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

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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 )
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