Removal of cobalt (II), copper (II), and nickel (II) ions from aqueoussolutions using phthalate - functionalized sugarcane bagasse : mono - and multicomponent adsorption in batch mode.

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ContentslistsavailableatScienceDirect

Industrial

Crops

and

Products

j ourna l h o m e pa g e : w w w . e l s e v i e r . c o m / l o c a t e / i n d c r o p

Removal

of

cobalt(II),

copper(II),

and

nickel(II)

ions

from

aqueous

solutions

using

phthalate-functionalized

sugarcane

bagasse:

Mono-and

multicomponent

adsorption

in

batch

mode

Stela

Nhandeyara

do

Carmo

Ramos,

Amália

Luísa

Pedrosa

Xavier,

Filipe

Simões

Teodoro,

Laurent

Frédéric

Gil,

Leandro

Vinícius

Alves

Gurgel

∗

GroupofPhysicalOrganicChemistry(GPOC),DepartmentofChemistry,InstituteofBiologicalandExactSciences(ICEB),FederalUniversityofOuroPreto, CampusMorrodoCruzeiro,Bauxita35400-000OuroPreto,MinasGerais,Brazil

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received3July2015

Receivedinrevisedform16October2015 Accepted16October2015

Availableonline18November2015 Keywords: Sugarcanebagasse Phthalicanhydride Adsorption Metalion Desorption

a

b

s

t

r

a

c

t

Thisstudydescribes,indetail,thechemicalmodiﬁcationofsugarcanebagassewithphthalicanhydride

toproduceacarboxylate-functionalizedsugarcanebagasse(SPA).Theoptimizedmodiﬁcation

condi-tionsyieldedSPAwithapercentweightgainof77.08%andanamountofcarboxylicacidgroupsof4.76

mmol/g.TheSPAwascharacterizedbyPZC,FTIR,elementalanalysis,TGA,andSEM-EDX.AnSPA

adsor-bentwasusedtoremoveCo2+_,_Cu2+_,_and_Ni2+_from_aqueous_solution_in_mono-_and_{multicomponent}

systemsinbatchmode.Theadsorptionkineticsfollowedapseudo-second-ordermodel.Theadsorption

rateconstantsassumedthefollowingorder:k_2,Co2+>k_2,Ni2+>k_2,Cu2+.Fourmono-andmulticomponent

isothermmodelswereusedtomodeltheadsorptiondata.Monocomponentdatawerewelldescribedby

theLangmuirmodel,whereasbicomponentdatawerewelldescribedbythemodiﬁedextended

Lang-muirandP-Factormodels.Themaximumadsorptioncapacities(Qmax,mono)fortheLangmuirmodelwere

0.561,0.935,and0.932mmol/gforCo2+_,_Cu2+_,_and_Ni2+_,_{respectively.}_Adsorption_in_bicomponent_systems

revealedthatCu2+_suppressed_the_adsorption_of_Co2+_and_Ni2+_,_whereas_Ni2+_suppressed_the_adsorption

ofCo2+_._Desorption_studies_revealed_that_the_SPA_adsorbent_could_be_fully_desorbed_with_a₁_mol/L_HNO

3

solution.

1. Introduction

Thepollutionofwaterbyheavymetalsisbecomingaserious globalproblem,astheyaretoxic,non-degradable,and bioaccumu-lative(Boamahetal.,2015;FuandWang,2011;Salmanetal.,2015). Thefastdevelopmentofindustriessuchasmetalplating,mining, fertilizers,tanneries,batteries,paper,pesticides,electronics, petro-chemical,textile,amongothershascontributedtoanincreaseinthe concentrationoftoxicmetalsinwastewaters,whicharedirectlyor indirectlydischargedintotheenvironment,mainlyindeveloping countries(Barakat,2011;FuandWang,2011;Kazemipouretal., 2008).Sometoxicmetalsofparticularconcerninthetreatmentof industrialwastewatersandacidminedrainageincludearsenic(III) and(V),cadmium(II),chromium(III)and(VI),cobalt(II),copper(II), lead(II),mercury(II),manganese(II),nickel(II),andzinc(II)(Barakat, 2011;Dumruletal.,2011;FuandWang,2011;Kursunluetal.,2009;

∗ Correspondingauthor.Tel.:+553135591744;fax:+553135591707. E-mailaddresses:[email protected],[email protected](L.V.A.Gurgel).

SheoranandSheoran,2006).Toxicmetalsarethoughtto biomag-nify,andbiomagniﬁcationisoneofthemostsigniﬁcantreasonsfor whichmetalsaredeemedharmfultotheenvironmentandliving organisms(Boamahetal.,2015;FuandWang,2011).Heavy met-alsarealsoknowntobecarcinogenicandtheiradverseeffectsin humanswerewelldocumentedbyKlaassen(2008).

Nowadays,theregulationsusedtocontroltheconcentration of toxic metals in industrial effluents are becoming more and morestringent,astheadverseeffectsoftoxicmetalsare becom-ingmorewell-known.Therefore,toxicmetalsmustberemoved fromwastewatersand acidmine drainsinorder toprotectthe environmentandlivingorganisms(FuandWang,2011;Sheoran andSheoran,2006).AccordingtoFuandWang(2011)aswellas Barakat(2011),variousmethodsarebeingusedtoremovetoxic metals.Theyincludechemicalprecipitation,ion-exchange, adsorp-tion,ultra-and nanofiltration,reverseosmosis,electrochemical, andelectrodialysistechniques.Amongthesemethods,adsorption hasbeenrecognizedasaneffectiveandeconomicmethodtotreat wastewaterscontainingtoxicmetals.Itoffersflexibilityindesign andoperation,astheadsorptionprocesscanbeoperatedinbatch http://dx.doi.org/10.1016/j.indcrop.2015.10.035

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orsemicontinuous(fixed-bedcolumn)modes(FuandWang,2011). Ingeneral,adsorptionalsoproduceshigh-qualitytreatedeffluents and,becauseadsorptionisoftenreversible,theadsorbentscanbe regeneratedthroughasuitabledesorptionprocessandreused,and theadsorbatecanalsoberecovered(FuandWang,2011).When theadsorbentisreusedandtheadsorbatecanberecovered,the adsorptionismore economicallyfavorable.Particularly, theuse oflow-costadsorbents,derived fromagriculturalcropresidues, naturalmaterials,industrialby-products,ormodified lignocellu-losic materials,with a metal ion-binding capacity hasrecently beenpursued,asitmakestheadsorptionprocessmoresuitable (Barakat, 2011).Amongthesebioadsorbents, sugarcanebagasse (SB)isapromisingalternative,asitislargelyavailablein coun-triessuchasBrazil,China,andIndia(Ferreiraetal.,2015;Yuetal., 2015).AccordingtoRochaetal.(2015),SBfromdifferent Brazil-ianregionsiscomposed,onaverage,ofcellulose(42.19±1.93%), hemicelluloses(27.60±0.88%),lignin(21.56±1.67%),inorganics (5.63±2.31%),andextractives(2.84±1.22%).

Cellulose, hemicelluloses, and lignin have different types of hydroxylgroups thatcan bemodifiedwithspecificorganic lig-ands,exhibiting a metal ion-binding capacity. Severalchemical methodshavebeenusedandreportedtomodifylignocellulosic materialsto improve their metalion-binding capacity, suchas esterification,etherification,halogenationfollowedby functional-izationwithorganicligandsbasedonamineandsulfurcompounds, oxidation, and chemical grafting modification (Sun, 2010; Wan NgahandHanafiah,2008).Inadditiontotheenhancedmetal ion-bindingcapacityprovidedbychemicalmodification,certaintypes oforganicligandscanprovidemodifiedadsorbentmaterialswith greaterselectivityfortheremovalofspecificmetalionsbyusing oxygen,amine,sulfur,amongotherelectron-donoratomsorgroups thatexhibitdifferentaffinitiestometalions(Da¸browskietal.,2004; HancockandMartell,1989).

Thisstudyaimedtoproduceanadsorbent basedonSBwith a metal ion-binding capacity through an esteriﬁcation reaction betweenphthalicanhydride(PA)andhydroxylgroupsofSB.PA isacommercialandstablereagent.Itisusetointroduceametal ion-binding capacity intoSB, and it is suitable becausePA is a cheaperreagentthanthoseoftenusedforthispurpose(Sun,2010). Inthisstudy,thechemicalmodiﬁcationofSBwithPAwas exten-sivelystudied andoptimized. Theproduced adsorbent material wasused tostudythe adsorptionof Co2+_,_Cu2+_, _and _Ni2+_from

spikedmono-andbicomponentaqueoussolutions.Theinﬂuences of contact time, pH of the solution, and initial metal-ion con-centrationonadsorptionwerealsostudiedin detail.A suitable desorptionprocessandthereuseofthespentadsorbentwerealso evaluated.

2. Materialandmethods

2.1. Material

CoCl2·6H2O,CuSO4·5H2O,monochlroaceticacid(99%),and

iso-propanolwerepurchasedfromSynth(Brazil).Phthalicanhydride (99%)(catno.125733)waspurchasedfromSigma-Aldrich(Brazil). NiCl2·6H2O, glacial acetic acid (99.5%), NaOH, HCl (37% w/w),

acetone,and pyridine (Py)were purchased fromVetec(Brazil). Quantitative ﬁlter papers (blue ribbon, JP-41, cat no. 3509-1, 12.5cm diameter, ash content of 0.00009g, and grammage of 80g/cm2₎_were_purchased_from_JProlab_(Brazil)._Py_was_reﬂuxed

ina2Lround-bottomedflaskwithNaOHpelletsfor12h,distilled, andstoredinflat-bottomedflaskscontainingNaOHpelletsbefore use.Allmetal-ionsolutionswerepreparedusingdeionizedwater (Millipore,modelMilli-Q®_Simplicity®_)._SB_stalks_were_collected_at

OuroPreto,MinasGerais,Brazil.SBstalkswerepreparedfor

chem-icalmodiﬁcationfollowingthemethodologyproposedbyRamos etal.(2015).

2.2. Modificationofsugarcanebagassewithphthalicanhydride TheesterificationreactionbetweenhydroxylgroupsofSBand PAwasstudiedasafunctionofreactiontimeandamountofPA. 2.2.1. Modificationofsugarcanebagasseasafunctionofthe amountofphthalicanhydride

SB(1.0g),PA(2.0,4.0,6.0,or8.0g),andanhydrousPy(15mL) wereaddedtoa100mLround-bottomedflasktogivea solid-to-liquidratioof1:15(w/v).Arefluxcondenserwithadryingtube filledwithanhydrouscalciumchloridewasattachedtotheflask. Thesuspensionwasheated at100◦C inanoilbathplacedona heating plate (Corning®_, _model _PC-420D) _under_constant

mag-netic stirringat300rpmfor 3h. Attheend ofthereaction,the modified SBwasleft tocoolfor 30minandthen transferredto a 250mLbeakercontaining125mLofisopropanol.It was mag-netically stirredfor 30min,after which it wastransferred toa 150mLglassBuchnerfunnel(discdiameterof60mmand poros-ity of3).The modifiedSB waswashed withisopropanol twice. Then,thephthalate-modifiedsugarcanebagasse(SPA)was sepa-ratedbyvacuumfiltrationandwashedsequentiallywithdistilled water(100mL),0.01mol/LaqueousHClsolution(100mL),distilled water(100mL),andisopropanol(50mL)(Koetal.,2010).TheSPA wasthendriedinanovenat95◦Cfor2.5h,placedinadesiccator tocool,andweighed.Thevaluesofpercentweightgain(pwg)and amountofcarboxylicacidgroups(nCOOH)weredetermined.

2.2.2. Modiﬁcationofsugarcanebagasseasafunctionofreaction time

SB(1.0g),PA(6.0or8.0g),andanhydrousPy(15mL)wereadded toa100mLround-bottomedflasktogiveasolid-to-liquidratio of1:15(w/v).Arefluxcondenserwithadryingtubefilledwith anhydrouscalciumchloridewasattachedtotheflask.The suspen-sionswereheatedat100◦Cinanoilbathplacedonaheatingplate (Corning®_,_model_PC-420D)_under_constant _magnetic_stirring_at

300rpm.Thereactiontimesstudiedfor6.0gofPAwere3and6h andfor8.0gtheywere1,3,6,12,18,and24h.Thepuriﬁcationand characterizationoftheSPAsfollowedtheproceduresdescribedin Section2.2.1.

2.3. Characterizationofmodiﬁedsugarcanebagassewith phthalicanhydride

Allsampleswerepreviouslydriedinanovenat90◦Cfor1hand storedinadesiccatorbeforeanalyses,withtheexceptionofthose usedforthedeterminationofthepercentweightgain.

2.3.1. Percentweightgain

The percent weight gain (pwg) after esteriﬁcation of the hydroxylgroupsofSBwithPAwascalculatedusingEq.(1): pwg=

wSPA−wSB

wSB

×100 (1)

wherepwg(%)isthepercentweightgain,andwSPAandwSB(g)are

theweightsofSPAandSB,respectively. 2.3.2. Amountofcarboxylicacidgroups

Theamountofcarboxylicacidgroups(nCOOH)releasedafterthe

esterificationofSBwithPAwasdeterminedbybacktitration.Two 0.1000gsamplesofSPAwereweighedinto 250mLErlenmeyer flasks,and100.0mLofstandardizedaqueousNaOHsolution(10 mmol/L)wasaddedtoeachflask.Theflasksweremechanically stirred (100rpm)(NovaÉtica, model109/2) for 60min at25◦C.

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Then,thesuspensionswerefilteredusingasinglefiltration(JP-41 filterpaper).Three20.0mLaliquotsfromeachflaskweretitrated withstandardizedaqueousHClsolution(10mmol/L)untilthe end-pointofphenolphthalein.ThenCOOHofSPAwascalculatedusingEq.

(2): nCOOH=

[CNaOHVNaOH−5 (CHClVHCl)]

wSPA

(2) wherenCOOH (mmol/g) istheamountof carboxylicacidgroups

ontheSPAsurfaceandCNaOHandCHCl(mmol/L)arethe

concen-trationsofNaOHandHClsolutions,respectively,VNaOH(L)isthe

volumeofNaOHsolutionandVHCl(L)isthevolumeofHClsolution

expendedwhentitratingexcessunconsumedNaOH,andwSPA(g)

istheweightofSPA.

2.3.3. Pointofzerocharge(PZC)

Three0.01mol/LNaNO3solutionswithpHvaluesof3,6,and11

werepreparedbyadjustingthepHofthesolutionsusing0.1mol/L HNO3andNaOHsolutions.DifferentamountsofSPAsampleswere

addedtoﬁve125mLErlenmeyerﬂasksand20.0mLofNaNO3

solu-tionofpH3,6,or11wasaddedtoeachﬂasktogivesuspensions of0.05, 0.1, 0.5,1, and 2%(w/v). Theﬂasks weremechanically stirred(130rpm)(NovaÉtica,model109/2)for24hat25◦C.Then, theequilibriumpHvaluewasmeasuredusingapHmeter(Hanna Instruments,modelHI223)toobtainthePZC.

2.3.4. Fourier-transforminfraredspectroscopy(FTIR)

SamplesofSB,SPA,andSPAloadedwithCo2+_,_Cu2+_,_and_Ni2+

(1.0mg)weremixedwith100mgofspectroscopy-gradeKBrand pressedinahydraulicpressat8tonsfor0.5mintoobtain13mm KBr pellets (Pike CrushIR, model 181-1110, Pike Technologies, Canada).TheFTIRspectrawererecordedonanABBBomenMB 3000FTIRspectrometer(Quebec,Canada)equippedwithopticsof ZnSeandaDTGSdetectorsetataresolutionof4cm−1from500to 4000cm−1with32scans.

2.3.5. Energy-dispersiveX-ray(EDX)spectroscopy

SamplesofSPAloadedwithCo2+_,_Cu2+_,_and_Ni2+_were_analyzed

onaVega3SBSEM-EDXspectrometer(Tescan/OxfordInstruments) usingaﬁlament voltageof20keVand abackscattered electron (BSE)detector.Samplesof100mgofSPAloadedwithmetalions werepressedinahydraulicpressat8tonsfor1mintoprepared pelletsof13mm.Then,theyweresputter-coatedwithcarbonin modularhigh-vacuumcoatingequipment(QuorumTechnologies, modelQ150RES).

2.4. Adsorptionexperiments

AdsorptionofCo2+_,_Cu2+_,_and_Ni2+_on_SPA_in_a_{monocomponent}

systemwasstudiedasafunctionofcontacttime(kinetics),pHofthe solution,andinitialconcentrationofmetalions(isotherm). Bicom-ponentadsorptionisothermscomposedofCo2+_–Cu2+_,_Co2+_–Ni2+_,

andCu2+_–Ni2+_systems_in_equimolar_{concentrations}_were_also

stud-ied.

2.4.1. AdsorptionofmetalionsonSPAasafunctionofcontact time(kinetics)

Samplesof100.0mLofeachmetal–ionsolution(0.79mmol/L), bufferedatpH5.5forCu2+_and_5.75_for_Co2+_and_Ni2+_(0.05_mol/L

CH3COOH/CH3COONa), wereaddedto250mLErlenmeyerﬂasks

andpre–thermostatedto25◦Cinashakerincubatorfor1h (Tec-nal,model TE–424,Piracicaba, SP,Brazil).Then, samplesofSPA (20.0mg)weighedincylindricalglasses(1.8mmheight_×2.2mm diameter)were added toeach Erlenmeyer ﬂask and stirred at 130rpmfordifferenttimeintervals.Then,thesolidswereﬁltered

offusing asinglefiltration(JP–41filterpaper)and the concen-trationofmetalionattimetwasdeterminedbyaflameatomic absorption spectrophotometer (FAAS) (Hitachi, model Z-8200) (Co=240.7mm,Cu=324.8mm,andNi=232mm).Theamount

ofeachmetalionadsorbedonSPAattimetwascalculatedusing Eq.(3): qt=

(Ci−Ct)_M2+VM2+

wSPA (3) whereqt(mmol/g)istheamountofmetalion(M2+)adsorbedper

unitweightofSPAatatimet,V_M2+(L)isthevolumeofmetal–ion solution,CiandCt(mmol/L)arethemetal–ionsolution

concentra-tionsat0andtimet,respectively,andwSPA (g)istheweightof

SPA.

2.4.2. AdsorptionofmetalionsonSPAasafunctionofsolutionpH SamplesofSPA(20.0mg)weredirectlyweighedinto250mL Erlenmeyer ﬂasks, and 100.0mL of each metal–ion solutions (0.79mmol/L)bufferedatpHvalues from2.0 to3.5(0.05mol/L ClCH2COOH/ClCH2COONa) and from 4.0 to 5.75 (0.05mol/L

CH3COOH/CH3COONa)were added.TheErlenmeyer ﬂaskswere

placed into a pre–thermostated shaker incubator at 25◦C and stirredat130rpmuntilequilibriumwasreached.Then,thesolids werefilteredoffthroughasinglefiltration(JP–41filterpaper)and theconcentrationsofmetalionsweredeterminedbyFAAS.The maximumpHvalueofeachmetal–ionsolutionrequiredtoavoid theformation of hydrolyzedspecies and precipitation was cal-culatedusingtheconcentrationof eachmetal–ion solutionand thesolubilityproduct constant(Ksp)forCo(OH)2 (5.92×10−15),

Cu(OH)2(1.8×10−20),andNi(OH)2(5.48×10−16)(Haynes,2014).

2.4.3. Adsorptionasafunctionoftheinitialmetal–ion concentration

2.4.3.1. Monocomponentisotherms. SamplesofSPA(20.0mg)were directlyweighedinto250mLErlenmeyerﬂasks,and100.0mLof eachmetal–ionsolution,bufferedatpH5.5forCu2+_and_5.75_for

Co2+ _and _Ni2+ _(0.05_mol/L _CH

3COOH/CH3COONa), with

concen-trationsrangingfrom0.05to1.12mmol/L,wereadded.Further experimentalprocedureswerethesameasthosedescribedin Sec-tions2.4.1and2.4.2.

2.4.3.2. Bicomponent isotherms. Samples of SPA (20.0mg) were directly weighed into 250mL Erlenmeyer ﬂasks, and 100.0mL ofthebinaryequimolarmetal–ion solutions,bufferedatpH5.5 (0.05mol/L CH3COOH/CH3COONa), with concentrations ranging

from0.05to1.12mmol/LforCo2+_–Cu2+_,_Co2+_–Ni2+_,_and_Cu2+_–Ni2+_,

wereadded.Furtherexperimentalprocedureswerethesameas thosedescribedinSections2.4.1and2.4.2.

2.5. Desorptionexperiments

SamplesofSPA(100mg)weredirectlyweighed into250mL Erlenmeyer ﬂasks, and 100.0mL of each metal–ion solution (3.15mmol/LforCo2+_,_Cu2+_,_and_Ni2+₎_buffered_at_pH_5.5_(0.05_mol/L

CH3COOH/CH3COONa) was added. The ﬂasks were stirred in a

pre–thermostated shaker incubator at 25◦C and 130rpm for 180min,usingtheoptimalexperimentalparametersdetermined fromtherespectiveadsorptionstudiesforCo2+_,_Cu2+_, _and_Ni2+_.

Then,thesolidswerefilteredoffthroughasinglefiltrationstep, washedwithanexcessofdeionizedwater,driedinanovenat60◦C for12h,andstoredinadesiccator.DriedsamplesofSPA(20.0mg) loadedwitheach metalion weredirectlyweighed into250mL Erlenmeyerflasks,and20.0mLof1.0mol/LHNO3wasadded.The

ﬂaskswereplaced intoa pre–thermostatedshakerincubatorat 25◦Cand130rpmfor5min.Furtherexperimentalprocedureswere

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Fig.1.(a)SchemeillustratingthesynthesisrouteusedtosynthesizetheSPAadsorbent,(b)suggestedadsorptionmechanismofmetalions(M2+₎_onto_the_SPA_adsorbent

[M=Co,Cu,orNiandmXn–₌_counter_ion_(SO

42–orCl–)],and(c)suggesteddesorptionmechanismofthemetalionsfromtheSPAsurface.

thesameasthosedescribedinSections2.4.1and2.4.2.The desorp-tionefﬁciency(DE)wascalculatedbyEq.(4):

DE=

_C e,M2+VHNO3 QT,maxwSPA

×100 (4)

whereDE(%)isthedesorptionefﬁciency,C_e,M2+(mmol/L)isthe equilibriumconcentrationofthemetalion(M2+_),_V

HNO3(L)isthe

volumeofnitricacidsolution,QT,max(mmol/g)isthetheoretical

maximumadsorptioncapacitydeterminedforeachmetalion,and wSPA(g)istheweightofSPAloadedwitheachmetalion.

2.5.1. ReuseofthespentSPA

SPAadsorbentsfromdesorptionstudieswerewashedwithan excessofdeionizedwaterandisopropanol,beforebeingdriedinan ovenat95◦Cfor2.5h.SamplesofSPA(20.0mg)wereweighedinto 250mLErlenmeyerﬂasks,and100.0mLofeachmetal–ionsolution (0.79mmol/L)wasadded.Theoptimalexperimentalparameters determinedfromadsorptionstudieswereadopted.Further exper-imentalprocedureswerethesameasthosedescribedinSections 2.4.1and2.4.2.There–adsorptionefﬁciency(RE)ofSPAforanew cycleofadsorptionofeachmetalionwascalculatedusingEq.(5): RE=

w_SPA,M2+−wSPA

QT,max

des+ (QmaxwSPA)re-ads

/wSPA

QT,max

×100 (5)

whereRE (%)isthere–adsorptionefﬁciency,w_SPA,M2+ (g)isthe weightof SPAloadedwithmetal ion,wSPA (g)(inside brackets

withsubscript“des”)istheweightofSPAafterdesorption,whichis determinedfromw_SPA,M2+andthedesorptionefﬁciency(DE),Qmax

(mmol/g)isthenewmaximumadsorptioncapacityobtainedfrom there–adsorptionstudy,andwSPA(g)istheweightofSPAusedin

there–adsorptiontest.Thesubscripts“des”and“re–ads”inEq.(5) arerelatedtothedesorptionandre–adsorptiontests,respectively.

3. Resultsanddiscussion

3.1. PreparationandcharacterizationofSPAadsorbent

ThecharacterizationofSBandSPAadsorbentby thermogravi-metricanalysis(TGA)(Fig.S1andTableS1)andscanningelectron microscopy(SEM)(Fig.S2)canbeaccessedintheSupplementary material.

3.1.1. Percentweightgainandnumberofcarboxylicacidgroups Fig.1presentsthesyntheticrouteusedtoobtaintheSPA adsor-bent,showingasuggestionoftheadsorptionmechanismthrough whichmetalions(M2+₎_are_removed_from_aqueous_solution_by_SPA

adsorbent,andasuggestionofthedesorptionmechanismthrough whichmetalionsaredesorbedfromtheSPAsurfaceandreleased intotheaqueoussolution.ThechemicalmodiﬁcationofSBwith PA usingPyasa solventand base/catalysttoproduce SPAwas optimizedthroughtheevaluationoftheeffectsofreactiontime andPAconcentrationonthevaluesofpwgandnCOOH.Theresults

ofoptimizingtheesteriﬁcationreactionarepresentedinTable1, whichshowsthat,astheamountofPAwasincreased,pwgand

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Table1

TheresultsofoptimizingthechemicalmodiﬁcationofSBwithPA.

Phthalicanhydride (PA)(g) Reaction time(h) SPAa pwg(%) nCOOH(mmol/g) 2.0 3 40.62±3.38 2.42±0.18 4.0 58.99±0.10 2.71±0.08 6.0 75.29±1.43 3.26±0.03 8.0 81.22±2.84 3.18±0.10 6.0 3 75.29±1.43 3.26±0.03 6 77.08±0.00 4.76±0.03 8.0 1 62.71±3.35 2.57±0.20 3 81.22±2.84 3.18±0.10 6 98.06±1.25 3.60±0.25 12 140.80±1.60 4.95±0.02 18 142.90±14.09 4.80±0.95 24 161.22±8.89 4.99±0.47

a_The_weight_of_SB_used_in_all_chemical_{modiﬁcations}_was_1.0_g._All_reactions_were

performedinduplicate.

nCOOHalsoincreased.TheincreaseintheamountofPAfrom2.0to

8.0gyieldedanincreaseinbothpwgandnCOOH.Asmallvariation

betweenthenCOOHvalueswasobservedforreactionsemploying

6.0and8.0gofPA,consideringthestandarddeviation.Therefore, whenstudyingtheesteriﬁcationasafunctionofreactiontime,6.0 and8.0gofPAwerealsoassessedwithaviewtoapossible reduc-tioninthepreparationcostsoftheSPAadsorbentonalargerscale. Forreactionsemploying8.0gofPA,asthereactiontimeincreased, pwgandnCOOHalsoincreased.Thesamebehaviorwasnoticedfor

reactionsusing6.0gofPA.However,thenCOOHforthereaction

employing6.0gofPAandareactiontimeof6hwassimilartothat whenusing8.0gofPAandreactiontimesequaltoorgreaterthan 12h.Thus,todecreasethepreparationcostsoftheSPAadsorbent, 6.0gofPA(PA/SBratio=6:1)andareactiontimeof6hwere cho-sentopreparealargeramountofSPAadsorbentinordertoperform theadsorptionexperiments.Thesereactionconditionswere cho-senbytakingintoaccountthehighestnCOOHobtainedusingthe

smalleramountofPAandshorterreactiontime. When increas-ingthereactionscaleto10.0gofSBand60.0gofPA,thepwgand nCOOHvalueswere74.85%and4.79±0.03mmo/g,respectively.The

scale–upofthereactionresultedinadecreaseinthepwgby2.98% andanincreaseinnCOOHby0.63%.Theestimatedpreparationcostof

SPA(PA/SB=6:1and6hreactiontime)basedontheinternational marketpricesofthechemicalsused,SB,andelectricityinBrazil wasUS$85.60perkilogramofSPA.ThepreparationcostofSPAdid notincludethechemicalsusedintheelaborationsteps(Section 2.2.1)andthepossibilityofrecoveringandrecyclingPyandexcess PA.

Liuetal.(2006)alsostudiedtheesteriﬁcationofSBwithPAusing Pyas a base/solvent (PA/SB ratio=1:1.5and Py/SB ratio=1:10) underultrasoundirradiationat115◦Cfor75min.Theyreporteda maximumpwgof39.1%whenusingthereactionconditions previ-ouslymentioned.IncomparisontotheresultsreportedbyLiuetal. (2006),SPAswereobtainedinthisstudywithhigherpwgvalues. 3.1.2. ElementalanalysisandFTIRspectroscopy

Elementalanalysiswascarriedout for SBand SPA.The car-bon,hydrogen,andnitrogencontentsforSBandSPAwere46.56 and48.62%,5.09and5.31%,0.30and0.19%,respectively.Fig.2a presentstheFTIRspectraforSBandSPA.Themajorchangesthat canbehighlightedinthespectrumofSBinrelationtoSPAare: (1)theappearanceofabandat3074cm−1,whichisattributedto anintramolecularbondbetweenthehydrogenofthecarboxylic acidgroupand theoxygenofthecarbonylestergroup; (2)the appearanceofbandsat 2638and2527cm−1,whichare related tothestretchingofhydrogenbondingexhibitedbydimmers of aromaticcarboxylicacid;(3)theappearanceofastrongbandat

1724cm−1,whichcorrespondstothestretchingofconjugated car-bonylester;(4)theappearanceofbandsat1600and1491cm−1, whicharerelatedtothestretchingofC Cbondsinthebenzenering ofphthalatemoietyandlignin;(5)abandat1286cm−1,related totheC Ostretching inesters and carboxylicacids; and (6) a bandat745cm−1 relatedtoC Hout–of–plane bendingforthe 1,2–substitutedbenzenoidring(Paviaetal.,2014).Theabsenceof bandsat1850–1800cm−1intheFTIRspectrumofSPA,which corre-spondtoasymmetricandsymmetricstretchingmodesofcarbonyl groupofcycliccarboxylicacidanhydride,indicatesthatSPAisfree ofunreactedPA(Liuetal.,2006;Paviaetal.,2014).Thesechanges intheFTIRspectraofSPAincomparisonwithSBconfirmthe intro-ductionofthephthalatemoietyintoSBthroughtheesterification reaction.SimilarIRbandassignmentswerereportedbydeMelo etal.(2010)andLiuetal.(2006)whostudiedtheesterificationof microcrystallinecelluloseandSBwithPA,respectively.

Fig.2bpresentstheFTIRspectraforSPAandSPAloadedwith Co2+_,_Cu2+_,_and_Ni2+_._The_adsorption_of_metal_ions_on_the_SPA

adsor-bentwascharacterizedbythesplittingofthebandintheregionof 1685cm−1(Smith,1998),whichcorrespondstothestretchingof C Oincarboxylicacidgroups.Thisbandappearstobeoverlapped withtheesterbandat1724cm−1.Whenthemetal–ion adsorp-tiontakesplace,thisbandissplitandtwonewbandsappearat 1592and 1552cm−1,correspondingtotheasymmetric stretch-ingof carboxylategroups. The bandat1592cm−1 alsoappears tobeoverlappedwiththestretchingofC Cbondsat1600cm−1 asseeninFig.2bforSPA-loadedwithCo2+_,_Cu2+_,_and_Ni2+_._The

splittingofthebandat1685cm−1isattributedtothecarboxylic acidgroupandisindicative thatthesegroupsaredeprotonated during the adsorption of metal ions and that the carboxylate groups are involved in the adsorption of metal ions(Łyszczek, 2007).ThesechangesintheFTIRspectraforSPAincomparison withSPAloadedwithCo2+_,_Cu2+_,_and_Ni2+_prove_that_the

adsorp-tionmechanisminvolvesanion–exchangephenomenon,inwhich hydroniumionsareexchangedbymetalionsfollowedby complex-ationofthemetalionswithcarboxylategroups,assuggestedin Fig.1b.

3.1.3. Pointofzerocharge(PZC)

ThePZCwasdeterminedtoassessthesurfacepropertiesofthe SPAadsorbent.ThepHthatisrequiredtoyieldanetzerosurface chargemayberelatedtotheacidityconstant(Ka)ofanadsorbent

materialcontainingacidic functionalgroups (Nohand Schwarz, 1990).Thus,thesurfaceofanadsorbenthasanetpositivesurface chargeatpH<PZC,whereas,atpH>PZC,ithasanetnegative sur-facecharge.ThePZCvaluefortheSPAadsorbentwas2.92±0.06. ThepKavalues ofPA,correspondingtothepositionofcarboxyl

groups1and2,are2.98and5.28(Fig.1),respectively(Braudeand Nachod,1955).AcomparisonofthepKavaluesforPAandthePZC

ofSPAindicatesthatelectroniceffectsprovidedbythepresenceof theesterbondbetweenthehydroxylgroupsofSBandthe phtha-latemoietyinﬂuencedtheacidityoftheremainingcarboxylgroups. ThePZCvaluefortheSPAadsorbentindicatesthattheadsorption ofmetalionswilloccuratpH≥2.92.

3.1.4. EnergydispersiveX–ray(EDX)spectroscopy

EDXspectroscopywasusedtomapthesurfaceoftheSPA adsor-bentinordertoevaluatetheadsorptionsitesoftheadsorbedmetal ionsandtheirdistributionalongthesurfaceoftheSPA.Fig.3a–c presentsthedistributionofCo2+_,_Cu2+_,_and_Ni2+_on_the_surface_of_the

SPAadsorbent.InFig.3a–c,anon–uniformdistributionofadsorbed Co2+_,_Cu2+_,_and_Ni2+_was_observed_at_the_adsorption_surface_sites_of

theSPAadsorbent.Someregionsexhibitedhigherconcentrations ofmetalionsthanothers.TheLangmuirmodelbetterdescribed theadsorptionofCo2+_,_Cu2+_,_and_Ni2+_on_the_SPA_adsorbent,_as

(6)

pre-4000 3500 3000 2500 2000 1500 1000 500 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80

)

%(

ec

na

tti

ms

na

r

T

Wavenumber (cm

-1

)

SB

(a)

4000 3500 3000 2500 2000 1500 1000 500 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 1600 2527 SPA

Tr

an

smit

tan

ce (%)

Wavenumber (cm

-1

)

2638 1724 745 1286 1491 3074 4000 3500 3000 2500 2000 1500 1000 500 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 1552 1592 (b)

)

%(

ec

na

tti

ms

na

r

T

Wavenu

mber (cm

-1

)

SPA-Ni2+ SPA-Cu2+ SPA-Co2+ SPA

Fig.2. FTIRspectraof(a)SBandSPAand(b)SPAandSPAloadedCo2+_,_Cu2+_,_and_Ni2+_{(transmittances}_of_SPA–Co2+_,_SPA–Cu2+_,_and_SPA–Ni2+_spectra_were_shifted_by_30,_60,

and90%).

sentedinSection3.2.3.1.Thissuggeststheformationofauniform adsorptionlayerontheSPAsurface.Conversely,inFig.3a–c,itcan beseenthattherearesomepreferentialadsorptionsites. How-ever,theLangmuirmodelwouldstillberepresentative,takinginto accountthatthereisnoexperimentalevidenceofmultilayer forma-tion,butmerelythepresenceofsparseandpreferentialadsorption sitesontheSPAsurface.SimilarresultswerereportedbyLiuand Xu(2007)forthebiosorptionofNi2+_on_aerobic_granules_and_by

VieiraandBeppu(2006)fortheadsorptionofHg2+_on_unmodiﬁed

andmodiﬁedcross–linkedchitosanﬁlms. 3.2. Adsorptionexperiments

3.2.1. EffectofthesolutionpHonmetal–ionremoval

TheeffectofthesolutionpHonmetal–ionremovalbySPAwas assessedat25◦Cwitha0.79mmol/Lmetal–ionsolution,130rpm, 0.2g/Ladsorbent,aswellas180minequilibriumtime(te),basedon

kineticdata(Table2)forCo2+_,_Cu2+_,_and_Ni2+_._Figure₄_presents_the

equilibriumadsorptioncapacity(qe)ofCo2+,Cu2+,andNi2+onthe

SPAadsorbentasafunctionofthesolutionpHfrom2.0to5.75.The removalofmetalionsfromaqueoussolutionisstronglycontrolled bythesolutionpH,asthesolutionpHaffectsboththesurfacecharge oftheSPAadsorbentandtheformofthemetalionsintheaqueous solution.Thenetsurfacechargeofanadsorbentinaqueous solu-tionisassociatedwithitsPZC.ThePZCvalueoftheSPAadsorbentis 2.92.Therefore,theremovalofmetalionswasfavoredatpH>PZC, wherethesurfaceoftheSPAadsorbentisnegativelycharged,owing tothedeprotonationofcarboxylicacidgroups.Fig.4showsthat theqeincreasedasthesolutionpHincreased,andaplateauwas

notattainedfortheremovalofCo2+_,_Cu2+_,_and_Ni2+_._At_low_pH

val-ues(pH<3),thecarboxylategroupsontheSPAadsorbentsurface wereprotonatedandtheremovalofmetalionsdidnotoccur.For pHvalueshigherthanthePZC(pH≥3),theremovalofCu2+_took

placeasaconsequenceofstrongattractionsbetweenthenegatively chargedcarboxylategroupscontainingnonbondingelectronpairs ontheSPAadsorbentsurfaceandpositivelychargedmetalionsin

(7)

Fig.3. SEM–EDXimageswithsurfacemappingfor(a)Co2+_,_(b)_Cu2+_,_and_(c)_Ni2+_adsorbed_on_the_SPA_surface. 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Co2+ Cu2+ Ni2+ qe ) g/l o m m( pH

Fig.4. EffectofpHontheadsorptionofCo2+_,_Cu2+_,_and_Ni2+_onto_the_SPA_(0.79

mmol/LM2+_,₁₃₀_rpm,₂₅◦_C,_0.2_g/L_SPA_and₁₈₀_min_of_shaking_for_Co2+_,_Cu2+_,_and

Ni2+_,_{respectively).}

solution.Conversely,theremovalofCo2+_and_Ni2+_took_place_at

pHvalueshigherthan4.Thisindicatesthatanalternativemethod toseparateCu2+_from_Co2+_and_Ni2+_is_to_control_the_solution_pH

atvaluesbelow4.ThemaximumqevaluewasattainedatpH5.5

forCu2+_and_5.75_for_Co2+_and_Ni2+_._Adsorption_at_pH_values_greater

than5.5forCu2+_and_5.75_for_Co2+_and_Ni2+_was_not_evaluated,_as_the

formationofmetal–hydrolyzedspecies[M(OH)+_]_could_occur_with

consequent precipitation of metal–ion hydroxides [M(OH)2(s)].

Therefore,pHvalues of5.5for Cu2+_and _5.75_for _Co2+ _and_Ni2+

werechosenasthemaximumpHvaluesforthefollowing adsorp-tionexperiments,bothasafunctionoftimeandinitialmetal–ion concentration.

3.2.2. Adsorptionkinetics

Theadsorptionkineticsrepresentsoneofthemostsigniﬁcant parametersfordesigningawastewatertreatmentplantbasedon thebatchadsorptionprocess(HoandMcKay,1999).Pseudo—ﬁrst— (see the Supplementary material) and pseudo–second–order kineticmodelswereusedtomodeltheadsorptionrateofmetal ionsontheSPAadsorbentsurfaceatpH5.5forCu2+_and_5.75_for

Co2+_and_Ni2+_at₂₅◦_C,₁₃₀_rpm,_0.79_mmol/L_metal–ion_solution,

and0.2g/Ladsorbent.

Thepseudo—second—orderkineticmodelofHoandMcKay (1998)alsoexpressestherateofadsorptionasafunctionofthe adsorptioncapacity;howeverit assumesthattherate—limiting stepmaybechemicaladsorption,involvingvalencyforcesthrough thesharingorexchangeofelectronsbetweenadsorbentand

(8)

adsor-Table2

Resultsofmodelingtheexperimentalkineticdata,monocomponentandbicomponentequilibriumadsorptiondatafortheadsorptionofmetalionsonSPA(25◦_C,₁₃₀_rpm,

0.2g/LSPA). Kineticmodel Experimental data Parameters Co2+ _Cu2+ _Ni2+ pH 5.75 5.5 5.75 te(min) 180 180 180 qe,e×p(mmol/g) 0.464±0.014 0.448±0.018 0.554±0.027

Pseudo–second–order qe,est(mmol/g) 0.443±0.016 0.432±0.022 0.525±0.025

k2(g/mmolmin) (4.20±0.82)×10−1 (1.54±0.35)×10−1 (1.89±0.43)×10−1 R2 _0.9218 _0.9222 _0.9404 2 red 0.0034 0.0051 0.0066 Monocomponentisotherms Experimental data Parameters Co2+ _Cu2+ _Ni2+ Qmax,exp(mmol/g) 0.462±0.021 0.845±0.006 0.701±0.017 pH 5.75 5.5 5.75 te(min) 180 180 180 CN 10.36 5.67 6.84 Ie(mol/L) 0.0512 0.0523 0.0526 e 0.886 0.886 0.886

Langmuir Qmax,est(mmol/g) 0.561±0.027 0.935±0.061 0.932±0.052

b(L/mmol) 7.24±0.90 7.09±1.21 3.01±0.33 R2 _0.9658 _0.9388 _0.9800 2 red 0.0022 0.0075 0.0022 adsG◦ −22.33±2.76 −22.28±3.79 −20.16±2.19 Freundlich KF[mmol/g/(L/mmol)1/n] 0.556±0.031 0.973±0.034 0.759±0.017 n 2.57±0.23 2.37±0.12 1.90±0.06 R2 _0.9198 _0.9685 _0.9892 2 red 0.0052 0.0039 0.0012 Bicomponentisotherms Experimental data

Parameters Co–Cu Cu–Co Co–Ni Ni–Co Cu–Ni Ni–Cu

Qmax,exp,i(mmol/g) 0.152±0.014 0.505±0.019 0.180±0.007 0.281±0.006 0.627±0.007 0.532±0.019 Qmax,exp,i+j(mmol/g) 0.657±0.024 0.461±0.009 1.159±0.020 Qmax,exp,i,multi/Qmax,exp,mono 0.329 0.598 0.390 0.401 0.742 0.759 pH 5.5 5.5 5.5 5.5 5.5 5.5 te,mono(min) 180 180 180 180 180 180 Ie(mol/L) 0.0536 0.0537 0.0557 e 0.886 0.886 0.886 Modiﬁed extended Lang-muir i 2.161±0.382 7.089±0.126 1.613±0.199 1.132±0.128 0.714±0.100 2.050±0.221 j 0.956±0.303 1.521±0.512 0.399±0.074 1.601±0.344 0.781±0.218 42.40±80.26 R2 _0.8069 _0.9289 _0.9126 _0.9511 _0.9404 _0.9100 2 red 0.0031 0.0034 0.0016 0.0014 0.0038 0.0063 P–Factor Pi 2.857 1.629 2.693 2.672 1.437 1.114 KL,i 6.20±1.28 12.72±1.72 6.958±0.974 6.653±0.708 11.69±1.43 1.545±0.179 aL,i(L/mmol) 11.05±3.21 13.61±2.70 12.40±2.25 7.14±1.18 12.50±2.07 1.658±0.441 R2 _0.8268 _0.9298 _0.9018 _0.9554 _0.9321 _0.9106 2 red 0.0028 0.0034 0.0018 0.0013 0.0044 0.0063

bate(HoandMcKay,1999).ThemodelofHoandMcKay(1998)is showninEq.(6):

dqt

dt =k2(qe−qt)

2 ₍₆₎

whereqeandqt(mmol/g)aretheadsorptioncapacitiesat

equilib-riumteandattimet(min),respectively,andk2(g/mmol.min)is

thepseudo–second–orderrateconstant.Rearrangingand integrat-ingEq.(6)usingboundaryconditionsqt=0att=0andqt=qtatt=t,

yieldsEq.(7). qt=

k2qe2t

1+k2qet

(7) The experimental kinetic data were modeled by nonlinear regressionanalysisusingpseudo —ﬁrst—andpseudo—second —orderkineticmodelswithMicrocalOriginPro®₂₀₁₅_software,

whichwassettousetheLevenberg–Marquardtiterationalgorithm andtheweightmethodstatistical[Eq.(8)].Theweightswereused tominimizethechi–squared(2₎_value_[Eq.₍₉₎_]_in_order_to_obtain

thebestﬁttingcurve.Boththecoefﬁcientofdetermination (R2₎

andreduced2_[Eq.₍₁₀₎_]_were_taken_into_account_to_evaluate_the

qualityofthenonlinearregressionanalysisandtodeﬁnethebest kineticmodelfordescribingtheadsorptionprocess.

wi=

1 yi

(8)

wherewiistheweightingcoefﬁcientandyiistheexperimental

datapoint. 2₌ N

i=1 wi

yi− ˆ yi

2 (9)

where ˆyiisthetheoreticaldatapointcalculatedbythemodel.

x2 red=

x2

(10)

where2

redisthereducedchi–squaredand

isthenumberof

degreesoffreedom.

Table 2 presents the kinetic parameters obtained by mod-eling the experimental kinetic data withpseudo–second–order kineticmodelusingnonlinearregressionanalysis.Resultsof mod-elingkineticdatawithpseudo–ﬁrst–ordermodelandexperimental kinetic dataare available inTables S2 and S3(see the Supple-mentaryMaterial).Fig.5a–cpresentsplotsofqt versustforthe

adsorptionof Co2+_, _Cu2+_,_and _Ni2+ _onto_the_SPA_adsorbent, _and

thecurvesﬁttedtotheexperimentaldatausingpseudo–ﬁrst–and pseudo–second–orderkineticmodelsareshown. Table2shows that both the values of R2 _and 2

red obtained by ﬁtting the

pseudo–second–ordermodeltotheexperimentaldatawerehigher andlowerincomparisonwiththepseudo–ﬁrst–ordermodel(Table S2),respectively.Additionally,whentheexperimentalequilibrium

(9)

0 50 100 150 200 250 300 350 400 450 500 550 600 650 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 Co2+

Pseudo-first-order model Pseudo-second-order model

qt ) g/l o m m( Tempo (min)

(a)

0 50 100 150 200 250 300 350 400 450 500 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 Cu2+

Pseudo-first-order model Pseudo-second-order model

(b)

0 50 100 150 200 250 300 350 400 450 500 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 Ni2+ Pseudo-first-order model Pseudo-second-order model

(c)

Fig.5. Adsorptionkineticsof(a)Co2+_,_(b)_Cu2+_,_and_(c)_Ni2+_on_the_SPA_at_pH_5.75

forCo2+_and_Ni2+_and_5.5_for_Cu2+_,_0.79_mmol/L_M2+_,₂₅◦_C,₁₃₀_rpm_and_0.2_g/L_SPA.

adsorptioncapacity(qe,exp)wascomparedtotheestimated

equi-libriumadsorptioncapacity(qe,est),itcouldbeseenthatthevalues

providedbythepseudo–second–ordermodelweremoresimilar thanthoseprovidedbythepseudo–ﬁrst–ordermodel.This sug-geststhatthepseudo–second–ordermodel betterdescribesthe

adsorptionkineticsofCo2+_,_Cu2+_,_and_Ni2+_on_the_SPA_adsorbent.

Theratiosofk2,Co/Cu,k2,Co/Ni,andk2,Ni/Cuwere2.73,2.22,and1.23,

respectively.ThismeansthatCo2+_is_adsorbed_by_SPA_faster_than

Cu2+_and_Ni2+_,_whereas_Ni2+_is_adsorbed_more_quickly_by_SPA_than

Cu2+_._These_ﬁndings_suggest_that_it_would_be_possible_to_make_a

kineticseparationofCo2+_from_Cu2+_and _Ni2+ _in_a

multicompo-nentionsolutionifalowagitationtimeisusedinabatch–mode system.TheseparationofCo2+_,_Cu2+_,_and_Ni2+_from_aqueous

solu-tionsisoftenrequiredinhydrometallurgyprocessing(Po´spiechand Walkowiak,2007)andindustrialwastes,forexample,inacidmine drainage(Yavuzetal.,2003).

3.2.3. Adsorptionisotherms

Adsorptionisotherms describehow differentkinds of pollu-tantsinteractwithadsorbentsand,therefore,areveryimportant intheclariﬁcationofadsorptionmechanismsaswellasin deter-miningtheequilibriumadsorptioncapacityanditsinﬂuenceonthe surfacepropertiesonadsorption.Hence,adsorptionisothermsare essentialinthedesignofbatchadsorptionsystems(Ferreiraetal., 2015;FooandHameed,2010).AdsorptionofCo2+_,_Cu2+_,_and_Ni2+

ontheSPAadsorbentwasinvestigatedinmono–andbicomponent aqueoussolutionsat25◦C,130rpm,0.2g/LSPA,0.05–1.12mmol/L metal–ionsolution,withanequilibriumtimeof180min(basedon thekineticstudiesofSection3.2.2).

3.2.3.1. Monocomponent adsorption. Monocomponent isotherm modelsofLangmuir,Freundlich,Sips,andRedlich–Peterson(R–P) havebeenusedtodescribetheequilibriumfeaturesofadsorption (Srivastavaetal.,2006).FortheoreticalapproachesoftheSipsand Redlich-PetersonisothermmodelsseetheSupplementary mate-rial.

The Freundlich isotherm model (Freundlich, 1906) assumes adsorptiononaheterogeneoussurfacewithanon–uniform dis-tributionofadsorptionheatovertheadsorbentsurface.Whereas, theLangmuirisothermmodel(Langmuir,1918)assumes adsorp-tiononahomogeneoussurfacewiththeadsorptionsiteshaving identicalenergyorequalafﬁnitytotheadsorbate(Srivastavaetal., 2006).

TheFreundlichisothermisgivenbyEq.(11): qe=KFCe

1

_⁄

_n

(11) whereKFistheFreundlichconstant[(mmol/g)(L/mmol)1/n]andn

istheheterogeneityfactor(Srivastavaetal.,2006). TheLangmuirisothermisgivenbyEq.(12): qe=QmaxbCe

1+bCe

(12) whereb(L/mmol)istheLangmuirconstantrelatedtothe adsorp-tionenergyorafﬁnityoftheadsorbatefortheadsorptionsiteand Qmaxisthemaximumadsorptioncapacity(mmol/g).

Thechangeinadsorptionfreeenergyofthesystem(adsG◦)can

becalculatedusingEq.(13):

adsG◦=−RTlnKa (13)

whereKaisthethermodynamicequilibriumconstant

(dimension-less), T (K) is the absolutetemperature, and R is the idealgas constant(8.314J/Kmol).

AccordingtoLiu(2009),thethermodynamicequilibrium con-stant can be calculated from Langmuir constant, b, using the approachshowninEq.(14):

Ka=

_b e (1mol/L)

(14) whereeistheactivitycoefﬁcientatequilibrium(dimensionless)

(10)

Iftheadsorbateisametalion,theactivitycoefﬁcientisstrongly affectedand promptlydecreasesastheionicstrengthincreases. Therefore,itiscrucialtocorrecttheactivitycoefﬁcientforeach adsorptionsystemevaluated usingtheextended Debye–Hückel law[Eq.(15)]toprovideacorrectcalculationofadsG◦(Liu,2009).

loge= −0.509z 2

_I

e

1+˛

Ie

_⁄

₃₀₅

(15) where z is the charge of the metal ion, Ie (mol/L) is the ionic

strengthat equilibrium, and␣ (pm)is theionsize (considered tobe600pmforCo2+_,_Cu2+_,_and _Ni2+₎₍_Harris,₂₀₁₀_)._The_ionic

strengthwascalculatedusingtheﬁrstequilibriumconcentration datapointoftheplateauofthemono–andbicomponent adsorp-tionisotherms.Monocomponentexperimentaldataweremodeled throughnonlinearregressionanalysisoftheLangmuir,Freundlich, Sips,and R–PisothermmodelsusingMicrocalOriginPro® ₂₀₁₅

software.Themonocomponentisothermmodelparameterswere foundbyminimizingthereduced2 _value,_as_shown_in_Section

3.2.2.Theequilibriummonocomponentadsorptionisothermsfor theremovalofCo2+_,_Cu2+_,_and_Ni2+_using_the_SPA_adsorbent_at₂₅◦_C,

130rpm,and0.2g/LSPAarepresentedinFig.6a–c. Monocompo-nentadsorptionwasevaluatedatpH5.5forCu2+_and_5.75_for_Co2+

andNi2+_._Table₂_shows_the_results_of_modeling_the

experimen-taladsorptiondatathroughnonlinearregressionanalysisforthe twomonocomponentisothermmodels.Theresultsofmodelingthe experimentaladsorptiondatawiththeSipsandRedlich-Peterson modelsandexperimentalisothermdataareavailableinTablesS2 andS4(seetheSupplementarymaterial).AsshowninTable2, con-sideringthevaluesofQmax,exp,Qmax,est,R2,and2red,aswellas

theshapeoftheisothermcurvesshowninFig.6a–c,the adsorp-tionofCo2+_,_Cu2+_,_and_Ni2+_was_best_described_by_the_Langmuir

model.However,consideringthevaluesofR2_and2

red,the

adsorp-tionof Cu2+_and _Ni2+ _seems_to_be_better_described_by _the_Sips

model.Conversely,theSipsmodelexhibitedahigherR2_value_and

smaller2

redvaluefortheadsorptionofCu2+andNi2+;thevalue

ofQmax,estincomparisonwithQmax,expwasoverestimatedbythis

model.Furthermore,alsobyanalyzingthevaluesofR2_and2 red,

theadsorptionofCu2+_and_Ni2+_seemed_again_to_be_better_described

bytheFreundlichandR–PmodelsincomparisontotheLangmuir model.However,theFreundlichandR–Pmodelsdonotseemto representtheplateauoftheisothermcurvesfortheadsorptionof Cu2+_and_Ni2+_very_well,_exhibiting_a_tendency_to_overestimate_the

maximumadsorptioncapacityoftheSPAadsorbent.Inaddition, theR–Pexponents(␤)fortheadsorptionofCu2+_and_Ni2+_were

closertounitythanzero,thuscloselyapproximatingtheLangmuir model.ThenparameterofSipsalsosuggestedthattheadsorption systemsinvolvingadsorptionofCu2+_and_Ni2+_presented_a_certain

degreeofheterogeneity.

Infact,theSPAadsorbenthasaheterogeneoussurface,where theadsorptionsitesaremostlycomposedofcarboxylicacidgroups fromthephthalatemoiety.Theotherfunctionalgroupspresenton theSPAsurface,whichcanpresentacapacityforadsorbingmetal ions,arethephenolicandcarboxylicacidgroupsinlignin. There-fore,thesurface oftheSPAadsorbent is not onlycomposedof adsorptionsitesthathaveidenticalafﬁnitytotheadsorbates.Thus, theinterpretationofsuchadsorptionsystemisnotsosimple,and theadsorptionofCo2+_,_Cu2+_,_and_Ni2+_cannot_be_seen_as_fully

Lang-muirian.Inaddition,theanalysisoftheScatchardplot(Fig.6a-c)for adsorptionofCo2+_,_Cu2+_,_and_Ni2+_on_the_SPA_adsorbent_reveals

pro-filesexhibitingmultilinearity,whicharerelatedtothepresenceof differentbindingsitespresentingdifferentaffinitiestowardmetal ions.Thesebindingsitesaredividedintotype-1(high-affinity,H) andtype-2(low-affinity,L)(AnirudhanandSuchithra,2010).The bindingconstants(b)andestimatedmaximumadsorption capac-ities(Qmax,est)of HandL bindingsiteswerecalculatedand are

given in TableS2.Asseen in TableS2,binding sitesexhibiting high-afﬁnitywererelatedtolow-capacitybindingsites,whereas thoseexhibitinglow-afﬁnitywererelatedtohigh-capacity. Accord-ingtoAnirudhanandSuchithra(2010),carboxylgroupsexhibiting lowpKavaluesanddifferentconformationsformetalbindingmay

exhibitdifferentafﬁnitiestowardmetalions,therebyconﬁrming theheterogeneityoftheadsorptionsitesontheSPAadsorbent. However,although theLangmuir modelfailed tofullydescribe the adsorptionsystem toa certaindegree, and its useto esti-mate theadsorptionconstantsfor comparisonpurposes isvery desirable.

ThevalueoftheSipsparameter,n,fortheadsorptionofCu2+

andNi2+_is_greater_than_unity,_which_differs_from_the_parameter

nfortheadsorptionofCo2+_._As_the_Sips_isotherm_constant,_n,_can

beusedasanestimationoftheheterogeneityoftheadsorption system,itsuggeststhattheadsorptionofCu2+_and_Ni2+_is_more

heterogeneousthanCo2+_._As_the_Sips_isotherm_is_a_combination_of

LangmuirandFreundlichisotherms,thevaluesofnforthe adsorp-tionofCo2+_,_Cu2+_,_and_Ni2+_suggest_that_the_adsorption_system_is

moreLangmuirianthanFreundlichian.Accordingtoexperimental equilibriumdataandtheLangmuirmodel,themaximummetal–ion adsorptioncapacityoftheSPAadsorbent adoptedthefollowing orderCu2+_>_Ni2+_»_Co2+_._As_the_{monocomponent}_adsorption

sys-temswerebestdescribedbytheLangmuirmodel,themodiﬁed Langmuirmodelswereusedtoevaluateandinterpretthe bicom-ponentadsorptiondata,whichareshowninthenextSection.

As shown in Table 2, thecoordination numbers (CNs) were higher than unity. Thisindicates that more than one carboxy-lategroupisbeingusedtoadsorbametalion.Thevaluesofthe CNs exhibited the following order Co2+_»_Ni2+_>_Cu2+_._The

differ-encebetweenphysisorptionandchemisorptionisapparentinthe magnitudeofthechangesinenthalpyandfreeenergyof adsorp-tion.Generally, themagnitudeofthechangein free energyfor physisorption is smaller than that of chemisorption (Yu et al., 2001). The change in free energy of adsorption for physisorp-tion rangesfrom −20to0kJ/mol and chemisorption from −80 to−400kJ/mol(Húmpolaetal.,2013;Yu etal.,2001).The val-uesofadsG◦ for theadsorptionofCo2+,Cu2+,and Ni2+onthe

SPA adsorbent were in the range from −20.2 to −22.3kJ/mol, whichareslightly higherthanthoseexpectedforphysisorption andmuchlowerthanthoseexpectedforchemisorption.This indi-catesthatamixedmechanismmaybecontrollingtheremovalof Co2+_,_Cu2+_,_and_Ni2+_from_aqueous_solution_by_the_SPA_adsorbent.

Furtherevidenceoftheadsorptionmechanismwasprovidedby FTIRspectrumofSPAincomparisonwithspectraofSPAloaded withCo2+_, _Cu2+_,_and _Ni2+_,_as _earlier _discussed_in _Section _3.1.2_.

BasedonthechangesinfreeenergyofadsorptionandFTIR spec-tra,it issuggestedthattheadsorptionmechanismis controlled by two steps: (1) ion–exchange, in which hydronium ions are exchangedwithmetalionsand(2)complexation,wheremetalions arecomplexedbycarboxylategroupsthroughthesharingof non-bondingelectronpairsofnegativelychargedoxygenatomswith metalions.However,asdiscussedinSection3.3,thedesorption dataofSPAloadedwithCo2+_,_Cu2+_,_and_Ni2+_indicate_that_fairly

weakbondsareformedbetweenthecarboxylategroupsandthe metalionsstudied.Thus, theseresultsconﬁrmthatthe adsorp-tionmechanisminvolvesacontributionofbothphysisorptionand chemisorption,asion–exchangeisacharacteristicphenomenonof physisorptionandcomplexationisacharacteristicphenomenon ofchemisorption(ÜnlüandErsoz,2006).Theenergyrequiredin theformer liesin themagnitude ofthephysisorption (Ferreira etal.,2015),whereastheenergyinvolvedinthelatteris depen-dent on the magnitude of chemisorption. Other evidence lies inthefact thattheactivationenergyrequired fordesorptionis largeandtheremovalofthechemisorbedspeciesfromthe sur-facemayonlybepossibleundersuitableconditions,forexample,

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 Co2+ Langmuir Freundlich Sips Redlich-Peterson qe ) g/l o m m( C_e (mmol/L)

(a)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Scatchard plot (Co2+₎

Type-1; H Type-2; L qe /C e (L/ g) qe (mmol/g) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Cu2+ Langmuir Freundlich Sips Redlich-Peterson qe ) g/l o m m( C_e (mmol/L)

(b)

0.0 0.10.2 0.3 0.40.5 0.6 0.70.8 0.9 1.0 0 1 2 3 4 5 6 7 8 9 10 11

Scatchard plot (Cu2+₎

Type-1; H Type-2; L qe /C e (L/ g) qe (mmol/g) 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Ni2+ Langmuir Freundlich Sips Redlich-Peterson qe ) g/l o m m( C_e (mmol/L)

(c)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Scatchard plot (Ni2+₎

Type-1; H Type-2; L qe /C e (L/ g) qe (mmol/g) 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 Co2+ Cu2+ EL MEL JS MEL (Co2+ ) JS MEL (Cu2+₎ P-Factor EL MEL JS MEL (Cu2+₎ JS MEL (Co2+₎ P-Factor qe ) g/l o m m( C_e (mmol/L)

(d)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 Co2+ Ni2+ EL MEL JS MEL (Co2+ ) JS MEL (Ni2+ ) P-Factor EL MEL JS MEL (Ni2+₎ JS MEL (Co2+ ) P-Factor qe ) g/l o m m( C_e (mmol/L)

(e)

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 Cu2+ Ni2+ EL MEL JS MEL (Cu2+ ) JS MEL (Ni2+ ) P-Factor EL MEL JS MEL (Ni2+₎ JS MEL (Cu2+ ) P-Factor qe ) g/l o m m( C_e (mmol/L)

(f)

Fig.6.Equilibriumadsorptionisothermsforthemonocomponentadsorptionof(a)Co2+_,_(b)_Cu2+_,_and_(c)_Ni2+_on_the_SPA_at₂₅◦_C,_pH_5.75_for_Co2+_and_Ni2+_and_5.5_for_Cu2+_,

180min,130rpm,0.2g/LSPAandbicomponentadsorptionof(d)Co2+_–Cu2+_,_(e)_Co2+_–Ni2+_,_and_(f)_Cu2+_–Ni2+_at_pH_5.5_and₁₈₀_min_(EL₌_Extended_Langmuir;_MEL₌_Modiﬁed

ExtendedLangmuir;JSMEL=JainandSnoeyinkModiﬁedExtendedLangmuir). through chemical treatment of the surface, which was per-formedinthis studybyusing 1.0mol/LHNO3 asthedesorbing

solution.

3.2.3.2. Bicomponent adsorption. Three bicomponent adsorption systemsCo2+_–Cu2+_,_Co2+_–Ni2+_,_and _Cu2+_–Ni2+ _were_studied_and

modeledusingfourmodels:(1)thenonmodiﬁedcompetitive Lang-muirmodel,(2) theJainand Snoeyink(J–S)modiﬁed extended

(12)

Langmuir model, (3) P–Factor, and (4) the modiﬁed extended Langmuirmodelwithaninteractionfactor.Thebinarymetal–ion systems were equimolar (Ci=Cj) with concentrations varying

from0.05 to1.12mmol/L.For theoreticalapproach of the non-modiﬁed competitive Langmuir and Jain and Snoeyink (J–S) modiﬁed extended Langmuir models see the Supplementary material.

TheP–FactorisamodeldevelopedbyMcKayandAlDuri(1987). ItassumesthesameconceptsastheLangmuirisothermmodeland isdescribedbyEq.(16): qe,i,multi= 1 Pi

KL,iCe,i,multi 1₊aL,iCe,i,multi

(16) wherePiwastermedbyMcKayandAlDuri(1987)asa“lumped”

capacityfactor,whichcanbedeﬁnedasshowninEq.(17): Pi=

KL,i/aL,i

mono

KL,i/aL,i

multi =Qmax,mono Qmax,multi (17) where(KL,i/aL,i)monoand(KL,i/aL,i)multiarethemaximumadsorption

capacitiesforcomponentiinamonocomponentand multicompo-nentadsorptionsystem,respectively.

ThemodiﬁedextendedcompetitiveLangmuirmodel(Mathews andWeber,1980)isalsoanextensionoftheLangmuirisotherm modelforacomponentiinanadsorptionsystemwithN compo-nents.However,thismodelincorporatesaninteractionfactor,,as showninEq.(18): qe,i= Qmax,ibi

Ce,i/i

1+ N

j=1 bj

Ce,j/j

(18)

where i and j are the interaction factors that can be

esti-matedfromthenonlinear regression ofcompetitiveadsorption datausingQmax,i,Qmax,j,bi,andbjfromthemonocomponent

Lang-muirisotherm.

Themulticomponentadsorptionisothermmodelswere eval-uated using themonocomponent parameters determined from thenonlinearregressionanalysisofthemonocomponent equilib-riumadsorptiondata(Table S4)usingtheLangmuirmodel[Eq. (12)], as shown in Tables 2 and S2.Microsoft Excel® ₃₆₅

soft-wareforWindows® _was_used_to_determine_the_values_of_R2 _and

2

redforthenonmodiﬁedcompetitiveLangmuirandJ–Smodiﬁed

extendedLangmuirmodelsusingthemulticomponentequilibrium adsorptiondata(TablesS2andS5)foreachmetalion.The inter-actionfactor()ofthemodiﬁed extendedLangmuirmodelwas determinedbynonlinearregressionanalysisofthe multicompo-nentequilibriumadsorptiondata(TableS5)byminimizing2

red

withMicrocalOriginPro®₂₀₁₅_software_and_using_the_procedures

describedinSection3.2.2.ThevalueofQmax,multiusedtodetermine

theparameterPiwasobtainedfromthenonlinearregression

anal-ysisofthemulticomponentequilibriumadsorptiondata,usingthe Langmuirmodel[Eq.(12)]andMicrocalOriginPro®₂₀₁₅_software,

aspreviouslymentioned.Theresultsofmodelingthe multicom-ponentequilibriumadsorptiondatawithmulticomponentmodels arepresentedinTables2andS2.

Bicomponentadsorption studies were performed at pH 5.5, 130rpm,25◦C,0.2g/LSPA,andwithanequimolarconcentrationof metalionsinbinarymixtures.Fig.6d–fpresentsthegraphsforthe binaryadsorptionsystemsCo2+_–Cu2+_,_Co2+_–Ni2+_,_and_Cu2+_–Ni2+_,

respectively. The equilibrium adsorption data for bicomponent metal–ionsystemscanalsobeanalyzedintermsofQmaxforametal

ion,i,inthepresenceofanothermetalion,j,orwhenmetalioniis presentaloneintheaqueoussolution(Qmax,exp,i,multi/Qmax,exp,mono).

When Qmax,exp,i,multi/Qmax,exp,mono>1, the adsorption of metal

ion i is promoted by the presence of the metal ion j. When Qmax,exp,i,multi/Qmax,exp,mono=1,thereisnoobservablenet

interac-tionbetweenmetalionsiandj.WhenQmax,exp,i,multi/Qmax,exp,mono

<1,theadsorptionofmetalioniissuppressedbythepresenceof metalionj(Rondaetal.,2013).AsshowninTable2,the adsorp-tionofallmetalionswassuppressedbythepresenceofanother metalion.However,inthebinarysysteminvolvingCo2+_and_Cu2+_,

therewasgreatertheadsorptionofCu2+_compared_to_the

adsorp-tionofCo2+_,_whereas_in_the_binary_system_involving_Co2+_and_Ni2+_,

theadsorptionofCo2+ _was_less _than_Ni2+_._In_the_binary_system

composedofCu2+_and _Ni2+_,_the_adsorption_of_Cu2+ _was_greater

thantheadsorptionofNi2+_._The_Pearson_parameter_(ı)_for_Co2+_,

Ni2+_, _and _Cu2+ _equals _0.130, _0.126,_and _0.104₍_Pearson, ₁₉₇₃_),

respectively.Thus,thehardnessofthemetalionsfollowstheorder Co2+_>_Ni2+_>_Cu2+_._The_carboxylate_group_of_the_phthalate_moiety

graftedontheSPAadsorbentbehavesasaveryweaklybasic, neg-atively charged,oxygen–donor group that can bestabilized by resonance.TheCo2+_,_Ni2+_,_and _Cu2+_ions_are_considered

border-lineLewisacids,whereasacarboxylateligand,suchasanacetate ion,isconsideredasahardLewisbase.Intermsofthehardandsoft acidbase(HSAB)concept,thecarboxylateligandsshouldexhibita preferenceforhardermetalions.However,accordingtotheIrving andWilliams(1953)series,whichdiscussestherelativestabilities ofcomplexesformedbetweentransition–metalionsandligands, carboxylateligandsformmorestablecomplexeswithmetalions inthefollowingorderCu2+_>_Ni2+_>_Co2+_._The_obtained_results_are

ingood agreementwiththeIrvingandWilliams series,andfor themonocomponentandmulticomponentadsorptionsystemsthe maximumadsorptioncapacitiesfollowtheorderCu2+_>_Ni2+_>_Co2+_.

Theadsorptiondata,asafunctionofsolutionpH,contacttime (kinetics),andinitialmetal–ionconcentration,indicatethatthereis thepossibilitytopartiallyseparateCu2+_from_Co2+_and_Ni2+_by

uti-lizingtheeffectofsolutionpHandNi2+_from_Co2+_through_kinetic

(k2,Co2+/k2,Ni2+=2.22) and thermodynamic control (Qmax,est,multi;

seeTable2).Themono–andbicomponentadsorptiondataobtained inthisstudycouldbeusedtodesignaﬁxed–bedcolumninorder tostudytheremovalofCo2+_,_Cu2+_,_and_Ni2+_from_mono–_and

mul-ticomponentaqueoussolutionsandtoevaluatethepossibilityof separatingthesemetalions.

TheadsG◦valuesforthemulticomponentsystemswere

sim-ilartothoseforthemonocomponentsystems.Thisindicatesthat multicomponentadsorptionwasalsofavorableandspontaneous. AsshowninTable2bycomparingthethreebicomponent adsorp-tionsystems,higherR2_and_lower_␹2

redvalueswereobtainedby

modelingtheequilibriumadsorptiondatawithmodifiedextended LangmuirandP–Factormodels.NonmodifiedextendedLangmuir and J–S modified extended Langmuir models showed a worse fit to the experimental data (see Table S2 in the Supplemen-tarymaterial).These resultsconfirm thatthere areinteractions betweenthemetalionsinthebicomponentsystemsstudied.Figure S3 presentsthe graphsof qe,est against qe,exp for the

bicompo-nentadsorptionsystemsstudied.TableS6presentstheslopesand interceptsoftheplotsofqe,estagainstqe,expforthebicomponent

systems studied. As shown in Fig. S3a–f, most of the equilib-riumadsorptiondatapointsaredistributedaroundthe45◦dashed line, witha 95%confidence interval,for themodified extended LangmuirandP–Factormodels.Theslopesandinterceptsforthe bicomponent systemscomposedof i–j and j–i metalions were closertounityandzero,respectively,forthemodifiedextended LangmuirandP–Factormodelsincomparisonwithnonmodified extendedLangmuirandJ–SmodifiedextendedLangmuirmodels. ThisindicatesthatthemodifiedextendedLangmuirandP–Factor modelscanpredictthevaluesofqe,estfromexperimentaldatavery

well.

Themaximummetal–ionadsorptioncapacityinthe bicompo-nentsystems(Qmax,exp,i+j)(Table2)werelowerthanthesumof

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Table3

ComparisonofQmaxfortheremovalofCo2+,Cu2+,andNi2+byvariousadsorbentsreportedintheliterature.

Adsorbent abbreviation

Typeofsolid support

Ligandtype pH Equilibrium time(min)

Qmax(mmol/g)a Reference

Co2+ _Cu2+ _Ni2+

SPA Sugarcanebagasse Carboxylicacid 5.5–5.75 180 0.561 0.935 0.932 Thisstudy

STA Sugarcanebagasse Carboxylicacid 5.5–5.75 180,250,75 1.140 1.197 1.563 Ramosetal.(2015)

CSIS Chitosan Amine/amide/azomethine 5.0 180 0.908 1.623 0.684 Monieretal.(2010)

PET–g–(MAA/AAm) Poly(ethylene terephthalate) (PET)

Carboxylicacid/amide 5.0 120 0.461 0.492 0.741 Cos¸kunetal.

(2006)

Sp–HPBA Sporopollenin(Sp)

ofLycopodium clavatum

Phenolic/imine 5-6 90 0.039 0.043 0.036 C¸imenetal.(2013)

Palygorskite (Brazil)

Palygorskite Silanol – 1440 1.27 1.57 1.72 Oliveiraetal.

(2013)

BANPA Banana

pseudostem

Carboxylicacid ∼6 120,120,60 0.56 0.49 0.55 Rodriguesetal.

(2013)

BANMA Banana

pseudostem

Carboxylicacid ∼6 180,120,120 0.27 0.29 0.47 Rodriguesetal.

(2013)

PAMMAm Syntheticpolymer

matrix

Nitrogenfrom aniline/alkylamide

5.5–6.0 30 0.122 0.201 0.104 Kushwahaetal.

(2013)

Sil–NSuc Silicagel Amine/carboxylicacid 5.5 – 1.85 1.04 1.89 Arakakietal.

(2013)

Celambiopolymer Cellulose Aliphaticamine/nitrogen

ofpyridine

∼6 – 0.097 0.102 0.077 SilvaFilhoetal.

(2013) a_Maximum_adsorption_capacities_obtained_from_the_Langmuir_model_at₂₅◦_C,_except_for_Palygorskite_adsorbent₍₃₀◦_C).

thesinglemaximumadsorptioncapacitiesforeachmetalion. Sim-ilarresultswerereportedbySrivastavaetal.(2006),whostudied theadsorptionofCd2+_and_Zn2+_on_bagasse_ﬂy_ash_in_mono–_and

bicomponentsystems.AccordingtoSrivastavaetal.(2006),the lowermaximumadsorptioncapacitiesobservedforbicomponent systemsincomparisonwithmonocomponentsystemsarearesult ofthepresenceofavarietyofadsorptionsitesontheadsorbent surface,whichexhibitspartiallyspecificaffinitytothesinglemetal ions.Theseobservationsseemtoinfringeupontheassumptions of the Langmuirtheory, that is, theadsorbent surface is com-posedof homogeneousadsorption sites,so there shouldbeno preferentialadsorptionorlateralinteractionsbetweenadsorbates, andtheadsorptionsitesexhibitequalaffinitiestotheadsorbate. AlthoughtheLangmuirtheoryfailstofullydescribethe adsorp-tionbehavior ofbicomponent systems,theuseofthe modified extendedLangmuirmodelseemedreasonable,astheinteraction factorincorporatedthedeviationsfromthenonmodifiedextended Langmuirmodelanditsassumptions.TheP–Factormodeldoesnot accountforcompetitionorinteractionsbetweenthemetalionsthat influenceadsorption(Choyetal.,2000).Thesecompetitionsand interactionsresultindeviationsintheestimationoftheequilibrium adsorptioncapacity(qe).However,theP–Factormodelprovideda

goodestimationofqe,estincomparisonwithqe,exp(TableS6).This

goodestimationcouldbeobtainedbecauseweusedthe“lumped” factor,Pi,whichprovidesamajorenhancementovertheextended

Langmuirmodel,asreportedbyChoyetal.(2000).Asshownin TableS6,theestimationoftheqevaluesbytheP–Factormodelwas

betterthanthosepredictedbythemodiﬁedextendedLangmuir model,whichagreeswiththeconclusionsreportedbyChoyetal. (2000).

3.3. DesorptionandreuseofthespentSPAadsorbent

DesorptionstudiesofSPAloadedwithCo2+_,_Cu2+_,_and_Ni2+_were

carriedoutinbatchmodeat25◦Cand130rpmwith20.0mLof 1.0mol/LHNO3,20.0mgofspentadsorbent,andacontacttimeof

5min.Thedesorptionefﬁciency(DE)was100%fortheSPA adsor-bentsloadedwithCo2+_,_Cu2+_,_and_Ni2+_.

ThepHofthe1.0mol/LHNO3 solutionwaszero.At thispH

value,there is a highconcentration ofhydronium ions (H3O+),

whichcanbeexchangedwithmetalionsthroughtheprotonation ofcarboxylategroupsinthephthalatemoiety.Therefore,the sug-gestedmechanismforthedesorptionprocessofmetalionsfrom SPAloadedwithCo2+_,_Cu2+_,_and_Ni2+_is_that_of_{ion–exchange,}_as

showninFig.1c.

There–adsorptionstudieswerecarriedoutusingtheSPA adsor-bents that were treated with1.0mol/L HNO3 for 5min in the

desorption studies described above. The re–adsorption studies werecarriedoutusingthebestadsorptionconditionsthatwere determinedfromearlieradsorptionstudiesasafunctionof solu-tionpH(pH5.5forCu2+_and_5.75_for_Co2+_and_Ni2+₎_and_contact

time (180min for Co2+_, _Cu2+_, _and _Ni2+₎ _with _100.0_mL _of _0.79

mmol/Lmetal–ionsolutionand20.0mgofthespentSPA adsor-bent. Re–adsorption studies demonstrate that the efﬁciency of re–adsorption(RE)was100%forallofthemetalionsstudied.These resultsindicatethattheSPAadsorbentcanberecoveredandreused withoutlossoftheadsorptioncapacity,andthatthemetalionscan alsoberecovered.ThepossibilityofrecoveringboththeSPA adsor-bentandthemetalionswouldcertainly improvetheeconomic feasibilityofanadsorptionprocessusingtheSPAadsorbent. 3.4. Comparisonswithpreviouslypublisheddata

Forcomparisonpurposes,Table3presentsseveraladsorbent materialsthathavepreviouslybeenreportedtoremoveCo2+_,_Cu2+_,

andNi2+_from_aqueous_solutions_in_batch_mode._Comparing_the_SPA

adsorbentwithsomepreviouslyreportedadsorbentsusedforthe adsorptionofCo2+_,_Cu2+_,_and_Ni2+_,_it_is_reasonable_to_suggest_that

theSPAadsorbentisoneofthemostefﬁcientadsorbentsrecently synthesizedforthispurpose.

4. Conclusions

The chemical modiﬁcation of sugarcane bagasse (SB) with phthalicanhydride(PA)wasstudiedasafunctionofPAamountand reactiontime.TakingintoaccountthepreparationcostsofSB mod-iﬁedwithPA(SPA),areactiontimeof6handaPA–to–SBratioof6:1 werechosen.ThisconditionyieldedSPAwithapercentweightgain of77.08%andanamountofcarboxylicacidgroupsof4.76mmol/g. TheSPAadsorbentwaseffectiveinremovingCo2+_,_Cu2+_,_and_Ni2+

Removal of cobalt (II), copper (II), and nickel (II) ions from aqueoussolutions using phthalate - functionalized sugarcane bagasse : mono - and multicomponent adsorption in batch mode.

Industrial

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