group663uu.com

PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1954ADSORPTIONCHARACTERIZATIONOFCo(II)IONSONTOCHEMICALLYTREATEDQUERCUSCOCCIFERASHELL:EQUILIBRIUM,KINETIC,ANDTHERMODYNAMICSTUDIESM.
HamdiKaraolu,*MehmetUurlu,andbrahimKulaQuercuscocciferashell(QCS),arelativelyabundantandinexpensivematerial,iscurrentlybeinginvestigatedasanadsorbenttoremovecobalt(II)fromwater.
Beforetheadsorptionexperiments,QCSwassubjectedtochemicaltreatmenttoprovidemaximumsurfacearea.
Then,thekineticsandadsorptionmechanismofCo(II)ionsonQCSwerestudiedusingdifferentparameterssuchasadsorbentdosage,initialconcentration,temperature,contacttime,andsolutionpH.
Theloadedmetalscouldbedesorbedeffectivelywithdilutehydrochloricacid,nitricacid,and0.
1MEDTA.
TheLangmuirandFreundlichmodelswereusedtodescribetheuptakeofcobaltonQCS.
TheequilibriumadsorptiondatawerebetterfittedtoLangmuiradsorptionisothermmodel.
Themaximumadsorptioncapacity(qm)ofQCSforCo(II)was33mgg-1.
Variouskineticmodelswereusedtodescribetheadsorptionprocess.
Theadsorptionfollowedpseudosecond-orderkineticmodel.
Theintraparticlediffusionwasfoundtobetherate-limitingstepintheadsorptionprocess.
Thediffusioncoefficientswerecalculatedandfoundtobeintherangeof3.
11*106to168.
78*106cm2s-1.
ThenegativeH*valueindicatedexothermicnatureoftheadsorption.
Keywords:Adsorption;Divalentcobalt;Quercuscocciferashell;Desorption;KineticsContactinformation:DepartmentofChemistry,FacultyofScience,MulaUniversity,Mula48000,Turkey,Correspondingauthor:fahamdi1972@hotmail.
comINTRODUCTIONEnvironmentalpollutionbytoxicmetalsarisesasaresultofmanyactivities,largelyindustrial,althoughsourcessuchasagricultureandsewagedisposalalsocontribute(MatheickalandYu1997;Karaoluetal.
2010a;Saretal.
2007).
Cobalt-containingcompoundsarewidelyusedinmanyindustrialapplicationssuchasmining,metallurgical,electroplating,paints,pigments,andelectronics(Bhatnagaretal.
2010).
Cobaltisalsopresentinthewastewaterofnuclearpowerplants.
TheincreaseduseofCo(II)innuclearpowerplantsandinmanyindustriessuchasmining,metallurgical,electroplating,paints,pigments,andelectronicindustrieshasresultedinCo(II)findingitswayintonaturalbodiesofwater.
ThetolerancelimitforCo(II)inpotablewaterhasbeenfixedas0.
05mgL1(Manoharetal.
2006).
Itshighdosescauseparalysis,diarrhoea,lowbloodpressure,lungirritations,andbonedefects.
Withabetterawarenessoftheproblemsassociatedwithcobalt,researchstudiesrelatedtothemethodsofremovingcobaltfromwastewaterhavedrawnattentionincreasingly(RengarajandMoon2002).
Inthiscontext,therecoveryofheavymetalsfromthewastewaterisamajortopicinwaterresearch,andseveralmethodshavebeenPEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1955commonlyusedforthispurpose(chemicalprecipitation,electrochemicalreduction,evaporation,reverseosmosis,membranefiltration,co-precipitation,electrodialysis,adsorption,biosorption,etc.
)(Guptaetal.
2005;GuptaandRastogi2009;SarandTuzen2008a).
Precipitation,ionexchange,solventextraction,andadsorptiononoxidesaretheconventionalmethodsfortheremovalofheavymetalionsfromaqueoussolutions,butduetohighmaintenancecostthesemethodsdonotsuittheneedsofdevelopingcountries(Karaoluetal.
2010a;Uluozluetal.
2008).
Inthisstudy,thebiosorbentQuercuscocciferashell(QCS)wasusedasabiosorbentduetoitslowcostandhighefficiency.
QuercuscocciferatreesareabundantintheMediterraneancountries,andboththistreeanditsshellareusuallyburnt.
Tothebestofourknowledge,thismaterialwasnotusedbeforeforthiskindofapplication.
UtilizationofQuercuscocciferashellnotonlyprovidesalowcostandeasilyavailablesorbentfortheremovalofheavymetalssuchasCo(II),butalsocontributestothepreventionofenvironmentalpollution.
EXPERIMENTALReagentsAllreagentsusedinpresentstudywereofanalyticalgrade.
CoCl26H2O,concentratedHCl,concentratedHNO3,andNaOHwereobtainedfromMerck(Germany),whileCo(II)atomicabsorptionspectrometerstandardsolution(1000mgL-1)waspurchasedfromGBCCompanychemicals(Australia).
Allglasswareandpolypropyleneflasksusedwereovernightimmersedin10%(v/v)HNO3andrinsedseveraltimeswithultrapurewater(18.
3mohm).
ForadjustingthepHofthemedium0.
1NsolutionsofNaOHandHClwereused.
PreparationofFormaldehyde-TreatedQCSQCSwascollectedfromalocalQuercuscocciferatreenearMula,Turkeyandwashedrepeatedlywithdeionizedwatertoremovethewater-solubleimpuritiesandothersurfaceadheredparticles.
Fortreatmentwithformaldehyde(FA)(CarloErba,Italy)toobtainFA-treatedbiomass,amixtureof17mLformalin30%and33mLHCl0.
1Mwasaddedto2.
5gofsmoothlycrushedbiomass.
Themixturewasleftatroomtemperaturewithgentlemixing.
After1h,thebiomasswasfilteredandwashedwithdistilledwater.
Subsequently,thebiosorbentwasincubatedwith50mLsodiumcarbonatesolution(0.
2M)for15min,filtered,washedwithdistilledwateranddriedovernightat80°C(Jalali-Radetal.
2004;Ebrahimietal.
2009).
Driedpowderwascrushedinarotarycrusherandsievedwith355μmsieve(Retsch,Germany;No:45)andstoredindesiccator.
BatchAdsorptionStudiesAbatchequilibriummethodwasusedtodeterminethesorptionofcobaltbyQCS.
Asetof250mLErlenmeyerflaskscontaining50mLofmetalsolutionwasusedintheexperiments.
QCS(0.
2g)wascontactedwiththemetalsolutions(50mL)for90minutesPEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1956onanincubatorshaker(Zhwy-200D,SouthKorea)at175rpmatroomtemperature(25°C).
Thecontentsoftheflaskswerefiltered,andthefiltrateswereanalyzedforresidualmetalconcentrationusingaGBCatomicabsorptionspectrometer(GBCAvanta,Australia)withdeuteriumbackgroundcorrector.
Allmeasurementswerecarriedoutinanair/acetyleneflame.
Theoperatingparametersfortheworkingelementweresetasrecommendedbythemanufacturer.
WTWmodelpH-meter(Germany)equippedwithacombinationpHelectrodewasusedtomeasurethepHofallsolutions.
Allthechemicalswereofanalyticalgrade.
Referencesolutionswerepreparedasrequiredbyfurtherdilutionwithultrapurewater.
DesorptionExperimentDesorptionstudieswerecarriedoutwithhydrochloricacid,nitricacid,andethylenedinitrilotetraaceticaciddisodiumsalt(Na2EDTA,C10H14N2Na2O82H2O)(Merck,Germany).
Thesedesorptionsolutionswereadjustedtoamolarityof0.
1M.
Todeterminethemosteffectivedesorptionsolution,0.
2gofQCSand50mLofdesorptionsolution(withoutanypHadjustment)waskeptincontactinanincubatorshaker(Zhwy-200D)at175rpmand25°Cfor90min.
Themixturewasfilteredafterdesorption,andthefiltratewasanalyzedfortheresidualCo(II)concentrationbyFlameAtomicAbsorptionSpectrophotometry(FAAS).
RESULTSANDDISCUSSIONCharacterizationofQCSFT-IRspectraforQCSinitsnaturalandchemicallytreatedformsarepresentedinFig.
1.
Astrongpeakat3330cm1representsthe–OHstretchingofthephenolgroupofcelluloseandlignin(Karaoluetal.
2010a).
Astrongandsharpbandat2921cm1isattributedtotheC–HstretchingvibrationfromCH2groupofcelluloseandhemicellulose.
Thepeakat1731cm1isduetoC–Ostretchingofcarbonylgroups(>C=O)inhemicellulose(Wengetal.
2009).
Thepeaksat1608and1505cm1aretheindicationofC=Caromaticstretchvibration(Senturketal.
2010).
Thebandinthe1233cm1isduetothebendingmodesofO–C–H,C–C–H,andC–O–H.
Thebandat1033cm1wasassignedtoC–Ostretching,whichalsoconfirmedthepresenceofligninontheQCS(Pavanetal.
2008).
TheFT-IRspectrumofchemicallytreatedQCSindicatesthatthepeaks,duetotheabovefunctionalgroups,wereaffectedintheirpositionandintensity.
QCSusedinthisstudywasanalyzedbyscanningelectronmicroscopy(SEM)inordertoexamineitsmorphology.
SEMimageofporoussurfaceofnaturalandchemicallytreatedbiomassisillustratedinFig.
2.
Theimagealsorevealsthattheexternalsurfacewasfullofcavities,whichsuggestthatchemicallytreatedQCSmaterialexhibitsahighsurfacearea.
Similarmorphologywasalreadyseenforatreatedolivestonewithsodiumhydroxide(Azizetal.
2009).
PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
19574000.
03000200015001000650.
050.
0556065707580859095100.
0cm-1%T3330292117311606150912331021(a)4000.
03000200015001000650.
056.
06065707580859095100.
0cm-1%T3330292015991234102717231505892(b)Fig.
1.
FTIRspectraof(a)rawQCSand(b)chemicallytreatedQCSPEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1958(a)(b)Fig.
2.
SEMmicrographsof(a)rawQCS;(b)chemicallytreatedQCSAdsorptionExperimentsAdsorptionofCo(II)ontoQCSwassystematicallyinvestigatedbyparameterssuchasadsorbentdosage,initialconcentration,temperature,contacttime,andnaturalpH.
Theexperimentalresultsandtherelevantobservationsarediscussedinthefollowingsections.
EffectofinitialconcentrationThemetaluptakemechanismwasespeciallydependentontheinitialheavymetalconcentration(Co),andatlowconcentrationsmetalswereadsorbedbyspecificsites.
Nevertheless,byincreasingmetalconcentrationsthespecificsitesbecomesaturated(Saeedetal.
2005).
TheCo(II)biosorptioncapacityofQCSispresentedasafunctionofmetalionconcentrationinFig.
3.
InitialconcentrationsofCo(II)ionswerestudiedbyvaryingtheconcentrationsfrom25to100mgL1.
TheamountofCo(II)ionsadsorbedperunitmassofthebiosorbentincreasedwiththeincreaseininitialconcentrationofmetalions.
ThemaximumamountofCo(II)adsorbedonthisbiosorbentwas17.
72mgg1.
EffectofadsorbentsdosageTheinfluenceofQCSdosageoncobaltbiosorptionwasexaminedbyvaryingdosagesfrom1.
0to4.
0gL-1(Fig.
4).
Theoptimumbiosorbentdoseselectedwas4.
0gL-1fortherestoftheexperimentalstudies.
Fromtheanalysisofexperimentaldataobtainedforcobaltion,itwasshownthattheremovalefficiencyincreasedwiththeincreaseinbiosorbentdosage.
Anincreaseinbiomassconcentrationgenerallyincreasesthebiosorbedmetalionsbecauseofanincreaseinsurfaceareaofthebiosorbent,whichinturnincreasesthebindingsites(Vijayaraghavanetal.
2006).
PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
195902468101214161820020406080100t(min)qt(mgg-1)25mg/L50mg/L100mg/LFig.
3.
InfluenceofinitialconcentrationoncobaltbiosorptionbychemicallytreatedQCS(biosorbentdossage:4g/L;pH:natural;temperature:298K;contacttime:90min).
0510152025020406080100t(min)qt(mgg-1)1.
0g/L2.
0g/L4.
0g/LFig.
4.
InfluenceofbiosorbentdoseoncobaltbiosorptionbychemicallytreatedQCS(cobaltconcentration:50mgl1;pH:natural;temperature:298K;contacttime:90min)PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1960EffectofpHonadsorptionprocessIndustrialwastewatersarecharacterizedbysubstantialvariationsinpHvalues,andhencetheinitialpHofthesolutionisanimportantfactortobeconsideredduringadsorptionstudies(Suhasinietal.
1999).
TheeffectofpHonCo(II)ionsbiosorptionisshowninFig.
5.
ThepHrangestudiedwasbetween3.
0and11.
0.
AtlowpHvalues,thehighhydrogenionsadsorbedattheinterfacemayrepelpositivelychargedmetalionselectrostaticallyandpreventstheirapproachtoQCSsurface(Cokunetal.
2006).
Thelowestcobaltsorptioncapacity(7.
33mgg1)wasfoundataninitialpHsolution(pH:3.
0).
AstheinitialpHincreasedto5,cobaltsorptioncapacityincreasedto12.
46mgg1.
Cobaltadsorptioncapacityvariedlittleandthesorptioncapacitywaskeptconstant(about12.
26mgg1)intheinitialpH7.
0.
WhentheinitialpHwas9,thesorptioncapacitywasincreasedto12.
50mgg1,whichcouldbeattributedtotheprecipitationofCo(OH)2.
SimilarresultshavealsobeenreportedbyKaraetal.
(2003)fortheadsorptionofCo(II)ionsontosepiolite.
5678910111213024681012pHqt(mgg-1)Fig.
5.
EffectofpHonadsorptionCo(II)ontochemicallytreatedQCS(Coconcentration:50mgL1;biosorbentdossage:4.
0gL-1;temperature:298K,time:90min.
)EffectofTemperatureTemperatureisoneoftheimportantparametersforsuccessfulbiosorptionapplication(SarandTuzen2008b).
TheeffectoftemperatureonCo(II)ionsbiosorptionisshowninFig.
6.
Overtherangeinvestigated(288to308K)temperature-relatedeffectswerenotsignificant.
Foreconomicconsiderations,roomtemperaturewaschosenastheoptimumfortheadsorptionofCo(II)ionsbyQCS.
PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
19614681012020406080100t(min)qt(mgg-1)288K298K308KFig.
6.
InfluenceoftemperatureoncobaltbiosorptionbychemicallytreatedQCS(Coconcentration:50mgL1;biosorbentdosage:4.
0gL1;pH:natural;contacttime:90min)DesorptionexperimentTheresultsofdesorptionexperimentsarepresentedinFig.
7,whichclearlyindicatesthateffectivedesorptionofCo(II)wasobservedwithmineralacids(HClandHNO3)andchelatingagentEDTAafter30min.
AsshowninFig.
7,desorptionratioofcobaltionsusingHClapproachedtoabout98%,whileEDTAandHNO3showedabout90%and86%oftheadsorbedcobaltions,respectively.
Consequently,HClwaschosenasthebestdesorbentforcobaltions.
708090100102030405060t(min)%Co(II),recovered0.
1MEDTA0.
1MHCl0.
1MHNO3Fig.
7.
Cobalt(II)recoverybydifferentdesorbentsPEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1962SorptionIsothermsTheequilibriumadsorptionisothermsareoneofthemostimportantformsofinformationbywhichonecanunderstandthemechanismoftheadsorptionsystems.
Theadsorptionequilibriumdatawerefurtheranalyzedusingtwowell-knownisothermmodels,theLangmuirandFreundlichmodels.
TheLangmuirisothermtheoryassumesmonolayercoverageofadsorbateoverahomogenousadsorbentsurface(Karaoluetal.
2009;Anayurtetal.
2009).
TheLangmuirisothermisgivenbyEqs.
1and2,eemeKCKCqq1(1)memeeqCKq1qC(2)whereqe(mgg-1)andCe(mgL-1)aretheamountofadsorbedCo(II)perunitweightofadsorbentandtheun-adsorbedCo(II)concentrationinsolutionatequilibrium,respectively,qmisthemaximumamountoftheCo(II)boundperunitweightofadsorbenttoformacompletemonolayeronthesurfaceathighCe,andKistheequilibriumconstantorLangmuirconstantrelatedtotheaffinityofbindingsites(Lmg-1).
qmandKwerecalculatedfromtheslopeandinterceptofthestraightlinesoftheplotCe/qevs.
Ce(TahirandRauf2006;Karaoluetal.
2010b).
TheempiricalFreundlichmodelcanbeappliedfornon-idealsorptiononheterogeneoussurfacesandmultilayersorption.
TheFreundlichequationmaybewrittenas(OhandTshabalala2007;Ghassabzadehetal.
2010),n1eFeCKq(3)eFelnCn1lnKlnq(4)whereKFisaFreundlichconstantthatshowsboththeadsorptioncapacityofanadsorbentandthestrengthoftherelationshipbetweenadsorbateandadsorbent.
Theslope1/n,rangingbetween0and1,isameasureofadsorptionintensityorsurfaceheterogenity,becomingmoreheterogeneousasitsvaluegetsclosertozero.
Ingeneral,KFincreasestheadsorptioncapacityofanadsorbentforagivenadsorbateincreases.
KFand(1/n)canbedeterminedfromthelinearplotoflnqevs.
lnCe(ErenandAcar2006;Karaoluetal.
2009).
ValuesofK,qm,KF,andnwerecalculatedfromtheinterceptandslopeoftheplots.
ThevaluesforqmandKarepresentedinTable1.
PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1963Table1.
CharacteristicParametersofSorptionProcessofCo(II)onQCSTemp.
(K)LangmuirisothermFreundlichisothermqm(mgg-1)K(Lmg-1)x103R2RLR229833.
008.
510,990.
670-0.
9860.
63TheresultsgiveninTable1showthattheLangmuirisothermdescribedcobaltbiosorptionbyQCSbetterthantheFreundlichisotherm.
FromTable1,itwasseenthattheFreundlichisothermwasnotsuitable(R21),linear(RL=1),favorable(0TheRLvaluesarereportedinTable1,whichshowstheadsorptionbehaviourofQCS.
ThevalueofRLwasfoundtobeintherangeof0to1,indicatingthattheadsorptionprocessisfavourableforCo(II).
AdsorptionKineticStudiesInordertoexaminethecontrollingmechanismoftheadsorptionprocess,pseudofirst,pseudosecond-orderequationsandintra-particlediffusionmodelwereusedtotesttheexperimentaldata(Chairatetal.
2005).
TheLagergrenpseudofirst-orderequationisgivenasfollows(SarandTuzen2009):tklnq)qln(q1ete(6)Thestraightlineoftheplotofln(qe-qt)versustimesuggeststheapplicabilityoftheLagergrenequationforthepresentsystem.
ThevaluesofR2weredeterminedfromtheslopeoftheplotsandaregiveninTable2.
Thepseudo-second-ordermodelcanberepresentedinthefollowingform(HoandMcKay1999;PrasadandAbdullah2010),e2e2tqtqk1qt(7)wherek2istheadsorptionrateconstant(gmg-1min-1).
Theplotoft/qtversustimeissimilarasshowninWeberandMorris.
Thevaluesofk2weredeterminedbytheslopeoftheplotandaregiveninTable2.
PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1964Table2.
First,Pseudo-secondOrderKinetics,andIntra-particleDiffusionModelParametersfortheAdsorptionSystemsintheStudyParametersFirst-orderPseudo-second-orderIntra-particlediffusionR2qe(calc)(mgg-1)qe(exp)(mgg-1)k2(g/mgmin)R2ki,1R2ki,2R2D(cm2s-1).
106t1/2(min)Temp.
(K)2880.
97012.
5912.
770.
05060.
9991.
7740.
0.
9950.
0600.
6816.
021.
5692980.
83012.
3312.
700.
03500.
9991.
2750.
9790.
0510.
8894.
072.
3203080.
96812.
1212.
540.
02720.
9992.
1040.
8920.
1920.
8283.
113.
039InitialCon.
(mgL-1)250.
6616.
256.
252.
81310.
9990.
5080.
9530.
5080.
953168.
780.
056500.
83012.
3312.
700.
03500.
9991.
2750.
9790.
0510.
8894.
072.
3201000.
79517.
7217.
760.
20580.
9990.
7840.
9550.
0320.
99834.
740.
272Ads.
dos.
(gL-1)1.
00.
76222.
7722.
980.
03340.
9992.
2120.
9780.
2340.
9957.
181.
3152.
00.
93817.
3317.
540.
03950.
9991.
1190.
9040.
1480.
9996.
461.
4614.
00.
83012.
3312.
700.
03500.
9991.
2750.
9790.
05130.
8894.
072.
320Thecoefficientsofdeterminationforthepseudo-first-ordermodelwereintherangeof0.
661to0.
968.
TheresultsindicatedthattheadsorptionofCo(II)ontoQCSdidnotfollowthepseudo-first-orderkinetics.
Asaresult,thekineticadsorptiondatawerefurtherfittedthepseudo-second-orderkineticmodel(R2:0.
999)(Table2).
Thehalf-adsorptiontime,t1/2,isdefinedasthetimerequiredfortheadsorptiontotakeuphalfasmuchQCSasitsequilibriumvalue.
Thistimeisoftenusedasameasureoftheadsorptionrate(Doanetal.
2009).
e21/2qk1t(8)InEq.
8k2isthesecond-orderadsorptionrateconstant(gmg-1min-1).
Thevaluesoft1/2determinedforthetestedparametersaregiveninTable2.
Afurtherverificationoftheexothermicnatureoftheprocesswasdonebycalculatingthehalf-lifeofprocessateachtemperature,whichwasfoundtoincreasewithincreasingtemperatures.
Anempiricallyfoundfunctionalrelationship,commontothemostadsorptionprocesses,isthattheuptakevariesalmostproportionallywitht1/2,theWeber–Morrisplot,ratherthanwiththecontacttimet(Weberetal.
1963;Alkanetal.
2007).
Ctkqdift(9)InEq.
9qtistheamountofCo(II)adsorbedattimet(mgg-1),Cistheintercept,andkdifistheintra-particlediffusionrateconstant(mgs-1/2g-1).
AccordingtoEq.
(9),aplotofqtPEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1965versust1/2shouldbeastraightlinewithaslopekdifandinterceptCwhentheadsorptionmechanismfollowstheintra-particlediffusionprocess.
Theintra-particlediffusionplotsaregiveninFigs.
8through10fortheeffectofparticlesize,initialCo(II)concentration,andtemperaturesonbiosorptionrate,respectively.
Thelinearityoftheplotsdemonstratedthatintra-particlediffusionplayedasignificantroleintheuptakeofCo(II)byQCS.
Inthisstudynoplotpassedthroughtheorigin.
Thisindicatesthatalthoughintra-particlediffusionwasinvolvedintheadsorptionprocess,itwasnotthesolerate-controllingstep.
ThisalsoconfirmsthatadsorptionofCo(II)onthebiosorbentwasamulti-stepprocess,involvingadsorptionontheexternalsurfaceanddiffusionintotheinterior(Doanetal.
2009).
FromFigs.
8through10,atallconditions,thesorptionprocesstendedtotakeplaceintwophases.
Itwasfoundthataninitiallinearportionendedwithasmoothcurve,followedbysecondlinearportion.
Thetwophasesintheintra-particlediffusionplotsuggeststhatthesorptionprocessproceedsfirstthroughsurfacesorptionandthenintra-particlediffusion.
Theinitialcurvedportionoftheplotindicatesaboundarylayereffect,whilethesecondlinearportionisduetointra-particleorporediffusion.
Sinceki,1valuesforfirstpartofplotwerehigh,thisstepisnotratelimitingstep.
Theslopeofsecondlinearportionoftheplothasbeendefinedastheintra-particlediffusionparameterki,2(mg/(gmin0.
5).
Ontheotherhand,theinterceptoftheplotreflectstheboundarylayereffect.
Valuesofinterceptgiveanideaaboutthethicknessofboundarylayer.
Thelargertheintercept,thegreateristhecontributionofthesurfacesorptionintherate-limitingstep(Alkanetal.
2008).
Thecalculatedintra-particlediffusioncoefficientki,1andki,2valuesatdifferentconditionsaregiveninTable2.
Theki,1andki,2expressdiffusionratesofthedifferentstagesintheadsorption.
Atthebeginning,Co(II)wasadsorbedbytheexteriorsurfaceofQCSparticle,sotheadsorptionratewasveryfast.
Whentheadsorptionoftheexteriorsurfacereachedsaturation,Cobalt(II)ionenteredintotheQCSparticlethroughtheporewithintheparticleandwasadsorbedbytheinteriorsurfaceoftheparticle.
Therate-limitingstepinadsorptionprocessisintra-particlediffusionduetolowki,2values.
Thevaluesofdiffusioncoefficientlargelydependonthesurfacepropertiesofadsorbents.
Thediffusioncoefficientsfortheintra-particletransportofCo(II)withintheporesofQCSparticleshavebeencalculatedundervariousconditionsbyfollowingtheequation,Dr030.
0t2021(10)wheret1/2isthehalflifeinsecondsascalculatedfromEq.
(10),r0istheradiusoftheadsorbentparticleincentimetres,andDisthediffusioncoefficientvalueincm2s-1.
Inthesecalculations,ithasbeenassumedthatthesolidphaseconsistsofsphericalparticleswithanaverageradiusbetweentheradiicorrespondingtoupper-andlower-sizefractions.
Thevalueofr0wascalculatedas1.
775*102cmforQCSsamples.
Calculatedvaluesoft1/2andDaregiveninTables2.
DvaluesfortheadsorptionofCo(II)underdifferentconditionsareintherangeof3.
11*106to168.
78*106cm2s-1.
SimilarresultswerefoundforCr(III)onvineyardpruningwaste(Karaoluetal.
2010a).
PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1966Fig.
8.
IntraparticlediffusionplotsfordifferentparticlesizesFig.
9.
Intraparticlediffusionplotsfordifferentinitialcobalt(II)PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1967Fig.
10.
IntraparticlediffusionplotsfordifferenttemperaturesThermodynamicStudyThermodynamicparameterswerecalculatedtoconfirmtheadsorptionnatureofthepresentstudy.
Thethermodynamicconstants,freeenergychange(G*),enthalpychange(H*),andentropychange(S*)werecalculatedtoevaluatethethermodynamicfeasibilityoftheprocess.
TheactivationenergyofadsorptionwasalsocalculatedfromthelinearizedArrheniusequation(BulutandAydn2006),TREklnklnga02(11)whereEaistheArrheniusactivationenergy,andk0istheArrheniusfactor.
Toextractk0andEafromkineticdata,weplottheseriesofrateconstantsmeasuredatdifferenttemperaturesinagraphoflnkversus1/T.
Thecorrespondingactivationenergywasdeterminedfromtheslopeofthelinearplot.
Theresultobtainedwas-22.
92kJmol-1forthebiosorptionofCo(II)ontoQCS.
Thethermodynamicparameterssuchaschangeinstandardenthalpy(H*)andentropy(S*)weredeterminedbyusingEyringequations,TR*ΔHR*ΔShklnTklnggb2(12)wherekbandhareBoltzmann'sandPlanck'sconstants,respectively.
AccordingtoEq.
(12),aplotofln(k/T)versus1/Tshouldbeastraightlinewithaslope-(ΔH*/Rg)andintercept[ln(kb/h)+(ΔS*/Rg)].
H*andS*werecalculatedfromtheslopeandinterceptPEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
1968ofline.
Gibbsenergyofactivationmaybewrittenintermsofentropyandenthalpyofactivation:*ST*HG*(13)G*wascalculatedat298KfromEq.
(13).
ItwasdeterminedthatthevaluesofthefreeenergyG*,enthalpy(H*),andentropy(S*)ofactivationwere81.
21kJmol-1,-25.
40kJmol-1,and-357.
76jmol-1K-1,respectively.
ThenegativeH*valueindicatedexothermicnatureoftheadsorption.
ComparisonoftheAdsorptionCapacityofQCSAdsorptioncapacityoftheadsorbent,QCS,wascomparedwiththatofthecapacitiesofothernonconventionallow-costadsorbents.
ItcanbeseenfromTable3thattheadsorptioncapacityofQCSissignificantandcomparabletothatofotheradsorbentsusedfortheremovalofCo(II).
TheresultsshowtheapplicabilityofQCSfortheremovalofCo(II)fromaqueoussolutions.
Table3.
TheComparisonofCo2+SorptionCapacitiesofVariousSorbentsAdsorbentsqmax.
(mgg-1)ReferencesLemonpeeladsorbent22.
00Bhatnagaretal.
2010Almondgreenhull45.
50Ahmadpouretal.
2009Areashellbiomass11.
53Dahiyaetal.
2008Coirpith12.
82Parabetal.
2006Blackcarrotresidues5.
35Güzeletal.
2008Marinegreenalga46.
10Vijayaraghavanetal.
2005aCrapshell20.
47Vijayaraghavanetal.
2005bRawQCS6.
29InthisstudyChem.
QCS33.
00InthisstudyCONCLUSIONS1.
AdsorptionstudieswithQuercuscocciferashell(QCS)asbiosorbentrevealedtheabilityofplantbiomaterialstoremoveCo(II)fromtheaqueousphase.
2.
QCSisaneffectiveadsorbentfortheremovalofCo(II)fromaqueoussolutions.
3.
TheLangmuirandFreundlichadsorptionmodelswereusedtoexpressthesorptionphenomenonofthesorbate.
TheequilibriumdatawerewelldescribedbytheLangmuirModel.
4.
Variouskineticmodelswereusedtodescribetheadsorptionprocess.
Thekineticdatashowedthatthepseudo-second-orderkineticmodelwasobeyedbetterthanpseudo-first-orderkineticmodel.
5.
ThenegativeH*valueindicatedanexothermicnatureoftheadsorption.
PEER-REVIEWEDARTICLEbioresources.
comKaraoluetal.
(2011).
"CobaltadsorptionbyQuercus,"BioResources6(2),1954-1971.
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Articlesubmitted:February17,2011;Peerreviewcompleted:March27,2011;Revisedversionreceivedandaccepted:April13,2011;Published:April18,2011.