Influence of Dynamical Conditions on the Reduction of UVI at the

Oct 28, 2009 - JOHN R. BARGAR,⊥ AND ANDREW R. FELMY+. Pacific Northwest National Laboratory, 902 Battelle Blvd.,. Richland, Washington 99352 ...
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Environ. Sci. Technol. 2010, 44, 170–176

Influence of Dynamical Conditions on the Reduction of UVI at the Magnetite-Solution Interface E U G E N E S . I L T O N , * ,+ J E A N - F R A N C ¸ OIS BOILY,‡ EDGAR C. BUCK,+ FRANCES N. SKOMURSKI,+ KEVIN M. ROSSO,+ CHRISTOPHER L. CAHILL,§ JOHN R. BARGAR,⊥ AND ANDREW R. FELMY+ Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, Washington 99352, Chemistry, Umeå University, SE-901 87 Umeå, Sweden, Chemistry, George Washington University, 725 21st St., Washington, D.C. 20052, and Stanford Synchrotron Radiation Laboratory, 22575 Sand Hill Rd., Menlo Park, California 94025

Received May 16, 2009. Revised manuscript received August 23, 2009. Accepted September 14, 2009.

The heterogeneous reduction of UVI to UIV by ferrous iron is believed to be a key process influencing the fate and transport of U in the environment. The reactivity of both sorbed and structural FeII has been studied for numerous substrates, including magnetite. Published results from UVI-magnetite experiments have been variable, ranging from no reduction to clear evidence for the formation of UIV. In this contribution, we used XAS and high resolution ((cryogenic) XPS to study the interaction of UVI with nanoparticulate magnetite. The results indicated that UVI was partially reduced to UV with no evidence of UIV. However, thermodynamic calculations indicated that U phases with average oxidation states below (V) should have been stable, indicating that the system was not in redox equilibrium. A reaction pathway that involves incorporation and stabilization of UV and UVI into secondary phases is invoked to explain the observations. The results suggest an important and previously unappreciated role of UV in the fate and transport of uranium in the environment.

Introduction Uranium mining operations and subsequent manufacturing processes, including nuclear weapons production, have made uranium a common contaminant in the environment and the most abundant radionuclide in the subsurface at U.S. Department of Energy facilities (1). The solubility, and hence mobility, of U in the subsurface is strongly governed by its oxidation state, which is influenced by numerous factors (2) such as the mineralogy and organic content of the system, bacterial activity, and solution composition. Under reducing conditions where UIV is stable, U forms sparingly soluble minerals such as uraninite (UO2) and coffinite (SiUO4). It is well-known, provided reducing conditions are maintained, that UIV is normally orders of magnitude less soluble than UVI. This difference has implications for the fate, transport, * Corresponding author phone: 509-371-6387; fax: 509-371-6354; e-mail: [email protected]. + Pacific Northwest National Laboratory. ‡ University of Umeå. § George Washington University. ⊥ Stanford Synchrotron Radiation laboratory. 170

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and remediation of U in the environment, where one strategy for restricting U mobility is to reduce it to and maintain it as UIV. Numerous studies, including the present one, have focused on the abiotic heterogeneous reduction of UVI by FeII in or at the surface of silicates and oxides (3-12). In fact, abiotic reduction of UVI to UIV by FeII does not occur readily in homogeneous solution but requires surfaces to facilitate the transformation, e.g., ref 9. The UVI-magnetite system has received appreciable attention due to magnetite being a ubiquitous ferrous bearing oxide, a metabolic byproduct of bacterial respiration, and a corrosion product of steel with ramifications for storage of nuclear waste (3, 4, 10, 12). Observations from studies of UVI(aq) interaction with magnetite have ranged from no observed reduction (3) to clear evidence of UIV (12), and suggestions for the formation of a mixed-valence UIV-UVI phase (10). The variation in results is conceivably due to different forms and stoichiometry of magnetite, as well as experimental conditions, which suggests that reaction pathways might be important. Consequently, it is relevant that our recent work, using X-ray photoelectron spectroscopy (XPS), has provided evidence that sorbed UV has a measurable lifetime during the reduction of UVI(aq) by annite, the ferrous endmember mica (5). There is also laboratory evidence for an intermediate UV species during bioreduction of UVI (13) and for growth of the UV-mineral wyartite on calcite (14). Further, recent synthetic efforts have produced a mixedvalence UV/UVI compound via reduction of UO22+ under hydrothermal conditions (15). These discoveries highlight questions concerning the role of UV in the overall UVI to UIV reduction reaction, such as whether sorbed UV disproportionates, or if a two electron transfer from the substrate is required to produce UIV. However, such considerations have historically been of little concern in the low temperature geochemistry of U, where the significance of the intermediate valence state, UV, has been discounted due to the well-known propensity of UV(aq) to disproportionate readily and rapidly under conditions of near-surface environmental interest (16). In the present study, we used a combination of thermodynamic modeling, high-energy resolution monochromatic ((cryogenic/hydrated) XPS, X-ray absorption spectroscopy (XAS), and transmission electron microscopy (TEM) to study the interaction of UVI(aq) with nanoparticulate magnetite under conditions of active dissolution and precipitation. Our interest was sparked by the cumulative body of work on the interaction of UVI with magnetite that was discussed above. Although prior XPS studies of the U-magnetite system did not identify UV species, UV was not considered in the spectral fits. Further, instruments were polychromatic with inherent low-energy resolution, which poses a problem for the identification of UV as the U4f binding energies of its primary and less intense, but distinctive, satellite peaks are intermediate to those of UVI and UIV.

Materials and Methods Materials. Nanoparticulate magnetite was prepared with ACS reagent grade chemicals and deionized (DI) water (18.2 MΩ cm) following Schwertmann and Cornell (17). The suspension was repeatedly washed, until a resistivity of the same order of magnitude as that of DI water was reached, and then stored (13.6 g/L) in a Teflon bottle nested in an opaque gastight container within an anaerobic chamber. Preparation of green rust is discussed in the Supporting Information (SI). Magnetite was characterized by XPS (SI Figure S1) and XRD (SI Figure S2). XRD indicated that the magnetite was highly crystalline 10.1021/es9014597

 2010 American Chemical Society

Published on Web 10/28/2009

with an average crystallite size of 58 ( 1 nm. No secondary products were detected by XRD. A 90 point N2(g) adsorption/ desorption isotherm (Micromiretics) revealed a BET surface area of 13.5 ( 0.1 m2/g. XPS analysis detected only Fe, O, and adventitious C at the near surface, giving FeII/Fe ) 0.30, which is slightly less than the stoichiometric value of 0.33. Maghemite was prepared by heating the nanoparticulate magnetite to 250 °C in air for 5 h. A 198 µmol/L UVI stock solution was prepared from reagent grade uranyl chloride and degassed DI water (18.2 MΩ cm). Experimental Section. A detailed review of the experimental procedures, including a schematic (SI Figure S3), is provided in the SI. In brief, nanoparticulate magnetite (or green rust) was reacted with uranyl containing solutions, with chloride as the counteranion, in 3 mL Vectaspin centrifuge tubes. The initial pH was adjusted with HCl, but not buffered. Carbonate and organics were excluded as the former stabilizes UVI against reduction at circum neutral to alkaline pH, and the latter because of potential surface contamination that would reduce the XPS signal. Further, it is not known whether organic buffers affect the stability of UV. The initial conditions are given in SI Table S1. Conditions were chosen in order to assess the effect of pH and variable U/magnetite ratios (i.e., oxidant/reductant ratios) on the reaction. Further, relatively low pH allowed us to load the experiments with high concentrations of UVI(aq) in order to obtain the signal-to-noise needed to assess U oxidation states from the XPS fine structure. All experiments and postexperimental handling of reaction products were performed in an anaerobic chamber at measured O2 < 0.1 ppm. Upon conclusion of an experiment, the sample was centrifugefiltered, and both the solid and fluid collected for analysis. Fe(aq) and U(aq) were analyzed on a PerkinElmer ICP-OES (optical emission spectrometer), model 2100DV. The pH was measured with an Orion 8103 Ross combination electrode. Solid State Analyses. For XPS analyses, samples were transferred to the instrument (see SI) using airtight containers and a N2 purged glovebag attached to fast entry port of the instrument. Regional scans of the U4f, O1s, Fe2p, and C1s region were recorded, and the energy scale was referenced to adventitious C1s at 285.0 eV. Standards for UVI, UV, and UIV were used to determine satellite structures and primary peak parameters; both the primary and satellite peaks for U4f were used in the fitting procedure (see SI). The UIV and UVI standards were synthetic stoichiometric UO2(c), as used by Schofield et al. (18), and schoepite, respectively. The U4f parameters for UV were derived by fitting the U4f spectrum of a synthetic UV-UVI oxyhydroxide phase, UV(H2O)2(UVIO2)2O4(OH)(H2O)4, with the UVI standard. The derived parameters are similar to those for monovalent UV alkali uranates (see SI). The synthesis and characterization (including XPS and XAS) of this compound has been described previously (15). U4f spectra were best fit by nonlinear least-squares using the CasaXPS curve resolution software, with parameters given in SI Table S1. For XAS analyses, centrifuged-filtered samples were shipped to the Stanford Synchrotron Radiation Lightsource (SSRL) under N2 (gastight triple containers) along with custom-made green rust oxygen “getters” in the outer and middle containers. All sample manipulation at SSRL was performed under an anaerobic atmosphere (5% H2, balance N2). The wet samples, as paste, were loaded in sample holders with Kapton windows. Samples were stored wet and anaerobic until analysis. Details concerning analysis procedures are given in the SI. Samples were prepared for TEM by dipping lacy carbon grids into methanol suspensions under ambient conditions. Details concerning the instrument and settings are given in the SI.

Results Results and initial conditions for the interaction of nanoparticulate magnetite and maghemite with aqueous UVI solutions, as well as for a magnetite dissolution experiment without UVI(aq), are given in SI Tables S2 and S3 for solution and XPS analyses, respectively. U4f XPS spectra of a green rust and an oxidized magnetite sample reacted with UVI(aq) are shown in SI Figure S4. Experiments Umag1-5 had identical initial conditions and reacted for 1 day, 1 week, 2 weeks, 3 weeks, and 5 weeks, respectively. Experiments Umag6-11 had lower U/magnetite ratios than Umag1-5, variable initial pH, and reacted for either 12 or 56 days. EXAFS results are tabulated in Table 1. Noncryogenic and Cryogenic XPS. The U4f and O1s spectra for a noncryogenic and cryogenic/hydrated sample are compared in Figure 1. As evidenced by the O1s spectra, cryogenic conditions preserved more H2O on the magnetite surface relative to noncryogenic conditions. U4f spectra for the noncryogenic and cryogenic magnetite samples are qualitatively similar, where all magnetite samples recorded broadening on the low binding energy (BE) side of the primary U4f spin-orbit split peaks (e.g., Figure 1). Further, the most intense satellite structures extend to higher BEs compared to monovalent UVI compounds. Collectively, the U4f XPS spectra record the presence of reduced uranium. Nonetheless, an analysis of the satellite peaks indicated that UIV was not detected in any of the spectra. The satellite structures, which are likely due to charge transfer (CT) from the O2p bonding orbital into the open 5f and/or 6d metal shells (19), are robust indicators of U oxidation states in (oxyhydr)oxide compounds (20). The spectra were initially curve resolved using the UVI and UIV standards which yielded acceptable fits to the primary peak envelopes in terms of shape, but not to the satellite structures. In essence, the BEs of the satellites for the UIV standard were too low to fit the intense satellite at about 7 eV above the composite primary peaks and, conversely, yielded too much intensity for the region around 6 eV above the composite primary peaks. Further, there was insufficient intensity around 4 eV above the composite primary peaks, which corresponds to the 4 eV UVI satellite region. Bonding character does affect the position and intensity of the UIV satellites; as a general rule, increasing ionicity increases the separation between CT satellites and the primary peaks, and diminishes their intensity (21). Consequently, UF4 puts an upper limit on the energy separation between the UIV satellites and their corresponding spin-orbit split peaks of ∼7.1 eV, which is only ∼0.2 eV greater than the ∼6.9 eV separation for our UIV standard. This indicates, and curve resolution showed, that UIV cannot model the intense, high BE satellite feature, regardless of bonding environment effects; this effectively rules out UIV. Because the U4f binding energies of the primary peaks for UVI can overlap those for UV (22, 23), we also tried to fit the spectra with two UVI components. As expected, the resulting fits were poor because the lower BE UVI component does not manifest the correct U4f satellite structure. In contrast, modeling the U4f spectra with the UVI and UV components, where UV has distinctive satellites at ∼7.9-8.4 eV above both primary spin-orbit split peaks (ref 22 and references therein; and ref 1 in the SI), does yield a good fit to the intense high BE satellites; where fits were optimized by marginally decreasing the UV satellite-primary peak binding energy separations by 0.1-0.2 eV relative to the standard (i.e., from ∆8.3 eV to ∆8.1-8.2 eV, see SI). However, there is still too little intensity in the region of the 4 eV UVI satellite. This issue is exaggerated for the noncryogenic versus cryogenic samples. The question is whether this missing intensity is due to a third component, or whether the satellite is more intense relative to our bulk UVI standard. VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Fitting Results from EXAFS (left) and Comparison of EXAFS Determined Nonuranyl/Utotal × 100 versus XPS Determined UV/ Utotal × 100 (right) Oax

Fe1

Fe2

EXAFS

XPS

%uranate

%UV

Umag8 N R (Å) σ2 (Å2)

1.5 (2) 1.79 (1) 0.003

3 (1) 2.39 (2) 0.006 (4)

2 (1) 3.45 (4) 0.007

2 (1) 3.66 (4) 0.007

25 (10)

39 (6)

Umag9 N R (Å) σ2 (Å2)

1.5 (2) 1.78 (1) 0.003

3 (1) 2.37 (2) 0.006 (4)

1.4 (9) 3.43 (4) 0.007

1 (1) 3.63 (5) 0.007

25 (10)

41 (6)

Umag10 N R (Å) σ2 (Å2)

1.3 (1) 1.774 (9) 0.003

6 (2) 2.37 (2) 0.014 (4)

1.6 (7) 3.42 (3) 0.007

1.7 (8) 3.63 (3) 0.007

35 (7)

48 (5)

Umag11 N R (Å) σ2 (Å2)

1.1 (1) 1.776 (9) 0.003

5 (1) 2.38 (1) 0.012 (3)

2.1 (6) 3.42 (2) 0.007

2.3 (7) 3.63 (2) 0.007

45 (9)

48 (5)

Oax

Oeq1

Umaghemite N R (Å) σ2 (Å2) a

Oeq

nd ) Not detected.

2.4 (1) 1.788 (6) 0.003 b

2.5 (2) 2.25 (2) 0.005

Oeq2 2.5 (2) 2.44 (2) 0.005

Fe 1.4 (6) 4.24 (2) 0.007

nda

b

Signal to noise too low for obtaining accurate valence state information.

FIGURE 1. U4f and O1s XPS of Umag4 (noncryogenic), top panel; and Umag4-cryo (hydrated cryogenic), bottom panel. To answer this question, we reacted UVI(aq) with a relatively oxidized (FeII/Fe ) 0.24) magnetite sample in the 172

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presence of nitrate, where the total amount of sorbed U was similar to the more reducing system. Curve fitting indicated

only one primary component and no detectable reduction; but in order to optimize the fit, the 4 eV satellite intensity was nearly doubled (SI Figure S4). We then refit the reduced samples with UV and UVI, where the UVI 4 eV satellite intensity was increased to match that for monovalent UVI sorbed to oxidized magnetite. The resulting curve fits very closely modeled the primary peaks and the entire satellite structure for the noncryogenic samples (e.g., Figure 1). If all three oxidation states are included in the fit, the UIV component decreased to nearly zero, with a detection limit at roughly 5% of total U. In fact, all the experimental run products yielded detectable UVI and UV, but not UIV. Modeling the U4f spectra taken under cryogenic/hydrated conditions yielded results in qualitative agreement with the noncryogenic counterpart (SI Table S3; Figure 1). The cryogenic sample was then dehydrated in the XPS and reanalyzed with similar results. XPS-determined binding energies, and UV/(U or Fe) and U/Fe atomic ratios are listed in SI Table S3. Most samples exhibited slow and systematic reduction during sequential analyses taken on the same spot (SI Figure S5). The valence states were obtained by extrapolating to time zero, where the difference between time zero and the first spectrum was less than 15%, which is on the order of the analytical error. XPS-determined atomic ratios for Umag1-5 are plotted as a function of reaction time in Figure 2a and b. Solution Analyses. The concentrations of U and Fe are correlated to reaction time, the U/magnetite ratios, and the pH (SI Table S2). For the Umag1-5 experiments, Fe and U increased and decreased, respectively, as a function of time (Figure 2c). The pH increased from an initial value of 4.0 to approximately 7 after 5 weeks of reaction. U LIII edge XAS. XANES analyses of Umag8-11 are given in SI Figure S6, and are compared to U sorbed by maghemite and standards for UIV and UVI. The LIII edge positions are generally consistent with that of UVI. Quantitative fits to the spectra (not shown) using mixtures of uranyl and UIV as UO2.0 floated to 0% UIV. We estimate a 5-10% detection limit in the present samples. These findings are consistent with the XPS results. However, the L-edge position for uranateV, where UV lacks the two short axial bonds typical of uranyl coordination, can be very close to that for UVI (24), and it can be difficult to use the position of the L-edge to distinguish uranateV in mixed valence UVI-UV compounds (15). Consequently, XANES cannot exclude the presence of nonuranyl UV. As discussed below, EXAFS indicates the presence of nonuranyl coordination, which in turn, is permissive of UV in the samples. EXAFS and corresponding Fourier transform (FT) magnitudes, along with fits to the data, for samples Umag8-11 are shown in Figure 3a and b, respectively. Qualitatively, the Umag8-11 spectra are similar to one-another, indicating general homology of the average local and intermediaterange structure around U. All are dominated by two strong U-O backscattering frequencies, labeled as Oax and Oeq (Figure 3b), corresponding to axial and equatorial oxygen positions at ca 1.78 and 2.38 Å, respectively, consistent with the transdioxo UVI cation. In addition, all spectra show three smaller but distinct EXAFS frequencies at >2.5 Å (R+dR), indicating the presence of significant ordering of the local and intermediate-range structure around U. The Umag spectra are compared to that of UVI sorbed to a redox-inert analog, maghemite in Figure 3. The U spectrum for maghemite is different from that for magnetite in two key aspects: (a) the Oax:Oeq frequency intensity ratio of 3:1 (typical for UVI sorbed on FeIII oxides) is strikingly larger than for the magnetite samples (ranging from 1.5:1 to almost parity); and (b) the maghemite sample does not exhibit significant EXAFS frequencies at >2.5 Å (R+dR). These observations indicate that the average local and intermediate-range structure

FIGURE 2. XPS and solution analyses of Umag1-5 as a function of reaction time. Panels a and b show XPS determined atomic ratios for Umag1-5. Panel c shows solution analyses. The pH values were 4.0 5.2, 5.6, 6.7, and 7.3 after 0, 1, 7, 21, and 35 days of reaction, respectively. around U in the Umag samples is markedly distinct from the surface complexes on maghemite. We note that the Umag8-11 spectra are distinctly different from the spectrum of schoepite precipitated at near-neutral pH (see SI), indicating that this phase is not a significant component of the samples. The results from quantitative fitting of the data are given in Table 1. A discussion of the fitting procedure is given below, with more details provided in the SI. The low intensity of the Oax shell relative to Oeq is atypical for urΦ5 (pentagonal dipyramidal) and urΦ6 (hexagonal dipyramidal) uranyl, which are indicated by the equatorial oxygen bond distance (25). Fits to the spectra were initially attempted assuming 100% pure uranyl (i.e., two axial oxygens required), but yielded inadequate fits to the oxygen shell region and produced Oax disorder parameter (σ2) values of ca 0.0085 Å2, which is 2- to 4-fold larger than typically observed, e.g., refs 26 and 27. Subsequently, fits were performed in which the Oax coordination number (CN) was allowed to float, obtaining values of 1.1-1.5 (Table 1), implying that 25-45% of uranium was nonuranyl. The Hamilton test (28) indicated that these fit improvements were significant at the 97.5% confidence interval, i.e., confirming the hypothesis that a nonuranyl phase is also present. VOL. 44, NO. 1, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. (a) EXAFS spectra and (b) Fourier transforms. Solid and dashed lines represent the data and fits, respectively. The 4.2 Å (R+dR) peak fits were performed using a U shell at 4.20 Å, as described in the text. Fits to the data are listed in Table 1. This multiple-species interpretation (i.e., uranyl and nonuranyl) is supported by qualitative inspection of the data. The FT amplitude of the valley between the Oax and Oeq peaks increases systematically with increasing final pH, whereas peak positions are invariant. This behavior suggests the existence of an interposed third peak, corresponding to uranium species with O shells of distances intermediate to ca 1.78 and 2.38 Å. This conclusion is consistent with the decrease in the Oax CN from 1.5 to 1.1 between Umag8 and 11 and, simultaneously, an increase in the Oeq σ2 value from 0.006 to 0.014 Å2 (cf., Table 1 and SI). Quantitative modeling of intermediate UsO shells (i.e., between Oax and Oeq) is challenging due to overlap of the corresponding frequencies. The Nyquist criterion dictates that the O shells cannot be resolved from one-another if they are closer than 0.21 Å ()π/2δk (29),) unless a-priori information, such as coordination numbers, can be applied to constrain the model. In this regard, both nonuranyl UV and nonuranyl UVI coordination polyhedra tend to exhibit 6-coordination. In the case of nonuranyl UVI, the average UsO distances lie between ∼2.05 and 2.10 Å (25). Nonuranyl UV records a wider range of UsO bond lengths with average values between 2.05 and 2.20 Å (SI Figure S7). With these challenges in mind, an EXAFS fitting model was constructed based on the following structural aspects: (a) an axial oxygen shell having CN ) 2, (b) an intermediate UsO shell having CN )6 (corresponding to nonuranyl UVI or UV), and (c) an Oeq shell having CN ) 5 or 6 (both were tested). The amplitudes of the Oax and Oeq shells for uranyl were scaled in proportion to the fitted fraction of uranyl present, whereas the amplitude of the nonuranyl shell was attenuated by the value equating to 1- (fraction of uranyl present). For sample Umag8, this fitting approach yielded a significant improvement in the fit (i.e., which was larger than that obtained by simply allowing CN for Oax to float, e.g., Table 1), with a uranyl fraction of 83 ( 20% and an intermediate O shell distance of 2.13 ( 0.09 Å, a reasonable distance in comparison to the literature (cf., SI Figure S7). However, stable and consistent fits could not be obtained for the other samples. These results can be explained if the presumed intermediate UsO shell has substantial static disorder (cf., SI Figure S7). The EXAFS results are therefore best explained by a mixture of uranyl (1/2 to 2/3 of total U) and a nonuranyl U species (1/2 to 1/3 of total U). 174

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Quantitative fitting of the FT frequencies beyond 2.5 Å (R+dR) indicated that the first two second shell FT peaks correspond to Fe neighbors at distances of ∼3.42 and 3.66 Å (Table 1). The third second shell FT peak can be fit with 2-3 Fe or U neighbors at 4.29 Å or 4.22 Å, respectively (not shown in Table 1). Details concerning the fitting procedure and interpretation are given in the SI. It is reasonable to conclude that each of the observed second-neighbor species can be assigned to one of the two dominant uranium species deduced from the O-shell analyses. Sample Umag11 appears to have an almost equal mixture of the two U species. It follows that the true CNs for the second-shells for sample Umag11 must be about twice as large as the fit-derived values, i.e., in the vicinity of 2-4 atoms. A similar conclusion is obtained for the other samples. Such CNs, as well as the relative abundance of second-shell frequencies (see SI), imply that uranium is present in an extended 2-D structure (e.g., a surface thin film) or 3-D structure (e.g., incorporated into a Fe oxide). In support of this conclusion, we note that the number and intensity of FT peaks at >2.5 Å (R+dR) in the Umag8-11 samples is greater than typically observed for UVI sorbed on other Fe oxides at near-neutral pH, e.g., (26). TEM analyses are consistent with this interpretation (see SI). Thermodynamic Modeling. Thermodynamic calculations, with a magnetite free energy from Sweeton et al. (30) and U thermodynamic data critically reviewed by the NEA (31, 32), were performed to simulate changes in Fe(aq), U(aq), and pH as a function of magnetite dissolution with and without UVI. The initial conditions used in the simulations, and both the predicted and experimental values, are listed in SI Table S2. The simulation of the U-free system closely matched the final Fe concentrations and pH values for magnetite dissolution in the absence of U, indicating that the magnetite was behaving as it should, with no evidence for a reactive contaminant. Consequently, the high Fe(aq) in the U-magnetite systems must be due to the interaction between U and magnetite. Simulations for Umag5 yielded UO2(c), U4O9, maghemite, and a Fe(aq) concentration in excellent agreement with the experimental data. Note that UO2(a) was not predicted to be stable. However, the calculated U concentrations and pH values were too low. We then suppressed the formation of UO2(c) and U4O9. In this case, the predicted Fe and U concentrations are close to the experimental results where both maghemite and schoepite

formed. Therefore, although there is sufficient free energy available to reduce UVI to UO2(c) and U4O9, the system is not likely in redox equilibrium. Instead, the final U concentration can be explained by the precipitation of a schoepite-like phase, driven by increases in pH from magnetite dissolution. This is consistent with TEM of Umag5 (SI). However, the predicted pH is still lower than the observed value. If the final experimental pH (∼7) is used in the thermodynamic model, then the calculated Fe(aq) concentration (0.7 µmol/ L) is far below the observed value, indicating that magnetite must have become isolated from the reacting solution. Collectively, these results are consistent with the XPS measurements which indicated that UIV did not form and that the majority of U is UVI. Unfortunately, it is not possible to predict the formation of UV as accurate thermodynamic data for well characterized phases of UV are limited.

Discussion Comparison of XPS and XAS. XANES did not detect UIV, which is consistent with the XPS results and thermodynamic modeling. EXAFS indicated that U had both uranylVI and nonuranyl coordination. The positive correlation between EXAFS- and XPS-determined nonuranyl U and UV, respectively, suggests that the nonuranyl component contains UV (Table 1). UV: Reality or Artifact? Several lines of evidence affirm that UV is not an artifact of sample manipulation and analysis. First, XPS of both monovalent sorbed UIV (produced by reduction of UVI by green rust; SI Figure S4a) and UVI (UVI sorbed to oxidized magnetite; SI Figure S4b) survived post experimental handling (i.e, no detectable UV); including drying, transport to the XPS, the glovebag environment, UHV conditions, and short to intermediate X-ray beam exposure time. Lastly, cryogenic XPS showed that UV was not an artifact of drying or dehydrated-UHV conditions. Comparison to Previous U-Magnetite XPS Studies. A striking result is that UIV was not detected either by XPS or XAS, whereas other XPS studies on the interaction of UVI(aq) with magnetite have interpreted spectra to contain UIV (and/or UVI) but not UV. However, previous studies did not consider the possibility of UV, nor did they include the satellite structures in the fits. U4f spectra in Scott et al. (12) certainly appear more reduced than in the present study, with the U4f7/2 composite peak positions near 380.6 eV or lower (readjusted to C1s at 285) and evidence for the distinctive UIV satellite at ∼6 eV above the primary peaks. Nonetheless, it is hard to exclude the presence of UV by visual inspection alone given the extended satellite structures and broad spectra. The study by Missana et al. (10) was closest in design to our own. Therefore, it is relevant that the composite U4f7/2 peak for their XPStailored experiment at pH 5 indicated a more oxidized U signature than in Scott et al. (12), with a binding energy of 381.2 eV after 15 days of reaction (readjusted to C1s at 285 eV), which is close to the BEs of the composite U4f7/2 spectra for Umag5 and Umag7 (SI Table S3). Missana et al. interpreted the spectrum to be composed of a mixedvalence UVI-UIV phase as the major component with both monovalent UVI and UIV species present as minor components. However, the spectrum does not include the satellite structures and simply consists of a broad, fairly symmetrical U4f7/2 peak. Consequently, one cannot exclude UV. They used 1 mmol/L UVI for the XPS-tailored experiments, which strongly suggests that precipitation of a uranyl phase should have occurred under their experimental conditions. As discussed below, this could have implications for reaction pathways and the attainment of redox equilibrium. Reaction Pathways. The discrepancy among previous U-magnetite studies (see Introduction) is conceivably due,

in part, to different forms of magnetite (including possible variations in magnetite stoichiometry) and experimental conditions. In the present experiments, the pH started in the acidic regime and evolved toward circum neutral values. The salient feature of our experiments is that UV formed but UIV was not detectable, despite sufficient free energy to drive the reaction: UO22+ + H2O + 2Fe3O4(mag) a UO2(c) (and U4O9) + 2H+ + 3Fe2O3(maghem). This implies that the system was not in redox equilibrium, where sorbed UV was stable for at least up to 56 days. Previous work demonstrated that UV was stabilized at the near surface of ferrous mica, possibly by polymerization with UVI and UIV (5). Analogous research on the solution chemistry of UV indicated that UVI-UV dimerization slowed the disproportionation of UV(aq) (33). It is plausible, therefore, that partial incorporation of U into a secondary Fe (oxyhydr)oxide phase (Umag8-11) and/or a uranyl precipitate (Umag1-5), as suggested by EXAFS, TEM and thermodynamic modeling, stabilized UV and prevented the attainment of redox equilibrium. Concomitant reduction and incorporation of U is also consistent with the drop in U/Fe and increase in UV/Fe ratios as recorded by XPS of samples Umag2-5 (Figure 2), despite the continual loss of U from solution (Table 1). It is possible that the lack of carbonate enlarged the stability field of UV relative to UVI at circum neutral pH. However, UV was also detected at ∼pH 5, where carbonate does not strongly affect U redox couples at ambient PCO2. Collectively, the evidence suggests that the reduction of UVI was influenced by dissolution/precipitation reactions, where the formation of secondary phases was driven by magnetite dissolution and increasing pH. These conditions set the stage for the competition between reduction kinetics and the formation of coordination environments that inhibited further reduction of UVI and UV. The alternative is that UV is stable under the experimental conditions, but such an assessment will require more accurate thermodynamic data. Virtually nothing is known about the environmental chemistry of UV as a stable species or intermediate. Further work is needed to elucidate the existence and roles of UV in environmental systems.

Acknowledgments Funding for this project was provided by DoE-OBES Geosciences and Chemical Sciences, and DoE OBER-ERSP through the PNNL Science Focus Area. Portions of this research were carried out at the Stanford Synchrotron Radiation Laboratory and the Environmental Molecular Sciences Laboratory at PNNL, national user facilities respectively operated by Stanford University on behalf of the U.S. DoE, OBES and by Battelle on behalf of the U.S. DoE, OBER.

Supporting Information Available Detailed description of procedures and approaches for both experimental and modeling efforts, and starting material characterization (including eight figures).This material is available free of charge via the Internet at http:// pubs.acs.org.

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