7 a novel conversion process for waste residue synthesis of zeolite kel 6

ContentslistsavailableatScienceDirect

Colloids

and

Surfaces

A:

Physicochemical

and

Engineering

Aspects

jo u r n al ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / c o l s u r f a

A

novel

conversion

process

for

waste

residue:

Synthesis

of

zeolite

from

electrolytic

manganese

residue

and

its

application

to

the

removal

of

heavy

metals

Changxin

Li,

Hong

Zhong

∗

,

Shuai

Wang

∗

,

Jianrong

Xue,

Zhenyu

Zhang

CollegeofChemistryandChemicalEngineering,CentralSouthUniversity,Changsha410083,China

h

i

g

h

l

i

g

h

t

s

•_{A novel conversionprocess forthe}

utilizationofEMRwasfirstproposed.

•_{EffectofSi/AlratioontheEMRZ}

syn-thesiswasinvestigated.

•_{EMRZshowedgoodadsorption}

prop-ertiesforremovalofMn2+_andNi2+

ions.

•_{The adsorption mechanism and}

kineticsweresystematicallystudied.

•_{Ion exchange was responsible for}

metalionsremoval.

g

r

a

p

h

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c

a

l

a

b

s

t

r

a

c

t

a

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t

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l

e

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n

f

o

Articlehistory:

Received1November2014

Receivedinrevisedform29January2015 Accepted2February2015

Availableonline9February2015

Keywords:

Electrolyticmanganeseresidue Zeolite

Heavymetal Removal Kinetics

a

b

s

t

r

a

c

t

Thezeolitematerialwasfirstsynthesizedfromelectrolyticmanganeseresidue(EMRZ),byatwo-step syntheticprocedureusingNaOHandNaAlO2withinashortagingperiod.Basedonthedetailed

analy-sesusingXRD,FT-IR,FE-SEM,XRF,CECandBETsurfaceareameasurement,theproductsynthesizedat Si/Alratio=1.5wasmainlycomposedofNa-Azeolitewithaspecificsurfaceareaof35.38m2_g−1_.Then,

theremovalcharacteristicsofMn2+_andNi2+_{ionsbyEMRZwereinvestigatedundervariousoperating}

variableslikecontacttime,solutionpHandinitialmetalconcentration.Theadsorptionequilibriumfor bothMn2+_andNi2+_{wasbestdescribedbytheLangmuirmodel,confirmingtheapplicabilityofmonolayer}

coverageofmetalionsontoEMRZparticles.ThemaximumsorptioncapacitiesforMn2+_andNi2+_shown

byEMRZwere66.93mgg−1_and128.70mgg−1_{,respectively.ItwasalsofoundthatadsorptionofMn}2+

andNi2+_{byEMRZfollowedsecond-orderkineticsandrateconstantsforMn}2+_andNi2+_sorptionwere

foundtobe0.001091and0.005668gmg−1_min−1_{,respectivelyat30}◦_{C.Theseresultsindicatethatthe}

synthesizedEMRZisapromisingandlow-costadsorbentforremovingheavymetalsfromwastewater duetohigheradsorptioncapacitythanotheradsorbents.

©2015ElsevierB.V.Allrightsreserved.

∗_{Correspondingauthors.Tel.:+8673188830654.}

E-mailaddresses:zhongh@csu.edu.cn(H.Zhong),wangshuai@csu.edu.cn (S.Wang).

1. Introduction

Rapidindustrializationhasledtoincreaseddisposalofheavy metalsintotheenvironment,causingserioussoilandwater pollu-tion[1].Meanwhile,heavymetalsarenotbiodegradableandtend

toaccumulatein livingorganisms causing variousdiseases and disorders[2,3].Moreover,heavymetalsareprioritytoxic pollut-antsthatseverelylimitthebeneficialuseofwaterfordomesticor industrialapplications.Thusconsiderableattentionhasbeenpaid tomethodsformetalremovalfromindustrialwastewatersasthe globalinterestsinthisissueincreaseoverthepastdecades[3–5].

To date, numerous processes exist for removing dissolved heavymetals,includingionexchange,precipitation, phytoextrac-tion,ultrafiltration,reverseosmosis,andelectrodialysis[6].Among them,ionexchangetechniquesusingsolidadsorbentshasbeena promisingmethodfortreatingwastewater,owingtoitsadvantages suchasoperationalsimplicity,lowcost,availabilityinlargeamount andabilitytotreatpollutantsinasufficientlylargescaleoperation

[6,7].Activatedcarbonadsorptionisconsideredtobeaparticularly competitiveandeffectiveprocessfortheremovalofheavymetals

[8];however,theuseofactivatedcarbonisnotsuitablein develop-ingcountriesduetothehighcostsassociatedwithproductionand regenerationofspentcarbon.Consequently,theuseofalternative low-costmaterialsaspotentialsorbentsfortheremovalofheavy metalshasbeenemphasizedrecently.

Electrolyticmanganeseresidue(EMR)isapotentiallyharmful industrialsolidwastethatcomesfromtheelectrolyticmanganese industryandhasrarelybeenrecycledinlargequantities[9].Inthe electrolyticmanganesemetal(EMM)industry,about6∼9tonsof EMRisdischargedintotheenvironmentpertonofproducedEMM

[10].ThecommonpracticeinChinaiscollectingtheEMRinopen sitesneartheplants.ItishighlyquestionableiftheEMRgenerated ismanagedproperly.BecauseEMRcontainssomeheavymetal ele-mentsandcompounds,theuntreateddischargecancauseserious pollutionofsurroundingsoilandreceivingwaterbodies[11,12]. Additionally,highvolumeEMRresultingfromlargescale indus-trialactivitieshavelongbeenconsideredtobeaburden,dueto thehighcostsfortheirassociatedpost-treatment,storageand dis-posal[10,11].Owingtoitsmineralcomposition, EMR hasbeen mostlyrecycledashydrauliccement,concreteaggregate,roadbase, brickmakingmaterialsincivilengineeringwork,aswellasforthe productionofchemicalfertilizersinagriculturalwork[13–16].Yet, problems associatedwith abovedisposal,such asthepotential environmentalimpactsandthesevereenvironmentalregulations, maketherecyclingofslagdifficult.Itis,therefore,essentialto con-tinuouslydevelopnewandadvancedrecyclingprocessesofEMR.

Recently, ourresearchgroup foundthat zeolite can be syn-thesizedusingEMRas SiandAlsources.Zeolitesareknownas ion-exchangematerials where theindigenous charge-balancing cations(typicallysodiumandcalcium)arenotrigidlyfixedtothe hydratedaluminosilicate framework and arereadily exchanged withmetalcationsinsolution[6,17].Meanwhile,thefactthat zeo-liteexchangeableionsarerelativelyinnocuous(sodium,calcium, andpotassiumions)makesthemparticularlysuitableforremoving undesirableheavymetalionsfromwastewaters[17,18].Moreover, zeolitescantransferaheavymetalcontaminationproblemofmany thousandsofliterstoafewkilosofeasilyhandledsolid.Thetoxic metalsarefirmlyheldinthecrystalstructureanddonotleach, how-everforultimateenvironmentalprotectionthesolidzeolitecan becementstabilizedorvitrified[2].Giventheseadvantages, zeo-lites(includingnatural,commercial,andchemicallysynthesized zeolites)havebeenintensivelystudiedrecentlyfortheremovalof heavymetals[17–19].Accordingly,makinguseofEMRZtoremove heavymetalscannotonlyreducethecost,butalsocomplywith therequirementofgreeneconomy.

Theobjectiveofthepresentstudywastoproposeanovel con-versionprocessofEMRtozeoliteinordertomakefulluseofthe eco-nomicpotentialofEMR,andtofindoutitspotentialuseaslow-cost adsorbentfortheremovalofheavymetalsfromaqueoussolution. Mn2+_andNi2+_{ionswerechosenasatargetcontaminantto} charac-terizetheadsorptivepropertiesofthesynthesizedzeolite.Forthe

presentstudy,theuptakeofheavymetalsonEMRZwasevaluated asafunctionofinitialmetalionsconcentration,pH,andcontact time.Further,modelstofittheadsorptionequilibriumandkinetic datawerepresentedtounderstandtheadsorptionmechanism.

2. Materialsandmethods

2.1. Materials

TheEMR investigatedinthis studywasmainlygeneratedin leachingprocessofthemanganeseore(withpartofresidue gen-eratedin electrolytic cellprocess)atan electrolytic manganese company,situatedinwesternofHunanprovince,China.The chem-icalcompositionofEMRusedinthisstudyislistedinTable1.This chemicalcompositionisfairlycommoninEMRproducedinChina’s EMMindustry.

Stock solutions of Mn2+ _{and Ni}2+ _{ions with concentration} at 1000mgL−1 _{were prepared by dissolving given amounts of} MnSO4·H₂OandNi(NO₃)₂·6H₂O,respectively,indistilled water.

Thesolutionsofdifferentconcentrationsusedinvarious experi-mentswereobtainedbydilutionofthestocksolutions.

2.2. Zeolitesynthesis

ThefabricationprocessofEMRZisshowninFig.1.Basedon ourpreviousrelatedstudies[20],afusionmethod,involving alka-linefusionfollowedbyhydrothermaltreatment,wasadoptedfor thesynthesisofzeoliteinthisresearch.Duringthehydrothermal reactionprocess,variousamountsofsodiumaluminate(NaAlO2) wereaddedintothereactionsystemtoinvestigatetheeffectofthe ratioofSi/Al.Attheendoftheprocessthesolidphasewas col-lectedbyfiltration,washedseveraltimeswithdistilledwater,and thendriedat100◦_{C.Theobtainedsampleswerelabeledbasedon} theirinitialSi/Alratios.Namely,thesampleswithinitialSi/Alratio of1.5,2.0and2.5werelabeledasEMRZ15,EMRZ20andEMRZ25, respectively.

2.3. Adsorptionstudies

Theadsorptionofheavymetalswasperformedbyshaking0.3g ofadsorbentswitha100mLsolutionofknownsolute(Mn2+_and Ni2+_{ions)concentrationrangingfrom100to600mgL}−1_at200rpm inaconstanttemperatureshakerbath.Aportionofthesolution wascollectedatpredeterminedtimeintervalsforkineticsandat equilibrium time forisotherms. Theresidual concentrationwas centrifugedat5000rpmfor2minandthendeterminedin tripli-cateusingThermoSOLAARatomicabsorptionspectrophotometers (Thermo Electron,USA). Theequilibrium adsorptioncapacity qe (mgg−1_{)wascalculatedusingthefollowingequation.}

qe= (C0−Ce)V

m (1)

whereC0andCe(mgL−1)aretheconcentrationofthetestsolution attheinitialstageandunderequilibriumconditions,respectively. V(L)isthevolumeofthetestsolutionandm(g)isthemassofthe adsorbentused.

Todeterminethe effectof experimentalparameterssuchas pHandcontacttimeonadsorptionprocess,thismethodwasalso appliedbyvaryingthecorrespondingcondition.ThepHofsolution investigatedwaschangedbetween1.0and6.0using0.5mol/LHCl solutions.

2.4. Characterization

Table1

MajorchemicalcompositioninEMR.

Constituent Na2O SiO2 Al2O3 MgO K2O CaO MnO Fe2O3 TiO2

Mass(wt%) 2.7 24.6 12.2 1.7 2.4 8.6 4.6 7.9 0.4

werecarriedout usinganXRFspectroscopy(Axios,PANalytical, Holland).Themorphologyandparticlesizeofthecrystalswere observedbyfieldemissionscanningelectronmicroscopy(FE-SEM) (Mira3,Tescan, CzechRepublic). Thefurrier transforminfra-red spectra (FT-IR) were recorded on a Thermo Scientific Nicolet 6700FT-IRspectrophotometer.Thespecificsurfacearea(SSA)was determinedbythenitrogenadsorptionmethod.Nitrogensorption experimentscarriedoutat77KusingASAP2020(Micromeritics, USA).TheBETequationwasappliedtodeterminethespecific sur-facearea.Cationexchangecapacity(CEC)ofsynthesizedzeolitewas measuredaccordingtothestudyofZhangetal.[21]andtheCEC wascalculatedanddenotedasmeqpergramofadsorbents.The pHvalueofsolutionswasmeasuredbyapHmeter(pHS-3C,Leici, China)withacombinationelectrode(E-201-C,Leici,China).

3. Resultsanddiscussion

3.1. SyntheticprocedureofEMRZanditscharacterization

3.1.1. Mineralogicalcomposition

Inpresentstudy,effectofSi/Alratiosonzeolitesynthesiswas investigatedbychangingtheSi/Alratioofstartingmixturefrom 1.5to2.5inthethreesynthesizedsamplesofEMRZ15∼EMRZ25.

Fig.2showstheXRDpatternsofsynthesizedzeolites,togetherwith leachingresidueasasilicasourceofzeolites.AsshowninFig.2, theXRD patterns of synthesized samplesof EMRZ15∼EMRZ25,

exhibitseveralsharpdiffractionpeaks,whicharedifferentfrom thosepresentintheuntreatedleachingresidue.AlthoughEMRZ15 showedadetectablediffractionpeakforzeoliteP(seeFig.2),no otherzeoliticphases,quartzorhydroxysodalitewithlow poros-itywereidentified.Thatistosay,thezeolitesynthesizedatSi/Al ratio=1.5,namelyEMRZ15,wasmainlycomposedofNa-Azeolite. Generally,zeolitewithlowerSi/Alratio,i.e.largerAl2O3content, showshigherCECandadsorptioncapacityduetothepresenceof exchangeablecations,whicharetrappedatthetetrahedral alu-minumsitesforchargecompensation.Forthisreason,zeoliteA, whoseSi/Alratioisinherentlyquitelow, hasbeenwidelyused asusefulcation-exchangersandadsorbents[22–24].Ontheother hand,theEMRZ15synthesized fromleaching residue exhibited remarkablediffractionpeaksattributabletoA-typezeolite,proving thattheleachingresidueisavailableenoughasasilicasourceon

zeolitiesynthesis.Thesepatternsfurtherindicatedthatthe crys-tallinityandphase purityof synthesizedsamplesaredecreased withincreasingSi/Alratiofrom1.5to2.5.

3.1.2. Chemicalcomposition

Meanwhile,adetailedanalysisofchemicalcompositionofEMRZ sampleswasperformedbyXRFmeasurementandtheresultsare summarizedinTable2.AsshowninTable2,smallamountsof impu-rityelements,suchasMg,FeandMn,werecommonlydetectedin thezeolitessynthesizedfromtheleachingresidue.Asnodetectable peaksattributabletoFe,MgandMncompoundswereobservedin theXRDpatterns,itisspeculatedthatthoseminormetalionsare uniformlydispersedwithinthestructureofEMRZbyreplacingthe Na+_{sites[23,25].Meanwhile,theinitialSi/Alratiosof1.5,2.0and} 2.5affordedEMRZ15withSi/Al=1.01,EMRZ20withSi/Al=1.09and EMRZ25withSi/Al=1.17respectively.Itisintriguingtonotethat theSi/Alratioofthefinalproductislessthanthatofthestarting reactionmixture.Andtheexplanationisdisplayedasbelow.Inthe experimentalconditionsadoptedinthiswork,thezeoliteAand PwithlowSi/Alratiowassynthesizedfromleachingresidue(see

Fig.2).Hence,thelowerSi/Alratioofthefinalproductcompared tothestartingreactionmixturewasduetotheformationofAlrich zeolite(zeoliteAorP).Namely,afterthenucleationofzeoliteAorP, AlspecieswereconsumedmorerapidlythanSiinthesolution[26]. ItiswellknownthatAlisthecontrollingspeciesinthesynthesisof zeolites.AndalmostallAlwasdepletedduringthesynthesisof zeo-lites,however,theextrawater-solublesilicatesextractedfromthe startingreactionmixtureswassweptawaybydistilledwaterinthe zeolitesynthesisprocess[27].Moreover,increasingofNa2O con-tentofthefinalproductupto18.7%(takingEMRZ15forexample), incomparisontotheleachingresidue,whichwasonly0.1%,isavery clearandpromisingevidenceofCECimprovementinthefinal prod-uct.TheremarkablechangesinNa2Ocontentcanbeconsidered anotherevidenceofsuccessfulconversion[28].

3.1.3. ResultsofBETsurfaceareameasurements

BETsurfaceareameasurementwasadoptedtodeterminethe SSAofthesynthesizedEMRZ.AndtheresultswereshowninTable3. AsshowninTable3,theSSAofEMRZ15∼EMRZ25wereestimated

tobe35.38, 28.64and 24.13m2_g−1_{,respectively. Althoughthe} zeolitizationprocessledtoanincreasein SSAandporevolume

Fig.2.XRDpatternsofsamplesEMRZ15,EMRZ20,EMRZ25andleachingresidue.

Table2

Theresultsofelementalanalysisofzeolitessynthesizedfromleachingresidue.

Sample Si/Alratio(startingreactionmixture) Chemicalcomposition/wt% Si/Alratio(finalproduct)

SiO2 Al2O3 Na2O Fe2O3 MgO MnO

EMRZ15 1.5 35.1 29.5 18.7 0.76 0.24 0.13 1.01

EMRZ20 2.0 35.4 27.5 16.8 0.87 0.27 0.15 1.09

EMRZ25 2.5 35.5 25.7 15.1 0.92 0.25 0.17 1.17

Leachingresidue 3.6 66.4 15.8 0.10 1.1 0.7 0.3 3.60

overthestartingmaterial,particularlyforEMRZ15obtainedbythe fusionmethod,theobtainedvalues ofSSAofEMRZ weremuch lowerthanthoseofcommercialortraditionalzeolitereportedin literaturesasexpected.However,theefficientutilizationofEMRas ingredientsofothermaterialsmaybeadvantageousinreductionof notonlywastegenerationbutalsomanufacturingcosts.Andlow SSAofthezeolitesynthesizedfromEMRmaybeattributedtothe lowcrystallinityofthesamples[29].Ontheotherhand,in partic-ular,thecationsor/andimpuritiesintheleachingresidue(e.g.Mg, Mn,Fe,etc.)ledtoalowersurfaceareathanalkalimetals(K,Li)did. Theseresultsmaycorrelatewithporefillingbylargecationsor par-tialfusionofthecationcompoundsduringthethermaltreatment

[30,31].

3.1.4. Resultsofcationexchangecapacity(CEC)

In order to evaluatethe ion exchange capability of the as-synthesizedzeolitesamples,CECsofthezeolitesweremeasured. InTable4,theCECsofthepreparedzeolitesarecomparedwiththe

Table3

Theresultofstructuralparametersofzeolitessynthesizedfromleachingresidue.

Sample Structuralparameters

SSA/m2_g−1 _V

BET/cm3g−1

EMRZ15 35.38 0.23

EMRZ20 28.64 0.21

EMRZ25 24.13 0.18

Leachingresidue 0.28 0.023

leachingresidue,andsomeotherrelevantzeolitesintheliteratures

[21,32–35].Theresultsobtainedrevealedthattheapplied zeoliti-zationprocessincreasedtheCECoftheEMRZ15∼EMRZ25to2.15,

1.84and1.38meq/g,respectively.Incomparisontozeolitereported intheliteratures,EMRZ15hadahigherCEC.Thismaybecausedby factthatlowSi/AlratioandhighphasepurityofEMRZ15is syn-thesizedonthebasisoffusion method.Moreover,thehighCEC valueofEMRZ15wasanevidenceofhighpotentialforremoving ammoniumandheavymetalsfromaqueoussolutions[32–35].

Therefore,giventheaboveanalysisincludingXRD,XRF,BETand CECs,aprecisecontrolofthesynthesisSi/Alratiointhisprocess drasticallyaltersthecomposition,typeandpropertyofthefinal product,andthatthemostsuitableSi/Alratioforthesynthesisof EMRZfromleachingresidueis1.5.Therefore,thezeolite synthe-sizedatSi/Alratio=1.5wasemployedintheheavymetalremoval processasshowninSection3.2.

3.1.5. FT-IRspectrometryanalysis

Furriertransforminfra-redspectroscopy(FT-IR)isapowerful technique for structural characterization of organic and inor-ganicmaterials.Hence,FT-IRanalysiswasdeterminedforsample EMRZ15,asbeingthebestsampleofthisresearchshowninFig.3. TheIRbandat870cm−1_{,whichcanbeassignedtoT–OHbondin} theamorphousprecursorofzeoliteA,wasnotobservedinFig.3

Table4

Cationexchangecapacity(CEC)oftheas-synthesizedzeolitesincomparisontosomerelatedtypicalzeolites.

Adsorbent Rawmaterial CEC/(meq/g) Reference

EMRZ15 LeachingresiduefromEMR 2.75 Thiswork

EMRZ20 LeachingresiduefromEMR 2.04 Thiswork

EMRZ25 LeachingresiduefromEMR 1.38 Thiswork

Leachingresidue EMR 0.015 Thiswork

ZeoliteX Coalflyash 2.79 [21]

ZeoliteA(Na) Coalflyash 0.87∼1.05 [32]

Na-P1 Coalflyash 0.99∼1.31 [33]

NaturalTurkishclinoptilolite Naturalzeolite 0.95∼1.40 [34]

NaturalChinese(Chende)zeolite Naturalzeolite 0.82 [35]

400 800 1200 1600 2000 2400 2800 3200 3600 4000 0 20 40 60 80 100

Tran

smi

tance(%

)

Wave

numbers

/cm

-1

34

20

1649

1001

6

64

556 462

Fig.3.FT-IRspectraofEMRZ15inthewavenumberrangeof400∼4000cm−1_.

theamorphousmaterialwastransformedintocrystallinezeoliteA. Thebandat556cm−1_{isrelatedtothepresenceofthedoublering} (D4R)thatischaracteristicinthezeoliteA[37,38].TheIRbandsat 462and664cm−1_{areassignedtotheinternallinkagevibrationsof} theTO4tetrahedraandtotheasymmetricstretching,respectively, ofzeoliteA.Thebandat1640cm−1_{hasbeenassociatedwiththe} characteristicH–Obendingmodeofwatermolecules.Moreover, theobservedsinglestrongbandat3420cm−1_{wasascribedtothe} presenceofhydroxylsinthesodalitecagesasthebuildingblocks ofzeoliteA.Ultimately,theintensitiesoftheseadsorptionbands areproportionatetopurityofsamplesandareinagreementwith theinterpretationofobtainedXRDresults.

3.1.6. SEManalysis

ThemorphologyandparticlesizeofEMRZsynthesizedunder optimalconditions(atSi/Alratio1.5)wereobservedbyFE-SEM (Fig.4).Thecubicparticleswithachamferedshape,alongwitha smallfractionofroundcrystalswereobservedintheFig.4a.And thecubicandroundcrystalscorrespondedtozeoliteAandzeoliteP, respectively[21,36],whichalsoconfirmedbytheXRDdatainFig.2. Meanwhile,thecrystalmorphologyofEMRZ15withanaveragesize of1␮mapproximatelywasmoreclearlyshowninFig.4b.Besides, itisworthtonotethatnoaggregateswereobservedintheSEM pictureswhichindicatethatthisshapetransformationcanprovide largesurfaceareawithtinypores.

3.2. RemovalofheavymetalbyEMRZmaterial

3.2.1. Effectofcontacttime

Beforeperformingthebatchuptakeequilibriumexperiments,it wasnecessarytodeterminethecontacttimerequiredfor adsorp-tion equilibrium. The adsorption of Mn2+ _{and Ni}2+ _{onto EMRZ} synthesizedunderoptimalconditions,namelyEMRZ15,asa func-tionofcontacttimewasstudiedat30◦_{Cbyvaryingthecontact} timefrom20to300minwhilekeepingallotherparameters con-stant.TheresultsareshowninFig.5.FromtheFig.5,itisshownthat theadsorptionofthestudiedheavymetalionsisincreasedfirstly andthenkeptincreasinggraduallyuntiltheequilibriumisreached andremainedconstant.Namely,theremovalratesforbothcations wereobviouslyfasteratinitialstage.Thefastremovalrateatthe initialstagewasduetothefactthat,initially,alladsorbentsites werevacantandthesoluteconcentrationgradientwashigh[21]. OverEMRZ15,Ni2+_{adsorptionimmediatelytookplacewithinthe} first60min,andthenreachedadsorptionequilibrium, demonstrat-ingthattheEMRZ15hasastrongabilityforbindingNi2+_cations. UnlikethecaseofNi2+_{adsorption,EMRZ15stillexhibited} mod-estincreasesintheMn2+_{uptakeevenafter60min,suggestinga}

300 250 200 150 100 50 0 30 40 50 60 70 80 90 100

Mn Ni

q(

m

g

·g

t

-1 )

Contact time (min)

Fig. 5.Effect of contact time on heavy metal adsorption(initial metal ions concentration=300mgL−1_{;adsorbentdose=0.3g/100mL;pHvalue=6.0;}

temper-ature=30◦_C).

sloweradsorptionratethanthatforNi2+_{adsorption.Thiscouldbe} attributedtothelargerionicradiusofMn2+_{thatwillpasswith} dif-ficultythroughthechannelsforzeolite[2,39].And180minafter initialreaction,theuptakeamountofMn2+_{becamealmoststable} at30◦_{C.Thus,acontacttimeof180minwasfoundtobesufficient} forallmetalstudiedtoachieveuptakeequilibrium,andthecontact timewasfixedat180minforthefollowingbatchexperiments.

3.2.2. Effectofinitialconcentration

Theeffectoftheinitialconcentrationontheretentionofmetal ions,atconcentrationlevelsrangingfrom100to600mgL−1_,was studiedat30◦_{Cwhilekeepingallotherparametersconstant.The} resultsareshowninFig.6.AscanbeseenfromFig.6,itisevident thattheadsorptioncapacityincreasedwithanincreaseinthe ini-tialconcentration.Theincreasesofheavymetaladsorptionwith anincreasinginitialconcentrationhavebeenwidelyobservedin manystudies[3,40].Andthefigurealsoshowsthatthe adsorp-tionoftheheavymetalionsatdifferentconcentrationsisincreased firstlyandthenkeptincreasinggraduallyuntiltheequilibriumis reachedandremainedconstant.Thisbehaviorcanbeexplainedby

thefactthatunderhigherinitialconcentration,moremetalions areleftun-adsorbedinsolutionduetothesaturationofbinding sites[40].Theseresultsindicatethatenergeticallylessfavorable sitesbecomeinvolvedwhentheconcentrationofmetalin solu-tionincreases[8,40,41].Theheavymetaluptakeisattributedto differentmechanismsofion-exchangeprocessesaswellastothe adsorptionprocess.Inthiscase,ICPanalysesofthetestsolution recoveredaftertheequilibriumadsorptionrevealedthatthe leach-ingamountofNa+_{waspredominatelyenhancedinthatcase.This} resultunambiguouslyindicatesthattheuptakeofmetalions pro-ceedsvia anion-exchangemechanism, i.e.,theweaklybonded, network-modifyingNa+_{ionsarereleasedtothesolutionsoasto} replacewithheavymetalions.Duringtheionexchangeprocess, diffusionwasfasterthroughtheporesandwasretardedwhenthe ionsmovedthroughthesmallerdiameterchannels.

3.2.3. EffectofpH

The pHof theaqueoussolution is animportant operational parameterintheadsorptionprocessbecauseitaffectsthesolubility ofthemetalions,concentrationofthecounterionsonthe func-tionalgroupsoftheadsorbentandthedegreeofionizationofthe adsorbateduringreaction[40,42].Asweknow,atpHvaluehigher than6.0,mostoftheheavymetalionstendtoformprecipitationas hydroxides.Andthe“true”adsorptioncapacityofEMRZ15couldbe maskedbyprecipitation[43].Thus,theadsorptionofheavymetal ontoEMRZ15atpHvaluerangingfrom1.0to6.0wasstudiedas showninFig.7.FromFig.7,anincreaseinpHcorrespondstoan increasein adsorption,reachingthemaximumcapacityata pH ofabout6.0.TheloweradsorptionofheavymetalatacidicpHis probablyduetothefollowingreasons.AtlowerpHvalues,where theconcentrationofH+ _{ishigh,thecompetitionbythenegative} sitesonthezeolitesurfaceisenhancedandthemetalsorptionis reducedaccordingly[40,44].Inaddition,zeolitecrystalsbeginto collapseordissolvewithdecreasingpHinaqueoussolutions, par-ticularlywhenthepHisbelow4.0[21].Meanwhile,theseresults areinagreementwiththeresultspreviouslyreported[40,43,44].

3.2.4. Adsorptionkinetics

Thekineticsofsorption ofmetalionsbyEMRZ15was stud-ied for its possible importance in treatment of metal-bearing industrialeffluents.Inthepresentstudy,pseudo-first-orderand

600 500

400 300

200 100

20 40 60 80 100 120 140

Mn Ni

q(

m

g

·g

t

-1

)

Initi

al metal ions concen

trati

on / (mg·L

-1

)

6 5 4 3 2 1 20 30 40 50 60 70 80 90 100

Mn Ni

q(

m

g

·g

t

-1 )

pH

Fig.7.Effectof pHvalueonheavy metaladsorption(initialmetalions con-centration=300mgL−1_{; contact time=180min; adsorbent dose=0.3g/100mL;}

temperature=30◦_C).

pseudo-second-orderkineticmodelswereemployedtotestthe experimental data [4,21]. The pseudo-first-order and pseudo-second-ordermodelsarerespectivelydescribedinthefollowing equations:

ln(qe−qt)=lnqe−k₁t (2)

t qt

= 1

k2q2e

+ t

qe

(3)

whereqe and qt aretheamountsofadsorbate adsorbedonthe adsorbents(mgg−1_{)atequilibriumandattimet,respectively.k}

1 (min−1_)andk

2 (gmg−1min−1)aretherateconstantsof pseudo-first-orderandpseudo-second-ordermodels,respectively.

Thekineticdatawerelinearizedusingthepseudo-first-order andpseudo-second-ordermodels,andplottedbetweenln(qe−qt) versustandt/qtversust,respectively.Theconstantswere calcu-latedfromtheslopeandtheinterceptoftheplots,andaregivenin

Table5andFig.8.TheresultsgiveninTable5showthat,forboth Mn2+_andNi2+_,theR2_values(R2_{=0.8616and0.2901)for} pseudo-firstordermodelweremuchlowerthanthoseobtainedusingthe pseudo-second-ordermodel(R2_{=0.9979and0.9998).Besides,the} qevaluepredictedfromthesecond-ordermodelismuchmore com-parabletotheexperimentalqevaluethanthatfromthefirst-order model.Thus,thepseudo-second-ordermodelexplainsthekinetic processesbetter.

3.2.5. Adsorptionisotherm

Inthiswork,twoofthemostcommonlyusedequilibrium mod-els,theLangmuirandFreundlichisotherms,werefittedwiththe experimentaldatatofindthemostsuitablemodel[5,21].

Langmuir’s isotherm model suggests that uptake occurs on homogeneoussurfacebymonolayersorptionwithoutinteraction betweensorbedmolecules.ThelinearformofLangmuir’sisotherm modelisgivenbythefollowingequation:

Ce qe

= 1

q0KL

+Ce

q0

(4)

whereKListheadsorptionequilibriumconstant(Lmg−1),q0isthe maximummonolayeradsorptioncapacity,andqe istheamount adsorbedonaunitmassoftheadsorbent(mgg−1_)whenthe equi-libriumconcentrationisCe (mgL−1).Theisothermexperimental data,whichwerecollectedatdifferentinitialmetalion concentra-tions,wereplotted betweenCe/qe versusCe (Fig.9a).Asseenin

Fig.9a,alinearplotisobtainedwhenCe/qe isplottedagainstCe overtheentireconcentrationrangeofmetalionsinvestigated.The Langmuirmodelparametersandthestatisticalfitsofthesorption datatothisequationaregiveninTable6.Regressionvalues(R2₎ presentedinTable6indicatedthattheadsorptiondataforheavy metalionsremovalfittedwelltheLangmuirisotherm.Meanwhile, theextremelyhighvalueofcorrelationcoefficient(R2_>0.99)forthe Langmuirisothermpredictsthemonolayercoverageofmetalions onEMRZ15particles.Ascanbeseenfromtheq0results(Table2), Ni2+_{adsorptionwashigherthanMn}2+_{adsorptionforEMRZ15.The} adsorptioncapacityofzeolitesgenerallydependson:(1)thecharge densityofcations,(2)theexchangeablesitesandporesizeonthe zeoliteframework,(3)theequilibriumtemperature[45–47].Since theionexchangewasconductedatthesameconditionforeach heavymetalspecies,thedifferentadsorptioncapacityofcations wasascribedtothedifferentchargedensityofcations.Thecharges ofthemetalcationarethesame(+2);thereforechargedensityfor Ni2+_{ionsishigherthanforthatofMn}2+_,sinceNi2+_{haslowerionic} radiusthanthatofMn2+_{.ThisdifferenceleadsNi}2+_{tobeattracted} tothesurfaceofzeolitemorestronglythanMn2+_[46,47].

OneoftheessentialcharacteristicsoftheLangmuirequation couldbeexpressedintermsofadimensionlessseparationfactor (RL).AndRLisdefinedasfollows[48]:

RL= 1

(1+KLC0)

(5)

whereC0isthehighestinitialsoluteconcentration(mgL−1),KLis theLangmuir’sadsorptionconstant(Lmg−1_).TheR

Lvalueimplies theadsorptiontobeunfavorable(RL>1),linear(RL=1),favorable (0<RL<1)orirreversible(RL=0).ForC0from50to250mgL−1used inthepresentstudy,thevaluesofRLrangefrom0.036to0.29for Mn2+_{and0.023to0.11forNi}2+_{.TheadsorptionofMn}2+_andNi2+ ontoEMRZ15canthusbeconsideredfavorable.Inotherwords,the preparedEMRZ15isasuitablesorbentforbothMn2+_andNi2+_.

Meanwhile,theresultsofmetalionsorptionontoEMRZ15were alsoanalyzedusingtheFreundlichmodeltoevaluateparameters associatedtothesorptionbehavior.TheFreundlichisothermisan empiricalequationemployedtodescribeheterogeneoussystems. ThelinearformofFreundlich’sisothermmodelisexpressedbythe followingequation:

ln(qe)=1

nln(Ce)+ln(KF) (6)

where KF ((mgg−1)(Lg−1)n) and 1/n are Freundlich constants relatedtosorptioncapacityandsorptionintensityofadsorbents. CeandqearesametothoseintheLangmuirisothermmodel.

FreundlichisothermalplotsarepresentedinFig.9b,andthe FreundlichconstantsarealsolistedinTable6.Ascanbeseenin

Fig.9bandTable6,theFreundlichmodeldoesnotfitthedataofboth Mn2+_andNi2+_{,asindicatedbythevaluesofR}2_{of0.8293and0.7583} forMn2+_andNi2+_{,respectively.Namely,theLangmuirisotherm}

Table5

Theconstantsandcorrelationcoefficientsofpseudo-firstorderandpseudo-secondorderkineticmodelsforadsorptionofheavymetalontoEMRZ15.

Metal C0/mgL−1 qe(exp)a/mgg−1 Pseudo-first-ordermodel Pseudo-second-ordermodel

qea/mgg−1 k1/min−1 R2 qea/mgg−1 k2/gmg−1min−1 R2

Mn2+ _{300 55.03 64.23 0.03293 0.8616 58.62 0.001091 0.9979}

Ni2+ _{300 92.91 0.7213 0.01966 0.2901 93.73 0.005668 0.9998}

a_qe(exp)andq

300 250 200 150 100 50 0 -5 -4 -3 -2 -1 0 1 2 3

4

(a) Pseudo-f

irst-ord

er model

Mn Ni

ln

(q

e-q

t)

t (min)

300 250 200 150 100 50 0

0 1 2 3 4 5

6

(b) Ps

eudo-s

econ

d-o

rder

mode

l

Mn Ni

t/

qt

t (min)

Fig.8.PlotsofkineticsofheavymetalremovalbytheEMRZ15(initialmetalionsconcentration=300mgL−1_{;adsorbentdose=0.3g/100mL;pHvalue=6.0;}

tempera-ture=30◦ C).

500 400

300 200

100 0

0 1 2 3 4 5 6 7 8

(a)

Mn Ni

Ce /qe

(g·

L

-1 )

C_e (mg·L-1)

6 5

4 3

2 1

3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4 4.6 4.8

(b)

Mn Ni

ln

qe

lnC_e

Fig.9. (a)Langmuirand(b)FreundlichisothermadsorptionmodeloftheMn2+_orNi2+_{ontoEMRZ15(adsorbentdose=0.3g/100mL;contacttime=180min;pHvalue=6.0;}

temperature=30◦ C).

modelfitstheexperimentaldatabettercomparedtothe Freund-lichmodel.Consequently,thesorptionofmetalionsonEMRZ15 followstheLangmuirisothermmodelwheretheuptakeoccurson homogeneoussurfacebymonolayersorptionwithoutinteraction betweensorbedions[48].

3.3. Comparisonbetweenourresultsandrelatedliterature

Table7liststhecomparisonofmaximumadsorptioncapacity ofMn2+_{and Ni}2+_{onvariousadsorbents.For Mn}2+_,theEMRZ15

hasagreatercapacitythannaturalclinoptilolite[8],natural zeo-lite[49],vermiculite[50],andNa-montmorillonite[51].ForNi2+_, theadsorptioncapacityofEMRZismuchhigherthanother low-cost adsorbents such as spirodela intermedia, Brazilian natural scolecite,pretreatedclinoptilolite,naturalbentonite,vermiculite and Na-montmorillonite [5,7,44,50–52]. Thus, it is evident that EMRZ15showsgreat potentialasanadsorbent formeatal ions. Besides, the adsorption of Mn2+ _{and Ni}2+ _{onto standard} zeo-lites (zeoliteA,Yand P)wasconductedin order toinvestigate which component is responsible for the superior performance

Table6

ParametersofadsorptionisothermsofheavymetalontoEMRZ15at30◦ C.

Metal pH Langmuirisotherm Freundlichisotherm

KL(Lmg−1) q0(mgg−1) R2 KF(mgg−1)(Lg−1)n 1/n R2

Mn2+ _{6.0 0.02373 66.93 0.9942 8.421 0.3468 0.8293}

Table7

Comparisonofmaximumadsorptioncapacityofmetalionsonvariousadsorbents.

Adsorbent q0(mgg−1) Reference

Ni2+ _{EMRZ15 128.70 Thiswork}

Spirodelaintermedia 122.10 [5]

Pretreatedclinoptilolite 15.55 [7]

Braziliannaturalscolecite 3.52 [44]

Vermiculite 25.33 [50]

Na-montmorillonite 3.63 [51]

Naturalbentonite 4.27 [52]

Mn2+ _{EMRZ15 66.93 Thiswork}

Naturalclinoptilolite 4.22 [8]

NaturalTurkishzeolite 2.42 [49]

Vermiculite 26.78 [50]

Na-montmorillonite 3.22 [51]

oftheEMRZ15,sincethesynthesizedEMRZ15wasamixtureof zeolites(seeFig.2).Andthetestsweremadeunderfollowing con-ditions:initialmetal ionsconcentration=300mgL−1_{; adsorbent} dose=0.3g/100mL; pH value=6.0; temperature=30◦_{C; contact} time=180min.ItwasfoundthattheuptakecapacityforNi2+_and Mn2+_{bycommerciallyavailablezeoliteA,PandYweredetermined} tobe108.57,80.86,56.62mgg−1 _{and65.59,48.26,35.42mgg}−1_, respectively.Hence,comparedtotheadsorptioncapacityofMn2+ andNi2+_{byEMRZ15underthesamecondition(seeFig.5),itcould} bestatedthatthezeoliteAinEMRZ15playsthemainroleinthe Mn2+_{and Ni}2+ _{uptake.This statementis alsosupportedbythe} resultsfromXRDanalysis(Fig.2)showingthatNa-Azeolitewas themajorconstituentofEMRZ15.

4. Conclusions

Inthepresentstudy,wehavefirstreportedonthesynthesisof zeoliteutilizingEMRasanabundantandcheapchemicalsource fromtheviewpointofthechemicalandeconomicaluseofEMR. AnalysesbymeansofXRD,FT-IR,FE-SEM,XRF,CECandBET sur-faceareameasurementindicatedthatthezeolitesynthesizedat Si/Alratio=1.5,namelyEMRZ15,wasmainlycomposedofNa-A zeoliteandthezeolitematerialhasspecificpropertiessuchashigh purityand CEC.Theadsorptionof Mn2+_{and Ni}2+_wasfoundto bedependentoninitialconcentration,pHandcontacttime.The adsorbedamountofMn2+_andNi2+_{bothincreasedwith} increas-inginitialconcentration,pHandcontacttime.Langmuirisotherm displayedabetterfittingmodelthanFreundlichisotherm,thus, indicatingtotheapplicabilityofmonolayercoverageofmetalions onthesurfaceofadsorbent.Thekineticprocesscanbepredictedby pseudo-second-ordermodelandrateconstantsforMn2+_andNi2+ sorptionwerefoundtobe0.001091and0.005668gmg−1_min−1_, respectively at30◦_{C. Moreover,themaximum sorption} capaci-tiesforMn2+_andNi2+_{shownbyEMRZ15were66.93mgg}−1 _and 128.70mgg−1_{,respectively. Theseresultsshow thatEMRZ15 is} a highlyefficient adsorbent formetal ionsremoval from aque-ous solution, which exhibits much higher adsorption capacity thanotherlow-costadsorbents.Thisconversionprocess, which enablesus to fabricate valuable zeolite materials from EMR at lowcostandthroughconvenientpreparationsteps,issurely ben-eficialfrom theviewpoint of thechemical and economicaluse ofEMR.

Acknowledgments

TheauthorsaregratefulfortheNationalNaturalScience Foun-dationofChina(No.21376273)andtheMajorProjectofScience andTechnologyofHunanProvince(No.2010FJ1011)byofferingthe researchfund.

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