Rapid Mobilization of Noncrystalline U(IV) Coupled with FeS Oxidation

Recent Progress in Molecular Oxygen Activation by Iron-Based Materials: Prospects for Nano-Enabled In Situ Remediation of Organic-Contaminated Sites

In situ chemical oxidation (ISCO) is commonly used for the remediation of contaminated sites, and molecular oxygen (O2) after activation by aquifer constituents and artificial remediation agents has displayed potential for efficient and selective removal of soil and groundwater contaminants via ISCO. In particular, Fe-based materials are actively investigated for O2 activation due to their prominent catalytic performance, wide availability, and environmental compatibility. This review provides a timely overview on O2 activation by Fe-based materials (including zero-valent iron-based materials, iron sulfides, iron (oxyhydr)oxides, and Fe-containing clay minerals) for degradation of organic pollutants. The mechanisms of O2 activation are systematically summarized, including the electron transfer pathways, reactive oxygen species formation, and the transformation of the materials during O2 activation, highlighting the effects of the coordination state of Fe atoms on the capability of the materials to activate O2. In addition, the key factors influencing the O2 activation process are analyzed, particularly the effects of organic ligands. This review deepens our understanding of the mechanisms of O2 activation by Fe-based materials and provides further insights into the application of this process for in situ remediation of organic-contaminated sites.

Degradation of Nitrobenzene by Mackinawite through a Sequential Two-Step Reduction and Oxidation Process

Mackinawite (FeS) has gained increasing interest due to its potential application in contaminant removal by either reduction or oxidation processes. This study further demonstrated the efficiency of FeS in degrading nitrobenzene (ArNO2) via a sequential two-step reduction and oxidation process under neutral conditions. In the reduction stage, FeS rapidly reduced ArNO2 to aniline (ArNH2), with nitrosobenzene (ArNO) and phenylhydroxylamine (ArNHOH) serving as the intermediates. X-ray photoelectron spectroscopy (XPS) analysis indicated that both Fe(II) and S(II) in FeS contributed electrons to the reduction of ArNO2. In the subsequent oxidation stage with oxygen, by addition of 0.5 mM tripolyphosphate (TPP), ArNH2 generated in the reduction process could be effectively oxidized to aminophenols by hydroxyl radicals (•OH), which would undergo eventual mineralization via ring-cleavage reactions. TPP exerted a favorable role in enhancing •OH production for ArNH2 degradation by promoting the formation of the dissolved Fe(II)-TPP complex, thus enhancing the homogeneous Fenton reaction. Additionally, TPP adsorption inhibited the surface oxidation reactivity of FeS due to the change of Fe(II) coordination. Finally, the effective degradation of ArNO2 by FeS in actual groundwater was demonstrated by using this sequential reduction and oxidation approach. These research findings provide a theoretical basis for a new FeS-based remediation approach, offering an alternative way for comprehensive removal of ArNO2.

Arsenic triggered nano-sized uranyl arsenate precipitation on the surface of Kocuria rosea

Arsenic (As) and uranium (U) frequently occur together naturally and, in consequence, transform into cocontaminants at sites of uranium mining and processing, yet the simultaneous interaction process of arsenic and uranium has not been well documented. In the present contribution, the influence of arsenate on the removal and reduction of uranyl by the indigenous microorganism Kocuria rosea was characterized using batch experiments combined with species distribution calculation, SEM-EDS, FTIR, XRD and XPS. The results showed that the coexistence of arsenic plays an active role in Kocuria rosea growth and the removal of uranium under neutral and slightly acidic conditions. U-As complex species of UO₂HAsO₄ (aq) had a positive effect on uranium removal, while Kocuria rosea cells appeared to have a large specific surface area serving as attachment sites. Furthermore, a large number of nano-sized flaky precipitates, constituted by uranium and arsenic, attached to the surface of Kocuria rosea cells at pH 5 through P=O, COO^-, and C=O groups in phospholipids, polysaccharides, and proteins. The biological reduction of U(VI) and As(V) took place in a successive way, and the formation of a chadwickite-like uranyl arsenate precipitate further inhibited U(VI) reduction. The results will help to design more effective bioremediation strategies for arsenic-uranium cocontamination.

Secondary Mineral Formation and Carbon Dynamics during FeS Oxidation in the Presence of Dissolved Organic Matter

Iron (Fe) minerals constitute a major control on organic carbon (OC) storage in soils and sediments. While previous research has mainly targeted Fe (oxyhydr)oxides, the impact of Fe sulfides and their subsequent oxidation on OC dynamics remains unresolved in redox-fluctuating environments. Here, we investigated the impact of dissolved organic matter (DOM) on FeS oxidation and how FeS and its oxidation may alter the retention and nature of DOM. After the anoxic reaction of DOM with FeS, FeS preferentially removed high-molecular-weight and nitrogen-rich compounds and promoted the formation of aqueous sulfurized organic molecules, according to Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) analysis. When exposed to O₂, FeS oxidized to nanocrystalline lepidocrocite and additional aqueous sulfurized organic compounds were generated. The presence of DOM decreased the particle size of the resulting nano-lepidocrocite based on Mössbauer spectroscopy. Following FeS oxidation, most solid-phase OC remained associated with the newly formed lepidocrocite via a monodentate chelating mechanism (based on FTIR analysis), and FeS oxidation caused only a slight increase in the solubilization of solid-phase OC. Collectively, this work highlights the under-appreciated role of Fe sulfides and their oxidation in driving OC transformation and preservation.

FeS-mediated mobilization and immobilization of Cr(III) in oxic aquatic systems

Reduction of soluble Cr(VI) into insoluble Cr(III) by iron sulfide (FeS) minerals under anoxic conditions has been widely observed in natural and engineered systems. Yet, information has been lacking on the FeS-mediated oxidation and remobilization potential of Cr(III) under varying environmental conditions. The objective of this study was to investigate FeS-mediated redox transformation of Cr(III) to Cr(VI) and the associated mobilization and immobilization when Cr(III)-FeS systems are exposed to atmospheric conditions. The results showed that FeS nanoparticles facilitated rapid and strong Fenton-like reactions during the early-stage oxygenation of FeS, resulting in rapid production of hydroxyl radicals (•OH). Consequently, Cr(III) was rapidly oxidized into Cr(VI). Yet, as the reactions proceeded, the oxidative potential was counteracted by competitive scavenging of •OH by Fe(II) and S(-II) from FeS and the reduction reactions by these electron donors. At equilibrium, all Cr(VI) was reduced back to Cr(III) at an FeS-Cr(III) molar ratio of 10:1, while a small fraction of Cr(VI) persisted in solid products of Cr(OH)₃(s) at an FeS-Cr(III) molar ratio of 1:1. Acidic conditions favored the generation of Cr(VI) and the equilibrium concentration of Cr(VI) in oxic FeS NPs systems at pH 5.0 was 1.7 times higher than at pH 9.0. Overall, the FeS-induced Fenton-like reactions and the oxidation of Cr(III) were favored in the early stage, but quenched in the later stage and outcompeted by the reduction of Cr(VI) if sufficient FeS was available. The findings provide new insights into the hydrochemical processes that can affect the speciation, toxicity, and mobility of Cr in aquatic systems containing FeS and Cr.

Uranium bioremediation with U(VI)-reducing bacteria

Uranium (U) pollution is an environmental hazard caused by the development of the nuclear industry. Microbial reduction of hexavalent uranium (U(VI)) to tetravalent uranium (U(IV)) reduces U solubility and mobility and has been proposed as an effective method to remediate uranium contamination. In this review, U(VI) remediation with respect to U(VI)-reducing bacteria, mechanisms, influencing factors, products, and reoxidation are systematically summarized. Reportedly, some metal- and sulfate-reducing bacteria possess excellent U(VI) reduction capability through mechanisms involving c-type cytochromes, extracellular pili, electron shuttle, or thioredoxin reduction. In situ remediation has been demonstrated as an ideal strategy for large-scale degradation of uranium contaminants than ex situ. However, U(VI) reduction efficiency can be affected by various factors, including pH, temperature, bicarbonate, electron donors, and coexisting metal ions. Furthermore, it is noteworthy that the reduction products could be reoxidized when exposed to oxygen and nitrate, inevitably compromising the remediation effects, especially for non-crystalline U(IV) with weak stability.

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.

Origin and stability of uranium accumulation-layers in an Alpine histosol

Uranium (U) accumulation in organic soils is a common phenomenon that can lead to high U concentration in montane wetlands. The stability of the immobilized U in natural wetlands following redox fluctuations and re-oxidation events, however, is not currently known. In this study, we investigated a saturated histosol that had accumulated up to 6000 ppm of U at 30 cm below ground level (bgl). Uranium in the waters feeding the wetland originates from the weathering of surrounding gneiss rocks, a process releasing trace amounts (<3 ppb) of soluble U into nearby streams. Redox oscillations in the first 20 cm bgl led to the accumulation of U, Ca, S in low permeability layers at 30 and 45 cm bgl. XRF measurements along the core showed that U strongly correlates with sulfur (S) and calcium (Ca), but not iron (Fe). We tested the stability of uranium in the histosol over a nine-month laboratory amendment of a large core of the histosol (∅ 30 cm; length 55 cm) with up to 500 ppm nitrate. Nitrate addition was followed by complete nitrate reduction and re-generation of oxidizing E_h conditions in the top 25 cm of the soil without U release to the soil pore waters above background levels (1-2 ppb). Our results demonstrate that, fast reduction of nitrate, sulfate, and Fe(III) occur in the soil without U release. The remarkable stability of sorbed U in the histosol may result from buffering by sulfide and S_n° and/or strong U(IV)-OM or U(VI)-OM enhanced by organic S moieties or bridging complexation by Ca. That U in the soil was immobile under nitrate addition for up to 9 months can inform remediation strategies based on the use of artificial wetlands to limit U mobility in contaminated sites.

Anaerobic Dissolution Rates of U(IV)-Oxide by Abiotic and Nitrate-Dependent Bacterial Pathways

The long-term stability of U(IV) solid phases in anaerobic aquifers depends upon their reactivity in the presence of oxidizing chemical species and microbial catalysts. We performed flow-through column experiments under anaerobic conditions to investigate the mechanisms and dissolution rates of biogenic, noncrystalline UO₂(s) by chemical oxidants (nitrate and/or nitrite) or by Thiobacillus denitrificans, a widespread, denitrifying, chemolithoautotrophic model bacterium. Dissolution rates of UO₂(s) with dissolved nitrite were approximately 5 to 10 times greater than with nitrate alone. In the presence of wild-type T. denitrificans and nitrate, UO₂(s) dissolution rates were similar to those of abiotic experiments with nitrite (from 1.15 × 10^-14 to 4.94 × 10^-13 mol m^-2 s^-1). Experiments with a T. dentrificans mutant strain defective in U(IV) oxidation supported microbially mediated U(IV) oxidation. X-ray absorption spectroscopy (XAS) analysis of post-reaction solids showed the presence of mononuclear U(VI) species rather than a solid U(VI) phase. At steady-state U release, kinetic and spectroscopic results suggest detachment of oxidized U(VI) from the UO₂(s) surface as the rate-determining step rather than electron transfer or ion diffusion. Under anaerobic conditions, production of nitrite by nitrate-reducing microorganisms and enzymatically catalyzed, nitrate-dependent U(IV) oxidation are likely dual processes by which reduced U solids may be oxidized and mobilized in the aqueous phase.

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.

Oxidative Degradation of Organic Contaminants by FeS in the Presence of O2

Reductive transformation of organic contaminants by FeS in anoxic environments has been documented previously, whereas the transformation in oxic environments remains poorly understood. Here we show that phenol can be efficiently oxidized in oxic FeS suspension at circumneutral pH value. We found that hydroxyl radicals (•OH) were the predominant reactive oxidant and that a higher O₂ content accelerated phenol degradation. Phenol oxidation depended on •OH production and utilization efficiency, i.e., phenol degraded per •OH produced. Low FeS contents (≤1 g/L) produced less •OH but higher utilization efficiency, while high contents produced more •OH but lower utilization efficiency. Consequently, the most favorable conditions for phenol oxidation occurred during the long-term interaction between dissolved O₂ and low levels of FeS (i.e., ≤1 g/L). Mössbauer spectroscopy suggests that FeS oxidation to lepidocrocite initially produced an intermediate Fe(II) phase that could be explained by the apparent preferential oxidation of structural S(-II) relative to Fe(II), rendering a higher initial •OH yield upon unit of Fe(II) oxidation. Trichloroethylene can be also oxidized under similar conditions. Our results demonstrate that oxidative degradation of organic contaminants during the oxygenation of FeS can be a significant but currently underestimated pathway in both natural and engineered systems.

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

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

Production of a Surface-Localized Oxidant during Oxygenation of Mackinawite (FeS)

The oxygenation of mackinawite (FeS) frequently occurs at anoxic-oxic interfaces in both natural and engineered systems such as intertidal sediment, in activated sludge in water treatment processes, and during sulfidized zero-valent iron particle corrosion. During reoxygenation events, FeS may drive a Fenton-like process leading to the production of strong oxidants though the details of this process are poorly understood. In this study, benzoic acid (BA) has been used to probe both the magnitude and identity of these strong oxidants under circumneutral pH conditions. The major product of BA oxidation during FeS oxygenation was found to be 2,5-dihydroxybenzoic acid (2,5-DHBA) rather than monohydroxybenzoic acids identified to be the major products in a range of hydroxyl radical (HO^·)-dominated systems. Based upon relative reactivity with other competitive probes and nature of the hydroxybenzoate product distribution, it is hypothesized that the strong oxidant must be a surface-localized entity such as high-valent iron or surface-associated hydroxyl or sulfur-based radicals with reactivity differing from those formed in free solution. The importance of both the reactivity of the oxidant and adsorption of the substrate to the surface is demonstrated.

Removal and Recovery of Uranium from Groundwater Using Direct Electrochemical Reduction Method: Performance and Implications

Removal of uranium from groundwater is of great significance as compared to in situ bioimmobilization technology. In this study, a novel direct electro-reductive method has been developed to efficiently remove and recover uranium from carbonate-containing groundwater, where U(VI)O₂(CO₃)₃^4- and Ca₂U(VI)O₂(CO₃)₃ are the dominant U species. The transferred electron calculations and XPS, XRD analyses confirmed that U(VI) was reduced to U(IV)O₂ and accumulated on the surface of the Ti electrode (defined as Ti@U(IV)O₂ electrode) with high current efficiencies (over 90.0%). Moreover, over 98.0% of the accumulated U(IV)O₂ could be recovered by soaking the Ti@U(IV)O₂ electrode in the dilute nitric acid. Results demonstrated that the accumulated U(IV)O₂ on the surface of the Ti electrode played a key role in the removal of U(VI), which can promote the electro-reduction of U(VI). Therefore, the electrode could be used repeatedly and has a high removal capacity of U(VI) due to the continuous accumulation of active U(IV)O₂ on the surface of the electrode. Significantly, the uranium in both real and high salinity groundwater can be efficiently removed. This study implies that the proposed direct electro-reductive method has great potential for the removal and recovery of uranium from groundwater and uranium-containing wastewater.

Behavior and fate of geogenic uranium in a shallow groundwater system

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

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.

Biomineralization mechanism of U(VI) induced by Bacillus cereus 12-2: The role of functional groups and enzymes

Current studies reveal that the biomineralization of U(VI) by anaerobes normally produces nano-sized U(IV) minerals that can easily re-migrate/re-oxidize, while the biomineralization of U(VI) by aerobes has been constrained because the general mechanism has not yet been fully characterized. The biomineralization of U(VI) by Bacillus cereus 12-2 was investigated in this work. The maximum biosorption capability of intact cells was 448.68 mg U/g biomass (dry weight) at pH 5, while a decrease over 60% was induced when phosphate, amino, and especially carboxyl groups were shielded. X-ray diffraction, electron microscopy, and tracing the concentration of soluble intracellular U(VI) demonstrated that extracellular amorphous uranium particles can directly enter cells as solid, and about 10 nm-sized (NH₄)(UO₂)PO₄·3H₂O was formed subsequently. It was also revealed that the biosorption capability was not affected by a high uranium concentration, while biomineralization was inhibited, suggesting that a high concentration of heavy metals may inhibit the enzyme activity involved in biomineralization. Besides, U(VI) could trigger the overexpression of proteins with a molecular weight of 22 kD, including various phosphatases, kinases, and other enzymes that are related to metabolism and stimulus response, which may contribute to the intracellular transformation of U(VI) compounds from amorphous to crystalline phase. Taken together, the immobilization of U(VI) by B. cereus 12-2 contains two major steps: (1) fast immobilization of U(VI) on the cell surface as amorphous compounds, in which the carboxyl groups served as the predominant coordination functional groups and (2) transport of amorphous particles to cells directly and enzyme-related formation of uramphite.

Variations of uranium concentrations in a multi-aquifer system under the impact of surface water-groundwater interaction

Understanding uranium (U) mobility is vital to minimizing its concentrations in potential drinking water sources. In this study, we report spatial-seasonal variations in U speciation and concentrations in a multi-aquifer system under the impact of Sanggan River in Datong basin, northern China. Hydrochemical and H, O, Sr isotopic data, thermodynamic calculations, and geochemical modeling are used to investigate the mechanisms of surface water-groundwater mixing-induced mobilization and natural attenuation of U. In the study site, groundwater U concentrations are up to 30.2 μg/L, and exhibit strong spatial-seasonal variations that are related to pH and Eh values, as well as dissolved Ca²⁺, HCO₃^-, and Fe(III) concentrations. For the alkaline aquifers of this site (pH 7.02-8.44), U mobilization is due to the formation and desorption of Ca₂UO₂(CO₃)₃⁰ and CaUO₂(CO₃)₃^2- caused by groundwater Ca²⁺ elevation via mineral weathering and Na-Ca exchange, incorporated U(VI) release from calcite, and U(IV) oxidation by Fe(OH)₃. U immobilization is linked to the adsorption of CaUO₂(CO₃)₃^2- and UO₂(CO₃)₃^4- shifted from Ca₂UO₂(CO₃)₃⁰ because of HCO₃^- elevation and Ca²⁺ depletion, U(VI) co-precipitation with calcite, and U(VI) reduction by adsorbed Fe²⁺ and FeS. Those results are of great significance for the groundwater resource management of this and similar other surface water-groundwater interaction zones.

Fate of Adsorbed U(VI) during Sulfidization of Lepidocrocite and Hematite

The impact on U(VI) adsorbed to lepidocrocite (γ-FeOOH) and hematite (α-Fe₂O₃) was assessed when exposed to aqueous sulfide (S(-II)_aq) at pH 8.0. With both minerals, competition between S(-II) and U(VI) for surface sites caused instantaneous release of adsorbed U(VI). Compared to lepidocrocite, consumption of S(-II)_aq proceeded slower with hematite, but yielded maximum dissolved U concentrations that were more than 10 times higher, representing about one-third of the initially adsorbed U. Prolonged presence of S(-II)_aq in experiments with hematite in combination with a larger release of adsorbed U(VI), enhanced the reduction of U(VI): after 24 h of reaction about 60-70% of U was in the form of U(IV), much higher than the 25% detected in the lepidocrocite suspensions. X-ray absorption spectra indicated that U(IV) in both hematite and lepidocrocite suspensions was not in the form of uraninite (UO₂). Upon exposure to oxygen only part of U(IV) reoxidized, suggesting that monomeric U(IV) might have become incorporated in newly formed iron precipitates. Hence, sulfidization of Fe oxides can have diverse consequences for U mobility: in short-term, desorption of U(VI) increases U mobility, while reduction to U(IV) and its possible incorporation in Fe transformation products may lead to long-term U immobilization.

Oxidizing Impact Induced by Mackinawite (FeS) Nanoparticles at Oxic Conditions due to Production of Hydroxyl Radicals

Mackinawite (FeS) nanoparticles have been extensively tested for reducing contaminants under anoxic conditions, while the oxidizing impact induced by FeS under oxic conditions has been largely underestimated. In light of previous findings that hydroxyl radicals (·OH) can be produced from oxygenation of sediment Fe(II), herein we revealed that ·OH can be produced efficiently from FeS oxygenation at circumneutral conditions, yielding 84.7 μmol ·OH per g FeS. Much more ·OH was produced from the oxygenation of FeS compared with siderite, pyrite, and zerovalent iron nanoparticles under the same conditions. The oxidation of FeS was a surface-mediated process, in which O₂ was transformed by the structural Fe(II) on FeS surface to ·OH with the generation of H₂O₂ intermediate. A small proportion of Fe(II) was regenerated from the reduction of Fe(III) by FeS and S(-II), but this proportion did not significantly contribute to ·OH production. We further validated that the ·OH produced from FeS oxygenation considerably contributed to the oxidation of arsenic. As the change of redox conditions from anoxic to oxic is common in both natural and artificial processes, our findings suggest that the oxidizing impact induced by FeS at oxic conditions should be concerned due to ·OH production.