Hydrogen Peroxide Decomposition by Pyrite in the Presence of Fe ...

P. CHIRIȚÃ, Hydrogen Peroxide Decomposition by Pyrite in the Presence of …, Chem. Biochem. Eng. Q. 23 (3) 259–265 (2009) 259Hydrogen Peroxide Decomposition by Pyrite in the Presence of Fe(III)-ligandsP. ChirițãDepartment of Inorganic, Analytical and Technological Chemistry,University of Craiova, Calea Bucureºti 107I, Craiova 200478, RomaniaOriginal scientific paperReceived: August 23, 2008Accepted: February 24, 2009The decomposition of hydrogen peroxide (H 2 O 2 ) by pyrite in the presence ofFe(III)-ligands (sulfosalicylate (SSAL), ethylenediaminetetraacetate (EDTA), and phosphate)has been investigated in aqueous acidic media (pH 1) at 25 °C. It was found thatH 2 O 2 decomposition by pyrite was inhibited by the presence of EDTA, SSAL and phosphate.On the other hand, pyrite oxidation by H 2 O 2 does not seem to be affected by thepresence of Fe(III)-ligands. The experimental results demonstrate that H 2 O 2 decompositionin the presence of Fe(III)-ligands is catalyzed by pyrite surface (a heterogeneousprocess). This process is first order with respect to [H 2 O 2 ]. It is expected that the rate-determiningstep of the reaction mechanism of H 2 O 2 decomposition in the presence ofFe(III)-ligands is one of the following two reactions:FeHO 2 2 Fe 2 HO 2•Fe(OH)(HO 2 ) Fe 2 HO 2 • HO –where denotes pyrite surface.Key words:Hydrogen peroxide, pyrite, Fe(III)-ligands, reaction mechanismIntroductionThe extraction of gold from pyrite matrix anddesulphurization of coal are based on pyrite oxidationby an efficient reagent. The use of hydrogenperoxide (H 2 O 2 ) as reagent for pyrite (FeS 2 ) oxidationis of particular interest. 1–4 H 2 O 2 is a powerfuloxidizing agent as depicted by the redox potentialof 1.77 V. 5,6 The oxidation of FeS 2 by H 2 O 2 inacidic media is characterized by the following overallreaction 1–4,7,8FeS 2 7.5 H 2 O 2 Fe 3 (aq) 2SO 2 4(aq) H 7H 2 O(1)The pyrite surface and the released ferric ironact as catalysts for H 2 O 2 decomposition into H 2 Oand O 2 (stable products of H 2 O 2 decomposition)causing a considerable loss of reagent 1–4,9H 2 O 2 H 2 O 0.5 O 2 (2)The H 2 O 2 decomposition by pyrite surface (aheterogeneous process) is first order with respect to[H 2 O 2 ], and H 2 O 2 decomposition by aqueous ferriciron (a homogeneous process) is first order with respectto both [H 2 O 2 ] and [Fe 3+ (aq)]. 4 Taking intoconsideration the catalytic effect of aqueous ferriciron on H 2 O 2 decomposition, it can be hypothesizedthat the process should be influenced by anycomplexing agent (Fe(III)-ligand) which is able toinfluence the catalytic activity of Fe 3+ (aq). Hence,the study of the H 2 O 2 decomposition by pyrite inpresence of Fe(III)-ligands may lead to the optimizationof the hydrometallurgical process of pyriteoxidation by H 2 O 2 .The objective of this study was to examineH 2 O 2 decomposition by pyrite in acidic media inpresence of Fe(III)-ligands, such as sulfosalicylate(SSAL), ethylenediaminetetraacetate (EDTA), andphosphate.Experimental methodsMaterialsAll chemicals were of analytical grade andwere used without further purification. Hydrochloricacid and hydrogen peroxide were used to obtainthe desired pH and, respectively, oxidant concentrationin each experiment. Doubly distilled water wasused to prepare all solutions. Pyrite from Baia Mareregion (Romania) was used for this study. Pyritewas characterized by wet chemical analysis andX-ray diffraction (XRD). The wet chemical analysiswas performed using typical method. 10 Five elements(Co, Ni, Cu, Zn and As) were measured. XRDanalysis was carried out on a CGR 60 diffractometerusing CuK monochromatic radiation(0.154 nm). The pyrite crystals were groundand classified into three size fractions by dryscreening: 71–80 m, 80–90 m and 90–125 m.The resulted fractions were treated for 1 min withnitric acid (c 1 mol L –1 ), washed repeatedly withdoubly distilled water by rapid suspension, and de-

260 P. CHIRIȚÃ, Hydrogen Peroxide Decomposition by Pyrite in the Presence of …, Chem. Biochem. Eng. Q. 23 (3) 259–265 (2009)canted in order to remove fine particles. Then, eachfraction was rinsed with fresh acetone several timesand stored under oxygen-free desiccator. Only the90125 m fraction was used for experiments. Thespecific surface area for this fraction, calculated for anassumed spherical geometry, is s 0.0112m 2 g –1 . 4Decomposition experiments andanalytical determinationsH 2 O 2 decomposition by pyrite was tested in thepresence of three Fe(III)-ligands at initial pH 1, 25 °Ctemperature, [H 2 O 2 ] o 0.35 mol L –1 , s 0.0112m 2 g –1 pyrite specific surface area, 250 mL of solutions,and m 0.3 g pyrite. EDTA (ethylenediaminetetraaceticacid disodium salt:dihydrate, Mr 372.2),SSA (sulfosalycilic acid, Mr 254.2), and phosphate(sodium dihydrogen phosphate:monohydrate,Mr 138) were used as Fe(III)-ligands. Their concentration(c 0.01 mol L –1 ) in the experimentswas always higher than the concentration of totaliron released during pyrite oxidation.All decomposition experiments were carriedout in a conventional gas meter placed in waterbatch controlled by a thermostat. In order to verifythe tightness of gas meter it was necessary to runcontrol experiments with pyrite and acidic solutionsin the absence of H 2 O 2 . The volumes of releasedoxygen were corrected under laboratory conditionsto find the corresponding decrease in hydrogen peroxideconcentration (c x /mol L –1 ) as results of its decompositioninto O 2 and H 2 O (eq. 2). 4 To avoid anincrease of H 2 O 2 decomposition, no stirring systemwas used.pH was monitored with an Ingold InLab 400combined pH glass electrode incorporated in thegas meter. Before use, the electrode was calibratedin Mettler pH 4.01 and 7.00 buffers.Duplicate experiments were carried out in orderto study the pyrite oxidation by H 2 O 2 . Periodically,liquid samples were extracted from the gasmeter with a syringe connected to a d 0.22 mfilter. Liquid samples were analyzed for dissolvedsulfate. The dissolved sulfate amounts were determinedcolourimetrically using the methylthymolblue method. 11 The oxidation degree of pyrite ()was calculated by dividing the amount (mol) of releasedsulfate in the moment of sampling by theamount (mol) of sulfur contained in the pyrite at thebeginning of experiments (i.e., 5 mmol S).At the end of the experiments, the resultingsolid residues were quickly rinsed with doubly distilledwater, filtered throughout d 0.22 m membranefilter, dried and stored in an evacuated desiccatorfree of oxygen until analyzed by Fouriertransform infrared spectroscopy (FTIR).FTIR spectroscopyFTIR spectroscopy was carried out using aBrucker ALPHA Fourier Transform InfraredSpectrometer, with a range of 5004000 cm –1and a resolution of 4 cm –1 ; 28 scans were performed.ResultsCharacterization of pyriteX-ray diffraction analysis of the pyrite sampleused in this study showed that it was cubic FeS 2 .No other phases could be detected. The wet chemicalanalysis showed that pyrite sample had acomposition close to the stoichiometric ratio expectedfor this mineral (v S/Fe 1.98 0.03). Themass fractions of the minor elements analyzedwere: w Co 0.02%; w Ni 0.02%; w Cu 0.01 %;w Zn 0.01 % and w As 0.13 %. Fig. 1 shows FTIRspectrum of pyrite sample after grinding (uncleansample), indicating presence of several oxidationproducts on mineral surface. There are various patternsin the 5001626 cm –1 region. The broadband around 3233 cm –1 includes peaks fromwater, ferric hydroxide and/or ferric oxy-hydroxide.12,13 The 5001200 cm –1 region shows that theunclean pyrite sample consists of products of sulfuroxidation (the signals at 1109, 1069, 1011, 665 and602cm –1 that may be assigned to sulfate, sulfite andthiosulfate 12,13 ) and, respectively, iron oxidation(the peaks at 811 and 523 cm –1 that may be attributedto amorphous or poorly crystalline ferric hydroxidesand oxides 12,13 ). The peak at 1454 cm –1can be assigned to carbonate ions. 14 FTIR spectrumof pyrite sample prepared by the previously describedcleaning procedure (clean sample) is alsoshown in Fig. 1. No oxidation compounds were observedin this pyrite sample.Fig. 1 – FTIR spectra for unclean and clean pyrite sample

P. CHIRIȚÃ, Hydrogen Peroxide Decomposition by Pyrite in the Presence of …, Chem. Biochem. Eng. Q. 23 (3) 259–265 (2009) 261H 2 O 2 decomposition by pyrite in the presenceof Fe(III)-ligandsThe results (c x /mol L –1 ) of the experiments performedin the presence of SSAL, EDTA and phosphate(c = 0.01 mol L –1 ) and in the absence ofFe(III)-ligands are shown in Fig. 2. At 25 °C, pH 1,and in the presence of Fe(III)-ligands, the c x vs. tcurves indicate a parabolic trend. In the absence ofFe(III)-ligands the c x vs. t curve indicates an exponentialtrend. Therefore, after a decomposition periodof 180 min, in the absence of Fe(III)-ligands, c xwas higher than that measured in the presence ofSSAL, EDTA or phosphate. These results indicatethat H 2 O 2 decomposition by pyrite is inhibitedby the presence of EDTA, SSAL, and phosphate(Fe(III)-ligands). The c x values registered in thecourse of experiments conducted in the presence ofEDTA and SSAL were higher than those registeredin the course of experiments carried out in the presenceof phosphate. This could be the result of a possibleligand concentration decrease due to theEDTA and SSAL (organic compounds) oxidationby H 2 O 2 /Fe system (Fe from pyrite). 15 However, noattempts were made in order to quantify the concentrationof EDTA and SSAL in the course of the experiments.We may note that during the H 2 O 2 decompositionexperiment conducted in the presenceof SSAL the intensity of solution colour (red, dueto Fe(SSAL) complex 16 ) gradually increased, suggestingthat there was sufficient SSAL in the solutionto react with the released ferric iron.Fig. 2 – Results of H 2 O 2 decomposition by pyrite in acidic media(in the presence and absence of Fe(III)-ligands)Pyrite oxidation by H 2 O 2 in the presenceof Fe(III)-ligandsThe oxidation degree of pyrite () in the presenceof Fe(III)-ligands is shown in Fig. 3. Fig. 3also shows the oxidation degree of pyrite registeredduring experiment without Fe(III)-ligands. As mayFig. 3 – Results of pyrite oxidation by H 2 O 2 in acidic media(in the presence and absence of Fe(III)-ligands)be seen from this figure, the values of , in thepresence and absence of Fe(III)-ligands, are practicallyidentical. The values registered after 180min of pyrite oxidation are: 0.077 (in absence ofligands), 0.076 (in presence of EDTA), 0.082(inpresence of SSAL) and 0.081 (in presence of phosphate).These observations suggest that the pyriteoxidation by H 2 O 2 is not significantly influenced bythe presence of Fe(III)-ligands.The pH measurements carried out during theH 2 O 2 decomposition experiments, in the presenceand absence of Fe(III)-ligands, showed that the protonconcentration (10 –pH ) remained constant aftert 180 min of reaction. These results indicate thatthe protons production or consumption, during theexperimental runs, is insignificant with respect totheir initial concentration (c 0 0.1 mol L –1 ).FTIR analysis of reacted pyrite samplesIn order to study the interaction between pyritesurface and Fe(III)-ligands, FTIR spectroscopy wasused. The FTIR spectra of reacted samples arepresented in Figs. 4a, 4b and 4c. FTIR spectra ofEDTA, phosphate and clean pyrite sample are alsogiven for comparison. No “testifier” spectrum wasrecorded for SSAL (an aqueous solution of w 30 %SSAL was used for the preparation of experimentalsolutions). The main feature noted from the analysisof collected FTIR spectra is that the spectrumof clean pyrite and that of pyrite reacted in thepresence of all Fe(III)-ligands were similar in the500-4000 cm –1 region. This observation indicatesthat there are no notable interactions betweenFe(III)-ligands and pyrite surface. This could be dueeither to the little number of adsorption centers forSSAL, EDTA and phosphate on pyrite surface or tothe low affinity of Fe(III)-ligands for pyrite surface.

262 P. CHIRIȚÃ, Hydrogen Peroxide Decomposition by Pyrite in the Presence of …, Chem. Biochem. Eng. Q. 23 (3) 259–265 (2009)Fe(III)-ligands do not influence pyrite oxidationby H 2 O 2 . Also, it is important to note that FTIRanalysis of reacted pyrite samples indicates that theadsorption degree of SSAL, EDTA and phosphateto pyrite surface is insignificant. All these resultssuggest an intricate interaction between pyrite andH 2 O 2 in the presence of Fe(III)-ligands. In the nextsections we will discuss the details of this interactionstarting from the above-mentioned experimentaldata.Rate-determining step of pyrite oxidationby H 2 O 2The data given in Fig. 3 were analyzed with theaid of the Shrinking core model controlled bychemical reaction at the unreacted particle surface5,17 131( 1 ) / k t r (3)where: is the fraction of oxidized pyrite, k r is ratecoefficient and t is time. Fig. 5 shows that theShrinking core model controlled by chemical reactionat the unreacted particle surface represented byeq. (3) describes very well the experimental dataobtained in this study. Correlation coefficients ofabove 0.99 were obtained in all of the cases. Thisfinding indicates that a surface chemical reaction isthe rate controlling step of pyrite oxidation by H 2 O 2both in the absence and presence of Fe(III)-ligands.Moreover, the obtained values of k r (Table 1)are practically equal indicating that the pyrite oxidationby H 2 O 2 is not influenced by the presence ofFe(III)-ligands after 180 min of reaction at 25 °Cand pH 1.Fig. 4– FTIR spectra for clean pyrite sample and reactedpyrite samples in the presence of (a) EDTA, (b) SSAL and (c)phosphate. FTIR spectra of (a) EDTA and (c) phosphate areshown for comparison.DiscussionThe experimental data presented above indicatethat, on one hand, the H 2 O 2 decomposition by pyriteis inhibited by the presence of SSAL, EDTA, andphosphate and, on the other hand, the presence ofFig. 5– Experimental data of pyrite oxidation by H 2 O 2in acidic media (in the presence and absence of Fe(III)-ligands)plotted according to eq. (5) (Shrinking core modelcontrolled by chemical reaction at the unreacted particle surface).

P. CHIRIȚÃ, Hydrogen Peroxide Decomposition by Pyrite in the Presence of …, Chem. Biochem. Eng. Q. 23 (3) 259–265 (2009) 263Kinetics of H 2 O 2 decomposition by pyritein the presence of Fe(III)-ligandsChirita 4 found that the data obtained in the experimentsof H 2 O 2 decomposition by pyrite inacidic media (in the absence of Fe(III)-ligands) canbe fitted by following equationwhere k is the rate coefficient, fits well with the experimentaldata obtained in experiments withFe(III)-ligands (Fig. 6b). The coefficients of correlation(r 2 ) were higher than 0.98. The rate coefficientsare listed in Table 1. These findings could beexplained starting from the proposed model forH 2 O 2 decomposition by pyrite in acidic media. 4 Accordingto this model the H 2 O 2 decomposition bypyrite (pH from 1 to 2and temperatures from 25to 45 °C) is double catalyzed by pyrite surface (aheterogeneous process) and Fe 3+ (aq) (a homogeneousprocess). Assuming that the homogeneous reactionis suppressed by the complexation of Fe 3+ (aq)with SSAL, EDTA and phosphate, it results that, inthe presence of Fe(III)-ligands, H 2 O 2 decompositionby pyrite is catalyzed only by pyrite surface(heterogeneous reaction). There are three main experimentalfindings that suggest this assumption.First, the parabolic trends of H 2 O 2 decompositioncurves registered in presence of SSAL, EDTA andphosphate vs. the exponential trend registered in theln R ka( 1 N[ HO 2 2] 0 ) t (4)( 1 N cxwhere R ) [ HO 2 2]0, N k b , k[ HO 2 2]0 cxk aa(min –1 ) and k b (L mol –1 min –1 ) are the rate coefficientsof heterogeneous and, respectively, homogeneousH 2 O 2 decomposition by pyrite in acidic media,[H 2 O 2 ] 0 is the initial concentration of H 2 O 2 , andc x (mol L –1 ) is the decrease in H 2 O 2 concentration asresults of its decomposition into O 2 and H 2 O. Indeed,eq. (4) fits very well the experimental dataobtained in the absence of Fe(III)-ligands (Fig. 6a).The coefficient of correlation was r 2 0.997 forN 45. The rate coefficients are shown in Table 1.Unfortunately, eq. (4) gave a poor fit to experimentaldata obtained in experiments conducted in thepresence of Fe(III)-ligands (for different N withinrange 10 –5 10 5 ). Instead, the following equation(first order kinetics equation) 4[ HO 2 2]0ln[ HO 2 2]0 c ktx(5)Fig. 6– (a) Experimental data of H 2 O 2 decomposition bypyrite in the absence of Fe(III)-ligands plotted according to thekinetic model proposed by Chirita. 4 (b) Experimental data ofH 2 O 2 decomposition by pyrite in the presence of Fe(III)-ligandsplotted according to eq. (5).Table 1 – Summary of experimental resultsSSAL EDTA Phosphate Without ligandsk r /min –1 1.2· 10 –4 (± 5.7 · 10 –6 ) 1.1·10 –4 (± 5.6 · 10 –6 ) 1.1·10 –4 (± 5.1 · 10 –6 ) 1.3·10 –4 (± 3.9 · 10 –6 )k a /min –1 1.3·10 –4 (± 4.5 · 10 –6 )k b /L –1 mol –1 min –1 5.9·10 –3 (± 1.78 · 10 –4 )k(k A )/min –1 1.4·10 –4 (± 3.8 · 10 –6 ) 1.9·10 –4 (± 4.5 · 10 –6 ) 0.9·10 –4 (± 1.7 · 10 –6 )

264 P. CHIRIȚÃ, Hydrogen Peroxide Decomposition by Pyrite in the Presence of …, Chem. Biochem. Eng. Q. 23 (3) 259–265 (2009)absence of Fe(III)-ligands (Fig. 2). The differenttrends suggest different reaction pathways. Second,the FTIR spectra of residual pyrite samples (Fig. 4)suggesting an unchanged pyrite surface in the presenceof Fe(III)-lignads. Third, the results obtainedfrom the experiment designated to study the pyriteoxidation by H 2 O 2 indicating a similar reactivity ofpyrite surface both in the absence and presence ofFe(III)-ligands.The heterogeneous H 2 O 2 decomposition is afirst order reaction with respect to [H 2 O 2 ]: 4r k A [ HO 2 2 ](6)where r is the rate of H 2 O 2 decomposition in presenceof Fe(III)-ligands, k A (min –1 ) is the rate coefficient,and [H 2 O 2 ] is the hydrogen peroxide concentration.Integrating eq. (7).r dH [ 2O2]dt k A [ HO 2 2 ] (7)and rearranging the terms of obtained relation we get:ln [ HO 2 2]C kAt (8)where C is a constant. From initial conditions t 0 0and [H 2 O 2 ] [H 2 O 2 ] 0 (for t 0 0) one obtains:C ln [ HO 2 2 ] 0(9)Combining eq. (8) and eq. (9), taking into accountthat [H 2 O 2 ] [H 2 O 2 ] 0 c x and rearrangingthe terms one obtains: [ ]lnHO[ HO ]2 2 02 2 0 kAtc x(10)The similarity between the theoretical equation(eq. (10)) and eq. (5) (that fits the experimentaldata) demonstrates the consistency of measured ratecoefficients with eq. (5) and supports the hypothesisthat H 2 O 2 decomposition by pyrite in thepresence of Fe(III)-ligands is catalyzed only bypyrite surface. Note that the quasi-equality betweenthe rate coefficients (average k A 1.4 · 10 –4 min –1 )of H 2 O 2 decomposition by pyrite in the presenceof Fe(III)-ligands and the rate coefficient (k a 1.3 · 10 –4 min –1 ) corresponding to heterogeneousdecomposition of H 2 O 2 by pyrite in the absenceof Fe(III)-ligands (Table 1) is also consistent withthe basic assumption of the proposed model ofH 2 O 2 decomposition by pyrite in the presence ofFe(III)-ligands.It is expected that the rate-determining step ofthe reaction mechanism of H 2 O 2 decomposition inthe presence of Fe(III)-ligands is one of the followingtwo reactionsFeHO 22Fe 2 HO 2· (11)Fe(OH)(HO 2 ) Fe 2 HO 2· HO – (12)where denotes pyrite surface. The rate-determiningstep, which supposes the decompositionof the surface complexes (FeHO 22orFe(OH)(HO 2 ) ) and regeneration of surface ferrousiron (Fe 2 ), was established taking into considerationthe results of previous studies concerningH 2 O 2 decomposition by ferric iron 18–20 and by pyrite.4ConclusionsThe H 2 O 2 decomposition by pyrite in acidicmedia (pH 1 at 25 °C) in the presence of Fe(III)-ligands(c 0.01 mol L –1 ) was studied. It was foundthat the reaction pathway of H 2 O 2 decompositionby pyrite is affected by the addition of SSAL,EDTA and phosphate; H 2 O 2 decomposition is inhibitedby the presence of Fe(III)-ligands. However,pyrite oxidation by H 2 O 2 does not seem to be influencedby the presence of Fe(III)-ligands, aftert 180 min of reaction.Because the ligands are able to sequester theaqueous ferric iron, the H 2 O 2 decomposition by pyritein the presence of SSAL, EDTA and phosphateis catalyzed only by the pyrite surface (a first orderreaction with respect to [H 2 O 2 ]).ACKNOWLEDGEMENTSI thank three anonymous reviewers for theirrigorous and valuable critique of the manuscript.List of symbolsc concentration, mol L –1c x extensive quantity for all concentration, mol L –1k a rate coefficient for a first order reaction, min –1k r rate coefficient for a first order reaction, min –1k(k A ) rate coefficient for a first order reaction, min –1k b rate coefficient for a second order reaction,L mol –1 min –1Mr molecular massm mass, gr 2 correlation coefficients specific surface area, m 2 g –1t time, minw mass percentage, % degree of dissolutionv stoichiometric ratio wavenumber, cm –1

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