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AnOverviewofnanoscalezero-valentironMorerecently,becauseofmyresearchrequirements,Ineedtoreadsomepapersaboutzero-valentiron,whicharesearchedfromChinaNationalKnowledgeInfrastructure(CNKI)orGoogleBooks.Thefollowingaresomeofmyreadingsandnotes.IntroductionZero-valentiron(Fe0(s)orZVI)hasbeenusedtoremediateavarietyofcontaminants.Inparticular,reductivereactionsofZVIhavebeenemployedinpermeablereactivebarrierstoremediategroundwatercontaminatedwithhalogenatedsolventsandreducibleinorganiccontaminants.Nanoscalezero-valentiron(nZVI)hasbeensuggestedanalternativemeansofexploitingthereactivityofZVIwithouttheneedtoconstructpassivebarriers.ThemainadvantagesofnZVIareitshighreactivityandthepotentialforintroducingtheparticlesdirectlyintocontaminatedsoilandgroundwater.Thepreparationofnanometerzero-valentironThepreparationofnanoscalezero-valentironingeneralcanbedividedintophysicalmethodandchemicalmethod.EvaporationcondensationmethodAlsocalledlowcondensationmethod.Itisintroducedinsupervacuumevaporationindoorlowpressureinertgas(heliumorargon),heatingthemetaliron,gasificationvaporizesatomicfog,fogatomiccollisionswithinertgasquantitylostagain,andnanoscaleironparticlesisformedafterquenching.Inhighvacuumevaporationcavityventilationwithhighpurity,withmetalmolybdenumasheatsourceofmetallicironheatingevaporation,thentheparticlesurfacepassivationofforalongperiodoftime.Thusmadeofironnanoparticlesarespherical,theaverageparticlesizewas,andstableperformanceintheair,nofurtheroxidation.LiquidphasereductionmethodLiquidphasereductionmethodisthemostcommonlymethodusedinthepreparationofnanometerzero-valentiron,butthecurrentpreparationprocessisdifficulttoabtaincleananddrynanometerzero-valentiron.Material,especiallythesurfaceofmaterialusuallycontainoxidefilm,whichhinderthereductionprocessofelectrontransfer,andisnotconducivetothereaction.Themoistureofnotdryingcompletelywillacceleratetheoxidationofnanometerzero-valentiron,andisnotconducivetothepreservationofnanometerzero-valentiron.Researchersadoptstwotimesreductiontechnologyforpreparationofpurenanometerzero-valentiron,whichhasnooxidefilminthesurface,anditisconducivetotheelectrontransferandstorage.TwicereductiontechnologyisusedtoreduceFeSO4solutionintonanometerzero-valentiron,thenusinghightemperaturehydrogendealwithnanometerzero-valentiron,hightemperaturehydrogenduringtheprocessofthepreparationisnotonlyatwicereductionprocessbutalsoadryingprocess.Theapplicationofthezero-valentironRecently,ithasbeendemonstratedthatZVIoxidizesorganiccompoundswhenitisusedinthepresenceofoxygen.ThebyproductsobservedduringZVI-mediatedoxidativedegradationofmolinateandphenolareconsistentwiththeactionofanonspecificoxidantsuchasthehydroxylradical.ThereactionsthatresultinoxidantproductionareinitiatedwhenFe。isoxidizedbyoxygen,whichlikelyformsareactiveoxygenspecieseitherontheparticlesurfaceorinsolution.TheoxidationofFe。byoxygenalsoresultsintheformationofalayerwithpropertiessimilartoFe2O3andFe3O4ontheparticlesurface,whicheventuallyleadstoareductioninthereactivityofthesurfaceaccompaniedbyadecreaseintherateofFe。oxidation.FormationofasurfacecoatingofironoxidesalsoappearstoberesponsibleforthedecreasedrateofFeo-mediatedoxidationoforganiccompoundsthatisobservedoverextendedtime.ForasysteminvolvinggranularFe0(s),thediminutionofreactivitycanbecounteractedbyadditionofachelatingagent(suchasEDTA)thatkeepstheoxidizedironinsolution.However,additionofchelatingagentslimitstheutilityofthetechniqueforinsitutreatment.ThehighsurfaceareaofnZVImayallowformoreefficientgenerationofoxidants,butasreportedinourpreviousstudy,adecreaseinreactivityassociatedwiththebuild-upofironoxidesonthesurfaceeventuallyslowsthereaction.IfthenZVIparticlescontinuetoproduceoxidantsafterasurfacecoatingforms,thecontinuedslowreleaseofoxidantsmayresultindegradationofcontaminantspresentinsoilandcontaminatedaquifers.ThehighsurfaceareaofthenZVIparticlesalsocouldprovideameansforselectiveoxidationofsurface-activecompounds.ToassessthepotentialapplicabilityofnZVIfortheoxidativetreatmentoforganiccontaminants,experimentswereperformedusingtheoxidationofbenzoicacid(BA)top-hydroxybenzoicacid(p-HBA)asaprobereactionforoxidantproduction.Thisreactionisparticularlyconvenientasp-HBAdoesnotadsorbstronglytoeitherFeOorironoxideproductsundertheconditionsofinterest,andthefurtheroxidationofp-HBAbyhydroxylradicalsisslowatthelowconcentrationsofp-HBAexpectedtobepresent.Analysisofthisoxidationreactionunderwell-definedconditionsprovidesinsightintotheeffectofenvironmentalconditionsonoxidationrates,theselectivityoftheoxidant,andtheoverallefficiencyoftheprocess.Additionally,insightsgainedfromthelaboratoryinvestigationsundercontrolledconditionsareusedtoassessthepotentialapplicationsofthistechnologyandtoidentifyareasforfurtherresearch.MaterialsandMethodsThereactionsofnZVIwerestudiedinpH-bufferedsolutionswithanionicstrengthadjustedto0.03MwithNaCl.BenzoicacidservedasthebufferatpH3andpH5while2mMbicarbonatewasusedtobuffersolutionsatpH8.Whennecessary,thepHofthesolutionswasadjustedusing0.1NHClor1NNaOH.BA,p-HBA,andphenolwereobtainedfromSigma-Aldrich;anilinewasobtainedfromAjaxFineChemLtd;andhumicacidwasobtainedfromAldrich.nZVIwassynthesizedasdescribedpreviously.TocomparethereactivityofthenZVIusedinthisstudywithotherformsofFe0(s),studieswerealsoundertakenwithMasterBuildersgranularZVI(OricaChemicals,Australia),electrolyticZVIpowder(KantoChemicalCo.,Inc,Japan),andAldrichfineZVIpowder.p-HBAconcentrationsweredeterminedbyhigh-performanceliquidchromatography(HPLC)usingaHewlett-Packard1100seriesHPLCsystemequippedwitha250_4.6mmWatersSpherisorbODS-25^column(Alltech,IL).Atwosolventgradientelution,consistingofwater(pH3)andacetonitrileataflowrateof1.0mL/minwasusedtoseparateBAandisomersofhydroxybenzoicacid.Thep-HBAisomerwasquantifiedat255nm,andBAwasquantifiedat270nm.Allstandardcurveswerelinearwithregressioncoefficientsof>0.9990inallcases.ThemethoddetectionlimitforBAandp-HBAwere2.5and0.1|iM,respectively.Allexperimentswerecarriedoutatroomtemperaturein100mLserumbottlesusingatotalvolumeof50mL.ThenZVIparticleswerekeptinsuspensionbyplacingthebottlesonanorbitalshakertableat175rotationsperminute.Toensuregasexchange,theserumbottleswereopentotheatmosphere.Throughoutthereaction,thesolutionpHwascontinuouslymonitoredwithapHelectrode.AdditionofnZVIusuallyresultedinaninitialpHincrease.TomaintainaconstantpH,smallquantitiesofHClwereaddedasneededduringthefirsthouroftheexperiments.Afterthefirsthour,thepHremainedconstant.ThepHofthesuspensionswasmaintainedwithin±0.2unitsoftheinitialvaluesduringtheentireexperiment.Toinitiateareaction,nZVIwasaddedfromastocksuspensiontobufferedsolutionscontainingbenzoicacid.Sampleswerecollectedatdifferenttimeintervalsina5mLglasssyringe.Sampleswerefilteredimmediatelythrougha0.45pmMillipore(MillexAP20)glasssyringefilter.Lossesofp-HBAonthefilterwerelessthan5%,asdeterminedbycomparisonoffilteredandunfilteredsamplescollectedpriortoadditionofnZVI.p-HBAwasanalyzedwithin24hofsamplecollection.TheratesofreactionofthenZVI-generatedoxidantwitharangeofprobecompounds(aniline,phenol,o-hydroxybenzoicacid,andhumicacid)weredeterminedbyexaminingtherateofproductionofp-HBAresultingfromBAoxidationinthepresenceofoneofthecompetingcompounds.ExperimentswerealsoperformedtoassesstherelativeratesofhydroxylradicaloxidationoftheseprobecompoundsunderconditionsidenticaltothoseusedinthenZVIexperiments.TheseexperimentswereperformedusingFenton’sreagentasasourceofhydroxylradicals.Experimentswereconductedinserumbottlesbyaddinganexcess(200pM)ofFe(II)toH2O2(20pM)inpH3,0.03Mionicstrengthsolutionscontaining10mMBA.After10min,whenlessthan0.02%oftheinitialH2O2remained,sampleswerecollectedandanalyzedforp-HBAbyHPLC.TotalferrousironwasquantifiedbymonitoringtheabsorbanceofanFe(II)-bipyridinecomplexat522nm.TheprocedureusedwasavariantofthatdescribedbyVoelkerandconsistedofpremixing0.8mLofpH6,0.5Mphosphatebufferand0.2mLof0.01Mbipyridinesolutionandadding4mLofunfilteredsamples,followedby0.04mLof0.1MNa2EDTA.After60s,thesamplestakenfromthesolutionatcertaintimeintervalswerefilteredthrougha0.22pmMillex-GSsyringefilter,andabsorbancewasmeasuredat522nm.

ResultsProductionofp-HBAfromBAexhibitedbiphasickinetics,withrapidinitialproductionofp-HBAfollowedbyaslowincrease(apartfromsomevariabilityatearlytimes)overperiodsofatleast1day.Theconcentrationofp-HBAproducedafter1hwasusedinallsubsequentexperimentstocomparetheratesofoxidationunderdifferentconditions.Undertheconditionsusedintheseexperiments,lessthan5%oftheBAinitiallypresentwastransformed;therefore,itservedasthemainsinkforoxidantswiththeresultthatoxidationofp-HBAwasnegligible.Thus,theconcentrationofp-HBAformedisrelatedtotheconcentrationofoxidizingspeciesthatcouldbeusedtotransformacontaminant.BAwastransformedintothreeisomersofhydroxybenzoicacid.Althoughitwasnotpossibletoquantifyeachoftheisomersduetodifficultyresolvingtheorthoandmetaforms,thethreeisomersappearedtobepresentatsimilarconcentrations.Thethreeisomersofhydroxybenzoicacidaccountfor90+5%oftheproductsofOH?reactionswithBAwiththeratioofo-HBA,m-HBA,andp-HBAproductsreportedtobeintheproportion1.7:2.3:1.2.FortheoxidationofBAbysolution-phaseOH-,theconcentrationofp-HBAhasbeenusedtoestimatecumulativeOH-productionusingeq1cu：]ii】2LiiL："EC*」=如;7 (1)AsimilarapproachisadoptedherewithresultsfromallnZVIexperimentsexpressedintermsofbothp-HBAconcentrationandcumulativeOH-production,withthelatterquantityestimatedusingeq1.ToassesscompetitionforoxidantsbetweenBAandotheroxidantscavengers,suchasFe(II),experimentswereconductedatpH3overarangeofBAconcentrations.Theyieldofp-HBAincreasedasBAconcentrationsincreasedfrom50口Mto5mMandthenremainedapproximatelyconstantupto20mM.SimultaneousmeasurementsofferrousironindicatedthattotalFe(II)rangedfrom190to200口Minalmostallcasesandwasrelativelyconstantoverthedurationoftheexperiment.ExperimentsconductedatpH3and10mMBAwithnZVIconcentrationsrangingfrom0.2to5mMindicatedanincreaseinpHBAproductionasnZVIincreased.AssumingthesamedistributionofHBAisomersinthenZVIsystemasthoseobservedinexperimentswithhomogeneousOH-,theseresultscanbeusedtoestimatetheefficiencyoftheinitialfastreaction(i.e.,theyieldofoxidantspermoleofnZVIadded).Thereactionefficiencyrangedfromapproximately5to25%withhighervaluesatthelowestZVIconcentration.Toelucidatetheselectivityoftheoxidant,competitionexperimentswereconductedwithaniline,o-hydroxybenzoicacid,phenol,andhumicacidatpH3and10mMBA.Inallcases,theyieldofp-HBAdecreasedastheconcentrationofthecompetitorincreased(seeFigure4aforatypicalresult).Therelativerateconstant(i.e.,therateconstantforeachprobecompoundrelativetotherateconstantforBA)wasestimatedbyaleast-squaresfitofthedatatoeq2:fcBA[BA|+^c[C]A;c[C] JC]_」1 ]十*泗麗whereFisthefractionofthep-HBAproducedinthepresenceofacertainconcentrationofrateconstantswerealsodeterminedforthecompoundsinasimilarmannerbutusingFenton’sReagentasasourceofOH-.ExperimentallydeterminedrelativerateconstantsforthefourprobecompoundsaregiveninTable2asareliteraturevaluesfortheabsoluterateconstantsforreactionoftheprobecompoundswithhydroxylradicals.thecompetitor(C)tothatintheabsenceofC,andkc/bAsthecompetitor(C)tothatintheabsenceofC,andkc/bAstherelativerateconstant.RelativeTherelativerateconstantsforaniline,apositivelychargedmoleculeatpH3,aresimilarinboththenZVIandFentonsystemssuggestingnoeffectofthenZVIsurfaceonreactionrate.Forphenol,therelativerateconstantobtainedwithnZVIistwiceashighasthatobtainedwithhydroxylradicalsgeneratedinthesolutionphasewhiletherelativerateconstantforhumicacidwithnZVI-mediatedoxidationisaboutone-thirdofthatfoundwithFentonreagent.Onepossibleexplanationforthelatterresultisthatthehumicacidcontainsacomponentofrelativelyrefractory,hydrophobicmaterialthatcoatstheZVIparticlesloweringtheirreactivity.Comparisonofthisresultwiththatobtainedusingapurifiedlowermolecularweightfulvicacidwouldbeofinterest.Someuncertaintysurroundsthequalityoftherelativeratedataforo-HBAsincetheBA/o-HBAsolutionturnedpurpleonaddingFe(II),presumablybecauseofcomplexationbetweenBAoro-HBAandFe(II).ConsistentwiththelowrelativerateconstantobtainedintheFentoncase,thiscomplexationmightbeexpectedtoretardtherateofreactionofFe(II)withHO2andthereforepreventthegenerationofOH.TheoxidizingabilityofnZVI(asmeasuredbytheextentofp-HBAproductionupto1h)asafunctionofpHandrelativetooxidizingabilityatpH3inthepresenceof50口Mand10mMBAisshowninFigure5.DespitethedecreaseinoxidantproductionrateaspHincreased,oxidationwasobservedatpHvaluesupto8.TheeffectofthetypeofZVIwasevaluatedatpH3with10mMBA.AllfourtypesofZVIparticleswerecapableofoxidizingBAandp-HBAproductionincreasedasthetotalZVIconcentrationincreased.WhilethenZVIproducedmorep-HBAthentheotherformsofFe0,theratesofproductionfortheotherformsofFe0wereverysignificant.Indeed,whennormalizedtothemeasuredsurfaceareaofeachparticletype,theAldrichparticleswerefoundtobeparticularlyeffectiveinoxidizingBA.DiscussionThefirststepintheoxidationofBAbynZVIinvolvestheoxidationofFe0byO2.OneinterpretationofcorrosionpositstheinitialoxidationofFe0asa2-electronprocessFe°+Og+2H+一Fe葉+H2O2H2O2producedineq3couldoxidizeanotherFe0:Fe°+H2O2一Fe2++2OH"Thisprocessresultsina4-electrontransferandanoverallstoichiometryof2molofFe。oxidized/molofO2.Alternatively,H2O2couldreactwithspeciessuchasFe(II):Fe(II)+H2O2一Fe3-+OH'+OH"ThesereactionscouldoccurontheFe0surfaceorcouldinvolvetransferofelectronsthroughanironoxidelayer,dependinguponthereactionratesandaffinityofthespeciesforsurfaces.Althoughthe4-electrontransferprocessisusuallythedominantmechanismofO2-mediatedcorrosion,the2-electron-transferprocesscanresultinproductionofsignificantamountsofO22-orH2O2,especiallyafteranoxidecoatinghasbeenformedonthesurface.Forexample,instudiesconductedonstainlesssteelrotatingdiskelectrodes,10-20%oftheoxygenreducedduringcorrosionwasconvertedinto皆-orH2O2.OnFe0surfaces,theinitialrateofthereactionisfast.However,asurfaceoxidelayerconsistingofq-Fe2O3andFe3O4-likeoxidesformsasthesurfaceisoxidized.ThispassivelayerprotectstheremainingFe0fromattackbyoxygenbyservingasabarrierbetweentheFe0andO2.TheinitialpulseofoxidantsproducedinthenZVIsystemisconsistentwithoxidationofthesurfaceFe0servingasthesourceofoxidant.Forexample,assumingthateachFe0occupiesanareaequalto1.6_10-19m2(i.e.,assuminganatomicradiusof1.4AandanFe-Febonddistanceof2.5A),amonolayerwouldcorrespondtoapproximately1.8%ofthetotalZVI.Thus,achievementofthe25%yieldofoxidants(deducedfromthemolesofoxidantformedpermoleofnZVIadded)thatisobservedatlownZVIdoseswouldrequirethatacorrosionlayerofabout14moleculesinthicknessisinvolvedintheoxidationprocess.Reaction5providesapossibleexplanationfortheobservedoxidationofBAandotherorganiccompoundsthatoccurswhennZVIisexposedtoO2.ThesimplestmodelforthisprocesswouldinvolvetheproductionofOH-insolutionfollowedbyahomogeneousreactionbetweenBAandOH-.Inthiscase,therateatwhichBAisoxidizedwoulddependontherelativeconcentrationsofdifferentOH-sinks.Undertheconditionsinoursystem,thedominantsinksareexpectedtobe:BA+OH*一intermediates1c=4.3x109M-1s-1Fe"+OH，—Fe3++OH"4.0x108M-1s-1WhenanaromaticcompoundreactswithOH-,acyclohexadienylradicalisformedthatreactswithoxygen,eventuallyresultingintheproductionofhydroxylatedproducts.AtthetotalFe(II)concentrationsmeasuredintheexperiments,benzoicacid(eq6)shouldbeadominantsinkforOH-ifthereactionoccursinthebulksolution.Asindicatedbythecalculatedfitdepicted,OH-isscavengedmoreeffectivelythanpredictedbyonlyFe(II).Undertheconditionsusedintheseexperiments,thepseudo-first-orderrateconstantforotherunknownsinkswouldhavetobeequalto2106s-1.IfthesinkisdissolvedFe(II),thiswouldcorrespondto5mMFe2+.AnalternativeexplanationfortheresultsdepictedinFigure2isthathydroxylradicalisgeneratedonoradjacenttothesurface.Undersuchconditions,Fe(II)adsorbedonthenZVIsurfaceorconcentratedintheareaimmediatelyadjacenttothesurfacecouldmoreeffectivelycompetewithBAforOH-.Ifthiswerethecase,wewouldexpectorganiccompoundswithahighertendencytoadsorbtothesurfacetobeoxidizedmorequicklythanthosecompoundswithalowaffinityforsurfaces.AtpH3,thesurfaceofthenZVIispositivelycharged.Therefore,positivelychargedsolutessuchasanilineshouldhavealoweraffinityforthesurfacethanneutralsolutesandsolutesthatcanformsurfacecomplexes(e.g.,BAorphenol).DespitethetendencyforBAtoexhibitahighersurfaceaffinitythananiline,therelativerateconstantfortheaniline(relativetoBA)isnearlyidenticalforthenZVIsystemandtheFenton’sreagentsystemirOWhiiscgEneratedinsolution.Therelativeratesfortheothersolutesalsodonotexhibitstrongselectivityforspeciesthatareexpectedtohaveahigheraffinityforthesurfaces.TheimportanceofthepresenceofoxygentotheZVImediatedoxidationprocessshouldbeemphasized.Itshouldalsobenotedthatdissolvedoxygenislikelytobeinitiallydepleted,particularlyathigherZVIconcentrations.Indeed,therateofproductionofoxidants(suchashydroxylradicals)may,insomeinstances,belimitedbyoxygendepletionandthesubsequentrelativelyslowdiffusionofoxygenfromtheatmosphere.SomeofthevariabilityinrateofoxidantproductionobservedinthefirstfewhourscouldberelatedtotheinhomogeneityofoxygenfluxtothesurfacefollowinginitialdepletionthoughotherfactorssuchastransientpHchangesresultingfromtheinitialcorrosionprocesscouldaccountfortheobservedchangesinoxidantgeneration.TheapparentlowselectivityobservedfornZVIcontrastswiththehighselectivityobservedinTiO2photocatalysis,inwhichtheoxidantisproducedatthesurface.Forexample,thetrichloroacetateanionispreferentiallydegradedatlowpHwhentheTiO2surfaceispositivelychargedwhereasthepositivelychargedcationchloroethylammoniumisdegradedpreferentiallyathighpHwhentheTiO2surfacehasanetnegativecharge.TheabsenceofarelationshipbetweenthechargeofthecompoundandrelativeratesofoxidationinthenZVIsystemsuggeststhatadirectinteractionbetweenthesurfaceandthesoluteisnotimportant.Asindicatedearlier,thedependenceoftheyieldon[BA]suggeststhattheoxidantisnotproducedinthebulksolution.Therefore,itispossiblethattheinitialoxidationoccursclosetothesurfaceinaregioninwhich[Fe(II)]ishigherthantheconcentrationsmeasuredinthebulksolution.ThedetectionofmixedFe(II)/(III)oxidessuchasmaghemiteatthenZVIsurfaceisconsistentwithoxidationofFe2+toFe(III)inthesesystems.Irrespectiveoftheidentityoftheoxidantorthelocationinwhichitisgenerated,theseresultsindicatethatoxidationofZVIbyoxygenresultsintheproductionofanoxidantthatiscapableoftransformingorganiccontaminants.Theinitialpulseofoxidantsmightbeusefulinremediationofcontaminatedsoilsandgroundwaterandmightserveasanalternativetootherinsitutreatmentoxidationprocesses,suchastheadditionofH2O2.Theslowerreleaseofoxidantsthatoccursafter30min,whichlikelycorrespondstothecontinuedcorrosionofthenZVIparticle,mightbeusefulforremediationofdifficult-to-reachgroundwater,providedthatoxygenwasnotdepletedfromthegroundwater.AdditionalresearchisneededtoassessthekineticsofoxidantproductionfromnZVIparticlesonthetimescaleofdaystomonths.AsnotedearlierandshowninFigure3,maximumyieldofoxidantspermoleofnZVIaddedisobservedatthelowerdosesofZVI.ThereasonforthereductioninyieldofoxidantonincreaseinZVIconcentrationisunclearbutmayberelatedtotheincreaseinconcentrationofferrousironthatwillaccompanytheincreaseinZVIconcentrationandtheincreaseincompetitivescavengingofhydroxylradicalsthatwilloccurasaresult.OtherreasonsforthisreductioninyieldonincreaseinZVIconcentrationincludethepossibilityoflessefficientoxidantproductionasaresultofoxygendiffusionlimitationsatthehigherZVIconcentrations;however,competitivescavengingfortheoxidantwouldseemthemostlikelycause.TheeffectofpHreflectedintheresultsshowninFigures1and5isinterestingandworthyoffurtheranalysis.Twokeyobservationsareworthmaking.First,theextentoftherapidinitialgenerationofoxidants(withinthefirsthour)decreasesmarkedlywithincreaseinpH.ThisphenomenonmayberelatedbothtothegreateraffinityoftheFe°(andironoxyhydroxide)surfaceforFe2+athigherpHandtotheabilityofO2tooutcompeteH2O2forFe2+atthehigherpHwitharesultantreductioninrateofgenerationofoxidizingradicalsandgreaterextentofgenerationofapassivatinglayer.Second,despitethesignificantreductioninextentofinitialoxidantproductionathigherpH,ongoingBAoxidationisobservedalbeitatalowerratethanatlowerpHvalues.Presumably,thisongoingoxidationoccursbecauseofongoingcorrosionoftheZVIsurfacedespitethepresenceofapassivatinglayerandongoingreductionofoxygentoH2O2.ItistobeexpectedthatanyoxidationthatoccursathigherpHwilloccuratorinveryclosevicinitytotheFe°surfacegiventhetimetakenforreactantstodiffusethroughthepassivatinglayer.TheproductionofoxidantsduringFe0oxidationisnotaphenomenonthatislimitedtonZVIparticles.Indeed,granularparticlesofthetypecommonlyusedinreductivebarriersareobservedtohavesimilarsurfaceareanormalizedoxidizingcapacityasthenZVIparticlesproducedhere,althoughtheverylowsurfaceareaoftheseparticleswouldresultinverylowoverallratesofoxidation.Interestingly,thecolloidalpart