Coupled Redox Transformation of Chromate and ... - ACS Publications

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Coupled Redox Transformation of Chromate and Arsenite on Ferrihydrite Elizabeth B. Cerkez,† Narayan Bhandari,†,⊥ Richard J. Reeder,‡ and Daniel R. Strongin*,† †

Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, United States Department of Geosciences, Stony Brook University, Stony Brook, New York 11794, United States

‡

S Supporting Information *

ABSTRACT: The redox chemistry of chromate (Cr(VI)) and arsenite (As(III)) on the iron oxyhydroxide, ferrihydrite (Fh), was investigated. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS) were used to determine the composition of the adsorbed layer on Fh during and after exposure to solution-phase Cr(VI) and As(III). The individual exposure of Cr(VI) or As(III) on Fh resulted in the adsorption of the respective species, and there was no change in the oxidation state of either species. In contrast, exposure of Fh simultaneously to Cr(VI) and As(III) led to an adsorbed layer that was primarily Cr(III) and As(V). This redox transformation occurred over various experimental conditions at pH 3, 5, and 7 and in the presence or absence of O2, as demonstrated by in situ ATR-FTIR results. A similar redox transformation was not observed at a solution of pH 9, due to minimal Cr(VI) adsorption. Postreaction XPS showed that the majority of adsorbed arsenic existed as As(V) at pH 3, 5, and 7, while As(III) was the main species detected at pH 9. At pH 3 the redox chemistry between Cr(VI) and As(III) led to a As(V) product surface loading of ∼600 mmol/kg. Experiments performed in the absence of dissolved O2 resulted in less As(V) on the surface compared to experiments in which O2 was present for equivalent reaction times.

1. INTRODUCTION The interaction of toxic inorganic contaminants in the environment is becoming an increasingly important area of study as abandoned industrial sites are reclaimed for new uses. The toxicity, mobility, and fate of various inorganic contaminants are often influenced by their redox speciation, and hence, a microscopic understanding of the critical redox chemical processes would be expected to help in the development of effective remediation strategies.1 Two inorganic contaminants relevant to the present contribution are chromium and arsenic. In general, chromium in the environment is in the form of trivalent [Cr(III)] and hexavalent [Cr(VI)] species. The toxic hexavalent species occur mainly as the oxyanions chromate, CrO42−, and dichromate, Cr2O72−. Cr(III) forms a sparingly soluble precipitate (Cr(OH)3) in the pH range of ∼6−11.5; only below pH 4 does it exist primarily as the Cr3+ ion. At pH values below 9, inorganic arsenite, As(III), mainly occurs as a neutral species (H3AsO3), while arsenate, As(V), generally exists as the oxyanions H2AsO4− (pH 2−6.8) and HAsO42− (pH 6.8−12). Unlike Cr compounds, all As species are generally considered toxic to human beings,2,3 although As(III) is more toxic and mobile than As(V).4 In the environment, Cr(VI) and As(III) are found in a variety of settings that include soil, groundwater, and industrial wastewater. An analysis of Cr and As speciation at particular acid mine drainage (AMD) sites, for example, has shown significant amounts of Cr(VI) and As(III) in the aqueous phase.5 Additionally, the US Environmental Protection © 2015 American Chemical Society

Agency estimates that Cr and As are two major heavy metal(loid)s present in most superfund sites. Significant prior research has investigated the individual redox chemistries of Cr(VI) and As(III) in solution,6−9 as well as the individual adsorption behaviors of Cr(VI) and As(III) on environmentally relevant surfaces that include iron (oxy)hydroxides.10−14 Research presented in this contribution builds on prior surface related studies and addresses the redox chemistry of Cr(VI) and As(III) on the environmentally relevant iron oxyhydroxide, ferrihydrite (Fh). The motivation for the current study is 2-fold. First, a study of the coadsorption of Cr(VI) and As(III) on environmentally relevant surfaces has potential relevance to the understanding of redox chemistry occurring in environmental settings that contain both contaminants.5,15−17 Second, on a more fundamental level, results presented in this contribution shed light on how a substrate can facilitate electron transfer between two redox active species. Composite redox reactions between Cr(VI) and As(III) can be written as 2HCrO4 − + 3H3AsO3 + 5H+ → 2Cr 3+ + 3H 2AsO4 − + 5H 2O Received: Revised: Accepted: Published: 2858

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November 19, 2014 February 5, 2015 February 6, 2015 February 6, 2015 DOI: 10.1021/es505666w Environ. Sci. Technol. 2015, 49, 2858−2866

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Environmental Science & Technology

in prior publications from our laboratory that investigated the redox chemistry of As(III) on iron oxyhydroxides.27,28 In brief, 50 mg of dry solid powder, Fh, was suspended in 199 mL of 2.5 mM NaCl solution and then sonicated for 5−10 min to disperse the solid phase into solution. Appropriate amounts of As(III) and Cr(VI) solutions (from individual 50 mM stock solutions prepared from NaAsO2 and K2Cr2O7) were added to the suspension, and the pH was maintained at a desired value using an autotitrator (718 STAT Titrino, Metrohm). Experiments were performed in solutions that were exposed to the ambient atmosphere and hence contained dissolved O2 and CO2, unless otherwise noted. Other experiments were performed in a glovebag with an O2 level below 0.10 mg/L, additionally the suspensions were purged with argon gas for 1 h to exclude dissolved O2. 2.3. Solution Analysis. Collected samples were centrifuged and filtered through a Millipore filter (0.2 μm). Aqueous phase Cr(VI) and As(V) were analyzed using an ion chromatograph (IC, Dionex ICS-1000) with a detection limit for each species of ∼5 and ∼2 μM, respectively. The IC was equipped with a Dionex IonPacAS22 (4 mm × 250 mm) analytical column and a conductivity detector. Details regarding the analytical methodology for As(III) and As(V) species determination were described previously.27 All the chemicals used in this study, including sodium arsenite (NaAsO2), ferric chloride (FeCl3), sodium hydroxide (NaOH), hydrochloric acid (HCl), aluminum chloride (AlCl3), and potassium dichromate (K2Cr2O7), were obtained from Sigma-Aldrich (analytical grade). All batch reactions were conducted in triplicate and calculated standard errors of estimation were within 2−7%. 2.4. X-ray Absorption Spectroscopy (XAS). Oxidation states of As and Cr species adsorbed on Fh were determined using As and Cr K-edge X-ray absorption near-edge structure (XANES) spectroscopy. As discussed in a previous contribution,29 prior XANES analyses of As-reacted iron (oxy)hydroxides revealed that partial oxidation of As(III) was possible upon exposure to the synchrotron beam over the time scale needed to collect a conventional near-edge spectrum. To avoid this experimental artifact, we have taken advantage of rapid data collection using quick-scanning X-ray absorption spectroscopy (Q-XAS) as implemented at beamline X18B at the National Synchrotron Light Source (Brookhaven National Laboratory). The Q-XAS technique developed at X18B has been described previously in detail.30,31 The monochromator was calibrated by assigning the first peak in the derivative spectrum of the aqueous As(III) reference sample an energy value of 11867 eV. Cr K-edge XANES data, which were found not to be subject to such artifacts, were collected by conventional edge-scan methods. Monochromator calibration was achieved by assigning the first peak in the derivative spectrum of a Cr metal foil to 5989 eV. At least two scans were averaged to improve signal/noise. Spectra from sorption samples (As and Cr) were collected in fluorescence mode using a partially implanted planar silicon detector. Spectra for reference samples were collected before and after the sorption samples to confirm no change in monochromator calibration. A linear pre-edge background was subtracted, and the XANES spectra were normalized using a single postedge point (11915 eV for As; 6075 eV for Cr) as described in a previous study.27 Sorption samples were prepared for analysis by centrifuging the reacted suspension. The collected solid was washed once

2CrO4 2 − + 3H3AsO3 + H 2O → 2Cr(OH)3 + 3HAsO4 2 − + 2H+

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The above reactions reflect the changes in species protonation as a function of pH; below pH ∼4, eq 1 predominantly applies while above pH ∼6.8 eq 2 applies. Research has generally shown that redox chemistry between aqueous Cr(VI) and As(III) in a variety of aqueous environments does not result in the formation of As(V) and Cr(III).18 Recent contributions illustrate that the redox chemistry between the two species in solution can be facilitated under certain circumstances. For example, a study has shown that redox chemistry between aqueous Cr(VI) and As(III) occurs if hydrogen peroxide is present.19 Other research has shown that the concurrent reduction of Cr(VI) and oxidation of As(III) occurs when the two species are contained within an ice matrix18 or in the presence of specific micro-organisms.20 Results from the former study suggested that the concentration of electron donor (i.e., As(III)) and protons in the grain boundaries of ice led to the efficient reduction of Cr(VI).18 The hypothesis tested in the current research is that coexposure to a solid surface will facilitate redox chemistry between Cr(VI) and As(III). To test the hypothesis, we exposed Fh to a solution containing both aqueous Cr(VI) and As(III). The surface would concentrate Cr(VI) and As(III) complexes through the adsorption process that would then facilitate electron transfer. Oxidation states of the adsorbed species before, during, and after exposure to Cr(VI) and As(III) were determined with in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS). We analyzed the effect of the order of As(III) and Cr(VI) exposure to the Fh surface and the effect of O2 on the system to help elucidate the mechanism by which the coupled oxidation−reduction occurs. Our experimental observations showed that the exposure of Fh to an aqueous mixture of Cr(VI) and As(III) resulted in the reduction of Cr(VI) to Cr(III) and the oxidation of As(III) to As(V) in both anoxic and oxic environments on the Fh surface.

2. EXPERIMENTAL SECTION 2.1. Synthesis and Characterization of Materials. Twoline Fh was prepared using a modified21,22 version of a method developed by Cornell and Schwertmann.23 Surface area measurements of synthesized Fh yielded an average value of 330 ± 20 m2/g, which agrees well with the literature.24 Transmission electron microscopy (TEM) analysis of the sample was also consistent with properties expected for twoline Fh (see the Supporting Information, Figure S1).24,25 Crystalline aluminum hydroxide (bayerite) used in this study was prepared by neutralizing an Al(III)-bearing solution with NaOH.26 Briefly, 1 M NaOH was added stepwise to a 0.1 M solution of AlCl3 (initial pH of ∼3) with constant stirring until the pH reached ∼7, at which point a white particle suspension was observed. The suspension was dialyzed with deionized water (18 MΩ cm−1) for 4−5 days to remove counterions and then centrifuged and air-dried. XRD characterization showed the material to be primarily bayerite (95%) and a minority amount of the gibbsite phase.26 The surface area of the material was determined to be 120 ± 1 m2/g. 2.2. Batch Studies. The methodology by which the batch reactions were carried out in this study was similar to that used 2859

DOI: 10.1021/es505666w Environ. Sci. Technol. 2015, 49, 2858−2866

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Figure 1. ATR-FTIR of Fh that was exposed to a solution containing both 0.10 mM Cr(VI) and 0.15 mM As(III) at pH 3, 5, 7, and 9 (A, B, C, and D, respectively) for (a) 0, (b) 5, (c) 10, (d) 30, (e) 60, (f) 120, (g) 180, (h) 240, and (i) 300 min. The spectra are offset from one another for clarity. Plot (e) also shows the fitting of modes for Cr(VI), As(V), and As(III). All plots were fitted in the same manner but omitted for clarity.

2.6. Attenuated Total Reflectance Fourier Transform Infrared (ATR-FTIR). The experimental apparatus and methodology were the same as described in prior publications.27,28 Briefly, a thin film of Fh (∼.05 mg) was prepared on a diamond ATR element by depositing a suspension of Fh in H2O and then dried under an argon environment. The sample was then enclosed by a Teflon flow cell which allowed the passage of a specified solution over the sample. Films were first exposed to H2O at the desired pH to equilibrate the film. Most experiments then involved flowing a solution containing 0.10 mM Cr(VI) and 0.15 mM As(III) at the selected pH over the sample at a rate of 1 mL/min. For experiments where O2 was excluded, the Fh film was dried under Ar and sealed to outside air within an Ar-purged glovebag. The solutions introduced to the flow cell were purged with Ar for 1 h to remove dissolved O2. Some ATR-FTIR spectra were fitted to determine the relative amount of different adsorbed species as a function of time and pH. Spectral fitting was carried out using Origin Lab software. Individually adsorbed Cr(VI), As(III), and As(V) on

with corresponding pH deionized water, dried in air, loaded into Lucite sample holders, and sealed with Kapton tape. 2.5. X-ray Photoelectron Spectroscopy (XPS). Oxidation states of As and Cr species adsorbed on Fh were also determined using a VG Scientific XPS, with a Mg Kα X-ray source operating at 280 W, 15 kV × 25 mA. Pass energy of 30 eV was used for all spectra. The Fe 2p and 3p peak positions were used to eliminate static charge effects, and all spectra were analyzed using CasaXPS software. Elements present were identified by their strongest binding energy peaks, Fe 2p (707 eV), As 3d (42 eV), and Cr 2p (572 eV). XPS-normalized peak intensities were calibrated to the surface loading of Cr and As species, determined by measuring the amount of Cr(VI) or As(III) from solution for a given mass of Fh (known surface area). The raw As and Cr peak areas were normalized against the Fe 3p peak area to account for changes in sample position and/or the amount of material loaded into the spectrometer. Furthermore, As 3d spectra were processed by subtracting XPS spectra for clean Fh (no adsorbate), obtained for the same binding energy window. 2860


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Figure 2. (A) ATR-FTIR of Fh (a) in pH 5 H2O, (b) after exposure to a flowing solution of 0.15 mM As(III) for 120 min, (c) after flushing with pH 5 H2O for 15 min, and after exposure to flowing 0.10 mM Cr(VI) solution for (d) 10, (e) 30, (f) 60, and (g) 120 min. (B) ATR-FTIR of Fh (a) in pH 5 water, (b) after exposure to a flowing solution of 0.10 mM Cr(VI) for 120 min, (c) after flushing with pH 5 H2O for 15 min, and after exposure to flowing 0.15 mM As(III) solution for (d) 10, (e) 30, (f) 60, (g) 120, and 180 min. All As(III) and Cr(VI) solutions were at pH 5.

and furthermore, plots of fitted peak areas (as a function of time) for the individual Cr(VI), As(III), and As(V) contributions to each experimental spectrum, at each pH, are shown in the Supporting Information (Figure S5). ATR-FTIR spectra associated with the simultaneous exposure of Fh to As(III) and Cr(VI) at pH 9 also show an increasing Cr(VI) vibrational mode intensity, weak compared to other pH conditions, at early reaction times and a loss of these modes at later reaction times. Unlike the lower pH data, the dominant As-derived modes are due to the presence of As(III), and only weak spectral weight is present that can be assigned to As(V) product. A comparison of the vibrational mode absorbance associated with adsorbed Cr(VI) at early reaction times (i.e., 10 min: spectra c in each pH data set in Figure 1) for the different pH data sets shows a decreasing Cr(VI) intensity/absorbance as the solution pH was raised from 3 to 9. This experimental observation is consistent with expected increased electrostatic repulsion between HCrO4−/ CrO42− and Fh (pzc of 8.2)35 as the pH is raised from 3 to 9. Based on the fitted vibrational spectra, the binding geometries of the three adsorbates, Cr(VI) (bidentate at low pH, monodentate at high pH13), As(III) (monodentate and outersphere33), and As(V) (primarily bidentate bridging36), are similar whether adsorbed independently or coadsorbed. ATR-FTIR data to this point show that As(V) is produced on the surface when Cr(VI) and As(III) are exposed to Fh. Also, the results so far suggest that there is an increasing amount of As(III) oxidation with decreasing pH based on the amount of spectral weight attributable to As(V). We expect that both the increasing Cr(VI) adsorbate concentration and increasing proton concentration with decreasing pH would facilitate the redox reaction (see eq 1). In order to help determine how As(V) oxidation occurs we note two other experimental observations: (1) the individual adsorption of As(III) on Fh does not result in As(V) formation27 and (2) the homogeneous solution reaction (in the absence of Fh) between Cr(VI) and As(III) does not result in As(V) formation at any of the solution pH conditions used in this study (see the Supporting Information, Figure S6). Therefore, the concom-

Fh were first obtained as a function of pH. The spectra associated with the individually adsorbed species were fitted to achieve an R2 of 0.99 at each pH, and our spectra agree well with vibrational data reported for the same three adsorbates in prior literature.13,32,33 [see the Supporting Information, Figure S2, for selected peak fits of Cr(VI), As(III), and As(V) at pH 5.] These individual fits were then used together to fit experimental data where one or all three species were present on Fh. When fitting experimental data associated with the exposure of Fh to As(III) and Cr(VI), only the peak areas of the individual synthesized spectra associated with Cr(VI), As(V), and As(III) were allowed to vary (energy positions were fixed). Typical fitted experimental spectra containing contributions from multiple adsorbed species had an R2 of 0.99.

3. RESULTS AND DISCUSSION 3.1. ATR-FTIR Studies of As(III) and Cr(VI) on Fh. 3.1.1. Effect of pH with Simultaneous Exposure. Figure 1 displays ATR-FTIR data obtained when Fh was exposed to a flowing solution containing 0.10 mM Cr(VI) and 0.15 mM As(III) at pH 3, 5, 7, and 9 (A, B, C and D, respectively) as a function of time. Each data set includes a representative spectrum that has been fitted with spectral components for Cr(VI), As(V), and As(III) (see section 2.6 for method). Data obtained for pH 3, 5, and 7 show similar behavior where the vibrational modes associated with Cr(VI) grow for a period of time, reach a maximum value (depending on the pH) and then show a continuous decrease with time (a complete set of curves for pH 3 is provided in the Supporting Information, Figure S3). We attribute the increase in the intensity of the Cr(VI) associated modes at early reaction times to its adsorption rate34 on Fh being faster than its consumption via redox reactions with As(III). In contrast to Cr(VI), the vibrational mode intensities associated with As(V) show a monotonic rise with time. Vibrational modes attributable to adsorbed As(III) are absent in the pH 3 data set, but are present (albeit weak) in the pH 5 and 7 data sets. To support these statements, selected time points are fitted with Cr(VI), As(III), and As(V) for pH 3, 5 and 7, as shown in the Supporting Information (Figure S4), 2861


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Environmental Science & Technology itant reduction of Cr(VI) to Cr(III) would be consistent with the loss of vibrational modes attributable to Cr(VI) at the later reaction times and As(III) oxidation. Due to lack of Cr(III) active vibrational modes, in the vibrational region investigated, the reduction cannot be confirmed by ATR-FTIR. The oxidation state of the Cr product is characterized with XANES and XPS analysis below. 3.1.2. Sequential Exposure of Fh to As(III) and Cr(VI). Further insight into the redox chemistry was obtained by investigating how the production of As(V) was affected by the sequence of exposure of Fh to As(III) and Cr(VI). Experiments were carried out that individually exposed Fh to flowing 0.15 mM As(III) solution or 0.10 mM Cr(VI) solution for 3 h at pH 5. After this initial 3 h exposure, the As(III) or Cr(VI) solution was flushed from the reaction cell with pH 5 water. The As(III)/Fh and Cr(VI)/Fh systems prepared in this way were then exposed to flowing 0.10 mM Cr(VI) and 0.15 mM As(III) solution, respectively. ATR-FTIR data were collected for both these exposure scenarios (Figure 2A,B). Figure 2A shows that the exposure of As(III)/Fh to Cr(VI) led to increases in both Cr(VI) and As(V) vibrational modes. Data presented in Figure 2B show that the exposure of Cr(VI)/Fh to a flowing solution of As(III) led to a decrease in modes attributable to Cr(VI) and an increase in modes associated with As(V) product. Although some Cr(VI) loss may be due to desorption, the increase of As(V) modes suggests that the electron reduction of Cr(VI) contributes to its loss in surface concentration. Comparing the two exposure scenarios, the Cr(VI)/Fh case produces more As(V) on the surface compared to As(III)/Fh (see the Supporting Information, Figure S7). We note that in both exposure scenarios, at the end of reaction, indicated by no further change in mode intensity, we observe that Cr(VI), As(III), and As(V) all coexist on the surface. This result suggests that only a fraction of Cr(VI) and As(III) on Fh react to form As(V). 3.1.3. Effect of O2 with Simultaneous Exposure. The effect of O2 on the redox chemistry between As(III) and Cr(VI) was investigated at pH 5 (Figure 3). In the anoxic case we observe the increase of Cr(VI) vibrational modes reaching a maximum, followed by a continuous decrease in intensity, similar to that observed in the oxic case (Figure 1, B). Also comparable is the observation of the continuous increase of As(V) vibrational modes. A comparison of the As(V) peak area derived from fitted spectra (Figure 3 inset), however, shows that less As(V) product forms after 3 h of reaction in the absence of dissolved O2 (i.e., anoxic case). This particular analysis suggests that the presence of dissolved O2 leads to more As(III) oxidation in the presence of Cr(VI) and Fh than in the absence of dissolved O2. 3.2. Characterization of the Adsorbed Layer on Fh with XANES and XPS. 3.2.1. Surface Analysis of Batch Studies. XANES spectroscopy was carried out to determine the oxidation states of Cr and As on Fh that had been exposed to a solution containing 0.10 mM Cr(VI) and 0.15 mM As(III) at pH 5. Based on the decrease in aqueous concentrations of Cr(VI) and As(III) the loading of Cr and As on Fh was 310 and 610 mmol/kg, respectively, and we note that in all batch experiments no As(V) was detected in the aqueous phase before or after reaction. Figure 4a compares Cr K-edge XANES spectra obtained for this sample to Fh that was individually exposed to aqueous Cr(III) and Cr(VI). A prominent pre-edge feature at ∼5993 eV is present in the XANES data for Fh that had been exposed to Cr(VI). This pre-edge feature is indicative of the presence of tetrahedrally coordinated Cr(VI) and

Figure 3. ATR-FTIR of Fh that was exposed to a flowing solution of 0.10 mM Cr(VI) and 0.15 mM As(III) at pH 5 for (a) 0, (b) 5, (c) 10, (d) 30, (e) 60, (f) 120, (g) 180, (h) 240, and (i) 300 min under anoxic conditions. Inset shows comparison of As(V) peak area (arbitrary units) at pH 5 anoxic (squares) and pH 5 oxic (circles) conditions. The anoxic case shows a reaction similar to that of the oxic case, growth, and then decrease in mode attributed to Cr(VI) coupled with the constant growth of As(V) vibrational modes, but less As(V) production.

consistent with chromate adsorbed on the surface.37 The feature is absent in the spectrum associated with Fh that had been exposed to solution containing both Cr(VI) and As(III), and the spectrum is instead similar to the Cr(III)/Fh reference spectrum. These data indicate that Cr(VI) is reduced to Cr(III) on Fh in the presence of As(III), as suggested by ATR-FTIR experimentation. Figure 4b exhibits complementary As K-edge XANES data collected for Fh that was exposed to a mixture of 0.10 mM Cr(VI) and 0.15 mM As(III) and for individual controls (either As(III) or As(V) adsorbed individually on Fh). XANES data associated with all of the Fh samples exhibit wellresolved As(III) or As(V) edge structures.38 A least-squares linear combination fit of the data, using reference spectra for As(III) and As(V) adsorbed individually on Fh, shows that As(III) and As(V) were both present on the Fh sample that was simultaneously exposed to Cr(VI) and As(III), with the majority As species being As(V) (80 ± 5%) at pH 5. While the surface As complex is not exclusively As(V), the results do support our interpretation of the ATR-FTIR data that the exposure of Fh to both Cr(VI) and As(III) results in the conversion of As(III) to As(V). The XANES data also show that this redox chemistry occurs together with the conversion of Cr(VI) to Cr(III). While our XANES results are limited to the pH 5 oxic experiment, we investigated with XPS the oxidation state of adsorbed Cr and As after Fh was exposed to solution phase As(III) and Cr(VI) at pH 3, 5, 7, and 9. We provide these spectra in the Supporting Information (Figures S8, and S9) and make oxidation state assignments based on comparison to 2862


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adsorbed As on Fh is As(V), while the relative proportion of As(V) is ∼80% (similar to the XANES result) under oxic conditions. These results also support our conclusions from in situ ATR-FTIR results that suggest more As(V) product is formed on Fh in the oxic circumstance (Figure 3). As mentioned before, pH 3 XPS data shows exclusively As(V) on Fh after reaction (consistent with ATR-FTIR). The Asloading on the particles was 610 mmol/kg, based on the amount of As(III) that was removed from solution in the presence of Fh. Hence, we estimate that in the pH 3 circumstance the As(V) loading after reaction is also 610 mmol/kg, since we do not detect any aqueous As(V) product (detection limit of 5 μM). 3.2.2. Characterization of Cr(III) Product. We carried out studies to characterize the nature of the Cr(III) product formed during the Cr(VI) and As(III) redox chemistry on Fh. At no point was aqueous phase Cr(III) experimentally observed during any of the experiments. This experimental observation suggests that the majority of Cr(III) product was present on the iron oxyhydroxide surface, possibly as a surface complex or as a Cr(III), Cr(III)-Fe or Cr(III)-As bearing secondary phase. To evaluate these possibilities, XRD was carried out on postreaction samples (see the Supporting Information, Figure S10). The X-ray diffraction data showed no experimentally resolvable differences between pure Fh and the reaction product after exposure to Cr(VI) and As(III). To further understand the nature of the surface Cr(III) we compared Cr XANES data from our reaction products to selected Cr(III) reference phases, including chromite (FeCr2O4), a Cr/Fh coprecipitated sample, and amorphous chromium hydroxide [Cr(III)(OH)3]. The latter two spectra were taken from the study by Tang et al.37 By comparison of the first derivatives of the relevant XANES spectra (See Supporting Information, Figure S11) the Fh coexposed to Cr(VI) and As(III) is found to be nearly identical to that of amorphous Cr(III) hydroxide, and slightly different than Cr(III) coprecipitated with Fh.37 Finally, analysis of the full EXAFS data did not support the presence of a Cr(III)-arsenate phase, although we cannot entirely rule out its possible formation. 3.3. Reaction Mechanism. 3.3.1. General Reaction Mechanism. Our experimental observations show that Cr(III) and As(V) product form when Fh is exposed to a solution containing both Cr(VI) and As(III) or if one reactant species is adsorbed (Cr(VI) or As(III)) and then exposed to a solution containing only the other reactant as an aqueous species (i.e., As(III) or Cr(VI), respectively). These experimental observations suggest that the surface is providing an energetically favorable pathway for electron transfer between Cr(VI) and As(III) that cannot be achieved when both species interact in the aqueous phase (in the absence of the solid substrate). Fh likely acts to concentrate one or both of the reactants (and intermediate species) in binding geometries that make the multielectron transfer process between Cr(VI) and As(III) energetically favorable. We emphasize, however, that our experimental results do not allow us to determine whether both Cr(VI) and As(III) need to be adsorbed to produce As(V) product. It is perhaps useful to point out that prior aqueous-based studies have suggested that the reaction of As(III), which is considered to be a two-electron reductant, and Cr(VI) to form As(V) and Cr(III) product would involve the generation of Cr(IV) intermediate species. Comproportionation and disproportionation reactions involving Cr(IV) species

Figure 4. (a) Normalized Cr K-edge XANES spectra of Fh that was individually exposed to Cr(VI), Cr(III), and of Fh that was exposed to a solution containing both Cr(VI) and As(III). (b) Normalized As Kedge XANES spectra of Fh that was individually exposed to As(III) and As(V) and of Fh that was exposed to a solution containing both Cr(VI) and As(III). All exposure times were 12 h, and the solution pH was 5 in all cases.

standards adsorbed on Fh. With regard to the oxidation state of adsorbed As, the fitted XPS spectra in general show that there is an increase in the relative proportion of As(V) as the pH of the solution decreases. The fitted pH 9 spectrum shows almost entirely As(III) (>90%), while at pH 3 the adsorbed As exists primarily as As(V). This particular result is consistent with our ATR-FTIR results (Figure 1A) at this pH that show no significant As(III) mode intensity. XPS also suggests that Cr(III) is present on the surface of Fh after exposure to Cr(VI) and As(III) at pH 3, 5, and 7, but the data also contain spectral weight that is attributable to Cr(VI) (Supporting Information, Figure S9). We do not detect Cr(III) with XPS in the pH 9 reaction product, but this is likely due to the concentration of adsorbed Cr being below our detection limit. The loading of Cr on Fh at pH 9 was calculated to be 5 mmol/kg based on an analysis of Cr(VI) loss from solution. As mentioned before, the ATR-FTIR experiments carried out at pH 9 (Figure 1D) also suggest that the amount of Cr(VI) adsorption is significantly less than at lower pH conditions investigated in this study. XPS data also allow us to compare the relative proportion of As(V) and As(III) on Fh after exposure to 0.10 mM Cr(VI) and 0.15 mM As(III) at pH 5 under both oxic and anoxic conditions. In both these cases, the loadings of Cr(VI) and As(III) were 310 and 610 mmol/kg, respectively. An analysis of the data shows that in the anoxic circumstance ∼52% of the 2863


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of the more immobile Cr(III) and As(V) products. Hence, it is conceivable that the addition of high surface area metal (oxy)hydroxide to contaminated water environments might be a useful remediation strategy, resulting in contaminant immobilization by adsorption, as well as through redox chemistry. Perhaps more importantly, the results and interpretations presented in this contribution add to our understanding of the fundamental principles that control redox chemistry at the mineral−water interface in environmental settings outside of the remediation framework.

would form Cr(V) and the reduction of this latter species by As(III) would lead to Cr(III) product:39,40 Cr(VI) + As(III) → Cr(IV) + As(V)

(3)

Cr(VI) + Cr(IV) → 2Cr(V)

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2Cr(IV) → Cr(III) + Cr(V)

(5)

Cr(V) + As(III) → Cr(III) + As(V)

(6)

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Such a mechanism might be expected to be facilitated by the colocalization of Cr and As species, presumably through their close proximity on a solid surface. Our experimental observations also show that dissolved oxygen, when present, increases the amount of As(V) product. At this time it is not possible to unequivocally determine the role of O2. However, based on prior studies, a possible enhancement in As(V) production could conceivably be due to the interaction of dissolved O2 with intermediate Cr41 and/or As19,33 valencestate species. The surface-facilitated redox chemistry may be also attributable to changes in the redox potentials of Cr(VI) (and intermediate species) and/or As(III) upon adsorption (compared to the respective Cr(VI) and As(III) aqueous phase species). While we cannot test this possibility in the present research, results from prior studies may be taken to be consistent with such a possibility. With regard to Cr(VI), prior research has shown that adsorbed Cr(VI) on Fe, Al, and Ti oxides demonstrates a much more facile reduction to Cr(III) by organic reductants than does aqueous Cr(VI).42,43 In a different study, reduction of U(IV) by Fe(II), which was only observed when the species were adsorbed on a hematite surface, was attributed to the change in redox potentials of adsorbed species.44 While the relevance of these prior studies to ours is limited, they do suggest that adsorbed oxidizing species (in our case Cr(VI)) can be more effective oxidizers than their solution phase counterparts in environmental redox reactions. 3.3.2. Role of the Electronic Properties of the Solid Substrate. Prior research has suggested that small band gap semiconductors, such as Fh, can act as conduits for electron transfer between a donor and acceptor.45,46 To test whether electron transfer between Cr(VI) and As(III) on Fh requires bulk conduction, we carried out additional experiments where we exposed dielectric nanodimensioned Al(OH)3 (mixture of bayerite and gibbsite) to Cr(VI) and As(III) under similar conditions to those described here. Post analysis of the surface composition after reaction by XANES (Supporting Information, Figure S12) shows that Cr(III) and As(V) are present, confirming that Cr(VI) is reduced to Cr(III) concomitant with As(III) oxidation to As(V), analogous to our findings for Fh. Hence, exposure of As(III) and Cr(VI) to Al(OH)3 is thought to be the primary factor allowing the experimentally observed redox chemistry between Cr(VI) and As(III). These results suggest that the electrical properties, i.e., positions of conduction and valence bands, do not have a primary effect on the ability of the surface to aide in the coupled redox of Cr(VI) and As(III) under our experimental conditions. However, future kinetic studies are planned that more carefully compare the rate of reaction on well-defined semiconductor and dielectric surfaces to determine whether electron transport in the substrate bulk/surface plays a significant role. In closing, our results demonstrate that the exposure of high surface area Fh to Cr(VI) and As(III) results in the formation

ASSOCIATED CONTENT

S Supporting Information *

TEM image of synthetic Fh, ATR-FTIR of Cr(VI), As(V), and As(III) individually adsorbed on Fh at pH 5, ATR-FTIR Cr(VI) and As(III) adsorbed on Fh at pH 3, ATR-FTIR fitted spectra of selected time points for pH 3, 5, and 7, ATR-FTIR peak areas of Cr(VI), As(V), and As(III) at pH 3, 5, and 7 versus time, Cr(VI) and As(III) concentration data of aqueous reaction without ferrihydrite, ATR-FTIR peak areas of Cr(VI), As(V), and As(III) at pH 5 versus time comparing order of exposure, XPS of As 3d region for batch reactions, XPS of Cr 2p region for batch reactions, XRD patterns of reaction products, 1st derivative Cr XANES data, XANES of Cr and As from pH 5 batch reaction with Al(OH)3, and XPS of As 3d and Cr 2p regions for As(III)/Fh reacted with aqueous Cr(VI). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ⊥

Brine Chemistry Consortium, Department of Civil and Environmental Engineering, Rice University, Houston, TX 77005. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Science Foundation (NSF) Collaborative Research in Chemistry grant (CHE0714121). We also thank Syed Khalid and Hyuck Hur for assistance with XAS data collection. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02−98CH10886.

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