Surface Passivation of Magnetite by Reaction with Aqueous Cr(VI

present in groundwaters due to industrial or mining opera- tions. Once in the groundwater, Cr(VI) is highly mobile, posing hazards to humans as both a...
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Research Communications Surface Passivation of Magnetite by Reaction with Aqueous Cr(VI): XAFS and TEM Results M A R I A L . P E T E R S O N , * ,† A R T F . W H I T E , ‡ G O R D O N E . B R O W N , J R . , †,§ A N D GEORGE A. PARKS† Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, U.S. Geological Survey, 345 Middlefield Road, M.S. 420, Menlo Park, California 94025, and Stanford Synchrotron Radiation Laboratory, Stanford, California 94309

Introduction Hexavalent chromium, Cr(VI), is a contaminant that may be present in groundwaters due to industrial or mining operations. Once in the groundwater, Cr(VI) is highly mobile, posing hazards to humans as both a toxin and a suspected carcinogen. Magnetite, an Fe2+-bearing oxide mineral common in many aquifers, has recently been studied for its capacity to reduce Cr(VI) to the less mobile Cr(III) in laboratory (1-3) and field (4, 5) studies. Magnetite persists over millions of years in rocks and soils and might be an excellent material for in-situ remediation of Cr(VI)-contaminated, magnetitebearing soils. However, we show that the magnetite surface passivates by reaction with aqueous Cr(VI) at pH 7, limiting the capacity of magnetite to act as an electron donor for Cr(VI) reduction at neutral pH. The local structure and the oxidation state of aqueous contaminants associated with mineral surfaces can be determined with minimal sample preparation by X-ray absorption fine structure (XAFS) spectroscopy. Many reviews of the technique and its applications in environmental sciences have been published, (e.g., refs 6-8) as have studies of oxidation-state determination using XAFS on metalcontaminated soils (e.g., refs 9-11) and on metal oxides in the laboratory (e.g. refs 2 and 12). Cr oxidation state and local structure may be determined by examining the X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) regions of the spectra. XANES spectra for Cr(VI) compounds include a prominent pre-edge feature that is attributed to a 1sf3d electronic transition, normally a forbidden transition, but one that is allowed by the lack of a center of inversion symmetry in the tetrahedral Cr(VI)O4 structure. In contrast, octahedral Cr(III)O6 compounds have only minor pre-edge features, allowing the size of the pre-edge feature to be calibrated for Cr(VI)/total Cr determination (e.g., refs 4 and 9). The EXAFS region also shows significant differences between Cr compounds in 3+ and 6+ oxidation states. Amplitude and frequency of oscillations in the EXAFS region are functions of the number, distance, and chemical identity of neighbor atoms to the absorber atom. Because Cr(VI) compounds tend to occur in tetrahedral coordination with * Author to contact for correspondence. Phone: 415-723-4152; fax: 415-725-2199; e-mail: [email protected]. † Stanford University. ‡ U.S. Geological Survey. § Stanford Synchrotron Radiation Laboratory.

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oxygen, with Cr-O distances of ≈1.6 Å, and Cr(III) compounds tend to occur in octahedral coordination with oxygen, with Cr-O distances of ≈2.0 Å, the EXAFS spectra of Cr(VI) and Cr(III) compounds have distinct amplitudes and frequencies. The EXAFS can be Fourier transformed, revealing first-shell Cr-O and second-shell and farther metal atom distances; the EXAFS can also be fit using theoretical modeling codes to determine the chemical identity, to within two elements of the central absorber (Z ( 1-2), and the number and distance of neighboring atoms. Thus, the Cr oxidation state can be determined by pre-edge peak size, Cr-O distance, or number of first-shell oxygen atoms around Cr, and the local coordination geometry of Cr out to ≈ 4-5 Å can be determined by EXAFS fitting. In this paper, we use a combination of XAFS and other analytical techniques to follow reactions between aqueous Cr(VI) and synthetic magnetite. A combination of XAFS and other analytical techniques was used to monitor solution Cr concentration, the oxidation state and local structure of surface-associated Cr, and changes in the surface structure of magnetite with reaction time.

Materials and Methods Magnetite was synthesized using a method described previously (2), modified from the method of Schwertmann and Cornell (13). Its surface area was 23.8 ( 0.1 m2 g-1 by BET N2 adsorption analysis, and its phase purity was verified by X-ray diffraction. High-resolution transmission electron diffraction (TEM) of the magnetite showed highly euhedral, octahedral crystals ranging in diameter from 25 to 150 nm. Two sets of Cr(VI) reduction kinetics experiments were performed: (1) short-term experiments, using an initial solution composition of 2 mM Na2CrO4‚4H2O and 100 mM NaCl, and (2) long-term experiments, using an initial solution composition of 50 mM Na2CrO4‚4H2O and 50 mM NaCl. For both sets of experiments, the solid/solution ratio was 20 g of magnetite L-1. The 2 mM chromate concentration in the short-term reaction was selected to examine Cr(VI) reduction and sorption to the magnetite surface at approximately monolayer sorption densities. Surface site density is approximated by postulating that the surface complex is CrOOH and that the Cr surface site density is between 1 and 7 sites nm-2 as for R-FeOOH (14). A higher chromate concentration, 50 mM, was chosen for the long-term reaction to determine the amount of Cr(VI) that could be reduced to Cr(III) by reaction with magnetite at neutral solution pH before the surface would become passivated by depletion of Fe2+. For this long-term reaction, the chromate concentration was chosen to be greater than the amount of Cr(VI) that could theoretically be reduced to Cr(III) by complete reaction with all of the Fe2+ in stoichiometric magnetite. The value was calculated based on the premise that 3 Fe2+ atoms are oxidized for each Cr(VI) ion reduced, as required by the half-reactions 3Fe2+ f 3Fe3+ + 3e- and Cr6+ + 3e- f Cr3+. Cr(VI) reduction by reaction with magnetite in anoxic solution occurs by a reaction such as

6Fe3O4 (magnetite) + 2Na2CrO4 + 4H+ ) 9γ-Fe2O3 (maghemite) + 2CrOOH + 4Na+ + H2O assuming that the Cr(III) phase which forms is CrOOH. The

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FIGURE 1. Removal of Cr from solution by reaction with magnetite for (a) short-term reaction and (b) long-term reaction at pH 7 (squares). Also shown are the initial Cr(VI) concentrations (dashed lines) and Cr(VI) concentrations in a blank experiment with no magnetite (open circles). A linear fit to the g24-h reaction values in the long-term experiment gives a line with a slope of -0.002 ( 0.002 mM h-1, indicating that the apparent negative slope to the data is not statistically different from a zero slope. iron oxide reaction product, γ-Fe2O3, has been observed to be the initial magnetite oxidation product in aqueous solution (15, 16). All experiments were performed under anaerobic conditions in a glovebox containing 95% N2 and 5% H2 at room temperature (27 ( 1 °C), pH-statted at 7.0, and the reaction vessels were covered with aluminum foil to exclude ambient light and preclude the possibility of photoredox reactions. Blank reactions were performed: (1) with no chromate; (2) with no magnetite, other conditions being identical. The reaction vessels were continuously stirred, and aliquots of the solid-solution slurry were taken periodically for atomic absorption spectroscopy of the solution and XAFS spectroscopy and high-resolution transmission electron microscopy (TEM) analysis of the solid. The solid and solution were separated by filtration through 0.1-µm filters. For the longterm reactions, the solid was further rinsed with 100 mM NaCl at pH 7 to remove most of the supernatant and the loosely adsorbed Cr(VI). The solid for XAFS analysis was kept moist, and a portion for TEM analysis was dried in the glovebox. XAFS spectroscopy was performed at the Stanford Synchrotron Radiation Laboratory on wiggler magnet beamlines 4-2 and 4-3, and fluorescent X-rays were monitored. The chromite model compound, from Black Lake, Quebec, Canada, was powdered and mixed with BN for transmission XAFS data collection. For EXAFS modeling, the theoretical single- and multiple-scattering code FEFF6 (17) and fitting software EXAFSPAK (18) were used. Detailed experimental setup and data processing protocols are given elsewhere (2, 5).

Results and Discussion Cr(VI) uptake and reduction to Cr(III) was complete within 6 h for the short-term reaction, with rapid reduction (g2.8 × 10-9 mol m-2 s-1) occurring within the first 15 min, followed by slower reduction (1.0 × 10-11 mol m-2 s-1), possibly explained by fast Cr(VI) adsorption followed by slower reduction. These reduction rates for Cr(VI) reacted with synthetic magnetite bracket the linear Cr(VI) reduction rate (1.3 × 10-10 mol m-2 s-1) found for Cr(VI) reacted with ground natural magnetite (3). In the short-term reaction, complete removal of Cr from solution was observed (Figure 1a). The long-term reaction also showed rapid reduction of solution Cr followed by slower reduction, occurring during the first 24 h. Solution Cr(VI) concentration then reached a steady-state value at 83% of the initial concentration (Figure 1b). In the short-term reaction, the amount of Cr(VI) reduced and sorbed to the magnetite surface as Cr(III) corresponds

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to a surface coverage of 4.4 µmol m-2. In the long-term reaction, the initial Cr(VI) concentration was 25-fold higher, 50 mM, and yet the steady-state surface coverage increased less than 5-fold, to 21 µmol m-2. This result suggests that there is a limit to the capacity of magnetite to reduce Cr(VI) at pH 7, since complete reduction of the initial Cr(VI) by all of the Fe2+ atoms in the magnetite present should have caused the final Cr(VI) content in solution to stabilize at 57% of the initial Cr(VI) content rather than the observed final Cr(VI) concentration measured to be 83% of the initial Cr(VI) content. Cr K-edge XAFS (Figure 2) were collected for the initial Cr(VI) solution used in the long-term reaction, for aliquots of magnetite periodically removed from the long-term reaction vessel, and for a Cr(III) model compound, chromite (Fe2+Cr3+2O4). Phase and amplitude parameters used to fit the experimental EXAFS were derived from FEFF6-generated theoretical phase and amplitude functions for the chromite model. The local environments around the octahedrally coordinated metal atoms in chromite, magnetite, and maghemite all have the spinel structure and are therefore structurally quite similar: the number of oxygen atoms coordinating the metal, NO, is 6 for each mineral phase at a distance, RCr-O, of 2.007 Å for chromite (19), RFe-O ) 2.059 Å for magnetite (20), and RFe-O ) 2.088 Å for maghemite (21); second-shell metal distances are also similar, with NCr or Fe ) 6 at a distance RCr-Cr ) 2.962 Å for chromite (19), RFe-Fe ) 2.968 Å for magnetite (20), and RFe-Fe ) 2.952 Å for maghemite (21). Moreover, the local structure of the Cr(III) sorption complex is locally similar to that of the octahedrally coordinated metal atoms in these model structures, as evidenced by XAFS fit parameters summarized below. EXAFS spectra for the Cr(VI)-reacted magnetite samples are quite similar; both from the short-term experiments, using 2 mM initial Cr(VI), and the long-term experiments, where XAFS spectra were collected periodically after 3 h to 5 weeks of reaction time. Fits to the EXAFS spectra of the Cr/magnetite sorption complexes show distances between Cr and its nearest shell of oxygen atoms, RCr-O, of 1.975 ( 0.01 Å and secondshell distances between Cr and Cr and/or Fe atoms, RCr-(Cr,Fe), of 3.01 ( 0.01 Å. The Debye-Waller values were fixed at 0.003 Å2 based on fits to model compounds (2). The preedge of the Cr(VI)-reacted magnetite, a useful indicator of Cr oxidation state (4, 9, 12), resembles that of the Cr(III) model chromite (Figure 2). Models for the Cr sorption complex that are consistent with these fit parameters and pre-edge include Cr in a local structure like chromite, magnetite, or maghemite, where Cr is in +3 oxidation state, and Cr is substituted for octahedral Fe3+ in the iron/chromium oxide structure. Additional possibilities include a CrOOH phase present as small surface hydroxy polymers, as seen by Charlet and Manceau on hydrous ferric oxide (22); polymeric Cr(III) species, reported by Eggleston and Stumm on hematite (23); or combinations of two or more of these phases. Several factors preclude the use of chemical and structural differences between the proposed phases to further refine the sorbate geometry model: (1) Cr and Fe, or any pair of elements with Z ( 1-2, cannot be differentiated by XAFS analysis because of the similarity in their phase and amplitude functions; (2) resolution limits for the fits of our data (k ) 2.5-11.7 Å-1) limit differentiation of atomic distances to (0.17 Å; and (3) the error in second-neighbor N values may be as high as ( 50% (2). The distance between the absorber Cr and its secondneighbor metals correspond to all of the proposed models; however, the number of second-shell metal atoms (Cr and/ or Fe) is approximately half that expected for Cr in a spinel or γ-CrOOH phase. The number of second-shell metal atoms is, however, consistent with those in an R-CrOOH phase or for small polymers of the above-mentioned species. High-resolution TEM images of unreacted magnetite grains show bulk magnetite structure, which extends to the edges of the grains (Figure 3a). In contrast, magnetite grains

FIGURE 2. Cr K-edge X-ray absorption fine structure spectra: (a) XANES; (b) EXAFS; and (c) Fourier-transformed EXAFS over k ) 2.5-11.7 Å-1. The initial Cr(VI) solution (top) shows XANES and EXAFS characteristic of Cr(VI), with a prominent pre-edge feature at 5993 eV, four first-shell oxygen neighbors, and Cr-O distances of ≈1.6 Å. The Cr(VI)-reacted magnetite (middle) and Cr(III) model compound chromite (bottom) show XANES and EXAFS characteristic of Cr(III), with small pre-edge features, six first-shell oxygen neighbors, and Cr-O distances of ≈2.0 Å. The Cr(VI)-reacted magnetite and chromite EXAFS and Fourier transforms also show evidence of Cr and/or Fe shells. The chromite Fourier transform is labeled with the absorber-backscatterer single-scattering pairs or multiple-scattering trios responsible for the Fourier transform peaks. The chromite fit has been described elsewhere (2). dFe(2+) is the density of Fe2+ atoms in the magnetite structure (atoms nm-3), and As is the magnetite surface area (m2 g-1) multiplied by 1018 to convert m2 to nm2. The surface reaction product is responsible for passivating the magnetite surface and explains why the magnetite does not fully oxidize to maghemite at pH 7 by reaction with Cr(VI).

FIGURE 3. High-resolution TEM images of magnetite crystals oriented along the 〈110〉 zone axis: (a) unreacted magnetite; (b) magnetite reacted for 5 weeks in 50 mM Cr(VI) at pH 7. that had reacted with 50 mM Cr(VI) for 3 h to 5 weeks show a surface layer that does not possess the bulk magnetite structure (Figure 3b). No color change of the magnetite was observed. The observed range of thickness of the oxidized surface layer (10-20 Å) is close to that expected for magnetite to maghemite oxidation due to electron transfer from Fe2+ in magnetite to Cr(VI). The oxidized surface layer thickness, T (nm), on magnetite was calculated to be 2.4 nm (24 Å) by

T ) {3([Cr]i - [Cr]f)Na}/(RsdFe(2+)As × 1018) where [Cr]i and [Cr]f are the initial and final solution Cr concentrations (M), multiplied by 3 to account for 3 Fe2+ atoms oxidized for every Cr(VI) ion reduced, Na is Avogadro’s number (atoms mol-1), Rs is the solid/solution ratio (g L-1),

Findings and implications of this study include the following: (1) Bulk magnetite exists for time scales of >5 weeks in 50 mM Cr(VI) solution at pH 7, protected from further oxidation by the passivating effect of a thin layer of maghemite at its surface. This passive layer may also explain why magnetite, which is thermodynamically unstable in most soil environments, persists over geologic time scales. (2) Magnetite acts as an electron donor for Cr(VI) reduction, but the mineral passivates after 10-20 Å of the surface has converted to maghemite in pH 7 solution. This was shown by the unchanging solution Cr concentration after 24 h of reaction with magnetite at pH 7, XAFS evidence of Cr(III) at the magnetite surface reduced from a Cr(VI) solution, and TEM evidence of an oxidized surface layer on bulk magnetite. (3) Similar problems with surface oxidation and passivation are likely to occur in the application of zero-valent metal remediation strategies. Recently, numerous papers have been published proclaiming the potential of zero-valent metals such as Fe(0) for remediation of contaminated groundwaters by reductive transformation of inorganic contaminants such as chromate, or organic contaminants such as chlorinated solvents or hydrocarbons [see the ACS Symposium devoted to this topic (24)]. In many of these papers, it is observed that the reductive capacity of the zero-valent metal degrades over time, and some authors postulate that ferrous or ferric reaction products precipitate on the metal, slowing contaminant reduction rates. The results reported in this paper demonstrate that, for the ferrous-ferric oxide magnetite, the surface does indeed passivate by reaction with Cr(VI), limiting the remediation capacity of magnetite for Cr(VI) reduction at pH 7.

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Acknowledgments We acknowledge the Stanford Center for Materials Research for TEM facilities, the staff of SSRL for their support, and the NSF (Grant EAR-9406490) for funding this research. SSRL is funded by the DOE and the NIH.

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Received for review October 9, 1996. Revised manuscript received January 7, 1997. Accepted January 13, 1997. ES960868I