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Research ArticleReceived: 03 September 2010 Revised: 19 October 2010 Accepted: 12 November 2010 Published online in Wiley Online Library: 24 December 2010

(wileyonlinelibrary.com) DOI 10.1002/jctb.2560

Decomposition of hydrogen peroxidein the presence of activated carbonswith different characteristicsAlmudena Aguinaco, Juan Pablo Pocostales, Juan F. Garcıa-Arayaand Fernando J. Beltran∗

Abstract

BACKGROUND: Catalytic ozonation promoted by activated carbon is a promising advanced oxidation process used in watertreatment. Hydrogen peroxide generated as a by-product from the reaction of ozone with some surface groups on the activatedcarbon or from the oxidation of some organic compounds present in the water being treated seems to play a key role in thecatalytic ozonation process. Hydrogen peroxide decomposition promoted by two granular activated carbons (GAC) of differentcharacteristics (Hydraffin P110 and Chemviron SSP-4) has been studied in a batch reactor. The operating variables investigatedwere the stirring speed, temperature, pH and particle size. Also, the influence of metals on the GAC surface, that can catalyzehydrogen peroxide decomposition, was observed.

RESULTS: Chemviron SSP-4 showed a higher activity to decompose hydrogen peroxide than HydraffinP110 (70 and 50% ofhydrogen peroxide removed after 2 h process, respectively). Regardless of the activated carbon used, hydrogen peroxidedecomposition was clearly controlled by the mass transfer, although temperature and pH conditions exerted a remarkableinfluence on the process. Catalytic ozonation in the presence of activated carbon and hydrogen peroxide greatly improvedthe mineralization of oxalic acid (a very recalcitrant target compound). About 70% TOC (total organic carbon) depletion wasobserved after 1 h reaction in this combined system, much higher than the mineralization achieved by the single processesused.

CONCLUSIONS: Of the two activated carbons studied, Chemviron SSP-4 with an acidic nature presented a higher activity todecompose hydrogen peroxide. However the influence of the operating variables was quite similar in both cases. Experimentscarried out in the presence of tert-butanol confirmed the appearance of radical species. A kinetic study indicated that theprocess was controlled by the internal mass transfer and the chemical reaction on the surface of the activated carbon. Thecatalytic activity of hydrogen peroxide in oxalic acid ozonation promoted by activated carbon (O3/AC/H2O2) was also studied.The results revealed the synergetic activity of the system O3/AC/H2O2 to remove oxalic acid.c© 2010 Society of Chemical Industry

Keywords: advanced oxidation; catalysis; mass transfer; ozone; reaction; environmental chemistry

INTRODUCTIONIn recent years, a number of ozone advanced oxidation technolo-gies have been investigated in order to improve the oxidizingcapacity and economic feasibility of ozonation processes for watertreatment.1 These advanced oxidation processes (AOPs) are basedon the formation of hydroxyl radicals as well as other oxidizingspecies which are powerful oxidizing agents capable of removinga large range of organic and inorganic water pollutants.2

Among the different AOPs used in water treatment the systemozone and activated carbon (O3/AC) has proved efficient forthe removal of water contaminants, especially in terms ofmineralization.3 – 6 The O3/AC process includes the importantadsorption capacity of the activated carbon, the high oxidizingcapacity of ozone and the synergetic capacity of the combinedsystem. According to the literature, activated carbon acts as apromoter of ozone decomposition rather than a true catalystsince it is modified during the ozonation process.7 – 9 As reported

in the literature, ozone reacts with some oxygen groups withbasic characteristics on the activated carbon surface which canyield hydrogen peroxide as well as other oxidizing species.9

Hydrogen peroxide is a well-known homogenous catalyst forthe decomposition of ozone into hydroxyl radicals so the role ofthis species in the catalytic process O3/AC is crucial. In addition,hydrogen peroxide may arise as a by-product of the reactionof ozone with some aromatic water pollutants, adding anothersource of this compound to the O3/AC system.10

∗ Correspondence to: Fernando J. Beltran, Departamento de Ingenier ıa Quımicay Quımica Fısica, Universidad de Extremadura, 06006 Badajoz, Spain.E-mail: fbeltran@unex.es

Departamento de Ingenier ıa Quımica y Quımica Fısica, Universidad deExtremadura, 06006 Badajoz, Spain

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Table 1. Some textural properties and chemical surface characterization of the activated carbons

Activated carbonVmi

cm3 g−1VT

cm3 g−1SBET

m2 g−1 pHPZC

Carboxylic groups(µeq g−1)

Lactones(µeq g−1)

Hydroxyls(µeq g−1)

Carbonyls(µeq g−1)

Acid groups(µeq g−1)

Basic groups(µeq g−1)

HydraffinP110 0.455 0.742 972 9.7 0 35 17 147 199 671

Chemviron SS4-P 0.500 0.760 1193 1.8 828 93 262 388 1571 0

Activated carbon can decompose hydrogen peroxide byinvolving the exchange of a hydroxyl oxygen group and ahydrogen peroxide anion. As reported in the literature, theperoxide formed at the surface of the activated carbon can reactwith other hydrogen peroxide molecules leading to regenerationof the activated carbon with release of O2 and H2O2.11,12 On theother hand, hydrogen peroxide can activate the carbon surfaceinvolving the generation of free radicals.13 This process is especiallyinteresting in the system O3/AC since it would favour ozonedecomposition, the free radical mechanism, and the oxidizingcapacity of this AOP.

With these premises the present paper studies the capacity oftwo activated carbons with different characteristics to adsorband decompose hydrogen peroxide. The influence of someoperating variables has been investigated (i.e. pH, presence ofhydroxyl radical scavenger, particle size of the activated carbon,temperature, agitation). In addition, this activity and its role inthe catalytic ozonation process promoted by activated carbon hasbeen studied using oxalic acid as a target compound.

EXPERIMENTALHydrogen peroxide solution (30%) was obtained from Aldrich(Spain) and its concentration was determined by a colorimetricmethod using a titanium salt.14 Two commercial activated carbonswere selected and used in the current work; HydraffinP110 (Lurgi,Germany) and Chemviron SS4-P (Chemviron Carbon, Belgium)with particle sizes in the range 0.8 to 1.6 mm. Before use and toremove some impurities the activated carbons were first washedin boiling ultrapure water for 2 h and then dried at 110 ◦C for 48 hin an oven.

Textural and chemical surface characterization of the activatedcarbons were carried out using the following techniques: adsorp-tion of nitrogen at 77 K (Quantachrome Autosorb-1 automated gasadsorption system, USA), mercury porosimetry (Thermo ScientificPascal 240 porosimeter, Italy), analysis of the concentration ofsurface oxygen groups15 and determination of the point of zerocharge (pHpzc).16 As seen in Table 1, both activated carbonspresent similar textural properties but very different chemicalsurfaces. The activated carbons show high surface areas and animportant contribution of microporosity to the total pore volume.According to the chemical surface characterization HydraffinP110is an activated carbon with a basic nature with high concentrationof basic groups and a pHpzc above 7. In contrast, Chemviron SS4-Ppresents a greater concentration of acidic groups, with practicallycomplete absence of basic groups and a very low pHpzc.

The reactor employed was a 500 cm3 cylindrical glass vesselsupplied with magnetic agitation and two sampling ports.Temperature was kept controlled by immersion of the reactor ina thermostatic bath. In some cases the solutions were buffered atthe desired pH in ultrapure water with NaH2PO4 and Na2HPO4 withan ionic strength of 0.05 mol L−1. Once the operating conditionswere established (temperature, agitation, activated carbon particle

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0Figure 1. Evolution of dimensionless hydrogen peroxide concentrationwith time during the course of decomposition experiments in the presenceof Chemviron SSP-4 (solid symbols) and HydraffinP110 (open symbols).Effect of stirring speed. Conditions; initial conc. H2O2 = 7.7 mg L−1;T = 25 ◦C; pH0 = 4.8; AC wt = 1 g; particle size = 1–1.25 mm; � 100 rpm;� 210 rpm; � 330 rpm.

size, pH, initial hydrogen peroxide concentration and, in somecases, added tert-butanol) the reactor was charged with 0.5 L of ahydrogen peroxide solution and samples were withdrawn steadilywith time. Before analysis samples were filtered using 0.45 µmfilters. Ozone was produced and measured in the gas phase ina Sander Laboratory Ozoniser (Germany) and an Ozomat GM-6000-PRO analyzer (Germany), respectively. An oxygen–ozonegas mixture at a rate of 20 L h−1 (Q) was fed through a porousplate placed at the bottom of the reactor with approximately20 mg L−1 ozone concentration (CO3g). TOC was measured with aShimadzu (Japan) TOC-VCSH analyzer.

RESULTS AND DISCUSSIONEffect of stirring speedThe effect of stirring speed on hydrogen peroxide decompositionin the presence of both activated carbons is shown in Fig. 1.As can be seen, increasing stirring speed leads to a slightimprovement in hydrogen peroxide decomposition after 2 h inthe presence of Chemviron SSP-4. An increasing agitation ratediminishes the external mass transfer resistance of the liquid–solidsystem thus favouring the decomposition of hydrogen peroxideon the activated carbon surface.17 Nevertheless, while this effectis clear in the case of Chemviron SSP-4, for HydraffinP110 noinfluence of stirring speed was observed under the experimentalconditions. This suggests that in this case hydrogen peroxideexternal diffusion is fast enough and does not exert any controlover the decomposition rate of hydrogen peroxide.

Effect of particle size of activated carbonsParticle size of the activated carbon plays an important role in thekinetics of hydrogen peroxide decomposition since it affects both

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Figure 2. Evolution of dimensionless hydrogen peroxide concentrationwith time during the course of decomposition experiments in thepresence of Chemviron SSP-4 (solid symbols) and HydraffinP110 (opensymbols). Effect of particle size. Conditions: initial conc. H2O2 = 7.7 mg L−1;T = 25 ◦C; AC wt=1 g; pH0 = 4.8; stirrer speed=330 rpm;�1.25–1.6 mm;� 1–1.25 mm; � 0.8–1 mm.

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Figure 3. Evolution of dimensionless hydrogen peroxide concentrationwith time during the course of decomposition experiments in the presenceof Chemviron SSP-4 (solid symbols) and HydraffinP110 (open symbols).Effect of temperature. Conditions: initial conc. H2O2 = 7.7 mg L−1; AC wt= 1 g; pH0 = 4.8; stirrer speed = 330 rpm; particle size = 1–1.25 mm; �T = 7 ◦C; � T = 25 ◦C; � T = 40 ◦C.

external and internal mass transfer resistances. As can be seenin Fig. 2, decreasing the particle size of the solid leads to higherhydrogen peroxide removal rates, regardless of the activatedcarbon, which indicates that the process is noticeably controlledby mass transfer. This is in agreement with other studies whichstate that lower particle sizes significantly decrease the externalliquid–solid and internal diffusion mass transfer resistances.18

Effect of temperatureThe influence of temperature was studied in the range 280–313 K.As Fig. 3 shows, regardless of the activated carbon, hydrogenperoxide decomposition rate is highly favoured by increasing thetemperature. These reactions were also carried out in the absenceof activated carbon and any effect of the temperature on hydrogenperoxide concentration under the conditions used was observedover 2 h of reaction. A higher temperature favours the chemicalstep of the hydrogen peroxide decomposition at the surface ofthe activated carbon, which would increase the control of masstransfer over the process.

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Figure 4. Evolution of dimensionless hydrogen peroxide concentrationwith time during the course of decomposition experiments in the presenceof Chemviron SSP-4 (solid symbols) and HydraffinP110 (open symbols).Effect of pH. Conditions: initial conc. H2O2 = 7.7 mg L−1; T = 25 ◦C; AC wt= 1 g; stirrer speed = 330 rpm; particle size = 1–1.25 mm; � pH 5; � pH 7;� pH 9; • pH 4.8, non-buffered.

Effect of solution pHAnother important variable is the pH of water since it stronglyaffects the stability of hydrogen peroxide and the nature of thechemical surface of the activated carbon. As Fig. 4 shows increasingpH conditions favour hydrogen peroxide removal in the pH rangeemployed and regardless of the activated carbon used. In thepresence of Chemviron SSP-4 about 29, 67 and 72% of hydrogenperoxide abatement was observed after 2 h at pH 5, 7 and 9,respectively. On the other hand, for HydraffinP110 the values forhydrogen peroxide conversion were 35, 43 and 49% under thesame conditions of time and pH. Nevertheless, higher hydrogenperoxide removal was observed in experiments carried out in theabsence of buffering salts. These results suggest that phosphateions might be absorbed onto the activated carbon, blocking someof the active sites of the solid and impeding the adsorption anddecomposition of hydrogen peroxide molecules.

On the other hand, hydrogen peroxide stability is also stronglyaffected by pH conditions. Hydrogen peroxide is a weak acidand the anionic (HO2

−) form is highly unstable (pKa = 11.3) inalkaline media. In the absence of other reactive species, HO2

−

can react with hydrogen peroxide to release oxygen. However, inthe presence of electrophilic sites such as those on the activatedcarbon surface the reaction between HO2

− and hydrogen peroxidecan be considered negligible.19 In acidic media hydrogen peroxideis more stable although, as seen from Fig. 4, higher capacity fordecomposition of the activated carbons was observed in theabsence of buffering salts, especially in the case of Chemviron SSP-4. It is assumed that hydrogen peroxide decomposition on theactivated carbon arises by the exchange of a hydroxyl group at thesurface of the solid with a hydrogen peroxide anion. This adsorbedperoxide, with presumably an oxidation capacity, would reactwith another hydrogen peroxide molecule releasing oxygen andregenerating the activated carbon group.12 These facts predict thehigher decomposition activity observed under basic conditionswith activated carbons with basic characteristics. On the otherhand, the presence of acidic groups, as present in Chemviron SSP-4, can slow down hydrogen peroxide decomposition, althoughresults also point out the higher activity of this solid to decomposehydrogen peroxide. As discussed later the presence of metals in

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Figure 5. Evolution of dimensionless hydrogen peroxide concentrationwith time during the course of decomposition experiments in the presenceof Chemviron SSP-4 (solid symbols) and HydraffinP110 (open symbols).Effect of tert-butanol. Conditions: initial conc. H2O2 = 7.7 mg L−1;T = 25 ◦C; pH = 4.8; AC wt = 1 g; stirrer speed = 330 rpm; particlesize = 1–1.25 mm; � tert-butanol; � absence of tert-butanol.

Chemviron SSP-4 is likely to be responsible for its capacity todecompose hydrogen peroxide.

Effect of tert-butanolTo determine the presence of hydroxyl radicals and theirimportance in hydrogen peroxide decomposition onto activatedcarbon, some experiments were carried out in the presence oftert-butanol. As seen in Fig. 5 the presence of tert-butanol clearlydecreased hydrogen peroxide decomposition regardless of theactivated carbon. The effect of the presence of this scavengerclearly indicates the role of hydroxyl radicals in the catalyticdecomposition of hydrogen peroxide onto activated carbon.Considering that hydroxyl radicals are mostly generated in themicropore structure of the solid, and tert-butanol, owing to itshigh molecule volume, cannot reach this space, the result of thepresence of this scavenger is not so remarkable as expected.

Pre-treated activated carbonsFrom the results above, the higher catalytic activity of the acidicactivated carbon Chemviron SSP-4 to decompose hydrogenperoxide seems to be clear. Nevertheless a number of authorshave proposed that activated carbons of basic characteristicssuch as HydraffinP110 must show a higher capacity to yieldhydrogen peroxide decomposition.12,20 For that reason, thepossible contribution of the presence of metals in the activatedcarbon with a capacity to decompose hydrogen peroxide must beconsidered. This has been shown for Fe(II) which can be present inactivated carbon and it is able to decompose hydrogen peroxideby Fenton reactions.21

To study the effect of metal content, activated carbons werewashed using different acidic solutions to remove possible metalcontent. Three different pre-treatments were carried out withboth activated carbons: 2 h wash in boiling ultrapure water(pre-treatment 1); 2 h wash in 0.1% HCl ultrapure water solution(pre-treatment 2); 2 h wash in 10% HCl ultrapure water solution(pre-treatment 3).

As seen from Fig. 6, the loss of activity of the activated carbonspre-treated by washing with 0.1% HCl solution is especiallyremarkable in the case of Chemviron SSP-4. After this treatmentmost of the metal content of the activated carbons was removed

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Figure 6. Evolution of dimensionless hydrogen peroxide concentrationwith time during the course of decomposition experiments in thepresence of Chemviron SSP-4 (solid symbols) and HydraffinP110 (opensymbols). Effect of activated carbon pre-treatments. Conditions: initialconc. H2O2 = 7.7 mg L−1; T = 25 ◦C; pH = 4.8; AC wt = 1 g; stirrer speed =330 rpm; particle size = 1–1.25 mm; � pre-treatment 1: 2 h wash in boilingultrapure water; � pre-treatment 2: 2 h wash in 0.1% HCl ultrapure watersolution; � pre-treatment 3: 2 h wash in 10% HCl ultrapure water solution.

Table 2. Mears’ criterion for the hydrogen peroxide decompositiononto activated carbons

t, min

CM, 330 rpm,(Chemviron

SSP-4)

Cwp, 330 rpm,(Chemviron

SSP-4)CM, 330 rpm,

(HydraffinP110)Cwp, 330 rpm,

(HydraffinP110)

0 5.80 10−2 14.0 0 3.40

15 5.98 10−2 14.4 0 3.65

30 5.92 10−2 14.3 0 3.97

45 5.55 10−2 13.4 0 4.23

60 5.17 10−2 12.4 0 4.58

75 5.07 10−2 12.2 0 5.04

90 4.98 10−2 12.0 0 5.48

105 5.88 10−2 14.1 0 6.00

120 7.76 10−2 18.7 0 6.63

and from the results the catalytic capacity observed in the case ofChemviron SSP-4 is probable ascribable to Fenton reactions. Onthe other hand, more severe pre-treatment with 10% HCL solutionmight also affect the surface chemistry of the activated carbons.

Kinetic considerationsTo study the importance of mass transfer resistances in hydrogenperoxide decomposition on activated carbon some criteria havebeen applied to the experimental data.

Mears’ criterion (MC) was used to confirm external diffusioncontrol in some experiments.22 This compares the relativeimportance of external diffusion rate with solid particles withinternal diffusion plus a surface chemical reaction rate. For MCvalues higher than 0.15 the process is controlled by externaldiffusion and not by the internal diffusion plus surface chemicalreaction. As stated (Fig. 1) in the case of HydraffinP110, no controlof the external mass transfer diffusion is observed with theagitation conditions used, while for Chemviron SSP-4 the oppositeconclusion is reached. These results are also confirmed by the MCvalues shown in Table 2. For Chemviron SSP-4 at the highest stirringspeed used MC was less than 0.15, which proves the absenceof external mass transfer resistance. However, under the other

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H2O

2/H

2O20

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1.00

TO

C/T

OC

0

t, min0 10 20 30 40 50 60

t, min

Figure 7. Evolution of dimensionless TOC (left) and hydrogen peroxide (right) with time during oxalic acid adsorption or oxidation by differentprocesses. T = 25 ◦C; pH0 = 3; stirring speed = 330 rpm; AC wt = 0.5 g; initial conc. H2O2 = 10 ppm (when applied unless indicated); Q = 20 L h−1;CO3g = 20 mg L−1 (when applied);� adsorption on HydraffinP110;�H2O2 oxidation;•H2O2-HydraffinP110 oxidation;×ozonation;+ O3-H2O2 oxidation;� O3-HydraffinP110 oxidation; ♦ O3-H2O2-HydraffinP110 oxidation; ♦ O3-H2O2-HydraffinP110 oxidation with initial concentration H2O2 = 20 ppm.

agitation conditions MC was above this value (values not shown)and, as also seen from Fig. 1, the external mass transfer resistancemust be taken into account. On the other hand, for HydraffinP110,MC values confirm the absence of external diffusion resistanceunder the conditions investigated as previously discussed.

With the aim of determining the importance of the internaldiffusion mass transfer resistance the criterion of Weisz–Prater(CWP) was applied. According to this criterion if CWP << 1 theinternal diffusion resistance can be neglected.23 As seen fromTable 2, CWP has values higher than 1 in both activated carbons,which indicates that internal diffusion is not negligible.

Catalytic ozonation of oxalic acid promoted by activatedcarbon in the presence of hydrogen peroxideThe main purpose of these experiments was to study the role ofhydrogen peroxide in catalytic ozonation processes promoted byactivated carbon as a water treatment. According to the literature,the presence of hydrogen peroxide in these systems can beattributable to two main pathways: from the reaction of ozonewith some surface groups of the activated carbon, and from thedestruction of some structures such as aromatic rings and/ordouble carbon bonds present in numerous water pollutants (i.e.,phenol derivatives, olefins, etc.).

In these experiments, oxalic acid was chosen as a modelcompound because it is one of the typical refractory end by-products formed in conventional chemical oxidation treatments.24

The accumulation of this compound in ozonation processes isascribable to its low reaction rate with ozone (k < 0.04 M−1s−1)25

so its mineralization is due to secondary oxidants such as hydroxylradicals (k in the order of 106 M−1 s−1).26

According to previous results, HydraffinP110 was selected to beused with the model compound since its catalytic activity was notaffected by the metal content of the solid. In addition, a phosphatebuffer was not used to avoid its adsorption onto the activatedcarbon and interference with the surface reactions.

TOC was used to follow the oxalic acid evolution since inall our experiments it was coincident with the organic carbondeduced from the oxalic acid concentration. Consequently,no intermediates were formed during the catalytic ozonation

process. As seen from Fig. 7, there was very little adsorptionof oxalic acid onto the activated carbon (about 6% of targetcompound removed after 1 h). Oxalic acid is a polar and hydrophiliccompound therefore its adsorption onto activated carbon is highlyunfavoured. Under the experimental conditions (pH 3) the surfaceof HydraffinP110 is positively charged (pH<pHPZC) and the limitedadsorption observed was due to the electrostatic forces betweenthe activated carbon surface and the anionic form of the carboxylicacid. On the other hand, single ozonation and hydrogen peroxideoxidation were not able to remove oxalic acid (less than 2%elimination after 1 h treatment). These results are in accordancewith the refractory nature of oxalic acid.25

As shown in Fig. 7, the combined process (hydrogen peroxideand activated carbon) removed about 14% of oxalic acid after 1 htreatment. Considering the limited target compound removal dueto adsorption onto the solid and oxidation by hydrogen peroxide,this mineralization can be attributed to the radical species formedduring the decomposition of hydrogen peroxide on the activatedcarbon surface. As a result about 26% of hydrogen peroxide wasremoved in the same period of time (Fig. 7). On the other hand,ozonation in the presence of hydrogen peroxide was able toachieve slight elimination of the target compound (about 14%after 1 h process). Hydrogen peroxide is a well-known promoterof ozone decomposition into hydroxyl radicals, although underthe pH conditions used (pH=3) its activity is highly restricted.27

Therefore, a modest decrease of hydrogen peroxide and TOC wasobserved during the reaction process (Fig. 7).

In the case of the combined process, ozonation in the presenceof activated carbon, and taking into consideration the provenlimited capacity of single ozonation and adsorption to remove thetarget compound, the remarkable mineralization observed (about50% of TOC removed after 1 h treatment) is ascribable to ozonedecomposition onto hydroxyl radicals promoted by activatedcarbon.7 – 9

Among the different processes used, ozonation in the presenceof activated carbon and hydrogen peroxide was the mosteffective for removing the persistent target compound. Evenconsidering the contribution of the homogenous catalysis dueto hydrogen peroxide, a synergetic effect of activated carbonand hydrogen peroxide to decompose ozone into more oxidizing

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species can be deduced for removal of the target compound(Fig. 7). In this system, in addition to ozone decompositionpromoted by the activated carbon, the contribution of thehydrogen peroxide decomposition onto the activated carbonsurface must be considered. This reaction can yield speciesthat initiate the decomposition of ozone. As also seen fromFig. 7, during the first 10 min of the process, the oxalic acidelimination with time was similar to that observed in the ozonationpromoted by activated carbon plus the contribution due to thehomogenous decomposition by hydrogen peroxide. Thus, whenthe adsorption and decomposition of hydrogen peroxide on theactivated carbon starts to be more significant and, consequently,the ozone decomposition promoted by this route, mineralizationof oxalic acid clearly increases. This effect was also observed in thehydrogen peroxide evolution shown in Fig. 7. After a lag period,the decomposition of hydrogen peroxide due to the presence ofactivated carbon and ozone was much more significant reachingabout 70% depletion after 1 h reaction. Additionally, in thiscombined system a positive influence on the mineralization of thetarget compound of the initial hydrogen peroxide concentrationwas also observed as Fig. 7 illustrates. The higher amount ofhydrogen peroxide decomposed at the surface of the activatedcarbon leads to higher decomposition of ozone promoted bythese reactions and, then, to a lower oxalic acid concentration.

CONCLUSIONSThe main conclusions reached in this work are:

1. Working under the same conditions, the activated carbonChemviron SSP-4 with an acidic nature showed a higheractivity to decompose hydrogen peroxide than HydraffinP110.Regardless of the activated carbon used the influence of theoperating variables studied was quite similar in both cases.

2. Hydrogen peroxide decomposition on activated carbon wasfavoured at higher pH conditions and negatively affected bythe presence of buffering salts in the medium.

3. The high capacity of Chemviron SSP-4 to decompose hydrogenperoxide is greatly due to the metal content presence of thesolid. In addition, experiments carried out in the presence oftert-butanol confirmed that hydroxyl radicals are involved inthe decomposition mechanism.

4. According to the Mears and Weisz–Prater criteria hydrogenperoxide decomposition on activated carbon is controlled byinternal mass transfer resistance and the surface chemicalreaction at the higher stirring speed used (300 rpm).

5. The presence of hydrogen peroxide in the catalytic ozonationprocess promoted by activated carbon clearly improved theremoval of oxalic acid. The decomposition of hydrogenperoxide on the activated carbon surface favoured ozonedecomposition into hydroxyl radicals, thus the oxidationcapacity of the O3/AC/H2O2 system.

ACKNOWLEDGEMENTSThis work has been supported by the CICYT of Spain and theEuropean Region Development Funds of the European Com-mission (Projects PPQ2006/04745 and CTQ2009/13459/C05/05).Mrs Aguinaco also thanks the Spanish Ministry of Science andEducation for a FPU grant.

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