Ion Exchange of Divalent Cobalt and Iron with Na–Y Zeolite

JournalofColloidandInterfaceScience232,126–132(2000)

doi:10.1006/jcis.2000.7165,availableonlineathttp://wendang.chazidian.com

on

IonExchangeofDivalentCobaltandIronwithNa–YZeolite:

BinaryandTernaryExchangeEquilibria

JongSungKim?andMarkA.Keane?,?,1

?DepartmentofChemicalEngineering,TheUniversityofLeeds,LeedsLS29JT,UnitedKingdom;and?DepartmentofChemicalandMaterialsEngineering,

UniversityofKentucky,Lexington,Kentucky,40506–0046

ReceivedMay1,2000;acceptedAugust11,2000

DivalentcobaltandironremovalfromaqueoussolutionsbybatchionexchangewithasyntheticNa–Yzeolitehasbeenstudiedundercompetitiveandnoncompetitiveconditions.ThebinaryCo/NaandFe/Naion-exchangeequilibriumisotherms,constructedat291±2Kandatotalsolutionpositivechargeconcentrationof0.1eqdm?3,exhibitedsigmoidalshapesthatareattributedtoanexchangesiteheterogeneity.ThesolutionpHandtheratioofsorbatetosor-bentareidenti?edforwhichminimalimbibitionofmetalhydroxideandmaintenanceofzeolitestructuralintegrityareensured.Anin-creaseinFeandCoconcentrationovertherange0.005–0.05moldm?3loweredtheremovalef?ciencybuttheexternalFewaspre-ferredtotheindigenoussodiumovertheentireconcentrationrange;therewasaswitchinpreferencefromCotoNaat[Co]inexcessof0.034moldm?3.ExchangedataforCu2+/Na+andNi2+/Na+binarysystemsareincludedforcomparativepurposes,andionex-changeaf?nityandNa–Yexchangecapacityarediscussedintermsofmetalionhydrationandionlocation.Theeffectofexchangetem-peraturehasbeenconsideredwherethemaximalFeexchangewastemperatureindependentwhileCoexchangewaspromotedwithincreasingtemperature.ACo/Fe/Na–Yternaryexchangeisothermwasconstructedfrom20pairsofexperimentalpointsandistreatedquantitativelyintermsofternaryandpseudo-binaryseparationfactors.Thepreferenceofthezeoliteforexchangewithironovercobaltundernoncompetitiveconditionsalsoextendedtosolutions

C2000AcademicPresscontainingbothmetals.??

KeyWords:Na–Yzeolite;ionexchange;iron;cobalt;binaryexchange;ternaryexchange;separationfactors;watertreatment.

INTRODUCTION

Cobaltandironareclassi?edasheavymetalsthatarecom-monlyassociatedwithwaterpollution(1).Intermsofapprovedwaterqualitystandards,cobaltandironarenutrientsatppb–ppmlevelsbutdisplayadegreeoftoxicityathigherlevels(2).Thepresenceofcobaltasawaterpollutantcanbearesultofleachingfromrock/soilorcommercialactivitiesrelatingtoagricultureormining/metallurgical/electronicsindustries,aby-productofelectroplatingorpigments/paints,orasspentcatalysts(3,4).ExceptinRussia(1ppm),internationalhealthauthoritieshave

Towhomcorrespondenceshouldbeaddressed.Fax:(859)323-1929.E-mail:makeane@engr.uky.edu.

1

notissuedamaximumacceptablecobaltcontaminantlevel(5).Nevertheless,concentrationsofcobalthigherthan1mgkg?1bodywtareconsideredtorepresentahealthhazardtohumans(5).Theoccurrenceofironinaqueousmediacanalsoarisefromleachingofironsaltsfromsoilandrocks,corrosionofpipes,anarrayofindustrialsources,andevenasaby-productofiron-containingbacteria(5).Whileironisfoundnaturallyinlargeconcentrationsinaninsolubleform,itcanbeconvertedtosol-ubleformsthatoftenresultinwatercontamination;theWorldOrganizationguidelineforironis0.3mgdm?3(6).

Themostcommonlyemployedtreatmentmethodforheavymetalremovalischemicalprecipitation(1,7).Althoughthisap-proachisrelativelysimpleandinexpensive,ithasthedecideddrawbackofgeneratingalargevolumeofsludgefordisposal.Alternativerecoverymethodsincludeelectrowinning,reverseosmosis,electrodialysis,solventextraction,evaporation,andionexchange(1,7).Zeolitesareprovenion-exchangemateri-alswheretheindigenous(typicallysodium)charge-balancingcationsarenot?xedrigidlytothehydratedaluminosilicateframeworkandarereadilyexchangedwithmetalcationsinso-lution(8,9).Thismethodhasthedecidedadvantageofmin-imalassociatedwastegeneration,processsimplicity,andeaseofmaintenance.Cobaltexchangefromaqueousmediausingbothsynthetic(10–17)andnaturallyoccurring(17–22)zeo-liteshasbeenreported.Theprocessofcobaltexchangewithzeoliteshasbeenundertakenlargelywiththeaimofpreparingsolidcatalysts(23–26).Theliteraturedealingwithironremovalbyzeoliteionexchangehaslikewiseconcentratedoncatalystsynthesis(27–29).Theapplicationofzeolitestoenvironmentalpollutioncontrolintermsofheavymetalremovalhasreceivedscantattention,possiblyduetothelowsolutionpHthatisof-tennecessary(particularlyinthecaseofiron)topreventmetalhydroxideprecipitationandensurethationexchangeisstoi-chiometric;zeolitescansufferstructuralbreakdownevenunderweaklyacidconditions(30,31).Hlavayetal.(32),however,haveinvestigatedtheef?ciencyofclinoptilolitefortheremovalofironfromdrinkingwater.TheapplicationofthehighsurfaceareazeoliteY,thefocusofthepresentstudy,totheremovalofCu,Ni,Cd,andPbfromwaterhasbeenreportedpreviously(33–37).WereporthereintheresultsofourinvestigationofNa–Yzeoliteasanagentsuitableforremovingdivalentcobaltand

126

DIVALENTCOBALTANDIRONIONEXCHANGE

127

ironfromaqueoussolutionsandpresentbinaryCo2+/Na+–YandFe2+/Na+–YandternaryCo2+/Fe2+/Na+–Yexchangeeq-uilibria;zeoliteef?ciencyandselectivityareexaminedandcom-paredwiththoseofcopperandnickelexchangesystems.

EXPERIMENTAL

ThestartingzeolitewasLindemolecularsieveLZ-52Y,whichhasthenominalanhydrousunitcellcompositionNa58(AlO2)58(SiO2)134:density=1.9gcm?3;freeaperture(an-hydrous)=0.74nm;unitcellvolume=0.15nm3;voidvol-ume=ca.50%;andmaximumCo2+andFe2+exchangecapac-ity=0.132and0.124gperganhydrouszeolite,respectively.Inordertoobtain,asfaraspossible,themonoionicsodiumform,3thezeoliteasreceivedwascontacted?vetimeswith1moldm?aqueoussolutionsofNaNO3.Thezeolitewasthenwashedbrie?ywithdeionizedwater,oven-driedat363K,andstoredoversaturatedNH4Clsolutionsatroomtemperature;thewatercontentwasfoundbythermogravimetry(Perkin–Elmerther-mobalance)tobe24.8%w/w.

Cobaltand/orironremovalfromaqueoussolutionbyionexchangewasconductedinthebatchmode.Theexchangeisothermswereconstructedat291±2Kand3atatotalexchangesolutionconcentrationof0.1moleqdm?,where1eqequals1molofpositivecharge.Binary(Co/NaorFe/Na)isothermpointswereobtainedbycontactingthezeolitewithaqueous(deionizedwater)solutionsofCo(NO3)2(H2O)6orFeCl2(H2O)4inthepresenceorabsenceofknownconcentrationsofNaNO3(Co/Nabinarysystem)orNaCl(Fe/Nabinarysystem)toensurethesameinitialsolutionphasechargeconcentration.Ternary(Co/Fe/Na)isothermpointswereobtainedinthepresenceofknownconcentrationsofNaNO3wheretheinitialcobaltand/orironsolutionconcentrationspannedtherange0.003–0.05moldm?3.TheNa–Yzeolite(sievedinthemeshrange50–70µm)wascontactedwiththeheavymetal(HM)/Nasolutions(thor-oughlypurgedwithHe)insealedpolyethylenebottlesandtheresultantslurrywasagitatedat100rpmonaGallenkampor-bitalshakerfor3days,atwhichpointequilibriumuptakehadbeenachieved;thelatterwasascertainedfromperiodicsamplingandanalysisoftheHMsolution.Theratiosofsorbatetosor-bentinthecaseofCo,Fe,andCo/Fetreatmentswere20,100,and100cm3g?1,respectively.Theseratioswerechosentoen-surethatthesolutionpHremainedwithintherangewhereHMprecipitationorstructuraldisintegrationofthezeolitedidnotoc-cur.ThesolutionphasepHbeforeandafterthezeolitetreatmentwasmeasuredwithaHannaHI9318programmableprintingpHbench-meter.TheequilibriumpHvaluesforthecobalt,iron,andcobalt/iron(ternary)solutionsfellwithin6.5–6.9,4.1–4.6,and4.2–4.6,respectively.Theeffectofion-exchangetemperaturewasconsideredbyheatingNa–Y/HMsuspensionsunderre?uxconditionsfor24h.

Thezeolitewasseparatedfromsolutionbyrepeated?ltrationandthemetalcontentinthe?lteredliquidsampleswasdeter-minedaftertheappropriatedilution.Inthecaseofferrousiron

determination,thesolutionforanalysiswasdilutedinacidi?ed(byadditionofnitricacidtopH4.1)deionizedwatertopreventoxidationofFe(II).Theliquid-phaseconcentrationsofCo,Fe,Na,Al,andSiweremeasuredbyatomicabsorptionspectropho-tometry(AAS,VarianSpectrAA-10);datareproducibilitywasbetterthan±2%.Structuralchangestothezeolitewereprobedbyscanningelectronmicroscopy(SEM)usingaHitachiS700?eldemissionSEMoperatedatanacceleratingvoltageof25kV.Samples(beforeandafterionexchange)foranalysiswerede-positedonastandardaluminumSEMholderanddoublecoatedwithgold.UnderthestatedconditionstherewasnoobservablelossofzeolitecrystallinityafterHMtreatment.Allthechemi-calsemployedinthisstudywereofanalyticalgradeandwereusedwithoutfurtherpuri?cation.

RESULTSANDDISCUSSION

SolutionpHChangesduringIonExchange

TheadditionofNa–Ytowaterwasaccompaniedbyanim-mediatesolutionpHincreaseasaresultofahydrolysisofthezeoliteaccordingto(38):

Na–Y+(H2O)x?? H–Y+(H2O)x?1+Na++OH?.[1]TheinitialpHvaluesofthe0.05moldm?3solutionsofcobaltandironwere3.2and6.2,respectively.Theratioofsorbatetosorbentemployedinthisstudywaschosentoavoidbothprecipitationoftheheavymetalandstructural?1disintegrationofthezeolite.Ahighratio(500cm3g)wasrequiredforcobalttreatmentinordertopreventprecipitation,whileamuchlowerratio(100cm3g?1)wassuf?cienttoensuremaintenanceofNa–Ystabilityduringtheirontreatment.TheequilibriumpHvaluesforthecobalt,iron,andcobalt/iron(ternary)solu-tionsfellwithintheranges6.5–6.9,4.1–4.6,and4.2–4.6,re-spectively.TheamountofHMhydroxidegeneratedduringtheexchangeprocesscanbeestimatedbasedtheassociated?17sol-ubilityequilibriumconstants(40):KFe=4.87×10(39);KCo=2.0×10?17.Wheretheinitialheavymetalconcentra-tion([HMs]i)equals0.05moldm?3,thepHatwhichhydroxideformationisinitiatedisslightlylower(6.6)forFethanforCo(6.9).TheoxidationofFe(II)totheferricformFe(III)anditssubsequenthydrolysisinwaterresultinimmediateprecipitationofironifthepHexceeds3.0evenwhentheFe(III)concentrationisverylow(41).TheamountofferricironformedduringionexchangewascalculatedusingtherateexpressionproposedbyStummandLee(31).TheresultsrevealedthatoxidationofFe(II)inthepHrange4.1–4.6wasnegligible;atpH4.5(T=293K),1%oxidationoftheFe(II)componentwasachievedafter98h,while5%oxidationrequired21days.Ontheotherhand,alowpHcanaffectthestabilityofthezeolite(42)andasolutionpHbelow4shouldbeavoided,ashydratedaluminabecomesappre-ciablysolubleatapHofca.4(42).TheAlandSicontentintheFesolution(where[Fes]i=0.05moldm?3)afteratreatmenttimeof72h,i.e.,themost“severe”conditions,wasmeasuredtoascertainthedegreeofstructuralcollapse.A0.34%removal

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KIMANDKEANE

ofthezeoliticaluminumcontentwasrecorded,whichcompareswithanevenmorenegligible(0.07%)elutionoftheindigenoussilicon;onthebasisofaroutineSEManalysis(43)oftheusedzeolite,theredidnotappeartobeanystructuraldamageafterionexchange.BinaryIonExchange

TheionexchangeofdivalentheavymetalionswithNa–Ycanberepresentedbytheequilibrium(8)

HM2z++2Na+z?? HM2z++2Na+s,

[2]

wheresandzrepresentthesolutionandzeolitephases,respec-tively.TheexchangeisothermsforthebinaryCo/NaandFe/Na

systemsareshowninFig.1,wheretheabscissaistheequi-libriumHMconcentrationinsolutionandtheordinaterepre-sentstheequilibriumconcentrationinthezeolitephase.Bothisothermsexhibitasigmoidalshape,whichisindicativeofex-changesiteheterogeneity.Whilethetwoisothermsaresimilaroverthelowerequilibriumsolutionconcentrationrange,cobaltuptakeisdistinctlygreaterandthesigmoidalresponseismoremarkedathigherconcentrations.Itiswellestablished(12)thation-exchangeselectivities/af?nitiesintherigidzeolitechannelsystemunderhydrousconditionsaregovernedbythestrengthofcoordinationoftheenteringcationswithwatermoleculesand/oraluminosilicateframeworkoxygens.TheYzeoliteemployedinthisstudyischaracterized(33)byanopenlatticestructureconsistingoftwoindependent,thoughinterconnecting,three-dimensionalnetworksofcavities:(i)theaccessiblesupercagesofinternaldiameter1.3nm,whicharelinkedbysharingrings

of

FIG.1.Iron(?)andcobalt(?)exchangeisotherms(T=291K)obtainedatatotalpositivechargeconcentrationof0.1eqdm?3.Inset:Variationofequi-libriumsolution-phasepositivechargeconcentrationwithinitialheavymetalconcentrationinsolution([HMs]i);symbolsasabove.

12tetrahedra(freediameter,0.7–0.8nm);(ii)thelessaccessiblesodaliteunits,whicharelinkedthroughadjoiningringsof6tetra-hedrawhichformthehexagonalprisms(freediameter,0.20–0.25nm).Thesigmoidalpro?lecanbeattributed(33,34)toaninitialpronouncedselectivityforHMexchangewithsodiumionsintheaccessiblesupercagesandincreasinginvolvementofexchangewithparentionslocatedinthesodaliteunitsathigherexternalferrousandcobaltionconcentrations.Amaxi-mumexchange?1of82%oftheindigenoussodiumcomplement(80mg3g)wasrecordedforcobalt(at[Cos]e=0.0473moldm?),whilea74%exchange(68mgg?1)wasachievedforiron(at[Fes]e=0.0378moldm?3).Thesevaluesexceedtheso-called“magicnumber”of0.68(68%)(8,12,33,34),whichisdiagnosticofapartialoccupancyofthesmallcages.Indeed,thediameterofthehydrateddivalentCoionsis0.208nm(44,45),whichisofthesameorderasthefreediameterofthehexagonalprisms,suggestingthatarestricted,butnotprohibited,entrytothesmallercageexchangesitesistobeexpected.ThediameterofthehydratedFe(II)isonlyslightlygreater,i.e.,0.212nm(44,45),andasimilarmaximumuptakewouldbeexpectedonthebasisofstericeffects.Moreover,valuesof0.236nm(46)and0.223nm(29)havebeenquotedforcobaltandiron,respectively,locatedinthehydratedsupercagesofzeoliteA.Previousstudieshaveshownthat,inthedehydratedzeoliteYunitcell,theFe(II)ionslocatepreferentiallyinthehexagonalprisms(27),whiletheCo(II)ionsprefersitesinthesodalitecages(47).ThemaximumlevelofCo(II)exchangerecordedinthisstudyissimilartobuthigherthanthosereportedpreviously,i.e.,74%(11)and80%(12),wheretheYzeoliteSi/Almolarratioswere3.8and?32.6,andsolutionconcentrationswere0.1and0.01moldm,re-spectively.ThemaximumFeexchangeisalsosomewhathigherthanrangeofvalues(64–70%)givenintheliterature(27,28)foracomparableYzeolite.Exchangewiththezeoliticsodiumandsitingwithinthealuminosilicateframeworkmustnecessi-tatesomeweakeningoftheion–dipoleinteractionsbetweenthein-goingHMionsandthecoordinatedwatermoleculeswherethehydrationsheathisstrippedandtheHMmetalionsaremosteffectivelysolvatedbythezeoliteframeworkoxygens.Ionex-changewithFeunderre?ux(at373K)deliveredamaximumremovalofthesodiumcomplementintherange66–70%,whichiscomparabletothatachievedatroomtemperature,andtheca-pacityofNa–YforexchangewithFeislargelyindependentoftemperatureand,therefore,ofthedegreeofFe(II)hydration.Incontrast,anincreaseinexchangetemperaturefrom291to373Kresultedinade?niteenhancementofcobaltexchangeasshowninTable1.ThemaximumrecordedCoremovalfromsolutionwaspromotedwithincreasingtemperature(291≤T≤373K)from80to88mgpergramofzeolite.Indeed,MaesandCremers(12)havereportedthatanincreaseintemperaturefrom298to318KgaverisetoanevenmoremarkedincreaseinCoexchange;i.e.,thepercentageexchangeofindigenousNawasraisedfrom80to98%at[Cos]i=0.005moldm?3.Thetotalpositivechargeconcentrationsintheequilibriumsolutionsareplottedasafunc-tionoftheinitialHMconcentration([HMs]i)intheinsetto

DIVALENTCOBALTANDIRONIONEXCHANGE

129

TABLE1

EffectofTemperatureontheDegreeofSodiumExchange

byCobaltatFourRepresentative[Cos]iValues

Sodiumexchange(%)

[Cos]i(moldm?3)

291K373K0.0582900.04268790.02654660.01

40

51

Fig.1.ThesolutionchargeconcentrationfortheCo/Nasystemwasessentiallyindependentof[Cos]i,lyingwithinthenarrowrange0.0994–0.1003moleqdm?3.ThechargeconcentrationinthecaseofFe/Navariedfrom0.998to0.1088moleqdm?3butthereisnoclearcutdependenceon[Fes]i.Thepredominantexchangeprocessinbothcasesinvolvedadirectreplacementofmonovalentsodiumbydivalentcobaltoriron.TherewasnoevidenceofanyappreciableoverexchangeorimbibitionofthemetalhydroxideandwhilecompetingprotonicexchangeoccurstosomedegreeinthecaseoftheNa/Fesystem,solution-phaseFe2+exchangewithzeoliticNa+isessentiallystoichiometric.Exchangeselectivitycanbequanti?edintermsofthesepa-rationfactor,α,

α=

[HZz]e[Nas]e

[HZs]e[Naz]e

,

[3]

whichforthissystemisde?nedasthequotientoftheequilib-riumconcentrationratiosofiron/cobaltorsodiuminthezeoliteandinsolution.Ifaparticularenteringmetalcationispreferred,thevalueoftheseparationcoef?cientisgreaterthanunityandtheconverseholdsifsodiumisfavoredbythezeolite.Therela-tionshipbetweentheseparationfactorand[HMs]eisshowninFig.2,wherethehighaf?nityexhibitedbythezeolitephasefortheenteringHMions(α>1)isimmediatelyevidentforbothCoandFe,particularlyatlowerconcentrations.Inbothcases,thevalueofαdroppedwithincreasing[HMs]eandwhileFewasfavoredoverNaateveryconcentration,theCo/Nasystemischaracterizedbyaswitchinpreferencefortheindigenoussodiumat[Cos]e>0.034moldm?3.Theheavymetalremovalef?ciencycanbeconvenientlyquanti?edusingtheexpressionRemovalef?ciency(%)={[HMs]i?[HMs]e}/[HMs]i×100.

[4]Theremovalef?cienciesexhibitedbyNa–Yareplottedasafunctionoftheinitialsolution-phaseratiosofFeandCoconcentrationtozeoliteinFig.3andcomparedwithdatafromNi/Na–YandCu/Na–Yequilibriageneratedunderthesameconditions.Itisevidentthatremovalef?ciencydeclinedwithincreasing[HMs]iforeachmetalovertherangeofconcentra-tionsthatwasconsidered.Exchangeef?ciencydecreasedintheorderCu≥Fe>Co≥Ni,asequencethat?ndssupportin

pre-

FIG.2.Separationfactor(α)fortheremovalofiron(?)andcobalt(?)asafunctionoftheequilibriumheavymetalconcentrationinsolution([HMs]e).

viousreports(11,12)wheretheorderwasCu>Co>Ni.Inthehydratedzeolite,theionswiththelowerchargedensity,i.e.,presentinalesshydratedstate,interactmorestronglywiththealuminosilicateframework.Theobservedsequenceofin-creasingexchangeef?ciencycanbeconsideredtore?ectanincreasingeffectivenessinneutralizingthenegativechargeonthealuminosilicateframework.MaesandCremers(12)haveviewedtheneutralizationofthezeolitenetworkchargeintermsofcomplexformation.Thedirectcoordinationofthe

divalent

FIG.3.Removalef?ciencyasafunctionoftheinitialheavymetaltozeoliteratiofortheFe/Na–Y(?),Co/Na–Y(?),Cu/Na–Y(?),andNi/Na–Y(?)exchangesystems.

130

KIMANDKEANE

ionwiththeframeworkoxygenisequivalenttoaninnerspherecoordination,whiletheinterpositionofwatermoleculesgivesrisetoanouterspherecomplexwithrespecttothezeolitelat-tice.TheconcentrationofinnerspherecomplexesoftransitionmetalionsinrelatedinorganicsystemsincreasesintheorderNi<Mn<Co<Zn<Cu<Cd(12);thisincreaseinchargeneutralizationef?ciencyrunsparalleltotheexchangeef?cien-ciesrecordedinthisstudy.TernaryIonExchange

TheCo/Fe/Na–Yternaryexchangeisothermwasconstructedfrom20pairsofexperimentalpoints.The3totalchargeconcentra-tionwasmaintainedat0.1moleqdm?buttheindividualmetalconcentrationswerevariedfrom0.003to0.042moldm?3,wherethesorbatetosorbentratiowas100cm3g?1;[Fes]iwasmain-tainedinexcessof0.005moldm?3topreventanyironoxidation.Theironandcobaltremovalef?cienciesinrepresentativebinaryandternarysolutionsaregiveninTable2.Itisclearthatthepres-enceofoneHMsuppressestheuptakeoftheother;i.e.,theFeremovalef?ciencydecreasedastheCoconcentrationwasraisedandviceversa.Theinitialsolutionmetalcomposition,intermsofequivalentfractions,isillustratedinFig.4.Theequivalentfractionof(say)Co(II)inthesolutionisde?nedas

Cocs=

2Co

2cCoFe[4]

Na

TABLE2

SelectedFeandCoRemovalEf?cienciesfromFe/NaandCo/NaBinaryandFe/Co/NaTernarySystems:TotalSolutionChargeCon-centration=0.1moleqdm?3

(a)Effectof[Cos]ionFeremovalef?ciency

[Fes]i(moldm?3)

[Cos]i(moldm?3)

Feremovalef?ciency(%)

0.0380260.008220.017

0400.005360.011310.017300.03240.008

0510.008410.017360.025350.038

29

(b)Effectof[Fes]ionCoremovalef?ciency

[Cos]i(moldm?3)

[Fes]i(moldm?3)

Coremovalef?ciency(%)

0.008

0.008400.033210.038160.025

0.005230.008220.025

19

FIG.4.Initialsolution-phasecompositionsusedtogeneratetheCo/Fe/Naternaryexchangeisotherms.

andthecorrespondingfractioninthezeolitephaseis

Comz=

2Co

2mCoFeNa

,

[5]

wherecCoandmCoare,respectively,thecobaltmolarityinsolu-tion(moldm?3)andinthezeolitephase(molkg?1).Itisimmedi-atelyevident,fromavisualinspectionofFig.4,thatanadequatecoverageofthepossiblerangeofsolutioncompositionswasachieved.Thetotalchargeconcentrationintheequilibriumsolu-tionswaswithin0.1±3×10?3;i.e.,exchangewasessentiallystoichiometric.Ingeneral,selectivitytrendsinternaryexchangesystemscanbequanti?edusingternaryseparationfactorsthattaketheform(35,48,49)

=(Az)2(Bs)(Cs)B,ACα

(As)2(Bz)(Cz)

,[6]

whereA,B,andCrepresentthethreecomponentions.Appro-priatecombinationsoftheternaryseparationfactorsgeneratepseudo-binaryfactorsthatdescribetheselectivityofthezeoliteforoneionoveranotherinthepresenceofathird(35,48,49),

??

B,ACα

??1/3ABα

=

[7]

A,B

C

α.Thethreepseudo-binaryseparationsFeCoα,FeNaα,andCo

afunctionofNaNaαareplottedinFig.5asz.Thezeoliteexhibitedanequivalentorgreaterpreferenceforironovercobalt(FeCoα>1)ateachternaryisothermpoint(Fig.5a).Thepreferenceofthezeoliteforexchangewithironovercobaltundernoncompetitiveconditions,asillustratedinFig.2,extendstosolutionscontain-ingbothmetals.Indeed,theratio[Cos]e/[Fes]ewasgreater(byafactorofupto1.24)thanthecorresponding[Cos]i/[Fes]iandFeremovalwasconsistentlypreferredoverCoremovalinmixed