Journal of Electroanalytical Chemistry 689 (2013) 158–167

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Journal of Electroanalytical Chemistry

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Anodic oxidation, electro-Fenton and photoelectro-Fenton degradation ofcyanazine using a boron-doped diamond anode and an oxygen-diffusion cathode

Núria Borràs a, Conchita Arias b, Ramon Oliver a, Enric Brillas b,⇑a Unitat de Química Industrial, Escola Universitària d’Enginyeria Tècnica Industrial de Barcelona, Universitat Politècnica de Catalunya, Comte d’Urgell 187, 08036 Barcelona, Spainb Laboratori d’Electroquímica dels Materials i del Medi Ambient, Departament de Química Física, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11,08028 Barcelona, Spain

a r t i c l e i n f o

Article history:Received 9 October 2012Received in revised form 1 November 2012Accepted 8 November 2012Available online 22 November 2012

Keywords:Anodic oxidationBoron-doped diamondCyanazineElectro-FentonOxidation productsPhotoelectro-Fenton

1572-6657/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.jelechem.2012.11.012

⇑ Corresponding author. Tel.: +34 934021223; fax:E-mail address: brillas@ub.edu (E. Brillas).

a b s t r a c t

The degradation of the s-triazinic herbicide cyanazine in 100 ml solutions of pH 3.0 has been compara-tively studied by electrochemical advanced oxidation processes (EAOPs) such as anodic oxidation withelectrogenerated H2O2 (AO–H2O2), electro-Fenton (EF) and photoelectro-Fenton (PEF) with a 6 W UVAlamp. All the electrolyses were performed in a cell containing a 3 cm2 boron-doped diamond (BDD) anodeand a 3 cm2 O2-diffusion cathode able to generate H2O2. Hydroxyl radicals (�OH) formed at the BDD sur-face in all EAOPs and in the bulk from Fenton’s reaction between added Fe2+ and generated H2O2 in EF andPEF, were the main oxidants. The PEF process was more potent than the EF one, allowing attaining analmost total mineralization with 98% total organic carbon decay due to the combined action of the aboveoxidants with the photolysis of intermediates by UVA light. The lower oxidation power was attainedusing AO–H2O2 owing to the lower production rate of �OH formed at the anode. The effect of current den-sity and herbicide concentration on the degradation behavior of all EAOPs has been examined. The decaykinetics for cyanazine always followed a pseudo-first-order reaction with increasing apparent rate con-stants in the sequence AO–H2O2 < EF < PEF. Heteroaromatic derivatives such as deisopropylatrazine, des-ethyldeisopropylatrazine, ammeline and cyanuric acid, as well as generated carboxylic acids such asformic and oxamic, have been quantified by reversed-phase and ion-exclusion HPLC, respectively. Inor-ganic ions like Cl�, NO�3 and NHþ4 lost during the degradation processes were detected by ionic chroma-tography. From these products, a reaction sequence for cyanazine mineralization by all EAOPs has beenproposed.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Over the last decade, a large variety of electrochemical advancedoxidation processes (EAOPs) have been developed to mineralizelow contents of persistent and toxic organics in waters [1–4]. Theyare considered environmentally friendly technologies able to elec-trogenerate hydroxyl radical (�OH), the second strongest oxidantknown after fluorine. The very high standard reduction potential(E�(�OH/H2O) = 2.80 V/SHE at 25 �C) of �OH allows them to reactnon-selectively with most organics giving dehydrogenated orhydroxylated derivatives, which can in turn be completely mineral-ized to carbon dioxide, water and inorganic ions [5,6].

The simplest and most common EAOP for water remediation iselectrochemical oxidation or anodic oxidation (AO) [1,2,4]. In thismethod, organics contained in a contaminated solution are oxi-dized by direct charge transfer at the anode (M) and at high currentdensities, they are majority destroyed with physisorbed hydroxyl

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radical (M(�OH)) formed as intermediate of O2 evolution fromwater oxidation as follows [1,2,7]:

MþH2O!Mð�OHÞ þHþ þ e� ð1Þ

It has been found that the use of active anodes like Pt, IrO2 andRuO2 in AO allows primordially the conversion of organics into car-boxylic acids since the corresponding M(�OH) is transformed into achemisorbed ‘‘superoxide’’ species with less oxidizing power [8,9].In contrast, the physisorbed M(�OH) radical becomes much morestable in non-active anodes like PbO2 and boron-doped diamond(BDD) and cause the mineralization of organics [7,10,11]. TheBDD thin-film anodes are preferred for AO owing to their techno-logically important characteristics such as an inert surface withlow adsorption properties, remarkable corrosion stability even instrongly acidic media and extremely high O2-evolution overvolt-age. These properties enhance the organic removal with reactiveBDD(�OH) [2,12], making the BDD anode potent enough to miner-alize aromatic pollutants [1,2,5–16] and their generated carboxylicacids [17,18] with much higher oxidation power than other com-mon anodes such as Pt [8,19] and PbO2 [20].

N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167 159

EAOPs based on H2O2 generation have also received great atten-tion for water treatment. These processes involve the continuoussupply of H2O2 to an acidic contaminated solution from the two-electron reduction of O2 gas [3]:

O2ðgÞ þ 2Hþ þ 2e� ! H2O2 ð2Þ

Good efficiencies for H2O2 generation from reaction (2) havebeen found for carbonaceous cathodes like carbon nanotubes-poly-tetrafluoroethylene (PTFE) [21,22], carbon nanotubes on graphite[23], carbon–PTFE gas (O2 or air) diffusion [10,24–29], carbon felt[30–34], activated carbon fiber [35], carbon sponge [36] and BDD[37,38]. When an undivided cell is used, the anodic oxidation treat-ment is called AO with electrogenerated H2O2 (AO–H2O2), in whichorganics are degraded by M(�OH) and other weaker oxidants likeH2O2 and hydroperoxyl radical (HO�2) formed from its oxidationat the anode [3,28]:

H2O2 ! HO�2 þHþ þ e� ð3Þ

The efficiency of AO–H2O2 can be enhanced using electro-Fen-ton (EF) [3,4,10,24–38], where a catalytic amount of Fe2+ ion isadded to the solution to react with H2O2 giving homogeneous�OH and Fe3+ ion from Fenton’s reaction (4). This reaction is cata-lytic and can be mainly propagated by the cathodic reduction ofFe3+ to Fe2+ ion from reaction (5) [3]:

Fe2þ þH2O2 ! Fe3þ þ �OHþ OH� ð4Þ

Fe3þ þ e� ! Fe2þ ð5Þ

Thus, the EF process in an undivided cell with a BDD anode in-volves the attack of organics by heterogeneous BDD(�OH) formedfrom reaction (1) and by homogeneous �OH produced in the bulkfrom Fenton’s reaction (4).

Other powerful EAOP is photoelectro-Fenton (PEF) in which thecontaminated solution treated under EF conditions is also irradi-ated with an UVA light of kmax = 360 nm [24–26,28,29,39–41].The enhancement of the mineralization process by this radiationcan be explained by: (i) the photolysis of FeðOHÞþ2 , the predomi-nant species of Fe3+ ion in the pH range 2.5–5.0, to regenerateFe2+ and produce more �OH as follows:

FeðOHÞ2þ þ hm! Fe2þ þ �OH ð6Þ

and (ii) the photolysis of complexes of Fe(III) with generatedcarboxylic acids by reaction (7):

FeðOOCRÞ2þ þ hm! Fe2þ þ CO2 þ R� ð7Þ

s-Triazinic herbicides are used worldwide to protect crops fromundesirable grassy and broadleaf weeds. They have been detectedin soils and ground and surface waters due to their large stabilityunder environmental conditions [42–45]. s-Triazines have beenrecognized as carcinogenic to humans by The International Agencyfor the Research of Cancer and consequently, research efforts areneeded to develop powerful oxidation processes like EAOPs forthe removal of these pollutants from waters, thus avoiding theirpossible dangerous health effects on humans and animals.

The major part of studies related to the degradation of s-tria-zines in waters have been focused to the treatment of atrazine(2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine), whichis the most frequently used herbicide of this family [46]. However,it has been found that the application of chemical, photochemicaland photocatalytic AOPs only allows its transformation into cyan-uric acid (2,4,6-trihydroxy-1,3,5-triazine), since this derivativecannot be destroyed by homogeneous �OH produced in the solutionbulk [47]. In contrast, recent work [28,34] has shown that the useof EAOPs with a BDD anode yields almost total mineralization ofatrazine because cyanuric acid can be removed by heterogeneous

BDD(�OH) at the anode surface. To check if these methods can beviable to mineralize other s-triazinic compounds, we have studiedthe degradation behavior of cyanazine (2-(4-chloro-6-ethylamino-1,3,5-triazin-2-ylamino)-2-methylpropiononitrile) by AO–H2O2, EFand PEF. This s-triazine is widely used as a pre- and post-emer-gence herbicide for corn, sorghum, soybeans, alfalfa, cotton andwheat. It has been detected in American and European soils andrivers at relatively high contents of lg L�1 [42–45,48–50] and itstoxicity to some bacteria and algae has been well proven [51–53]. The degradation of cyanazine by O3/UV and H2O2/UV [54]and by chemicals like free chlorine and permanganate [55] hasbeen described, only considering its removal rate but withoutdetection of oxidation products.

In this paper, we present the results obtained for the compara-tive treatment of acidic cyanazine solutions by means of AO–H2O2, EF and PEF. All experiments were made using an undividedBDD/O2-diffusion cell and the oxidation power of EAOPs was exam-ined from the effects of current density and herbicide concentrationon the mineralization rate and mineralization current efficiency(MCE). The decay of cyanazine and the time course of its oxidationintermediates, like heteroaromatics, generated carboxylic acids andreleased inorganic ions, were followed by several chromatographictechniques. From detected products, a reaction sequence has beenproposed for cyanazine mineralization by the EAOPs tested.

2. Experimental

2.1. Chemicals

Cyanazine and four of its possible oxidation products like deiso-propylatrazine, desethyldeisopropylatrazine, ammeline (dese-thyldeisopropylhydroxy atrazine) and cyanuric acid were ofreagent grade supplied by Sigma and used as received. Formicand oxamic acids were of reagent grade purchased from Panreac.Anhydrous sodium sulfate, used as background electrolyte, and fer-rous sulfate heptahydrate, used as catalyst, were of analyticalgrade supplied by Merck and Fluka, respectively. Solutions wereprepared with ultra pure water obtained from a Millipore Milli-Qsystem with resistivity >18 MX cm at 25 �C. Their initial pH wasadjusted to 3.0 using analytical grade sulfuric acid from Merck. Or-ganic solvents and other chemicals used were of either HPLC oranalytical grade supplied by Aldrich, Lancaster, Merck and Panreac.

2.2. Electrolytic systems

All electrolyses were conducted in an open and undivided cylin-drical glass cell of 150 ml capacity with a double jacket for circula-tion of external thermostated water to regulate the solutiontemperature. The anode was a BDD thin-film electrode purchasedfrom Adamant Technologies (La-Chaux-de-Fonds, Switzerland)and synthesized by the hot filament chemical vapor depositiontechnique on single-crystal p-type Si(100) wafers (0.1 X cm, Sil-tronix). The cathode was a carbon–PTFE O2-diffusion electrodesupplied by E-TEK (Somerset, NJ, USA), mounted as described else-where [56]. It was fed with pure O2 at 12 ml min�1 for continuousH2O2 generation from reaction (2). The geometric area of both elec-trodes in contact with the solution was 3 cm2 and the interelec-trode gap was about 1 cm. All the experiments were performedat constant current density (j) provided by an Amel 2053 potentio-stat–galvanostat. Before the use of both electrodes in the electro-lytic assays, they were polarized in 100 ml of 0.05 M Na2SO4 at100 mA cm�2 for 180 min to remove the impurities of the BDD an-ode surface and activate the O2-diffusion cathode.

Comparative degradations of 100 ml of cyanazine solutions in0.05 M Na2SO4 of pH 3.0 were performed by AO–H2O2, EF and

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CE

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Fig. 1. (a) TOC abatement and (b) mineralization current efficiency calculated fromEq. (9) vs. specific charge for the degradation of 100 ml of a 110 mg l�1 cyanazinesolution in 0.05 M Na2SO4 of pH 3.0 using an open and undivided cell with a 3 cm2

boron-doped diamond (BDD) anode and a 3 cm2 O2-diffusion cathode at100 mA cm�2 and 35 �C. (�)Anodic oxidation with electrogenerated H2O2 (AO–H2O2), (h) electro-Fenton (EF) with 0.5 mM Fe2+ and (M) photoelectro-Fenton (PEF)with 0.5 mM Fe2+ under 6 W UVA irradiation of kmax = 360 nm.

160 N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167

PEF. For the two latter EAOPs, 0.5 mM Fe2+ was also added to thetreated solution as catalyst. This solution pH and Fe2+ concentrationwere chosen because they were found optimal for analogous degra-dations of other aromatics [28,29,39–41]. The effect of j in the range33.3–150 mA cm�2 and cyanazine content between 55 and145 mg l�1 on the oxidation power of each treatment was exam-ined. The solution was kept at 35 �C, which was the maximum tem-perature that can be used in the cell without significant waterevaporation during prolonged electrolysis [5]. The solution was al-ways vigorously stirred with a magnetic bar at 800 rpm to ensuremixing and the transport of reactants towards/from the electrodes.For the PEF process, a Philips TL/6 W/08 fluorescent black light bluetube placed at 7 cm above the solution was employed. The tubeemitted UVA light in the wavelength region 320–400 nm withkmax = 360 nm, yielding a photoionization energy of 5 W m�2 as de-tected with a Kipp & Zonen CUV 5 radiometer. The cyanazine con-tent did not vary when the electrodes were introduced in theinitial solution without passing current, indicating that it was notadsorbed on the carbon–PTFE O2-diffusion cathode.

2.3. Instruments and product analysis procedures

The solution pH was measured with a Crison GLP 22 pH-meter.Samples were withdrawn at regular time intervals from the treatedsolution, then alkalinized to stop the degradation process andmicrofiltered with 0.45 lm PTFE filters from Whatman beforeanalysis. The mineralization of solutions was monitored from theirtotal organic carbon (TOC) abatement using a Shimadzu VCSN TOCanalyzer. Reproducible TOC values with ±1% precision were foundby injecting 50 ll aliquots into the TOC analyzer.

Cyanazine removal and the evolution of its heteroaromatic oxi-dation products were followed by reversed-phase HPLC using aWaters 600 LC fitted with a Spherisorb ODS 5 lm,150 mm � 4.6 mm, column at room temperature, coupled to aWaters 996 photodiode array detector set at k = 240 nm. Theseanalyses were made with a 40:60 (v/v) acetonitrile/water (phos-phate buffer of pH 3) mixture at 0.6 ml min�1 as mobile phase,showing well-defined peaks for cyanazine (retention time(tr) = 8.03 min), deisopropylatrazine (tr = 3.66 min), desethyldeiso-propylatrazine (tr = 2.83 min), ammeline (tr = 2.34 min) and cyan-uric acid (tr = 2.21 min). Generated carboxylic acids wereidentified by ion-exclusion HPLC using the above LC fitted with aBio-Rad Aminex HPX 87H, 300 mm � 7.8 mm, column at 35 �C,the photodiode array detector selected at k = 210 nm and a mobilephase of 4 mM H2SO4 at 0.6 ml min�1. These chromatograms onlydisplayed peaks related to oxamic (tr = 11.7 min) and formic(tr = 13.4 min) acids.

Inorganic ions were quantified by ionic chromatography using aShimadzu 10 Avp HPLC coupled to a Shimadzu CDD 10 Avp con-ductivity detector. A Shodex IC YK-421, 125 mm � 4.6 mm, cationcolumn under circulation of a solution with 5.0 mM tartaric acid,2.0 mM dipicolinic acid, 24.2 mM boric acid and 15.0 mM crownether at 1.0 ml min�1 and 40 �C as mobile phase was employedto quantify NHþ4 ion (tr = 8.27 min). A Shim-Pack IC-A1S,100 mm � 4.6 mm, anion column and 1.0 mM p-hydroxybenzoicacid and 1.1 mM N,N-diethylethanolamine of pH = 7.9 at1.5 ml min�1 and 40 �C as mobile phase were used for the mea-surements of Cl� (tr = 2.17 min) and NO�3 (tr = 3.73 min) ions.

3. Results and discussion

3.1. Comparative degradation of cyanazine by EAOPs

The comparative oxidation power of the methods tested wasclarified by electrolyzing 110 mg l�1 cyanazine solutions (corre-sponding to 50 mg l�1 TOC) in 0.05 M Na2SO4 in the absence and

presence of 0.5 mM Fe2+ at pH 3.0, 100 mA cm�2 and 35 �C. In thesetrials performed for 480 min, the solutions remained colorless andtheir pH remained practically unchanged up to the end of electrol-yses. Fig. 1a depicts the TOC abatement found as a function of spe-cific charge (Q, in Ah l�1). As can be seen, TOC decayed morerapidly up to the consumption of 12 Ah l�1 (240 min) in all pro-cesses, whereupon its removal was gradually inhibited up to24 Ah l�1 (480 min). These results also evidence an increase inthe relative oxidation power of the EAOPs in the order AO–H2O2 < -EF < PEF. The AO–H2O2 treatment only allowed 90% mineralizationat 24 Ah l�1. Its relatively slower mineralization ability can be ac-counted for by the hard removal of organic matter mainly withBDD(�OH) formed from reaction (1), along with a much smallerparticipation of other weaker oxidants such as H2O2 and HO�2 pro-duced from reactions (2) and (3), respectively. In contrast, thecyanazine solution was more quickly degraded by EF attaining95% mineralization as a result of the quicker parallel destructionof organics with �OH generated from Fenton’s reaction (4). The syn-ergistic action of UVA irradiation in PEF enhanced the TOC abate-ment of the herbicide solution allowing reaching an almost totalmineralization with 98% TOC removal. The higher oxidation powerof PEF can then be associated with the additional action of reaction(6) and photolysis of some intermediates, like carboxylic acidsfrom reaction (7).

Note that the incineration of cyanazine to CO2 involves the re-lease of Cl� and NO�3 as major primary ions, as will be discussed be-low. Based on these considerations, its overall mineralizationreaction can be expressed as follows:

C9H13ClN6 þ 36H2O! 9CO2 þ Cl� þ 6NO�3 þ 85Hþ þ 78e� ð8Þ

The mineralization current efficiency for each assay at current I(A) and time t (h) was then estimated from the following equation[39]:

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Fig. 2. Influence of current density on TOC removal with (a) electrolysis time and(b) specific charge for the PEF treatment of 100 ml of a 110 mg l�1 cyanazinesolution in 0.05 M Na2SO4 and 0.5 mM Fe2+ of pH 3.0 at 35 �C. Current density: (d)33.3 mA cm�2, (M) 100 mA cm�2 and (�) 150 mA cm�2. Plot (c) presents thecorresponding mineralization current efficiency.

N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167 161

MCEð%Þ ¼nFVsDðTOCÞexp � 100

4:32� 107mItð9Þ

where n is the number of electrons consumed per molecule (78electrons from reaction (8)), F is the Faraday constant(96,487 C mol�1), Vs is the solution volume (l), D(TOC)exp is theexperimental TOC decay (mg l�1), 4.32 � 107 is an homogenizationfactor (3600 s h�1 � 12,000 mg mol�1) and m is the number of car-bon atoms of cyanazine (nine atoms).

Fig. 1b highlights the MCE values calculated from Eq. (9) for theassays of Fig. 1b. As expected, the efficiency drops in the sequencePEF > EF > AO–H2O2, in agreement with the relative oxidationpower of these EAOPs. Decreasing maximum MCE values of 8.1%for PEF, 7.2% for EF and 6.6% for AO–H2O2 can be observed at6 Ah l�1 (120 min), which further decayed slowly to 3.6–3.9% atthe end of all treatments. This trend suggests an initial fast conver-sion of several intermediates into CO2 and the formation of morepersistent oxidation products that are slowly removed with pro-longing electrolysis. This latter fact along with the presence of lessorganic matter in solution [2] could explain the slower TOC re-moval and lower efficiency found at long electrolysis time (seeFig. 1a and b).

3.2. Effect of current density and herbicide concentration on TOC decayand MCE

The applied current density is a key parameter that affects theproduction of hydroxyl radicals and hence, the oxidation abilityof EAOPs tested [2,3]. This influence was examined for 110 mg l�1

cyanazine solutions at 33.3, 100 and 150 mA cm�2. As an example,Fig. 2a shows the TOC removal with electrolysis time for these tri-als in the case of the more powerful PEF treatment. A gradual in-crease in degradation rate with increasing j can be observed upto 240 min, but at longer times TOC dropped in a quite similarway for 100 and 150 mA cm�2 due to the low amount of organicmatter present in solution in both cases. At 480 min, the solutiononly reached 88% TOC decay for 33.3 mA cm�2, whereas it was al-most totally mineralized with ca. 98% TOC reduction at both higherj values of 100 and 150 mA cm�2.

A similar increase in oxidation power with raising j was alsofound for the AO–H2O2 and EF treatments. This can be deducedfrom the percentage of TOC removal at the different j values testedafter 240 min of electrolysis collected in Table 1. As can be seen, at33.3 mA cm�2 the mineralization degree only rose slightly from37% for AO–H2O2 to 45–47% for EF and PEF. This suggests that inthe two latter EAOPs, the removal of organics by �OH only corre-sponds to about 27% of their destruction with BDD(�OH), which isthen the main oxidant, and that the action of UVA light on the min-eralization of the small concentration of intermediates formed inPEF at 33.3 mA cm�2 is insignificant. The high increase up to 66%TOC decay for AO–H2O2 determined at 100 mA cm�2 can then berelated to the quicker oxidation of recalcitrant intermediates bygreater quantities of BDD(�OH) generated from acceleration ofreaction (1). Nevertheless, a much higher mineralization of 82%was obtained for EF at this j, indicating a larger production of�OH from Fenton’s reaction (4) as a result of the concomitant elec-trogeneration of more H2O2 from reaction (2) [25,26]. The higheroxidation power of the comparative PEF process with 90% TOC de-cay can be accounted for by the more rapid photolysis by UVA lightof the higher contents of intermediates originated from the greatergeneration of both, BDD(�OH) and �OH at 100 mA cm�2, which canalso be enhanced from the photocatalytic reaction (6). However,Table 1 shows a very small increase in TOC decay when j rises from100 to 150 mA cm�2 for all EAOPs, suggesting that the process iscontrolled by mass transfer of intermediates towards the anodeand that the increase in j accelerates the waste reactions of

hydroxyl radicals. These reactions involve, for example, the anodicoxidation of BDD(�OH) by reaction (10), the dimerization of �OH byreaction (11) and its reaction with H2O2 by reaction (12) or withFe2+ ion by reaction (13), which can compete efficiently with loworganic concentration [2,3,5]. The relative proportion of BDD(�OH)formed can also be diminished by the production of more amountsof weaker oxidants at the anode, like S2O2�

8 ion from SO2�4 ion of the

background electrolyte by reaction (14) and O3 by reaction (15)[2,7].

2BDDð�OHÞ ! 2BDDþ O2ðgÞ þ 2Hþ þ 2e� ð10Þ

2�OH! H2O2 ð11Þ

H2O2 þ �OH! HO�2 þH2O ð12Þ

Fe2þ þ �OH! Fe3þ þ OH� ð13Þ

2SO�4 ! S2O2�8 þ 2e� ð14Þ

3H2O! O3 þ 6Hþ þ 6e� ð15Þ

Table 1Percentage of TOC removal and mineralization current efficiency obtained after 240 min of degradation of 100 ml of cyanazine solutions in 0.05 M Na2SO4 at pH 3.0 and 35 �C bydifferent electrochemical advanced oxidation processes using a BDD/O2-diffusion cell of 3 cm2 electrode area under selected experimental conditions.

Method Cyanazine (mg l�1) j (mA cm�2) % TOC removal % MCE

OA-H2O2a 55 100 68 2.8

110 33.3 37 8.9100 66 5.2150 69 3.7

145 100 59 6.1

EFb 55 100 87 3.5110 33.3 47 11.3

100 82 6.6150 87 4.7

145 100 78 8.1

PEFc 55 100 91 3.7110 33.3 45 10.8

100 90 7.2150 93 5.0

145 100 85 8.8

a Anodic oxidation with electrogenerated H2O2.b Electro-Fenton with 0.5 mM Fe2+.c Photoelectro-Fenton with 0.5 mM Fe2+ and 6 W UVA light of kmax = 360 nm.

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Fig. 3. Effect of cyanazine content on (a) TOC decay and (b) mineralization currentefficiency for the PEF degradation of 100 ml of herbicide solutions in 0.05 M Na2SO4

and 0.5 mM Fe2+ at pH 3.0, 100 mA cm�2 and 35 �C. Cyanazine concentration: (.)55 mg l�1, (4) 110 mg l�1 and (j) 145 mg l�1.

162 N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167

The action of these reactions is evidenced in Fig. 2b, where aprogressive spent of more Q to remove a similar TOC can be ob-served from 33.3 to 150 mA cm�2. That means that the increasein j consumes more applied specific charge for mineralizationdue to the acceleration of parasitic reactions like (10)–(15), therebyyielding less relative organic oxidation events. This fact can alsojustify the gradual loss in MCE for all EAOPs when j rises, as shownin Table 1. The MCE-Q plots obtained for the PEF experiments ofFig. 2b are presented in Fig. 2c. Decreasing efficiencies at higher jvalues can be observed, more remarkable when passing from33.3 to 100 mA cm�2, owing to the production of smaller relativeamounts of oxidants BDD(�OH) and �OH because of the concomitantrise in rate of their waste reactions. The application of the lower j of33.3 mA cm�2 yielded a practically constant MCE value between9.4% and 11.2%, indicating that organics are mineralized at almostconstant rate during all the electrolysis time by the low productionof hydroxyl radicals. For 100 and 150 mA cm�2, however, maxi-mum efficiencies close to 8% were found at the beginning of elec-trolysis, followed by a dramatic drop in MCE due to the loss inorganic matter and the formation of more persistent intermediates.

The oxidation power of EAOPs was also tested by studying theeffect of cyanazine content on their degradation rate and MCE.An inspection of Table 1 allows concluding that after 240 min ofall EAOPs at 100 mA cm�2, the percentage of TOC removal de-creased slightly when the herbicide content varied from 55 to145 mg l�1, but with higher MCE values. This behavior can be bet-ter explained from the changes in TOC removal and MEC for thePEF trials depicted in Fig. 3a and b, respectively. Fig. 3a shows thatthe solution of 55 mg l�1 cyanazine attained an almost total miner-alization (>95% TOC decay) at 18 Ah l�1 (360 min), whereas a high-er Q of ca. 24 Ah l�1 (480 min) was needed for 110 and 145 mg l�1,as expected from the decay in mineralization degree. At a giventime, however, higher quantity of TOC was removed in the pres-ence of more cyanazine. For example, at 12 Ah l�1 (240 min), 22,45 and 55 mg l�1 of TOC are mineralized for 55, 110 and 145 mg l�1

of cyanazine, respectively. Accordingly, the efficiency progressivelyincreased, as can be observed in Fig. 3b. For all concentrations, themaximum efficiency was found to about 6 Ah l�1 (120 min), whenthe most oxidizable products were mineralized, remaining in solu-tion intermediates that are more slowly destroyed with hydroxylradicals and UVA irradiation giving rise to a further decay inMCE, which is also enhanced by the presence of less organic mat-ter. The increase in efficiency at higher cyanazine content for thesame j in all EAOPs points to a faster removal of organics by higher

amounts of BDD(�OH) and �OH, which is possible if their parasiticreactions (10)–(15) are decelerated since both radicals react in lar-ger extent with the higher amounts of organics present in solution.

3.3. Decay kinetics for cyanazine

The reaction of cyanazine with electrogenerated oxidants(mainly BDD(�OH) and �OH) in all the EAOPs tested was analyzedby reversed-phase HPLC. The action of these agents was confirmedfrom the insignificant removal found for 110 mg l�1 of this herbi-cide in a solution of pH 3.0 with and without 20 mM H2O2 underUVA irradiation for 60 min. This evidences that cyanazine is not

0

20

40

60

80

100

120[C

yana

zine

] / m

g l -1

0

1

2

3

4

0 10 20 30 40 50 60 70

ln (

C /

C )

0

Time / min

a

b

Fig. 4. (a) Decay of cyanazine concentration with electrolysis time for thedegradation of 100 ml of a 110 mg l�1 herbicide solution in 0.05 M Na2SO4 of pH3.0 at 100 mA cm�2 and 35 �C. (s) AO–H2O2, (h) EF and (4) PEF with a 6 W UVAirradiation of kmax = 360 nm. For the two latter methods, the solution contained0.5 mM Fe2+ as catalyst. (b) Kinetic analysis considering a pseudo-first-orderreaction for cyanazine.

N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167 163

directly photolyzed and can only be attacked by the differenthydroxyl radicals generated in the EAOPs.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 60 120 180 240 300 360 42

0.0

0.3

0.6

0.9

1.2

0 30 60 90 120 150

a

b

Time

Con

cent

rati

on /

mg

l-1

Fig. 5. Time-course of the heteroaromatic products detected during the treatment of 10035 �C by (s) AO–H2O2, (h) EF with 0.5 mM Fe2+ and (4) PEF with 0.5 mM Fe2+. Compoucyanuric acid.

The abatement of cyanazine concentration at pH 3.0 and100 mA cm�2 for all treatments is depicted in Fig. 4a. As can beseen, the oxidation of the herbicide with BDD(�OH) in AO–H2O2

was very fast and its concentration was reduced by 91% in60 min. A quicker disappearance of cyanazine can be observedusing the EF process, leading to 95% reduction of its content atthe same time. This enhancement can be explained by its parallelreaction with �OH produced in the bulk from Fenton’s reaction(4). The PEF treatment destroyed much more rapidly the herbicideup to its total disappearance in ca. 40 min, indicating a very posi-tive oxidation with additional �OH generated from the photocata-lytic reaction (6) since it is not directly photolyzed by UVA light,as pointed out above. Comparison of Figs. 1 and 4 evidences thatthe mineralization process for EAOPs needs a much longer timethan the corresponding removal of cyanazine, suggesting the for-mation of intermediates that are hardly attacked by both BDD(�OH)and �OH.

The above concentration decays were well-fitted to a pseudofirst-order kinetic equation, as can be seen in Fig. 4b. From thisanalysis, a pseudo first-order rate constant (k1) of 6.6 � 10�4 s�1

(square regression coefficient (R2) = 0.996) for AO–H2O2,8.0 � 10�4 s�1 (R2 = 0.995) for EF and 1.6 � 10�3 s�1 (R2 = 0.997)for PEF was determined. This suggests a constant production ofoxidant BDD(�OH) from reaction (1) in AO–H2O2, along with a con-stant generation of �OH from Fenton’s reaction (4) in EF and underthe additional action of reaction (6) in PEF.

3.4. Time course of heteroaromatic intermediates, generatedcarboxylic acids and released inorganic ions

Reversed-phase HPLC of the above electrolyzed solutions exhib-ited additional peaks to that of cyanazine. Taking into account theproducts detected for other s-triazines [28,29,36], these peakswere related to heteroaromatics like deisopropylatrazine,

0

0

1

2

3

4

5

6

7

0 60 120 180 240 300 360 420 480

c

d

/ min

0

10

20

30

40

50

0 60 120 180 240 300 360 420

ml of a 110 mg l�1 cyanazine solution in 0.05 M Na2SO4 at pH 3.0, 100 mA cm�2 andnds: (a) Deisopropylatrazine, (b) desethyldeisopropylatrazine, (c) ammeline and (d)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 60 120 180 240 300 360 420 480

a

b

Time / min

Con

cent

rati

on /

mg

l-1

Fig. 6. Evolution of (a) formic and (b) oxamic acids under the same conditions ofFig. 5. (s) AO–H2O2, (h) EF and (4) PEF.

0

2

4

6

8

10

12

14

[Cl

] / m

g l

−-1

0

5

10

15

20

[NO

3− ] / m

g l-1

0.0

0.5

1.0

1.5

2.0

2.5

0 60 120 180 240 300 360 420 480

Time / min

[NH

4+]

/ mg

l-1

a

b

c

Fig. 7. Time-course of (a) chloride, (b) nitrate and (c) ammonium ions releasedduring the treatments shown in Fig. 5. (s) AO–H2O2, (h) EF and (4) PEF.

164 N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167

desethyldeisopropylatrazine, ammeline and cyanuric acid, formedfrom dealkylation, deamination, dechlorination and/or hydroxyl-ation reactions involving the generated hydroxyl radicals. Thepresence of these derivatives was confirmed in the reversed-phasechromatograms by comparing their UV–Vis spectra, measured onthe diode array detector, and retention times with those of purecompounds. Their concentration in each EAOP was then obtainedvia calibration with external standards.

Fig. 5a–d illustrates the evolution of the detected heteroaromat-ics for the treatments of 110 mg l�1 cyanazine solutions at pH 3.0and 100 mA cm�2. Fig. 5a shows that small concentrations ofdeisopropylatrazine were only accumulated in AO–H2O2, reachinga maximal near 1 mg l�1 at about 90 min of electrolysis and disap-pearing in 120 min. That means that this heteroaromatic is directlyproduced from cyanazine and rapidly removed with BDD(�OH),although it is much more quickly destroyed by �OH, reason forwhich it was not detected using EF and PEF. Low contents of itsderivative desethyldeisopropylatrazine were also determined inall EAOPs, as can be observed in Fig. 5b. For this compound, max-imum contents near 1.16 mg l�1 at 120 min of AO–H2O2,1.14 mg l�1 at 60 min of EF and 0.68 mg l�1 at 60 min of PEF werefound. This trend can be related to the oxidation action of the dif-ferent generated hydroxyl radicals. Thus, in AO–H2O2, desethylde-isopropylatrazine is slowly formed and destroyed by BDD(�OH),persisting during 360 min. In contrast, the much rapid parallel oxi-dation with �OH favors its quicker accumulation and total disap-pearance in 240 min using EF, which is even much more rapidlyremoved in PEF, in only 180 min, as a result of the higher produc-tion of �OH via the photocatalytic reaction (6).

Fig. 5c and d shows a very different behavior for ammeline andcyanuric acid, respectively, which are accumulated in much largerextent. Note that maximum accumulations were obtained atapproximately 120 min for ammeline and at 60 min for cyanuricacid, regardless of the EAOP tested. This is feasible if their evolutionis not affected by �OH production and consequently, both hetero-aromatics are only oxidized by BDD(�OH), as previously reportedfor cyanuric acid [28,34]. Nevertheless, decreasing maximum

concentrations of 6.2 mg l�1 for AO–H2O2, 3.0 mg l�1 for EF and1.9 mg l�1 for PEF were determined for ammeline (see Fig. 5c). Asimilar trend was also found for cyanuric acid, but with much high-er maximal concentration of 46, 42 and 34 mg l�1, respectively (seeFig. 5d), indicating that it is the main heteroaromatic formed in alltreatments. Accordingly, both compounds disappeared from solu-tion in the sequence PEF < EF < AO–H2O2 because longer timewas required to generate more amounts of BDD(�OH) for their oxi-dation. This behavior suggests that �OH formed in EF and in largerextent in PEF converts their precedent heteroaromatics into otherproducts, thereby diminishing the production of ammeline andcyanuric acid. Comparison of Fig. 5b–d allows inferring that foreach EAOP, desethyldeisopropylatrazine disappears at similar timeto cyanuric acid, but ammeline is much more persistent. This sug-gests that the former heteroaromatic can generate the two latterones by independent ways and that the oxidation of ammelinedoes not yield cyanuric acid, at least as predominant product.Cyanuric acid could then be rather formed from deisopropylatr-azine degradation involving deamination, dechlorination andhydroxylation reactions.

Ion-exclusion chromatograms recorded for the above electro-lyzed solutions only displayed well-defined peaks associated withformic (HCOOH) and oxamic (COOH-CO(NH2)) acids. Formic acidcan be formed from the oxidation of all carbon atoms, includingthose coming from the cleavage of the heteroaromatic ring,whereas the oxamic acid could proceed from the oxidation of theN–C bonds of heteroaromatic intermediates, as well as of the

Fig. 8. Proposed reaction sequence for the mineralization of cyanazine in acidic medium by anodic oxidation, electro-Fenton and photoelectro-Fenton with a BDD anode andan O2-diffusion cathode.

N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167 165

ethylamino group lost in the deamination of deisopropylatrazine.Both short-liner carboxylic acids are ultimate products since theyare directly mineralized to CO2 [25,39,40]. Note that in EF andPEF, they exist in the form of complexes of Fe(III), an ion largelygenerated from Fenton’s reaction (4).

Fig. 6a shows a rapid accumulation of formic acid at 60–90 minof all EAOPs, with contents lower than 1.3 mg l�1, followed by aslow decay to disappear in 360 min in EF and PEF and at longertime in AO–H2O2. The slightly quicker removal of Fe(III)-formatecomplexes in the two former treatments indicates that they aremore rapidly destroyed with hydroxyl radicals than the free acid.Fig. 6b highlights that oxamic acid was much more poorly accumu-lated, depending its concentration and persistence on the EAOP ap-plied. In AO–H2O2, it disappeared in 180 min after attaining0.051 mg l�1, whereas in EF it persisted up to 240 min with evenlower contents. In contrast, the PEF process yielded a much higher

accumulation of 0.12 mg l�1 oxamic acid at 180 min, whereupon itwas slowly removed to 0.025 mg l�1 at 420 min. The photolysis ofprecedent intermediates giving more oxamic acid under the actionof UVA irradiation could explain its large enhancement in PEF. Thelarge persistence of Fe(III)-oxamate complexes can be related to itslow reactivity with BDD(�OH) and slow photolysis by UVA light[40].

Note that after 420 min of electrolysis at 100 mA cm�2

(Q = 21 Ah l�1), all heteroaromatic derivatives (see Fig. 5) and car-boxylic acids (see Fig. 6) are removed from the solution or existin very small content, while its TOC is only reduced by 87% inAO–H2O2, 94% in EF and 96% in PEF (see Fig. 1a). This confirmsthe existence of other undetected intermediates still remainingin the electrolyzed solutions. One can then infer that the combinedaction of BDD(�OH), �OH and UVA light favors the mineralization ofthese intermediates enhancing the efficiency of PEF.

166 N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167

Cl�, NO�3 and NHþ4 ions formed during the above treatmentswere quantified by ionic chromatography. No other inorganicnitrogen ions like NO�2 and CN� were detected by this technique.Fig. 7a evidences a rapid dechlorination of cyanazine in all EAOPs,in agreement with the fast accumulation of cyanuric acid as themain heteroaromatic derivative (see Fig. 5d). However, the re-leased Cl� ion was slowly removed from the solution at similar ratein AO–H2O2 and EF but slightly more rapidly in PEF, due to its oxi-dation to Cl2 mainly with BDD(�OH) [2,3]. On the other hand, theinitial N of cyanazine (38.4 mg l�1) was progressively convertedinto NO�3 ion along with NHþ4 ion in lesser extent, although theirevolution was a function of the EAOP applied. As can be seen inFig. 7b and c, both ions were accumulated primordially duringthe first 180–240 min of all treatments, and after 420 min of elec-trolysis their contents in the medium were of 20.0 mg l�1 NO�3(11.8% of initial N) and 2.5 mg l�1 NHþ4 (5.1% of initial N) for AO–H2O2, 9.5 mg l�1 NO�3 (5.6% of initial N) and 1.6 mg l�1 NHþ4 (3.2%of initial N) for EF and 10 mg l�1 NO�3 (5.9% of initial N) and1.2 mg l�1 NHþ4 (2.4% of initial N) for PEF. Since only 16.9%, 8.8%and 8.4% of the initial N is converted into inorganic nitrogen ionsin AO–H2O2, EF and PEF, respectively, one can infer that N-deriva-tives are poorly mineralized with BDD(�OH) and even in less extentwith �OH, giving rise to by-products that are released from themedium, probably N2 and volatile NxOy species.

3.5. Proposed reaction sequence

On the basis of detected intermediates, the reaction sequence ofFig. 8 is proposed for the mineralization of cyanazine in acidicmedium by the EAOPs tested. The main oxidant is considered tobe the electrogenerated BDD(�OH) in AO–H2O2, along with �OH inEF and PEF, although parallel removal with other weaker oxidizingagents (H2O2, HO�2, O3, etc.) is also feasible. The UVA light in PEF en-hances the �OH production from photocatalytic reaction (6). Theprocess is initiated by the dealkylation of the methylpropiononitri-le group of cyanazine to yield deisopropylatrazine, which is subse-quently either dealkylated with loss of the ethyl group leading todesethyldeisopropylatrazine or deaminated and dechlorinatedwith hydroxylation giving cyanuric acid. These dechlorinationand deamination reactions lead to the release of Cl�, NO�3 andNHþ4 ions. Further oxidation of desethyldeisopropylatrazine alsoproduces cyanuric acid, whereas its direct hydroxylation with lossof Cl� ion gives ammeline. The degradation of ammeline and cyan-uric acid causes the opening of their heteroaromatic rings to yieldlinear compounds that evolve to formic and oxamic acids with re-lease of NO�3 and NHþ4 ions. These acids are directly mineralizedwith BDD(�OH) in AO–H2O2. While in EF Fe(III)-formate andFe(III)-oxamate species are formed and mainly destroyed withBDD(�OH), in PEF these complexes are also photolyzed by UVA lightwith loss of Fe2+ ion [3,40,41].

4. Conclusions

It has been demonstrated that EAOPs with a BDD/O2-diffusioncell are able to effectively mineralize cyanazine solutions of pH3.0 and then, can be viable for the decontamination of waters con-taining s-triazines. Almost total mineralization with 98% TOC decaywas achieved by PEF due to the combination of BDD(�OH) and �OH asoxidants with the photolytic action of UVA light. This EAOP is morepotent than EF because of the generation of more oxidant �OH fromphotolytic reaction (6) and the photolysis of intermediates underUVA irradiation. The lower oxidation power was attained usingAO–H2O2 owing to the lower production rate of BDD(�OH). For allEAOPs, the mineralization rate increased with increasing j anddecreasing herbicide content, but with the concomitant drop in

mineralization current efficiency by the acceleration of parasiticreactions of hydroxyl radicals. The decay kinetics for cyanazine al-ways followed a pseudo-first-order reaction with increasing appar-ent rate constants in the sequence AO–H2O2 < EF < PEF. Reversed-phase HPLC revealed the formation of heteroaromatic derivativeslike deisopropylatrazine, desethyldeisopropylatrazine, ammelineand cyanuric acid. The latter compound was the main intermediatein all EAOPs. Low contents of short-linear carboxylic acids like for-mic and oxamic were also detected by ion-exclusion HPLC. Cl�, NO�3and NHþ4 ions released during the degradation processes werequantified by ionic chromatography. Cl� ion was oxidized to Cl2

with BDD(�OH) and the initial N was mainly mineralized to NO�3ion, although its major part was lost as volatile N-derivatives. Aplausible reaction sequence for cyanazine mineralization involvingall detected products is proposed.

Acknowledgments

The authors thank the financial support from MICINN (Ministe-rio de Ciencia e Innovación, Spain) through project CTQ2010-16164/BQU co-financed with FEDER funds.

References

[1] C.A. Martínez-Huitle, S. Ferro, Chem. Soc. Rev. 35 (2006) 1324.[2] M. Panizza, G. Cerisola, Chem. Rev. 109 (2009) 6541.[3] E. Brillas, I. Sirés, M.A. Oturan, Chem. Rev. 109 (2009) 6570.[4] I. Sirés, E. Brillas, Environ. Int. 40 (2012) 212.[5] C. Flox, J.A. Garrido, R.M. Rodríguez, F. Centellas, P.L. Cabot, C. Arias, E. Brillas,

Electrochim. Acta 50 (2005) 3685.[6] A. Özcan, Y. S�ahin, A.S. Koparal, M.A. Oturan, Water Res. 42 (2008) 2889.[7] B. Marselli, J. García-Gomez, P.A. Michaud, M.A. Rodrigo, Ch. Comninellis, J.

Electrochem. Soc. 150 (2003) D79.[8] L. Liu, G. Zhao, M. Wu, Y. Lei, R. Geng, J. Hazard. Mater. 168 (2009) 179.[9] E. Brillas, S. Garcia-Segura, M. Skoumal, C. Arias, Chemosphere 79 (2010) 605.

[10] S. Ammar, R. Abdelhedi, C. Flox, C. Arias, E. Brillas, Environ. Chem. Lett. 4(2006) 229.

[11] G. Zhao, Y. Zhang, Y. Lei, B. Lv, J. Gao, Y. Zhang, D. Li, Environ. Sci. Technol. 44(2010) 1754.

[12] M. Panizza, G. Cerisola, Appl. Catal. B: Environ. 75 (2007) 95.[13] C. Flox, P.L. Cabot, F. Centellas, J.A. Garrido, R.M. Rodríguez, C. Arias, E. Brillas,

Chemosphere 64 (2006) 892.[14] C. Sáez, M. Panizza, M.A. Rodrigo, G. Cerisola, J. Chem. Technol. Biotechnol. 82

(2007) 575.[15] M. Klavarioti, D. Mantzavinos, D. Kassinos, Environ. Int. 35 (2009) 402.[16] V. Santos, J. Diogo, M.J.A. Pacheco, L. Ciríaco, A. Morão, A. Lopes, Chemosphere

79 (2010) 637.[17] P. Cañizares, J. García-Gómez, J. Lobato, M.A. Rodrigo, Ind. Eng. Chem. Res. 42

(2003) 956.[18] P. Cañizares, R. Paz, C. Sáez, M.A. Rodrigo, Electrochim. Acta 53 (2008) 2144.[19] M. Hamza, R. Abdelhedi, E. Brillas, I. Sirés, J. Electroanal. Chem. 627 (2009) 41.[20] C. Flox, C. Arias, E. Brillas, A. Savall, K. Groenen-Serrano, Chemosphere 74

(2009) 1340.[21] M. Zarei, A.R. Khataee, R. Ordikhani-Seyedlar, M. Fathinia, Electrochim. Acta 55

(2010) 7259.[22] M. Iranifam, M. Zarei, A.R. Khataee, J. Electroanal. Chem. 659 (2011) 107.[23] A.R. Khataee, M. Safarpour, M. Zarei, S. Aber, J. Electroanal. Chem. 659 (2011)

63.[24] B. Boye, M.M. Dieng, E. Brillas, Electrochim. Acta 48 (2003) 781.[25] I. Sirés, F. Centellas, J.A. Garrido, R.M. Rodríguez, C. Arias, P.L. Cabot, E. Brillas,

Appl. Catal. B: Environ. 72 (2007) 373.[26] C. Flox, J.A. Garrido, R.M. Rodríguez, P.L. Cabot, F. Centellas, C. Arias, E. Brillas,

Catal. Today 129 (2007) 29.[27] M. Panizza, G. Cerisola, Water Res. 43 (2009) 339.[28] N. Borràs, R. Oliver, C. Arias, E. Brillas, J. Phys. Chem. A 114 (2010) 6613.[29] N. Borràs, C. Arias, R. Oliver, E. Brillas, Chemosphere 85 (2011) 1167.[30] S. Hammami, N. Oturan, N. Bellakhal, M. Dachraoui, M.A. Oturan, J. Electroanal.

Chem. 610 (2007) 75.[31] N. Oturan, M. Panizza, M.A. Oturan, J. Phys. Chem. A 113 (2009) 10988.[32] I. Sirés, N. Oturan, M.A. Oturan, Water Res. 44 (2010) 3109.[33] M.A. Oturan, N. Oturan, M.C. Edelahi, F.I. Podvorica, K. El Kacemi, Chem. Eng. J.

171 (2011) 127.[34] N. Oturan, E. Brillas, M.A. Oturan, Environ. Chem. Lett. 10 (2012) 165.[35] A. Wang, J. Qu, H. Liu, J. Ru, Appl. Catal. B: Environ. 84 (2008) 393.[36] A. Özcan, Y. Sahin, A.S. Koparal, M.A. Oturan, J. Electroanal. Chem. 616 (2008)

71.[37] K. Cruz-González, O. Torres-López, A. García-León, J.L. Guzmán-Mar, L.H.

Reyes, A. Hernández-Ramírez, J.M. Peralta-Hernández, Chem. Eng. J. 160(2010) 199.

N. Borràs et al. / Journal of Electroanalytical Chemistry 689 (2013) 158–167 167

[38] K. Cruz-González, O. Torres-López, A. García-León, E. Brillas, A. Hernández-Ramírez, J.M. Peralta-Hernández, Desalination 286 (2012) 63.

[39] E. Isarain-Chávez, C. Arias, P.L. Cabot, F. Centellas, R.M. Rodríguez, J.A. Garrido,E. Brillas, Appl. Catal. B: Environ. 96 (2010) 361.

[40] E.J. Ruiz, C. Arias, E. Brillas, A. Hernández-Ramírez, J.M. Peralta-Hernández,Chemosphere 82 (2011) 495.

[41] L.C. Almeida, S. Garcia-Segura, N. Bocchi, E. Brillas, Appl. Catal. B: Environ. 103(2011) 21.

[42] M.J. Homes, J.R. Frankenberger, B.A. Engel, J. Am. Water Resour. Assoc. 37(2001) 987.

[43] M.A. Locke, K.N. Reddy, L.A. Gaston, R.M. Zablotowicz, Soil Sci. 167 (2002) 444.[44] M. Waria, S.D. Comfort, S. Onanong, T. Satapanajaru, H. Boparai, C. Harris, D.D.

Snow, D.A. Cassada, J. Environ. Qual. 38 (2009) 1803.[45] A. Daneshvar, K. Aboulfadl, L. Viglino, R. Broseus, S. Sauve, A.S. Madoux-

Humery, G.A. Weyhenmeyer, M. Prevost, Chemosphere 88 (2012) 131.[46] B. Balci, N. Oturan, R. Cherrier, M.A. Oturan, Water Res. 43 (2009) 1924.[47] G. Zhangi, Y. Shaogui, T. Na, T.N. Sun, J. Hazard. Mater. 145 (2007) 424.

[48] J.L. Hatfield, D.B. Jaynes, M.R. Burkart, C.A. Cambardella, T.B. Moorman, J.H.Prueger, M.A. Smith, J. Environ. Qual. 28 (1999) 11.

[49] S. Usenko, D.H. Landers, P.G. Appleby, S.L. Simonich, Environ. Sci. Technol. 41(2007) 7235.

[50] P.C. Von der Ohe, V. Dulio, J. Slobodnik, E. De Deckere, R. Kuehne, R.U. Ebert, A.Ginebreda, W. De Cooman, G. Schueuermann, W. Brack, Sci. Total Environ. 409(2011) 2064.

[51] P.Y. Caux, L. Menard, R.A. Kent, Environ. Pollut. 92 (1996) 219.[52] J. Ma, W. Liang, Bull. Environ. Contam. Toxicol. 67 (2001) 347.[53] J. Ma, S. Tong, P. Wang, J. Chen, Int. J. Environ. Res. 4 (2010) 347.[54] F.J. Benitez, J. Beltran-Heredia, T. Gonzalez, J.L. Acero, Ozone: Sci. Eng. 17

(1995) 237.[55] E. Chamberlain, H. Shi, T. Wang, Y. Ma, A. Fulmer, C. Adams, J. Agric. Food

Chem. 60 (2012) 354.[56] E. Brillas, M.A. Baños, S. Camps, C. Arias, P.L. Cabot, J.A. Garrido, R.M. Rodríguez,

New J. Chem. 28 (2004) 314.