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

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