Identification of simultaneous U(VI) sorption complexes and U(IV) nanoprecipitates on the magnetite (111) surface

Sequestration of uranium (U) by magnetite is a potentially important sink for U in natural and contaminated environments. However, molecular-scale controls that favor U(VI) uptake including both adsorption of U(VI) and reduction to U(IV) by magnetite remain poorly understood, in particular, the role of U(VI)-CO(3)-Ca complexes in inhibiting U(VI) reduction. To investigate U uptake pathways on magnetite as a function of U(VI) aqueous speciation, we performed batch sorption experiments on (111) surfaces of natural single crystals under a range of solution conditions (pH 5 and 10; 0.1 mM U(VI); 1 mM NaNO(3); and with or without 0.5 mM CO(3) and 0.1 mM Ca) and characterized surface-associated U using grazing incidence extended X-ray absorption fine structure spectroscopy (GI-EXAFS), grazing incidence X-ray diffraction (GI-XRD), and scanning electron microscopy (SEM). In the absence of both carbonate ([CO(3)](T), denoted here as CO(3)) and calcium (Ca), or in the presence of CO(3) only, coexisting adsorption of U(VI) surface species and reduction to U(IV) occurs at both pH 5 and 10. In the presence of both Ca and CO(3), only U(VI) adsorption (VI) occurs. When U reduction occurs, nanoparticulate UO(2) forms only within and adjacent to surface microtopographic features such as crystal boundaries and cracks. This result suggests that U reduction is limited to defect-rich surface regions. Further, at both pH 5 and 10 in the presence of both CO(3) and Ca, U(VI)-CO(3)-Ca ternary surface species develop and U reduction is inhibited. These findings extend the range of conditions under which U(VI)-CO(3)-Ca complexes inhibit U reduction.     

Influence of magnetite stoichiometry on U(VI) reduction

Hexavalent uranium (U(VI)) can be reduced enzymatically by various microbes and abiotically by Fe(2+)-bearing minerals, including magnetite, of interest because of its formation from Fe(3+) (oxy)hydroxides via dissimilatory iron reduction. Magnetite is also a corrosion product of iron metal in suboxic and anoxic conditions and is likely to form during corrosion of steel waste containers holding uranium-containing spent nuclear fuel. Previous work indicated discrepancies in the extent of U(VI) reduction by magnetite. Here, we demonstrate that the stoichiometry (the bulk Fe(2+)/Fe(3+) ratio, x) of magnetite can, in part, explain the observed discrepancies. In our studies, magnetite stoichiometry significantly influenced the extent of U(VI) reduction by magnetite. Stoichiometric and partially oxidized magnetites with x ≥ 0.38 reduced U(VI) to U(IV) in UO(2) (uraninite) nanoparticles, whereas with more oxidized magnetites (x < 0.38) and maghemite (x = 0), sorbed U(VI) was the dominant phase observed. Furthermore, as with our chemically synthesized magnetites (x ≥ 0.38), nanoparticulate UO(2) was formed from reduction of U(VI) in a heat-killed suspension of biogenic magnetite (x = 0.43). X-ray absorption and M?ssbauer spectroscopy results indicate that reduction of U(VI) to U(IV) is coupled to oxidation of Fe(2+) in magnetite. The addition of aqueous Fe(2+) to suspensions of oxidized magnetite resulted in reduction of U(VI) to UO(2), consistent with our previous finding that Fe(2+) taken up from solution increased the magnetite stoichiometry. Our results suggest that magnetite stoichiometry and the ability of aqueous Fe(2+) to recharge magnetite are important factors in reduction of U(VI) in the subsurface.     

Influence of calcium carbonate on U(VI) sorption to soils

The high stability of calcium uranyl carbonate complexes in the circumneutral pH range has a strong impact on U(VI) sorption in calcareous soils. To quantify this influence, sorption of U(VI) to soils in the presence of naturally occurring calcium carbonate was investigated by conducting batch experiments in which either U(VI) concentration or solution pH was varied. Two soils containing different calcium carbonate concentrations were selected, one from Oak Ridge, TN, and another from Altamont Pass, CA. The results show that the presence of calcium carbonate in soils strongly affects U(VI) sorption. Higher concentrations of soil calcium carbonate lead to a pronounced suppression of the pH-dependent sorption curve in the neutral pH range because of the formation of a very stable neutral complex of calcium uranyl carbonate in solution. A surface complexation model considering both strong and weak sites for ferrihydrite and ionizable hydroxyl sites for clay minerals was compared with experimental results, and U(VI) binding parameters were reasonably estimated. Fair agreement was found between the model predictions and sorption data, which span a wide range of U(VI) concentrations and pH. The results also show that appropriate solution-to-solid ratios need to be used when measuring distribution coefficients in calcareous soils to avoid complete CaCO3 dissolution and consequent dilution of calcium uranyl carbonate complexes.     

Non-uraninite products of microbial U(VI) reduction

A promising remediation approach to mitigate subsurface uranium contamination is the stimulation of indigenous bacteria to reduce mobile U(VI) to sparingly soluble U(IV). The product of microbial uranium reduction is often reported as the mineral uraninite. Here, we show that the end products of uranium reduction by several environmentally relevant bacteria (Gram-positive and Gram-negative) and their spores include a variety of U(IV) species other than uraninite. U(IV) products were prepared in chemically variable media and characterized using transmission electron microscopy (TEM) and X-ray absorption spectroscopy (XAS) to elucidate the factors favoring/inhibiting uraninite formation and to constrain molecular structure/composition of the non-uraninite reduction products. Molecular complexes of U(IV) were found to be bound to biomass, most likely through P-containing ligands. Minor U(IV)-orthophosphates such as ningyoite [CaU(PO(4))(2)], U(2)O(PO(4))(2), and U(2)(PO(4))(P(3)O(10)) were observed in addition to uraninite. Although factors controlling the predominance of these species are complex, the presence of various solutes was found to generally inhibit uraninite formation. These results suggest a new paradigm for U(IV) in the subsurface, i.e., that non-uraninite U(IV) products may be found more commonly than anticipated. These findings are relevant for bioremediation strategies and underscore the need for characterizing the stability of non-uraninite U(IV) species in natural settings.     

Microbial reduction of U(VI) at the solid-water interface

Microbial (Geobacter sulfurreducens) reduction of 0.1 mM U(VI) in the presence of synthetic Fe(III) oxides and natural Fe(III) oxide-containing solids was investigated in pH 6.8 artificial groundwater containing 10 mM NaHCO3. In most experiments, more than 95% of added U(VI) was sorbed to solids, so that U(VI) reduction was governed by reactions at the solid-water interface. The rate and extent of reduction of U(VI) associated with surfaces of synthetic Fe(III) oxides (hydrous ferric oxide, goethite, and hematite) was comparable to that observed during reduction of aqueous U(VI). In contrast, microbial reduction of U(VI) sorbed to several different natural Fe(III) oxide-containing solids was slower and less extensive compared to synthetic Fe(III) oxide systems. Addition of the electron shuttling agent anthraquinone-2,6-disulfonate (AQDS; 0.1 mM) enhanced the rate and extent of both Fe(III) and U(VI) reduction. These findings suggest that AQDS facilitated electron transfer from G. sulfurreducens to U(VI) associated with surface sites atwhich direct enzymatic reduction was kinetically limited. Our results demonstrate that association of U(VI) with diverse surface sites in natural soils and sediments has the potential to limit the rate and extent of microbial U(VI) reduction and thereby modulate the effectiveness of in situ U(VI) bioremediation.     

Kinetics of microbial reduction of Solid phase U(VI)

Sodium boltwoodite (NaUO2SiO3OH x 1.5 H2O) was used to assess the kinetics of microbial reduction of solid-phase U(VI) by a dissimilatory metal-reducing bacterium (DMRB), Shewanella oneidensis strain MR-1. The bioreduction kinetics was studied with Na-boltwoodite in suspension or within alginate beads in a nongrowth medium with lactate as electron donor at pH 6.8 buffered with PIPES. Concentrations of U(VI)tot and cell number were varied to evaluate the coupling of U(VI) dissolution, diffusion, and microbial activity. Microscopic and spectroscopic analyses with transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and laser-induced fluorescence spectroscopy (LIFS) collectively indicated that solid-phase U(VI) was first dissolved and diffused out of grain interiors before it was reduced on bacterial surfaces and/or within the periplasm. The kinetics of solid-phase U(VI) bioreduction was well described by a coupled model of bicarbonate-promoted dissolution of Na-boltwoodite, intragrain uranyl diffusion, and Monod type bioreduction kinetics with respect to dissolved U(VI) concentration. The results demonstrated that microbial reduction of solid-phase U(VI) is controlled by coupled biological, chemical, and physical processes.     

Cr(VI) reduction and immobilization by magnetite under alkaline pH conditions: the role of passivation

This study investigated Cr(VI) reduction and immobilization by magnetite under alkaline pH conditions similar to those present at the Hanford site. Compared to acidic and neutral pH, chromium(VI) reduction by magnetite at high pH conditions is limited (<20% of potential reduction capacity), and the extent of reduction does not vary significantly with increasing NaOH concentration. This is due to the formation of maghemite, goethite, and/or Fe1-xCrxOOH, which may form a passivation layer on the magnetite surface, stopping further chromate reduction. Maghemite is formed in lower NaOH concentrations. The extent of goethite formation increases with NaOH concentration. Goethite may be formed through two mechanisms: (i) dissolution of magnetite leads to the precipitation of goethite and/or (ii) dissolution of newly formed maghemite intermediate, followed by precipitation of goethite. Extended X-ray absorption fine structure spectroscopy shows that Cr has a similar structural environment at alkaline pH as at acidic and circumneutral conditions.     

Inhibition of a U(VI)- and sulfate-reducing consortia by U(VI)

The stimulation of microbial U(VI) reduction is currently being investigated as a means to reduce uranium's mobility in groundwater, but little is known about the concentration at which U(VI) might inhibit microbial activity, or the effect of U(VI) on bacterial community structure. We investigated these questions with an ethanol-fed U(VI)- and sulfate-reducing enrichment developed from sediment from the site of an ongoing field biostimulation experiment at Area 3 of the Oak Ridge Field Research Center (FRC). Sets of triplicate enrichments were spiked with increasing concentrations of U(VI) (from 49 microm to 9.2 mM). As the U(VI) concentration increased to 224 microM, the culture's production of acetate from ethanol slowed, and at or above 1.6 mM U(VI) little acetate was produced over the time frame of the experiment. An uncoupling inhibition model was applied to the data, and the inhibition coefficient for U(VI), Ku, was found to be approximately 100 microM U(VI), or 24 mg/L, indicating the inhibitory effect is relevant at highly contaminated sites. Microbial community structure at the conclusion of the experiment was analyzed with terminal restriction fragment length polymorphism (T-RFLP) analysis. T-RFs associated with Desulfovibrio-like organisms decreased in relative abundance with increasing U(VI) concentration, whereas Clostridia-like T-RFs increased.     

Quantitative separation of monomeric U(IV) from UO2 in products of U(VI) reduction

The reduction of soluble hexavalent uranium to tetravalent uranium can be catalyzed by bacteria and minerals. The end-product of this reduction is often the mineral uraninite, which was long assumed to be the only product of U(VI) reduction. However, recent studies report the formation of other species including an adsorbed U(IV) species, operationally referred to as monomeric U(IV). The discovery of monomeric U(IV) is important because the species is likely to be more labile and more susceptible to reoxidation than uraninite. Because there is a need to distinguish between these two U(IV) species, we propose here a wet chemical method of differentiating monomeric U(IV) from uraninite in environmental samples. To calibrate the method, U(IV) was extracted from known mixtures of uraninite and monomeric U(IV) and tested using X-ray absorption spectroscopy (XAS). Monomeric U(IV) was efficiently removed from biomass and Fe(II)-bearing phases by bicarbonate extraction, without affecting uraninite stability. After confirming that the method effectively separates monomeric U(IV) and uraninite, it is further evaluated for a system containing those reduced U species and adsorbed U(VI). The method provides a rapid complement, and in some cases alternative, to XAS analyses for quantifying monomeric U(IV), uraninite, and adsorbed U(VI) species in environmental samples.     

Effect of grain size on uranium(VI) surface complexation kinetics and adsorption additivity

The contribution of variable grain sizes to uranium adsorption/desorption was studied using a sediment from the US DOE Hanford site. The sediment was wet sieved into four size fractions: coarse sand (1-2 mm), medium sand (0.2-1 mm), fine sand (0.053-0.2 mm), and clay/silt fraction (<0.053 mm). For each size fraction and their composite (sediment), batch and flow-cell experiments were performed to determine uranium adsorption isotherms and kinetic uranium adsorption and subsequent desorption. The results showed that uranium adsorption isotherms and adsorption/desorption kinetics were size specific, reflecting the effects of size-specific adsorption site concentration and kinetic rate constants. The larger-size fraction had a larger mass percentage in the sediment but with a smaller adsorption site concentration and generally a slower uranium adsorption/desorption rate. The same equilibrium surface complexation reaction and reaction constant could describe uranium adsorption isotherms for all size fractions and the composite after accounting for the effect of adsorption site concentration. Mass-weighted, linear additivity was observed for both uranium adsorption isotherms and adsorption/desorption kinetics in the composite. One important implication of this study is that grain-size distribution may be used to estimate uranium adsorption site and adsorption/desorption kinetic rates in heterogeneous sediments from a common location.     

Modeling the inhibition of the bacteral reduction of U(VI) by beta-MnO2(s)

Pyrolusite (beta-MnO2(s)) was used to assess the influence of a competitive electron acceptor on the kinetics of reduction of aqueous uranyl carbonate by a dissimilatory metal-reducing bacterium (DMRB), Shewanella putrefaciens strain CN32. The enzymatic reduction of U(VI) and beta-MnO2(s) and the abiotic redox reaction between beta-MnO2(s) and biogenic uraninite (UO2(s)) were independently investigated to allow for interpretation of studies of U(VI) bioreduction in the presence of beta-MnO2(s). Uranyl bioreduction to UO2(s) by CN32 with H2 as the electron donor followed Monod kinetics, with a maximum specific reduction rate of 110 M/h/10(8) cells/mL and a half-saturation constant of 370 microM. The bioreduction rate of beta-MnO2(s) by CN32 was described by a pseudo-first-order model with respect to beta-MnO2(s) surface sites, with a rate constant of 7.92 x 10(-2) h(-1)/10(8) cells/mL. Uraninite that precipitated as a result of microbial U(VI) reduction was abiotically reoxidized to U(VI) by beta-MnO2(s), with concomitant reduction to Mn(II). The oxidation of biogenic UO2(s) coupled with beta-MnO2(s) reduction was well-described by an electrochemical model. However, a simple model that coupled the bacterial reduction of U(VI) and beta-MnO2(s) with an abiotic redox reaction between UO2(s) and beta-MnO2(s) failed to describe the mass loss of U(VI) in the presence of beta-MnO2(s). Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) revealed that the particle size and spatial distribution of the biogenic UO2(s) changed dynamically in systems with, as compared to without, beta-MnO2(s)). These observations suggested that the surface properties and localization of UO2(s) in relation to the cell and beta-MnO2(s) surfaces was an important factor controlling the abiotic oxidation of UO2(s) and, thus, the overall rate and extent of U(VI) bioreduction. The coupled model that was modified to account for the "effective" contact surface area between UO2(s) and beta-MnO2(s) significantly improved the simulation of microbial reduction of U(VI) in the presence of beta-MnO2(s).     

Kinetic desorption and sorption of U(VI) during reactive transport in a contaminated Hanford sediment

Column experiments were conducted to investigate U(VI) desorption and sorption kinetics in a sand-textured, U(VI)-contaminated (22.7 micromol kg(-1)) capillary fringe sediment from the U.S. Department of Energy (DOE) Hanford site. Saturated column experiments were performed under mildly alkaline conditions representative of the Hanford site where uranyl-carbonate and calcium-uranyl-carbonate complexes dominate aqueous speciation. A U(VI)-free solution was used to study contaminant U(VI) desorption in columns where different flow rates were applied. Sorbed, contaminant U(VI) was partially labile (11.8%), and extended leaching times and water volumes were required for complete desorption of the labile fraction. Uranium-(VI) sorption was studied after the desorption of labile, contaminant U(VI) using different U(VI) concentrations in the leaching solution. Strong kinetic effects were observed for both U(VI) sorption and desorption, with half-life ranging from 8.5 to 48.5 h for sorption and from 39.3 to 150 h for desorption. Although U(VI) is semi-mobile in mildly alkaline, subsurface environments, we observed substantial U(VI) adsorption, significant retardation during transport, and atypical breakthrough curves with extended tailing. A distributed rate model was applied to describe the effluent data and to allow comparisons between the desorption rate of contaminant U(VI) with the rate of shortterm U(VI) sorption. Desorption was the slower process. We speculate that the kinetic behavior results from transport or chemical phenomena within the phyllosilicate-dominated fine fraction present in the sediment. Our results suggest that U(VI) release and transport in the vadose zone and aquifer system from which the sediment was obtained are kinetically controlled.     

Surface complexation modeling of U(VI) sorption to Hanford sediment with varying geochemical conditions

A series of U(VI) sorption experiments with varying pH, ionic strength, concentrations of dissolved U(VI), and alkalinity was conducted to provide a more realistic database for U(VI) sorption onto near-field vadose zone sediments at the proposed Integrated Disposal Facility (IDF) on the Hanford Site, Washington. The distribution coefficient (Kd) for U(VI) in a leachate that is predicted to result from the weathering of vitrified wastes disposed in the IDF is 0 mL/g due to the high sodium and carbonate concentrations and high pH of the glass leachate. However, when the pH and alkalinity of the IDF sediment native pore water increases during mixing with the glass leachate, U(VI) uptake is observed and the value of the U(VI) Kd increases 4.3 mL/g, because of U(VI) coprecipitation with newly formed calcite. A nonelectrostatic, generalized composite approach for surface complexation modeling was applied and a combination of two U(VI) surface species, monodentate (SOUO2+), and bidentate (SO2UO2(CO3)2-), simulated the measured U(VI) sorption data very well. The generalized composite surface complexation model, when compared to the constant or single-valued Kd model, more accurately predicted U(VI) sorption under the varying geochemical conditions expected at the IDF.     

Role of surface alteration in determining the mobility of U(VI) in the presence of citrate: implications for extraction of U(VI) from soils

In the present study, the adsorption of U(VI) by a natural iron-rich sand in the presence of citrate was studied over a range of citrate concentrations and pH values. Adsorption of U(VI) on the iron-rich sand decreased in the presence of increasing concentrations of citrate. Adsorption of citrate to the sand was weak under most conditions studied. Several explanations for the adsorption behavior of U(VI) and citrate were investigated, including aqueous complexation of U(VI) by citrate, competition of U(VI) and citrate for adsorption sites, and extraction of Fe and Al from the sorbent surface by citrate (surface alteration). Although aqueous complexation of U(VI) by citrate may still play a significant role, both competitive adsorption and aqueous complexation proved to be inadequate explanations of the adsorption behavior. Both physical surface alteration (i.e., loss of surface area) and chemical surface alteration (i.e., change in the chemical composition of the sand surface) were investigated, with chemical surface alteration controlling the bulk of U(VI) adsorption. Considering these results, remediation schemes that involve organic complexing agents should address the possibility of surface alteration affecting radionuclide adsorption and mobility.     

Using chromium stable isotope ratios to quantify Cr(VI) reduction: lack of sorption effects

Chromium stable isotope values can be effectively used to monitor reduction of Cr(VI) in natural waters. We investigate effects of sorption during transport of Cr(VI) which may also shift Cr isotopes values, complicating efforts to quantify reduction. This study shows that Cr stable isotope fractionation caused by sorption is negligible. Equilibrium fractionation of Cr stable isotopes between dissolved Cr(VI) and Cr(VI) adsorbed onto gamma-Al2O3 and goethite is less than 0.04 per thousand (53Cr/52Cr) under environmentally relevant pH conditions. Batch experiments at pH 4.0 and pH 6.0 were conducted in series to sequentially magnify small isotope fractionations. A simple transport model suggests that adsorption may cause amplification of a small isotope fractionation along extreme fringes of a plume, leading to shifts in 53Cr/52Cr values. We therefore suggest that isotope values at extreme fringes of Cr plumes be critically evaluated for sorption effects. A kinetic effect was observed in experiments with goethite at pH 4 where apparently lighter isotopes diffuse into goethite clumps at a faster rate before eventually reaching equilibrium. This observed kinetic effect may be important in a natural system that has not attained equilibrium and is in need of further study. Cr isotope fractionation caused by speciation of Cr(VI) between HCrO4- and CrO4(2-) was also examined, and we conclude that it is not measurable. In the absence of isotope fractionation caused by equilibrium speciation and sorption, most of the variation in delta53Cr values may be attributed to reduction, and reliable estimates of Cr reduction can be made.     

Cryogenic laser induced U(VI) fluorescence studies of a U(VI) substituted natural calcite: implications to U(VI) speciation in contaminated Hanford sediments

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) and imaging spectromicroscopy (TRLFISM) were used to examine the chemical speciation of uranyl in contaminated subsurface sediments from the U.S. Department of Energy (U.S. DOE) Hanford Site, Washington. Spectroscopic measurements for contaminant U(VI) were compared to those from a natural, uranyl-bearing calcite (NUC) that had been found via X-ray absorption spectroscopy (XAS) to include uranyl in the same coordination environment as calcium. Spectral deconvolution of TRLFS measurements on the NUC revealed the unexpected presence of two distinct chemical environments consistent with published spectra of U(VI)-substituted synthetic calcite and aragonite. Apparently, some U(VI) substitution sites in calcite distorted to exhibit a local, more energetically favorable aragonite structure. TRLFS measurements of the Hanford sediments NP4-1 and NP1-6 were similar to the NUC in terms of peak positions and intensity, despite a small CaCO3 content (1.0 to 3.2 mass %). Spectral deconvolution of the sediments revealed the presence of U(VI) in calcite and aragonite structural environments. A third, unidentified U(VI) species was also present in the NP1-6 sediment. TRLFISM measurements at multiple locations in the different sediments displayed only minor variation, indicating a uniform speciation pattern. Collectively, the measurements implied that waste U(VI), long-resident beneath the sampled disposal pond (32 y), had coprecipitated within carbonates. These findings have major implications for the solubility and fate of contaminant U(VI).     

Adsorbed U(VI) surface species on muscovite identified by laser fluorescence spectroscopy and transmission electron microscopy

Time-resolved laser-induced fluorescence spectroscopy (TRLFS) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) were applied to investigate the species of uranyl(VI) adsorbed onto muscovite platelets and muscovite suspensions (grain size: 63-200 microm). TRLFS provided evidence for the presence of two adsorbed uranium(VI) surface species on edge-surfaces of muscovite. The two species showed different positions of the fluorescence emission bands and different fluorescence lifetimes indicating a different coordination environment for the two species. HAADF-STEM revealed that nanoclusters of an amorphous uranium phase were attached to the edge-surfaces of muscovite powder during batch sorption experiments. These U-nanoclusters were not observed on {00/} cleavage planes of the muscovite. The surface species with the shorter fluorescence lifetimes are interpreted as truly adsorbed bidentate surface complexes, in which the U(VI) binds to aluminol groups of edge-surfaces. The surface species with the longer fluorescence lifetimes are interpreted to be an amorphous U(VI) condensate or nanosized clusters of polynuclear uranyl(VI) surface species with a particle diameter of 1 to 2 nm. Depending on the size of these clusters the fluorescence lifetimes vary; i.e., the larger the nanosized clusters, the longer is the fluorescence lifetime.     

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

Abiotic reduction of 0.1 mM U(VI) by Fe(II) in the presence of synthetic iron oxides (biogenic magnetite, goethite, and hematite) and natural Fe(III) oxide-containing solids was investigated in pH 6.8 artificial groundwater containing 10 mM NaHCO3. In most experiments, more than 95% of added U(VI) was sorbed to solids. U(VI) was rapidly and extensively (> or = 80%) reduced in the presence of synthetic Fe(III) oxides and highly Fe(II) oxide-enriched (18-35 wt % Fe) Atlantic coastal plain sediments. In contrast, long-term (20-60 d) U(VI) reduction was less than 30% in suspensions of six other natural solids with relatively low Fe(III) oxide content (1-5 wt % Fe). Fe(II) sorption site density was severalfold lower on these natural solids (0.2-1.1 Fe(II) nm(-2)) compared tothe synthetic Fe(lII) oxides (1.6-3.2 Fe(II) nm(-2)), which may explain the poor U(VI) reduction in the natural solid-containing systems. Addition of the reduced form of the electron shuttling compound anthrahydroquinone-2,6-disulfonate (AH2DS; final concentration 2.5 mM) to the natural solid suspensions enhanced the rate and extent of U(VI) reduction, suggesting that AH2DS reduced U(VI) at surface sites where reaction of U(VI) with sorbed Fe(II) was limited. This study demonstrates that abiotic, Fe(II)-driven U(VI) reduction is likely to be less efficient in natural soils and sediments than would be inferred from studies with synthetic Fe(III) oxides.     

Engineering biogenic magnetite for sustained Cr(VI) remediation in flow-through systems

In this work, we report a route to enhance the reactivity and longevity of biogenic magnetite in Cr(VI) remediation under continuous-flow conditions by combining functionalization of the biomagnetite surface with a precious metal catalyst, nanoscale palladium, and exposure to formate. Column influent conditions were varied to simulate oxic, anoxic, and nitrate cocontaminated environments. The addition of sodium formate as an electron donor for Pd-functionalized magnetite increased capacity and longevity allowing 80% removal of Cr(VI) after 300 h in anoxic conditions, whereas complete breakthrough occurred after 60 h in anoxic nonformate and nonfunctionalized systems. Removal of Cr(VI) was optimized under anoxic conditions, and the presence of oxidizing agents results in a modest loss in reductive capacity. Examination of reacted Pd-functionalized magnetite reveals close association of Fe with Cr, suggesting that Pd-coupled oxidation of formate serves to regenerate the reactive surface. XMCD studies revealed that Cr(III) is partially substituted for Fe in the magnetite structure, which serves to immobilize Cr. No evidence for a mechanistic interference by nitrate cocontamination was observed, suggesting that this novel system could provide robust, effective and sustained reduction of contaminants, even in the presence of common oxidizing cocontaminants, outperforming the reductive capacity of nonfunctionalized biogenic magnetite.