Redox Properties of Structural Fe in Clay Minerals - American

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Redox Properties of Structural Fe in Clay Minerals: 3. Relationships between Smectite Redox and Structural Properties Christopher A. Gorski,†,‡ Laura E. Klüpfel,§,‡ Andreas Voegelin,‡ Michael Sander,*,§ and Thomas B. Hofstetter*,‡,§ †

Department of Civil and Environmental Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, United States ‡ Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland § Institute of Biogeochemistry and Pollutant Dynamics (IBP), Swiss Federal Institute of Technology, ETH Zürich, Zürich, Switzerland S Supporting Information *

ABSTRACT: Structural Fe in clay minerals is an important redox-active species in many pristine and contaminated environments as well as in engineered systems. Understanding the extent and kinetics of redox reactions involving Fe-bearing clay minerals has been challenging due to the inability to relate structural Fe2+/ Fetotal fractions to fundamental redox properties, such as reduction potentials (EH). Here, we overcame this challenge by using mediated electrochemical reduction (MER) and oxidation (MEO) to characterize the fraction of redox-active structural Fe (Fe2+/ Fetotal) in smectites over a wide range of applied EH-values (−0.6 V to +0.6 V). We examined Fe2+/Fetotal − EH relationships of four natural Fe-bearing smectites (SWy-2, SWa-1, NAu-1, NAu-2) in their native, reduced, and reoxidized states and compared our measurements with spectroscopic observations and a suite of mineralogical properties. All smectites exhibited unique Fe2+/Fetotal − EH relationships, were redox active over wide EH ranges, and underwent irreversible electron transfer induced structural changes that were observable with X-ray absorption spectroscopy. Variations among the smectite Fe2+/Fetotal − EH relationships correlated well with both bulk and molecular-scale properties, including Fetotal content, layer charge, and quadrupole splitting values, suggesting that multiple structural parameters determined the redox properties of smectites. The Fe2+/Fetotal − EH relationships developed for these four commonly studied clay minerals may be applied to future studies interested in relating the extent of structural Fe reduction or oxidation to EH-values.

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INTRODUCTION Iron-bearing clay minerals are important redox-active phases in the subsurface that affect many biogeochemical processes and nutrient cycles as well as the fate and toxicity of several classes of contaminants.1−12 Assessing the role that structural Fe in clay minerals plays in redox reactions in pristine and contaminated environments as well as in engineered systems remains difficult, largely due to experimental and conceptual challenges in linking observed redox reaction reactions to thermodynamic parameters of structural Fe, such as reduction potential (EH) values.13−19 As we have recently shown, the experimental challenges can be overcome by employing mediated electrochemical analysis techniques that utilize dissolved electron transfer mediators to determine the extent of structural Fe reduction or oxidation as a function of applied EH.19,20 We previously showed that mediated electrochemical reduction (MER) and oxidation (MEO) could be used to quantify the fractions of structural Fe in four smectites that were redox-active under strongly oxidizing (EH = +0.61 V) and strongly reducing (EH = −0.60 V) conditions.19 The fraction of structural Fe that was redox-active varied among samples and © 2013 American Chemical Society

was likely related to the structural differences among the minerals. We also showed by MER and MEO that the extent of structural Fe reduction (Fe2+/Fetotal) in a particular smectite (ferruginous smectite, SWa-1, 12.6 wt % Fe) varied as a function of applied EH and the redox history of the mineral. The electron transfer to and from SWa-1 occurred over a wide range of EHvalues (≈1.2 V) and induced reversible and irreversible changes in both the mineral structure and Fe2+/Fetotal − EH relationship.20 Our detailed electrochemical and spectroscopic characterization of SWa-1 highlighted the complexities of electron transfer to and from structural Fe in clay minerals. Electron transfer could not be described using the Nernst equation; instead, it occurred over a much wider EH range than expected for a single Nernstian Fe3+/Fe2+ redox couple and exhibited considerable irreversibility over cycles of chemical reduction and reoxidation. This nonideal Received: Revised: Accepted: Published: 13477

August 28, 2013 November 4, 2013 November 12, 2013 November 12, 2013 dx.doi.org/10.1021/es403824x | Environ. Sci. Technol. 2013, 47, 13477−13485

Environmental Science & Technology

Article

chemical cells used, and the experimental details of MER and MEO experiments are described in our earlier paper.19 Potentials were measured against a Ag/AgCl reference electrode, but are reported here versus the standard hydrogen electrode (SHE). MER and MEO experiments were conducted to quantify the number of electrons transferred to or from structural Fe in the smectites at a constant applied potential (EH). In these experiments, electron transfer mediating compounds (i.e., mediators) were used to facilitate electron transfer between the working electrode (WE) set at a constant applied EH and structural Fe in the smectite. The extent of structural Fe reduction or oxidation was determined by integrating the current peak response (eq 1):

behavior was attributed to the formation of metastable Fe sites during redox cycling, consistent with spectroscopic observations of redox-induced structural alterations due to the size differences of Fe3+ and Fe2+, irreversible reactions occurring within the clay mineral structure (e.g., reduction-induced dehydroxylation), and Fe atom migration within the octahedral sheet.12,14,21−30 What remained unclear from the previous work was whether the wide EH range of redox-activity and irreversible electron transfer are also observed for other clay minerals and to what extent these observations depend on measurable structural parameters. Previous works have argued that EH-values at which reduction and oxidation occur can vary as function of several variables, including (i) the total Fe content,31−35 (ii) the layer charge (i.e., the net negative charge in the mineral structure),1,34−36 (iii) elemental layer composition and ordering,27,33,37−40 (iv) Fe−O bond lengths,2 and (v) the relative abundances of cis- and trans-vacant sites in the mineral structure.37,41,42 Determining the relative impact that these parameters have on structural Fe reduction and oxidation is difficult, as many of these parameters vary as a function of the extent of structural Fe reduction and oxidation (e.g., the layer charge) and are interdependent (e.g., the relative abundances of cis- and trans-vacant sites in the octahedral layer depends on the structural Fe content).37 In light of these complexities, the goals of this study were (i) to determine the Fe2+/Fetotal − EH relationships for four smectites, (ii) determine the reversibility of electron transfer to and from structural Fe in these smectites by examining Fe2+/Fetotal − EH relationships as a function of redox history, and (iii) determine how smectite Fe2+/Fetotal − EH relationships were influenced by the aforementioned structural properties. MER and MEO were used to measure Fe2+/Fetotal − EH relationships for four natural Fe-bearing smectites (i.e., ferruginous smecite SWa-1, Wyoming montmorillonite SWy-2, and Australian nontronites NAu-1 and NAu-2; structural Fe contents ranging from 2.3 to 21.2 wt % Fe). We examined the reversibility of electron transfer to and from structural Fe in these smectites by chemically reducing and reoxidzing the minerals prior to MER and MEO analysis. Measured Fe2+/Fetotal − EH relationships were related to several structural parameters that were collected using X-ray absorption and cryogenic 57Fe Mössbauer spectroscopies or taken from the literature to elucidate possible trends between electrochemical and structural properties.

q=

1 · F

∫t

t2

I ·dt

(1)

1

−

where q [mol e ] is the number of electrons transferred, F is the Faraday constant [96 485 C/mol e−], I [C/sec] is the current, and t [sec] is time. Eight mediators were selected to cover a wide EH-range (−0.65 to +0.65 V; Table 1, cyclic voltammograms of all mediators are in Table 1. Electron Transfer Mediators Used in This Studya acronym TQ ZiV FMN RSR NQ DCPIP FeCN ABTS

electron transfer mediator

n

E0H [V]

triquat (1,1′-trimethylene-2,2′-bipyridyl) zwitterionic viologen [4,4′-bipyridinium-1,1′-bis(2ethylsulfonate)] flavin mononucleotide resorufin 1,4-napthoquinone 2,6-dichlorophenolindophenol ferri/ferro-cyanide [Fe2+/3+(CN)6] 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)

1 1

−0.54 −0.38

1 2 2 2 1 1

−0.21 −0.06 +0.06 +0.22 +0.43 +0.70

a

All mediators except triquat are zwitter- or anionic in their oxidized and reduced forms. EH0 -values were determined using cyclic voltammetry at pH 7.5, as detailed in Figure S2. n refers to the number of electrons transferred to/from the mediator per molecule reduced/oxidized.

Figure S2). One- and two-electron transfer mediators were used in MER and MEO measurements to be within ±0.12 V and ±0.06 V of their E0H-values, respectively, to ensure proper mediation.43,44 Six mediators were commercially available, while zwitterionic viologen (ZiV) and triquat (TQ) were synthesized according to methods described in the SI and ref 20, respectively. Mediators were selected here to (i) rapidly reach reversible apparent redox equilibration with the WE and structural Fe in the smectites, (ii) be stable over the course of an experiment (up to 24 h), (iii) be sufficiently water-soluble, and (iv) be anionic or zwitterionic in their oxidized and reduced states (with the exception of TQ). Negatively and neutrally charged mediators were needed because of preferential sorption of some cationic mediator species to SWy-2, which resulted in measurement artifacts. The use of TQ, which is cationic, did not result in artifacts. We performed all MER and MEO experiments in this study using this new set of mediators and confirmed that the new and old20 mediator set yielded virtually identical redox profiles for native NAu-1 (Figure S3). Quantification of Apparent Standard Reduction Potentials, E⌀H. To interpret redox profiles of the four clay minerals quantitatively, we fit each redox profile using a modified form of the Nernst equation (eq 2):20,45−49

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MATERIALS AND METHODS Materials Used. Section S1 in the Supporting Information (SI) contains a complete list of the clay minerals and chemicals used in this study. Methods describing the purification and size fractionation of SWa-1, SWy-2, NAu-1, and NAu-2, triquat synthesis, and details regarding anaerobic conditions are provided in our earlier work.19 Smectites that were neither reduced nor oxidized chemically prior to use in experiments are referred to being in their “native” redox state. Dithionite reduction and hydrogen peroxide reoxidation of smectites was carried out using our previously described approach.20 The synthesis and characterization of zwitterionic viologen (ZiV) is described in Section S2. All aqueous solutions were prepared using nanopure water (resistivity σ > 18 MΩ·cm; Nanopure Diamond Water System). All experiments were carried out in a pH 7.5 buffered solution (0.1 M NaClO4, 0.01 M 3-(Nmorpholino)propanesulfonic acid [MOPS] buffer, pKa = 7.2). Mediated Electrochemical Reduction (MER) and Oxidation (MEO). A description of the potentiostat, electro13478

dx.doi.org/10.1021/es403824x | Environ. Sci. Technol. 2013, 47, 13477−13485


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Table 2. Bulk, Redox, Molecular-Scale, And Hyperfine Properties of Native Smectitesa bulk properties smectite

wt % Fe

layer charge

SWy-2 SWa-1 NAu-1 NAu-2

2.3 12.6 21.2 19.2

0.5551 0.91−1.0951−54 1.0536 0.7236

redox parameters E⌀H

hyperfine parameters

β

Fe−O [Å]

DW

CS [mm/s]

QS [mm/s]

0.23 ± 0.03 0.30 ± 0.02 0.52 ± 0.07 0.36 ± 0.02

2.006 2.002 2.004 2.006

4.9 × 10−3 6.7 × 10−3 5.6 × 10−3 6.4 × 10−3

0.43 0.47 0.48 0.49

0.96 0.64 0.45 0.63

[V]

−0.03 ± 0.02 −0.41 ± 0.01 −0.45 ± 0.01 −0.37 ± 0.01

local fe-binding

a

Structural Fe contents (wt % Fe) were collected in our previous work.19 Layer charge values were calculated from smectite unit formulas taken from the literature using established methods.55 E⌀H and β values (i.e., “redox parameters”) were collected from fitting redox profiles using a modified Nernst equation (eq 2), as described in the text. Mean Fe−O bond lengths and distributions (Debye Waller, DW factors) were taken from first-shell fits of EXAFS spectra (Figure 3). Hyperfine parameters were previously collected by fitting 13 K Mössbauer spectra of the smectites.19

E H = E H⌀ −

1 RT {Fe 2 +} · ln β nF {Fe3 +}

the top axis in Figure 1. The curves relating the Fe2+/Fetotal fraction to applied EH-values are referred to here as redox prof iles. As expected, the Fe2+/Fetotal fractions in all smectites increased as the EH-value became increasingly more negative (i.e., conditions became more reducing). At sufficiently negative EH-values of −0.6 V, all the structural Fe3+ was reduced to Fe2+ in SWy-2, SWa-1, and NAu-2, while a small fraction (≈7%) of structural Fe in NAu-1 remained oxidized as Fe3+.

(2)

where β is a unitless, fractional value (0 < β < 1), with wider EH distributions resulting in smaller β values and β = 1 corresponding to the ideal, Nernstian conditions; R,T, n, and F have their usual meanings; and curly brackets denote activities. Least-squares fitting was done on the data as shown in Figures 1 and 2 by minimizing the difference between the modeled and experimental Fe2+/Fetotal ratios. X-ray Absorption Spectroscopy. For X-ray absorption spectroscopic (XAS) analysis, smectite suspensions were dried in the anaerobic chamber (reduced samples) or in air (native or reoxidized samples), mixed with cellulose and boron nitride, and pressed into pellets of 10 mm (native and reoxidized) or 7 mm diameters (reduced). Fe K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra were measured at the XAS beamline at the Angströmquelle Karlsruhe (ANKA, Karlsruhe, Germany). The Si(111) double crystal monochromator was calibrated by setting the first inflection point of the absorption K-edge of a Fe metal foil to 7112 eV. Spectra were recorded at room temperature in transmission mode (except reduced NAu-2 in fluorescence mode) using a N2-purged sample holder. The spectra were processed and analyzed using the software codes Athena and Artemis.50 Further details on data extraction and analysis are provided in section S5. Mössbauer Spectroscopy. A description of the sample preparation, Mössbauer spectrometer, and modeling approach can be found in ref 19. We previously found that all the structural Fe in the four smectites was present as octahedral Fe3+, with the exception of NAu-1, which contained approximately 2 wt % tetrahedral Fe3+.

Figure 1. Redox profiles of native SWy-2, SWa-1, NAu-1, and NAu-2 collected over a series of applied potentials (EH). Each data point represents the average of several (≥3) replicate smectite spikes, with errors bars showing standard deviations. All experiments were conducted in pH 7.5 buffer with 0.1 M NaClO4 as the background electrolyte. The mediator used is each experiment is shown on the top axis (see Table 1 for mediator details). SWa-1 data was taken from ref 20.

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RESULTS AND DISCUSSION Mediated Electrochemical Reduction of Native Smectites. We characterized the Fe2+/Fetotal − EH relationships of four natural Fe-bearing smectites (SWy-2, SWa-1, NAu-1, NAu-2; Table 2) in their native redox states (≈100% Fe3+) using mediated electrochemical reduction (MER). Known amounts of a smectite were spiked into electrochemical cells with reductively poised working electrodes, resulting in reductive current responses that were integrated to determine the number of electrons transferred to Fe3+ and hence the extent of structural Fe reduction (eq 1, see ref 19 for details). Several such MER experiments were conducted for each smectite over a range of applied EH-values from −0.6 V to +0.6 V (vs. SHE). The resulting curves that relate the smectite Fe2+/Fetotal fraction to the applied EH are shown in Figure 1. The electron transfer mediators used in each experiment are indicated by their operational EH range on

The redox profiles of the four smectites differed in two major ways. The first difference was the onset of Fe3+ reduction, which occurred at different EH-values for each smectite. This was most apparent for Wyoming montmorillonite SWy-2, which was reduced at much higher EH-values, and hence less reducing conditions, than the other three smectites (Figure 1). Variation in the onset potential of reduction was also apparent, albeit to a lesser degree, among the other three smectites. The second difference was that the width of EH range over which structural Fe3+ was reduced (i.e., from 99% Fe3+ to 99% Fe2+) varied considerably among the smectites. The EH ranges were ≈0.8 V for SWy-2, ≈0.5 V for SWa-1 and NAu-1, and ≈0.6 V for NAu-2. For all four smectites, these ranges were much wider than the 0.24 V expected for a single Nernstian Fe3+/Fe2+ redox couple undergoing reversible electron transfer. 13479



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Figure 2. Redox profiles of redox cycled smectites (SWy-2, SWa-1, NAu-1, and NAu-2). All experiments were conducted in pH 7.5 buffer with 0.1 M NaClO4 as the background electrolyte. Data for SWa-1 is taken from ref 20.

being reduced in an MER experiment conducted at EH = −0.56 and 81% of the structural Fe being oxidized in an MEO experiment at EH = +0.62 V (Figure 2). The observation that structural Fe in NAu-1 and NAu-2 was not fully reduced by dithionite is consistent with our previous findings.19 The incomplete Fe3+ reduction in NAu-1 and NAu-2 by dithionite (i.e., 52% for NAu-1 and 19% for NAu-2) was accounted for in the construction of redox profiles shown in Figure 2. Note that we previously found that only 81% of the structural Fe in NAu-2 was redox-active with MER and MEO,19 while here we observed that approximately 100% was redox-active; the origin of this discrepancy remains unclear. Reoxidation of the structural Fe2+ with hydrogen peroxide oxidized virtually all the structural Fe for all four smectites. The redox profiles of the redox-cycled smectites were fit using eq 2 to determine how reduction and reoxidation influenced observed E⌀H-values and β factors (Figure 2; fit parameters in Table S4). The model accurately fit the redox profiles, with minor uncertainties associated with the fittings of reduced NAu-1 and reduced NAu-2. All four smectites exhibited different redox profiles for native, reduced and reoxidized specimen (Figure 2). We previously saw similar differences for SWa-1, which were attributed to redox cycling of structural Fe exhibiting two forms of irreversibility.20 (i) During the first reduction−oxidation cycle, E⌀H-values and β factors of native and reoxidized SWa-1 differed from each other. This change was attributed to permanent alterations in the Fe coordination environments in SWa-1 as observed with X-ray absorption and Mössbauer spectroscopies,20 consistent with previous observations of structural changes in

To quantitatively interpret these differences, we used eq 2 to determine the apparent standard reduction potentials (E⌀H) and widths of the redox profiles (β) (Table 2). E⌀H represents the reduction potential at which there are equal amounts of Fe2+ and Fe3+. β factors are used to account for the widened redox profiles, with a β factor of 1 representing an ideal, Nernstian profile, and β factors less than 1 describing widened redox profiles. This model accurately fit all the redox profiles (lines shown in Figure 1). Both the fitted E⌀H-values and the β factors varied significantly among the smectites, consistent with visual observations. The E⌀H of Wyoming Montmorillonite SWy-2 (−0.03 ± 0.02 V) was substantially higher than that of the Fe-rich clay minerals (E⌀Hvalues between −0.37 and −0.45 V, Table 2), demonstrating that electrons were transferred to structural Fe3+ in SWy-2 under less reducing conditions than the other smectites. β-factors did not trend with E⌀H-values. SWy-2 exhibited the smallest β factor (0.23), meaning the redox profile was the widest of the four smectites, while NAu-1 had the largest β factor (0.52) and narrowest redox profile. Redox Profiles of Redox-Cycled Smectites. Redox profiles were collected for the four smectites at different stages of redox cycling to assess the reversibility of electron transfer (Figure 2, photographs of redox cycled smectites in Figure S5). Chemical reduction reduced virtually all the structural Fe3+ in SWy-2 and SWa-1, while significant fractions of Fe3+ remained unreduced in NAu-1 and NAu-2. The initial Fe2+ content of reduced NAu-1 was determined to be 48% by Mössbauer spectroscopy (Figure S6). The initial Fe2+ content of reduced NAu-2 was found to be 81%, based on 19% of the structural Fe 13480



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Figure 3. Fourier-transformed k3-weighted extended X-ray absorption fine structure (EXAFS) spectra of native, reduced, and reoxidized smectites. Corresponding X-ray absorption near edge structure (XANES) and k3-weighted EXAFS spectra and results from first-shell fits are provided in the SI.

other smectites undergoing redox cycling.12,14,21−30 (ii) In subsequent redox cycles, all structural changes were reversible, but the redox profiles for “reduced” (i.e., reduced and rereduced) and “oxidized” (i.e., reoxidized and re-reoxidized) SWa-1 significantly differed from one another (i.e., E⌀H values varied by 0.4 V). This latter form of irreversibility was attributed to the formation of metastable Fe states during redox cycling. Here, both of these forms of irreversibility were observed in the redox profiles of all four smectites: The reduced and native redox profiles differed from each other, although the magnitude of the difference varied. Wyoming montmorillonite SWy-2 exhibited close to reversible electron transfer (Figure 2a), while the other smectites exhibited pronounced irreversibility (Figure 2, panels b-d). The magnitude of the differences trended well with structural Fe content, consistent with the hypothesis that these differences arise as a result of structural reorganizations of the Fe local environments that occur during redox cycling of the smectites. Differences were also observed between the native and reoxidized redox profiles for some of the smectites (Figure 2), indicating that some of the changes induced by redox cycling the smectites persisted after complete reoxidation. How the native and reoxidized redox profiles differed from each other varied among the smectites, with reoxidized samples being reduced at more positive EH-values for SWa-1 and NAu-2, the same EHvalues for NAu-1, and slightly more negatives EH-values for SWy2. The lack of a systematic trend between native and reoxidized redox profiles suggests that the redox-cycling induced changes were mineral-dependent. To relate the differences among the

native, reduced, and reoxidized redox profiles to structural changes that occurred in the smectites during redox cycling, we examined the smectites at each stage of redox cycling using X-ray absorption spectroscopy. X-ray Absorption Spectroscopy (XAS) of Redox Cycled Smectites. Native, reduced, and reoxidized smectites were analyzed by K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy to assess structural changes during redox cycling (Figure 3). X-ray absorption near edge structure (XANES) spectra, EXAFS spectra in r-space, and parameters for first-shell fits, and are provided in the SI (Figure S4, Table S3). For all four smectites, the EXAFS spectra of the native and reduced sample varied (Figure 3). These differences are most apparent for the peak intensities at approximately 1.5 Å, which represents the first shell amplitudes. Differences are also present for the second shell amplitudes, which are observed at approximately 2.8 Å for all the smectites except SWy-2 (which has a sufficiently low structural Fe content such that there are negligible Fe−Fe next nearest neighbors). Similar decreases in first and second shell peak intensities have been observed for other reduced smectites, and have been generally attributed to increased structural distortion in reduced smectites that arise from octahedral Fe2+ having a substantially larger ionic radius than octahedral Fe3+.56 We also observed significant differences in the EXAFS spectra of the native and reoxidized spectra for all the smectites (Figure 3), which again are most apparent for the first and second shell peak amplitudes at 1.5 and 2.8 Å, respectively. These differences 13481



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Figure 4. Scatterplot matrix of smectite structural properties, E⌀H-values, and β factors constructed from the values in Table 2. Shaded subplots contain linear relationships between structural properties and E⌀H-values or β factors that exhibited high correlation factors. 95% confidence intervals for the linear fits are shown as dotted lines. Error bars represent the standard deviation of either measured values or one-sigma statistical uncertainties calculated during the fitting of EXAFS and Mössbauer spectra. Error bars for wt % Fe, CS, QS, and E⌀H are contained within the markers.

linear correlations. We acknowledge, however, that nonlinear trends are also conceivable and presumably more appropriate. Within this large data set, E⌀H-values linearly correlated well with one molecular-scale (QS) and one bulk (layer charge) parameter. Due to the limited number of data points, our discussion emphasizes the two parameters which exhibited the strongest linear correlations with both E⌀H- and β-values. At the molecular-scale, more positive E⌀H-value were observed for smectites having larger quadrupole splitting (QS) values (Figure 4, pink shaded area). For example, SWy-2 exhibited a considerably more positive E⌀H-value (−0.03 V) as compared to the other smectites (−0.37 to −0.45 V) as well as a much larger QS value (0.96 mm/s) relative to the others (0.45 to 0.64 mm/s). This trend is consistent with previous observations that structural Fe in smectites exhibiting larger QS values was preferentially reduced over Fe sites having smaller QS values.20,58,60,61 QS values arise from symmetry distortions of the local electronic environment at the nucleus, and are analogous to 3d orbital splitting values that arise from crystal field splitting.62 Further interpretation of large QS values as indicators for higher E⌀H, however, is beyond the scope of this study, due to the recognition that (i) many coexisting factors can simultaneously influence QS values for minerals and (ii) QS values cannot be predicted for mineral structures.62−66 Also, the observed trend may be limited to smectites; illites often exhibit relatively high QS values (≈0.6 mm/s),67 but are typically reduced to a much smaller extent than smectites under similar reducing conditions.1,34 Among bulk structural parameters, E⌀H-values were found to be more positive for smectites having lower layer charge values (Figure 4, yellow shaded area). A similar trend has been observed for illites,34,58 and was attributed to higher layer charges making it more difficult for interlayers to expand and be accessible to charge transferring

indicate that redox cycling of Fe in the smectites induced permanent structural changes in the Fe coordination environment, which is consistent with previous observations on redoxcycled smectites by infrared (IR) and Mössbauer spectroscopies.12,20,25,27−29 These changes have been linked to irreversible dehydroxylation reactions within the smectites during the reduction step and to redox-induced migration of Fe atoms within the octahedral sheet. We observed that the spectrum of reoxidized NAu-1 closely resembled the spectrum of native NAu-2 (see SI), possibly indicating that NAu-1 and NAu-2 may have similar structural origins, but different redox histories.57 Relationship between Smectite Redox Profiles to Mineralogy. To determine if differences in measured E⌀H and β values for the smectites may be attributable to differences in mineralogy, we related these values to several structural properties for each of the smectites in their native redox state (Figure 4). The structural properties included: (i) bulk properties (structural Fe content and layer charge) and (ii) molecular-scale properties (mean and distributions of Fe−O bond lengths and Fe hyperfine parameters). This broad range of parameters was selected because previous studies have linked the rate and extent of redox reactions involving structural Fe in smectites to an array of both bulk1,2,13,17,34,58 and molecularscale9,25,26,33,36,37,59 properties. To visually compare trends among E⌀H-values, β factors, and structural parameters and to guide the following discussion, we plotted the variables against each other in a scatterplot matrix (Figure 4). Note the large uncertainty of this comparison, which is due to the fact that most of the properties examined change as a result of Fe3+ reduction (i.e., layer charge, Fe−O1, CS, QS). Moreover, since only four data points were available, we decided to examine exclusively 13482



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EH-values yields β factors similar to those determined for smectites here.46,85 Previous works have also observed that electron transfer to and from hematite (α-Fe 2 O 3 ) is thermodynamically irreversible,86,87 consistent with our findings for smectites. Collectively, these results indicate that complex, non-Nernstian redox behavior may be prevalent for Fe-bearing minerals, which underscores their importance as redox buffers and requires future consideration in assessing biogeochemical Fe cycling.

compounds and absorbable cations needed to maintain charge balance during structural Fe3+ reduction.34 Using the same comparative approach, we also found trends between β factors and structural parameters. Redox profile widths narrowed, that is they exhibited more Nernst-like behavior and β approached 0.5, as structural Fe contents increased (Figure 4, blue shaded area) and QS values decreased (Figure 4, green shaded area). Determining why these parameters may correlate with β factors is challenging, since the underlying origin for the widening of the redox profiles remains unclear. To date, similar widened redox profiles have been observed for reduced and oxidized organic polymers45,47−49,68−73 and magnetite.46 Wide redox profiles of organic polymers were attributed to intrapolymeric interactions of redox-active sites. Our observations here are not fully consistent with this conceptual model since smectites with higher Fe contentsand hence increased Fe−Fe interactions exhibited the opposite trend (i.e., redox profiles narrowed with increased Fe content). In the study examining magnetite mentioned above, redox profile widening was attributed to Fe atoms within the magnetite lattice having different activity coefficients and/or reduction potential values. It is well established from spectroscopic studies that Fe can indeed exist in a range of local chemical environments in smectites,4,9,25−27,33,59 although it is unclear if and how this range would be related to structural Fe content. Whether or not the activity coefficients of Fe2+ and Fe3+ differ from one another remains unclear; activity coefficients of elements in minerals have been shown to deviate from unity,74−79 but no work has specifically explored how oxidation state affects them. Collectively, these observations suggest that widening of redox profiles is most likely due to Fe existing in a range of local coordination environments, although the possibility that Fe2+ and Fe3+ exhibit different activity coefficients cannot be ruled out.

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ASSOCIATED CONTENT

S Supporting Information *

The clay minerals and chemicals used in this study, details regarding the synthesis and characterization of zwitterionic viologen, a comparison of the new and old mediator sets with native NAu-1, cyclic voltammograms of the mediators, details and fit parameters for XAS analyses, photographs of the redox cycled smectites, and fitted E⌀H-values and β factors for the redox cycled smectites. This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*(M.S.) E-mail: [email protected]. *(T.B.H.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Bart Baeyens and Maria Marques Fernandes (Paul Scherer Istitut, Switzerland) for purifying and size-fractionating SWa-1, and Olivier Leupin (Nagra, Switzerland) and Anke Neumann (University of Iowa) for fruitful discussions. We acknowledge the Angströmquelle Karlsruhe (ANKA, Karlsruhe, Germany) for the provision of beamtime at the XAS beamline and Stefan Mangold for his assistance during data collection. This work was supported by the Swiss National Science Foundation (grant no. 200021-129476/1 to T.B.H. and M.S.) and Nagra (project no. 7246 and 9009).

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ENVIRONMENTAL IMPLICATIONS Our data collectively indicate that structural Fe in smectites is (i) redox-active over wide EH ranges, (ii) undergoes irreversible coordination changes during redox cycling, and (iii) exhibits E⌀Hvalues and β factors that vary as a function of mineralogy. The measured E⌀H-values and β factors linearly correlated with structural parameters that can be readily measured, particularly QS values, suggesting that it may be possible to use structural properties to predict Fe2+/Fetotal relationships. Such approximations will be useful for attempting to model the roles that smectites play in biogeochemical redox cycles and contaminant transformation in natural and engineered systems. For instance, the redox profiles developed here can be used to compare the extents of biological reduction of different smectites on the basis of EH-values, which may offer additional insights into whether thermodynamics controls the reduction and oxidation of structural Fe in smectites and other clay minerals (e.g., ref 1 and refs therein). Studies comparing the extent of biological NAu-1 and NAu-2 reduction agree with trends in E⌀H-values measured here.58,80 Our findings that smectites exhibit complex, non-Nernstian redox profiles also raises the question if and to what extent other mineral phases exhibit similar complexities. While addressing this question is challenging, there is ample evidence to suggest that environmentally relevant compounds, such as natural organic material81−84 and Fe oxides, also display wide EH distributions and electron transfer irreversibility. For magnetite (Fe3‑δO4), a common Fe oxide, the relationship between Fe2+/Fe3+ ratios and

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Redox Properties of Structural Fe in Clay Minerals - American

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