Complexation by Organic Matter Controls Uranium Mobility in Anoxic Sediments

Coupling effect of phytic acid and chlorite on uranium removal by Shewanella putrefaciens under simulated natural environment

Understanding the transport and transformation of uranium species in surface and underground water environments, including biological migration, is a critical issue in nuclear environmental research. While it is known that uranium is primarily immobilized through reduction and mineralization with clay minerals and microorganisms, the coupled processes involving these motifs and their molecular mechanisms under multiple environmental factors remain elusive. In this work, using phytic acid (denoted as IP6 hereinafter) as a typical organic ligand and chlorite (a layered hydrous aluminum silicate with Mg2+, Fe2+, etc.) as a typical inorganic clay mineral, we investigate the immobilization of UO22+ [abbreviated as U(VI) here in after] by S. putrefaciens under simulated natural aerobic conditions. The experimental results show that in aerobic environment, the reduction of uranium by S. putrefaciens is inhibited, and only mineralized products are observed. In the chlorite-U(VI) system, UO22+ enters the chlorite interlayer due to internal channel restriction effects and becomes more firmly fixed over time. When there are decomposed S. putrefaciens, UO22+ is fixed by its stronger binding sites and adsorbed on the surface of chlorite. After adding phytic acid, the process of uranium biomineralization by S. putrefaciens is blocked. In the simulated aerobic four component system, some Fe2+ and Mg2+ ions dissolve from the chlorite and replace the UO22+ adsorbed by IP6. The released UO22+ combines with the decomposing S. putrefaciens, and the complex deposits on the surface of the chlorite. At low pH values (pH<5.5), IP6 dissolves the metal ions of the chlorite and reduces their coupling with uranium coordination. Quantum-chemical methods are utilized to understand the experimental results at the molecular level and to provide an analysis of the interlayer structure of uranium in clay minerals. This study delves into the migration and fixation mechanisms of uranium in complex aerobic water environments, providing a theoretical basis for in-situ remediation of uranium contaminated water environments and understanding the biogeochemical behavior of nuclides in aquatic ecological environments.

Sequestration process and mechanism of U(VI) on montmorillonite-aspergillus niger composite

The existence state and spatiotemporal evolution process of uranium in mineral-microbe complex systems are important factors that constrain its ecotoxicity. This study investigated the sequestration of U(VI) by montmorillonite-Aspergillus niger (MTA) composite using bioassay and spectroscopies approaches. The results demonstrate that the sequestration process and mechanism of U(VI) on MTA differ substantially from those of individual components. Under neutral conditions, the sorption of U(VI) decreased from 92.4 ± 4.6 % on MT to 73.2 ± 2.4 % on MTA4 and 74.9 ± 6.3 % on MTA10, respectively, while the stability of U(VI) species on MTA increased obviously compared to MT. In the case of MTA formed over 4 days (MTA4), the biosorption effect of A. niger hyphae dominated the sequestration of U(VI). In contrast, for MTA formed over 10 days (MTA10), the interactions between MT and A. niger became more pronounced, and the hyphae of Aspergillus niger played a pivotal role in U(VI) sequestration, immobilizing U(VI) through complexation with organic ligands and bioreduction reactions. The high expandability of MT facilitated the penetration of extracellular polymeric substances (EPS) from A. niger into its interlayer of MT, enhancing U(VI) complexation and reduction. These processes significantly contributed to the effective sequestration of U(VI) by the MTA composite.

Inoculation with Bacillus thuringiensis reduces uptake and translocation of Pb/Cd in soil-wheat system: A life cycle study

Microbial inoculation is an important strategy to reduce the supply of heavy metals (HMs) in soil-crop systems. However, the mechanisms of microbial inoculation for the availability of HMs in soil and their accumulation/transfer in crops remain unclear. Here, the inhibitory effect of inoculation with Bacillus thuringiensis on the migration and accumulation of Pb/Cd in the soil-wheat system during the whole growth period was investigated by pot experiments. The results showed that inoculation with Bacillus thuringiensis increased soil pH and available nutrients (including carbon, nitrogen, and phosphorus), and enhanced the activities of nutrient-acquiring enzymes. Dominance analysis showed that dissolved organic matter (DOM) is the key factor affecting the availability of HMs. The content of colored spectral clusters and humification characteristics of DOM were significantly improved by inoculation, which is conducive to reducing the availability of Pb/Cd, especially during the flowering stage, the decrease was 12.8 %. Inoculation decreased Pb/Cd accumulation in the shoot and the transfer from root to shoot, with the greatest decreases at the jointing and seedling stages (27.0-34.1 % and 6.9-11.8 %), respectively. At the maturity stage, inoculation reduced the Pb/Cd accumulation in grain (12.9-14.7 %) and human health risk (4.1-13.2 %). The results of Pearson correlation analysis showed that the availability of Pb/Cd was positively correlated with the humification of DOM. Least square path model analysis showed that Bacillus thuringiensis could significantly reduce Pb/Cd accumulation in the grain and human health risks by regulating DOM spectral characteristics, the availability of HMs in soil and metals accumulation/transport in wheat at different growth stages. This study revealed the inhibition mechanism of Bacillus thuringiensis on migration of Pb/Cd in a soil-wheat system from a viewpoint of a full life cycle, which offers a valuable reference for the in-situ remediation of HM-contaminated soil and the safe production of food crops in field.

Reduction capacity in the transmissive zones fueled by the embedded low-permeability lenses: Implications for contaminant transformation in heterogeneous aquifers

Redox conditions play a decisive role in regulating contaminant and nutrient transformation in groundwater. Here we quantitatively described and interpreted the temporal and spatial variations of aquifer reduction capacity formation in lens-embedded heterogeneous aquifers in 1-D columns. Experimental results indicated that the aquifer reduction capacity exported from the low-permeability lens permeated into the downstream sandy zones, where it subsequently accumulated and extended. Reactive transport modeling suggested that reduction capacity within the lens preferentially diffused to the transmissive zones around the lens-sand interface, and was then transported via convection to downstream transmissive zones. A low-permeability lens of the same volume, but more elongated in the flow direction, led to less concentrated reduction capacity but extended further downgradient from the lens. The increased flow velocity attenuated the maintenance of aquifer reduction capacity by enhancing mixing and diluting processes in the transmissive zones. The reduction zones formed downstream from the low-permeability lens were hotpots for resisting the oxidative perturbation by O2. This study highlights the important role of low-permeability lenses as large and long-term electron pools for the transmissive zones, and thus providing aquifer reduction capacity for contaminant transformation and remediation in heterogeneous aquifers.

Molecular features of uranium-binding natural organic matter in a riparian wetland determined by ultrahigh resolution mass spectrometry

Tims Branch riparian wetland located in South Carolina, USA has immobilized 94 % of the U released >50 years ago from a nuclear fuel fabrication facility. Sediment organic matter (OM) has been shown to play an important role in immobilizing U. Yet, uranium-OM-mineral interactions at the molecular scale have never been investigated at ambient concentrations. The objectives of this study were to extract, purify, and concentrate U-bound sediment OM along the stream water pathway and perform molecular characterization using Fourier transform ion cyclotron resonance mass spectrometry (FTICRMS). Out of 9614 identified formulas, 715 contained U. These U-containing formulas were enriched with Fe, N, and/or S compared to the total OM. Lignin-like and protein-like molecules accounted for 40 % and 19 % of the U-containing formulas, respectively. Phosphorus-containing formulas were found to exert an insignificant influence on complexing U. U-containing formulas in the 'mobile' (groundwater extractable) OM fraction had lower (reduced) nominal oxidation states of carbon (NOSC); and less aromatic moieties than OM recovered from the 'immobile' (sodium pyrophosphate extractable) OM fraction. U-containing formulas in the redox interfacial zones (stream banks) compared to those in nearby up-slope zones tended to have smaller molecular weights; lower NOSC; higher contents of COO and/or CONO functional groups; and higher abundance of Fe-containing formulas. Fe was present in 38 % of the U-containing formulas but only 20 % of the total OM formulas. It is postulated that Fe played an important role in stabilizing the structure of sedimentary OM, especially U-containing compounds. The identification for the first time of hundreds of Fe-U-OM formulas demonstrates the complexity of such system is much greater than commonly believed and numerically predicting U binding behavior in OM-rich systems may require greater use of statistical or artificial intelligence approaches rather than deterministic approaches limited to measuring metal complexation with well-defined individual analogue organic ligands.

Machine learning surrogates for surface complexation model of uranium sorption to oxides

The safety assessments of the geological storage of spent nuclear fuel require understanding the underground radionuclide mobility in case of a leakage from multi-barrier canisters. Uranium, the most common radionuclide in non-reprocessed spent nuclear fuels, is immobile in reduced form (U(IV) and highly mobile in an oxidized state (U(VI)). The latter form is considered one of the most dangerous environmental threats in the safety assessments of spent nuclear fuel repositories. The sorption of uranium to mineral surfaces surrounding the repository limits their mobility. We quantify uranium sorption using surface complexation models (SCMs). Unfortunately, numerical SCM solvers often encounter convergence problems due to the complex nature of convoluted equations and correlations between model parameters. This study explored two machine learning surrogates for the 2-pK Triple Layer Model of uranium retention by oxide surfaces if released as U(IV) in the oxidizing conditions: random forest regressor and deep neural networks. Our surrogate models, particularly DNN, accurately reproduce SCM model predictions at a fraction of the computational cost without any convergence issues. The safety assessment of spent fuel repositories, specifically the migration of leaked radioactive waste, will benefit from having ultrafast AI/ML surrogates for the computationally expensive sorption models that can be easily incorporated into larger-scale contaminant migration models. One such model is presented here.

Adsorption of uranium (VI) complexes with polymer-based spherical activated carbon

Adsorption processes with carbon-based adsorbents have received substantial attention as a solution to remove uranium from drinking water. This study investigated uranium adsorption by a polymer-based spherical activated carbon (PBSAC) characterised by a uniformly smooth exterior and an extended surface of internal cavities accessible via mesopores. The static adsorption of uranium was investigated applying varying PBSAC properties and relevant solution chemistry. Spatial time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed to visualise the distribution of the different uranium species in the PBSAC. The isotherms and thermodynamics calculations revealed monolayer adsorption capacities of 28-667 mg/g and physical adsorption energies of 13-21 kJ/mol. Increasing the surface oxygen content of the PBSAC to 10 % enhanced the adsorption and reduced the equilibrium time to 2 h, while the WHO drinking water guideline of 30 µgU/L could be achieved for an initial concentration of 250 µgU/L. Uranium adsorption with PBSAC was favourable at the pH 6-8. At this pH range, uranyl carbonate complexes (UO2CO3(aq), UO2(CO3)22-, (UO2)2CO3(OH)3-) predominated in the solution, and the ToF-SIMS analysis revealed that the adsorption of these complexes occurred on the surface and inside the PBSAC due to intra-particle diffusion. For the uranyl cations (UO22+, UO2OH+) at pH 2-4, only shallow adsorption in the outermost PBSAC layers was observed. The work demonstrated the effective removal of uranium from contaminated natural water (67 µgU/L) and meeting both German (10 µgU/L) and WHO guideline concentrations. These findings also open opportunities to consider PBSAC in hybrid treatment technologies for uranium removal, for instance, from high-level radioactive waste.

Uranium: an overview of physicochemical properties, exposure assessment methodologies, and health effects of environmental and occupational exposure

Uranium is chemo- and radiotoxic element which can cause multifactorial health hazards. Natural and anthropogenic uranium contamination raises concerns about potential public health problems. Natural contamination plays a significant role with regard to uranium exposure in the general population, whereas anthropogenic contamination leads to occupational uranium exposure, particularly in nuclear industry workers. In this review, we present a state-of-the-art status concerning uranium-induced health risks with a focus on epidemiological findings of uranium processing and enrichment plant workers. We provide a general overview of physicochemical properties of uranium and analytical methods for measuring or monitoring uranium, describe environmental and occupational exposure scenarios, and discuss the challenges for objectively investigating risks from uranium exposure.

Competitive adsorption of lead and cadmium on soil aggregate at micro-interfaces: Multi-surface modeling and spectroscopic studies

Aggregates are the basic structural units of soils and play a crucial role in metal migration and transformation. Combined contamination of lead (Pb) and cadmium (Cd) is common in site soils, and the two metals may compete for the same adsorption sites and affect their environmental behavior. Herein, the adsorption behavior of Pb and Cd on aggregates of two soils and contributions of soil components in single and competitive systems were studied by combining cultivation experiments, batch adsorption, multi-surface models (MSMs), and spectroscopic techniques. The results demonstrated that < 2 µm size aggregate was the dominant sink for Pb and Cd competitive adsorption in both soils. Compared with Pb, the adsorption capacity and behavior of Cd were affected greatly under competition. MSMs prediction revealed that soil organic matter (SOM) contributed the most to Cd and Pb adsorption on aggregates (> 68.4%), but the dominant competitive effect occurred on different sites for Cd adsorption (primarily on SOM) and Pb adsorption (primarily on clay minerals). Further, 2 mM Pb coexistence caused 5.9 - 9.8% of soil Cd conversion to unstable species (Cd(OH)₂). Thus, the competitive effect of Pb on Cd adsorption cannot be ignored in soils with high content of SOM and fine aggregates.

Nitrate-Stimulated Release of Naturally Occurring Sedimentary Uranium

Groundwater uranium (U) concentrations have been measured above the U.S. EPA maximum contaminant level (30 μg/L) in many U.S. aquifers, including in areas not associated with anthropogenic contamination by milling or mining. In addition to carbonate, nitrate has been correlated to uranium groundwater concentrations in two major U.S. aquifers. However, to date, direct evidence that nitrate mobilizes naturally occurring U from aquifer sediments has not been presented. Here, we demonstrate that the influx of high-nitrate porewater through High Plains alluvial aquifer silt sediments bearing naturally occurring U(IV) can stimulate a nitrate-reducing microbial community capable of catalyzing the oxidation and mobilization of U into the porewater. Microbial reduction of nitrate yielded nitrite, a reactive intermediate, which was further demonstrated to abiotically mobilize U from the reduced alluvial aquifer sediments. These results indicate that microbial activity, specifically nitrate reduction to nitrite, is one mechanism driving U mobilization from aquifer sediments in addition to previously described bicarbonate-driven desorption from mineral surfaces, such as Fe(III) oxides.

Decontamination of neutron-activated radioactive concrete waste by separating Eu, Co, Fe, and Mn-containing sand particles using dense medium separation

Neutron-activated concrete waste is one of the most challenging radioactive wastes to decontaminate because the radionuclides exist in a chemically stable binding state, and it is very difficult to break those bindings with the conventional acid decontamination method. Here, we suggest a new dense medium separation (DMS) of felsic and mafic minerals from simulated neutron-activated concrete waste using sodium-polytungstate (SPT) solution because most elements (Eu, Co, Fe, and Mn) that can be activated by neutrons are concentrated in mafic minerals. We also determined the optimal density of the SPT solution as ∼ 2.70 g/cm³, and a high degree of decontamination was achieved for sand particles ranging from 75 to 500 µm in size. Under these optimized conditions, DMS (80.02%) exhibits much higher radionuclide removal efficiency (RRE) than 5 M acid decontaminations (23.27-31.29%) for Eu. Furthermore, DMS (59.38-63.36%) shows similar RRE to 5 M acid decontaminations (41.67-73.94%) for Fe, Mn, and Co. We believe this DMS process could be useful and applicable to the decontamination of neutron-activated concrete wastes because it is possible to perform a large-scale process compared to conventional acid decontamination methods, which is also advantageous in reducing secondary waste generation and facile radionuclide recovery.

Nano-scale analysis of uranium release behavior from river sediment in the Ili basin

Due to the limitations of the conventional water sample pretreatment methods, some of the colloidal uranium (U) has long been misidentified as "dissolved" phase. In this work, the U species in river water in the Ili Basin was classified into submicron-colloidal (0.1-1 μm), nano-colloidal (0.1 μm-3 kDa) and dissolved phases (< 3 kDa) by using high-speed centrifugation and ultrafiltration. The U concentration in the river water was 5.39-8.75 μg/L, which was dominated by nano-colloidal phase (55-70%). The nano-colloidal particles were mainly composed of particulate organic matter (POM) and had a very high adsorption capacity for U (accounting for 70 ± 23% of colloidal U). Sediment disturbance, low temperature, and high inorganic carbon greatly improved the release of nano-colloidal U, but high levels of Ca²⁺ inhibited it. The simulated river experiments indicated that the flow regime determined the release of nano-colloidal U, and large amounts of nano-colloidal U might be released during spring floods in the Ili basin. Moreover, global warming increases river flow and inorganic carbon content, which may greatly promote the release and migration of nano-colloidal U.

Effect of humic acid derived from leonardite on the redistribution of uranium fractions in soil

Humic acids (HAs) are complex organic substances with abundant functional groups (e.g., carboxyl, phenolic-OH, etc.). They are commonly distributed in the soil environment and exert a double-edged sword effect in controlling the migration and transformation of uranium. However, the effects of HAs on dynamic processes associated with uranium transformation are still unclear. In this study, we used HAs derived from leonardite (L-HA) and commercial HA (C-HA) as exogenous organic matter and C-HA as the reference. UO₂, UO₃, and UO₂(NO₃)₂ were used as the sources of U to explore the fractionations of uranium in the soil. We also studied the behavior of the HA. The incubation experiments were designed to investigate the effects of HA on the soil pH, uranium fraction transformation, dynamic behavior of exchangeable, weak acid, and labile uranium. The observations were made for one month. The results showed that soil pH decreased for L-HA but increased for C-HA. Under these conditions, uranium tended to transform into an inactive fraction. The dynamic behavior of exchangeable, weak acid, and labile uranium varied with the sources of HA and uranium. This study highlighted that HA could affect soil pH and the dynamic redistribution of U fractions. The results suggest that the sources of HA and U should be considered when using HA as the remediation material for uranium-contaminated soils.

Sediment spiking and equilibration procedures to achieve partitioning of uranium similar to contamination in tropical wetlands near a mine site

The derivation of sediment quality guideline values (SQGVs) presents significant challenges. Arguably the most important challenge is to conduct toxicity tests using contaminated sediments with physico-chemistry that represents real-world scenarios. We used a novel metal spiking method for an experiment that ultimately aims to derive a uranium SQGV. Two pilot studies were conducted to inform the final spiking design, i.e. percolating a uranyl sulfate solution through natural wetland sediments. An initial pilot study that used extended mixing equilibration phases produced hardened sediments not representative of natural sediments. A subsequent percolation method produced sediment with similar texture to natural sediment and was used as the method for spiking the sediments. The range of total recoverable uranium (TR-U) concentrations achieved was 8-3200 mg/kg. This reflected the concentrations found in natural wetlands and water management ponds found on a uranium mine site and was above natural levels. Dilute-acid extractable uranium (AE-U) concentrations were >80% of total concentrations, indicating that much of the uranium in the spiked sediment was labile and potentially bioavailable. The portion of TR-U extractable as AE-U was similar at the start and end of the 4.5-month field-deployment. Porewater uranium (PW-U) analyses indicated that partition coefficients (Kd) were 2000-20,000 L/kg, and PW-U was greater in post- than pre-field-deployed samples when TR-U was ≤1500 mg/kg, indicating the binding became weaker during the field-deployment period. At higher spiked-U concentrations, the PW-U was lower post-field-deployment. Comparing the physico-chemical data of the spiked sediments with environmental monitoring data from sediments in the vicinity of a uranium mining operation indicated that they were representative of sediments contaminated by mining and that the U-spiked sediments had a clear U concentration gradient. This confirmed the suitability of the spiking procedure for preparing sediments that were suitable for deriving a SQGV for uranium.

Uranium stability in a large wetland soil core probed by electron acceptors, carbonate amendments and wet-dry cycling in a long-term lysimeter experiment

Understanding the hydro-biogeochemical conditions that impact the mobility of uranium (U) in natural or artificial wetlands is essential for the management of contaminated environments. Field-based research indicates that high organic matter content and saturation of the soil from the water table create favorable conditions for U accumulation. Despite the installation of artificial wetlands for U remediation, the processes that can release U from wetland soils to underlying aquifers are poorly understood. Here we used a large soil core from a montane wetland in a 6 year lysimeter experiment to study the stability of U accumulated to levels of up to 6000 ppm. Amendments with electron acceptors showed that the wetland soil can reduce sulfate and Fe(III) in large amounts without significant release of U into the soil pore water. However, amendment with carbonate (5 mM, pH 7.5) resulted in a large discharge of U. After a six-month period of imposed drought, the re-flooding of the core led to the release of negligible amounts of U into the pore water. This long-term experiment demonstrates that U is strongly bound to organic matter and that its stability is only challenged by carbonate complexation.

Uranium (U) source, speciation, uptake, toxicity and bioremediation strategies in soil-plant system: A review

Uranium(U), a highly toxic radionuclide, is becoming a great threat to soil health development, as returning nuclear waste containing U into the soil systems is increased. Numerous studies have focused on: i) tracing the source in U contaminated soils; ii) exploring U geochemistry; and iii) assessing U phyto-uptake and its toxicity to plants. Yet, there are few literature reviews that systematically summarized the U in soil-plant system in past decade. Thus, we present its source, geochemical behavior, uptake, toxicity, detoxification, and bioremediation strategies based on available data, especially published from 2018 to 2021. In this review, we examine processes that can lead to the soil U contamination, indicating that mining activities are currently the main sources. We discuss the relationship between U bioavailability in the soil-plant system and soil conditions including redox potential, soil pH, organic matter, and microorganisms. We then review the soil-plant transfer of U, finding that U mainly accumulates in roots with a quite limited translocation. However, plants such as willow, water lily, and sesban are reported to translocate high U levels from roots to aerial parts. Indeed, U does not possess any identified biological role, but provokes numerous deleterious effects such as reducing seed germination, inhibiting plant growth, depressing photosynthesis, interfering with nutrient uptake, as well as oxidative damage and genotoxicity. Yet, plants tolerate U toxicity via various defense strategies including antioxidant enzymes, compartmentalization, and phytochelatin. Moreover, we review two biological remediation strategies for U-contaminated soil: (i) phytoremediation and (ii) microbial remediation. They are quite low-cost and eco-friendly compared with traditional physical or chemical remediation technologies. Finally, we conclude some promising research challenges regarding U biogeochemical behavior in soil-plant systems. This review, thus, further indicates that the combined application of U low accumulators and microbial inoculants may be an effective strategy for the bioremediation of U-contaminated soils.

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.

Uranium Deposits of Erlian Basin (China): Role of Carbonaceous Debris Organic Matter and Hydrocarbon Fluids on Uranium Mineralization

The relationship of sedimentary organic matter, oil-gas and sandstone-type uranium (U) deposits is the key problem of U-mineralization. Whether migrate hydrocarbons participate in U-mineralization is still a controversy. Typical U deposits of the Erlian Basin in northeast China have been investigated through detailed petrography, mineralogical, micro spectroscopic, organic geochemical and C-isotope studies. Petrographic observations, Microscopic Laser Raman Spectroscopic, Infrared Spectroscopic and Scanning Electron Microscope analyses indicated there are three types of organic matter (including carbonaceous debris and migrated hydrocarbons). A significant amount of uranium was associated with pyrites, clay minerals and carbonaceous debris organic matter, either coexisted with hydrocarbon fluids. There are at least two stages mineralization events, stage I is related to sedimentary organic matter (syngenetic pre-enrichment stage), and stage II is related to mobile hydrocarbon fluids (main mineralization stage). Therefore, our results support that migrated hydrocarbons were involved as a reducing agent for the main uranium mineralization after synsedimentary mineralization.

Combined Effects of Fe(III)-Bearing Clay Minerals and Organic Ligands on U(VI) Bioreduction and U(IV) Speciation

Reduction of U(VI) to U(IV) drastically reduces its solubility and has been proposed as a method for remediation of uranium contamination. However, much is still unknown about the kinetics, mechanisms, and products of U(VI) bioreduction in complex systems. In this study, U(VI) bioreduction experiments were conducted with Shewanella putrefaciens strain CN32 in the presence of clay minerals and two organic ligands: citrate and EDTA. In reactors with U and Fe(III)-clay minerals, the rate of U(VI) bioreduction was enhanced due to the presence of ligands, likely because soluble Fe³⁺- and Fe²⁺-ligand complexes served as electron shuttles. In the presence of citrate, bioreduced U(IV) formed a soluble U(IV)-citrate complex in experiments with either Fe-rich or Fe-poor clay mineral. In the presence of EDTA, U(IV) occurred as a soluble U(IV)-EDTA complex in Fe-poor montmorillonite experiments. However, U(IV) remained associated with the solid phase in Fe-rich nontronite experiments through the formation of a ternary U(IV)-EDTA-surface complex, as suggested by the EXAFS analysis. Our study indicates that organic ligands and Fe(III)-bearing clays can significantly affect the microbial reduction of U(VI) and the stability of the resulting U(IV) phase.

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.

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.

Thermodynamic properties of the uranyl carbonate minerals roubaultite, fontanite, widenmannite, grimselite, čejkaite and bayleyite

The thermodynamic properties of six important uranyl carbonate minerals, roubaultite, fontanite, widenmannite, grimselite, čejkaite and bayleyite, are determined as a function of temperature using first principles methods.

The crystal structures and mechanical properties of the uranyl carbonate minerals roubaultite, fontanite, sharpite, widenmannite, grimselite and čejkaite

The structure, hydrogen bonding, X-ray diffraction pattern and mechanical properties of six important uranyl carbonate minerals, roubaultite, fontanite, sharpite, widenmannite, grimselite and čejkaite, are determined using first principles methods.