Decolorisation of aqueous crystal violet solution by a new nanoporous carbon: Equilibrium and kinetic approach

Decolorisation

of

aqueous

crystal

violet

solution

by

a

new

nanoporous

carbon:

Equilibrium

and

kinetic

approach

Fuat

Gu¨zel

a,

*

,

Hasan

Sayg˘ılı

b

,

Gu¨lbahar

Akkaya

Sayg˘ılı

a

,

Filiz

Koyuncu

a a_{DepartmentofChemistry,FacultyofEducation,DicleUniversity,21280Diyarbakır,Turkey}

b

DepartmentofChemistry,FacultyofScience&Arts,BatmanUniversity,72060Batman,Turkey

1. Introduction

Dyesarepresentinthewastewaterstreamsofmanyindustrial sectorssuchas,dyeing,textile,tanneryandthepaintindustry.The dyemoleculesortheirmetabolites(e.g.,aromaticamines)maybe highlytoxic,potentiallycarcinogenic,mutagenicandallergenicon exposedorganisms.Theycontaminatenotonlytheenvironment but also traverse through the entire food chain, leading to biomagnifications [1]. Today there are more than 10,000 dyes withdifferentchemical structuresavailablecommercially. Dyes arebroadlyclassifiedasanionic,cationicandnon-ionicdepending on theionic charge on the dyemolecules [2,3]. Among them, cationicdyesaremoretoxicthananionicdyes,andtheirtinctorial valuesareveryhigh(lessthan1.0mgL1_{)[4].Amongthevarious}

available dyes, crystal violet (CV) dye, a member of the

triphenylmethanegroup,which iscationicdye,isawell-known dye that is used in a variety of ways: as a biological stain, dermatologicalagent,veterinarymedicine,additivetopoultryfeed toinhibit propagation of mold, intestinal parasites and fungus, textiledyingandpaperprinting,etc.Itiscarcinogenicandhasbeen classifiedasarecalcitrantmoleculesinceitispoorlymetabolized bymicrobes,isnon-biodegradable,andcanpersistinavarietyof environments[5].HencetheCVdyeremovalfromthewaterbodies becomesessential.

Variousphysical,chemicalandbiologicaltreatmenttechniques

can be employed to remove dyes from wastewater. The most

widelyusedmethodsforwastewatertreatmentareflocculation, coagulation, precipitation,adsorption,membrane filtration, and electrochemicaltechniques[6].Thesetechnologiesdonotshow significant effectiveness or economic advantage. Among them, adsorptionhasbeenfoundtobesuperiortoothertechniquesfor dyewastewatertreatmentintermsofcost,simplicityofdesign, easeofoperationandinsensitivitytotoxicsubstances[7,8].Most oftheinvestigationsarebasedoncommercialactivatedcarbonand activatedcarbonderivedfromvarioussourcesanditfoundtobe moreeffectiveforcolorremoval.Commercialactivatedcarbonis usuallyderivedfromnaturalmaterialssuchaswoodorcoaland thereforeitis stillconsideredexpensive [9].Thishasledtothe searchforcheapersubstitutes.Hence,low-costactivatedcarbons basedonagriculturalsolidwastesareinvestigatedforalongtime. Thishasledtothesearchforcheapersubstitutes.Hence,low-cost activatedcarbonsbasedonagriculturalsolidwastesare investi-gatedforalongtime.Agriculturalproductsandwastematerials usedfortheproductionofactivatedcarbonsincludealmondand hazelnutshells[10,11],peanutshells[12,13],sourcherrypits[14], andolivestones[15,16],jutefibercarbon[17],cassavapeel[18], bagasse [19],waste apricot[20],cocoa (Theobroma cacao) shell [21],date’sstones[22],etc.Inthispaper,wereporttheuseofa novelprecursor,thetomatowaste(TW),whichisaveryabundant andinexpensivematerialinMediterraneancountries.Tothebest ofourknowledge,thepresentstudyisthefirstonetostudythe adsorptionof CVby TWNC.According totherecords ofUnited NationsFoodand AgricultureOrganization(FAO), tomatoisthe mostwidelygrownproductinfreshvegetablesaroundtheworld withaproductionof145.6milliontons.Turkeyranksfourthwith productionof10milliontonsoftomatointheworld[23,24]. ARTICLE INFO

Articlehistory:

Received9October2013 Accepted9December2013 Availableonline16December2013 Keywords:

Tomatopastewaste Nanoporouscarbon Crystalvioletadsorption Equilibrium

ABSTRACT

Anewnanoporouscarbonfromtomatopastewaste(TWNC)wasprepared.Thesurfacearea,totalpore

volume,averageporediameterofTWNCwas foundas722.17m2_g1_,0.476cm3_g1_and2.644nm,

respectively.TheeffectsofsolutionpH,adsorbentdose,initialconcentration,ionicstrength,contact

time,andtemperaturewerestudied.Adsorptionkineticswasfoundtobebestrepresentedbythepseudo

secondordermodel.Isothermdata werefittedwelltothebothLangmuir andFreundlich models.

Maximumadsorptioncapacitywasfoundas68.97mgg1_{at508C.Thermodynamicparametersshowed}

thattheprocesswasspontaneousandendothermic.

ß 2013TheKoreanSocietyofIndustrialandEngineeringChemistry.PublishedbyElsevierB.V.Allrights

reserved.

* Correspondingauthor.Tel.:+904122488377;fax:+904122488257. E-mailaddresses:fguzel@dicle.edu.tr,guzelfuat@gmail.com(F.Gu¨zel).

ContentslistsavailableatScienceDirect

Journal

of

Industrial

and

Engineering

Chemistry

j o urna l hom e pa ge :ww w. e l s e v i e r. c om/ l o ca t e / j i e c

1226-086X/$–seefrontmatterß2013TheKoreanSocietyofIndustrialandEngineeringChemistry.PublishedbyElsevierB.V.Allrightsreserved.

In this study, we prepared a new nanoporous carbon from tomato waste as low-cost and abundantly available precursor, whichiswasteoftomatojuiceandpastefactories,withchemical activationbyzincchlorideasadehydratingagent.TheTWNCis characterized with nitrogen adsorption/desorption isotherms, Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), FourierTransformInfrared(FT-IR)analysisandsomeadsorptive

tests such as methylene blue and iodine numbers. Optimum

adsorptionconditionsforCVfromasyntheticaqueoussolutionare determinedasafunctionofpH,adsorbentdose,initial concentra-tion, contact time, and temperature. The adsorption kinetics, thermodynamicandisothermswerealsodiscussed.

2. Materialsandmethods 2.1. Materials

TWwasprovidedfromtomatopastefactoryinAdana,Turkey. Firstly,theTWwaswashedanddriedinanairovenat708Cfor 24h and then crushed and sieved to the desired particle size

(between 0.177mm and 0.4mm) for using in the chemical

activation experiment. Zinc chloride (purchased from Sigma-Aldrich)ofpurity99.9%wasusedaschemicalactivator.CV,abasic dye,C.I.42555,

l

max=590nm,molecularformulaC25H30N3Cl,also

knownashexamethylpararosanilinechloride,waspurchasedfrom Merck company and its general characteristics are shown in Table1 [25].Stock solutionof dyewasprepared bydissolving accuratelyweigheddyein distilled watertoa concentration of 1000mgL1_{.Theexperimentalsolutionofthedesired}

concentra-tionswasobtainedbysuccessivedilutions.Freshdilutionswere usedin each experiment.The pHofdye solutionwasadjusted using0.1NHClorNaOH.Allchemicalsusedwereof analytical grades.

2.2. PreparationofTWNC

TheTWwasmixedwithzincchloride(TW/ZnCl2weightratioof

1:1)andtherequiredamountofdistilledwaterwasaddedtothis mixture.Then,themixturewasdriedat1058Cinanairovento

obtain an impregnated sample. The impregnated sample was

placed in a stainless-steel tubular reactor (7.0cm diame-ter100cmlength),andthenheatedtotheactivation tempera-tureof5008Cfor1hundernitrogenatmosphere(99.99%)flow (100mLmin1_{) at the rateof 108Cmin}1_{. After the activation}

process,theobtainproductwascooleddownundernitrogenflow andthen0.2Nhydrochloricacidwasaddedonit.Thismixturewas filtered and washed with distilled water for several times to removeresidualchemicalsandchlorineuntilfiltratedsolutiondid notgiveanyreactionwithsilvernitrate.Itwasdriedat1058Cfor 24handgroundandsievedtounder40–80meshsizes.Finally,the resulting product was stored in desiccators for further use in adsorptionexperiments.Theyieldwascalculatedastheratioofthe dryweightofresultantactivatedcarbontotheweightofthe air-driedoftherawprecursor.

2.3. CharacterizationofTWNC

The proximate analysis was conducted according to ASTM D3173-3175standards[26]andtheresultsweregivenasmoisture, ash,volatilematter,andfixedcarboncontents.Todeterminethe contentsofC,H,N,andSintheTWandTWNC,ultimateanalysis wasperformedinanElementalAnalyzer(ThermoScientificFlash 2000,CHNSAnalyzer,Italy).Resultswereobtainedaspercentages ofcarbonandnitrogen,andtheoxygencontentwasdeterminedby difference.

ThesurfacephysicalmorphologiesofTWandTWNCbeforeand afteradsorptionwereidentifiedbyusingSEMtechnique (Jeol/jsm-6335F,USA).

Table1

Generalcharacteristicsofcrystalvioletdye. Chemicalstructure N CH3 H3C N CH3 H3C N CH3 CH3

Cl

-Molecularformula C25H30ClN3 Dyecontent(%) 90

C.I.number 42555 Molecularweight(g/mol) 407.979

C.I.name BasicViolet3 Molecularsurfacearea(A˚2

/molecule)a

585.90

Chemicalclass Cationicdye Width(A˚´)a

14

Chromophore Methylgroups Depth(A˚´)a

14

Ionization Basic Watersolubility 16gL1_(258C)

lmax(nm) 590

a

Ref.[25].

F.Gu¨zeletal./JournalofIndustrialandEngineeringChemistry20(2014)3375–3386 3376

Surface area and poresize distribution weredeterminedby nitrogen adsorption–desorption isotherms measured at 77K (Micromeritics,ASAP2020).Priortothemeasurements,theTWNC wasoutgassedat423Kundernitrogenflowfor4h.Thenitrogen adsorption–desorption data were recorded at liquid nitrogen temperature(77K)andwasmeasuredoverarelativepressure(P/ P0)rangefromapproximately106–1. Thespecificsurface area

(SBET)wasdeterminedbymeansofthestandardBET(Brunauer–

Emmett–Teller) equation [27] applied in the relative pressure rangefrom0.05to0.35[28].Thisstudyassumesthatthe cross-sectionalareaofanitrogenmoleculeis0.162nm2_.Theexternal

surfacearea(includingonlymesopores,Sext), microporevolume

(Vm)andmicroporearea(Sm)werecalculatedbyt-plotmethod.The totalporevolume(VT)wasestimatedbyconvertingtheamountof

nitrogengas adsorbed(expressed in cm3g1_{STP) at a relative}

pressureof0.95toliquidvolumeofthenitrogenadsorbate[29]. Themesopore volume (Vm)wasdetermined bysubtracting the

microporevolumefromthetotalporevolumewhilethemicropore fraction [Vm (%)=Vm/VT100] and mesopore fraction [Vm

(%)=Vm/VT100] were based on the total pore volume. The

averageporediameter(Dp)wasestimatedfromtheBETsurface

area and totalpore volume (Dp=4VT/SBET)assuming an

open-endedcylindrical poremodel withoutpore networks[30]. This

study assumes that micropores are less than 2nm wide,

mesopores are 2–50nm wide, and macropores are more than

50nmwide[28,29].Theporesizedistributionwasdeterminedby usingBarrett–Joyner–Halenda(BJH)model[31].

Thedeterminationofsurfaceacidfunctionalgroupswasbased on theBoehmtitrationmethod[32].The variousacidic groups were determined with the following assumption that NaHCO3

neutralized carboxyl groups, Na2CO3 neutralized carboxyl and

lactone groups and NaOH neutralized carboxyl, lactone and phenolicgroups.

Surface functional groups was detected using the pressed potassiumbromide(KBr)pelletscontaining5%ofcarbonsample by FT-IRspectrometer (PerkinElmerspectrum 100, USA) in the scanningrangeof4000–400cm1_.

ThepHpzcindicatestheacidorbasiccharacterofthecarbon

surface.The combinedinfluenceof all thefunctional groupsof activatedcarbondeterminespHpzc,i.e.,thepHatwhichthenet

surfacechargeoncarbonwaszero.ThepHpzcwasdeterminedby

themethoddescribedbyPreethiandSivasamy[33].Thedifference between theinitial pH(pHi)and

D

pH (pHipHf)values were

plottedagainstthepHi.Thepointofintersectionoftheresulting

curvewithabscissa,wherepHwaszero,givesthepHpzc.

TheX-raypowderdiffraction(XRD)patternswerecollectedon an X-ray powder diffractometer (Bruker, D8 Discovery EVA, Germany).XRDfortheTWNCwasmeasuredusingCuK

a

radiation at40kVand40mAovertherange5–558(2

u

)atascanspeedof 68min1_.

Adsorptive capability was preliminarily characterized by measuringbothiodineandmethylenebluenumbers.Theiodine

number and methylene blue number tests were conducted as

describedinASTMD4607-94[34]andChinaNationalstandards [35].Theiodinenumber(mgofiodineadsorbed/gofcarbon,IN) andmethylenebluenumber(mgofmethyleneblueadsorbed/gof carbon,MN)areconsideredasameasureofadsorptioncapability ofactivatedcarbon.Normally,iodinenumberdenotestheamount of micropore (less than 10A˚ in diameter) and methylene blue number (equal or greater than 15A˚ in diameter) denotes the amountofmesoporeofactivatedcarbon[36,37].

2.4. Crystalvioletremovalstudies

Removalexperimentswerecarriedoutin100mLflasksandthe totalvolumeofthereactionsolutionwaskeptat50mL.Theflasks

wereshaken at 120rpmfor therequiredtime in a water bath shaker(Daihan-WSB-30,Korea).Theeffectsofvariousoperating parameters, solution pH (3–10), adsorbent dosage (0.1–0.9g), initialconcentration(25–200mgL1_{),contacttime(5–270min),}

temperature(20–508C)andionicstrength(0–0.1molL1_)onthe

adsorptionwerestudied.Thetemperaturewascontrolledbyusing anisothermalshaker.Aftereachadsorptionprocess,thesamples werecentrifuged(5000rpm,10min)for solid–liquidseparation andtheresidualdyeconcentrationinsolutionwasanalyzedbya

UV–vis spectrophotometer (Perkin Elmer-Lamda 25, USA) at

590nm.Inthepresentstudy,adsorptionisothermswerecarried outbyusingseveralsolutionswithdifferentconcentrations.The amount of dye adsorbed onto per gram of adsorbent (qe)was

calculatedbyusingEq.(1). qe¼

ðC0CeÞV

m (1)

where C0 and Ce are initial and equilibrium concentrations,

respectively(mgL1_{),Vissolutionvolume(L),misthemassof}

adsorbent(g).

Removalisothermexperimentswerecarriedoutbyagitating dye solutions of different concentrations (20–350mgL1_)with

0.1g TWNC at different temperatures (208C, 308C, 408C and 508C).

Removalkineticstudieswereusedtoinvestigatetheeffectof contact time and initial concentrations and determine kinetic parameters. For these experiments, 0.1g TWNC was added to 50mLCVsolutionswithdifferentinitialconcentrations(25,75, 100,150and200mgL1_{).Themixturewasstirredinwaterbathat}

258Cand120rpm.Atpredeterminedtimeintervals(5–270min),

10mL samples were taken out and filtered. The amount of

adsorptionqt(mgg1),attimet(min),wascalculatedby:

qt¼

ðC0CtÞV

W (2)

whereCtisconcentrationofCVattimet(mgL1).

2.5. Erroranalysis

NonlinearChi-squaretest(

x

2

)andaveragerelativeerror(ARE) tocheckconsistencyofadsorptionkineticandisothermmodels wereperformedinadditiontodeterminationthelinearregression correlationcoefficient(R2).Theexpressionsoftheerrorfunctions aregivenbelow:

x

2_¼X N i¼1 ðqe;expqe;calÞ2 qe;exp (3) ARE¼100 N XN i¼1 jqe;expqe;calj qe;exp " # i (4) Table2

ProximateandultimateanalysesofTWandTWNC. Proximate analysis(wt%) Ultimate analysis (wt%) TW TWNC TW TWNC Moisture 2.95 7.48 C 59.84 72.93 Ash 1.58 1.47 H 8.79 3.49 Volatilematter 82.67 22.73 S 0.26 0.33 Fixedcarbon 12.80 68.32 N 4.08 3.42 Burnoff – 78.63 Oa _{27.03 19.83} Yield – 21.37 Chemicalrecovery(ZnCl2%) – 96.42 a Bydifference.

whereqe,expandqe,calrepresenttheexperimentallyandcalculated

adsorbedamountsofCV(mgg1_{),respectively.Nisthenumberof}

observationsintheregressionmodel[38]. 3. Resultsanddiscussion

3.1. CharacteristicsofTWNC

3.1.1. Proximateandultimateanalysis

TheproximateandultimateanalysisoftheTWandTWNCare summarizedinTable2.Table2disclosesthatTWhavehighfixed carbon and low ash content which confirms its suitability as activatedcarbonprecursor.However,sincethecarboncontentand fixedcarboncontentincreasefurtherthroughcarbonization,the contentswillbeevenhigher andtherawmaterial isbetterfor producingactivatedcarbon.

3.1.2. Surfacemorphologyandtexturalstructure

SEMmicrographsofTWandTWNCbeforeandafteradsorption areshowninFig.1.AscanbeseenfromFig.1a,thesurfacetexture of the TW was regular and undulating withonly a few pores availableonthesurface.However,afterZnCl2activationtreatment,

manyvarioussizesporesinahoneycombcanbeobservedonthe samplesurfaceasshowninFig.1b.Duringactivationprocess,the ZnCl2–carbon reaction occurred, which enhances the pores

development thus, the surface area and porosity increase. In addition, almost heterogeneous type of pores structure was distributedontheTWNCsurface.Itisclearthat,TWNCappears tohavenumbersofporeswhere,thereisagoodpossibilityfordye tobetrappedandadsorbedintothesepores.SEMimagesofTWNC (Fig. 1b) showed bright dark color on the surface. After CV adsorptionthesurfaceofTWNCwasturnedtolightcolor(Fig.1c). Thismaybedue totheadsorptionofCV onthe surfaceof the activatedcarbon.

Fig.2ashowsthenitrogenisothermat1968C.Asseenfrom Fig.2a, it exhibits adsorptionisothermof type IV according to IUPAC [39]. The type IV isotherm characteristically shows the simultaneouspresenceofmicroandmesopore.Theinitialpartof theisothermfollowsthesamepathlikethecorrespondingtypeII

isotherm and therefore the result of monolayer–multilayer adsorption on the mesopore walls [40]. The BET surface area (SBET), external surface area (mesopore surface area, Sext),

micropore surface area (Sm), totalpore volume (VT), micropore

volume (Vm), mesopore volume (Vm), micropore fraction(Vm%), mesoporefraction(Vm%)andaverageporediameter(Dp)results

obtainedbyapplyingtheBETequationandt-methodtonitrogen adsorption at 1968C are listed in Table 3. The pore size distribution of the prepared activated carbon is shown in Fig. 2b. It can be foundfrom Fig. 2b that the sample exhibits multimodal distribution in both the micropore and mesopore domainsbutdoesnothavemacropores.Thiswasconfirmedbythe microporeandmesoporefractionvaluesinTable3.Porosityresults suggestthatthemoremesoporedominantTWNCissuitablefor dyeadsorption.ThisideaisalsosupportedbythevaluesofINand MNvalues(Table3).Therefore,thehighBETsurfaceareaofthe TWNCrenderthemtobesuitableaseffectiveadsorbentforthe removalofbothairandwastewaterpollutants.Acomparisonof thetextural propertiesoftheactivatedcarbon preparedinthis

study with other reported values for some commercial and

vegetable-basedactivatedcarbonsarelistedinTable4. 3.1.3. Surfacechemistry

The surface chemistry is important characteristics of the activatedcarbons since it determinesthe surface properties of thecarbonsandhassignificantimplicationsontheirbehaviorsas ion exchangers, adsorbents, catalysts, and catalyst supports [48,49]. For a better understanding of the surface chemistry differencesoftheTWNC,FT-IRspectraarerepresentedinFig.3.In thespectraofnativeTWNC,thebandobservedat3245.68cm1

presentbonded–OHgrouponthesurface.Thisbanddisappeared afterCVadsorption,indicatedthat–OHgroupsplayanimportant roleintheadsorptionofCV.Thebandsobservedat2922.96and 2853.11cm1_{correspondtosymmetric–CH}

2vibrationand–CH2

stretching vibration, respectively. These peaks shifted after CV adsorption.Thepeakat1741.68cm1_{whichistheindicative of}

C55Ostretchingofcarboxylicacidsdisappearedafteradsorption. Thepeakobservedat1592.65cm1_{isassignedtoaromaticC55C}

stretching.Thispeakshiftedto1575.80cm1_{withasignificance}

Fig.1.SEMmicrographsofrawTW(a),TWNC(b),andTWNCafterCVadsorption(c). F.Gu¨zeletal./JournalofIndustrialandEngineeringChemistry20(2014)3375–3386 3378

differenceof16.85cm1_{afterCVadsorption.Thepeakslocatedat}

1374.41and1155.16cm1_{presentlactonegroupsonthesurface.}

These peaks shifted to 1352.91 and 1165.50cm1 _{after CV}

adsorptionwithavariationof21.50and10.34cm1_{,respectively.}

Thebandsappearedat1090.63and720.87cm1_{indicatedSi–O}

stretching vibrations. New bands appeared in the fingerprint region at1434.60, 938,873.50and 744.52cm1_{on thesurface}

afteradsorption.ThesebandsconfirmtheadsorptionofCVonthe surface.

The surface acidity and basicity is an important criterion describing thesurface chemistryof thecarbon adsorbents.The surfacepropertiesoftheactivatedcarbonareaffectedsignificantly bythetypeandquantityofthesurfacefunctionalgroups.Table5

showsquantitativesurfacechemistryanalyses, whichconsistof amountofacidsandbasicfunctionalgroupsofsurface.According tothe results, it exhibitedan acidic behavior,with thesurface acidity of 1.33mequiv.g1 _{with the maximum composition of}

phenolic group (0.26mequiv.g1_{) with traces of lactonic}

(0.53mequiv.g1_{)andcarboxylic(0.54mequiv.g}1_)groups,and

0.95mequiv.g1 _{as surface basicity. In parallel withthis, FTIR}

spectraconfirmedthepresenceofgroupscontainingoxygensuch ascarboxylicandphenolicgroups.

3.1.4. Crystalstructure

XRDtechniqueisapowerfultooltoanalyzecrystallinenatureof materials. Fig.4 shows XRDprofiles of the TWand TWNC.As Fig.2.Nitrogenadsorption(filledsymbols)–desorption(emptysymbols)isotherms(a)andporesizedistribution(b)fortheTWNC.

Table4

Comparisonoftexturalpropertiesofsomecommercialandvegetable-basedactivatedcarbons.

Activatedcarbons SBET(m2g1) VT(cm3g1) DP(nm) Ref.

Filtrasorb400 793 0.486 2.44 [41]

Mosobambooactivatedcarbon 486.80 0.235 1.93 [42]

Mabambooactivatedcarbon 589.65 0.276 1.87 [42]

CalgonCPG-LF 648.50 0.177 222.5 [43]

Chestnutshellactivatedcarbon 1319 0.567 36.5 [44]

Grapeseedactivatedcarbon 916 0.392 30.5 [44]

CecaAC-40 1294 0.650 2.02 [45]

Sorbo-Noritactivatedcarbon 1143 0.570 2.02 [45]

Teaindustrywasteactivatedcarbon 1066 0.580 2.18 [46]

WSC-470activatedcarbon 1143 0.505 1.77 [47]

WS-490activatedcarbon 1324 0.599 1.81 [47]

Tomatopastewasteactivatedcarbon 722.17 0.476 2.64 Thiswork

Table3

TexturalandadsorptivecharacteristicsofTWNC.

SBET(m2g1) Sm(m2g1) Sm(m2g1) VT(cm3g1) Vm(cm3g1) Vm(%) Vm(cm3g1) Vm(%) DP(nm) IN(mgg1) MN(mgg1)

showninFig.4,therewereverybroaddiffractionpeaksinbothof thesamples.TW shows binary peak between 2

u

=108 and 268 ranges.TWNCexhibitsinwide2

u

ranges(between2

u

=108and 208).TheXRDpatternrevealstheamorphousstateoftheobtained samples.Activatedcarbonswhichwereachievedfromagricultural wastes show usually amorphous structure. Possible functional

groups which existed on the activated carbon surface may

complicateformationofthecrystallization[50].

3.2. EffectofvariousoperatingparametersforCVadsorptionon TWNC

3.2.1. EffectofpHpzcandpH

pHpzc determines combined influence of all the functional

groupsofsurface.Fig.5ashowsthatat5.30,

D

pH=0.Thereforethe pHpzcofTWNCis5.30.AtpH<pHpzc,thecarbonsurfacehasanet

positivecharge,whileatpH>pHpzcthesurfacehasanetnegative

charge [51]. The pHpzc of TWNC indicated that the surface is

negativelychargedatpHvaluesabove5.30.Theaqueoussolution pHhasbeenreported to present a significantinfluenceon the adsorptiveuptakeofdyemoleculesduetoitsimpactonboththe surfacebinding-sitesoftheadsorbentandtheionizationprocessof thedyemolecule[52].TheeffectofinitialpHwasdeterminedat different pH values (3–10). The pH values of solutions were adjusted by drop wise addition of 0.1M HCl and 0.1M NaOH solution.Initialconcentration,shakingtime,temperatureandthe amountofadsorbentwerefixedat100mgL1_{,120min,258Cand}

0.1g,respectively.TheeffectofpHwasillustratedinFig.5a.Itwas observedthatadsorptioncapacityincreasedbyincreasingpHfrom 3to10.ThemaximumadsorptionwasfoundaspH8.0,indicating theadsorptionwasstronglypH-dependent.Therefore,pH8.0was considered more effective pH, and it was used for further adsorption experiments. At low pH values, protonation of the

functionalgroupspresentonthesurfaceeasilyoccurs.Thesurface oftheadsorbentbecomespositivelycharged,andthisdecreases the adsorption of the positively charged dye ions through electrostatic repulsion.AsthepHofthedyesolutionincreases, a proportional increase in adsorption takes place due to the consecutive deprotonationof positively chargedgroups on the adsorbentandelectrostaticattractionbetweennegativelycharged sites on the TWNC and CV+ _{ions. Similar observations were}

reportedfortheadsorptionofCVbysomeearlierresearchers[53– 56]. ðTWNCÞOH!H þ ðTWNCÞOHþ2þCV þ !ðTWNCÞOHþ2$CV þ (5) ðTWNCÞOH!OH  ðTWNCÞOþCVþ!ðTWNCÞO... CVþ (6)

ThebindingmechanismoftheCV(atpH8)ontoTWNCisgiven inFig.6.

3.2.2. EffectofTWNCdose

The dependence of CV adsorption on TWNC dosage was

investigatedintherangeof2–18gL1_{.Thedependenceondosage}

ofCVadsorptionwasstudiedbyvaryingtheamountofadsorbents inthemediumfrom2to18gL1_{,whilekeepingotherparameters}

constantsuchasinitialconcentration(100mgL1_{),pH8.0,stirring}

rate120rpmandcontacttime2h.Theeffectofadsorbentdosage isshowninFig.5b.Theadsorptioncapacitydecreasedfrom10.97 to 7.65mgg1 _{with an increase in adsorbent dose from 2 to}

18gL1_{.Thus,theadsorptioncapacityofCVdecreasedwiththe}

increaseintheadsorbentdoseandreachedmaximumadsorption valuearound0.1gof adsorbentdosage.Itwasusedforfurther adsorption experiments. As presented in Fig. 5b, with the increasing amountof adsorbent, qe (mgg1)values, which are

the amount of CV adsorbed per unit weight of adsorbent at Fig.3.FTIRspectraofTWNC(a)andTWNCafteradsorptionofCV(b).

Table5

SurfacechemicalcharacteristicsoftheTWNC.

Carboxylic(mequiv./g) Phenolic(mequiv.uiv./g) Lactonic(mequiv./g) Totalacidity(mequiv./g) Totalbasicity(mequiv./g) pHPZC

0.54 0.26 0.53 1.33 0.95 5.30

F.Gu¨zeletal./JournalofIndustrialandEngineeringChemistry20(2014)3375–3386 3380

equilibrium, were decreased. This may be attributed to the decrease in totaladsorption surface area available to dye ions resulting from overlapping or aggregation of adsorption sites. Similarobservationswerepreviouslyreportedbysome research-ers[53,57].

3.2.3. Effectofcontacttimeandinitialconcentration

Fig.5cshowstheeffectsofcontacttimeandinitial concentra-tiononthedyeuptakeat258C.Itwasfoundthattheadsorption wasfastatinitialstageof120min,thereafteritbecameslower until it reached a constant value where no more dye can be removedfrom the solution.The rapid adsorption at the initial contacttimeisduetothehighlynegativelychargedsurfaceofthe TWNCforadsorptionofCVinthesolutionatpH8.Thelaterslow rate of adsorption probably occurred due to the electrostatic hindranceorrepulsionbetweentheadsorbedpositivelycharged adsorbate species onto the surface and the available cationic adsorbatespeciesinthesolutionaswellastheslowporediffusion of the solute ions into the bulk of the adsorbent. Similar observations werereported by some otherresearchers [58,59].

The equilibrium was attained at 150min when the maximum

adsorptionwasreachedanditisselectedasequilibriumcontact time forfurther adsorption experiments.Theincrease in initial concentrationalsoenhancestheinteractionbetweendyeionsand surface.Therefore,anincreasein initialconcentration enhances theadsorptioncapability.Theeffectofinitialconcentrationinthe rangeof25–200mgL1_{isshowninFig.5c.Theadsorbedamount}

in the equilibrium state (qe) was increased from 2.83 to

21.89mgg1_{withtheincrease ininitialconcentration from25}

to200mgL1_{.Thismaybeattributedtoanincreaseinthedriving}

force between the aqueous and solid phases and increase the numberofcollisionsbetweendyeionsandadsorbent[60]. 3.2.4. Effectoftemperature

Theeffectoftemperaturewasstudiedat20,30,40and508C, and theresultsare shown in Fig.5d. Asshown in Fig. 5d, the amountofdyeadsorptionwithincreaseintemperaturewasfound toincrease from51.55 to 68.97mgg1_{. This suggests that the}

adsorptionprocessisendothermicinnature.Thismaybedueto increaseinthedyemobilitytopenetrateinsidethesampleporesat hightemperature.Besides,itmightalsobeduetotheincreasein

chemicalinteractionbetweentheadsorbateandsurface function-alitiesoftheadsorbent[61].

3.2.5. Effectofionicstrength

The ionic strengthof the solutionis one of thefactors that control both electrostatic and non-electrostatic interactions betweentheadsorbateandtheadsorbentsurface[62].Theeffect of ionic strength was analyzed in the NaCl solutions with concentrationsrangingfrom0.0to1.0molL1_{at258C, pH8.0,}

120rpmand100mgL1_{initialdyeconcentration.Fig.5eshows,}

theinfluenceofthepresenceofsodiumchlorideontheadsorption capacityofCV+_{ionsontothesurface.AsseeninFig.5e,increasing}

the ionic strength of solution decreases the adsorption. This behavior could be attributed to the electrical double layer surrounding thesurface waspressed and herebyresulted in a decrease in adsorption. Similar observations were reported by someearlierresearchers[1,63].

3.3. Kineticsstudies

Inordertoanalyzetheadsorptionkinetics,twokineticmodels; pseudo first order [64] and pseudo second order [65] kinetic modelswereappliedtotheexperimental data.In addition,the intraparticlediffusionmodelwastestedtodeterminethediffusion mechanism of the adsorption process. The best fit model was selectedbasedonthelinearregressioncorrelationcoefficient(R2₎

values.Thelinearizedpseudofirstorderandpseudosecondorder kineticequationsare:

logðqeqtÞ¼logqe k1 2:303t (7) t qt ¼ 1 k2q2e þ1 qe t (8)

where k1 (min1) and k2 (gmg1min1) are rate constants of

pseudo first order and pseudo second order, respectively. The valuesofthepseudofirstorderandpseudosecondorderkinetic parameterswerecalculatedfromtheslopeandinterceptoftheir respectiveplots(Fig.7aandb).Table6liststheresultsofkinetic parametersofbothmodelsatdifferentconcentrations.Forpseudo first order,theqe,calvalues werelow as comparedto theqe,exp

values,suggestingthatthepseudofirstordermodelwasnotfitfor describing the adsorption process although higher correlation coefficients were achieved (R2=0.978). Pseudo second order

model described the adsorption process more effectively

(R2_{=0.997) withthe q}

e,cal values matched well withthe qe,exp

values. These resultsimpliedthat theadsorptioncouldbebest describedbythepseudosecondordermodel.

AsshowninTable6,thevaluesofrateconstantk2decreasewith

increasing initial concentration and surface loading. At lower concentrations,CV+_{ionspresentintheadsorptionmediumcould}

interactwiththebindingsites; hencehigherrateconstantsare obtained.Athighersurfaceloadingswouldresultinlessdiffusion efficiencyandahighcompetitionofCV+_{ionsforafixedreaction}

sites,consequentlylowerk2valueswereobserved.Similarresults

havebeenobservedbyothersomeresearchers[6,66]. 3.4. Adsorptionmechanism

Thepseudofirstorderandpseudosecondorderkineticmodels cannotidentifythediffusionmechanismand thekineticresults werethenanalyzedbyusingtheintraparticlediffusionmodel.The mechanismofadsorptionprocessisusuallydemonstratedbyfour steps:(i)bulkdiffusion;(ii)filmdiffusion;(iii)porediffusionor intra-particle diffusion; (iv) adsorption of adsorbate on the adsorbent surface. Because the first step is not involved with Fig.4.XRDprofilesofTWNCandTW.

Fig.5.EffectsofsolutionpHandpHpzc(a),adsorbentdose(b),initialdyeconcentrationandcontacttime(c),solutiontemperature(d)andionicstrength(e)ontheadsorption

ofCVontoTWNC.

F.Gu¨zeletal./JournalofIndustrialandEngineeringChemistry20(2014)3375–3386 3382

adsorbentandthefourthstepisaveryrapidprocess,theydonot belongtotheratecontrollingsteps.Therefore,theratecontrolling stepsmainlydependoneithersurfaceorporediffusion[67,68]. WeberandMorrismodelisawidelyusedintra-particlediffusion model,Eq.(9),topredicttheratecontrollingstep[69].

qt¼kidt1=2þC (9)

wherekidistheintraparticlediffusionrateconstant(mgg1min1/ 2_{)andCisthethicknessoftheboundarylayer.Ifthemechanismof}

adsorptionprocessfollowstheintraparticlediffusion,theplotofqt

versus t1/2 _{would be a straight line and the k}

id and C can be

calculatedfromtheslopand interceptof theplot,respectively. Accordingtothismodel,theplotshouldbelinearifintraparticle diffusionisinvolvedintheadsorptionprocessandiftheselines passthrough theoriginthen intraparticlediffusion is the rate-controllingstep.Whentheplotsdonotpassthroughtheorigin, thisisindicativeofsomedegreeofboundarylayercontrolandthis furthershowsthattheintraparticlediffusionisnottheonly rate-limitingstep,butalsootherkineticmodelsmaycontroltherateof adsorption,allofwhichmaybeoperatingsimultaneously[70,71]. Fig.7cshowstheintraparticlediffusionplotsat258Cforvarious dyeinitialconcentrations.AsseenfromFig.7c,therewasalinear relationshipwiththreestagesoverthewholetimerange,butplots didnotpassthroughtheorigin.Therefore,intraparticlediffusion wasnotonlyratelimitingstepandboundarylayercontrolmaybe involvedintheadsorptionprocess.

Thevaluesofkid,CandR2obtainedforthethreeregionsfrom

theplotsaregiveninTable7.Itshowedthatthekidvaluesforthe

threeregionsincreasewithincreaseininitialconcentration.The resultsdisclosedthattheincreaseindyeconcentrationresultsin an increase in thedrivingforce, which is an indicationfor the increaseofthethicknessoftheboundarylayer.

3.5. Isothermstudies

The interaction between adsorbate and adsorbent can be estimatedbytheadsorptionisotherm.Theshapeoftheisothermis the first experimental tool used to diagnose thenature of the adsorptionphenomenon.Twofamousisothermmodels,namely the Langmuir [72] and Freundlich [73] were applied to the experimentaldataatdifferenttemperaturesfrom20to508C.The usedlinearizedisothermequationswere:

Ce qe ¼ 1 qmb þCe qm (10) logqe¼logKFþ 1 nlnCe (11)

wherebisaconstantrelatedtotheenergyofadsorptionandqm

constant represents the maximum binding at the complete

saturationofadsorbentbindingsites.Fromtheslopeandintercept of straight portion ofthelinear plotobtained by plotting Ce/qe

against 1/Ce (Fig. 8a), the values of Langmuir constants were

C O OH O C O OH _OH TWNC Surface C O HO OH C O TWNC Surface N CH3 H3C N H3C CH3 N H3C CH3 Cl N H3C CH3 N CH3 H3C N CH3 CH3 Cl H OH O H

calculated. KF and n are the Freundlich adsorption isotherm

constants(Eq.(11)).KF(mgg1(Lmg1)1/n)isFreundlichconstant

and taken as an indicator of adsorption capacity, and 1/n is a measureoftheadsorptionintensity.Fromtheslopeandintercept ofstraightportion ofthelinearplotobtainedby plottinglogqe

againstlogCe(Fig.8b),thevaluesofFreundlich constantswere

calculated.ThecalculatedLangmuirandFreundlichconstantsand regressioncoefficients(R2)aresummarizedinTable8.Thebest-fit modelwasselectedbasedontheR2_values.

AsobservedfromTable8,althoughtheequilibriumdatafitted well with high R2 values at studied temperatures to both the

Langmuir and Freundlich adsorption isotherm models, the

Freundlichmodelexhibitedaslightlybetterfittotheadsorption datathantheLangmuirmodel.Themaximumqmobtainedfrom

theLangmuirequationat508C,andis 68.97mgg1_.Asseenin

Table 8, the constants qm and b increased withincreasing the

temperature,indicatingthattheadsorptiondensitywashigherat higher temperatures. The value of the Freundlich constant, KF

Fig.7.Theplotsofpseudofirstorder(a),pseudosecondorder(b)andintraparticlediffusion(c)kineticmodelsatvariousconcentrationsforCVadsorptionontoTWNC.

Table6

KineticparametersforCVadsorptionontoTWNCatdifferentinitialconcentrations.

Co(mgL1) 25 75 100 150 200 Pseudo-firstorder qe,exp(mgg1) 3.88 6.85 12.66 17.79 21.30 qe,cal(mgg1) 3.18 3.08 9.30 11.77 13.11 k1(min1) 0.0235 0.0196 0.0177 0.0150 0.0113 R2 0.9638 0.9734 0.9778 0.9848 0.9867 x2 0.13 2.15 0.89 2.04 3.15 ARE 2 6.19 3.79 4.83 5.49 Pseudo-secondorder qe,cal(mgg1) 3.76 6.84 13.21 17.57 20.28 k2103(gmg1min1) 17.5 8.1 4.7 4.4 4.3 R2 _{0.9883 0.9974 0.9940 0.9879 0.9895} x2 0.00 0.00 0.02 0.00 0.05 ARE 0.34 0.18 0.62 0.18 0.68

F.Gu¨zeletal./JournalofIndustrialandEngineeringChemistry20(2014)3375–3386 3384

representsthedegreeofadsorption.TheincreaseofKFvaluesat

higher temperatures suggests that the adsorption process was favorable at higher temperatures. Furthermore, the Freundlich intensityparameter,1/n,indicatesthedeviationoftheadsorption isothermfromlinearity. Ifn=1,theadsorptionislineari.e.,the

adsorption sites are homogeneous and there is no interaction between the adsorbed species. If 1/n<1, the adsorption is favorable;theadsorptioncapacityincreasesandnewadsorption sites appear. If 1/n>1, the adsorption is unfavorable; the

adsorption bonds become weak and the adsorption capacity

decreases[74].Inparticular,thevalueofnissignificantlylower thanunityatallthetemperaturesstudied.Thevaluesof1/nforCV dyewhichislessthan1(Table8)indicatefavorableadsorption.

TheessentialcharacteristicsoftheLangmuirisothermmaybe expressed in terms of dimensionless separation parameters RL,

whichisindicativeoftheisothermshapethatpredictswhetheran adsorptionsystemisfavorableorunfavorable.RLisdefinedas[75]:

RL¼

1 1þbC0

(12)

Thevalue ofRLindicates theshape ofisothermtobeeither

unfavorable(RL>1)orlinear(RL=1)orfavorable(0<RL<1)or

irreversible(RL=0).Here,RLvaluesobtainedfordyearelistedin

Table8.AsseenfromTable8,alltheRLvaluesareintherangesof

0.173–0.10at20–508Ctemperatures,andconfirmedthefavorable adsorption.

3.6. Thermodynamicsstudies

Theincreaseinadsorptionwithariseintemperaturerevealsan endothermicprocesswhichcanbeexplainedthermodynamically byevaluatingthermodynamicparameterssuchaschangeinfree energy(

D

G8),enthalpy(

D

H8)andentropy(

D

S8).Theseparameters werecalculatedusingthefollowingequations:

D

G_¼RTlnK

L (13)

whereKLequalstoqmb,theequilibriumconstantoftheadsorption

process (Lmg1_{). R and T are gas constant and absolute}

temperature, respectively. According to thermodynamics, the

D

G8isalsorelatedtothe

D

H8and

D

S8atconstanttemperature

Fig.8.Langmuir(a)andFreundlich(b)linearadsorptionisothermsofCVonto TWNCatdifferenttemperatures.

Table8

Isotherm parameters obtained for adsorption of CV onto TWNC at different temperatures. Temperatures(8C) 20 30 40 50 Langmuir qm(mgg1) 51.55 63.29 64.52 68.97 b(Lmg1_{) 0.014 0.015 0.020 0.026} RL 0.173 0.160 0.127 0.100 R2 0.9888 0.9862 0.9873 0.9880 x2 0.36 0.21 0.20 0.74 ARE 0.94 0.74 0.58 1.28 Freundlich KF(mgg1(Lmg1)1/n) 2.880 3.263 4.392 7.425 1/n 0.51 0.49 0.47 0.39 R2 _{0.9970 0.9971 0.9925 0.9990} x2 0.02 0.05 0.09 0.00 ARE 0.21 0.24 0.37 0.06 Table7

Intra-particlediffusionmodelparametersfortheadsorptionofCVdyeontoTWNCatdifferentinitialconcentrations. Co(mgL1) Intra-particlediffusionmodel

kid,1 C1 R12 kid,2 C2 R22 kid,3 C3 R32

25 0.15 0.95 0.9772 0.19 0.34 0.9972 0.02 3.63 0.9998

75 0.57 2.24 0.9924 0.30 4.24 0.9942 0.06 6.09 0.9999

100 0.84 2.86 0.9697 0.64 4.99 0.9991 0.14 9.24 0.9835

150 1.01 4.59 0.9942 0.90 7.07 0.9896 0.23 15.84 0.9732

bythevan’tHoffequation: lnKL¼

D

G RT ¼

D

H RT þ

D

S R (14)

Inordertodeterminethethermodynamicparameters, experi-mentswerecarriedoutatdifferenttemperaturesintherangeof 20–508C.TheplotoflnKLasafunctionof1/T(figurenotshown)

yieldsastraightline(R2_{=0.998)fromwhich}

_D

_H8and

_D

_S8were

calculatedfromtheslopeandintercept,respectively.

D

G8(0.8474,0.1309,0.6242and1.5372kJmol1_at20,30,

40and508C,respectively)athightemperaturesrevealedfavorable andspontaneousnatureoftheprocess.Thepositivevalueof

D

H8 (25.38kJmol1_{)indicates that theprocess is endothermic. The}

positive value of

D

S8 (83.33Jmol1_K1_{) demonstrated the}

increased randomness which displayed good affinity between CVandthesurfaceofTWNCduringtheadsorptionprocess. 4. Conclusion

ThepresentstudyexaminestheadsorptionofCVontoTWNC. Theresultsofthisworkcanbesummarizedasfollows:

1.TheN2adsorptionisothermistypeIV.ThevaluesofSBET,Vt,V

m

,

Vext and average pore size are 722.17m2g1, 0.457cm3g1,

0.201cm3_g1_,0.276cm3_g1_{and26.44A˚,respectively.Results}

showthatTWNCincludesmicroporesandmesopores. 2.Theadsorptionof CVwasfoundtoincrease withincrease in

initialsolutionpH,dosage,contacttime,solutiontemperature anddecreaseswithincreasingtheionicstrengthofsolution. 3.Equilibrium data fitted well with high R2 _{values at studied}

temperaturestoboththeLangmuirandFreundlichadsorption isotherm models, the Freundlich model exhibited a slightly betterfittotheadsorptiondatathantheLangmuirmodel.The maximumadsorptioncapacity (qm)increaseswithincreasing

temperature,whichalsoindicatesthattheprocessis

endother-mic. The maximum monolayer adsorption capacity was

obtained as 68.97mgg1 _{at 508C. The separation factors of}

Langmuir isotherm RL were in the range of 0–1, and the

Freundlichconstants1/nweresmallerthan1,whichindicated thattheprocessisfavorable.

4.The adsorption of CV from aqueous solution onto TWNC

proceeds accordingtothepseudosecondorder modelwhich provides thebestcorrelationofthedatainall casesand the experimentalqe,expvaluesagreewiththecalculatedones.Inthis

adsorptionprocess, notonly intra-particlediffusion,but also boundarylayerdiffusiontakesplace.

5.Thermodynamic studies showed that the adsorptionprocess wasspontaneousandendothermic.

6.Tomatowasteactivatedcarbonwithzincchlorideimpregnation canbeusedeffectivelyforremovalofbasicdyesinindustrial effluents.

Acknowledgements

TheauthorsacknowledgetheScientificResearchFundofDicle Universityforfinancialsupport(ProjectNo:12-ZEF-95). References

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