Reversible electrokinetic adsorption barriers for the removal of atrazine and oxyfluorfen from spiked soils

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ContentslistsavailableatScienceDirect

Journal

of

Hazardous

Materials

jou rn a l h om ep ag e :w w w . e l s e v i e r . c o m / l o c a t e / j h a z m a t

Reversible

electrokinetic

adsorption

barriers

for

the

removal

of

atrazine

and

oxyﬂuorfen

from

spiked

soils

E. Vieira

dos

Santos

a

_,

_C.

_Sáez

b

_,

_P.

_{Ca ˜}

_nizares

b

_,

_C.A.

_{Martínez-Huitle}

c

_,

_M.A.

_Rodrigo

b,∗ a_School_of_Science_and_Technology,_Federal_University_of_Rio_Grande_do_Norte,_Campus_{Universitario,}_59078-970_Natal,_Brazil

b_Department_of_Chemical_Engineering,_Enrique_Costa_Building,_Campus_{Universitario}_s/n,₁₃₀₇₁_Ciudad_Real,_Spain c_Institute_of_Chemistry,_Federal_University_of_Rio_Grande_do_Norte,_Campus_{Universitario,}_59078-970_Natal,_Brazil

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g

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s

•REKABtechnologyefﬁcientlyremovesatrazineandoxyﬂuorfenfromsoil.

•SynergisticinteractionbetweenelectrokineticsandGACpermeablereactivebarrier.

•Signiﬁcantcarbonbed-adsorptionmechanismforatrazineandoxyﬂuorfen.

•VolatilizationofherbicidesispreventedintheREKABtechnology.

•ReversiblechangesinthepolaritycontrolavoidextremepHvalues.

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Articlehistory: Received26July2016 Receivedinrevisedform 23September2016 Accepted14October2016 Availableonline17October2016 Keywords:

Electrokinetic REKAB Soilremediation Permeablereactivebarrier Atrazine

Oxyﬂuorfen

a

b

s

t

r

a

c

t

Thisstudydemonstratestheapplicationofreversibleelectrokineticadsorptionbarrier(REKAB)

tech-nologytosoilsspikedwithlow-solubilitypollutants.Apermeablereactivebarrier(PRB)ofgranular

activatedcarbon(GAC)wasplacedbetweentheanodeandcathodeofanelectrokinetic(EK)soil

remedi-ationbench-scalesetupwiththeaimofenhancingtheremovaloftwolow-solubilityherbicides(atrazine

andoxyfluorfen)usingasurfactantsolution(sodiumdodecylsulfate)astheflushingfluid.This

innova-tivestudyfocusedonevaluatingtheinteractionbetweentheEKsystemandtheGAC-PRB,attempting

toobtaininsightsintotheprimarymechanismsinvolved.Theobtainedresultshighlightedthe

success-fultreatmentofatrazineandoxyﬂuorfenincontaminatedsoils.Theresultsobtainedfromthetestsafter

15daysoftreatmentwerecomparedwiththoseobtainedusingthemoreconventionalelectrokineticsoil

ﬂushing(EKSF)technology,andveryimportantdifferenceswereobserved.Althoughbothtechnologies

areefﬁcientforremovingtheherbicidesfromsoils,REKABoutperformsEKSF.Afterthe15-daytreatment

tests,onlyapproximately10%ofatrazineandoxyﬂuorfenremainedinthesoil,andadsorptionontothe

GACbedwasanimportantremovalmechanism(15–17%ofherbicideretained).Theevaporationlosesin

REKABwerelowerthanthoseobtainedinEKSF(45–50%comparedto60–65%).

1. Introduction

Overthepastdecades,concernsabouttheapplicationof herbi-cideshavebeenincreasing,particularlyabouttheireffectsonthe environment[24].Currently,theuseofherbicidesisverycommon inmostagriculturalregionsoftheworld,providinggreat advan-tagesrelated toimprovedcropproduction.Consequently, since themid-1940s,theindustrialproductionoforganicherbicideshas continuouslyincreased, and even now it continues to

progres-∗ Correspondingauthor.

E-mailaddress:manuel.rodrigo@uclm.es(M.A.Rodrigo).

sivelyincrease.Thenegativeenvironmentalimpactofherbicides is related towater and soilcontamination, which hasreceived a considerableamountof attentionfromthescientific commu-nity[23,10,9,5,7,26].Fromanenvironmentalperspective,avery interestingclassificationofherbicidesisrelatedtotheirsolubility. Low-solubilityherbicidesaretypicallyformulatedasemulsionsto favourtheirapplication.Thetransportpropertiesofsuchherbicides when theybecomepollutants stronglydepend onthematrices used in the commercialformulation. Among thelow-solubility herbicides,there aretwo thatare ofparticularinterest andare the focusof this manuscript:oxyfluorfen and atrazine. Oxyflu-orfen(2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl) benzene)isadiphenyl-etherherbicidethatisusedforbroad

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and grassy weeds [15]. This herbicide has low water solu-bility (0.1mg/dm−3 at 22◦C), low vapour pressure (0.026mPa at 25◦C), high Koc (log Koc=3.46–4.13) and high Kow (log Kow=4.86). Atrazine (1-chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine) is characterized by a low biodegradability, long half-life,lowvapourpressure(0.0385mPaat25◦C)andlow sol-ubilityinwater(33mgdm−3 at22◦C)[14,20],andbecauseofits hazardousness,itwasbannedintheEuropeanUnion,althoughit remainsinuseinmanyothercountriesaroundtheworld.

Oneoftheworsttypesofeventsrelatedtothepollutioncaused bytheseherbicidesisassociatedwithaccidental leakage,which maybecomeamajorsourceofdiffusepollution.Toprevent seri-ousenvironmentalproblemsunderaccidentaldischargesofthese species,itisveryimportanttotakerapidactionsagainstaccidental dischargesofhazardousspeciesusingefficienttechnologiesthat helptorapidlyremediatethesoil[22].Electrokinetic(EK) remedi-ationintegratedwithpermeablereactivebarriers(PRBs)hasbeen investigatedbyseveralauthorsinrecentyears[21,2,3,31,16],and thistechnologyappearstobeaverypromisingalternative.This technologicalapproach wasimplemented primarilybecauseEK alsoenablestheuseofPRBinlow-permeabilitysoils[11].Thus, whena PRBis coupledwithEKremediation, theflow of pollu-tantsthroughthePRBisnotprovidedbythetransportdrivenby thehydraulicgradientofgroundwater;rather,itisdrivenbythe electro-osmoticflowofsoilporefluid,electromigrationor elec-trophoresis(particularly inlow-permeability soils)[6,30,13].As thecontaminatedgroundwaterpassesthroughthePRB, contam-inantsmaybedegradedor sequesteredand cleangroundwater exitsthePRB.Thereactivematerialscommonlyconsideredinclude reductionsusingelementalmetals,adsorptionwithporous high-surface-areamaterials,ionexchangewithresin-basedmaterials, biological degradation, limestone, çhydroxyapatite, active car-bonand zeolites[11].Theuseofinexpensivereactivematerials aspermeablebarriers,suchas granularactivatedcarbon(GAC), shouldcontributetowardsimprovingthecosteffectivenessofthe combinedtreatmentandincreasingenvironmentalsustainability

[12].Consequently,PRBshavebeenextensivelyproposedforthe remediationofinorganicand organicpollutantsingroundwater

[28,30,13,8].

RegardingtheuseofEK-PRBwithherbicides,afirstexperience intheapplicationofthecombinationofEKSFwithadsorption bar-rierswasreportedfortheremovaloftrichlorophenol(notexactlya herbicidebuthighlyrelatedtothesetoxicpollutantsfroma chemi-calperspective)fromspikedsoils[25],whereitshighefficiencyand easyperformanceweredemonstrated.Anotherinteresting expe-riencecamefromtheuseofbiobarriersintheremovalofdiesel, whereitwasconcludedthattheapplicationofpolarityreversal allowsforconsiderablybetterperformance[16,18,19].The advan-tagesofreversiblechangesinpolaritywerealsonotedbymany otherauthors[1].Thereversiblechangesinpolarityhelpto sup-pressacidificationandbasificationofthecontaminatedsoilinthe closevicinityoftheelectrodes’surfacesandpreventsthedepletion ofionicspecies.Thisfactisofspecialrelevanceincombinations ofEKSFwithbiologicalbarriersbecausenon-reversibleprocesses leadtotheexhaustionofnutrientsinsoil.

Todate,nostudiesontheremovalofatrazineandoxyﬂuorfen fromsoilbyEK-PRBcouplinghavebeenreported.Consequently, theauthorsofthepresentworkconsideredthatitwouldbe inter-estingtoinvestigatethefeasibilityofcouplingEKSFandGAC-PRBto removeatrazineandoxyﬂuorfenfromlow-permeabilitysoilusing aninnovativeprocesscalledreversibleelectrokineticadsorption barrier(REKAB)technology.Forthesettingofthisprocess,the pre-viousexperienceofourgroup,gainedinthedevelopmentofeasier technologiessuchasEKcombinedwithnonreversibleadsorption permeablebarriersandbiobarriershasbeenused.Inthiscontext,

fromclaysoilsusingelectrokineticallyassistedsoilflushing(with sodiumdodecylsulfate(SDS)astheflushingsolution)coupledwith aPRBconsistingofbedsofGACandtoassesstheinfluenceofthe electricfieldontheefficiencyofthistechnology.

2. Materialsandmethods

2.1. Chemicals

Kaolinite, provided by Manuel Riesgo Chemical Products (Madrid,Spain),wasusedasamodelofclaysoil[17].Atrazineand oxyﬂuorfen(Sigma-Aldrich),wereofanalyticalgradeandusedas received.HPLC-gradeacetonitrile(Sigma-Aldrich,Spain)wasused asthemobilephaseinhigh-performanceliquidchromatography (HPLC)analyses.Hexaneandethylacetate(Sigma-Aldrich,Spain) wereusedassolventsfortheextractionofliquidandsolid sam-ples.Granularactivatedcarbon(granulesizeof1.25–3.15nm)was purchasedfromPanreac(Spain).Graphiteelectrodes(100.0cm2₎

providedbyCarbosystem(Madrid,Spain)wereusedasthe elec-trodematerial.

2.2. Experimentalprocedure

Thebenchsetupusedinthisworkwasconstructedfrom trans-parentmethacrylateanddividedintosevencompartments(Fig.1)

[29].

Thecentralcompartmentwitha lengthof20cmwasloaded with herbicide-polluted soil and an active carbon PRB, which wasmanuallycompactedandseparatedfromtheelectrode com-partmentsbya0.5mmnylonmesh.Oneofthesecompartments containedtheanode,andtheothercontainedthecathode.Each electrode compartment was connected to additional compart-ments to collect theliquid overﬂowing from the wells that is transportedduetotheEKprocesses.Theexperimentswere per-formedinpotentiostaticmode,i.e.,settingavoltageof1.0Vcm−1. Thedurationoftheexperimentswas15days,whichislongenough toprovideaclearoverviewofthemainprocessoccurringinthe soilandshortenoughtoavoidthedepletionofherbicides(thiswas intentionallysoughttoevaluatetheremovalmechanisms).

BecausethecarbonPRBareaswereplacedinanintermediate sectionoftheinstallation,farawayfromtheelectrodes,itwas nec-essarytouseasurfactanttopromotethetransportoftheatrazine andoxyfluorfen.Aflushing fluidthatconsistedofaSDS surfac-tantsolution(1000mgdm−3)wasusedasthesolubilizingagent. BecauseSDSisananionicsurfactant,thesuperficialchargeofthe atrazine-SDSoroxyfluorfen-SDSmicellesisexpectedtobe nega-tive[27,4],andconsequently,theyareexpectedtobetransported fromthecathodiczonetowardstheanodiccompartment,withthe atrazineand oxyfluorfenbeingadsorbedwhen passingthrough thecarbonPRB.Thepolarityoftheelectricfieldappliedbetween theelectrodeswasreversedonceaday(valueselectedarbitrarily), andthisperiodicchangewillhelptoexplainthezig-zagchanges observedinthepHvaluesoftheelectrolytescontainedinthewells throughouttheexperiment.

Themodelsoilwasspikedwith960mLofherbicideaqueous solutions (100mgdm−3)until aninitialpollutantconcentration of 30mg per kg of soil was obtained. In the case of atrazine andoxyfluorfensolutions,SDSwasusedasthesolubilizingagent (1000mgdm−3).Theinitialtargetmoisturelevelofthesoilwas 30%.Thepollutedsoilwasmanuallycompactedinanattemptto avoidtheformationofheterogeneitiesinthesoil,whichmayresult inpreferentialpathsforfluidtransport. Thelevelsof theanode wellsweremaintainedusinga levelregulationloop.Waterwas pumpeddailyfromthecathodewellandelectroosmoticflux(Jeo)

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Fig.1.(a)ExperimentalsetupusedtostudytheREKABprocessand(b)samplingpointforpostmortemanalysis.

wasestimatedbyequation1,whereVisthevolumeofwater recov-ered(cm3_),_t_is_the_time_(d)_and_S_is_the_section_of_the_soil_(cm2_).

Evaporationlosswasestimatedbymassbalance. Jeo=

dV dt 1

S (1)

Electricalcurrent,temperature,pH,andpollutantconcentration intheelectrolytecompartmentsweremonitoreddaily.Moreover, attheendoftheexperiments,anin-depthsectionedanalysisof thecompletesoilsectionandofthePRBusedwereperformed(see

Fig.1b).Thishelpstoestimatetheamountofpesticidetransported byelectromigrationandelectroosmosis,andretainedinthePRB. Theamountofpesticidetransferredtoatmospherewasestimated bymassbalance.

2.3. Adsorptionequilibria

Tobetterunderstandtheremovalofatrazineandoxyfluorfenby REKAB,adsorptionequilibriumisotherms(25◦C)wereobtainedby conductingseveralbatchtestsusingagitatedvessels(0.25L)with 0.1Lsolutionsof100mgL−1atrazineandoxyfluorfenand increas-ingamounts(2–100mgL−1)ofactivatedcarbonuntilequilibrium wasreached.Areactiontimeof24h(asestablishedby prelimi-narykinetictests)wasusedforsubsequentequilibriumtests.After thespecifiedtime,thesampleswerefilteredthrough0.45␮m cel-luloseacetatesyringemembranefiltersandanalysedbyHPLCas discussedinSection2.5.ThesolutionpHwasnotadjustedtokeep theequilibriumsystemssimple,limitingtheeffectsofionsfrom acidorbaseaddition,andbecausetheimpactofthesolutionpHon theuptakeofatrazineoroxyfluorfenwasconsideredtobe insignif-icant.

2.4. Analyticaltechniques

Moisture measurements were performed gravimetrically by dryingthesoilsamplesin anovenfor24hat105◦C. To deter-minethepHandconductivityofthesoilsamples,thestandard

method(E.P.A.-9045C,1995)forsaturatedsoilwasused.ThepH measurementswereperformedusingaWTWinoLabpHmeter. Conductivity wasmeasuredusing a GLP 31 conductivitymeter (CrisolInstruments,Spain).Allthesamples(pre-andpost-mortem) werefilteredthrough0.45_␮mnylonfilterspriortoanalysis.The atrazineandoxyfluorfenconcentrationsintheliquidsampleswere determined daily usinga liquid-liquid(L-L) extractionmethod. Theatrazineandoxyfluorfenconcentrationsweredeterminedby HPLCusinganAgilent1100(AgilentTechnologies,PaloAlto, Cal-ifornia,EEUU)equippedwithaUVdetectoranda150×3.0mm Gemini5LC18110acolumn(Phenomenex,Ref.00F-4435-YYO), withacetonitrile/water(45:55V/V)ataflowrateof0.3cm3_min−1

as themobile phase and at 223nmfor atrazineand with ace-tonitrile/water(70:30V/V)ataﬂowrateof0.25cm3_min−1_as_the

mobilephaseandat220nmforoxyﬂuorfen.Thetotalorganic car-bon(TOC)concentrationwasmonitoredusingaMultiN/C3100 AnalytikJenaanalyser.Thesoiltemperaturewasmonitoredusing aDigitalSoilthermometer.

3. Resultsanddiscussion

Inthiswork,toevaluatetheperformanceoftheREKABprocess, two testswereconducted,onefor each modelherbicidetested (atrazine andoxyﬂuorfen).Alloperating conditionswere main-tainedconstantinthetwotests.

PartaofFig.2presentsinformationaboutthechangesinthe appliedelectriccurrentasaconsequenceoftheapplicationofthe REKABprocesstobothspikedsoils.Duringthe15-daydurationof thetests,thecurrentintensityprogressivelyincreased10%, show-ingvaluesslightlyover4mAcm−2.Thisincreasecanbeassociated withasmallincreaseintheconductivityofthewellsandinthe moistureofthesoil(discussedafterwards),andhence,tothe result-inglowerohmicloses.PartbofFig.2focusesonthevariationsin pHandconductivityobtainedinthewells.AsinParta,thereare noconsiderabledifferencesbetweenthetwotests(eachofthem withdifferentherbicides),clearlyindicatingtherobustnessofthe

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0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 2 4 6 8 10 12 14 0 5 10 15

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0 1 2 3 4 5 0 5 10 15

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Time /

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Fig.2.Changesinthemainoperatingparametersduringtheremediationofthesoilbyelectroremediationpoweredbyapowersupplyat20Vcm−1_for_both_pesticides.

Atrazine(fullsymbols)andoxyﬂuorfen(emptysymbols)(a)currentdensity(䊏,䊐).(b)anodiccompartmentpH(䊏,䊐),cathodiccompartmentpH(䊉,),anodicreservoir conductivity(,♦),andcathodicreservoirconductivity(,).

0 1 2 3 4 5 6 7 8 9 10 0 1 2 3 4

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0 200 400 600 800 1000 1200 1400 1600 1800 2000 0 1 2 3 4

Co

nductivity

/

mS

cm

-1

Position

Barrier Barrier

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Fig.3.(a)pHmapand(b)conductivitymapofthesoilaftertheremediationtestforatrazine(fullsymbols)andoxyﬂuorfen(emptysymbols)remediation.Upperright position(䊉,),upperleftposition(䊏,䊐),bottomrightposition(,♦)andbottomleftposition(,)ofthesoilaftertheremediationtest.

resultsobtainedinthiswork.Notethatthepolarityofthe elec-tricﬁeldappliedbetweentheelectrodeswasreversedonceaday (timespanforthepolarityreversalwasarbitrarilyselected),and thisperiodicchangehelpstoexplainthezig-zagchangesobserved inthepHvaluesoftheelectrolytescontainedinthewells through-outtheexperiment.Consequently,italsohelpstodemonstratethat nopHgradientswereformedinthesoilbetweentheelectrodes (becauseofthedailyneutralization).Notethatabuffersolutionwas notaddedtotheelectrolyteduringthetests;theonlystrategythat wasappliedtoregulatethepHwasthedailyreversalofthe polar-ity.Asshowninthisﬁgure,thepHintheanodicandcathodicwells doesnottendtoextremevaluesasistheexpectedtrendinEKSF processes;rather,ittendstomoreneutralvalueswithintherange of7–11.Typically,asingleEKprocessgeneratesanacidicfrontin theanodicwellandabasicfrontinthecathodicwellasa conse-quenceof(i)theproductionofprotons(duringwateroxidation) andhydroxylanions(duringwaterreduction)intheanodeand cathodewells,respectively,asshowninequations2(anode)and 3(cathode),and(ii)thetransportofH+_and_OH−_ions_by_the

elec-tromigrationphenomenon.Hence,polarityreversalcontributesto betterEKremediationperformancebypreventingtheoccurrence ofextremepHchangesinregionsofthesoilclosetheelectrode wells.Thisisapositiveaspectbecauseaccordingtotheliterature,

[23,30,13]controllingthepHattheanodeandcathodeappearsto resultinfavourablesoilpHfordesorptionandelectro-osmoticﬂux

2H2O →O2(gas)+4H+(aq)+4e−E◦ = −1.229V (2)

4H2O+4e−→ 2H2(gas)+4OH−(aq)E◦ = −0.828V (3)

Regardingtheconductivity,aslightincreaseinboththeanolyte andcatholytewasobservedduringbothREKABtests,indicating thationsareremovedfromthesoilandtransportedtothewells. Thisincreaseinconductivityisprimarilyduetothe electromigra-tionofionstowardstheanodicandcathodicwellsandduetothe productionofprotonsandhydroxylionsduringwateroxidation andreduction,respectively.Asaconsequenceoftheﬁrstprocess, thereisadecreaseintheconductivityofthesoil.Thetransport oftheSDSaddedtofavourthemobilityoftheherbicidesisalso expectedtohaveaninﬂuenceonthechangesintheconductivity.

Fig.3presentsa3-DmapofthepHandconductivitydistribution attheendofthetests(post-mortemanalysis).Theﬁrstimportant aspecttonoteistheverysimilarbehavioursofbothtests.Likewise, notethatdespitethesigniﬁcanceoftheprocessesoccurringinthe electrodes,bothparametersaremaintainedalmostconstantinthe soilanddispersionisverylow.

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0 0.01 0.02 0.03 0.04 0.05 0.06 0 2 4 6 8 10 12 14 16

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Fig.4. Timecourseoftheelectroosmoticflux(䊏,䊐)andevaporationflux(䊉,)monitoredduringtheremediationprocess.Moisturemapofthesoilfollowingtheremediation test.Upperrightposition(䊉,),upperleftposition(䊏,䊐),bottomrightposition(,♦)andbottomleftposition(,)ofthesoilaftertheremediationtest.Atrazine(full symbols)andoxyfluorfen(emptysymbols).

Table1

Comparisonofsteady-statepH,conductivityandmoistureobtainedafterapplying15-daylongremediationtest(EKSF[29]orREKAB)tosoilspikedwithatrazine(ATZ)and oxyﬂuorfen(OXY).

EKSF(ATZ) EKSF(OXY) REKAB(ATZ) REKAB(OXY)

J(Am−2) 2.87 3.41 4.71 4.59 Moisture(%) 25.55 32.82 33.99 34.99 pHanode/cathode 1.98/14.55 2.04/14.01 8.59/10.00 7.95/11.00 Conductivity(␮Scm−1₎_{anode/cathode} _6417/18942 _5326/9389 _1154/915 _569/769 Post-mortempH 7.63 7.74 8.08 7.91 Post-mortemConductivity(␮Scm−1₎ _157.90 _183.90 _238.5 _241.04

Flowrateadded(cm3_h−1₎ _3.6 _7.3 _4.7 _4.5

FlowratecollectedEK(cm3_h−1₎ _3.2 _4.5 _4.1 _4.2

Fig.4showsthetransientchangesofthefluxesandthe mois-turemapattheendofthetests.Theelectroosmoticflowrates(fluid addedtotheelectrodeactingastheanode)increaseoverthetests toavalueintherangeof0.04–0.05cm3_cm−2_d−1_._Moreover,_the

waterevaporationﬂuxesaremaintainedapproximatelyconstantat avaluecloseto0.01cm3_cm−2_d−1_._According_to_part_b,_the

electro-osmoticﬂuxeshelptomaintainthemoistureinthesoilatvalues over30%(initialvalueset)despitetheevaporationbecausethereis nomoistureproﬁleintheanode-cathodedirectionandtheaxial dispersionislow. Again,thereproducibilitybetweenbothtests isaclearindicatoroftherobustnessofthemethodologyusedto evaluatetheREKABprocessatthebenchscale.

ToobtainmoreinformationabouttheperformanceoftheREKAB processintheremovaloflow-solubilitypollutants,Table1 com-paresthemainchangesobservedduringtheREKABtestswiththose obtainedinapreviousstudyinwhichtheremovalofatrazineand oxyfluorfenusing EKSFwasevaluated [29]withsimilar operat-ingparameters.ComparisonofthepHinthesteady-statevalues obtainedin theREKABand intheEKSFindicatesthat thereare importantdifferencesbetweenbothEKconfigurations,withmore extremepHvaluesinthecaseoftheEKSFtechnologywithboth herbicides.Aclearinfluenceoftheacidicandalkalinefronts com-pletelymovingthesoilmatrixisobservedinthepH.Adecrease inthe pHvaluesis generallyobserved in thesectionsnearthe anode,andasignificantincreaseisobservedinthesectionsnear thecathode.However,aspreviouslyexplained,withthe applica-tionofREKAB,thesemoreextremepHvaluesarenotachieved, andinthiscase,itcanbeinferredthatthepolarityreversal con-tributestoefficientperformance.NotethatthecombinationofEK withPRBisoccasionallyconsideredforeliminatingtheobstruction

ofthePRBsystemcausedbymineralprecipitationorobstruction ofactivesites[1].

Regarding conductivity, during the two tests, significant changeswereobservedintheelectrolytecontainedinthewells. ThesechangesaremoreimportantintheEKSFwithoutpolarity reversalthanintheREKABprocess,andtheyareconsistentwiththe fluctuationsobservedinthesoilintheEKSF,withhigheraxial dis-persionsintheproximityoftheelectrodewells.Thisbehaviourcan clearlybeattributedtothetransportofionicspeciesandthe fluc-tuationsinthepHvalues.ComparingthevaluesobtainedinREKAB withthoseobtainedinEKSF,theconductivityvalueishigherinthe processstudiedinthiswork.Thishighervaluecanbeexplained becauseincontrasttoEKSF,theperiodicreversalofthepolarity preventsthewashingupofionscontainedinthesoil.Thisisalso observedbycomparingtheconductivityoftheelectrolytes con-tainedinthewells,whichissignificantlyhigherinthecaseofEKSF. However,inthiscase,notonlythetransportofionsbutalsothe moreextremechangesinthepHshouldbetakeninto considera-tion.

Regardingthemoistureof thesoil,noimportantdifferences were observed between the two processes, despite the large changesin thevalues oftheﬂowrates,in whichthedifference (associatedwithevaporation)ishigherfortheREKABapproach.

Fig.5showstheconcentrationofherbicidesmeasuredinboth electrode wellsduring thetestsand the3-D mapof the pollu-tantdistributionattheendofthetests.Regardingthepollutant distribution,partashowstheconcentrationmapofatrazineand oxyfluorfeninthetests.Asshown,after15days oftreatment,a decreaseintheconcentrationofpesticidesremaininginthesoil isregistereduntiltheconcentrationislowerthan5mgperkgof soil,andbothpollutants,atrazineandoxyfluorfen,wereefficiently

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0 5 10 15 20 25 30 0 1 2 3 4

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0 5 10 15 20 25 30 0 2 4 6 8 10 12 14 16

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b)

Fig.5.(a)Concentrationmapofthesoilaftertheremediationtestforbothatrazine(fullsymbols)andoxyﬂuorfen(emptysymbols).Upperrightposition(䊉,),upper leftposition(䊏,䊐),bottomrightposition(,♦)andbottomleftposition(,)ofthesoilaftertheremediationtest.(*)initialconcentrationofpollutants.(b)Changesin theconcentrationsofatrazine(fullsymbols)andoxyﬂuorfen(emptysymbols)thatarriveattheelectrodewellsduringtheremediationtest.Concentrationintheanodic chamber(䊏,䊐),concentrationinthecathodicchamber(䊉,).

Fig.6.Adsorptionisothermsof()atrazineand(䊏)oxyﬂuorfenontotheactivated carbonused.Experimentalconditions:batchtestsusingagitatedvessels(0.25L) with0.1Lsolutionsof100mgL−1_atrazine_and_oxyﬂuorfen_and_increasing_amounts

(2–100mgL−1)ofactivatedcarbonduring24huntilequilibrium.

removed,achievingapproximately90%removalduringthe treat-mentofsoil.Thesignificantlyhighconcentrationsofatrazineand oxyfluorfeninthecentralregionofthesoilafterthetreatmentcan beattributedtothepresenceofGAC-PRB.Regardingtheherbicide collectedintheelectrodewells(partb),notethatthedailychange intheelectrodepolarityallowsbothwellstoactaseithertheanode orthecathodedependingontheday.Thefinalconcentrationsof atrazineandoxyfluorfencollectedintheelectrodewellsarevery low,whichmeansthatpolarityreversalisnotaverygoodoption forefficientEKtransport.Thisobservationwasalsonotedwiththe conductivity.However,itwasnotthemainremovalmechanism soughtwiththeREKABprocess;rather,thisprocessutilizedthe adsorptionofthepollutantintheactivatedcarbonbedplacedin thesoilcolumn.

ThedifferentadsorptioncapabilitiesofGACtowardsatrazine andoxyfluorfencanbeeasilyunderstoodbyexaminingthe adsorp-tionisotherms(Fig.6),whichwereestimatedfortheconcentration rangeofinterestbymixingsolutionscontaining100mgdm−3 of herbicideanddifferentamountsofactivatedcarbon.Asshown,the adsorptioncapacityoftheactivatedcarbonisnotsaturatedforthe concentrationrangetested,andtheadsorptioncapacityof oxyflu-orfenishigherthanthatofatrazine,whichisconsistentwiththe resultsobtainedintheREKAB process.Likewise,itisconfirmed

Fig.7. Averageprofilesofthepesticidesattheendoftests.Atrazine(,䊉)and Oxyfluorfen(,).Reversibleelectrokineticadsorptionbarrier(REKAB):empty points;electrokineticsoilflushing(EKSF):fullpoints.

thatactivatedcarboniseffectivefortheremovalofbothherbicides, despitetheirlowsolubility.

Fig.7comparestheaverageconcentrationprofileinthesoilafter 15daysoftreatmentintheremovalofthetwoherbicidesbyREKAB andEKSF.Similaritiesbetweenthebehavioursforbothpesticides werealsoobservedduringtheapplicationoftheEKSFprocess,and itisveryinterestingtocomparetheprofilesofpesticidesinsoilafter theapplicationofthetworemediationtechnologiesbecauseitmay helptounderstandthetransportmechanism,andhence,itcould beusedtooptimizetheremoval.InthecaseoftheREKABprocess, theherbicideconcentrationprofilesafterthetestsaremore simi-lar,clearlyindicatingthatabetterremovalofatrazineisachieved, whichisincontrasttothebetterremovalofoxyfluorfenobserved inthecaseofEKSF.Thisbehaviourcanbeexplainedbythemore efficientadsorptionindicatedbytheadsorptionisotherms.

Insummary,themechanismsfortheremovalofherbicidesin bothtechnologiesareverydifferent.Thepercentagesofherbicides removedbythedifferentmechanismsandtheherbicides remain-inginthesoilafter15daysoftreatmentareshowninFig.8.This ﬁgurecomparestheresultsfortheremovaloftheherbicidesinthis workwiththoseofapreviousworkfocusedonatrazineand oxyﬂu-orfenusingthesametechnologyandexperimentaldevice[29].In

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0%

20%

40%

60%

80%

100%

REKAB oxyfluorfen REKAB atrazine EKSF oxyfluorfen EKSF atrazine

removal pesticide/ %

adsorption evaporated transport cathode transport anode remaining soil

Fig.8.Averageproﬁlesoftheherbicidesattheendoftests.

thecaseofatrazineandoxyfluorfen,theirremovalissignificantly favouredwhenthesepollutantsaremobilizedbyelectromigration andelectro-osmoticfluxes.

Despite being similar herbicides, this ﬁgure highlights the importanceofbench-scalestudiesforunderstandingthe perfor-manceofEKsoilremediationtechnologies.Evenfortwoherbicides thatareinsolubleinwater,themassbalanceindicatesthatremoval stronglydependsontheadsorptionandvolatilityproperties.Even whentheevaporationphenomenonappearstobeimportantduring thesoiltreatment,theevaporatedfractionsdonotdependonthe herbicidecontainedinthesoilbutratheronlyonthe characteris-ticsofthesoil.Notethatthistypeofphenomenonnaturallyoccurs, andconsideringthisphenomenon,innovativealternativesforsoil remediationcanbeapplied.Theseexperimentsareinprogress,and theirresultswillbereportedindetailinaseparatepaperinthenear future.

4. Conclusions

Fromthiswork,thefollowingconclusionscanbedrawn:

-REKABis anefﬁcienttechnologyfortheremovalofnon-polar herbicidesfromsoils.

-The combination of EKSF withadsorption-PRB technology in shortperiods(15days)appearstobeaninnovativealternative todepollutesoils.

-ThereareimportantdifferencesbetweenREKABandEKSF.The adsorptionofpesticideontoactivatedcarbonprevents evapora-tion.

-Atrazineandoxyﬂuorfenareefﬁcientlytransportedinthe com-binedREKABtechnologybyelectromigrationandelectro-osmotic processes.

-PolarityreversalhasapositiveeffectonregulatingthepHandon preventingthewashingupofthesaltscontainedinthesoil.

Acknowledgements

TheauthorsacknowledgeﬁnancialsupportfromtheEUand Spanish Government through the MINECO Project CTM2013-45612-R.FinancialsupportfromtheNationalCouncilforScientiﬁc andTechnologicalDevelopment–Brazil(CNPq–446846/2014-7) isgratefullyacknowledged.

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