Speciation-dependent kinetics of uranium(VI) bioreduction

The kinetics of uranium(VI) reduction by Shewanella oneidensis strain MR-1 was studied for varied pH and concentrations of dissolved inorganic carbon (DIC) and calcium. These are key variables affecting U(VI) speciation in aqueous systems. For all conditions studied, a nearly log-linear decrease of [U(VI)] suggested pseudo-first-order kinetics with respect to U(VI). The reduction rate constants (k) decreased with increasing DIC and calcium concentration, and were sensitive to pH. A positive correlation was found between k and the logarithm of the total concentration of U(VI)-hydroxyl and U(VI)-organic complexes. Linear correlations of the rate constant with the redox potential (E-H) of U(VI) reduction and the associated Gibbs free energy of reaction (Delta G(r)) were found for both Ca-free and Ca-containing systems. Both E-H and Delta G(r) are strong functions of aqueous U(VI) speciation. Because the range in Delta G(r) among the experimental conditions was small, the differences in k are more likely to be due to differences in EH or to differences in individual rate constants of U(VI) species. Calculation of conditional reduction rate constants for the major groups of U(VI) complexes revealed highest constants for the combined groups of U(VI)-hydroxyl and U(VI)-organic species, lower rate constants for the U(VI)-carbonate group, and much lower constants for the Ca-U(VI)-carbonate group. Mechanistic explanations for these findings are discussed

Relative Reactivity of Biogenic and Chemogenic Uraninite and Biogenic Non-Crystalline U(IV)

Aqueous chemical extractions and X-ray absorption spectroscopy (XAS) analyses were conducted to investigate the reactivity of chemogenic uraninite, nanoparticulate biogenic uraninite, and biogenic monomeric U(IV) species. The analyses were conducted in systems containing a total U concentration that ranged from 1.48 to 2.10 mM. Less than 0.02% of the total U was released to solution in extractions that targeted water-soluble and ion exchangeable fractions. Less than 5% of the total U was solubilized via complexation with a 0.1 M solution of NaF. Greater than 90% of the total U was extracted from biogenic uraninite and monomeric U(IV) after 6 h of reaction in an oxidizing solution of 50 mM K2S2O8. Additional oxidation experiments with lower concentrations (2 mM and 10 mM) of K2S2O8 and 8.2 L-1 dissolved oxygen suggested that monomeric U(IV) species are more labile than biogenic uraninite; chemogenic uraninite was much less susceptible to oxidation than either form of biogenic U(IV). These results suggest that noncrystalline forms of U(IV) may be more labile than uraninite in subsurface environments. This work helps fill critical gaps in our understanding of the behavior of solid-associated U(IV) species in bioremediated sites and natural uranium ore deposits

Comparative dissolution kinetics of biogenic and chemogenic uraninite under oxidizing conditions in the presence of carbonate

The long-term stability of biogenic uraninite with respect to oxidative dissolution is pivotal to the success of in situ bioreduction strategies for the subsurface remediation of uranium legacies. Batch and flow-through dissolution experiments were conducted along with spectroscopic analyses to compare biogenic uraninite nanoparticles obtained from Shewanella oneidensis MR-1 and chemogenic UO2.00 with respect to their equilibrium solubility, dissolution mechanisms, and dissolution kinetics in water of varied oxygen and carbonate concentrations. Both materials exhibited a similar intrinsic solubility of similar to 10(-8) M under reducing conditions. The two materials had comparable dissolution rates under anoxic as well as oxidizing conditions, consistent with structural bulk homology of biogenic and stoichiometric uraninite. Carbonate reversibly promoted uraninite dissolution under both moderately oxidizing and reducing conditions, and the biogenic material yielded higher surface area-normalized dissolution rates than the chemogenic. This difference is in accordance with the higher proportion of U(V) detected on the biogenic uraninite surface by means of X-ray photoelectron spectroscopy. Reasonable sources of a stable U(V)-bearing intermediate phase are discussed. The observed increase of the dissolution rates can be explained by carbonate complexation of U(V) facilitating the detachment of U(V) from the uraninite surface. The fraction of surface-associated U(VI) increased with dissolved oxygen concentration. Simultaneously, X-ray absorption spectra showed conversion of the bulk from UO2.0 to UO2+x. In equilibrium with air, combined spectroscopic results support the formation of a near-surface layer of approximate composition UO2.25 (U4O9) coated by an outer layer of U(VI). This result is in accordance with flow-through dissolution experiments that indicate control of the dissolution rate of surface-oxidized uraninite by the solubility of metaschoepite under the tested conditions. Although U(V) has been observed in electrochemical studies on the dissolution of spent nuclear fuel, this is the first investigation that demonstrates the formation of a stable U(V) intermediate phase on the surface of submicron-sized uraninite particles suspended in aqueous solutions. (C) 2009 Elsevier Ltd. All rights reserved

Speciation and reactivity of uranium products formed during in situ bioremediation in a shallow alluvial aquifer

In this study, we report the results of in situ U(VI) bioreduction experiments at the Integrated Field Research Challenge site in Rifle, Colorado, USA. Columns filled with sediments were deployed into a groundwater well at the site and, after a period of conditioning with groundwater, were amended with a mixture of groundwater, soluble U(VI), and acetate to stimulate the growth of indigenous microorganisms. Individual reactors were collected as various redox regimes in the column sediments were achieved: (i) during iron reduction, (ii) just after the onset of sulfate reduction, and (iii) later into sulfate reduction. The speciation of U retained in the sediments was studied using X-ray absorption spectroscopy, electron microscopy and chemical extractions. Circa 90% of the total uranium was reduced to U(IV) in each reactor. Noncrystalline U(IV) comprised about two-thirds of the U(IV) pool, across large changes in microbial community structure, redox regime, total uranium accumulation, and reaction time. A significant body of recent research has demonstrated that noncrystalline U(IV) species are more suceptible to remobilization and reoxiation than crystalline U(IV) phases such as uraninite. Our results highlight the importance of considering noncrystalline U(IV) formation across a wide range of aquifer parameters when designing in situ remediation plans

Uranium redox transition pathways in acetate-amended sediments

Redox transitions of uranium [from U(VI) to U(IV)] in low-temperature sediments govern the mobility of uranium in the environment and the accumulation of uranium in ore bodies, and inform our understanding of Earth's geochemical history. The molecular-scale mechanistic pathways of these transitions determine the U(IV) products formed, thus influencing uranium isotope fractionation, reoxidation, and transport in sediments. Studies that improve our understanding of these pathways have the potential to substantially advance process understanding across a number of earth sciences disciplines. Detailed mechanistic information regarding uranium redox transitions in field sediments is largely nonexistent, owing to the difficulty of directly observing molecular-scale processes in the subsurface and the compositional/physical complexity of subsurface systems. Here, we present results from an in situ study of uranium redox transitions occurring in aquifer sediments under sulfate-reducing conditions. Based on molecular-scale spectroscopic, pore-scale geochemical, and macroscale aqueous evidence, we propose a biotic-abiotic transition pathway in which biomass-hosted mackinawite (FeS) is an electron source to reduce U(VI) to U(IV), which subsequently reacts with biomass to produce monomeric U(IV) species. A species resembling nanoscale uraninite is also present, implying the operation of at least two redox transition pathways. The presence of multiple pathways in low-temperature sediments unifies apparently contrasting prior observations and helps to explain sustained uranium reduction under disparate biogeochemical conditions. These findings have direct implications for our understanding of uranium bioremediation, ore formation, and global geochemical processes

Uranium speciation and stability after reductive immobilization in sediments

It has generally been assumed that the bioreduction of hexavalent uranium in groundwater systems will result in the precipitation of immobile uraninite (UO2). In order to explore the form and stability of uranium immobilized under these conditions, we introduced lactate (15 mM for 3 months) into flow-through columns containing sediments derived from a former uranium-processing site at Old Rifle, CO. This resulted in metal-reducing conditions as evidenced by concurrent uranium uptake and iron release. Despite initial augmentation with Shewanella oneidensis, bacteria belonging to the phylum Firmicutes dominated the biostimulated columns. The immobilization of uranium (similar to 1 mmol U per kg sediment) enabled analysis by Xray absorption spectroscopy (XAS). Tetravalent uranium associated with these sediments did not have spectroscopic signatures representative of U-U shells or crystalline UO2. Analysis by microfocused XAS revealed concentrated micrometer regions of solid U(IV) that had spectroscopic signatures consistent with bulk analyses and a poor proximal correlation (mu m scale resolution) between U and Fe. A plausible explanation, supported by biogeochemical conditions and spectral interpretations, is uranium association with phosphoryl moieties found in biomass; hence implicating direct enzymatic uranium reduction. After the immobilization phase, two months of in situ exposure to oxic influent did not result in substantial uranium remobilization. Ex situ flow-through experiments demonstrated more rapid uranium mobilization than observed in column oxidation studies and indicated that sediment-associated U(IV) is more mobile than biogenic UO2. This work suggests that in situ uranium bioimmobilization studies and subsurface modeling parameters should be expanded to account for non-uraninite U(IV) species associated with biomass. (C) 2011 Elsevier Ltd. All rights reserved

Time Scales for Sorption−Desorption and Surface Precipitation of Uranyl on Goethite

The sorption of uranium on mineral surfaces can significantly influence the fate and transport of uranium contamination in soils and groundwater. The rates of uranium adsorption and desorption on a synthetic goethite have been evaluated in batch experiments conducted at constant pH of 6 and ionic strength of 0.1 M. Adsorption and desorption reactions following the perturbation of initial states were complete within minutes to hours. Surface−solution exchange rates as measured by an isotope exchange method occur on an even shorter time scale. Although the uranium desorption rate was unaffected by the aging of uranium−goethite suspensions, the aging process appears to remove a portion of adsorbed uranium from a readily exchangeable pool. The distinction between sorption control and precipitation control of the dissolved uranium concentration was also investigated. In heterogeneous nucleation experiments, the dissolved uranium concentration was ultimately controlled by the solubility of a precipitated uranyl oxide hydrate. The X-ray diffraction pattern of the precipitate is characteristic of the mineral schoepite. Precipitation is kinetically hindered at low degrees of supersaturation. In one experiment, metastable sorption controlled dissolved uranium concentrations in excess of the solubility limit for more than 30 d