JournalofTaibahUniversityforScience10(2016)664–674
Availableonlineatwww.sciencedirect.com
ScienceDirect
Removal
of
Basic
Fuchsin
dye
from
water
using
mussel
shell
biomass
waste
as
an
adsorbent:
Equilibrium,
kinetics,
and
thermodynamics
Mohammadine
El
Haddad
∗EquipedeChimieAnalytique&Environnement,DépartementdeChimie,FacultéPoly-disciplinaire,UniversitéCadiAyyad,BP4162,46000Safi, Morocco
Received23May2015;receivedinrevisedform31July2015;accepted14August2015 Availableonline10November2015
Abstract
Thecurrentstudyfocusedontheequilibrium,kinetics,andthermodynamicsofBasicFuchsindyeadsorptionfromaqueoussolution usingmusselshellsasanadsorbent.OptimumadsorptionconditionswereidentifiedbyvaryingthesolutionpH,adsorbentdose, initialdyeconcentration,andcontacttime.EquilibriumdatawerefittedbytheLangmuir,FreundlichandDubinin–Radushkevich isothermmodels,andapseudo-second-ordermodelbestdescribedthekinetics.ThermodynamicdatashowedthatBasicFuchsin dyeadsorptionontomusselshellswasafeasible,spontaneousandendothermicprocess.Statisticalanalysiswasperformedusing theChi-square(χ2)andmeansquareerror(MSE)testmethodstoevaluatethebestfitofthemodeltotheexperimentaldata.The
adsorptionofBasicFuchsinbycalcinedmusselshellsindicatestheirpotentialapplicationasanadsorbentfortheremovalofdyes fromaqueoussolutions.
©2015TheAuthor.ProductionandhostingbyElsevierB.V.onbehalfofTaibahUniversity.Thisisanopenaccessarticleunder theCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords:Dyeremoval;Adsorption;Calcinedmusselshells;Basicdye
1. Introduction
Adsorption is one of the most common methods for the removal dyes from wastewaters. The environ-ment is contaminated by many hazardous chemical species,especiallywithchemicaldyes.Environmental
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E-mailaddress:[email protected]
PeerreviewunderresponsibilityofTaibahUniversity
http://dx.doi.org/10.1016/j.jtusci.2015.08.007
1658-3655©2015TheAuthor.ProductionandhostingbyElsevierB.V.onbehalfofTaibahUniversity.Thisisanopenaccessarticleunderthe CCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
protectionrequiresconscientiousnessregardingtheuse ofdyeswithrespecttoregulationsandthetreatmentof effluent discharge because manyindustries use chem-ical dyes to treat their finished products. Chemical dyes reduce the light penetration in water, interfering with photosynthesis, and most of them contain sus-pected carcinogens [1,2].Therefore, itis necessaryto reduce or eliminate these life-threatening compounds fromwastewaterbeforeitisdischarged.
BasicFuchsindyebelongstothetriarylmethaneclass dyes, which is inflammable in nature and possesses anaesthetic, bactericidal, and fungicidal properties. It is widely used as a colouring agent for textile and leathermaterialsandinthestainingofcollagen,muscle,
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mitochondria, and tubercle bacillus. Due to its prop-erties ofpoor biodegradation,toxicity, carcinogenicity andunsightliness, the removal of Basic Fuchsin from wastewatersystems isof greatconcern andshouldbe testedandimplementedpromptly.
The literature is rich with techniques and proce-duresforremovingchemicaldyesfromaqueousmedia. Variousauthorshaveevaluatedsuppliesofanimalor veg-etableorigin[3–7]forthedecontaminationofaqueous mediabychemicaldyes.
Inrecentyears,developingeconomicaladsorbentsto treatdyesinwastewaterhasattractedgreatinterest.The application ofinexpensiveadsorbentsfor dyeremoval hasbeenreviewed,andmanynon-conventional,low-cost adsorbentshavebeenreported.Inourongoingresearch programme,wehavefocusedondescribingnew alterna-tivemethodsforthebestremovalofchemicaldyesfrom aqueousmediausinglow-costandefficientadsorbents. In this study, the removal of Basic Fuchsin dye fromaqueoussolutionsontocalcinedmusselshellswas investigated.Theeffectsofdifferentparameterssuchas pH, contact time, adsorbent dose, initial dye concen-trationandtemperaturewereinvestigated.Theremoval ratekinetics, thermodynamicsandisothermsfor Basic Fuchsin adsorption onto calcined mussel shells were alsostudied,andthecharacterizationofcalcinedmussel shellsasanadsorbentispresented.
2. Materialsandmethods
Themusselshellswerecollectedandwashedwithtap waterfollowedbydistilledwater,thendriedat378Kfor 12h.Musselshellswerepowderedtosmallgrainsand then calcinedat 1173K for 2h. The obtained residue waswashedwithdistilledwateranddriedat353Kfor 24h.The residuewasfinelychoppedandground into smallparticlesofsizesintherangeof75–100m,milled inan agate mortar, washedwith distilledwater, dried overnightat378K,andthencalcinedataheatingrateof 2K/minto673Kandmaintainedatthistemperaturefor 4h.Theresultingcalcinedmusselshellmaterial(CMS) wasstoredinaglassbottleforfurtheruse.
The CMS adsorbent was characterized using ele-mentalanalysis, FT-IRand XRD.FT-IR spectra were obtainedusinganATIMattsonGenesisseriesFTIRTM UNICAMinstrument.XRDofCMSadsorbentwas con-ducted usingan Xpert ProX-ray diffractometerwith acopperanticathodeλ(Cu)=1.5418 ˚A.Thezeropoint chargepHoftheCMSadsorbentwasdeterminedusing thepHdriftmethodasdescribedinthereference[8].
The chemical structure of Basic Fuchsin treated as cationic dye is shown in Fig. 1. Batch adsorption
NH2 N H2 NH2 + Cl
-Fig.1.ChemicalstructureofBasicFuchsin.
experimentswereperformedbyshakinganappropriate weightofCMSadsorbentwith100mLaqueoussolution ofBasicFuchsinofknownconcentrationinaseriesof 250mLconicalflasksplacedinatemperaturecontrolled shakingwaterbathatdifferentconcentrations(between 50and200mgL−1),pHvalues(between2and12), tem-peratures(between298and428K)andadsorbentdoses (between 100mg and 500mg) at a constant shaking rateof350rpm.Afterthedesiredcontacttime,samples werewithdrawnfromthemixtureusingamicropipette andcentrifugedfor5minat5000rpm.After centrifuga-tion,supernatantswereanalysedtodeterminethefinal concentration of Basic Fuchsin using a UV–vis spec-trophotometeratawavelengthof544nm.Theamount of equilibrium adsorption qe (mgg−1) was calculated
usingtheformula
qe= C0−Ce
W V (1)
whereCe(mgL−1)istheconcentrationofBasicFuchsin
aqueoussolutionattheequilibriumstate,C0(mgL−1)
istheinitialconcentrationofBasicFuchsininaqueous solution,Visthevolumeofthesolution(L)usedinthe experiments,and W is the weight of the CMS adsor-bent (g). The Basic Fuchsin removal percentage was calculatedfromthefollowingexpression:
Removal(%)= C0−Ce
C0
×100 (2)
where C0 is the initial Basic Fuchsin concentration
(mgL−1)andCe istheconcentrationofBasicFuchsin
(mgL−1)atequilibrium.
Forstatisticalanalysis, thetestswereperformed in duplicatetoensurethereliabilityandreproducibilityof theresultsobtained,andthedatawerereportedasthe mean±SD.Themodelparametersandconstantswere analysedbylinearregressionusingExcel2010.In addi-tion to the correlationcoefficient (r2), the Chi-square (χ2),the mean squareerror (MSE)andthe validation bynormalizedstandarddeviationq(%)testmethods werealsoused toevaluatethe bestfit ofthemodel to
Fig.2.X-raydiffractionofCMSadsorbent.
theexperimentaldatausingEqs.(3),(4)and(5), respec-tively. χ2= n i=1 (qe,exp−qe,cal)2 qe,cal (3) MSE=1 n n i=1 (qe,cal−qe,exp)2 (4) q(%)= n
i=1((qe,exp−qe,cal)/qe,exp)2
n−1 (5)
wherenisthenumberofdatapoints,qe,expisthe
obser-vationfromtheexperiment,andqe,calisthecalculation
from the models. Smaller values correspond tobetter curvefitting.
3. Resultsanddiscussion
3.1. Characterization
TheresultsofelementalanalysisshowthattheCMS adsorbentcontainssignificantquantitiesofCa(60.24%) andSi(3.57%)andloweramountsofotherspeciessuch asMg(0.90%),Al(0.41%),P(0.20%)andSr(0.11%). The analysis by XRD depicted in Fig. 2 showed thepresenceofcalciteandportlandite,syn.TheFT-IR analysisshowsfunctionalgroupsexistinginthe CMS adsorbent(Fig.3).Thebandsat3643cm−1areassigned tohydroxygroupstretchingmodes,andthestretching
andfoldingofthecarbonategroupareassignedtothe bands at 1437cm−1 and 874cm−1. The SEM image shown inFig.4 shows that the CMSadsorbent parti-cleshaveirregularformsanddifferentsizes.Weseealso thatthesurfacemorphologyisnothomogenous,withthe existenceofsomepores.
3.2. Effectofadsorbentdose
The effect of adsorbent dose on the removal % of Basic Fuchsin dyefrom aqueous solutionsontoCMS adsorbent was investigated by contacting 100mL of 60mgL−1dyesolutionatroomtemperature(20◦C)for 240min ata constant stirring speed of 350rpm. Dif-ferentamountsof CMS(100–500mg)weretested for this study. Fig. 5 depicts the variation of removal % versusadsorbentdose.Inlightoftheseresults,wenote thatincreasingamountsofCMSadsorbentincreasethe removal%of BasicFuchsin.Thisbehaviourisrelated totheincreasednumberofsitesavailableontheCMS adsorbent for dye adsorption. This phenomenon was alreadydescribedinourpreviousworksondyeremoval withcalcinedbonesastheadsorbent[9].Forthe follow-ingexperiments,weused500mgofCMSper100mL ofBasicFuchsinsolution.
3.3. EffectofpH
ThepHvaluesofthesolutionareanimportant param-eterduringtheadsorptionprocesses,andtheinitialpH
Fig.3.IRspectraofCMSadsorbent.
Fig.4.SEMimageoftheCMSadsorbent.
0 10 20 30 40 50 60 70 80 90 100 100 mg 200 mg 300 mg 400 mg 500 mg Removal % Adsorbent dose (mg)
Fig. 5.Effect of adsorbent dose on the removal % of Basic Fuchsin,concentrationdye60mgL−1,temperature25◦C,contacttime 240min.
0 10 20 30 40 50 60 70 80 90 100 2 4 6 8 10 12 Removal % pH
Fig.6.EffectofpHontheremoval%ofBasicFuchsinontoCMS adsorbent,concentrationdye60mgL−1,adsorbentdose500mg, tem-perature25◦C,contacttime240min.
valueofthesolutionhasgreaterinfluencethanthefinal pH[10,11].In thiscase,Fig.6 showsthevariationof removal % of Basic Fuchsin from aqueous solutions versuspH from2 to12.Itis veryimportant to deter-minethepHofthezeropointchargeoftheCMS,which isequalto9.2,asdepictedinFig.7.ForpHvalueshigher than 9.2, the surface of the CMS adsorbent becomes negativelycharged,whileitisintheoppositestatefor pH<9.2. Fig. 6 shows that the removal % of Basic FuchsinontoCMSishighestforpHZPCabove9.2,which
indicatesthatthenegativeformof CMSisresponsible foradsorptioninthisrange.ForacidicpH,increasingthe valueincreasestheremovalefficiencyofBasicFuchsin. Thelowestremoval%atacidicpHisduetotherepulsive electrostaticforcesbetweenthepositivechargedsurface ofCMSandthecationicdye.Theremoval%ofBasic FuchsinismaximalforpHZPC,at90%.
-1.5 -0.5 0.5 1.5 2.5 2 4 6 8 10 12 ΔΔ pH Initial pH
Fig.7.DeterminationofzeropointchargepHofCMSadsorbent.
3.4. Effectofinitialdyeconcentrationandcontact
time
Fig.8showstheeffectoftheBasicFuchsinsolution concentration on adsorption ontothe CMSadsorbent. Intheseexperiments,theamountofCMSwasconstant (500mgofCMSin100mLofdyesolution),whilethe dye concentration was varied from 50 to 200mgL−1 withdifferenttimeintervals.Byplottingtheremoval% versusdyeconcentrationandcontacttime,itcanbeseen thattheremoval%ofthedyeincreaseswithincreasing concentrationandcontacttime.Themaximum adsorp-tionofBasicFuchsinisobservedat60minforalldye concentrations.Thus,thereisalmostnofurtherincrease in the adsorption,andthe equilibrium contact timeis fixed. Atthistime,removal% increasesfrom72.45% to91.2%whentheconcentrationofthedyeischanged from50mgL−1to200mgL−1.
At higher concentration, the ratio of the initial number of Basic Fuchsin molecules to the available surfaceareaishigh,andthefractionaladsorption subse-quentlybecomesdependentontheinitialconcentration. However, at lower concentration, the available sites of adsorption are fewer, and hence the dye removal dependsuponconcentration.Inlightoftheseresults,it appearsthatforthefirst40min,theadsorptionuptakeis rapid,afterwhichitproceedsatasloweradsorptionrate and finally attains saturation at 60min. The obtained removal curves were single, smooth and continuous, indicatingmonolayercoverageofthedyeonthesurface oftheCMSadsorbent.
3.5. Adsorptionkinetics
Theadsorptionkineticexperimentaldatawerefitted withthefollowingmodels.Thepseudo-first-ordermodel isexpressedbythefollowinglinearizedform[12]: log(qe−qt)=log(qe)− k1
2.303t (6)
whereqe (mgg−1)andqt (mgg−1)arethe amountof
adsorbeddyepergramofCMSadsorbentatequilibrium andattimet,respectively,andk1(min−1)isthe
pseudo-first-orderrateconstant.
Hence, a linear trace is expected between the two parameters, log (qe−qt) and t, if the adsorption
fol-lows first-orderkinetics. The valuesof k1 andqe can
bedeterminedfromtheslopeandintercept.
The pseudo-second-order model proposed by Ho and McKay [13] is based on the assumption that the adsorption follows second-order chemisorption. The
0 10 20 30 40 50 60 70 80 90 100 0 20 40 60 80 100 120 140 160 180 Removal % Time (min) 50 mg/L 100 mg/L 150 mg/L 200 mg/L
Fig.8.Effectofinitialdyeconcentrationontheremoval%ofBasic Fuchsin,adsorbentdose500mg,temperature25◦C.
linearizedformofthepseudo-second-ordermodelisas follows: t qt = 1 k2q2e + 1 qet (7) where k2 is the pseudo-second-order rate constant
(gmg−1min−1).
The experimental andcalculated (q) valueson the kineticfittingcurvescanbecomparedtoestablishthe adapted kinetics model. In addition, the correlation coefficients(r2)obtainedwereusedtovalidatethebest kineticsmodelanddescribetheadsorptionkinetics.
Theparametersofthepseudo-first-orderand pseudo-second-order kineticmodels were estimated by linear regression. The resulting data and the correlation coefficients(r2)aregiveninTable1.Fromtheseresults, the correlation coefficients for the pseudo-first-order model of Basic Fuchsinadsorption were low,andthe valuesofχ2andMSEwerehigh.Thetheoreticalqedid
notgiveacceptablevaluescomparedtotheexperimental ones.
Aplotoft/qtandtshouldgivealinearrelationshipif
theadsorptionfollowsthepseudo-second-ordermodel. Theqeandk2canbecalculatedfromtheslopeand
inter-ceptofthe plotobtainedinFig.9.Allthedetermined modelparametersandconstantswiththestatistical anal-ysisvaluesaregiveninTable1.Thecalculatedvalues forthepseudo-first-ordermodelarenotwellfittedwith theexperimentaldata,basedonthelowcorrelation coef-ficientr2andthehighvaluesofχ2,MSEandq(%). Thus,theadsorptionofBasicFuchsinontoCMSdoes
Table1
KineticparametersfortheadsorptionofBasicFuchsinontoCMSadsorbent. BasicFuchsinconcentration
50mgL−1 100mgL−1 150mgL−1 200mgL−1
Adsorptioncapacityqe,exp(mgg−1) 141.65 166.43 176.78 186.67
Pseudo-first-ordermodel qe,cal(mgg−1) 121.456 135.78 146.82 157.39 k1(min−1) 0.0654 0.0524 0.0497 0.0576 r2 0.9845 0.9876 0.9867 0.9897 χ2 1.9786 2.6573 3.2987 3.5673 MSE 3.7658 7.5438 9.5647 11.5794 q(%) 2.7857 3.6574 4.5324 5.7649 Pseudo-second-ordermodel qe,cal(mgg−1) 145.45 167.69 178.92 185.78 k2(×10−4gmg−1min−1) 35.67 28.78 24.93 19.67 r2 0.9995 0.9997 0.9996 0.9998 χ2 0.4563 0.3876 0.4356 0.5437 MSE 0.6754 1.2314 1.1138 1.2546 q(%) 0.4353 0.6759 0.6549 0.7658
Intraparticlediffusionmodel
kid(mgg−1min−1) 7.89 11.65 13.67 15.07 Ci 2.43 2.36 2.65 2.39 qe,cal(mgg−1) 63.55 92.61 108.53 119.13 r2 0.9933 0.9923 0.9918 0.9921 χ2 3.6754 3.9854 4.2346 3.7685 MSE 7.7685 11.7654 12.6543 14.6765 q(%) 6.6754 8.7658 8.6548 8.7984
0 1 2 3 0 20 40 60 80 100 t/ qt (m in .g/mg) Time (min) 50 mg/L 100 mg/L 150 mg/L 200 mg/L
Fig.9.Pseudo-second-orderplotforadsorptionofBasicFuchsinonto CMS.
notfollowthepseudo-first-ordermodel. Incontrast to thefirstmodel,wefoundahighcorrelationcoefficient
r2 (very closetounity)as wellas small χ2 andMSE values for the pseudo-second-order model, indicating thattheadsorptionprocessofBasicFuchsinontoCMS obeyspseudo-second-ordermodelkineticsatallinitial dyeconcentrations.Thevaluesofq(%)arestillhigh, butlessso.TherapiduptakeofBasicFuchsinindicates thatthe rate-determiningstep couldbechemisorption, asdescribedbymanyresearchers[14,15].
Theintraparticle diffusionmodel indicates that the rate-limitingstepisthetransportofthesolutefromthe bulkofsolutionintoadsorbentporesthroughan intra-particleprocess[16].Theequationisasfollows:
qt =kidt1/2+Ci (8)
where kid is the intraparticle diffusion rate constant
(mgg−1min1/2),andCiisthethicknessoftheboundary
layer.
It should be noted that the intraparticle diffusion modelalsodemonstratedahighcorrelationcoefficient
r2valueaswellassmallχ2andMSEvalues,indicating thatporediffusionalsoaffectstherateofBasicFuchsin adsorption.However,they-interceptCi isnotequalto
zero,whichimpliesthatthismechanismdoesnotsolely limit the overalladsorption process. The rate-limiting stepmaybe acomplexcombinationof chemisorption andintraparticletransport[17].
3.6. Adsorptionisotherms
Theadsorptionisothermisagraphicalrepresentation ofthe relationshipbetweentheamountadsorbed bya unitweightof adsorbent andtheamountof adsorbate
remaining inthe testmedium atconstant temperature underequilibriumconditions.Theisothermmodelsused toanalysetheexperimentalequilibriumdataareas fol-lows:
• Langmuirisotherm:
The Langmuirisotherm[18]assumesthat mono-layer adsorption occur at bindings sites with homogenous energy levels, with no interactions betweenadsorbed moleculesandno transmigration ofadsorbedmoleculesontheadsorptionsurface.The linearLangmuirequationisgivenasEq.(9):
Ce qe = 1 qmKL + 1 qmC e (9)
whereCe(mgL−1)istheequilibriumconcentration
ofBasicFuchsin,qe(mgg−1)istheamountofBasic
Fuchsinadsorbedperunitmassofadsorbent,andqm
(mgg−1) andKL (Lmg−1) are Langmuirconstants
relatedtoadsorptioncapacityandrateofadsorption, respectively.Astraight line withslopeof 1/qm and
interceptof1/qmKL isobtainedwhen Ce/qe is
plot-ted against Ce. The essential characteristics of the
Langmuir equation can be expressed in terms of a dimensionlessseparationfactor,RL,definedas
RL=
1 1+KLC0
(10) whereC0istheinitialconcentrationofBasicFuchsin.
The RL value indicates whether the adsorption is:
Unfavourable: RL>1; Linear: RL=1; Favourable:
0<RL<1;Irreversible:RL=0.
• Dubinin–Radushkevichisotherm:
TheDubinin–Radushkevich(D–R)isothermmodel [19]focusesontheheterogeneityofthesurface ener-giesandhasthefollowingexpression:
Ln(qe)=Ln(qm)−βε2 (11) ε=RT Ln 1+ 1 Ce (12) whereqmisthemaximumbiosorptioncapacity,βisa
coefficientrelatedtothemeanfreeenergyof biosorp-tion(mol2J−2),εisthePolanyipotential(Jmol−1), Ris the gas constant (8.314Jmol−1K−1), T is the temperature(K),andCeisthedyeequilibrium
con-centration(mgL−1). The D–R constants qm andβ
canbedeterminedfromtheinterceptandslopeofthe plotbetweenLn(qe)andε2.Theconstantβprovides
informationonthemeanfreeenergyE(kJmol−1)of adsorptionpermoleofthedyewhenitistransferred tothesurfaceofthesolidfrominfinityinthesolution
Table2
IsothermsparametersfortheadsorptionofBasicFuchsinontoCMSadsorbent. Temperature 298K 308K 318K 328K Langmuirisotherm qm(mgg−1) 141.65 132.78 123.78 111.86 KL(Lmg−1) 0.081 0.075 0.068 0.057 r2 0.9993 0.9997 0.9998 0.9996 χ2 0.3425 0.4237 0.2876 0.7658 MSE 1.7865 1.8965 1.9879 1.8764 q(%) 0.8789 0.5674 0.6754 0.7654 D–Risotherm qm(mgg−1) 127.65 116.93 105.77 94.89 β10−8(×10−8mol2J−2) 0.457 0.559 0.615 0.681 E(kJmol−1) 10.457 9.453 9.013 8.568 r2 0.9398 0.9326 0.9345 0.9342 χ2 0.9765 1.0345 0.9872 1.0654 MSE 2.6748 2.6785 2.9854 2.8976 q(%) 1.8976 1.9876 2.5468 2.7865 Freundlichisotherm Kf(mgg−1)(Lg−1)1/n 23.57 21.55 19.67 17.87 1/n 2.435 2.212 2.089 1.897 r2 0.9876 0.9856 0.9876 0.9878 χ2 1.2567 1.2765 1.2667 1.2845 MSE 3.0675 3.1764 3.0234 3.0543 q(%) 2.6754 3.5678 4.6574 5.6419
andcanbecalculatedusingthefollowingrelationship [20]:
E=√1
2β (13)
IfthemagnitudeofEisbetween8and16kJmol−1, the adsorption process is indicated to proceed via chemisorption,whileforvaluesofE<8kJmol−1,the adsorptionprocessisphysicalinnature[20]. • Freundlichisotherm:
The Freundlichmodel [21]isan empirical equa-tionbasedonsorptiononaheterogeneoussurfaceor surfacesupportingsitesofvariedaffinities.The loga-rithmicfromofFreundlichisgivenbythefollowing equation:
log(qe)=log(Kf)+
1
n log(Ce) (14)
where qe is the equilibrium dye concentration on
biosorbent (mgg−1), Ce is the equilibrium dye
concentration in solution (mgL−1), Kf (mgg−1)
(Lg−1)1/nistheFreundlichconstantrelatedto adsorp-tioncapacity,andnistheheterogeneityfactor.Kfand
1/narecalculatedfromtheinterceptandslopeofthe straightlineoftheplotlog(qe)versuslog(Ce).
In thisinvestigation,the Freundlich, theLangmuir, andtheDubinin–Radushkevich(D–R)modelswereused to describe the equilibrium data acquired at different temperatures.TheresultsareshowninTable2.
TheLangmuir isotherm constantsKL andqm were
calculated from the slope and intercept of the plot betweenCe/qeandCe.Theisothermshowedagoodfitto
theexperimentaldatawithhighcorrelationcoefficients at all temperatures, as shown in Table 2. The maxi-mumdyeadsorptioncapacitybyCMSwasfoundtobe 141.65mgg−1at298K.ThevaluesofKLdecreasedwith
increasingtemperature,indicatingthatincreasing tem-peratureinducedalowermaximumadsorptioncapacity. Todistinguishbetweenphysicalandchemical adsorp-tionontheheterogeneoussurfaces,theequilibriumdata were tested with the D–R isotherm model. The plots betweenLn(qe)andε2gavestraightlinesatall
tempera-tures;thevaluesofconstantsqmandβthusobtainedare
giveninTable3.TheestimatedvaluesofEforthisstudy werefoundintherangeexpectedforchemical adsorp-tion,asshowninTable2.Thus,theadsorptionofBasic FuchsinonthesurfaceofCMSischemicalinnature.
TheFreundlichconstantsKfand1/nwerecalculated
fromtheinterceptandslopeofthestraightlineoftheplot log(qe)versuslog(Ce).Table2showsthattheadsorption
Table3
ThermodynamicparametersfortheadsorptionofBasicFuchsinontoCMS.
Temperature(K) G◦(Jmol−1) H◦(kJmol−1) S◦(Jmol−1K−1)
298 −1444 5.247 22.45
308 −1668
318 −1893
328 −2217
magnitudeofn givesameasureofthefavourabilityof adsorption.Valuesofnbetween1and10(1/nlessthan1) representfavourableadsorption.Forthisstudy,thevalue ofn presentedthe sametrend,indicatingafavourable adsorption.
Bycomparingthevaluesofr2,χ2andMSEforthe threeisothermslistedinTable2,itcouldbeconcluded thattheadsorptionofBasicFuchsinontoCMSwasbest fittedtotheLangmuirisothermequationinthe temper-aturerangestudied.Thefitoftheadsorptiondatatothe Langmuirisothermimpliedthatthebindingenergyon thewholesurfaceoftheadsorbentwasuniform.Italso indicatedthattheadsorbeddyemoleculesdidnot inter-actor competewitheachotherandwereadsorbedby formingamonolayer.
3.7. Adsorptionthermodynamic
Thespontaneity of theadsorption processis deter-mined bythe crucial thermodynamicparameters. The standard Gibbs free energy changes (G◦), standard enthalpychanges(H◦),andstandardentropychanges (S◦)canbecalculatedfromthefollowingequations:
G◦=−RTLn(K 0) (15) Ln(K0)=S ◦ R − H◦ RT (16)
whereG◦isthe freeenergychange(kJmol−1),Ris the universal gas constant (8.314Jmol−1K−1), K0 is
thethermodynamicequilibrium constant,andT isthe absolutetemperature(K). ValuesofK0 maybe
calcu-latedfromtherelationLn(qe/Ce)versusqeatdifferent
temperatures,extrapolatingtozero.
The thermodynamic parameters are listed in Table3.ThenegativeG◦values(−1.444kJmol−1to −2.217kJmol−1)indicatethespontaneityand feasibil-ityoftheadsorptionprocessofBasicFuchsinontoCMS. The adsorptionischemicalinnature,indicatedbythe increaseinG◦valueswhenthetemperaturewasraised from298to328K.Thesameresultsareobtainedin sim-ilarstudies[22,23].H◦andS◦werethenestimated fromtheslopeandinterceptoftheplotofLnK0versus
1/TbasedonEq.(15).ThedataarepresentedinTable3. The positive value of H◦ (5.247kJmol−1) suggests theendothermicnatureoftheadsorptionprocess,while thepositiveS◦(22.45Jmol−1K−1)valuesuggeststhe increaseinadsorbate concentrationinthe solid–liquid interface, therebyindicatingthe decrease inadsorbate concentrationontothesolidphase.Italsoconfirmsthe increasedrandomnessatthesolid–liquidinterface dur-ingadsorption.Thisresultisanormalconsequenceofthe chemicaladsorptionphenomenon,whichoccursthrough electrostaticinteractions.
3.8. Comparisonoftheadsorptioncapacitiesof
certainadsorbentsfortheremovalofdyesfrom
aqueousmedia
Table4depictsthevaluesofadsorptioncapacitiesof variousmarine-originadsorbents.CMScanbeclassified asagoodadsorbentcomparedwithotherexamples,with anadsorptioncapacityof141.65mgg−1.
3.9. ReusabilityofCMSadsorbent
The stability and regeneration ability of the adsor-bent duringthe adsorption process areessential toits
Table4
Comparisonoftheadsorptioncapacitiesofsomeadsorbentsforremovaldyesfromaqueousmedia.
Adsorbent Dye Adsorptioncapacity(mgg−1) Reference
Gastropodshell CongoRed 99.87 [24]
Brownmacroalgae AcideBlack1 29.79 [25]
MarineAspergilluswentii BrilliantBlueG 312.5 [26]
Fungalpellets AcidBrilliantRed 86.5 [27]
Fig.10.Effectofregenerationcyclesontheadsorptioncapacityof dyeontoCMS.
practicalapplication.Interestingly,the CMSadsorbent couldberegeneratedbysimplecalcinationat500◦Cin airfor 60min.The calcinationof theadsorbent could retainthehighremovalefficiencyduringfivesuccessive cycles.Toachievethiseffect,weexaminedstudiedthe variationofadsorptioncapacityversuscyclenumber.As depictedinFig.10,theadsorptioncapacitiesdecreasefor eachnewcycleafterregeneration.Theoriginal adsorp-tioncapacityofCMSforBB41is167.68mgg−1.After five cycles,theadsorptioncapacityof CMSfor BB41 drops to 10.12mgg−1. Therefore,CMS shows excel-lent adsorption performance andregeneration, and its usecanbe extendedtoenvironmentalapplications for wastewatertreatment.
4. Conclusion
The ability of calcined mussel shells as an adsor-benttoremove Basic Fuchsinfrom aqueoussolutions was investigated. Operational parameterssuch as pH, initialdyeconcentration,adsorbentdose,contacttime, andtemperaturewerefoundtoaffecttheadsorption effi-ciencyofcalcinedmusselshells.Thekineticdatashow that the adsorption process follows a pseudo-second-ordermodel,indicatingchemisorption.Thesameresult wasfoundbytheD–Risothermmodel,andthe adsorp-tion data present the best fit to the Langmuir model, suggesting that adsorptionoccurs bythe formation of amonolayer.Thethermodynamicparametersshowthat adsorption is spontaneous and endothermic in nature andincreasedrandomnessatthesolid–liquidinterface, overall showing the feasibility of the process. This studyshowedthatcalcinedmusselshellscouldbeused as agood and inexpensiveadsorbent for dye effluent treatment.
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