Bio-precipitation of uranium by two bacterial isolates recovered from extreme environments as estimated by potentiometric titration, TEM and X-ray absorption spectroscopic analyses

This is the post-print version of the final paper published in Journal of Hazardous Materials. The published article is available from the link below. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. Copyright @ 2011 Elsevier B.V.This work describes the mechanisms of uranium biomineralization at acidic conditions by Bacillus sphaericus JG-7B and Sphingomonas sp. S15-S1 both recovered from extreme environments. The U–bacterial interaction experiments were performed at low pH values (2.0–4.5) where the uranium aqueous speciation is dominated by highly mobile uranyl ions. X-ray absorption spectroscopy (XAS) showed that the cells of the studied strains precipitated uranium at pH 3.0 and 4.5 as a uranium phosphate mineral phase belonging to the meta-autunite group. Transmission electron microscopic (TEM) analyses showed strain-specific localization of the uranium precipitates. In the case of B. sphaericus JG-7B, the U(VI) precipitate was bound to the cell wall. Whereas for Sphingomonas sp. S15-S1, the U(VI) precipitates were observed both on the cell surface and intracellularly. The observed U(VI) biomineralization was associated with the activity of indigenous acid phosphatase detected at these pH values in the absence of an organic phosphate substrate. The biomineralization of uranium was not observed at pH 2.0, and U(VI) formed complexes with organophosphate ligands from the cells. This study increases the number of bacterial strains that have been demonstrated to precipitate uranium phosphates at acidic conditions via the activity of acid phosphatase

The effects of bicarbonate and mineral surfaces on uranium immobilization under anaerobic conditions

For four decades, from 1940 through 1980, the U.S. Department of Energy (DoE) extensively mined and processed uranium at various sites. As a result, widespread uranium contamination exists in subsurface sediments and aquifers. In subsurface environments, uranium primarily exists as U(VI) or U(IV), oxidized and reduced species, respectively. U(VI) is highly soluble and toxic, U(IV), while relatively toxic, is insoluble which greatly reduces its exposure pathways. We seek to examine the role of ferric iron on U(VI) reduction by adsorbing U(VI) onto ferric and non-ferric mineral surfaces in the presence of a reductant. Further, we seek to understand the role that NaHCO3, a natural groundwater buffer, has in the reductive geochemical transformations of U(VI) adsorbed on ferric and non-ferric mineral surfaces. Bench top studies were performed using 100 uM U(VI) and the reductant AHQDS, in the presence and absence of Fe-Gel (amorphous ferric oxyhydroxide) and gamma-Al2O3. In the presence of a HEPES buffer at pH 8, results demonstrate direct homogeneous reduction in several hours in the absence of Fe-Gel or gamma-Al2O3, and reduction within a 48-hour period in the presence Fe-Gel or gamma-Al2O3. While adsorbed to both ferric and non-ferric mineral surfaces, U(VI) reduction is inhibited. U(VI) reduction in the presence of NaHCO3 buffer also inhibits U(VI) reduction

CO2 conversion to phenyl isocyanates by uranium(vi) bis(imido) complexes.

Uranium(vi) trans-bis(imido) complexes [U(κ4-{(tBu2ArO)2Me2-cyclam})(NPh)(NPhR)] react with CO2 to eliminate phenyl isocyanates and afford uranium(vi) trans-[O[double bond, length as m-dash]U[double bond, length as m-dash]NR]2+ complexes, including [U(κ4-{(tBu2ArO)2Me2-cyclam})(NPh)(O)] that was crystallographically characterized. DFT studies indicate that the reaction proceeds by endergonic formation of a cycloaddition intermediate; the secondary reaction to form a dioxo uranyl complex is both thermodynamically and kinetically hindered

Electrochemical Redox Processes of Uranium In Aqueous Solutions of Acetylacetone

Electrochemical redox processes of uranium (VI) and uranium (V) in aqueous solutions of acetylacetone have been studied by means of various electrochemical techniques. The presence of acetylacetone accelerates the rate of disproportionation of uranium (V), even under conditions such that uranium (VI) is not in the form of the acetylacetonato complex. The corresponding rate constants of disproportionation were determined. The influence of the acetylacetonato ion concentration on the potential and on the rate of the uranium (VI) - uranium (V) electron transfer was investigated by means of cyclic voltammetry and squa,re-wave polarography. Both reduction and oxidation of uranium (V) were investigated by using the Kalousek commutator technique. A mechanism for those processes is proposed. It was proved by electrocapillary measurements that besides the adsorption of acetylacetone, adsorption of several uranium acetylacetonato species plays an important role in the overall mechanism. Taking into account the experimental results, the redox processes of uranium (VI) and uranium (V) in the presence of acetylacetone can be explained in terms of an ECE mechanism

Importance of c-Type cytochromes for U(VI) reduction by Geobacter sulfurreducens

BACKGROUND: In order to study the mechanism of U(VI) reduction, the effect of deleting c-type cytochrome genes on the capacity of Geobacter sulfurreducens to reduce U(VI) with acetate serving as the electron donor was investigated. RESULTS: The ability of several c-type cytochrome deficient mutants to reduce U(VI) was lower than that of the wild type strain. Elimination of two confirmed outer membrane cytochromes and two putative outer membrane cytochromes significantly decreased (ca. 50–60%) the ability of G. sulfurreducens to reduce U(VI). Involvement in U(VI) reduction did not appear to be a general property of outer membrane cytochromes, as elimination of two other confirmed outer membrane cytochromes, OmcB and OmcC, had very little impact on U(VI) reduction. Among the periplasmic cytochromes, only MacA, proposed to transfer electrons from the inner membrane to the periplasm, appeared to play a significant role in U(VI) reduction. A subpopulation of both wild type and U(VI) reduction-impaired cells, 24–30%, accumulated amorphous uranium in the periplasm. Comparison of uranium-accumulating cells demonstrated a similar amount of periplasmic uranium accumulation in U(VI) reduction-impaired and wild type G. sulfurreducens. Assessment of the ability of the various suspensions to reduce Fe(III) revealed no correlation between the impact of cytochrome deletion on U(VI) reduction and reduction of Fe(III) hydroxide and chelated Fe(III). CONCLUSION: This study indicates that c-type cytochromes are involved in U(VI) reduction by Geobacter sulfurreducens. The data provide new evidence for extracellular uranium reduction by G. sulfurreducens but do not rule out the possibility of periplasmic uranium reduction. Occurrence of U(VI) reduction at the cell surface is supported by the significant impact of elimination of outer membrane cytochromes on U(VI) reduction and the lack of correlation between periplasmic uranium accumulation and the capacity for uranium reduction. Periplasmic uranium accumulation may reflect the ability of uranium to penetrate the outer membrane rather than the occurrence of enzymatic U(VI) reduction. Elimination of cytochromes rarely had a similar impact on both Fe(III) and U(VI) reduction, suggesting that there are differences in the routes of electron transfer to U(VI) and Fe(III). Further studies are required to clarify the pathways leading to U(VI) reduction in G. sulfurreducens

Dominant Mechanisms of Uranium(VI)‒Phosphate Interactions in Subsurface Environments: An in situ remediation perspective

Anthropogenic activities associated with the production of nuclear materials have resulted in uranium contaminated soil and groundwater. The carcinogenic and toxic effects of uranium contamination pose a significant risk to the environment and human health. Phosphate addition to uranium-contaminated subsurface environments has been proposed as a strategy for in situ remediation. Addition of phosphate amendments can result in uranium sequestration in its oxidized +VI state without sustaining reducing conditions as is needed for in situ immobilization via chemical or biological reduction of U(VI) to less soluble U(IV) species. Phosphate addition can be used as a stand-alone process or as a complementary process to bioremediation-based methods, especially for sites with naturally oxic conditions. Although recent studies have reported phosphate-induced precipitation of U(VI)-phosphates in laboratory and field-scale tests, the fundamental mechanisms controlling U(VI) immobilization are not well known. Hence understanding the mechanisms at the microscopic and molecular levels is imperative to successfully designing and implementing phosphate-based in situ uranium immobilization. Interactions with phosphate can result in uranium immobilization through various processes. This study investigated the dominant mechanisms of U(VI)-phosphate reactions using an integrated approach of aqueous phase and solid phase characterization techniques. Batch experiments were performed to study the effect of pH and co-solutes (dissolved inorganic carbon (DIC), Na+ and Ca2+) on the products and solubility of uranium(VI) precipitated with phosphate. The results suggested that in the absence of co-solute cations, chernikovite [H3O(UO2)(PO4)*3H2O] precipitated despite uranyl orthophosphate [(UO2)3(PO4)2*4H2O] being thermodynamically more favorable under certain conditions. The presence of Na+ as a co-solute led to the precipitation of sodium autunite [Na2(UO2)2(PO4)2], and the dissolved U(VI) concentrations were generally in agreement with equilibrium predictions of sodium autunite solubility. In the calcium-containing systems, the observed concentrations were below the predicted solubility of autunite [Ca(UO2)2(PO4)2]. Consequently, specific batch studies were conducted to investigate the dependence of U(VI) uptake mechanisms on the starting forms of calcium and phosphate at concentrations relevant to field sites. Depending on the experimental conditions, uranium uptake occurred through adsorption on calcium-phosphate solids, precipitation of autunite, or incorporation into a calcium-phosphate solid. Extended X-ray absorption fine structure (EXAFS) spectroscopy analysis using structural model fittings and linear combination fitting allowed quantification of the contribution of each uranium uptake mechanism mentioned above. Following the batch experiments with simple systems, the effect of phosphate amendment on uranium immobilization was evaluated for sediments obtained from a field site in Rifle, Colorado using batch sorption studies and column experiments. Batch sorption studies showed that phosphate addition increased the U(VI) adsorption, however the net uranium uptake was limited due to the dominance of the aqueous speciation by Ca-U(VI)-carbonate complexes. Column experiments were performed under conditions that simulated the subsurface environment at the Rifle site. Remobilization experiments showed increased retention of uranium when phosphate was present in uranium-free influent. The response of dissolved uranium concentrations to stopped-flow events and the comparison of experimental data with a simple reactive transport model indicated that uranium transport was controlled by non-equilibrium processes. Intraparticle diffusion is thought to be acting as the rate-limiting process. Sequential extractions and laser induced fluorescence spectroscopy (LIFS) analysis indicated that adsorption was the dominant mode of uranium immobilization. When uranium and phosphate were added concurrently to columns packed with sediments, significant uptake of uranium continued as long as phosphate was present in the influent. Even when phosphate was removed from the influent, the columns retained significant amounts (~ 67 %) of the accumulated uranium. Sequential extractions showed that the uranium accumulated transformed into less easily extractable (i.e., more immobile) species with the relative amounts of accumulated uranium extracted in the acetic acid and hot acid digestion step being highest for the column that was treated with phosphate for the longest duration. The uranium retained in the sediments after the phosphate was removed from the influent was primarily in a form that could be extracted with acetic acid and ammonium acetate. The extraction results, aqueous phase analysis and LIFS analysis showed that uranium uptake occurred through multiple processes. For select conditions, EXAFS analysis was used to quantify the contribution of uranium uptake which confirmed that uranium uptake occurred through a combination of precipitation and adsorption. The information gained from this research project improved our understanding of U(VI)-phosphate reactions that can be used to identify and manipulate the conditions that lead to the greatest decreases in U(VI) mobility. The results illustrate that precipitation of uranyl-phosphates is not the only means of in situ uranium remediation and that a wide range of uranium immobilization mechanisms can control uranium mobility following phosphate addition. Although phosphate addition led to significant retardation of uranium release and also resulted in increased net uptake of uranium for conditions of the Rifle site, phosphate amendments could be more beneficial at sites with lower pH and dissolved inorganic carbon concentrations

Electrochemical Reduction of Uranium (VI) to Uranium (IV) in Carbonate Solutions

The purpose of this work was to investigate the possibility of preparation of uranium dioxide by electrochemical reduction of uranium(VI) from alkali carbonate solutions. There are many interesting features in deciding whether or not the electrochemical process is applicable to certain conditions of solution equilibria in the uranium-carbonate system. The study ·of the reduction of uranium(VI) from carbonate solutions has been undertaken to determine the reduction potential-current density relationship, the current efficiency, and the O/U ratio in the precipitated uranium dioxide, for which only scarce data are available in the literature

Влияние комплексообразователей на процессы сорбционной очистки вод, содержащих уран

Исследовано влияние комплексообразователей: динатриевых солей этилендиаминтетрауксусной кислоты (ЭДТА) и нитрилотриуксусной кислоты (НТА),уксусной кислоты (НАс), фульвокислот (ФК), карбоната натрия ( Na2CO3 ) на сорбцию U(VI) монтмориллонитом. Установлен сложный характер зависимости величин сорбции U(VI) от рН в присутствии комплексообразователей, что указывает на определяющую роль форм нахождения урана в очищаемыхводах. По величине влияния на процесс сорбции U(VI) монтмориллонитом исследованные комплексообразователи можно расположить в ряд: ЭДТА >НТА > Na2CO3 > HAc > ФК.Досліджено вплив комплексоутворювачів: динатрієвих солей етилендиамінтетраоцтової кислоти (ЕДТА) та нітрилотриоцтовоїкислоти (НТА), оцтової кислоти (НАс), фульвокислот (ФК), карбонату натрію (Na2CO3) на сорбцію U(VI) монтморилонітом. Встановлено складний характер залежності величин сорбції U (VI) від рН в присутності комплексоутворювачів, що вказує на визначальну роль форм знаходження урану y водах, що підлягають очистці. За величиною впливу на процес сорбції U(VI) монтморилонітом досліджені комплексоутворювачіможно розташувати в ряд: ЕДТА > НТА > Na2CO3 > HAc > ФК.Effect of complexing agents: ethylendiaminetetraacetic acid (EDTA) and nitrilotriacetic acid (NTA) disodium salts, acetic acid (HAc), fulvic acids (FA), sodium carbonate (Na2CO3) on the uranium (VI) sorption by montmorillonite was investigated. Complex character of dependence of uranium (VI) sorption values vs. pH in presence of complexing agents was established indicating important role of uranium species in waters under purification. It is possible to arrange complexing agents investigated according to magnitude of their effect on the uranium (VI) sorption by montmorillonite: EDTA > NTA > Na2CO3 > HAc > FA