Journal of Electroanalytical Chemistry 738 (2015) 84–91

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Journal of Electroanalytical Chemistry

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Electrochemical promotion of strong oxidants to degrade Acid Red 211:Effect of supporting electrolytes

http://dx.doi.org/10.1016/j.jelechem.2014.11.0301572-6657/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +52 01 477 7100011x1528.E-mail address: jperalta@ciatec.mx (J.M. Peralta Hernández).URL: http://www.ciatec.mx.

Aldo Uranga-Flores a, Catalina de la Rosa-Júarez a, Silvia Gutierrez-Granados b,Dayanne Chianca de Moura c, Carlos A. Martínez-Huitle c, Juan M. Peralta Hernández a,⇑a Centro de Innovación Aplicada en Tecnologías Competitivas (CIATEC), Departamento de Investigación y Posgrado, Omega-201, Fraccionamiento Industrial Delta, León37545, Guanajuato, Mexicob Universidad de Guanajuato, Departamento de Química, Cerro de la Venada S/N, Pueblito de Rocha, Guanajuato, Gto. CP. 36040, Mexicoc Universidade Federal do Rio Grande do Norte, Instituto de Química, Lagoa Nova CEP 59078-970, Natal, RN, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 September 2014Received in revised form 18 November 2014Accepted 19 November 2014Available online 27 November 2014

Keywords:Electro-oxidationElectro-FentonWastewater treatmentSupporting electrolyteOxidants

Solutions of the synthetic dye Acid Red 211 (AR211), commonly used in many industries, were compar-atively degraded using different supporting electrolytes under current-controlled electrolysis conditions.The efficiency of the electro-oxidation (EOx) approach with boron doped diamond electrodes (EOx/BDD)was examined by applying 100, 200, 300 and 400 mA with HClO4, H2SO4 or Na2SO4 as a supportingelectrolyte. Under similar experimental conditions, a diamond electro-Fenton (EF/BDD) system was usedfor treating synthetic dye solutions and was compared with the removal efficiencies obtained by EOx/BDD. The results clearly showed that when AR211 was oxidized by employing the EF/BDD process withgeneration of free �OH from the electrogeneration production of hydrogen peroxide (H2O2) and additionof Fe2+, the oxidizing power increased to 97% under the best conditions, with higher current efficienciesand lower energy consumption than those obtained by EOx/BDD. In this study, we also attempt to discusscritical evidence regarding the viability of the use of different supporting electrolytes because the resultsobtained in this research clearly demonstrated that the effect of the electrogenerated strong oxidantspecies depends on parallel mechanisms followed on the BDD surface, as well as the particularproduction of strong oxidants, such as the case of BDD(�OH) or BDDðSO��4 Þ.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Worldwide, different industries produce a large quantity ofwastewater that is highly colored and contains high levels oforganic matter [1]. The use of large quantities of dyestuffs duringthe dyeing stages of leather manufacturing processes is a causeof such pollution that causes major aesthetic and environmentalproblems [2,3].

Technologies such as biological, physical and chemicalprocesses have been carried out for organic matter and colorreduction [4]. However, most disposed synthetic dyes are of anon-biodegradable nature, and direct biological treatment of thecolored effluents is not effective [2,4]. Therefore, it is necessaryto propose effective techniques to treat these effluents, both interms of limited water resource management and the need for

environment preservation. Among these techniques, so-calledadvanced oxidation processes (AOPs) are powerful treatmenttechnologies which generate highly reactive free hydroxyl radicals(�OH) able to mineralize several toxic and non-biologicallydegradable compounds [5,6]. Nevertheless, in recent years, electro-chemical advanced oxidation processes (EAOPs) have been consid-ered promising technologies which offer favorable approaches forthe prevention of pollution problems from industrial effluents[2,3,7–12]. The most important EAOP is electrochemical oxidation(EOx), where organics are oxidized via �OH produced from waterdischarge at the surface [9–11,13]. The anodes (with a high O2-overpotential) principally used for this electrochemical approachare PbO2, SnO2 and, more recently, boron doped diamondelectrodes (BDD) [9–11], according reaction (1). BDDs are consid-ered the best anodes for EOx due to their wide electrochemicalwindow, low adsorption ability, large chemical stability and higherO2-overpotential than other electrodes [13].

BDDþH2O! BDDð�OHÞ þHþ þ e� ð1Þ

A. Uranga-Flores et al. / Journal of Electroanalytical Chemistry 738 (2015) 84–91 85

Thanks to these properties, organics can be incinerated by thelarge amounts of weakly physisorbed �OH generated from wateroxidation at high current. On the other hand, indirect electro-oxidation methods based on Fenton’s reaction chemistry such aselectro-Fenton (EF) and solar photoelectro-Fenton (SPEF) are beingdeveloped for the remediation of wastewaters with organic pollu-tants [12–16]. In the case of EF, H2O2 is electrochemically producedby the two-electron reduction of injected O2 at the appropriatecathode from reaction (2) [12,13]:

O2 þ 2e� þ 2Hþ ! H2O2 ð2Þ

However, efficient production of H2O2 is achieved when carbona-ceous cathodes are used, such as carbon nanotubes–polytetrafluo-roethylene (PTFE) [17,18] carbon-felt [19–21] carbon sponge [21],graphite-felt [22] carbon-PTFE gas O2 or air diffusion electrodes[23–27] and BDD electrodes [28,29]. Moreover, the oxidation powerof H2O2 is improved by adding catalytic quantities of Fe2+ to pro-duce Fe3+ and, after that, the production of free �OH is promotedin the bulk from the Fenton reaction (3), under an optimum rangeof pH from 2.8 to 3.0 [12,28]. The advantages of this EAOP comparedwith the chemical Fenton reagent is the in situ H2O2 production andthe regeneration of Fe2+ catalysts at the cathode from Fe3+ reductionby reaction (4) [12,24–28]:

Fe2þ þH2O2 þHþ ! Fe3þ þH2Oþ ð�OHÞ ð3ÞFe3þ þ e� ! Fe2þ ð4Þ

When an undivided cell with a BDD anode and BDD cathode is used(BDD/BDD), organics are mineralized into water, carbon dioxide andinorganic ions by both BDD(�OH) formed from reaction (1) and �OHproduced in the bulk from the Fenton reaction (3).

In light of this information, the present work reports a prelimin-ary study on the degradation of the synthetic dye AR211 by usingdifferent supporting electrolytes (H2SO4, HClO4 and Na2SO4). Oneof the factors that influences the production of free radicals isthe electrolytic medium in which they are generated; severalstudies have shown that a change of pH in the solution affects boththe rate of degradation of organic compounds and the rate of freeradical generation; however, �OH production has not beenspecifically studied under the influence of the supporting electro-lyte. The effect of the media and the pH on the process efficiencywas evaluated by total organic carbon (TOC) measurements duringcurrent-controlled electrolysis of synthetic dye AR211 solutions.

2. Experimental

2.1. Chemicals

Synthetic dye AR211 (Table 1) was provided by local industry.The chemicals used as H2SO4, Na2SO4, and ferrous iron (FeSO4�7H2

O – catalyst) were supplied by Baker, while HClO4 and titanium(IV) isopropoxide-97% were from Aldrich. All other reagents were

Table 1Physico-chemical properties of the synthetic dye AR211.

Chemical formulation C16H14ClN5O4SChemical structure

Molecular weight (g/mol) 407.83Registration number (CAS) 12239-05-3

of analytical grade, and these reagents were used without furtherpurification. Solutions were prepared with high-purity water froma Millipore-Elix system, with resistivity P18 MX cm at 25 �C.

2.2. Electrochemical reactor and procedures

Electrochemical experiments were performed at room temper-ature in a 0.5 L undivided filter flow press reactor equipped withtwo electrodes. The anode and cathode electrodes have a geomet-rical area of 25 cm2 of niobium plate with a diamond-coated filmthickness of 2–7 lm. BDD electrodes were provided by MetakemGmbH™, Germany. For the EOx process, BDD anodes and stainlesssteel cathodes were used. The inter-electrode gap was 5 mm. Thesynthetic solutions were placed in the reservoir and recirculatedthrough the system using a diaphragm pump, as seen in Fig. 1.The flow rate is kept constant at 200 mL min�1. The degradationof 100 mg L�1 aqueous synthetic dye AR211 solutions was carriedout under current-controlled electrolysis conditions at 100, 200,300 and 400 mA using a BK Precision–1688B power supply. Assayswere monitored by TOC using a Shimadzu TOC-L. Prior to theelectrolysis, compressed air was bubbled for 30 min through thesolution. A catalytic amount of ferrous iron of 0.5 mM was addedto solutions before starting the electrolysis. Supporting electrolyteconcentrations were 0.1 mol L�1 for HClO4 and H2SO4, in the case ofNa2SO4 0.05 mol L�1 were utilized. The perchloric acid mediumwas chosen in accordance with the literature revised [30], sinceno ferric or ferrous complexes have been determined in thepresence of ClO�4 ion.

Evaluation of H2O2 accumulation in the system was carried outby testing H2O2 generation in the different electrolyte solutions.The H2O2 concentration was determined using the Ti(SO4)2

titration method and spectrophotometric analysis at k = 407 nm.

3. Results and discussion

3.1. EOx/BDD experiments

EOx experiments applying different currents (100, 200, 300 and400 mA) were carried out to understand the effect of thesupporting electrolyte (H2SO4, HClO4 and Na2SO4) during the elec-trochemical degradation of AR211. As can be observed in Fig. 2,TOC decay was followed, as a function of time, at different current

Fig. 1. Flow circuit for the electrochemical reactor.

(a) HClO4

(b) H2SO4

(c) Na2SO4

Fig. 2. Mineralization obtained by EOx/BDD during the treatment of 100 mg L�1

AR211 under different electrolytic solutions (a) HClO4, 0.1 mol L�1, (b) H2SO4,0.1 mol L�1 and (c) Na2SO4, 0.05 mol L�1 at 100 (}), 200 (h), 300 (4), and 400 (�)mA, using a 25 cm2 electrode area and 0.5 L solution.

86 A. Uranga-Flores et al. / Journal of Electroanalytical Chemistry 738 (2015) 84–91

densities for the different supporting electrolytes. It was observedthat the electrochemical mineralization was dependent on thesupporting electrolyte used, possibly affecting the degradationmechanism of AR211. In the case of HClO4, TOC was slowlyremoved, obtaining slight improvement when an increase in theapplied current was attained (Fig. 2a). In fact, at 100 mA of current(}), TOC removal was only 32%, but when the current increased,TOC removal values were 38% (200 mA (h)), 42% (300 mA (4)),and 47% (400 mA (�)). HClO4 was chosen as the supportingelectrolyte because it does not generate some oxidizing speciesliable to react with organics, as occurs using a Cl� medium (i.e.,generation of Cl2) or a SO2�

4 medium (i.e., production of S2O2�8 )

[31]. Using this supporting electrolyte with a BDD electrode, whichpresents an inert surface with low adsorption properties and awide potential window for water discharge, the organics and theirintermediates are oxidized by the EOx/BDD approach to CO2 byBDD(�OH) radicals electrogenerated from water discharge, accord-ing with reaction (1) [32].

Although the increase of current produces enhanced TOCreduction, it is not a significant improvement in the oxidation rate,as expected, and this behavior indicates that under these experi-mental conditions, the oxidation of AR211 is probably limited by

a secondary reaction where hydroxyl radicals are decomposed tooxygen [27]:

2BDDð�OHÞ ! BDDþ O2 þ 2Hþ þ 2e� ð5Þ

For the case of H2SO4 as supporting electrolyte (Fig. 2b), thedegradation rate of TOC removal increased slightly when theapplied current density was increased during the electrolysis ofAR211, passing from 31% of TOC decay at 100 mA (Fig. 2b(})) to53% when 400 mA was applied (Fig. 2b(�)). However, a modestimprovement was observed with H2SO4 when compared with theresults obtained with HClO4 (Fig. 2a). For example, at 400 mA,53% of TOC decrease is achieved (Fig. 2b(�)) using H2SO4, while47% of TOC removal is attained when HClO4 is used as the support-ing electrolyte under similar conditions. This behavior is due to theproduction of other oxidants which further free BDD(�OH), increas-ing with current density, which eventually increases the pollutantdegradation [32–34]. In fact, with H2SO4, the main reaction is theformation of peroxodisulfuric acid due to direct anodic oxidationof the sulfuric acid supporting electrolyte (Eq. (6)) [34], in contrastto HClO4, where the relevant reactions are the production of hydro-xyl radicals and oxygen evolution due to water oxidation (Eqs. (1)and (5), respectively).

2H2SO4 ! H2S2O8 þ 2e� þ 2Hþ ð6Þ

Nevertheless, several authors have suggested that the mineral-ization process is efficiently improved by the use of a supportingelectrolyte containing sulfate ions [31,34–36], but our resultsclearly showed that this effect was not significant when H2SO4

was used.For this reason, a new set of experiments was performed to

increase understanding of electrochemical treatment involvingthe electrochemical production of reactive oxidant species by usingdifferent supporting electrolytes. To address this question, bulkelectrolysis was performed using 0.1 M Na2SO4.

Fig. 2c shows the anodic oxidation of an AR211 dye solutionwith Na2SO4 as the supporting electrolyte by applying differentcurrents at the BDD electrode. Organic load is more rapidlyreduced, increasing the applied current. The faster TOC removalwith increasing current can then be ascribed to the concomitantgreater production of BDD(�OH) from reaction (1), whichaccelerates the oxidation rate of all organics as well as an efficientelectrogeneration of peroxodisulfate species (Eq. (7)) [37,38]:

2SO2�4 ! S2O2�

8 þ 2e� ð7Þ

These powerful oxidizing agents can oxidize organic matter by achemical reaction whose rate depends on the amount of sulfateions in solution or/and temperature [38]. In fact, as seen inFig. 2c, the efficiencies achieved using this supporting electrolyteconsiderably improved with respect to those shown in the aboveassays (HClO4 (Fig. 2a) and H2SO4 (Fig. 2b)). At 100 mA, TOCremoval efficiency was approximately 43% (Fig. 2c(})); while for200, 300 and 400 mA, the TOC elimination achieved values ofapproximately 50% (Fig. 2c(h)), 54% (Fig. 2c(4)) and 70%(Fig. 2c(�)), respectively.

The results presented above confirm that the EOx/BDD is usefulin degrading AR211 dye in different supporting electrolytes.However, the oxidation power of the system depends strongly onthe electrolytic medium used to promote the generation ofBDD(�OH) free radicals or other oxidants on the BDD surface.

To explain these behaviors, we can consider that the first step,in all supporting electrolytes, is water discharge on BDD with theformation of hydroxyl radicals (Eq. (1)) [12,32–34]; after that,these oxidant species can be involved in parallel reactions.

In the case of H2SO4 as the supporting electrolyte, it can bedirectly oxidized to the BDD surface, producing peroxodisulfuric

(a) HClO4

(b) H2SO4

(c) Na2SO4

Fig. 3. EOx/BDD mineralization current efficiency (MCE) under different electro-lytic solutions (a) HClO4, 0.1 mol L�1, (b) H2SO4, 0.1 mol L�1 and (c) Na2SO4,0.05 mol L�1 at 100 (}), 200 (h), 300 (4), and 400 (�) mA, using a 25 cm2 electrodearea and 0.5 L solution.

A. Uranga-Flores et al. / Journal of Electroanalytical Chemistry 738 (2015) 84–91 87

acid, as indicated in reaction (6). However, electrogeneratedBDD(�OH) can also react with sulfuric acid giving peroxodisulfuricacid (Eq. (8)) by the parallel mechanism [36]:

2H2SO4 þ 2�OH! H2S2O8 þ 2H2Oþ ð8Þ

This second reaction may promote a decrease in the efficiencyof degradation via free BDD(�OH) because these species areconsumed when peroxodisulfuric acid is formed. This assertionjustifies the modest improvement on TOC decay as AR211 isdegraded, when compared with a supporting electrolyte thatfavors hydroxyl radicals as the main oxidant species (HClO4).

On the other hand, when HClO4 is used as the supportingelectrolyte, only two reactions are probably favored: theproduction of hydroxyl radicals (Eq. (5)), and, as HClO4 is stabletowards BDD(�OH) oxidation, the second reaction is O2 evolutionprobably via H2O2 formation [34]. In fact, electrogeneratedBDD(�OH) radicals may react with each other to form hydrogenperoxide near the electrode (Eq. (9)); this then diffuses into thebulk of the electrolyte (Eq. (10)) or is oxidized to oxygen(Eq. (11)) [34].

2�OH! H2O2 ð9ÞBDDðH2O2Þ ! ðH2O2Þsolution ð10ÞH2O2 ! O2 þ 2Hþ þ 2e� ð11Þ

These reactions (Eqs. (5), (9), (10) and (11)) explain the modestTOC removal efficiencies obtained when HClO4 is used assupporting electrolyte because the electrogenerated BDD(�OH)are consumed by parallel reactions instead of favoring the rapidand complete mineralization of AR211.

Conversely, when Na2SO4 is used as the supporting electrolyte,the production of peroxodisulfates is the main reaction attained,involving two mechanisms [13,37–39]. The first reaction considersa direct oxidation process at the BDD electrode, as indicated inreaction (7) [39]. But, due to the influence of the pH on the kinetics,the protonation of sulfate must be taken into account [40]. How-ever, the first mechanism should include both species SO2�

4 andHSO�4 . The following equation must be taken into account:

2HSO�4 ! S2O2�8 þ 2e� þ 2Hþ þ 2e� ð12Þ

The second model involves an indirect oxidation process inaqueous solutions; after that, the very reactive hydroxyl radicalsare produced on the BDD surface (Eq. (1)). The electrogeneratedBDD(�OH) can react with the electrolyte, following mediatedsurface chemical steps that are responsible for the formation ofperoxodisulfate [38,40]:

BDDð�OHÞ þ SO2�4 ! BDDðSO��4 Þ þ OH� ð13Þ

BDDðSO��4 Þ þ SO2�4 ! S2O2�

8 þ e� ð14Þ

The surface site BDDðSO��4 Þ is not as oxidative as SO��4 but issufficient to take part in the S2O2�

8 formation [39]. If an organic pol-lutant is present in solution when the BDDðSO��4 Þ surface is formedor SO��4 is in bulk solution, the mineralization rate is improved[39,41]. This outcome is in agreement with the TOC removalefficiencies observed during AR211 oxidation using Na2SO4 as thesupporting electrolyte. Finally, the overall peroxodisulfate forma-tion process corresponds to reaction (7) with the participation ofBDD(�OH). The whole process is highly dependent on the mediatedoxidation surface generation of intermediate BDDðSO��4 Þ, which ismore feasible than the direct electrochemical formation of S2O2�

8 .Additionally, it is important to consider that the decompositionof S2O2�

8 to H2O2 is feasible, reducing its oxidation power efficiencyin the bulk solution [39].

3.2. Mineralization current efficiency for EOx/BDD experiments

The efficiency of the applied current is a key parameter thataffects the production of oxidants and, hence, the oxidation abilityof EOx/BDD. Note that the incineration of AR211 to CO2 involvesthe release of Cl�, SO2�

4 and NO�3 as major primary ions. Based onthese considerations, its overall mineralization reaction can beexpressed as follows:

C16H14ClN5O4Sþ 47H2O! 16CO2 þ Cl� þ 5NO�3 þ SO2�4

þ 108Hþ þ 108e� ð15Þ

The mineralization current efficiency (MCE), in terms ofpercentage, for each assay at current I (A) and time t (h) was thenestimated from the following equation [42]:

MCE ð%Þ ¼nFVsDðTOCÞexp

4:32� 107mIt� 100 ð16Þ

where F is the Faraday constant (96,487 C mol�1), Vs is thesolution volume (in L), D(TOC)exp is the experimental TOC decay(in mg/L), 4.32 � 107 is a conversion factor to homogenize units

(a) HClO4

(b) H2SO4

(c) Na2SO4

Fig. 4. Time-course of the concentration of H2O2 electrogeneration during theelectrolysis of 0.5 L under different electrolytic solutions (a) HClO4, 0.1 mol L�1, (b)H2SO4, 0.1 mol L�1 and (c) Na2SO4, 0.05 mol L�1 at 100 (}), 200 (h), 300 (4), and400 (�) mA, using a 25 cm2 electrode area.

88 A. Uranga-Flores et al. / Journal of Electroanalytical Chemistry 738 (2015) 84–91

(3600 s/h � 12,000 mg/mol) and m is the number of carbon atomsof AR211 (16 atoms).

Fig. 3 highlights the MCE values estimated from Eq. (16) for theEOx/BDD of AR211 when different supporting electrolytes areused. As expected, the efficiency drops in the sequence Na2SO4 -P HClO4 > H2SO4, in agreement with the relative oxidation powerof these EAOPs. For example, decreasing maximum MCE valuesof 40% for anodic oxidation of AR211 using Na2SO4 was attained(Fig. 3c), while values were 39% and 25% for EOx/BDD when usingHClO4 (Fig. 3a) and H2SO4 (Fig. 3b), respectively, at the beginning ofelectrolysis, which further decayed slowly to 35–25% at the end ofall treatments. This behavior was also observed at 200, 300 and400 mA, but the MCE decreases in accordance with the increasein applied current (Fig. 3). This trend suggests that the currentsapplied are consumed during the production of oxidants that favorthe conversion of AR211 to CO2 and the formation of more persis-tent oxidation products that are slowly removed with the progressof the electrolysis [13]. This latter fact along with the presence ofless organic matter in solution could explain the slower TOCremoval and lower efficiency found at long electrolysis time (seeFig. 1).

3.3. Hydrogen peroxide production

Based on the experimental results and the possible mechanismfollowed according to the supporting electrolyte used, a side reac-tion is also possible due to the coupling of hydroxyl radicals toform hydrogen peroxide in the electrolyte. For this reason, theH2O2 was determined when individual electrolysis, in absence oforganic pollutants, was performed for each one of the supportingelectrolytes. Fig. 4 shows the results of H2O2 concentrations whenthe BDD/BDD system was used for electrolyzing 0.5 L of differentelectrolytic solutions at constant currents of 100, 200, 300 and400 mA for 60 min. In Fig. 4a, corresponding to the assays carriedout in 0.1 mM of HClO4, it is possible to observe that the concentra-tion of accumulated H2O2 increased slowly up to values of 22.97,24.94, 27.76 and 30.15 mg L�1 after 60 min of electrolysis at 100(curve 3a(})), 200 (curve 3a(h)), 300 (curve 3a(4)) and 400 mA(curve 3a(�)), respectively. These results clearly evidence thatHClO4 is a suitable solution to efficiently produce hydrogen perox-ide [34]. Conversely, this efficiency decreases when H2SO4 and Na2-

SO4 were used as supporting electrolytes. For the H2SO4 medium,accumulation of H2O2 is achieved, but concentrations range from3.87 to 19.15 mg L�1 when the applied current increases from100 to 400 mA. The most popular medium for hydrogen peroxideproduction is Na2SO4 because many researchers have performedelectrochemical approaches that use it [22–29,42,43]. In this case,the concentration of Na2SO4 and pH were evidently maintained asconstant to favor the efficient production of H2O2. As can beobserved in Fig. 4c, an increase in current applied improves thehydrogen peroxide production; however, lower concentrationswere produced. At 100 mA, 8.06 mg L�1 was reached (curve4c(})); while 12.1, 13.1 and 15.1 mg L�1 were achieved by apply-ing 200 (curve 4c(})); 300 (curve 4c(})) and 400 mA (curve4c(})), respectively. The tentative formation of a plateau in threecases studied is indicative of the parallel destruction of electrogen-erated H2O2, which became faster as the concentration of thisspecies rose, so that the quasi-steady content in each currentwas reached just when its electrogeneration and destruction rateswere equal [41]. The rapid removal of H2O2 in each one of the elec-trolytic systems can be accounted for by its decomposition to O2 atthe BDD anode surface [27,29,42].

The above results confirm that during the production of oxidantspecies such as �OH, H2S2O8, SO��4 and S2O2�

8 when HClO4, H2SO4

and Na2SO4 are used as supporting electrolytes, parallel mecha-nisms to produce H2O2 may be attained. These reactions can be

achieved due to the combination of �OH or via decomposition ofsynthesized oxidant species [31,32,36,39]. Then, these assump-tions are in agreement with the behaviors observed in the presenceof AR211 in solution during TOC removal, showing that the oxida-tion efficiency is radically dependent on the stability of the strongoxidants produced as well as the rate of reaction between pollutantand oxidant. While the electrogeneration of peroxodisulfatesfavors a faster electro-conversion/combustion than that observedfrom �OH-based mechanism of incineration, the possibility ofenhancing or reducing the efficiency of the electrochemical treat-ment by the decomposition of these oxidants is interesting.

3.4. Degradation of AR211 by EF/BDD process

The possibility of continuously producing H2O2 during the useof the BDD/BDD-undivided flow cell (by reactions 8,9, by decompo-sition of strong oxidants, such as H2S2O8 and S2O2�

8 , as well as byoxygen reduction at BDD cathode surface (reaction 1)) is able togenerate free radical �OH in the bulk solution from Fenton’s typereaction (3) by EF processes [12,22,27,42,43]. Then, the higher pro-duction of H2O2 observed when different supporting electrolytes

A. Uranga-Flores et al. / Journal of Electroanalytical Chemistry 738 (2015) 84–91 89

were used can be a suitable alternative for removing AR211 by theEF approach.

Experimental assays for the degradation of the solutionscontaining AR211 by the EF/BDD process were performed. Fig. 5illustrates the TOC abatement efficiency for the electrolysis of a100 mg L�1 AR211 solution, in terms of TOC, using HClO4, H2SO4

and Na2SO4 at different current densities by adding 0.3 mM of ironferrous ion at 25 �C. The results showed that, similar to EOx/BDD,lower removal efficiency was observed in the HClO4 medium(Fig. 5a) than those achieved at H2SO4 (Fig. 5b) and Na2SO4

(Fig. 5c). However, the EF/BDD approach was more efficient in min-eralizing AR211 than EOx/BDD using HClO4. For example, at100 mA, 61% of TOC abatement was reached (curve 5a(})), whilefor 200, 300 and 400 mA, TOC removal values approximately 67%(curve 5a(h)), 72% (curve 5a(})) and 75% (curve 5a(�)) were reg-istered, respectively. The results in Fig. 5b show that AR211 isquickly degraded during the first 40 min under these conditions,yielding 87% degradation efficiency at 100 mA (curve 5b(}));whereas at the same time, the use of 200 mA led to 93% (curve5b(h)); 300 mA reached 97% (curve 4b(4)); and 400 mA was close

(a) HClO4

(b) H2SO4

(c) Na2SO4

Fig. 5. TOC abatement vs. electrolysis time for 0.5 L of a 100 mg L�1 AR211 solutionin different electrolytic solutions (a) HClO4, 0.1 mol L�1, (b) H2SO4 0.1 mo L�1 and(c) Na2SO4 0.05 mol L�1 adding 0.3 mM Fe2+ at 25 �C degraded by EF/BDD process ata flow rate of 200 mL h�1. Applied current: 100 (}), 200 (h), 300 (4), and 400 (�)mA.

to 99% TOC removal (curve 4b(�)) when H2SO4 was used as thesupporting electrolyte. This behavior clearly indicates that thehydrogen peroxide produced by anodic or cathodic reactions isefficiently transformed to �OH via homolytic decomposition ofH2O2 [12,22,27,42,43]. Similar behavior was observed by Diagneet al. [30], when they study the degradation of methyl parathionin different media. After that, �OH attacks dye molecules to com-pletely mineralize them. Meanwhile, similar results were obtainedfor Na2SO4; TOC removal values of approximately 88%, 93%, 97%and 100% at 100 mA, (curve 5c(})), 200 mA (curve 4c(h)),300 mA (curve 4c(4)) and 400 mA (curve 4c(�)), respectively,were achieved after 20 min of electrolysis. This trend indicates thatthe electrolytic system becomes more powerful in the Na2SO4

medium because larger amounts of EF/BDD(�OH) react with theorganic matter, enhancing the degradation process [12,13,19,27],because current remains constant and, hence, EF/BDD(�OH)production is constant. At the same time, the production of S2O2�

8

species is feasible, accelerating the oxidation rate of organicpollutants, and, as a consequence, the increase in the TOC removalover short times. This fact can also justify the higher rise in MCEwhen current rises for all EF/BDD experiments using different

(a) HClO4

(b) H2SO4

(c) Na2SO4

Fig. 6. EF/BDD mineralization current efficiency (MCE) under different electrolyticsolutions (a) HClO4, 0.01 mol L�1, (b) H2SO4, 0.01 mol L�1 and (c) Na2SO4 0.05 molL�1 at 100 (}), 200 (h), 300 (4), and 400 (�) mA, using a 25 cm2 electrode area and0.5 L solution.

Table 2Electrochemical characteristics determined for the treatment of 0.5 L of AR211solution by EOx/BDD under selected conditions.

EOx/BDD approach

Supportingelectrolyte

Current applied(mA)

Energy consumption(kW h g�1 TOC)

Cost(USD)

HClO4 100 2.07 19.8200 4.04 38.7300 8.27 79.4400 12.08 116.0

H2SO4 100 2.24 21.4200 5.54 53.2300 7.87 75.6400 11.10 106.6

Na2SO4 100 2.01 19.3200 4.08 39.1300 7.00 79.3400 13.05 125.4

Table 3Electrochemical characteristics determined for the treatment of 0.5 L of AR211solution by EF/BDD under selected conditions.

EF/BDD approach

Supportingelectrolyte

Current applied(mA)

Energy consumption(kW h g�1 TOC)

Cost(USD)

HClO4 100 2.14 20.6200 4.38 42.1300 7.96 76.4400 10.84 104.1

H2SO4 100 2.00 19.2200 4.08 39.2300 9.38 90.2400 11.69 112.4

Na2SO4 100 2.29 22.1200 5.16 49.5300 8.08 77.7400 11.18 107.4

90 A. Uranga-Flores et al. / Journal of Electroanalytical Chemistry 738 (2015) 84–91

electrolytes, as shown in Fig. 6. The MCE-time plots obtained forthe EF/BDD electrolysis show that decreasing efficiencies at higheri values can be observed, more remarkable when passing from 100to 400 mA, owing to the production of smaller relative amounts ofoxidants BDD(�OH) and �OH from H2O2 because of the concomitantrise in the rate of their waste reactions. The application of thelower current of 100 mA yielded practically 100% of MCE after10 min of electrolysis, indicating that organics are rapidlymineralized at the beginning of the electrolysis time by the higherproduction of hydroxyl radicals and in minor proportion by otheroxidants. After that, a dramatic drop in MCE is attained due tothe loss of organic matter and the formation of more persistentintermediates. In fact, higher TOC removal values are achieved dur-ing the first 30 min of electrolysis (Fig. 5), being more evident withNa2SO4 as the supporting electrolyte. For 300 and 400 mA, how-ever, maximum efficiencies of close to 30% and 18% for HClO4,32% and 25% for H2SO4 and 15% and 9% for Na2SO4 were found atthe beginning of electrolysis, decreasing successively.

3.5. Current efficiency and energy cost estimate

The energy consumption per unit TOC mass (EC) was calculatedfrom Eq. (17):

ECTOC ¼ ðkW h g�1TOCÞ ¼ EcellItDðTOCÞexpVs

ð17Þ

where Ecell is the average cell voltage (V).

Tables 2 and 3 show that both the AR211 treatments by EAOPsrequired high ECTOC values because of their slow mineralizationrate as a result of the large persistence of intermediates andaccumulation of different carboxylic acids. The deceleration ofthe mineralization process with loss in MCE as electrolysis timerose was also reflected by a gradual increase in ECTOC values inall cases. For example, at 400 mA under the Na2SO4 medium,13.05 kW h g�1 TOC were consumed in EOx/BDD, which droppedto 11.18 kW h g�1 TOC (14% reduction) in EF/BDD. An interestingresult is shown in the change of the supporting electrolyte, whereit is possible to observe that, when we used H2SO4 and Na2SO4,energy consumption is very similar [12,13]. These results confirmthat sometimes EOx/BDD or EF/BDD approaches can be unfeasibleindustrial techniques due to the higher consumptionrequirements.

4. Conclusions

Based on the results obtained, the elimination of AR211 can beattained by EOx/BDD or EF/BDD technologies. However, the EF/BDD approach is more efficient in removing the organic load con-tained in a solution of 100 mg L�1 of AR211. Based on the resultsobtained, a significant amount of �OH is produced during EOx/BDD when HClO4, H2SO4 or Na2SO4 are used as supporting electro-lytes, but the coexistence of parallel mechanisms for producingstrong oxidants reduces the power efficiency of this approach. Anevaluation of the concentration of H2O2 in bulk solution, viaelectrolysis of each one of the supporting electrolytes, confirmsthat the �OH formed during EOx/BDD is consumed by formationof H2O2 and O2, and these also participate in the formation ofH2S2O8 and S2O2�

8 species. This behavior indicates that theefficiency of EOx/BDD approach depends on the nature of the elec-trode material [2,3,9–12], as already discussed in authoritativereviews, but also depends on the supporting electrolyte as wellas the stability/reactivity of �OH [13,31,32,44]. In light of theseresults, we can see that the tendency of accumulation of H2O2 indifferent electrolytes is opposite to the destruction of the TOC inEOX and EF process, which is attributed to the greater influenceof BDD (�OH) with respect to the (�OH) to destroy the organic mat-ter. The comparisons between H2SO4 and Na2SO4 as supportingelectrolytes open new considerations about the real mechanismfollowed to form S2O2�

8 species because the mechanism was thesame independent of sulfate electrolyte used, but the efficienciesare different.

Conflict of interest

No conflict of interest.

Acknowledgment

This study was supported by CONACYT with Grant No: GTO-2012-C01-192117.

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