Cl-mediated electrochemical oxidation for treating an effluent using platinum and diamond anodes

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ContentslistsavailableatScienceDirect

Journal

of

Water

Process

Engineering

j o u r n a l ho me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / j w p e

Short

Communication

Cl-mediated

electrochemical

oxidation

for

treating

an

efﬂuent

using

platinum

and

diamond

anodes

Dayanne

Chianca

de

Moura

a

_,

_Chrystiane

_do

_Nascimento

_Brito

a

_,

_Marco

_Antonio

_Quiroz

b

_,

Sibele

B.C.

Pergher

a

_,

_Carlos

_A.

_{Martinez-Huitle}

a,∗

a_Federal_University_of_Rio_Grande_do_Norte,_Instituto_of_Chemistry,_Campus_{Universitario}_s/n,_Lagoa_Nova,_59078-970_Natal,_RN,_Brazil

b_Universidad_de_las_Américas_Puebla,_Grupo_de_{Investigación}_en_Energía_y_Ambiente,_ExHda._Sta._Catarina_Martir_s/n,_Cholula_72820,_Puebla,_Mexico

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received10June2014

Receivedinrevisedform8November2014

Accepted13November2014

Availableonline3December2014

Keywords:

Oxygenevolutionreaction

Polarizationcurves Diamondelectrode Realefﬂuent Hydroxylradicals Activechlorine

a

b

s

t

r

a

c

t

Inthiswork,theroleofchloridewithrespecttotheoxygenevolutionreactionbyusingBDDandPt/Ti anodeswasinvestigatedtounderstandtheperformanceofbothanodesfortheelectrochemicaltreatment ofarealsampleofwastewater,withlowerorganicload,generatedatFederalUniversityofRioGrandedo Norte.Bulkexperimentswereperformedunderrealdischargedefﬂuentconditions(pHand conductiv-ity)inordertoverifytheapplicabilityofdirectandmediatedelectrochemicaloxidationbyapplying 2.5and5.0mAcm−2.Resultsclearlyshowedthat,apartialeliminationofchemicaloxygendemand (COD)wasachieved,dependingonappliedcurrentdensity.UsingPt/Ti,byapplying2.5and5.0mAcm−2_,

CODremovalefficienciesof25.2%and30.49%wereobtained.Underthesameconditions,aminorCOD removalwasachievedbyusingBDDelectrode.However,theremovalefficiencieswereimprovedwhen Cl-mediatedoxidationapproachwasemployed,obtaining63.7%and60%forPt/Tibyapplying2.5and 5.0mAcm−2,respectively;whileforBDDelectrode,CODremovalswereabout15%and47%,underthe sameconditions.Thesefiguresareinagreementwiththepolarizationcurvesobtained,whereitwas pos-sibletoconfirmthattheconcentrationofhalideinsolutionincreasestheimportanceofCl2/oxy-chloro

radicalssystemdependingontheelectrocatalyticmaterialandthisbehaviorplaysanimportantrolein relationwiththeoxygenevolutionreaction,inﬂuencingontheefﬁciencyofelectrochemicalapproach adopted.

1. Introduction

The day-to-day human and industrial activities have influ-encedtheflowandstorageofwaterandthequalityofavailable freshwater[1].Theremediationofurbanandindustrial waste-watercontainingorganicpollutantscanbecarriedoutbydifferent methods,includingchemical,physicochemicalandbiological treat-ments[2,3].Themostmethodusedistheconventional-biological treatment,butitistimeconsuming,needlargeoperationalarea andisnotcompletelyeffectiveforeffluentscontainingwith bio-refractorypollutants[4].

Physical–chemicalmethods(filtration,coagulation,adsorption, and flocculation), chemical oxidation (use of chlorine, ozone, hydrogenperoxide,wet air oxidation), and advanced oxidation processes(AOP)(Fenton’sreaction,ozone+UVradiation, photo-chemistry)are currentlyused totreat industrialeffluents [3,5].

∗ Correspondingauthor.Tel.:+558432119224;fax:+558432119224.

E-mailaddress:[email protected](C.A.Martinez-Huitle).

However, all these methods have some major drawbacks. For example, filtration and adsorption are not always sufficient to achieve thedischarge limits[6]; coagulationand flotation gen-erate a large amount of sludge; chemical oxidations have low capacityratesandneedtransportationandstorageofdangerous reactants;andadvancedoxidationprocessesrequirehigh invest-mentcosts.Consequently,anurgentchallengeisthedevelopment of new environmentally benign technologies able to mineral-ize completelynon-biodegradableorganicmatterand eliminate pathogens.

Inthiscontext,oxidativeelectrochemicaltechnologiesofferan alternativesolutiontoseveralenvironmentalproblemsregarding thewastewatertreatment,becauseelectronsprovideaversatile, efﬁcient,cost-effective,easilyautomation,andcleanreagent[2–5]. Someyearsago,theeffectiveapplicationofelectrochemical tech-nologiesforthetreatmentoforganicpollutantshasbeenrelatively small[2,3].Butnowadays,thankstointensiveinvestigationsthat haveimprovedtheelectrocatalyticactivity,stabilityofelectrode materialsaswellasoptimizedreactorgeometry;electrochemical technologieshavereachedapromisingstateofdevelopmentand

http://dx.doi.org/10.1016/j.jwpe.2014.11.005

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canbeeffectivelyusedfordisinfectionandpuriﬁcationof waste-waterpollutedwithorganiccompounds[7–14].

Themainobjectoftheelectrochemicalwastewatertreatment isthecompleteoxidationoforganicstoCO2or,atleast,their

con-versiontobiodegradablecompounds[7,8].Inthisframe,several organicsubstrateshavebeenconsideredastargetpollutantsfor directandindirectelectrochemicaloxidationsbyusingdifferent experimentalconditionsand anodematerials[3–11].Regarding theelectrodes, ahighnumberof themhavebeentested [3,5,7]

includingpolypyrrole,granularactivatedcarbon,ACF,glassy car-bon,graphite,Pt/TiandPt,doped-PbO2andmixedmetaloxidesof

Ti,Ru,Ir,Sn,TaandSb.However,alargevarietyofstudieshave demonstratedtheefﬁciencyofnon-activeanodes[7,8,11],suchas PbO2andBDDelectrodesbytheirbetteroxidativeperformancedue

toaccumulationof•OHradicalsattheirsurfaces.

Ontheotherhand,themostpopularmethodofelectrochemical treatmentiselectro-chlorination(basedonindirect electrochem-icaloxidation). Its main advantage is theon-site generation of disinfectants,thusavoidingtheproblemsofcommonchlorination suchastransportandstorageofdangerouschlorine[3,5,9].There aretwotypesofelectro-chlorinationproceduresinvolvingeither thesynthesisoffreechlorinefrombrineinanelectrolyticgenerator orthedirectproductionofoxidantsfromthewatertobetreated throughtheelectrolyser.

ActivechlorinespeciessuchasCl2,HOCl,ClO−andClO2 have

beenwidelyrecognizedaskeyoxidantsresponsiblefor degrad-ingorganicpollutantsorinactivatingcellsinelectro-chlorination

[9,15].Thesespeciescanbeproducedattheanodeviathefollowing totalreactions[9]:

H2O+Cl−→ClOH• +H++2e− (1)

H2O+ClOH• +Cl−→Cl2+O2+3H++4e− (2)

Cl2+2OH−→ H2O+OCl−+Cl− (3)

ClOH• +Cl2→ ClO2+3H++2Cl−+e− (4)

Someresearchershavepointedoutthatthedisinfectingefﬁcacy ofthismethodismuchhigherthanchlorinationduetothe com-petitiveelectrogenerationofotheroxidants[11].Recentstudies have also attributed the higher disinfecting power of electro-chlorination to the additional oxidant role of reactive oxygen species(ROS)suchashydroxylradical(•OH),atomicoxygen(•O), hydrogenperoxideandozone,whichcanbegeneratedfromwater dischargeattheanode,asfollows[16–18]:

H2O→ •OH+H++e− (5)

•_OH_→ •_O₊_H+₊_e− ₍₆₎

2•O→ O2 (7)

2•OH →H2O2 (8)

O2+•O→ O3 (9)

The most used types of electrode for this electrochemical approacharethemetaloxideelectrodesthat,generally,arethe derivativesoffourmetaloxides,SnO2,PbO2,RuO2andIrO2[19].

However,Pt/Ti(active)andBDD(non-active)anodesarealsoable toproduceappreciableamountsofROSandotheroxidizingspecies suchasactivechlorine[9],peroxodisulfate,peroxodicarbonateand peroxodiphosphatecomingfromtheoxidationofionspresentin thesolution,alsoallowingafastandpermanentdisinfection.

Forthisreason,takingintoconsiderationtheabove informa-tion,ourinvestigationfocussestostudytheroleofchloriderespect totheoxygenevolutionreactionbyusingBDDandPt/Tianodes tobetterunderstandtheeffectsduringdirectandindirect electro-chemicalincinerationofarealsampleofwastewater,withlower

Fig.1. ElectrochemicalﬂowcellconﬁgurationfortheoxidationexperimentsatPt/Ti

andBDDanodes:(1)thermoregulatedreservoir,(2)electrochemicalﬂowcell,(3)

powersupplyand(4)pump.

organicload,generatedatFederalUniversityofRioGrandedoNorte (UFRN).

2. Materialsandmethods

2.1. Reagents

Ultrapurewaterwasobtainedusingawaterpuriﬁcationsystem (MilliQ).Thechemicalreagentsusedwereofthehighestquality availableonthemarket,withoutadditionalpuriﬁcation.NaCland Na2SO4wereobtainedfromFluka.

2.2. Wastewaterefﬂuentcharacteristics

Theefﬂuentsample wascollected, beforephysical–chemical management(chlorination)inthetreatmentstationattheUFRN locatedin Natal (Northeast of Brazil). The efﬂuentcontained a concentrationofchemicaloxygendemand(COD)of341mgdm−3. Itselectricconductivitywasaround500␮Scm−1 anditspHwas around6.0±0.2. Through analysisbyIon Chromatography (IC), it waspossibleto characterize someimportant ionspresent in the wastewater as Cl− (208.6mgdm−3), NO3− (85.9mgdm−3),

Ca2+ _(11.1_mg_dm−3_), _Na+ _(126.2_mg_dm−3_), _Mg2+ _(6.5_mg_dm−3₎

andSO42−(21.5mgdm−3).

2.3. Electrochemicalmeasurements

Electrochemical analyses were performed with an Autolab modelPGSTAT320N(Metrohm).Quasi-steadypolarizationcurves werecarriedoutatascanrateof5mVs−1andwitha0.45mVstep potential,insolutionsof NaClatdifferentconcentrations,using Na2SO4 tofurtherincrease theelectrolyteconductivity.

Experi-mentswerecarriedoutinaconventionalthree-electrodesystem, andmeasurementswereperformedbetween0and2.5V.Pt/Tiand BDD,withanexposedgeometricareaofca.0.75cm2_,_were_used

astheworkingelectrode,whileaplatinumwireandanAg/AgCl (KCl3moldm−3)electrode wereemployedastheauxiliary and referenceelectrodes,respectively.

2.4. Electro-oxidationexperiments

The electro-oxidation experiments were made in a single compartmentusinganelectrolyticﬂowcellwithparallelplate elec-trodesfortreating2dm3_of_wastewater₍_Fig.₁_)._Disk_anodes_(Pt/Ti

andBDDwith10cmindiameter)wereused,exposingtothe aque-oussolutionanominalsurfaceareaof63.5cm2_._Pt/Ti_or_BDD_were

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wassuppliedbyIndustrieDeNoraS.p.A.(Milan,Italy).BDD elec-trodewassuppliedbyAdamantTechnologies(Switzerland)and itwassynthesizedasdescribedinpreviousworks[20] maintain-ingthequalityparameters(single-crystalwithathicknessof1␮m (±5%)andaresistivityof15mcm(±30%)withaboron concentra-tionof5000ppm,p-siliconwafers(1–3mcmand1mmthick)). Inordertostabilizeitssurface(hydrophilicnature)andtoobtain reproducibleresults,theBDDelectrodewaspre-treatedat25◦Cby anodicpolarizationin1MHClO4at10mAcm−2for30min[21].

Inordertounderstandtheeffectofcurrentdensityduringdirect andindirecttreatment,experimentswereperformedapplying2.5 and5.0mAcm−2during2h.Inthecaseofindirect electrochemi-caltreatmentbyelectro-chlorination,itwasevaluatedbyadding 1.25gdm−3ofNaClinordertocomparewiththeelectrochemical depollutionofefﬂuentasreceived.

2.5. Analyticalmethods

Decontaminationofrealefﬂuentwasmonitoredfromthe abate-mentofitsCOD.Valueswereobtained,usingaHANNAHI83099 spectrophotometerafterdigestionofsamplesinaHANNA thermo-reactor.Fromthesedata,thepercentageofCODdecayisestimated fromthefollowingequation:

%CODremoval= COD0−CODf

COD0 ×

100 (10)

whereCOD0andCODfrepresentthevaluesbeforeandattheend

oftheelectrolysis,respectively.

Totalcurrentefficiency(TCE,in%)fordirectandindirect elec-trochemicaloxidationsoftheeffluentwasestimatedbyusingthe initialandfinalCODvalues,followingrelationship:

%TCE=FV

COD0−CODt

8_It

×100 (11)

whereIisthecurrent(A),FtheFaradayconstant(96,487Cmol−1), Vistheelectrolytevolume(dm3_),₈_is_the_oxygen_equivalent_mass

(geq.−1)andtistheelectrolysistime,allowingforaglobal deter-minationoftheoverallefﬁciencyoftheprocess.

Additionally,thelimitingcurrentwasestimatedfromthevalue ofCODusingEq.(12)forelectrochemicaltreatmentofareal waste-water,asindicatedbyPanizzaandCerisola[21]:

Ilim(t)=4FAkmCOD(t) (12)

whereIlim(t) isthelimiting current (A)at agiven timet, 4the

numberofexchangedelectrons,Atheelectrodearea(m2_),_F_the

Faraday’sconstant,km theaveragemasstransportcoefﬁcientin

theelectrochemicalreactor(ms−1)andCOD(t)thechemicaloxygen

demand(molO2m−3)atagiventimet.

Theenergy consumptionpervolume oftreated efﬂuentwas estimatedandexpressedinkWhm−3.Theaveragecellvoltage dur-ingtheelectrolysis(cellvoltageisreasonablyconstantwithjust someminoroscillations,forthisreasoniscalculatedtheaverage cellvoltage),istakenforcalculatingtheenergyconsumptionby expression:

Energyconsumption= Ec₁₀₀₀×I×t×Vs (13)

wheretisthetimeofelectrolysis(h);_Ec(V)andI(A)arethe averagecellvoltageandtheelectrolysiscurrent,respectively;and Vsisthesamplevolume(m3).

Fig.2.Currentdensity–potentialcurvesforthePtelectrodeinthepresenceof

dif-ferentamountsofNaClon(a)Pt/Tiand(b)BDDanodes.Na2SO4assupporting

electrolyte;scanrate:5mVs−1_and₂₅◦_C.

3. Resultsanddiscussion

3.1. Polarizationcurvesinthepresenceofhalideion

Priortodirectandindirectelectrochemicaloxidations,the pos-sible effect of halide on the oxygen evolution reaction (o.e.r.) wasstudied.Quasi-steadypolarizationcurves wererecordedin background solutions containing 0.25moldm−3 Na2SO4, in the

absence and in the presence of differentconcentrations of Cl− (1×10−3moldm−3 to0.4moldm−3).Theresultsobtainedinthe presenceofchlorideions,atbothanodematerials,areshownin

Fig.2.InthecaseofthePt/Tianode,polarizationcurve concern-ing totheo.e.r. is modestlyshiftedto morepositive potentials (Fig.2a),whenalittleincreasinginthechlorideconcentrationis attained(0.001moldm−3);abovethisvalue,areversalofthetrend isobserved.Theinversion ofthetrenduponincreasingchloride concentrationabove0.01moldm−3 isduetotheincreaseofthe importanceoftheCl2/H2Osystem.Undertheseconditions,afast

incinerationofanumberoforganicsubstratescanbefavoreddueto theproductionofreactivehydroxylradicals,inconcomitancewith activechlorinespecies,asintermediatesinthechlorineevolution reaction.

SimilarexperimentswerecarriedoutusingBDDanodeinthe presenceofCl−,asshowninFig.2b,employingthesamerange ofCl−concentrations.Inthatcase,atverysmallNaCl concentra-tion(0.001moldm−3),arelevantshiftofJ/Ecurvesinthepositive directionisobserved.Above0.02moldm−3,theanodepotential becomesincreasinglybufferedbythehalideelectroactivity.This behaviorcanbeattributedtoaninteractionbetweenhydroxyl radi-cals(physisorbed)and Cl− toformactive chlorinespecies (e.g., Cl−+•OH→ClO−+H+₊_e−₎ _on _BDD _surface _[22,23]_. _It _may

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Fig.3. (a)%ofCODremovaland(b)%TCE,asafunctionofappliedcurrentdensity

(2.5and5.0mAcm−2),duringdirectelectrochemicaloxidationofrealefﬂuentby

usingPt/TiandBDDanodes.

undesirable[9,11],suchasCl•,Cl2,ClO2−andClO3−,ClO4−,

respec-tively)withthisnon-activematerial.Afterthat,whenanincrease onthe NaCl concentrationwas attained (from 0.4moldm−3 to 0.6moldm−3), polarization curves are shifted to less positive potentials(Fig.2b).

Basedontheresultsobtained,theconcentrationofhalide in solutionincreasestheimportanceofCl2/oxy-chlororadicalssystem

dependingontheelectrocatalyticmaterialusedandthisbehavior playsanimportantroleinrelationwiththeo.e.r.[24],inﬂuencing ontheefﬁciencyofelectrochemicalapproachadopted[25,26].

3.2. CODremovalbyelectrochemicaltreatment

Fig.3a shows%ofCODremovalusingPt/Tianodeby apply-ing2.5and 5.0mAcm−2 after2hofelectrolysis.Results clearly showedthatatPt/Tielectrode,amodestCODremovalwasobtained independentonthecurrent density(2.5and 5.0mAcm−2), cor-respondingto 25.2% and 30.49%, respectively. Under the same experimentalconditions,usingBDDelectrode,itoccurredasmall CODdecay(1%and4.5%at2.5and5.0mAcm−2,respectively).These resultsclearlyindicatethat,thelowconductivity,saltscontentand organicmatterdissolvedintheeffluentcomplicatethedepuration treatment[11].Ahighercharge isconsumedforcomplete min-eralizationduringtheelectrochemicalprocessbecausearelative greateramountof•OHiswastedinparasitenon-oxidizingreactions suchasoxygenevolution[21].Itcanbeconfirmedfromthecurrent efficiencies(TCE,in%)obtainedforeachcurrentdensityapplied

undertheseconditions(Fig.3b).Ifthisbehaviorisattained,itis fre-quentlycharacteristicofelectrolysisundermasstransportcontrol whentheelectrolysisisperformedapplyingacurrenthigherthan thelimitingone,asalreadyindicatedbyotherauthors[8,21,27].

Forarecirculationrateof250dm3_h−1_,_the_mass_transfer

coef-ﬁcientwas2.5×10−5ms−1andconsequently,thelimitingcurrent (Ilim)wasapproximately0.78A,accordingEq.(12).However,this

currentishigherthanthecurrentsapplied(Iappl)inthiswork(0.16

and0.32A),indicatingthatthepreliminaryassumptionisincorrect. Asacommoncondition,whenIapplisminorthanIlim;the

cur-rentefﬁciencyiscloseto100%,andtheCODdecreaseslinearlywith time,suggestingthattheoxidationundertheseconditionscould beoccurringundercurrentcontrol[8].However,lowerTCE val-ues(Fig.3b)indicatethatthelastconditionwasnotattained.This behaviorcouldbeduetotheuseofappliedcurrenttofavorother electrochemicalprocesses,suchastheproductionofoxidantsand theoxidation/reductionofions[22].Infact,theexisting concentra-tionsofCl−,NO3−andSO42−intheefﬂuent(as-obtained)promote

theoccurrenceofparallelreactions,consumingtheapplied cur-rent.IntheparticularcaseofCl−,initialconcentrationof208.6mg/L (0.00356moldm−3)isenoughtoshifttheo.e.r.topositive poten-tialsatbothelectrodes,beingmoreevidentthisbehaviortoBDD anode.Itsuggeststhatotherelectrochemicalreactionscantake placewhileanodicoxidationoccurs.

Undertheseconditions,althoughtheapplicabilityofthis treat-mentseemsfeasible,longtimeswouldberequiredtocomplete organicmatterremoval.Forthisreason,newsetofexperiments was performed to generate efﬁciently reactive oxidant species (activechlorine),plushydroxylradicals.

3.3. EffectofNaCldissolvedintheefﬂuent

CODdecaywasalsomonitoredbyapplying2.5and5.0mAcm−2 ofcurrentdensitywhenanamountof2.5gofNaClwasdissolvedin 2dm3_of_the_efﬂuent_(0.021_mol_dm−3_)._Fig.₄_shows_the_inﬂuence

ofthecurrentdensityonthepercentageofCODremovalduring indirectelectrochemicalapproachfortreatingarealwastewater usingbothanodes,at25◦C.ResultsclearlydemonstratedthatPt/Ti anodewasmoreefficientthanBDDelectrode,achieving63.7%and 60%for2.5and5.0mAcm−2,respectively.WhereasatBDD elec-trode,%CODremovalswereabout15%and47%underthesame conditions.Fromthesefigureswecaninferthat,whileatdirect oxidationlowerefficiencieswereachieved(Fig.3),moreefficient processwasperformedwhenaconsiderableamountofCl− was addedtotheeffluent(Fig.4),exceptingforBDDat2.5mAcm−2.

As already stated by other researchers, the change from Pt toBDD shouldnotinvolvedramaticchangesintheincineration mechanismbecausetheoxidation reactionsshouldbemainlya setofvolumeratherthansurfacereactionsanditdependsmainly onoxidantspeciesproduced[28],inthecaseofchloride media-tion.However,restrictingnowourresultstothepotentiodynamic measurements,theprocessseemstobetheconsequenceofsome speciﬁcrolesplayedbythehalogensaltontheanodesurfaceto generateactivechlorinespecies.

ForPt/Ti,theeffectofchloridesmaybeahybridoftwo mecha-nismswheretheanionmaypartiallychangethestoichiometryand microstructureoftheoxideﬁlmthatgrowsonthePtelectrode sur-faceatstronglypositivepotentials[24];asaconsequence,theo.e.r. islessfavoredandtheelectrochemicalincinerationisconsequently privileged.Infact,thequasi-statepolarizationcurvesindicatethat, between0.001and0.01moldm−3ofCl−concentrations,theo.e.r.is inhibitedbuttheproductionofstrongoxidantsismodest.Then,if theelectrogenerationofstrongoxidantsispartiallyattained,the oxidationoccursprincipallyinthevicinityoftheanodesurface wherethemixtureofoxidantsreactswiththeorganicpollutant, justifyingtheenhancementintheCODremovalinchloridemedia.

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Table1

EnergyconsumptioncalculatedfromEq.(13),pervolumeoftreatedefﬂuentduringdirectandmediatedoxidationofarealefﬂuent.

Experimentalconditions Energyconsumption(kWhm−3) Cost

Anode Appliedcurrentdensity(mAcm−2₎

Pt/Ti 2.5 0.54 0.16a_(0.07)b Pt/Ti 5.0 3.86 1.15a_(0.52)b BDD 2.5 1.50 0.45a_(0.20)b BDD 5.0 5.11 1.53a_(0.69)b Pt/Ti 2.5+NaCl1.25gdm−3 _0.75 _0.23a_(0.10)b Pt/Ti 5.0+NaCl1.25gdm−3 _2.01 _0.60a_(0.27)b BDD 2.5+NaCl1.25gdm−3 1.00 0.30a_(0.13)b BDD 5.0+NaCl1.25gdm−3 2.72 0.82a_(0.37)b

a_Brazilian_currency_(real).

b_US_currency_(dollar).

Fig.4.(a)%ofCODremovaland(b)%TCE,asafunctionofappliedcurrentdensity

(2.5and5.0mAcm−2_),_during_Cl-mediated_oxidation_of_real_efﬂuent_by_using_Pt/Ti

andBDDanodes.

Conversely,atBDDanode,thepotentiodynamicmeasurements supporttheideathatstrongoxidantsaregeneratedinaﬁrststage, andconsumedinasolutionregionstrictlyconﬁnedaroundthe elec-trodesurface.Thegeneratedoxidantsmaybesimply•OH,S2O82−

andHClO/ClO−(whenlowerCl−concentrationisused).But,forCl− concentrationsabove0.1moldm−3,amorecomplexsituationcan beattainedbecausetheproductionofchloroandchloro-oxy radi-calsaswellastheirreactivityarefavoredduetotheincreaseofthe importanceofCl2/oxy-chlororadicalssystematBDDsurface.The

occurrenceofHClOcanbeconsidered,onlywithintheNernstlayer asaconsequenceofaslightacidicconditionatanodesurfacebythe

productionofoxygen;andthisactivechlorinespeciesparticipate intheoxidationoforganicmatter.However,theenhancementon theeliminationoforganicmatterwasattainedathighercurrent density(5.0mAcm−2),indicatingthattheactivechlorinespecies areefficientlyproducedundertheseexperimentalconditions.This behaviorexplainsthelowerCODremovalefficiencyat2.5mAcm−2. Conversely, at Pt/Tielectrode, higher efficiencieswere attained by applying 2.5 and 5.0mAcm−2 because an efficient produc-tion of active chlorine was attained. These assumptions are in accordancewiththeperformancesobtainedduringCODremoval, at different appliedcurrent density. Another important feature wasthat,ICanalysisshowedthattheconcentrationofsomeions decreasedafterelectrochemicaltreatment.Forexample,atPt/Ti anodebyapplying2.5mAcm−2at25◦C,theconcentrationswere of2.6mgdm−3ofNO3−,1.2mgdm−3ofCa2+,4.1mgdm−3ofNa+,

2.7mgdm−3ofMg2+_and_12.6_mg_dm−3_of_SO₄2−_.

3.4. Energyconsumptionestimation

BasedontheCODvaluesobtainedatdifferentappliedcurrent densities,energyconsumptionwasestimatedbyEq.(13).Table1

presentstheelectricalenergyrequiredpervolumeoftreated efﬂu-ent at both anodes. For example, in the case of Pt/Ti without additionofNaCl,itincreasesfrom0.54to3.86kWhm−3ofefﬂuent treatedwhenthecurrentdensitypassesfrom2.5to5.0mAcm−2.

Moreover,inthesameconditionsbutwiththeadditionofNaCl (1.25mgm−3),itcausedaslightincreaseinenergyconsumption from0.54to0.75kWhm−3(Table1),whileinothercasetheNaCl additionresultedinacleardecreaseofenergyconsumption.For BDDanode,theadditionofNaClintheeffluentpromotesan impor-tant decrease in the energy consumption, and consequently, a reductiononthecosts.Forexample,1.50kWhm−3areconsumedin theabsenceofCl−intheeffluent,butitisreducedto1.00kWhm−3 whenanamountofNaClof1.25gdm−3,wasadded.Comparing theelectrical energyconsumed byPt/Tiand BDDanodeswhen 2.5mAcm−2areapplied,intheabsenceorpresenceofNaClinthe effluent,theresultsclearlyindicatedthatBDD ismoreefficient, butlowerefficienciesonorganicmatterremovalwereachieved (seeFig.4a).

Finally,takinginto considerationanelectricalenergy costof aboutR$0.3(Brazilianprice,taxesexcluded)perkWh (Agência NacionaldeEnergiaElétrica,Brazil),theprocessexpenditurewas estimatedandreportedinTable1inordertoshowtheviabilityof thisprocessasagreenalternativeforthetreatmentofurban waste-water.Thispricewasalsoconvertedtodollarandreportedinthe sametable.

4. Conclusions

On the basis of the results obtained for direct and medi-atedoxidationofarealwastewaterefﬂuent,theelectrochemical

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technologyisefficientwithalowoperationcostwhenthe addi-tionofasmallamountofNaClisperformed.Theresultspointout thehighperformancePt/Tianodicoxidationfor treating waste-watereffluentcomparedtoBDDanode.Although,theuseofNaCl favorstheproductionofstrongoxidantspeciesthattogethertoROS (suchas,hydroxylradicals)oxidizedissolvedorganicmatterinreal effluent(closetoanodesurfaceandinthereactioncage)[28–30]; particularattentionandexperimentalobservationsmustbetaken intoconsiderationbeforeorduringitsuseduetotheproduction oforganochloridecompounds(pollutantswerenotdetectedafter electrolysis).Finally,theresultsreportedinthepresentworkhave recentlyallowedtostartthedesignandimplementationofapilot electrochemicalcellinthetreatmentstationattheUFRN.These experimentsareinprogressandtheirresultswillbereportedin detailinaseparatepaperinanearfuture.

Acknowledgements

Theauthorsthanktheﬁnancial supportprovidedby CAPES-REUNIstudentfellowshipsandtheyalsothankIndustrieDeNora S.p.A.(Milan,Italy)forprovidingthePt/Tielectrodes.

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