journal of colloid and interface science · 2019. 11. 26. · data with thales 3.16 software. eis...

Journal of Colloid and Interface Science 467 (2016) 51–59

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Journal of Colloid and Interface Science

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Mechanism of the cathodic process coupled to the oxidation of ironmonosulfide by dissolved oxygen

http://dx.doi.org/10.1016/j.jcis.2016.01.0100021-9797/� 2016 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail address: [email protected] (P. Chiriță).

Mădălina I. Duinea a, Andreea Costas b, Mihaela Baibarac b, Paul Chiriță a,⇑aUniversity of Craiova, Department of Chemistry, Calea Bucuresti 107I, Craiova 200478, RomaniabNational Institute of Materials Physics, Laboratory of Optical Processes in Nanostructured Materials, P.O. Box MG-7, Bucharest R077125, Romania

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 October 2015Revised 5 January 2016Accepted 5 January 2016Available online 6 January 2016

Keywords:FeS oxidationFeS/water interfaceEISSEM/EDXRaman spectroscopy

a b s t r a c t

This study investigated the mechanism of iron monosulfide (FeS) oxidation by dissolved oxygen (O2(aq)).Synthetic FeS was reacted with O2(aq) for 6 days and at 25 �C. We have characterized the initial andreacted FeS surface using Scanning Electron Microscopy coupled with Energy Dispersive X-ray (SEM/EDX) analysis, Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR). It was found thatduring the aqueous oxidation of FeS new solid phases (disulfide, polysulfide, elemental sulfur, ferric oxy-hydroxides and Fe3O4) develop on the mineral surface. The results of potentiodynamic polarizationexperiments show that after 2 days of FeS electrode immersion in oxygen bearing solution (OBS) at initialpH 5.1 and 25 �C the modulus of cathodic Tafel slopes dramatically decreases, from 393 mV/dec to86 mV/dec. This decrease is ascribed to the change of the mechanism of electron transfer from cathodicsites to O2 (mechanism of cathodic process). The oxidation current densities (jox) indicate that mineraloxidative dissolution is not inhibited by pH increase up to 6.7. Another conclusion, which emerges fromthe analysis of jox, is that the dissolved Fe3+ does not intermediate the aqueous oxidation of FeS. Theresults of electrochemical impedance spectroscopy (EIS) show that after 2 days of contact between elec-trode and OBS the properties of FeS/water interface change. From the analysis of the EIS, FTIR spec-troscopy, Raman spectroscopy and SEM/EDX data we can conclude that the change of FeS/waterinterface properties accompanies the formation of new solid phases on the mineral surface. The newcharacteristics of the surface layer and FeS/water interface do not cause the inhibition of mineraloxidation.

� 2016 Elsevier Inc. All rights reserved.

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52 M.I. Duinea et al. / Journal of Colloid and Interface Science 467 (2016) 51–59

1. Introduction

Iron monosulfide minerals (pyrrhotite, troilite and mackinaw-ite) are present in various geological environments [1]. Also, ironmonosulfide phases (FeS) can be present on the iron/steel surfaceas result of the reaction between H2S (product of sulfate reduction[2]) and Fe2+ (product of iron/steel corrosion [3]). By its products ofoxidation (ferric iron and protons), FeS oxidation can produce sev-ere environmental problems [4,5]. In acidic media, the ferric iron isan effective oxidant which dissolves new amounts of FeS and othermineral sulfides [6] releasing toxic species such as Cu, Cd, Hg, Pb orAs. Since the oxidation of FeS phases is fast (much faster than thatof pyrite [7]), the understanding of the mechanism of aqueous oxi-dation of FeS is very important. Badica and Chirita [8] have shownthat the aqueous oxidation of troilite by dissolved oxygen (O2(aq))occurs after an electrochemical mechanism. The oxidative dissolu-tion of FeS in the presence of oxygen (the most common oxidant atthe Earth’s surface) generates a surface layer which incorporateferric iron, oxygen, disulfide, polysulfide and elemental sulfur[4,9]. Also, FeS oxidation produces soluble species such as ferrousiron and sulfate:

FeSþ 1=2O2 þ 2Hþ ¼ Fe2þ þ SþH2O ð1Þ

FeSþ 2O2 ¼ Fe2þ þ SO2�4 ð2ÞThe aqueous oxidation of FeS is always accompanied by the

non-oxidative dissolution, which is faster [4,10]:

FeSþ 2Hþ ¼ Fe2þ þH2S ð3ÞThe non-oxidative dissolution is an acid-consuming process.

The released Fe2+ is oxidized by O2(aq) to Fe(III). If the reactionoccurs at acidic pH, the soluble Fe(III) (Fe3+) will oxidize FeS pro-ducing Fe2+ [11,12]:

FeSþ 2Fe3þ ¼ 3Fe2þ þ S ð4Þ

FeSþ 8Fe3þ þ 4H2O ¼ 9Fe2þ þ SO2�4 þ 8Hþ ð5ÞAs in the case of pyrite [13], it can appear a cycle in which the

Fe2+ is oxidized to Fe3+, and the Fe3+ subsequently oxidizes the FeSand produces Fe2+. Fe2+/Fe3+ cycle can be interrupted at pH > 4 [4](in many cases, the pH of oxidizing solutions) by the precipitationof Fe3+

Fe3þ þ 2H2O ¼ FeOOHþ 3Hþ ð6Þ

Fe3þ þ 3H2O ¼ FeðOHÞ3 þ 3Hþ ð7ÞThe reactions (6) and (7) are acid-producing processes. The pre-

cipitation of Fe3+ can affect the oxidation of FeS by, on the onehand, the decrease of Fe3+ concentration [12], and, on the otherhand, the hindering of the access of O2(aq) to mineral surface[14]. Because the great majority of the experimental studies wereperformed in acidic media [4,15], the details of the reaction mech-anism (the rate determining step(s), intermediates, reaction prod-ucts, etc.) of FeS oxidation with O2(aq) at high pH in aqueous mediahave not yet established.

In this paper, the aqueous oxidation of FeS in oxygen bearingsolution (OBS) at initial pH 5.1 and 25 �C was studied by varioustechniques. At pH 5.1 the ferric iron solubility is very low(�10�11 M) and it continues to decreases when pH increases upto 8 (10�12.26 M) [16]. Hence, in this pH range, Fe3+ cannot be theoxidant of FeS. To observe any behavioral changes, the aqueousoxidation of FeS was monitored by means of electrochemical tech-niques (potentiodynamic polarization and electrochemical impe-dance spectroscopy (EIS)) over a period of 6 days of FeS electrodeimmersion in OBS. The reaction products were analyzed using

chemical methods, Scanning Electron Microscopy coupled withEnergy Dispersive X-ray (SEM/EDX) analysis, Raman spectroscopyand Fourier transform infrared spectroscopy (FTIR). For compar-ison, we carried out a series of electrochemical experiments withpyrite as electrode material.

2. Experimental

2.1. Materials

The characteristics of synthetic FeS (96 wt.% of troilite and 4 wt.% of elemental iron) and natural FeS2 (pyrite) used in this studyhave been previously described in detail elsewhere [8,17–19]. Allchemicals were of analytical grade and all solutions were preparedwith distilled water.

2.2. Electrochemical experiments

The electrochemical measurements were performed in a stan-dard three-electrode cell with an electrochemical workstation Zah-ner Elektrik IM6e. The working electrode (WE) was prepared byencapsulation of FeS (or FeS2) in epoxy resin. Only one side, withthe effective area of 0.5 cm2 (or 1 cm2 for the pyrite electrode)was exposed. It has been polished with 600, 2000 and 3000 gradesilicon carbide paper. After polishing the electrode surface wasrinsed with distilled water and acetone. The counter electrodewas platinum foil, while reference electrode was saturated calomelelectrode (SCE). The working electrode was immersed in OBS atinitial pH 5.10 and 25 �C. The initial pH was adjusted to 5.10 byaddition of diluted HCl, without any other background electrolyte.A constant flow of air was bubbled continuously through the solu-tion. The initial pH and pH before each electrochemical measure-ment was measured with a combined glass electrode (Consort).Before each measurement the pH electrode was calibrated againsttwo commercial pH buffers (pH 4.01 and pH 7.00).

Potentiodynamic polarization measurements were carried outwith a scan rate of 1 mV/s in the potential range from �250 to+250 mV relative to the open circuit potential (OCP). Before thepolarization measurement corresponding to 0 days, the electrodewas allowed to equilibrate with the oxygen bearing solution for�40 min. The electrochemical parameters (oxidation potential(Eox), oxidation current density (jox), anodic and cathodic Tafelslopes (ba and bc)) were determined by analyzing the polarizationdata with Thales 3.16 software. EIS measurements were carried outat OCP over a frequency range of 3 MHz–10 mHz with a signalamplitude perturbation of 10 mV. The impedance data were vali-dated using Z-HIT transform test (Fig. S1 of the Supplementarymaterial) [8,17] and then fitted with an equivalent electric circuitusing Thales software.

2.3. Aqueous batch experiment

In order to characterize the solid layer developed on the FeSsurface during its aqueous oxidation the unreacted FeS powderand the residual solid resulted from an aqueous batch experimentwere analyzed by FTIR spectroscopy. FeS powder was prepared bycrushing in an agate mortar the same material used for the con-struction of WE. The specific surface area of the FeS powder wasdetermined using BET method with a Micromeritics TriStar 3000equipment and found to be 2.02 m2/g. The aqueous batch experi-ment was performed by the suspension of 1 g FeS powder in0.5 L oxygen bearing HCl solution (initial pH 5.10) at 25 �C. Theexperiment lasted for 6 days, and a constant flow of air was bub-bled continuously through the solution. At the end of aqueousbatch experiments aliquots of suspension were removed with a

Table 1Time dependencies of electrochemical parameters obtained by Tafel polarization forFeS (or FeS2) electrodes in OBS at 25 �C and different pH values. The error for the joxranged from 2% to 8%, being the lowest for the FeS2. For error evaluation, at least fourindependent experiments were performed for each electrode, and at each initial pH.

Time(days)

Electrode pH Eox (V) jox(lA cm�2)

ba(V dec�1)

bc(V dec�1)

0a FeS 5.10b �0.337 16.8 0.359 �0.3930a FeS 6.50b �0.316 15.8 0.287 �0.3322 FeS 6.41c �0.198 20.4 0.397 �0.0864 FeS 6.30c �0.250 24.0 0.344 �0.0846 FeS 6.70c �0.257 26.8 0.396 �0.0800a FeS2 5.10b 0.216 0.283 0.310 �0.1152 FeS2 5.15c 0.267 0.389 0.300 �0.1244 FeS2 5.11c 0.270 0.298 0.277 �0.1139 FeS2 5.17c 0.256 0.468 0.316 �0.126

a Immediately (�40 min) after immersion of FeS (or FeS2) electrodes in OBS.b Initial pH.c pH before the electrochemical measurements.

M.I. Duinea et al. / Journal of Colloid and Interface Science 467 (2016) 51–59 53

syringe connected to 0.2 lm filter. The filtrates were analyzed fortotal dissolved iron (Fetotal) and sulfate (SO42�). The concentrationof total dissolved iron ([Fetotal]) was determined (after the reduc-tion of aqueous Fe(III) by a solution of 10% hydroxylamine) byspectrophotometry (PG Instruments T70-UV–vis spectrophotome-ter) using the 2,20-dipirydyl method at 522 nm [20]. The concentra-tion of sulfate ([SO42�]) was analyzed by turbidimetry at 420 nm[21].

2.4. SEM/EDX analysis

The morphology and composition of initial FeS surface (beforeits immersion in OBS) and FeS surface oxidized for 6 days in OBSwere investigated using a Tescan LYRA3 XMU Scanning ElectronMicroscope (SEM) with Schottky field emission cathode havingEnergy Dispersive X-ray (EDX) analysis Bruker Quantax 200, Peltiercooled X-ray detector as accessory.

2.5. Raman spectroscopy

Raman spectra were recorded using a T64000 Raman spec-trophotometer from Horiba Jobin Yvon endowed with a Ar laser.The resolution of Raman spectra was of 1 cm�1.

2.6. FTIR measurements

The FTIR spectra were recorded with a Bruker Alpha spectrom-eter using KBr technique. They were collected in the rangebetween 375 and 4000 cm�1, with a resolution of 4 cm�1, and arethe average of 64 scans. In order to limit the sample oxidation,the FTIR spectra were recorded immediately after pelletpreparation.

3. Results and discussion

3.1. Potentiodynamic polarization measurements

The potentiodynamic polarization curves of FeS electrode at dif-ferent times of contact between mineral and OBS are presented inFig. 1. The calculated electrochemical parameters are listed inTable 1. One can be seen that after 2 days of contact between FeSand OBS the Eox shifts to positive direction. Initial Eox is�336.7 mV, and after 2 days of immersion of FeS electrode inOBS it increases up to �197.9 mV. After 6 days of contact betweenFeS and OBS the Eox becomes �256.8 mV. The increase of the oxi-dation current densities (jox) after 2 days of FeS electrode immer-sion in OBS indicates that the FeS oxidation is not inhibited bythe prolonged contact of mineral with OBS. The anodic Tafel slopes(ba) show a moderate oscillation between 287 mV/dec and

Fig. 1. Potentiodynamic polarization curves recorded at (a) initial pH 5.10 and differenrespectively, immediately after FeS electrode immersion in OBS. SHE = Standard hydrog

397 mV/dec. Very interestingly, the modulus of cathodic Tafelslopes (bc) dramatically decreases after 2 days of contact betweenFeS and OBS, from 393 mV/dec to 86 mV/dec. The trend of Tafelslopes indicates that, on the one hand, the mechanism of anodicprocess (the oxidation of FeS) is not affected by the 6 days of con-tact between FeS electrode and OBS and, on the other hand, themechanism of cathodic process (the reduction of O2) occurringon the FeS electrode changes after 2 days of WE immersion inOBS. The decrease of the modulus of bc indicates that the productacn (the only variable in bc, part of its denominator) increases[3,22]. ac is the cathodic charge transfer coefficient and n is thenumber of the electrons transferred during the rate determiningstep of cathodic process. The increase of acn means that after2 days of reaction either the activated complex adopts predomi-nantly the structure of the reduced species (ac shifts toward 1)[3], the rate determining step of cathodic process involves thetransfer of more electrons than immediately after the contact withOBS [22] or both.

In order to assess the role of pH increase during the oxidativedissolution experiments (Table 1), we have compared the electro-chemical parameters determined for FeS electrode immediately(�40 min) after its immersion in aerated solutions (25 �C) at initialpH 5.1 and, respectively, initial pH 6.5 (Table 1). The initial pH wasadjusted to 6.50 by addition of diluted HCl and NaOH solutions.Although there are some differences between the cathodic Tafelslopes obtained for polarized FeS electrode immediately after itsimmersion in OBS at pH 5.1 and 6.5, respectively, these differencesare not comparable with those registered between the cathodicTafel slopes determined for the polarized FeS electrode immedi-ately (�40 min) after its immersion in OBS and, respectively, after

t times of contact between FeS electrode and OBS and (b) initial pH 5.10 and 6.50,en electrode.

Fig. 2. Potentiodynamic polarization curves recorded at initial pH 5.10 anddifferent times of contact between FeS2 and OBS. SHE = Standard hydrogenelectrode.


at least 2 days of immersion in OBS. These findings indicate thatthe pH does not cause the change of the mechanism of cathodicprocess occurring on the mineral surface. It is important to men-tion that, the behavior of FeS electrode was not observed for pyriteelectrode. Although the cathodic current densities measured forFeS2 electrode increase after 2 days of immersion in OBS (Fig. 2)the anodic and cathodic Tafel slopes show only a minor variationduring the 9 days of reaction and indicate that the mechanism ofFeS2 interaction with O2(aq) does not change. Also, if we take intoconsideration the jox values, our results confirm that the reactivityof FeS is higher than that of pyrite. It is interesting to stress that thebc values obtained for FeS2 electrode are close to the values

Fig. 3. Nyquist plots (10 mHz–3 MHz) for FeS electrode (a) immediately (�40 min) afterOBS. More details regarding the fitting procedure can be found in the Supplementary m

obtained for FeS electrode after 2 days of contact with OBS(Table 1).

3.2. EIS measurements

The Nyquist plots of the FeS electrode in aerated HCl solution atOCP and 25 �C are shown in Fig. 3. As we can easily observe theimpedance behavior of FeS electrode immediately after its immer-sion in OBS (Fig. 3a) is very different from the impedance behaviorregistered after 2, 4 and 6 days of contact between FeS and OBS(Fig. 3b–d). The observed differences show that the interfacial prop-erties of FeS electrode after 2, 4 and 6 days of contact with OBS arechanged relative to interfacial properties of the FeS electrodeimmediately (�40 min) after its immersion in OBS. These differ-ences can be an explanation for the observed change of the mecha-nism of electron transfer from cathodic sites on FeS surface to O2.

The electrical circuit used to fit the EIS data for the oxidationprocess of FeS electrode immediately (�40 min) after its immer-sion in OBS is shown in Fig. 4a. This circuit is a slight variation ofthe one used in a previous study by members of our group [8]and was chosen because it is in accordance with the general mech-anism of FeS oxidative dissolution (which involves a mix of surfacereaction and diffusion [8,17,23]) and give the best statistically fit ofthe experimental data. The equivalent circuit shown in Fig. 4b wasused to fit the impedance data for the oxidation process of FeS elec-trode after 2 days of immersion in OBS. This circuit is also a mod-ification of the one used by Badica and Chirita [8] to characterizeFeS/water interface. It gives the best statistically fits of the exper-imental data obtained after 2 days of FeS immersion in OBS. Thedifferences between equivalent electric circuit used by Badicaand Chirita [8] and the circuits of Fig. 4 can be explained by the

contact with OBS and, respectively, after (b) 2, (c) 4 and (d) 6 days in contact withaterial. h Measure samples and s fitting (simulated data).

Fig. 4. Equivalent electric circuits used to fit EIS data of FeS electrode (a)immediately (�40 min) after its immersion in OBS and (b) after at least 2 days ofimmersion in oxidizing solution.


low ionic strength of the initial solution with pH 5.1 or by the oxi-dation of the electrode surface after 2 days of contact with OBS.

Rs represents the solution resistance, Rsl is the resistive compo-nent of the mass transport through the surface layer, Rct is thecharge transfer resistance, Csl is the capacitance exerted by the sur-face layer and Cdl is the double layer capacitance. For better fittingresults, in the case of the experimental data for the oxidation pro-cess of FeS electrode after 2 days of immersion in OBS Csl and Cdlwere substituted with the constant phase elements CPEsl and CPEdl,respectively. Cdl and CPEdl reveal the existence of a capacitive loopat low frequencies while Csl and CPEsl can be associated to a capac-itive loop at high frequencies. Both equivalent circuits are com-pleted with an inductive component L. This inductance can beassigned either to the increase of mineral dissolution rate as resultof surface layer dissolution or to adsorption processes produced onthe electrode surface [8]. The equivalent circuit in Fig. 4b alsoincludes a Warburg element. The impedance parameters obtainedfrom the fitting of the experimental data using the equivalent elec-tric circuits of Fig. 4 are summarized in Table 2.

In Table 2 Qsl and Qdl are the parameters of CPEsl and CPEdl, andnsl and ndl are the corresponding constant phase element expo-nents [24]. nsl and ndl could be attributed to non-uniform currentdistribution and surface roughness [25]. The high value of Rsl indi-cates that a surface layer is initially present on FeS surface and isformed during the polishing of the electrode. Because the initialRsl is higher with approximately two orders of magnitude thanRct it results that immediately (�40 min) after immersion of FeSelectrode in OBS the overall oxidative dissolution of FeS is con-

Table 2Time dependencies of impedance parameters for FeS immersed in OBS at 25 �C and differe2 days of immersion in OBS and 33% for the parameters derived immediately (�40 min) a

Time(days)

pH Rct(kX cm2)

Cdl(mF cm�2)

Rsl(kX cm2)

Csl(pF cm�2)

CPE

Qdl(lF

0a 5.10b 0.07 22.4 4.0 367.8 –2 6.41c 0.66 – – – 3344 6.30c 0.49 – – – 4846 6.70c 0.44 – – – 428

a Immediately (�40 min) after immersion of FeS electrode in OBS.b Initial pH.c pH before the electrochemical measurements.

trolled by diffusion process across the surface layer [8]. After2 days of reaction of FeS with OBS the rate determining step ofthe mineral oxidation becomes the charge transfer process (Rct ishigher with approximately one order of magnitude than Rsl, anda Warburg diffusion element is associated to Rct (Fig. 4b)). TheWarburg element can be correlated with low frequencies diffusionprocess of electroactive species to the mineral interface from solu-tion [26,27]. In comparison with Cdl, Qdl decreases with approxi-mately two orders of magnitude (by between 46 and 67 times)after 2 days of FeS electrode immersion in OBS. The situation isreversed in the case of Qsl. It increases with approximately oneorder of magnitude (by between 4 and 12 times) in comparisonwith Csl. These findings indicate important modifications of thecharacteristics (area of the plates, distance between plates and/orthe composition of the dielectric medium) of the correspondingcapacitors.

3.3. Reaction products

3.3.1. SEM/EDX analysisAnother explanation for the observed change of the cathodic

slopes after 2 days of FeS electrode immersion in OBS may residein the formation of new solid phases on the mineral surface. Ana-lyzing the SEM/EDX data of initial FeS surface (before its immer-sion in OBS) (Figs. 5a and S2a of the Supplementary material)and FeS surface oxidized for 6 days in OBS (Figs. 5b, c and S2b ofthe Supplementary material) one can observe that the main com-ponents of initial surface are iron and sulfur, and the outermostlayer developed on the FeS surface mainly incorporates oxygenand iron. This layer formed on the electrode surface is heteroge-neous (Figs. 5b and c). The acicular morphology which can be seenin Figs. 5b and c is typical of goethite (a-FeOOH) [28].

3.3.2. Raman spectroscopyAdditional information concerning the layer developed on the

FeS surface are shown in Fig. 6. Raman spectrum of the FeS elec-trode shows: (i) in the low frequencies spectral range two linesof very low intensity situated at 212 and 276 cm�1, that areassigned to the asymmetric and symmetric stretching vibrationalmodes of FeS, respectively [29], and (ii) in the high frequenciesspectral range a Raman line of very high intensity peaked at3079 cm�1, which corresponds to the OH stretching vibrationalmode of water and hydroxyl groups [30]. A characteristic of theFeS electrode in the initial state is the value of the ratio betweenthe relative intensities of the Raman lines situated in the high fre-quencies spectral range and low frequencies spectral range, equalwith 12. According with Fig. 6, a new Raman line in the low fre-quencies spectral range is observed at 661 cm�1, which is not verylong situated of Raman line of Fe3O4 (665 cm�1) [31]. In this lastcase, the value of the ratio between the relative intensities of the

nt pH values. The error is lower than 8% for the impedance parameters obtained afterfter FeS electrode immersion in OBS.

dl DW(cm�2)

L(lH cm2)

Rsl(X cm2)

CPEsl

cm�2)ndl Qsl

(nF cm�2)nsl

– – 3400 – – –.8 0.719 568 32 56.5 4.52 0.864

0.728 414 14.9 54 1.68 0.992.8 0.726 514.8 10 41.5 1.91 0.997

Fig. 5. SEM images obtained (a) before and (b and c) after 6 days of immersion in OBS and their corresponding EDX analysis.


Raman lines situated in the high frequencies spectral range andlow frequencies spectral range is equal with 1.

3.3.3. FTIR spectroscopyThe chemical analysis of the solution resulted from aqueous

batch experiment indicates that iron (Fetotal) was preferentiallyreleased relative to sulfur (as SO42�) from the mineral surface

([Fetotal]:[SO42�] = 145:22 lM:lM). This finding is in line with theresults of previous studies [4,32–34] and indicates that under thenew formed Fe(III)/Fe(II) bearing phases (outermost layer) thereis a sulfur rich layer (SRL) which may incorporate species such asdisulfide, polysulfide and elemental sulfur. It should be noted thatdetails regarding the speciation of aqueous iron and sulfur can befound in Table S1 of the Supplementary material.

Fig. 6. Raman spectra of the FeS electrode (a) before and (b) after 6 days ofimmersion in OBS.

Scheme 1. Proposed electron transfer mechanism from the FeS to O2(aq) via thesurface layer (i.e., Fe(III)/Fe(II) and/or S–S bonds). Aqueous Fe(III) (i.e., Fe3+) does notintermediate the mineral oxidation. It is formed by Fe(II) oxidation and is in balancewith Fe(III) species present on FeS surface.


In order to obtain additional information regarding the natureof solid phases formed on FeS electrode, the solid residue collectedafter 6 days of contact between FeS powder (the same material likethat used for the electrode construction) and an aerated solutionwith initial pH 5.1 was analyzed by FTIR spectroscopy. The FTIRspectra of initial and reacted FeS powder are presented in Fig. 7.The spectrum of initial FeS sample shows peaks at 1021, 1117,1632 and 3444 cm�1. The peaks at 1021 and 1117 can be assignedto sulfate ions formed during the preparation of FeS powder [8,23].The peak at 1632 cm�1 can be assigned to H–O–H bending modes[23]. The broad band centered at about 3444 cm�1 results fromstretching modes of surface water or iron hydroxo groups[23,35]. It is plausible that similar chemical species appear onthe FeS electrode during polishing procedure. The FTIR spectrumof reacted FeS contains all the four signals present in the FTIR spec-trum of initial FeS (but they are more intense) and additional sig-nals at 473, 569, 801, 884, 2923 and 3223 cm�1. The peak at473 cm�1 indicates the presence of elemental sulfur, polysulfide

Fig. 7. FTIR spectra of the initial FeS powder and FeS powder reacted for 6 days in OB

and disulfide which are incorporated in the sulfur rich layerformed on FeS surface during its oxidation [23,35]. The peak at569 cm�1 can be ascribed to stretching vibrations of disulfidegroups formed on the oxidized FeS surface [36,37]. The other peaks(801, 884, 2923 and 3223 cm�1) can be associated to Fe(III) oxyhy-droxides. The first two peaks result from the Fe–O–H bendingmodes of Fe(OH)3 and goethite [35,38], while the last two are pro-duced by the stretching modes of Fe(OH)3 and goethite [23,35]. Thesmall peaks observed in the range of 1350–1500 cm�1 can be asso-ciated with carbonate species [23].

3.4. Mechanism of cathodic process

If we take into account the experimental data, it is reasonable toassume that the variation of the characteristics of surface layer andFeS/water interface is largely responsible for the change of themechanism of cathodic process. Since the rate of aqueous oxida-tion of FeS (which is directly proportional to jox [8]) does notdecrease, although the pH increases, it results that aqueous oxida-tion of FeS minerals at high pH values is not intermediated by thedissolved ferric iron (Fe3+(aq)) (whose concentration decreases, whenthe pH increases) [16]. Fe(III) oxyhydroxides and Fe3O4 (a mixedvalence Fe(III)/Fe(II) oxide), which are incorporated in the surfacelayer and separates the O2(aq) and cathodic sites, may enable thetransfer of electrons [39] between unreacted FeS and solution(Scheme 1). Also, the transfer of electrons between cathodic sitesand oxygen may be facilitated by the S–S bonds in polysulfide anddisulfide species [8]. For the latter mechanism of electron transfer(through S–S bonds) advocates the bc values obtained for pyrite elec-

S. The experimental conditions were initial pH 5.10 and a temperature of 25 �C.


trode. EIS results indicate that the initial surface layer formed duringthe preparation (polishing) of the electrode surface does not inter-mediate the transfer of the electrons and it acts as a physical barrierfor the diffusion of the soluble reactants/reaction products fromsolution/surface to surface/solution.

4. Conclusions

In this study, SEM/EDX analysis, Raman spectroscopy and FTIRspectroscopy were used to investigate the morphology and specia-tion of initial and oxidized surface of FeS at pHP 5.1 and 25 �C. Itwas found that during the aqueous oxidation of FeS new solid spe-cies (disulfide, polysulfide, elemental sulfur, ferric oxyhydroxidesand Fe3O4) develop on the mineral surface. The results of potentio-dynamic polarization experiments indicate that after 2 days ofimmersion of FeS electrode in OBS the mechanism of electrontransfer from cathodic sites to O2 (mechanism of cathodic process)changes. The new formed solid phases on FeS surface at pHP 5.1change the mechanism of cathodic process, but do not inhibit themineral oxidation. Impedance data demonstrate that the formationof solid oxidation products is accompanied by the change of FeS/water interface properties.

The mechanism of anodic process (oxidation of FeS) remainspractically unchanged over the 6 days of FeS immersion in OBS.

Our findings have important implications for understanding FeSbehavior at pHP 5.1. The results of potentiodynamic polarizationexperiments show that at pHP 5.1 the FeS oxidation is not inter-mediated by Fe3+(aq). It is likely that after 2 days of FeS immersion inOBS the new solid phases developed on the mineral surface interme-diate the electron transfer from cathodic sites to O2. A practicalimplication of the finding that Fe3+ does not intermediate FeS oxida-tion is that the mineral oxidative dissolution cannot be inhibited byFe3+ sequestration with inorganic and/or organic ligands.

Future studies should be conducted to evaluate the aqueousoxidation of pyrite at high pH and over a large time scale. Takinginto account the differences between FeS2 and FeS reactivity, a sub-stantial surface layer on pyrite should develop slower than in thecase of FeS. Knowledge of the variation of the FeS2 reactivity withtime will certainly help to have a better image on the mechanismof aqueous oxidation of pyrite.

Acknowledgements

This work was supported by a grant of the Romanian NationalAuthority for Scientific Research, CNDI-UEFISCDI, project number51/2012.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcis.2016.01.010.

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Mechanism of the cathodic process coupled to the oxidation of iron monosulfide by dissolved oxygen1 Introduction2 Experimental2.1 Materials2.2 Electrochemical experiments2.3 Aqueous batch experiment2.4 SEM/EDX analysis2.5 Raman spectroscopy2.6 FTIR measurements

3 Results and discussion3.1 Potentiodynamic polarization measurements3.2 EIS measurements3.3 Reaction products3.3.1 SEM/EDX analysis3.3.2 Raman spectroscopy3.3.3 FTIR spectroscopy

3.4 Mechanism of cathodic process

4 ConclusionsAcknowledgementsAppendix A Supplementary materialReferences

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