Anodic Reactivity of Ferrous Sulfide Precipitates Changing over Time

Article pubs.acs.org/est

Anodic Reactivity of Ferrous Sulfide Precipitates Changing over Time due to Particulate Speciation Elena Mejia Likosova,*,† Richard N. Collins,‡ Jurg Keller,† and Stefano Freguia† †

The University of Queensland, Advanced Water Management Centre (AWMC), St Lucia, Brisbane, Queensland 4072, Australia UNSW Water Research Centre, School of Civil and Environmental Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia

‡

ABSTRACT: The disposal of ferric phosphate (FePO4) sludge, routinely generated in wastewater and drinking water treatment, has a major impact on the overall treatment cost. Iron sulfide (FeSx) precipitation via sulfide addition to ferric phosphate (FePO4) sludge has been proven to be an effective method for phosphate recovery. Electrochemical oxidation of FeSx can then be utilized to recover ferric iron for reuse back in the phosphate removal process. In this study, the reactivity of FeSx particles for anodic oxidation at pH 4 was studied as a function of time after FeSx precipitate generation at a S/Fe molar ratio of 1.75. Cyclic voltammetry showed high reactivity for fresh FeSx particles, but the reactivity diminished significantly over a period of 1 month. X-ray absorption spectroscopy (XAS) revealed that this reduced reactivity with time is a consequence of the transformation of the FeSx particles in suspension from mackinawite (FeS) to pyrite (FeS2).

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INTRODUCTION The use of ferric salts, mainly ferric chloride (FeCl3) or ferric sulfate (Fe2(SO4)3), is very effective for phosphorus removal and organics coagulation in sewage treatment and recycled water production, but it has a major impact on the overall treatment cost due to both chemical dosing and sludge disposal. As a result, the development of an efficient and cost-effective method to recover and recycle ferric iron would be highly beneficial and enable a more sustainable overall process. Phosphorus recovery from FePO4 via iron sulfide generation has been proposed and studied by several authors.1−3 After the addition of sulfide to the iron phosphate sludge, phosphate is released into solution and FeSx sludge is generated. Iron and sulfur can theoretically be separated and recovered for reuse within the process; i.e., iron can be recycled as Fe(II/III) into the precipitation process and sulfur can be recovered as S2− and reused in the phosphate recovery stage.2 Compared to other methods for the removal of wastewater sulfide, electrochemical techniques offer advantages in terms of selectivity, energy efficiency, and chemical consumption. Sulfide is an electro-active compound and can be removed by electrochemical oxidation on a variety of anode materials.4−16 Depending on the anode materials and imposed potentials, aqueous sulfide can be electrochemically oxidized to elemental sulfur (S0), polysulfides (Sn2−), sulfate (SO42−), sulfite (SO32−), and thiosulfate (S2O32−).17 Elemental sulfur has been the only final oxidation product in a number of studies with carbon/ graphite anode materials.4,7,11,12 The electrochemical oxidation © 2013 American Chemical Society

of pyrite (FeS2) electrodes in aqueous acidic solutions has also been demonstrated.17,18 Liu and co-workers reported very slow oxidation and reduction rates for pyrite between −0.36 and +0.84 V vs SHE with current densities of less than 0.048 mA/ cm2. However, an increase in the oxidation rate above +0.84 V vs SHE was reported.18 In addition to elemental sulfur, thiosulfate and sulfate have been found as oxidation products of pyrite depending on the values of the anode potentials and the types of electrodes used.19 The electrochemical oxidation of pyrite leads to the release of ferric ions into solution, which could be recycled to the precipitation/coagulation process. Although the electrochemical oxidation of sulfide for sulfur recovery has been well studied,4,6,11,12,16,17,20 the process has never been proven with solid FeSx particles, which are the main form of sulfide in the FeSx sludges generated in wastewater treatment. The use of graphite as electrode material for such a process has also not been demonstrated, although it would be very advantageous due to its low cost, chemical stability, and common availability. The intrinsic unknown variables here are the reactivity of FeSx solid particles with the electrode and the effect of far higher concentrations of sulfide compared to previous studies. Moreover, a study on the factors affecting the electrochemical reactivity of the FeSx particulates has yet to be Received: Revised: Accepted: Published: 12366

July 5, 2013 October 1, 2013 October 5, 2013 October 5, 2013 dx.doi.org/10.1021/es402967e | Environ. Sci. Technol. 2013, 47, 12366−12373

Environmental Science & Technology

Article

reactor to perform the cyclic voltammetry (CV) experiments (Figure 1). Two separate graphite rods were used as the current

undertaken. Several electrode materials, such as graphite rods, plates and granules,6,15,21,22 and electrodes containing precious metals such as platinum,17,23,24 nickel,9,25 and titanium,14,16,26,27 have been studied for electrochemical sulfide oxidation at laboratory scale. Similarly, work on the electrochemical detection and characterization of FeS nanoparticles on mercury and gold electrodes has recently been published.28−30 However, considering a future scale-up of the process and the high capital cost associated with full-scale electrochemical systems, the use of graphite and carbon as electrode materials would be ideal given their low cost.31,32 In addition to the electrode material, the S/Fe molar ratio and the time after formation of the FeSx sludge are factors that could also affect the electrochemical performance of the FeSx sludge. In this study, the reactivity of FeSx sludge formed at pH 4 and at S/Fe molar ratio of 1.75 (these conditions were found to be optimal for FeSx formation and separation)2 on graphite electrodes was investigated as a function of the time after FeSx precipitation. Cyclic voltammetry integrated with liquid phase sampling was undertaken to understand the sequence of electrochemical reactions, and X-ray absorption spectroscopy (XAS) was used to determine the speciation of both, Fe and S, in the sludge as a function of time to understand the FeSx transformations related to changes in the reactivity with time.

Figure 1. Modified Schott bottle used for the CV experiments. Three electrodes in a single chamber set up with pH control and continuous recirculation of the FeSx suspension via a peristaltic pump. Graphite granules were used as the working electrode (WE), and graphite rods were used as current collector (CC) and counter electrode (CE). An in-house prepared Ag2S reference electrode (RE) was used to perform the CV at a scan rate of 0.1 mV/s between −0.2 and +1.3 V vs SHE.

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MATERIALS AND METHODS Preparation of Synthetic FeSx Sludge and Solutions for Cyclic Voltammetry (CV) Experiments. FeSx sludge was prepared over a 30 min period by reaction of 0.1 M ferric phosphate with appropriate quantities of a 0.8 M sodium sulfide solution to obtain a 1.75 S/Fe molar ratio. The pH was controlled at pH 4 using 3 M HCl. The initial reaction of ferric phosphate with sulfide, at a S/Fe molar ratio of 1.75, can theoretically be represented (in mildly acidic solutions) by the following stoichiometry:1,2

collector and counter electrode. A volume of 150 mL of the FeSx suspension (0.41 g Fe/L) in 0.1 M acetate buffer (pH 4.0) (see Preparation of Synthetic FeSx Sludge and Solutions for Cyclic Voltammetry (CV) Experiments) was used to fill the reactor and was continuously recirculated at 29 mL/min by means of a peristaltic pump to avoid settling of solid matter at any stage during the experiment. The solution was deoxygenated with nitrogen prior to starting the experiment. A glass tube sealed at the bottom with a cation exchange membrane (CEM) was used to separate the counter electrode from the FeSx suspension. An Ag/Ag2S reference electrode (RE)33 (E = −420 (±30) mV vs SHE) was prepared immediately before every experiment with the purpose of assuring the stability of the RE. Cyclic voltammograms (CV) were recorded with a potentiostat (VSP Modular 5 channels potentiostat, BioLogic Science Instrument, France) at a scan rate of 0.1 mVs−1 between −0.2 and +1.3 V vs SHE. During the CV, samples were taken to determine soluble sulfur speciation by ion chromatography (IC). Chemical Analyses. Ion chromatography (IC, Dionex 2010i) was used to measure the different anionic sulfur species, i.e., sulfide (S2−), sulfate (SO42−), sulfite (SO3−), and thiosulfate (S2O32−), according to the method developed by Keller-Lehmann et al.34 Samples were preserved in sulfide antioxidant buffer (SAOB) solution prior to IC analyses. The SAOB solution was prepared following the procedure explained elsewhere.34,35 Preparation of Synthetic FeSx Sludge for Fe and S KEdge XAS Analysis. The FeSx sludges were also analyzed (in terms of time since FeSx generation) for iron and sulfur speciation using Fe and S K-edge X-ray absorption spectroscopy (XAS). FeSx sludge was prepared by reaction of synthetic 0.1 M ferric phosphate and a 0.8 M sodium sulfide solution to obtain a 1.75 S/Fe molar sludge at pH 4 as explained above (Preparation of Synthetic FeSx Sludge and Solutions for Cyclic Voltammetry (CV) Experiments). The FeSx sludge was generated and then deposited in a 6 mL tube without leaving

Fe IIIPO4(s) + 1.75H 2S → Fe IIS(s) + 0.5S(s)0 + 0.25H 2S + H 2PO4 − + H+

(1)

The resulting synthetic FeSx sludge was divided into two 50 mL Falcon tubes without any headspace and centrifuged at 2,100g for 5 min to facilitate phase separation of the FeSx particles. The supernatant with residual soluble sulfide was removed, and the remaining FeSx sludge was washed and resuspended in 70 mL of 0.1 M acetate buffer (pH 4). No headspace was left in the 70 mL container to avoid oxidation of the FeSx particles while left quiescent to allow sedimentation of the particles for 1 and 2 days, 1 week, and 1 month, before analysis at these four time points. A FeSx suspension (0.41 g Fe/L) was prepared using the settled FeSx particles further diluted in a 0.1 M acetate buffer (pH 4.0). As measured by dynamic light scattering,2 the point of zero charge of the FeSx particles is at pH 4. At this pH, the particles settle rapidly after reaction of the ferric phosphate sludge with sulfide. Massive (or crystalline) pyrite (FeS2), obtained from the mineral collection of the Australian Museum, Sydney, was ground to a fine powder with an agate mortar and pestle and used to prepare a 0.41 g Fe/L suspension in a 0.1 M acetate buffer (pH 4) as described above. Reactor Design for Cyclic Voltametry (CV) Experiments. A modified 250 mL Schott bottle, filled with graphite granules (El Carb 100, Graphite Sales, Inc., USA) as the working electrode up to the 100 mL mark, was used as the 12367

dx.doi.org/10.1021/es402967e | Environ. Sci. Technol. 2013, 47, 12366−12373


Article

Table 1. Nonlinear Least-Squares Fitting Parameters of the Fe K-edge EXAFS Spectra Obtained for the Standards and Samplesa FeSx species pyrite

b,c,d

path

ΔE

Fe−S Fe−S Fe−Fe Fe−S−Fe

−4.6

CN e

σ2

R-factor

bond length (Å) (crystallography)

0.0005 0.0012 0.0002 0.0016

0.01

2.264 3.445 3.830 4.17

0.0026 ± 0.0004 0.0086 ± 0.0013

0.04

2.256 2.598

interatomic distance (Å) ± ± ± ±

6 6e 12f 29 ± 8

2.26 3.45 3.82 4.13

0.01 0.01 0.01 0.01

4.1 ± 0.2e 4.1 ± 0.2e

2.26 ± 0.01 2.69 ± 0.01

0.0023 0.0051 0.0027 0.0001

± ± ± ±

mackinawiteg

Fe−S Fe−Fe

greigiteh

Fe−S Fe−S Fe−Fe Fe−Fe

fresh

Fe−S Fe−Fe

3.0

3.8 ± 0.3e 3.8 ± 0.3e

2.25 ± 0.01 2.68 ± 0.02

0.0032 ± 0.0008 0.0084 ± 0.0018

0.04

1 day

Fe−S Fe−Fe

3.9

4.0 ± 0.3e 4.0 ± 0.3e

2.26 ± 0.01 2.68 ± 0.01

0.0025 ± 0.0005 0.0080 ± 0.0012

0.02

2 day

Fe−S Fe−Fe

3.3

4.2 ± 0.4e 4.2 ± 0.4e

2.25 ± 0.01 2.68 ± 0.01

0.0024 ± 0.0006 0.0088 ± 0.0016

0.02

1 week


3.1

3.8 ± 0.6e 3.8 ± 0.6e 7.6f 24 ± 37

2.24 3.45 3.82 4.14

± ± ± ±

0.01 0.03 0.02 0.05

0.0023 0.0060 0.0050 0.0035

± ± ± ±

0.0012 0.0067 0.0015 0.0119

0.07

1 month


1.4

4.2 ± 0.4e 4.2 ± 0.4e 8.4f 24 ± 12

2.24 3.44 3.82 4.13

± ± ± ±

0.01 0.02 0.01 0.01

0.0045 0.0065 0.0051 0.0005

± ± ± ±

0.0007 0.0023 0.0007 0.0030

0.04

2.4

4 (1/3 occupancy) 6 (2/3 occupancy) 4 2

2.147 2.464 3.492 4.094

ΔE = photoelectron kinetic energy origin correction; CN = coordination number; σ2 = Debye-Waller factor. bCrystallographic data from Bayliss et al.39 cCN of single scattering pathways fixed during fitting. dReference used to determine the amplitude reduction factor, S02 = 0.70. eCN equal for both paths during fitting. fCN twice that of Fe−S paths. gCrystallographic data from Lennie et al.38 hCrystallographic data from Skinner et al.48

a

any headspace to avoid oxidation of the FeSx sludge, centrifuged at 2,100g for 15 min to allow separation of the solid and liquid phases, and left to age for up to one month. The samples were left to age in an anaerobic chamber (gas composition: 4% H2, 6% CO2, and 90% N2) to avoid oxidation of the sludge. Before XAS analyses, and inside the anaerobic chamber, the supernatant was removed and the tubes with the solids were heated to 70 °C for 4 h by means of a heating block in order to remove any extra moisture present in the FeSx sludge. By XAS analyses of wet pastes and dried samples, it was confirmed that this drying process did not have an impact upon sample integrity. The samples were finally stored and transported in an anaerobic gas jar with an Anaerogen Sachet (Oxoid) to the Photon Factory in Tsukuba, Japan (for Fe Kedge extended X-ray absorption fine structure, EXAFS, spectroscopy) or the National Synchrotron Radiation Research Centre, Hsinchu, Taiwan (for S K-edge X-ray absorption near edge structure, XANES, spectroscopy). The “fresh” and 1- and 2-day old FeSx sludge samples were prepared directly on site according to the same procedure used in the initial studies (described above), i.e., identical setup and reaction procedure, with the same centrifuging and anaerobic storage conditions being followed. X-ray Absorption Spectroscopy (XAS) Sample Preparation and Analysis. Measurements across the Fe K-edge of

samples and reference minerals were undertaken on the bending magnet beamline BL-20B (Australian National Beamline Facility) at the Photon Factory (KEK Tsukuba, Japan). The storage ring was operated at 2.5 GeV in continuous injection mode with a current of 430 mA. The incident X-ray beam was detuned to ∼50% of maximum intensity to minimize higher-order harmonic reflections. The beam energy was selected using a Si(111) monochromator. All data were collected in transmission mode with the use of Oken ionization chambers. During the XAS analyses, all samples were maintained at ∼12 K in a closed-cycle helium cryostat to maintain sample integrity (i.e., to prevent oxidation). Elemental Fe foil was used for energy calibration of sample and reference mineral spectra, and both energy calibration and multiple scan merging were carried out with the software program Average.36 The S K-edge data were collected at room temperature on beamline 16A at the National Synchrotron Radiation Research Centre (NSRRC) in Hsinchu, Taiwan. The storage ring was operating at 1.5 GeV in continuous injection mode, with the energy selected using a Si(111) monochromator. Total X-ray fluorescence data were collected with a Lytle detector. The incident X-ray beam was detuned to ∼50% of maximum intensity to minimize higher-order harmonic reflections. The beam energy was calibrated to the peak of the first derivative of the Mo foil edge (K-edge: 2.52 keV). Samples and standards 12368



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FeS → Fe 2 + + S0 + 2e−

were measured as powders and appropriately diluted with boron nitride to avoid self-absorption effects. Data processing was performed with the standard features of the ATHENA software,37 and background subtraction was performed with the Autobk algorithm.37 ARTEMIS36 was used to generate theoretical phase and amplitude functions for single and multiple scattering pathways from the crystallographic data of mackinawite38 and pyrite.39 These functions were then used for nonlinear least-squares fitting of the Fe K-edge EXAFS data (fitting details are provided in Table 1). A crystalline pyrite reference, as used by Collins et al,40 was used both as a reference and to obtain the amplitude reduction factor (S02) for fitting of the data. A mackinawite (FeS) standard was generated by reduction of ferric iron with excess H2S. Although this produced a quantitative FeS standard for Fe K-edge XAS, the S(s)0 produced from this reaction was present in the FeS standard and, therefore, the presence of S(s)0 was only assessed qualitatively for the S K-edge XAS analyses.

(2)

On the basis of a solubility constant of Ks = 8 × 10−19 for crystalline FeS at 25 °C41 and a standard redox potential of −0.476 V for the reaction S0 + 2e− → S2−,12 a theoretical redox potential of 0.06 V vs SHE for the FeS oxidation reaction was calculated. Three oxidation steps can be observed with the “young” sludges (fresh to 1-week old). The first, small peak exhibits a formal potential around +0.1 V and may be associated with the presence of some free sulfide in solution; the second step is a sigmoidal wave which inflects at +0.45 V and may indicate FeS oxidation according to reaction 2; the third oxidation step produces a peak which is putatively associated with the oxidation of Fe2+ to Fe3+ at higher potentials (E0′ = +0.77 V for Fe3+ + e− → Fe2+).42 With this interpretation, it would appear that the oxidation of FeS particles is affected by large overpotentials, but this can be expected for a reaction between a solid substrate and a solid electrode. The liquid phase samples confirmed the formation of sulfate (SO42−) above +0.8 V vs SHE in all CVs (data not shown). As oxidation to sulfate is irreversible, operating at potentials greater than +0.8 V would lead to permanent loss of sulfur. A gradual change in the color of the FeSx suspension liquid phase from transparent to red was observed above +0.6 V vs SHE during the oxidative current (forward and reverse scan) and back to transparent in the reductive current (reverse scan), suggesting the oxidation of ferrous to ferric iron during the CV above this potential and reduction back to ferrous at the end of the reverse scan (Figure 3C). In Figure 3A, the profile of the fraction of iron that is soluble (necessarily ferrous at this pH)43 during the CV for the 1-day old FeSx is presented. As FeS oxidizes to elemental sulfur, it liberates Fe2+ and an increase in the soluble iron fraction is expected. This was confirmed by the soluble iron profile during the forward scan between −0.2 and +0.55 V vs SHE (blue solid line A−B, Figure 3). Subsequently, as the solubilized ferrous iron oxidizes to ferric iron (at this pH insoluble, nominally as Fe(OH)3), the soluble iron fraction is expected to decrease. This was supported by the soluble iron profile along the forward scan, starting at +0.6 V up to +1.3 V vs SHE and continuing into the reverse scan down to +0.5 V vs SHE (orange dashed line B−C, Figure 3). Finally, the reductive peak observed between +0.5 and −0.2 V can be associated to ferric (oxy)hydroxide reduction back to soluble ferrous, as suggested by the fact that an increase in soluble iron was observed during the reverse scan between 0 and −0.2 V vs SHE (red dotted line C−A, Figure 3). This was confirmed quantitatively by integration of the reductive peak, which through Faraday’s law predicts an increase of soluble iron by 0.29 g Fe(sol)/g Fe(total), very close to the 0.27 g Fe(sol)/g Fe(total) measured in the liquid phase. Chemical Speciation of FeSx Sludge as a Function of Time after Precipitate Generation. The Fe K-edge EXAFS data of the FeSx sludge are presented in Figure 4A. The spectra for the “fresh” and 1- and 2-day old FeSx are very similar to mackinawite, but spectral features associated with pyrite are evident in the latter two samples, particularly around 8 Å−1. The spectra of the 1-week and 1-month old sludge samples are significantly different and, although not identical to the crystalline pyrite standard, have similar peak positions to this standard. Furthermore, the EXAFS oscillations of the 1-week and 1-month samples are dampened compared to the crystalline pyrite standard, and this may result from either

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RESULTS AND DISCUSSION Reactivity of FeSx Sludge on Graphite Electrodes as a Function of Time after FeSx Precipitation. Figure 2 shows

Figure 2. Cyclic voltammograms of FeSx suspensions (0.41 g Fe/L), after acetate wash of the FeSx prepared at 1.75 S/Fe molar ratio and pH 4 with a scan rate of 0.1 mVs−1 between −0.2 and +1.3 V vs SHE. Blue solid line: 1-day old FeSx; green dashed line: 2-day old FeSx; yellow dotted line: 1-week old FeSx; red dashed-dotted line: 1-month old FeSx; gray solid line: crystalline pyrite (FeS2); black dashed line: acetate background.

the cyclic voltammograms (CV) of 1- and 2-day, 1-week, and 1month old FeSx and crystalline pyrite suspensions (0.41 g Fe/ L) on graphite granules at pH 4. On the basis of the oxidative current generated during the CV at E > 0 V, it can be inferred that the FeSx particles are reactive at pH 4 for anodic oxidation. The presence of a reductive current during the reverse (positive to negative) scan can be attributed to ferric reduction to ferrous, as explained below. The reactivity of the FeSx particles gradually decreased with time after FeSx precipitation, as indicated by the decreasing peak currents for the older sludge samples. This was found to be coincident with changes in the chemical speciation of the FeSx sludge, as discussed below in Chemical Speciation of FeSx Sludge as a Function of Time after Precipitate Generation. The oxidation of mackinawite (FeS) particles can be represented by the following stoichiometry: 12369



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Figure 3. (A) Soluble iron profile (defined as g Fe(Sol)/g Fe(Total)) during the CV of the 1-day old FeSx between −0.2 and +1.3 V vs SHE. Blue solid line: Forward scan between −0.2 and +0.55 V vs SHE, assigned to the oxidation of FeS to ferrous iron. Orange dashed line: Forward scan between +0.55 and +1.3 V, plus reverse scan from +1.3 V until +0.5 V vs SHE, assigned to the oxidation of ferrous iron to ferric hydroxide. Red dotted line: reverse scan from +0.5 V until −0.2 V vs SHE, attributed to the reduction of ferric iron to ferrous. (B) Soluble iron profile and (C) solution color profile, during the CV of 1-day old FeSx between −0.2 and +1.3 V vs SHE. A−B solid blue line: change in color between −0.2 and +0.55 V; B−C orange dashed line: change in color in the forward scan between +0.55 and +1.3 V and reverse scan back to +0.55 V; C−A red dotted line: change in color in the reverse scan between +0.55 and −0.2 V vs SHE. Error bars calculated as the standard deviation with n = 3.

Figure 4. (A) Fe K-edge EXAFS and (B) phase-uncorrected Fourier-transformed EXAFS data (k = 2−14 Å−1) modeled between R = 1−4.4 Å−1 for pyrite and 1-week and 1-month FeSx samples and R = 1−3 Å−1 for mackinawite and the fresh and 1- and 2-day FeSx samples. The vertical dashed line indicates the nearest Fe neighbor in mackinawite.

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Figure 5. (A) S K-edge XANES data of 1.75 S/Fe molar ratio FeSx sludge as a function of time after precipitate generation, produced in anoxic conditions at pH 4 and the mackinawite + S0, pyrite, and FeSO4 standards. (B) Corresponding data presented as the 1st derivative. The contribution of FeS in the 1-week sample is noted by the arrow, and the black dashed lines are the linear combination fits of the respective samples.

Together, these data indicate that the dampening of the EXAFS oscillations, in relation to the crystalline pyrite standard, is most likely the result of the samples containing nanoparticulate/disordered pyrite rather than being diluted by significant concentrations of residual mackinawite or intermediate greigite. Similar to the Fe K-edge EXAFS results, a transformation of the FeSx particles from mackinawite to pyrite as a function of time after precipitate generation is suggested by the S K-edge XANES data (Figure 5). In Figure 5A, the presence of mackinawite (FeS) and elemental S in the fresh FeSx sludge (1.75 S/Fe molar ratio) as suggested by eq 1 can be seen. As mentioned in X-ray Absorption Spectroscopy (XAS) Sample Preparation and Analysis, the presence of elemental S in the standard was expected as a result of the sulfide overdose during preparation and, therefore, only used for qualitative comparison. Nevertheless, it can be observed in Figure 5A that the peaks corresponding to mackinawite and elemental S appear to be largely absent from the 1-week and 1-month FeSx samples (vertical dashed lines), although some contribution from mackinawite to the 1-week sample spectrum is evident as a shoulder in the main pyrite peak. This contribution is seen more clearly in the first derivative of the XANES data for this sample (pointed out by the arrow in Figure 5B), and linear combination fitting of this region resulted in a best fit of 12 ± 1% FeS and 88 ± 1% FeS2 (R-factor = 0.008). Although, linear combination fitting of the 1-month sample resulted in a best fit combining FeS and FeS2, the estimated contribution from FeS was very minor at 6 ± 4% (R-factor = 0.024). Given that this percentage is so small, with a corresponding high error value, this result tends to indicate that FeS, if present, is only a very minor species in the 1-month sample. For example, previous work has recognized the need of an oxidizing agent for mackinawite to react to pyrite,46,49,50 i.e., an oxidized intermediate sulfur species (such as elemental sulfur or polysulfide) and/or surface oxidized monosulfide species (oxidized mackinawite, greigite).46 In these experiments, the principle reactive species driving pyrite formation would be residual elemental sulfur, formed from the initial reduction of

(1) the presence of residual FeS, or other intermediate FeSx species such as greigite (Fe3S4), or (2) that the FeS2 formed is nanoparticulate (i.e., lattice relaxed) or more disordered compared to the crystalline FeS2 standard. The corresponding Fourier-transformed EXAFS data are shown in Figure 4B, and it is clear that the samples until 2-days old are essentially mackinawite with Fe−S and Fe−Fe coordination numbers and interatomic distances are identical to the mackinawite standard (Table 1). While the Fe−S bond distances are similar to those reported previously for EXAFS studies on nanocrystalline mackinawite,44 the Fe−Fe interatomic distances observed here (∼2.69 Å) are approximately 0.06 Å longer. This suggests that both the mackinawite standard and samples until 2-days old have a smaller crystallite size45 than that reported for the samples generated by Jeong et al.44 Indeed, the similarity of the fitting parameters for these samples (Table 1) indicate that little change, as detected by EXAFS, has occurred to the short-range structure of these particles within 2 days. The next nearest Fe neighbor in mackinawite is absent in the spectra from the 1-week and 1-month old FeSx samples, indicating that it is undetectable by EXAFS (with a detection limit estimated at ∼15−20%). In addition, at least two further peaks appear at similar interatomic distances (and relative ratios) as that observed for the pyrite standard (Table 1). However, as observed in the EXAFS data, the peak intensities (which are related to the amplitude of the EXAFS oscillations) of these samples are lower than that for the pyrite standard. As mentioned previously, this may result either if pyrite is not the only mineral present or if the pyrite formed in the samples is nanoparticulate/disordered. The absence of the peak at 2.69 Å, due to Fe−Fe in mackinawite, indicates that dampening has not occurred due to the presence of this mineral. Greigite has also been shown to be a possible intermediate mineral species during the transformation of mackinawite to pyrite,46,47 and, therefore, potentially could result in the EXAFS oscillation dampening observed here. Although we did not analyze a greigite standard, it is clear from the crystallographic data reported in Table 1 that the Fe−S bond distances typical for this mineral (2.147 and 2.464 Å) are also not present. 12371



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ferric phosphate by sulfide (reaction 1). Furthermore, the presence of some dissolved oxygen during the preparation of the initial ferric phosphate suspension may also have resulted in the oxidation of some sulfide to elemental sulfur. Thus, the reaction of mackinawite with elemental sulfur is suggested to be responsible for mackinawite conversion to pyrite. Together, the Fe EXAFS and S XANES results are consistent and indicate that the change in the anodic reactivity of the FeSx sludge with time is a result of changes in the chemical speciation of the FeSx sludge; i.e., the greater anodic reactivity of the fresh FeSx sludge is a result of mackinawite (FeS) being the predominant compound. Conversely, the pyrite (FeS2) that is formed over time is likely responsible for the reduced anodic reactivity of the FeSx sludge. This study proves the feasibility of electrochemical recovery of soluble iron (as ferrous or ferric) from FeSx suspensions formed from sulfide addition to FePO4 sludges generated in wastewater treatment. These findings offer important insights into the development of electrochemical methods for the recycling of ferric iron, used as a coagulant in water and wastewater treatment plants. The reactivity of FeSx particles for anodic oxidation is however suppressed by the progressive transformation of mackinawite to pyrite over a period of approximately one week or more, thus limiting options for the storage of FeSx sludge. Consequently, we highlight the importance of the rapid use of the formed FeSx sludge in the electrochemical reaction, in order to prevent the loss of FeSx reactivity. This loss of reactivity is not of great concern for Fe recovery from ferric sludges generated in drinking water or wastewater postprecipitation processes, as the FeSx sludge can be oxidized immediately after generation. However, this may be a critical issue in reactions where FeSx is formed more slowly (e.g., sludge digestors) or where there is greater potential for elemental S formation, which may further increase the conversion rate of mackinawite to pyrite.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We sincerely thank Daniel Boland from UNSW for his significant contribution during the synchrotron experiments at the National Synchrotron Radiation Research Centre (NSRRC) in Taipei, Taiwan. Part of this research was undertaken at the Australian National Beamline Facility at the Photon Factory in Japan, operated by the Australian Synchrotron. We acknowledge the Australian Research Council (LE110100174) for financial support and the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Japan, for operations support. Travel funding to the National Synchrotron Radiation Research Centre, Taipei, Taiwan, was provided by the International Synchrotron Access program (ISAP) managed by the Australian Synchrotron and funded by the Australian Government. Elena Mejia Likosova thanks the University of Queensland for scholarship support. The authors thank Veolia Water and Seqwater for the funding support. The Australian Research Council (LP100200122) funded this work. Richard Collins is a recipient of an Australian Research Council Future Fellowship (FT110100067). 12372



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Anodic Reactivity of Ferrous Sulfide Precipitates Changing over Time

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