Chemical reduction of U(VI) by Fe(II) at the solid-water interface using natural and synthetic Fe(III) oxides

Effects of Fe(II) and humic acid on U(VI) mobilization onto oxidized carbon nanofibers derived from the pyrolysis of bacterial cellulose

The effects of Fe(II) and humic acid on U(VI) immobilization onto oxidized carbon nanofibers (Ox-CNFs, pyrolysis of bacterial cellulose) were investigated by batch, spectroscopic and modeling techniques, with results suggesting that, Ox-CNFs exhibited fast adsorption rate (adsorption equilibrium within 3 h), high adsorption performance (maximum adsorption capacity of 208.4 mg/g), good recyclability (no notable change after five regenerations) in the presence of Fe(II) towards U(VI) from aqueous solutions (e.g., 40 % reduction and 10 % adsorption at pH 8.0), which was attributed to the various oxygen-containing functional groups, excellent chemical stability, large specific surface area and high redox effect. U(VI) adsorption increased with increasing pH from 2.0 to 5.0, then high-level plateau and remarkable decrease were observed at 5.0-6.0 and at pH > 6.0, respectively. According to FT-IR and XPS analysis, a negative correlation between U(VI) reduction and organic in the presence of Fe(II) implied that U(VI) reduction was driven by Fe(II) while inhibited by humic acid. The interaction mechanism of U(VI) on Ox-CNFs was demonstrated to be adsorption and ion exchange at low pH and reduction at high pH according to XPS and surface complexation modeling. These findings filled the knowledge gaps pertaining to the effect of Fe(II) on the transformation and fate of U(VI) in the actual environment. This carbon material with distinctive performance and unique topology offers a potential platform for actual application in environmental remediation.

The redox reactions of U(VI)/UO2 on Tamusu claystone: Effects of Fe2+/Fe3+ and organic matters

The claystone-based Tamusu area in the Bayingebi Basin, Inner Mongolia, is preselected as a China's high-level radioactive waste (HLRW) repository site. This study investigated the redox reactions of U(VI)/UO2 on Tamusu claystone. Five Tamusu claystone samples collected from boreholes Tzk1 and Tzk2 at different depths were used for batch experiments at pH ∼5.0, ∼7.0, and ∼9.0. These claystones contain considerable amounts of organic matters and Fe2+-containing minerals such as pyrite, fluorannite, and ankerite. Results showed that aqueous U(VI) could be partially reduced to U(IV) and/or U(V)-containing precipitates (U3O8, U4O9, etc.) by these Tamusu claystones, and the reaction is more favorable under acidic condition. We proposed that leaching of the structural Fe2+ followed by surface adsorption and interface reaction, is the primary mechanism responsible for U(VI) reduction. Under alkaline condition, organic matters might dominate the partial reduction of aqueous U(VI). Besides, the phosphorus-containing spots on Tamusu claystone surfaces are the reactive sites for U aggregation, implying the possible formation of U(VI)- and/or U(IV)-phosphate minerals. It is important to note that, due to the presence of minor Fe3+ in Tamusu claystones, the high-purity UO2 could undergo partial oxidation to U4O9 and/or U3O8. Therefore, insoluble UO2+x (0 < x ≤ 0.67) is proposed to be the most thermodynamically stable form in Tamusu claystone. This study enhances our comprehension of the essential geochemical processes of uranium in claystone surroundings, but also offers crucial information for the safety evaluation of China's HLRW repository.

Sustainable removal of uranium from acidic wastewater using various mineral raw materials

Various types of plutonic and volcanic rocks and their alteration products from Greece (serpentinite, magnesite and andesite), have been used for sustainable removal of Uranium (U) from the acidic drainage of Kirki mine, as well as for the pH increase of the polluted solutions. In this light, this study aims at the further understanding and improvement of the ecofriendly reuse of sterile, natural raw materials (including those remaining through industrial processing and engineering testing of aggregate rocks), for remediation of acid mine drainage. The selected rocks constitute such residues of sterile materials were used as filters in experimental continuous flow devices in the form of batch-type columns, in order to investigate acidic remediation properties with special focus on U removal. The initial pH of the wastewater was 2.90 and increased after seven (7) days of experimental application and more specifically from the fourth day onwards. Uranium removal became quantitatively significant once pH reached the value of 5.09. The volcanic rocks appeared to be more effective for U removal than the plutonic ones because of microtextural differences. However, optimum U removal was mainly achieved by serpentinite: while the raw materials rich in Mg strongly reacted and remediated the pH of the drainage water waste. Furthermore, the increase of pH values due to the presence of mineral raw materials, provided increased oxidation potential which deactivated the toxic load of metals, particularly U. Consequently, batch-type serpentinite reaction with the tailing fluid caused a drop in U concentration from an initial value of 254 ppb to the one of 8 ppb, which corresponds to 97% of removal. Andesite presented the second best reactant for experimental remediation, especially when it was mixed with magnetically separated mineral fractions. Despite the fact that the proposed methodology is currently at a relatively low Technology Readiness Level (TRL), it carries the potential to become an extremely effective and low-cost alternative to conventional environmental restoration technologies.

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.

Partial Oxidation of FeS Nanoparticles Enhances Cr(VI) Sequestration

Iron sulfide nanoparticles (nano-FeS) have shown great potential for in situ remediation of Cr(VI) pollution by reducing Cr(VI) to the less soluble and toxic Cr(III). However, material oxidation that inevitably occurs during storage and application alters its reactivity. Herein, we show that partial oxidation of nanoparticulate mackinawite (FeS) significantly enhances its capability in sequestering Cr(VI). Oxidation of nano-FeS increases its binding affinity to Cr(VI), likely due to preferential inner-sphere complexation of Cr(VI) oxyanions to ferric over ferrous iron in mackinawite/lepidocrocite (FeS/γ-FeOOH) nanocomposites. A trade-off is that oxidation mitigates Cr(VI) reduction by lowering the electron-donating potential of the material and the electron transfer at a solution-material interface and consequently hinders the transformation of adsorbed Cr(VI) to Cr(III). Notably, the rate-limiting step of Cr(VI) sequestration transitions from adsorption to reduction during oxidation, as demonstrated with batch experiments coupled with kinetic modeling. Thus, an optimum oxidation degree exists, wherein the gain in the overall performance from enhanced adsorption overcompensates the loss from inhibited reduction, resulting in maximum sequestration of aqueous Cr(VI) as solid-phase Cr(III). Our findings inform better assessment and design of nanomaterials for Cr(VI) remediation and may be extended to interactions of other oxyanions with natural and engineered nanoparticles during oxidative aging.

Semiconducting hematite facilitates microbial and abiotic reduction of chromium

Semi-conducting Fe oxide minerals, such as hematite, are well known to influence the fate of contaminants and nutrients in many environmental settings through sorption and release of Fe(II) resulting from microbial or abiotic reduction. Studies of Fe oxide reduction by adsorbed Fe(II) have demonstrated that reduction of Fe(III) at one mineral surface can result in the release of Fe(II) on a different one. This process is termed “Fe(II) catalyzed recrystallization” and is believed to be the result of electron transfer through semi-conducting Fe (hydr)oxides. While it is well understood that Fe(II) plays a central role in redox cycling of elements, the environmental implications of Fe(II) catalyzed recrystallization require further exploration. Here, we demonstrate that hematite links physically separated redox reactions by conducting the electrons involved in those reactions. This is shown using an electrochemical setup where Cr reduction is coupled with a potentiostat or Shewanella putrefaciens, a metal reducing microbe, where electrons donated to hematite produce Fe(II) that ultimately reduces Cr. This work demonstrates that mineral semi-conductivity may provide an additional avenue for redox chemistry to occur in natural soils and sediments, because these minerals can link redox active reactants that could not otherwise react due to physical separation.

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.

Fe(II) Redox Chemistry in the Environment

Iron (Fe) is the fourth most abundant element in the earth's crust and plays important roles in both biological and chemical processes. The redox reactivity of various Fe(II) forms has gained increasing attention over recent decades in the areas of (bio) geochemistry, environmental chemistry and engineering, and material sciences. The goal of this paper is to review these recent advances and the current state of knowledge of Fe(II) redox chemistry in the environment. Specifically, this comprehensive review focuses on the redox reactivity of four types of Fe(II) species including aqueous Fe(II), Fe(II) complexed with ligands, minerals bearing structural Fe(II), and sorbed Fe(II) on mineral oxide surfaces. The formation pathways, factors governing the reactivity, insights into potential mechanisms, reactivity comparison, and characterization techniques are discussed with reference to the most recent breakthroughs in this field where possible. We also cover the roles of these Fe(II) species in environmental applications of zerovalent iron, microbial processes, biogeochemical cycling of carbon and nutrients, and their abiotic oxidation related processes in natural and engineered systems.

Biomineralization of Uranium-Phosphates Fueled by Microbial Degradation of Isosaccharinic Acid (ISA)

Geological disposal is the globally preferred long-term solution for higher activity radioactive wastes (HAW) including intermediate level waste (ILW). In a cementitious disposal system, cellulosic waste items present in ILW may undergo alkaline hydrolysis, producing significant quantities of isosaccharinic acid (ISA), a chelating agent for radionuclides. Although microbial degradation of ISA has been demonstrated, its impact upon the fate of radionuclides in a geological disposal facility (GDF) is a topic of ongoing research. This study investigates the fate of U(VI) in pH-neutral, anoxic, microbial enrichment cultures, approaching conditions similar to the far field of a GDF, containing ISA as the sole carbon source, and elevated phosphate concentrations, incubated both (i) under fermentation and (ii) Fe(III)-reducing conditions. In the ISA-fermentation experiment, U(VI) was precipitated as insoluble U(VI)-phosphates, whereas under Fe(III)-reducing conditions, the majority of the uranium was precipitated as reduced U(IV)-phosphates, presumably formed via enzymatic reduction mediated by metal-reducing bacteria, including Geobacter species. Overall, this suggests the establishment of a microbially mediated "bio-barrier" extending into the far field geosphere surrounding a GDF is possible and this biobarrier has the potential to evolve in response to GDF evolution and can have a controlling impact on the fate of radionuclides.

Studies of aqueous U(iv) equilibrium and nanoparticle formation kinetics using spectrophotometric reaction modeling analysis

Hydrolysis of tetravalent uranium (U(iv)) and U(iv)-nanoparticle formation kinetics were examined over a wide range of temperatures using spectrophotometric reaction modeling analysis. The characteristic absorption bands representing U⁴⁺, U(OH)³⁺, and a proposed oxohydroxo species were newly identified in the UV region (190–300 nm). Dynamic absorption band changes in the UV and visible regions (360–800 nm) were explored to reevaluate the binary ion interaction coefficients for U(iv) ions and the thermodynamic constants of the primary hydrolysis reaction, including complexation constants, enthalpy, and entropy. No further hydrolysis equilibrium beyond the formation of U(OH)³⁺ was identified. Instead, an irreversible transformation of U(iv) ions to U(iv)-nanoparticles (NPs) was found to occur exclusively via the formation of a new intermediate species possessing characteristic absorption bands. The kinetic analysis, based on a two-step, pseudo-first-order reaction model, revealed that the rate of the initial step producing the intermediates is highly temperature-dependent with the measured kinetic energy barrier of ∼188 kJ mol⁻¹. With additional experimental evidence, we conclude that the intermediates are oligomeric oxohydroxo U(iv) species occurring from the condensation of U(iv) ions and simultaneously participating in the nucleation and growth process of UO₂(cr)-NPs.

The primary hydrolysis equilibrium of U⁴⁺ and the kinetics of U(iv)-nanoparticle formation were investigated by using spectrophotometric reaction modeling analysis and the spectral data collected in the UV and visible regions.

Factors influencing the reduction of U(VI) by magnetite

Under suboxic and anoxic environments, magnetite is one corrosion product of iron being used in nuclear waste canisters. Previous studies have reported a complete reduction of U(VI) on the surfaces of biogenic and natural magnetite crystals, while incomplete reductions to U(V)/U(IV)-containing species have been observed on chemosynthetic magnetite. To date, the reasons behind such disparities remain poorly studied. This study shows that uranyl nitrate or uranyl acetate is mainly reduced to UO_2+x oxides (e.g., U₄O₉, U₃O₈, etc.) by chemosynthetic magnetite under acidic conditions. When extra zero valent-iron was added, the reaction rate was significantly increased, and an improved but still incomplete U(VI) reduction was observed. Nitrate and ferric ions are ubiquitous in natural environment. Results demonstrate that the nitrate ion associated with uranyl and the ferric ion contained in magnetite or generated from U(VI) reduction have a non-negligible oxidative effect on the final products, which could mainly account for the incomplete reduction of U(VI) by chemosynthetic magnetite in the absence or presence of extra zero valent-iron observed in this study. Furthermore, the surface loading of uranium in U-Fe systems can, in part, unravel the discrepancies in various observations. An enhanced understanding of the U-Fe reaction mechanism can facilitate predictions of the extent of uranium mobility with respect to nuclear waste disposal and radioactive decontamination.

Role of Carbonate in Thermodynamic Relationships Describing Pollutant Reduction Kinetics by Iron Oxide-Bound Fe2

The reduction of environmental pollutants by Fe²⁺ bound to iron oxides is an important process that determines pollutant toxicities and mobilities. Recently, we showed that pollutant reduction rates depend on the thermodynamic driving force of the reaction in a linear free energy relationship that was a function of the solution pH value and the reduction potential, E_H, of the interfacial Fe³⁺/Fe²⁺ redox couple. In this work, we studied how carbonate affected the free energy relationship by examining the effect that carbonate has on nitrobenzene reduction rates by Fe²⁺ bound to goethite (α-FeOOH). Carbonate slowed nitrobenzene reduction rates by inducing goethite particle aggregation, as evidenced by surface charge and particle size measurements. We observed no evidence for carbonate affecting Fe³⁺/Fe²⁺ reduction potentials or the mechanism of nitrobenzene reduction. The linear free energy relationship accurately described the data collected in the presence of carbonate when we accounted for the effect it had on the reactive surface area of goethite. The findings from this work provide a framework for determining why common groundwater constituents affect the E_H-dependence of reaction rates involving oxide-bound Fe²⁺ as a reductant.

Nanoscale mechanism of UO2 formation through uranium reduction by magnetite

Uranium (U) is a ubiquitous element in the Earth's crust at ~2 ppm. In anoxic environments, soluble hexavalent uranium (U(VI)) is reduced and immobilized. The underlying reduction mechanism is unknown but likely of critical importance to explain the geochemical behavior of U. Here, we tackle the mechanism of reduction of U(VI) by the mixed-valence iron oxide, magnetite. Through high-end spectroscopic and microscopic tools, we demonstrate that the reduction proceeds first through surface-associated U(VI) to form pentavalent U, U(V). U(V) persists on the surface of magnetite and is further reduced to tetravalent UO2 as nanocrystals (~1-2 nm) with random orientations inside nanowires. Through nanoparticle re-orientation and coalescence, the nanowires collapse into ordered UO2 nanoclusters. This work provides evidence for a transient U nanowire structure that may have implications for uranium isotope fractionation as well as for the molecular-scale understanding of nuclear waste temporal evolution and the reductive remediation of uranium contamination.

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.

Abiotic reduction of uranium(VI) with humic acid at mineral surfaces: Competing mechanisms, ligand and substituent effects, and electronic structure and vibrational properties

Abiotic reduction represents an attractive technology to control U(VI) contamination. In this work, an abiotic route of U(VI) reduction with humic acid at mineral surfaces is proposed and reaction mechanisms are addressed by periodic density functional theory calculations. Different influencing factors such as ligand effect, content of CO₃^2- ligands and substituent effect are inspected. The coordination chemistry of uranyl(VI) surface complexes relies strongly on substrates and ligands, and the calculated results are in good agreements with experimental observations available. For the OH^- ligand, two competitive mechanisms co-exist that respectively produce the U(IV) and U(V) species, and the former is significantly preferred because of lower energy barriers. Instead, the NO₃^- ligand leads to the formation of U(V) while for the Cl^- ligand, the U(VI) surface complex remains very stable and is not likely to be reduced because of very high energy barriers. The U(V) and U(IV) complexes are the predominant products for low and high CO₃^2- contents, respectively. Accordingly, the abiotic reduction processes with humic acid are efficient to manage U(VI) contamination and become preferred under basic conditions or at higher CO₃^2- contents. The U(VI) reduction is further promoted by introduction of electron-donating rather than electron-withdrawing substituents to humic acid. Electronic structure analyses and vibrational frequency assignments are calculated for the various uranium surface complexes of the reduction processes, serving as a guide for future experimental and engineered studies. The molecular-level understanding given in this work offers an abiotic route for efficient reduction of U(VI) and remediation of U(VI)-contaminated sites at ambient conditions.

Isotopic Fingerprint of Uranium Accumulation and Redox Cycling in Floodplains of the Upper Colorado River Basin

Uranium (U) groundwater contamination is a major concern at numerous former mining and milling sites across the Upper Colorado River Basin (UCRB), USA, where U(IV)-bearing solids have accumulated within naturally reduced zones (NRZs). Understanding the processes governing U reduction and oxidation within NRZs is critical for assessing the persistence of U in groundwater. To evaluate the redox cycling of uranium, we measured the U concentrations and isotopic compositions (δ²³⁸U) of sediments and pore waters from four study sites across the UCRB that span a gradient in sediment texture and composition. We observe that U accumulation occurs primarily within fine-grained (low-permeability) NRZs that show active redox variations. Low-permeability NRZs display high accumulation and low export of U, with internal redox cycling of U. In contrast, within high-permeability NRZs, U is remobilized under oxidative conditions, possibly without any fractionation, and transported outside the NRZs. The low δ²³⁸U of sediments outside of defined NRZs suggests that these reduced zones act as additional U sources. Collectively, our results indicate that fine-grained NRZs have a greater potential to retain uranium, whereas NRZs with higher permeability may constitute a more-persistent but dilute U source.

Behavior and fate of geogenic uranium in a shallow groundwater system

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

Uranium storage mechanisms in wet-dry redox cycled sediments

Biogeochemical redox processes that govern radionuclide mobility in sediments are highly sensitive to forcing by the water cycle. For example, episodic draining and intrusion of oxidants into reduced zones during dry seasons can create biogeochemical seasonal hotspots of enhanced and changed microbial activity, affect the redox status of minerals, initiate changes in sediment gas and water transport, and stimulate the release of organic carbon, iron, and sulfur by oxidation of solid reduced species to aqueous oxic species. In the Upper Colorado River Basin, water-saturation of organic-enriched sediments locally promotes reducing conditions, denoted 'Naturally Reduced Zones' (NRZs), that accumulate strongly U(IV)_sol. Subsequently, fluctuating hydrological conditions introduce oxidants, which may reach internal portions of these sediments and reverse their role to become secondary sources of U_aq. Knowledge of the impact of hydrological variability on the alternating import and export of contaminants, including U, is required to predict contaminant mobility and short- and long-term impacts on water quality. In this study, we tracked U, Fe, and S oxidation states and speciation to characterize the variability in redox processes and related U_sol solubility within shallow fine-grained NRZs at the legacy U ore processing site at Shiprock, NM. Previous studies have reported U speciation and behavior in permanently saturated fine-grained NRZ sediments. This is the first report of U behavior in fine-grained NRZ-like sediments that experience repeated redox cycling due to seasonal fluctuations in moisture content. Our results support previous observations that reducing conditions are needed to accumulate U_sol in sediments, but they counter the expectation that U_sol predominantly accumulates as U(IV)_sol; our data reveal that U_sol may accumulate as U(VI)_sol in roughly equal proportion to U(IV)_sol. Surprisingly high abundances of U(VI)_sol confined in transiently saturated fine-grained NRZ-like sediments suggest that redox cycling is needed to promote its accumulation. We propose a new process model, where redox oscillations driven by annual water table fluctuations, accompanied by strong evapotranspiration in low-permeability sediments, promote conversion of U(IV)_sol to relatively immobile U(VI)_sol, which suggests that U_sol is accumulating in a form that is resistant to redox perturbations. This observation contradicts the common idea that U(IV)_sol accumulated in reducing conditions is systematically re-oxidized, solubilized and transported away in groundwater.

Uranium Retention in a Bioreduced Region of an Alluvial Aquifer Induced by the Influx of Dissolved Oxygen

Reduced zones in the subsurface represent biogeochemically active hotspots enriched in buried organic matter and reduced metals. Within a shallow alluvial aquifer located near Rifle, CO, reduced zones control the fate and transport of uranium (U). Though an influx of dissolved oxygen (DO) would be expected to mobilize U, we report U immobilization. Groundwater U concentrations decreased following delivery of DO (21.6 mg O₂/well/h). After 23 days of DO delivery, injection of oxygenated groundwater was paused and resulted in the rebound of groundwater U concentrations to preinjection levels. When DO delivery resumed (day 51), groundwater U concentrations again decreased. The injection was halted on day 82 again and resulted in a rebound of groundwater U concentrations. DO delivery rate was increased to 54 mg O₂/well/h (day 95) whereby groundwater U concentrations increased. Planktonic cell abundance remained stable throughout the experiment, but virus-to-microbial cell ratio increased 1.8-3.4-fold with initial DO delivery, indicative of microbial activity in response to DO injection. Together, these results indicate that the redox-buffering capacity of reduced sediments can prevent U mobilization, but could be overcome as delivery rate or oxidant concentration increases, mobilizing U.

Effect of bicarbonate on aging and reactivity of nanoscale zerovalent iron (nZVI) toward uranium removal

Bicarbonate, ubiquitous in natural and waste waters is an important factor regulating the rate and efficiency of pollutant separation and transformation. For example, it can form complexes with U(VI) in the aqueous phase and at the solid-water interface. In this work, we investigated the effect of bicarbonate on the aging of nanoscale zero-valent (nZVI) in the context of U(VI) reduction and removal from wastewater. For fresh nZVI, over 99% aqueous uranium was separated in less than 10 min, of which 83% was reduced from U(VI) to U(IV). When nZVI was aged in water, its activity for U(VI) sequestration and reduction was significantly reduced. Batch experiments showed that for nZVI aged in the presence of 10 mM bicarbonate, only 20.3% uranium was reduced to U(IV) after 6 h reactions. Characterizations of the iron nanoparticles with spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) suggest that in fresh nZVI, uranium was concentrated at the nanoparticle center; whereas in nZVI aged in bicarbonate, uranium was largely deposited on the outer surface of the nanoparticles. Furthermore, aged nZVI without bicarbonate contained more lepidocrocite (γ-FeOOH) while aged nZVI in the presence of bicarbonate had more magnetite/maghemite (Fe₃O₄/γ-Fe₂O₃). This could be attributed to the formation of carbonate green rust and pH buffer effect of . Primary mechanisms for U(VI) removal with nZVI include reduction, sorption and/or precipitation. Results demonstrate that bicarbonate alter the aging products of nZVI, and reduces the separation efficiency and reduction capability for uranium removal.

Uranyl Peroxide Nanocluster (U60) Persistence and Sorption in the Presence of Hematite

The presence of uranium-based nanomaterials in environmental systems may significantly impact our current understanding of the fate and transport of U(VI). Sorption of the uranyl peroxide nanocluster [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀) to hematite (α-Fe₂O₃) was studied using batch sorption experiments with varying U₆₀, hematite, and alkali electrolyte (i.e., NaCl, KCl, and CsCl) concentrations. Data from electrospray ionization mass spectrometry and centrifugal microfiltration revealed that U₆₀ persisted in the presence of hematite and the background electrolyte for at least 120 days. K⁺ ions were removed from solution with uranium whereas Li⁺ ions remained in solution, indicating that the U₆₀ cluster behaved like an anion and that the Li⁺ ions did not play a significant role in the sorption mechanism. Analysis of the reacted mineral surface using X-ray photoelectron and Raman spectroscopies confirmed the presence of U(VI) and uranyl species with bridged peroxo groups associated with the mineral surface. These results indicate that uranyl peroxide nanoclusters may persist in the aqueous phase under environmentally relevant conditions for reasonably long periods of time, as compared to that of the uranyl cation.

The influence of Si(iv) on the reactivity of [Fe(iii)]/[Fe(ii)] couples for 2-nitrophenol reduction in γ-Al2O3 suspensions

In a natural environment, Fe(ii) adsorbed onto the surfaces of natural particles to form various surface complex species can influence the transformation of contaminants. The reductive reactivity of the [Fe(iii)]/[Fe(ii)] couples are close correlated with the surrounding conditions. In this study, we investigated the effects of Si(iv) on the reductive reactivity of [Fe(iii)]/[Fe(ii)] couples adsorbed onto γ-Al₂O₃. Experiments were conducted under different conditions to investigate the effects of Si(iv) on the reactivity of [Fe(iii)]/[Fe(ii)] couples for 2-nitrophenol (2-NP, selected as the model pollutant) reduction in γ-Al₂O₃ suspensions. Kinetics results revealed that chemical adsorption is the rate limiting step in Fe(ii) and Si(iv) adsorption processes and the reduction of 2-NP is an endothermic reaction. The linear correlations between the reduced peak oxidation potential (E_p) (versus SCE) and 2-NP reduction rate (ln k), and between the adsorbed Fe(ii) density (ρ_Fe(II)) and ln k, illustrated that E_p and ρ_Fe(II) are two key factors in the inhibiting effects of Si(iv) on the reductive reactivity of Fe(iii)/Fe(ii) couples on γ-Al₂O₃. The results of Fe K-edge X-ray absorption spectroscopy revealed that the increase of Si(iv) concentration resulted in the gradual change in the composition of the adsorbed Fe species from pure AlOFe⁺ (γ-Al₂O₃ surface-bound Fe(ii) species with higher reductive reactivity) to a mixture of AlOFe⁺ and SiOFe⁺ (SiO₂ surface-bound Fe(ii) species with lower reductive reactivity), leading to the decrease in ρ_Fe(II), the positive shift in E_p, the increase in activation energy (E_a), and consequently the decrease in the reduction rate (ln k) of 2-NP.

In a natural environment, Fe(ii) adsorbed onto the surfaces of natural particles to form various surface complex species can influence the transformation of contaminants.

Redox Controls over the Stability of U(IV) in Floodplains of the Upper Colorado River Basin

Aquifers in the Upper Colorado River Basin (UCRB) exhibit persistent uranium (U) groundwater contamination plumes originating from former ore processing operations. Previous observations at Rifle, Colorado, have shown that fine grained, sulfidic, organic-enriched sediments accumulate U in its reduced form, U(IV), which is less mobile than oxidized U(VI). These reduced sediment bodies can subsequently act as secondary sources, releasing U back to the aquifer. There is a need to understand if U(IV) accumulation in reduced sediments is a common process at contaminated sites basin-wide, to constrain accumulated U(IV) speciation, and to define the biogeochemical factors controlling its reactivity. We have investigated U(IV) accumulation in organic-enriched reduced sediments at three UCRB floodplains. Noncrystalline U(IV) is the dominant form of accumulated U, but crystalline U(IV) comprises up to ca. 30% of total U at some locations. Differing susceptibilities of these species to oxidative remobilization can explain this variability. Particle size, organic carbon content, and pore saturation, control the exposure of U(IV) to oxidants, moderating its oxidative release. Further, our data suggest that U(IV) can be mobilized under deeply reducing conditions, which may contribute to maintenance and seasonal variability of U in groundwater plumes in the UCRB.

Mechanical investigation of U(VI) on pyrrhotite by batch, EXAFS and modeling techniques

The interaction mechanism of U(VI) on pyrrhotite was demonstrated by batch, spectroscopic and modeling techniques. Pyrite was selected as control group in this study. The removal of U(VI) on pyrite and pyrrhotite significantly decreased with increasing ionic strength from 0.001 to 0.1mol/L at pH 2.0-6.0, whereas the no effect of ionic strength was observed at pH >6.0. The maximum removal capacity of U(VI) on pyrite and pyrrhotite calculated from Langmuir model was 10.20 and 21.34mgg^-1 at pH 4.0 and 333K, respectively. The XPS analysis indicated the U(VI) was primarily adsorbed on pyrrhotite and pyrite and then approximately 15.5 and 9.8% of U(VI) were reduced to U(IV) by pyrrhotite and pyrite after 20 days, respectively. Based on the XANES analysis, the adsorption edge of uranium-containing pyrrhotite located between U^IVO₂(s) and U^VIO₂²⁺ spectra. The EXAFS analysis demonstrated the inner-sphere surface complexation of U(VI) on pyrrhotite due to the occurrence of U-S shell, whereas the U-U shell revealed the reductive co-precipitates of U(VI) on pyrrhotite/pyrite with increasing reaction times. The surface complexation modeling showed that outer- and inner-surface complexation dominated the U(VI) removal at pH<4 and pH>5.0, respectively. The findings presented herein play a crucial role in the removal of radionuclides on iron sulfide in environmental cleanup applications.

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

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

Review of soluble uranium removal by nanoscale zero valent iron

Uranium (U) has been released to surface soil and groundwater through military and industrial activities. Soluble forms of U transferred to drinking water sources and food supplements can potentially threaten humans and the biosphere due to its chemical toxicity and radioactivity. The immobilization of aqueous U onto iron-based minerals is one of the most vital geochemical processes controlling the transport of U. As a consequence, much research has been focused on the use of iron-based materials for the treatment of U contaminated waters. One material currently being tested is nanoscale zero-valent iron (nZVI). However, understanding the removal mechanism of U onto nZVI is crucial to develop new technologies for contaminated water resources. This review article aims to provide information on the removal mechanism of U onto nZVI under different conditions (pH, U concentration, solution ion strength, humic acid, presence of O₂ and CO₂, microorganism effect) pertinent to environmental and engineered systems, and to provide risk or performance assessment results with the stability of nZVI products after removal of U in environmental restoration.

In situ mobility of uranium in the presence of nitrate following sulfate-reducing conditions

Reoxidation and mobilization of previously reduced and immobilized uranium by dissolved-phase oxidants poses a significant challenge for remediating uranium-contaminated groundwater. Preferential oxidation of reduced sulfur-bearing species, as opposed to reduced uranium-bearing species, has been demonstrated to limit the mobility of uranium at the laboratory scale yet field-scale investigations are lacking. In this study, the mobility of uranium in the presence of nitrate oxidant was investigated in a shallow groundwater system after establishing conditions conducive to uranium reduction and the formation of reduced sulfur-bearing species. A series of three injections of groundwater (200 L) containing U(VI) (5 μM) and amended with ethanol (40 mM) and sulfate (20 mM) were conducted in ten test wells in order to stimulate microbial-mediated reduction of uranium and the formation of reduced sulfur-bearing species. Simultaneous push-pull tests were then conducted in triplicate well clusters to investigate the mobility of U(VI) under three conditions: 1) high nitrate (120 mM), 2) high nitrate (120 mM) with ethanol (30 mM), and 3) low nitrate (2 mM) with ethanol (30 mM). Dilution-adjusted breakthrough curves of ethanol, nitrate, nitrite, sulfate, and U(VI) suggested that nitrate reduction was predominantly coupled to the oxidation of reduced-sulfur bearing species, as opposed to the reoxidation of U(IV), under all three conditions for the duration of the 36-day tests. The amount of sulfate, but not U(VI), recovered during the push-pull tests was substantially more than injected, relative to bromide tracer, under all three conditions and further suggested that reduced sulfur-bearing species were preferentially oxidized under nitrate-reducing conditions. However, some reoxidation of U(IV) was observed under nitrate-reducing conditions and in the absence of detectable nitrate and/or nitrite. This suggested that reduced sulfur-bearing species may not be fully effective at limiting the mobility of uranium in the presence of dissolved and/or solid-phase oxidants. The results of this field study confirmed those of previous laboratory studies which suggested that reoxidation of uranium under nitrate-reducing conditions can be substantially limited by preferential oxidation of reduced sulfur-bearing species.

Character of Humic Substances as a Predictor for Goethite Nanoparticle Reactivity and Aggregation

Natural organic matter (NOM) is ubiquitous in surface water and groundwater and interacts strongly with mineral surfaces. The details of these interactions, as well as their impacts on mineral surface reactivity, are not well understood. In this work, both the reactivity and aggregation of goethite (α-FeOOH) nanoparticles were quantified in the presence of well-characterized humic substances. Results from monitoring the kinetics of reductive degradation of 4-chloronitrobenzene (4-ClNB) by Fe(II) adsorbed onto the goethite nanoparticles with and without added humic substances demonstrates that, in all cases, humic substances suppressed Fe(II)-goethite reactivity. The ranking of the standards from the least to most inhibitive was Pahokee Peat humic acid, Elliot Soil humic acid, Suwannee River humic acid, Suwannee River NOM, Suwannee River fulvic acid I, Suwannee River fulvic acid II, and Pahokee Peat fulvic acid. Correlations between eight characteristics (molecular weight, carboxyl concentration, and carbon, oxygen, nitrogen, aliphatic, heteroaliphatic, and aromatic content) and 4-ClNB degradation rate constants were observed. Faster kinetic rates of reductive degradation were observed with increased molecular weight and nitrogen, carbon, and aromatic content, and slower rates were observed with increased carboxyl concentration and oxygen, heteroaliphatic, and aliphatic content. With these correlations, improved predictions of the reactivity of Fe(II)-goethite with pollutants based on properties of the humic substances are possible.

Application of diffusive gradients in thin films and core centrifugation methods to determine inorganic mercury and monomethylmercury profiles in sediment porewater

A diffusive gradient in thin films (DGT) is an in situ sampling technique for the quantitative analysis of contaminant concentrations that is based on the diffusion and adsorption of contaminants on to resin gels. In the present study, a DGT technique was applied to measure total mercury (Hg) and monomethylmercury (MMHg) concentrations in lake and coastal sediment porewaters and compare them with those from ex situ sediment centrifugation. To calculate the total Hg and MMHg concentrations in porewater using the DGT method, the diffusion coefficients of Hg species in a diffusive gel medium was first determined, and then total Hg and MMHg depth profiles were measured using the experimentally determined diffusion coefficients. Using the diffusion coefficients for artificial lake and estuarine waters containing inorganic salts, rather than those for lake and estuarine waters containing Suwannee River humic acid (∼5 mg C L(-1) ), the DGT method demonstrated similar Hg and MMHg profiles to those using the centrifugation method. Based on the need for fine vertical resolution and high metal concentrations to be collected, DGT is suggested to be a reliable method for determining Hg(II) and MMHg depth profiles in sediment porewater.

Influence of clay minerals on sorption and bioreduction of arsenic under anoxic conditions

Adsorption of As(V) on various clay minerals including kaolinite (KGa-1), montmorillonite (SWy-1) and nontronites (NAU-1 and NAU-2), and subsequent bioreduction of sorbed As(V) to As(III) by bacterium Shewanella putrefaciens strain CN-32 were investigated. Nontronites showed relatively higher sorption capacity for As(V) primarily due to higher iron oxide content. Freundlich equation well described the sorption of As(V) on NAU-1, NAU-2 and SWy-1, while As(V) sorption isotherm with KGa-1 fitted well in the Langmuir model. The bacterium rapidly reduced 50% of dissolved As(V) to As(III) in 2 h, followed by its complete reduction (>ca. 98%) within 12 h. In contrast, sorption of As(V) to the mineral surfaces interferes with the activity of bacterium, resulting in low bioreduction of As(V) by 27% for 5 days of incubation. S. putrefaciens also promoted the reduction of Fe(III) present in the clay mineral to Fe(II). This study indicates that the sorption and subsequent bioreduction of As(V) on clay minerals can significantly influence the mobility of As(V) in subsurface environment.

Uranium(VI) reduction by nanoscale zero-valent iron in anoxic batch systems: the role of Fe(II) and Fe(III)

The role of Fe(II) and Fe(III) in U(VI) reduction by nanoscale zerovalent iron (nanoFe0) was investigated using two iron chelators 1,10-phenanthroline and triethanolamine (TEA) under a CO2-free anoxic condition. The results showed that U(VI) reduction was strongly inhibited by 1,10-phenanthroline and TEA in a pH range from 6.9 to 9.0. For instance, at pH 6.9 the observed U(VI) reduction rates decreased by 81% and 82% in the presence of 1,10-phenanthroline and TEA, respectively. The inhibition was attributed to the formation of stable complexes between 1,10-phenanthroline and Fe(II) or TEA and Fe(III). In the absence of iron chelators, U(VI) reduction can be enhanced by surface-bound Fe(II) on nanoFe0. Our results suggested that Fe(III) and Fe(II) possibly acted as an electron shuttle to ferry the electrons from nanoFe0 to U(VI), therefore a combined system with Fe(II), Fe(III) and nanoFe0 could facilitate U(VI) reductive immobilization in the contaminated groundwater.

Fe(II)-catalyzed recrystallization of goethite revisited

Results from enriched (57)Fe isotope tracer experiments have shown that atom exchange can occur between structural Fe in Fe(III) oxides and aqueous Fe(II) with no formation of secondary minerals or change in particle size or shape. Here we derive a mass balance model to quantify the extent of Fe atom exchange between goethite and aqueous Fe(II) that accounts for different Fe pool sizes. We use this model to reinterpret our previous work and to quantify the influence of particle size and pH on extent of goethite exchange with aqueous Fe(II). Consistent with our previous interpretation, substantial exchange of goethite occurred at pH 7.5 (≈ 90%) and we observed little effect of particle size between nanogoethite (average size of 81 × 11 nm; ≈ 110 m(2)/g) and microgoethite (average size of 590 × 42 nm; ≈ 40 m(2)/g). Despite ≈ 90% of the bulk goethite exchanging at pH 7.5, we found no change in mineral phase, average particle size, crystallinity, or reactivity after reaction with aqueous Fe(II). At a lower pH of 5.0, no net sorption of Fe(II) was observed and significantly less exchange occurred accounting for less than the estimated proportion of surface Fe atoms in the particles. Particle size appears to influence the amount of exchange at pH 5.0 and we suggest that aggregation and surface area may play a role. Results from sequential chemical extractions indicate that (57)Fe accumulates in extracted Fe(III) goethite components. Isotopic compositions of the extracts indicate that a gradient of (57)Fe develops within the goethite with more accumulation of (57)Fe occurring in the more easily extracted Fe(III) that may be nearer to the surface.

Inhibition of U(VI) reduction by synthetic and natural pyrite

Reductive precipitation is an effective method of attenuating the mobility of uranium (U) in subsurface environments. The reduction of U(VI) by synthetic and naturally occurring pyrite was investigated at pH 3.0-9.5. In contrast to thermodynamic calculations that were used to predict UO2(s) precipitation, a mixed U(IV) and U(VI) product (e.g., U3O8/U4O9/U3O7) was only observed at pH 6.21-8.63 and 4.52-4.83 for synthetic and natural pyrite, respectively. Under acidic conditions, the reduction of UO2(2+) by surface-associated Fe(2+) may not be favored because the mineral surface is nearly neutral or not negative enough. At high pH, the sorption of negatively charged U(VI) species is not favored on the negatively charged mineral surface. Thus, the redox reaction is not favored. Trace elements generally contained within the natural pyrite structure can affect the reactivity of pyrite and lead to a different result between the natural and synthetic pyrite. Because UO2(s) is extremely redox-sensitive toward U(VI), the observed UO2+x(s) phase reduction product indicates a surface reaction that is largely controlled by reaction kinetics and pyrite surface chemistry. These factors may explain why most laboratory experiments have observed incomplete U(VI) reduction on Fe(II)-bearing minerals.

Uranium bioreduction rates across scales: biogeochemical hot moments and hot spots during a biostimulation experiment at Rifle, Colorado

We aim to understand the scale-dependent evolution of uranium bioreduction during a field experiment at a former uranium mill site near Rifle, Colorado. Acetate was injected to stimulate Fe-reducing bacteria (FeRB) and to immobilize aqueous U(VI) to insoluble U(IV). Bicarbonate was coinjected in half of the domain to mobilize sorbed U(VI). We used reactive transport modeling to integrate hydraulic and geochemical data and to quantify rates at the grid block (0.25 m) and experimental field scale (tens of meters). Although local rates varied by orders of magnitude in conjunction with biostimulation fronts propagating downstream, field-scale rates were dominated by those orders of magnitude higher rates at a few selected hot spots where Fe(III), U(VI), and FeRB were at their maxima in the vicinity of the injection wells. At particular locations, the hot moments with maximum rates negatively corresponded to their distance from the injection wells. Although bicarbonate injection enhanced local rates near the injection wells by a maximum of 39.4%, its effect at the field scale was limited to a maximum of 10.0%. We propose a rate-versus-measurement-length relationship (log R' = -0.63 log L - 2.20, with R' in μmol/mg cell protein/day and L in meters) for orders-of-magnitude estimation of uranium bioreduction rates across scales.

Geochemical and microbiological responses to oxidant introduction into reduced subsurface sediment from the Hanford 300 Area, Washington

Pliocene-aged reduced lacustrine sediment from below a subsurface redox transition zone at the 300 Area of the Hanford site (southeastern Washington) was used in a study of the geochemical response to introduction of oxygen or nitrate in the presence or absence of microbial activity. The sediments contained large quantities of reduced Fe in the form of Fe(II)-bearing phyllosilicates, together with smaller quantities of siderite and pyrite. A loss of ca. 50% of 0.5 M HCl-extractable Fe(II) [5-10 mmol Fe(II) L(-1)] and detectable generation of sulfate (ca. 0.2 mM, equivalent to 10% of the reduced inorganic sulfur pool) occurred in sterile aerobic reactors. In contrast, no systematic loss of Fe(II) or production of sulfate was observed in any of the other oxidant-amended sediment suspensions. Detectable Fe(II) accumulation and sulfate consumption occurred in non-sterile oxidant-free reactors. Together, these results indicate the potential for heterotrophic carbon metabolism in the reduced sediments, consistent with the proliferation of known heterotrophic taxa (e.g., Pseudomonadaceae, Burkholderiaceae, and Clostridiaceae) inferred from 16S rRNA gene pyrosequencing. Microbial carbon oxidation by heterotrophic communities is likely to play an important role in maintaining the redox boundary in situ, i.e., by modulating the impact of downward oxidant transport on Fe/S redox speciation. Diffusion-reaction simulations of oxygen and nitrate consumption coupled to solid-phase organic carbon oxidation indicate that heterotrophic consumption of oxidants could maintain the redox boundary at its current position over millennial time scales.

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

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

Fe-phyllosilicate redox cycling organisms from a redox transition zone in Hanford 300 Area sediments

Microorganisms capable of reducing or oxidizing structural iron (Fe) in Fe-bearing phyllosilicate minerals were enriched and isolated from a subsurface redox transition zone at the Hanford 300 Area site in eastern Washington, USA. Both conventional and in situ “i-chip” enrichment strategies were employed. One Fe(III)-reducing Geobacter (G. bremensis strain R1, Deltaproteobacteria) and six Fe(II) phyllosilicate-oxidizing isolates from the Alphaproteobacteria (Bradyrhizobium japonicum strains 22, is5, and in8p8), Betaproteobacteria (Cupriavidus necator strain A5-1, Dechloromonas agitata strain is5), and Actinobacteria (Nocardioides sp. strain in31) were recovered. The G. bremensis isolate grew by oxidizing acetate with the oxidized form of NAu-2 smectite as the electron acceptor. The Fe(II)-oxidizers grew by oxidation of chemically reduced smectite as the energy source with nitrate as the electron acceptor. The Bradyrhizobium isolates could also carry out aerobic oxidation of biotite. This is the first report of the recovery of a Fe(II)-oxidizing Nocardioides, and to date only one other Fe(II)-oxidizing Bradyrhizobium is known. The 16S rRNA gene sequences of the isolates were similar to ones found in clone libraries from Hanford 300 sediments and groundwater, suggesting that such organisms may be present and active in situ. Whole genome sequencing of the isolates is underway, the results of which will enable comparative genomic analysis of mechanisms of extracellular phyllosilicate Fe redox metabolism, and facilitate development of techniques to detect the presence and expression of genes associated with microbial phyllosilicate Fe redox cycling in sediments.

U(VI) reduction in sulfate-reducing subsurface sediments amended with ethanol or acetate

An experiment was conducted with subsurface sediments from Oak Ridge National Laboratory to determine the potential for reduction of U(VI) under sulfate-reducing conditions with either ethanol or acetate as the electron donor. The results showed extensive U(VI) reduction in sediments supplied with either electron donor, where geochemical and microbiological analyses demonstrated active sulfate reduction.

Impact of microbial Mn oxidation on the remobilization of bioreduced U(IV)

Effects of Mn redox cycling on the stability of bioreduced U(IV) are evaluated here. U(VI) can be biologically reduced to less soluble U(IV) species and the stimulation of biological activity to that end is a salient remediation strategy; however, the stability of these materials in the subsurface environments where they form remains unproven. Manganese oxides are capable of rapidly oxidizing U(IV) to U(VI) in mixed batch systems where the two solid phases are in direct contact. However, it is unknown whether the same oxidation would take place in a porous medium. To probe that question, U(IV) immobilized in agarose gels was exposed to conditions allowing biological Mn(II) oxidation (HEPES buffer, Mn(II), 5% O2 and Bacillus sp. SG-1 spores). Results show the oxidation of U(IV) to U(VI) is due primarily to O2 rather than to MnO2. U(VI) produced is retained within the gel to a greater extent when Mn oxides are present, suggesting the formation of strong surface complexes. The implication for the long-term stability of U in a bioremediated site is that, in the absence of competing ligands, biological Mn(II) oxidation may promote the immobilization of U(VI) produced by the oxidation of U(IV).

Abiotic reductive immobilization of U(VI) by biogenic mackinawite

During subsurface bioremediation of uranium-contaminated sites, indigenous metal and sulfate-reducing bacteria may utilize a variety of electron acceptors, including ferric iron and sulfate that could lead to the formation of various biogenic minerals in situ. Sulfides, as well as structural and adsorbed Fe(II) associated with biogenic Fe(II)-sulfide phases, can potentially catalyze abiotic U(VI) reduction via direct electron transfer processes. In the present work, the propensity of biogenic mackinawite (Fe 1+x S, x = 0 to 0.11) to reduce U(VI) abiotically was investigated. The biogenic mackinawite produced by Shewanella putrefaciens strain CN32 was characterized by employing a suite of analytical techniques including TEM, SEM, XAS, and Mössbauer analyses. Nanoscale and bulk analyses (microscopic and spectroscopic techniques, respectively) of biogenic mackinawite after exposure to U(VI) indicate the formation of nanoparticulate UO2. This study suggests the relevance of sulfide-bearing biogenic minerals in mediating abiotic U(VI) reduction, an alternative pathway in addition to direct enzymatic U(VI) reduction.

Evaluating ion exchange resin efficiency and oxidative capacity for the separation of uranium(IV) and uranium(VI)

BACKGROUND

Previously described methods to separate dissolved U(IV) from dissolved U(VI) under acidic anoxic conditions prior to laboratory analysis were ineffective with materials currently available commercially. Three strong anion exchange resins were examined for their efficiency in separating, recovering, and preserving both redox states during separation.

RESULTS

Under oxic conditions, recovery of U(VI) from three exchange resins (Bio-Rad AG® 1x8 Poly-Prep® prefilled columns, Bio-Rad AG® 1x8 powder, and Dowex® 1x8 powder) ranged from 72% to 100% depending on the dosed mass, eluent volume, and resin selected. Dowex® 1x8 resin was the only resin found to provide 100% recovery of U(VI) with fewer than 5 bed volumes of eluent. Under anoxic conditions, all three resins oxidized U(IV) in aqueous solutions with relatively low U(IV) concentrations (<3x10-6 M). Resin-induced oxidation was observed visually using a leuco dye, safranin-o. Oxidants associated with the resin were irreversibly reduced by the addition of Ti(III). After anoxic resin pre-treatment, a series of U(IV)/U(VI) mixtures at micro-molar levels were prepared and separated using the Dowex® 1x8 resin with 100% recovery of both U(IV) and U(VI) with no resin-induced changes in oxidation state.

CONCLUSIONS

Currently available anion exchange resins with apparently identical physical properties were found to have significantly different recoveries for hexavalent uranium at micro-molar concentrations. A novel qualitative technique was developed to visually assess oxidative capacities of anion exchange resins under acidic anoxic conditions. A protocol was developed for pre-treatment and use of currently available anion exchange resins to achieve quantitative separation of U(IV) and U(VI) in aqueous solutions with low U(IV) concentrations. This method can be applied to future work to quantitatively assess dissolved U(IV) and U(VI) concentrations in both laboratory and field samples.

Effects of solution chemistry on the removal reaction between calcium carbonate-based materials and Fe(II)

Elevated iron concentrations have been observed in the groundwater underlying and surrounding several Florida landfill sites. An in situ groundwater remediation method for iron (present as soluble ferrous iron) using a permeable reactive barrier composed of calcium carbonate-based materials (CCBMs), such as limestone, was examined as a potentially effective and low-cost treatment technique. The effects of various environmental factors (i.e., pH, co-existing cations, and natural organic matter (NOM)) on the removal reaction were investigated using laboratory batch studies. Solution pH had a minor effect on iron removal, with superior iron removal observed in the highest pH solution (pH of 9). Sodium and calcium tended to impede the iron removal process by increasing the ionic strength of the solution. Manganese competes with iron ions at the adsorption sites on CCBMs; therefore, the presence of manganese prohibits iron removal and reduces removal effectiveness. NOM was found to decrease Fe(II) uptake by CCBMs and reduce the removal effectiveness by complexing Fe(II), most likely through the carboxyl group, thereby maintaining Fe(II) mobility in the aqueous phase.

Sorption coefficients and molecular mechanisms of Pu, U, Np, Am and Tc to Fe (hydr)oxides: a review

Pu, U, Np, Am and Tc are among the major risk drivers at nuclear waste management facilities throughout the world. Furthermore, uranium mining and milling operations have generated an enormous legacy of radioactively contaminated soils and groundwater. The sorption process of radionulcides onto ubiquitous Fe (hydr)oxides (FHOs; hematite, magnetite, goethite and ferrihydrite) is one of the most vital geochemical processes controlling the transport and fate of radionuclides and nuclear wastes in the subsurface zones. Meanwhile, understanding molecular-level chemical speciation of radionuclides onto FHOs is crucial to model their behavior in subsurface environments, and to develop new technologies for nuclear waste treatment and long-term remediation strategies for contaminated soils and groundwater. This review article aims (1) to provide risk or performance assessment modelers with macroscopic distribution coefficient (K(d)) data of Pu, U, Np, Am and Tc onto FHOs under different conditions (pH, radionuclide concentration, solution ion strength, sorbent loading, partial pressure of CO(2) (P CO(2)), equilibrium time) pertinent to environmental and engineered systems, and (2) to provide a microscopic or molecular-level understanding of the chemical speciation and sorption processes of these radionuclides to FHOs.

Pore-scale characterization of biogeochemical controls on iron and uranium speciation under flow conditions

Etched silicon microfluidic pore network models (micromodels) with controlled chemical and redox gradients, mineralogy, and microbiology under continuous flow conditions are used for the incremental development of complex microenvironments that simulate subsurface conditions. We demonstrate the colonization of micromodel pore spaces by an anaerobic Fe(III)-reducing bacterial species (Geobacter sulfurreducens) and the enzymatic reduction of a bioavailable Fe(III) phase within this environment. Using both X-ray microprobe and X-ray absorption spectroscopy, we investigate the combined effects of the precipitated Fe(III) phases and the microbial population on uranium biogeochemistry under flow conditions. Precipitated Fe(III) phases within the micromodel were most effectively reduced in the presence of an electron shuttle (AQDS), and Fe(II) ions adsorbed onto the precipitated mineral surface without inducing any structural change. In the absence of Fe(III), U(VI) was effectively reduced by the microbial population to insoluble U(IV), which was precipitated in discrete regions associated with biomass. In the presence of Fe(III) phases, however, both U(IV) and U(VI) could be detected associated with biomass, suggesting reoxidation of U(IV) by localized Fe(III) phases. These results demonstrate the importance of the spatial localization of biomass and redox active metals, and illustrate the key effects of pore-scale processes on contaminant fate and reactive transport.

Removal of U(VI) from aqueous solutions by the nano-iron oxyhydroxides

The nano-iron oxyhydroxides (α- and γ-FeOOH) were synthesized by oxidation of the various raw materials (FeSO4 and FeCl2) as iron precursors under alkaline conditions. Morphologies of nano-iron oxyhydroxides were characterized by high resolution transmission electron microscope (HRTEM), X-ray powder diffraction (XRD) and specific surface area (SSA), respectively. The removal of U(VI) from aqueous solution by nano-iron oxyhydroxides was also investigated as a function of contact time, U(VI) concentration, pH, ionic strength and temperature under ambient conditions. The occurrence of needle-like shape of nano-goethite and rod-like shape of nano- lepidocrocite were attributed to oxidization of Fe2+ under different conditions in terms of XRD analysis and HRTEM images. The sorption results indicated that the removal of U(VI) strongly depended on pH and were independent of ionic strength. Langmuir model was employed to model the sorption isotherms of U(VI) at three various temperatures of 293, 313 and 333 K. The thermodynamic parameters (Δ Hº, Δ Sº and Δ Gº) computed from the temperature showed that sorption process of U(VI) on nano-iron oxyhydroxides was endothermic and spontaneous. The mechanism of U(VI) sorption on nano-iron oxyhydroxides was assumed to outer-sphere surface complexation at low pH (pH < 5.0) and at low ionic strength conditions (IS = 0.01 M NaClO4), whereas the uptake of U(VI) was dominated by surface precipitation at high pH conditions. Thereby nano-iron oxyhydroxides can be used as an efficient backfill materials for the in situ disposal of U(VI)-bearing contaminated groundwater.