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TransportandDevelopmentofMicroemulsion-andSurfactantStabilizedIronNanoparticlesfor
InSituRemediation
By
DennisHsu
Athesissubmittedinconformitywiththerequirements
forthedegreeofMasterofAppliedScience
DepartmentofChemicalEngineeringandApplied
ChemistryUniversityofToronto
©CopyrightbyDennisHsu(2017)
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ColumnTransportStudyofSurfactant-andMicroemulsion-StabilizedIronNanoparticlesinPorous
Media
DennisHsu
MastersofAppliedScience
GraduateDepartmentofChemicalEngineeringandAppliedChemistryUniversityofToronto
2017
Abstract
Thisworkdescribesthemobilityassessmentsofmicroemulsion-stabilizedironoxide
nanoparticlesandanionicsurfactantsodiumdiethylhexylphosphate(SDEHP)-stabilized
nanoscalezerovalentiron(NZVI)particlesinlaboratoryporousmedia.Thetwoformulations
testedinthisworkachievedstableironnanoparticlesuspensionsformonthsandpreparedviaa
simple“one-pot”synthesismethoddevelopedbyWangetal.Bothformulationsweretested
underfieldscalevelocityof5m/daywithnomechanicalaidduringtheinjection.Athree-
compartmentmodel,involvingcolloiddiffusiontheory,diffusiontheoryandtailingwasapplied
todescribethebreakthroughcurvesofthestudies.Theobtainedbreakthroughcurvesofboth
formulationsimpliedexcellenttransportinporousmediawithsteadyplateauC/Coat0.8-0.9
andrecoveryofupto0.95forSDEHPstabilizedNZVI.Postanalysisontheretentionofironon
theporousmediaimpliedidealtransportwithconsistentdatatothebreakthroughcurves.
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Acknowledgement
Iwouldliketogivemydeepestandsincerestgratitudetomysupervisor,ProfessorEdgar
Acosta,forhisguidance,mentoringandsupportoverthelasttwoyearsofmystudy.Ihave
neverreceivedmoreinspirationsandencouragementsinlife,careerandacademicallatonce
fromanyoneinmylife.Ihavebecomeabetterthinkerinacademiaandlifewithhisguidance.It
wasanhonourtobehisstudent.
IalsowanttothankProfessorSleep,Dr.MondalandDr.LimafromtheRENEWprogram.Iam
verygratefultobeinthisprogramforbetterresearchandcareerdevelopment.Iwanttothank
ProfessorSleeptoprovidemetheaccesstotheuseofglovebox.Itwasavitalpartofthisstudy.
IwanttothankDr.MondalforgivingmesuggestionsinmyresearchandDr.Limaformaking
myMaster’sexperiencemorerewarding.
Iwanttothankallmycolleaguesandfriends,FrancisChoi,AmericoBoza,SilviaZarate,Aurelio
Stammitti,MehdiNouraei,AshuBhanotandSasanMehrabianforhelpingmewithmyresearch.
IparticularlywanttothankSilviaforhelpingmetostartmyresearch,Americoforprovidinghis
knowledgeincolloidalscienceandFrancisforcollaboratinghisworkwithme.
Iwouldalsoliketothankmyparentsandmybrotherfortheirunconditionalsupport.
Finally,IwanttothankmysoulmateJessicaKokforbeingthereformewheneverIneedher
duringmystudy.
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TABLEOFCONTENT
CHAPTER1:INTRODUCTION.................................................................................................................11.2REFERENCE:...........................................................................................................................................8
CHAPTER2:TRANSPORTOFMICROEMULSION-STABILIZEDIRONOXIDEINPOROUSMEDIA..............11ABSTRACT.................................................................................................................................................112.1INTRODUCTION....................................................................................................................................122.2METHODOLOGY...................................................................................................................................16
2.2.1SynthesisofMicroemulsionIronOxide.....................................................................................162.2.2Determiningthestabilityofmicroemulsionironoxide.............................................................172.2.3Viscositystudyofmicroemulsionironoxideformulations........................................................172.2.4Sizecharacterizationofmicroemulsionironoxideandmicroemulsionformulations..............172.2.5Columnstudy............................................................................................................................182.2.6.BreakthroughCurveModeling.................................................................................................222.3.1StabilityTest.............................................................................................................................262.3.2RheologicalProperties..............................................................................................................272.3.3SizeCharacterization................................................................................................................302.3.4μEIronOxideTransport............................................................................................................312.3.5IronDistributionAnalysis..........................................................................................................40
2.4CONCLUSIONS.....................................................................................................................................42
CHAPTER3:DEVELOPMENTANDTRANSPORTOFPHOSPHATESURFACTANT,SDEHP-STABILIZEDNZVIINPOROUSMEDIAFORINSITUREMEDIATION..................................................................................47
3.0ABSTRACT...........................................................................................................................................473.1INTRODUCTION....................................................................................................................................483.2METHODOLOGY...................................................................................................................................53
3.2.1SynthesisofSodiumDiEthylHexylPhosphate(SDEHP)Surfactant...........................................533.2.2CriticalMicelleConcentrationofSDEHPwithdissolvediron....................................................543.2.3TotalOrganicCarbon(TOC)ofirondissolvedSDEHP...............................................................543.2.4SynthesisofSDEHPNZVI...........................................................................................................543.2.5pHAnalysis...............................................................................................................................563.2.6StabilityAnalysis.......................................................................................................................563.2.7ViscosityAnalysis......................................................................................................................573.2.8ColumnExperimentProcedure.................................................................................................573.2.9NZVIColumnDistributionAnalysis...........................................................................................59
3.3RESULTSANDDISCUSSION.....................................................................................................................593.3.1DeterminingtheOptimalSynthesisFormulation......................................................................593.3.2SynthesisResultsandStabilityofFeSO4-basedSDEHPNZVIat100mMand1g/L.................643.3.3pHandViscosityAnalysisandImplication................................................................................683.3.4SizeAnalysisofSDEHPNZVI......................................................................................................693.3.5MobilityofSDEHPNZVIat100mMand1g/L..........................................................................713.3.7ImplicationsforinsituRemediation.........................................................................................75
3.4CONCLUSION.......................................................................................................................................76
CHAPTER4:CONCLUSIONANDRECOMMENDATIONS........................................................................844.2REFERENCES:.......................................................................................................................................89
APPENDIXA–FERRICCHLORIDEBASEDSODIUMDIETHYLHEXYLPHOSPHATE(SDEHP)-STABILIZEDNZVI...................................................................................................................................................90
A.1INTRODUCTION...................................................................................................................................90
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A.2METHODOLOGY...................................................................................................................................91A.2.1PreparationofSurfactantSDEHPandFeCl3-basedNZVI.........................................................91A.2.2FormulationdesignofFeCl3-basedNZVI..................................................................................91A.2.3CharacterizationAnalysis:SizeandStability............................................................................92
A.3RESULTSANDDISCUSSIONS...................................................................................................................92A.3.1FormulationDesignImplication...............................................................................................92A.3.2StabilityandSizeAnalysis.........................................................................................................93
A.4FUTUREWORKS..................................................................................................................................95A.5REFERENCES:.......................................................................................................................................97
APPENDIXB:COMPARISONBETWEENCARBOXYLMETHYL-CELLUOSESTABILIZEDIRONOXIDENANOPARTICLESWITHMICROEMULSION-STABILIZEDNANOPARTICLES.............................................98
B1.BACKGROUND:....................................................................................................................................98B2.RESULTS:............................................................................................................................................98B2.1.STABILITY.........................................................................................................................................98B2.2.MOBILITYCOMPARISON.....................................................................................................................99
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ListofFigures
Figure1.1ReactionschematicsummaryofNZVI,adaptedfromFuetal....................................2Figure1.2A.SchematicofwormlikemicelleandB.theinteractionbetweenwormlikemicelles
byWangetal.andnanoparticles..........................................................................................6Figure2.1Columnexperimentconfiguration.............................................................................19Figure2.2Injectionscheduleofthecolumnstudies...................................................................21Figure2.3Schematicofthethree-compartmentmodelusedtorepresentthereversible
adsorptionofparticle,advection/dispersioncolumntransport,andparticleattachment.22Figure2.4TimelapsephotocomparisonsofμEironoxideandbareironoxide:(A)10g/LμE
ironoxide.(B)5g/LμEironoxide.(C)10g/Lbareironoxidenanoparticles......................27Figure2.5Viscosityprofilegraph(logscaled)ofμEironoxide(a)andME(b),comparison
betweentheoriginalformulationsanddilutionwithNaClbrinesolution(10g/100mL)at1:1ratio...................................................................................................................................29
Figure2.6TEMimagingofMicroemulsionironoxideat5g/Lwith100nmasscale(a)andmicroemulsionNZVIat1g/LbyWangetal.........................................................................31
Figure2.7Transportofironoxidesuspensionsin1-cmdiameter(highaspectratio)columnat5m/dayporevelocity(a)10g/Lironoxide(b)5g/Lironoxide.............................................33
Figure2.8Breakthroughcurvesof5g/L(asFe)μEsuspensionofironoxideinjectedat5m/day(porevelocity)throughcolumnswithaspectratioof15(left)and6(right).Thesolidlinesshowthesolutionofthe3-compartmentmodelusingtheconstantssummarized............34
Figure2.9Breakthroughcurvesobtainedfor5g/LμEironoxideinjectedat5m/day(porevelocity)througha2.5cmx15cmcolumn(aspectratioof6),usingscheduleA(10%NaClconditioning/rinsingfluid)andscheduleB(deionizedwaterconditioning/rinsingfluid.....37
Figure2.10BreakthroughcurvesobtainedfordilutedμEs(noironoxide)injectedat5m/day(porevelocity)througha2.5cmx15cmcolumn(aspectratioof6),usingscheduleA(10%NaClconditioning/rinsingfluid)andscheduleB(deionizedwaterconditioning/rinse).......38
Figure2.11Ironoxidedepositedonsandcolumnaftertheinjectionof1.5PVof5g/L(asFe)ironoxidenanoparticlesat5m/day(porevelocity)througha2.5cmx15cmcolumn(aspectratioof6),usingscheduleA(10%NaClconditioning/rinsingfluid)andscheduleB(deionizedwaterconditioning/rinsingfluid).Thesolidlinerepresentsthepredictionofdepositedironfromthe3-compartmentmodel..................................................................41
Figure3.1StructureofanionicphosphatesurfactantSDEHP....................................................51Figure3.2Illustrationofthe“one-pot”synthesisprocedureofSDEHP-stabilizedNZVI.The
procedurewasconductedintheglovebox..........................................................................56Figure3.3ColumnstudysetupforSDEHP-stabilizedNZVI.........................................................59Figure3.4Surfacetensionmeasurementsof1g/LofNZVIdissolved:Curve1showsthe
surfacetensionmeasurementoftheoriginalSDEHPconcentrationandCurve2displayedthecorrectedconcentrationofSDEHP................................................................................62
Figure3.5DissolvedSDEHPequilibriumconcentrationwithironVS.addedironsulfateconcentrationsfordifferentinitialSDEHPconcentrations..................................................63
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Figure3.6TEMimagingof10mMSDEHP-stabilizedNZVIat1g/Lwithdifferentscaleat500nmscale...............................................................................................................................64
Figure3.7SetA,TimelapsephotosofSDEHP-stabilizedNZVIat0.5g/LofNZVIatvariousSDEHPconcentrations:a.30mMofSDEHPb.50mMofSDEHPandc.100mMofSDEHP...............................................................................................................................................65
Figure3.8SetB,Stabilitytimelapsepictureof100mMatNZVIconcentration1,1.5and2g/Loveraperiodof24hours:a.1hourandb.24hoursaftersynthesisandre-suspension....67
Figure3.9ColumnstudybreakthroughcurveofhighlystableSDEHP-stabilizedNZVI,at100mMSDEHPand1g/LofNZVIat5m/daywiththemodeldescribedinchapter2(solidline)...............................................................................................................................................73
FigureA1.FigureA1.SurfacetensionmeasurementsofSDEHPatvariousconcentrationwith
ironchloride(1g/LequivalenceofNZVI)dissolved............................................................93FigureA2.FigureA2.Ferricchloride-basedNZVIat100mMofSDEHP@0.3g/Lofiron
concentration.1houraftersynthesis..................................................................................95FigureB1.A.TimelapsedphotosofCMCandmicroemulsionstabilizedironoxideat2.5g/L.B.
EvidenceofsettlingofCMCironoxideafter80hoursuponsuspension.............................99FigureB2.Comparisonofpressuredropmonitoringresultsatthepost-flushingstagebetween
CMCandmicroemulsionironoxide...................................................................................101FigureB3.Iron-sandgrainanalysiswithmicroscopepicturesforA.Microemulsionironoxideat
2.5g/LandB.CMCironoxideat2.5g/L............................................................................102
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LiterofTables
Table2.1Summaryofbreakthroughcurveparameters.............................................................40
Table3.1LiteraturesummaryofcolumnstudiesandstabilitybehaviourfordifferenttypesofsurfacemodifiedironoxideandZVInanoparticles..............................................................52
Table3.2SynthesisresultandcharacterizationofSDEHP-stabilizedNZVIatvariousNZVIandsurfactantconcentrations....................................................................................................67
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Chapter1:Introduction
Nano-scalezerovalentiron(NZVI)arereactivemetalnanoparticlesartificiallyreducedfrom
Fe2+orFe3+thatholdsahighredoxpotential(E0=−0.44V)duetothezero-valentarounditas
showninfigure1[1].Becauseofthehighredoxpotential,NZVIarecapableofreducingawide
rangeofchemicals,includingchlorinatedorganiccompounds,nitricaromaticcompoundsand
heavymetals[1][2][3],asdemonstratedinfigure1.1.NZVIparticlesalsoholdhighsurfaceareas
duetothenano-scalesizerangingfrom1to250nmthatincreasetherateofreduction
reactions[4][5].Becauseoftheabovefeatures,inthelasttwodecades,NZVIhasbeen
identifiedasapotentialefficientinsitugroundwaterremediationtechnologycomparingto
otherexistingtechnologies[6].ItisexpectedthatwiththedirectcontactbetweentheNZVI
particlesandthesourceofthecontaminantcanactivelyandrapidlyreducethecontaminant
zoneconcentrationandachievedfullremediation[7].Inspecific,inanindustrial-scaleinsitu
NZVIremediation,stabilizedNZVIaretobeinjectedthroughmultipleinjectionwellsvia
differentinjectiontechnologysuchaspressure-pulseorgravityinjection[8][9].Theamountof
NZVIinjectedisdeterminedbasedonthecontaminantconcentrationsfromthe
characterizationofthesitepriortoinjection.Uponinjections,theNZVIistobeleftinthesoil
fortreatingthecontaminantsoveraperiodfromweekstomonths;forbetterresults
recirculationofthegroundwaterisoftenscheduledperiodicallytopromotethemobility.
Duringthisperiod,concentrationoftheironandcontaminantsaremonitoredfrommonitoring
wellsforprogressandhydrauliccontrol.Extractionwellsareinstalledatdownstreamtocollect
transportedNZVIparticles.
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However,injectingNZVIparticlessuspensionintothesoiltoachieveeffectivegroundwater
remediationisacomplexprocedure;O’Carrolletal.summarizedintothefollowing3steps:(1)
Transportingthereactiveparticlesthroughthesoilmatrix(2)Formingcontacttothe
contaminantzoneand(3)Reactingwiththecontaminantstoachieveremediation[10].NZVI
suspensionholdanundesirablepropertyoffastaggregationandsedimentationduetothehigh
stabilitycontributedbythemagneticattractionforcesbetweentheparticle[11].Thisfeature
failsNZVItoachievethefirststepofconductingtheinsituremediation—thelarger,
aggregatedzero-valentparticleswillexperiencefiltrationinthesoilmatrixandironparticles
willattachtothesoilgrain,keepingtheNZVIparticlesfromreachingthedeepercontaminant
zone[12].Thetransportandmobilityoftheironparticlesintheporousmediaisidentifiedas
themajorobstacleofNZVIinsituremediation.Tothisdate,NZVIinjectionhasremainedasan
state-of-the-arttechnologyandresearchhasbeenactivelydoneonimprovingthetransport.
Figure1.1ReactionschematicsummaryofNZVI,adaptedfromFuetal.[1].
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LiteraturereviewsuggestedthatimprovingthestabilityoftheNZVIsuspensioncanimprovethe
mobilityofNZVIintheporousmedia[10],[11].Inspecific,astablenanoparticlesuspension
eliminatestheissueofaggregationandsedimentation:ThismeansthatastableNZVI
nanoparticleparticlecanremaininthenano-scalesize,travelbetweensandgrainswithout
filtrationandmaximumamountofNZVIcanbetransported.Currently,themostdirectand
commontechniqueofimprovingthestabilityofNZVIistoapplysurfacemodificationstothe
surfaceofNZVI.Applyingsurfacemodifiers,therepulsionforcesbetweentheironmetal
nanoparticlescanbereducedduetotheadditionofthebarrierandachievehigherstability.
Surfacemodifiersareproventohavepositiveinfluencesonimprovingthestabilityasearlyas
10yearsago[4],[13];however,theresearchonimprovingthestabilityandmobilityisstill
ongoing.Recentstudieshaveshownthatpolymeradsorptionisthemostcommonwayto
stabilizingNZVI,foodgradepolymersuchascarboxyl-methylcellulose[11],[14]–[16],PV3A[17]
andPAA[17].Ontheotherhand,biodegradablesurfaceactiveagentssuchasTween80[18],
SDBS[19]andbiodegradablesurfactants[20]havealsoshownsomeprogressinthisfield.Itis
worthmentioningthatemulsioninducedNZVIhasdrawnalotofattentionasanalternative
wayofstabilizingZVIparticleswithoutsurfaceadsorption[21],[9],[22].However,outofthe
above,theinstabilitywasstillobservedintheabovestudies,forexample,carboxyl-methyl
cellulosebasedNZVIcanremainstableforabout80hourswhileaggregationandsedimentation
areconstantlyobserved[16].Furthermore,Tween80surfactant-basedNZVIalthoughclaimed
remainingstableformonthsinstoragecondition,onceinfieldcondition,thestabilityis
disturbed[18].EmulsioninducedNZVIontheotherhandholdsakineticallystabilityof8hours
whilerequiringmechanicalforceduringinjection[9].Despitescholarshaveinputgreatefforts
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intostabilizingNZVItoimprovemobility,therehasnotbeenaNZVIsurfacemodifierthat
completelyeliminatesaggregationandsedimentation.
Laboratory1-Dcolumnstudyisusuallythefirststepinevaluatingthemobilityofasurface
modifiedNZVIbeforescalinguptoafieldremediation.ThesurfacemodifiedNZVIsuspensions,
includingtheabovedescribed,weretestedinvarioussimilarbenchscalecolumnsettingsthat
generallyimpliedthreeissues:1.Flowvelocity:mostofthelaboratorycolumnstudieswere
conductedatarelativelyhighflowratefrom8to200m/day[23][17][18],thisisunrealistic,
consideringtypicalfieldapplicationsareconductedat0.25-4m/day[8].2Performance:as
mentioned,instabilitywasstillobservedinallthesurfacemodifiedNZVIthusfar,itwas
constantlyreportedthatthehighestbreakthroughpeakcanreachover0.9athigherlaboratory
flowvelocities[24].However,poorrecoveryandbreakthroughpeakatflowvelocities
approachingtothe4m/day(Tiraferri&Sethi,2009;Xin,Tang,Zheng,Shao,&Kolditz,2016).3.
Mixing:someofthecolumnstudiesintegratedmechanicalmixingintheirsetting[23],field
remediationisoftenconstrainedtointegratesuchfeature.However,carboxyl-methylcellulose-
basedandemulsioninducedNZVIhavesuccessfulacquiredadequateresultsatfieldflow
velocities[16][21].Full-scalefieldstudieswereconductedwiththetwoNZVIsuspensions;
however,unsuccessfulNZVItransportwasreported[11][22].
Theobjectiveofthisthesisistostudythemobilityofmicroemulsion-basedNZVIandtodevelop
anoptimizedsurfactant-basedNZVI.Chapter2and3inthisstudyaretwoscientificarticlesthat
criticallyexaminetwohighlystabilizedNZVIsurfacemodifiersontheirmobilityinporousmedia
andtheirpotentialtoafull-scaleremediation.
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Inchapter2,themobilityofmicroemulsion-stabilizedNZVIwithastabilityofover6months
developedbyWangetal.isaccessedusingironoxidenanoparticlesasastableanalogytoNZVI.
Highlyconcentratedmicroemulsionironnanoparticleswithidenticalcolloidalpropertiesto
microemulsionNZVIischaracterizedbysize,rheologyandstability.Laboratory1-Dcolumn
studyareconductedatdifferentconditionswithdifferentconcentrations,atlaboratoryand
fieldvelocitiesanddifferentsalinityenvironment.Thebreakthroughresultsaredemonstrated
andmodelledusingColloidFiltrationTheory(CFT).Excellenttransportresultswereobserved;
however,thehighsalinitysensitivityandthepropertyofthesurfactantimplythat
microemulsion-stabilizedNZVIisnotsuitableforafieldtest.
Inchapter3,basedontheimplicationsfromchapter2,anenvironmentalfriendlyphosphate
surfactant,isselectedasasurfactantstabilizer.Aframeworktodeterminethemoststable
nanoparticlessuspensionisadaptedfromWangetal.todeterminethemostoptimized
surfactant-basedNZVI.Itisimportanttonotethat,thusfar,nosophisticatedframeworkhas
beenappliedtodeterminetheformulationforaNZVIstabilizer.Theoptimizedsurfactant
yieldedastabilityofover2monthsandisexaminedwithalaboratory1-Dcolumnstudyatfield
velocity.Itisreportedthatthedevelopedformulationcanrecoverover90%ofNZVIwith
breakthroughpeaksat1.Itisexpectedthatthedevelopedphosphateformulationcanleadto
potentialfieldtestandeventuallyasuccessfulfieldapplication.
Itishypothesizedthattheprolongedstabilityobservedinthemicroemulsion-stabilizediron
nanoparticlesandsurfactant-stabilizedNZVIiscontributedbytheformationofwormlike
micellesinthesystems.Wormlikemicelles,asshowninfigure1.2,arelongandentangled
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aggregatesofmicellesthatdemonstratestructuresandproperties(suchasviscosity)similarto
polymers[27].Theformationofwormlikemicelleisdependentontheconcentrationofthe
surfactants,surfactantgeometry,curvaturesandpacking.Uponreachingtoacriticalassembly
concentration,themicellewillself-assembleintowormlikemicelles[28][29].Studieshave
suggestedthattheinteractionbetweennanoparticlesandwormlikemicellecanpromotethe
stabilityofthesuspension[27].
A.
B.
Figure1.2A.Schematicofwormlikemicelle,pictureprovidedbyMr.FrancisChoi[29]andB.
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theinteractionbetweenwormlikemicellesbyWangetal.andnanoparticles[27].
Overall,thisstudycontributedaphosphatesurfactantbasedNZVIsuspensionthatissuitable
forafieldapplicationbasedontheperformanceofmicroemulsion-basedNZVI.Thisstudyalso
suggestedthefeasibilityofmicroemulsionasatransportvehicleforinsituNZVIremediation.
Futurestudyon1.amoredetailedreactivitystudyofthephosphatesurfactantbasedNZVIand
2.TargetdeliverystudywithDNAPLinalargercolumnorsandboxarerecommendedpriorto
thefieldapplication.
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[22] J.Quinn,C.Geiger,C.Clausen,K.Brooks,C.Coon,S.O’Hara,T.Krug,D.Major,W.S.Yoon,A.Gavaskar,andT.
Holdsworth,“FielddemonstrationofDNAPLdehalogenationusingemulsifiedzero-valentiron,”Environ.Sci.Technol.,
vol.39,no.5,pp.1309–1318,2005.
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nanoironinclayusingdirectelectriccurrent,”Water.Air.SoilPollut.,vol.224,no.12,pp.1–12,2013.
[24] C.Mystrioti,N.Papassiopi,A.Xenidis,D.Dermatas,andM.Chrysochoou,“Columnstudyfortheevaluationofthe
transportpropertiesofpolyphenol-coatednanoiron,”J.Hazard.Mater.,vol.281,pp.64–69,2015.
[25] J.Xin,F.Tang,X.Zheng,H.Shao,andO.Kolditz,“Transportandretentionofxanthangum-stabilizedmicroscalezero-
valentironparticlesinsaturatedporousmedia,”WaterRes.,vol.88,pp.199–206,2016.
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[26] A.TiraferriandR.Sethi,“Enhancedtransportofzerovalentironnanoparticlesinsaturatedporousmediabyguargum,”
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forMolecularSelf-Assembly,”Nat.Mater.,vol.15,no.September,pp.1–9,2015.
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Chapter2:TransportofMicroemulsion-StabilizedIronOxideinPorous
Media
Abstract
ThefullpotentialofNZVItechnologiesisoftenlimitedbythetransportNZVIparticlesthrough
porousmedia,whichinturnislimitedbythecolloidalstabilityoftheNZVIsuspension.Previous
workhasshownthatstableNZVIsuspensionscanbeproducedusingmicroemulsions(μEs)as
synthesis and suspensionmedia. In thiswork, Ironoxide nanoparticles, used as non-reactive
analogstoNZVI,wereusedtoevaluatethetransportintheμEusedtosynthesizeandsuspend
NZVI.Thetransportofthesesystemswasexaminedatfullstrength(10g/LFe)andasadiluted
(5g/L Fe) suspension using column studies. The nanoparticle injection protocol was also
evaluated(watervs.brineconditioning/rinsingfluid).Theresultingbreakthroughcurveswere
analyzed via a 3-compartment transport model that accounts for reversible and irreversible
attachmenttothesandpackedinthecolumn.Itwasdeterminedthatlargepressuredropswere
observedwithconcentratedsuspensions(10g/LFe),whichisexplainedbythelargeviscosityof
thesesystems.Thedilutedsuspensions(5g/LFe),havingalowerviscosity,couldbeinjectedin
thesystem,producinghighparticlerecoveries(~90%)whenthesolutionusedtoconditionand
rinsethecolumnwasabrinewiththesamesaltconcentrationastheμE.However,whenusing
deionizedwatertoconditionandrinsethecolumn, lowerrecoveries (~60%)wereobtained,
likelyduetophasetransitionsintheμEthatresultedinthedepositionofparticles.
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2.1Introduction
Nanoscalezero-valentiron(NZVI)particlesareeffectivereducingagentsforavarietyof
contaminants,includingheavymetals,dyes,chlorinatedorganiccompounds,aromaticand
arsenic-containingcompounds[1][2].ThenanoscalesizeofZVIparticles,rangingfrom10to
200nm,yieldshighsurfaceareasandthusincreasedcontactwiththecontaminantsmakingthe
reductionreactionsmoreefficient[3].NZVIcaneffectivelyreducecommoncontaminantssuch
astrichloethylene(TCE)toethylenewithindaysorweeks[1][4][5][6].NZVIinjectionhasshown
promiseintreatingcontaminantsinnumerousfield-scalestudiesconductedinEuropeand
NorthAmericainthelastdecade[7][8][9].
NZVIaquiferremediationisstillhinderedbythelimitedcolloidalstabilityofsuspensions
currentlyinuse,resultinginrapidaggregationandsettling[8][9][10][11][12].Therapid
aggregationandfastsettlingfeatureofNZVIiscausedbythehighsurfacemagneticpotential
[4][13].Duringthetransport,aggregatedNZVIaremorelikelytoexperiencefiltrationbythe
sandporesduetothelargesize.Settlingofaggregatedparticlesalsocontributetothe
depositionofNZVIontheporousmedia[13].Schricketal.andTiraferietal.reportedthatpoor
transportperformanceofunstableNZVIareobservedinlaboratorycolumnstudiesat
consideratelylowconcentrations(0.05-0.1g/L)[10][14].
TheintroductionofthesurfacemodificationscanreducetheaggregationofNZVIparticlesand
thuscontributetoimproveNZVItransportinporousmedia[10][15][16].Inspecific,surface
modificationsareservedassurfacestabilizers,theycreatedanenergybarrierintheNZVI
suspensiontokeeptheparticlesfromattractingandaggregating.Commonsurfacemodifiers
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13
includeemulsions,anionicsurfactants,polymers,organicacidsandpolymericsurfactants[4]
[7][16][12][17][18][19][20].
Laboratorystudiesandfieldtrialshavereportedsomeprogressinimprovingthemobilityof
NZVIinsoil;however,theaggregationofNZVIhasnotbeenfullyaddressed[17][19][21][22].
MostofthestabilizedNZVIcolumnstudies,suchasthoseconductedusingxantumgum-
stabilizedNZVIandpolyphenol-stabilizedNZVIreportedhighNZVIrecovery,over85%,atpore
velocitiesbetween20-200m/daywhileonly10%tonegligiblerecoveryobservedatvelocities
below10m/day[22][23][17][24][25].O’Carrolletal.andKocuretal.indicatedthattypical
injectionvelocitiesforafield-scalein-situremediationrangebetween0.25and4m/day
[20][19][18].Thehighrecoveryreportedbymosttheliteraturestudiesareanoptimistic
estimationoftheabilityofthesurfacestabilizersapplied.Stabilizersthatallowadequate
transportofNZVIatlowporevelocitiesarestillneeded.
ColumnstudiesconductedbyBergeetal.andKocuretal.withemulsionNZVIand
carboxylmethyl-cellulose(CMC)NZVI,respectively,reportedNZVIrecoveriesof90%for
injectionvelocitiesrelevanttofieldapplications[19][26].ItwasreportedthatemulsionNZVIis
kineticallystableandCMCNZVIholdsarelativelyhighstabilityof80hours[7][19].However,
thefieldresultsintheCMCNZVIimpliedthatastabilityof80hoursmaynotbesufficient.In
2014,Kocuretalconductedafield-scaleremediationwithCMCNZVIatasiteinSarnia[2].CMC
stabilizedNZVIat1g/Lironconcentrationwasinjectedatgroundwaterflowvelocitiesof0.02-
0.8m/dayusinggravityinjection.After10daysofcontactingperiod,CMCNZVIwasobservedat
monitoringwells1meterdownstream,implyingthattheminimumtransportdistanceofCMC
NZVIisatleast1meter.However,therecoveryoftheinjectedNZVIwasonly1%[27].Similar
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14
findingswerereportedintheemulsionNZVIfieldstudy[10].Additionally,fielddemonstrations
ofNZVIremediationreportedshorttraveldistancesbetween0.5to2.4meters[19][28].
Furthermore,someofthereportedNZVIcolumnstudiesrequiredcontinuousmechanical
mixingtopreventNZVIfromsettlingintheinjectionpoint[17].Mechanicalmixingmightbe
impracticaltoscaleup.TheseobservationsshowthatdespitetheimprovedstabilityofNZVI
suspensionbystabilizers,thedelayedaggregationmechanismwasnotfullyeliminated.
O’Carrolletal.suggestedthatanidealNZVIformulationshould:(1)maintainstabilitysuchthat
theparticlesdonotaggregateandsettle;(2)musthaveahighconcentrationofironwhile
maintainingasmallsize;(3)sustaincertainamountofmobilitywhilebeinginjectedinsoil[19].
ThemostsuccessfulsystemsthusfarareCMCNZVIandbimetallicNZVI[27][28].Despitegood
remediationresults,allthefieldscalestudiesofNZVIreportedthatlittletonoNZVIparticles
wererecovereduponcompletion,implyingimmobilizationandemphasizingtheneedforbetter
transportoftheparticles.
Wangetal.introducedtheuseofmicroemulsions(μEs)asbothsynthesissolventand
suspendingmediainaone-potsynthesisprocedure[29].μEsarethermodynamicallystable
systemscontainingoiland/orwaternano-domains(typicallyof10to100nm)thatare
stabilizedbysurfactant(s)adsorbedattheoil-waterinterface.TheformulationoftheμE-based
synthesis/suspensionmediawasdesignedviatheHydrophilic-Lipophilic-Difference(HLD)
framework,usedbyChoietal.andWangetal.todeterminethecombinationofsurfactant,
electrolyte,oil,andtemperaturethatproducesbicontinuousnet-zerocurvaturesystems
(whereHLD=0),whichalsoleadstotheformationofsuspensionsthatarestableforseveral
months,andareeasilyre-suspendedwithmildmixing[16][29][30].
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15
DespitetheadvancesmadewithμEsynthesis/suspensionmediaforNZVI,nostudieshavebeen
conductedtovarytheirtransportthroughporousmedia.Microemulsionsontheirownhave
beenappliedaspartofthesurfactantenhancedrecoveryremediation[31].Thesurfactant
enhancedaquiferremediation(SEAR)hasbeensuccessfullyappliedfortheremediationof
mediumtolowdensitynon-aqueousphaseliquids(LNAPLs),butdownwardmobilizationofhigh
densityplumesofchlorinatedsolvents(DNAPLs)havelimitedtheiruseforthoseapplications
[32].
Inprinciple,μE-NZVImeetsthestandardsrequiredforanidealstabilizer,buttheireffectiveness
havenotbeenevaluatedviacolumnstudies.Thus,itistheinterestofthisworktoexaminethe
transportpropertiesofμE-NZVIinone-dimensional(1-D)columnstudiesandassessits
potentialusefulnessinfieldapplications.
Forthepurposeofanalyzingtheintrinsicabilityofmicroemulsionasatransportvehicle,iron
oxideisusedinthisresearchasananalogytoNZVI[13][16][33][34].Microemulsionironoxide
developedbyChoietal.hasidenticalcolloidalstructureandsizetoμE-NZVI[30].Inadditional,
ironoxideisusedasasynthesisbasistoproduceNZVIinsomecasesandcanbecategorized
withNZVIintermsofenvironmentalapplication[34][35].Transportofironoxideinporous
mediaisalsointerestofenvironmentalandbiomedicalresearch.
TheobjectiveofthisworkistodetermineandassessthepotentialandabilityofμEsasastabilizer
and delivery vehicle for NZVI in porousmedia using iron oxide nanoparticles as analog. The
studieswereconductedusingfield-relevantconditionsthatmosttheothercolumnstudiesdon’t
typically consider: high concentration of suspended iron and a low Darcy velocity. Several
preliminary and post analytical strategies including size determination using dynamic light
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16
scattering(DLS)andTEM,viscositytestandparticlecolumndistributionareconductedtoprovide
a better understanding on the filtration mechanism between the porous media and the
nanoparticles.Colloidfiltrationtheoryisusedtodescribeandestimatethetransportefficiency
anddistanceofthemicroemulsionironoxide.Theresultofthispaperimpliedandcontributed
thepotentialofanewandefficientsurfacemodificationtechniqueforNZVIinsituremediation.
2.2Methodology
2.2.1SynthesisofMicroemulsionIronOxide
Microemulsionironoxidesuspensionwassynthesizedbybatchfromtheproceduredeveloped
byChoietal.[30]:0.103gramsironoxidenanoparticles(purchasedfromSigmaAldrich,
98%,No.637106),0.327gramsfoodgradeoilethylcaprate(purchasedfromSigma
Aldrich,98%,No.W243205),2.976mLsurfactantAlfoteraK3-4SC10H21O(CH3CH2(CH3)O)4SO4Na
(donatedbySasolNorthAmerica,32.5wt%,lotno.4130115),3.23mLNaCl(Bioshop,Reagent
Grade,SOD002.205)brinesolution(30g/100ml)and3.47mLwater.Thechemicalswere
addedbyweightusinganelectronicscale(DenverInstrumentXX7020023AnalyticalBalance
LabScale±0.0001g)andbyvolumeviapipettingwithanautomaticpipet(FisherBrand,Elite,
AdjustableVolumePipetter,0.5-5mL).Theformulationmixturewasmixedusingavortexer
(VWR,minivorterxer,WM-3000)at5,000rpmfor1minuteandsonicateusingasonicatorbath
(Cole-Parmer,8891)for1minute.Themixingtagewastoensurefullandevensuspensionof
thenanoparticles.Thecompletedsuspensionyieldedabrowncolouruniformly.Eachbatchof
theironoxidesuspensionprovides10mlof10g/Lironoxide(equivalentto7g/LFe)
suspensionwithasalinityof10gNaCl/100ml(10%NaClbrine).FormakingμEironoxide
suspensionwithlowerconcentrationat5g/Lthe10g/Lformulationwasdilutedwith10%NaCl
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brinesolution(10g/100mL).Keepingthesamesalinityensuredthattheformulationretained
itsstructure,thuspreventinganyphasebehaviorchange.Itisimportanttonotethatforthe
purposeoftrackingthetransportofμE,solventbluedye(Sigma-aldrich,98%,17354-14-2)was
dissolvedinethylcaprateataconcentrationof5000ppm.
2.2.2Determiningthestabilityofmicroemulsionironoxide.
Todeterminethestabilityofmicroemulsionironoxide,timelapsephotosoftheironoxide
formulationsandbareironoxideweretakenoveraperiodof1year.Thetimelapsephotos
weretakendailyinthefirstmonth,bi-weeklylaterandmonthlyfortheremainingperiod.The
collectedphotosofthesampleswereanalyzedvisuallyforsignsofaggregation,settlingand
instability.
2.2.3Viscositystudyofmicroemulsionironoxideformulations
TodeterminetheviscosityoftheμEsandtheironoxide–loadedμEsarheometer(TA
instrument,CSL2500)wasused.Theviscositiesweremeasuredatdynamicshearrates
increasingfrom3-500(1/S).
2.2.4Sizecharacterizationofmicroemulsionironoxideandmicroemulsionformulations
Transmissionelectronmicroscopy(TEM,HitachiHF-3300)wasusedtoassessthesizeandstate
ofaggregrationoftheironnanoparticlesin5g/Lsuspensions.ItisimportanttonotethatTEM
imagingwasnotabletocapturetheμEcomponentsofthesuspension,onlytheiron
nanoparticles.Dynamiclightscattering(DLS)measurementswereconductedonμEironoxide
at5and10g/LusingaBrookhavenParticleSizeAnalyzer90Plus.Priortosizeanalysis,the
sampleswerediluted50timeswith10%NaClbrine.
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2.2.5Columnstudy.
Twosetsofcolumntransportexperimentswereconductedwithsimilarproceduresand
identicalsetupforanalyzingthetransportatdifferentflowconditions.Thesetupofthecolumn
experimentsisshowninFigure2.1.Inshort,theconditioning/rinsingfluidandμEironoxide
wereinjectedwithaperistaticpump(ColeParmer,MasterFlexL/S)inanupwarddirectionand
wereswitched,asneeded,viaathree-way-valve.Theflowpassedthroughapressuregaugefor
pressuredropmonitoringbeforeenteringthesandcolumn.Theeffluentfromthecolumnwas
collectedbyafractioncollector(RediFrac,BioscienceAmersham).
Thefirstsetsofcolumnexperimentswereconductedwithaglasscolumn(1.0x15cm,Kontes
ChromaflexColumns,KimbleChase,Vineland,NJ.)havinganaspectratioof15andthesecond
setofcolumnexperiments,wereconductedwithalargerglasscolumn(2.5x15cm,Kontes
ChromaflexColumns,KimbleChase,Vineland,NJ.)withidenticallengthbutbiggerinner
diameter,thusaloweraspectratioof6.Bothcolumnswerewet-packedhomogeneouslywith
acidwashedOttawasand(FisherScientific,~500micrometersinradius)and1wt%Alfoterra
K3-4Ssurfactantsolutionusedtoremoveairpocketstrappedinthesandmedia.Theweightof
thecolumn,sandandamountofsolutionweremeasuredwithanelectronicbalance(Denver
Instrument,TP-214)beforeandafterthepackingprocess.Themeasuredweightswereusedin
massbalancetodeterminetheporevolume.Itwasdeterminedthatsmallandlargecolumns
have4.7and30.2mLporevolume,respectively.
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Figure2.1Columnexperimentconfiguration
Thefirstsetsofcolumnexperimentswereconductedwithaglasscolumn(1.0x15cm,Kontes
ChromaflexColumns,KimbleChase,Vineland,NJ.)equivalenttoanaspectratioof15andthe
secondsetofcolumnexperiments,wereconductedwithalargerglasscolumn(2.5x15cm,
KontesChromaflexColumns,KimbleChase,Vineland,NJ.)withidenticallengthbutbiggerinner
diameter,thusaloweraspectratioof6.Bothcolumnswerewet-packedhomogeneouslywith
acidwashedOttawasand(FisherScientific,~500micrometersinradius)with1wt%K3-4S
surfactantsolutionforeliminatingairpocketstrappedinthesandmedia.Theweightofthe
column,sandandamountofsolutionweremeasuredwithanelectronicbalance(Denver
Instrument,TP-214)beforeandafterthepackingprocess.Theweightsoftheabovewereused
inmassbalancetodeterminetheporevolumes.Itisdeterminedthatsmallandlargecolumn
holda4.7and30.2mLofporevolumes,respectively.
Forexperimentsconductedwiththesmallcolumn,10porevolumesofconditioningsolution
werepumpedfromthebottomofthecolumnbyaperistaticpump,thisisknownasthepre-
flushingorconditioningstage.Theperistaticpumpwasoperatingat0.5mL/minequivalenttoa
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20
Darcyvelocityof20m/dayandforalowervelocityasyringepump(notshowninFigure2.1)
wasusedtoproduceaDarcyvelocityof5m/day.Duringtheconditioningstage,thepressure
dropwasrecordedeveryporevolume.Brine(10%NaCl)solutionwasusedastheconditioning
fluidtokeepthesalinityconsistentwiththeμEironoxideformulation.Aftertheconditioning
stage,thepressuregaugewasbypassedtoavoidμEentrainmentinthepressuregaugeline.A
totalof1.5porevolumesofμEironoxideatconcentrationsof5or10g/L(asironoxide)were
injectedtothecolumn.Thefractioncollectorwasthenstartedandsettocollect3minutesof
flowpersample(i.e.1.5mL/sample)forthehigherDarcyvelocityand5minutes/sample
(0.5mL/sample)forthelowerDarcyvelocity.Uponthecompletionoftheironoxideinjection,
another10porevolumesofthesamesolutionusedduringtheconditioningstagewerethen
introducedintothecolumnasarinsingstep.Thecollectionofsampleswasstoppedattheend
oftherinsingstage.
ThesamplescollectedwereanalyzedviaaciddigestionwithHCl6N(BDH,BDH7204-1)witha
sampletoHClvolumeratioof1:14.5,aspertheprocedureofRadetal.[1].Theaciddigested
ironoxidesampleswereanalyzedunderUV-VISspectrometer(80-2092-26,LKBBiochrom
England)at398nmafterareactionperiodof3days.Acalibrationcurvewascreatedusingthe
μEironoxidesuspensioninjectedintothecolumn.
Forthelargecolumn(aspectratioof6),experimentswereconductedat5m/day.Inthese
experiments,twoconditioning/rinsingscheduleswereevaluatedassummarizedinFigure2.2.
ScheduleA,inFigure2.2,followsthesameconditioning/rinsingstepsusedwiththesmall
columnwhere10%NaClwasusedastheconditioning/rinsingsolution.InScheduleBstudies,
deionizedwaterwasusedastheconditioning/rinsingsolvent.ScheduleAisbeneficialbecause
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21
itmaintainstheionicstrengthofthemicroemulsion,reducingthechangesforphasechanges.
ScheduleBrunstheriskofμEphasechanges,butitslowionicstrengthismoreconsistentwith
thatofgroundwater.Uponcompletionoftheconditioningstage,1.5porevolumesofμEiron
oxidesuspensionscontaining5and10g/L(asironoxide)wereinjectedintothecolumn.The
fractioncollectorwassetto3min/sample(1.5ml/sample).Fractionalcollectorsettingat3
min/sample(1.5ml/sample)wasusedtocollectthesample.Thecollectedsampleswerethen
analyzedforironcontentusingtheaciddigestionprocedurepreviouslydescribed.Afterthe
rinsingstep,thesandinthecolumnwascollectedanddividedintofivesegments,eachbeing
approximately3cminlengthtodeterminetheresidualirondistributionleftonthecolumn.The
irondistributionanalysiscombinedmicroscopeimagingandaciddigestionsofsandataweight
ratioof0.3gofsandto3mLofHCl6Nacid.Finally,selectedeffluentsamples,correspondingto
thepeakofthebreakthroughcurve,wereanalyzedunderDLStoassesspotentialparticle
aggregation.
Figure2.2Injectionscheduleofthecolumnstudies.
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2.2.6.BreakthroughCurveModeling.
Aswillbediscussedlater,thebreakthroughcurvesobtainedinthisworkhavefeatures
characteristicofthreedifferentphenomena;diffusion/dispersion,reversible
“chromatographic”adsorption,andtheirreversible“attachment”orfiltrationofparticles.To
representthesefeatures,Figure2.3presentsa3-compartmenttransportmodel.
Figure2.3Schematicofthethree-compartmentmodelusedtorepresentthereversible
adsorptionofparticle,advection/dispersioncolumntransport,andparticleattachment.
Thecentralcompartmentcorrespondstothetransportoftheparticlesthroughthecolumn,
withmassbalanceequation:
Ci,t-1Ci-1,t-1
Crev i,t-1Crev i-1,t-1
Catti,t-1Catti-1,t-1
Ci+1,t-1
Crev i+1,t-1
Catti+1,t-1
Flow
Ci,tCi-1,t
Crev i,tCrev i-1,t
Catti,tCatti-1,t
Ci+1,t
Crev i+1,t
Catti+1,t
Flow
Time“t-1”
Time“t”
Page 31
23
!"!#= −𝑣 !"
!(+ 𝐷 !+"
!(+− 𝑘-##𝑐 − 𝑓012
!"345!#
(1)
Where“C”istheconcentrationoftheparticleatagiventime“t”andasectionofcolumn“i”,
andwouldcorrespondtothevariableCi,tintheschematicofFigure2.3.Theterm“v”isthe
pore(Darcy)velocity,“D”istheeffectivediffusivityoftheparticlesinthecolumn.Itshouldbe
clarifiedthatthisdiffusivityincludesback-mixingordispersioneffectsinthecolumn,beyond
theintrinsicdiffusivityoftheparticles.Thevariable“z”isthecolumnlengthaxis,represented
bythecolumnlocationindex“i”inFigure2.3.Theparameterkattisusedtorepresentthe
particleattachmentprocessasafirstorderirreversiblereaction,inasimilarwaythatthe
colloidfiltrationtheorydoes[36][37].
Thereversibleadsorptioncompartmentisusedtoaccountforthesametypeofreversible
exchangethattakesplaceinchromatographicseparation[38][39].Inthecolumnstudies,this
reversibleexchangeresultsin“tailing”effectsinthebreakthroughcurvethatcannotbe
simulatedwiththediffusivity(dispersion)term.Asitwillbeshownintheresultsection,such
tailingeffectsareobservedinourresultsandotherresultspresentedintheliterature.To
considerreversibleadsorption,Figure2.3presentsamathematicalconstructionofa
compartmentwithanequivalentvolume“Vrev”wheretheconcentrationoftheparticlesis
“Crev”.Theterm“frev”istheratiobetweenthevolumeofthereversibleadsorption
compartmentandthevolumeofthecolumn(frev=Vrev/V).Themassbalanceoftheparticlesin
thereversiblecompartmentis:
!"345!#
= 𝐾 78 012
𝑐 − 𝑐012 = 𝑘012 𝑐 − 𝑐012 (2)
Page 32
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whereKcanbeinterpretedasthemasstransfercoefficientinbetweenthecolumnandthe
reversiblecompartment,andA/Vcanbeinterpretedasthesurfaceareatovolumeratioforthe
transportintothereversiblecompartment.TheoveralltermK*A/Vresultsinafirstorder
constant,krev.ItmustbeclarifiedthatthesimplemasstransportexpressionofEquation2
impliesalinearadsorptionbehavior.Morecomplexadsorptionbehavior,suchasLangmuir
adsorptioncouldbeused,butaswillbeshownlater,thesimplemodelofEquation2was
enoughtoreproducethetailfeaturesofthebreakthroughcurves.
Finally,themassbalancefortheirreversiblyadsorbed(attached)particlecompartmentis:
!9:;;!#
= 𝑉-##!":;;!#
= 𝑉𝑘-##𝑐 (3)
Asinthecaseofthereversibleadsorption,theirreversibleparticleattachmentcompartmentis
amathematicalsimplificationofanequivalentcompartmentofvolume“Vatt”,aspresentedin
Figure2.3.Theuseofthereversibleadsorptionandattachmentcompartmentssimplifiesthe
numericalsolutionofthemassbalancesandavoidsintroducingpartitioncoefficientsor
adsorptionisothermsthatwouldintroducemorefittingparameters.
Tosolvethedifferentialequations,finitedifferencesintime(t)andspace(z)were
implemented,asillustratedinFigure2.3.Theinitialtime-baseconditionwasallthe
concentrationsinthethreecompartmentsbeingzeroattimezero.Theinitialspace-base
conditionwasintroducedinawaythatitrepresentedtheinjectionprotocol,inotherwords,C0,t
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=Co(feedconcentration)foraslongastheinjectionoccurred,andzerootherwise.Thenon-
dimensionalfinitedifferenceformsofthebalanceequationsare:
𝐶012#,?∗ = 𝐶012#AB,?∗ + 𝑘012∗ 𝐶#AB,?∗ − 𝐶012#AB,?∗ 𝛥𝑡∗ (4)
𝐶#,?∗ = 𝐶#AB,?∗ − 𝐶#AB,?∗ − 𝐶#AB,?AB∗ ∆#∗
∆(∗+ 𝐷∗ ";FG,HIG
∗ A";FG,H∗
∆(∗− ";FG,H
∗ A";FG,HFG∗
∆(∗∆#∗
∆(∗− 𝑘-##∗ 𝐶#AB,?∗ ∆𝑡∗ − 𝑓012 𝐶012#,?∗ − 𝐶012#AB,?∗ (5)
𝑚-###,?∗ = 𝑚-###AB,?
∗ + 𝑘-##∗ 𝐶#,?∗ ∆𝑡∗ (6)
wherealltheconcentrationtermsarenormalizedbytheinitialconcentration(C*=C/Co),the
subindex“t”representthesolutionatagiventime,and“t-1”representsthesolutioninthe
previoustimestep.ThedimensionlessintervaloftimeisΔt*=Δt/τ,where“τ”istheresidence
timeinthecolumn.Thedimensionlessreversibleadsorptionrateconstantisk*rev=krev·τ.The
dimensionlessintervalofspaceisΔz*=Δz/L,where“L”isthelengthofthecolumn.The
dimensionlessdiffusioncoefficientisD*=D·τ/L2.Thedimensionlessattachmentrateconstantis
k*att=katt·τ.Thedimensionlessattachedmassismatt*=matt/(V·Co).
ThesefinitedifferenceequationsweresolvedinExcel,usinganspatialstep,Δz*=Δz/L=0.01,
andantemporalstepΔt*=Δt/τ=0.005.
Theattachmentconstant,katt,canbeusedincolloidfiltrationtheorytoestimatethedistance
towhich1%oftheinitialparticles(Lmax)arestillpresentinthefluid,usingtheequation[2]:
𝐿9-L = − 2M:;;
ln(0.01) (7)
Usingthesingle-collectorcontactefficiency(ηo)correlationdevelopedbyTufenkjietal.[37],
onecanassesstheattachmentefficiency(α),usefultocomparetoothercolumn
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26
studies[10][19][20][21][40]:
𝑘-## =U(BAV)WXYZ
𝜂\𝛼𝑣 (8)
Inequation8,𝜖representstheporosityofthesystemand𝑑`arepresentstheaveragegrainsize.
Consideringv=4m/day,d50=0.5mm,𝜂\~0.0005fromthecorrelationofTufenkjietalusinga
Hamakerconstantof1E-19J,a270nmparticleandafluidwith250cPviscosity[3].Aswillbe
shownlater,thesearetheconditionsthatapplytothecolumnstudiescarriedoutwithdiluted
μEsystemscontaining5g/Lironoxidenanoparticles.
2.3ResultsandDiscussion
2.3.1StabilityTest.
Figure2.4showsthetimelapsephotosofbareironoxide,μEironoxideat10g/Land5g/L
(Dilutionratio1:1withNaClbrinesolution(10g/100ml))overaperiodof1year.Asexpected,
thebareironoxidesuspensiondisplayedcolloidalinstability,settlingsoonaftermixingasseen
inFigure2.4(c).Bareironoxidenanoparticlessettledwithin10minutes.Incontrast,theμE-
stabilizedironoxideformulationsattheoriginalconcentrationof10g/Landatthediluted
concentrationof5g/Lremainedasasinglephaseafter1year.Visualinspectionindicatedno
signsofaggregationandsettlingbehaviour,asshowninFigure2.4.Theprolongedstability
demonstratedbytheμEironoxideisconsistentwithμENZVIasreportedbyWangetal.[29].
Thesimilarityinthehighstabilitybetweenthetwosuspensionsemphasizesthesimilarity
betweenthetwotypesofsuspensions.Thecompellingstabilitydemonstratedbythe
microemulsionironoxideformulationmatchedtherequirementsproposedbyO’Carroll,as
mentionedpreviously,inferringtraitsofgoodNZVI/ironmobilityinsoil.
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27
Choietal.proposedthatinsystemsthattendtoformbicontinuousμEs(thecaseforthe
formulationconditionsusedhere,andthoseofWangetal.),butwhoseextremelylowoil/water
ratiodoesnotallowtheformulationofbicontinuousstructures,worm-likemicellesareformed
instead,producinganetworkcapableofsuspendingnanoparticles[30].Theobservedstability
observedwithμEironoxidenanoparticlesaresubstantiallylongerthanonethemostsuccessful
NZVIsurfacestabilizations(about80hourswithCMC)[19][18][34].
Figure2.4TimelapsephotocomparisonsofμEironoxideandbareironoxide:(A)10g/LμE
ironoxide.(B)5g/LμEironoxide.(C)10g/Lbareironoxidenanoparticles
2.3.2RheologicalProperties.
Figure2.5showstheviscosityvs.shearratefortheoriginalμEformulationandthe50%diluted
formulation,with(topFigure)andwithout(bottomFigure)ironnanoparticles.Fromthegraphs
inFigure2.5,theviscositydecreaseswithanincreaseinshearrate,confirmingashear-thinning,
non-NewtonianbehaviourforbothμEandμEironoxide.TointerprettheresultsofFigure2.5,
oneneedstoconsiderthattheshearrate(γ̊~v/d50)foracolumnoperatingataporevelocity
ofv=5m/dayandparticlesized50=0.5mm,thentheshearrateisintheorderof0.1s-1.The
Page 36
28
viscositiespresentedinFigure2.5areforhighershearrates,butextrapolatingthepowerlaw
functionstotheexpectedshearrate,γ̊~0.1s-1,thentheviscosityforthefullstrengthiron
oxide(10g/LFe2O3)μEsuspensionwouldbecloseto1200cP,andforthedilutedsuspension
(5g/LFe2O3)wouldbecloserto250cP.FortheμEalone(withoutironoxide),theviscosity
seemtoplateaucloseto200cPatlowshearrates.TheviscosityforthedilutedμE(withoutiron
oxide)atashearrateof0.1s-1isexpectedtobearound30cP,andthelowvalueofthe
exponentofthepowerfunctionsuggeststhatthebehaviorofthisparticularsystemiscloserto
thatofaNewtonianfluid.AtleastforthedilutedμEsystems,isclearthattheadditionof
nanoparticlesproducedanincreaseintheviscosityofthesystem.
TheviscosityofthesurfacemodifiednZVIsuspensionsisidentifiedasacriticalvariablethat
mayinfluencethetransportintheporousmediaandmobilizationofDNAPL[2][8][19][27].The
trendsinchangesinviscositywithchangesinconcentrationandparticleadditionobservedin
thisstudyareconsistentwithotherstudiesonemulsiontransportinporousmediaForthecase
ofpolymer-suspendednanoparticles,suchasCMCNZVI,increasingtheNZVIparticlecontent
doesnotinfluencetheviscosityofthesolution[19][41].AμEviscosity,atabout100-1000cP
dependingonthedilution,wasalsolargerthantheviscosityofCMCNZVI,atabout10-50cP
[19].TherelativelyhighviscosityoftheconcentratedμEironoxidemayhinderμEironoxide
transportthroughporousmediaandmaycontributetohighpressuredrop,asobservedinO/W
emulsiontransportstudies[42].
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29
(a)
(b)
Figure2.5Viscosityprofilegraph(logscaled)ofμEironoxide(a)andME(b),comparison
betweentheoriginalformulationsanddilutionwithNaClbrinesolution(10g/100mL)at1:1
ratio.
y=0.4784x-0.432
y=0.1128x-0.366
0.001
0.01
0.1
1
10 100 1000
Viscosity
Pa.s
ShearRate(1/s)
MicroemulsionFeo10g/LMicroemulsion5g/L
y=0.0204x-0.191
y=2.0728x-0.594
0.001
0.01
0.1
1
10 100 1000
Viscosity
Pa.S
ShearRate(1/S)
50%DilutionNoDilution
Page 38
30
2.3.3SizeCharacterization.
Figure2.6demonstratestheapproximatesizerangeofμEironoxideandμENZVIunderTEM.
TheTEMimaginggivesinformationaboutthesizeofindividualparticlesandparticleclusters,
butitisnotsuitabletoindicatethesizeofoil-swollen-micellesadsorbedonthenanoparticles.
Analternativetechniquewouldhavebeentousecryo-TEMthatcouldillustratetheinteraction
ofmicelles(presumablyworm-likemicelles)andthenanoparticles.However,thattechnique
wasnotavailableinourfacilities.Figure2.6thatμENZVI-stabilizedironoxideparticlesarein
therangeof50to100nm,whichconfirmingthesimilarityinsizewithμENZVIbyWangetal.
Sincedynamiclightscatteringcanmeasurethehydrodynamicradiusoftheironoxideparticles
withinthemicellesystem,abetterdescriptionoftheaggregatesizecanbeobtainedviaDLS
measurements.ThemeasureddiameterofμEironoxideat10g/Lis270+/-10nm,this
measuredsizefellwithintherangeofestimatedoptimaltransportofNZVIandabouthalfthe
sizeofCMCNZVI[2][4].SizemeasurementswerealsoconductedtotheμEironoxideeffluent
fromthelowaspectratiocolumn,findinganaggregatesizeof530+/-70nm.Similarresults
werealsoobtainedwithμEironoxideat5g/Lsystem.Eventhoughthesizeofthe
nanoparticlesalmostdoubleduponelutionfromthecolumn,thesizesarestillconsideredinthe
optimalrange.SimilaraggregationeffectshavebeenobservedinthecolumneffluentofCMC
NZVIstudies[2][4].
Page 39
31
(a)
(b)
Figure2.6TEMimagingofMicroemulsionironoxideat5g/Lwith100nmasscale(a)and
microemulsionNZVIat1g/LbyWangetal.
2.3.4μEIronOxideTransport
Figure2.7presentpicturesofthecolumnstudies(1cmx15cmcolumn)obtainedwithμEiron
oxidesuspensionscontaining10g/L(left)and5g/L(right)Fe,whentheporevelocitywas5
m/day.The10g/Lsystemshowsaclearaccumulationofironinthebottomhalfofthecolumn.
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32
Atthatpoint,theflowwasstoppedbecauseofthelargepressuredropobtainedwiththis
system(morethan100inchesofwater).Thepictureontheleftshowstheironcompletely
distributedthroughoutthecolumnafterthefirstporevolumebrokethroughthecolumn.As
indicatedintherheologicalstudies,thelargeviscosity(1200cP)couldbethereasonforthe
largepressuredrop.OnecanusetheKozeny–Carmanequationtoestimatethepressuredrop
forapackingofsmoothmonodispersespheres[43]:
∆𝑃 = (BAV)+BcadV+XYZ+
𝑣 ∙ 𝐿 (9)
Usingaporevelocity(v)of5m/day,aviscosity(μ)of1200cP,aparticlediameter(d50)of0.5
mm,abedporosity(ε)of0.35,acolumnlength(L)of15cm,thepressuredropshouldhave
been104”water.AlthoughthisisconsistentwiththepressurereportedinTable2.1,the
pressurewassubstantiallylargerthan100”water.Evenafterbypassingthepressuregauge,no
flowcouldbeinjectedthroughthecolumn.Itispossiblethatchangesinthestructureofthe
worm-likemicellesystemcouldhaveledtochangesinviscosityintheleadingedgeoftheμE
suspensionfrontincontactwiththeconditioningphase(a10%NaClbrineforthesystemsof
Figure2.7).Infact,accordingtoChoietal.[30],wormlikemicellesproducedwithsuspensions
similartothoseusedinthisworkdisplaygel-likepropertiesatlowenoughshear.
Page 41
33
a. b.
Figure2.7Transportofironoxidesuspensionsin1-cmdiameter(highaspectratio)columnat5
m/dayporevelocity(a)10g/Lironoxide(b)5g/Lironoxide.
Toovercomethegel-likebehavioratlowshearrateforthe10g/Lsystem,theporevelocitywas
increasedto20m/day.Thisincreaseinvelocitydisruptedthegel-likestructureformedwiththe
10g/Lsystem,allowingtherecoveryof60%ofironoxide,asindicatedinTable2.1.However,
porevelocitiescloserto5m/dayarepreferred,closertotheporevelocitiesusedinaquifer
remediation.Tomaintainaporevelocityof5m/day,thediluted5g/Lsuspensionwas
consideredfortherestofthestudies.Figure2.8presentsthebreakthroughcurvesobtained
withthe5g/Lsuspension,injectedat5m/day,usingcolumnswithaspectratioof15(1cmx
15cm)and6(2.5cmx15cm).The3-compartmentmodelparameterscorrespondingtothese
systems(50%dilutedμE,5g/L,scheduleA)arepresentedinTable2.1.Thepressuredrop
Page 42
34
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4
C/Co
Porevolume
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4
C/Co
Porevolume
5g/L2.5cmx15cmcolumn
5g/L1cmx15cmcolumn
calculatedfromEquation9was22”water,lowerthanthereportedvalueof38”inTable2.1,
howeveritistobeexpectedgivenotherpressurelossesintheentireconfiguration.
Figure2.8Breakthroughcurvesof5g/L(asFe2O3)μEsuspensionofironoxideinjectedat
5m/day(porevelocity)throughcolumnswithaspectratioof15(left)and6(right).Thesolid
linesshowthesolutionofthe3-compartmentmodelusingtheconstantssummarized.
AsshowninFigure2.8,whenthesameexperimentwasconductedintwodifferentcolumn,the
outcomewasverysimilar.Whilebothcolumnstudiesappeartodisplayasecondarypeakatthe
end,thisfeaturecouldonlybereproducedasatailingeffectthroughthereversibleadsorption
compartment.Thistailingeffectissimilartootherreportedstudies[6][22][24]anditisbelieved
tobeduetoreversibledeposition.Table2.1showsthatthefittingparametersusedforboth
modelswerethesamewiththeexceptionofaslightlyhigherkrevforthecolumnwithaspect
ratioof6.Thehighercoefficientofdetermination(R2)obtainedwiththe2.5cmx15cmcolumn
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35
(aspectratio6)wasmainlyduetothelargernumberofsamplespointsthatonecancollectfor
thatsystem.
Becauseofthelargernumberofsamplingpoints,andthemorehomogeneousflowwithinthe
column,the2.5cmx15cmcolumn(aspectratio6)wasusedfortherestofthestudies.
ForthesebaselinesystemsofFigure2.8,theattachmentefficiency(α)wasoftheorderof10-3,
whichisinlinewiththelowerrangereportedbyKocuretal.forlowporevelocities[12].
However,whencomparingthesystemswiththeclosestcharacteristics(2.5g/LNZVI,v=4
m/day),theirattachmentefficiency(α)wascloseto0.2,almosttwoorderofmagnitudehigher
thanthesystemsexploredinFigure2.1.Thissuggestthatatleastfortheconditionsofthe
curvesinFigure2.8,theuseofμEassuspendingmediaimprovestheabilitytotransportthe
particles.AnotherinterestingobservationintheworkofKocuretal[19],andtheworkof
Tufenkjietal.[37]isthattheoptimalsizetominimizethesinglecollectorefficiency(ηo)isclose
to500nm,whichisthefinalsizeoftheaggregatesintheeffluentofthecolumn.Thedrawback
ofusinglargerparticlesizeisthattheycansinkundertheeffectofgravityandaccumulateat
thebottomofthecolumn(ortheaquifer).Thesettlingvelocity(vgr)ofsphericalparticlesin
dilutesuspensions,inlaminarflowcanbeestimatedusingtheStokesequation:
𝑣f0 =(ghAgi)∙f∙XYZ+
Bcd (10)
whereρpisthedensityoftheironoxideparticle(5200kg/m3)andρfisthedensityofthefluid
(assumedwater,1000kg/m3).Consideringaparticleof270nmina250cPfluid,thesettling
velocityfromEquation10is5.8E-5m/d,wellbelowtheporevelocityusedinthiswork.Particle
aggregatesinbetween100nmand1000nmaretoobigtodiffuseviarandommotion,butat
thesametime,toosmalltosettle,thusthesuitabilityofparticlesinthisrangetofacilitatethe
Page 44
36
transportoftheparticles.Theintrinsicdiffusivity(Dint)ofthe270nmparticlesina250cPfluid
canbeestimatedusingtheStokes-Einsteinequation:
𝐷?j# =kl∙m
UndXYZ (11)
wherekBisBoltzmann’sconstantandTisthetemperatureofthesystem(298K).Atthese
conditionstheintrinsicdiffusivityoftheparticlesis6.5·10-11cm2/s.Thisvalueissubstantially
smallerthanthevalueoftheorder10-5to10-6cm2/sfortheeffectivediffusivityofparticlesin
Table2.1.Potentialback-mixing(non-idealplugflow)mayberesponsibleforthelarger
effectivediffusivity.Infact,thelaminarnatureoftheflowcreatesadispersioneffectasthe
particlestravellingclosertothesurfaceoftheporeswillhavealowervelocitythantheparticles
travellingalongthecenterofthepore.Theeffectivediffusion(Deff),ordispersion,coefficient
(duetolaminarflowsegregation)canbeestimatedusingTaylor’sdispersionequation:
𝐷1oo =2∙pYZ+
+
qcrHs; (11)
For270nmparticles,injectedat5m/day,withDint=6.5·10-11cm2/s,Deff=2·10-6cm2/s,a
valuethatisclosertotheeffectivediffusioncoefficientfoundinTable2.1.
AlthoughtheresultsaboveshowthatthetransportofparticlesisfavouredinthediluteμE
systems,thereisapracticalissueinvolvedwithusingtheconditioning/rinsingprescribedin
scheduleA(Figure2).ScheduleAprescribesinjecting10PVofa10%NaClsolutionbeforeand
aftertheinjectionofthemicroemulsion.Thishighconcentrationofsaltcanimpactthe
chemistryandecosystemoftheaquifer.Tothisend,itwouldbebesttoinjectalowionic
strengthsolutionthatwouldbemorecompatiblewithexistinggroundwater.However,thisis
likelytohaveanimpactonthephasebehavioroftheμEironoxidesuspension.Toassessthis
Page 45
37
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4
C/Co
Porevolume
ScheduleB
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4
C/Co
Porevolume
ScheduleA
potentialimpact,abreakthroughcurvewasobtainedusingscheduleB(deionizedwater
conditioning/rinsing).ThecomparisonbetweentheseschedulesispresentedinFigure2.9.
AsshowninFigure2.9,theintroductionofdeionizedwater,insteadof10%NaCl,producesa
significantreductionontherecoveryofironoxidenanoparticles,fromvaluescloseto90%
(scheduleA)tocloseto60%(scheduleB).ForScheduleB,theattachmentefficiency(α)
increaseto0.023thatis,still,oneorderofmagnitudelowerthanthatofKocuretal.[19].
Figure2.9Breakthroughcurvesobtainedfor5g/LμEironoxideinjectedat5m/day(pore
velocity)througha2.5cmx15cmcolumn(aspectratioof6),usingscheduleA(10%NaCl
conditioning/rinsingfluid)andscheduleB(deionizedwaterconditioning/rinsingfluid.
TheresultsofFigure2.9confirmthehypothesisthatchangesinsalinitymayinducephase
changesthatdestabilizetheparticles.Togainabetterunderstandingofthetransportofthe
Page 46
38
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4
C/Co
Porevolume
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4
C/Co
Porevolume
ScheduleA ScheduleB
microemulsionitself,theexperimentsofFigure2.9wererepeated,butintheabsenceofiron
oxidenanoparticles.Figure2.10presentsthebreakthroughcurvesfordilutedμEsinjectedusing
schedulesAandB.
Figure2.10BreakthroughcurvesobtainedfordilutedμEs(noironoxide)injectedat5m/day
(porevelocity)througha2.5cmx15cmcolumn(aspectratioof6),usingscheduleA(10%NaCl
conditioning/rinsingfluid)andscheduleB(deionizedwaterconditioning/rinse).
Whentheparametersofthe3-compartmentmodel(Table2.1)fortheparticle-freedilutedμEs
(Figure2.10)arecomparedtothosecontaining5g/Lironoxide(Figure2.9)onefindsthatthey
arealmostthesame,withtheexceptionthatthevolumeratioofthereversiblecompartmentis
larger(frev)whichresultsinamorepronouncedtailingeffect.Furthermore,theattachment
constant(Katt)iszerofortheparticle-freeμEinjectedwithscheduleAanditissmallforthe
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39
particle-freeμEinjectedwithscheduleB.Evenintheabsenceofironoxideparticles,thelarge
changesinsaltconcentrationexperiencedduringscheduleBinduceirreversiblelosesofμEto
thecolumn.
Despitethelowerrecovery,scheduleBisstillmorefavourableforpotentialapplicationas
deliverystrategyduetothelowerriskofimpactingthechemistryandecosystemofthe
reservoir.Also,undesirabledensenon-aqueousphaseliquid(DNAPL)mobilizationcouldalso
occurifthesystemiskeptatconditionsthatcanproduceultralowinterfacialtensions,asisthe
caseforscheduleA[10][16].
AfinalpointofinterestregardingthepotentialuseofμE–basedsuspensionsistheestimated
traveldistance(Lmax),accordingtoEquation7.EvenwhenusingScheduleB,themaximum
penetrationdistanceisintheorderofhundredsofmeters.Thisdistanceissubstantiallylarger
thanthetypicalwelltowelldistanceinNZVIremediation,andlargerthanthosereportedfrom
otherstudies(intherangeof1-10meters)[44].
μE,% 100 100 50 50 50 50 50Fe2O3,g/L 10 10 5 5 5 0 0vpore,m/day 5 20 5 5 5 5 5Aspectratio 15 15 15 6 6 6 6Schedule A A A A B A BDeff·10-5,cm2/s - - 8.7 8.7 0.87 8.7 8.7krev·10-6,1/s - - 5.8 7.7 7.7 7.7 7.7frev - - 0.1 0.1 0 0.2 0.2Katt·10-6,1/s - - 0.31 0.31 1.4 0 0.19Recovery,% 0 58 88 88 56 100 93Cmax/Co - - 0.9 0.9 0.7 0.94 0.91α·10-3 - - 5.4 5.4 23.9 ND NDLmax,m - - 863 863 197 ND 1382R2fit 0.8 0.92 0.98 0.94 0.92ΔPmax,“H2O >100 >100 >100 38 35 36 35
Page 48
40
Table2.1Summaryofbreakthroughcurveparameters
2.3.5IronDistributionAnalysis
Figure2.11comparesthedistributionofironoxidedepositedonthecolumnbetweenthe
transportofthe5g/LironoxidenanoparticlesfollowingschedulesAandB.Forthecaseof
scheduleA,becauseofthehighrecovery,theamountofretainedironperweightofthesandis
low.Thepredictedattachediron(fromthe3-compartmentmodel)matchesthemeasurediron
attachedtosand.ToputthenumbersofFigure2.11inperspective,Xinetal.[24],injected5PV
of3g/L(Fe)NZVIat8.3m/day(conditionssomewhatsimilartoourstudy),obtainingan
attachedironof5mg/g,morethanoneorderofmagnitudelargerthantheironattached
obtainedinFigure2.11.
ComparedtoscheduleA,moreironwasattachedinscheduleB.The3-compartmentmodel
predictioncoincideswiththeretentioninthebottomofthecolumnoperatedwithscheduleB,
butnotintherestofthecolumn,wheretheamountofironattachedwaslowerthanthe
predictedamount.ThiscouldsuggestthatnotalltheattachmentinthecaseofscheduleBwas
attachmenttothesand,butapartitiontogel-likestructuresthatwerenotcollectedwhenthe
sandwassampledfromthecolumn.
ThepicturesforthescheduleAstudyshowbarelyvisiblespecksonthesurfaceoftheparticles
homogenouslydistributedthroughoutthegrains.ForthecaseofscheduleB,largerspecksare
shown,likelytheresultofaggregation,especiallyatthebottomofthecolumn.
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41
Figure2.11Ironoxidedepositedonsandcolumnaftertheinjectionof1.5PVof5g/L(as
Fe2O3)ironoxidenanoparticlesat5m/day(porevelocity)througha2.5cmx15cmcolumn
(aspectratioof6),usingscheduleA(10%NaClconditioning/rinsingfluid)andscheduleB
(deionizedwaterconditioning/rinsingfluid).Thesolidlinerepresentsthepredictionof
depositedironfromthe3-compartmentmodel.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 3 6 9 12 15
Attached
iron
oxide
,mgF
e/gs
and
Columnlength,cm
ExperimentalPredicted
1mm
ScheduleA
ScheduleB
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 3 6 9 12 15
Attached
iron
oxide
,mgF
e/gs
and
Columnlength,cm
ExperimentalPredicted
1mm
Page 50
42
2.4Conclusions
Theoriginalintentofthisworkwastoevaluatethetransportofmicroemulsion(μE)stabilized
ironnanoparticlesthroughporousmedia,usingironoxidenanoparticlesasmodelsystem.The
original10g/L(asFe2O3)suspensiondespitebeinghighlystable,ithadrheologicalproperties
thatpreventeditsuseatporevelocitiesconsistentwiththoseusedinaquiferremediation.
Dilutingthemicroemulsiontoa5g/Lsuspensionachievedreasonablepressuredropsatpore
velocitiesof5m/day,consistentwiththoseusedinaquiferremediation.
Thecolumnstudiesconsideredinthisworkwereanalyzedusinga3-compartmenttransport
modelthataccountforthetransportinthecolumnandtheexchangeofparticleswitha
reversibleadsorptioncompartmentandanirreversibleattachmentcompartment.
TheμEusedtosuspendthenanoparticleswaspreviouslydesignedtoformbicontinuous
systemsthatupondilutionin10%NaClbrinesolutionwouldyieldworm-likemicelles.When
usinga10%NaClbrinesolutiontoconditionandrinsethecolumnaftertheinjectionofthe
suspension,alargefractionofnanoparticleswasrecoveryandtheparticleattachment
experiencedinthecolumnwaslessthan1/10theattachmentobtainedwithothersuspension
mediareportedintheliterature.Whendeionizedwaterwasusedasconditioning/rinsing
solutionmoreparticleswereretainedbythecolumn,likelybecausephasetransitions
experiencedbytheμEphaseduetothelargechangeinelectrolyteconcentration.However,
eventhislessdesirabletransportwasstillmoreefficientthanothersuspensionsreportedinthe
literature.FuturestudiesshouldconcentrateinproducingμEsuspensionswithlowelectrolyte
concentrationsuchthatthecolumncanbeconditionedandrinsedwithlowenoughionic
strengthsolutionsthatwouldnotinducesubstantialchangesintheμEphasebehavior.
Page 51
43
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Chapter3:Developmentandtransportofphosphatesurfactant,SDEHP-
stabilizedNZVIinporousmediaforinsituremediation
3.0Abstract
Nanoscalezero-valentiron(NZVI)particleshasbeenidentifiedasefficientreducingagentsfora
widerangeofgroundwatercontaminants;however,itsapplicationislimitedbythepoor
transportanddeliveryintheporousmedia.Inthiswork,sodiumDiEthylHexylPhosphate
(SDEHP)surfactantwasusedasasurfacemodifiertoimprovethemobilityofNZVIinsoil.An
optimalSDEHP-stabilizedNZVIformulation,100mMSDEHP1g/LNZVI,wasidentifiedthrough
criticalmicelleanalysisandtotalorganiccarbonanalysis.Simple“one-pot”NZVIsynthesis
procedurewasadaptedandmodifiedtosynthesizeNZVIformulations.TheSDEHPsurface
modifierimprovedthestabilityofbareNZVIfromminutestomonths.Theoptimized
formulationyieldedahydrodynamicdiameterabout240nmunderdynamiclightscattering
(DLS)and100nmindiameterundertransmissionelectronmicroscope(TEM).Acolumnstudy
ofthestableformulationisconductedwitha2.5x15cmglasscolumnfilledwithOttawasand
atafieldscaleflowvelocityof1.5m/daywithnomechanicalstirring.Theresultofthecolumn
studyshowedthatover95%ofNZVIarerecoveredwithasteadyplateauC/Copeakreachingto
1.AdiscussionsuggestedthatthedevelopedSDEHPsurfacemodifierholdsdesirabletraitsfor
afieldscaleinsituremediation.
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3.1Introduction
Nanoscalezero-valentiron(NZVI)particlesarerecognizedasanusefulgroundwater
remediationtechnologybecauseofitshighefficiencyinreducingawiderangeofcontaminants,
includingbutnotlimitedtoDNAPLandheavymetals[1][2][3][4].However,thistechnologyis
limitedbythepoormobilityinsoil[5][6].TheunmodifiedNZVIparticles,commonlyknownas
bareNZVI,duetohighmagneticandVanderWaalforces[3][7],aggregateandsettlewithin
minutesofbeingsynthesized.AggregatedZVIdemonstratedlosesitsoriginalhighsurfacearea
andbecomelessreactivetotargetcontaminants.TheseLargerparticlesarefilteredbythesand
grain,hinderingtheZVIparticlesfromreachingthecontaminantzoneandimmobilizedinsoil
[8].
TransportofNZVIinporousmediaisinfluencedbyvariousfactorssuchastransportvelocity,
pH,particlesizes,ionicstrength,soilmatrixandthecompositionofgroundwater[9][10].NZVI
particlestendtoaggregateandsettlebeforeencounteringthesoilmatrixandgroundwateror
clogattheearlystageofsubsurface[11][12].Addressingthestabilityissuebyeliminatingor
extendingthesettlingtimeandkeepingtheoriginalsizesoftheparticleshasbeenshownto
improvenanoparticlesmobility[13][14].Fortunately,particlesizesandstabilityofthe
suspensionarealsocloselyrelatedandcaneasilybeengineeredthroughtheapplicationof
surfacestabilizers[15][16][11][17],namelysurfacestabilizers.Currently,extensiveresearch
effortshavebeencommittedtotheapplicationofsurfacestabilizersonNZVI,includingto
polymers,surfactantsandemulsionsareused[13][18][15].However,surfacestabilizationon
NZVIparticlescanreducethereactivityofthereactiveparticlesbycreatingabarrierbetween
contaminantsandNZVI[19];alongwithothertrade-offssuchasconcentration[20][13][21]and
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injectiontechnique[8][22],stabilizedNZVIarenotalwayscandidatesforfieldremediation.
O’Carrolletal.summarizedthreefundamentalcharacteristicsthatanefficientlystabilizedNZVI
shouldhold:1.Demonstrateprolongparticlestability2.SufficientNZVIconcentration(1-12
g/L)toachievesuccessfulremediationand3.Maintainanadequatereactively[3].Applying
surfacestabilizersthatprovidethethreefundamentalcharacteristicshasbeentheobjectiveof
thisandotherstudies.
Laboratorycolumnexperimentsareoftenthefirststeptodetermineandassesstheabilityofa
stabilizedNZVI.Table3.1summarizes,theperformanceofselectednanoparticlesstabilizers
andtheircolumntransportresults[6][13][23][18][22][24][25][26][27][28][29].Outofthese
stabilizers,carboxyl-methylcelluloseNZVIat1g/LwasfieldtestedinSarnia,ONin2014by
Kocuretal.[30]Overall,thesurfacestabilizerscanimprovethestabilityandthusthemobilityof
theironnanoparticlesuspensionwithgreaterrecoveryandtransportchrematistics;however,
furtherimprovementsarehinted.7outofthe10listedcolumnstudieslistedwereconducted
ataflowvelocitybetween7.8to198.7m/day,wellabovethetypicalgroundwaterflowvelocity
of0.25to0.4m/day[3],implyingacompatibilityissueduringtheprocessofinjection[31].
Additionally,adequatebreakthroughperformanceofNZVIareonlyobservedatthelow
concentrations(below1g/L)forcarboxyl-methylcellulose,guargumandpoly(acrylicacid)
basedNZVI[29][8][16];whilealltheNZVIstabilizersincludingsurfactantTween-80yielded
poorbreakthroughresultsathigherconcentrationswithlowmaximumpeakanduneven
breakthroughcurves.Thismeansthatmoresevereagglomerationandinstabilityareobserved
athigherconcentrations[7].Furthermore,polymerstabilizerssuchascarboxyl-methylcellulose
althoughclaimedaprolongedstabilizationof80-hours,aggregationswerenotfullyeliminated
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andsettlingisobservedconstantlyoverthe80-hourperiod.Intheaforementionedfieldstudy
ofcarboxyl-methylcelluloseNZVI,itwasreportedthatNZVIcantravelatleast1meterwitha
recoveryrateof1%[30][32].ItisclaimedbySoukupovaetal.thatevena10dayperiodof
negligibleaggregationisnotenoughforafullscaleremediation[27].Non-ionicsurfactant
stabilizedNZVI,Tween-80,demonstrateda2-monthstabilityunderstoragecondition;however,
instabilitywasobserveduponcontactingwiththeporousmedia.Ontheotherhand,emulsion
encapsulatedNZVI,eventhoughyieldedahighandsteadyrecoverypeak(Cmax/Co)ata
desirablefieldflowvelocity(0.4m/day);theinjectionofemulsionNZVIrequiresconstantly
mechanicalstirringandcreateddifficultyininjection.Althoughapplyingsurfacestabilizers
improvedthemobilityofNZVIinporousmedia,laboratorycolumnstudiesshowedthatthere
hasn’tbeenasurfacestabilizerstrongenoughforanefficientfull-scaleremediation.Other
words,asurfacestabilizerthatcansatisfythebasicNZVIcharacteristicswithnoaggregation
andsettlingovertimehasyetbeenfound.
Surfactantsaresometimespreferredoverpolymersasstabilizingagentsbecausetheyhavea
highertendencytoadsorbontothesurfaceofnanoparticles[27].Biodegradablenon-ionic
surfactantssuchasTween-20,Tween-80andAlkylethanolamidesareoftenadaptedas
alternativestabilizersforNZVIforenvironmentalreasonsandsmoothsynthesis[33][34][27].It
isalsoreportedthatsurfactantscanincreasethereductionrateofthecontaminantsdueto
synergisms[35][36].Eventhoughnon-ionicsurfactantsholdseveraladvantages,theystill
producepoorstability[12].Wangetal.showedthatanionicsurfactantscanformelectrostatic
repulsionsbetweencoatedparticlesandsand,creatinganenergybarrierforNZVI
agglomeration[15].Itisalsoproventhatanionicsurfactantironnanoparticleswithstrong
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chargedcoatingsholdmoderatestabilityandmobility,assummarizedinTable1,namelyoleate
ionstabilizedironoxidenanoparticles[23].
Figure3.1StructureofanionicphosphatesurfactantSDEHP.
Inthisstudy,sodiumdiethylhexylphosphate(SDEHP),ananionicsurfactanthasbeenselected
asthestabilizerbecauseofitsnon-toxic,mildnatures[37].AndthefactthatWangetal.
demonstratedthatsodiumdodecylphosphate(SDP),anotherphosphatebasedsurfactant,can
actasastabilizerandpromotethereductionofchlorinatedhydrocarbons[38].Figure3.1
showsthestructureofsurfactantSDEHP,withphosphategroupbeingthehydrophilicheadand
doublecarbonchains.ItwasreportedthatSDP,anotherphosphatesurfactant,doesnotreact
withNZVIasastabilizerbutinfactpromotethedechlornatingreactions.Inadditional,itis
importanttonotethatphosphonategroupinanionicsurfactantsplayamajorroleinforming
morestablesuspensionwithmetalnanoparticles[39].Withthisknowledge,surfactantswith
phosphategroup[39]thatsharesimilarpropertiesareexpectedtohavesimilarperformancein
suspension.TheSDEHPanionicsurfactant,withthereactivityadvantageoverthetypical
biodegradablenon-ionicsurfactants,wasselectedasapotentialstabilizerofNZVIfor
developinganefficientalternativeforNZVIinsituremediation.
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Table3.1Literaturesummaryofcolumnstudiesandstabilitybehaviourfordifferenttypesof
surfacemodifiedironoxideandZVInanoparticles.
TheprimaryobjectiveofthispaperistodevelopaSDEHPsurfactant-basedformulationthat
SystemNo.
Max.Conc.(g/L)
Stability TransportVelocity(m/Day)
Viscosity Cmax/Co Size(nm)
Reference
Carboxyl-methylCelluloseNZVI
1 0.1-2.5 80hourswithaggregation
0.25,2,4 13.8-72.8
0.85-0.75(Decreasingtrend)
25-61 [13]
Polyeletrolyte-stabilizedNZVI
2 0.085-1.7
Stirringwhileinjection
6.4 0.97 0.8 85(particle)185(hydrodynamic)
[40]
EmulsionNZVI 3 2.5 Kineticallystable~8hours/Mixingduringinjection
0.4 9300 0.8-1 1000(droplet)12(particles)
[41]
PolyacrylicAcid-StabilizedNZVI
4 0.1,0.3,4
3hoursAgitationrequired
6.28-15.67 N/A ~1 ~100nm [6]
Polyphenol-basedNZVI
5 1 >10days 7.4 N/A 0.5 n/a [28]
GuarGumNZVI
6 0.154 Days 2.38-11.92 0.89-1.35
0.21(lowflow)-0.87(highflow)
320 [20]
XantumGumNZVI
7 3 72hours 8.4-198.72 ShearThinning10,000
0.4-0.6(increasingtrend,lowflow)0.8-1(fastestflow)
microscale [26]
Tween80NZVI
8 20%(w/w)0.32(columnstudy)
2monthsunderstorageconditions
24.32 N/A N/A 40-80 [27]
Oleate(OL)ionsstabilizedironoxide
9 5 >2weeks 14 N/A 0.93 N/A [23]
MicroemulsionIronoxide
10 5and10 >6months 5,20 20-400 0.9 120 Chapter2
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demonstratesprolongedstabilityandminimumaggregationusingtheframeworkproposedby
Wangetal.[15].Thesecondaryobjectiveistoexamineandassessthemobilityinporousmedia
oftheoptimalSDEHP-basedNZVIviaa1Dcolumnstudy.
3.2Methodology
Unlessotherwiseindicated,alltheproceduresandmaterialswerepreparedandconductedat
standardambienttemperatureandpressure(SATP).
3.2.1SynthesisofSodiumDiEthylHexylPhosphate(SDEHP)Surfactant
TheprocedureofproducingSDEHPwasadaptedfromthepublicationsofLuanetal.withsome
modification[37].Inshort,30,50and100mMofSDEHPwassynthesizedviatheneutralization
reactionbetweensodiumhydroxideandhydrogendiethylhexylphosphateacid(HDEHPA,
Sigma-Aldrich,237825,97%).3gofHDEHPAismeasuredbyweightusinganelectronicbalance
(Sartorius,Germany,33904396)ina20mLvialtomake100mMofSDEHP.Themeasured
HDEHPAisthentransferredtoa100mLvolumetricflaskwiththeflushingofDIwater.1M
Sodiumhydroxide(NaOH,Caledon,lot#89075)solutionwasaddedtothevolumetricflaskat
1.2timesthestoichiometricamount.BalancetherestofthevolumetricflaskwithDIwaterand
inducevigorousmixingmanually.Afterthesolutionturnedclear,stoppedthemixingandplace
thevolumetricflaskfor24hourstoreachcompleteequilibrium.Thecompletionofthe
neutralizedSDEHPsolutionwasthenconfirmedwithapHprobe(Vernier,Model:LD2-LE)to
determinetheacidity.Itisrecommendedtoaddseveraldropsofthe1MNaOHsolutionifthe
pHoftheSDEHPsolutionisbelowpH9,thehigherpHenvironmentguranteefullconversionof
theHDEHP.
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3.2.2CriticalMicelleConcentrationofSDEHPwithdissolvediron
SurfacetensionmeasurementsusingKSVSigmaTensiometer(model700)wasusedto
determinethecriticalmicelleconcentration(CMC)forSDEHP.Ironsulfate(FeSO4[H2O]7,Fisher
Scientific,7782-63-0)wasdissolvedintoSDEHPsurfactantsolutionatthefollowing
concentrations:10,25,30,50,80and100mM.Atotalof0.2gofironsulfate(equivalentto1
g/LNZVI)wasaddedto40mLofeachSDEHPsolutionatdifferentconcentrationwithgentle
mixing.Thesurfactant-ironsolutionwasthenmixedandplacefor1hourtoreachequilibrium
beforemeasurement.Thepurposeofdissolvingironsulfateistosimulatethesynthesis
condition.Identicalprocedurewasconductedforferricchloride(FeCl3,SigmaAldrich,7705-08-
0)andthoseresultsareshownanddiscussedinAppendixA.
3.2.3TotalOrganicCarbon(TOC)ofirondissolvedSDEHP
Totalorganiccarbon(TOC)analysiswasconductedusingatotalcarbonanalyzer(TOC-Vcpn,
SHIMADZU)fordeterminingtheamountofsurfactantadsorptiontotheiron.SDEHPsurfactant
solutionwaspreparedatthefollowingconcentrations:10,30,50,80and100mMfor10mLin
a20mLvial.DifferentconcentrationsofironsulfatesatdifferentNZVIequivalence
concentrationsrangesfrom0.3,0.5,1,1.5,2,2.5,3,4to5g/Lareadded.Thesampleswere
thencappedandmixedfor2minutesusingavortexmixerat1000rpm,thencentrifuged(Cole-
Parmer,1741423)at4000rpmfor45minutes.Thesupernatantwasdecantedandfilteredwith
anano-filter(PallCorp.AcrodiscSyringeFilter,450nm).Thefilteredsolutionwasdilutedto12.5
timesbeforeanalysis.
3.2.4SynthesisofSDEHPNZVI
NZVIiscommonlysynthesizedfromthereductionreactionbetweensodiumborohydride
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(NaBH4)andiron[42][43][44][2].TherearetwocommonNZVIsynthesismethods,namely
chloride-basedandsulfate-basedsynthesis[43].Inthiswork,sulfate-basedmethodisselected
forreasonsthatwillbefurtherexplainedinresultanddiscussion.ThereactionofNZVIsulfate-
synthesisisasthefollowing:
2𝐹𝑒Ww +𝐵𝐻qA + 3𝐻W𝑂 → 2𝐹𝑒a + 𝐻W𝐵𝑂UA + 4𝐻w + 2𝐻W (1)
The“one-pot”synthesisprocedurewasadaptedfromtheoriginalsynthesisprocedure
describedbyWangetal.forsynthesizingmicroemulsionNZVI[45].Specifically,surfactant
solutionsatthedesiredconcentrations(30,50and100mM)werepreparedusingdeaeratedDI
water.Thesurfactantsolutionsandotherreactantswereplacedinanitrogenfilledgloveboxfor
threehourstoremoveadditionaloxygen.Afterthethreehours,20mLofthesurfactant
solutionwastransferredtoa250mLbeaker.Toproducea1g/Lironsolution,0.1gofiron
sulfatewasweightedusinganelectronicbalanceinsidethegloveboxandtransferredtothe250
mLbeaker.Usingaglassstirringrod,manualgentleagitationwasusedtodissolvetheiron
sulfateforabout15minutes.Atotalof0.04gwasofsodiumborohydride(NaBH4,Sigma-
Aldrich,16940-66-2)wasweightedintheglovebox.Thesodiumborohydridewasslowlyadded
intothesolutionwithin30minutestopreventexcessivegasevolutionfromthereaction.After
this,thesolutionwasleftfor1hourinthegloveboxtoletthesolutiontocooldowntoroom
temperature.Thereductionyielded20mLof1g/LNZVIat30,50or100mMofSDEHP
surfactant.Similarprocedureswereusedtoproduce0.3,0.5,2and5g/LNZVI.Figure3.2
illustratedthedescribedone-potsynthesisprocessofNZVI.
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Figure3.2Illustrationofthe“one-pot”synthesisprocedureofSDEHP-stabilizedNZVI.The
procedurewasconductedintheglovebox.
3.2.5pHAnalysis
ApHmeterprobewasusedtoanalyzethepHofSDEHPsurfactantat100mMandSDEHP
stabilizedNZVIatdifferentironconcentrationsandsurfactantconcentrations2monthupon
synthesis.
3.2.6StabilityAnalysis
Uponsynthesis,SDEHPNZVIatdifferentformulationsandbareNZVIaretakenoutsideofthe
glovebox(synthesiscondition)forfurthercharacterization.Toassesscolloidalstability,the
synthesizedformulationswerere-suspendedwithasonicationbathfor1minute.Pictureswere
takenattimeintervalof15,30,60,120and180minutesforthefirstthreehoursanddailyfor
oneweekafterre-suspension.
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3.2.7ViscosityAnalysis
Tomeasuretheviscosityofthesynthesizedsolutions,syringeaspirationtimemethodwasused
[46].ThreeSDEHPNZVIformulations,SDEHPat100mMandDIwateraremeasured.Thewater
viscositymeasurementwasusedasthereferencepointtodeterminetheviscosityofthe
formulations.
3.2.8SizeAnalysis
Dynamiclightscattering(DLS)andtransmissionelectronmicroscope(TEM)areusedtoanalyze
thehydrodynamicandparticlesizesofthesynthesizedSDEHPNZVI,respectively.ForDLS
analysis,thefreshlysynthesizedNZVIatdifferentconditionswerediluted10timeswithDI
waterina20mLvialinthesynthesisconditionwithagloveboxfilledwithmixedair(95%N2
andbalanceCO2).PriortoDLSanalysis,thesamplewasmixedwithavortexmixerfor30
secondsandsonicatedusingasonicationbath(Cole-Parmer,8891)for1minute.Thesample
wasthenanalyzedviaaDLSparticlesizeanalyzer(BrooklynInstrumentCrop,90Plus)
measurementsfor10minutes.ForTEManalysis,identicalproceduretoDLSanalysiswas
followedforsamplepreparationpriortoanalysis.ATEMmicroscope(Hitachi,HF3300)was
usedforanalyzingtheironparticlesizes.
3.2.8ColumnExperimentProcedure
Aglasscolumn(15x2.5cm,KontesBrand,ChromaFlex,No.420830-1S1D)wasfilledwithacid-
washedOttawasand(Silicondioxide,Sigma-Aldrich,60676-86-0)asporousmediumfor1-D
transportanalysis.Thesandmediumwaswet-packedandstirredduringpackingtoremoveair
trappedinthesand.Thecolumnwasweightedbeforeandafterpackingandmassbalancewas
performedtodeterminetheporevolume(1porevolumeorPV=32mL).Aperistaticpump
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(Cole-Parmer,MasterFlexL/S)wasusedforthisexperimentandtheflowratewassatatthe
lowestsettingat0.5mL/min(equivalentto1.5m/dayasflowvelocity).Thepumpisconnected
toathree-way-valvetoallowswitchingbetweentheflushingfluidandthenanoparticle
solutions.TominimizeNZVIoxidiation,anenclosurewasinstalledaroundtheSDEHPNZVI
solutiontoeliminateoxygenfromtheambient.Inspecific,aN2gascylinderwasconnectedto
anexpandablesmallgloveboxwithapurgestreamataconstantrateof20psi.Thecontinuous
purgingofthenitrogensimulatesthesynthesizingconditionintheglovebox.Theexperiment
startswiththeflushingstage,aninjectionof10porevolumes(320mL)offlushingfluid(DI
water)fromthebottomofthecolumn.Uponcompletionoftheflushingstage,2porevolumes
(64mL)of100mMSDEHPNZVIsolutionwasinjectedintheidenticalcondition.The
transportedsolutionwascollectedwithanautomaticfractionalcollectoratarateof1.5
mL/sample(3minutes)startingfromtheNZVIinjectionstage,thesamplingiscontinueduntil
theendoftheexperiment.Another10porevolumesofDIwaterwereinjectedtothecolumn
towashouttheNZVI.Thecollectedsamplesfromthecollectorismixedwith6Nhydrochloric
acid(HCl,BDH,BDH7204-1)atvolumeratioof1:4(Nanoparticlessolution:HCl6N)for
concentrationandbreakthroughcurveanalysis.Theoverallexperimentalschematicwas
illustratedinFigure3.3.
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Figure3.3ColumnstudysetupforSDEHP-stabilizedNZVI.
3.2.9NZVIColumnDistributionAnalysis
Uponcompletionofthecolumnstudy,thesandwasrecoveredfromthecolumntodetermine
thedistributionoftheironretainedinthecolumn.Thecolumnwasdividedintofivesections,
witheachonesectionbeing3cminlength.Thecolumnwasdisconnectedandaspatulawas
usedtoretrievedfromeachsection.Theretrievedsandwasanalyzedunderamicroscopefor
particleadsorptionandunderUV/VISspectroscopyforconcentration.Priortothe
concentrationanalysis,thesandwasdigestedinHCl6Nfor3dayswasingasand-acidratioof1
g/5mL.
3.3ResultsandDiscussion
3.3.1DeterminingtheOptimalSynthesisFormulation
Aframeworktodeterminingtheoptimalsurfactant-nanoparticlesformulationisvitalfora
stablesuspension.Inthepast,severalstudieshaveattemptedtouseanionicandnon-ionic
surfactanttosuspendmetalnanoparticlesincludingNZVI[47][39][48][49][30].Ontheother
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hand,Wangetal.successfullyoptimizedwithaframeworkfortwoanionicsurfactantstabilized
ironnanoparticlestoyieldastablesuspensionforover24hours[23].Asimilarformulationfor
ironnanoparticlesathighconcentrationwasconcludedtobemobileinporousmediathrougha
columnstudybyWangetal[23].Tothisday,nostudyhasusedanyframeworkorstrategyto
determinethemostefficientsurfactantstabilizedNZVIformulation.
ThisstudyadaptedtheformulationstrategyproposedbyWangetal.thatastablenanoparticle
suspensioncanbeachievedifthesurfactantconcentrationisabovethecriticalmicelle
concentration(CMC)afterreachinganequilibriumwithnanoparticles[15].Thedetermination
oftheoptimalformulationofNZVIwasthusdividedintotwoparts:1.TodeterminetheCMCof
SDEHPwiththepresenceironsulfateand2.Todeterminethehighestironconcentration
possibleinthelowestSDEHPconcentrationsolutiontosynthesizeNZVI.
Figure3.4showsthesurfacetensionmeasurementsatdifferentconcentrationofSDEHPfrom
10to100mMwith1g/Lequivalenceofironsulfatedissolved.Theadditionofironsulfate
simulatesNZVIsynthesisconditionwiththesurfacemodifierpriortothereaction.Ferrous
sulfatemayaltertheoriginalCMCconsideringtheinteractionoftheferrousionwiththe
anionicsurfactantthatleadtotheformationofferroussalts,someofwhichprecipitatefrom
solutions.Figure3.4presentstwosurfacetensionmeasurementcurvesfortheaforementioned
scenario.Curve1fromFigure3.4representstheoriginalSDEHPconcentrationandcurve2
representsthecorrectedconcentrationofnon-adsorbedSDEHPbasedonTOCanalysis.From
curve1inFigure3.4,itcanbeobservedthatthesurfacetensiondecreasedlinearlyfrom45
mN/mto25mN/matconcentrations10,20,30and50mMofSDEHP.From50mMonwardsto
100mM,thesurfacetensionreachedtoaplateauatapproximately25mN/m.Thegeneral
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trendimpliedinfigure3.4isatypicalsurfacetensioncurveofasurfactantandisconsistent
withotherreportedCMCmeasurementsbytheSDEHPstudyinliterature[37][50].Through
logarithminterpolationbetweenthedescendingandplateausectionofthesurfacetension
curve,itwasdeterminedthattheCMCofSDEHPwith1g/LequivalenceNZVIofironsulfate
dissolvedisabout57mM.Thisisabout30-40mMhigherthanthevaluesreportedbyother
studieswithpureSDEHPsurfactant[37][50][51].ThedramaticraiseintheCMCimpliesthatthe
presenceofironsulfatedidhaveanimpactonthebehaviourofthesurfactantlikelyduetothe
precipitationofferroussaltsofDEHP.Inotherwords,thedissolvedironfromtheironsulfate
areactingasadditionalsurfacesforthesurfactantmolecules.Ontheotherhand,theCMC
impliedincurve2isclosertothereportedliteraturevalue.Theadditionalsurfacesprovidedby
theironparticlesdelayedtheformationofemptymicellesandthusshouldbetakenaccount
whenformulatingforthestabilizationofNZVIsuspension.Overall,theresultofCMCbasedon
thestrategyproposedbyWangetal.impliesthattheminimumSDEHPinitialsynthesis
concentrationshouldbewellabove57mMwiththesurfactantadsorptionbeingconsidered.
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Figure3.4Surfacetensionmeasurementsof1g/LofNZVIdissolved:Curve1showsthe
surfacetensionmeasurementoftheoriginalSDEHPconcentrationandCurve2displayedthe
correctedconcentrationofSDEHP.
ThesecondpartofdetermininganoptimalconditionforsuspendingNZVIistoidentifySDEHP
formulationsthatcanholdthehighestNZVIconcentration.Totalorganiccarbon(TOC)wasused
todeterminetheconcentrationofun-adsorbedSDEHPinthemixtureofironsulfateand
SDEHP.TOCcananalyzethedissolvedSDEHPinthesolutionconsideringthatthesurfactantis
theonlyorganicmaterialintheformulation.Figure3.5showstheequilibriumsurfactant
concentrationasafunctionoftheaddedSDEHPmixingwithironsulfateconcentrationsfor
variousaddedSDEHPconcentration(initial).30mMSDEHPwith1g/LFedissolvedfromferrous
sulfatewasusedasthe‘worstcasescenario’forstability/synthesiscomparison.100mMSDEHP
0
5
10
15
20
25
30
35
40
45
50
1 10 100
SurfaceTensionm
N/M)
SDEHPConc.(mM)
SurfaceTensionofSDEHPw/1g/LofFeSO4dissolved
Curve1:Original Curve2:"Corrected"
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isthehighestconcentrationtestedduetotheexponentialraiseinviscositybeyond100mM.As
expected,anegativecorrelationisobservedbetweenthefreesuspendingSDEHPconcentration
andtheamountofironsulfatedissolved.Inotherwords,thehigherconcentrationsofiron
sulfateparticlesprovidemoresurfacesforsurfactantmoleculestoadsorbon,thusgivingless
suspendingSDEHP.ConnectingbacktotheSDEHPCMCfindingsof41mMfromsurfacetension
measurement,stablesuspensionsareexpectedtobefoundatironsulfateconcertationfrom
0.3to1.5g/Lfor80and100mMSDEHPand0.3to1g/Lironsulfateconcentrationsfor50mM
SDEHP.ConsideringthattheamountoffreesuspendingSDEHPareenoughtoformempty
micellesandmeettheminimumstandardoftheframework.
Figure3.5DissolvedSDEHPequilibriumconcentrationwithironVS.addedironsulfate
concentrationsfordifferentinitialSDEHPconcentrations.
0
20
40
60
80
100
120
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
DISSOLVED
SDE
HPCONC.(M
M)
CONCENTRATIONOFFE2SO4
100mMSDEHP 80mMSDEHP 50mMSDEHP 30mMSDEHP
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3.3.2SynthesisResultsandStabilityofFeSO4-basedSDEHPNZVIat100mMand1g/L
Figure3.6TEMimagingof10mMSDEHP-stabilizedNZVIat1g/Lwithdifferentscaleat500
nmscale.
The100mMSDEHP-stabilizedNZVIat1g/LwasidentifiedasthemostsuccessfulNZVI
suspensionwithastabilityofover2months.Table2summarizedthelistofcandidatesthat
wereselectedforNZVIsynthesiswiththegoalofachievingprolongedcolloidalstability.Figure
3.6displaystheTEMimagingofthemostsuccessfulSDEHP-stabilizedNZVIformulationand
furtherdiscussedinsection3.3.4.
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a. b.
c. Figure3.7SetA,TimelapsephotosofSDEHP-stabilizedNZVIat0.5g/LofNZVIatvarious
SDEHPconcentrations:a.30mMofSDEHPb.50mMofSDEHPandc.100mMofSDEHP.
Figure3.7and3.8demonstratetwosetsofstabilitytimelapsephotos.Infigure3.7,thefirstset
(setA)ofphotos,30,50and100mMofSDEHPNZVIformulationsaredisplayedandmonitored
forstabilityover2days(48hours).Fromfigure3.7,ashypothesized,theNZVIparticles30mM
formulationwasquicklyaggregatedandformedsedimentationwithin30minutesuponre-
suspensionandsynthesis.The100mMNZVIformulation,withfreesuspendingSDEHP
concentrationofcloseto100mM,demonstratedprolongedstabilityinsuperiortotheother
twocandidates.Nosignsofsedimentationsandstable,evendistributionoftheparticleswere
observed48hoursafterre-suspension.Thesuspensionremainedstabletwomonthsafterre-
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suspensionandphaseseparationwasobservedduetooxidation.Thehighconcentrationsof
theSDEHPsurfactantalongwiththeadditionoftheNZVIparticlesformwormlikemicellesdue
tothelowpackingandthenatureoftheSDEHPsurfactant[52].Thewormlikemicellescan
anchorontothesurfaceofthenanoparticlesandprovideprolongedstability[53].Signsofthe
formationofwormlikemicellescanbeobservedthroughtheformationofliquidcrystal[54]in
the100mMSDEHPNZVIformulationandnotobservedin30and50mMincontrast(not
shown).Inaddition,anincreaseinviscosityisalsoasigninformationofthewormlikemicelle
asdemonstratedinTable2.Basedontheabove,100mMSDEHPNZVIformulationisselected
forfurtherconcentrationanalysisduetothesuperiorstabilityat0.5g/L.
a.
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b.
Figure3.8SetB,Stabilitytimelapsepictureof100mMatNZVIconcentration1,1.5and2g/L
overaperiodof24hours:a.1hourandb.24hoursaftersynthesisandre-suspension.
30mM0.5g/L
50mM0.5g/L
100mM0.5g/L
100mM1g/L
100mM2g/L
Stability <1hour ~1hour >2months >2months ~1hourHydrodynamicSize(nm)
482+/-150 792+/-89 287+/-26 244+/-30 412+/-22
Viscosity(cP) - - 1.4+/-0.03 1.4+/-0.03 1.2+/-0.09pH - - 8.8 9.3 7.8
Table3.2SynthesisresultandcharacterizationofSDEHP-stabilizedNZVIatvariousNZVIand
surfactantconcentrations.
Figure3.8showsthestabilityresultsof100mMNZVIat1,1.5and2g/L.Outofthethree,2g/L
showedaggregationandsedimentationupon1hourafterre-suspension,while1.5g/Llasted
justoveronehour.The1g/LofNZVIformulationdemonstratedidenticalstabilityasthe0.5g/L
100mMNZVIformulationdiscussedpreviously.Theresultsareconsistentwiththefactthat
increasingtheironsulfateconcentrationreducesdissolvedSDEHP(figure3.5),limitingitsability
toformtheworm-likemicellerequiredtostabilizetheparticles.
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InadditionaltotheironsulfatebasedNZVIsynthesis,ferricchloride-basedNZVIwasalso
attemptedfordeterminingastableNZVIstableformulation.Ferricchloride-basedNZVIat
identicalconditionsastheironsulfate-basedNZVIdidnotdemonstratethesamestability.
Rapidaggregationandsettlingwereobservedwithin45minutesto1houraftersynthesis.
DetailedresultsarepresentedanddiscussedinappendixA.
3.3.3pHandViscosityAnalysisandImplication
Viscosityismeasureduponsynthesistosomeoftheformulationsbasedontheirstability.
ViscosityofthestabilizedNZVIsuspensionneedstobemonitoredbecauseitcancritically
impactthemobilitywhenitistoolowortoohigh[41][16][55],asdiscussedinchapter2.The
viscosityofthesecondsetsofsynthesis,withdifferentNZVIconcentrationsalongwith0.5g/L
100mMSDEHPformulationfromthefirstsetweremeasured.TheoriginalSDEHP,without
additionofironsulfateandanysynthesisreactions,heldasimilarviscositytowaterat100mM,
approximately1cP.Uponsynthesis,theviscosityof1and0.5g/L100mMSDEHPNZVIshared
similarviscosity,approximately1.4cP.Thisisslightlyhigherthantheviscosityoftheoriginal
SDEHPsolutionby0.4cPat100mM.Theraiseinviscositycanbeexplainedbytworeasons:1.
Thecontributionoftheadditionofinorganicsolidnanoparticlesandcolloidtothesolution[53]
and2.Theformationofwormlikemicelle[56].Incontrast,the1.5g/Lalthoughdemonstrates
lowerstability,themeasurementswereconductedimmediatelyuponresuspensionbutwitha
lowerviscositymeasuredby0.2cP.Itisexpectedthattheincreaseofnanoparticleswill
increasetheviscosity;however,thisisnotthecasehere.Thisimpliesthattheformationof
wormlikemicellemaycontributetotheviscosityovertheadditionoftheparticles.Theviscosity
resultimpliesthepresenceofwormlikemicellesin0.5and1g/LSDEHP-stabilizedNZVIthat
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demonstratedhighstability.
ItisbelievedthatpHishighlycorrelatedwiththestabilityduetotheionicinteractionsbetween
thesurfacestabilizersandthehydrogenions[57].Usingzeta-potentialasanindicator,several
studiesreportedthatmodifiedmetalnanoparticlessuspensionexperiencedadecreasein
stabilityatpH6-7[12][23][55][43].Inotherwords,ahigherpHisexpectedforahighly
stabilizedsuspension.pHisalsoavariableoftencontrolledinotherstudiesanddeemedto
promoteimpactinreactivitywithlowerpHs[56][57].ThepHresultsaresummarizedinTable
2.Inthisstudy,0.5,1and2g/Lof100mMSDEHP-stabilizedNZVIformulationsaretestedfor
pH.Theoriginal100mMSDEHPsolutionisalsotested.FortheSDEHPonlypHmeasurement,
thepHisabout11implyingabasicenvironmentduetotheexcessNaOHusedinsynthesis.In
0.5and1g/Lsamples,thepHvariesbetween8.8-9.2,showingadecreasefromthepure
surfactant.Thisisbecausethesynthesisreaction,aslistedintheexperimentalsectionwill
produce2molesofhydrogenionsforeverymoleofNZVIproduced.Thehydrogenionsundergo
aneutralizationreactionwiththebasicsurfactantsolution,causingadropinthepH.Onthe
otherhand,2g/LsampleshowedalowerpHof7.8.Thisisconsistentwiththeotherdata,
consideringthathigherconcentrationofNZVIproducesmorehydrogenions,thusmore
neutralization.Theraiseinhydrogenionscanalsodisturbtheformationofwormlikemicelleor
theformationofemptymicellesingeneral,causinginstabilityofthesuspensionasobserved.
3.3.4SizeAnalysisofSDEHPNZVI
ControloftheparticlesizeduringsynthesisiscriticaltoasuccessfulNZVIsurfacestabilizer
[30][3],[14],[61],asmentionedinthethreestandards[3].Thestabilizersmodifiedorinduced
ontothesurfaceoftheNZVIwillincreasetheoverallparticlesizeandinfluencemobility.
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Specifically,higherparticlesizeswillincreasecontactwiththesandgrainandpromote
mechanismssuchasporestrainingandnegativelyimpactthemobility[40][20].Itwasreported
thatstrainingiscommonamongpolymer-stabilizedNZVIandmorelikelytooccurinfinerpore
sizeswithlargerparticlesizes[21][40].ItisimportantfortheSDEHPNZVItobeintheoptimal
transportsizebetween100-1000nm[61][62].Thecombinationofhydrodynamicandparticle
diametercanprovidethefullimageoftheparticlesizeofSDEHP-stabilizedNZVI[63].The
hydrodynamicdiameterprovidesthemeasurementofthefullparticlesizeofNZVIparticles
includingtheSDEHPanchoring.TheTEMimagesprovidemeasurementsoftheparticles
withoutthewormlikemicelleanchoring.Thehydrodynamicdiameterof0.5,1and2g/LSDEHP-
stabilizedNZVIaresummarizedinTable2.Thehydrodynamicsizesareabout278and244nm
for0.5and1g/L100mMSDEHP-stabilizedNZVI,respectively.Asignificantincreasein
hydrodynamicsizewasobservedat2g/L,holdinganaverageof412nmindiameter.Thesizeof
thenanoparticleisbelievedtoreflectthestabilityofthesuspension.Specifically,athigher
concentrations,inthiscase,2g/L,thelackofthepresenceofwormlikemicellecontributesto
therapidaggregationofthenanoparticles,causingtheaveragediametertobemorethan1.7
timeshigherthanthestableNZVIsuspensions.Conveniently,thehydrodynamicdiameterof
thestableSDEHP-NZVIsuspensionfellintotheoptimalsizeoftransport[61].TEMimagingof
theparticleforthemoststableformulation,1g/L100mMSDEHPisshowninFigure3.6.The
picturedemonstratedthattheNZVIparticleswithoutthesurfactantcoatingisabout100nm,
meetingthesizestandardproposedintheliterature.Surprisingly,net-likecoatingsare
observedintheTEMimagingaroundtheNZVInanoparticles.Thecoatingisbelievedtobethe
surfactantcoatingofthenanoparticlesisalsoobservedaroundthenanoparticles,givinga
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largerdiameterofapproximately300nm,whichisconsistenttothehydrodynamicdiameter.
Overall,theparticlesizeofthe1g/LSDEHP-stabilizedNZVIhighlystableformulation
demonstratedsuitablesizebythestandards.
3.3.5MobilityofSDEHPNZVIat100mMand1g/L
Figure3.9showsthebreakthroughcurveof1g/Liron,100mMSDEHP-stabilizedNZVI
transportingatthefieldflowrateof5m/daywithfittingfromthemodeldescribedinchapter2.
Thefittedmodelincludedthedispersionmechanismbutnoparticleattachmentmechanism.In
otherwords,particleattachmentsarenotapplicableinthetransportofSDEHP-stabilizedNZVI
duetothegoodtransport,thisisthefirstsignofgoodtransport.Theperformanceofthe
breakthroughcurveisanindicationontheperformanceofthemobilityofthesynthesized
NZVI-suspension.TwofeaturesonthebreakthroughcurveshowthatthehighstabilitySDEHP-
stabilizedNZVIholdsanexcellentmobility:1.Highrecoveryand2.Highandsteadyplateau
concentrationpeak.TherecoveryiscalculatedthroughtheratiooftheinjectedNZVIpore
volumeandtherecoveredporevolumeasthefollowing:
~8Hs�4�;4p~834��5434p
(2)
Therecoveryiscalculatedtobeashighas95%.Thehighrecoveryisbetterthanthevalue
reportedbyLinetal.,achievingarecoveryofalmost90%atahigherflowvelocityof15.87
m/day[29].ComparingtotheemulsionNZVIresult,therecoveryisinthesamerangeas
SDEHP-stabilizedNZVIandtheflowvelocityisconductedatlowervelocityof0.4m/day[22];
however,mechanicalstirringisintegratedfortheemulsifiedNZVI.Theresultsarecomparable
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tothepolyelectrolyte-stabilizedNZVIreportedbyRaychoudhuryetal.atacomparableflow
rateof6.4m/day[40].However,SDEHP-stabilizedNZVIachievedthehighrecoveryandplateau
valuesathigherironconcentrationswithoutconstantmxingduringinjection.SDEHP-stabilized
NZVIcouldachieveahighrecoveryatafieldflowratewhilenomechanicalstirringisneeded,
thisexcellentperformancewasneverreportedbyotherNZVIsuspension.Thehighandsteady
plateaurecovery(C/Co)isascloseas1,thisismuchbetterthanotherstudiesaswell.
Specifically,carboxyl-methylcelluloseNZVItransportstudyconductedbyKocuretal.achieved
asteadyplateauconcentrationat0.8and0.9C/Co,at0.1and2.5g/LNZVIconcentration,
respectively[13]at4m/day.Comparingtothisstudy,SDEHP-stabilizedNZVIachievedaslightly
higherC/CoatalowerNZVIconcentrationof1g/Latacomparableof1.5m/day.Thealmost
perfectplateauconcentrationalsoimpliedthatminimumretentionoccurredbetweenthesand
andtheNZVIparticles.TheminorresidualoftheNZVIinthecolumnisfoundonlyinthebottom
ofcolumnfromtheextractedsand,believedtobecausedbygravitationalsedimentationthat
wasobservedseverelyinotherstudies[64][28].Itcanbeconcludedbasedontheabovetwo
featuresthat,thedevelopedSDEHP-stabilizedNZVIcansuccessfullytransportthroughporous
mediawithoutdifficulty.
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Figure3.9ColumnstudybreakthroughcurveofhighlystableSDEHP-stabilizedNZVI,at100
mMSDEHPand1g/LofNZVIat5m/daywiththemodeldescribedinchapter2(solidline).
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 1 2 3 4
C/Co
Porevolume
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3.3.6NZVIColumnDistributionAnalysis
Figure3.1NZVIdistributionincolumngraphfor100mMSDEHP1g/LNZVIwithmicroscope
imagingat3,9and15cmsectionofthecolumn.
Figure3.10illustratesthedistributionofNZVIparticlesuponcompletionofalltheinjectionand
flushingintheexperimentalphase.Althoughthebreakthroughcurvefromfigure3.9implieda
highNZVIrecovery,tracesofNZVIretentionswereshownfromthedistributioncurveandthe
sandgrainmicroscopeimaging.Overall,theNZVIshowedadecreasingretentiontrendfromthe
bottomtothetopofthecolumn.Thetrendofthecolumndistributionprofileisconsistentwith
thefindingsreportedbyXinetal.[26].Itisimportanttonotethatdespitethesimilarityinthe
overallNZVIdistributiontrend,theamountofironretainedinthesandreportedinthisstudy
wasmagnitudesloweroftheresultbyXinetal.atsimilarconcentrationsandamuchlower
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flowvelocity[26].Comparingtotheirondistributioninchapter2,theresultsindicatedisabout
1timeslower,althoughconductedattheconcentrationinthisstudyis5timesless.Inother
words,figure3.10confirmsthecompellingtransportabilityofSDEHP-stabilizedNZVIinporous
media.Inadditional,visuallynolargeparticleswerefoundaroundtheporeinthesandanalysis,
concludingstrainingwastheleastlikelyretentionmechanism.
Comparingamongthethreemicroscopepicturesalongthecolumn,sometraceofNZVIwas
remainedonsandbutonlyatthebottomsectionofthecolumn(3cm).Thisisconsistentwith
thefindingsoftheretentionprofilethathighestNZVIwasfoundinthebottom.However,the
amountoftheNZVIremainedatthebottomofthesandwasconsideredminimalincomparison
tothefindingsinchapter2,thetransportofmicroemulsionironoxide.Thiscouldbe
interpretedasasignofgoodtransport,becausetherewasnegligibleformationofNZVI
blockageamongthesandgraininthecolumn.ItisbelievedthatthestrainedNZVIparticles
wereonlythelargerparticleswhensynthesized.Inshort,itcanbeconcludedbasedonthe
sandgrainimagingthatSDEHP-stabilizedNZVIsuspensiondidnotaggregateandsettlewhen
contactwiththesand.
3.3.7ImplicationsforinsituRemediation
Theultimategoalofthisstudyistodevelopasurfacestabilizerforafull-scaleremediation.
Basedonthefunctionalityandpropertiesofthestabilizationmethod,SDEHP-stabilizedNZVIis
expectedtobethebestcandidateoutofallthesurfacestabilizationtechniquesthusfar.In
termsoffunctionality,SDEHP-stabilizedNZVIsuspensioncanbeinjectedataflowvelocity
similartotheremediationvelocitywithoutanymechanicalstirring.TheSDEHP-stabilizedNZVI
holdsahighstabilitywithnosedimentationobservedforovertwomonths,thisgreatly
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improvesthechanceforasuccessfulinsituremediation.InthelatestfieldstudiesofCarboxyl-
methylcellulose-stabilizedNZVI,itisreportedthatnodirecttraceofNZVIwasfoundin
downstream,implyingagainthetransportissue.SDEHP-stabilizedNZVIdemonstratedideal
transportin1-DcolumnstudywithresultsthatweresuperiorcomparewithotherNZVI
suspensions.Inadditionaltothefunctionality,thepropertiesoftheappliedsynthesismethod
arealsoinfavourofafull-scaleremediation.ItiswidelyacceptedthatonsiteNZVIsynthesis
yieldsbetterremediationresultsincomparisontopre-synthesizedNZVI[3][30][62].One-pot
synthesisNZVItechniqueprovidesasimpleprocedureofsynthesizingSDEHP-stabilizedNZVI
consideringitssimplemethod.Lastly,surfactantSDEHPisachemicalthatiswidelyusedinthe
miningindustrywithnohistoryofenvironmentalconcerns[51].Thephosphategroupcanbe
naturallydegradeduponcontactwithsoil.Theamountofsurfactantusedintheformulation,
100mM,isabout3.5wt%andconsideredalowconcentrationcomparingtoemulsionNZVI,
thusnosideeffectonthetoxicity,whichisconcernofinsituNZVIremediation[65].Itis
concludedthatSDEHPisagoodtransportvehicleforNZVI,morestudiesinvolvingreactivityis
requiredandeventuallyapilot-scaleinsituremediationisneeded.
3.4Conclusion
ThisworksuccessfullydevelopedaSDEHP-NZVIformulationusingtheconceptproposedby
Wangetal.ThroughthedeterminationofCMC,therangeofthemoststableformulationis
narroweddown.TheTOCresultsfurtherprovidedarangeofiron-surfactantratioforsynthesis.
Thesynthesisandstabilityresultssuggestedthatthehighstabilitycanbeachievedbywormlike
micellesanchoringontheNZVIparticlesurfaceswhenthedissolvedSDEHPconcentrationis
closeto100mM.Inthemobilitystudy,becauseofthehighstability,1g/Liron,100mMSDEHP-
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stabilizedNZVIyieldedremarkablemobilityintheporousmediaatafieldflowvelocityof1.5
m/day.ThehighNZVIrecoveryandasteadypeakofthebreakthroughcurveimplythatthe
nanoparticlesexperienceminimalfiltrationduringtransport.Itisimpliedthatthecompelling
mobilityinporousmediademonstratedbySDEHP-stabilizedNZVIcanleadtostudiesatthe
higherscale.
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3.6References:
[1] A.B.Cundy,L.Hopkinson,andR.L.D.Whitby,“Useofiron-basedtechnologiesincontaminatedlandandgroundwater
remediation:Areview,”Sci.TotalEnviron.,vol.400,no.1–3,pp.42–51,2008.
[2] A.CorreiadeVelosaandR.F.PupoNogueira,“2,4-Dichlorophenoxyaceticacid(2,4-D)degradationpromotedby
nanoparticulatezerovalentiron(nZVI)inaerobicsuspensions,”J.Environ.Manage.,vol.121,pp.72–79,2013.
[3] D.O’Carroll,B.Sleep,M.Krol,H.Boparai,andC.Kocur,“Nanoscalezerovalentironandbimetallicparticlesfor
contaminatedsiteremediation,”Adv.WaterResour.,vol.51,pp.104–122,2013.
[4] F.Fu,D.D.Dionysiou,andH.Liu,“Theuseofzero-valentironforgroundwaterremediationandwastewater
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Chapter4:ConclusionandRecommendations
Stabilityiscommonlyconcludedtobepositivelycorrelatedwiththemobilityofnanoscalezero
valentiron(NZVI)particles[1][2].ImprovingthestabilityofbareNZVIsuspensionsusingsurface
stabilizerscanpromotethemobilityofNZVIintheaquiferandeventuallyasuccessful
remediation[3].Currently,varioussurfacemodifiedNZVIsuspensionshavebeendeveloped
andtestedinbenchscale[3][4]andindustrialscale[5][6][7];however,aggregationand
sedimentationsarestillobservedandnosuccessfultransportreported.Overall,thisstudy
concludedtwoalternativetransportvehiclestoimprovethemobilityandstabilityofNZVI.
AlthoughNZVIremediationapplicationshavebeenconductedthroughacademicfieldstudies,
theextentofNZVIapplicationincontaminatedlandsisstilllimited.Inspecific,NZVIinsitu
remediationduetomobility,isfavouredinsoilwithhigherhydraulicconductivity.Theresults
fromthisstudyimpliedtwoalternativeNZVItransportvehicleandpotentiallyimprovethe
extentofNZVIremediation.
Inchapter2,microemulsion-stabilizednanoparticlesaresuggestedtobeanalternative
transportvehicleforNZVIinsituremediationbyWangetal.[8].Toassesstheintrinsicmobility
andeffectivenessofmicroemulsion-stabilizedNZVIdevelopedbyWangetal.[8],ironoxide
nanoparticlesareusedasananalogytoeliminatetheoxidizingfactorofNZVI.Inthecolumn
experiment,microemulsion-stabilizedironoxidenanoparticlesatveryhighconcentrations(2.5,
5and10g/L)arecharacterizedintermsofviscosity,hydrodynamicsize,particlesizeand
stability.Itisfoundthatmicroemulsion-stabilizedironoxidenanoparticlesholdidentical
stabilityperiodwithmicroemulsion-stabilizedNZVIwithsizesinthesimilarrangeasconfirmed
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bythesizeanalysis.However,therheologyanalysisimpliedthathigherironoxide
concentrations,10g/Land7g/L,hadahighviscosityofover400CPanddemonstratednon-
Newtonian,shear-thinningbehaviour.However,theviscositydrasticallydecreasedat5g/Liron
oxideconcentrationto20CPandreturnedtoNewtonianbehaviour.Itwasfoundthatthehigh
viscosityat10g/Lcontributedtothepoortransportobservedatthefieldof5m/day.In
contrast,underthesamecondition,at5g/Lironoxideformulation,highrecoveryandhigh
plateaupeakwasobserved.Inaddition,bychangingthesalinityconditionofthesandinthe
columnbetweenDIwatersaturatedandbrinesaturated(10g/L)atthesameconcentrationas
theformulation,thetransportresultischanged.Byshiftingthesalinitytozero,theplateau
peakisdecreasedsharply,implyingthehighsaltsensitivityofthemicroemulsion-stabilizer.
Microemulsion-stabilizedironnanoparticlesalthoughdemonstratedpromisingtransportresults
duetothehighstability,furtherimprovementsarerequiredforreducingthesensitivityofsalt
andviscosity.Itisconcludedthatmicroemulsioncanholdupto5g/Lofironoxide
nanoparticleswhiledemonstratingadequatetransportbehaviour.Viscosityandsalinitywere
identifiedascriticalvariablesforimprovingthefeasibilityoflargerscaleremediationstudies.
BasedonthefindingsinChapter2,surfactantSDEHP-stabilizedNZVIisdevelopedbasedona
formulationdesignframeworksuggestedbyWangetal.Chapter3discussedthedevelopment
ofahighlystabilizedanionicsurfactantstabilized-NZVIandtheassessmentonthemobilityof
theNZVIformulation.Inthedevelopment,formulationdesignframeworkbyWangetal.is
appliedforthefirsttimeinsurfactant-stabilizedNZVI[9].Anionicsurfactant,SDEHPisselected
forNZVIstabilizationbasedonthepackingpropertiesandenvironmentalfriendliness.Through
determiningcriticalmicelleconcentration(CMC)andtheratiobetweenSDEHPandironsulfate,
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1g/LNZVIat100mMSDEHPisidentifiedasthemostoptimalformulationyieldingastabilityof
over2months.TheNZVIformulationischaracterizedwithsizeandviscosityanalysis;withthe
resultsshowingsignsofidealNZVI.Thecolumnstudyatfieldvelocityresultsdemonstrated
excellenttransportofSDEHP-stabilizedNZVIwithhighrecoveryandplateaupeaks.The
developedformulationwasevaluatedforfieldstudypotentialandhighlightedtobeastrong
candidateforremediationduetotheoverallpropertiesofSDEHP-stabilizedNZVI.
DespitethefindingsinChapter3impliedthattheSDEHP-stabilizedNZVIishighlyfeasiblefora
fullscaleremediationapplication,somefutureworkisrecommendedforfurtherunderstanding
andoptimizingtheformulation.Firstly,theunderstandingofthereactivityoftheSDEHP-
stabilizedNZVIisconsideredpreliminary.Itwasunderstoodfromthepreviousreactivitystudy
fromWangetal.thatsodiumdodecylphosphate(SDP)-stabilizedNZVIduetothephosphate
group(PO43-)holdskineticsratewithreactiveblack5(RB5)andcarbontetrachloride(CT)close
totherateofbareNZVI[10].Inotherwords,thephosphategroup,thatpresentsinSDEHPas
well,maypromotethereactivitywithotherchlorinatedsolvents,makingSDEHP-stabilizedNZVI
moreefficient.Withthisimplication,areactivitystudyandtargetdeliverystudyis
recommendedforfuturework.Secondly,thestabilizationmechanismandthebehaviourof
surfactantaggregationarenotyetwellunderstoodintheSDEHP-stabilizedNZVIformulation.
Eventhoughsignsshowingthattheformationofwormlikemicelleplayedanimportantrolein
stabilization,theexactmechanismonthenanoscaleisstillhypothesized.Theformationof
wormlikemicelleswasnotfoundintheidenticalNZVIformulationwhentheinitialironsource
isfromironchloride(FeCl3).AsdemonstratedinAppendixB,thestabilitybehaviourofFeCl3
basedNZVIformulationistotallydifferentthantheironsulfatebasedformulationdescribedin
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Chapter3.Finally,theconcentrationoftheSDEHP-stabilizedNZVI(1g/L)discussedinChapter3
despitesufficientintermsofremediation,couldnotbescaledupduetothelimitationofthe
formulation.Themicroemulsion-basedNZVIisanimprovementoftheformulationtestedin
Chapter2andabletoholdhigherNZVIconcentrations.However,furtherunderstandingin
termsoftransportandmobilizationwithDNAPLisrequired.
Despitethestrongmobilityinporousmediawasdemonstratedbymicroemulsion-stabilized
ironnanoparticlesandSDEHP-stabilizedNZVI,furtherimprovementsarerecommendedfor
bothstabilizers.Formicroemulsion-stabilizedironnanoparticles,thehighsalinityofthe
formulation(10g/100mL)mustbeaddressedtopreventsaltcontaminationtolandsbefore
applications.Thehighsaltconcentration,asdiscussedinchapter2,wasnecessarytoproducea
one-phasemicroemulsion,wherethehydrophilic-lipophilic-differenceiszero(HLD=0)withthe
extendedsurfactantused.Loweringthesaltconcentrationwiththeexgtendedsurfactantwill
shifttheHLDoftheformulationandthuslosingthestability.BasedontheHLDequation,
alternativesurfactantswithahighercharacteristiccurvature(Cc)canreducethesalinity.
Phosphatesurfactantfromchapter3,SDEHPmayreducethesalinityrequiredtobalancethe
HLD,aSDEHPmicroemulsionsystemcanbedevelopedfollowingtheprocedureofWantetal.
FortheSDEHP-stabilizedNZVIdevelopedinchapter3,eventhoughsuccessfultransportwas
observedat1g/LNZVIconcentrationandmettheminimumremediationcriteria,higherNZVI
concentrationsarepreferredformoreefficientremediation.However,basedontheresultsin
chapter3,1g/ListhemaximumNZVIconcentrationpossible,consideringanysurfactant
concentrationhigherthan100mMwasnotfeasible.Basedontheresultsinchapter2,
microemulsionsystemscanholdironoxidenanoparticlesupto10g/L.TheSDEHP
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microemulsionsystemcanachievehigherconcentrationsandcanbeexpectedwithvariationof
thesynthesisprocedure.
Thesignificanceofthisstudyliesinthefollowing:1.ThetransportstudyontheSDEHP-
stabilizedNZVIconfirmedanimprovementonthemobilitytoporousmedia.Currently,thefull-
scaleremediationresultsofpolymer-stabilizednZVIsuggestedthatlongerdistanceandhigher
recoveryarerequired.ItcanbeexpectedthatSDEHP-stabilizedNZVIyieldedalongertravel
distanceandhigherrecoveryinthefieldtestfromtheresultsinthisstudy.2.Themobilitystudy
ofmicroemulsion-stabilizedironnanoparticlessuggestedthatgreaterconcentrationsofNZVI
canbetransportedinaneffectivefashion.MostofthereportedNZVItransportvehiclescan
onlyholdupto2.5g/Lofiron[1],whichlimitedthecosteffectivenessoftheremediationin
highlycontaminatedlands.MicroemulsioncanbeappliedasanalternativestabilizerforNZVIat
contaminantssitethatarenotsuitableforthecurrentNZVItechnology.
Forclosure,thisstudysuccessfullyassessedexaminedtwoNZVIstabilizationsystems.The
SDEHP-stabilizedNZVIisanimprovementbasedonthetransportimplicationsfromthe
microemulsion-stabilizedironoxidenanoparticles.Thisworkemphasizedtheimportanceof
stabilitytoNZVItransportintheporousmediaandhighlightedSDEHP-stabilizedNZVIasa
potentialcandidateforfuturegroundwaterremediation.
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applications,”Chem.Eng.J.,vol.287,pp.618–632,2016.
[3] D.O’Carroll,B.Sleep,M.Krol,H.Boparai,andC.Kocur,“Nanoscalezerovalentironandbimetallicparticlesfor
contaminatedsiteremediation,”Adv.WaterResour.,vol.51,pp.104–122,2013.
[4] F.Fu,D.D.Dionysiou,andH.Liu,“Theuseofzero-valentironforgroundwaterremediationandwastewater
treatment:Areview,”J.Hazard.Mater.,vol.267,pp.194–205,2014.
[5] A.I.A.Chowdhury,M.M.Krol,C.M.Kocur,H.K.Boparai,K.P.Weber,B.E.Sleep,andD.M.O’Carroll,“NZVIinjection
intovariablysaturatedsoils:Fieldandmodelingstudy,”J.Contam.Hydrol.,vol.183,pp.16–28,2015.
[6] C.M.Kocur,A.I.Chowdhury,N.Sakulchaicharoen,H.K.Boparai,K.P.Weber,P.Sharma,M.M.Krol,L.Austrins,C.
Peace,B.E.Sleep,andD.M.O’Carroll,“CharacterizationofnZVImobilityinafieldscaletest,”Environ.Sci.Technol.,
vol.48,no.5,pp.2862–2869,2014.
[7] S.O’Hara,T.Krug,J.Quinn,C.Clausen,andC.Geiger,“FieldandlaboratoryevaluationofthetreatmentofDNAPL
sourcezonesusingemulsifiedzero-valentiron,”Remediat.J.,vol.16,no.2,pp.35–56,2006.
[8] Z.Wang,“SYNTHESISOFSTABLEANDREACTIVEMICROEMULSIFIEDZERO-VALENTIRONNANOPARTICLES(MENZVI)
USINGEXTENDEDSURFACTANT,”2015.
[9] Z.Wang,A.Lam,andE.Acosta,“SuspensionsofIronOxideNanoparticlesStabilizedbyAnionicSurfactants,”J.
SurfactantsDeterg.,vol.16,no.3,pp.397–407,2013.
[10] E.Wang,Ziheng;Choi,FrancisandAcosta,“EffectofSurfactantsonZero-ValentIronNanoparticles(NZVI)Reactivity.”.
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AppendixA–FerricChlorideBasedSodiumDiethylHexylPhosphate
(SDEHP)-stabilizedNZVI
A.1Introduction
NZVIcanbeartificiallysynthesizedfromtwomainsources,ferricchloride(FeCl3)andiron
sulfate(FeSO4)[1].Bothsourcesofiron,namelyferricsourceandsulfatesource,undergoa
reductionreactionwithNaBH4,reducingfromiron(II)sulfateandiron(III)chloridetothezero-
valentform,aslistedinthefollowing:
(1) 2𝐹𝑒Uw + 3𝐵𝐻qA + 3𝐻W𝑂 → 2𝐹𝑒a + 𝐻W𝐵𝑂UA + 4𝐻w + 2𝐻W
And
(2) 4𝐹𝑒Uw + 3𝐵𝐻A + 9𝐻W𝑂 → 4𝐹𝑒a + 3𝐻W𝐵𝑂A + 12𝐻w + 6𝐻W
respectively.Fromtheliterature,bothironsourceswereappliedextensivelyassynthesis
sourcesforNZVIstudieswiththeironsulfatebeingslightlyhigher[1]whilethechloridesource
wasthemostoriginalmethod.Itisalsoindictedthatironsulfateisconsideredamore
environmentalfriendlysourceofNZVIincomparisontoferricchloride[1].Inbothcases,bare
NZVIparticlesaresynthesizedwithidenticalpropertiessuchasinstabilityandatsizeswithin
similarrange.
Forimprovingthemobility,surfacemodifiersarecommonlyaddedtobareNZVIsuspensions.In
general,surfacemodifiersareappliedtoNZVIintwoways:1.Re-suspension[2]and2.
Synthesiswiththepresenceofthesurfacemodifiers[3].Inthere-suspensiontypesurface
modifications,thesourceoftheNZVImayplayanegligibleimpacttotheoverallsuspension;
however,someimpactsareexpectedwhentheNZVIsynthesistakesplacewiththemodifiers.
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Inspecific,thereactionsproductsfrombothferricandsulfatemethodmayinteractwiththe
surfacemodifiersandtheoverallsuspensiondifferently.Currently,therehasnostudies
focusedonthestabilityandmobilitydifferencebetweentheNZVIfrombothsources.In
chapter3,ironsulfate-basedNZVIparticlesweresynthesizedusingthe“one-pot”technique
andanalyzedatvarioussurfactantandNZVIconcentrations.AnNZVIformulationat100mM
SDEHPwasreportedtodemonstrateaprolongedofover2monthswithexcellentmobilityin
porousmedia.ItisobservedthatwithdifferentsourcesofNZVIappliedinthe“one-pot”
synthesismethoddescribedinthisthesis,differentqualityofNZVIsuspensionswereobtained.
Theobjectiveofthisworkistoinvestigatethebasicproperties,suchasstabilityandsizesof
ferricchloride-based,SDEH-stabilizedNZVIformulationsincomparisontotheiron-sulfate
basedformulation.Thesecondaryobjectiveistodeterminethecausebehindthedifferences
betweenchlorideandsulfatesynthesisinSDEHP-stabilizedNZVI.
A.2Methodology
Unlessotherwisespecified,allproceduresandmaterialsareconductedandpreparedatroom
temperatureandconditions.
A.2.1PreparationofSurfactantSDEHPandFeCl3-basedNZVI
TheprocedureofsynthesizingSDEHPisdescribedinsection3.2.1andadaptedfromprocedure
ofLuanetal.[4]TheNZVIwassynthesizedinthesame“one-pot”synthesisfashionasdescribed
insection3.2.2withthefundamentalideaadaptedfromWantetal.[5]
A.2.2FormulationdesignofFeCl3-basedNZVI
Surfacetensionmeasurementwasappliedtodeterminethecriticalmicelleconcentration
(CMC)ofSDEHPwith1g/LNZVIequivalenceofferricchloridedissolved.Themeasurementwas
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conductedwithatensiometeratdifferentconcentrationsofSDEHP.Detailedprocedureis
describedinsection3.2.4.
A.2.3CharacterizationAnalysis:SizeandStability
Transmissionelectronicmicroscope(TEM)anddynamiclightscattering(DLS)areusedto
analyzeparticleandhydrodynamicsizes,respectively.Samplepreparationforbothsizeanalysis
isdescribedinsection2.3.4.Forstabilityanalysis,identicalprocedurewasfollowedas
describedinsection2.3.3.
A.3ResultsandDiscussions
A.3.1FormulationDesignImplication
BasedontheformulationdesignframeworkproposedbyWangetal.,theconcentrationofthe
surfactantmustbehigherthantheCMCtoachievehigherstability[6].Theapproachwas
reportedsuccessfulwithprolongedstabilityinthesulfate-based,SDEHP-stabilizedNZVIat1
g/L,100mMsurfactantconcentration.FigureA1showsthesurfacetensionmeasurementsat
differentconcentrationofSDEHP.TheCMCisidentifiedatabout35mMbasedonthe
interpretationofline-of-the-best-fit.TheCMCoftheferricchloridedissolvedSDEHP(35mM)is
lowerthanironsulfatedissolvedSDEHP(57mM).Thisimpliesthebindingofthesurfactantand
ironaredifferentinthetwocasesduetothepresenceofanions,chlorideorsulfate.Inthecase
ofthesulfatemethod,thehigherCMCcanbeexplainedbythepossibilitythatthesurfactants
aremorelikelytobindtothesurfaceoftheiron,leavinglessfreesurfactantstoformempty
micelles.Ontheotherhand,thehigherCMCinthechloridemethodimpliestheopposite,the
surfactantislesslikelytobindontothesurfaceoftheiron,thusmorefreesurfactantsare
availabletoformmicellesatthesameconcentrations.TherootcauseofthedifferenceinCMC
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ishowever,unclear,itissuspectedthatthepresenceofthedifferentionsorthedifferencein
theironcharges.Thisisexpectedtohavesomeimpactstothestabilityaftertheproductionto
theformulation.
FigureA1.FigureA1.SurfacetensionmeasurementsofSDEHPatvariousconcentrationwith
ironchloride(1g/LequivalenceofNZVI)dissolved.
A.3.2StabilityandSizeAnalysis
FigureA2.showsthestabilitytimelapsephotoat1hourofchloridemethodbased,SDEHP-
stabilizedNZVIat30,50and100mM.Allthethreeconcentrationsexperiencedaggregation
andsedimentafteronehouruponsynthesis.Outofthethree,100mMSDEHPdemonstrated
lesssettlingthanthe30and50mM,implyingthathigherconcentrationofSDEHPmayimprove
15
20
25
30
35
40
45
50
1 10 100
SurfaceTension(m
N/M)
SDEHPConc.(mM)-LogScale
SurfaceTensionofSDEHPwith1g/LFeCl3Dissolved
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thestabilityinthecaseofchloridebasedmethod.Incontrast,atthesameconcentrationof
NZVIandSDEHP,sulfatemethoddemonstratedastabilityofover2months.Thedifferencein
thesynthesiscanberelatedtotheCMCdifferencediscussed,wheremolecularlythe
interactionsbetweentheanions,surfactantandironaredifferentbetweenthesulfateand
chloridemethod.Furthermore,thereactionfromthechloridemethodproducesabout3times
morehydrogenionsthanthesulfatemethod.ThismeansthatthepHlevelofthechloride
methodNZVIsuspensioncanbealotlowerthanthepHofthesulfatemethod.Thedecreasein
pHisreportedtoimpactthezetapotentialthuscontributiontolowerstability.
Forsizeanalysis,figure3showstheTEMimagingoftheNZVIparticlesizeofthesynthesized
NZVIat100mM.Overall,thehardparticlesizeofthechloridebasedNZVIisabout100-200nm,
whichisinthesamerangetothereportedvalueinChapter3.However,thesurfactantcoating
aroundtheNZVIisalotmoreemphasizedincomparisontotheTEMimagingforthesulfate
method.Thisisreflectedinthehydrodynamicdiameterofthechloridemethod-basedNZVIas
well.Assummarizedintable3.2,thehydrodynamicdiameterfor30,50and100mMSDEHPare
400,420and380,respectively.Theyarefarlargerthanthesulfatemethod-basedNZVIas
reportedinChapter3.Thedifferenceinhydrodynamicsizemayimpliedthedifferencein
molecularinteractionbetweenthechlorideandthesurfactant.Thiscouldbeanimplicationon
themechanismforsurfactantsuspendingandtheformationofwormlikemicelle.
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[email protected] /Lofiron
concentration.1houraftersynthesis.
A.4FutureWorks
Differentstabilitybehavioursareobservedforchloride-andsulfate-basedNZVIunderthesame
synthesiscondition.Thisappendixprovedthatwithdifferentsourcesofironanddifferent
reactionpathways,synthesiswiththepresenceofsurfacemodifiers.Fromthestudy,itis
shownthatthesulfatemethodismoreinfavouroThiscouldbeanimplicationon
improvementsofotherNZVIsurfacestabilizationsandmobilityinporousmedia,onecan
improvetheabilityoftheNZVIsuspensionbychangingthesourceofsynthesisiron.Itis
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importanttoalsonotethatwhetherthetwoironsourceswilldemonstrateanydifferencein
reactivityduetothedifferenceinsynthesisreactions.
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A.5References:
[1] W.ZhangandD.W.Elliott,“Applicationsofironnanoparticlesforgroundwaterremediation,”Remediat.J.,vol.16,no.
2,pp.7–21,2006.
[2] J.Soukupova,R.Zboril,I.Medrik,J.Filip,K.Safarova,R.Ledl,M.Mashlan,J.Nosek,andM.Cernik,“Highly
concentrated,reactiveandstabledispersionofzero-valentironnanoparticles:Directsurfaceandsiteapplication,”
Chem.Eng.J.,vol.262,pp.813–822,2015.
[3] S.Z.Yu,Y.Cheng,X.F.Fan,andL.P.Xu,“PreparationofCoatedCMC-nZVIUsingRheologicalPhaseReactionMethod
andResearchonDegradationofChloroforminWater,”Mater.Sci.Forum,vol.847,pp.230–233,2016.
[4] Y.Luan,G.Xu,S.Yuan,L.Xiao,andZ.Zhang,“Comparativestudiesofstructurallysimilarsurfactants:Sodiumbis(2-
ethylhexyl)phosphateandsodiumbis(2-ethylhexyl)sulfosuccinate,”Langmuir,vol.18,no.22,pp.8700–8705,2002.
[5] Z.WangandE.Acosta,“FormulationdesignfortargetdeliveryofironnanoparticlestoTCEzones,”J.Contam.Hydrol.,
vol.155,pp.9–19,2013.
[6] Z.Wang,A.Lam,andE.Acosta,“SuspensionsofIronOxideNanoparticlesStabilizedbyAnionicSurfactants,”J.
SurfactantsDeterg.,vol.16,no.3,pp.397–407,2013.
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AppendixB:ComparisonbetweenCarboxylmethyl-celluosestabilized
ironoxidenanoparticleswithmicroemulsion-stabilizednanoparticles
B1.Background:Carboxylmethyl-celluose(CMC)isbyfarthemostsuccessfulNZVIstabilizerintheliterature.The
mobilityofCMC-stabilizedNZVIhadwelltestedinthelabscaleandfullscaleremediationwas
launchedin2014.Inthissection,amobilitycomparisonbetweenCMC-stabilizedironoxideand
microemulsionironoxidewastestedfollowingidenticalexperimentalsetupdescribedin
chapter2.TheformulationofCMC-stabilizedNZVIwasmodifiedfromtheonedescribedby
Kocuretal.(2.5g/Lofiron,CMCMW=90,000at0.8wt%)[1]withNZVIbeingreplacedbyiron
oxideforconsistencyofcomparison.2.5g/Lequivalenceofironoxidewassuspendedinthe
50%dilutedmicroemulsionformulationasdiscussedinchapter2.
B2.Results:
B2.1.StabilityThestabilityofmicroemulsionandCMC-stabilizedironoxideat2.5g/Lwerecomparedusing
timelapsephotoscomparisons.AsindicatedinfigureB1,CMC-stabilizedironoxidestartedto
showsignsofaggregationandparticlesettlingafter15hours.Majorsedimentationwas
observedatthebottomofthevialforCMC-stabilizedironoxideafter80hours.Ontheother
hand,nosignificantsettlingwasobservedwithmicroemulsionironoxide,consistenttothe
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findingsinchapter2.Thisindicatedthatmicroemulsionironoxideholdsamuchstronger
stabilitythanCMCironoxide.Theminimumsettlingandaggregationimpliedthat
microemulsion-basedironsuspensionexperienceasmoothertransportprocessinsandin
comparisontoCMC-basedironparticles.
A.
FigureB1.A.TimelapsedphotosofCMCandmicroemulsionstabilizedironoxideat2.5g/L.B.
EvidenceofsettlingofCMCironoxideafter80hoursuponsuspension.
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B2.2.MobilityComparisonTransportstudywasconductedonbothmicroemulsionironoxideandCMCironoxideat2.5
g/L.Forthemicromeulsionironoxidetransportat2.5g/L,thetransportresultwasreproducible
comparingtothetransportresultsinchapter2.ForCMCironoxide,despitesuccessful
completionoftheexperimentanddataanalysis,abreakthroughcurvewasnotabletobe
obtained.However,fromthepostanalysessuchaspressuredropmonitoringandsandgrain
analysis,itcanbeconfirmedthatmicroemulsionironoxidedemonstratedbettertransportthan
CMCironoxide.
Pressuredropwasrecordedateveryporevolumeduringtheflushingstage(foratotalof5
porevolumes).FigureB2demonstratedthepressuredropmoniroingresultcomparison
betweenCMCandmicroemulsionironoxide.Thepressuredropmonitoringresultshowsthat
CMCironoxidestartedwithahighpressuredropof22.5psiandlinearlydecreasedto3psi
whilemicroemulsionironoxideremainedat0.5psithroughouttheflushingstage.Thisimplies
thatduringthetransportofCMCNZVI,potentialcloggingofthesandporesincreasedthe
difficultyofflowinthecolumn,thusexperiencingahighpressuredropinthebeginning.The
decreaseinthepressuredropmayimplythatthecloggedparticleswerebeingflushedoutof
thecolumnandthusinferredreversibleadsorptionoftheironparticles.Ontheotherhand,
microemulsionironoxidedemonstratednegligiblepressuredropintheflushingstage,implying
minimumcloggingandretentionoftheparticles.
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FigureB2.Comparisonofpressuredropmonitoringresultsatthepost-flushingstagebetween
CMCandmicroemulsionironoxide.
SimilarironretentionanalysiswasconductedafterthecolumnstudyforCMCand
microemulsionironoxideasdemonstratedinfigureB3.FigureB3showsconsistentresultsand
conclusionasfigureB2,whereCMCironoxideexperiencedmoreretentionanddifficulties
duringtransport.InfigureB3,CMCironoxidedemonstratedadecreasingtrendintheiron
retentionanalysissimilartotheresultsofscheduleBinchapter2.Whereasmicroemulsioniron
oxidedemonstratedreproducibleresultstoscheduleA.Thisagainconfirmsthehypothesisthat
duetoinstabilityoftheCMCironoxide,moreretentionofironwasobserved.
0
5
10
15
20
25
13 14 15 16 17 18 19 20
PressureDrop(Psi)
PoreVolumeInjected
PressureDropMonitoring- PostFlushingStage
ME CMC
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A.
B.
FigureB3.Iron-sandgrainanalysiswithmicroscopepicturesforA.Microemulsionironoxideat
2.5g/LandB.CMCironoxideat2.5g/L.
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B3.References:
[1] C.M.Kocur,D.M.O’Carroll,andB.E.Sleep,“ImpactofnZVIstabilityonmobilityinporousmedia,”J.Contam.Hydrol.,
vol.145,pp.17–25,2013.