Biogenic non-crystalline U(IV) revealed as major component in uranium ore deposits

Enforcing energy consumption promotes microbial extracellular respiration for xenobiotic bioconversion

Extracellular electron transfer (EET) empowers electrogens to catalyse the bioconversion of a wide range of xenobiotics in the environment. Synthetic bioengineering has proven effective in promoting EET output. However, conventional strategies mainly focus on modifications of EET-related genes or pathways, which leads to a bottleneck due to the intricate nature of electrogenic metabolic properties and intricate pathway regulation that remain unelucidated. Herein, we propose a novel EET pathway-independent approach, from an energy manipulation perspective, to enhance microbial EET output. The Controlled Hydrolyzation of ATP to Enhance Extracellular Respiration (CHEER) strategy promotes energy utilization and persistently reduces the intracellular ATP level in Shewanella oneidensis, a representative electrogenic microbe. This approach leads to the accelerated consumption of carbon substrate, increased biomass accumulation and an expanded intracellular NADH pool. Both microbial electrolysis cell and microbial fuel cell tests exhibit that the CHEER strain substantially enhances EET capability. Analysis of transcriptome profiles reveals that the CHEER strain considerably bolsters biomass synthesis and metabolic activity. When applied to the bioconversion of model xenobiotics including methyl orange, Cr(VI) and U(VI), the CHEER strain consistently exhibits enhanced removal efficiencies. This work provides a new perspective and a feasible strategy to enhance microbial EET for efficient xenobiotic conversion.

Effect of Competing Metals and Humic Substances on Uranium Mobilization from Noncrystalline U(IV) Induced by Anthropogenic and Biogenic Ligands

Anthropogenic and biogenic ligands may mobilize uranium (U) from tetravalent U (U(IV)) phases in the subsurface, especially from labile noncrystalline U(IV). The rate and extent of U(IV) mobilization are affected by geochemical processes. Competing metals and humic substances may play a decisive role in U mobilization by anthropogenic and biogenic ligands. A structurally diverse set of anthropogenic and biogenic ligands was selected for assessing the effect of the aforementioned processes on U mobilization from noncrystalline U(IV), including 2,6-pyridinedicarboxylic acid (DPA), citrate, N,N'-di(2-hydroxybenzyl)ethylene-diamine-N,N'-diacetic acid (HBED), and desferrioxamine B (DFOB). All experiments were performed under anoxic conditions at pH 7.0. The effect of competing metals (Ca, Fe(III), and Zn) on ligand-induced U mobilization depended on the particular metal-ligand combination ranging from nearly complete U mobilization inhibition (e.g., Ca-citrate) to no apparent inhibitory effects or acceleration of U mobilization (e.g., Fe(III)-citrate). Humic substances (Suwannee River humic acid and fulvic acid) were tested across a range of concentrations either separately or combined with the aforementioned ligands. Humic substances alone mobilized appreciable U and also enhanced U mobilization in the presence of anthropogenic or biogenic ligands. These findings illustrate the complex influence of competing metals and humic substances on U mobilization by anthropogenic and biogenic ligands in the environment.

Vadose-zone alteration of metaschoepite and ceramic UO2 in Savannah River Site field lysimeters

Uranium dioxide (UO₂) and metaschoepite (UO₃•nH₂O) particles have been identified as contaminants at nuclear sites. Understanding their behavior and impact is crucial for safe management of radioactively contaminated land and to fully understand U biogeochemistry. The Savannah River Site (SRS) (South Carolina, USA), is one such contaminated site, following historical releases of U-containing wastes to the vadose zone. Here, we present an insight into the behavior of these two particle types under dynamic conditions representative of the SRS, using field lysimeters (15 cm D x 72 cm L). Discrete horizons containing the different particle types were placed at two depths in each lysimeter (25 cm and 50 cm) and exposed to ambient rainfall for 1 year, with an aim of understanding the impact of dynamic, shallow subsurface conditions on U particle behavior and U migration. The dissolution and migration of U from the particle sources and the speciation of U throughout the lysimeters was assessed after 1 year using a combination of sediment digests, sequential extractions, and bulk and μ-focus X-ray spectroscopy. In the UO₂ lysimeter, oxidative dissolution of UO₂ and subsequent migration of U was observed over 1-2 cm in the direction of waterflow and against it. Sequential extractions of the UO₂ sources suggest they were significantly altered over 1 year. The metaschoepite particles also showed significant dissolution with marginally enhanced U migration (several cm) from the sources. However, in both particle systems the released U was quantitively retained in sediment as a range of different U(IV) and U(VI) phases, and no detectable U was measured in the lysimeter effluent. The study provides a useful insight into U particle behavior in representative, real-world conditions relevant to the SRS, and highlights limited U migration from particle sources due to secondary reactions with vadose zone sediments over 1 year.

Evidence of microbial activity in a uranium roll-front deposit: Unlocking their potential role as bioenhancers of the ore genesis

Uranium (U) roll-front deposits constitute a valuable source for an economical extraction by in situ recovery (ISR) mining. Such technology may induce changes in the subsurface microbiota, raising questions about the way their activities could build a functional ecosystem in such extreme environments (i.e.: oligotrophy and high SO₄ concentration and salinity). Additionally, more information is needed to dissipate the doubts about the microbial role in the genesis of such U orebodies. A U roll-front deposit hosted in an aquifer driven system (in Zoovch Ovoo, Mongolia), intended for mining by acid ISR, was previously explored and showed to be governed by a complex bacterial diversity, linked to the redox zonation and the geochemical conditions. Here for the first time, transcriptional activities of microorganisms living in such U ore deposits are determined and their metabolic capabilities allocated in the three redox-inherited compartments, naturally defined by the roll-front system. Several genes encoding for crucial metabolic pathways demonstrated a strong biological role controlling the subsurface cycling of many elements including nitrate, sulfate, metals and radionuclides (e.g.: uranium), through oxidation-reduction reactions. Interestingly, the discovered transcriptional behaviour gives important insights into the good microbial adaptation to the geochemical conditions and their active contribution to the stabilization of the U ore deposits. Overall, evidences on the importance of these microbial metabolic activities in the aquifer system are discussed that may clarify the doubts on the microbial role in the genesis of low-temperature U roll-front deposits, along the Zoovch Ovoo mine.

Radionuclide Reduction by Combinatorial Optimization of Microbial Extracellular Electron Transfer with a Physiologically Adapted Regulatory Platform

Microbial extracellular electron transfer (EET) is the basis for many microbial processes involved in element geochemical recycling, bioenergy harvesting, and bioremediation, including the technique for remediating U(VI)-contaminated environments. However, the low EET rate hinders its full potential from being fulfilled. The main challenge for engineering microbial EET is the difficulty in optimizing cell resource allocation for EET investment and basic metabolism and the optimal coordination of the different EET pathways. Here, we report a novel combinatorial optimization strategy with a physiologically adapted regulatory platform. Through exploring the physiologically adapted regulatory elements, a 271.97-fold strength range, autonomous, and dynamic regulatory platform was established for Shewanella oneidensis, a prominent electrochemically active bacterium. Both direct and mediated EET pathways are modularly reconfigured and tuned at various intensities with the regulatory platform, which were further assembled combinatorically. The optimal combinations exhibit up to 16.12-, 4.51-, and 8.40-fold improvements over the control in the maximum current density (1009.2 mA/m²) of microbial electrolysis cells and the voltage output (413.8 mV) and power density (229.1 mW/m²) of microbial fuel cells. In addition, the optimal strains exhibited up to 6.53-fold improvement in the radionuclide U(VI) removal efficiency. This work provides an effective and feasible approach to boost microbial EET performance for environmental applications.

Biostimulation as a sustainable solution for acid neutralization and uranium immobilization post acidic in-situ recovery

Major uranium (U) deposits worldwide are exploited by acid leaching, known as 'in-situ recovery' (ISR). ISR involves the injection of an acid fluid into ore-bearing aquifers and the pumping of the resulting metal-containing solution through cation exchange columns for the recovery of dissolved U. Rehabilitation of ISR-impacted aquifers could be achieved through natural attenuation, or via biostimulation of autochthonous heterotrophic microorganisms due to the associated acid neutralization and trace metal immobilization. In this study, we analyzed the capacity of pristine aquifer sediments impacted by diluted ISR fluids to buffer pH and immobilize U. The experimental setup consisted of glass columns, filled with sediment from a U ore-bearing aquifer, through which diluted ISR fluids were flowed continuously. The ISR solution was obtained from ISR mining operations at the Muyunkum and Tortkuduk deposits in Kazakhstan. Following this initial phase, columns were biostimulated with a mix of molasses, yeast extract and glycerol to stimulate the growth of autochthonous heterotrophic communities. Experimental results showed that this amendment efficiently promoted the activity of acid-tolerant bacterial guilds, with pH values rising from 4.8 to 6.5-7.0 at the outlet of the stimulated columns. The reduction of sulfate, nitrate, and metals as well as dissimilatory nitrate reduction to ammonia induced the rise in pH values, in agreement with geochemical modelling results. Biostimulation efficiently promoted the complete immobilization of U, with the accumulation of up to 3343 ppm in the first few centimeters of the columns. Synchrotron analysis and SEM-EDS revealed that up to 60% of the injected hexavalent U was immobilized as tetravalent non-crystalline U onto bacterial cell surfaces. 16S rDNA amplicon analysis and qPCR data suggested a predominant role played for members of the Phylum Firmicutes (from the genera Clostridium, Pelosinus and Desulfosporosinus) in biological U reduction and immobilization.

Reduction of U(VI) on Chemically Reduced Montmorillonite and Surface Complexation Modeling of Adsorbed U(IV)

Adsorption and subsequent reduction of U(VI) on Fe(II)-bearing clay minerals can control the mobility of uranium in subsurface environments. Clays such as montmorillonite provide substantial amounts of the reactive surface area in many subsurface environments, and montmorillonite-containing materials are used in the storage of spent nuclear fuel. We investigated the extent of reduction of U(VI) by Fe(II)-bearing montmorillonite at different pH values and sodium concentrations using X-ray absorption spectroscopy and chemical extractions. Nearly complete reduction of U(VI) to U(IV) occurred at a low sodium concentration at both pH 3 and 6. At pH 6 and a high sodium concentration, which inhibits U(VI) binding at cation-exchange sites, the extent of U(VI) reduction was only 70%. Surface-bound U(VI) on unreduced montmorillonite was more easily extracted into solution with bicarbonate than surface-bound U(IV) generated by reduction of U(VI) on Fe(II)-bearing montmorillonite. We developed a nonelectrostatic surface complexation model to interpret the equilibrium adsorption of U(IV) on Fe(II)-bearing montmorillonite as a function of pH and sodium concentration. These findings establish the potential importance of structural Fe(II) in low iron content smectites in controlling uranium mobility in subsurface environments.

From Adsorption to Precipitation of U(VI): What is the Role of pH and Natural Organic Matter?

We investigated interfacial reactions of U(VI) in the presence of Suwannee River natural organic matter (NOM) at acidic and neutral pH. Laboratory batch experiments show that the adsorption and precipitation of U(VI) in the presence of NOM occur at pH 2 and pH 4, while the aqueous complexation of U by dissolved organic matter is favored at pH 7, preventing its precipitation. Spectroscopic analyses indicate that U(VI) is mainly adsorbed to the particulate organic matter at pH 4. However, U(VI)-bearing ultrafine to nanocrystalline solids were identified at pH 4 by electron microscopy. This study shows the promotion of U(VI) precipitation by NOM at low pH which may be relevant to the formation of mineralized deposits, radioactive waste repositories, wetlands, and other U- and organic-rich environmental systems.

Biogenic Sulfidation of U(VI) and Ferrihydrite Mediated by Sulfate-Reducing Bacteria at Elevated pH

Globally, the need for radioactive waste disposal and contaminated land management is clear. Here, gaining an improved understanding of how biogeochemical processes, such as Fe(III) and sulfate reduction, may control the environmental mobility of radionuclides is important. Uranium (U), typically the most abundant radionuclide by mass in radioactive wastes and contaminated land scenarios, may have its environmental mobility impacted by biogeochemical processes within the subsurface. This study investigated the fate of U(VI) in an alkaline (pH ∼9.6) sulfate-reducing enrichment culture obtained from a high-pH environment. To explore the mobility of U(VI) under alkaline conditions where iron minerals are ubiquitous, a range of conditions were tested, including high (30 mM) and low (1 mM) carbonate concentrations and the presence and absence of Fe(III). At high carbonate concentrations, the pH was buffered to approximately pH 9.6, which delayed the onset of sulfate reduction and meant that the reduction of U(VI)_(aq) to poorly soluble U(IV)_(s) was slowed. Low carbonate conditions allowed microbial sulfate reduction to proceed and caused the pH to fall to ∼7.5. This drop in pH was likely due to the presence of volatile fatty acids from the microbial respiration of gluconate. Here, aqueous sulfide accumulated and U was removed from solution as a mixture of U(IV) and U(VI) phosphate species. In addition, sulfate-reducing bacteria, such as Desulfosporosinus species, were enriched during development of sulfate-reducing conditions. Results highlight the impact of carbonate concentrations on U speciation and solubility in alkaline conditions, informing intermediate-level radioactive waste disposal and radioactively contaminated land management.

Uranium Isotope Fractionation during the Anoxic Mobilization of Noncrystalline U(IV) by Ligand Complexation

Uranium (U) isotopes are suggested as a tool to trace U reduction. However, noncrystalline U(IV), formed predominantly in near-surface environments, may be complexed and remobilized using ligands under anoxic conditions. This may cause additional U isotope fractionation and alter the signatures generated by U reduction. Here, we investigate the efficacy of noncrystalline U(IV) mobilization by ligand complexation and the associated U isotope fractionation. Noncrystalline U(IV) was produced via the reduction of U(VI) (400 μM) by Shewanella oneidensis MR-1 and was subsequently mobilized with EDTA (1 mM), citrate (1 mM), or bicarbonate (500 mM) in batch experiments. Complexation with all investigated ligands resulted in significant mobilization of U(IV) and led to an enrichment of ²³⁸U in the mobilized fraction (δ²³⁸U = 0.4-0.7 ‰ for EDTA; 0.3 ‰ for citrate; 0.2-0.3 ‰ for bicarbonate). For mobilization with bicarbonate, a Rayleigh approach was the most suitable isotope fractionation model, yielding a fractionation factor α of 1.00026-1.00036. Mobilization with EDTA could be modeled with equilibrium isotope fractionation (α: 1.00039-1.00049). The results show that U isotope fractionation associated with U(IV) mobilization under anoxic conditions is significant and needs to be considered when applying U isotopes in remediation monitoring or as a paleo-redox proxy.

Uranium(VI) attenuation in a carbonate-bearing oxic alluvial aquifer

Uranium minerals are commonly found in soils and sediment across the United States at an average concentration of 2-4 mg/kg. Uranium occurs in the environment primarily in two forms, the oxidized, mostly soluble uranium(VI) form, or the reduced, sparingly soluble reduced uranium(IV) form. Here we describe subsurface geochemical conditions that result in low uranium concentrations in an alluvial aquifer with naturally occurring uranium in soils and sediments in the presence of complexing ligands under oxidizing conditions. Groundwater was saturated with respect to calcite and contained calcium (78-90 mg/L) with elevated levels of carbonate alkalinity (291-416 mg/L as HCO₃^-). X-ray adsorption near edge structure (XANES) spectroscopy identified that sediment-associated uranium was oxidized as a uranium(VI) form (85%). Calcite was the predominant mineral by mass in the ultrafine fraction in uranium-bearing sediments (>16 mg/kg). Groundwater geochemical modeling indicated calcite and/or a calcium-uranyl-carbonate mineral such as liebigite in equilibrium with groundwater. The δ¹³C (0.57‰ ± 0.15‰) was indicative of abiotic carbonate deposition. Thus, solid-phase uranium(VI) associated with carbonate is likely maintaining uranium(VI) groundwater levels below the maximum contaminant level (MCL; 30 µg/L), presenting a deposition mechanism for uranium attenuation rather than solely a means of mobilization.

Diagenetic formation of uranium-silica polymers in lake sediments over 3,300 years

The long-term fate of uranium-contaminated sediments, especially downstream former mining areas, is a widespread environmental challenge. Essential for their management is the proper understanding of uranium (U) immobilization mechanisms in reducing environments. In particular, the long-term behavior of noncrystalline U(IV) species and their possible evolution to more stable phases in subsurface conditions is poorly documented, which limits our ability to predict U long-term geochemical reactivity. Here, we report direct evidence for the evolution of U speciation over 3,300 y in naturally highly U-enriched sediments (350-760 µg ⋅ g^-1 U) from Lake Nègre (Mercantour Massif, Mediterranean Alps, France) by combining U isotopic data (δ²³⁸U and (²³⁴U/²³⁸U)) with U L ₃ -edge X-ray absorption fine structure spectroscopy. Constant isotopic ratios over the entire sediment core indicate stable U sources and accumulation modes, allowing for determination of the impact of aging on U speciation. We demonstrate that, after sediment deposition, mononuclear U(IV) species associated with organic matter transformed into authigenic polymeric U(IV)-silica species that might have partially converted to a nanocrystalline coffinite (U^IVSiO₄·nH₂O)-like phase. This diagenetic transformation occurred in less than 700 y and is consistent with the high silica availability of sediments in which diatoms are abundant. It also yields consistency with laboratory studies that proposed the formation of colloidal polynuclear U(IV)-silica species, as precursors for coffinite formation. However, the incomplete transformation observed here only slightly reduces the potential lability of U, which could have important implications to evaluate the long-term management of U-contaminated sediments and, by extension, of U-bearing wastes in silica-rich subsurface environments.

Biological Reduction of a U(V)-Organic Ligand Complex

Metal-reducing microorganisms such as Shewanella oneidensis MR-1 reduce highly soluble species of hexavalent uranyl (U(VI)) to less mobile tetravalent uranium (U(IV)) compounds. The biologically mediated immobilization of U(VI) is being considered for the remediation of U contamination. However, the mechanistic underpinnings of biological U(VI) reduction remain unresolved. It has become clear that a first electron transfer occurs to form pentavalent (U(V)) intermediates, but it has not been definitively established whether a second one-electron transfer can occur or if disproportionation of U(V) is required. Here, we utilize the unusual properties of dpaea^2- ((dpaeaH₂═bis(pyridyl-6-methyl-2-carboxylate)-ethylamine)), a ligand forming a stable soluble aqueous complex with U(V), and investigate the reduction of U(VI)-dpaea and U(V)-dpaea by S. oneidensis MR-1. We establish U speciation through time by separating U(VI) from U(IV) by ion exchange chromatography and characterize the reaction end-products using U M₄-edge high resolution X-ray absorption near-edge structure (HR-XANES) spectroscopy. We document the reduction of solid phase U(VI)-dpaea to aqueous U(V)-dpaea but, most importantly, demonstrate that of U(V)-dpaea to U(IV). This work establishes the potential for biological reduction of U(V) bound to a stabilizing ligand. Thus, further work is warranted to investigate the possible persistence of U(V)-organic complexes followed by their bioreduction in environmental systems.

Role of Iron Sulfide Phases in the Stability of Noncrystalline Tetravalent Uranium in Sediments

Uranium (U) in situ bioremediation has been investigated as a cost-effective strategy to tackle U contamination in the subsurface. While uraninite was believed to be the only product of bioreduction, numerous studies have revealed that noncrystalline U(IV) species (NCU(IV)) are dominant. This finding brings into question the effectiveness of bioremediation because NCU(IV) species are expected to be labile and susceptible to oxidation. Thus, understanding the stability of NCU(IV) in the environment is of crucial importance. Fe(II) minerals (such as FeS) are often associated with U(IV) in bioremediated or naturally reduced sediments. Their impact on the stability of NCU(IV) is not well understood. Here, we show that, at high dissolved oxygen concentrations, FeS accelerates NCU(IV) reoxidation. We hypothesize that either highly reactive ferric minerals or radical S species produced by the oxidation of FeS drive this rapid reoxidation of NCU(IV). Furthermore, we found evidence for the contribution of reactive oxygen species to NCU(IV) reoxidation. This work refines our understanding of the role of iron sulfide minerals in the stability of tetravalent uranium in the presence of oxygen in a field setting such as contaminated sites or uranium-bearing naturally reduced zones.

Effect of Bicarbonate, Calcium, and pH on the Reactivity of As(V) and U(VI) Mixtures

Natural or anthropogenic processes can increase the concentration of uranium (U) and arsenic (As) above the maximum contaminant levels in water sources. Bicarbonate and calcium (Ca) can have major impacts on U speciation and can affect the reactivity between U and As. We therefore investigated the reactivity of aqueous U and As mixtures with bicarbonate and Ca for acidic and neutral pH conditions. In experiments performed with 1 mM U and As mixtures, 10 mM Ca, and without added bicarbonate (pCO2 = 3.5), aqueous U decreased to <0.25 mM at pH 3 and 7. Aqueous As decreased the most at pH 3 (∼0.125 mM). Experiments initiated with 0.005 mM As and U showed similar trends. X-ray spectroscopy (i.e., XAS and EDX) and diffraction indicated that U-As-Ca- and U-Ca-bearing solids resemble uranospinite [Ca(UO2)2(AsO4)2·10H2O] and becquerelite [Ca(UO2)6O4(OH)6·8(H2O)]. These findings suggest that U-As-Ca-bearing solids formed in mixed solutions are stable at pH 3. However, the dissolution of U-As-Ca and U-Ca-bearing solids at pH 7 was observed in reactors containing 10 mM bicarbonate and Ca, suggesting a kinetic reaction of aqueous uranyl-calcium-carbonate complexation. Our study provides new insights regarding U and As mobilization for risk assessment and remediation strategies.

Microbial U Isotope Fractionation Depends on the U(VI) Reduction Rate

U isotope fractionation may serve as an accurate proxy for U(VI) reduction in both modern and ancient environments, if the systematic controls on the magnitude of fractionation (ε) are known. We model the effect of U(VI) reduction kinetics on U isotopic fractionation during U(VI) reduction by a novel Shewanella isolate, Shewanella sp. (NR), in batch incubations. The measured ε values range from 0.96 ± 0.16 to 0.36 ± 0.07‰ and are strongly dependent on the U(VI) reduction rate. The ε decreases with increasing reduction rate constants normalized by cell density and initial U(VI). Reactive transport simulations suggest that the rate dependence of ε is due to a two-step process, where diffusive transport of U(VI) from the bulk solution across a boundary layer is followed by enzymatic reduction. Our results imply that the spatial decoupling of bulk U(VI) solution and enzymatic reduction should be taken into account for interpreting U isotope data from the environment.

Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments

Uranium contamination threatens the availability of safe and clean drinking water globally. This toxic element occurs both naturally and as a result of mining and ore-processing in alluvial sediments, where it accumulates as tetravalent U [U(IV)], a form once considered largely immobile. Changing hydrologic and geochemical conditions cause U to be released into groundwater. Knowledge of the chemical form(s) of U(IV) is essential to understand the release mechanism, yet the relevant U(IV) species are poorly characterized. There is growing belief that natural organic matter (OM) binds U(IV) and mediates its fate in the subsurface. In this work, we combined nanoscale imaging (nano secondary ion mass spectrometry and scanning transmission X-ray microscopy) with a density-based fractionation approach to physically and microscopically isolate organic and mineral matter from alluvial sediments contaminated with uranium. We identified two populations of U (dominantly +IV) in anoxic sediments. Uranium was retained on OM and adsorbed to particulate organic carbon, comprising both microbial and plant material. Surprisingly, U was also adsorbed to clay minerals and OM-coated clay minerals. The dominance of OM-associated U provides a framework to understand U mobility in the shallow subsurface, and, in particular, emphasizes roles for desorption and colloid formation in its mobilization.

Formation of a U(VI)-Persulfide Complex during Environmentally Relevant Sulfidation of Iron (Oxyhydr)oxides

Uranium is a risk-driving radionuclide in both radioactive waste disposal and contaminated land scenarios. In these environments, a range of biogeochemical processes can occur, including sulfate reduction, which can induce sulfidation of iron (oxyhydr)oxide mineral phases. During sulfidation, labile U(VI) is known to reduce to relatively immobile U(IV); however, the detailed mechanisms of the changes in U speciation during these biogeochemical reactions are poorly constrained. Here, we performed highly controlled sulfidation experiments at pH 7 and pH 9.5 on U(VI) adsorbed to ferrihydrite and investigated the system using geochemical analyses, X-ray absorption spectroscopy (XAS), and computational modeling. Analysis of the XAS data indicated the formation of a novel, transient U(VI)-persulfide complex as an intermediate species during the sulfidation reaction, concomitant with the transient release of uranium to the solution. Extended X-ray absorption fine structure (EXAFS) modeling showed that a persulfide ligand was coordinated in the equatorial plane of the uranyl moiety, and formation of this species was supported by computational modeling. The final speciation of U was nanoparticulate U(IV) uraninite, and this phase was evident at 2 days at pH 7 and 1 year at pH 9.5. Our identification of a new, labile U(VI)-persulfide species under environmentally relevant conditions may have implications for U mobility in sulfidic environments pertinent to radioactive waste disposal and contaminated land scenarios.

Effect of Aging on the Stability of Microbially Reduced Uranium in Natural Sediment

Reductive immobilization of uranium has been explored as a remediation strategy for the U-contaminated subsurface. Via the in situ biostimulation of microbial processes, hexavalent U is reduced to less soluble tetravalent species, which are immobilized within the sediment. Although the mineral uraninite (UO₂) was initially considered the dominant product of biological reduction, non-crystalline U(IV) species (NCU(IV)) are found to be abundant in the environment despite their greater susceptibility to oxidation and remobilization. However, it has been recently proposed that, through aging, NCU(IV) might transform into UO₂, which would potentially enhance the stability of the reduced U pool. In this study, we performed column experiments to produce NCU(IV) species in natural sediment mimicking the environmental conditions during bioremediation. Bioreduced sediment retrieved from the columns and harboring NCU(IV) was incubated in static microcosms under anoxic conditions to allow the systematic monitoring of U coordination by X-ray absorption spectroscopy (XAS) over 12 months. XAS revealed that, under the investigated conditions, the speciation of U(IV) does not change over time. Thus, because NCU(IV) is the dominant species in the sediment, bioreduced U(IV) species remain vulnerable to oxidation and remobilization in the aqueous phase even after a 12-month aging period.

Full crystal structure, hydrogen bonding and spectroscopic, mechanical and thermodynamic properties of mineral uranopilite

The determination of the full crystal structure of the uranyl sulfate mineral uranopilite, including the positions of the H atoms in the corresponding unit cell, has not been feasible to date due to the poor quality of its X-ray diffraction pattern.

Effect of Bicarbonate and Oxidizing Conditions on U(IV) and U(VI) Reactivity in Mineralized Deposits of New Mexico

We investigated the effect of bicarbonate and oxidizing agents on uranium (U) reactivity and subsequent dissolution of U(IV) and U(VI) mineral phases in the mineralized deposits from Jackpile mine, Laguna Pueblo, New Mexico, by integrating laboratory experiments with spectroscopy, microscopy and diffraction techniques. Uranium concentration in solid samples from mineralized deposit obtained for this study exceeded 7000 mg kg-1, as determined by X-ray fluorescence (XRF). Results from X-ray photoelectron spectroscopy (XPS) suggest the coexistence of U(VI) and U(IV) at a ratio of 19:1 at the near surface region of unreacted solid samples. Analyses made using X-ray diffraction (XRD) and electron microprobe detected the presence of coffinite (USiO4) and uranium-phosphorous-potassium (U-P-K) mineral phases. Imaging, mapping and spectroscopy results from scanning transmission electron microscopy (STEM) indicate that the U-P-K phases were encapsulated by carbon. Despite exposing the solid samples to strong oxidizing conditions, the highest aqueous U concentrations were measured from samples reacted with 100% air saturated 10 mM NaHCO3 solution, at pH 7.5. Analyses using X-ray absorption spectroscopy (XAS) indicate that all the U(IV) in these solid samples were oxidized to U(VI) after reaction with dissolved oxygen and hypochlorite (OCl-) in the presence of bicarbonate (HCO3 -). The reaction between these organic rich deposits, and 100% air saturated bicarbonate solution (containing dissolved oxygen), can result in considerable mobilization of U in water, which has relevance to the U concentrations observed at the Rio Paguate across the Jackpile mine. Results from this investigation provide insights on the reactivity of carbon encapsulated U-phases under mild and strong oxidizing conditions that have important implication in U recovery, remediation and risk exposure assessment of sites.

Metaschoepite Dissolution in Sediment Column Systems-Implications for Uranium Speciation and Transport

Metaschoepite is commonly found in U-contaminated environments and metaschoepite-bearing wastes may be managed via shallow or deep disposal. Understanding metaschoepite dissolution and tracking the fate of any liberated U is thus important. Here, discrete horizons of metaschoepite (UO₃·nH₂O) particles were emplaced in flowing sediment/groundwater columns representative of the UK Sellafield Ltd. site. The column systems either remained oxic or became anoxic due to electron donor additions, and the columns were sacrificed after 6- and 12-months for analysis. Solution chemistry, extractions, and bulk and micro/nano-focus X-ray spectroscopies were used to track changes in U distribution and behavior. In the oxic columns, U migration was extensive, with UO₂²⁺ identified in effluents after 6-months of reaction using fluorescence spectroscopy. Unusually, in the electron-donor amended columns, during microbially mediated sulfate reduction, significant amounts of UO₂-like colloids (>60% of the added U) were found in the effluents using TEM. XAS analysis of the U remaining associated with the reduced sediments confirmed the presence of trace U(VI), noncrystalline U(IV), and biogenic UO₂, with UO₂ becoming more dominant with time. This study highlights the potential for U(IV) colloid production from U(VI) solids under reducing conditions and the complexity of U biogeochemistry in dynamic systems.

A New Frontier for Palaeobiology: Earth's Vast Deep Biosphere

Diverse micro-organisms populate a global deep biosphere hosted by rocks and sediments beneath land and sea, containing more biomass than any other biome except forests. This paper reviews an emerging palaeobiological archive of these dark habitats: microfossils preserved in ancient pores and fractures in the crust. This archive, seemingly dominated by mineralized filaments (although rods and coccoids are also reported), is presently far too sparsely sampled and poorly understood to reveal trends in the abundance, distribution, or diversity of deep life through time. New research is called for to establish the nature and extent of the fossil record of Earth's deep biosphere by combining systematic exploration, rigorous microanalysis, and experimental studies of both microbial preservation and the formation of abiotic pseudofossils within the crust. It is concluded that the fossil record of Earth's largest microbial habitat may still have much to tell us about the history of life, the evolution of biogeochemical cycles, and the search for life on Mars.

Organic Functional Group Chemistry in Mineralized Deposits Containing U(IV) and U(VI) from the Jackpile Mine in New Mexico

We investigated the functional group chemistry of natural organic matter (NOM) associated with both U(IV) and U(VI) in solids from mineralized deposits exposed to oxidizing conditions from the Jackpile Mine, Laguna Pueblo, NM. The uranium (U) content in unreacted samples was 0.44-2.6% by weight determined by X-ray fluorescence. In spite of prolonged exposure to ambient oxidizing conditions, ≈49% of U(IV) and ≈51% of U(VI) were identified on U L_III edge extended X-ray absorption fine structure spectra. Loss on ignition and thermogravimetric analyses identified from 13% to 44% of NOM in the samples. Carbonyl, phenolic, and carboxylic functional groups in the unreacted samples were identified by fitting of high-resolution X-ray photoelectron spectroscopy (XPS) C 1s and O 1s spectra. Peaks corresponding to phenolic and carbonyl functional groups had intensities higher than those corresponding to carboxylic groups in samples from the supernatant from batch extractions conducted at pH 13, 7, and 2. U(IV) and U(VI) species were detected in the supernatant after batch extractions conducted under oxidizing conditions by fitting of high-resolution XPS U 4f spectra. The outcomes from this study highlight the importance of the influence of pH on the organic functional group chemistry and U speciation in mineralized deposits.

The Role of Organic Matter on Uranium Precipitation in Zoovch Ovoo, Mongolia

The Zoovch Ovoo uranium deposit is located in East Gobi Basin in Mongolia. It is hosted in the Sainshand Formation, a Late Cretaceous siliciclastic reservoir, in the lower part of the post-rift infilling of the Mesozoic East Gobi Basin. The Sainshand Formation corresponds to poorly consolidated medium-grained sandy intervals and clay layers deposited in fluvial-lacustrine settings. The uranium deposit is confined within a 60- to 80-m-thick siliciclastic reservoir inside aquifer driven systems, assimilated to roll-fronts. As assessed by vitrinite reflectance (%Rr < 0.4) and molecular geochemistry, the formation has never experienced significant thermal maturation. Detrital organic matter (type III and IV kerogens) is abundant in the Zoovch Ovoo depocenter. In this framework, uranium occurs as: (i) U-rich macerals without any distinguishable U-phase under SEM observation, containing up to 40 wt % U; (ii) U expressed as UO2 at the rims of large (several millimeters) macerals and (iii) U oxides partially to entirely replacing macerals, while preserving the inherited plant texture. Thus, uranium is accumulated gradually in the macerals through an organic carbon–uranium epigenization process, in respect to the maceral’s chemistry and permeability. Most macerals are rich in S and, to a lesser extent, in Fe. Frequently, Fe and S contents do not fit the stoichiometry of pyrite, although pyrite also occurs as small inclusions within the macerals. The organic matter appears thus as a major redox trap for uranium in this kind of geological setting.

Redox Fluctuations and Organic Complexation Govern Uranium Redistribution from U(IV)-Phosphate Minerals in a Mining-Polluted Wetland Soil, Brittany, France

Wetlands have been proposed to naturally attenuate U transfers in the environment via U complexation by organic matter and potential U reduction. However, U mobility may depend on the identity of particulate/dissolved uranium source materials and their redox sensitivity. Here, we examined the fate of uranium in a highly contaminated wetland (up to 4500 mg·kg^-1 U) impacted by former mine water discharges. Bulk U L_III-EXAFS and (micro-)XANES combined with SEM-EDXS analyses of undisturbed soil cores show a sharp U redox boundary at the water table, together with a major U redistribution from U(IV)-minerals to U(VI)-organic matter complexes. Above the water table, U is fully oxidized into mono- and bidentate U(VI)-carboxyl and monodentate U(VI)-phosphoryl complexes. Minute amounts of U(VI)-phosphate minerals are also observed. Below the water table, U is fully reduced and is partitioned between U(IV)-phosphate minerals (i.e., ningyoite and a lermontovite-like phase), and bidentate U(IV)-phosphoryl and monodentate U(IV)-carboxyl complexes. Such a U redistribution from U-minerals inherited from mine water discharge deposits could result from redox cycling nearby the water table fluctuation zone. Oxidative dissolution of U(IV)-phosphate minerals could have led to U(VI)-organic matter complexation, followed by subsequent reduction into U(IV)-organic complexes. However, uranium(IV) minerals could have been preserved in permanently waterlogged soil.

Reduction spheroids preserve a uranium isotope record of the ancient deep continental biosphere

Life on Earth extends to several kilometres below the land surface and seafloor. This deep biosphere is second only to plants in its total biomass, is metabolically active and diverse, and is likely to have played critical roles over geological time in the evolution of microbial diversity, diagenetic processes and biogeochemical cycles. However, these roles are obscured by a paucity of fossil and geochemical evidence. Here we apply the recently developed uranium-isotope proxy for biological uranium reduction to reduction spheroids in continental rocks (red beds). Although these common palaeo-redox features have previously been suggested to reflect deep bacterial activity, unequivocal evidence for biogenicity has been lacking. Our analyses reveal that the uranium present in reduction spheroids is isotopically heavy, which is most parsimoniously explained as a signal of ancient bacterial uranium reduction, revealing a compelling record of Earth’s deep biosphere.

Red beds contain reduction spheroids that formed underground millions of years ago and whose origin remains poorly constrained. Here the authors use uranium isotopes to identify ancient fingerprints of bacteria in these features, confirming that they were produced by subsurface life in the geological past.

Transformation of uranium species in soil during redox oscillations

Redox oscillation is commonly found in near-surface environment, where soils are often polluted with many redox active contaminants, including uranium (U). In order to investigate the transformation of U species in near-surface soil under redox oscillations conditions, redox oscillations and reduction experiments were performed, biogeochemical parameters and native microbial community composition were monitored, main elements on the surface of solid-phase were analyzed by XPS, and labile U(IV) species and stable U(IV) species in solid-phase were provisionally defined using an anoxic 1 M sodium bicarbonate extraction. It was found that redox oscillations slightly increased the water-soluble U but significantly increased the stable U(IV) species (P < 0.05) in soil. In reduction experiment, there was upper limit value for percentage of stable U(IV) species, and the labile U(IV) species could not transform to stable U(IV) species in a short period of time under reduction conditions. The redox transition of Fe enriched on the surface of soil and the conversion of microbial community composition played a major role in speciation transformation of U under redox oscillations conditions. In addition, sequential extraction revealed that the increase of stable U(IV) species content reflected the U speciation transition from acetate extract to more recalcitrant hydroxylamine extract. The finding provides a potential method for improving the stability of U when bio-reduction is used to remediate the U-contaminated soils.

Characterization of uranium redox state in organic-rich Eocene sediments

The presence of organic matter (OM) has a profound impact on uranium (U) redox cycling, either limiting or promoting the mobility of U via binding, reduction, or complexation. To understand the interactions between OM and U, we characterised U oxidation state and speciation in nine OM-rich sediment cores (18 samples), plus a lignite sample from the Mulga Rock polymetallic deposit in Western Australia. Uranium was unevenly dispersed within the analysed samples with 84% of the total U occurring in samples containing >21 wt % OM. Analyses of U speciation, including x-ray absorption spectroscopy and bicarbonate extractions, revealed that U existed predominately (∼71%) as U(VI), despite the low pH (4.5) and nominally reducing conditions within the sediments. Furthermore, low extractability by water, but high extractability by a bi-carbonate solution, indicated a strong association of U with particulate OM. The unexpectedly high proportion of U(VI) relative to U(IV) within the OM-rich sediments implies that OM itself does not readily reduce U, and the reduction of U is not a requirement for immobilizing uranium in OM-rich deposits. The fact that OM can play a significant role in limiting the mobility and reduction of U(VI) in sediments is important for both U-mining and remediation.

Expanding beyond canonical metabolism: Interfacing alternative elements, synthetic biology, and metabolic engineering

Metabolic engineering offers an exquisite capacity to produce new molecules in a renewable manner. However, most industrial applications have focused on only a small subset of elements from the periodic table, centered around carbon biochemistry. This review aims to illustrate the expanse of chemical elements that can currently (and potentially) be integrated into useful products using cellular systems. Specifically, we describe recent advances in expanding the cellular scope to include the halogens, selenium and the metalloids, and a variety of metal incorporations. These examples range from small molecules, heteroatom-linked uncommon elements, and natural products to biomining and nanotechnology applications. Collectively, this review covers the promise of an expanded range of elemental incorporations and the future impacts it may have on biotechnology.

Uranium dispersion from U tailings and mechanisms leading to U accumulation in sediments: Insights from biogeochemical and isotopic approaches

Uranium contamination is a worldwide problem that grows proportionally to human demands for energy and armory. Understanding U cycling in the environment is of eminent interest, mostly concerning ecosystems directly impacted by point sources. In Bow Lake (Ontario, Canada), which is located adjacent to a former U mine, exceptionally high concentrations of U are related to U dispersion from tailings and biogeochemical processes such as biotic reduction and adsorption. This has been shown by a U-Pb isotope composition model. In this study, we use U isotope fractionation (δ²³⁸U) to highlight U cycling and the role of bacteria (Geobacteraceae and sulfate-reducing bacteria) in affecting U cycling. Bacteria affected U cycling directly via biotic U reduction and indirectly via reductive dissolution of carrier phases. All the processes are interconnected through diagenetic reactions with the supply of bioavailable organic matter being the primary driving force of the diagenesis. This study is the first to use multiple biogeochemical and isotopic approaches to track U cycling from a contamination point source to U storage in lake sediments.