Influence of Magnetite Stoichiometry on UVI ReductionDrew E. Latta,*,†,‡ Christopher A. Gorski,†,§ Maxim I. Boyanov,‡ Edward J. O’Loughlin,‡

Kenneth M. Kemner,‡ and Michelle M. Scherer†

†Department of Civil and Environmental Engineering, University of Iowa, Iowa City, Iowa 52242, United States‡Biosciences Division, Argonne National Laboratory, Argonne, Illinois 60439, United States§Environmental Chemistry, Eawag, Swiss Federal Institute of Aquatic Science and Technology,Ueberlandstrasse 133, 8600 Duebendorf, Switzerland

*S Supporting Information

ABSTRACT: Hexavalent uranium (UVI) can be reduced enzymatically byvarious microbes and abiotically by Fe2+-bearing minerals, including magnetite,of interest because of its formation from Fe3+ (oxy)hydroxides via dissimilatoryiron reduction. Magnetite is also a corrosion product of iron metal in suboxic andanoxic conditions and is likely to form during corrosion of steel waste containersholding uranium-containing spent nuclear fuel. Previous work indicateddiscrepancies in the extent of UVI reduction by magnetite. Here, we demonstratethat the stoichiometry (the bulk Fe2+/Fe3+ ratio, x) of magnetite can, in part,explain the observed discrepancies. In our studies, magnetite stoichiometrysignificantly influenced the extent of UVI reduction by magnetite. Stoichiometricand partially oxidized magnetites with x ≥ 0.38 reduced UVI to UIV in UO2(uraninite) nanoparticles, whereas with more oxidized magnetites (x < 0.38) andmaghemite (x = 0), sorbed UVI was the dominant phase observed. Furthermore,as with our chemically synthesized magnetites (x ≥ 0.38), nanoparticulate UO2 was formed from reduction of UVI in a heat-killedsuspension of biogenic magnetite (x = 0.43). X-ray absorption and Mossbauer spectroscopy results indicate that reduction of UVI

to UIV is coupled to oxidation of Fe2+ in magnetite. The addition of aqueous Fe2+ to suspensions of oxidized magnetite resulted inreduction of UVI to UO2, consistent with our previous finding that Fe2+ taken up from solution increased the magnetitestoichiometry. Our results suggest that magnetite stoichiometry and the ability of aqueous Fe2+ to recharge magnetite areimportant factors in reduction of UVI in the subsurface.

■ INTRODUCTIONMagnetite (Fe3O4) is expected to play an important role inseveral aspects of the technological and natural uranium (U)cycles. Biostimulation of metal-reducing microbial communitieshas been considered and implemented in field-scale trials as astrategy to immobilize subsurface U contamination resultingfrom U processing.1−3 Immobilization of U by dissimilatorymicrobial metal metabolism is thought to occur both via directenzymatic reduction of the more soluble UVI species to the lesssoluble UIVO2

4,5 and via indirect reduction of UVI to UV andUIV species by sorbed and structural Fe2+.6−12 Reduction of Uby structural Fe2+in magnetite is of particular interest because itis a commonly observed end product of dissimilatory iron (Fe)reduction of several iron oxides, including lepidocrocite andferrihydrite,13−15 as well as a corrosion product from Fe canistersused to contain U-bearing wastes in both short-term storage andlong-term geologic repositories.16,17

Previous reports investigating the reduction of UVI bymagnetite showed results varying from complete reduction ofUVI to UIV9,18 to partial reduction17,19−23 to no reduction.24,25

The majority of these studies found that, in the presence ofmagnetite, U added to solution is reduced to a mix of valencestates (UVI, UV, and UIV) in the solid phase. Most of these

studies have reported the formation of mixed UVI/UIV phasesfrom fitting of U 4f X-ray photoelectron spectroscopy (XPS)data.17,19−21 Among the studies, no trend in U speciation isdiscernible across solution conditions, which ranged from pHvalues of 4 to 9,17,19−22 nor has a significant effect from theaddition of bicarbonate been observed.21 The magnetite used inthese studies has been of various origins, including syntheticprocedures17,19,21,22,26 and natural crystals.20

The studies that observed complete reduction of UVI in thesolid phase were conducted with microbially producedmagnetite, with U speciation determined by X-ray absorptionspectroscopy (XAS)18 and/or bicarbonate extraction of UVI.9

Formation of solid-phase UO2 was also observed in anotherstudy that used microbially produced magnetite, but themajority of the U remained as aqueous UVI.23 In contrast, noreduction of UVI was observed with magnetites synthesized byoxidation of FeSO4 under normal atmospheric conditions andin the presence of KNO3 in solution24 or with commercially

Received: July 19, 2011Revised: November 30, 2011Accepted: December 8, 2011Published: December 8, 2011

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purchased magnetite.25 In the latter study, reduction of UVI wasobserved in the presence of H2(g), although whether the H2

reduced UVI directly or reduced the magnetite was unclear.25

We hypothesize that variations in magnetite stoichiometrymay explain some of the discrepancies reported in the rate andextent of UVI reduction by magnetite, on the basis of ourprevious work demonstrating a dramatic effect of magnetitestoichiometry on nitrobenzene reduction rates.27,28 Stoichio-metric magnetite contains Fe2+ and Fe3+ in a ratio of 1:2(Fe2+:Fe3+). Removal of all Fe2+ from the structure of magnetiteby oxidation or dissolution results in the formation ofmaghemite (γ-Fe2O3), which contains only Fe3+.29 Partiallyoxidized magnetites can exist between the end members ofstoichiometric magnetite and maghemite; they can bedistinguished by their Fe2+ content (x):

=+

+

+ +xFe

Fe Fe

oct 2

oct 3 tet 3 (1)

The stoichiometry (x) of magnetites has considerableinfluence on their properties, such as conductivity and redoxpotential. A slight change in stoichiometry has a dramatic effecton the conductivity of magnetite samples,30,31 and severalstudies indicate significant variation in redox potential withvarying stoichiometry.28,29,32 We have also recently shown thatthe stoichiometry of magnetite has a dramatic influence onreduction rates of substituted nitrobenzenes,27,28 and severalrecent studies have suggested that magnetite stoichiometry mayinfluence the reactivity of magnetite with UVI18,22 as well asother metals.33 To test our hypothesis, we measured the extentof UVI reduction by magnetites of varying stoichiometries atnear-neutral pH in the presence and absence of (bi)carbonateanion.

■ MATERIALS AND METHODSMagnetite Synthesis and Characterization. Magnetite

was synthesized according to published methods.27,28,34 Briefly,the solids were prepared by precipitating magnetite with NaOHfrom a solution containing a 1:2 ratio of Fe2+/Fe3+. Care wastaken to limit washing of the solids to prevent leaching of Fe2+

from the magnetite. Partially oxidized magnetite was made byadding hydrogen peroxide (approximately 30% H2O2) tooxidize Fe2+ to Fe3+. Maghemite was produced by oxidation ofmagnetite in air at 200 °C. Magnetite stoichiometry wascharacterized by acidic dissolution (xd) in 5 M HCl, followedby measurement of aqueous Fe2+ colorimetrically by 1,10-phenanthroline complexation, with Fe3+ masking by fluo-ride.34,35 Total Fe was measured after reduction of Fe3+ byhydroxylamine hydrochloride. The magnetite stoichiometrywas also measured by Mossbauer spectroscopy (xMS).

34 Thespecific surface area, obtained via N2 adsorption Brunauer−Emmett−Teller analysis, was found to be 63 ± 7 m2·g−1,consistent with the average particle diameter of approximately20 nm, as measured by transmission electron microscopy.27

Uranium Uptake and Reduction Experiments. Ura-nium uptake by magnetites with various stoichiometries wasmeasured in 5 g·L−1 suspensions in either 50 mM 3-(N-morpholino)propanesulfonic acid buffer (MOPS, pKa = 7.2) or2 mM NaHCO3 buffer. Both were adjusted to an initial pH of7.2. Experiments were done inside an anoxic glovebox with anatmosphere of 93% N2 and 7% H2. Uranium was added to anominal concentration of 500 μM UVI from uranyl acetate[UO2(CH3COO)2·2H2O] dissolved in 0.1 M HCl. Initial U

concentrations were measured prior to initiation of the reac-tion by addition of magnetite, and final concentrations weremeasured after 24 h of reaction time, prior to XAS. Samplesfor long-term kinetics were stored in the dark in an anoxicglovebox until analysis. Samples were filtered through a 0.22-μmfilter, and dissolved U was measured colorimetrically by use of2-(2-thiazolylazo)-p-cresol.36,37 In all systems, except for themaghemite (x = 0) system, the amount of Fe2+ added by themagnetite was much greater than the theoretical amountneeded to reduce all of the added UVI, with the highest amountbeing 22 mM Fe2+ (x = 0.50) and the lowest being 9.2 mM(x = 0.17).

Fe2+ Uptake Experiments. For Fe2+ uptake experimentson partially oxidized magnetite (x = 0.28), an aliquot of FeCl2stock was added to 60 mL of MOPS buffer. In one experiment,11 mM Fe2+ was added, on the basis of the total amount of Fe2+

needed to bring the partially oxidized magnetite to x = 0.50. Inaddition, a second experiment with 5.7 mM aqueous Fe2+

determined whether partial restoration of magnetite stoichi-ometry induced UVI reduction. The effect of aqueous Fe2+ onUVI reduction was tested by replacing the aqueous phase of onereactor containing 11 mM added Fe2+ with fresh MOPS bufferprior to UVI addition. The amount of Fe2+ in solution wasmeasured once prior to addition of 5 g·L−1 partially oxidizedmagnetite and again after 20 h, prior to addition of UVI.Aqueous Fe2+ was measured after filtration through a 0.22-μmfilter by the 1,10-phenanthroline method described above. Theamount of Fe2+ removed from solution was used to calculatethe stoichiometry of the magnetite after Fe2+ sorption and wasbased on mass balance calculations.27

X-ray Absorption Spectroscopy Measurements. TheU LIII-edge X-ray absorption fine structure (XAFS) measure-ments were carried out at the MRCAT/EnviroCAT insertiondevice beamline, sector 10, at the Advanced Photon Source, byusing a previously described setup.38 Briefly, the beamlineundulator was tapered and fixed, and the incident energy wasscanned by using the Si(111) reflection of the double-crystalmonochromator in quick-scanning mode (approximately2 min/scan for the extended region and 30 s/scan for the near-edge region). Centrifuged wet paste samples were mounted indrilled Plexiglas slides and sealed inside the anoxic chamberwith Kapton film windows. Linear-combination spectralanalyses of X-ray absorption near-edge structure (XANES)data with several standards were performed by use of theprogram SIXpack.39 The UIV standards included a crystallineUO2,

40 biogenic UIV nanoparticles,4 and UIV nanoparticlesproduced by sulfate green rust.12 UVI sorbed to goethite wasused as a standard for UVI sorbed to an iron oxide. Furtherdetails on sample preparation and standards used in the analysisare given in the Supporting Information.

Magnetite Oxidation by UVI. Experiments to monitorFe2+ oxidation in magnetite during UVI reduction wereconducted in 50 mM (pH 7.2) MOPS buffer with a solidsloading of 1.5 g·L−1 of nearly stoichiometric magnetite, with astoichiometry of 0.49 as measured by dissolution (xd) and 0.45as measured by Mossbauer spectroscopy (xMS). A spike of500 μM UVI was added from the uranyl acetate stock, and thereactors were readjusted to pH 7.2. After 20 h, the solution wasfiltered to collect the solids on a 0.45-μm filter. The solidswere then mounted between two pieces of Kapton tape forMossbauer spectroscopy measurements.

Biogenic Magnetite. Biogenic magnetite was pre-pared through the bioreduction of lepidocrocite by

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Shewanella putrefaciens CN32 as described previously.13 Thecultures were incubated for 3 days after inoculation. Thesuspension was then pasteurized for 1 h at 70 °C. To avoiddissolution of Fe2+ from the magnetite, the material was notwashed to remove cell debris. The material was confirmed to bemagnetite by X-ray diffraction and Mossbauer spectroscopy. UVI

was reacted with these solids in a manner similar to that for thesynthetic magnetites.Mossbauer Spectroscopy. Transmission Mossbauer

spectroscopy was used to measure the stoichiometry ofbiogenic and synthetic magnetite before and after reactionwith U. Mossbauer spectroscopy was done with a variable-temperature, He-cooled system with a 1024-channel detector.A 57Co source (∼ 50 mCi) embedded in Rh was used at roomtemperature. All center shifts reported are calibrated relative toan α-Fe foil at room temperature. Samples were kept anoxic bymounting them between pieces of adhesive Kapton tape and byminimizing the duration of air exposure prior to mounting inthe spectrometer cryostat. Collected Mossbauer spectra werefitted by using the Recoil software package (University ofOttawa, Ottawa, Canada) with extended-Voigt-based fitting.34

The relative peak areas of the sextets were constrained to theideal 3:2:1:1:2:3 ratios. Center shift (CS), quadrupole shift(QS), and hyperfine field (H) parameters were allowed to floatduring the fitting procedure. Sextets were fitted with twohyperfine field components with individual Gaussian distribu-tions of hyperfine field parameters and relative areas.

■ RESULTS AND DISCUSSION

Uranium Reduction by Stoichiometric Magnetite. Wereacted stoichiometric magnetite (x = 0.50) with UVI anddetermined the oxidation state of U in the solids by XANESand the magnetite stoichiometry (x) by Mossbauer spectros-copy (xMS). Stoichiometric magnetite (x = 0.50) completelyreduced UVI to UIV in 2 mM NaHCO3 buffer at pH 7.2 (Figure 1),

as well as in 50 mM MOPS buffer at pH 7.2 (Figure S2 inSupporting Information). Evidence for complete reduction isshown in Figure 1, where U reacted with stoichiometricmagnetite has an edge position similar to that for the UIV

standard (in this case, biogenic nanoparticulate uraninite). Inaddition, the absence of the resonance feature beyond theabsorption edge (denoted by the vertical arrow) indicates thelack of axial uranium−oxygen bonds (Oax) that would bepresent in a UVI or UV cation in the uranyl geometry.Reduction of U is coupled with oxidation of Fe2+ in mag-

netite, as observed by Mossbauer spectroscopy. A Mossbauerspectrum of a nearly stoichiometric magnetite (xd = 0.49from dissolution and xMS = 0.45 from Mossbauer spectroscopy)before reaction with U is shown as the top spectrum in Figure 2.

The spectrum consists of two magnetically ordered sextets, withthe outer sextet corresponding to tetrahedrally and octahedrallycoordinated Fe3+ (oct,tetFe3+) in the magnetite lattice. The innersextet corresponds to octahedrally coordinated Fe2+ and Fe3+

pairs that appear as an octFe2.5+ sextet because of fast electronhopping between Fe2+ and Fe3+.27,34

After reaction with UVI, Mossbauer spectroscopy indicatesthat the Fe remains as magnetite but that the octFe2.5+ sextetarea decreases and the oct,tetFe3+ sextet area increases, by about7% in both cases, confirming that Fe2+ has been oxidized toFe3+ in magnetite (Figure 2, bottom spectrum; Table S1,Supporting Information). The measured stoichiometry (xMS) ofthe magnetite after reaction is 0.38, which is within theexpected range of x = 0.34−0.38 if all of the added UVI wasreduced to UIV. (Detailed calculations are given in SupportingInformation.) The change in magnetite stoichiometry indicatesthat electron transfer from structural Fe2+ in magnetite to UVI

results in stoichiometric oxidation of Fe2+; this is consistentwith XANES data shown in Figure 1, as well as with previouswork reporting reduction of UVI to UIV coupled to Fe2+ oxida-tion on the surface of freshly cleaved single-crystal magnetite

Figure 1. U LIII edge XANES for UVI reacted with magnetites ofdifferent stoichiometries (xd = Fe2+/Fe3+), including a biogenicmagnetite with x = 0.43 produced by the bioreduction of lepidocrocite.Horizontal arrows indicate the absorption edge positions of UIV andUVI, and the vertical arrow indicates the postedge resonancecharacteristic of uranyl species. Solution conditions: 5 g·L−1 magnetiteadded to 500 μM UVI in a 2 mM bicarbonate buffer, pH 7.2.

Figure 2. Mossbauer spectrum of 1.5 g·L−1 magnetite (xd = 0.49,xMS = 0.45), (A) before and (B) after reaction with 500 μM UVI in50 mM MOPS buffer, pH 7.2. The stoichiometry of the magnetite inspectrum B (after reaction with UVI) was determined to be xMS = 0.38.

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with a near-stoichiometric Fe2+/Fetotal ratio (but at a muchlower pH value of 4.1).20

Effect of Magnetite Stoichiometry on UVI Reduction.On the basis of our previous observations that reduction ratesof substituted nitrobenzenes are highly dependent on thestoichiometry (x) of magnetite,27,28 we investigated the effectof magnetite stoichiometry on the extent of U reduction. Wereacted magnetite of various stoichiometries with UVI in 2 mMNaHCO3 buffer (pH 7.2) and determined the oxidation state ofU in the solids by XANES. As for stoichiometric magnetite (x =0.50), partially oxidized magnetites (x = 0.48 and x = 0.42)resulted in near-complete reduction of UVI (Figure S1,Supporting Information). In contrast, for more oxidizedmagnetites with x ≤ 0.33, little to no reduction was observedover approximately 1 week (Figure 1; Figure S1, SupportingInformation). Similar results were seen in 50 mM MOPS buffer(pH 7.2) (Figure S2, Supporting Information). The position ofthe absorption edge and the presence of the resonance featureabove the absorption edge are consistent with the UVI standard(in this case, UVI sorbed on goethite), indicating that themajority of the U in these oxidized magnetite samples remainsin the +6 valence state as the uranyl cation (Figure 1; Figure S1,Supporting Information).We quantified the proportion of UIV and UVI in the solid

phase (as UIV/Utotal) remaining after reaction with magnetite byfitting the XANES spectra with linear combinations of endmember spectra. Based on the speciation of U in the solidphase (see below), nanoparticulate UIVO2 and UVI sorbed togoethite standards were used as UIV and UVI end members,respectively. The percentage UIV/Utotal as a function of initialmagnetite stoichiometry (Table S2 in Supporting Information)is plotted in Figure 3. An abrupt shift in the amount of reduced

U in the solid phase is observed between x = 0.42 and x = 0.33.The amount of UIV/Utotal decreases from 84% in the x = 0.42magnetite to only 13% in the x = 0.33 magnetite sample.

We also note a decrease in the final dissolved U concentrationwhen reduction to UIV occurred (Table S2, SupportingInformation), consistent with the low solubility ofUO2.

9,41−43 The abrupt decrease in magnetite reactivity withU at x = 0.33 occurs in both 2 mM bicarbonate buffer and50 mM MOPS buffer at pH 7.2, despite the stoichiometricexcess of solid-phase Fe2+ and the significant differences in UVI

speciation in the different buffer solutions. In the absence ofmagnetite, U is soluble in the bicarbonate buffer due toaqueous uranyl carbonate complex formation, whereas in the50 mM MOPS buffer, UVI precipitation as a schoepite-like colloid(UO3·2H2O) is expected on the basis of the solubility productfor schoepite [pKs0(schoepite) = 5.994].44 A precipitate wasvisually evident after addition of uranyl acetate to 50 mMMOPS buffer, prior to addition of magnetite. The presence ofmagnetite, however, provides UVI adsorption sites and alters theUVI speciation in both the MOPS and carbonate buffer systemsby formation of UVI−magnetite surface complexes (see TableS6, Supporting Information, and the speciation of solid-phase Ubelow).In an earlier study, we measured significant differences in

rates of nitrobenzene reduction by magnetites with differentstoichiometries. Nearly stoichiometric magnetite reducednitrobenzene much faster than more oxidized magnetites(e.g., for x = 0.50, t1/2 = 1.2 min, and for x = 0.31, t1/2 =90 days).28 To determine whether the shift from sorbed UVI toreduced UIV products with increasing x was kinetically limited,we conducted a longer-term experiment in which x = 0.28magnetite reacted with UVI for 22 days and 129 days. Similar toour short-term experiment (about 3−5 days), little U reductionwas observed, indicating either that the rate was too slow to bemeasurable over a 4-month period or possibly a thermody-namic limitation.In the absence of a measurable kinetic effect, we explored

whether a thermodynamic argument could be made to explainthe abrupt change in magnetite reactivity between x = 0.33 andx = 0.42. We note that the variation in nitrobenzene reductionrates and 15N kinetic isotope effects as a function of magnetitestoichiometry have been successfully explained by a thermody-namic model for magnetite reactivity but not by a oxidizedshell-reduced core model.28 Neither measured nor theoreticalredox potentials are readily available for magnetite particles ofdefined stoichiometries. We have, however, previously collabo-rated with others to measure the electrochemical open-circuit potentials (EOCP) of these same magnetite particles,and we use them here as an estimate of magnetite reduc-tion potentials.28 A redox ladder comparing the measuredmagnetite EOCP values with calculated Eh values for sev-eral U redox couples is shown in Figure 4. The U specieswere selected from the dominant UVI species present in2 mM bicarbonate solution (pH 7.2) and using UO2(am)(amorphous UO2) as the reduced species (thermodynamicparameters tabulated in Table S3, Supporting Information).The most abundant U species predicted to exist in thebicarbonate buffer (using the Visual MINTEQ database)45 isthe (UO2)2CO3(OH)3

− species (∼78%) (Table S4, SupportingInformation). We note, however, that UVI forms a surfacecomplex with maghemite (see below), and metal cation−metaloxide surface complexes are often noted to have different reduc-tion potentials than their aqueous counterparts.46−50

Most of the magnetites used in this study, with the exceptionof x = 0.17, have an EOCP value more negative than the −0.05 Vfor the (UO2)2CO3(OH)3

−/UO2(am) couple, suggesting that

Figure 3. Influence of magnetite stoichiometry (x = Fe2+/Fe3+) on theextent of UVI reduction to UIV under two buffer conditions(bicarbonate and MOPS). The percent UIV/Utotal was determined byusing linear-combination fitting of U XANES spectra. Long-term x =0.28 experiments were run for 22 and 129 days in 2 mM bicarbonatebuffer.

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reduction of (UO2)2CO3(OH)3− species is thermodynamically

favorable. Additionally, the use of a crystalline UO2 reducedspecies in the thermodynamic calculations shifts the UVI/UIV

couples by +0.19 V, making reduction thermodynamicallyfavorable over the whole range of magnetite stoichiometriesused in this study. We note that similar redox potentials arecalculated for U species in the absence of bicarbonate (Table S5,Supporting Information). The −0.05 V redox potential (versusthe standard hydrogen electrode) for the (UO2)2CO3(OH)3

−/UO2(am) couple corresponds to an x value of 0.24 as thetipping point for thermodynamic favorability. This value islower than our measured data, which showed a transition fromsorption to reduction somewhere between x = 0.33 andx = 0.42. Recognizing the limitations of EOCP as a proxy forredox potentials,28 as well as the uncertainty in the relevant Ucouple and the reversibility of U reduction,51 we speculated thatthe observed transition from oxidized to reduced U providessome indication that a thermodynamic limitation is inhibitingreduction of U by partially oxidized magnetites. Our pre-vious work conclusively demonstrated that the dependence ofmagnetite oxidation kinetics coupled to nitrobenzene reduc-tion was due to a change in the thermodynamic driving forceand not site availability, surface passivation, or another masstransfer process.28 The difference between the calculated andobserved UIV−UVI transition points in this study was mostlikely due to the error associated with using EOCP values asa proxy for reduction potentials and the uncertainty inusing aqueous U redox couples as estimates for sorbed Uspecies.

Uranium Speciation in the Solid Phase. The abruptchange in magnetite reactivity with UVI, from completereduction to UIV for magnetite with x ≥ 0.42 to no reductionat lower stoichiometries (x < 0.42), is also reflected in the ULIII-edge extended X-ray absorption fine structure (EXAFS)spectra. The U EXAFS spectra for magnetites with varyingstoichiometries are compared with a bulk uraninite standardand with UVI adsorbed to maghemite in Figure 5. The Fourier

transforms of the U EXAFS spectra for the nearly stoichio-metric magnetites (x ≥ 0.42) reacted with UVI contain a peak at3.6 Å, indicating the presence of bidentate UO2U interac-tions, consistent with formation of the UIV product uraninite[UO2(s)]. The smaller amplitude of this U−U peak in theFourier transform data, relative to that of bulk uraninite, isconsistent with the presence of nanoparticulate uraninite.23

Figure S3 (Supporting Information) demonstrates that thereduced UIV species are very similar to the biogenicnanoparticulate uraninite formed in the absence of Feminerals.4 No indication of UIV−Fe coordination wasfound in the x = 0.42−0.50 spectra (Figure S3, SupportingInformation),38 suggesting the formation of segregateduraninite particles rather than a thin uraninite coating on theFe minerals. For the more oxidized magnetites (x ≤ 0.33)(Figure 5), the EXAFS spectra contain features similar to thatof UVI reacted with maghemite, which are consistent with anFe−O−U surface complex similar to those observed for UVI

adsorption onto hematite and goethite.52−54 In the EXAFSspectra, surface complexation of UVI as the uranyl cation isevident by the U−Oax bond at R + Δ = 1.4 Å and an equatorialuranium−oxygen bond (U−Oeq) at R + Δ = 2.0 Å, indicatingthat the solid-phase U in the more oxidized magnetite samplesremains predominantly as sorbed UVI. (The smaller Oax

amplitude in samples x = 0.28 and x = 0.33 are discussedbelow.) As with the XANES data, the shift from predominantly

Figure 4. Redox ladder comparing the thermodynamically derivedreduction potential of several UVI/UIV redox couples to the measuredopen-circuit potential (EOCP) of magnetites with varying stoichiome-tries (x).28 The reduction potentials for the UIV species were calculatedon the basis of 500 μM UVI for each component in 2 mM bicarbonatebuffer at pH 7.2. UO2(am) represents a nanoparticulate uraninite endmember from UO2 thermodynamic data. Use of a well-crystallizeduraninite [UO2(cr)] end member results in a shift of all Eh valuesby +0.19 V. Thermodynamic data are from Grenthe et al.,62 the mostpositive magnetite EOCP value is from White et al.29 at pH ∼7, theFe3O4/Fe

2+ redox couple value is from Straub et al.63 for pH 7.2, andthe range of Fe3O4/γ-Fe2O3 couples at pH 7.2 is calculated fromreported E0 values (arrows pointing to gray bar).32,64,65.

Figure 5. U LIII edge EXAFS spectra for UVI reacted with syntheticmagnetites of different stoichiometries (x) and with biogenicmagnetite of x = 0.43 stoichiometry. Fourier transforms wereperformed on k2-weighed EXAFS data over the range Δk = 2.2−11.5 Å−1 with a 1.0-Å−1 Hannig window. Solution conditions: 5 g·L−1

magnetite added to 500 μM UVI in a 2 mM bicarbonate buffer, pH 7.2.

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UVI to UIV species in the EXAFS data is distinct betweenx = 0.33 and x = 0.42.The formation of UIV species for x > 0.42 magnetites

observed here differs from a previous report of the formation ofUVI/UV products with XPS and XAS after reaction of U withmagnetite.22 In the previous study, XANES analysis of the UV

phase formed during reaction of U with magnetite showed thesame edge position as in a UVI reference spectrum.18 Because ofthe similarities between the UV phase and the UVI reference inthe XANES spectra, EXAFS spectra were used in conjunctionwith the XPS spectra to infer the presence of a non-uranylcomponent that was interpreted as UVI and/or UV uranatespecies.The observation of UV in the previous study versus UIV in

our study may be due to important differences in experimentalconditions, including solution pH, precipitation of separate Uphases, and methods for determining magnetite stoichiometry.Specifically, our experiments were buffered at near-neutral pH,whereas the earlier study was conducted in unbuffered solutionwith an initial pH between 3.2 and 4.7 that rose with time,likely because of the dissolution of Fe2+ from magnetite.22

Another possible explanation is that the increase in pHthroughout the experiment may have favored the precipitationof a mixed UV/UVI phase. Ilton et al.22 found that U-richprecipitates were formed when magnetite reacted with UVI atthe lower pH values. Stabilization of UV in the structure of amineral has been reported previously for a mixed UVI/UV

oxyhydroxide.55

Another explanation for the observed differences may be themethods of determining the stoichiometries of the magnetitesin each study. In the study by Ilton et al.,22 the magnetitestoichiometry was reported to be between x = 0.37 and x = 0.52(starting material Fe2+/Fetotal = 0.30, corresponding to x =0.43), on the basis of XPS fitting of the Fe 2p peak for Fe2+ andtotal Fe. The variability in x reported by Ilton et al. cannot bedue to oxidation of the magnetite by UVI, as the expecteddecrease in x due to formation of a mixed UV/UVI phase undertheir experimental conditions is approximately 0.01. Theobserved variability in stoichiometry may be due to the useof XPS to determine Fe2+ contents or due to differences in themeasurement of bulk Fe2+ relative to the surface-onlymeasurement of XPS. Other researchers have reportedsignificant variability in magnetite stoichiometries measuredby XPS, with large ranges reported (e.g., x = 0.33−0.67;17x = 0.28−0.5420).Because the factors contributing to the presence of mixed

UV/UVI phases in the previous study are not fully known, weinvestigated the potential for stoichiometry to control theformation of non-uranyl UVI or UV phases. The EXAFS datafrom the partially oxidized (x = 0.28 and x = 0.33) magnetitesystems show a decrease in Oax peak amplitude at R + Δ = 1.4 Årelative to the x = 0 (maghemite) sample (Figure 5; Figure S4,Supporting Information), similar to the Oax amplitudedecrease observed with magnetite by Ilton et al.22 Loss ofOax amplitude can result from transformation of part of theuranyl UVI species to reduced UIV and/or to non-uranyl UVI/UV

species.To quantify the possible non-uranyl spectral component in

the x = 0.28 and x = 0.33 samples, we constructed spectracontaining different proportions of the UVI and UIV endmember spectra corresponding to U speciation in the x = 0.00and x = 0.50 magnetite systems, respectively, and fitted themwith a two-shell (Oax and Oeq) EXAFS model. Figure S4

(Supporting Information) shows the fitted Oax coordinationnumbers in these model spectra and compares them to the Oaxcoordination numbers obtained for the x = 0.28 and x = 0.33magnetites (further details on fitting are provided in theSupporting Information in Figure S7 and Table S6). The fitsindicate greater loss of Oax amplitude than would be expectedfrom the average valence state determined by XANES. Thisbehavior suggests the presence of non-uranyl UVI or UV species.The presence of an additional O shell at intermediate distancesto the Oax and Oeq shells (1.95−2.20 Å, as observed by Iltonet al.22 and Renshaw et al.56) could not be definitively estab-lished in order to infer the presence of UV. Figure 5 also showsthe lack of the defined Fourier transform structure observedbeyond R + Δ = 3.0 Å that was present in the spectra of Iltonet al.,22 suggesting that different U species were formed in thex = 0.28 and x = 0.33 magnetites of our study. As discussed byIlton et al., it is possible that a UVI/UV mineral was formed inthat study, whereas adsorbed or disordered non-uranyl UVI/UV

species were formed in our study as a small fraction (10−15%)of the total U balance. As noted above, a potential reason forthe different species in the two studies could be the differentformation conditions; however, given the significant differencesbetween the two studies, it is difficult to assign a definite causefor the formation of mixed UVI/UV phases in the Ilton et al.study relative to the 10−15% non-uranyl UV or UVI found inthe presence of the partially oxidized magnetites in our study.

Implications for UVI Reduction by Magnetite. Ourfindings support the hypothesis that magnetite stoichiometryinfluences whether UVI is reduced to UIV at near-neutral pHvalues and may, in part, explain the reported variations inmagnetite reactivity.18,22 The presence of atmospheric oxygenor dissolved nitrate, or the leaching of Fe2+ by washing themagnetite solids during magnetite and sample preparation,could have resulted in partial oxidation of the magnetite used inprevious studies.13,22,26,28−30 Variability in the reactivity of UVI

with magnetite observed in the laboratory can be extended toimportant processes that could modify reactivity in theenvironment. In the subsurface, magnetite reactivity might bemodified by oxidation due to O2, NO3

−, and other oxidantsdissolved in groundwater. Continual exposure to the flushingaction of groundwater lacking dissolved Fe2+ might removeFe2+ from the structure of magnetite, also decreasing itscapacity to reduce UVI. Furthermore, the results of this studyalong with our thermodynamic analysis of UVI reduction bymagnetite provide additional evidence for the validity of theredox model describing magnetite reactivity.28,32,57

In addition to reactions that remove Fe2+ from magnetite,structural Fe2+ in magnetite is likely to be recharged byreductants. We have previously shown that aqueous Fe2+ canrecharge oxidized magnetite and re-establish rapid rates ofnitrobenzene reduction.27 We hypothesized that the reactivityof magnetite with UVI could also be restored through reactionof oxidized magnetites with aqueous Fe2+. To test thishypothesis, we allowed x = 0.28 oxidized magnetite to reactwith 5.7 mM and 11 mM aqueous Fe2+ and observed uptake ofFe2+ (5.1 mM and 8.0 mM, respectively) from solution; thisamount of Fe2+ uptake would increase the magnetitestoichiometry from x = 0.28 to x = 0.38 and x = 0.44,respectively, consistent with our previous work showing thataqueous Fe2+ reduced octahedral Fe3+ in magnetite to Fe2+.27

Addition of UVI to these recharged magnetites results in nearlycomplete reduction to UIV, with over 95% of the total U oc-curring as UIV for the 11 mM Fe2+ addition (Figure 3; Table S2

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and Figure S5, Supporting Information), and with 84%reduction for the 5.7 mM addition. Removal of the majorityof aqueous-phase Fe2+ by resuspension in fresh buffer solutionhad no effect on the ability of the magnetite to reduce UVI.The restored reactivity of the Fe2+-reacted magnetites

indicates that the effects of microbial respiration on Fe mightalso be important for recharging magnetite particles. Wemeasured the stoichiometry and reactivity of magnetiteproduced from lepidocrocite reduction by S. putrefaciensCN32. The suspensions were heat-killed by pasteurization tolimit biological UVI reduction. Mossbauer spectroscopyindicated x = 0.43 for the biogenic magnetite (Figure S6 andTable S1, Supporting Information). Our result is consistentwith previously measured stoichiometry of biogenic magnetitefrom reduction of ferrihydrite (along with trace goethite andsiderite),18 but it is slightly less than the value measured byusing X-ray magnetic circular dichroism,58 and it contains noresidual lepidocrocite, despite its deficiency of Fe2+.59

Linear-combination analysis of the XANES spectrum revealsthat the majority (63%) of added UVI was reduced to UIV uponreaction with biogenic magnetite in 2 mM bicarbonate buffer(Figure 1; Table S2, Supporting Information); however, datafrom the linear-combination XANES analysis indicate that only33% of total U was reduced to UIV in the MOPS suspension(Figure 3; Table S2, Supporting Information). EXAFS analysisof the biogenic magnetite allowed to react with UVI inbicarbonate buffer indicated that a portion of the reduced Uin the sample had U−U coordination at approximately 3.8 Å(R + Δ), indicative of nanoparticulate uraninite (Figure 5). Wespeculate that the difference in the amount of reduction bybiogenic magnetite relative to synthetic magnetite may be dueto the presence of cell debris in the suspension, which could actas a secondary sink for UVI in these samples, changing thethermodynamics or kinetics of the reactions between magnetiteand UVI. A previous report on reduction of UVI to UIV bybiogenic magnetite over approximately a week18 suggests that asimilar kinetic effect might be present in our study.The recharge of structural Fe2+ in magnetite by aqueous Fe2+

is likely to be an important process in mediating its reactivitywith contaminants, as shown for UVI here and nitroaromaticcompounds previously.27,28 Solid-state cycling of structural Fe2+

and Fe3+ in magnetite might also be an important process forthe electron-transfer mechanisms that have been hypothesizedto occur between sediment-dwelling bacteria and inorganicelectron acceptors at a distance.60,61 Termed the “biogeobatterymodel,” this phenomenon is hypothesized to occur over largedistances (meter scale).61 Along with bacterial reduction of ironoxides, it might ultimately drive the fate of Fe and U duringredox cycling in the subsurface. Recharge of magnetitereactivity by dissolved Fe2+, and potentially by bacteria, suggeststhat magnetite might be an important and rechargeablereductant for UVI in the environment.

■ ASSOCIATED CONTENT

*S Supporting InformationAdditional text, six tables, and seven figures describing XASdata collection, standards, and fitting; Mossbauer fittingparameters; results of calculation of magnetite stoichiometry;thermodynamic data, U LIII XANES data used to create Figure 3;EXAFS comparison of UIV products; Mossbauer spectrum ofbiogenic magnetite; and EXAFS fitting. This material is availablefree of charge via the Internet at http://pubs.acs.org/.

■ AUTHOR INFORMATION

Corresponding Author*Phone: (630)252-3985; E-mail: dlatta@anl.gov.

■ ACKNOWLEDGMENTSWe thank T. Shibata for help during EXAFS data collection andKaren Haugen for editorial comments. We also thank threeanonymous reviewers for their helpful comments. Researchwas done under the Subsurface Science Focus Area programat Argonne National Laboratory and is supported by theSubsurface Biogeochemical Research Program, Office ofBiological and Environmental Research, Office of Science,U.S. Department of Energy (DOE), under Contract DE-AC02-06CH11357. Use of the Advanced Photon Source, an Office ofScience User Facility operated for the U.S. Department ofEnergy (DOE) Office of Science by Argonne NationalLaboratory, was supported by the U.S. DOE under ContractDE-AC02-06CH11357. MRCAT/EnviroCAT operations aresupported by DOE and the member institutions.

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S1

Supporting Information

Influence of Magnetite Stoichiometry on UVI

Reduction

Drew E. Latta,1,2

Christopher A. Gorski,1,3

Maxim I. Boyanov,2 Edward J. O’Loughlin,

2

Kenneth M. Kemner,2 Michelle M. Scherer

1

1Department of Civil and Environmental Engineering, University of Iowa,4105 Seamans Center,

Iowa City, Iowa 52242

2Biosciences Division, Argonne National Laboratory, Argonne, Illinois 60439

3Environmental Chemistry, Eawag, Swiss Federal Institute of Aquatic Science and Technology,

Ueberlandstrasse 133, 8600 Duebendorf, Switzerland

Pages 19

Tables 6

Figures 7

S2

XAS Data Collection and Standards

X-ray absorption spectroscopic (XAS) data were collected for samples that had been

centrifuged and mounted between Kapton film windows in a drilled Plexiglas slide. Samples

were prepared in an anoxic chamber (Coy®

) with an atmosphere of 5% H2 and 95% N2, with a Pt

O2 scrubbing catalyst. Previous studies of the sample holders indicated anoxic integrity for up to

8 h.1 The sealed slides were transferred from an O2-free transport container to the N2-purged

detector housing for room temperature measurements of the hydrated samples.

Possible sample heterogeneity, spectral background issues, and beam-induced changes in the

samples were monitored by collecting multiple spectra from 4-6 fresh locations on the sample.

Spectra that were consistent between locations and did not show evolution with beam exposure

were used in the final averaged spectrum.

Several UVI

and UIV

standards were used for XANES and EXAFS analysis. An acidic (pH 3)

solution of uranyl chloride was used as the standard for hydrated UVI

, and a basic (pH 11)

solution of U:carbonate = 1:50 was used as the standard for carbonate-complexed UVI

. The UIV

standards included a crystalline UO2 purchased from Alfa Aesar and diluted 1:100 in SiO2,2

biogenic UIV

nanoparticles produced by S. oneidensis MR-1 and characterized in a previous

study,3 and U

IV nanoparticles produced abiotically by reduction with sulfate green rust.

1 U

VI

sorbed to goethite, used as a standard for UVI

sorbed to an Fe oxide, contained 250 µM of UVI

in

a suspension of 1.5 g L-1

of goethite buffered at pH 7.4 with 0.1 M TAPS and 2 mM NaHCO3.

EXAFS Data Reduction and Analysis

Raw data from the beamline were aligned on the energy axis by the reference channel

and averaged. Standard data processing and data analysis procedures were followed.4

Background was subtracted by using the program AUTOBK.5 EXAFS scattering amplitudes and

phases were obtained from ab initio calculations by using the program FEFF8.6 The ATOMS

program generated an input cluster for FEFF8 from the crystal structure of UVI

-carbonate7. Self-

consistent scattering potentials were used to estimate the Fermi energy. Data were Fourier

transformed (FT) and refined by using the FEFFIT program.8

Identical parameters (e.g., k-

weighting, Fourier transform and fitting ranges, and Fourier transform window functions) were

used for all standards and unknown spectra. Definitions of the goodness-of-fit indicators (2, R-

factor, degrees of freedom) and explanation of the error calculation routines can be found in the

program documentation.

As seen on Figure 5 and explained in the main text, the samples with x=0.28 and x=0.33

magnetite stoichiometry showed a significantly smaller Oax peak amplitude in the Fourier

transform than the x = 0 sample, suggesting the possibility of a non-uranyl species as part of the

total solid phase U budget in these samples. To test for a non-uranyl species in a quantitative

way, we determined the expected decrease in refined axial O coordination numbers resulting

only from the transformation of maghemite-adsorbed UVI

species (U in the x = 0 magnetite

system) to reduced UIV

species in the magnetite system (U in the x = 0.50 magnetite system) and

compared it to the axial O coordination numbers determined in the x = 0.28 and x = 0.33

magnetite samples. The experimental EXAFS spectra from the endmember species in the x = 0

magnetite (1, UVI

adsorbed on maghemite) and in the x=0.5 magnetite (2, UIV

in nano-

particulate uraninite) were combined linearly as (k)=(1-y)*1(k) + y*2(k) (0y1) and the

S3

resulting (k) spectra were fit with a two shell Oax and Oeq EXAFS model. No detailed analysis

was done to determine the structure of the equatorial O shell, it was included as a single shell in

the EXAFS fit to account for the cumulative spectral leakage of all O atoms in the equatorial UVI

environment or in the UIV

environment into the axial O spectral region. The correlation between

coordination number and Debye-Waller factor parameters for the axial O shell was addressed by

determining the Debye-Waller factor value in the UVI

-only spectrum (y=1.0) where the

coordination number is known (=2.0) and then fixing the the Debye-Waller parameter to that

value in all other spectra (i.e., all y<1.0 spectra and the experimental spectra for x=0.28 and

x=0.33 magnetite). The Oax coordination number refined for that value of the Debye-Waller

parameter was then used as a measure of Oax peak amplitude loss. The linear combinations of

the endmember experimental spectra and their fits with an Oax+Oeq EXAFS model are shown in

Figure S7, showing good reproduction of the data. The refined numerical parameters are listed in

Table S6, and demonstrate decreasing Oax coordination numbers with increasing proportion of

the UIV

component. Fits of the x = 0.28 and x = 0.33 magnetite samples spectra with the same

Oax+Oeq EXAFS model are also shown in Figure S7 and parameters are listed in Table S6.

Based on the refined Oax coordination numbers obtained by fitting the linearly combined

endmember spectra (Table S6), we created a calibration graph of expected Oax coordination

number vs. proportion of the UIV

/UVI

endmembers in the system (Figure S5). Overplotting the

Oax coordination numbers obtained by fitting the spectra from the x = 0.28 and x = 0.33

magnetite samples vs. the proportion of UIV

component determined by XANES shows that the

refined Oax coordination numbers are less than what would be expected from a simple change in

the proportion of UVI

/UIV

endmembers and suggests the presence of additional non-uranyl UV or

UVI

species in the solid-phase associated U budget.

S4

Table S1. Mössbauer Parameters at 140 K for Magnetite, before and after Reaction with

UVI

, and for Biogenic Magnetite

OctFe

2.5+ Oct,TetFe

3+

Sample CS

(mm s-1

) ε

(mm s-1

) H

(T) Area

(%) CS

(mm s-1

) ε

(mm s-1

) H

(T) Area

(%) xMSb

Unreacted

Magnetitea

0.74 -0.02 46.0 62.3 0.38 0.001 49.3 37.7 0.45

Magnetite +

500 µM UVI

0.75 -0.01 46.6 54.8 0.38 -0.001 49.2 45.2 0.38

Biogenic

Magnetite 0.77 -0.01 46.3 60.3 0.38 -0.003 50.3 39.8 0.43

a Magnetite xd = 0.49.

b xMS = ½ (

OctFe

2.5+)/(½

OctFe

2.5+ +

Oct,TetFe

3+).

S5

Table S2. Summary of Results from Magnetite + UVI

Experiments in this Study

x = Fe2+

/Fe3+

[U]initial (µM) [U]finala (µM) % U

IV/UTotal

b

2 mM NaHCO3 buffer

0.50 – Magnetite 405 < 9 99

0.48 558 < 9 87

0.43 – Biogenicc 435 < 9 63

0.42 358 < 9 84

0.33 512 52 13

0.28 493 46 12

0.28 – 22 days n.m.d

n.m. 4

0.28 – 129 days n.m. n.m. 7

0.22 550 84 12

0.17 541 38 7

0 – Maghemite 522 52 4

50 mM MOPS buffer

0.50 – Magnetite 380 < 9 95

0.48 480 < 9 86

0.43 – Biogenicc 398 24 33

0.42 375 < 9 88

0.33 541 < 9 9

0.28 554 22 11

0.22 504 < 9 3

0 – Maghemite 525 < 9 0

0.28 353 < 9 17

0.28 → 0.44e 289 < 9 99

0.28 → 0.44f 343 < 9 96

0.28 → 0.38g

286 < 9 84 a [U]final was measured after approximately 24 h of reaction. Samples with [U]final < 9 µmoles L

-1

had final solution U concentrations below the detection limit of the colorimetric U analysis

method used in this study, determined to be 9 µmoles L-1

. b U

IV/(U

IV + U

VI) ratios were obtained by linear-combination analysis of the U XANES spectra.

UVI

end member, UVI

sorbed to goethite; UIV

end member, biogenic nanoparticulate uraninite. c Fe

2+/Fe

3+ ratio determined by Mössbauer spectroscopy.

d Not measured.

e Non-stoichiometric magnetite with x = 0.28 reacted with 11 mM of aqueous Fe

2+. The

magnetite sorbed Fe2+

(1.59 mmoles g-1

). The final x value of 0.44 was calculated with the

formulae given in the Supporting Information of Gorski and Scherer,9 and is based on the

amount of Fe2+

removed from solution. f Same as in note “e” above, but with the aqueous Fe

2+ removed prior to addition of U

VI. The

magnetite sorbed a similar quantity of Fe2+

(1.62 mmoles g-1

). g x = 0.28 magnetite reacted with 5.7 mM of aqueous Fe

2+. The magnetite removed Fe

2+ (1.03

mmoles g-1

) from solution. The final value of x is 0.38.

S6

Calculation of Magnetite Stoichiometry

To calculate the change in magnetite stoichiometry for reduction of 500 µM of U(VI), we

used the formulas derived in our previous work to calculate the Fe2+

and Fe3+

contents of the

magnetite from the Fe2+

/Fe3+

ratio x.9

For 1.5 g L-1

stoichiometric magnetite:

For stoichiometric magnetite: x =

= 0.50

Let Fe3+

= y. Then:

19.44 mM of Fe = (0.5y + y)

Fe3+

= y = 12.96 mM of Fe3+

Fe2+

= 19.44 – y = 6.48 mM of Fe2+

For maghemite:

The magnetite used has a stoichiometry from dissolution of xd = 0.49.

Using the scheme from our previous work,9 where the relative proportions of maghemite (Mh)

and magnetite (M) are used to calculate Fe2+

and Fe3+

concentrations:

M = fraction magnetite

Mh = fraction maghemite

[Fe2+

] = M*6.48

[Fe3+

] = 18.79*Mh + 12.96*M

Mass balance: M + Mh = 1

[Fe3+

] = 18.79*(1 - M) + 12.96*M

[Fe3+

] = 18.79 - 5.83*M

x =

=

S7

Rearranging for M:

M =

⁄

M =

⁄

= 0.986

[Fe2+

] = 6.48*0.986 = 6.39 mM Fe2+

[Fe3+

] = 18.79 – 5.83*0.986 = 13.04 mM Fe3+

We assume, on the basis of our XANES results, that the reaction with 500 µM of UVI

results in

complete reduction to 500 µM of UIV

, which corresponds to transfer 1 mM of e- equivalents to

the U from Fe2+

.

Therefore the final x is as follows:

x =

=

= 0.384

If xinitial = xMS 0.45, as determined by Mössbauer fitting, the calculated final x is as follows:

x = 0.341

S8

Calculation of UVI

/UIV

Redox Couples

Table S3. Gibbs Free Energies of Formation (∆Gf0, kJ/mol) for the Chemical Species Used

in Thermodynamic Calculations10

Chemical Species ∆Gf0 (kJ mol

-1)

UO2 (am) -995.8

UO2 (cr) -1031.8

UO22+

(aq) -952.5

UO2CO30 (aq) -1537.2

UO2(CO3)22-

(aq) -2103.2

UO2(CO3)34-

(aq)

-2660.9

(UO2)2CO3(OH)3-

-3139.5

UO3∙2H2O (cr) -1636.5

UO2+ (aq) -961.0

UO2(CO3)35-

(aq) -2587.0

H+ (aq) 0

H2 (g) 0

H2O (l) -237.1

HCO3- (aq) -586.9

CO32-

(aq) -527.9

OH- (aq) -157.2

(UO2)3(OH)5+ -3954.6

(UO2)4(OH)7+ -5345.2

UO2OH+ -1160

UO2(OH)20 ~-1368

Schoepite (UO3-2H2O) -1636.8a

a Schoepite data from Ref.

11

S9

Table S4. Calculated Reduction Potentials (versus standard hydrogen electrode) of UVI

/UIV

Couples in 2 mM NaHCO3- Solution at pH 7.2

a

Chemical Species Concentration (M)b Eh (V)

b Concentration (M)

c Eh (V)

c

UVI

/UIV

UO22+

/UO2 (am) 5 × 10-4

0.126 5 × 10-4

0.126

(UO2)2CO3(OH)3-/UO2 (am) 2.5 × 10

-4 -0.048 2.24 × 10

-4 -0.049

UO2CO30/UO2 (am) 5 × 10

-4 0.008 5.51 × 10

-6 -0.050

UO2(CO3)22-

/UO2 (am) 5 × 10-4

-0.022 3.33 × 10-5

-0.051

UO2(CO3)34-

/UO2 (am) 5 × 10-4

0.006 1.13 × 10-5

-0.042

UVI

/UV

UO22+

/UO2+ [U

VI] = 5 × 10

-4

[UV] = 10

-6

0.247 [UVI

] = [UV]

0.0878

UO2(CO3)34-

/UO2(CO3)35- [U

VI] = 5 × 10

-4

[UV] = 10

-6 -0.592 [UVI

] = [UV] -0.752

a For calculations including carbonate, the concentrations from equilibrium speciation modeling

using Visual MINTEQ12

are [H2CO3*] = 1.97 × 10

-4 M, [HCO3

-] = 1.47 × 10

-3 M, [CO3

2-] = 1.28

× 10-6

M. b Concentrations were assumed to be equal to added U

VI concentration of 500 µM for these

calculations, as the exact speciation is not known when a surface for sorption is added. These

numbers are used in Figure 4. c Major aqueous species calculated using Visual MINTEQ. Total CO3 = 2 mM, pH 7.2.

S10

Table S5. Calculated Reduction Potentials (versus standard hydrogen electrode) of UVI

/UIV

couples in MOPS buffer (no bicarbonate) at pH 7.2.

Chemical Species Concentration (M)a Eh (V)

a Concentration (M)

b Eh (V)

b

UVI

/UIV

UO22+

/UO2 (am) 5 × 10-4

0.126 8.6 × 10-9

-0.015

(UO2)3(OH)5+

/ UO2 (am) 1.67 × 10-4

-0.015 2.1 × 10-5

-0.024

(UO2)4(OH)7+

/ UO2 (am) 1.25 × 10-4

-0.016 9.2 × 10-6

-0.024

UO2OH+

/ UO2 (am) 5 × 10-4

0.008 4.3 × 10-7

-0.023

UO2(OH)20/ UO2 (am) 5 × 10

-4 -0.196 6.9 × 10

-7 -0.28

UO3-2H2O(schoepite)

/UO2 (am)

- -0.061 - -

a Concentrations were assumed to be equal to added U

VI concentration of 500 µM for these

calculations, as the exact speciation is not known when a surface for sorption is added. b Major aqueous species calculated using Visual MINTEQ. Schoepite precipitation allowed. 400

µM UVI

precipitates as schoepite.

S11

Figure S1. U LIII edge XANES spectra for U

VI reacted with magnetites of different

stoichiometries (x) in 2 mM bicarbonate buffer, pH 7.2.

S12

Figure S2. U LIII edge XANES spectra for U

VI reacted with magnetites of different

stoichiometries (x) in 50 mM MOPS buffer at pH 7.2.

S13

Figure S3. Fourier-transformed EXAFS spectra from U

IV formed by nearly stoichiometric

magnetite, compared to previously characterized UIV

phases: (1) nanoparticulate uraninite

formed in the absence of Fe oxides by S. oneidensis CN32,3 and (2) a uraninite-like U

IV species

produced by Fe2+

, in which inner-sphere coordination between UIV

and Fe atoms was determined

to cause the amplitude dampening and the change in amplitude ratio between the U-U doublet

peaks.13

Spectra from the nearly stoichiometric magnetite systems are identical (within noise) to

data from biogenically formed uraninite. This suggests the predominant formation of segregated

nanoparticulate uraninite in these systems, as opposed to a thin uraninite coating associated with

the Fe oxide surface or as opposed to a combination of segregated uraninite particles and a

substantial fraction of single UIV

atoms adsorbed inner-sphere to the magnetite surface.

S14

Figure S4. Dependence of the fitted Oax coordination number on the proportion of U

IV in

synthetic spectra created by linear combinations of experimental spectra from a nanoparticulate

uraninite UIV

end member and a UVI

sorbed to maghemite end member. The EXAFS model used

to fit the model spectra was a two-shell (Oax and Oeq) model, in which the Debye-Waller factor

of the axial O shell was fixed to that obtained in fits of the UVI

end member with a fixed Oax

coordination number of 2.0. The dashed lines represent polynomial interpolation of the fit

uncertainty in Oax coordination numbers; they provide a visual band of the uncertainty. The Oax

coordination numbers for the open data points are from Oax + Oeq model fits of the EXAFS

spectra from UVI

reacted with x = 0.28 and x = 0.33 magnetite, plotted versus the UVI

fraction

determined by XANES. (Horizontal error bars represent approximately 15% uncertainty in the

XANES determination.) The vertical difference between the calibration band and the open data

points suggests the presence of a small amount (10-15%) of non-uranyl UVI

or UV species in the

x = 0.28 and x = 0.33 samples. The XANES fits were made with the assumption of UIV

and UVI

end members. Allowing for the presence of UV in the XANES fits would slightly decrease the

proportion of UIV

atoms and would shift the open points slightly to smaller x-axis values, making

the vertical difference with the calibration band slightly larger.

0 20 40 60 80 100

0.0

0.5

1.0

1.5

2.0F

itte

d O

ax c

oord

ination

num

ber

Percentage UIV

/(UIV

+UVI

)

S15

Figure S5. XANES U LIII edge XANES spectra for UVI

reacted with oxidized magnetite (x =

0.28), recharged by addition of 5.7 mM and 11 mM of aqueous Fe2+

. One of the 11 mM samples

was magnetically separated, and the solution containing aqueous Fe2+

was replaced with fresh

buffer. Experimental conditions: pH 7.2, 50 mM MOPS buffer.

S16

Figure S6. Mössbauer spectrum of unwashed biogenic magnetite produced via the reduction of

lepidocrocite by S. putrefaciens CN32. Fitting of the Mössbauer spectrum indicates that the x

value of this magnetite is 0.43.

S17

Figure S7. A) Fourier transforms (FT) of different linear combinations of the experimental

EXAFS data in the x = 0.00 and x = 0.50 magnetite systems (i.e., UVI

sorbed to maghemite and

UIV

nanoparticles produced by UVI

reduction with x = 0.50 magnetite, respectively). B) Data

(symbols) and fit (line) of the data in A) and of the experimental data in the x=0.28 and x=0.33

samples, using an Oax and Oeq shell as described in the text. Fitting was done in R-space over

the range R+= 0.9 – 2.1 Å. Fourier transforms of the k3-weighed EXAFS data are over the

range k=2.5 – 10.5 Å-1

with a 1.0 Å-1

Hanning window. The same transform parameters were

used for all Fourier transforms. The position of the peak corresponding to the Oax atoms in

uranyl UVI

atoms is noted by the vertical dashed line.

0 1 2 3 4 5 6

0

2

4

6

FT

Am

plit

ud

e o

f k

3(k

)

R+ (A)0 1 2 3 4 5 6

data

fit

R+ (A)

FT of (k)=(1-y).1(k)+y.2(k)

0y1

1(k)=UVI sorbed to maghemite

2(k)=nanoparticulate UIV

y

1.0

0.8

0.6

0.4

0.2

0.0

A B

y

0.0

0.2

0.4

0.6

0.8

1.0

Linear combination

spectra

UVI+magnetite spectra

x=0.28

x=0.33

OaxOax

S18

Table S6. Numerical Results from Modeling of the EXAFS Data in Figure S7a

a

When uncertainties are not given for a parameter, the parameter is held fixed to the indicated value. DF = degrees of freedom in the fit. The R-factor is a normalized sum of the squared differences between data

and fit. 2 is a goodness-of-fit parameter accounting for the amount of data and number of variables in

the fit. More information on these parameters can be found in the FEFFIT manual. b

Debye-Waller parameters held fixed to that refined in the x=0.00 magnetite sample with a fixed N(Oax)=2.0.

Path DF R-factor 2

y=0.0, UVI

in the x=0.00 magnetite system, fixed N(Oax)

U-Oax 2.0 1.80 ± 0.02 0.0017 ± 0.0005 1.6 ± 4.4 2 0.004 53

U-Oeq 5.6 ± 3.4 2.38 ± 0.04 0.0165 ± 0.0086 1.6 ± 4.4

y=0.0, UVI

in the x=0.00 magnetite system, fixed s2(Oax)

U-Oax 2.0 ± 0.1 1.80 ± 0.01 0.0017b

2.5 ± 3.6 2 0.003 43

U-Oeq 5.0 ± 2.7 2.39 ± 0.03 0.0154 ± 0.0074 2.5 ± 3.6

y=0.20

U-Oax 1.6 ± 0.2 1.77 ± 0.02 0.0017b

-4.7 ± 6.2 2 0.006 87

U-Oeq 8.3 ± 3.9 2.33 ± 0.05 0.0180 ± 0.0057 -4.7 ± 6.2

y=0.40

U-Oax 1.1 ± 0.1 1.76 ± 0.01 0.0017b

-6.3 ± 3.3 2 0.005 89

U-Oeq 8.0 ± 2.1 2.32 ± 0.03 0.0158 ± 0.0033 -6.3 ± 3.3

y=0.60

U-Oax 0.7 ± 0.1 1.76 ± 0.01 0.0017b

-5.7 ± 2.3 2 0.006 132

U-Oeq 7.0 ± 1.4 2.32 ± 0.02 0.0132 ± 0.0025 -5.7 ± 2.3

y=0.80

U-Oax 0.4 ± 0.1 1.74 ± 0.02 0.0017b

-5.1 ± 1.9 2 0.007 237

U-Oeq 6.4 ± 1.1 2.33 ± 0.01 0.0111 ± 0.0022 -5.1 ± 1.9

y=1.00

U-Oax 0.2 ± 0.1 1.67 ± 0.03 0.0017b

-3.8 ± 1.6 2 0.009 340

U-Oeq 5.7 ± 1.0 2.34 ± 0.01 0.0090 ± 0.0021 -3.8 ± 1.6

UVI

in the x=0.28 magnetite system

U-Oax 1.5 ± 0.2 1.78 ± 0.03 0.0017b

-5.5 ± 8.9 2 0.008 77

U-Oeq 6.3 ± 4.6 2.33 ± 0.07 0.0171 ± 0.0080 -5.5 ± 8.9

UVI

in the x=0.33 magnetite system

U-Oax 1.3 ± 0.1 1.77 ± 0.02 0.0017b

-4.6 ± 6.8 2 0.006 66

U-Oeq 4.5 ± 2.8 2.32 ± 0.05 0.0137 ± 0.0066 -4.6 ± 6.8

N R(Å) s2 (Å

2) E(eV)

S19

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