Electron Transfer between Iron Minerals and Quinones: Estimating the

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Electron Transfer between Iron Minerals and Quinones: Estimating the Reduction Potential of the Fe(II)-Goethite Surface from AQDS Speciation Silvia Orsetti,* Christine Laskov, and Stefan B. Haderlein Eberhard Karls Universität Tübingen, Center for Applied Geosciences (ZAG), Hölderlinstrasse 12, 72074 Tübingen, Germany S Supporting Information *

ABSTRACT: Redox reactions at iron mineral surfaces play an important role in controlling biogeochemical processes of natural porous media such as sediments, soils and aquifers, especially in the presence of recurrent variations in redox conditions. Ferrous iron associated with iron mineral phases forms highly reactive species and is regarded as a key factor in determining pathways, rates, and extent of chemically and microbially driven electron transfer processes across the iron mineral−water interface. Due to their transient nature and heterogeneity a detailed characterization of such surface bound Fe(II) species in terms of redox potential is still missing. To this end, we used the nonsorbing anthraquinone-2,6-disulfonate (AQDS) as a redox probe and studied the thermodynamics of its redox reactions in heterogeneous iron systems, namely goethite-Fe(II). Our results provide a thermodynamic basis for and are consistent with earlier observations on the ability of AQDS to “shuttle” electrons between microbes and iron oxide minerals. On the basis of equilibrium AQDS speciation we reported for the first time robust reduction potential measurements of reactive iron species present at goethite in aqueous systems (EH,Fe‑GT ≈ −170 mV). Due to the high redox buffer intensity of heterogeneous mixed valent iron systems, this value might be characteristic for many iron-reducing environments in the subsurface at circumneutral pH. Our results corroborate the picture of a dynamic remodelling of Fe(II)/Fe(III) surface sites at goethite in response to oxidation/reduction events. As quinones play an essential role in the electron transport systems of microbes, the proposed method can be considered as a biomimetic approach to determine “effective” biogeochemical reduction potentials in heterogeneous iron systems.

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conditions,14,15 in real world applications including heterogeneous systems such measured redox potentials are often poorly reproducible and provide at best a rough estimate rather than a thermodynamically based measure of the actual redox conditions. Quinones are major redox active groups in natural organic matter (NOM),16−19 and, given the numerous molecular environments in NOM, such quinones comprise a variety of reduction potentials. Therefore, the reduction potential of NOM cannot be described by a single value but comprises a range which varies with the sample origin, fractionation procedure, etc.18,19 Quinones are considered as extracellular electron shuttles in microbial respiration20,21 and pollutant degradation.22 Several studies have addressed the role of quinones as e-shuttles between microbes and poorly soluble iron oxides regarding their effects on rates of dissimilatory Fe(III) reduction. Both inhibitory or stimulating effects of quinones on Fe(III) reduction rates were observed, depending on the reduction potential of the specific quinone23 and

INTRODUCTION In anoxic aquifers, the iron redox system often plays a major role in biogeochemical redox processes including transformation processes of pollutants1−3 due to the ubiquitous presence of iron in the lithosphere and the wide range of reduction potentials that Fe(II)/Fe(III) couples can cover.4,5 Under iron reducing conditions, ferrous iron typically is present sorbed to iron(III) minerals as well as in aqueous solution. It has been reported that the sorption of aqueous Fe(II) onto iron minerals increases its reducing reactivity, by lowering its standard redox potential compared to that in solution.6 However, redox conditions in environmental systems are far from the standard state7,8 and determining speciation and activities of sorbed ferric and ferrous iron at in situ conditions is experimentally challenging. Hence, a reliable experimental determination of the reduction potential of iron redox couples in heterogeneous aqueous systems would represent a major step forward toward a suitable characterization of such complex systems in terms of activity and energetics of the often transient iron species involved. Despite of its well-documented limitations,9−11 direct measurements of the redox potential in aqueous systems is mostly done by potentiometry using platinum electrodes.12,13 Even though this approach can produce accurate potentials under certain well-defined © 2013 American Chemical Society

Received: Revised: Accepted: Published: 14161

August 17, 2013 October 8, 2013 November 15, 2013 November 15, 2013 dx.doi.org/10.1021/es403658g | Environ. Sci. Technol. 2013, 47, 14161−14168

Environmental Science & Technology

Article

experimental conditions such as quinone concentrations.24 These studies focused on the kinetics of electron transfer processes rather than on thermodynamics at equilibrium conditions, and there is a lack of a systematic investigation of the reaction between anthraquinone-2,6-disulfonate (AQDS) and iron mineral in well-defined conditions. Other studies24,25 investigated abiotic electron transfer reactions between iron oxides and humic substances (HS) demonstrating that HS are capable of reducing Fe(III) in iron oxides, even if the HS were used in native state without reduction prior to the experiment. The systems studied included suspended iron oxides at anoxic conditions but in the absence of initially sorbed Fe(II). Also, quinones act as nonprotein electron transport carriers within the membrane of microorganisms. Being of hydrophobic nature, quinones can diffuse through the lipid membrane and carry electrons from iron−sulfur proteins to cytochromes.26 Therefore, the reactivity of a quinone in solution could be taken as a “biomimetic” electrochemical sensor, reflecting how microbes are affected by geochemical redox conditions of a heterogeneous system. Surface bound ferrous iron is commonly present in anoxic aquifers and dramatically changes the chemistry and the redox state of iron mineral surfaces. Fe(III)-bearing oxides and hydroxides (such as goethite (α-FeOOH)) with sorbed Fe(II) thus provide reactive surface sites that promote electron transfer to oxidants at the mineral−water interface.5,27−29 A thermodynamic characterization of the redox properties of such Fe(II) surface species is still missing. Thus, a profound understanding of the nature and thermodynamics of the electron exchange process between these two major players in biogeochemical redox processes, quinones and Fe(II)/iron oxides (see Scheme 1), is highly

surface characteristics of Fe(II)-goethite. Previous studies evaluated the thermodynamics regarding redox reactions between quinones and dissolved iron species but did not consider heterogeneous reactions.32,33 The redox dependent speciation of AQDS can easily be measured by UV−vis spectrometry in solution as absorbance spectra of the oxidized and reduced AQDS species differ significantly. Once the redox probe has equilibrated in the Fe(II)-goethite system and the activity ratio of AQDSred/ AQDSox is known, the apparent reduction potential (EH,AQDS) of the system can be calculated from this redox couple using the Nernst equation.

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EXPERIMENTAL SECTION Reagents. Anthraquinone-2,6-disulfonic acid disodium salt (AQDS, minimum 98%, Sigma-Aldrich, Steinheim, Germany) was used as received. Goethite (α-FeOOH, Bayferrox 920 Z) was provided by LANXESS; specific surface area (N2−BET): 9.2 m2/g ; pHpzc = 6.5.34 Fe(II) stock solutions (0.5 M in 1 M HCl) were prepared from Fe(0) powder as described elsewhere.34 Fe(II) Determination. Fe(II) concentrations were measured with the ferrozine assay in duplicates. Aqueous Fe(II) was measured in filtered (0.45 μm) samples. Fe(II) associated with goethite was determined by first removing dissolved Fe(II) from the sample by filtrating an aliquot of the suspension and rinsing the solids on the filter 5 times with Millipore water. Next, the solid material on the filter was resuspended in 1 M HCl for 24 h. Fe(II) in this filtrate was considered as “sorbed” Fe(II). The procedure was performed in anoxic conditions and in duplicates. The total Fe(II) was calculated as the sum of the aqueous and sorbed Fe(II): Fe(II)TOT = Fe(II)AQ + Fe(II)SORBED

Scheme 1. Scheme of the Two-Electron Transfer Reaction between Goethite-Fe(II) and AQDSox

(1)

The loss of Fe(II) by oxidation is determined by the difference between initial and final total Fe(II), according to eq 2: Fe(II)LOSS = Fe(II)0TOT − Fe(II)FTOT

(2)

It is important to point out that total elimination of the quinone/hydroquinone is needed before sorbed iron is extracted with HCl; otherwise, a shift in the reaction takes place (Fe(III) reduction and quinone oxidation) due to the low pH (data shown in SI). This would result in a strong artifact in the Fe(II) determination. Electrochemical Reduction of Quinones. In order to obtain the UV−vis spectra of all reduced AQDS species, dissolved AQDSox was reduced electrochemically at pH 7 in the presence of 0.1 M KCl in a phosphate buffer using a glassy carbon working electrode, a Pt wire auxiliary electrode, and an Ag/AgCl reference electrode (all from Bioanalytical Systems Inc., West Lafayette, IN). A potential of −500 mV vs SHE was applied with an Autolab PGSTAT101 potentiostat (Metrohm, Germany). After reduction of AQDSox was completed, the pH was adjusted with NaOH or HCl to pH 3, 9, or 12 and the spectra of the reduced AQDS species were recorded. The electrochemical reduction of AQDS for further use in the experiments was carried out using the same procedure as previously described, but in the absence of phosphate buffer to avoid interferences with the iron chemistry. The pH was adjusted to 7 prior to the reduction with HCl/NaOH, and the process was conducted in a flow through system with an

relevant for the characterization of the redox state of many natural systems. Considering reactive quinones as redox probes and determining their equilibrium speciation offers a novel and alternative approach to characterize and quantify the redox potential in such complex heterogeneous systems. The aim of this study is therefore to investigate abiotic electron transfer processes between dissolved quinones in various redox states and goethite, in the presence and absence of Fe(II), with respect to (i) resulting quinone speciation to estimate the apparent reduction potential of the iron mineral suspension, (ii) ability of the electron transfer processes to proceed from both starting points (“backwards” reaction), and (iii) validation of the approach at equilibrium conditions. We chose anthraquinone-2,6-disulfonate (AQDS: E0H(pH 7) = −184 mV)30 as a model quinone as it has been widely used as surrogate of redox moieties in NOM in previous studies on electron shuttling between microbes and iron oxides.24,31 Most important with regard to our study, however, is that sorption of AQDS to goethite is expected to be negligible, since lack of significant sorption to ferryhidrite has been reported.23 Therefore, presence of AQDS is not expected to change the 14162

dx.doi.org/10.1021/es403658g | Environ. Sci. Technol. 2013, 47, 14161−14168


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Figure 1. AQDS speciation (left) and absorbance spectra of the relevant species in this study (right).

automatic titrator, following the work of Aeschbacher et al.19 A portion of 0.1 M HCl was added in order to keep the pH constant. All electrochemical reductions were carried out inside an anoxic glovebox. Preparation of GT- and GT-Fe(II) Stock Suspensions. The final concentration of goethite in the suspensions corresponded to a surface area of 100 m2/L. Goethite was suspended in Millipore water and rinsed three times by shaking the suspension overnight and eliminating the supernatant after decanting, in order to remove ions from the surface. This suspension was transferred to a serum glass bottle, sealed with butyl rubber stopper, purged with N2, and then transferred into the glovebox where the pH was adjusted and kept to 7.0 ± 0.1 with HCl and/or NaOH. To this GT suspension, Fe(II) was added from the stock solution in three steps under constant stirring to achieve a final total Fe(II) concentration of around 3.0 mM. After each step, the pH was readjusted to pH 7 (GTFe(II) suspension). Experimental Assays and Procedures. All experiments and air sensitive procedures were conducted inside an Unilab anoxic glovebox (M. Braun, Germany) with an O2 content below 1 ppm (N2 atmosphere, H2 free). Three different experimental setups were used in order to address the following: (i) whether AQDSox can be reduced by Fe(II) associated with goethite and to determine its speciation at equilibrium conditions; (ii) whether AQDSred can be oxidized by Fe(III) in goethite in the absence of initial Fe(II); (iii) minimal disturbance of the Fe(II)-goethite redox equilibrium, by adding a mixture of AQDSox and AQDSred at a ratio close to that in apparent redox equilibrium with GTFe(II) system. The terminology and composition of these three setups were as follows: (i) GT-Fe(II) + AQDSox: final total concentrations of AQDSox were 100, 500, and 800 μM. (ii) GT + AQDSred: final total concentrations of AQDSred were 50, 100, and 200 μM. (iii) GT-Fe(II) + AQDSox/AQDSred: a 1 mM solution containing 68% AQDSox and 32% AQDSred as initial concentrations (prior to reaction with GT-Fe(II)) was used. A Pt ring electrode (Inlab 501, Mettler Toledo) was used to determine a measured redox potential of this suspension in order to compare these values to the ones obtained from AQDS as in situ redox probe.

General Procedure. Batches with 25 mL aliquots of the previously described GT- or GT-Fe(II) stock suspensions were equilibrated with either AQDSox, AQDSred, or AQDSox/ AQDSred in a 1:1 dilution, resulting in the final quinone concentrations specified above and a goethite surface area of 50 m2/L. Controls contained only the correspondent GT suspension and Millipore water. In the systems with (total or partially) prereduced AQDSred, controls with only stock quinone solution and Millipore water served to check the absence of oxygen in the glovebox during the time of the experiment. All batches were prepared in duplicates. The pH was adjusted to 7.0 ± 0.1 with HCl or NaOH. Redox reactions of AQDS in the suspensions generally were in the time scale of seconds to minutes (data not shown), but to ensure equilibrium conditions, 24 h of contact time was allowed before the suspension was filtered and the absorbance spectra of the supernatant were recorded using airtight 1 cm quartz cuvettes and a photoLab 6600 UV−vis spectrophotometer (WTW, Germany). Also, aqueous and sorbed Fe(II) were determined.

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RESULTS AND DISCUSSION Quantification of the AQDS Redox Probe. The scheme in Figure 1 shows the general speciation of AQDS. In addition to the redox speciation, acid/base equilibria exist for the semiquinone and hydroquinone redox states. At the conditions of our study (pH 7), however, only the quinone (AQDS) and the hydroquinone species (AQDSH2; AQDSH−; AQDS2−) are present; the semiquinone redox state (AQDSH; AQDS−) is not stable at neutral pH.35 The right panel in Figure 1 shows significant differences for the absorbance spectra of the prevalent AQDS species confirming that a spectroscopic determination of their concentrations is feasible. Our recorded UV−vis spectra of the various AQDS species are consistent with those reported in the literature.35 At neutral pH, a mixture of the di- and monoprotonated hydroquinone is present. Since the absorbance of the reduced AQDS depends on the pH, quantification of the hydroquinone in the following experiments was performed considering the difference of the peak intensity at 328 nm (oxidized form) before and after aerating the sample. Complete oxidation under these conditions occurs within a few seconds. Speciation of the AQDS Redox Probe in GT- and GTFe(II) Suspensions. Figure 2 (left panel) shows the 14163



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Figure 2. Reaction of AQDSox with goethite and Fe(II) at pH = 7 in anoxic aqueous suspensions. (left panel) Absorbance spectra of AQDS (filtrate). The color-coded lines correspond to total concentrations of AQDS, regardless of the redox state. (right panel) Electron turnover and distribution of Fe(II) as a function of AQDSox added.

Figure 3. Reaction of AQDSred with goethite only (no Fe(II) added) at pH = 7 in anoxic aqueous suspensions. (left panel) Absorbance spectra of AQDS species (filtrate). The color-coded lines correspond to total concentrations of AQDS, regardless of the redox state. (right panel) Electron turnover and distribution of Fe(II) as a function of AQDSred added.

and redox properties of regenerated Fe(II)-surface sites differed from those originally present. To investigate whether the “backward” reaction occurs, fully reduced quinone (i.e., hydroquinone; AQDSred) was added to an anoxic suspension of pure goethite (no initial Fe(II)). After apparent equilibrium (24 h), the absorbance spectra of the AQDS species in filtered samples as well as aqueous and sorbed ferrous iron concentrations were determined (Figure 3). The results show for the first time that the redox equilibrium between the redox probe AQDS and reactive iron species present in goethite or goethite/Fe(II) systems can be achieved starting from pure goethite and quinone, in the absence of initial Fe(II). Goethite completely oxidized AQDS in the experiments where 50 or 100 μM of AQDSred was initially added, while 13% remained in the reduced form at 200 μM initial concentration of AQDSred. Accordingly, the measured Fe(II) concentrations increased with the amount of AQDS oxidized. Most of the Fe(II) was present in sorbed state (right panel in Figure 3). Thus, saturation of the goethite with regard to Fe(II) sorption was by far not reached and only at the highest AQDSred spike (200 μM) the concentration of dissolved Fe(II) became significant. Again, the trend in redox equivalents of Fe(II) species corresponded to the turnover of AQDS similar to the data

absorbance spectra of AQDSox after reaction with Fe(II) associated with goethite. Three concentration levels of AQDSox (100, 500, and 800 μM) were studied in duplicates and gave very consistent results. AQDSox was completely reduced when added at 100 μM but only partially when added at 500 or 800 μM initial concentrations. It is important to note that Fe(II) in aqueous solution at pH 7 is not able to reduce AQDSox (data not shown). No decrease of the total AQDS aqueous concentration was observed after reaction with the mineral; therefore, no significant sorption occurred, as expected. The same was observed in the following setups. The electron equivalents calculated from the measured reduction of AQDS correlate well with the decrease of dissolved Fe(II) in these GT-Fe(II) systems whereas the amount of surface bound (i.e. “sorbed”) Fe(II) remained fairly constant as more AQDSox was reduced (Figure 2, right panel). To a large extent, consumption of reactive Fe(II) surface sites at goethite due to reduction of AQDSox was compensated by adsorption of Fe(II) from aqueous solution at previously oxidized reactive Fe(II)-surface sites of goethite. Although the amount of sorbed Fe(II) at goethite was in excess compared to AQDSox in all experiments, reduction of AQDSox was progressively incomplete in the experiments with higher concentrations of initial AQDSox indicating that the nature 14164



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Table 1. Reduction Potentials (EH,Fe‑GT; According to Equations 3, 5, and 6) for Anoxic Suspensions of the Various GoethiteFe(II) Systems Studieda Systems Containing Ferrous Iron and Goethite (GT-Fe(II) Systems) before Addition of AQDS Redox Probes sample GT-Fe(II) + AQDSox A B GT-Fe(II) + AQDSox/ AQDSred

total AQDS (μM)

AQDSox (μM)

AQDSred (μM)

AQDSred/ AQDSox

pH

EH,Fe‑GT (mV)

ΔEPt (mV)

Fe(ll)tot initial (mM)

electrons transferred (μM)

800 500 500

472 249 155

343 287 345

0.73 1.15 2.22

6.74 6.70 6.74

−166.0 + 0.5 −170 ± 2 −180.0 ± 0.5

ND ND −161

1.40 1.40 1.46

686 498 330

System Containing Initially Only Goethite (No Ferrous Iron) before Addition of AQDS Redox Probes

GT + AQDSred 200 μM a

total AQDS (μM)

AQDSox (μM)

AQDSred (μM)

AQDSred/ AQDSox

pH

EH,Fe‑GT(mV)

Fe(ll)final (mM)

electrons transferred (μM)

200

180

25

0.14

7.16

−169.0 ± 0.7

0.24

360

ΔEPt corresponds to a measured redox potential using a conventional Pt electrode ring electrode. All potentials are expressed vs SHE.

ΔE = E H,AQDS − E H,Fe ‐ GT = 0

shown in Figure 2 (right panel). There is a gap in the e-balance, probably due to incomplete recovery of goethite bound Fe(II) using 1 M HCl extraction. In the GT-Fe(II)/AQDSox experiment (Figure 2, right panel), the calculation of iron redox equivalents was based on the loss of total Fe(II), while in the GT/AQDSred experiment iron redox equivalents were calculated from the Fe(II) formed. Therefore, an underestimation of mineral associated Fe(II) consistently explains the differences between the AQDS and the Fe(II) equivalents in both sets of experiments. Gorski and Scherer (2011) suggested that a fraction of the electrons in sorbed Fe(II) at goethite are transferred into the conducting band (CB) of the bulk mineral leading to different follow-up processes: (i) localization of electrons in a trapping site (generated from defects, impurities, surface effects, etc); (ii) electron transfer to an oxidized species in solution (here: AQDSox); or (iii) reduction of Fe(III)-surface sites, triggering reductive dissolution and Fe(II) exchange.36 Such trapping sites are only present in nonperfect materials, generating energy levels between the valence band (VB) and the conducting band. Therefore, the type and amount of impurities and defects in the mineral will determine the presence and position of such extra energy levels, where electrons from Fe(II) can be located. When sorbed Fe(II) is released by “soft” acidic extraction (using 1 M HCl), it is likely that not all electrons are recovered from the CB and/or trapping sites, since during this extraction only a small fraction of the mineral gets dissolved. The amount of electrons “trapped” in the bulk mineral (i.e., not available for reducing another compound and/or not being released as Fe(II) after 1 M HCl treatment) will depend on the specific characteristics of the material, thus another type of goethite might exhibit a higher or lower ability of trapping electrons from Fe(II). In the GT-Fe(II)/AQDSox experiment, the recovery of the added Fe(II) in the blank (no added AQDS), where no loss of total Fe(II) is expected, was high (94%) but not complete. Therefore, a perfect match of redox equivalents is not expected when extracting Fe(II) without complete dissolution of the iron mineral. Using AQDS as Redox Probe to Determine the Apparent Reduction Potential of Surface Bound Fe(II) Species. We determined the reduction potential of the various GT-Fe(II) systems according to the following equations, providing apparent equilibrium between the reactive iron species at goethite and considering the resulting speciation of the quinone as redox probe:

(3)

were EH,AQDS and EH,Fe‑GT correspond to the apparent reduction potential of each redox couple at given conditions. EH,Fe‑GT corresponds to the apparent reduction potential of all redox active iron species associated with goethite, for the reduction from ferric to ferrous redox state. The Nernst equation for each of these reduction potentials can be written as follows: 0′ E H,Fe − GT = E H,Fe − GT −

0′ E H,AQDS = E H,AQDS −

RT ⎛ [Fe(II)‐GT] ⎞ ln⎜ ⎟ F ⎝ [Fe(III)‐GT] ⎠

RT ⎛ [AQDSred ] ⎞ ⎟⎟ ln⎜⎜ 2F ⎝ [AQDSox ] ⎠

(4)

(5)

E0H,Fe‑GT ′

Here, is the standard reduction potential at given pH; Fe(III)-GT and Fe(II)-GT represent the activities of ferric and ferrous ions associated with the mineral. The activity of Fe(II)GT could be, in principle, estimated from the measured concentration of sorbed Fe(II) assuming only one type of surface-bound Fe(II) species and an unity activity coefficient, but the activity of Fe(III)-GT is unknown. Hence using AQDS as a redox probe is needed. E0H,AQDS ′ is the standard reduction potential of AQDS at a given pH, according to eq 6:32 0′ 0 E H,AQDS = E H,AQDS +

RT red ln({H+}2 + K a1 {H+} 2F

red red + K a1 K a2 )

(6)

[AQDSred] is the total activity of fully reduced AQDS species (hydroquinone redox state), regardless of the pH speciation and [AQDSox] is the activity of the fully oxidized quinone (each of these activities are considered to be close to the corresponding concentrations). A standard redox potential of AQDS E0H,AQDS of +228 mV was used for calculations.30 As acidity constants of the hydroquinone form of AQDS vary in literature (pKa,1: 7.2−8.7; pKa,2: 10.4−11.9),35 we used the intermediate values reported in the work of Clark (1960)30 (pKa,1 ≈ 8.1 and pKa,2 ≈ 10.5) for purpose of consistency with literature for calculations in eq 5. At the pH of our experiments, only the first pKa is relevant. An estimation of the effect of the pKa value on the calculated reduction potential can be done by choosing the minimum and maximum values (7.2 and 8.7) in eq 6. This gives, for pH =7, −186 and −180 mV, respectively. Therefore, the effect of different pKa values only has a very minor effect (max. 6 mV) on the presented results. 14165



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Figure 4. Comparison of the reduction potentials of some environmental relevant redox couples, at pH 7 and vs SHE (E0H′). The estimated EH,Fe‑GT in our work is included in biogeochemical redox couples (highlighted, α-FeOOH). Elliot HA corresponds to the reported range of reducible moieties in Elliot Humic Acid.38 The E0H′ of quinones correspond to the 2 e− transfer process.29 Data for iron couples (left panel) was obtained from ref 14.

for sorbed Fe(II) extraction (details in SI). Therefore, we would expect a higher extent of the forward reaction as the pH increases, considering no precipitation of Fe(II) (hydr)oxides. For the GT-Fe(II) + AQDS/AQDSred system, the reduction potential was also measured using a commercially available Pt ring electrode (ΔEPt, see Table 1). This ΔEPt substantially deviated (by 20 mV) from the EH,Fe‑GT value of the same system obtained from the quinone redox speciation. The readings differed with the condition of the electrode and were poorly reproducible. The limitations of the Pt ring electrode have been discussed in the literature9−11 and include irreversible reactions and/or formation of precipitates at the electrode surface,30 mixed potentials, and different electrode activities of the redox species, especially in heterogeneous systems. Thus, in contrast of the quinone redox probe measurements presented here, interpretation of redox potential determinations with Pt ring electrodes in heterogeneous iron systems at circumneutral pH values provide at best a rough estimate rather than a thermodynamically based measure of the actual redox conditions. Pt readings of batch control systems (GT-Fe(II) in the absence of quinone) were also performed (data shown in SI Table S1). These readings were troublesome, since measurements at different days gave readings as far as 50 mV apart and, sorption of the mineral on the Pt ring surface was observed. This aggregation and sorption effect has been studied by Shi et al (2011) in nano zerovalent iron (nZVI) nanoparticle suspensions, by performing the measurements under controlled experimental conditions (using rotating disk electrodes).11 Since the redox potential of a heterogeneous system is a mixed potential (with weighed contributions of each redox active couple),11 in a goethite suspension with added ferrous iron no significant amount of Fe(III) is present in solution. Therefore, there is no redox couple in solution for equilibrating with the Pt electrode and no meaningful readings can be obtained. Environmental Significance. To our best knowledge, this is the first systematic study on the thermodynamics of the redox reaction between AQDS and heterogeneous iron systems, namely goethite-Fe(II). Our results are consistent with and provide a thermodynamic basis for earlier observations on the ability of AQDS to shuttle electrons between microbes and iron oxide minerals. Besides reversibility of the redox reactions, essential prerequisites of an effective

For every experimental system where detectable amounts of both quinone and hydroquinone species were present after equilibration, apparent reduction potentials were determined using eq 3, 5, and 6 and measured AQDSox/AQDSred ratios. These values are compiled in Table 1 and comprised a surprisingly narrow range from −166 to −180 mV vs SHE. Interestingly, this value is close to the potential calculated for (pure) goethite in the presence 10 μM aqueous Fe2+ at pH 7 (ignoring adsorption of Fe(II) and the surface chemistry of the system (ref 14 and Figure 4)). Even for the systems that differed most in composition, i.e., GT-Fe(II) + AQDSox and GT (no initial ferrous ion) + AQDSred, the apparent reduction potentials were quite similar (ca. − 170 mV) and differed less than 5 mV. These findings indicate that Fe(II)-goethite systems show a high redox-buffer intensity even after substantial disturbance of the systems due to significant amounts of electrons transferred by reactions of the redox probe (Table 1). To minimize system disturbance and to obtain an estimate of the apparent reduction potential of an almost “pristine” Fe(II)/ GT system, a mixture of reduced (32%) and oxidized (68%) AQDS was added to a GT-Fe(II) suspension (see bottom line of Table 1a and data in SI). A somewhat lower potential of −180 mV was obtained, which is consistent with a lower consumption of Fe(II) surface sites during equilibration with the redox probe (330 μM electrons transferred). This result could indicate that in undisturbed goethite-Fe(II) systems a small pool of highly reactive sites with low reduction potential is present that cannot be recovered readily due to readsorption of Fe(II) from aqueous solution within the time frame of our experiments (24 h). The EH,Fe‑GT values presented in Table 1 are conditional and thus only valid at the conditions present in our experiments. The main parameters which could affect such values are pH, Fe(II) loading, and the presence of ions competing with Fe(II) for sorption sites. The pH of the solution would not only influence the standard potentials of both iron and AQDS (E0′ in eqs 4 and 5) but the Fe(II) sorption behavior: an increase of pH would lead to a higher amount of sorbed Fe(II). Since the reduction potential of AQDS decreases with increasing pH,30 the effect of pH on the AQDSox/AQDSred ratio is not straightforward to assess. However, we observed a complete shift toward reactants in experiment i (GT-Fe(II) + AQDSox) when an aliquot of the suspension was in contact with 1 M HCl 14166



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electron shuttle are absence of significant adsorption and thus interference of the shuttle with the surface chemistry of iron minerals as well as a suitable redox potential close to that of the reactive surface species. Using AQDS speciation as a redox probe, we reported for the first time robust reduction potential measurements of reactive iron species present at goethite in aqueous systems (EH,Fe‑GT ≈ −170 mV). As quinones play an essential role in the electron transport systems of microbes, the proposed method can be considered as a biomimetic approach to determine the “effective” biogeochemical redox potential in heterogeneous iron systems. The quinone redox probe method is capable of tracking reduction potentials in the range of surface bound ferrous iron species during redox perturbations. Our results corroborate the picture of a dynamic remodeling of Fe(II)/Fe(III) surface sites at goethite in response to oxidation/reduction events.29,37 Due to the high redox buffer intensity of such heterogeneous mixed valent iron systems, the determined EH,Fe‑GT ≈ −170 mV might be characteristic for many iron-reducing environments in the subsurface. For comparison, Figure 4 provides a compilation of reduction potentials of important biogeochemical redox couples including recent data on quinone/NOM systems which also exhibit significant redox buffering. Further work using quinones as redox probes for reactive and transient surface species in heterogeneous iron systems is needed and currently in progress in our laboratory to put the measured reduction potentials of the investigated pure Fe(II)/ goethite systems into a wider perspective and to determine potential effects of surface coatings and precipitates.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Detailed descriptions of the procedure for determining sorbed Fe(II) and analysis of AQDS interferences (Figure SI1 and Figure SI2); cyclic voltammetry of AQDS in phosphate buffer pH 7 (Figure SI3); spectra of AQDS in experiment iii; Pt ring electrode readings of a control batch consisting of GT-Fe(II) in the absence of AQDS. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: 0049 (0)7071 2973135. Fax: +49 7071 295059. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the following: Dr. Michael Sander (ETH Zurich) for advice on the electrochemical reduction of AQDS; Dr. Jorge E. Yanez Heras (Universität Tübingen) for cyclic voltammetry of AQDS; Zhengrong Xue and Stephanie Spahr (Universität Tübingen) for experimental assistance. Funding was provided by the German Research Foundation (DFG) through grant HA 3453/3-2 within the research group “Electron Transfer Processes in Anoxic Aquifers” (FOR 580). We thank all members for fruitful discussions.

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ABBREVIATIONS AQDS anthraquinone-2,6-disulfonate GT goethite 14167



Article

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Electron Transfer between Iron Minerals and Quinones: Estimating the

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