Nanogoethite Formation from Oxidation of Fe(II) Sorbed on Aluminum

Apr 21, 2010 - A specific surface area of 6 m2 g−1 was measured by N2 adsorption ..... formed from heterogeneous oxidation of Fe(II) can influence r...
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Environ. Sci. Technol. 2010, 44, 3765–3771

Nanogoethite Formation from Oxidation of Fe(II) Sorbed on Aluminum Oxide: Implications for Contaminant Reduction P H I L I P L A R E S E - C A S A N O V A , * ,† DAVID M. CWIERTNY,‡ AND MICHELLE M. SCHERER Department of Civil and Environmental Engineering, University of Iowa, 4105 Seamans Center, Iowa City, Iowa 52242-1527

Received October 17, 2009. Revised manuscript received March 31, 2010. Accepted April 1, 2010.

Ferrous iron [Fe(II)] bound to mineral surfaces has been shown to reduce several important groundwater contaminants, but little is known of the nature of the newly formed, insoluble ferric iron [Fe(III)] and whether it influences the heterogeneous contaminant reduction process. To explore how the formation and evolution of the Fe oxidation products influences contaminant reduction, we measured the kinetics of nitrobenzene reduction by Fe(II) sorbed on R-Al2O3 while simultaneously characterizing the Fe oxidation product with Mo¨ssbauer spectroscopy and electron microscopy. After a brief period of slow kinetics, the onset of nitrobenzene reduction coincided with a change in particle suspension color from white to yellowocher due to formation of nanogoethite rods (R-FeOOH) from oxidation of sorbed Fe(II). Formation of nanogoethite on the R-Al2O3 particles appears to promote the rapid reduction of nitrobenzene. Our results show that nanogoethite crystals can form rapidly by heterogeneous Fe(II) oxidation, and formation of goethite can profoundly influence contaminant reduction rates by Fe(II).

Introduction Fe(II) in anoxic subsurface environments is an important source of electrons for reducing toxic anthropogenic contaminants. Reduction of organic contaminants can occur by various forms of Fe(II), including dissolved Fe(II) (1), complexed Fe(II) (2), surface-complexed Fe(II) on minerals (3-7), and structural Fe(II) in minerals (8). Complexing ligands, whether dissolved or surficial, are fortuitous to contaminant reduction because they can dramatically increase the reaction rate and extent by relieving thermodynamic or kinetic restraints that hinder electron transfer from aqueous Fe(II) alone (9, 10). In consideration of the abundance of iron in the subsurface, the prevalence of Fe(II) due to microbial respiration of Fe(III), and implications of Fe(II) as a reductant within contaminant plumes (11), a considerable body of work has examined the ability of mineral surfaces including iron, aluminum, and silicon oxides (3, 6) to couple the sorption * Corresponding author phone: (203) 436-4049; fax: (203) 4323134; e-mail: [email protected]. † Present address: Department of Geology & Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520-8109. ‡ Present address: Department of Chemical & Environmental Engineering, University of CaliforniasRiverside, B0363 Bourns Hall, Riverside, CA 92521. 10.1021/es903171y

 2010 American Chemical Society

Published on Web 04/21/2010

of aqueous Fe(II) and the reduction of organic contaminants including nitroaromatics (3, 7, 8, 12-14), chlorinated solvents (13, 15, 16), and pesticides (4, 6). The process of heterogeneous contaminant reduction by Fe(II) begins with the sorption of aqueous Fe(II) onto structural oxygen ligands and the formation of an innersphere Fe(II) surface complex. The Fe(II) surface complex may be electronically stable when formed on a nonconducting mineral (such as aluminum or silicon oxides) or, when it is sorbed to a semiconducting substrate (such as iron or manganese oxides), may result in electron transfer to a metal cation within the underlying crystal lattice (17, 18). The contaminant can then form an inner- or outer-sphere complex with the surface and receive electrons from surfacecomplexed Fe(II) or through the semiconducting substrate. Regardless of the electron transfer pathway, the rate of contaminant reduction can be rapid (within minutes) or very slow (within weeks). Because a contaminant can exhibit a wide range of reaction rates among mineral substrates with sorbed Fe(II) (3, 6, 13), individual minerals are thought to have unique inherent “reactivities”, often described by physical properties such as the amount or density of sorbed Fe(II) (6), the number and type of surface sites, and the speciation of modeled Fe(II) surface complexes (4) as well as electrochemical properties such as potential and resistance (10). Mathematical models for heterogeneous contaminant reduction also allow insight to the reaction mechanism, where contaminant and Fe(II) concentrations are monitored in the solution phase and electron transfer processes are inferred at the mineral surface. Reaction rates are usually described with (pseudo)first-order (3, 4, 6, 7, 13, 14) or second-order (19, 20) expressions with respect to contaminant or Fe(II) concentrations. Proposed reaction mechanisms, however, have been difficult to validate because of several observations that defy conventional models. These observations have led to speculation that more complex interfacial physical or electronic processes are occurring, such as conduction through the bulk structure (21) or changes in the mineral surface. As an example of a changing mineral surface, Klausen et al. (ref 3, Figure 10) observed an autocatalytic rate of nitrobenzene reduction by Fe(II) on aluminum oxide while also noting the development of a yellow-ocher color appearance, presumably an iron oxide, in the particle suspension. Pecher et al. (16) also observed a color change from Fe(II) exposure to goethite and posited a suite of surface “remodeling” reactions. Elongation and tip roughening of goethite crystals, but no color change, was also reported (22) after multiple cycles of Fe(II) sorption and oxidation by nitrobenzene and a concomitant decrease in nitrobenzene reaction rates. It is clear from these studies that the newly formed Fe(III) atoms from Fe(II) sorption and oxidation likely influenced reaction rates by surface processes that possibly include altering the mineral surface structure, forming secondary iron mineral precipitates, or otherwise creating new reactive surface sites. Identifying subtle changes in mineral surfaces as a result of Fe(II) sorption and oxidation, however, has been challenging because it is difficult to detect small amounts of Fe(III) and distinguish it from the underlying mineral substrate. The yellow-ocher solid observed by Klausen et al. (3) was not detectable by X-ray diffraction nor by Fourier transform infrared spectroscopy (FTIR). Microscopic techniques, such as atomic force microscopy, have been useful for assessing the morphology of the oxidation production but do not provide direct information on the VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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identity of Fe(III) surface precipitates (15). Oxidation products of aqueous Fe(II) typically include ferrihydrite, lepidocrocite, and goethitesdepending on solution pH, oxidation rate, and presence of foreign or complexing anions or cationssand the Fe(III) hydroxide evolved on mineral substrates is likely one or more of these forms. Here, we take advantage of the isotope specificity of 57 Fe-Mo¨ssbauer spectroscopy to track the evolution of the heterogeneous 57Fe(II) oxidation product on R-Al2O3 during reaction with nitrobenzene. Aluminum oxide was chosen because they have been used as sorbents (3, 5-7, 10, 14, 23, 24). Our results show that nanogoethite crystals form rapidly by heterogeneous Fe(II) oxidation and provide evidence that secondary mineral formation can significantly impact rates of contaminant reduction by Fe(II).

Experimental Section Chemicals. R-Al2O3 powder was purchased from Alfa Aesar and stored in an anoxic chamber (95% N2, 5% H2) for approximately 2 months prior to use to allow sorbed O2 to diffuse away. More details on the anoxic chamber are given in Supporting Information. Deionized water was deoxygenated by N2 sparging and used for all reactor solutions and aqueous stock solutions. An acidic 57Fe(II) solution (96% purity) was prepared by dissolving 57Fe(0) metal in anoxic 0.5 M HCl solution and filtering with a 0.2 µm poly(tetrafluoroethylene) (PTFE) filter. Nitrobenzene and aniline stock solutions were prepared in deoxygenated methanol. Fe(II) Sorption and Nitrobenzene Reduction Experiments. All experiments were conducted in an anoxic chamber. Nearly identical batch reactors were assembled containing a Fe(II) solution, R-Al2O3, and nitrobenzene and were run staggered and not simultaneously. Fe(II) solutions were prepared by spiking Fe(II) stock solution to a final concentration of about 1 mM in a solution of 25 mM KBr electrolyte (to poise ionic strength) and 25 mM N-(2hydroxyethyl)piperazine-N′-2-ethanesulfonic acid (HEPES) (to buffer pH), with pH adjusted to 7.5 by addition of 0.5 M KOH or HCl. The Fe(II) solution was allowed to equilibrate for at least 12 h before filtration through a 0.2 µm PTFE filter to remove any possible precipitates formed from Fe(II) addition [this filtration step showed no change in aqueous Fe(II) concentration within other solutions that tested this step]. Aluminum oxide was added to a concentration of 6 g L-1, and Fe(II) sorption was allowed for 2 h prior to nitrobenzene addition. Samples for nitrobenzene and Fe(II) and were filtered through 0.2 µm PTFE filters before analysis. At the end of the allotted time period, a portion of the solids were filtered for Mo¨ssbauer, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) analyses. Due to the high R-Al2O3 concentration, this filtering step took about 4-12 min, and the reported sample time is the midpoint of filtering. Solution conditions for each batch reactor are listed in Table S1 in Supporting Information. Chemical Analyses. Samples for aqueous Fe(II) were preserved in 0.5 M HCl and stored in the chamber until analysis, which was performed colorimetrically with the 1,10phenanthroline method (25) immediately after removal from the chamber. Samples for nitrobenzene and aniline were stored within the chamber until termination of the batch reactor and were analyzed shortly thereafter by HPLC with a C-18 column and an eluent of 70% acetonitrile and 30% 0.5 g L-1 ammonium acetate aqueous solution at 1 mL min-1 flow rate. Detection was with a UV-vis detector at 254 nm for nitrobenzene and 235 nm for aniline. Solids Characterization. Samples for TEM imaging were prepared by filtering solids from batch reactors and resuspending them in deoxygenated methanol, from which samples were deposited on carbon-coated copper grids and allowed to dry. Solids for SEM imaging were filtered and 3766

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FIGURE 1. Disappearance of nitrobenzene in the presence of aqueous Fe(II) and r-Al2O3 within the 93-min batch reactor. The r-Al2O3 suspension contained 939 µM initial aqueous Fe(II), 132 µM nitrobenzene, 25 mM KBr, and 25 mM HEPES buffer at pH 7.5 pre-equilibrated for 2 h with 6 g L-1 r-Al2O3 [149 uM Fe(II) had sorbed]. The control solution contained 1021 µM initial aqueous Fe(II), 111 µM nitrobenzene, 25 mM KBr, and 25 mM HEPES buffer at pH 7.5. X-marks on the time axis indicate approximate sampling times for sacrificed batch reactors whose Mo¨ssbauer spectra appear in Figure 2. Nitrobenzene disappearance profiles for all batch reactors appear in Figure S3 in Supporting Information. placed on aluminum stubs and allowed to dry. All sample preparation and storage took place in the anoxic chamber. The prepared samples were transported to the microscopes within an anoxic vessel, and sample transfer from the vessel to the microscope chamber allowed brief exposure to air. Scanning electron microscopy was performed on dry solids with a Hitachi microscope. Transmission electron microscopy was performed on solids with a Jeol microscope. The purity of the aluminum oxide was examined by powder X-ray diffraction with a Cu KR radiation source (the pattern is in Figure S1 in Supporting Information), and narrow reflections were observed, indicative of high crystallinity. A specific surface area of 6 m2 g-1 was measured by N2 adsorption with Brunauer-Emmett-Teller analysis. The samples taken for 57Fe Mo¨ssbauer analysis were preserved between two layers of oxygen-impermeable Kapton tape and immediately placed within the spectrometer cryostat, which operated in a helium atmosphere that prevented Fe(II) oxidation by O2. Mo¨ssbauer analysis was performed with a 57Co source at room temperature with linear acceleration in transmission mode. The temperature of the sample was varied with a Janis cryostat, and spectra collection lasted up to about 10 h per temperature. Spectra were calibrated against spectra of R-Fe(0) and modeled by use of Voigt-based models within Recoil software.

Results and Discussion Kinetics of Nitrobenzene Reduction by Fe(II) Sorbed on r-Al2O3. Nitrobenzene was rapidly reduced by Fe(II) sorbed on R-Al2O3 at pH 7.5 after 93 min (Figure 1). Nitrobenzene reduction by aqueous Fe(II) alone was negligible. The formation of aniline was observed in a separate batch reactor, confirming reduction of the nitro group and accounting for 57% of reacted nitrobenzene (Figure S2 in Supporting Information). An intermediate compound was also observed within HPLC chromatograms but was not quantified. The kinetics of nitrobenzene reduction by Fe(II) sorbed on R-Al2O3 appears autocatalyticsan initial slow kinetic

FIGURE 2. Mo¨ssbauer spectra of 57Fe(II) sorbed on r-Al2O3 (0 min) and of 57Fe(II) reacted with nitrobenzene on r-Al2O3 (2-93 min). Circles are data points, and solid lines are model fits. Hatched-filled sextets are goethite. Unfilled doublets are Fe(II) and Fe(III) doublets and are distinguished by the indicated line positions. The scale bar represents 0.5% absorption and differs in length for each spectrum. Analysis temperature was 13 K. Parameters for model fits are listed in Table 1. Conditions for the batch reactors are listed in Table S1 in Supporting Information. phase followed by rapid kinetics when a product of the reaction catalyzes, or promotes, further reaction. The rate of reaction is initially negligible and the aqueous concentration of nitrobenzene remains constant for about 30 min. Following this slow kinetics phase, nitrobenzene is rapidly reduced and follows first-order kinetics as indicated by the straight line on the log C/C0 versus time plot (Figure 1). The transition between the slow kinetics and rapid kinetics for nitrobenzene reduction was reproducible among multiple batch reactors containing R-Al2O3 (Figures S2 and S3 in Supporting Information). Moreover, the loss of aqueous Fe(II) mirrored both the slow and rapid kinetics of nitrobenzene reduction (Figure S2), suggesting aqueous Fe(II) is a necessary reservoir for continued Fe(II) sorption and surface-promoted Fe(II)-driven reduction of the contaminant (16, 18, 19). The amount of Fe(II) initially sorbed (143 µM) was less than the required amount of Fe(II) (346 µM) to form aniline, based on the stoichiometric requirement of 6 µmol of Fe(II) for each µmol of aniline formed (as a measure of nitrobenzene reduced).

The apparent autocatalytic kinetics for nitrobenzene reduction and aqueous Fe(II) loss are similar to the kinetic behavior reported by Klausen et al. (3) for nitrobenzene reduction by Fe(II) with γ-Al2O3 Aluminoxid C, for pentachloronitrobenzene reduction by heterogeneous Fe(II) (26), and for Fe(II) oxidation by O2 (27, 28). In fact, the autocatalytic kinetic model for Fe(II) oxidation by O2 (27), which contains a homogeneous and a heterogeneous reaction term, reasonably fits the dual-phase nitrobenzene reduction here (Figure S4 in Supporting Information). The autocatalytic kinetic behavior observed here and by Klausen et al. (3), however, has not been observed by others for reduction of other contaminants by Fe(II) on aluminum oxides. Others report pseudo-first-order kinetics with respect to contaminant concentration without an initial slow kinetic phase (5-7, 10, 14), even with the same γ-Al2O3 Aluminoxid C substrate (6, 10). It is unclear why some Al2O3 exhibit an initial slow phase and others do not. A likely explanation is differences in speciation of the surface-complexed Fe(II), such as strong versus weak sites or hydroxylated versus nonhydroxylated (4, 10, 14, 24), which are controlled by the solution conditions and oxide type and may have different electrochemical activities such as redox potential, charge transfer resistance, or interfacial charge distribution as suggested by computational models (29) and recent electrochemical evidence (10). Other possibilities include differences in amount or density of sorbed Fe(II), in the reduction mechanism for each contaminant, or in the substrate’s ability to form new Fe(III) surfaces. Another observation common to our study and that of Klausen et al. (3) is we both observed a distinct change in the color of the suspension. After the slow kinetic phase, the white aluminum oxide suspension acquired a yellow-ocher color indicating the presence of a Fe(III) hydroxide, such as goethite (R-FeOOH). Klausen et al. (3) found the yellowocher solids, after filtration, to be invisible to identification with powder X-ray diffraction and FTIR spectroscopy, possibly because the yellow-ocher solids were too poorly crystalline for characterization. Likewise, powder X-ray diffraction did not reveal the identity of our yellow-ocher solid after filtration of solids at the end of the reaction (data not shown), likely due to the small amount (∼1 mg as FeOOH) of iron precipitated. Evolution of Heterogeneous Fe(II) Oxidation Product. Our use of 57Fe isotopes allows us to track the evolution of the heterogeneous Fe(II) oxidation products during nitrobenzene reduction using Mo¨ssbauer spectroscopy despite the small mass of iron sorbed on the aluminum oxide. Filtered solids were collected from sacrificed batch reactors before and after the onset of nitrobenzene reduction to identify Fe(III) products and to examine if a particular Fe(III) form can explain the rapid onset of nitrobenzene reduction. Prior to nitrobenzene addition, sorbed 57Fe(II) was observed on filtered R-Al2O3 solids as a nearly symmetrical doublet with model parameters consistent with ferrous iron in a highspin configuration and an octahedral coordination (〈CS〉 1.31 mm s-1, 〈QSD〉 2.76 mm s-1) (30) (Table 1 and Figure 2, t ) 0). Both the 〈CS〉 and 〈QSD〉 of the Fe(II) doublet differ significantly from those of frozen aqueous Fe(II) (∼1.39, ∼3.22) and solid ferrous hydroxide [Fe(OH)2] (∼1.24, ∼3.00), indicating that frozen aqueous Fe(II) and solid Fe(OH)2 were not present in our filtered samples. There was also no spectral evidence for any Fe(III) species, indicating that oxidation of Fe(II) by trace O2(g) was not significant within the batch reactor or within the cryostat. The Fe(II) doublet has parameters similar to values previously attributed to sorbed Fe(II) species on aluminum oxide (18), titanium oxide (18), hematite (31), Illite (32), and montmorillonite (33). This Fe(II) doublet may represent Fe(II) sorbed as one type of surface complex or as a collection of different complexes. The average amount of Fe(II) sorbed during the pre-equilibration step for all batch VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Model Parameters for Mo¨ssbauer Spectra (13 K) in Figure 2 Fe(II) doublet 〈CS〉, mm s-1 a

sample d

0 min 4 min 12 min 37 min 60 min 93 min goethiteh

1.31 1.31 1.35 ni 1.24 ni

〈QSD〉, mm s-1

Fe(III) doublet

b

2.76 2.74 2.65 ni 2.86 ni

RA,c % 100 100 9.0 ni 5.0 ni

Fe(III) sextet

〈CS〉, mm s-1

〈QSD〉, mm s-1

RA, %

g

ni ni 0.79 0.65 ni ni

ni ni 27.8 4.3 ni ni

ni ni 0.47 0.50 ni ni

〈CS〉, mm s-1

〈QSD〉, mm s-1

〈H〉,d T

Hp,e T

RA, %

identification

χ2,f %

ni ni 0.47 0.48 0.49 0.49 0.48

ni ni -0.19 -0.22 -0.22 -0.23 -0.25

ni ni 45.7 42.0 41.1 47.1 50.6

ni ni 49.1 49.6 49.7 49.7 50.6

ni ni 63.2 95.7 95.0 100

ni ni unidentified goethite goethite goethite

0.6 0.7 1.0 1.1 3.1 11.5

a 〈CS〉 is average center shift relative to R-Fe(0). b 〈QSD〉 is average quadrupole splitting distribution. c RA is relative area. 〈H〉 is average hyperfine field. Samples 12, 37, and 93 min contained two hyperfine field components, and sample 60 min contained three hyperfine field components. Multiple components were necessary to account for the broadened inner lines of the Fe(III) sextet peaks, which are attributed to Fe(III) in goethite undergoing magnetic relaxation effects. e Hp is the most probable hyperfine field. f χ2 is the reduced chi-squared value, which is an indication of the overall model goodness-of-fit. g The phase was not identified in the spectrum. h Mo¨ssbauer parameters are listed for synthetic goethite prepared by thermal conversion of ferrihydrite at pH 12 (36). Note that goethite 〈QSD〉 notably differs from those of ferrihydrite (∼0.02) and lepidocrocite (∼-0.04). Also, goethite 〈H〉 (and Hp) differs from that of hematite (∼54.0). d

reactors was 3.1 ( 1.6 Fe(II) atoms nm-2. This value is similar to a calculated value of 2.7 sites nm-2 (34); therefore all surface sites were likely occupied by Fe(II). Here, we assume the sorbed Fe(II) atoms formed inner-sphere surface complexes with R-Al2O3 because Fe(II) was shown to form inner-sphere complexes on TiO2, γ-AlOOH, and γ-Al2O3 on the basis of invariant sorption extent with respect to varying ionic strength (24). The Fe(II) doublet remained unchanged for at least the first 4 min after nitrobenzene reduction (Table 1 and Figure 2, t ) 4 min). Prior to the onset of rapid nitrobenzene reduction but later in the slow kinetic phase, some of the Fe(II) was oxidized to Fe(III), but the Fe(III) species were not identifiable (Figure 2, t ) 12 min). Qualitatively, both a Fe(III) sextet signal (indicating magnetic order) and a paramagnetic Fe(III) doublet (indicating a lack of long-range magnetic order) are apparent in addition to the Fe(II) doublet. Clear identification of these two Fe(III) species was not possible due to very low signal intensity and broad absorption peaks, but candidates do include (superparamagnetic)goethite, ferrihydrite, lepidocrocite, clusters of Fe(III)-hydroxide polymers, and (for the Fe(III) doublet only) sorbed Fe(III) monomers. Goethite is the likeliest possibility for the Fe(III) sextet on the basis of its similar QSD with goethite. Nevertheless, the observation of long-range magnetic order within the Fe(III) sextet signal is definitive evidence for oxygen-linked Fe(III)-Fe(III) solids. We suspect that, by 12 min, a Fe(III) (oxy)hydroxide formed and congregated on the R-Al2O3 surface but was not present in sufficient amount to promote the more rapid Fe(II) sorption and nitrobenzene reduction that occurred after 30 min. Around this time, only about 5 µM nitrobenzene and 100 µM Fe(II) disappeared (∼17 µmol m-2), which is similar to the few micromoles of nitrobenzene reduced during the slow kinetic phase observed in ref 3. After the onset of rapid nitrobenzene reduction, goethite was definitively identified. At about 37 min, the Mo¨ssbauer spectrum reveals a magnetic Fe(III) solid that closely resembles goethite but with broadened peaks and slightly diminished magnetic order. The inner line of each peak broadens toward the spectrum’s midpoint, indicating the incomplete magnetic ordering at this temperature due to nanosized crystallites or structural imperfections (e.g., defects or foreign elements) (35). We do not suspect the peak broadening is due to sample density because all 57Fe densities were below the ideal Mo¨ssbauer density for 57goethite (Table S1 in Supporting Information). At this point, nanosized crystallites are expected due to the small amount (73 µM) of Fe(II) oxidized. After more Fe(II) was oxidized at 60 min, goethite was more clearly present, along with a minor Fe(II) 3768

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doublet that could be Fe(II) sorbed to R-Al2O3 or goethite or even Fe(II) incorporated within the goethite structure. Goethite was the only iron species identified at 93 min when the reaction was stopped, and its peaks were narrow and

FIGURE 3. Temperature profile of Mo¨ssbauer spectra of the solids collected from the batch reactor containing r-Al2O3, 57 Fe(II), and nitrobenzene after 93 min. The superparamagnetic ferric iron doublet at 298 K transitions to a magnetically ordered sextet by 13 K. Circles are data points, and the solid line at 13 K is a model fit of goethite.

FIGURE 4. Scanning electron micrographs (left column) and transmission electron micrographs (right column) of solids collected from suspensions of r-Al2O3, 57Fe(II), and nitrobenzene after 4 and 93 min of reaction time. Solution conditions are listed in Table S1 in Supporting Information. White arrows indicate needle-shape solids apparently following the curved features of the parent r-Al2O3. well-defined. The identification of goethite within the 37-, 60-, and 93-min samples is certain, based on their quadrupole splitting distribution values being similar to the characteristic value of goethite and not lepidocrocite or ferrihydrite. The possible nanocrystalline nature of the formed goethite was further explored by examining the temperature dependence of the hyperfine magnetic field. The well-resolved goethite spectrum at 13 K acquires a diminished hyperfine magnetic field at warmer temperatures (Figure 3), which is not usually observed for synthetic goethite formed from standard mineral recipes (36). The antiferromagnetic sextet gradually collapses to a superparamagnetic Fe(III) doublet as temperature increases from about 77 to 298 K. The goethite fails to sustain its crystallite magnetic domains above 77 K because the ambient thermal energy overcomes the magnetic anisotropy energy, a phenomenon of superparamagnetism common to goethite nanoparticles or to goethite with physical defects (35). The disappearance of peak broadening over time in Figure 2 is consistent with maturing size or crystallinity of goethite particles and supports the notion that superparamagnetism due to nanosize was responsible for the peak broadening at 13 K at earlier times. To determine the goethite crystallite size and morphology, SEM and TEM images were collected prior to the onset of nitrobenzene reduction (t ) 4 min) and after the rapid nitrobenzene reduction (t ) 93 min) (Figure 4). At 4 min, early in the slow kinetic phase and before the onset of nitrobenzene reduction, no rodlike features were present in SEM and TEM images. Some small, irregular-sized (10-100 nm) particles could be seen in the SEM image among the larger parent aluminum oxide particles (about 0.2-2 µm in size), but these were all present in the as-received R-Al2O3. At 93 min, clear acicular rod crystals consistent with goethite were ubiquitous. The rod lengths, which were between 50 and 200 nm and sometimes curved with the surface topology of the underlying aluminum oxide, were certainly small enough to explain the observed superparamagnetism. No clear rod crystals were observed in any other TEM or SEM samples prior to 93 min (data not shown).

Observing nanogoethite at the beginning of and during rapid nitrobenzene reduction (but not during the slow kinetic phase) provides compelling evidence that the newly formed nanogoethite is responsible for promoting the reaction rate. The reactive Fe(II) species probably moves from an Al2O3bound Fe(II) species (at early time points when the reaction is slow) to a Fe(III) solid-associated Fe(II) species (at later time points when the reaction is rapid and first-order). Progressive formation of insoluble Fe(III) results in Fe(III) hydroxide nucleation and crystal growth. Nanogoethite formation could promote nitrobenzene reduction by providing either additional surface reactive sites, surface reactive sites with a greater inherent reactivity, or a combination of the two. Klupinski et al (26) showed autocatalysis of nitrobenzene reduction with Fe(II) and a small amount of iron oxide nanoparticles, presumably because an increasing number of reactive sites were being created during reaction progress, as evidenced by an increase in particle abundance observed with photon correlation spectroscopy. Goethite may have a greater inherent reactivity compared to Al2O3 for coupling Fe(II) sorption and contaminant reduction, as pseudo-first-order kinetics with goethite can be much faster (13, 23) [although not always (6)] than with non-iron substrates for a given contaminant. Here, the autocatalytic kinetic model (in Supporting Information) reveals the reaction term associated with goethite is over 700 times greater than that with R-Al2O3. The formation of goethite through this heterogeneous Fe(II) oxidation is somewhat unexpected when it is considered that lepidocrocite is usually produced by the homogeneous oxidation of purely aqueous Fe(II) with molecular oxygen at circumneutral pH without significant foreign ions (36-38). With the same chemical formula but different packing structures, goethite (R-FeOOH) and lepidocrocite (γ-FeOOH) are similar enough to possibly form simultaneously during oxidation of Fe(II) (38), but their abundances can be manipulated by analytes, such as carbonate species (37), that interfere with structures during crystal formation and growth. With our finding of strictly goethite, we speculate VOL. 44, NO. 10, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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the aluminum oxide surface could be acting as a template for goethite formation. Once surface-complexed Fe(II) is oxidized to Fe(III), the oxygen surface atoms of R-Al2O3 might be arranged in a way that promotes combination of surfacecomplexed Fe(III) to favor the corner-sharing rows of Fe(III)-hydroxide octahedra of goethite as opposed to the edge-sharing rows of Fe(III)-hydroxide octahedra of lepidocrocite (37). Others have observed a templating effect during heterogeneous sorption and oxidation of Fe(II) on aluminum or aluminosilicate substrates. Hematite was formed on R-Al2O3 after sorption of Fe(II) and oxidation with Tc(VII) or O2, possibly because R-Al2O3 and hematite (R-Fe2O3) are isostructural and have similar interatomic distances for surficial oxygen atoms (5). One possible reason for the discrepancy between goethite here and hematite in ref 5 is that Tc(VII) is likely reduced via an inner-sphere electron transfer mechanism whereas nitrobenzene is likely reduced via an outer-sphere electron transfer mechanism (12). The electron transfer mechanism may influence the hydrolysis and precipitation of Fe(III) (15), as different Fe(III) product morphologies were formed by Fe(II) sorption and oxidation on a phlogopite single crystal with different classes of contaminants. The oxidants Hg(II) and As(V), via inner-sphere electron transfer, produced 20-30 nm size spheres, whereas trichloroethene, presumably via outer-sphere electron transfer, produced 50 nm size needles, similar to the goethite produced here by reaction with nitrobenzene. Moreover, the needles observed by Charlet et al. (15) appeared oriented along crystallographic bond chain directions on the (001) phlogopite face, indicating epitaxial growth. Goethite epitaxy on R-Al2O3 might be occurring here (Figure 4) where some needles align with the curved aluminum oxide surfaces. Overall, the autocatalytic reduction rate of nitrobenzene reveals a complex kinetic regime that likely results from different types of Fe(II) reductants at different timessFe(II) sorbed on Al2O3 at early times and Fe(II) reacted with nanogoethite at later time. Our results and others (3, 16, 22, 26) demonstrate that the insoluble Fe(III), and specifically goethite, formed from heterogeneous oxidation of Fe(II) can influence rates of contaminant reduction. Precisely how the formed Fe(III) and the parent mineral structure interact, and how the different electron transfer mechanisms direct different Fe(III) forms, requires further study. Our results and those in ref 15 also indicate that surface-mediated Fe(II) sorption and oxidation may be a viable pathway for iron oxide nanoparticle formation in subsurface environments. Iron oxide nanoparticle nucleation by heterogeneous oxidation of Fe(II), coupled with the reduction of aqueous oxidants, might compete with nucleation by homogeneous oxidation of Fe(II) (e.g., by O2). A rapid heterogeneous Fe(II) oxidation process might explain the recent reports of widespread occurrence of nanogoethite in sediments (39), although its formation can also be explained by Fe(III) precipitation and rearrangement processes occurring at much slower rates (39, 40). For Fe(III) oxide formation via Fe(II) oxidation, the presence of a templating substrate may direct Fe(III) oxide product and particle size distributions, which, considering the many size-dependent physical and chemical properties of oxides, may have implications for the type of reactive surfaces for contaminant transformation, the exchangability of iron between dissolved and solid forms (41), and even the bioavailability of iron for microbial respiration.

Acknowledgments This work was supported by a National Science Foundation NIRT Grant (EAR-0506679) and an NSF Graduate Research Fellowship to P.L.-C. We thank Timothy Strathmann at the University of Illinois Urbana-Champaign for helpful dis3770

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cussions and Christopher Gorski at the University of Iowa for electron microscopy assistance.

Supporting Information Available Solution conditions, anoxic chamber conditions, an X-ray diffraction pattern, additional kinetic data, and a model of nitrobenzene reduction by Fe(II) with R-Al2O3. This material is available free of charge via the Internet at http:// pubs.acs.org.

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