Carbon isotope fractionation of 1,1,1-trichloroethane during base-catalyzed persulfate treatment

ContentslistsavailableatSciVerseScienceDirect

Journal

of

Hazardous

Materials

j o ur na l h o 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 h a z m a t

Carbon

isotope

fractionation

of

1,1,1-trichloroethane

during

base-catalyzed

persulfate

treatment

Massimo

Marchesi

_,

_Neil

_R.

_Thomson

_,

_Ramon

_Aravena

_,

Kanwartej

S.

Sra

_,

_Neus

_Otero

_,

_Albert

_Soler

a_{DepartmentofCivilandEnvironmentalEngineering,UniversityofWaterloo,Waterloo,Ontario,CanadaN2L3G1}

b_{DepartmentofEarthandEnvironmentalSciences,UniversityofWaterloo,Waterloo,Ontario,CanadaN2L3G1}

c_{GolderAssociatesInc,Toronto,Ontario,CanadaL5N5Z7}

d_{DepartamentdeCristal.lographia,MineralogiaiDipositsMinerals,FacultatdeGeologia,UniversitatdeBarcelona,Barcelona,Spain08028}

h

i

g

h

l

i

g

h

t

s

•TreatabilityandCfractionationof1,1,1-TCAbybase-catalyzedS2O82−wasstudied. •Therateofdegradationof1,1,1-TCAincreasedwithahigherOH−:S2O82−ratio. •Base-catalyzedS2O82−canpotentiallytreatrecalcitrantcompoundlike1,1,1-TCA. •Anenrichmentfactorof−7.0‰independentoftheOH−:S2O82−ratiowasobtained. •CarbonisotopecanpotentiallybeusedtoestimatetheISCOtreatmentefficacy.

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Articlehistory:

Received6December2012

Receivedinrevisedform1May2013

Accepted7May2013

Available online 14 May 2013 Keywords:

Insituchemicaloxidation(ISCO)

1,1,1-Trichloroethane(1,1,1-TCA)

Base-catalyzedpersulfate

Isotopefractionation

a

b

s

t

r

a

c

t

Theextentofcarbonisotopefractionationduringdegradationof1,1,1-trichloroethane(1,1,1-TCA)bya base-catalyzedpersulfate(S2O82−)treatmentsystemwasinvestigated.Significantdestructionof

1,1,1-TCAwasobservedatapHof∼12.AnincreaseintheNaOH:S2O82−molarratiofrom0.2:1to8:1enhanced

thereactionrateof1,1,1-TCAbyafactorof∼5toyieldcomplete(>99.9%)destruction.Anaverage car-bonisotopeenrichmentfractionationfactorwhichwasindependentoftheNaOH:S2O82−molarratioof

−7.0±0.2‰wasobtained.Thissignificantcarbonisotopefractionationandthelackofdependenceon changesintheNaOH:S2O82−molarratiodemonstratesthatcarbonisotopeanalysiscanpotentiallybe

usedinsituasaperformanceassessmenttooltoestimatethedegradationeffectivenessof1,1,1-TCAby abase-catalyzedpersulfatesystem.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

1,1,1-Trichloroethane(1,1,1-TCA,C2H3Cl3)isacommon

chlo-rinated organiccompound foundin soil and groundwater, and has been identified at more than 50% of the hazardous waste sitesidentifiedontheEPANationalPrioritiesList(NPL)[1].It fre-quentlyco-existsatcontaminatedsiteswithchlorinatedethenes (e.g.,trichloroethene(TCE))sinceitsindustrialuseissimilar[2]. However,1,1,1-TCAismorerecalcitrantthancommonchlorinated ethenesduetothepresenceasingleC Cbond[3].

∗ Correspondingauthorat:DepartmentofCivilandEnvironmentalEngineering,

UniversityofWaterloo,Waterloo,Ontario,CanadaN2L3G1.

Tel.:+15198884567x38244;fax:+15197467484.

E-mailaddress:[email protected](M.Marchesi).

Onepotentialremediationtechnologyisinsituchemical oxi-dation(ISCO)whichinvolvestheinjectionorreleaseofachemical reagentintothesubsurfacewiththecapabilitytodegradethetarget organiccompound(s).Amongthenumerousoxidantsand activa-tionsystemsavailable,theemergenceofpersulfate(S2O82−)andits

novelactivationstrategieshascreatedthepotentialfortreatment ofchlorinatedmethanesandethanes(like1,1,1-TCA)[4]. Gates-Andersonetal.[5]observedverylittledegradationof1,1,1-TCA witheitherperoxideorpermanganatewhileHuangetal.[4]and Liangetal.[6]foundthatthermallyactivatedpersulfate(>40◦C) completelydegradedchlorinatedsolventcompounds like 1,1,1-TCA.Numerousmethodsareavailabletoactivatepersulfatesuch theusetransitionmetals,peroxide,andheat,orestablishing alka-lineconditions.Themostfrequentlyappliedactivationmethodis base-catalyzedpersulfate(pH>11)whichhasbeenusedat_∼60%of thesiteswherepersulfatehasbeenemployed[7].Base-catalyzed persulfatetreatmentconsistsofestablishinghighpHconditions

0304-3894/$–seefrontmatter © 2013 Elsevier B.V. All rights reserved.

byaddingastrongbase(e.g.,sodiumhydroxide)tothesubsurface alongwithpersulfate.Blocketal.[8]andBrownetal.[9]suggested thatsome chlorinatedmethanes and ethanescan bedestroyed usingbase-catalyzedpersulfate.

Furmanetal.[10,11]investigatedthebaseactivation mecha-nismandtheeffectofalkalinityonpersulfatereactivityandshowed completedegradationoftheprobecompoundhexachloroethane. Furmanetal.[10]reportedthatthebaseactivationmechanism forpersulfatestartswiththebase-catalyzedhydrolysisofa per-sulfatemoleculetogeneratethehydroperoxideanion,areducing species(HO2−,Eh=−0.9V).Thehydroperoxideanionthenreduces

anotherpersulfatemoleculegeneratingthesulfateradical(SO•₄−) andthehydroperoxideanionisoxidizedtothesuperoxide radi-cal(O•₂−,Eh=−2.4V),anotherstrongreducingspecies.Theoverall

reactionisgivenby[10]: 2S2O2−8 +2H2O OH− −→SO•4−+O •₋ 2 +3SO2−4 +4H+ (1)

The production of these reducing species is what enables thebase-catalyzedpersulfatesystemtodegradehighlyoxidized organiccompounds[10,11].Furmanetal.[11]alsodemonstrated thatthegenerationofthesereducingspecieswashighlydependent onthebasetopersulfatemolarratioemployed,indicatingthatfor basetopersulfatemolarratiosabove3:1theproductionofreducing speciesincreasedconsiderably.

Theevaluation of ISCOperformance for organiccompounds followingconventionalconcentrationormass-basedestimatesis generallydifficultandpronetonumerousuncertainties[12].For chlorinatedorganiccompounds,estimationofdestruction effec-tivenessisusuallybasedonthedecreaseinconcentration(ormass load)ofthetargetcontaminantand/oranincreaseinchloride(Cl−) concentrationatamonitoringnetworkdowngradientofthesource zone.However,changesintheorganiccompoundconcentrations betweenthetargetsourcezoneandmonitoringlocationscanalso occurthrough non-chemicaloxidation/reduction processes(e.g., displacementofcontaminated groundwaterbytheinjected oxi-dantsolution).Furthermore,eventhoughoxidation/reductionof thetargetcontaminantmayhaveextensivelyoccurred,the concen-trationmaystillremainelevatedduetothecontinuousdissolution fromresidualnon-aqueousphaseliquids(i.e.,rebound[13]). Inter-pretationofCl−datamayalsobeambiguousinthepresenceofhigh backgroundconcentrationlevels,and,inthecaseofpersulfate,Cl− canreactwiththesulfateradical[14].Hence,theuseof concentra-tiondataalonemaybeinsufficienttoevaluatetheeffectivenessof anISCOsystem[15].

Stablecarbonisotopeanalysishasbeenincreasinglyappliedas acomplementarytooltomonitortheinsituefficacyofabioticand biotictreatmentsystems(e.g.,Refs.[15]and[16]).Thistechnique isbasedonchangesintheisotopiccompositionofthetarget con-taminantand itsdegradationproducts.Duetomass-dependent differencesinactivationenergiesofthedifferentisotopes13_Cand 12_{C, thelighterisotope(}12_{C)reactsfaster than theheavier}

iso-tope(13_{C),leadingtofractionationand anenrichmentofheavy}

isotopesintheresidualcontaminantasthereactionproceeds[17]. Consequentlytheuseofstablecarbonisotopeanalysisto comple-menttheconcentrationdatacanbeveryhelpfulindistinguishing between chemical oxidation and non-oxidation processes, and thereforeinappropriatelyquantifyingtheeffectivenessof chem-icaloxidation [15].Inordertotestthecarbonisotopeapproach asapotentialtooltoassessISCOperformanceatthefieldscale,a seriesofbench-scaleexperimentsweredesignedinthisstudy.The experimentalobjectiveswere(a)toevaluatethecarbonisotope fractionationfactor,and(b)toassesswhethertheuseofdifferent initialhydroxidetopersulfatemolarratiosresultsindifferencesin thecarbonisotopicfractionationandalsoin1,1,1-TCAdegradation rates.

2. Experimentalmethodology

2.1. Materialsandexperimentalconditions

Todeterminethetreatabilityandisotopicfractionationof 1,1,1-TCA, a series of aqueous batch experiments were conducted. Persulfateand1,1,1-TCAconcentrations,alongwithpHwere mon-itoredtoevaluatetheextentofdegradationandtoobtainkinetic information.Theisotopiccompositionof1,1,1-TCAwasalso quan-tified,andusedtocalculatethecarbonisotopefractionationfactor andtoinvestigateitsdependenceupontreatmentconditions.

Allchemicalsusedwerereagentgrade:1,1,1-TCA(99.9%purity, FisherScientific);sodiumpersulfate(Na2S2O8,98%,AlfaAesar)and

sodiumhydroxide (NaOH, 99.5%, Alfa Aesar). A1,1,1-TCA stock solutionwaspreparedtoaconcentrationof25mg/L(0.19mM). This1,1,1-TCAstocksolutionwasmixedsuccessivelywitha pre-determined amount of persulfate to obtain a final persulfate concentration of 100mM (∼16g/L), and in turn with different amountsofNaOHtoobtain0.2:1,0.5:1,2:1,and8:1NaOH:S2O82−

molarratiosolutions.Nominal40mLreactorscappedwithTeflon septawereusedand werestored in thedarkatambientroom temperature (i.e., 20◦C). Reactors were completely filled (no headspace)tominimizevolatilization.Theinitialconcentrationin thereactorswas20,50,200and800mMforNaOH,and0.15mM (20mg/L)for1,1,1-TCA.Triplicatereactorswerepreparedforall experimentaltrials.TheinfluenceoftheS2O82−:1,1,1-TCAmolar

ratioontreatmenteffectivenesswasnotinvestigatedheresince thepersulfateconcentrationusedwasconsideredtobeinexcess (theapproximateS2O82−:1,1,1-TCAinitialmolarratiowas>600:1).

Preliminaryexperimentsindicatedthatathree-weeklong reac-tionperiodwasnecessarytoobtain∼90%destructionof1,1,1-TCA. SamplingwasperformedatDay1,3,8,15,and22.Thereactors weresacrificedandquenchedinanicebath,andthenaliquotswere takenfor:persulfateanalysis(1mL),␦13_{Cdetermination(22mL),}

quantificationof1,1,1-TCAconcentrationandpotential degrada-tionproducts(2mL),andtomeasurepH.Preliminaryexperiments showedthattheuseoftheicebathwasadequatetoessentiallystop thereactionovera1–2dayperiod,sincenosignificantvariationin 1,1,1-TCAconcentrationwasobservedoverthistimeframe(data notshown).Allanalyseswereconductedwithin24hofsampling.

Basedonpreviousstudies[8],minimaldegradationof 1,1,1-TCAwasexpectedfortheNaOH:S2O82−molarratioof0.2:1trial

andhenceisotopicanalyseswerenotperformed.Thepurposeof thistrialwastoinvestigatetheeffectivenessofthebase-catalyzed persulfatesystematalowerNaOH:S2O82− molarratiothanthe

recommendedratioof0.4:1[8].

Asidefromchemicaldegradationbythereducingspecies gen-erated in the base-catalyzed persulfate system, destruction of 1,1,1-TCAmayoccurbyhydrolysis[18].For example,1,1,1-TCA mayreactwithOH−topartiallydegradeintochloroethenewhich canthenbecompletelymineralizedinthepresenceofactivated persulfate[8,14].Myamoto andUrano [19]observed significant degradationof1,1,1-TCAatneutralpHat80◦C;however,the half-lifeat15and20◦CestimatedbyusingtheArrheniusequationis between2and 5years[19].Toinvestigatetheroleof hydroly-sisoverthe22-dayreactionperiod,anotherseriesofexperiments (controls)wererunataninitialpHof7,9,11,and13withoutany persulfateadded.

2.2. Analyticalmethods

The concentration of 1,1,1-TCA and potential chlorinated degradationproducts(e.g.,1,1-dichloroethaneandchloroethane) was determined by pentane extraction (2mL sample to 2mL pentane) and subsequent analysis on a Hewlett-Packard 6890 Plusgaschromatograph(GC)equippedwithanHP-624column

(30m×0.32mm,1.8␮mfilmthickness)andamicro-ECD detec-tor. Carbon isotope analysis of 1,1,1-TCA was performed on a GC(Hewlett-Packard6890;Agilent)coupledthrougha combus-tioninterfacetoanisotoperatiomassspectrometer(Micromass Isochrom)asdescribedbyHunkelerandAravena[16].Residual per-sulfateanalysiswasconductedfollowingtheproceduredescribed byHuangetal.[20],andthesolutionpHwasdeterminedusinga pHmeter(WTWpH3300i).

2.3. Quantificationofisotopefractionation

Carbonisotopeenrichmentfactorshavebeendeterminedfor many chlorinated organic carbon compounds under different abiotic processes, such as: oxidation by permanganate [15,21], hydrogenperoxide[22],and iron-activatedpersulfate[23]; and transformation by reductive dechlorination by zerovalent iron [24,25]andreductionbypalladiumcatalystswithhydrogen[26]. To applythis technique in situ, theenrichmentfactor mustbe sufficiently high and the fractionation signature must not be affectedbyreactantconcentrationvariationsinthesubsurface(e.g., NaOH:S2O82−:1,1,1-TCAratio). Thesevariationscanarisedueto

mixingoftheinjectionvolumeofpersulfate(andactivatorwhen used)withgroundwater,andduetotheusualheterogeneous con-taminantdistributioninthesubsurface.

Thecarbonisotopicratioisusuallyexpressedusingthe␦13_C

notationinunitsofpermil(‰)deviationfromtheinternational standardViennaPeeDeeBelemnite(VPDB)asexpressedby ␦13_C= 13C/12Cc−13C/12Cs

13_C/12_Cs ×1000 (2)

where 13_C/12_C

c and 13C/12Cs are theratiosfor the sample and

thestandard,respectively.Byusingthe␦13_{Cnotationforthe}

iso-topiccomposition,Fforthefractionofthecompoundremaining (definedastheratiooftheconcentrationattime ttotheinitial concentration),and_␦13_{Cforthedifferencebetweentheisotope}

compositionattimetandinitialconcentration,thefractionation factor(˛)canbeexpressedbytheRayleighequation

ln





coisthecarbonisotopicratioattheinitialconcentration.

TheRayleighequationcanbeusedbyassumingthattheabundance oftheheavierisotopeissmallcomparedtothelighterisotope,and that␣remainsconstantduringtreatment[27].Amorepractical waytodescribeisotopefractionationwhen˛isverysmallistouse theenrichmentfactorεdefinedasε(‰)=(˛−1)×1000.

3. Resultsanddiscussion

3.1. Destructionof1,1,1-TCA

Fig.1(a)showsthe1,1,1-TCAtemporalconcentrationprofiles asaresultoftreatmentbybase-catalyzedpersulfate(eachdata pointrepresentstheaveragefromtriplicatereactors).The experi-mentalcontrols(withoutpersulfate)underallinitialpHconditions yieldedarelativelystable1,1,1-TCAconcentrationoverthe22-day reactionperiod.Thesedataindicatethatat20◦Cthedegradation of1,1,1-TCAbyalkalinehydrolysisisinsignificant,confirmingthe observationsofJeffersetal.[18]andMiyamotoandUrano[19]who estimatedahalf-lifebetween1.1and2yearsforthehydrolysisof 1,1,1-TCA.

For all theNaOH:S2O82− molar ratiosexploreda significant

decreaseintheconcentrationof1,1,1-TCAwasobservedandis con-sistentwithotherstudies[8,10,11].Anincreaseinthehydroxide concentrationproducedahigherrateof1,1,1-TCAdegradationand

Fig.1. (a)Variationof1,1,1-TCAconcentrations,and(b)␦13_{Cvaluesfordifferent}

NaOH:S2O82−molarratiosandthecontrolexperiments.

almostcompletedestruction(>99.9%)wasachievedatthehigher molarratiosof2:1and8:1.Partialdegradationof1,1,1-TCAwas observedforthemolarratios0.2:1(∼70% byDay15)and 0.5:1 (>95%byDay22).Furman etal.[11] observedsimilarbehavior fortheirprobecompound(hexachloroethane).Ahigher concen-trationofOH−promotestheformationofahigherconcentration ofreductivespecies(O•₂−),resultinginanincreasedrateof degra-dation.Nochlorinateddegradationproductsweredetectedinthe treatmentandcontrolreactors.Overthereactionperiodpersulfate concentrationdecreasedby∼44% inthe8:1molarratiosystem andby∼5%inthe0.2:1molarratiosystem.Theoveralltreatment performanceefficiency(ratiooffractionalchangein1,1,1-TCA con-centrationtothefractionalchangeinpersulfateconcentration)was highest(∼15)forthelowermolarratioof0.2:1anddecreasedasthe molarratioincreased(Table1).Thelowerperformanceefficiency athigherNaOH:S2O82−molarratiosindicatesthatagreatermass

ofpersulfateisneededtodegradeagivenmassof1,1,1-TCAwhen higherNaOH:S2O82−molarratiosareused.

Asmall(<0.19unit)decreaseinpHwasobservedoverthe reac-tionperiodandthisreductionwaslargestforthelowestmolarratio employed.Overall,thepHinalltrialsexploredwassuitablefor alkalineactivation(pH>10.5)throughouttheexperimentalperiod (Table1).ItwasexpectedthatthesolutionpHwoulddecreaseover timeasthedegradationofpersulfateproducesprotonsunder alka-lineconditions(seeEq.(1)).However,thepresenceofahighinitial concentrationofOH−(from20to800mM)preventedasignificant decreaseinpH.

Table1

InitialandfinalpH,S2O82−consumed,treatmentefficiency,andreactionratecoefficient(k)forthedifferentNaOH:S2O82−molarratiosandthecontrolexperiments.

NaOH:S2O82− InitialpH FinalpH S2O82−consumed(%) Efficiencya k(day−1M−1)b

0.2:1 12.06 11.87 4.6 15.0 1.11 0.5:1 12.28 11.81 20.5 7.5 1.93 2:1 12.68 12.52 35.9 4.2 4.87 8:1 12.94 12.90 44.1 3.1 5.31 pH7 7.12 7.13 – – – pH9 9.22 9.19 – – – pH11 10.98 10.96 – – – pH13 13.07 13.06 – – –

a_{Definedasratioofthefractionalchangeinthe1,1,1-TCAconcentrationtothefractionalchangeinpersulfateconcentration.}

b _SeeEq.(4).

AssumingthatthepHwasconstantforeachexperimentaltrial, thefollowingkineticmodelwasdeemedmostappropriateto rep-resentthedata

r1,1,1−TCA=−k[1,1,1−TCA]n





(4) wherek(day−1M−1)isthereactionratecoefficient,andnandm arethereactionorderswithrespectto1,1,1-TCAandpersulfate, respectively.Eq.(4)waslinearizedandaleast-squaresmethodwas usedtodeterminetheratelawparameters.Thebest-fitreaction ordersforboth1,1,1-TCAandpersulfatewereunity,andkvaried from1.11to5.31day−1M−1asthemolarratioincreased(Table1). This5foldincreaseinkisdirectlyrelatedtotheincreaseinOH−; however,it appearsthatkmaybestabilizingasthemolarratio approacheshigher values. Thissuggeststhat anincrease inthe concentrationofOH−causesthegeneratedreactivespeciestobe involvedincompetingreactionswhichresultsinarelatively con-stant1,1,1-TCAdegradationrate.Additionalresearchisrequiredto elucidatethecompetingreactionsandhowtheyimpactthe treat-mentefficiency.

3.2. Extentofcarbonfractionation

Theexperimentalcontrols(withoutpersulfate) fortherange ofinitialpHtrialsinvestigatedshowednolossin1,1,1-TCAmass (<5%), and no variation in the ␦13_{C 1,1,1-TCA value (<0.5‰)}

throughouttheexperimentalperiod(Fig.1(b)).Theisotopicdata fromthecontrols(n=12)whereusedtoestablishabaseline␦13_C

valuefor1,1,1-TCAof−28.0±0.4‰(95%confidenceinterval). The isotope composition of the residual 1,1,1-TCA became moreenrichedintheheavierisotope(13_{C)overtimefor allthe}

NaOH:S2O82−molarratiosexplored(Fig.1(b)).Forexample,the

␦13_{C1,1,1-TCAvalueincreasedfrom−28.0‰atthebeginningof}

theexperimentto+3.7‰after15days(99%degradationof 1,1,1-TCA)forthe2:1NaOH:S2O82−molarratio.Forthe8:1and0.5:1

NaOH:S2O82−molarratios,the␦13Cvalueincreasedto−0.9and

−12.1‰after98and88%degradationof1,1,1-TCA,respectively. The isotopic enrichmentpattern confirmed the normal isotope effectassociatedwithdegradation[17,28].Thisincreaseinthe␦13_C

valueintheremaining1,1,1-TCAallowsforthepotentialto esti-matethedegradationprogressduringbase-catalyzedpersulfate treatment.

While the 1,1,1-TCA concentration decreased expo-nentially with time, the ␦13_{C profiles were essentially}

linear (Fig. 1(b)). To estimate the isotopic enrichment fac-tor, Eq. (3) was fit to the data by linear regression using 1000ln[(␦13_C

co+␦13C+1000)/(␦13Cco+1000)] versus lnF, and

forcing the intercept through the origin (Fig. 2). The resulting enrichmentfactorsforthedifferentNaOH:S2O82−ratiosare

pre-sentedinTable2.Thepatternof␦13_{Cfor1,1,1-TCA(Fig.2)together}

withtheexcellentfits(r2 _{variedfrom0.98to1.00),isconsistent}

withtheRayleighisotopicevolutionmodel.Thisclearlyindicates thattheobserved enrichmentfractionationfactor wasconstant

throughoutthereactionprocessoratleastuntil99%degradation ofthe1,1,1-TCAmassoccurred.Theisotopicenrichmentfactors for1,1,1-TCArangedfrom−6.9±0.4‰to−7.1±0.2‰andwere consistentacrossalltrialswithanaverageenrichmentfactorof −7.0±0.2‰.

ThetrendshownonFig.2usinganaverageenrichmentfactor of−7.0±0.2‰canhypotheticallybeusedtoestimatethedegree ofdegradationforthebase-catalyzedpersulfatetreatmentof1,1,1 TCA.Forexample,a␦13_{Cenrichmentof1.4‰isassociatedwith}

20%degradationwhereasa19.7‰enrichmentisassociatedwith 95%degradation. Bothofthese ␦13_{C variations aresignificantly}

largerthantheuncertaintyinthe_␦13_{Cdetermination(±0.5‰isthe}

typicalerrorincarbonisotoperatiomeasurementby compound-specificisotopeanalysis(CSIA)forchlorinatedsolvents[27]).This indicatesthatthemagnitudeoftheenrichmentfractionationfactor issignificantlylargeenoughtominimizeerrorsinthe interpreta-tionofisotopicfielddata.

ItisimportanttonotethatavariationintheNaOH:S2O82−molar

ratiosfrom0.5:1to8:1resultedinaveryslightdifferenceinthe enrichmentfactor(0.2‰),confirmingthattheinvestigatedmolar ratiosdidnothaveanyinfluenceontheisotopicfractionationin ourexperimentaldesign.Thisfindingisessentialtouseisotopic compositiontomonitortheeffectivenessofbasecatalyzed persul-fateinsitu.Isotopiccompositionchangescanalsobeusedtohelp understandthedynamics associatedwithcontaminantrebound duringandaftertreatment[15],andtodifferentiatebetween dilu-tion(noisotopicchanges)anddegradation(enrichmentpattern) atlocationsawayfromtheinjectionzonewhereadecreasein con-centrationcanoccurduetodisplacementordilution.

Fig.2.Linearized13_{Cenrichmentof1,1,1-TCAaccordingtoEq.(3),fordifferent}

Table2

Enrichmentfactors,␦13_{C(‰),1,1,1-TCAdegradation,andfinal␦}13_{Cfor1,1,1-TCAforthedifferentNaOH:S}

2O82−molarratiotrials.

NaOH:S2O82− Degradation Final␦13C(‰) ␦13C(‰) εc bulk‰ r2

0.5:1 88% −12.1 15.1 −7.2±0.3 0.998

2:1 99‰ +3.7 31.7 −6.9±0.4 0.999

8:1 98% −0.9 27.2 −7.2±0.3 0.984

Average – – – −7.0±0.3 0.990

Unlikebioticsystemswherethecomplexityofdifferent

degra-dationpathways is derived from differentmicroorganisms and

resultsin alargevariabilityinisotopefractionationfactors[28]

(e.g.,theenrichmentfractionationfactorformicrobialreductionof TCEcanvaryfrom−2.5to−13.8‰[27]),theenrichment fraction-ationfactorsforabioticdegradationpathwaysshouldvaryovera muchsmallerrangeforaspecificcompoundunderafixedchemical reaction(e.g.,reductivedehalogenationof1,1,1-TCAbyCr,Feand Cu/Fe[29]).WenotethatHunkeleretal.[15]obtainedan enrich-mentfractionationfactorslightlydifferentthanthatreportedby PoulsonandNaraoka[21](−21.4‰comparedto−26.8‰)forthe oxidationofTCEbypermanganate;however,thisdiscrepancyis likely attributedtodifferencesin theexperimentalsystem(the presenceofheadspaceand thusvolatilizationprocesses).Inthe experimentsperformedinthisstudy,no1,1,1-TCAvolatilization occurredandthusthegeneratedenrichmentfractionationfactor representsareferencevalueforthetreatmentof1,1,1-TCAbya base-catalyzedpersulfatesystem.

Incontrasttochlorinatedethenes,thebodyofliteraturerelated totheenrichmentfractionationfactorfor1,1,1-TCAdegradation underbioticandabiotictreatmentprocessesislimited.Elsneretal. [29] obtaineda carbon isotope fractionation factor of −13.6to −15.8‰forreductivedehalogenationof1,1,1-TCAbyCr,Feand Cu/Fe,suggestingaconsistencywithapotentialmechanismthat involvestheCCl3carboncenter.Thevalueof−7.0‰obtainedin

thisstudyisalsoconsistentwithareduction mechanismwhich likelyinvolvestheCCl3carboncenterthroughanelectrontransfer

mechanism.However,nodegradationproductsorintermediates wereobservedandfurthersystematicresearchisneeded.

Finally, a carbon isotope fractionation factor of −1.8‰ was reportedforbiodegradationof1,1,1-TCAbySherwoodLollaretal. [30],whichismuchsmallerthantheisotopefractionationfactorof −7.0‰obtainedforabioticdegradationof1,1,1-TCAinthisstudy. Thiscreates the possibility of using compound-specific isotope analysistodistinguishbetweenthesetwodegradationpathways.

4. Conclusions

Thisinvestigationdemonstratedthat1,1,1-TCAcanbedegraded byemployingbase-catalyzedpersulfateandthatasignificant car-bonisotopefractionationoccurs.HigherNaOH:S2O82−molarratios

increasedtherateofdegradationof1,1,1-TCA.Althoughthedata arenotconclusive,itappearsthatthe1,1,1-TCAreactionrate sta-bilizesastheNaOH:S2O82−molarratioincreased.Whileahigher

NaOH:S2O82−molarratiowasabletoincreasethe1,1,1-TCA

degra-dationrate,itreducedthetreatmentperformanceefficiency.An averageenrichmentfractionationfactorof−7.0±0.2‰was deter-mined,andthisenrichmentfractionationfactorwasindependent oftheNaOH:S2O82−molarratiosemployed.

Although additional research is required to determine the impact of other environmental parameters (e.g., temperature, bufferingcapacity)onthefractionationfactor,theresultsobtained fromthepresent investigationshow thatthe carbonisotope is potentiallya usefultracerthat canbeusedtoassess the effec-tivenessof1,1,1-TCAdegradationinsituwhen abase-catalyzed persulfatesystemisemployed.

Acknowledgements

Thisworkwasfunded bytheNaturalScienceand Engineer-ing Research Council (NSERC) of Canada, the Ontario Ministry of Science and Innovation, the Spanish Government (MICINN, projectsCICYT-CGL2011-29975-C04-01),andtheCatalan Govern-ment(2009SGR103).WethanktheUniversitatdeBarcelonafora AjutdePersonalInvestigadorenFormació(APIF)Doctoral Fellow-shiptoM.Marchesi.Theauthorsalsothankthevaluedcomments fromfouranonymousreviewerswhichimprovedthequalityofthis manuscript.

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