Organic complexation of U(VI) in reducing soils at a natural analogue site: Implications for uranium transport

Fuller, Adam J.; Leary, Peter; Gray, Neil; Davies, Helena; Mosselmans, J. F. W.; Cox, Filipa; Robinson, Clare H.; Pittman, Jon K.; McCann, Clare M.; Muir, Michael R.; Graham, Margaret C.; Utsunomiya, Satoshi; Bower, William R.; Morris, Katherine; Shaw, Samuel; Bots, Pieter; Livens, Francis R.; Law, Gareth T. W.

doi:10.1016/j.chemosphere.2020.126859

Cited by 41 publications

(23 citation statements)

References 64 publications

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“…31 For instance, organic ligands can form strong complexes with UO 2 2+ and inhibit UO 2 2+ reduction. 4,32,33 Humic substances were observed to enhance the U(VI) bioreduction rate, possibly by acting as an electron shuttle between U(VI) and microorganisms. 34 In the presence of organic ligands, such as citrate, ethylene diamine tetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and humic substances, the U(IV) resulting from the microbial reduction of U(VI) typically forms soluble complexes with these organic compounds.…”

Section: ■ Introductionmentioning

confidence: 99%

“…Uranium contamination is a growing problem because of the operation of nuclear power plants and the mining of uranium-rich ores . The mobility of uranium in aquatic and terrestrial environments is largely controlled by its complexation with organic matter and its valence state. − Uranium has two dominant oxidation states: hexavalent uranium [U(VI)] and tetravalent uranium [U(IV)]. − U(VI), usually in the form of a soluble uranyl cation (UO 2 2+ ), is predominant under oxidizing conditions. , Under reducing conditions, U(VI) can be reduced to U(IV) (uranous ion, U 4+ ), which occurs as sparingly soluble uraninite (UO 2 ), amorphous U(IV)-phosphate phases, monomeric U(IV) adsorbed to metal oxide surfaces, or persistent colloids. ,− …”

Section: Introductionmentioning

confidence: 99%

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

Zhang

Chen

Xia

et al. 2021

Environ. Sci. Technol.

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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 Fe 3+ − and Fe 2+ −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.

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Section: ■ Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Zhang

Chen

Xia

et al. 2021

Environ. Sci. Technol.

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“…Our findings suggest that bioreduction processes play a marginal role in the kinetics of labile U VI . Studies on organic soils naturally enriched in U by Fuller et al (2020) and Regenspurg et al (2010) have demonstrated that U was largely complexed with soil organic matter as U VI despite reducing conditions; binding to organic matter appeared to prevent U VI bioreduction. Similarly, Burgos et al (2007) suggested that complexation of U VI with humic substances may interrupt electron transport to U VI thereby decreasing the potential for reduction.…”

Section: Ue Kineticsmentioning

confidence: 99%

“…Echevarria et al (2001) found a linear relationship between soil pH and U sorption, whilst soil texture and organic matter were not significant. However, Cumberland et al (2016) and Fuller et al (2020) demonstrated the relevance of U complexation with humic substances, which can influence U mobility and Kd values. Thus, Kd values are subject to uncertainties that may compromise disposal risk assessments if site-specific factors are not considered.…”

Section: Introductionmentioning

confidence: 99%

Kinetics of uranium(VI) lability and solubility in aerobic soils

et al. 2020

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Uranium may pose a hazard to ecosystems and human health due to its chemotoxic and radiotoxic properties.The long half-life of many U isotopes and their ability to migrate raise concerns over disposal of radioactive wastes. This work examines the long-term U bioavailability in aerobic soils following direct deposition or transport to the surface and addresses two questions: (i) to what extent do soil properties control the kinetics of U speciation changes in soils and (ii) over what experimental timescales must U reaction kinetics be measured to reliably predict long-term of impact in the terrestrial environment? Soil microcosms spiked with soluble uranyl were incubated for 1.7 years. Changes in U VI fractionation were periodically monitored by soil extractions and isotopic dilution techniques, shedding light on the binding strength of uranyl onto the solid phase. Uranyl sorption was rapid and strongly buffered by soil Fe oxides, but U VI remained reversibly held and geochemically reactive. The pool of uranyl species able to replenish the soil solution through several equilibrium reactions is substantially larger than might be anticipated from typical chemical extractions and remarkably similar across different soils despite contrasting soil properties. Modelled kinetic parameters indicate that labile U VI declines very slowly, suggesting that the processes and transformations transferring uranyl to an intractable sink progress at a slow rate regardless of soil characteristics. This is of relevance in the context of radioecological assessments, given that soil solution is the key reservoir for plant uptake.

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“…Uranium is typically found in two oxidation states: U(VI) and U(IV). Uranium(IV) predominates under sulfate-reducing or iron-reducing conditions and is relatively immobile in near-neutral pH groundwater. − Uranium(VI) predominates in oxic to suboxic groundwater where it occurs as the uranyl aquocation [U(VI)O 2 2+ ]. ,, Uranyl mobility is controlled primarily by sorption (e.g., onto oxyhydroxides of Fe, Mn, and Al and organic matter). − Under pH conditions typical of most groundwaters (e.g., pH ∼ 6–8) and in the presence of dissolved Ca and inorganic carbon (DIC), uranyl mobility is enhanced by formation of the aqueous complexes Ca 2 UO 2 (CO 3 ) 3 and CaUO 2 (CO 3 ) 3 2– (henceforth named “CaUC” complexes) that have a high thermodynamic stability and a low affinity for sorption sites. ,, Crystalline bedrock aquifers with high concentrations of dissolved Ca and DIC may therefore be especially prone to geogenic U concentrations exceeding water-quality guidelines. − …”

Section: Introductionmentioning

confidence: 99%

Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking ²³⁴U-²³⁵U-²³⁸U, Groundwater Ages, and Hydrogeochemistry

Skierszkan

Dockrey²,

Helsen³

et al. 2021

ACS Earth Space Chem.

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Uranium (U) contamination in groundwater from geogenic sources affects water quality globally. Here, we use a multifaceted isotopic and geochemical approach to elucidate U sources and controls on geogenic U release to groundwater and surface water at a prospective subarctic gold deposit in Yukon, Canada, that is characterized by permafrost, fractured bedrock, and cold (<2 °C) groundwater. X-ray absorption spectroscopy, sequential extractions, and micro X-ray fluorescence mapping show extensive subsurface oxidation and solid-phase U present in its hexavalent and mobile form. Limited 238U/235U isotope fractionation and predominance of U(VI) in rocks suggest U(VI) sorption–desorption is the main driver of U mobilization. Groundwater U concentrations are appreciable (median 38 μg/L, range 1.2–535 μg/L) and are explained by high-alkalinity, Ca-rich groundwater produced from oxidative weathering of sulfide and carbonate-mineralized structures around the deposit. Minor 238U/235U isotope fractionation in groundwater indicates that limited U(VI) reduction occurs beneath permafrost despite groundwater redox conditions below Fe(III) and S(VI) reduction, and groundwater ages inferred from 3H and 14C to be on the order of thousands of years. The complexation of U as uranyl–calcium–carbonate complexes and the resilience of these complexes to U(VI) reduction contributes to high U(VI) mobility under cold groundwater conditions. This study provides insight into processes and time scales of U transport in subarctic groundwater at a pivotal time when hydrogeochemical changes may be anticipated in cold regions worldwide due to permafrost degradation.

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Organic complexation of U(VI) in reducing soils at a natural analogue site: Implications for uranium transport

Cited by 41 publications

References 64 publications

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

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

Kinetics of uranium(VI) lability and solubility in aerobic soils

Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking ²³⁴U-²³⁵U-²³⁸U, Groundwater Ages, and Hydrogeochemistry

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Organic complexation of U(VI) in reducing soils at a natural analogue site: Implications for uranium transport

Cited by 41 publications

References 64 publications

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

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

Kinetics of uranium(VI) lability and solubility in aerobic soils

Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking 234U-235U-238U, Groundwater Ages, and Hydrogeochemistry

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Product

Resources

About

Persistence of Uranium in Old and Cold Subpermafrost Groundwater Indicated by Linking ²³⁴U-²³⁵U-²³⁸U, Groundwater Ages, and Hydrogeochemistry