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Biochemical Engineering Journal
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Analysis of mass transfer kinetics in the biosorption of synthetic dyes onto Spirulina platensis nanoparticles
G.L.Dotto, L.A.A.Pinto∗
UnitOperationLaboratory,SchoolofChemistryandFood,FederalUniversityofRioGrande–FURG,475EngenheiroAlfredoHuchStreet,96203-900RioGrande,RS,Brazil
a r t i c l e i n f o
Articlehistory:
Received24April2012
Receivedinrevisedform9July2012 Accepted15July2012
Available online 21 July 2012
Keywords:
Biotnumber Biosorption Diffusion Dyes Masstransfer Microalgae
a b s t r a c t
Inthisresearch,themasstransferkineticsforthebiosorptionofsyntheticdyes(acidblue9andFD&C redno.40)bySpirulinaplatensisnanoparticleswasanalyzedunderdifferentexperimentalconditions.
Theexternalmasstransfermodel(EMTM)andthehomogeneoussoliddiffusionmodel(HSDM)were employedtostudythemasstransferkineticsandalsotoestimatethevaluesofexternalmasstransfer coefficient(kf)andintraparticlediffusioncoefficient(Dint).TheBiotnumber(Bi)wasusedtoverifythe importanceofexternalmasstransferinrelationtointraparticlediffusion.Thevaluesofexternalmass transfercoefficient(kf)rangedfrom1.67×10−6to11.40×10−6cms−1 andtheintraparticlediffusion coefficient(Dint)rangedfrom0.70×10−14to4.30×10−14cm2s−1.TheBiotnumbers(0.53≤Bi≤10.33) showedthat,forbothdyes,thebiosorptionontoS.platensisnanoparticleswascontrolledsimultaneously byexternalmasstransferandintraparticlediffusion.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Theinadequatedisposalofwastewatersoriginatedfromdye productionandapplicationpresentaveryseriousenvironmental problem[1],beingarisktotheaquaticecosystemsandtohuman health[2,3].Manymethodshavebeenemployedtoremovesyn- theticdyesfromindustrialeffluents[3–5].Amongthesemethods, thebiosorptionisanemergent,competitive,effectiveandinexpen- sivetechnologywhichreducestheconcentrationofsyntheticdyes toacceptablelevels[5–9].Forthispurpose,variousbiosorbents, suchas,fungi,bacteria,chitosan,peatandalgaearereportedin theliterature[2,5–11].Recently,themicroalgaeSpirulinaplatensis wasproposedasanalternativebiosorbenttoremovaldyesfrom wastewater[9,12,13],however, researchesaboutitsapplication arelimited.
S.platensisisavailableinlargequantities,islargelycultivated throughoutworldwideandrelativelycheap[14–16].Inaddition,its biomasscontainsavarietyoffunctionalgroups,suchas,carboxyl, hydroxyl,sulfateandotherchargedgroupswhichcanberesponsi- bleforpollutantsbinding[9,12,16–18].Thesecharacteristicsshow thattheS.platensisisapromisingbiosorbent[16–18].However,all propertiesofS.platensisarenotaccessibleinthenaturalformofthe
∗Correspondingauthor.Tel.:+555332338645;fax:+555332338745.
E-mailaddresses:guilherme[email protected](G.L.Dotto), [email protected](L.A.A.Pinto).
biomass.Inthisway,ourresearchgrouphaspreparedS.platensis nanoparticlestoimproveitsbiosorptionproprieties[12].In our recent study[12], theequilibrium andthermodynamics for the biosorptionoffooddyesontoS.platensisnanoparticleswereeluci- dated,butthereisnoinformationintheliteratureaboutthemass transferkineticsofdyesbiosorptionontoS.platensisnanoparticles.
Inbiosorptionsystems,itisfundamentalthestudyofthemass transferkineticsandthepotentialratecontrollingsteps[5].From thisanalysis,thesoluteuptakerate,whichdeterminestheresi- dencetimerequiredforcompletionofbiosorptionreaction,may beestablished[5,19].Biosorptionmasstransferkineticsmustcon- siderthethreefollowingsteps:externalmasstransfer,intraparticle diffusionanduptake ofmoleculesbytheactivesites[5,20–22].
Generally,fordyeremovalbybiosorbents,thislaststepisveryfast [5,7,9,12],so,theprocessiscontrolledbyexternalmasstransfer or/andintraparticlediffusion.Thesestepscanbeaffectedbyvari- ousfactors[5,7,11,20,22],beingimportantverifyitsinfluencesfor designpurposes.
Thisworkaimedtoelucidatethemasstransferkineticsofthe synthetic dyes (acid blue 9 and FD&C red no. 40) biosorption ontoS.platensis nanoparticles.Thenanoparticleswereobtained fromS.platensisdeadbiomassandcharacterizedbydynamiclight scattering(DLS),N2-adsorptionisotherms(BETmethod)andscan- ningelectronmicroscopy(SEM). Themasstransferkineticswas analyzedunderdifferentconditionsofpH(2–4)andstirringrate (50–400)bytheexternalmasstransfer(EMTM)andthehomoge- neoussoliddiffusion(HSDM)models.
1369-703X/$–seefrontmatter© 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.bej.2012.07.010
Fig.1. Optimizedthree-dimensionalstructuralformulaeofthedyes:(a)acidblue 9and(b)FD&Credno.40.
2. Materialsandmethods
2.1. Dyes
Thesyntheticdyesacidblue9(triphenylmethanedye,molec- ular weight 792.8gmol−1, color index 42,090, max=408nm, pKa=5.6and6.6)andFD&Credno.40(azodye,molecularweight 496.4gmol−1,colorindex16,045,max=500nm,pKa=11.4)were suppliedbylocalmanufacturer,PluryChemicalLtd.,withapurity higherthan85%.Theoptimizedthree-dimensionalstructuralfor- mulaeof thedyes (obtained fromChemBio 3D 11.0.1software (CambridgeSoft,USA))areshowninFig.1.Thedyeswavelength isconstantwiththepH.Distilledwaterwasusedtoprepareall solutions.Allreagentswereofanalytical-grade.
2.2. PreparationandcharacterizationofS.platensisnanoparticles S. platensis nanoparticles were prepared by a previously reportedmechanicalagitationmethod[12].Insummary,S.platensis strain LEB-52 was cultivated in a 450L open outdoor photo- bioreactors,beingthebiomassrecoveredwithamoisturecontent of 0.76kgkg−1 (wet basis) [23]. The wet biomass was dried [24],ground(WileyMillStandard,no.03,USA)andsieveduntil thediscrete particlesize rangedfrom 68 to75m. Thesieved biomass(250mg)wasaddedin distilled water(90mL) andthe pHwasadjusted (2,3and 4)using10mLof abufferdisodium phosphate/citricacidsolution(0.1molL−1).Thesuspensionwas
agitated(Dremel,1100-01,Brazil)at10,000rpmfor20min[12].
Detailedinformationcanbeobtainedintheliterature[12,23,24].
Themeandiameter(dp)andpolydispersityindex(PDI)ofthe nanoparticleswereobtainedbydynamiclightscattering(DLS)[25].
Thedynamiclightscatteringequipmentwasconstitutedbyalaser (Spectra-physics,127,USA)coupledtoagoniometer(Brookheaven, BI-200M,USA)andadigitalcorrelator(Brookheaven,BI-9000AT, USA).
Thespecificsurfacearea,porevolumeandaverageporeradius of thenanoparticles wereobtainedby a volumetric adsorption analyzer(QuantachromeInstruments,NewWin2,USA)usingthe Bennett,EmmetandTeller(BET)method[26].Theapparentden- sityandvoidfractionofthenanoparticleswereestimatedbythe followingequations[26]:
Vp= 1 p− 1
s (1)
εp=1−p
s
(2) wherepistheapparentdensity(kgm−3),sisthesoliddensity (kgm−3),Vpistheporevolume(m3kg−1)andεpisthevoidfraction.
InordertoverifythesurfacemorphologyofS.platensisnanopar- ticles,andalsotoconfirmitsmeandiameter,imageswereobtained fromscanningelectronmicroscopy(SEM)(Jeol,JSM-6060,Japan) [8].
2.3. Biosorptionexperiments
Thebiosorptionexperimentswerecarriedoutusingbatchsys- temsatdifferentvaluesofpH(2,3and4)andstirringrate(50,225 and400rpm)(Thesevaluesweredeterminedfromtheliterature andpreliminarytests.). Firstly,100mLofa suspensioncontain- ing250mgofS.platensisnanoparticleshadthepHadjusted(2,3 and4)(Mars,MB10,Brazil)throughthe50mLofbufferdisodium phosphate/citricacidsolution(0.1molL−1),whichdidnotpresent interactionwiththedyes[10].After,50mLofasolutioncontain- ing10gL−1ofdyewasaddedtoeachS.platensissuspension,and itwascompletedto1Lwithdistilledwater,thus,theinitialdye concentrationwasapproximately500mgL−1.
Theexperimentswerecarriedoutinajartest(Novaetica,218 MBD,Brazil),underambienttemperature(25±1◦C)[12].Aliquots werewithdrawninpresettimeintervals(2,4,6,8,10,15,20,25,30, 40,60,80,100and120min).Thebiomassandbiosorbeddyeswere removedfromtheliquidthroughafiltrationwithWhatmannFilter Paperno.40,whichdidnotpresentinteractionwiththedyes[10], andthedyesconcentrationwasdeterminedbyspectrophotometry (Quimis,Q108,Brazil).Allexperimentswerecarriedoutinreplicate (n=3)andblankswereperformed.
Themean biosorptioncapacity attime “t” (qt)(mgg−1)was calculatedasfollows:
qt= C0−Ct
m V (3)
whereC0istheinitialdyeconcentrationinliquidphase(mgL−1), Ctisthedyeconcentrationinliquidphaseattimet(mgL−1),mis biosorbentamount(g)andVisthevolumeofsuspension(L).
2.4. Analysisofmasstransferkinetics
Inthiswork,themasstransferkineticswasanalyzedasfollows:
Firstly,toidentifythedifferentmasstransferstepsthatoccurinthe biosorptionprocess,theexperimentalvaluesof“qt”wereplottedas afunctionof“t0.5”[27].After,EMTM(externalmasstransfermodel) andHSDM(homogeneoussoliddiffusionmodel)modelswerefit- tedwiththeexperimentaldata(firstandsecondlinearportions,
respectively)inordertoestimatetheexternalmasstransfercoef- ficient(kf)andtheintraparticlediffusioncoefficient(Dint).Finally, theBiotnumber(Bi)wascalculatedtoverifytherelativeimpor- tanceofexternalmasstransfertointraparticlediffusion[28,29].
TheEMTMisbasedupontheassumptionthattheexternalmass transportiscontrollingtheoverallrateofbiosorption[26].Accord- ingtoSuzuki [28], inthis case, thesolute concentrationinthe particleisassumedtobeuniform,andthemasstransfercanbe representedbythefollowingequation:
dCt
dt =−kfSA(Ct−Cs) (4)
whereCsisthesoluteconcentrationattheexternalsurfaceofthe biosorbent(mgL−1), kf is theexternal mass transfercoefficient (cms−1)andSA(cm−1)isdefinedasfollows:
SA= 6(m/V)
dpp(1−εp) (5)
Whent→0,Cs→0andCt→C0,thismanner,Eq.(4)canbeinte- gratedleadingto[28]:
Ct
C0 =exp(−kfSAt) (6)
Inthisway,Eq.(6)canbefittedtotheexperimentaldatatocheck thebiosorptionmechanism,andalsothekfvaluescanbeestimated.
Whenmasstransferresistanceisinternal,intraparticlediffusion controlstheprocess.Inthiscase,theHSDMcandescribethemass transferinanamorphousandhomogeneoussphere[28]forauni- directionalandisothermalprocess[29].Ifintraparticlediffusivity isconsideredconstant,theHSDMcanbepresentedintheformof [28]:
∂q
∂t =D
int
r2
∂
∂r
r2∂q
∂r
(7) whereDintistheintraparticlediffusioncoefficient(cm2s−1),ris theradialposition(cm),andqthebiosorptionquantityofsolutein thesolid(mgg−1)varyingwithradialpositionattimet.Theinitial andboundaryconditionsarepresentedasfollows[22,28,29]:
q(r,0)=0 (8)
q
dp
2,t
=qe (9)
∂q
∂r
r=0
=0 (10)
whereqeisthebiosorptioncapacityatequilibrium(mgg−1).
ThebalanceforthebatchsystemisgivenbyEq.(11)[28]:
V
dC
dt
=−mdq
t
dt
(11) In the concentration range of this work, the Henry model showed a good fit with the equilibrium experimental data in thebiosorptionof bothdyesontoS.platensisnanoparticles(see Supplementarymaterial).Then,therelationshipbetweentheequi- libriumconcentrationsofthefluidphaseandbiosorbedphasecan beconsideredlinear.Inthiscase,forfinitevolumeprocess,Crank [30]developedasolutionforEq.(7),whichcanbeapproximated tothefirsttermofserieswhentheFouriernumberishigherthan 0.2:
qt
qe =1−
6˛(˛+1)exp(−q2nDintt/R2p) 9+9˛+q2n˛2
(12) where Rp is the particle radius (cm), ˛ is the effective vol- umeratio,expressed asa function of theequilibrium partition
Table1
CharacteristicsofS.platensisnanoparticles.
Characteristic Valuea
Polydispersityindex(PDI) 0.150±0.010
Meandiameter(dp)(nm) 215±10
Specificsurfacearea(SS)(m2g−1) 14.0±0.1 Porevolume(Vp)(m3kg−1)×106 6.9±0.1
Averageporeradius(AR)(Å) 23.0±0.2
Soliddensity(s)(kgm−3) 1391.5±1.2
Apparentdensity(p)(kgm−3) 1378.3±1.5
Voidfraction(εp) 0.010±0.002
aMean±standarderrorintriplicate.
coefficient(solid/liquidconcentrationratio)(Ce/C0−Ce)andqncan beobtainedasfollows:
tan qn= 3qn
3+˛q2n (13)
Thismanner,fromEq.(8)itispossibletoestimatetheintra- particlediffusioncoefficient(Dint)values.
Theinfluenceofeach masstransfersteponthedyebiosorp- tionresistancewasfoundbyanon-dimensionalBiotnumber(Bi), whichreflectstherelativeimportanceofexternalmasstransferto intraparticlediffusion.TheBiotnumberisdefinedasfollows[31]:
Bi= kfdpC0
2pDintq0
(14) whereq0(mgg−1)isthesolidphaseconcentrationinthebiosorbent inequilibriumwitharesidualhypotheticalliquidconcentration.
2.5. Non-linearregressionanalysis
Themasstransfer coefficients(kf andDint)weredetermined fromthefitofthemodelstotheexperimentaldatabynon-linear regression, using Statistic 7.0 software (Statsoft, USA) through Quasi-Newtonestimationmethod.Thefitqualitywasmeasured bythecoefficientofdetermination(R2)andaveragerelativeerror (ARE).
3. Resultsanddiscussion
3.1. CharacteristicsofS.platensisnanoparticles
Thecharacteristicsof S.platensisnanoparticlesare shownin Table1.ThePDIvalue(Table1)showsthattheS.platensisnanopar- ticleswerestable,relativelymonodisperseandpresentedalittle variationinthesize.AccordingtoBruceandPecora[32],anear- monodispersesystemwouldhaveaPDIvalueof0.2orlower.Fig.2 showstheSEMimagesofS.platensisnanoparticles.InFig.2isshown thatS.platensisnanoparticlespresentedasizedistributioninthe rangefrom50to500nm.Nanoparticlesarecommonlydescribed assolidcolloidalparticles,ranginginsizefrom10nmto1000nm [32].Inaddition,itcanbeobservedinFig.2thatthenanoparticles werehomogeneous,withellipsoidal–sphericalforms.
3.2. Masstransferkinetics
Toidentifythemasstransferstepsthatoccurinthebiosorption process,thebiosorptioncapacityasafunctionofthesquarerootof timewasplotted[27,33].AccordingtoWeberandMorris[27],the plotqtversust0.5showsmulti-linearity,andeachportionrepre- sentsadistinctmasstransferstep.Thefirstportionistheexternal masstransferorinstantaneousadsorptionstep.Thesecondportion isthegradualadsorptionstepwheretheintraparticlediffusioncan beratecontrolling.Thethirdportionisthefinalequilibrium[27].
TheWeber–Morrisplotsoftheacidblue9andFD&Credno.40
Fig.2. SEMimagesofS.platensisnanoparticles:(a)33,000×;(b)13,000×.
biosorptionontoS.platensisnanoparticlesunderdifferentcondi- tionsofpHandstirringrateareshowninFigs.3and4,respectively (thecurvesofCtversustimeatthesameconditionsarepresented inSupplementarymaterial).
Figs.3and 4presentedthemulti-linearity withtwo distinct phases.Theinitialportionisrelativetotheexternalmasstransfer.
Thesecondportiondescribesthegradualbiosorptionstep,where intraparticlediffusioncontrolisratelimiting.Thisshowsthatthe externalmasstransferandintraparticlediffusionoccurredsimul- taneouslyduringthebiosorptionofacidblue9andFD&Credno.
40ontoS.platensisnanoparticles.
Inordertoestimatetheexternalmasstransfercoefficient(kf) andtheintraparticlediffusioncoefficient(Dint),theexperimental dataofthefirstportionofWeber–Morrisplotwerefittedwiththe EMTM(Eq.(6)),andexperimentaldataofsecondportionwerefitted withHSDM(Eq.(12)).Themasstransfercoefficients(kfandDint) andBiotnumber(Eq.(14))forthesyntheticdyesbiosorptiononto S.platensisnanoparticlesareshowninTable2.
As can be seen in Table 2, the external mass transfer model(EMTM)presentedgoodfitwiththeexperimentaldatain
Fig.3. Weber–Morrisplotsofacidblue9biosorptionontoS.platensisnanoparticles:
(a)pHeffect(400rpm)and(b)stirringrateeffect(pH2).
relationtothefirstportionoftheWeber–Morrisplot(R2>0.95 andARE<4.00%).In thesameway,theHSDMmodel presented goodfitwiththeexperimentaldataforthesecondportionofthe Weber–Morrisplot(R2>0.95 andARE<8.00%).InTable2,three aspectscanbenotedinrelationtothekf values.Firstaspect,in general,thekfvalueswereincreasedwiththepHdecrease.This suggeststhatthepHdecreaseleadtoanincreaseinbiosorption rate,andconsequently,thecontributionofexternalmasstransfer isdecreased.ThisoccurredbecauseinlowvaluesofpH,thedyes sulfonatedgroupsweremorerapidlydissociated;inaddition,the S.platensissurfaceiseasilyprotonated,consequently,theelectro- staticattractionwasincreased,facilitatingthemasstransferinthe externallayer.ThesamedependenceofkfwiththepHwasdemon- stratedbyPiccinetal.[34].Secondaspect,thestirringrateincrease causedanincreaseinthekfvalues.Thisoccurredbecausethestir- ringrateincreasecausesanincreaseintheenergydissipationand turbulenceinthemixingzone,leadingtoanincrease insystem mobility.Thismanner,adecreasein theexternalfilmthickness occurs,decreasingtheexternalresistanceandconsequently,facil- itatingthetransferenceacrosstheexternallayer.Similarbehavior wasobtainedbyotherresearches[11,20,26].Thirdaspect,thekf valuesfortheacidblue9werelowerthankfvaluesfortheFD&C redno.40.Thiscanbeoccurredbecauseacidblue9hadahigher andmoreramifiedmolecularstructure(Fig.1).Asconsequence,its moleculardiffusivityislower[28,29],hinderingthetransference acrosstheexternallayer.
Theintraparticlediffusioncoefficientvaluesnotpresentedten- dencyinrelationtothepHandstirringrate,however,ingeneral werehigherfortheFD&Credno.40(Table2).Thiscanbeoccurred becauseFD&Credno.40hadalowermolecularstructure(Fig.1), facilitatingitstransferenceinsideoftheS.platensisnanoparticles.
