Article pubs.acs.org/est
Redox Properties of Structural Fe in Clay Minerals. 1. Electrochemical Quantification of Electron-Donating and -Accepting Capacities of Smectites Christopher A. Gorski,† Michael Aeschbacher,‡ Daniela Soltermann,§,‡ Andreas Voegelin,†,‡ Bart Baeyens,§ Maria Marques Fernandes,§ Thomas B. Hofstetter,*,†,‡ and Michael Sander*,‡ †
Eawag, Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland Institute of Biogeochemistry and Pollutant Dynamics (IBP), ETH Zürich, Zürich, Switzerland § Laboratory for Waste Management, Paul Scherrer Institut (PSI), Villigen, Switzerland
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‡
S Supporting Information *
ABSTRACT: Clay minerals often contain redox-active structural iron that participates in electron transfer reactions with environmental pollutants, bacteria, and biological nutrients. Measuring the redox properties of structural Fe in clay minerals using electrochemical approaches, however, has proven to be difficult due to a lack of reactivity between clay minerals and electrodes. Here, we overcome this limitation by using oneelectron-transfer mediating compounds to facilitate electron transfer between structural Fe in clay minerals and a vitreous carbon working electrode in an electrochemical cell. Using this approach, the electron-accepting and -donating capacities (QEAC and QEDC) were quantified at applied potentials (EH) of −0.60 V and +0.61 V (vs SHE), respectively, for four natural Fe-bearing smectites (i.e., SWa-1, SWy-2, NAu-1, and NAu-2) having different total Fe contents (Fetotal = 2.3 to 21.2 wt % Fe) and varied initial Fe2+/Fetotal states. For every SWa-1 and SWy-2 sample, all the structural Fe was redox-active over the tested EH range, demonstrating reliable quantification of Fe content and redox state. Yet for NAu-1 and NAu-2, a significant fraction of the structural Fe was redox-inactive, which was attributed to Fe-rich smectites requiring more extreme EH-values to achieve complete Fe reduction and/or oxidation. The QEAC and QEDC values provided here can be used as benchmarks in future studies examining the extent of reduction and oxidation of Fe-bearing smectites.
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INTRODUCTION
Developing a thorough understanding of clay mineral redox properties in the context of biogeochemistry has, however, proven to be challenging. Structural Fe reduction and oxidation is coupled to structural rearrangements in the clay mineral lattice that occur to maintain charge balance and accommodate the atomic size difference between Fe3+ and Fe2+.20−25 As a result, clay mineral redox reactions are dependent on both the clay mineral properties (i.e., the total Fe content, the Fe binding environment, the layer charge, and mineral structure) and the solution chemistry (e.g., pH and the reductant or oxidant used). In addition to the complexity of clay minerals, methodological limitations have also impaired our understanding of clay mineral redox properties at a mechanistic level. Previous experimental approaches have been unable to directly quantify the number of electrons transferred to and from a clay mineral over the course of a reaction under well-defined
The Fe2+/Fe3+ redox couple is an important redox buffer in the environment that affects biogeochemical element and nutrient cycling and controls the partitioning and redox transformations of organic and inorganic contaminants.1−5 Much of the Fe in the lithosphere is present as a structural component of clay minerals (i.e., phyllosilicates),1 where it can participate in a variety of electron transfer reactions. Structural Fe2+ in clay minerals may reduce heavy metals, radionuclides, and organic contaminants, altering their mobility, (bio)availability, and toxicity.6−12 Structural Fe3+, in contrast, may act as a terminal electron acceptor for Fe-reducing bacteria, providing a mechanism by which structural Fe2+ can be (re)generated.13 The structural Fe redox state also affects the physicochemical properties of clay minerals, which can include the cation exchange capacity and the swelling capacity.14−18 These changes can alter the fate and (bio)availability of redox-inactive contaminants and nutrients (e.g., K+, Ca2+)18 as well as dictate the viability of clay minerals in engineered systems (e.g., clay mineral backfill in radioactive waste repositories).19 © 2012 American Chemical Society
Received: Revised: Accepted: Published: 9360
May 20, 2012 July 16, 2012 July 24, 2012 July 24, 2012 dx.doi.org/10.1021/es3020138 | Environ. Sci. Technol. 2012, 46, 9360−9368
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Scheme 1. Mediated Electrochemical Reduction and Oxidation of SWa-1
Table 1. Fe Contents and Electron Accepting and Donating Capacities (QEAC and QEDC, respectively) of Natural Fe-Bearing Clay Minerals As Determined by MER and MEO at EH = −0.60 V and EH = +0.61 V, Respectively Fe content
a
electron capacity [mmole−/gclay]
clay mineral
wt %
mmolFe/gclay
redox treatment
SWa-1
12.6 ± 0.1
2.26 ± 0.02
SWy-2
2.3 ± 0.2
0.41 ± 0.04
NAu-1
21.2 ± 1.1
3.80 ± 0.20
NAu-2
19.2 ± 0.3
3.44 ± 0.05
native dithionite-reduced partially reduced partially re-oxidized native dithionite-reduced native dithionite-reduced native dithionite-reduced
QEAC 2.21 0.05 1.46 1.71 0.40 0.00 3.11 0.68 3.41 0.24
± ± ± ± ± ± ± ± ± ±
0.01 0.01 0.07 0.02 0.04 0.00 0.10 0.12 0.14 0.01
QEDC 0.00 2.24 0.76 0.44 0.00 0.39 0.00 2.37 0.00 2.55
± ± ± ± ± ± ± ± ± ±
0.00 0.02 0.03 0.12 0.00 0.01 0.00 0.16 0.00 0.02
Qtotal 2.21 2.29 2.22 2.15 0.40 0.39 3.11 3.06 3.41 2.78
± ± ± ± ± ± ± ± ± ±
0.01 0.03 0.08 0.12 0.04 0.01 0.10 0.20 0.14 0.03
redox-active Fe [%]a 97.8 101.4 98.6 95.3 96.5 94.5 81.8 80.5 99.3 81.0
± ± ± ± ± ± ± ± ± ±
1.0 1.6 3.6 5.4 13.6 9.6 5.0 6.8 4.2 1.5
Total standard deviations in Fe content and Qtotal were calculated from at least five and four replicate measurements, respectively.
WE. This approach has several unique features that would be beneficial to studying the redox properties of structural Fe in clay minerals: (i) the number of electrons transferred to and from the clay mineral can be directly quantified, (ii) the experiments can be conducted under well-controlled solution and EH conditions, (iii) the effect of solution pH on the structural Fe redox properties can be readily elucidated because the electron transfer to and from the selected mediators is unaffected by pH, and (iv) the analysis can be performed on the time scale of minutes to hours. The goal of this work was therefore to evaluate the applicability of MER and MEO to Fe-bearing clay minerals and to determine the electron-accepting capacities (QEAC) and electron-donating capacities (QEDC) of clay minerals under well-defined solution conditions and EH-values. To this end, we adapted the MER and MEO approach to quantify QEAC and QEDC values at strongly reducing (EH = −0.60 V) and strongly oxidizing (EH = +0.61 V) redox potentials, respectively, for four natural Fe-bearing smectites (i.e., SWa-1, SWy-2, NAu-1, and NAu-2). These smectites were selected because they have been studied extensively in the past and because their structural Fe content varies considerably, ranging from 2.3 (SWy-2) to 21.2 (NAu-1) wt % Fe (Table 1). The minerals were characterized using 57Fe Mössbauer spectroscopy to confirm their purity and to explore relationships between QEAC and QEDC values and the local coordination environment of structural Fe atoms. The results from this study build the foundation for the companion paper, in which electron transfer to and from structural Fe in
solution conditions. Instead, changes in the redox states of structural Fe were inferred from spectroscopic measurements and/or labor-intensive acidic dissolution studies.9,13,26−29 Other studies monitored the reduction of probe compounds to assess the reactivity of structural Fe in the clay minerals.6−8,11,30−32 While such approaches provide information on the relative reactivity of structural Fe in different clay minerals, results from these studies may be dependent on the experimental conditions used. To broaden our understanding of clay mineral redox properties, the measurement of thermodynamic values is clearly desirable. To arrive at such values, novel experimental approaches are required that allow for a direct quantification of electron transfer to and from structural Fe as a function of solution conditions and redox potential (i.e., EH). Our group has recently demonstrated direct quantification of the number of electrons transferred to and from natural organic matter (NOM) samples using mediated electrochemical reduction (MER) and oxidation (MEO).33−37 MER and MEO rely on the use of one-electron-transfer mediating compounds (i.e., mediators) to facilitate electron transfer between the sample of interest and a working electrode (WE) (Scheme 1). In this approach, an electrochemical cell containing a pH-buffered solution is set to a constant EHvalue while the current is measured over time. In the presence of a mediator in redox equilibrium with the WE, known amounts of a sample are added to the electrochemical cell, resulting in current responses that can be integrated to directly determine the number of electrons transferred to or from the 9361
dx.doi.org/10.1021/es3020138 | Environ. Sci. Technol. 2012, 46, 9360−9368
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SWa-1 is explored as a function of EH.38 Together, the companion papers illustrate the applicability of MER and MEO to assess the redox properties of Fe in solid-phase samples.
was added (3 times the clay mineral mass in suspension), and the suspension was stirred at 70 °C overnight. A portion of the suspension was then taken and put into a prewashed and dried dialysis tube (MWCO 12400 Da, Sigma Aldrich), and added to 1 L of deoxygenated 0.1 M NaClO4. The suspension was equilibrated for at least 8 h, at which point the dialysis tubing was placed into fresh NaClO4 buffer. This process was repeated four times. Native clay mineral samples were produced in an identical manner, except that no sodium dithionite was added to the citrate−bicarbonate buffer. Partially reoxidized SWa-1 was obtained by purging dithionite-reduced SWa-1 with air outside of the glovebox for approximately 18 h, at which point it was purged with Ar for 2 h and transferred back into the glovebox. The clay mineral concentration was determined gravimetrically by drying the suspension at 105 °C while accounting for the NaClO4. The Fe content of the clay minerals was measured according to an established acidic digestion method30 adapted from earlier procedures,42,43 using ferrous ethylenediammonium sulfate tetrahydrate as the Fe standard. 57 Fe Mö ssbauer Spectroscopy. Samples for Mössbauer analysis were filtered, dried, and ground in the anaerobic glovebox prior to analysis. Measurements were made using a previously described setup.44 Spectra were fit using the commercial software Recoil (Ottawa, Canada) and a Voigtbased model (fit parameters shown in Table S3).45 57Fe Mössbauer spectra were collected for each smectite at 13 K after the purification procedure to determine the structural coordination and oxidation state of the Fe (Figure 1). Spectra collected at 40 K were consistent with 13 K spectra (Figure S4). All spectra were characteristic of Fe3+ in clay minerals, with no indication of any Fe2+ phases or Fe3+ (oxyhydr-)oxide impurities (detection limit ≈1−2% Fe).46 The fitted hyperfine parameters of SWa-1, SWy-2, and NAu-1 indicated that the minerals contained only cis-octahedral Fe3+ (OctFe3+; i.e., the two binding hydroxyl groups were adjacent to one another). The NAu-2 spectrum was more complex: ≈2% of the structural Fe3+ was tetrahedrally coordinated (TetFe3+) and a second octahedral Fe3+ site (i.e., OctFe3+ [2], 32% of area) was observed. The OctFe3+ [2] site has previously been interpreted as both trans-octahedral Fe3+ (i.e., two hydroxyl groups opposite each other)47−51 and distorted cis-octahedral Fe3+ near a tetrahedral Fe3+ atom28,52,53 (Figure 1). Electrochemistry. Electrochemical experiments were controlled with a 630D electrochemical analyzer (CH Instruments, Austin, TX). Potentials were measured versus an Ag/AgCl reference electrode but are reported versus the standard hydrogen electrode (SHE). MER and MEO experiments were carried out in electrochemical cells containing 80 mL of pH 7.5 buffer, a cylindrical vitreous carbon working electrode (WE), a Pt wire counter electrode separated from the WE compartment by a glass frit, and an Ag/AgCl reference electrode (all electrodes from Bioanalytical Systems, UK) (Scheme 1). Cyclic voltammetry (CV) experiments were conducted in a 8−10 mL solution of buffer containing a 3.0 mm diameter glassy carbon disk working electrode, a platinum wire counter electrode, and an Ag/AgCl reference electrode (all electrodes from Bioanalytical Systems Inc.). CV scans were done at 10 mV·s−1 over a potential range of approximately ±0.3 V of the expected E0H-values. CV was used to measure the redox potentials (E0H) of diquat (DQ2+/DQ•+, −0.35 V), triquat (TQ2+/TQ•+, −0.54 V), and ABTS (ABTS2−/ABTS•−, +0.70
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MATERIALS AND METHODS Materials. Section S1 of the Supporting Information contains a complete list of chemicals used in this study. Ferruginous smectite (SWa-1), Na-rich montmorillonite (SWy2), and two nontronites (NAu-1 and NAu-2) were purchased from the Source Clay Minerals Repository (Purdue University, West Lafayette, IN). Literature unit cell formulas of the clay minerals are provided in Table S2. ABTS (2,2′-azinobis(3ethylbenzothiazoline-6-sulfonic acid), 99%) and diquat (1,1′ethylene 2,2′-bipyridyl dibromide, DQ, 100%) were purchased from Sigma-Aldrich (St. Louis, MO). Triquat (1,1′-trimethylene 2,2′-bipyridyl dibromide, TQ) was synthesized following an established method39 and was recrystallized in MeOH, as described in the Section S2. NMR, high-resolution mass spectrometry (thermoexactive MS), and cyclic voltammetry were used to confirm the identity and high purity of the triquat (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 MOPS, pKa = 7.2). Anaerobic Conditions. All experiments were conducted in an anaerobic glovebox ( +0.61 V), QEDC values decreased. These observations were attributed to mediated electrolysis of H+ and H2O, respectively. The QEAC of native SWa-1 was measured by sequentially spiking increasing masses of SWa-1 (39 to 780 μg) to a cell that was pre-equilibrated with triquat at EH = −0.60 V (Figure 3, top). Each spike resulted in sharp, well-defined current peak that could be readily isolated from the background current, with the peak currents increasing concurrently with larger masses of SWa-1 added. SWa-1 current peaks were integrated with respect to time to determine the number of electrons (q, mole−) transferred from the WE to the native SWa-1 (eq 1). The mediator current peak was also integrated to confirm that the measured extent of mediator reduction agreed with the expected value calculated at the applied EH using the Nernst equation.33 A strong linear correlation was observed between q and gSWa−1 added (R2 = 1.00; Figure 3, top inset), demonstrating that the current peak area was directly proportional to the amount of native SWa-1 added. The slope of this correlation provided a QEAC value of 2.21 ± 0.01 mole−/gSWa‑1 (Table 1). The QEAC of dithionite-reduced SWa-1 was also measured under the same experimental conditions. Its QEAC value (0.05 ± 0.01 mole−/gSWa‑1) was much smaller than that of the native SWa-1, which was consistent with the dithionite-reduced SWa1 containing negligible Fe3+ after the dithionite reduction step (Table 1). In a similar manner, the QEDC (i.e., donating capacity) of dithionite-reduced SWa-1 was determined from replicate spikes containing sequentially increasing masses (73 to 549 μg) to a
Figure 3. (Top) Mediated electrochemical reduction of native SWa-1 at EH = −0.60 V using triquat to mediate electron transfer between the WE and the SWa-1. (Bottom) Mediated electrochemical oxidation of reduced SWa-1 at EH = +0.61 V using ABTS as the mediator. Each peak is labeled with the mass of SWa-1 added. The inset contains the integrated peak area (q) plotted against gSWa‑1.
cell equilibrated with ABTS at EH = +0.61 V (Figure 3, bottom). A strong linear correlation was observed between q and gSWa−1 added (R2 = 0.97; Figure 3, bottom inset), demonstrating that the current response was directly proportional to the amount of dithionite-reduced SWa-1 added. The dithionite-reduced SWa-1 QEDC value was 2.24 ± 0.02 mmole−/ gSWa‑1, which was identical within experimental error to the QEAC of native SWa-1. Native SWa-1 exhibited no electrondonating capacity (i.e., QEDC = 0 mmole−/gSWa‑1; Table 1), which was consistent with the native SWa-1 Mössbauer spectrum exhibiting no indication of structural Fe2+ (Figure 1). Note that preferential sorption of one mediator redox species over the other was ruled out as a source of the current responses observed for the following reasons: (i) QEAC and QEDC values were approximately 1 order of magnitude greater than the theoretical maximum current response that could have resulted from preferential sorption of one mediator species to the SWa-1 surface (i.e., 0.27 mole-/gSWa‑1, see Section S3) and (ii) if the current response was induced by preferential mediator sorption, similar current response for the native and dithionite-reduced SWa-1 samples at any applied EH-value would be expected, which was not the case. Qtotal values for the native and dithionite-reduced SWa-1 were calculated by summing QEAC and QEDC values (native SWa-1: 2.21 ± 0.01 mmole−/gSWa‑1, dithionite-reduced SWa-1: 2.29 ± 0.03 mmole−/gSWa‑1; Table 1). When these values were 9364
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Figure 4. Electron-donating and -accepting capacities of Fe-bearing smectites with different redox histories. Reduction and oxidation values were calculated by relating QEAC and QEDC values, respectively, to the clay mineral total Fe content (Table 1). Nat. = native, Red. = dithionite-reduced, Part Red. = partially reduced (by dithionite), Part Re-ox = partially re-oxidized (by air).
(E0H = +0.77 V at pH 7) was likely due to slow Fe2+ re-oxidation kinetics by O2 because of its low first-electron reduction potential (E0H = −0.16 V at pH 7).57 Electron-Accepting and -Donating Capacities of Smectites. QEAC, QEDC, and Qtotal values were quantified for three additional Fe-bearing smectites (SWy-2, NAu-1, and NAu-2) in their native and dithionite-reduced redox states to assess the applicability of MER and MEO to smectites that span a wide range of structural Fe contents (2.3 to 21.2 wt % Fe) and have different structural Fe coordination environments. Analysis of all three smectites showed substantial QEAC and QEDC values (Table 1), indicating that triquat and ABTS facilitated electron transfer between structural Fe in these clay minerals and the WE. As expected, Qtotal values increased concurrently with the amount of structural Fe in the smectites (Table 1). For SWy-2 (2.3 wt % Fe), Qtotal values for both the native (0.40 ± 0.04 mmole−/gSWy‑2) and dithionite-reduced (0.39 ± 0.01 mmole−/gSWy‑2) samples were identical within experimental error to the SWy-2 structural Fe content (0.41 ± 0.04 mmolFe/gSWy‑2, Table 1, Figure 4). The nearly identical Qtotal values for the native and dithionite-reduced SWy-2 indicated that the dithionite reduction step did not alter the fraction of redox-active Fe in SWy-2. For NAu-1, Qtotal values of the native (3.11 ± 0.10 mmole−/ gNAu‑1) and dithionite-reduced (3.06 ± 0.20 mmole−/gNAu‑1) samples were identical within experimental error, again indicating that the number of redox-active Fe sites was unaltered by the dithionite reduction step. When the Qtotal values of NAu-1 were compared to its structural Fe content (3.80 mmolFe/gNAu‑1), however, only about 81% of the structural Fe was found to be redox-active (Table 1, Figure 4). Because Qtotal values were equal to the structural Fe content for SWa-1 and SWy-2, the observed fraction of redox inactive Fe (19%) in NAu-1 was not considered to be an experimental artifact, but instead indicated that this fraction of Fe was not reduced or oxidized at the applied EH-values over the duration of the experiments. The observed fraction of redox-inactive Fe
compared to the Fe content of SWa-1 measured using acidic digestion (2.26 ± 0.02 molFe/gSWa‑1), the calculated percentage of redox-active structural Fe detectable with MER and MEO was approximately 100% for both the native (97.8 ± 1.0%) and dithionite-reduced (101.4 ± 1.6%) SWa-1 (Table 1, Figure 4). The similarity between the Qtotal values of native and dithionitereduced SWa-1 demonstrated that the amount of redox-active Fe was not altered by the dithionite reduction step and that MER and MEO could be used to quantify the QEAC, QEDC, and Qtotal of SWa-1. Because all the structural Fe was redox-active, the QEAC and QEDC values could be used to calculate the percentage of Fe2+ in the samples (native: 0%, dithionitereduced: 98%). Note that in this study, negligible reductive dissolution of Fe (
