Hydrogen peroxide-promoted-CWAO of phenol with activated carbon

Applied Catalysis B: Environmental 93 (2010) 339–345

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Hydrogen peroxide-promoted-CWAO of phenol with activated carbon A. Quintanilla *, J.A. Casas, J.J. Rodriguez Chemical Engineering Section, Universidad Auto´noma de Madrid, Campus de Cantoblanco, 28049 Madrid, Spain

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 August 2009 Received in revised form 1 October 2009 Accepted 7 October 2009 Available online 13 October 2009

The effect of hydrogen peroxide as radical promoter in the wet air oxidation of phenol with activated carbon catalysts is studied in a trickle-bed reactor at 127 8C, 8 atm and 20–320 gAC h/gPhenol. The modifications that hydrogen peroxide induces on the radical reaction mechanism and the consequences on the effluent ecotoxicity are analyzed. The synergistic effect between oxygen and hydrogen peroxide in the initiation step of the reaction is verified. Hydroperoxy radicals, produced by the reaction between hydrogen peroxide and adsorbed oxygen, initiate the reaction on the carbon and in the liquid phase. They are responsible for the increased initial activity which provokes a faster removal of phenol and aromatic intermediates and, therefore, of the toxicity. The adsorbed oxygen on the carbon is crucial for the efficient consumption of hydrogen peroxide. ß 2009 Elsevier B.V. All rights reserved.

Keywords: Promoted oxidation Ecotoxicity Catalytic wet air oxidation Hydrogen peroxide Activated carbon Phenol

1. Introduction Catalytic wet air oxidation (CWAO) can be a cost-effective process when it is coupled with a subsequent biological treatment [1,2]. In this case, the biodegradability of the effluent after the chemical oxidation step is a determining factor for the potential application of this combined strategy. Consequently, biodegradability and ecotoxicity have been emphatically considered in the analysis of the wet oxidation processes in the recent years [3–10]. Activated carbons have proved their catalytic activity in the wet air oxidation of toxic organic pollutants such as phenol [11–15]. When compared with the earlier copper catalysts, a higher mineralization, namely complete oxidation of phenol to CO2 and H2O, has been observed [16], in detriment of the highly toxic oxidation intermediates such as hydroquinone, catechol and pbenzoquinone, main responsible for the effluent toxicity [4,6]. Nevertheless, though these aromatic intermediates are detected in lower concentrations with activated carbon catalysts, the milder reaction conditions required to minimize the carbon combustion [17] lead to a longer life-times of these species. Therefore, it is convenient to accelerate the oxidation of these highly toxic species to avoid the use of oversized reactors or long space–time values. In this sense, a free-radical promoter such as hydrogen peroxide can be used to initiate/assist the oxidation reaction. This strategy has already been applied in non-catalytic wet air oxidation treatments [18–22] and in homogeneously catalyzed

* Corresponding author. Tel.: +34 91 4972878; fax: +34 91 4973516. E-mail address: [email protected] (A. Quintanilla). 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.10.007

wet air oxidation processes [21,23–26]. In these works, it is widely accepted that the enhanced activity is caused by the action of hydroxyl radicals, produced from hydrogen peroxide, in the initiation step of the reaction. In the non-catalytic processes, hydrogen peroxide decomposes to hydroxyl radicals by thermal scission of the oxygen–oxygen bond [20,21]: T

H2 O2 !2HO

(1)

and in the homogeneously catalyzed reactions, hydroxyl radicals are mainly produced in the redox reaction with the metallic ions: H2 O2 þ Mnþ ! HO þ HO þ Mnþ1

(2)

Also, at the temperatures of the wet air oxidation treatments (>100 8C), hydrogen peroxide can be decomposed into water and oxygen: T

2H2 O2 !2H2 OþO2

(3)

This reaction (3) reduces the efficiency of hydrogen peroxide in the removal of the organic pollutants because the operating conditions of the wet air oxidation (oxygen flow rate, temperature and pressure) already assure the oxygen saturation in the liquid phase. The feasibility of wet air oxidation with activated carbon catalysts promoted by hydrogen peroxide has been scarcely investigated [5,27]. The efficiency of the treatment was accepted based on the increased conversion of the organic pollutant, total organic carbon (TOC) and chemical oxygen demand (COD). At the conditions employed (140–170 8C, 2–3.4 atm and 0.23–23 gCAT h/gpollutant in a trickle-bed reactor), the biodegradability of the

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effluents measured by respirometric techniques, as the readily biodegradable COD fraction, was still not sufficient for a subsequent biological oxidation. Stuber et al. [28] speculates that the improved conversions are obtained by the promoting effect of the hydroxyl radicals formed on the carbon surface: H2 O2 þ  ! 2HO

(4)

RH þ  , RH

(5)

HO þ RH ! R þ H2 O þ 

(6)

These reactions could be linked to the presence of ashes in the activated carbon (3.7 wt.%) more than the surface of the carbon itself. However, the later work of Rey et al. [29] studying the selectivity of the hydrogen peroxide decomposition reaction on an activated carbon with a higher content of ashes (8.35 wt.%) verifies that activated carbon surface promotes the decomposition towards oxygen by a non-radical via (the selectivity to oxygen was 99%): H 2 O2 þ  ! H 2 O2 

(7)

2H2 O2  ! 2H2 O þ O2 

(8)

If the introduction of hydrogen peroxide in the CWAO treatment improves pollutant TOC and COD conversions is because reaction (7) must be somehow inhibited and reaction (3) must be favored. Likely, in this sense, oxygen may play a role. Herein, we study the effect of the addition of hydrogen peroxide on the carbon activity and oxidation intermediate distribution that will determine the detoxification of the effluent, also measured. For this purpose, the wet oxidation of phenol will be explored in a wide range of space–time values (20–320 gCAT h/gPhenol) at fixed temperature and pressure. The results are explained based on the reaction mechanism proposed and elucidated from experiments carried out with hydrogen peroxide in presence and absence of oxygen. To evaluate the efficiency of the hydrogen peroxide-promotedCWAO, the results will be compared with two oxidation treatments already proposed to improve the CWAO with activated carbons: the CWAO [9] and two-step sequential CWPO–CWAO treatment [30] both with iron on activated carbon (Fe/AC) catalysts.

2.2. Experimental set-up and chemical analyzes The oxidation experiments were conducted in a trickle-bed reactor at lab-scale (1/200 o.d.). The liquid and gas phases were passed through the bed (activated carbon mass, WAC = 2.5 g) in cocurrent down-flow. The inlet gas was pure oxygen. Detailed information about the components and operation procedure of this unit has been reported elsewhere [9]. The selected operating conditions are: T = 127 8C, PT = 8 atm and space–time, t = 20– 320 gCAT h/gphenol. The initial conditions of the gas and liquid are: Cinlet phenol = 1 g/L, C inletH2 O2 ¼ 5 g=L, pHinlet = 3.5 and Q O2 ¼ 91:6 N mL=min: For some particular experiments, the temperature employed is 65 8C. The progress of the reaction was followed by taking periodically liquid samples from the reactor outlet once the steady-state was reached. The liquid samples were analyzed by different procedures. Phenol and aromatic compounds were determined by HPLC (Varian, mod. ProStar), low molecular weight acids by IC (Metrohm, mod. 761 Compact IC) and total organic carbon (TOC) using a TOC analyzer (O.I. Analytical, model 1010). Hydrogen peroxide concentration was quantified by colorimetric titration using the Ti(SO4)2 method [31]. Complete hydrogen peroxide conversion was achieved in all the experiments and therefore, no additional ecotoxicity results from this reagent. The ecotoxicity of the liquid effluent, expressed as TU50, was determined by a bioassay in a Microtox M500 Analyzer (Azur Environmental) following the standard Microtox test procedure (ISO 11348-3, 1998). This bioassay is based on the decrease of light emission by Photobacterium phosphoreum as the result of its exposure to a toxicant. The toxicity units of the sample are calculated as: TU50 ¼

100 IC50

(9)

IC50 is defined as the sample dilution percentage that yields the 50% reduction of the light emitted by the microorganisms. Before measuring the toxicity, the pH values of all the samples were readjusted between 6 and 7 to prevent the pH effect. The microorganisms were purchased from Microtox Acute Reagent supplied by I.O. Analytical. More detailed information about this assay and its application to CWAO effluents can be found in Santos et al. [4]. 3. Results and discussion

2. Experimental 3.1. CWAO promoted by H2O2 2.1. Catalyst characterization A granular activated carbon commercialized by Merck (Ref.: 102514, 4.63 wt.% content of ashes) was used as received (particle size ranged from 0.5 to 1 mm). This catalyst presents the following properties: BET area (SBET) = 974 m2/g, external area (At) = 85 m2/g, micropore volume (Vmicro) = 0.42 cm3/g and mesopore volume (Vmes) = 0.13 cm3/g. The textural properties were determined from the 77 K N2 adsorption/desorption isotherms performed in an Omnisorp apparatus (mod. 100CX). The micropore volume (Vmicro) and the external or non-micropore surface area (At) were obtained by the t-method. The oxygen surface groups were analyzed by temperatureprogrammed desorption (TPD). In the TPD experiments, a sample of 100 mg of activated carbon was placed in a quartz tube and heated at 10 K/min up to 1173 K. A flow of 200 N mL/min of He was continuously passed as carrier gas. The evolved CO and CO2 were continuously analyzed by NDIR in a SIEMENS (mod. Ultramat 22) gas analyzer.

3.1.1. Activity The phenol and TOC conversion-time profiles obtained in presence (AC + O2 + H2O2) and in absence (AC + O2) of hydrogen peroxide are given in Fig. 1. The corresponding initial rates, ri, are shown in Fig. 2. They are calculated as follows, where i refers either to phenol or TOC, WAC is the activated carbon mass and QL the liquid flow rate: ri ¼ 

dC i 1 ðgi g1 AC h Þ dðW AC =Q L Þ

(10)

The higher phenol and TOC conversion values for a given space– time (Fig. 1) and the higher oxidation and mineralization initial rates (Fig. 2) for the AC + O2 + H2O2 system compared to the AC + O2, indicates that the addition of hydrogen peroxide significantly favors phenol and TOC abatement (Fig. 1), resulting in a substantially higher initial activity (Fig. 2). Therefore, hydrogen peroxide is a source of free radicals.

A. Quintanilla et al. / Applied Catalysis B: Environmental 93 (2010) 339–345

Fig. 1. Phenol and TOC conversion values as a funtion of space–time in different oxidation treatments. (&) AC + H2O2 + O2, (*) AC + O2, *Fe/AC + O2, (~) sequential CWPO–CWO with Fe/AC.

3.1.2. Oxidation intermediates The difference between the phenol and TOC conversion values is higher when hydrogen peroxide is used as promote (Fig. 1), which means a higher amount of intermediate in the reactor effluent. The identified intermediates were p-benzoquinone and low molecular weight acids such as maleic, acetic, oxalic and formic. The TOC values calculated from these identified intermediates and the remaining phenol fit fairly well with the measured TOC values. Thus it can be concluded that all the oxidation intermediates have been identified and successfully quantified. The colorless of the samples supports the absence of aromatic condensation products in the liquid phase. The concentration profiles of the intermediates in terms of normalized concentration expressed in carbon units (ratio

Fig. 2. Comparison of initial rates in different oxidation treatments.

341

Fig. 3. Oxidation by-products distribution in different treatments. (&) AC + H2O2 + O2, (*) AC + O2, *Fe/AC + O2, (~) sequential CWPO–CWO with Fe/AC.

between the concentration sum of the i species and inlet phenol concentration, both in mg/L of carbon, SCCi/CCO) are shown as a function of the space–time in Fig. 3. The oxidation by-products have been grouped into aromatics and low molecular weight acids. In the AC + O2 system, hydroquinone, p-hydroxybenzoic acid, in addition to p-benzoquinone contribute to the aromatic byproducts and the acid group also includes malonic acid. As can be observed in Fig. 3, the aromatic species are detected in lower concentrations and are more rapidly destroyed in the hydrogen peroxide-promoted-CWAO. p-Benzoquinone is easily oxidized into acids. Most of the acids are quite resistant to the oxidation because the decay in their concentration profile is only observed at high space–time values. Acetic is the only completely refractory species at the conditions employed [32]. Therefore, the differences between phenol and TOC conversions (Fig. 1) are due essentially to the remaining organic acids. Direct mineralization of phenol and p-benzoquinone cannot be ruled out. Fig. 4 shows the reaction pathway proposed for the hydrogen peroxide-promoted-CWAO of phenol with activated carbon. Phenol is oxidized to p-benzoquinone, which yields low molecular weight acids and CO2. Neither hydroquinone nor p-hydroxybenzoic acid was detected, which are typical intermediates in CWAO with activated carbon-based catalysts [13,14,31]. In presence of hydrogen peroxide promoter, the hydroxylation of phenol to hydroquinone only occurs on the carbon surface (adsorbed hydroquinone is represented by the ROH* species in the reaction mechanism, see below). Likely, the carboxylation of phenol to p-hydroxybenzoic acid, a quite slow reaction compared to hydroxylation [32], is now inhibited. 3.1.3. Effluent ecotoxicity Looking at the toxicity of the intermediates, hydroquinone is the most toxic one followed by p-benzoquinone, both with a

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Fig. 4. Reaction pathway for the hydrogen peroxide-promoted-CWAO of phenol with activated carbon.

significantly higher ecotoxicity than phenol (EC50 = 0.04, 0.1 and 17 mg/L, respectively). The carboxylic acids at the observed concentrations are non-toxic. Fig. 5 shows the evolution of the ecotoxicity, expressed as TU50, with space–time. The toxicity curves always show a maximum according to the concentration profile of the aromatic intermediates (Fig. 3). In presence of hydrogen peroxide, the maximum in the toxicity appears at shorter space–time values and its value is significantly lower than in the non-promoted reaction. This is due to the absence of hydroquinone and the lower concentration of p-benzoquinone and phenol, as a result of their faster oxidation. Fig. 5 evidences the beneficial effect of hydrogen peroxide in the detoxification of the phenolic wastewater. 3.1.4. Reaction mechanism The role of hydrogen peroxide and oxygen in the CWAO mechanism is studied in order to understand the positive effect of its addition, meaning higher initial phenol and TOC rates as well as lower concentrations and faster oxidation of the toxic aromatic species. A control experiment is carried out at the selected operating conditions of the hydrogen peroxide-promoted-CWAO but substituting oxygen for nitrogen (AC + H2O2 + N2) to maintain the same fluid dynamic conditions. The initial activity of the AC + H2O2 + N2 system was slightly higher than that of the non-

promoted-CWAO (AC + O2) but significantly lower compared with the AC + H2O2 + O2 system (see Fig. 2). These results prove that oxygen also plays a role in the initiation of the reaction and that there is a synergistic effect of both oxidants, hydrogen peroxide and oxygen. This synergistic effect can be explain by (i) the high carbon occupancy by oxygen which reduces the adsorption of hydrogen peroxide and its subsequent decomposition into oxygen and water (reactions (7) and (8)) and (ii) the production of hydroperoxy radicals in the reaction between adsorbed oxygen and hydrogen peroxide: O2 þ  , O 2 

(11)

O2  þ H2 O2 ! HO2  þ HO2 

(12)

Mainly the adsorbed, not dissolved, oxygen participates in the initiation step as can be deduced from a comparison of the results of Fig. 2 and those reported by Kolaczkowski et al., [20]. The latter shows that dissolved oxygen does not influence on the initial phenol conversion in non-catalytic wet promoted oxidation. Hydroperoxy radicals (reaction (12)) along with hydroxyl radicals (reaction (1)) in the liquid phase may extend the phenol oxidation in this phase. To prove this assumption, experiments at the same space–time but different residence time are performed. As shown in Fig. 6, for a given space–time, the higher the residence time the higher the phenol conversion. However, TOC conversion remains unaffected by the residence time. There is some homogeneous-phase contribution in phenol conversion but not in mineralization. These observations indicate that the initiation step, in which phenol (R) is converted into phenoxy radicals (R), takes place on the carbon surface by adsorbed hydroxyl radicals and in the liquid phase by hydroperoxy and hydroxyl radicals. On the contrary, the propagation step, where phenol is eventually oxidized to low molecular weight acids and CO2, mainly occurs on the carbon surface. Then, phenoxy radicals in the liquid phase must react with adsorbed oxygen giving rise to peroxyphenyl radicals also adsorbed: R þ O2  ! ROO

Fig. 5. Ecotoxicity evolution with space–time in different oxidation treatments. (&) AC + H2O2 + O2, (*) AC + O2, *Fe/AC + O2, (~) sequential CWPO–CWO with Fe/AC.

(13)

In addition, there is also a homogeneous contribution in the hydrogen peroxide conversion (Fig. 6) which confirms that hydrogen peroxide can be consumed not only by the reaction with the adsorbed oxygen (reaction (12)) but also by its decomposition into hydroxyl radicals. The latter occurs as a consequence of the reaction temperature (above 100 8C) and the presence of iron in the stainless steel reactor wall [30] as it is the

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Termination: R þ R ! R  R þ 

(26)

R þ HO ! ROH

(27)

2HO ! H2 O2

(28)





HO þ HO2 ! H2 O þ O2

(29)

HO þ HO2  ! H2 O þ O2 

(30)

Side reactions: HO þ H2 O2 ! HO2  þ H2 O 2H2 O2  ! 2H2 O þ O2  T

2H2 O2 !2H2 OþO2 Fig. 6. Homogeneous-phase reaction contribution in the H2O2-promoted-CWAO with activated carbon at 65 8C. (&): t = 33.3 gCAT h/gphenol, (~): t = 67 gCAT h/ gphenol. Close symbols for phenol, symbols with a  for TOC and open symbols for hydrogen peroxide.

case in the experiments of Fig. 6 carried out at 65 8C (dissolved iron was detected in the reactor effluent by TXRF). According to the above discussion, the following reaction mechanism is proposed: Initiation: O2 þ  , O 2 

(11)

O2  þ H2 O2 ! HO2  þ HO2 

(12)

RH þ  , RH

(14)

HO2  þ RH ! R þ H2 O2 

(15)

HO2  þ RH ! R þ H2 O2

(16)

HO2  þ RH ! R þ H2 O2

(17)

T=wall

H2 O2 ! 2HO

(1)

HO þ RH ! R þ H2 O

(18)

HO þ RH ! R þ H2 O

(19)

Propagation (oxidation): R þ O2  ! ROO þ 

(20)

R þ O2  ! ROO

(13)

ROO þ RH ! ROOH þ R

(21)

ROOH ! RO þ HO

(22)

RO þ RH ! ROH þ R

(23)

R þ H2 O2 ! ROH þ HO

(24)

ROH þ ðO2  ; HO ; HO2  ; HO2  Þ ! ½::::::: ! CO2 þ H2 O þ acetic acid

(25)

(31) (7) (3)

Reactions (17) and (19) account for the homogeneous-phase reaction contribution (Fig. 6). The inefficient consumption of hydrogen peroxide by decomposition into oxygen (reaction (7)) is minimized but not completely depleted because in the reaction between the adsorbed organic molecule and hydroperoxy radical yielding the adsorbed organic radical (reaction (15)), hydrogen peroxide is regenerated on the carbon surface where it can be subsequently decomposed into oxygen and water. 3.1.5. Catalyst stability A long-term experiment was performed to learn about the stability of the activated carbon in presence of the hydrogen peroxide promoter. Carbon activity, analyzed in terms of phenol and TOC conversion, remained constant during the 135 h time on stream of the experiments. Values of 83  4% and 45  3.6% were maintained for phenol and TOC conversions, respectively, at 127 8C, 8 atm and 20 gCAT h/gphenol space–time (the standard deviation was calculated from experiments at three different times on stream: 10, 75 and 135 h). In spite of this good stability, the activated carbon surface can be physically and chemically modified due to the oxidation operating conditions as was previously reported in the nonpromoted-CWAO process [9]. Now, in the promoted oxidation, hydrogen peroxide can have an additional effect. The evolution of porous structure and oxygen surface groups of the carbon with the time on stream are given in Table 1. As shown, the oxidation process dramatically affects to the BET area and micropore volume. The amount of oxygen surface groups substantially increases. An important fraction of these new oxygen surface groups can be placed at the entrance of the micropores blocking the accessibility of the nitrogen molecules in the BET analyzes. As in the non-promoted-CWAO, these changes on the carbon surface do not affect the catalyst activity. We can conclude that the addition of hydrogen peroxide as a source of free radicals in CWAO with activated carbon provides an improved solution for the detoxification of phenolic wastewaters thus allowing a subsequent biological treatment. The initiation of the reaction by hydroperoxy radicals has a positive effect on the rates of oxidation of phenol and the most toxic intermediates, leading to a fast detoxification of the wastewater. In previous works, we showed that another two different CWAO treatments: the CWAO [9] and two-step sequential CWPO–CWAO treatment [30] both with iron on activated carbon (Fe/AC) catalysts were also efficient solutions for the same type of wastewaters. Next, a comparison between the hydrogen peroxide-promoted-CWAO and the other two treatments is

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Table 1 Evolution of porous structure and oxygen surface groups of the activated carbon with time on stream at 80 gCAT h/gphenol space–time and 65 8C in hydrogen peroxidepromoted-CWAO. Sample

TOS (h)

SBET (m2/g)

At (m2/g)

Vmicro (cm3/g)

mmol CO/g C

mmol CO2/g C

Fresh AC Used AC

0 25 37 134

974 256 113 148

85 n.m. 87 120

0.42 n.m. 0.01 0.01

289 948 1617 1939

72 354 807 1099

n.m.: not measured.

carried out in terms of activity, intermediate distribution and ecotoxicity. 3.2. H2O2-promoted-CWAO vs. CWAO with Fe/AC CWAO of phenol with a home-made Fe/AC catalyst was studied in previous works at the same operating conditions [9,15,32]. The results have been included in Figs. 1–3 and 5. Fe/AC was presented as a promising catalyst because significantly improves the efficiency of CWAO with bare activated carbons as shown by the higher phenol and TOC conversion values (Fig. 1) and higher oxidation and mineralization initial rates (Fig. 2). For a similar amount of carbon oxygen surface groups, Fe/AC provided higher phenol and TOC conversions than AC [9] which proves the co-catalytic role of the Fe in the oxidation process. Also, the removal of the toxic intermediates with Fe/AC catalysts is faster, as shown in Fig. 3, which leads to a faster detoxification (Fig. 5). However, the hydrogen peroxide-promoted-CWAO with activated carbon studied in this work is clearly a more efficient process for the decontamination of phenolic wastewater than the CWAO with Fe/AC as shown by the improved phenol and TOC conversions and enhanced activity (Figs. 1 and 2). The toxic intermediates are faster oxidized (Fig. 3) resulting in a much more efficient detoxification (Fig. 5). 3.3. H2O2-promoted-CWAO vs. sequential CWPO–CWAO with Fe/AC An integrated treatment consisting in a CWPO (at 25 8C, 1 atm and 100% of the stoichiometric dose of hydrogen peroxide) followed by CWAO treatment at mild conditions (100 8C and 8 atm) both using a home-made Fe/AC catalyst was also reported in a previous work as an efficient system for decontamination of phenolic wastewater [30]. The results have also been collected in this work for comparison, in Figs. 1–3 and 5. Phenol and TOC conversions are improved in the sequential treatment with respect to those values obtained in CWAO with bare activated carbons and with Fe/AC. The results are comparable to those presented in this work with the AC + H2O2 + O2 system (Fig. 1), even higher phenol and TOC initial rates can be observed (Fig. 2). This is explained by the presence of hydroxyl radicals in the first step of the treatment (CWPO) where a heterogeneous system (supported-Fe + H2O2) acts according to reaction (2). Nevertheless, 20% of the total load of the iron catalyst was leached out after 50 h of time on stream [30]. The formation of iron–organic complex occurs (i.e. with oxalic acid and catechol intermediates) resulting in more ecotoxic effluents than expected (Fig. 5). Therefore, hydrogen peroxide-promoted-CWAO with bare activated carbon results to be a more efficient process. 4. Conclusion Hydrogen peroxide-promoted-CWAO using activated carbon catalysts is a highly efficient solution for a fast detoxification of

phenolic wastewaters. The effluents only contain p-benzoquinone and phenol as toxic compounds at very low concentrations. The faster oxidation and mineralization rates are due to the initiation of the reaction by hydroperoxy radicals. These radicals are produced on the carbon surface and in the liquid phase by the reaction between hydrogen peroxide and adsorbed oxygen. Consequently, homogeneous and heterogeneous-phase reactions contribute to the overall phenol oxidation. On the contrary, only heterogeneous contribution accounts for the overall TOC oxidation. Due to the oxygen occupancy of the carbon sites, the adsorption of hydrogen peroxide is inhibited and therefore its inefficient consumption by decomposition into oxygen and water is minimized. Further research is now focused on the optimization of temperature and hydrogen peroxide dose to minimize the operation costs. Acknowledgements We acknowledge financial support from MICINN through the projects CTQ2007-61748/PPQ and CTQ2008-03998/PPQ and from the Comunidad Auto´noma de Madrid through the project S-0505/ AMB/0395.

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