Adsorbent ability of lignin-based activated carbons for the removal of p-nitrophenol from aqueous solutions

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Chemical Engineering Journal

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

Adsorbent ability of lignin-based activated carbons for the removal of p-nitrophenol from aqueous solutions

Luis M. Cotoruelo

^a,∗

, María D. Marqués

^a

, Francisco J. Díaz

^a

, José Rodríguez-Mirasol

^a

, Juan J. Rodríguez

^b

, Tomás Cordero

^a

aDepartamentodeIngenieríaQuímica,FacultaddeCiencias,UniversidaddeMálaga,29071Málaga,Spain

bIngenieríaQuímica,FacultaddeCiencias,UniversidadAutónomadeMadrid,28049Madrid,Spain

a r t i c l e i n f o

Articlehistory:

Received16October2011 Receivedinrevisedform 23December2011 Accepted3January2012

Keywords:

p-Nitrophenol Activatedcarbon Adsorption Equilibrium Thermodynamics Kinetics

a b s t r a c t

Thispaperreportsthep-nitrophenol(PNP)removalfromaqueoussolutionsbyadsorptionontoactive carbons(ACs).NineACswerepreparedfromacid-precipitatedeucalyptuskraftligninfollowingatwo- stepprocessconsistinginCO2partialgasification(750–850^◦C)aftercarbonization(350–800^◦C)inN2

atmosphere.Theamountadsorbedrangedfrom1to4.4mmol/g,anditisrelatedtotheinitialconcentra- tionofadsorbate,temperature,pH,burnoffoftheactivatedcarbons,andcontacttime.Theequilibrium resultswerefittedbytheTemkin,Dubinin-Radushkevich,andGuggenheim-Anderson-deBoerequa- tions.Themainthermodynamicmagnitudeswereestimatedaswell,andtheirvaluesindicatedthatthe adsorptionprocesseswerespontaneousandexothermic.Thekineticstudyshowedthattheprocesses areofapparentsecondorderrelatedtotheconcentrationsoftheemptyactivesitesontheACssurface.

Thevaluesoftheeffectivediffusivitieshavebeencalculatedandtheyhavesuggestedthattheinternal diffusioncontrolsthenetmasstransfer.Theresultsobtainedinthepresentworkcanbeforthebenefit ofthepreparationofnewadsorbents,aswellastheprimarydesignoftheadsorptionequipmentwith separationorenvironmentalpurposes.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Progressiveincreasesin theindustrialactivityproduce large amountsoftoxiccompounds.Amongthem,p-nitrophenol(PNP) canentertheenvironmentduringtheproductionofdyes,pesticides anddrugs.Nitrophenolsaretoxictoplantsandanimals.Duetoits harmfuleffects,wastewaterscontainingphenoliccompoundsmust betreated beforetotheirdischargetothereceivingwaterbod- ies.Manydifferenttypesofadsorbentmaybeevaluatedtoremove substitutedphenolsfromwastewaters.Theyinclude,amongoth- ers,XADpolymericresins[1]andactivatedcarbons,whichstillare theadsorbentswidestusedtothisend[2–12].Ontheotherhand, aswehavereportedinpreviousworks[13,14],thepreparationof activatedcarbonsfromkraftligninisaninterestingpossibilityfor theligninbenefitasanalternativewayfortheblackliquorsfrom thecellulosepulpindustry,astheyareusuallyprocessed.So,the objectiveofthisworkhasbeentostudytheadsorptionofPNPfrom aqueoussolutionsusingACspreparedinourlaboratory.TheACs derivedfromeucalyptuskraftligninhavebeenchosenfor their valuableproperties basedontheirwelldevelopedsurface area,

∗ Correspondingauthor.Tel.:+34952132037;fax:+34952132038.

E-mailaddress:[email protected](L.M.Cotoruelo).

surfacechemicalnature,andtheirthermo-chemicalstability.Also, theuseofPNPadsorptionfromwaterororganic(benzene,hep- tane,andxylene)solutionsbecameareferencedprocedureinthe surfacestudyofvariedadsorbents,asalumina,carbons,cement, metalpowders,silica,titania,pigments,sugar,andwool[15–17].

2. Materialsandmethods

Theexperimentalproceduresfollowedinthepreparationand characterizationstudiesoftheACsasadsorbents(herenamedas:

A40,B20,B40,C50,D40,E4,E12,E16,andE20)werepreviously describedindetailelsewhere[18].Therawmaterialwaseucalyp- tuskraftligninsuppliedbytheEmpresaNacionaldeCelulosasas obtainedbyacidprecipitationofkraftblackliquors.Thisligninwas carbonizedunderN2atmosphereandthenitwasactivatedbypar- tialgasificationwithCO₂duringdifferenttimestoobtainactivated carbonswithdifferentburnoff.

TheporousstructuresoftheACswerecharacterizedbymeansof adsorption–desorptionofN2at77KandCO2at273KusingaQuan- tachromeapparatus(Autosorb-1model)andmercuryporosimetry usingaCarloErbamercuryporosimeter4000.Ultimateanalyses oftheACswerecarriedoutinaPerkin-Elmer(model2400CHN) analyzer.

1385-8947/$–seefrontmatter © 2012 Elsevier B.V. All rights reserved.

doi:10.1016/j.cej.2012.01.026

Table1

PreparationconditionsandfinalpropertiesoftheACs.

AC A40 B20 B40 C50 D40 E4 E12 E16 E20

ABET(m²/g)^a 1450 1123 1343 1319 1613 822 1273 1348 1853

AS(m²/g)^b 725 354 510 373 367 36 341 393 504

Vmicrop.CO₂(cm³/g)^c 0.380 0.354 0.366 0.372 0.384 0.304 0.339 0.351 0.376

Vmicrop.N₂(cm³/g)^d 0.528 0.426 0.567 0.510 0.576 0.331 0.483 0.531 0.701

Vmesop.(cm³/g)^e 0.992 0.714 0.740 0.490 0.770 0.060 0.480 0.780 0.860

Vmacrop.(cm³/g)^f 0.510 0.189 0.343 0.240 0.310 0.020 0.110 0.150 0.530

Carbonizationtemp.(K) 550 800 800 350 350 350 350 350 350

Activationtemp.(K) 800 800 800 750 800 850 850 850 850

Activationtime(h) 40 20 40 50 40 4 12 16 20

Burnoff(%)^g 66.7 54.8 71.3 67.3 70.5 21.3 54.5 63.5 76.8

Yield(%weight) 14.5 17.8 11.3 12.7 11.3 31.5 18.2 13.9 9.4

Ultimateanalyses(%dryashfreebasis)

C(%) 92.8 96.4 94.5 93.1 94.5 95.5 94.8 94.3 93.7

O(%) 6.65 2.88 4.98 6.21 4.87 3.92 4.69 5.23 5.86

H(%) 0.55 0.72 0.52 0.69 0.63 0.58 0.51 0.47 0.44

Inorganicmaterial(%) 12.1 7.5 10.2 8.2 11.0 4.2 6.8 8.5 11.2

aDeterminedbyadsorptionofN2at77K(BETequationwithAm=0.162nm²).

b ExternalsurfacedeterminedfromtheN2adsorptionisothermby␣Smethod.

c DeterminedbyadsorptionofCO2at273K(Dubinin-Radushkevichequation).

d DeterminedfromtheN2adsorptionisothermby␣Smethod.

e(From2to8nmwidth)estimatedfromtheN2adsorptionisothermbytakingvolumeadsorbedatP/Po=0.950asliquidand(from8to50nmwidth)estimatedbymercury porosimetrybytakingcumulativeporevolumeatr=25nm.

f Estimatedbymercuryporosimetrybytakingcumulativeporevolume.

gCalculatedasthesampleweightlossrelativetocompletelydevolatilizedkraftlignin.

For theliquid phase adsorption,a previousexperimentation includeda program for the influencesof the initialconcentra- tionofPNP,aswellastheadsorbentdose.Anadsorbentdoseof 0.100gAC/L(drybasis),andarangefrom50to300mgPNP/Lwere setasselected forthe testsin this present work.So, theequi- libriumandthekineticexperimentswerecarriedoutbyputting thedosesofactivatedcarbonweightedintoglassflasks.Astock solutionofPNP (Merck,99.5%purity) waspreparedin distilled waterandthedesiredconcentrationswereobtainedbysuccessive dilutions.100mLofaqueoussolutionsofdifferentinitialconcen- trations(Co:0.036–2.157mmol/L)wereaddedtotheglassflasks tocontacttheACs,andmechanicallyagitatedat180rpmusingan orbitalincubator(Gallenkamp,modelINR-250).Thesampleswere keptatdifferenttesttemperaturesintherange283K–313K.The contacttimesvariedbetween10and14days(dependingonthe samples).Thesolutionsfromthekinetictestswereanalyzedfor differentcontacttimesupto2h.ToanalyzethePNPconcentra- tions,acalibrationplotwaspreparedbymeasuringtheabsorbance ofsampleswithdifferentpredeterminedconcentrations(UV–vis spectrophotometerVarian,modelCary1E,max:315nm).Toavoid thepHinfluenceontheabsorbanceofthesamples,allsampleswere previouslyacidifiedbyadding0.1MHCl.

3. Resultsanddiscussion

3.1. Characterizationoftheactivatedcarbons

Eucalyptuskraftligninisnotaporousmaterial(A_BET<1m²/g) assupplied,andithasnoadsorptioncapacityasis.Afirstther- mochemicalprocessisnecessarytodevelopanyspecificsurface area,and it hastobefollowedby aphysical activation.Ahigh gradeofporosity(microporosityandacertaingradeofmesoporos- ity)isdevelopedbytheactivationprocess.TheresultingACshave heterogeneousphysicalandchemicalstructure.Theformerarises fromtheexistenceofporeswithdifferentsizes.Thelatterisorig- inated froma variety offunctional groups (mainly in theform ofcarbon–oxygen)onthesurface. Table1summarizessomeof thecharacteristicsoftheACs,whichhavebeenextensivelydis- cussedinapreviouswork[18].Thesurfaceareaandthemicropore

volumesubstantiallyincreasewithburnoff;awideningofmicro- pores,andanincreasingmesoporescontributionareobservedand, athighburnofflevels,asignificantdevelopmentofmacroporos- itycanbedetected.EseriesincludesACswithlow,intermediate, andhighactivationdegrees.ThechemicalnatureoftheACssur- facehasbeenalsostudiedandreportedinapreviouswork[19].

XPSandTPDanalysessuggestedthatthesurfaceoxygengroups formedduringgasificationwerethefollowing:hydroxyl,ether,car- bonyl,carboxylicacid,lactones,phenols,quinone,andanhydrides ofcarboxylicacids.

3.2. Adsorptionequilibriumisotherms

3.2.1. Equilibriumresults

Theadsorptionisothermisanexpressionforthevariationofthe adsorbentadsorptioncapacity(qe,mmol/g)whichisinequilibrium withtheconcentrationofadsorbate(Ce,mmol/L)inthebulkofthe solutionataconstanttemperature.

GraphicalpresentationsofthePNPadsorptionisothermsonall ACsat293KaredepictedinFig.1.Fig.1bdisplaystheEseries isothermsfortheACspreparedwhentheactivationtimevaried.

Theshapeoftheisothermsindicatesahighadsorbentaffinityto thePNPatlowconcentrations.Theisothermsareconcave(favor- able)withrespecttotheCe-axisfortheentireconcentrationrange studied.ThehighestadsorptioncapacitiescorrespondedtoD40and E20,inagreementwiththeirhighesttotalsurfacedevelopment.

TheadsorbedamountprogressedwiththeincreaseoftheACburn off,asaconsequenceofthegreatersurfaceareaandmesoporos- itydevelopment.TheisothermofE20revealedahighadsorption capacityevenatlowequilibriumconcentrations.Incontrast,the E4isothermprovidedaweakenlargeonitscapacityvaluestoward thehighestPNPconcentrationsinthesolution.

Tohaveanideaabouttheexpectedyieldfromtheadsorption system,it is important atfirst, toestablish themostappropri- atecorrelationsfortheequilibriumstatecurves.Variousisotherm equation types like those from Guggenheim-Anderson-de Boer (GAB),Dubinin-Radushkevich(D-R)andTemkin,havebeentested todescribetheequilibriumdata.

0 1 2 3 4

2.0 1.5

1.0 0.5

0.0 0 1 2 3 4 5

A40 B20

B40 C50 D40

a

q (mmol/g)e

b

Ce (mmol/L) E20

E16 E12 E4

Fig.1. Experimentaldata(dots)andGABfits(curves)fortheadsorptionisotherms ofPNPonACsat293K:(a)A40,B20,B40,C50,andD40;(b)Eseries.

3.2.2. Guggenheim-Anderson-deBoermodel TheGABmodel[20]maybeexpressedas:

qe= K1MGABCe

(1−K₂C_e)(1+(K₁−K₂)C_e) (1) whereK1 andK2 (L/mmol)aretheisothermconstantsandM_GAB (mmol/g)istheequilibriumconcentrationofadsorbateonthesolid phasecorrespondingtoacompletecoverageoftheadsorptionsites.

ThisequationcanbecomeaLangmuirisothermforK2=0.Fig.1and Table2showtheresultsofthefitstotheGABmodel(includedthe valuesof²:averagequadraticdeviation).

3.2.3. Dubinin-Radushkevichmodel TheD-Requation[21]isexpressedas:

qe=MD-Rexp(−ˇε²) (2)

Eq.(2)canbelinearizedas:

Lnqe=LnMD-R−ˇε² (3)

8 6

4 2 0

0.3 0.6 0.9 1.2 1.5 0.3 0.6 0.9 1.2

b Ln qe (qe in mmol/g)

ε² (J²/mmol²)

E20 E16 E12 E4

A40 B20 B40 C50 D40 a

Fig.2.Experimentaldata(dots)andD-Rfits(linear)fortheadsorptionisotherms ofPNPonACsat293K:(a)A40,B20,B40,C50,andD40;(b)Eseries.

whereM_D-R(mmol/g)representsthetheoreticalsaturationcapac- ity;ˇ(mmol²/J²)isaconstantrelatedtotheadsorptionenergy;

and ε(J/mmol) isthePolanyi potential,which isrelated tothe equilibriumconcentrationasfollows:

ε=RTLn



1+



₁

Ce



(4)

whereRisthegasconstant(0.008314J/mmolK)andTistheabso- lute temperature (K). The Polanyi sorption potential (ε) is the workrequiredtoremoveamoleculefromitslocationinthesorp- tionspacetoinfinity,independentofthetemperature.Thismodel assumestheheterogeneityofsorptionenergieswithinthisspace.

ThevaluesofMD-RandˇmaybeestimatedbyplottingLnqevs.ε²; i.e.,fromtheinterceptandtheslopeofthestraightlinesdepicted byEq.(3).Table2showstheresultsofthefitstotheD-Requation andthevaluesofr²(determinationcoefficient).Experimentaldata andtheirD-RfitsareplottedinFig.2.

Table2

GAB,D-RandTemkinisothermconstantsat293Kandcoveragefactors.

AC A40 B20 B40 C50 D40 E4 E12 E16 E20

GAB

MGAB(mmol/g) 1.783 1.333 2.057 1.789 2.373 1.399 1.586 2.025 2.556

K1(L/mmol) 302.0 104.1 362.9 315.9 354.2 156.8 222.9 331.5 395.7

K2(L/mmol) 0.217 0.061 0.232 0.218 0.238 0.088 0.131 0.220 0.269

² 2.9E-2 4.9E-3 2.7E-2 2.1E-2 5.2E-2 3.4E-2 1.3E-2 6.2E-2 9.9E-2

D-R

MD-R(mmol/g) 3.031 1.523 3.428 2.956 3.804 1.795 2.061 3.019 4.674

ˇ(mmol²/J²) 0.068 0.020 0.051 0.056 0.038 0.058 0.022 0.026 0.047

r² 0.995 0.915 0.925 0.939 0.977 0.999 0.970 0.947 0.977

E(J/mmol) 2.71 5.00 3.13 2.99 3.63 2.94 4.77 4.39 3.26

Temkin

MT(mmol/g) 0.620 0.138 0.632 0.544 0.605 0.299 0.201 0.435 1.029

KT(L/mmol) 51.60 2.9E4 100.1 94.25 255.6 170.8 1.2E4 599.1 42.66

b(Jg/mmol²) 3.927 17.644 3.853 4.476 4.025 8.144 12.114 5.598 2.366

r² 0.967 0.960 0.969 0.957 0.995 0.989 0.967 0.943 0.983

EquivalentareaoccupiedandcoveragefactorsforCo:2.157mmol/L

APNP(m²/gAC) 907 480 1082 910 1173 544 658 952 1398

C.F.(APNP/ABET) 0.63 0.43 0.81 0.69 0.73 0.66 0.52 0.71 0.75

1 2 3 4

1.0 0.5 0.0 -0.5 -1.0 -1.5 -2.0 1 2 3 4

A40 B20 B40 C50 D40

a

qe (mmol/g)

Ln C_e (C_e in mmol/L) E20

E16 E12 E4

b

Fig. 3.Experimental data (dots) and Temkin fits (linear) for the adsorption isothermsofPNPonACsat293K:(a)A40,B20,B40,C50,andD40;(b)Eseries.

Theconstantˇgivesthemeanadsorptionfreeenergy(E)when theadsorbateistransferredfromtheinfinityinthesolutiontothe surfaceofthesolid.Itcanbecalculatedas:

E=



1 2ˇ

(5)

ThemagnitudeofE(J/mmol)isusefulforestimatingthemech- anismoftheadsorptionstep(Table2).Inthepresentcase,with E<8kJ/mol, it isassumed thatthephysical forcesare themain affectingtheadsorptionmechanism.

3.2.4. Temkinmodel

TheTemkinisotherm[21]describesthebehaviouroftheadsorp- tionsystemsonheterogeneoussurfaces.Thismodelassumesthat:

(1)theadsorptionheatofallthemoleculesinthelayerdecreases linearlywiththecoverageduetotheadsorbate–adsorbateinterac- tionsand(2)theadsorptionischaracterizedbyahomogeneous distributionofbinding energies,upto somemaximumbinding energy.TheTemkinequationmaybeexpressedas:

qe=



_RT

b



Ln(KTCe) (6)

IfRT/b=M_T;thenEq.(6)canbelinearizedas:

qe=MT LnKT+MTLnCe (7)

whereK_T(L/mmol)isthebindingequilibriumconstantcorrespond- ingtothemaximumbindingenergy,andMT(mmol/g)isrelatedto theheatofadsorption.Aplotofqe vs.LnCe makespossiblethe determinationofM_TandK_Tfromtheslopeandintercept,respec- tively.Table2andFig.3showtheresultsofthefitstotheTemkin equationandthevaluesofr².

Thevalues oftheequivalentareaoccupied(A_PNP), aswellas thecorrespondingcoveragefactor(C.F.:A_PNP/A_BET)reachedatthe maximuminitialconcentrationused(Co:2.157mmol/L)havebeen calculated.AccordingtoEq.(8),anyq_e valuecanbetransformed toPNPequivalentsurfacearea(A_PNP)whichwouldcover1gofAC assumingthatthePNPmoleculestakeaparallelorientation(flat

2.4 2.0 1.6 1.2 0.8 1 0.4

2 3 4 5

E20

B40 D40 E16 A40 C50

B20 q (mmol/g)e E12

V_pore (cm³/g) E series

q_e = 0.741 + 1.641 V_pore r² = 0.920

E4

Fig.4.AmountofPNPadsorbedasfunctionoftheACstotalporevolume.

adsorption)ontheACsurfaceandthesurfaceareacoveredbyeach moleculeadsorbedis52.5 ˚A²[15]:

A_PNP



m²_PNP g_AC



=316.2



m²_PNP mmolPNP



·qe



_mmol

PNP

g_AC



(8)

Table2showsthevaluesofthecoveragefactorsforthePNP adsorption.Thoseareintherangefrom40to80%oftheBETspe- cificsurfaceareas.WhencomparingthedifferentACs(exceptE4 withE12),wehaveobservedthatthecoveragefactorsincreaseas burnoffsincrease.Thisistheconsequenceoftheactivationgrade influence,ascombination ofthemicroandmesoporositydevel- oped.

Fig. 4 shows the maximum adsorption capacity (Co: 2.157mmol/L) of PNP vs. the total pore volume for all the studiedACs.A linear-liketrend hasbeenobtainedforE series;

it confirms thefavorable effectof theactivationstep. E4 hasa verylow capacity (1.720mmol/g)while E20 showsa very high capacity(4.420mmol/g).Thisisinagreementwiththeirinternal surfaceareadevelopment,andmoreespeciallywiththevolumeof mesopores.

3.3. EffectofthepHontheadsorption

TheadsorptionofPNPatdifferentpH(from3to11)isdepicted inFig.5.TheremovalofPNPwasthemaximumatacidpH,butit wasslightlydecreasing,andwhenthepHofthesolutionincreased tovalueshigherthan7,especiallyuptoalkaline,theadsorptionof PNPdecreasedsharply.

PNP (a water soluble solid) is moderately acid in water (pKa=7.15). Whenthesolution pHis acid;for example pH=2;

then:[p-nitrophenol]/[p-nitrophenolate]=1.4×10⁵,almostallthe

12 10 8 6 4 2 1.0 1.5 2.0 2.5 3.0 3.5

PNP on E20 C_o: 0.719 mmol/L T: 293 K w: 0.1 g/L qe (mmol/g)

pH

Fig.5. pHeffectontheadsorptionequilibriumofPNP.

120 100 80 60 40 20 0 0.0 0.4 0.8 1.2 1.6 2.0

pH : 11.0 pH : 8.9 pH : 7.9 pH : 6.6

qt (mmol/g)

t (min)

pH : 2.0

Fig.6.KineticofPNPadsorptiononE16atdifferentpH.Co:0.719mmol/L;T:293K;

w:0.100g/L.

moleculesofPNPinsolutionareintheirmolecularform.AtlowpH, thereisnearlynoelectrostaticrepulsion;so,theadsorbedamount isthemaximum.ThelargestamountadsorbedatlowpHmaybethe resultofthestrongchemicalbondbetweenthelonelypairofelec- tronsonthehydroxylgroupinPNPandthesurfaceofAC,aswell astheexistingforcesfromthedispersiveinteractionsbetweenthe aromaticringofthePNPandspecificsitesontheactivatedcarbon surface.AthighpHvalues,thePNPmoleculesaremoreionizedto p-nitrophenolateanions;thenumberofnegativelychargedsites increasesand thenumber ofpositively chargedsites decreases.

ThesurfacegroupsontheACbecomeanionic(negativecharge), ascarboxilate( COO⁻)andphenolate(Ph O⁻)ions,andtheyare notfavorablefortheadsorptionofanions.So,thesharpdecrease intheadsorptioncapacityatbasicpHmaybeattributedtothe electrostatic repulsion betweenthe p-nitrophenolate molecules andthenegativelychargedadsorbentsurface.Moreover,thelow adsorptionathighpHisprobablyduetothepresenceofOH⁻ions competingwiththephenolatesfortheresidualpositivelycharged adsorptionsitesontheACssurface.

Asacomplementarystudy,thekineticadsorptiondataforPNP onE16atdifferentpHhavebeenobtained(Fig.6).Itcanbeobserved thatatlowpH,theadsorptioncapacityandtherateincreaseasa consequenceoftheoccurringstrongattractionforces.Neverthe- less,itmaynotbepracticaltomaintainextremepHvaluesbecause itwouldneedadditionofchemicalstolowerthepHandtheeffluent wouldneedafinalneutralizationstepbeforeitsdischarge.

3.4. Effectofthetemperatureonadsorption

The temperature influencesthe adsorption equilibrium and itsvariationsproduceadisplacementfromortowardthephase adsorbed. Also, an increase in temperature generally improves thesolubilityofthemolecules(ifinliquidphase)andtheirdif- fusionwithintheporesoftheadsorbentmaterials.Fig.7shows theadsorptioncapacityofB20andE12atdifferenttemperatures.

Theisothermsshowsimilarbehaviours,withstrongaffinitiesof thePNP molecules for thesurfaces,as evidenced bythe initial slopes.TheamountsadsorbedofPNP rosewiththedecreaseof thetemperature,indicatingtheapparentexothermicnatureofthe entireprocess.Similareffecthasbeenreportedbyotherauthors [6,22–24].Ingeneral,physicaladsorptionprocessesaretypically exothermic;anincreaseintemperaturecausesareactioninthe oppositedirection,i.e.,thisproducesdesorptionofmoleculesfrom thelessenergeticactivecentres,resultingadisplacementinthe equilibriumtowardthefluidphase.Thethermodynamicanalysis providesinformationabouttheprocessabilitytotakeplaceand thestabilityoftheadsorbedphaseaswell.Thechangesonthefree energy(G)havebeenestimatedat293KforalltheACs,andat

0.0 0.4 0.8 1.2 1.6

2.0 1.6 1.2 0.8 0.4 0.0 0.0 0.4 0.8 1.2 1.6 2.0 2.4

a

q (mmol/g)e

b

Ce (mmol/L)

Fig.7. Experimentaldata(dots)andGABfits(curves)fortheadsorptionisothermsof PNPatdifferenttemperatures:(a)B20;(b)E12.MGABin(mmol/g),K1,K2in(L/mmol).

fourtemperaturesforB20andE12.Inaddition,fortheselatterACs, theenthalpy(H)andtheentropy(S)changeshavebeenalso calculated.

The free energy of adsorptionhas beencalculated from the expressionderivedfromtheGibbsadsorptionequation[25].Ifwe referthefreeenergychangepermolofsoluteadsorbed,thefollow- ingfinalexpressioncanbeobtained:

G=−RT q_e

Ce

0

qe

C_edCe (9)

whereqeisrelatedtoCebytheadsorptionisotherm.Inthepresent work,asacriterion,thefitsoftheGABequationhavebeenusedfor thecalculationoftheequilibriumrelations.TheGvalues(kJ/mol) havebeencalculatedusingEq.(9)bynumericalintegration.Inthis case,Gdependsonthetemperatureandthegradeofcoverage (qe).Table3showsthevaluesofGatvariouslevelsofsurface coverage(Ce:0.800,1.200,and2.000mmol/L).Thenegativevalues ofGindicatethefeasibilityoftheprocessesandthespontaneous natureofthePNPadsorptiononalltheACs.Althoughtheincrease inCe inducesa higheramountadsorbedintheequilibrium,we havefoundthat,insomeofcases,theabsolutevaluesofGget lower.AthighCe,theaveragereleasedenergydescendswhencov- eringagreatvarietyofmoreandlessactivesites.Bycontrast,for B20,E4andE12,withlowburnoff,ABET,andporositydeveloped,

Gincreases(averagemolarvalues)alittlewhenCeincreases.It seemsthatitisaconsequenceoftheiractivesitesdistributionfor adsorption,whichhastotakeplaceonthemostactivesites,mainly locatedthemicroporosityplacesinside.On theotherhand,B20 andE12haveshowedalittleornegligibleincreaseinGwhenthe temperaturedecreased.Thesecomplexphenomenaareduetothe adsorbentssurfaceheterogeneityaswellasthevarietyofpossible combinedmechanismstoorganizeandtodrivethemasstransfer steps.

Table3

Thermodynamicsforadsorptionatdifferentequilibriumconcentrations.Ce(mmol/L),T(K),GandH(kJ/mol),S(J/molK).

AC T Ce=0.800 Ce=1.200 Ce=2.000

G H S T(G=0) G H S T(G=0) G H S T(G=0)

A40 293 −11.1 −10.9 −9.4

B20 283 −9.3 −16.1 −24 671 −10.0 −15.6 −20 780 −10.6 −13.4 −10 1340

293 −9.0 −9.7 −10.4

303 −8.7 −9.5 −10.3

313 −8.6 −9.4 −10.3

B40 293 −15.9 −15.0 −12.2

C50 293 −11.5 −11.3 −9.7

D40 293 −19.8 −18.3 −14.5

E4 293 −5.9 −6.6 −7.3

E12 283 −5.4 −7.1 −5.9 1203 −6.0 −7.0 −3.4 2059 −6.4 −6.9 −1.9 3632

293 −5.3 −5.9 −6.4

303 −5.3 −5.9 −6.3

313 −5.2 −5.9 −6.3

E16 293 −16.0 −15.2 −12.6

E20 293 −24.7 −22.3 −16.2

TheenthalpyandtheentropychangesfortheadsorptionofPNP onB20andE12inarangeoftemperaturebetween283Kand313K havebeenestimatedbytheGibbs–Helmholtzequation:

G=H−TS (10)

ThevaluesHandShavebeendeterminedfromtheinter- ceptandtheslope,respectively,byplottingGvs.T(Fig.8).The negativevaluesofHconfirmtheexothermicnatureoftheover- allprocess(Table3).ContrarytoG,theabsolutevaluesofH decreasedalittleasthecoveragelevelgrew.Thissituationisfre- quentintheadsorptiononnon-homogeneoussurfaces.Itcanbe explainedbecausetheprocessiseasieronthesurfacesitesatlow coveragelevel,attendingtoenergyreasons.ThefitsfromEq.(10) havedeterminednegativevaluesofSwhichresultfromalower gradeoffreedomofthemoleculesintheadsorbedphasethanin thefluidphase.Inthesecases(H<0;S<0),theprocesscanonly occurwhen|TS|<|H|.Theprocesshastobecomeexothermic enoughtoovertakethedisadvantageduetothenegativechange

-10.5 -10.0 -9.5 -9.0 -8.5

318 312 306 300 294 288 282 -6.4 -6.0 -5.6 -5.2

C_e: 2.0 mmol/L r²=0.833 Ce: 1.2 mmol/L r²=0.952 Ce: 0.8mmol/L r²=0.960

a

C_e: 2.0 mmol/L r²=0.749 C_e: 1.2 mmol/L r²=0.741 C_e: 0.8 mmol/L r²=0.894

ΔG (kJ/mol)

T (K)

b

Fig.8.ThermodynamicestimationsfortheadsorptionofPNPonB20andE12at differentsurfacecoverage:(a)B20;(b)E12.

ofentropy.Ifthetemperaturevalueisenoughhigh,thetermTS cansurpassthetermH,andtheprocessdoesnottakeplace.The highestvaluesofTforthedifferentcasesanalyzedaccordingtoa spontaneousprocesshavebeenreportedasT_(G=0)inTable3.

Nevertheless,thelowaccuracyobtainedfromthesimplicityof Eq.(10)asused,withitslinearregressions,allowthinkingthat theseresultsarenumericalestimationsandrelativetrendsforthe thermodynamicmagnitudesand their dependence onthevari- ables.Oneoftheserelatestotheporousstructureoftheadsorbents, because its accessibilityin energy terms varies as the adsorp- tiontakesplace.Therefore,thevaluescalculatedforG,Hand

Smayrepresenttheapparentbehavioursassociated withthe adsorptionprocessesastheyhavebeenstudied;attheintervalof temperaturesandtheotherexperimentalconditionsset.

3.5. Kineticstudy

3.5.1. Empiricalmodelforthefluidphase

Thesolid–liquidadsorptionprocessesonporousmaterialsmay bedescribedasaseriesofsteps:externalmasstransferfromtheliq- uidphasetothesolidsurfacethroughtheboundarylayer,internal diffusionwithintheporousparticle,andadsorptiononthesurface.

Forthepresentstudy,theresultsfromD40andE12wereselected asexamples,onthebasisoftheirsurfacepropertiesandadsorptive behaviours.

Theamountqt(mmol/g)ofPNPadsorbedontheACwascalcu- latedasthechangeintheaqueousphaseconcentrationfromthe initialvalueby:

qt= Co−Ct

w (11)

whereCt(mmol/L)istheadsorbateconcentrationatacertaincon- tacttimet.AtthesightoftheshapeofthekineticplotsforD40 and E12,asshowedin Fig.9,an empiricalmodel wasusedfor theadsorbateconcentrationdecaycurvesintheaqueousphase.

TheexperimentalresultsforthePNPadsorptionwerefittedtothe followingsecondorderexponentialdecayequation:

Ct=yo+A₁ exp(−B1t)+A₂exp(−B2t) (12) dqt

dt =A₁B₁

w exp(−B1t)+A₂B₂

w exp(−B2t) (13)

whereyo,A1,B1,A2,andB2areempiricalcoefficients.Theirvalues areshown in Fig.9. TheplotattachedtoFig. 9shows theval- uesderivedfromEq.(13).Itisobservedasharpdecreaseinthe adsorptionrateduringthefirstminutesofcontacttime.Thisstep

120 100 80 60 40 20 0 0.51 0.54 0.57 0.60 0.63 0.66 0.69 0.72

Ct (mmol/L)

t (min)

Fig.9.Experimentaldata(dots)andfits(curves)forkineticofPNPadsorptionon D40andE12.Co:0.719mmol/L;T:293K;w:0.100g/L.yo,A1,A2in(mmol/L);B1,B2

in(min⁻¹).

correspondstothesurfacecoverageinthewidestporesandthe mostactivesites.Asecondstepdevelopsfor15minapproximately withanintermediatedecreasingrate.Thisstepisthemostinter- estingtothekineticcontrolforitsrelationtothecoverageinto thenarrowmesoporesandmicropores.Thelastpartdevelopeda negligibleadsorptionratewhichleadstotheequilibriumconcen- trations,soextendedasseveraldaysofcontacttime.Theadsorption ratewashigheronD40thanonE12.Itcanbeexplainedbecauseof thefirst’shighersurfaceareaandporevolume(Table1).

3.5.2. Kineticmodelforthesolidphase

ThekineticdatahavebeenanalyzedbytheLagergrenmodel [26]forordern;itis:

dqt

dt =k(qm−qt)ⁿ (14)

wherekistheadsorptionrateconstantreferredtotheadsorbate concentrationonthesolid,nistheapparentorderoftheprocess relatedtotheavailableadsorbentconcentration(qm−qt),whichis thedrivingforceoftheprocess,thatis,thevacantandaccessible activesitesinthesurfaceoftheadsorbent,foranytimeofcontact.

Inthiswork,qm(mmol/g)representstheasymptoticqtvaluein thefunctioninthegivenexperimentalconditions.However,the selectedequation(withitsparameters evaluatedfor 120minof contacttime)cannotbeusedtopredicttheequilibriumconcentra- tions(dataasfaras8–12days).Thevaluesofqmmaybecalculated as:

qm=Co−yo

w

D40:1.97mmol/g ∴ E12:1.26mmol/g



(15) Inaddition,byintegrationofEq.(14)forn=2,wehave[27]:

t qt = 1

kq²_m+ 1

qmt (16)

Thevaluesofk(g/mmolmin)andqm,canbeobtainedfromthe interceptandtheslopeofthestraightlinesintheplots(t/qtvs.t;

Fig.10).Thevalues(qt,t)werethosesuppliedbytheexperimental fittingsofthedecaymodel.ThelinearregressionsfromEq.(16) wereacceptable,andtheyprovidedqmvaluesveryclosetothose fromEq.(15),whichverifytheapplicabilityofthiskineticequation foranapparentsecondorderintheadsorptionprocessofPNP.

3.5.3. Effectivediffusioncoefficients

A number ofmodels tothink aboutthe mainmass transfer resistances(externalandinternaldiffusion)havebeenpostulated, whichdifferintheirdescriptionofthediffusionprocessintothe pores[28].

Thediffusionprocessinthehomogeneousspheremodelcanbe expoundedbymeansofanequationinvolvingconstantdiffusivity,

120 100 80 60 40 20 0 0 15 30 45 60 75 90 105

t/qt (min g/mmol)

t (min) D40 q_m:1.982 k:0.211 E12 q_m:1.269 k:1.261 linear fits r²:0.999

Fig.10.KineticorderplotsforPNPadsorptiononD40andE12.Co:0.719mmol/L;

T:293K;w:0.100g/L;qmin(mmol/g);kin(g/mmolmin).

thatistosay,asingleparameter,effectiveorapparentthatwewill denominateD_i;whatisrepresentativeofallthepositionsintothe particle.Thisapproachavoidstodifferentiateamongdiffusionin theporespaces(porediffusion),diffusionacrossthesurfaceswhich canoccursubsequenttoadsorption(surfacediffusion),anddiffu- sioninthesolidmaterialitself.Crank[29]developedthefollowing equationforsphericalparticles:

qt

qe =1− 6

²



∞ n=1

1 n² exp



−Din²²t R²_P



(17)

whereRPistheradiusoftheadsorbentparticle.

Thisequationprovidesandexactsolutionforthesituationoften describedas“infinitebath”,whichassumesaconstantsurfacecon- centrationforthesolute,andanegligibleexternalfilmresistance.

Forshorttimes(St),ormoreprecisely:qt/qe<0.3;DibecomesDSt, andEq.(17)maybeapproximatedto:

D_St=



_q

t

q_e



2R²_P

36t (18)

For “longtimes”(i.e.Lt;DLt), thehighest termsin theseries ofEq.(17)becomeextremelysmall,andonlythefirsttermhas prevalence.Wehave

D_Lt= R²_P

²tLn

6/² 1−(qt/qe)



(19)

DuetomathematicalreasonsDLtvaluesonlymaybepositive if(q_t/q_e)>[1−(6/²)].Inthepresentwork,wehaveusedEqs.(18) and(19)todeterminetheD_ivalues(D_Stand/orD_Lt)fromthekinetic

120 100 80 60 40 20 0 1E-12 1E-11 1E-10

D (cm2 /s)

t (min)

Fig.11. Effectivediffusioncoefficients.Variationwiththecontacttimeandwith theloadadsorbed(enclosed).

datashowedinFig.9.Theqtvaluescorrespondingtothediffer- enttimesweresuppliedbythesecondorderexponentialdecay model.Fig.11plotstheeffectivediffusivityvaluesasafunctionof thecontacttimeandtheamountadsorbed.Theequationsdidnot predictconstantvalues.Afterseveralminutesofcontacttime,the apparentdiffusioncoefficientsdecreased,accordingtotheincreas- ingdifficultythatthemoleculesfindtopenetrateintotheporous structures.Multiplecollisionsamongmoleculesandtheporewalls, aswellasthepartialobstructionoftheporemouthscanoccuras thesametime.Thesurfaceareasoftheporesbecomegraduallyless attainabletoadsorption.

4. Conclusions

Activatedcarbonsobtainedfromlignin,atdifferentexperimen- tal conditions, have been successfully employed as adsorbents forquantitativeremoval ofPNP fromaqueoussolution.Chemi- cal and physical characteristics,as surface areas,pore volumes andporesizes, determinetheyieldsand ratesexpectedforthe potentialuseoftheACsasadsorbents.Ithasbeenfoundthatthe activatedcarbonsburnoffinfluencesontheadsorptioncapacity.

Thethermodynamicanalysis indicatedthat thePNP adsorption wasspontaneousandexothermic.Theresultsfromthesorption energyparameters estimationshowedthephysisorptionnature oftheadsorptionprocesses.Thekineticanalysisrevealedadsorp- tion rates with second apparent order related to the available activesitesontheactivatedcarbonssurfaces.Theeffectivediffu- sioncoefficientswerelow,intherangefrom10⁻¹²to10⁻¹⁰cm²/s, decreasingwiththetime,whatsuggeststhattheinternalporosity controlstherateofthemasstransfertowardtheultimateequilib- riumstate.

Acknowledgments

TheauthorsacknowledgetheSpanishDGICYT-MEC(ProjectCTQ 2009-14262)andtheJuntadeAndalucía(TEP-184)forthefinancial support.

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