and organics (acetate, malonate, oxalate and citrate) on the enzymatic reduction of U(VI) by this bacterium. Furthermore, methods were evaluated to selectively precipitate uranium or heavy metals from test solutions. Selective precipitation can significantly lower disposal costs by reducing the volume of mixed-waste sludge produced during treatment. Results indicated that sulfate/nitrate concentrations up to 5000 mg/l did not appreciably interfere with U(VI) reduction, however, anion levels greater than 10,000 mg/l significantly slowed the rate of U(VI) reduction. U(VI) was readily reduced by the bacterium when 10 mg/l of Zn or Ni was present, but Cu inhibited uranium reduction. U(VI) was reduced rapidly in the presence of a monodentate organic ligand (acetate) whereas reduction was slower in the presence of multidentate ligands. Initial results from selective precipitation experiments indicated two potential treatment approaches for isolating either uranium or the test heavy metals using D. desulfuricans. The first method involved free energy differences for U(VI) and sulfate reduction, while the second method involved complexation of reduced uranium by a chelator during metal–sulfide precipitation.

Effects of aqueous complexation on reductive precipitation of uranium by Shewanella putrefaciens

Effects of aqueous complexation on reductive precipitation of uranium by Shewanella putrefaciens

Reductive Precipitation of Uranium(VI) by Zero-Valent Iron

This study was undertaken to determine the effectiveness of zero-valent iron (Fe0) and several adsorbent materials in removing uranium (U) from contaminated groundwater and to investigate the rates and mechanisms that are involved in the reactions. Fe0 filings were used as reductants, and the adsorbents included peat materials, iron oxides, and a carbon-based sorbent (Cercona Bone-Char). Results indicate that Fe0 filings are much more effective than the adsorbents in removing uranyl (UO22+) from the aqueous solution. Nearly 100% of U was removed through reactions with Fe0 at an initial concentration up to 76 mM (or 18 000 mg of U/L). Results from the batch adsorption and desorption and from spectroscopic studies indicate that reductive precipitation of U on Fe0 is the major reaction pathway. Only a small percentage (<4%) of UO22+ appeared to be adsorbed on the corrosion products of Fe0 and could be desorbed by leaching with a carbonate solution. The study also showed that the reduced U(IV) species on Fe0 surfaces could be reoxidized and potentially remobilized when the reduced system becomes more oxidized. Results of this research support the application of the permeable reactive barrier technology using Fe0 as a reactive media to intercept U and other groundwater contaminants migrating to the tributaries of Bear Creek at the U.S. Department of Energy's Y-12 Plant located in Oak Ridge, TN.