Enhancement of p-nitrophenol adsorption capacity throughN2-thermal-based treatment of activated carbons

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ContentslistsavailableatScienceDirect

Applied

Surface

Science

j o u r n a l ho me p ag e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Enhancement

of

p-nitrophenol

adsorption

capacity

through

N

2

-thermal-based

treatment

of

activated

carbons

S. Álvarez-Torrellas

a,b,∗

_,

_M.

_{Martin-Martinez}

b

_,

_H.T.

_Gomes

b

_,

_G.

_Ovejero

a

_,

_J.

_García

a,b

a_Grupo_de_Catálisis_y_Procesos_de_Separación_(CyPS),_Departamento_de_Ingeniería_Química,_Facultad_de_Ciencias_Químicas,_Universidad_Complutense_de

Madrid,Avda.Complutenses/n,28040Madrid,Spain

b_Laboratory_of_Separation_and_Reaction_Engineering_–_Laboratory_of_Catalysis_and_Materials_(LSRE-LCM),_Department_of_Chemical_and_Biological

Technology,SchoolofTechnologyandManagement,PolytechnicInstituteofBraganc¸a,CampusdeSantaApolónia,5300-253Braganc¸a,Portugal

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received27December2016

Receivedinrevisedform27February2017 Accepted6April2017

Availableonline14April2017 Keywords: Activatedcarbon Adsorption Fixed-bedcolumn 4-Nitrophenol

a

b

s

t

r

a

c

t

Inthisworkseveralactivatedcarbonsshowingdifferenttexturalandchemicalpropertieswereobtained

bychemicalandphysicalactivationmethods,usingalignocellulosicmaterial(peachstones)asprecursor.

Theactivatedcarbonresultingfromthechemicalactivation,namelyasCAC,revealedthebesttextural

properties(SBET=1521m2g−1,porevolume=0.90cm3g−1)andanacidiccharacter.Itwasfoundthatthe

activatedcarbonobtainedat300◦C(underairatmosphere,PACair),andthosesynthesizedat750◦Cin

presenceofN2ﬂowwithbubblingofwater/12MH3PO4solution(PACN2(H2O)/PACN2(H3PO4)),

respec-tively,revealedworsetexturalproperties,comparedtoCAC.Twofunctionalizationtreatments,byusing

sulphuricacidatboilingtemperature(PACS)andnitricacid-urea-N2heatingat800◦C(PAC-NUT),were

appliedtoPACair,inordertoenhancetheadsorptionabilityofthecarbonmaterial.Severaltechniques

werecarriedouttocharacterizethephysicalandchemicalpropertiesoftheobtainedcarbonmaterials.

Themodiﬁcationtreatmentshadinﬂuenceon thecarbonsurface properties,sincethe nitric

acid-urea-N2heatingtreatmentledtoacarbonmaterialwithhighly-improvedproperties(SBET=679m2g−1,

pHIEP=5.3).

Accordingly,theoriginaland modiﬁed-carbonmaterialsweretestedasadsorbentstoremove

4-nitrophenol(4-NP),assessingbatchandﬁxed-bedcolumnadsorptiontests.PAC-NUTcarbonofferedthe

bestadsorptionbehavior(qe=234mgg−1),showingahighabilityfortheremovalof4-NPfromwater.

1. Introduction

Nitro-aromaticcompoundsareaclassofcompoundswithone

ormorenitrosubstituents.Amongthem,4-nitrophenol(4-NP)isa

hazardouspollutantmainlyproducedduringchemicalprocesses,

suchaspetrochemicalmanufacturing,oilreﬁning,rubber,wood

preservationoperations, pulpand papermills aswellas inthe

productionofpesticides,paintsandplastics[1].Thepresenceof

phenolanditsderivativesintheaquaticenvironmenthasbecome

agreatconcerninrecentyearsduetotheirincreasingdischarge,

toxicnatureandpotentialadverseeffectsonthereceivingwater

bodies[1,2].

∗ Correspondingauthorat:GrupodeCatálisisyProcesosdeSeparación(CyPS), DepartamentodeIngenieríaQuímica,FacultaddeCienciasQuímicas,Universidad ComplutensedeMadrid,Avda.Complutenses/n,28040Madrid,Spain.

E-mailaddresses:satorrellas@ucm.es(S.Álvarez-Torrellas), mariamartin@ipb.pt(M.Martin-Martinez),htgomes@ipb.pt(H.T.Gomes), govejero@ucm.es(G.Ovejero),jgarciar@ucm.es(J.García).

Phenoliccompoundsareknowntohaveatoxiceffectonaquatic

life,plantsandevenhumanhealth.Ingestionofphenolata

concen-trationlevelof10–240mgL−1foralongperiodcausesmouthsores,

diarrhea,excretionofdarkurineandimpairedvision.Accordingly,

thelethalbloodconcentrationforphenolisaround4.7–130mgL−1.

Duetoitstoxicity,evenatlowconcentrations,phenolic

com-pounds have been classiﬁed as priority pollutants by the U.S.

EnvironmentalProtectionAgency(EPA).Pertherecommendation

oftheWorldHealthOrganization(WHO),thethreshold

concen-trationofphenolindrinkingwatershouldfallbelow1.0␮gL−1.

Therefore,theUSEPArecommendsamaximumlevelof1␮gmL−1

oftotalphenoliccompoundsinwatersupplies.

Due toits harmful effects, wastewaters containing phenolic

compoundsshouldbespeciﬁcallytreatedbeforebeingdischarged

tothereceivingwaterbodies.

Thetertiarytreatmentsfortheremovalofpersistentpollutants

from wastewater include advanced oxidation processes (AOPs)

withtheirvariations(ozone,ultravioletradiation,gamma

radia-tion),membranebioreactors,micro/nanoﬁltrationandadsorption

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S.Álvarez-Torrellasetal./AppliedSurfaceScience414(2017)424–434 425 [3,4].Adsorptionisaneffectiveseparationprocessduetoits

sim-pledesign,operationﬂexibility,suitabilityforbatchandcontinuous

mode,possibilityofregenerationandreuseofthesaturated

adsor-bents and low capital costs [5]. Theselection of the adsorbent

foreachspeciﬁcapplicationisacriticalaspectintheadsorption

processes.Zeolites,duetotheirrelativelyhighsurfaceareaand

tailored-porosity (micro, meso and macro pores) have

demon-stratedusefulnessasadsorbentmaterialsfortheremovaloforganic

pollutants [6]. Accordingly, metal oxides, such as MnO, have

beenwidelyusedforthedepletionofhazardousandundesirable

compoundsfromwastewater.Therefore, metaloxidesareoften

characterizedbysmallspeciﬁcsurfacearea,andconsequently,they

couldbemodiﬁedbyseveraltreatmentsinordertoobtain

bet-teradsorptionproperties.Forexample,MnxOy-SiO2mixedoxides

materialsshowbettertexturalpropertiesandadsorptionafﬁnity

thanmanganeseoxides,exhibitinghigheradsorptioncapacity[7].

In general,activatedcarbonsoffera highadsorption

perfor-manceforawiderangeoforganicandinorganicpollutantsdueto

theirhighsurfaceareaandporevolume[8].Low-cost,quite

abun-dance,renewability,andhighlignocellulosiccontentofagricultural

biomassmakethempromisingprecursorsforcost-effective

acti-vatedcarbons[9].Awidenumberoflignocellulosicresidues,such

asbamboo,wood,nuts,sawdust,cherrystones,ricehusk,peach

stones[10],coffee wastes,potato peels,almondshells[11] and

peanutshells[10,11],amongmanyothers,areofparticular

inter-estas by-productsfromfood processingindustries, in orderto

obtainactivatedcarbonswithgoodmechanicalstrengthanda

well-developedporousstructure.

Physicalandchemicalmethodshavebeenadoptedforthe

con-versionofthesebiomasswastestoactivatedcarbons[12].Physical

activationinvolvesthepyrolysisofthecarbonaceousprecursorat

hightemperaturesfollowedbyactivationofthecharinthe

pres-enceofanoxidizinggas(CO2orwatersteam)[13,14].Inchemical

activation,therawmaterialisﬁrstlyimpregnatedwithstrong

acti-vatingagentssuchasZnCl2,H2SO4,NaOH,K2CO3,H3PO4,KOH,

among others, followed by thermal activation at temperatures

rangingfrom400to600◦C[15].

Inthechemicalactivationmethod,thedehydratingeffectofthe

activatingagent,forexample,H3PO4solution,hindersthe

forma-tionoftar,leadingtohigheryieldandthepossibilitytouselow

activationtemperaturestoproducehighergradecarbons[16].Both

inphysicalandchemicalreactions,itisnecessarytheactivation

steptoenhancetheporevolume,porediameterandsurfacearea

oftheresultingmaterial[17].Ingeneral,longeractivationtimesare

favorableforthedevelopmentofhighermicroporosityandspeciﬁc

surfacearea.

Thetexturalandchemical propertiesoftheactivatedcarbon

mainlyconditionthespeciﬁcapplicationandtheresultsobtained.

Generally,forliquid-phaseadsorption,ahighsurfaceareais

desir-able and a mesoporous texture enhances the diffusion of the

pollutantwithintheporousstructureoftheadsorbent[18].

Inthepresentwork,activatedcarbonsfrompeachstoneshave

beenpreparedasfollows:(i)chemical activation:impregnation

with12MH3PO4(s)andfurtherthermaltreatment(400◦C,4h,air

ﬂow);theresultingmaterialwasnamedasCAC;(ii)physical

acti-vationby thefollowing methods:(1)activation inN2 presence

(800◦C,4h)followedbytreatmentinairﬂow(300◦C,1h);

car-bonnamelyasPACair;(2)activationunderN2atmosphere(600◦C,

1h)and furtherunder N2 saturatedin H2O(750◦C,6h);

mate-rialnamelyasPACN2(H2O);(3)activationunderinertatmosphere

(600◦C,1h)andsubsequentlyitwasannealedinN2saturatedwith

12MH3PO4(s)(750◦C,6h);theresultingmaterialwasnamedas

PACN2(H3PO4).Additionally,PACairwasfunctionalizedinorder

toenhancetheadsorptionabilityofthecarbonmaterial.The

car-bonwastreatedwithconcentratedH2SO4 solution(200◦C,3h),

resultinginPACSactivatedcarbon.Accordingly,thebasic

function-alizationtoobtainPAC-NUTcarbonconsistedofthetreatmentof

PACairwith5MHNO3(s)(83◦C,3h),furtheritwastreatedwith1M

ureasolutionatautogenouspressure(200◦C,2h)andtheresulting

materialwasannealedat800◦Cfor4h.

Thetextural,chemicalandmorphologicalpropertiesofallthe

testedcarbonmaterialswerestudied.Theywerefurthertestedas

adsorbentsregardingkineticandequilibrium4-NPadsorptiontests

bothinbatchandﬁxed-bedcolumnoperation.

2. Experimental

2.1. Reactants

Peachstoneswerecollectedfromalocalagriculturalcompany,

producingpeachcansinsyrup,beingthefruitstonesabiomass

wastedailyoriginated.

4-Nitrophenol(4-NP,O3NC6H5,98wt.%)and phosphoricacid

(85wt.%) were purchased from Sigma–Aldrich. Sulphuric acid

(98wt.%)wasobtainedfromPanreac.Nitricacid(69.5wt.%)and

acetonitrile (HPLC grade) were provided by Carlo Erba. Urea

(65wt.%)was purchased fromRiedel-de-Haën. Methanol (HPLC

grade) and glacial acetic acid (analytical reagent grade) were

obtainedfromFisherchemical.Allreagentswereusedasreceived.

Ultrapurewaterwasusedthroughouttheresearch.

2.2. Preparationoftheactivatedcarbons

Inthechemicalactivationmethod,ﬁrstly,animpregnationstep

wascarriedoutplacing30gofprecursor(0.5–1.0mm)with12M

H3PO4(s)inaround-bottomﬂaskreactorat85◦Cfor6h.Afterthat,

thesolidwasﬁlteredandcarbonizedinaverticalquartzreactor

under ﬂowing air (50cm3_min−1₎ _at₄₀₀◦_C _for ₄_h₍₅◦_C_min−1_).

Theresultingcarbonwasthoroughlywashedseveraltimeswith

ultrapurewateruntilapHclosetoneutralitywasreached.Then,

theactivatedcarbonwasdriedovernightanditwassievedatthe

requiredparticlesizerange(100–250␮m)[19].Theresulting

mate-rialwasnamelyasCAC.

Physicalactivation methodin ordertoobtainPACair

mate-rialwascarriedoutasfollows,adaptingthemethoddevelopedby

Ribeiroetal.[20]:30gofprecursorwasannealed(100cm3_min−1₎

inaverticalquartzreactorat120,400and600◦Cduring1hateach

temperature,andﬁnallyat800◦Cfor4h(2◦Cmin−1).Theresulting

materialwasfurtheractivatedunderairﬂow(100cm3_min−1₎_for

1hat300◦C.

Finally, PACN2(H2O) and PACN2(H3PO4) activated carbons

were obtainedas thefollowing method: 30g of precursor was

heatedunderN2ﬂow(100cm3min−1)fromroomtemperatureto

600◦Candmaintainedfor1h(5◦Cmin−1).Theactivationofthe

obtainedcharwascarriedoutat750◦C(15◦Cmin−1)underN2

sat-uratedwithwateror12MH3PO4(s),maintainingthetemperature

for6h[21].

2.3. Modiﬁcationoftheactivatedcarbon

PACairactivatedcarbonwasmodiﬁedbytwofunctionalization

treatmentsinordertostudythebehaviorofthemodiﬁedmaterials

on4-NPadsorption,followingtheproceduresreportedelsewhere

byGomesetal.andRochaetal.[22,23],respectively.

Firstly,asuspensioncontaining50gL−1ofPACairin

concen-trated H2SO4(s) (18M) was kept for 3h at 200◦C in a 250mL

round-bottomﬂask;theresultingsolidswerethoroughlywashed

withultrapurewateruntilneutralityandfurtherdriedinanoven

overnight.ThisactivatedcarbonwasnamelyasPACS.

In the second procedure, a suspension of PACair material

(50gL−1)wastreatedusing5MHNO3solutionfor3hatboiling

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oftherinsingwatersandfurtherdriedovernight.2gofthis

mate-rialwasputincontactto50mLofureasolution(1M)ina125mL

stainlesssteelautoclavebatchreactorunderautogenouspressure

at200◦Cfor2h.Finally,athermaltreatmentwasappliedtothe

recoveredsolids,underN2ﬂow(100cm3min−1)at120,400and

600◦Cduring1hateachtemperatureand800◦Cfor4h,resulting

intheﬁnalmaterial(PAC-NUT).

2.4. Characterizationoftheactivatedcarbons

2.4.1. Texturalcharacterization

The textural characterization of the activated carbons was

studied by N2 adsorption–desorption isotherms at 77K in a

Micromeritics ASAP 2020 equipment. The speciﬁc surface area

(SBET)wascalculatedbyusingBETequation[24]intherangeof

P/P0₌_0.05–0.15,_external_surface_area_(S

ext)wasestimatedbyt-plot

methodandDubinin–Radushkevichequationwasusedtocalculate

microporevolume(V0)[25].Mesoporevolumewasdetermined

bysubtractingthevalueofV0fromthetotalN2amountadsorbedat

P/P0₌_0.95_[26]_._Pore_size_{distributions}_(PSD)_of_the_tested_carbons

wereobtainedbydensityfunctionaltheory(DFT)method.

2.4.2. Surfacechemistrycharacterization

Chemicalsurface characterizationwas performedby Fourier

TransformedInfraredSpectroscopy(FTIR),usingaThermo

Nico-letFTIRspectrophotometer.Infraredspectrawererecordedin a

wavelengthrangerangingfrom400to4600cm−1.

Thepointofzerocharge(pHPZC)ofthesynthesizedactivated

carbonswasdeterminedfollowingapHtitrationprocedure.20mL

ofNaCl0.01Msolutionwaspouredintoseveral25mL-vessels.pH

withineach vesselwasadjustedtoavaluebetween2.5and 11

byadditionofHCl0.02MorNaOH0.02Msolutions.Then,0.05gof

samplewasaddedtothevesselsandtheywerelocatedinanorbital

shakerunderagitationat320rpm.TheﬁnalpHwasmeasuredafter

48h,usingacalibratedpHmeter(ModelGLP21;Crison

Instru-mentsSA;Barcelona,Spain)withanaccuracyof±0.01pH.ACrison

5015Telectrode,withtwoceramicdiaphragmswasused.pHPZC

valueisdeﬁnedasthepointwherethepHﬁnalcurvevs.pH

ini-tialcrossesthemaindiagonaloftheplot[27].Theisoelectricpoint

(pHIEP)ofthesampleswasdeterminedbyelectrophoretic

migra-tionmeasurementsusingaZetasizerNanoZSequipment(Malvern

Instruments). A suspension of 0.050g of sample (10–20␮m of

particlesize)in20mLofultrapurewaterwasusedforthe

mea-surements.pHsolutionwasmodiﬁedwitheither0.1MHClorNaOH

solutions.pHIEPvaluewascalculatedbythegraphical

representa-tionofzetapotentialvaluesversuspHofsolution.

Elementalcompositionoftheactivatedcarbons(C,H,NandS,

%)wasdeterminedbyelementalmicroanalysisusingaLECO

CHNS-932analyzer.Activatedcarbonsample(0.6–1.6mg)washeldina

furnaceat1000◦C,wherethecombustionofthesampleoccurred.

Finally,thermogravimetricanalyses(TG)oftheactivatedcarbons

wereperformed.SamplesweremeasuredinanEXTAR6000Seiko

thermal analyzer,operating withN2 ﬂow (100cm3min−1), and

heatingfrom35to900◦Cataheatingrateof10◦Cmin−1.

2.4.3. Morphologicalcharacterization

MorphologicalstudieswerecarriedoutwithaJEOLJSM6400

microscope,equipped witha thermoionic cathodeand a 25kV

detector.Previously,carbonsampleswerecoatedwithagoldlayer,

usingaBalzersSCD004apparatus,for180s.

2.5. Equilibriumadsorptionstudies

The adsorption rate and the equilibrium time are strongly

dependentontheadsorbent particlesize.The activatedcarbon

Fig.1.N2adsorption–desorptionisothermsat77Kofthetestedadsorbents.

samplesweregroundandsieved;onlythesizefractionsmaller

than100␮mwasusedintheadsorptionexperiments.

Inordertostudythekinetic4-NPadsorption,60mgof

adsor-bent was placed in contact to 25mL of contaminant solution

(C0=100mgL−1)inDeltalabplastic vessels inan orbitalshaker

(LabMate). Temperature and stirring ratewere constant in the

experiments(30◦C,250rpm).Attherequiredintervaltimes,the

vessels wereremoved, solutionwas ﬁlteredby using 0.45

␮m-nylonsyringeﬁltersand4-NPconcentrationwasanalyzedinaHigh

LiquidPressure chromatograph(Jasco),equipped witha UV/vis

detector (UV-2075Plus) and a quaternary gradient pump

(PU-2089 Plus) for solventdelivery. The analyses were carried out

usingaKromasil100-5-C18column(15cm×4.6mm;5␮m

par-ticlesize),workingatroomtemperature.Themobilephaseused

wasmethanol(3%aceticacid,1%acetonitrile):ultrapurewater(3%

aceticacid)(40:60,v/v)ataﬂowrateof1mLmin−1.

Intheequilibriumadsorptiontests,differentweightsof

adsor-bent(5–800mg)wereplacedincontactto25mLof4-NPsolution

(C0=100mgL−1)inaLabMateorbitalshaker(30◦C,250rpm).After

reachingthe equilibrium time, samples were ﬁlteredand

ana-lyzedasdetailedabove.Theanalyseswerecarriedoutinduplicate;

additionally,blanktestswereaccomplishedinordertoevaluate

possible contaminant removal by other mechanisms, obtaining

negativeresults.

2.6. Fixed-bedcolumnadsorptionexperiments

Adsorptiontestsin ﬁxed-bed columnwere performedusing

glasstubesadaptedwithametalliclayeratthebottomassupport

oftheadsorbentandre-ﬁlledwithglassballsinordertoavoiddead

volumeandpreferentialchannelsinthebed.0.8gofPACair

car-bon(particlesize:106–250␮m)wasusedintheexperiments.4-NP

solution(C0=10.0mgL−1)waspumpedusingaperistalticpump

ata selectedvolumetric ﬂow rate(Q=1.33and 2.50mLmin−1),

operatingindown-ﬂowmode.1.5mLefﬂuentsampleswere

peri-odicallycollectedandanalyzedbyusinghighperformanceliquid

chromatography(HPLC),asdescribedabove.

3. Resultsanddiscussion

3.1. Characterizationoftheactivatedcarbons

3.1.1. Texturalcharacterizationoftheactivatedcarbons

N2adsorption–desorptionisothermsat77Kandporesize

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S.Álvarez-Torrellasetal./AppliedSurfaceScience414(2017)424–434 427

Fig.2. Poresizedistributionsofthetestedadsorbents.

andFig.2,respectively.SBET,Sext,micropore(V0)andmesopore

(Vm)volumesweredetermined.

CACactivatedcarbon presented a highlydeveloped porosity

network of micropores in combination witha high percentage

ofmesopores,offeringmuchhigheramountofadsorbedN2than

thephysically-activated activatedcarbons,which canbe

classi-ﬁedasstrictlymicroporousmaterials.Thus,PACair,PACN2(H2O)

and PACN2(H3PO4) samples presented a strong microporous

nature, with lower SBET and Vm than CAC activated carbon

(SBET CAC/PACair=1521/412m2g−1; Vm=0.379/0.004cm3g−1)

(Table1).

CACN2adsorption–desorptionisothermcouldbeclassiﬁedas

Type I–IV, according to IUPAC classiﬁcation [28], since a large

mesoporous percentage is contributing onits porousstructure.

Thepresenceofahysteresisloop(TypeH4)athighP/P0 _values

(0.40–0.95)issuggestingtheexistenceofabundantmesoporosity,

associatedtocapillarycondensationoccurringinthemesopores.

Ontheotherhand,bothphysically-activatedcarbonsand

modiﬁed-materialsN2 adsorptionisothermscouldbeclassiﬁedasTypeI,

indicativeofstrictlymicroporousmaterials.

ThemesoporouscharacterofCACactivatedcarbonwas

con-ﬁrmed in the PSD diagrams (Fig. 2), showing a pore diameter

rangingfrom27to50 ˚A,whiletheothermicroporouscarbonshad

anaverageporediametersmallerthan15 ˚A.

Textural properties of PACS material were not modiﬁed in

a great extent, comparing to the original carbon (PACair).

Thus, an increasing in the speciﬁc surface area and pore

volume was observed in PAC-NUT carbon (SBET=679m2g−1,

VTotal=0.309cm3g−1)(Table1).Thisbehaviormaybeattributed

totheremovalofthemaincontainingoftheoxygenatedsurface

functionalities,increasingtheaccessiblecarbonsurfaceareaforN2

molecules[23].

3.1.2. Chemicalsurfacecharacterizationoftheactivatedcarbons

Thechemicalsurfacepropertiesofthetestedactivatedcarbons

werestudiedthroughelementalcomposition,FT-IR,pointofzero

charge(pHPZC)andisoelectricpoint(pHIEP)measurements.Both

pHPZCandpHIEPvaluesshowedthatCACactivatedcarbonpresents

highersurfaceaciditythanPACair,beingtheseresultsin

accor-dancetothoseobtainedintheelementalmicroanalysis(highO%

compositionandO/CratioofCACsample,thislatterindicativeof

thepolarityofthecarbonmaterials[29])(Table2).Accordingly,as

itcanbeexpected,resultsshowedthatPACN2(H3PO4)carbonis

offeringaslightlymoreacidiccharacterthanPACN2(H2O).Thus,

themodiﬁed-activatedcarbons(PACS,PAC-NUT)werethemost

acidicandbasiccarbons,respectively,obtainedinthisstudy(pHIEP

PACS/PAC-NUT=1.2/5.3).

Meanwhile,asithasbeenreportedbyLeónyLeónandRadovic

[30],thedifferenceobservedbetweenpHPZCandpHIEPvaluescould

beinterpretedasameasurementofthechargesdistributionatthe

carbonsurface.Positivevaluesof{pHPZC−pHIEP}areindicativeof

morenegativelychargedexternalthaninternalparticlesurfaces,

thisis,a heterogeneousdistributionofthesurfacecharges.This

wasobservedinallthetestedmaterials(Table2).

FT-IRspectraoftheactivatedcarbons(Fig.3aandb)revealed

theabsorptionbandscharacteristicof thecarbonaceous

materi-als.AbsorptionspectraofCACactivatedcarbon(Fig.3a)showthe

highestintensityintheabsorptionbands(phenolicandcarbonyl

groups),duetoitshighacidiccharacter,comparedtomorebasic

activatedcarbons(PACmaterials).Accordingly,FT-IRspectrumof

PACSshowedtheincorporationofahighamountofoxygenated

groupsonthecarbonaceoussurface.Lowintensityinthe

absorp-tionbandswasobservedinPAC-NUTFT-IRspectrum,attributedto

theremovalofahighquantityofoxygenated-functionalitiesduring

thethermaltreatment.

Theabsorptionbandat∼3400cm−1ischaracteristicoftheO H

stretchingvibration.Thus,thevibrationat1731cm−1(PACS)can

beattributedtotheC Ovibrationofcarbonylincarboxylicorester

groups.Thebandat1118cm−1couldbeassociatedtoC Obonding

andtheabsorptionpeaksat∼1340cm−1seemstoberesponsibleof

thelactonegroups,at_∼1645cm−1totheconjugated C Ogroups

andat∼1560cm−1and1645cm−1tothecarbonatefunctionalities.

ThemaindifferencesfoundinFT-IRspectraofPACairandPACS

aremainlyobservedat3400,1731,1118and1340cm−1

absorp-tionbands.Theseresultsareinaccordancetothedatareported

intheliteratureaboutcharacterizationofactivatedcarbonsfrom

lignocellulosicwastes[31–33].

Inthethermogravimetricanalysisofthetestedcarbon

materi-als,showninFig.4,couldbeobservedamasslossduetowater

steamrangingfrom1.0to7.5%attemperaturelowerthan100◦C.

Bothphysically-activatedcarbonsandPAC-NUTexhibitedhigh

thermalstability,showingonlyamasslossintherangefrom4to

18%at900◦C,indicatingacompletedegradationofthe

lignocel-lulosicmaterialsduringthecarbonizationandfurtheractivation

procedures.Meanwhile, CAC andPACS materialsshoweda low

thermal stability, withmass lossupto 37.5and 47.5%,

respec-tively.Thiscouldbeattributedtothepresenceofacidsolutions

onthecarbonssurface,whichdecomposeatlowertemperatures

thanthoseattheacidicfunctionalitiesincorporatedintotheinner

porousstructureoftheothercarbons.Theseresultsarein

agree-Table1

Texturalproperties(speciﬁcsurfacearea,externalsurfacearea,totalporevolume,microporevolumeandmesoporevolume)ofthetestedcarbonaceousmaterials.

Carbon SBET(m2_g−1₎ _Sext_(m2_g−1₎ _VTotal_(cm3_g−1₎ _V0_(cm3_g−1₎ _Vm_(cm3_g−1₎ _Vm/VTotal

CAC 1521 1172 0.902 0.522 0.379 0.579 PACair 412 60 0.186 0.182 0.004 0.978 PACN2(H2O) 403 58 0.182 0.178 0.0038 0.978 PACN2(H3PO4) 428 57 0.191 0.189 0.002 0.990 PACS 401 62 0.181 0.177 0.004 0.978 PAC-NUT 679 110 0.309 0.297 0.012 0.961

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Table2

Elementalmicroanalysis,pointofzerochargeandisoelectricpointofthetestedcarbonaceousmaterials.

Carbon N(%) C(%) H(%) O(%) S(%) O/Cratio pHPZC pHIEP {pHPZC−pHIEP}

CAC 0.33 68.70 3.91 27.04 0.02 0.394 4.3 1.9 2.4 PAC air 0.63 84.68 1.69 12.98 0.02 0.153 5.9 2.5 3.4 PACN2(H2O) 0.77 90.83 1.42 6.96 0.02 0.077 6.4 2.4 4.0 PACN2(H3PO4) 0.69 88.60 1.53 9.13 0.05 0.103 6.3 2.0 4.3 PACS 0.44 67.59 1.63 24.60 5.74 0.364 3.8 1.2 2.6 PAC-NUT 2.63 79.57 1.25 16.50 0.05 0.207 6.0 5.3 0.7

Fig.3. FTIRspectraofthetestedadsorbents.

Fig.4.TGanalysisofthetestedadsorbents.

menttothosereportedintheliteraturestudyingactivatedcarbons

obtainedfrombiomasswastes[34].

3.1.3. Morphologicalcharacterizationoftheactivatedcarbons

Themorphologicalpropertiesofthesynthesizedactivated

car-bonshave beenstudiedby SEMmicrographs(Fig.5),providing

complementaryinformationtothetexturalpropertiesofthe

mate-rials.

The more opened-porous structure of CAC activated carbon

(Fig.5a)canbeattributedtothewideporesizedistributionofthe

carbon,withahighquantityofporesinthemesoporousrange,in

accordancetothepreviousdiscussionabouttexturalproperties.On

theotherhand,asurfacedamage,maybeattributedtothesevere

treatmentconditions,couldbeobservedin SEMmicrographsof

PACSandPAC-NUT(Fig.5e–f,respectively).

3.2. Kineticadsorptiontests.Modelingoftheexperimentaldata

Kineticadsorption experimentswerecarried outin orderto

evaluatethekineticrateof4-NPadsorptionontothesynthesized

adsorbents.InFig.6isdepicted4-NPadsorptioncapacityversus

operationtime.

AdsorptionequilibriumontoCACactivatedcarbonwasattained

in8h,revealingthefastestkineticadsorptionamongthetested

systems.Thiscanbeattributedtoitsopened-texturalstructure,

highSBETandporesize,comparedtophysically-activatedcarbons.

Asithasbeenreportedintheliterature,4-NPisamoleculeshowing

highmobilityandsmallsize[35],obtaininghighadsorptionrates

ontohighsurfacearea-activatedcarbons.

Physically-activated carbons showed long equilibrium times

(48h)duetotheirstrongmicroporouscharacter,withhighly

nar-rowPSD andsmallporesize.ThetreatmentwithH2SO4 (PACS)

increasedtheacidicfunctionalitiescontentonthecarbonsurface,

mainlythiol,sulphoxideand/orsulphonegroups,which

decom-posereleasingSO2 species andprevailingovercarboxylicacids,

lactones,phenolandquinonegroups[36].Highhydrophilic

char-acterofPACScarbondisfavoredtheadsorptionof4-NP,suggesting

thatthecompetitionbetweenwaterandpollutantadsorbingin

pores is mainly responsible of the observed decreasing in the

adsorptioncapacity.Thisledtoaslowadsorptionrate(96h)anda

verylowafﬁnitytoward4-NPmolecule.

Generally,ithasbeenwidelyreportedintheliteraturethatbasic

carbonsshowhighafﬁnitytowardphenoliccompounds[37].Thus,

PAC-NUTshowedthebestadsorptionbehavior(q4-NP=35mgg−1at

equilibriumtimeof4h).Thetreatmentswithnitricacidand

subse-quentureasolutionoriginatedanincreasinginthenitrogencontent

andadecreaseoftheoxygenatedfunctionalitiesonthecarbon

sur-face,inferringthatsomecarboxylicacidgroupsweresuppressed

bytheincorporationofnitrogenfunctionalities(cyano oramine

groups).

Themodeling oftheexperimentalkineticdatawasassessed

byusingpseudo-ﬁrst-orderandpseudo-secondorderequations.

Pseudo-ﬁrstorderorLagergren’smodel[38]isgivenbyEq.(1).

(6)

S.Álvarez-Torrellasetal./AppliedSurfaceScience414(2017)424–434 429

Fig.5.SEMmicrographsofthetestedactivatedcarbons:(a)CAC;(b)PACair;(c)PACN2(H2O);(d)PACN2(H3PO4);(e)PACS;(f)PAC-NUT.

whereqeandq(mgg−1)aretheequilibriumadsorptioncapacity

andtheadsorptioncapacityattimet,respectively;k1 (min−1)is

thepseudo-ﬁrst-ordermodelrateconstant.

Pseudo-secondorderequation[39]isgivenbyEq.(2).

q= q2e·k2·t

1₊_qe·k2·t

(2)

where qe and q(mgg−1)are theequilibrium adsorption

capac-ity and the adsorption capacity at time t, respectively; k2

(gmg−1min−1)isthepseudo-second-orderrateconstant.

Thekineticparametersobtainedforbothmodels,andthe

cor-respondingcorrelationcoefﬁcients(R2_,_SSE,_Eq.₍₃₎_),_this_latter_as

thesumofthesquareoftheresidualsminimizedintheﬁttingof

thenon-linearizedequations,areshowninTable3.

SSE=

(qexp−qcal)2 (3)

whereqexpandqcalaretheexperimentalandcalculatedadsorption

capacityvalues,respectively.

Both pseudo-ﬁrst order and pseudo-second order models

(7)

Table3

Kineticadsorptiondatapredictedbypseudo-ﬁrstorderandpseudo-secondordermodelsforadsorptionof4-NPbycarbonaceousmaterials.

Carbon Pseudo-ﬁrstorderkinetic Pseudo-secondorderkinetic

qeexp(mgg−1₎ _qe_cal_(mg_g−1₎ _k1_(min−1₎ _R2 _SSE _qe_cal_(mg_g−1₎ _k2_(g_mg−1_min−1₎ _R2 _SSE

CAC 27.3 27.3 0.205 0.9475 91.3 27.2 0.0061 0.9996 32.8 PACair 38.7 38.7 0.050 0.9386 521.5 38.6 0.0005 0.9988 259.5 PACN2(H2O) 38.3 38.3 0.034 0.9197 634.9 38.2 0.00043 0.9990 340.4 PAC N2(H3PO4) 38.1 38.1 0.031 0.9108 662.1 37.9 0.00042 0.9988 358.3 PACS 35.6 35.6 0.001 0.9203 621.3 33.3 8.7×10−5 _0.9646 _408.5 PAC-NUT 38.4 38.4 0.059 0.9754 60.4 38.3 0.0012 0.9999 24.0

Fig.6.Experimentaladsorptionkineticcurvesof4-NPontothetestedadsorbents.

valueswereobtainedforthepseudo-secondordermodel,

espe-cially high for the adsorption onto PAC-NUT activated carbon.

The literature establishes that the suitabilityof pseudo-second

ordermodelsuggestingthattheadsorptionprocessisgoverned

bychemisorptionforces, thatis, bytheestablishmentofvalent

bondsbetweensurfaceoftheadsorbentandadsorbate,asitwill

bediscussedfurther.

Ingeneral,pseudo-ﬁrstandpseudo-secondordermodelsaccord

tophysicalandchemicaladsorptiondomination,respectively,but

thedivisionbetweenthemisnotsoclearandfurtherstudiesare

necessarytodiscussthisaspect.

Similarresultshave beenfoundbyTangetal. [40]studying

theadsorptionof4-NPontocarbonﬁbersandbyAhmaruzzaman

andLaxmiGayatri[41]onthestudyof4-NPremovalbyactivated

carbonobtainedfromjutestick.

Attendingtotheexperimentalresults,texturalpropertiesofthe

testedcarbonsarenotseemhighlyinﬂuencingonthekineticrate;

so,itseemsthattheacidity/basicitypropertiesofthecarbon

mate-rialsplayacriticalroleontheadsorptionrate.Thus,pHIEPvaluesof

thephysically-activatedcarbonscanbecorrelatedtok1 constant

(Fig.7).

Fromtheﬁgure,itcanbededucedthattheadsorptionrateis

stronglydependentonthechemicalsurfacepropertiesofthe

acti-vatedcarbon;thus,thedeterminedapparentrateconstantof4-NP

adsorptionontoPAC-NUTwas0.06min−1,whichis60timeshigher

thanthatachievedbyPACS.

3.3. Equilibriumadsorptiontests.Modelingoftheexperimental

data

Fromtheequilibriumadsorptiontestscouldbeinferredthatthe

surfacechemistryoftheactivatedcarbonsplaysanimportantrole

ontheadsorptionperformance,asdetailedabove.Theadsorption

Fig.7. Pseudo-ﬁrstkineticrateconstantsvs.isoelectricpointforthe physically-activatedcarbons(originalandmodiﬁedcarbons).

Fig.8. Experimentaladsorptionisothermsof4-NPontothetestedadsorbents.

isothermsofallthestudiedsystemscanbeclassiﬁedasL-3orIII

typeisotherms,accordingtoGilesclassiﬁcation[42](Fig.8).

AtypeIIIisothermappearswhenthebindingenergyforthe

ﬁrstlayeris lower thanthe bindingenergy betweenadsorbate

molecules.Thisassumptionwillbeconﬁrmedbelowthroughthe

adsorptionmodelingoftheexperimentaldata.

Double–ormore–sigmoidadsorptionisothermsareindicating

thattheenergyofadsorptionisconcentrationdependent.So,itcan

beobservedasmallinitialplateaufollowedbyasuddenriseinthe

curveathigheraqueousconcentration[43].Thisriserepresentsa

reorientationoftheadsorbedmoleculesinadirectionofamore

(8)

S.Álvarez-Torrellasetal./AppliedSurfaceScience414(2017)424–434 431 Table4

BET,GAB,LangmuirandFreundlichmodelparametersfortheadsorptionof4-NPbycarbonaceousmaterials.

Carbon BETmodel GABmodel

qs(mgg−1₎ _CBET_(L_mg−1₎ _Cs_(mg_L−1₎ _SSE _qm_(mg_g−1₎ _K1_(L_mg−1₎ _K2_(L_mg−1₎ _SSE

CAC 19.4 40.9 55.4 1328 16.3 928.8 0.018 1945

PACair 38.4 683.6 40.0 1393 38.4 17.1 0.025 1393

PACN2(H2O) 14.6 5.3×105 _37.4 ₄₄₄₀ _14.6 ₆₅₃₄ _0.026 ₄₄₄₀

PAC N2(H3PO4) 7.0 5.3×105 _36.7 ₅₁₄₉ _7.0 ₆₅₃₆ _0.027 ₅₁₄₉

Carbon Langmuirmodel Freundlichmodel

qsat(mgg−1₎ _b_(L_mg−1₎ _SSE _KF_(L_g−1₎ _nF _SSE

PAC-NUT 279.1 0.1 4484 60.6 2.7 2456

spaceforadsorption.Thisassumptionisinaccordancetothe

rela-tionbetween4-NPmolecularsize(0.684nm×0.417nm[44])and

theaverageporewidthofthetestedadsorbentmaterials.According

toPelekaniandSnoeyink[45],theaverageporesizeofthe

adsor-bentshouldbeabove1.2timesofthesecondwidestdimensionof

theadsorbatemoleculetoalloweffectiveadsorption.Inthiswork,

theporewidth-molecularsizeproportionwasfoundtobeinthe

rangefrom3to6.Sincetheoccurrenceofsterichindranceisnot

expectable,thereaccommodationoftheadsorbateinthewidest

poresisprobablygeneratingthesigmoid-typeisotherms.

Theinitialpartoftheadsorptionisothermisindicatingalow

interactionbetween4-NPandthesolidsurfaceatlow

concentra-tions.However,astheconcentrationintheliquidphaseincreased,

adsorptionwasmore readily,occurringmultilayeroraggregate

formation. This behavior is namely as cooperative adsorption,

asynergisticeffectwiththeadsorbedmoleculesfacilitatingthe

adsorptionofadditionalmoleculesfromtheaqueoussolution,as

resultoftheimportantroleofadsorbate–adsorbateinteractionsin

theadsorptionprocess[46].

Thus,atlowaqueousconcentrationrange,high4-NPmolecule

afﬁnity toward PAC-NUT carbon was found, so it could be

established that the basic surface groups favor the adsorption

of 4-NP due to ␲–␲ interactions between the carbon surface

and the adsorbate molecule. Accordingly, the adsorption onto

the chemically-activatedcarbon was hindered due to its more

hydrophiliccharacter,which maybeoriginates theformationof

watermoleculesclusterslocatedinthecarbonporousstructure

andtheblockageofsomeporeentrances(solventeffect).Through

thisphenomenon,someporesmaybeinaccessibletotheformed

phenol-wateraggregates[47].

Such sigmoid trend in the adsorption isotherms has been

reported for phenol adsorption on some activated carbons by

Terzyk[36],whichpostulatedthattheadsorptionmechanismof

phenoliccompoundsisbasedontheformationofdonor-acceptor

complexesbetweentheoxygenatedsurfacefunctionalities

(elec-trondonors)andthearomaticringofphenolactingasacceptor.

AccordingtoTerzyk,carboxylicgroups,i.e.,themostacidic,playan

importantroleintheadsorptionofphenoliccompoundsatsmall

concentration,dramaticallydecreasingtheadsorptioncapacity.

Indeed, phenolic compounds,due totheirplanar shape and

delocalized␲-bonds,interactstronglywiththeadsorbentsurface,

bytheso-called␲–␲interactions,causingareorientationofthe

moleculeintothepores.Thisimpliesthattheadsorptionis

occur-ringonsurfaceactivecenterslocatedinlargermicropores(orin

mesopores),leadingtothechangesobservedintheshapeofthe

isotherms.Thus,theadsorbedmoleculesgeneratebinding

inter-actionswiththemoleculesinsolution,increasingtheadsorption

capacityathigherconcentrations.

Furthermore,ithasalsobeenreportedthatoxygenand

nitro-gen groups of 4-NP molecule react withboth oxygenated and

nitrogenated-functionalities on the carbon surface (PAC-NUT),

resultingintheformationofcovalentbonding[48,49],whichis

inaccordancetothegoodﬁttingfoundtothepseudo-secondorder

model.

Zhu and Gu [50] found aggregateformation on the

adsorp-tionprocesses,indicatingthatthistypeofadsorptioninvolvestwo

phases;intheﬁrstphase,adsorptionisoccurringonthecarbon

sur-face,andinthesecondphase,theinteractionbetweentheadsorbed

moleculesisinvolved.

Meanwhile,accordingtopKavalueof4-NP(7.15[44])andpHPZC

andpHIEPofthecarbons,rangingfrom1.0and6.0,itisexpectedthat

electrostaticforcesarenotinvolvedin4-NPadsorptionprocess.

BETmodelwasappliedtoadequatelydescribethemulti-layer

adsorptionisotherms[51,52]: qe= qs·CBET·Ce (Cs−Ce)·

1+(CBET−1)·(Ce/Cs)

(4)

where qe (mgg−1)and Ce (mgL−1)areadsorptioncapacity and

aqueous-phase concentration at equilibrium, respectively; CBET

(Lmg−1) is the characteristic parameter of BET equation; Cs

(mgL−1), thesaturationconcentrationonthemonolayerand qs

(mgg−1),thetheoreticaladsorptioncapacityfollowingBET

equa-tion.

Thus, a modiﬁed form of BET model is the

Guggenheim-Anderson-DeBoer(GAB)equation,whichpostulatesthatthestate

oftheadsorbatemoleculesonthesecondandsubsequentlayersis

equalbetweenthem,butitisdifferenttothosemoleculesthatare

inliquidphase.Accordingly,inthiscase,theadsorptionis

occur-ringonaﬁnitenumberoflayers,whereastheBETequationdeals

withaninﬁnitenumber[53].GABisothermmodelisexpressedas

presentedinEq.(5).

qe= qm·K1·Ce

(1−K2·Ce)·[1+(K1−K2)·Ce]

(5)

where qe (mgg−1) is the equilibrium adsorption capacity; Ce

(mgL−1)istheequilibriumconcentrationintheaqueousphase;

qm(mgg−1)isthemaximumadsorptioncapacityontheﬁrstlayer

andK1andK2(Lmg−1)aretheequilibriumconstantsfortheﬁrst

andsecondlayer,respectively.

BothBETandGABmodelssatisfactorilyreproducedthe

experi-mentaladsorptiondataofthenon-modiﬁedactivatedcarbons.The

modelparametersaresummarizedinTable4,includingSSEvalues.

K1parametersweremuchhigherthanK2forallthestudied

sys-tems.Thiscanbeattributedtothefactthattheadsorbateismore

stronglyattachedtothecarbonsurfaceatlowconcentrationsand

weaklateralinteractionsoccurredathigheraqueous

concentra-tions[54].

Thus, Langmuir model [55], suitable for monolayer

adsorp-tion (Eq.(6)), and Freundlich equation[56], commonly applied

toheterogeneousprocesses(Eq.(7)), wereusedtodescribethe

adsorptiondataof4-NPontoPAC-NUT.Therefore,the

(9)

Fig.9. Experimentalandtheoreticaladsorptionisothermsof4-NPonto(a)non-modiﬁedand(b)modiﬁedcarbonmaterials.

Table5

Resultsonadsorptionofphenoliccompoundsbylignocellulosic-activatedcarbonsreportedintheliterature.

Adsorbent Contaminant qexp(mgg−1₎ _Reference

Apricotstones-basedactivatedcarbon Phenol 150.0 [55,58]

4-Nitrophenol 145.0

Activatedcarbonderivedfromricestraw 4-Chlorophenol 24.0 [56,59]

Physicallyandchemically-activatedcarbonsfromjuteandcoconutﬁbers Phenol 150.0 [57,60] 75.0

180.0 112.0

Activatedcarbonfromdatepits(activatedwithH3PO4) 4-Nitrophenol 108.7 [25,35]

Microwave-assistedactivatedcarbonsfromwoodchips Phenol 175.0 [58,61]

Activatedcarbonpreparedfromwastetea Phenol 37.0 [59,62]

Physically-activatedcarbonfrompeachstonesandfurthertreatedwithurea(PAC-NUT) 4-Nitrophenol 234.3 Thiswork

thetestedmodels,e.g.,BET,GAB,Langmuir,FreundlichandSips

equations.

qe= qsat·b·Ce

(1+b·Ce)

(6)

qe=KF·Ce1/nF (7)

where qe (mgg−1) is the equilibrium adsorption capacity; Ce

(mgL−1),theequilibriumconcentration;qsat(mgg−1)isthe

max-imum adsorptioncapacity onthe monolayer;b (Lmg−1)is the

adsorptionequilibriumconstantforLangmuirmodel;KF(Lg−1)is

theadsorptionafﬁnitycoefﬁcientand1/nFindicatestheintensityof

theadsorptionprocessintheFreundlichequation.BothLangmuir

andFreundlichparametersareshowninTable4.

Theexperimentaldataobtainedforthenon-modiﬁedcarbons

andthenon-linearizedmodelﬁttingareplottedinFig.9a,while

experimentalandtheoreticaladsorptiondataofmodiﬁed-carbons

aredepictedinFig.9b.

PAC-NUTadsorptiondatawerebetterdescribedbyFreundlich

model.1/nFvaluewasfoundtobelessthan1,suggestingthat4-NP

adsorptiononPAC-NUTcarbonisafavorableprocess.Good

corre-lationofbothLangmuirandFreundlichmodelstoadsorptiondata

ofphenoliccompoundshasbeenwidelyreportedintheliterature

[47,57].

Severalexperimentalresultsfromstudies ontheadsorption

ofphenolic compounds bylignocellulosic-activated carbonsare

summarizedinTable5.Itwasfoundthattheresultsobtainedfor

PAC-NUTactivatedcarbonarehighlycompetitiveintheremovalof

4-NPfromaqueoussolution.

Fig.10.Breakthroughcurvesof4-NPontoPACairadsorbentattheoperation con-ditions:C0=10mgL−1,Q=2.0mLmin−1andmassofadsorbent=0.8g.

3.4. Fixed-bedadsorptionexperiments.Estimationofadsorption

parameters

Fixed-bed column adsorption studies of 4-NP were

accom-plished with PACair activated carbon. Breakthrough curves at

different volumetric ﬂow rates (Q=1.33 and 2.50mLmin−1),

C0=10.0mgL−1of4-NPand0.8gofactivatedcarbonwereobtained

(Fig.10).Generally,ﬂatterproﬁlesforbothbreakthroughcurves

wereobserved,mainlyattributedtoahighinﬂuenceofthemass

transferresistanceintheadsorptionprocess.

Breakthroughtimes(tb),calculatedatC/C0=0.05,werefoundto

(10)

maxi-S.Álvarez-Torrellasetal./AppliedSurfaceScience414(2017)424–434 433 Table6

Adsorptioncapacities,MTZ,FBUandremovalpercentageforadsorptionof4-NPadsorptionbycarbonaceousmaterials.

Q(mLmin−1) tb(h) ts(h) qb(mgg−1) qs(mgg−1) MTZ(cm) FBU Y(%)

1.33 6.4 88.7 4.4 20.4 6.3 0.21 68.4

2.50 2.1 88.7 3.3 27.2 7.0 0.12 85.1

mumsaturationobtainedinthecolumnoperatingat1.33mLmin−1

wasC/C0=0.69(ts=88.6h);meanwhile,theadsorptionbed

work-ingataﬂowrateof2.50mLmin−1reachedaC/C0 valueof0.89,

forthesamesaturationtime.Thissuggestedthatwhentheﬂow

ratewashigher,thetimerequiredtoremovethesamequantityof

pollutantwasshorterduetothebedwassaturatedearlier.Thus,

athighervolumetricﬂowrate,adecreasinginthebreakthrough

timewasobserved,andresultinginasteeperbreakthroughcurve

(Fig.10),indicatingadecreaseinthemasstransferresistanceofthe

adsorptionprocess[63].

Adsorptionparameters,e.g.,breakthroughtime(tb),saturation

time(ts),adsorptioncapacityatbreakthroughtime(qb,mgg−1),

adsorptioncapacityatsaturationtime(qs,mgg−1),masstransfer

zonelength(MTZ,cm),fractionalbedutilization(FBU,

dimension-less)andadsorbateremovalpercentageatbreakthroughtime(Y,

%)weredeterminedandsummarizedinTable6.

Itcanbeconcludedthatatalowerﬂowrate,theresidencetime

oftheadsorbateinthecolumnishigherandthus,theadsorbent

isgettingmoretimetobondwith4-NPmoleculeefﬁciently.This

resultedinaslightincreaseintheadsorptioncapacityat

break-throughtime,indicatingthattheadsorptionprocessiscontrolled

byintra-particlediffusionmasstransfer.Asimilartendencyhas

beenfoundbyotherresearchers[64,65].

4. Conclusions

In this study, it was found that 4-NP adsorption is highly

conditionedbythechemicalsurfacepropertiesoftheadsorbent.

Accordingly, the best adsorption results, in terms of

adsorp-tion capacity and molecule afﬁnity, were those obtained for

the more basic activated carbon (PAC-NUT). Additionally, the

chemically-activatedcarbon(CAC)showedthehighestspeciﬁc

sur-face area, total porevolume and pore size values. Thiscarbon,

withhydrophiliccharacter,offeredgoodkineticproperties,dueto

itsmoreopened-porousstructure,butlow afﬁnitytoward4-NP

molecule.Thus,physically-activatedcarbonsshowedmuchmore

hinderedtexturalproperties(totallymicroporousmaterials).

Experimental kinetic data were successfully ﬁtted to the

pseudo-secondordermodel,suggestingthatchemisorptionforces

couldbeinvolvedin4-NPadsorptionprocess.Thestrong

depen-dency oftheadsorption rateonthechemical properties ofthe

adsorbentwascorroboratedthroughtherelationshipbetweenthe

apparentkineticrateconstantsandpHIEPvaluesofthecarbons.

Theadsorptionisothermsshowedasigmoidtrend,attributedto

abilayer/multilayeradsorption,beingwellreproducedbyBETand

GABmodels.Breakthroughcurvesof4-NPontoPACairat

differ-entvolumetricﬂowrateswereobtained,showingalatterproﬁle

andnotatotalcolumnsaturation.Finally,thebreakthroughcurve

obtainedathighﬂowrate(Q=2.5mLmin−1)showedahigherslope,

attributedtoadecreasinginthemasstransferresistance.

Acknowledgements

The authors would like to thank the ﬁnancial support

of Ministry of Economía and Competitividad of Spain

(Con-tractCTQ2014-59011-RREMEWATERandTRAGUANETNetwork

CTM2014-53485-REDC), the Regional Government of Madrid

provided through Project REMTAVARES S2013/MAE-2716 and

the European Social Fund, and Project

POCI-01-0145-FEDER-006984 – Associate Laboratory LSRE-LCM, funded by FEDER

throughCOMPETE2020–ProgramaOperacionalCompetitividade

eInternacionalizac¸ão(POCI)–andbynationalfundsthroughFCT

– Fundac¸ão para a Ciéncia e a Tecnologia. M.Martin-Martinez

acknowledgesﬁnancialsupportfromtheFCTPostdoctoralgrant

SFRH/BPD/108510/2015. S. Álvarez-Torrellas acknowledges also

thefunding provided bythe SpanishMinistry ofEconomy and

Competitiveness(MINECO)throughJuandelaCierva-Formacion

Contract.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbefound,in

theonlineversion,athttp://dx.doi.org/10.1016/j.apsusc.2017.04.

054.

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