Reduction of uranium(VI) by soluble iron(II) conforms with thermodynamic predictions

Uranium Repartitioning during Microbial Driven Reductive Transformation of U(VI)-Sorbed Schwertmannite and Jarosite

This study exposes U(VI)-sorbed schwertmannite and jarosite to biotic reductive incubations under field-relevant conditions and examines the changes in aqueous and solid-phase speciation of U, Fe, and S as well as associated microbial communities over 180 days. The chemical, X-ray absorption spectroscopy, X-ray diffraction, and microscopic data demonstrated that the U(VI)-sorbed schwertmannite underwent a rapid reductive dissolution and solid-phase transformation to goethite, during which the surface-sorbed U(VI) was partly reduced and mostly repartitioned to monomeric U(VI)/U(IV) complexes by carboxyl and phosphoryl ligands on biomass or organic substances. Furthermore, the microbial data suggest that these processes were likely driven by the consecutive developments of fermentative and sulfate- and iron- reducing microbial communities. In contrast, the U(VI)-sorbed jarosite only stimulated the growth of some fermentative communities and underwent very limited reductive dissolution and thus, remaining in its initial state with no detectable mineralogical transformation and solid-phase U reduction/repartitioning. Accordingly, these two biotic incubations did not induce increased risk of U reliberation to the aqueous phase. These findings have important implications for understanding the interactions of schwertmannite/jarosite with microbial communities and colinked behavior and fate of U following the establishment of reducing conditions in various acidic and U-rich settings.

Fe-MMT/WO3 composites for chemical and photocatalysis synergistic reduction of uranium (VI)

The preparation of Fe-MMT/WO3 composites by the hydrothermal method has been explored in this study for the construction of a chemical and photocatalytic catalyst for the reduction of U (VI). This research found that the visible light absorption and reduction potential of the Fe-MMT/WO3 composites were relatively superior compared to Fe-MMT and WO3 alone. Based on an evaluation of the performance of the Fe-MMT/WO3 composites under visible light irradiation, it was discovered that they had greater uranium extraction capacity, where the maximum extraction capacity of U (VI) was determined to be 1862.69 mg g-1, with removal efficiency reaching 93.32%. To investigate the electron transfer and U (VI) to U (IV) reduction mechanisms after the composite, XPS and DFT calculations were conducted. Results showed that Fe (II) is converted to a higher state Fe (III) and WO3 produce photoelectrons which together reduce U (VI) to U (IV). Moreover, the photoelectrons partially transferred to Fe-MMT with low reduction potential to reduce Fe (III) to Fe (II), allowing iron cycling during uranium extraction to be achieved.

Enhanced sequestration of uranium by coexisted lead and organic matter during ferrihydrite transformation

The dynamic reactions of uranium (U) with iron (Fe) minerals change its behaviors in soil environment, however, how the coexisted constituents in soil affect U sequestration and release on Fe minerals during the transformation remains unclear. Herein, coupled effects of lead (Pb) and dissolved organic matter (DOM) on U speciation and release kinetics during the catalytic transformations of ferrihydrite (Fh) by Fe(II) were investigated. Our results revealed that the coexistence of Pb and DOM significantly reduced U release and increased the immobilization of U during Fh transformation, which were attributed to the enhanced inhibition of Fh transformation, the declined release of DOM and the increased U(VI) reduction. Specifically, the presence of Pb increased the coprecipitation of condensed aromatics, polyphenols and phenols, and these molecules were preferentially maintained by Fe (oxyhydr)oxides. The sequestrated polyphenols and phenols could further facilitate U(VI) reduction to U(IV). Additionally, a higher Pb content in coprecipitates caused a slower U release, especially when DOM was present. Compared with Pb, the concentrations of the released U were significantly lower during the transformation. Our results contribute to predicting U sequestration and remediating U-contaminated soils.

Kinetic reactive transport explains distinct subsurface deposition patterns of pollutants in sediments. The case of the Sellafield-derived 236U, 137Cs and 239,240Pu in the Esk Estuary, UK

The kinetics of the uptake of pollutants by solids in sediments interacts with transitional eddy diffusivity in the pore fluid, leading to different depth-distribution patterns. This work aims to gain insights into the still poorly understood behaviour in the marine environment of the anthropogenic ²³⁶U, a recently postulated tracer of water masses. It is hypothesized that the transition from mobile U(VI) to highly particle-reactive U(IV) in the anoxic zone of the sediment produces a subsurface deposition pattern. A novel numerical model for kinetic reactive transport in sediments, which merges diagenetic processes for transport and box models for the uptake, is used for concept demonstration. It is applied to synthetic environments with high eddy diffusivity to obtain the singular depth-distribution patterns of pulsed inputs of tracers that mimic the anthropogenic ^239,240Pu, ¹³⁷Cs, and ²³⁶U. While the first is retained in the upper cm, the second shows an exponential penetration pattern over few cm, and ²³⁶U is deposited with a Gaussian-like pattern centred below the transition to the anoxic zone. These patterns are then merged into a diagenetic model to compute the depth distribution at decadal or centennial scales of dissolved and particle-bound inputs of these radiotracers. It is successfully applied to a real case using literature data for a sediment core from the Esk Estuary, UK, affected by radioactive releases from the Sellafield nuclear reprocessing plant. This work provides insight into until now poorly understood field data and provides a novel view of broad implications in the study of the behaviour of pollutants in surficial aquatic sediments.

Molecular Mechanisms Underlying Bacterial Uranium Resistance

Environmental uranium pollution due to industries producing naturally occurring radioactive material or nuclear accidents and releases is a global concern. Uranium is hazardous for ecosystems as well as for humans when accumulated through the food chain, through contaminated groundwater and potable water sources, or through inhalation. In particular, uranium pollution pressures microbial communities, which are essential for healthy ecosystems. In turn, microorganisms can influence the mobility and toxicity of uranium through processes like biosorption, bioreduction, biomineralization, and bioaccumulation. These processes were characterized by studying the interaction of different bacteria with uranium. However, most studies unraveling the underlying molecular mechanisms originate from the last decade. Molecular mechanisms help to understand how bacteria interact with radionuclides in the environment. Furthermore, knowledge on these underlying mechanisms could be exploited to improve bioremediation technologies. Here, we review the current knowledge on bacterial uranium resistance and how this could be used for bioremediation applications.

Ferrihydrite Reduction Increases Arsenic and Uranium Bioavailability in Unsaturated Soil

Redox driven mobilization and plant uptake of contaminants under transiently saturated soil conditions need to be clarified to ensure food and water quality across different irrigation systems. We postulate that solid-phase iron reduction in anoxic microsites present in the rhizosphere of unsaturated soil is a key driver for mobilization and bioavailability of contaminants under nonflooded irrigation. To clarify this, two major crops, corn and soybean differing in iron uptake strategies, were grown in irrigated synthetic soil under semiarid conditions with gravimetric moisture content ∼12.5 ± 2.4%. 2-line ferrihydrite, which was coprecipitated with uranium and arsenic, served as the only iron source in soil. Irrespective of crop type, reduced iron was detected in pore water and postexperiment rhizosphere soil confirming ferrihydrite reduction. These results support the presence of localized anoxic microsites in the otherwise aerobic porous bulk soil causing reduction of ferrihydrite and concomitant increase in plant uptake of comobilized contaminants. Our findings indicate that reactive iron minerals undergo reductive dissolution inside anoxic microsites of primarily unsaturated soil, which may have implications on the mobility of trace element contaminants such as arsenic and uranium in irrigated unsaturated soils, accounting for 55% of the irrigated area in the US.

Redox mechanism and stability of uranyl phosphites at mineral surfaces: Cooperative proton/electron transfer and high efficacy for Uranium(VI) reduction

Uranium phosphites have recently emerged as promising materials to remediate radioactive contamination. In this study, the redox mechanisms of uranyl phosphites at mineral surfaces have been addressed by periodic DFT calculations with dispersion corrections. Different from other ligands, the phosphite anions (H₂PO₃^-, HPO₃^2-) are efficient reducing agents for uranyl reduction, and the redox reactions are divided into three steps, as isomerization between two phosphite anion isomers (Step 1), conformational transition (Step 2) and dissociation of the water molecule (Step 3). A second water molecule is critical to lower the activation barriers of Step 1, and all activation barriers are moderate so that the redox reactions occur favorably under normal conditions, which are further dramatically accelerated by the highly exergonic Step 3. Accordingly, formation of uranyl phosphites becomes an effective approach to manage uranium pollution. Moreover, the lower activation barriers for H₂PO₃^- rather than HPO₃^2- rationalize the superior reduction activities of uranyl phosphites and the enhanced stability of U(IV) products at lower pH conditions. Owing to the cooperative proton/electron transfer, the U(VI) reduction to U(IV) and P(III) oxidation to P(V) are completed within one step, with transition states being featured by the U(V) and P(IV) species.

Calcium-Uranyl-Carbonato Species Kinetically Limit U(VI) Reduction by Fe(II) and Lead to U(V)-Bearing Ferrihydrite

Reaction conditions and mechanisms promoting or inhibiting U reduction exert a central control on U solubility and, therefore, U transport and its associated risks. Here, we vary and track common aqueous uranium species to show that a kinetic restriction inhibits homogeneous reduction of the calcium-uranyl-carbonato species (CaUO₂(CO₃)₃^2- and Ca₂UO₂(CO₃)₃) by Fe(II)_(aq), while ferrihydrite surface-catalyzed reduction of all aqueous uranyl by Fe(II) proceeds. Using U L₃ high energy resolution fluorescence detection (HERFD) X-ray absorption near edge structure (XANES) spectroscopy, U L₃ extended X-ray absorption fine structure (EXAFS) spectroscopy, and transmission electron microscopy (TEM), we also show that U(V) is generated and incorporated into ferrihydrite formed from homogeneous U(VI) reduction by Fe(II)_(aq). Through elucidation of the mechanisms that inhibit reduction of the calcium-uranyl-carbonato species and promote stabilization of U(V), we advance our understanding of the controls on U solubility and thus improve prediction of U transport in surface and subsurface systems.

Low concentration of Fe(II) to enhance the precipitation of U(VI) under neutral oxygen-rich conditions

Immobilization of U(VI) by naturally ubiquitous ferrous ions (Fe(II)) has been considered as an efficient and ecofriendly method to retard the migration of aqueous U(VI) at many nuclear sites and surface environments. In this study, we conducted Fe-U coprecipitation experiments to investigate the mechanism and stability of uranium (U) precipitation induced by a small quantity of Fe(II) under oxygen-rich conditions. The experimental results suggest that the sedimentation rates of U(VI) by Fe(II) under neutral oxygen-rich conditions are more than 96%, which are about 36% higher than those without Fe(II) and 16% higher than those under oxygen-free conditions. The Fe-U coprecipitates were observed to remain stable under slightly acidic to neutral and oxygen-rich conditions. Fe(II) primarily settles down as low-crystalline iron oxide hydroxide. U(VI) mainly precipitates as three forms: 16-20% of U forms uranyl hydroxide and metaschoepite, which is absorbed on the surface of the solids; 52-56% of U is absorbed as discrete uranyl phases at the internal pores of iron oxide hydroxide; and 27-29% of U is probably incorporated into the FeO(OH) structure as U(V) and U(VI). The U(V) generated via one-electron reduction is somewhat resistant to the oxidation of O₂ and the acid dissolution. In addition, nearly 70% of U and only about 15% of Fe could be extracted in 24 h by a hydrochloric acid solution with the H⁺ concentration ([H⁺]) of 0.01 M, revealing that U(VI) immobilization by low concentration of Fe(II) combined with O₂ has potential applications in the separation and recycling of aqueous uranium.

Uranium sequestration in sediment at an iron-rich contaminated site at Oak Ridge, Tennessee via. bioreduction followed by reoxidation

This study evaluated uranium sequestration performance in iron-rich (30 g/kg) sediment via bioreduction followed by reoxidation. Field tests (1383 days) at Oak Ridge, Tennessee demonstrated that uranium contents in sediments increased after bioreduced sediments were re-exposed to nitrate and oxygen in contaminated groundwater. Bioreduction of contaminated sediments (1200 mg/kg U) with ethanol in microcosm reduced aqueous U from 0.37 to 0.023 mg/L. Aliquots of the bioreduced sediment were reoxidized with O₂, H₂O₂, and NaNO₃, respectively, over 285 days, resulting in aqueous U of 0.024, 1.58 and 14.4 mg/L at pH 6.30, 6.63 and 7.62, respectively. The source- and the three reoxidized sediments showed different desorption and adsorption behaviors of U, but all fit a Freundlich model. The adsorption capacities increased sharply at pH 4.5 to 5.5, plateaued at pH 5.5 to 7.0, then decreased sharply as pH increased from 7.0 to 8.0. The O₂-reoxidized sediment retained a lower desorption efficiency at pH over 6.0. The NO₃^--reoxidized sediment exhibited higher adsorption capacity at pH 5.5 to 6.0. The pH-dependent adsorption onto Fe(III) oxides and formation of U coated particles and precipitates resulted in U sequestration, and bioreduction followed by reoxidation can enhance the U sequestration in sediment.

Behavior and fate of geogenic uranium in a shallow groundwater system

To unveil behavior and fate of uranium (U) in the Quaternary aquifer system of Datong basin (China), we analyzed sediment and groundwater samples, and performed geochemical modeling. The analyses for sediments were implemented by a sequential extraction procedure and measurements including X-ray power diffraction, scanning electron microscopy, and energy dispersive X-ray spectroscopy. Concentrations of main elements and U, and ²³⁴U/²³⁸U activity ratios for groundwater were determined. Results show that sediment U contents range from 1.93 to 8.80 (average 3.00 ± 1.69) mg/kg. In relation to the total U, average fractions of residual U (probably as betafite) and U(VI) bound to carbonates and FeMn oxides are 74.4 ± 18.7%, 17.2 ± 13.3%, and 4.3 ± 2.9%, respectively. Lower average fractions were determined for both organic matter- and sulfide-bound U (mainly as U(IV), e.g., brannerite) (2.0 ± 0.7%) and exchangeable U(VI) (2.0 ± 2.8%). For the groundwater (pH 7.36-8.86), Ca₂UO₂(CO₃)₃⁰, CaUO₂(CO₃)₃^2-, and UO₂(CO₃)₃^4- constitute >99.5% of the total dissolved U; and elevated U concentrations occur mainly in shallow aquifers (3-40 m deep below land surface) of the west flow-through and discharge areas, with 50% of the sampled points exceeding 30 μg/L. We argue that betafite and carbonate weathering and U(VI) desorption from ferrihydrite are the primary geochemical processes responsible for U mobilization, with a minor contrition from U(IV) oxidation. Abiotic U(IV) oxidation may be induced mainly by dissolved oxygen under oxic/suboxic conditions (e.g., in the recharge and flow-through areas), but significantly linked to amorphous ferrihydrite under Fe(III)- and sulfate-reducing conditions. Abiotic U(VI) reduction could be caused principally by siderite and mackinawite. Under alkaline conditions, higher HCO₃^- concentrations and lower Ca²⁺/HCO₃^- molar ratios (<~0.2) cause formation of CaUO₂(CO₃)₃^2- and UO₂(CO₃)₃^4-, and U(VI) desorption. With increases in concentrations of Ca²⁺ and Ca²⁺/HCO₃^- ratios (>~0.2), these anionic forms may shift to neutral Ca₂UO₂(CO₃)₃⁰, which can facilitate further desorption of U(VI). Our results improve the understanding of U environmental geochemistry and are important for groundwater resources management in this and similar other Quaternary aquifer systems.

Uranium speciation in coal bottom ash investigated via X-ray absorption fine structure and X-ray photoelectron spectra

Similar to chromium contamination, the environmental contamination caused by uranium in radioactive coal bottom ash (CBA) is primarily dependent on the chemical speciation of uranium. However, the relationship between uranium speciation and environmental contamination has not been adequately studied. To determine the relationship between uranium speciation and environmental contamination, X-ray absorption fine structure (XAFS) and X-ray photoelectron spectra (XPS) analyses were performed to determine the uranium speciation in CBA exposed to different chemical environments and simulated natural environments. The leachability of the different forms of uranium in the CBA was studied via a simulated acid rain leaching experiment, and the results showed that 57.0% of the total uranium was leached out as U(VI). The results of a linear combination fit (LCF) of the X-ray absorption near edge structure (XANES) spectrum revealed that in the raw CBA, the uranium mainly occurred as U₃O₈ (71.8%). However, in the iron-rich particles, the uranium mainly occurred as UO₂ (91.9%) after magnetic separation. Magnetite is a ubiquitous ferrous-bearing oxide, and it was effective for the sorption of U(IV). The result of FeSO₄ leaching experiment indicated that 96.57% of total uranium was reduced from U(VI) to U(IV) when infiltrated with the FeSO₄ solution for 6months. This result clearly demonstrated the changes in chemical valence of uranium in the coal ash and provided a conceptual principle for preventing uranium migration from ash to the surrounding soil and plants.

Bacterial Community Shift and Coexisting/Coexcluding Patterns Revealed by Network Analysis in a Uranium-Contaminated Site after Bioreduction Followed by Reoxidation

A site in Oak Ridge, TN, USA, has sediments that contain >3% iron oxides and is contaminated with uranium (U). The U(VI) was bioreduced to U(IV) and immobilized in situ through intermittent injections of ethanol. It then was allowed to reoxidize via the invasion of low-pH (3.6 to 4.0), high-nitrate (up to 200 mM) groundwater back into the reduced zone for 1,383 days. To examine the biogeochemical response, high-throughput sequencing and network analysis were applied to characterize bacterial population shifts, as well as cooccurrence and coexclusion patterns among microbial communities. A paired t test indicated no significant changes of α-diversity for the bioactive wells. However, both nonmetric multidimensional scaling and analysis of similarity confirmed a significant distinction in the overall composition of the bacterial communities between the bioreduced and the reoxidized sediments. The top 20 major genera accounted for >70% of the cumulative contribution to the dissimilarity in the bacterial communities before and after the groundwater invasion. Castellaniella had the largest dissimilarity contribution (17.7%). For the bioactive wells, the abundance of the U(VI)-reducing genera Geothrix, Desulfovibrio, Ferribacterium, and Geobacter decreased significantly, whereas the denitrifying Acidovorax abundance increased significantly after groundwater invasion. Additionally, seven genera, i.e., Castellaniella, Ignavibacterium, Simplicispira, Rhizomicrobium, Acidobacteria Gp1, Acidobacteria Gp14, and Acidobacteria Gp23, were significant indicators of bioactive wells in the reoxidation stage. Canonical correspondence analysis indicated that nitrate, manganese, and pH affected mostly the U(VI)-reducing genera and indicator genera. Cooccurrence patterns among microbial taxa suggested the presence of taxa sharing similar ecological niches or mutualism/commensalism/synergism interactions.IMPORTANCE High-throughput sequencing technology in combination with a network analysis approach were used to investigate the stabilization of uranium and the corresponding dynamics of bacterial communities under field conditions with regard to the heterogeneity and complexity of the subsurface over the long term. The study also examined diversity and microbial community composition shift, the common genera, and indicator genera before and after long-term contaminated-groundwater invasion and the relationship between the target functional community structure and environmental factors. Additionally, deciphering cooccurrence and coexclusion patterns among microbial taxa and environmental parameters could help predict potential biotic interactions (cooperation/competition), shared physiologies, or habitat affinities, thus, improving our understanding of ecological niches occupied by certain specific species. These findings offer new insights into compositions of and associations among bacterial communities and serve as a foundation for future bioreduction implementation and monitoring efforts applied to uranium-contaminated sites.

pH-dependent speciation and hydrogen (H2 ) control U(VI) respiration by Desulfovibrio vulgaris

In situ bioreduction of soluble hexavalent uranium U(VI) to insoluble U(IV) (as UO₂ ) has been proposed as a means of preventing U migration in the groundwater. This work focuses on the bioreduction of U(VI) and precipitation of U(IV). It uses anaerobic batch reactors with Desulfovibrio vulgaris, a well-known sulfate, iron, and U(VI) reducer, growing on lactate as the electron donor, in the absence of sulfate, and with a 30-mM bicarbonate buffering. In the absence of sulfate, D. vulgaris reduced  >90% of the total soluble U(VI) (1 mM) to form U(IV) solids that were characterized by X-ray diffraction and confirmed to be nano-crystalline uraninite with crystallite size 2.8 ± 0.2 nm. pH values between 6 and 10 had minimal impact on bacterial growth and end-product distribution, supporting that the mono-nuclear, and poly-nuclear forms of U(VI) were equally bioavailable as electron acceptors. Electron balances support that H₂ transiently accumulated, but was ultimately oxidized via U(VI) respiration. Thus, D. vulgaris utilized H₂ as the electron carrier to drive respiration of U(VI). Rapid lactate utilization and biomass growth occurred only when U(VI) respiration began to draw down the sink of H₂ and relieve thermodynamic inhibition of fermentation.

Mechanisms and Rates of U(VI) Reduction by Fe(II) in Homogeneous Aqueous Solution and the Role of U(V) Disproportionation

Molecular-level pathways in the aqueous redox transformation of uranium by iron remain unclear, despite the importance of this knowledge for predicting uranium transport and distribution in natural and engineered environments. As the relative importance of homogeneous versus heterogeneous pathways is difficult to probe experimentally, here we apply computational molecular simulation to isolate rates of key one electron transfer reactions in the homogeneous pathway. By comparison to experimental observations the role of the heterogeneous pathway also becomes clear. Density functional theory (DFT) and Marcus theory calculations for all primary monomeric species at pH values ≤7 show for UO₂²⁺ and its hydrolysis species UO₂OH⁺ and UO₂(OH)₂⁰ that reduction by Fe²⁺ is thermodynamically favorable, though kinetically limited for UO₂²⁺. An inner-sphere encounter complex between UO₂OH⁺ and Fe²⁺ was the most stable for the first hydrolysis species and displayed an electron transfer rate constant k_et = 4.3 × 10³ s^-1. Three stable inner- and outer-sphere encounter complexes between UO₂(OH)₂⁰ and Fe²⁺ were found, with electron transfer rate constants ranging from k_et = 7.6 × 10² to 7.2 × 10⁴ s^-1. Homogeneous reduction of these U(VI) hydrolysis species to U(V) is, therefore, predicted to be facile. In contrast, homogeneous reduction of UO₂⁺ by Fe²⁺ was found to be thermodynamically unfavorable, suggesting the possible importance of U(V)-U(V) disproportionation as a route to U(IV). Calculations on homogeneous disproportionation, however, while yielding a stable outer-sphere U(V)-U(V) encounter complex, indicate that this electron transfer reaction is not feasible at circumneutral pH. Protonation of both axial O atoms of acceptor U(V) (i.e., by H₃O⁺) was found to be a prerequisite to stabilize U(IV), consistent with the experimental observation that the rate of this reaction is inversely correlated with pH. Thus, despite prevailing notions that U(V) is rapidly eliminated by homogeneous disproportionation, this pathway is irrelevant at environmental conditions.

The application of iron-based technologies in uranium remediation: A review

Remediating uranium contamination is of worldwide interest because of the increasing release of uranium from mining and processing, nuclear power leaks, depleted uranium components in weapons production and disposal, and phosphate fertilizer in agriculture activities. Iron-based technologies are attractive because they are highly efficient, inexpensive, and readily available. This paper provides an overview of the current literature that addresses the application of iron-based technologies in the remediation of sites with elevated uranium levels. The application of iron-based materials, the current remediation technologies and mechanisms, and the effectiveness and environmental safety considerations of these approaches were discussed. Because uranium can be reduced and reoxidized in the environment, the review also proposes strategies for long-term in situ remediation of uranium. Unfortunately, iron-based materials (nanoscale zerovalent iron and iron oxides) can be toxic to microorganisms. As such, further studies exploring the links among the fates, ecological impacts, and other environmentally relevant factors are needed to better understand the constraints on using iron-based technologies for remediation.

Se Isotopes as Groundwater Redox Indicators: Detecting Natural Attenuation of Se at an in Situ Recovery U Mine

One of the major ecological concerns associated with the in situ recovery (ISR) of uranium (U) is the environmental release of soluble, toxic selenium (Se) oxyanions generated by mining. Post-mining natural attenuation by the residual reductants in the ore body and reduced down-gradient sediments should mitigate the risk of Se contamination in groundwater. In this work, we investigate the Se concentrations and Se isotope systematics of groundwater and of U ore bearing sediments from an ISR site at Rosita, TX, USA. Our results show that selenate (Se(VI)) is the dominant Se species in Rosita groundwater, and while several up-gradient wells have elevated Se(VI), the majority of the ore zone and down-gradient wells have little or no Se oxyanions. In addition, the δ⁸²Se_VI of Rosita groundwater is generally elevated relative to the U ore up to +6.14‰, with the most enriched values observed in the ore-zone wells. Increasing δ⁸²Se with decreasing Se(VI) conforms to a Rayleigh type distillation model with an ε of -2.25‰ ± 0.61‰, suggesting natural Se(VI) reduction occurring along the hydraulic gradient at the Rosita ISR site. Furthermore, our results show that Se isotopes are excellent sensors for detecting and monitoring post-mining natural attenuation of Se oxyanions at ISR sites.

Tetra- and Hexavalent Uranium Forms Bidentate-Mononuclear Complexes with Particulate Organic Matter in a Naturally Uranium-Enriched Peatland

Peatlands frequently serve as efficient biogeochemical traps for U. Mechanisms of U immobilization in these organic matter-dominated environments may encompass the precipitation of U-bearing mineral(oid)s and the complexation of U by a vast range of (in)organic surfaces. The objective of this work was to investigate the spatial distribution and molecular binding mechanisms of U in soils of an alpine minerotrophic peatland (pH 4.7-6.6, E_h = -127 to 463 mV) using microfocused X-ray fluorescence spectrometry and bulk and microfocused U L₃-edge X-ray absorption spectroscopy. The soils contained 2.3-47.4 wt % organic C, 4.1-58.6 g/kg Fe, and up to 335 mg/kg geogenic U. Uranium was found to be heterogeneously distributed at the micrometer scale and enriched as both U(IV) and U(VI) on fibrous and woody plant debris (48 ± 10% U(IV), x̅ ± σ, n = 22). Bulk U X-ray absorption near edge structure (XANES) spectroscopy revealed that in all samples U(IV) comprised 35-68% of total U (x̅ = 50%, n = 15). Shell-fit analyses of bulk U L₃-edge extended X-ray absorption fine structure (EXAFS) spectra showed that U was coordinated to 1.3 ± 0.2 C atoms at a distance of 2.91 ± 0.01 Å (x̅ ± σ), which implies the formation of bidentate-mononuclear U(IV/VI) complexes with carboxyl groups. We neither found evidence for U shells at ∼3.9 Å, indicative of mineral-associated U or multinuclear U(IV) species, nor for a substantial P/Fe coordination of U. Our data indicates that U(IV/VI) complexation by natural organic matter prevents the precipitation of U minerals as well as U complexation by Fe/Mn phases at our field site, and suggests that organically complexed U(IV) is formed via reduction of organic matter-bound U(VI).

Isotopic Evidence for Reductive Immobilization of Uranium Across a Roll-Front Mineral Deposit

We use uranium (U) isotope ratios to detect and quantify the extent of natural U reduction in groundwater across a roll front redox gradient. Our study was conducted at the Smith Ranch-Highland in situ recovery (ISR) U mine in eastern Wyoming, USA, where economic U deposits occur in the Paleocene Fort Union formation. To evaluate the fate of aqueous U in and adjacent to the ore body, we investigated the chemical composition and isotope ratios of groundwater samples from the roll-front type ore body and surrounding monitoring wells of a previously mined area. The (238)U/(235)U of groundwater varies by approximately 3‰ and is correlated with U concentrations. Fluid samples down-gradient of the ore zone are the most depleted in (238)U and have the lowest U concentrations. Activity ratios of (234)U/(238)U are ∼5.5 up-gradient of the ore zone, ∼1.0 in the ore zone, and between 2.3 and 3.7 in the down-gradient monitoring wells. High-precision measurements of (234)U/(238)U and (238)U/(235)U allow for development of a conceptual model that evaluates both the migration of U from the ore body and the extent of natural attenuation due to reduction. We find that the premining migration of U down-gradient of the delineated ore body is minimal along eight transects due to reduction in or adjacent to the ore body, whereas two other transects show little or no sign of reduction in the down-gradient region. These results suggest that characterization of U isotopic ratios at the mine planning stage, in conjunction with routine geochemical analyses, can be used to identify where more or less postmining remediation will be necessary.

Formation and Geological Sequestration of Uranium Nanoparticles in Deep Granitic Aquifer

The stimulation of bacterial activities that convert hexavalent uranium, U(VI), to tetravalent uranium, U(IV), appears to be feasible for cost-effective remediation of contaminated aquifers. However, U(VI) reduction typically results in the precipitation of U(IV) particles less than 5 nanometers in diameter, except for environmental conditions enriched with iron. Because these tiny particles are mobile and susceptible to oxidative dissolution after the termination of nutrient injection, in situ bioremediation remains to be impractical. Here we show that U(IV) nanoparticles of coffinite (U(SiO4)1-x(OH)4x) formed in fracture-filling calcium carbonate in a granitic aquifer. In situ U-Pb isotope dating demonstrates that U(IV) nanoparticles have been sequestered in the calcium carbonate for at least 1 million years. As the microbiologically induced precipitation of calcium carbonate in aquifer systems worldwide is extremely common, we anticipate simultaneous stimulation of microbial activities for precipitation reactions of calcium carbonate and U(IV) nanoparticles, which leads to long-term sequestration of uranium and other radionuclides in contaminated aquifers and deep geological repositories.

Effects of Titanium Doping in Titanomagnetite on Neptunium Sorption and Speciation

Neptunium-237 is a radionuclide of great interest owing to its long half-life (2.14 × 10(6) years) and relative mobility as the neptunyl ion (NpO2(+)) under many surface and groundwater conditions. Reduction to tetravalent neptunium (Np(IV)) effectively immobilizes the actinide in many instances due to its low solubility and strong interactions with natural minerals. One such mineral that may facilitate the reduction of neptunium is magnetite (Fe(2+)Fe(3+)2O4). Natural magnetites often contain titanium impurities which have been shown to enhance radionuclide sorption via titanium's influence on the Fe(2+)/Fe(3+) ratio (R) in the absence of oxidation. Here, we provide evidence that Ti-substituted magnetite reduces neptunyl species to Np(IV). Titanium-substituted magnetite nanoparticles were synthesized and reacted with NpO2(+) under reducing conditions. Batch sorption experiments indicate that increasing Ti concentration results in higher Np sorption/reduction values at low pH. High-resolution transmission electron microscopy of the Ti-magnetite particles provides no evidence of NpO2 nanoparticle precipitation. Additionally, X-ray absorption spectroscopy confirms the nearly exclusive presence of Np(IV) on the titanomagnetite surface and provides supporting data indicating preferential binding of Np to terminal Ti-O sites as opposed to Fe-O sites.

Uranium incorporation into aluminum-substituted ferrihydrite during iron(ii)-induced transformation

Uranium retention processes (adsorption, precipitation, and incorporation into host minerals) exert strong controls on U mobility in the environment, and understanding U retention is therefore crucial for predicting the migration of U within surface and groundwater. Uranium can be incorporated into Fe (hydr)oxides during Fe(ii)-induced transformation of ferrihydrite to goethite. However, ferrihydrite seldom exists as a pure phase within soils or sediments, and structural impurities such as Al alter its reactivity. The presence of Al in ferrihydrite, for example, decreases the rate of transformation to goethite, and thus may impact the retention pathway, or extent of retention, of U. Here, we investigate the extent and pathways of U(vi) retention on Al-ferrihydrite during Fe(ii)-induced transformation. Ferrihydrite containing 0%, 1%, 5%, 10%, and 20% Al was reacted with 10 μM U and 300 μM Fe(ii) in the presence of 0 mM and 4 mM Ca(2+) and 3.8 mM carbonate at pH 7.0. Solid reaction products were characterized using U L3-edge EXAFS spectroscopy to differentiate between adsorbed U and U incorporated into the goethite lattice. Uranium incorporation into Al-ferrihydrite declined from ∼70% of solid-phase U at 0% and 1% Al to ∼30% of solid phase U at 20% Al content. The decrease in U incorporation with increasing Al concentration was due to two main factors: (1) decreased transformation of ferrihydrite to goethite; and, (2) a decrease of the goethite lattice with increasing Al, making the lattice less compatible with large U atoms. However, uranium incorporation can occur even with an Al-substituted ferrihydrite precursor in the presence or absence of Ca(2+). The process of U incorporation into Al-goethite may therefore be a potential long-term sink of U in subsurface environments where Al-substituted iron oxides are common, albeit at lower levels of incorporation with increasing Al content.

Reduction of U(VI) by Fe(II) during the Fe(II)-accelerated transformation of ferrihydrite

X-ray absorption spectroscopy has been used to study the reduction of adsorbed U(VI) during the Fe(II)-accelerated transformation of ferrihydrite to goethite. The fate of U(VI) was examined across a variety of pH values and Fe(II) concentrations, with results suggesting that, in all cases, it was reduced over the course of the Fe(III) phase transformation to a U(V) species incorporated in goethite. A positive correlation between U(VI) reduction and ferrihydrite transformation rate constants implies that U(VI) reduction was driven by the production of goethite under the conditions used in these studies. This interpretation was supported by additional experimental evidence that demonstrated the (fast) reduction of U(VI) to U(V) by Fe(II) in the presence of goethite only. Theoretical redox potential calculations clearly indicate that the reduction of U(VI) by Fe(II) in the presence of goethite is thermodynamically favorable. In contrast, reduction of U(VI) by Fe(II) in the presence of ferrihydrite is largely thermodynamically unfavorable within the range of conditions examined in this study.

Uranium immobilization in an iron-rich rhizosphere of a native wetland plant from the Savannah River Site under reducing conditions

The hypothesis of this study was that iron plaques formed on the roots of wetland plants and their rhizospheres create environmental conditions favorable for iron reducing bacteria that promote the in situ immobilization of uranium. Greenhouse microcosm studies were conducted using native plants (Sparganium americanum) from a wetland located on the Savannah River Site, Aiken, SC. After iron plaques were established during a 73-day period by using an anoxic Fe(II)-rich nutrient solution, a U(VI) amended nutrient solution was added to the system for an additional two months. Compared to plant-free control microcosms, microcosms containing iron plaques successfully stimulated the growth of targeted iron reducing bacteria, Geobacter spp. Their population continuously increased after the introduction of the U(VI) nutrient solution. The reduction of some of the U(VI) to U(IV) by iron reducing bacteria was deduced based on the observations that the aqueous Fe(II) concentrations increased while the U(VI) concentrations decreased. The Fe(II) produced by the iron reducing bacteria was assumed to be reoxidized by the oxygen released from the roots. Advanced spectroscopic analyses revealed that a significant fraction of the U(VI) had been reduced to U(IV) and they were commonly deposited in association with phosphorus on the iron plaque.

Growth of Desulfovibrio vulgaris when respiring U(VI) and characterization of biogenic uraninite

The capacity of Desulfovibrio vulgaris to reduce U(VI) was studied previously with nongrowth conditions involving a high biomass concentration; thus, bacterial growth through respiration of U(VI) was not proven. In this study, we conducted a series of batch tests on U(VI) reduction by D. vulgaris at a low initial biomass (10 to 20 mg/L of protein) that could reveal biomass growth. D. vulgaris grew with U(VI) respiration alone, as well as with simultaneous sulfate reduction. Patterns of growth kinetics and solids production were affected by sulfate and Fe(2+). Biogenic sulfide nonenzymatically reduced 76% of the U(VI) and greatly enhanced the overall reduction rate in the absence of Fe(2+) but was rapidly scavenged by Fe(2+) to form FeS in the presence of Fe(2+). Biogenic U solids were uraninite (UO2) nanocrystallites associated with 20 mg/g biomass as protein. The crystallite thickness of UO2 was 4 to 5 nm without Fe(2+) but was <1.4 nm in the presence of Fe(2+), indicating poor crystallization inhibited by adsorbed Fe(2+) and other amorphous Fe solids, such as FeS or FeCO3. This work fills critical gaps in understanding the metabolic utilization of U by microorganisms and formation of UO2 solids in bioremediation sites.

Sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions

The sorption and reduction of selenite on chlorite surfaces in the presence of Fe(II) ions were investigated as a function of pH, Se(IV) concentration, and Fe(II) concentration under an anoxic condition. The sorption of Se(IV) onto chlorite surfaces followed the Langmuir isotherm regardless of the presence of Fe(II) ions in the solution. The Se(IV) sorption was observed to be very low at all pH values when the solution was Fe(II)-free or the concentration of Fe(II) ions was as low as 0.5 mg/L. However, the Se(IV) sorption was enhanced at a pH > 6.5 when the Fe(II) concentration was higher than 5 mg/L because of the increased sorption of Fe(II) onto the chlorite surfaces. XANES (X-ray absorption near edge structure) spectra of the Se K-edge showed that most of the sorbed Se(IV) was reduced to Se(0) by Fe(II) sorbed onto the chlorite surfaces, especially at pH > 9. The combined results of field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD) also showed that elemental selenium and goethite were formed and precipitated on the chlorite surfaces during the sorption of selenite. Consequently it can be concluded that Se(IV) can be reduced to Se(0) in the presence of Fe(II) ions by the surface catalytic oxidation of Fe(II) into Fe(III) and the formation of goethite at neutral and particularly alkaline conditions. Thus the mobility of selenite in groundwater is expected to be reduced by the presence of a relatively higher concentration of Fe(II) in subsurface environments.

Kinetics of homogeneous and surface-catalyzed mercury(II) reduction by iron(II)

Production of elemental mercury, Hg(0), via Hg(II) reduction is an important pathway that should be considered when studying Hg fate in environment. We conducted a kinetic study of abiotic homogeneous and surface-catalyzed Hg(0) production by Fe(II) under dark anoxic conditions. Hg(0) production rate, from initial 50 pM Hg(II) concentration, increased with increasing pH (5.5-8.1) and aqueous Fe(II) concentration (0.1-1 mM). The homogeneous rate was best described by the expression, r(hom) = k(hom) [FeOH(+)] [Hg(OH)2]; k(hom) = 7.19 × 10(+3) L (mol min)(-1). Compared to the homogeneous case, goethite (α-FeOOH) and hematite (α-Fe2O3) increased and γ-alumina (γ-Al2O3) decreased the Hg(0) production rate. Heterogeneous Hg(0) production rates were well described by a model incorporating equilibrium Fe(II) adsorption, rate-limited Hg(II) reduction by dissolved and adsorbed Fe(II), and rate-limited Hg(II) adsorption. Equilibrium Fe(II) adsorption was described using a surface complexation model calibrated with previously published experimental data. The Hg(0) production rate was well described by the expression r(het) = k(het) [>SOFe((II))] [Hg(OH)2], where >SOFe((II)) is the total adsorbed Fe(II) concentration; k(het) values were 5.36 × 10(+3), 4.69 × 10(+3), and 1.08 × 10(+2) L (mol min)(-1) for hematite, goethite, and γ-alumina, respectively. Hg(0) production coupled to reduction by Fe(II) may be an important process to consider in ecosystem Hg studies.

Reaction of U(VI) with titanium-substituted magnetite: influence of Ti on U(IV) speciation

Reduction of hexavalent uranium (U(VI)) to less soluble tetravalent uranium (U(IV)) through enzymatic or abiotic redox reactions has the potential to alter U mobility in subsurface environments. As a ubiquitous natural mineral, magnetite (Fe3O4) is of interest because of its ability to act as a rechargeable reductant for U(VI). Natural magnetites are often impure with titanium, and structural Fe(3+) replacement by Ti(IV) yields a proportional increase in the relative Fe(2+) content in the metal sublattice to maintain bulk charge neutrality. In the absence of oxidation, the Ti content sets the initial bulk Fe(2+)/Fe(3+) ratio (R). Here, we demonstrate that Ti-doped magnetites (Fe3 - xTixO4) reduce U(VI) to U(IV). The U(VI)-Fe(2+) redox reactivity was found to be controlled directly by R but was otherwise independent of Ti content (xTi). However, in contrast to previous studies with pure magnetite where U(VI) was reduced to nanocrystalline uraninite (UO2), the presence of structural Ti (xTi = 0.25-0.53) results in the formation of U(IV) species that lack the bidentate U-O2-U bridges of uraninite. Extended X-ray absorption fine structure spectroscopic analysis indicated that the titanomagnetite-bound U(IV) phase has a novel U(IV)-Ti binding geometry different from the coordination of U(IV) in the mineral brannerite (U(IV)Ti2O6). The observed U(IV)-Ti coordination at a distance of 3.43 Å suggests a binuclear corner-sharing adsorption/incorporation U(IV) complex with the solid phase. Furthermore, we explored the effect of oxidation (decreasing R) and solids-to-solution ratio on the reduced U(IV) phase. The formation of the non-uraninite U(IV)-Ti phase appears to be controlled by availability of surface Ti sites rather than R. Our work highlights a previously unrecognized role of Ti in the environmental chemistry of U(IV) and suggests that further work to characterize the long-term stability of U(IV) phases formed in the presence of Ti is warranted.

The design of long-term effective uranium bioremediation strategy using a community metabolic model

Acetate amendment at uranium contaminated sites in Rifle, CO. leads to an initial bloom of Geobacter accompanied by the removal of U(VI) from the groundwater, followed by an increase of sulfate-reducing bacteria (SRBs) which are poor reducers of U(VI). One of the challenges associated with bioremediation is the decay in Geobacter abundance, which has been attributed to the depletion of bio-accessible Fe(III), motivating the investigation of simultaneous amendments of acetate and Fe(III) as an alternative bioremediation strategy. In order to understand the community metabolism of Geobacter and SRBs during artificial substrate amendment, we have created a genome-scale dynamic community model of Geobacter and SRBs using the previously described Dynamic Multi-species Metabolic Modeling framework. Optimization techniques are used to determine the optimal acetate and Fe(III) addition profile. Field-scale simulation of acetate addition accurately predicted the in situ data. The simulations suggest that batch amendment of Fe(III) along with continuous acetate addition is insufficient to promote long-term bioremediation, while continuous amendment of Fe(III) along with continuous acetate addition is sufficient to promote long-term bioremediation. By computationally minimizing the acetate and Fe(III) addition rates as well as the difference between the predicted and target uranium concentration, we showed that it is possible to maintain the uranium concentration below the environmental safety standard while minimizing the cost of chemical additions. These simulations show that simultaneous addition of acetate and Fe(III) has the potential to be an effective uranium bioremediation strategy. They also show that computational modeling of microbial community is an important tool to design effective strategies for practical applications in environmental biotechnology.

U(VI) sorption and reduction kinetics on the magnetite (111) surface

Sorption of contaminants onto mineral surfaces is an important process that can restrict their transport in the environment. In the current study, uranium (U) uptake on magnetite (111) was measured as a function of time and solution composition (pH, [CO(3)](T), [Ca]) under continuous batch-flow conditions. We observed, in real-time and in situ, adsorption and reduction of U(VI) and subsequent growth of UO(2) nanoprecipitates using atomic force microscopy (AFM) and newly developed batch-flow U L(III)-edge grazing-incidence X-ray absorption spectroscopy near-edge structure (GI-XANES) spectroscopy. U(VI) reduction occurred with and without CO(3) present, and coincided with nucleation and growth of UO(2) particles. When Ca and CO(3) were both present no U(VI) reduction occurred and the U surface loading was lower. In situ batch-flow AFM data indicated that UO(2) particles achieved a maximum height of 4-5 nm after about 8 h of exposure, however, aggregates continued to grow laterally after 8 h reaching up to about 300 nm in diameter. The combination of techniques indicated that U uptake is divided into three-stages; (1) initial adsorption of U(VI), (2) reduction of U(VI) to UO(2) nanoprecipitates at surface-specific sites after 2-3 h of exposure, and (3) completion of U(VI) reduction after ~6-8 h. U(VI) reduction also corresponded to detectable increases in Fe released to solution and surface topography changes. Redox reactions are proposed that explicitly couple the reduction of U(VI) to enhanced release of Fe(II) from magnetite. Although counterintuitive, the proposed reaction stoichiometry was shown to be largely consistent with the experimental results. In addition to providing molecular-scale details about U sorption on magnetite, this work also presents novel advances for collecting surface sensitive molecular-scale information in real-time under batch-flow conditions.

Soluble electron shuttles can mediate energy taxis toward insoluble electron acceptors

Shewanella species grow in widely disparate environments and play key roles in elemental cycling, especially in environments with varied redox conditions. To obtain a system-level understanding of Shewanella's robustness and versatility, the complex interplay of cellular growth, metabolism, and transport under conditions of limiting carbon sources, energy sources, and electron acceptors must be elucidated. In this paper, population-level taxis of Shewanella oneidensis MR-1 cells in the presence of a rate-limiting, insoluble electron acceptor was investigated. A novel mechanism, mediated energy taxis, is proposed by which Shewanella use riboflavin as both an electron shuttle and an attractant to direct cell movement toward local sources of insoluble electron acceptors. The cells secrete reduced riboflavin, which diffuses to a nearby particle containing an insoluble electron acceptor and is oxidized. The oxidized riboflavin then diffuses away from the particle, establishing a spatial gradient that draws cells toward the particle. Experimental and modeling results are presented to support this mechanism. S. oneidensis MR-1 cells inoculated into a uniform dispersion of MnO(2) particles in dilute agar exhibited taxis outward, creating a clear zone within which riboflavin was detected by mass spectrometry. Cells inoculated into dilute agar containing oxidized riboflavin similarly exhibited taxis, rapidly forming an expanding zone of reduced riboflavin. A mathematical model based on the proposed mechanism was able to predict experimental trends, including how concentrations of riboflavin and insoluble electron acceptors (e.g., MnO(2)) affected tactic cell migration.