Depassivation of Aged Fe0 by Ferrous Ions: Implications to

Nov 6, 2013 - These findings provide new insight into the molecular-scale interaction of aged Fe0 and ferrous iron with particular implications for su...
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Depassivation of Aged Fe0 by Ferrous Ions: Implications to Contaminant Degradation Tongxu Liu,†,‡ Xiaomin Li,† and T. David Waite*,† †

School of Civil and Environmental Engineering, University of New South Wales, Sydney, NSW, Australia 2052 Guangdong Institute of Eco-Environmental and Soil Sciences, Guangzhou, Guangdong, P. R. China 510650

‡

S Supporting Information *

ABSTRACT: Investigation of the effects of ferrous iron (Fe(II)) on the ability of aged (iron oxide coated) Fe0 to degrade trichloroethylene (TCE) has revealed that, while neither aged Fe0 nor Fe(II) separately were able to degrade TCE, approximately 95% of the TCE present was degraded after exposure to a mixture of aged Fe0 and Fe(II) for 21 days. The rates of TCE degradation increased with an increase in Fe(II) concentration from 0 to 1.6 mM and then reached a relative plateau. Results of Fe(II) “adsorption” studies revealed that the equilibrium pH decreased significantly with an increase in Fe(II) concentration. Proton release during adsorption of Fe(II) to iron oxide coatings was identified as being responsible for promotion of surface dissolution and, concomitantly, enhancement in extent of TCE reduction by aged Fe0. Results of open circuit potential analysis and Tafel plot measurement showed that the corrosion potential of aged Fe0 (Ecorr) in the presence of Fe(II) decreased to levels similar to that of Fe0/Fe2+, while significant increase in corrosion current (Icorr) and decrease in polarization resistance (Rp) were found with an increase in Fe(II) concentration. The fact that the effects of different Fe(II) concentrations on the Ecorr, Icorr, and Rp was decoupled from their effects on TCE degradation by aged Fe0 suggested that the enhancement of TCE degradation in the presence of Fe(II) was attributable to the dissolution of the Fe(III) oxyhydroxide layer coating the aged Fe0. While the presence of Fe(II) may also lead to transformation of the Fe(III) (oxy)hydroxide coating to more crystalline phases, the rate of reduction of compounds such as TCE by Fe(II) associated with the Fe(III) (oxy)hydroxide coating is substantially slower than that mediated by Fe0. These findings provide new insight into the molecular-scale interaction of aged Fe0 and ferrous iron with particular implications for sustaining the reactivity of Fe0-mediated degradation of contaminants in iron-bearing environments.

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INTRODUCTION Zero valent iron (Fe0) can degrade or transform a wide range of contaminants including chlorinated organics,1 heavy metals,2 nitroaromatics,3 and nitrate4 with the result that permeable reactive barriers of Fe0 have been successfully applied for the remediation of contaminated groundwater.5 However, several laboratory and field studies involving the long-term performance of Fe0 have shown that contaminant degradation rates decrease with time6 because of the formation and accumulation of Fe(III) oxides such as goethite (α-FeOOH), maghemite (γFe2O3), and hematite (α-Fe2O3) on the outer layer of aged Fe0 though it appears that contaminant degradation can be sustained when Fe(II)/Fe(III) hydroxides such as magnetite (Fe3O4) and green rust are formed on the Fe0 surface due to the high conductivity of these minerals.7−9 Passivation of the Fe0 surface occurs when reactive sites are covered with the Fe(III) oxides resulting in significant reduction in efficiency of electron transfer from Fe0 to the contaminant. A number of approaches have been developed to recover the reactivity of aged Fe0 including dissolution of iron oxide coatings using an improved flash-drying protocol after acid washing10 and application of strong reductants such as sodium © XXXX American Chemical Society

borohydride to reduce Fe(III) minerals to the active zerovalent state.11 Additionally, reductants such as dithionite and ironreducing bacteria such as Shewanella alga have been applied to reconvert the ferric iron present in coatings to its ferrous form thereby prolonging the reduction of chlorinated organic compounds (COCs) by Fe0.12−14 However, not surprisingly, the mechanism by which the ferrous species sustains the reactivity of Fe0 is unclear given the complexity of the Fe0-iron oxide system. Fortunately, a number of studies in recent years on the interaction between Fe(II) and mineral surfaces have provided insights of considerable value in aiding our understanding of factors influencing the reactivity of passivated Fe0 surfaces.15−17 Ferrous iron adsorption onto mineral surfaces is an important environmental process because the surface sorbed Fe(II) has a relatively negative redox potential which promotes reductive transformation of organic/inorganic contaminants18 such as nitroaromatic compounds,19,20 pentachloronitrobenReceived: August 20, 2013 Revised: October 30, 2013 Accepted: November 6, 2013

A

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zene,21 and DDT.7,8 A general consensus is that the mineral surface provides hydroxyl groups (represented here as >SOH0) to stabilize Fe(II), leading to the formation of Fe(II) surfacecomplexes such as >SOFe(II)+ and >SOFe(II)OH0 which exhibit lower redox potential than aqueous Fe(II) species.22 Nano and Strathmann23 showed that the negative shift of the redox potential was indicative of the enhancement of Fe(II) reactivity. In addition, it is now recognized that Fe(II) may catalyze the transformation of amorphous ferric oxides (AFO) to more crystalline phases such as goethite with the effect that the redox potential of the system will drop as a result of the lowered ferric ion activity.24,25 As such, the effects of Fe(II)mediated crystallization of AFO should also be considered in a reaction system containing both Fe(II) and aged Fe0. Based on the aforementioned reports, the previously observed enhancement in COCs degradation that occurs when Fe(II) is added to aged Fe0 may simply be related to the elevated reducing capacity of Fe(II) adsorbed to the iron oxide coatings on the aged Fe0 surface. While this may be the case, there has been little analysis of this phenomenon from a thermodynamic (or speciation-oriented) perspective and, as such, no justification of such a hypothesis. In a recent study,26 we reported that the presence of Mg2+ ions (at concentrations typical of seawater) can depassivate aged Fe0 as a result of promotion of surface dissolution arising from proton release during formation of the surface complex >FeOMg+ and, as a result, can enhance trichloroethylene (TCE) degradation by aged Fe0. Since the dissolution, hydrolysis, and precipitation properties of Fe2+ are quite similar to those of Mg2+, the surface complex >FeOFe(II)+ might also be formed on the aged Fe0 surface in the presence of ferrous iron, resulting in possible surface dissolution after proton release. However, no reports have been found to confirm whether the presence of ferrous iron can also drive proton release which, in turn, may facilitate the surface dissolution of aged Fe0, thereby enhancing its reactivity. In this study, the effects of ferrous iron on TCE degradation by aged Fe0 were investigated with different concentrations of ferrous ion and different dosages of aged Fe0. Adsorption of ferrous iron onto the surface of aged Fe0 was studied as well as resulting changes in extent of Fe0 dissolution and pH. The corrosion properties of aged Fe0 were also evaluated using open circuit chronopotentiometric analysis and Tafel plot measurement in the presence of ferrous iron. The thermodynamics of critical species were also analyzed through speciation calculation and application of the Nernst equation. Results obtained have enabled elucidation of the mechanism responsible for depassivation of aged Fe0 by ferrous iron with new insights obtained of value in predicting the likely performance of Fe0 as a dechlorination technology in ironbearing environments, especially under anoxic conditions.

indicated a very broad particle size distribution with an average size of around 167 μm. X-ray diffraction (XRD) analysis confirmed that the material was principally Fe0 but with traces of oxides (Fe3O4 and FeO) present, presumably as a surface layer. Scanning electron micrographs of the particles suggested the presence of surface deposits or coatings with electron dispersive spectroscopic analysis indicating that these deposits contain oxygen as well as iron atoms. Trichloroethylene (TCE), FeSO4, and Na2SO4 were obtained from Sigma-Aldrich in Australia. All solutions were prepared in Milli-Q water. Highperformance liquid chromatography-grade methanol and nhexane (Sigma-Aldrich) were used without further purification. Experimental Procedure. The experimental procedure involved purging Milli-Q water for more than three hours prior to transferring the Ar-purged Milli-Q water into the anaerobic chamber. The preweighted aged Fe0 was added into serum bottles and then transferred into the anaerobic chamber. In the anaerobic chamber, the Ar-purged Milli-Q water was added to the serum bottles containing aged Fe0, and then a preweighted amount of FeSO4 powder was added into the serum bottle. Following dissolution of FeSO4, the pH of the mixtures were well below those at which Fe(OH)2(s) would be expected to precipitate (i.e., 8.9, 8.5, and 8.2 for Fe(II) concentrations of 1 mM, 10 mM, and 100 mM, respectively). As the pH was always less than these values in the Fe(II)-modified suspensions, Fe(OH)2(s) formation was not expected (or observed) at any stage. TCE in methanol stock solution was added to the reaction solution at a final concentration of 0.22 mM before the serum bottles were sealed with Teflon-coated butyl rubber stoppers and crimp seals. All studies were conducted in duplicate with vials incubated in a shaker at 180 rpm at 25 °C. As preliminary experiments suggested that the TCE can be completely degraded in three weeks, sampling was undertaken on days 1, 2, 3, 5, 7, 10, 14, and 21. The extent of adsorption of Fe(II) on aged Fe0 was determined from batch experiments undertaken in the dark. Aged Fe0 particles (0.10 g) were mixed with 10 mL of Fe(II) solution of varying concentrations (0−100 mM) in serum vials (25 mL) which were sealed and agitated at 180 rpm in a thermostatic shaker bath and maintained at a temperature of 25 ± 1 °C for 24 h enabling adsorption−desorption equilibrium to be reached. After adsorption, the pH and Fe(II) concentration in the supernatant solution were measured after the suspensions were centrifuged at 1,000 × g for 5 min at 25 °C. The adsorbed amount of Fe(II) on aged Fe0 was calculated based on mass balance. Analytical Methods. The unfiltered samples (0.15 mL suspension) were extracted with 1.5 mL of n-hexane for TCE determination with analyses undertaken using a gas chromatograph (Agilent 6890) equipped with an ECD detector and Trace TR-5MS silica fused capillary column (30 m × 0.25 mm i.d., 0.25 μm film thickness). The injector temperature was 250 °C, and the helium flow rate was 1.0 mL min−1. The column temperature was set at 100 °C for 2 min, increased at a rate of 15 °C min−1 to 160 °C, and then switched to a rate of 5 °C min−1. The temperature was finally increased to 250 °C and maintained isothermally for 10 min. TCE external standards were prepared in n-hexane with standard curves found to be linear. The average of duplicate determinations of concentration is reported with relative percent differences found to be typically less than 10%. As TCE is volatile and partitions between the liquid and gas phases, the total moles of TCE

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EXPERIMENTAL SECTION Materials. Zero valent iron (Fe0, powder, ∼70 mesh, 99%) was obtained from Acros Organics (Product No. 197815000) and aged in air for around three months prior to use. Preliminary experiments indicated minimal change in reactivity after this aging time though, for consistency, any set of studies was undertaken on material that had been aged for the same period.27 The aged Fe particles have been characterized by a variety of techniques with the results of these characterization studies reported in our previous related work.26 In brief, analysis of particle size using a Malvern Mastersizer 2000C B

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system of aged Fe0+FeSO4 (Figure S1a) with acetylene (C2H2), ethane (C2H4), and ethane (C2H6) found to be the major degradation products (Figure S1b). No significant TCE degradation was observed in the system of aged Fe0+Na2SO4 with equivalent sulfate concentration to that in the FeSO4 case, suggesting that TCE could not be degraded by the aged Fe0 in the presence of sulfate ions. These results indicated that some possible interaction between aged Fe0 and Fe(II) might contribute to the observed TCE degradation by aged Fe0 in the presence of Fe(II). Results of studies of the effects of Fe(II) concentrations on the kinetics of TCE degradation (Figures S2a) showed that both the rate and extent of TCE degradation (Figure 1a)

remaining in both liquid and gas phases were calculated using a Henry’s Law constant for TCE of 0.00937 atm m3 mol−1 (the proportion of moles of TCE in the liquid phase was typically on the order of 0.54). Gas chromatographic/flame ionization detection (GC-FID) was employed for direct quantification of TCE reduction products (acetylene (C2H2), ethane (C2H4), and ethane (C2H6)) present in the headspace of the serum bottles. The Henry’s Law constant and FID response factors of individual products were used to quantify their concentrations. For determination of dissolved Fe(II) and Fe(III) concentrations, the samples were centrifuged at 1000 × g for 5 min at 25 °C, and the supernatant was then filtered (Millipore, 0.22 μm, PTFE) in the anaerobic chamber. Dissolved Fe(II) was measured colorimetrically by the 1,10-phenathroline colorimetric method, and total dissolved Fe was determined in the same way after adding 10% hydroxylamine hydrochloride to reduce all filterable Fe(III) to Fe(II)(aq). Filterable Fe(III) concentrations were obtained from the difference between total filterable Fe concentrations and Fe(II) concentrations. Open circuit chronopotentiometric measurement was carried out in a conventional two-electrode electrochemical cell using a CHI 650D Electrochemical Workstation (Texas, USA). A platinum electrode (2 mm in diameter) was used as the working electrode with a Ag/AgCl electrode as the reference electrode.6 Unless otherwise stated, all the reported voltages were versus Ag/AgCl. Prior to each measurement, the Pt electrode was polished with emery paper followed by Al2O3 powders of 1.0 and 0.06 mm particle sizes and was thoroughly rinsed with Milli-Q water between the two polishing steps. The Pt electrode was then cleaned successively with Milli-Q water. The aged Fe0 (1 mg) was dispersed in a dilute Nafion solution (0.5 wt.%, 50 μL) by vortex mixing for 1 min. An aliquot (2 μL) of the above suspension was then coated on the clean Pt electrode using a microsyringe and air-dried for 5 min before being placed in the O2-free salt solution. The sampling rate for open circuit chronopotentiometry was 30 s−1, but the data in the reported figures are presented with markers spaced every 100 data points for visual clarity. While the use of Nafion to bind Fe0 to the surface of the Pt disk electrode differs from the packed powder electrode approach used by Tratnyek and colleagues,28,29 Nafion is recognized to be highly conductive and has been used previously in a similar manner to that adopted here.26 Tafel plot measurements were directly performed after 4 h of open circuit chronopotentiometry at a scan rate of 1 mV s−1 with a platinum wire electrode used as a counter electrode. Corrosion potential (Ecorr), corrosion current (Icorr), and polarization resistance (Rp) were directly obtained using CHI 650D software. Equilibrium speciation of Fe(II) and Fe(III) species and surface complexes was carried out using the thermodynamic package Visual MINTEQ 3.0 with speciation modeling also used in experimental design to ensure that mineral precipitation was avoided at the salt concentrations used. While the systems under investigation here are not at equilibrium in all aspects, speciation modeling is restricted to conditions where an iron oxyhydroxide surface coating is present on the Fe0 surface, and pseudoequilibrium for major ions and iron species can be reasonably assumed.

Figure 1. Pseudo-first-order rate constants (k) of TCE reduction by aged Fe0 in solutions of FeSO4 and final pH after 21-day reaction under different conditions, (a) effects of FeSO4 concentrations in the range of 0−100 mM with 25 g L−1 of aged Fe0, (b) effects of aged Fe0 dosages in the range of 0−25 g L−1 with 10 mM of FeSO4.

increased significantly with an increase in Fe(II) concentrations from 0 mM to 1.6 mM though a further increase in Fe(II) concentration (up to 100 mM) resulted in a little further increase in the rate of TCE degradation. Studies of the effects of Fe0 dosages on the rate of TCE degradation were conducted in the presence of 1 mM Fe(II) (Figure S2b) and the pseudo-firstorder rate constants (k) for TCE degradation (Figure 1b) found to be linearly correlated to the dosage of aged Fe0. As shown in Figure 1a, final pHs ranged from 10.5 to 6.42 for Fe(II) concentrations from 0 mM to 1.6 mM and was relatively uniform (at around 5.6) for higher Fe(II) concentrations. The final pH of the mixtures increased slightly from 5.92 to 6.38 with an increase in addition of aged Fe0 from 0 to 25 g L−1 in the presence of 10 mM Fe(II) (Figure 1b). The results of XRD analyses undertaken at the conclusion of replicated TCE dechlorination experiments (Figure S3) indicate that the height of the Fe0 peaks decreases with increasing Fe(II) concentration, while the height of the magnetite (Fe3O4) peaks increases,

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RESULTS AND DISCUSSION TCE Degradation by Aged Fe0 in the Presence of Fe(II). While no significant TCE degradation was observed with aged Fe0 or FeSO4 alone, the TCE concentrations decreased over time with 95% of initial TCE degraded by day 21 in the C

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Fe(II). While the aged Fe0 in the absence of Fe(II) could not degrade TCE in the systems with initial pH ≥ 3, significant TCE degradation by aged Fe0 was observed in the system with an initial pH of 2 (Figure S5a). According to Nurmi et al.,10 dissolution of the iron oxide coatings on the surface of aged Fe0 can occur under low pH conditions. The dissolved Fe(II) concentrations and final pH (for particular initial pH) were measured after 21 days of reaction in both the absence and presence of added (10 mM) Fe(II) (Figure 3). While no

especially in the presence of 100 mM Fe(II). There was no evidence of the formation of any other minerals over the time course of the reaction. Fe(II) Adsorption on Aged Fe0. The adsorption of Fe(II) on the surface of aged Fe0 was investigated with results shown in Figure 2. In this figure, “Adsorbed Fe(II)” represents the

Figure 2. Difference between Fe(II)Initial and Fe(II)Final in solution (black line) and final pH (red line) versus different initial Fe(II) concentrations after adsorption−desorption equilibrium of Fe(II) on 0.10 g aged Fe0 for 24 h.

difference in concentration between Fe(II)Initial and Fe(II)Final in solution after adsorption−desorption equilibrium for 24 h. The adsorbed Fe(II) concentrations increased steeply to 0.68 mM with the increase in initial (added) Fe(II) concentration from 0 mM to 1.5 mM and then increased slowly to 1.78 mM when the initial Fe(II) increased to 50 mM. As such, the Fe(II) concentration at which TCE degradation plateaus (1.5−3.2 mM) in Figure 1a coincides with that required for saturation coverage of passivated ZVI. Interestingly, a decrease in the concentration of adsorbed Fe(II) was observed when the concentration of initial Fe(II) exceeded 50 mM with a net release of Fe(II) to solution at these high added Fe(II) concentrations (for example, a final Fe(II) solution concentration of 108 mM was achieved for an initial (added) Fe(II) concentration of 100 mM). These results indicate that the added Fe(II) does not simply adsorb onto the surface of aged Fe0 but facilitates the dissolution of iron oxide coatings from the aged Fe0 surface. In addition, results of pH analysis after 24 h of equilibration with added Fe(II) (Figure 2) showed that the pH dropped dramatically from 10.5 to 6.02 with initial Fe(II) concentrations from 0 mM to 1.5 mM and then decreased gradually to 4.14 with an increase in Fe(II) concentrations to 100 mM. This decrease in pH was presumably associated with proton (H+) release in the presence of Fe(II) with this proton release presumably enabling the dissolution of the passivating film on the aged Fe0 surface with concomitant release of dissolved iron into solution. In addition, the pH of the aged Fe0 suspension in the presence of 10 mM Fe(II) was measured throughout the experiment with results shown in Figure S4. The pH increased from 7.4 to 10.4 in the first minute after mixing with aged Fe0 and then decreased over about one hour to ∼6.5 following Fe(II) addition. The pH remained relatively stable (at ∼6.5) over the next 21 days. Effect of pH. The decrease in the equilibrium pH (Figure 2) appears to be negatively correlated to the increase in rate of TCE degradation (Figure 1a). As such, particular consideration was given in these studies to the effect of pH on TCE degradation by aged Fe0 in the absence and presence of 10 mM

Figure 3. TCE reduction by aged Fe0 in solutions in the absence or presence of 10 mM FeSO4 under different initial pH, (a) pseudo-firstorder rate constants (k), (b) dissolved Fe(II) (after 21 days of reaction), (c) final pH (after 21 days of reaction).

significant changes in pH and dissolved Fe(II) (Figure 3b,c) were observed in the non-Fe(II) dosed systems with initial pH ≥ 3, the dissolved Fe(II) concentration increased from 0 to 2.55 mM (Figure 3b) and the pH increased from 2 to 6.87 (Figure 3c) for an initial pH of 2. These results suggest that significant dissolution of iron oxide coatings on the aged Fe0 surface driven by H+ occurred at pH as low as 2 and resulted in significant TCE degradation by the activated Fe0. Surprisingly, in all the aged Fe0+Fe(II) systems with initial pH 2−12 (Figure 3a), very similar k values for TCE D

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degradation were observed (k only varied in the range 0.097− 0.124 d−1) suggesting that the reactivity of aged Fe0 was relatively pH-independent in the presence of Fe(II). Interestingly, the final pH in all these aged Fe0+Fe(II) systems was similar at around 6.0 (Figure 3c) in spite of the large variation of the initial pH (from 2−12). As shown in Figure 3b, the dissolved Fe(II) concentrations decreased gradually in these aged Fe0+Fe(II) systems with an increase in initial pH. As an aid to interpretation of these observations, iron speciation was calculated using Visual MINTEQ for the system of Fe(II)-aged Fe0 under different pH (2−12) by assuming the aged Fe0 surface was coated by hydrous ferric oxide (HFO), an amorphous form of iron (oxy)hydroxide used widely in previous speciation calculations.30 While we have no conclusive evidence that HFO was present at the surface of the aged Fe0, the likelihood of this ubiquitous amorphous phase being present is high given that the Fe0 was exposed to oxygen for a significant time prior to use in the studies described here. The calculated concentrations of all the major iron species present versus pH are shown in Figure S6. For pH 2, Fe2+ and FeSO40 are the dominant aqueous species (data not shown) with their total concentrations very close to the initial input of Fe(II), and, in accord with the results presented in Figure 3b, there is only a minor contribution of surface bound Fe(II) species. When the pH was close to 6 or higher, the concentrations of Fe(II) surface complexes such as >FeOFe(II)+ and >FeOFe(II)OH increased gradually with the increase in pH. These results confirmed that, as expected, the adsorption of Fe(II) species is strongly pH-dependent. At an initial pH of 2, the fact that similar TCE degradation kinetics and final pH values were obtained in the absence and presence of Fe(II) suggests that the high concentration of H+ dominated the recovery of aged Fe0 at pH 2 regardless of the presence of Fe(II) with the passivating film undergoing acid dissolution thereby resulting in exposure of reactive Fe0. TCE degradation rates in the presence of Fe(II) with initial pH ≥ 3 are close to or even higher than that in the absence of Fe(II) and with initial pH of 2. These results indicate that the presence of Fe(II) leads to similar efficiency of TCE degradation by aged Fe0 to that achieved by acid-induced depassivation at pH 2 in the absence of Fe(II), regardless of the circumstantial pH. However, it should be noted that the presence of Fe(II), even with concentration as high as 100 mM, only leads to a decrease in solution pH to around 4.0 (Figure 2), which is not low enough to result in significant acid-induced dissolution of the iron oxide passivating layer (Figure 3a). It is clear that some other mechanism(s) of depassivation needs to be considered. Particular consideration is given below to the possibility of the generation of localized acidity from proton release as a result of either hydrolysis or surface complexation processes at the surface of aged Fe0. Proton Release from Aged Fe0 in the Presence of Fe(II). Complete solution and surface species composition including the extent of proton release to solution was calculated using Visual MINTEQ for different concentrations of FeSO4 (or Na2SO4 as comparison) with the surface of aged Fe0 assumed to be covered by HFO (1 g L−1, 600 m2 g−1). While low concentrations of H+ released to solution were obtained in the presence of Na2SO4 (0−100 mM), the concentrations of aqueous H+ increased gradually with the increase in FeSO4 concentration (Figure 4a). The changing trends of the calculated pH in the insert figure of Figure 4a in the systems with different concentrations of FeSO4 appeared to

Figure 4. Model predicted results for (a) released H+ (b) changes of all the iron species in suspension of hydrous ferric oxide (HFO, 1 g L−1, 600 m2 g−1) containing different salts (FeSO4 or Na2SO4) in the range of 0−100 mM.

be reasonably consistent with the detected decreases of pH in Figure 2 though the measured pHs were lower than the calculated values, possibly as a result of additional proton release resulting from the autoreduction of surface-located Fe(III) species by the underlying Fe0 (as discussed in more detail below). Although the concentration of H+ was quite low (predicted to be in the μM range), higher local acidity on the particle surface is a possibility31 with an accumulation of protons at the solid−liquid interface of aged Fe0 though direct confirmation of a low local pH environment at the interface has not been obtained (but would be of value determining if at all possible). The reactions related to proton release in the aged Fe0 system containing added ferrous iron are presented below with the extent of release controlled by a variety of processes including hydrolysis and surface-located acid−base and complexation reactions. Reactions 1 and 2 represent the acid−base reactions of surface Fe(III) hydroxyl groups present on the HFO assumed to cover the aged Fe0 with proton release expected to occur at pHs greater than the acidity constants (pKa’s) of these groups.32 >FeOH+2 ⇌ >FeOH 0 + H+ 0

−

>FeOH ⇌ >FeO + H

+

log K = − 7.29

log K = − 8.93

(1) (2)

32

As shown in reactions 3−5, proton release will also accompany Fe2+ hydrolysis though, at the pHs of interest here, little release of H+ to solution occurs as a result of these hydrolysis reactions since Fe2+ dominates at pH < 10. In addition, adsorption of Fe2+ to iron oxyhydroxide surface sites >FeOH0 can drive proton release as a result of the formation of E

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surface complexes such as >FeOFe(II)+ though the actual extent of proton release will be very much dependent upon the pH at which adsorption occurs and the precise nature of the surface complex (reactions 6−8).33 Fe2 + + H 2O ⇌ FeOH+ + H+

log K = 4.5

Fe2 + + 2H 2O ⇌ Fe(OH)2 (aq) + 2H+

of dissolution of metal oxides has been shown in many instances to be directly proportional to the concentration of the fully protonated surface species >FeOH+2 .36 While an increase in iron oxide dissolution rate is typically observed on decrease in pH, proton-mediated dissolution is recognized to be relatively slow, at least compared to that observed on reduction of surface-located Fe(III).36 Indeed, in the studies described here, the metal ion released to solution was in the form of Fe(II) with reduction of Fe(III) present in the relatively unstable >FeOH+2 species presumably induced by reaction with underlying Fe0; i.e.

(3)

log K = 7.4 (4)

Fe

2+

+ 3H 2O ⇌

Fe(OH)−3

+ 3H

+

log K = 11.0

(5)

Fe2 + + >FeOH 0 ⇌ >FeOFe(II)+ + H+

Fe2 + + >FeOH 0 ⇌ >FeOFe(II)+ + H+

log K = − 2.98 Fe

2+

+

>FeOH+2

(6) +

⇌ > FeOFe(II) + 2H

>FeOH 0 + H+ ⇌ >FeOH+2

+

log K = − 10.27

(10)

2+ Fe0 + > FeOH+2 → Feaq + H+ + regenerated > FeOH 0

(11)

(7)

The resultant removal of the iron oxyhydroxide coating from the Fe0 surface results in exposure of the solution to the underlying Fe0 which then enables effective dechlorination of TCE. Note that additional proton release is expected to result from the autoreduction of surface-located Fe(III) species by the underlying Fe0 (eq 11) with this process possibly accounting for the lower measured pHs in the Fe2+-Fe0 system (Figure 2) than calculated for the Fe2+-HFO system (Figure 4a). In addition to the process of Fe(II) sorption-mediated proton release with subsequent inducement of surface coating dissolution, the sorption of Fe(II) would be expected to be followed by interfacial electron transfer37 and, potentially, atom exchange,38 which might destabilize the thin oxide film somewhat leading to film breakdown and release of “buried” Fe(II) with resultant exposure of underlying Fe0 enabling reaction with TCE. The possibility also exists of conversion of the amorphous iron oxide coating to conducting magnetite (Fe3O4) though this would seem less likely at the relatively acidic pHs generated on Fe(II) addition. Corrosion Properties of Aged Fe0 in the Presence of Fe(II). The corrosion potential of Fe0 (Ecorr), which can be characterized using open circuit chronopotentiometry, reflects the changes in redox properties at the surface of the Fe0 deposited on the platinum disk electrode.10,28,29 The Ecorr value in the absence of Fe(II) was determined to be around −0.05 V vs Ag/AgCl after 4 h (Figure S7a), indicating that the aged Fe0 was fully passivated. With the addition of different concentrations of Fe(II), the Ecorr decreased dramatically to a relatively negative value of −0.7 to −0.8 V vs Ag/AgCl after 4 h, with these values similar to the typical value of fresh Fe0 in the active state.28 The position of the peak in the Tafel plot also provides a measure of the corrosion potential of aged Fe039 with the Ecorr values in the presence of different concentrations of Fe(II) deduced from the Tafel plots (Figure S7b) showing similar changes to those derived from open circuit chronopotentiometry. These results suggest that the active Fe0 may be exposed in the presence of Fe(II), presumably as a result of breakdown of the passivating film on the surface of the aged Fe0.10,28,29 In addition, the corrosion rate calculation of the data presented in the Tafel plots revealed that corrosion currents (Icorr) increased significantly with an increase in Fe(II) concentration (Figure 5), while the corrosion resistance (Rp) decreased. These two parameters (Icorr and Rp) reflect the conductivity of the surface oxide coatings with higher Icorr and lower Rp representing higher conductivity and therefore lower

Fe2 + + > FeOH 0 + H 2O ⇌ > FeOFe(II)OH + 2H+ log K = − 11.96

(9)

(8)

The concentrations of the various iron species vs pH are shown in Figure S6 for a fixed initial concentration of Fe2+ of 10 mM. Addition of different concentrations of ferrous iron results in change in pH (Figure 4a) with concomitant change in concentration of the various iron species present (Figure 4b). As noted above, pH change may result from both the hydrolysis reactions of dissolved Fe(II) species (reactions 3−5 above) and as a result of Fe(II) sorption to HFO surface sites (>FeOH0). Low concentrations of FeOH+, Fe(OH)20, and Fe(OH)3− were predicted on addition of FeSO4 with the concentrations of these species not changing significantly with increasing Fe(II) concentrations, suggesting that these species had little contribution to the total proton release. In comparison, the concentration of the surface complex >FeOFe(II)+ increased dramatically on increase in Fe(II) concentrations with concomitant extensive release of protons to solution during the formation of this surface complex. The ferrous ion binds significantly to the >FeOH0 sites on the HFO surface covering Fe0 thereby facilitating proton release. The calculated concentration of the surface complex >FeOFe(II)+ (which represents monolayer coverage of passivated ZVI) showed very similar dependency on added Fe(II) concentration to that of the measured extent of Fe(II) sorption (Figure 2) and firstorder rate constant for TCE degradation (Figure 1a). Based on the speciation analysis for ferrous solutions presented above, it is apparent that formation of >FeOFe(II)+ is critical to the extent of proton release. A consequence of the resultant decrease in pH of the solution (and, perhaps even more significantly, at the HFO surface) is the resultant increase in concentration of the surface species >FeOH2+ (Figure 4b). Analogous to the results with Mg2+, the adsorption of Fe2+ on oxide coatings resulted in an increase in concentration of the surface species >FeOH2+, leading to the enhanced tendency for dissolution of any iron oxyhydroxide mineral phase present on the Fe0 surface.26 This is potentially critical to depassivation as there is ample evidence that protonation of surface sites leads to enhanced tendency for dissolution of minerals such as iron oxides as it is recognized to lead to the generation of highly polarized interatomic bonds in the immediate proximity of the surface central ions which facilitates the detachment of cationic surface groups and their release to solution.34,35 Indeed, the rate F

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Fe0, and Fe0 with an HFO coating. Herein, the surface areas of HFO and aged Fe0 were assumed to be 200 m2 g−1 and 2 m2 g−1, so 0.25 g L−1 of HFO will have the same surface area as that of 25 g L−1 of aged Fe0. Results shown in Figure S9 suggest that while HFO alone showed no significant TCE reduction (data not shown), the presence of Fe(II) did enhance the TCE degradation though the reaction was very slow. An increase in the Fe(II) concentration led to an increase in the rate of TCE reduction with pseudo-first-order rate constants of 0.0019 d−1, 0.0056 d−1, and 0.0128 d−1 obtained for 1 mM, 10 mM, and 100 mM Fe(II), respectively. It is noteworthy that these k values are substantially lower than those obtained for depassivated Fe0 where rate constants of >0.1 d−1 were obtained. As the XRD of HFO in the presence of Fe(II) in Figure S10 suggested that goethite was formed after 21 days’ reaction, a decrease in redox potential was expected to accompany the transformation of the iron oxide to a more crystalline form as a result of the associated reduction in ferric ion activity.25 The process of surface Fe(II) complex-mediated TCE dechlorination in Fe0 systems has previously been proposed by Scherer et al.31 in line with evidence that Fe(II) associated with iron oxides (goethite, hematite, lepidocrocite, ferrihydrite) can reduce a variety of organic pollutants.14,41−43 In accord with the Fe(II)-catalyzed amorphous iron oxides transformation process described above, Colón et al.42 explored the reduction of nitroaromatics by surface bound Fe(II) and observed a distinct increase in reactivity with increasing crystallinity (and thus decreasing solubility) of the underlying Fe(III) oxyhydroxide phase. The ongoing release of Fe(II) on oxidation of the underlying Fe0 may lead to crystallization of any HFO present on the Fe0 surface though, as noted above and shown in Figure S9, the rate of degradation of TCE by Fe(II) associated with either amorphous or crystalline oxide coatings on Fe0 is likely to be slow, at least compared to the rate of reduction by Fe0 (unless, of course, the new coating is conducting in which case electron transfer from the underlying Fe0 may be enhanced). Environmental Significance. Based on the aforementioned discussion, the presence of Fe(II) can drive proton release following formation of an Fe(II) surface complex on the surface of aged Fe0. This proton release may, in turn, lead to a reduction in local pH with formation of the relatively easily reduced >FeOH+2 surface species. Dissolution of the iron (oxy)hydroxide coating then leads to exposure of the underlying Fe0 solid to solution with subsequent reductive degradation of TCE. While the Fe(II) generated on oxidation of Fe0 may also lead to transformation of the Fe(III) (oxy)hydroxide coating to more crystalline phases, the rate of reduction of compounds such as TCE by Fe(II) associated with the Fe(III) (oxy)hydroxide coating is substantially slower than that mediated by Fe0. While the solubility of ferric iron is extremely low in natural waters in the circumneutral pH range,44 relatively high concentrations of Fe(II) may be present, particularly under anoxic conditions as the result of the reductive dissolution of Fe(III) (hydr)oxides by strong reductants and/or iron-reducing bacteria.14,45 The findings reported here that depassivation of aged Fe0 can be influenced by the presence of ferrous ion provide new insight into the effect of ferrous ion on the application of Fe0-based technologies for the degradation of chlorinated contaminants in iron-bearing environments, especially under anoxic conditions.

Figure 5. Final corrosion potentials (Ecorr), corrosion current (Icorr), and polarization resistance (Rp) at the fourth hour of electrodes modified by aged Fe0 in solutions of different concentrations of FeSO4 (0−100 mM).

electron transfer barrier and, concomitantly, more facile electron transfer from Fe0 to the substrates on/near the surface. These results suggest that the presence of higher concentrations of Fe(II) could lead to higher electron flow capacity from the aged Fe0 to the electrode and higher degree of depassivation of the aged Fe0, which may explain the increasing release of dissolved iron into solution in the presence of high concentrations of Fe(II). However, the effects of different Fe(II) concentrations on the values of Ecorr, Icorr, and Rp appear to be decoupled from their effects on TCE degradation by aged Fe0. As such, it is clear that additional processes need to be considered. Fe(II) Species in Aged Fe0-Fe(II) System. It is clear that not only aqueous Fe(II) but also surface-located Fe(II) species exist in the aged Fe0 system as a result of the adsorption of Fe(II) onto the surface of aged Fe0 and concomitant formation of Fe(II) surface complexes with the possible aqueous and surface-located Fe(II) species including Fe2+, FeSO40, FeOH+, Fe(OH)20, Fe(OH)3−, >FeOFe(II)+, and >FeOFe(II)OH. While the concentrations of FeOH+, Fe(OH)20, and Fe(OH)3− are low, Fe2+, FeSO40, >FeOFe(II)+, and >FeOFe(II)OH are present at high concentrations and, according to speciation calculations, dominate the chemistry of Fe(II) in these systems. Using the Nernst equations and calculated concentrations of Fe(II) and Fe(III) species (assuming initial FeSO4 of 10 mM and HFO of 1 g L−1 with surface area of 600 m2 g−1), it is readily seen (Table S1 and Figure S8) that, while not as reducing as Fe0, both Fe2+(aq) and adsorbed Fe(II) species (>FeOFe(II)+ and >FeOFe(II)OH) in equilibrium with HFO are capable of reducing TCE to dichloroethylene, vinyl chloride, and ethylene. The actual redox potential for the system will be set by the couple present at highest concentration (the “dominant couple”) which, in the absence of an Fe0 passivating layer, will most likely be Fe0/Fe2+ but, in the presence of an iron oxide coating on Fe0, will either be Fe2+ or one of the adsorbed Fe(II) species in equilibrium with HFO with the actual dominant couple (and, indeed, the redox potential) dependent upon the concentration of Fe2+ present and the pH. To further illustrate the capacity of Fe(II) associated with oxide surface coatings to degrade TCE, additional experiments were conducted with Fe(II) in the presence of HFO. As the surface area of Fe0 is very low (0.05−4.17 m2 g−1)40 in comparison with that of HFO (159−840 m2 g−1),30 care needs to be taken in comparing systems containing pure HFO, pure G

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ASSOCIATED CONTENT

S Supporting Information *

Kinetics of TCE transformation by aged Fe0 and HFO under different treatments or different pH; XRD patterns of aged Fe0 or HFO with Fe(II) after 21 days’ reaction with TCE; pH vs time in the aged Fe0 suspension in the presence of 10 mM Fe(II); open circuit potential vs time (OCPT) and Tafel plot of aged Fe0 with FeSO4; model predicted concentrations of iron species versus pH; and Nernst equations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 61293855060. Fax: 61293856139. E-mail: d.waite@ unsw.edu.au. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The author (T.X.L.) acknowledges the award of a UNSW ViceChancellor’s Post-Doctoral Research Fellowship with Supplementary Research Support Grant (No. RG114816). The authors also acknowledge support provided through ARC Linkage Project LP100100852. We also thank Xiaoming Ma and Yongjia Xin in the School of Civil and Environmental Engineering, University of New South Wales for their assistance with GC-ECD and electrochemistry measurements.

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