8 characterization of lead sorption by the natural and feiii modiefac81ed zeolite kel 7

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ContentslistsavailableatSciVerseScienceDirect

Applied

Surface

Science

j o ur na l ho m e p age :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Characterization

of

lead

sorption

by

the

natural

and

Fe(III)-modified

zeolite

Milan

Kragovi ´c

a

,

Aleksandra

Dakovi ´c

a,∗

,

Marija

Markovi ´c

a

,

Jugoslav

Krsti ´c

b

,

G.

Diego

Gatta

c

,

Nicola

Rotiroti

c

a_Institute_for_Technology_of_Nuclear_and_Other_Mineral_Raw_Materials,_Franche_d’Epere_86,₁₁₀₀₀_Belgrade,_Serbia b_Institute_of_Chemistry,_Technology_and_Metallurgy,_University_of_Belgrade,_Njegoˇseva_12,₁₁₀₀₀_Belgrade,_Serbia c_Dipartimento_di_Scienze_della_Terra,_Università_degli_Studi_di_Milano,_Via_Botticelli_23,_I-20133_Milano,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received25April2013

Receivedinrevisedform28June2013 Accepted6July2013

Available online 13 July 2013

Keywords: Sorption Desorption Lead Naturalzeolite Fe(III)-modifiedzeolite

a

b

s

t

r

a

c

t

Theinfluenceofcontacttime,temperatureandparticlesizeonleadsorptionbythenaturaland

Fe(III)-modifiedzeoliteswasinvestigated.CharacterizationofthenaturalandFe(III)-modifiedzeolitebefore

and afterleadsorption was performedbydetermination oftextural properties,byscanning

elec-tronmicroscopyandX-rayspectroscopyinenergy-dispersivemode(SEM–EDS),transmissionelectron

microscopy(TEM)andX-raypowderdiffraction(XRPD)analysis.Leadsorptionkineticsat303–333K,

bestrepresentedbythepseudo-secondordermodelandactivationenergy(13.5and8.5kJ/molforthe

naturalandFe(III)-modifiedzeoliterespectively)confirmedanactivatedchemicalsorption.Desorption

experimentsindicatedthatleadwasirreversiblysorbedonbothzeolites.XRPD,TEMandSEMresults

showedthatmodificationofthenaturalzeolitewithFe(III)ionsdidnotchangeitscrystalstructureand ironismainlylocatedatthezeolitesurface,likelyinformofamorphousironoxy-hydroxides.Specific surfaceareasignificantlyincreasesaftermodificationofthenaturalzeolitewithFe(III)ions(from30.2 forthenaturalto52.5m2_/g_for_{Fe(III)-modified}_zeolite)._{Characterization}_of_both_lead_saturated_sorbents

suggestedthatbesidesionexchange,leadisbothchemisorbedandprecipitatedattheirsurfaces,and

presenceofamorphousironinFe(III)-modifiedzeolitefavorssorptionoflead.

1. Introduction

Heavymetals aredangerousfor livingorganisms becauseof theirstability, toxicityand tendencytoaccumulate inthe envi-ronment.Withtherapiddevelopmentofindustriessuchasplating facilities,miningoperations,fertilizers,tanneries,batteries,paper, andpesticidesindustries,heavymetalswastewatersaredirectly orindirectlydischargedintotheenvironmentincreasingly, espe-ciallyindevelopingcountries.Unlikeorganiccontaminants,heavy metals are not biodegradable and tend toaccumulate in living organismsandmanyheavymetalsionsareknowntobetoxicand carcinogenic[1,2].Toxicheavymetalsofparticularconcernin treat-mentofindustrialwastewatersincludeZn,Cu,Ni,Hg,Cd,Pband Cr.

Regardingacutetoxicity, cadmium,leadalong withmercury form“thebigthree”ofheavymetalswiththegreatestpotential haz-ardtohumanandenvironment[3].Leadhasbeenfoundtobeacute toxictohumanbeingswhenpresentinhighamounts(e.g.>15␮g

∗_{Corresponding}_author._Tel.:₊₃₈₁₁₁₃₆₉₁_722;_fax:₊₃₈₁₁₁₃₆₉₁_583. E-mailaddresses:a.dakovic@itnms.ac.rs,dakovica@yahoo.com(A.Dakovi ´c).

indrinkingwater)[4,5].Also,itiswellknownthatleaddamages thekidney,liverandreproductivesystem,basiccellularprocesses andbrainfunctions.Leadtoxicsymptomsareanemia,insomnia, headache,dizziness, irritability,weaknessofmuscles, hallucina-tionandrenaldamages.Therefore,theremovalofexcessleadions fromwastewaterisessential.Treatmentprocessesusuallyinclude chemicalprecipitation,adsorption,solventextraction, ultrafiltra-tionandionexchange.Zeolitesareinexpensivenaturalmaterials withahighandselectivecationexchangecapacity.Amongnatural zeolites,clinoptiloliteisthemostabundantandcommonlyusedas ionexchangerorsorbentforinorganicions.Exchangeablecationsin zeolitestructure,e.g.K,Na,Ca,andMgarenottoxic,makingzeolites especiallysuitablefortertiaryprocessesofwastewatertreatment [6].

Theefficiencyof thenaturalzeolite for leadremovalis well documented.Karatas[7]reportedasorptioncapacityofraw clinop-tilolite for Pb2+ _from _aqueous _solution _of _16.8_mg/g, _whereas

Sprynskyyetal.[8]determinedthesorptioncapacityof clinoptilo-liteforPb2+_in_{multi-component}_aqueous_solution_to_be_27.8_mg/g.

Althoughtherearealargenumberofstudiesontheremovalof Pb2+_ion_from_aqueous_solutions_using_natural_zeolites,_any_given

zeoliticmaterialrequiresspecificexperimentsinthisrespect[9].

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Becauseit is wellknown that oxides of Feand Mn are the potential carriers of heavy metals (e.g. lead) [10], in order to enhancethesorption capacity of thenatural zeolitesfor heavy metalions,someexperimentshavebeendevotedtomodifytheir surfacebythesemetaloxides[11].Kragovi ´cetal.[12]reported a moreefficient sorbentforPb obtainedthrough the modifica-tionofanatural zeoliticsamplewithFe(III)ionsunderstrongly basicconditions.Theyobservedanincrease oflead sorptionby thenatural and Fe(III)-modified zeolite withincreasing of sor-bentdoseandtheinitialleadconcentration.Maximumleadsorbed amount (SCmax), under applied experimental conditions, was

66mg/gforthenaturalzeoliteand133mg/gforFe(III)-modified zeolite.

The aim of this paper is a further and extended investiga-tion on the abilities of natural and Fe(III)-modified zeolite to remove lead from aqueous solutions. The influence of specific parameters,suchascontact time,temperatureandparticlesize onsorptionof leadbybothsorbents wasstudied.Furthermore, sincedesorptionbehavior ofcontaminantsisat leastas impor-tantastheirsorption,estimationofthereversibilityofsorption and possible mobility of metals in an aqueous solution is of great interest. Therefore, desorption of sorbed lead from the naturaland Fe(III)-modifiedzeolitewasalsoinvestigated. Char-acterizationofthenaturalandFe(III)-modifiedzeolitebeforeand aftersorption of lead was performed bydetermination of tex-turalpropertiesandbyscanningelectronmicroscopyandX-ray spectroscopyinenergy-dispersivemode(SEM/EDS),transmission electronmicroscopy(TEM),andX-raypowderdiffraction(XRPD) analysis.

2. Materialsandmethods

2.1. Samplespreparation

Thenaturalzeolite-richrocksampleusedinthisstudycomes fromtheVranjskaBanja(Serbia)deposit.Thesamplewascrushed upandsievedtoparticlesize<0.043mm.Samplewasrinsedwith distilledwatertoremovepossibleimpurities,driedat60◦_C _and storedinthedesiccator.TheFe(III)-modifiedzeolitewasobtained combiningthemethodforpuregoethitepreparation[13]andthat forthepreparationofFe-coatedzeolite[14].Briefly,50gof natu-ralzeolitewasmixedwith25mLof10%FeCl3·6H₂O(p.a.Aldrich)

solutionand700mLof0.1mol/LKOH(pH=10)andaddedina2L container.Thecontainerwascappedandthesuspensionwasaged for20daysatroomtemperature.Afterthereactionperiod, sus-pensionwasfiltered,rinsedwithdistilledwater,driedat60◦_C_and storedinthedesiccator[12].

2.2. Leadsorptionexperiments

Theeffectoftemperatureonthekineticsandsorption capac-ityforleadonnaturalandFe(III)-modifiedzeolitewasstudiedby adding50mLof4000mgPb2+_/L_solution_(for_kinetic)_and₅₀_mL_of

1000,3000,4500or6000mgPb2+_/L_(for_{thermodynamic}

experi-ments)to1.0000gofbothsorbentsat303,313,323and333Kand attimeinterval0–2880min(forkinetic)andin1440min(for ther-modynamicstudy),respectively.Inordertoevaluatekineticand thermodynamicdata,separateflaskswerepreparedforeachtime intervalandonlyoneflaskwastakenfordesiredtime.InitialpH (pHi)inallexperimentswas4.24.

Inall experiments,after reaction,solutionand each sorbent wereseparatedthroughstandardfilterandconcentrationsofnon sorbedleadinsupernatant weredetermined byatomic absorp-tionspectrophotometry(AAS)usingan“AnalyticJenaSpekol300”. Sorbedamountofleadwascalculated fromdifferencebetween

startingconcentrationofleadanditsconcentrationinthe super-natant.

2.3. Sorption–desorptionexperiment

Sorption–desorption experiments were conducted using the protocol described by Hamidpour et al. [3]. For sorption experiments,in100mLcontainers,1.0000gofthenaturalor Fe(III)-modifiedzeolite wasdispersed in50mLof0.01mol/LCa(NO3)2

solutions,containingpredeterminedamountsofleadinthe con-centration rangeof 25–100% ofPb-maximum sorption capacity (SCmax)of each sorbent.The initiallead concentrations ranged

from 350to 1330mgPb2+_/L _for _the_natural _zeolite, _and ₆₉₀_to

2660mgPb2+_/L_for _{Fe(III)-modified} _zeolite._The _initial_pH_of_the

solutionswasadjusted to4.24byadding negligiblevolumesof 0.1mol/LofKOHorHNO3.Afteradditionoflead,suspensionswere

shakenfor24h.Afterreactiontime,suspensionswerecentrifuged (10,000rpmfor10min)andonehalfofthesupernatantvolume (25mL)waspipettedoutofthecontainertodeterminelead con-centration.Theotherhalfofsupernatants,alongwithsolids,was keptfordesorptionexperiments.

Immediatelyafterremovalof25mLofeachsupernatant,therest ofsupernatantstogetherwithsolidswasquantitativelytransferred intonew containersand mixed withthe samevolume (25mL) ofPb2+_-free_0.01_mol/L_Ca(NO

3)2 solution(withthesamepHas

insorptionexperiments)for24h.Afterdesorptioncycle, suspen-sionswerecentrifuged,25mLofthesupernatantswerekeptfor leadanalyses,whilesolidswithanother25mLofthesupernatants weretreatedagainwiththesamevolume(25mL)ofmetalfree backgroundelectrolyte.Desorptioncycleswererepeatedfour suc-cessivetimesforeachsample.Desorptionisothermswereprepared byplottingtheamountsofleadremainedinthesolidphaseafter eachdesorptioncycleversusthecorrespondingequilibriumlead concentrationsinsolution.

2.4. Characterizationofleadsaturatednaturaland Fe(III)-modifiedzeolite

2.4.1. Scanningelectronmicroscopyandelementalanalysis

Thesurfacemorphologyandthechemicalcompositionof sam-pleswereinvestigatedusinga“JEOLJSM-6610”scanningelectron microscope(SEM)equippedwithX-rayenergydispersive detec-tor(EDS).Observationsaimedtodescribethemorphologyofthe crystalswereperformedwithsamplescoatedwithgold.Forthe experimentsaimedtoperformchemicalmicro-analysisinenergy dispersivemode,thesamplesweresputteredwithcarbon.EDS ele-mentalanalyseswereperformedonasignificantlyhighnumberof points,inordertodetectapotentialheterogeneousdistributionof theelementsonthesurfaces.

2.4.2. XRPDanalysis

X-raypowderdiffractiondata werecollectedusing aPhilips X’Pert Pro diffractometer equipped with a multi-channel X’celeratordetectorandNi-filteredCuK␣radiation.Eachpattern

wascollectedwithangularrange2=5–70◦_,_steps_of₂₌_0.016◦_, and a countingtime of450s/step.Sampleswere grinded inan agatemortar,withafinalgrainsizeof1–5␮m.TheRietveld/RIR

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Fig.1..Effectofcontacttimeonsorptionofleadbythenatural(filledsymbols)and Fe(III)-modifiedzeolite(emptysymbols).

structuremodelsusedfortheRietveldfitwereobtainedfromthe MSACrystalStructureDatabase[19].

2.4.3. Transmissionelectronmicroscopy

Samplesfortransmissionelectronmicroscopywerepreparedby gentlegrindinginanagatemortar,suspensioninisopropylalcohol anddepositiononaholeycarboncoatedgrid.TEMobservations havebeencarriedoutonafieldemissiongunFEITecnaiF20super twinelectronmicroscopeequippedwithGatanSlowScanCCD794 andoperatedat200kVattheDST-MI.Themicroscopeisequipped withanEDSdetector,forqualitativeandsemi-quantitative chem-icalanalysis.

2.4.4. Texturalproperties

Adsorption–desorptionisothermsofallsampleswereobtained bynitrogenadsorptionat77KusingaSorptomatic1990Thermo Finnigandevice.Priortoadsorption,thesamplesweredegassed firstfor1hatroomtemperatureundervacuumandthenfor16hat 383Katthesameresidualpressure.Theresultingisothermswere analyzedbySoftwareADPVersion5.13ThermoElectron.Values ofthetotalporevolume(Vtot)andthespecificsurfacearea(SBET)

ofthesamplesweredeterminedbyapplyingGurevitsch’sruleat arelativepressurep/p0=0.98(pandp0representtheequilibrium

andsaturationpressuresofnitrogenatthetemperatureof adsorp-tion)andaccordingtotheBrunauer,Emmet,Teller(BET)method fromthelinearpartofthenitrogenadsorptionisotherms, respec-tively[20].TheDubinin–Radushkevich(DR)equationwasapplied tothenitrogenadsorptionisothermstoobtainthemicropore vol-ume(Vmic-DR)[21].Mesoporevolume(Vmeso)wasdeterminedby

theDollimoreandHeal(DH)method[22].

3. Resultsanddiscussion

3.1. LeadsorptionbythenaturalandFe(III)-modifiedzeolite

3.1.1. Effectofcontacttime

Theeffectofcontacttimeonleadretentionontonaturaland Fe(III)-modifiedzeolitewasstudiedatdifferenttemperaturesand isshownatFig.1.Thecontacttimevariedintherange0–2880min andtheinitialmetalconcentrationwas4000mgPb2+_/L._The_pH

iof

suspensionswereadjustedto4.24andthefinalpHs(pHf)were

from4.80at303Kto4.96at333Kforthenaturalzeolite,andfrom 4.96at303Kto5.25at333KforFe(III)-modifiedzeolite.

Table1

KineticsdataforsorptionofleadbythenaturalandFe(III)-modifiedzeolite.

Temperature(K) Parameters Naturalzeolite Fe(III)-modified zeolite

303

qe,exper(mg/g) 54.00 76.00 qe(mg/g) 55.34 76.33 k2(g/(mgmin)) 2.77×10−4 _7.56×10−4 h(g/(mgmin)) 0.85 4.88 R2 _0.999 _0.999

313

qe,exper(mg/g) 61.90 83.70 qe(mg/g) 62.81 84.03 k2(g/(mgmin)) 3.89×10−4 _8.78_×₁₀−4 h(g/(mgmin)) 1.53 6.20 R2 _0.999 _0.999

323

333

AspresentedinFig.1,theremovalofleadbysorptiononboth zeolitesshowedsteadyincreaseforthefirst230min,followedby anappreciablestabilizationtrendattainingamaximumvalueat about900minforthenaturalzeoliteandabout300minfor Fe(III)-modifiedzeolite.ItisclearfromFig.1thatFe(III)-modifiedzeolite isaquickerandefficientsorbentofleadthanthenaturalzeolite.

Leadsorptiondata(Fig.1)werefittedaccordingtothe pseudo-second order kinetics becauseit wasshown tobe more likely topredictthebehavior overthewholerange ofsorption being basedontheassumptionthattherate-determiningstepmaybe achemical sorptioninvolvingvalence forcesthroughsharingor exchangeofelectronsbetweensorbentandsorbate[23]. Pseudo-secondorderkineticmodelisexpressedas[24]:

dqt

dt =k2(qe−qt)

2 ₍₁₎

whereqtandqearesorbedamountattimetandequilibrium(mg/g)

andk2istherateconstantofpseudo-secondordersorptionoflead

(g/(mgmin)).IntegrationofEq.(1)leadstoEq.(2):

t

qt

= 1

kq2

e

+ 1

qet (2)

Fromthepseudo-secondordermodelitispossibletodefinethe initialsorptionrate,h(mg/(gmin)),ast→0:

h=kq2_e (3)

ThenEq.(3)becomes:

t

qt

= 1

h+

1

qet (4)

Thevaluesofqeandk2werecalculatedfromtheslopeand

inter-ceptofthestraightlineobtainedbyplottingt/qtagainstt(curves

notshown)andwerecollectedinTable1.

AscanbeseenfromTable1,foralltemperatures,theexcellent correlationcoefficients(R2₌_0.999),_as_well_as_the_good_agreement

of calculated and experimental qe for the natural and

Fe(III)-modifiedzeolite,indicatedthevalidityofthepseudo-secondorder modelforleadsorptionbythesesorbents.

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Fig.2.TheplotoflnKvs.1/Tforthenatural(filledsymbols)andFe(III)-modified zeolite(emptysymbols)fordifferenttemperatures:303K(blackline),313K(red line),323K(greenline)and333K(blueline).

Thepseudo-secondorderkineticssupportsthatthechemical sorptioncouldbetherate-determiningstepcontrollinglead sorp-tionprocess,andisinagreementwiththeresultsreportedbyother researchers[24].Therateconstantk2andqeincreasedwith

increas-ingthetemperaturefor thenaturalzeolite and Fe(III)-modified zeolitewhichindicatethatthehightemperaturefavorslead sorp-tion. Predicted initial lead sorption rate, h, for Fe(III)-modified zeolitewasfourtimeshigherthanforthenaturalzeolite.Themuch fasterleaduptake(higherinitialrate)byFe(III)-modifiedzeolite

maybeanadditionalevidencethatthismodificationincreasethe numberofactivesurfacesitesavailableforsorptionoflead.

Theactivationenergiesformetalionsorptiononnaturaland Fe(III)-modifiedzeolitewerecalculatedaccordingtotheArrhenius equation[25]:

k2=k0exp

−Ea

RT

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wherek0(g/(mgmin))isthetemperatureindependentfactor,Ea

(kJ/mol)istheactivationenergyofthereactionofsorption,Risthe gasconstant(8.314J/(Kmol))andTisthesorptionabsolute tem-peratureinK.Themagnitudeofactivationenergymaygiveanidea aboutthetypeofsorption.Therearetwomaintypesofsorption: physicaland chemical.Usually,theactivationenergy for physi-calsorptionisnomorethan4.2kJ/mol,sincetheforcesinvolved inthis processareweak. Chemicalsorptionis specific,involves forcesmuchstrongerthaninphysicalsorptionandmaybe acti-vatedornon-activated.Activatedchemical sorptionmeansthat theratevarieswithtemperatureaccordingtoafiniteactivation energy(8.4–83.7kJ/mol)intheArrheniusequation,whilefor non-activatedchemicalsorption,theactivationenergyisnearzero[26]. Theenergiesofactivationdeterminedfromslopesoflnk2versus

1/Tforbothsorbents(figurenotshown)werefoundtobe13.46 for thenatural and 8.53kJ/mol for Fe(III)-modifiedzeolite con-firmingactivatedchemicalsorption.Thepositivevaluesofenergy activationsuggestthatriseintemperaturefavorssorptionandthis processisanendothermicinnature.

3.1.2. Thermodynamicstudy

Thedatacollectedforleadsorptiononthenaturaland Fe(III)-modified zeolite, at four different initial concentrations (1000, 3000,4500and 6000mgPb2+_/L)_and _at_303,_313, ₃₂₃_and ₃₃₃_K

wereusedforestimationofsomethermodynamicparameters.The pHiofeachsolutionwasadjustedto4.24andthefinalpHs(pHf),for

Table2

ThermodynamicparametersforleadsorptiononnaturalandFe(III)-modifiedzeolite.

Co(mg/L) T(K) G◦

(kJ/mol) H◦

(kJ/mol) S◦

(J/(Kmol)) R2

Naturalzeolite

1000

303 −3.00

30.53 110.64 0.998

313 −4.18 323 −5.11 333 −6.38

3000

303 1.58

28.39 88.72 0.981

313 0.61 323 −0.60

333 −0.94

4500

303 2.88

23.96 69.64 0.994

313 2.18 323 1.30 333 0.86

6000

303 3.75

21.72 59.29 0.989

313 3.23 323 2.37 333 2.07

Fe(III)-modified zeolite

1000

303 −5.06

22.25 90.47 0.991

313 −6.21 323 −7.05 333 −7.78

3000

303 −0.83

20.75 54.95 0.982

313 −1.76 323 −2.54

333 −2.93

4500

303 0.64

11.52 35.87 0.999

313 0.32 323 −0.08 333 −0.43

6000

303 1.50

5.96 8.35 0.999

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theinitialconcentrationof1000mgPb2+_/L_were_from_5.21_at₃₀₃_K

to6.47at333Kforthenaturalzeolite,andfrom5.96at303Kto6.38 at333KforFe(III)-modifiedzeolite.Atthehighestinitial concen-tration,pHfwasfrom4.73at303Kto4.80at333Kforthenatural

zeolite,andfrom4.81at303Kto5.09at333KforFe(III)-modified zeolite.

Thestandardfreeenergyofsorption(G◦_),_standard_enthalpy

(H◦₎_and _entropy₍_S◦₎_were_calculated _from_following

equa-tions:

G◦

=−RTlnK (6)

lnK= S ◦

R − H◦

RT (7)

whereRisthegasconstant(8.314J/(Kmol))andKisthe equilib-riumconstantatthetemperatureT.TheconstantKwascalculated astheratiooftheequilibriumleadsorbedamountandequilibrium concentrationinsolution.ThevaluesofH◦_and_S◦_were

calcu-latedfromtheslopeandtheinterceptoflinearregressionoflnK vs.1/T(Fig.2).TheobtainedparametersarepresentedinTable2.

FromTable2,itisobservedthatforthelowerinitiallead concen-trations,G◦_was_negative_at_all_{temperatures,}_for_both_the_natural and Fe(III)-modified zeolite, indicating that sorption process is spontaneousinnature.Withincreasingtheinitiallead concentra-tion,G◦_was_positive_suggesting_that_at_these_lead_{concentrations} sorptionprocessbecomesnon-spontaneous.ForFe(III)-modified zeolite,G◦_is_lower_than_for_the_natural_one,_and_more_negative inthewiderconcentrationrange,suggestingthehighernumber of active sites availablefor spontaneous sorption of lead. Also,

G◦_becomes_more_negative_with_increasing_temperature, suggest-ingthatsorptionismorefavorableathightemperature.Changing in enthalpy (H◦₎ _values _was_positive _showing _that _the sorp-tion of lead at both the natural and Fe(III)-modified zeolite is endothermicinnature.Itisobservedthatforthehighestinitial leadconcentration,H◦_is_much_lower_for_the_{Fe(III)-modified} zeo-lite(5.96kJ/mol)thanforthenaturalone(21.7kJ/mol),confirming morefavorablesorptionbyFe(III)-modifiedzeolite.Thepositive valuesofentropyS◦ _suggests_an_increase_in_randomness_at_the solid–solutioninterfacebythefixationofleadionsontoboth sor-bents.Namely,basedonpHmeasurements,evenathigherinitial concentrations,leadispresentinsolutionpredominantlyinthe cationicforms[27],thusleadionsaresurroundedbyafirmlybound hydrationlayerwherewatermoleculesaremorehighlyordered. Whenleadionsenterthehydrationsurfaceofsorbent,theordered watermoleculesdisturbed,thatcausesincreaseofentropyofwater molecules.Sorptionofleadalsoinvolvesreleaseofcationsfrom sorbents,which influenceincreaseintheentropy.Furthermore, theobserveddecreaseinentropywithincreasingtheinitial con-centrationofleadindicatesthatchemisorptioncausedecreasein randomnessatthesolid–solutioninterface.Thiseffectismuch pro-nouncedforFe(III)-modifiedzeolite.

Basedontheamountofreleasedcationsduringleadsorption bythenatural andFe(III)-modifiedzeolite, sorptioninclude ion exchangeatlowerinitialconcentrationsoflead,whileathigher ini-tialconcentrations,chemicalsorptionofleadatsurfacewasmuch pronouncedthanionexchange[9].Intheleadinitialconcentration rangewhenionexchangewasdominant,theamountsofreleased cations were higher than amount of sorbed lead, while when chemisorptionmainlyoccurredtheoppositetrendwasobserved. Thus, since theamounts of releasedcations fromboth zeolites aresimilarinwholeinitialleadconcentrationrange,theobserved decreasein entropywithincreasingtheinitialconcentrationof leadindicatesthatchemisorptioncausedecreaseinrandomness atthesolid–solutioninterface.Thiseffectismuchpronouncedfor Fe(III)-modifiedzeolite.

Table3

RefinedparametersforFreundlichequationfittotheleadsorptiondata.

Natural zeolite

ksorb(L/mg) 14.36 28.81

nsorb 0.203 0.183

R2 _0.977 _0.982

3.1.3. Effectoftheparticlesize

Sorptionisothermswerepreviouslydeterminedforthe natu-ral andFe(III)-modified zeolitesfor twodifferent particlesizes: <0.043mmand0.6–0.8mm[12,28].Forbothfractionofthe nat-uralandFe(III)-modifiedzeolite,sorptionofleadincreasedwith increasingtheinitialconcentrationoflead.Theresultsofsorption werefittedtotheLangmuirandFreundlichsorptionmodelandthe bestfitwasobtainedusingtheFreundlichmodel.Undertheapplied experimentalconditions,fortheparticlesizesof<0.043mm,the maximumsorbedamount(SCmax)ofleadwas66mg/gforthe

nat-uraland133mg/gforFe(III)-modifiedzeolite.Fortheparticlesize 0.6–0.8mm,SCmaxwas62and102mg/gforthenaturaland

Fe(III)-modifiedzeolite,respectively.Theseresultsshowedthatsorption ofleadbythenaturalzeoliteslightlydecreasewithincreasing par-ticlesize,whilefor Fe(III)-modifiedzeolite,this phenomenonis muchmorepronouncedastheresultofdecreasingofsurfacearea ofsorbent.Still,much highersorptionofleadwasobservedfor Fe(III)-modifiedzeolite,forbothparticlesizes.Itwasalready men-tionedthatsorptionofleadbyFe(III)-modifiedzeoliteincludeion exchangeandchemisorptionandmostprobablyirondepositedat thesurfacecreatesactivesitesresponsibleforenhancedlead sorp-tion.Thus,becausesorptionisasurfacephenomenon,thesmaller sorbentsizeofferedcomparativelylargeravailablesurfaceareafor ironduringmodificationandhencehighernumberofactivesites forleadremovalatequilibrium[29].Thehighestinitiallead concen-trationpointonthesorptionisothermswasusedforpreparationof leadsaturatedsorbentsforthecharacterizationexperiments (Sec-tion3.3).

3.2. Sorption–desorptionoflead

Thesorption–desorptionisothermswerecalculatedusingthe Freundlichequations[3].

qe=ksorbCensorb (8)

qe=k_desorbCn_edesorb ₍₉₎

whereqe,Ce,ksorb(kdesorb),nsorb(ndesorb)arethequantityofleadin

solidphase(mg/g),equilibriumliquidphaseconcentration(mg/L), theFreundlichboundingconstant(L/mg)forsorption(desorption) andsorption(desorption)coefficient,respectively.

Leadsorption–desorption isotherms fittedby theFreundlich modelforthenaturaland Fe(III)-modifiedzeolitearepresented inFig.3,andcharacteristicparametersoftheisothermsaregiven atTable3forsorptionandTable4fordesorptionofleadfromboth sorbents.

As can been seen from Tables 3 and 4 the correlation coefficientsforsorptionand desorptionofleadaregreaterthan 0.900 confirming Freundlich model very well described lead sorption–desorptionprocessonbothsorbents.Usingofthe Freund-lich modelfor describing sorption processfor heavy metals on differentsorbentsiswellknown.Forexample,Hamidpour etal. [3]usedFreundlichequationforinvestigationofdesorptionoflead andcadmiumfromnaturalzeoliteandbentonite.

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Fig.3. Leadsorption(blackline)-desorptionisothermsatthreemetalion con-centrations:50%(redline),75%(greenline),and100%(blueline)ofSCmax,forthe natural(filledsymbols)andFe(III)-modifiedzeolite(opensymbols).

Table4

RefinedparametersforFreundlichequationfittotheleaddesorptiondata.

Initialleadload (%ofSCmax)

Parameter Natural zeolite

50

kdesorb(L/mg) 31.05 61.28

ndesorb 0.009 0.006

R2 _0.990 _0.992

75

ndesorb 0.012 0.010

R2 _0.911 _0.964

100

ndesorb 0.016 0.013

R2 _0.972 _0.987

sorption–desorptionofleadfrombothsorbentswerenotreversible (Fig.3).Attheinitialleadconcentrationsof50%,75%and 100% ofSCmax,thequantityoflead desorbed,afterfourcycleswere:

0.95%,1.99% and2.95% forthenatural zeolite and0.41%, 1.22% and 2.02% for Fe(III)-modified zeolite. These results are very comparablewithresultsobtainedbyHamidpouretal.[3].They foundthat 1%,3%and 5%of leadwasreleasedfromzeolite for initialconcentrationofleadof50%,75%and100%ofSCmax and

release of sucha relativelylow proportion of sorbedlead was attributedtothestrongbondingtothehighaffinitysitesinzeolite. The Freundlich constant k (a quantity parameter representing theamountofleadretainedonthesolidphase)foreachsorbent obtainedfromthedesorptionisothermswassignificantlyhigher thanthatcalculatedfromsorptionisotherms(Tables3and4),and also,thisconstant for Fe(III)-modifiedzeolite wasalmosttwice

higherthanforthenaturalzeolite,confirmingpositivehysteresis andindicatingapproximatelyslightlyhigherdesorptionofsorbed leadfromthenaturalthanfromFe(III)-modifiedzeolite.

3.3. CharacterizationofthenaturalandFe(III)-modifiedzeolite aftersorptionoflead

3.3.1. Scanningelectronmicroscopyandelementalanalysis

SEMimagesofthenaturalandFe(III)-modifiedzeolitesbefore andaftersorptionofleadareshowninFigs.4and5.

Fieldobservationandmicroscopicstudyofthesamplerevealed thatthenaturalzeoliteoccursmainlyaswell-formedfinesized crystals.Manyofthezeolitecrystalsshowatabularmorphology, asexpectedfora(monoclinic)clinoptilolite(Fig.4a).Aswe previ-ouslyreported[12],aftermodificationwithFe(III)ions,formand sizearepreserveddespitethestronglybasicconditionsusedfor modification(Fig.4b).

SEMcoupledwithEDSprovidesasemiquantitativeelemental analysisofthecrystals surface.In theEDSspectraof the natu-ralzeolite,Si,Al,O,Mg,Ca,K,Na,TiandFeweredetected.Iron iscertainlydueto(non-zeolitic)Fe-bearingmineralsinthe vol-canicrocks [30], while Ca,Na, Kand Mg are mainlythe extra frameworkcationsthatcompensatethenetnegativechargeofthe zeolite(clinoptilolite–heulandite)network[31].Aftermodification ofthenaturalzeolite withFe(III)ions,iron andpotassium con-tentincreased(Fe2O3=2.03%forthenaturalandFe2O3=5.70%for

Fe(III)-modifiedzeolite;K2O=1.41%forthenaturalandK2O=5.81%

for Fe(III)-modified zeolite), while content of sodium and cal-ciumdecreased(Na2O=1.27%forthenaturalandNa2O=0.49%for

Fe(III)-modifiedzeolite;CaO=3.26%forthenaturalandCaO=2.18% forFe(III)-modifiedzeolite).

InFe(III)-modifiedzeolite,theamountofironexceedscation exchangecapacityofthenaturalzeolite,whichtogetherwithEDS resultsconfirmthat,inadditiontotheionexchangebetweenthe exchangeablecationsfromthezeolitenetworkandtheFefromthe solution,mostoftheironisdepositedonthesurfaceofthezeolite crystalsandmaybeintheformofFe-oxides[32].

Forleadsaturatedsamples,thecontentofleaddeterminedby EDSwas6.30%forthenaturaland14.22%forFe(III)-modified zeo-liteanditwasclosetothemaximumcapacitiesofbothsamples obtainedfromsorptionexperiments.Forthisinitialconcentration of lead used for preparation saturated samples, it was postu-latedthat sorption of lead include ion exchange together with chemisorption of lead on the surface [12]. Sorption complexes of Pb(II) with various adsorbents including alumina, clay min-erals(suchasmontmorillonite),manganese(III,IV)oxyhydroxide aswell asPb(II) mononuclearbi dentateinner spheresorption complexes(withhematiteandgoethiteoverawiderangeof con-ditions),werereportedintheliterature[33].Thus,similarresults ofEDS analysisandmaximum sorptioncapacity suggestedthat forbothzeolitesbesidesionexchangedlead,itisalsoretainedon

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Fig.5. SEMmicrographsofleadsaturated:(a)naturaland(b)Fe(III)-modifiedzeolite.

hydroxylsurfacesitesparticularlyat≡FeOHsitesassociatedwith

Fe-oxyhydroxidemineralphases,and/or>SOH,whichrepresents bothzeolitemineraledgesandAl-oxyhydroxidefunctionalgroups [34].Besidestheseprocesses,themixedeffectofsorptionwith co-precipitationoreven coagulationofleadatthemineralsurface mayoccurred[35].TheSEMmicrographsofbothleadsaturated zeolites(fraction0.6–0.8mm)presentedatFig.5,showrare ran-domlydistributed agglomerateswithsignificantamountoflead (60.16wt%–thenaturalzeoliteand71.50wt%–Fe(III)-modified zeolite).

Luetal.[36]reportedthatcoprecipitationofPb2+_with_ferric

oxihydroxidesoccurredatpH∼4,anditwasmoreefficientthan

sorptionofleadfromaqueoussolutionsatsimilarsorbate/sorbent ratiosandpH.Theirdesorptionexperiments showedthat more Pb2+_was_released_from_loaded_sorbents_collected_from_sorption

experimentsthanfromPbtoFecoprecipitates.Theyhypothesized thatPb2+_was_first_sorbed_on_the_{nanometer-sized}_metastable

Fe-oxyhydroxide polymersand, as thesenano-particles assembled intolargerparticles,somePb2+_was_trapped_in_the_{Fe-oxyhydroxide}

structureandre-arranged.Lowdesorptionofleaddescribedinthe previoussection,similarcontentofleadinbothsamplesobtained fromEDSandfromsorptionexperiments,aswellasthepresence ofrandomagglomeratesofleadatthemineralsurfaces,confirmed complexleadsorptionmechanismincludingionexchange,surface complexationandco-precipitation.Themuchhighersorptionof leadbyFe(III)-modifiedzeolitethanbythenaturalone,suggested thattheseleadsorptionprocessesaremorepronouncedwheniron ispresentatthezeoliticsurface.

3.3.2. XRPDanalyses

XRPDpatternsofthelead-saturatednaturalandFe(III)-modified samplesareshowninFig.6.

The refined amount of the crystalline phases (wt%) by Rietveld/RIRmethodare: clinoptilolite66(1)%, mordenite3(1)%, plagioclase18(1)%,andquartz12(1)%forthenaturalsampleand clinoptilolite 73(1)%, mordenite 3(1)%, plagioclase 14(1)%, and quartz10(1)%fortheFe(III)-modifiedsample.Theestimated frac-tionoftheamorphousphasewasbelowthedetectionlimit(i.e. <3wt%).Thetwodiffractionpatternsshowthatthereisnodrastic changeof thecrystal-chemistry ofthe sample afterthe Fe(III)-treatment.AstheFe(III)-modifiedzeolitewaspreparedaccording totheprotocolforpuregoethite,weexpectedtofindevidence,in thediffractionpattern,ofironhydroxidesand/orironoxides. Nev-ertheless,noFe-richcrystallinephasewasdetected.However,a carefulinspectionoftheprofilesshowedthat:(a)the full-width-at-half-maximumofthediffractionpeaksofclinoptiloliteexperienced increased(∼8–10%)intheFe(III)-modifiedsample,(b)a general

slightdecreaseoftheofthediffractionpeaksintensityisobserved

forallthecrystallinephases,thoughmorepronouncedfor clinop-tilolite.

Additionally, crystal Fe–Pb-bearing oxides/hydroxides were also not observed in XRPDpattern of both lead saturated nat-ural and Fe(III)-modified zeolite. The polyphase nature of the samples, coupled with the quality of the data collected with a conventional diffractometer, hindered high-quality structure refinements of clinoptilolite in order to prove a variation of theextra-frameworkpopulationbetweenthenaturaland Fe(III)-modifiedsample,respectively.

3.3.3. Transmissionelectronmicroscopy

ATEMinvestigationofthelead-saturatedFe(III)-modified zeo-liteshowedahighfractionofidiomorphiccrystalsofclinoptilolite (Fig.7), alongwithand plagioclaseandquartz,andasecondary fractionofnano-aggregateswhichhavebeenfoundtobe amor-phous(Fig.8).TheEDSanalysesshowedthat:(1)Theamountof FeandPbinidiomorphiccrystalsofclinoptiloliteisnegligible(i.e. belowtheEDSdetectionlimit),andsoFeandPbarenot concen-tratedinzeolitestructure(i.e.asextra-frameworkpopulation);(2) FeandPbappeartobemainlyconcentratedintotheamorphous fraction.Theseexperimentalfindingscanexplaintheabsenceof Fe–Pb-bearingoxides/hydroxides(e.g.goethite,limonite,etc.)in theX-raydiffraction patternsofthelead-saturated natural and Fe(III)-modifiedsamples.

Afurtherexperimentalfindingconcernsthepresenceof spher-ical nano-clusters onthe crystal faces,as shown in Fig.9.The dispersionoftheclustersdoesnotallowustohavearepresentative chemicalcompositionoftheclusters,buttheimagecontrast sug-geststhattheclustershaveadifferentcompositionwithrespect tothehostcrystalsofzeolite. Inotherwords,clusterscouldbe made by Fe or Pb oxides, but we cannot prove that. The TEM high-resolutionimagessuggestthatthesphericalnano-clustersare amorphous(Fig.9).

3.3.4. Texturalproperties

InFig.10,thenitrogenadsorption–desorptionisothermsof nat-uralzeolite,Fe(III)-modifiedzeoliteandpuregoethite1_are_shown.

Theobtainedisotherms,althoughsimilar,accordingtoIUPAC nomenclaturecanbedefinedastwoseparateclasses.Isothermof puregoethite1_,_due_to_existence_of_barely_noticeable_but_visible

plateau at high relative pressure, narrow hysteresis loop and almost vertical and nearly parallel hysteresis branches, with the desorption partend at relative pressure closeto 0.85, can be classified as Type IV with hysteresis loop of type H1. This type of hysteresis is associated with porous materials which

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Fig.6.X-raypowderdiffractionpatternoftheleadsaturated:(a)naturalzeoliteand(b)Fe(III)-modifiedzeolite.

consistof agglomeratesor compacts of approximatelyuniform spheresinfairlyregulararrayandhencethesematerialshavea narrowporesizedistribution.Overall,itappearsthatsynthesized puregoethitebehaveslikeamesoporousmaterial.Theporesize distribution (results not presented) estimated from desorption branchesofisothermshowsthemaximumat44.7nm,confirming mesoporosityofsynthesizedgoethite.

For the natural and Fe(III)-modified zeolite, the obtained isothermsdonothaveaplateauathighrelativepressureandfor bothsorbentshysteresisloopsendatp/p0≈_0.42._The_position_of

hysteresisloopendingforbothsamplesistypicalformeasurements withnitrogenat77Kandisnottheconsequenceofthepore sys-temnetwork ofsorbents.IsothermscanbeclassifiedasTypeII, typicalfornonporousmaterials.Additionally,hysteresisloopof H3typecanberecognizedandischaracteristicfortheaggregates ofplate-likeparticles.

TheisothermsofnaturalandFe(III)-modifiedzeoliteafterlead sorption(resultsnotshown)areverysimilartoisothermsof corre-spondingmaterialsbeforeleadsorption,althoughsomereduction oftotalamountofadsorbednitrogenandslightlywideningof hys-teresisloopcanberecognized.Thetypeofisotherms(typeII)and hysteresisloops(typeH3)stayunchanged.

Theresultsofcalculatedtexturalparametersofthenaturaland Fe(III)-modifiedzeolitebeforeandaftersorptionoflead,aswellas forpuregoethite1_,_are_presented_in_Table₅_.

Resultsshowedthatspecificsurfaceareasignificantlyincreases after modificationof the natural zeolite withFe(III) ions,from 30.20m2_/g_for_the_natural_zeolite_to_52.50_m2_/g_for_{Fe(III)-modified}

zeolite.Fromthechemicalanalysis,themodificationofthe nat-uralzeolitewithFe(III)ionsunderbasiccondition,increasedthe iron content for only 1.1% (from Fe2O3=2.30% for the natural

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Fig.7.Idiomorphicsingle-crystalsofclinoptilolite(left)andadiffractionpatternofoneindividual(right).TheEDSchemicalanalysisoftheidiomorphiccrystalsshowsFe andPbcontentbelowthedetectionlimit.

Fig.8.Amorphousaggregate(left)anditsdiffractionpattern(right).TheEDSchemicalanalysisatP6yields:Fe=15.2(3)%andPb=7.5(8)%(givenasatomic%).

Fig.9.High-resolutionimagesofsub-sphericalnano-clusterswhichappeartobeamorphous(left)andlyingonthesurfaceofthecrystalsofclinoptilolite(right).

Table5

TexturalparametersofthenaturalandFe(III)-modifiedzeolitebeforeandafterleadsorptionandofpuregoethite.a

Sample SBET(m2_/g) _Vtot_(cm3_/g) _Vmeso_(cm3_/g) _V

mic(cm3/g)

Naturalzeolite 30.20 0.135 0.078 0.012

Naturalzeolite+lead 29.70 0.082 0.042 0.011

Fe(III)-modifiedzeolite 52.50 0.151 0.092 0.019

Fe(III)-modifiedzeolite+lead 46.50 0.131 0.058 0.017

Goethitea _55.50 _0.528 _0.489 _0.020

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Fig.10.Nitrogenisothermsofnaturalzeolite(Z),Fe(III)-modifiedzeolite(FeZ)and puregoethite(Puregoethitewassynthesizedunderthesameprocedureas Fe(III)-modifiedzeolite)(G);filledsymbolsadsorption,emptysymbolsdesorptionpoints.

suchsmall amount of iron elevated SBET close to the value of

puregoethite1 _(55.50_m2_/g)._Increase_of_the_specific_surface_area

observedbyJiménez-Cedilloetal.[32]forzeolitestreatedwithboth ironandmanganese,indicatedthatmetallicspeciesarenotonly exchangeablebythecationsfromthezeolitenetworkbutarealso depositedonthesurfaceofthezeoliticmaterials.Doula[11]also reportedforFe-modifiedclinoptilolitethatonlyasmallamountof Fecouldbeexchangedwiththecationsofextra-framework popu-lationofthezeolitestructure.Sheindicatedthat,asaresultofthe presenceofironinthezeolitestructurechannels,thetextural prop-ertiesofthismaterialchangednotably.Comparedtothenatural zeolite,theincreaseofthespecificsurfaceareaofFe(III)-modified zeolitesthrough the increase of micropores volume (especially onultra-micropores withdiameter less than 1nm)may be an additionalevidenceofionexchange,becausemicroporevolume previouslyinaccessibletothemoleculeofnitrogenbecomes acces-sibleafterexchangeoflargercations(K+_,_Na+_,_Ca2+_,_Mg2+₎_with

considerablesmalleroneFe3+_[37]_._On_the_other_hand,_the_increase

inmesoporevolumecanbecorrelatedwiththeformationofanew phaseatthezeoliticsurfacewhichisitselfmesoporous.Thus,the Fe(III)-modifiedzeoliteshowsbothincreaseofmicro-and meso-poresvolume,confirmingformationofnewFe-bearingphaseson zeolitebesidetothemechanismofionexchange.

Afterlead sorption, specific surface areaof the natural and Fe(III)-modified zeolite, compared to the corresponding source materials, slightly decreased. In fact, all textural parameters decrease,especiallymesoporevolume.Thedecrease ofVtot and Vmesomayfurthersuggestthatleadisconsiderablydepositedon

thesurfaceofbothsorbents,confirmingthat(forthisinitial con-centration)besidesionexchange,chemisorptionofleadoccurred. Finally, since it is well known that amorphous Fe-oxides havelargersorptioncapacitiesformetalcontaminantsthan crys-talline oxides such as goethite [33], to support observations thatamorphousironatthezeoliticsurfaceincreasedlead sorp-tion,we didpreliminarylead sorption experiment (initial lead concentration∼1300mg/L,amountofsolidphase1.0000g/50mL

and pH=4.24) with pure goethite.1 _The _result _was _compared

with lead sorption, under the same conditions, by the natu-raland Fe(III)-modifiedzeolite. We observedthatlead sorption increased in the following order: goethite1 _(17.5%)_<_the

natu-ralzeolite(61.5%)<Fe(III)-modifiedzeolite (91.6%).Thus,results on lead sorption by the natural and Fe(III)-modified zeolite, along withcharacterization of materials beforeand after sorp-tion of lead reported in this study, confirmedthat amorphous

Fe-oxy/hydroxides at the surface of Fe(III)-modified zeolite are responsibleforenhancedsorptionoflead.

4. Conclusion

Resultsreportedinthispapershowedthatsignificantlyhigher sorption of lead was achieved with modification of the nat-ural zeolite with Fe(III) ions under basic conditions. Sorption experimentsaswellascharacterizationofboth thenaturaland Fe(III)-modifiedzeolitebeforeandaftersorptionofleadconfirmed complexleadsorptionmechanismincludingionexchangeaswell aschemisorptionandprecipitationofleadatthezeoliticsurface. CharacterizationofFe(III)-modifiedzeoliteaftersorptionoflead confirmedenhancedleadsorptionisduetopresenceofamorphous phaseatthesurfaceofmodifiedzeolite.Highleadsorptionandhigh stabilityofleadsaturatedmodifiedzeolitemakeFe(III)-modified zeoliteaspromisingmaterialforwaterpurification.

Acknowledgements

TheseinvestigationswereconductedundertheProjects172018 and 34013 funded by the Ministry of Education, Science and Technological Development of theRepublic of Serbia. G.Diego GattaandNicolaRotirotithanktheItalianMinistryofEducation, MIUR-Project:“ImPACT-RBFR12CLQD”.Theauthorsacknowledge theagreementforinternationalcooperationamongtheUniversità degliStudidiMilanoandtheInstituteforTechnologyofNuclearand OtherMineralRawMaterialsinBelgrade.Thecompany “Nemet-ali”fromVranjskaBanja,Serbiakindlyprovidedthenaturalzeolitic sample.

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