2. Fundamentals of Adsorption

2FundamentalsofAdsorptionontoActivatedCarboninWaterandWastewaterTreatmentOzgurAktasandFerhanCecen2.1ActivatedCarbonActivatedcarbonsareporouscarbonaceousadsorbents.Alargevarietyoforganicsolutes(Table2.1)canberemovedfromwaterandwastewaterbyadsorptionontoactivatedcarbon,andsomeinorganicsolutescanalsoberemovedbythismeans.Theporoussurfaceofactivatedcarbonadsorbsandretainssolutesandgases,theamountofmaterialadsorbedbeingpotentiallyverylargebecauseofthegreatinternalsurfaceofactivatedcarbon.Activatedcarbonhasahighadsorptivesurfacearea(500–1500m2g�1),whiletheporevolumerangesbetween0.7and1.8cm3g�1.Itismainlyusedintheformofpowderedactivatedcarbon(PAC)orgranularactivatedcarbon(GAC).2.1.1PreparationofActivatedCarbonsCommerciallyavailableactivatedcarbonsarepreparedfrommaterialshavingahighcarboncontentsuchascoal,lignite,wood,peat,nutshell,coconutshell,lignin,petroleumcoke,andsynthetichighpolymers.Themanufacturingprocesscomprisestwophases,carbonizationandactivation.Thecarbonizationprocessincludesdryingandheatingtoremoveundesirablebyproductssuchastarandotherhydrocarbons.Thecarbonaceousmaterialsarethenpyrolyzedandcarbonizedwithinatemperaturerangeof400–6001Cinanoxygendeficientatmosphere.Thisremovesthevolatilelowmolecularweightfractionandcausesthematerialtoundergoanactivationprocess.Activationcanbeachievedthermallybytheuseofoxidationgasessuchassteamatabove8001Corcarbondioxideathighertemperatures.Chemicalactivation,ontheotherhand,involvesimpregnationoftherawmaterialwithchemicalssuchasphosphoricacid,potassiumhydroxide,andzincchloride[1–3].ThetermactivationreferstothedevelopmentoftheActivatedCarbonforWaterandWastewaterTreatment:IntegrationofAdsorptionandBiologicalTreatment.FirstEdition.FerhanC¸ec¸enandOzgurAktasr2011WILEYVCHVerlagGmbHCo.KGaA,Weinheim.Published2011byWILEYVCHVerlagGmbHCo.KGaAc028July2011;14:22:26|13adsorptionpropertiesofcarbon.Microporesareformedduringtheactivationprocess,theyieldofmicroporeformationbeingusuallybelow50%[3].Therawmaterialhasaverylargeinfluenceonthecharacteristicsandperformanceofactivatedcarbonobtained.Rawmaterialssuchascoalandcharcoalhavesomeadsorptioncapacity,butthisisgreatlyenhancedbytheactivationprocess[4].ThecommonmaterialsusedintheproductionofactivatedcarbonandthebasicpropertiesoftheactivatedcarbonsproducedaresummarizedinTable2.2.2.1.2CharacteristicsofActivatedCarbonActivatedcarboniscomposedofmicrocrystallitesthatconsistoffusedhexagonalringsofcarbonatoms,thisstructurebeingquitesimilartothatofgraphite.Thespacesbetweentheindividualmicrocrystallitesarecalledpores.Microporesofactivatedcarbon,wheremostoftheadsorptiontakesplace,areintheformoftwodimensionalspacesbetweentwoparallelcrystallineplaneswithaninterlayerdistanceof3–35A˚.Thediameterofthemicrocrystallitesisroughlyninetimesthewidthofonecarbonhexagon.Functionalgroupsthatterminatethemicrocrystalliteplanesinterconnectthemicrocrystallites.AdsorptionoccursontheTable2.2Basicpropertiesofcommonmaterialsusedinthemanufactureofactivatedcarbon(adaptedfrom[5]).RawmaterialCarbon(wt%)Volatiles(wt%)Ash(wt%)Wood40–4555–600.3–1.1Nutshells40–4555–60Lignite55–7025–405–6Coal65–955–302–15Petroleumcoke70–8515–200.5–0.7Table2.1Classesoforganiccompoundsadsorbedonactivatedcarbon(adaptedfrom[5]).OrganicchemicalclassExamplesAromaticsBenzene,Toluene,Ethylbenzene,XylenePolynuclearAromaticsNaphthalene,Anthracenes,BiphenylsChlorinatedAromaticsChlorobenzene,PolychlorinatedbiphenylsPhenolicsPhenol,Cresol,Chlorophenols,NitrophenolsHighmolecularweighthydrocarbonsGasoline,KeroseneChlorinatedaliphaticsTrichloroethylene,CarbontetrachlorideAliphaticandaromaticacidsTaracids,BenzoicacidsKetones,esters,ethers,andalcoholsHydroquinone,PolyethyleneglycolSurfactantsAlkylbenzenesulfonatesSolubleorganicdyesMethyleneblue,Indigocarmine14|2FundamentalsofAdsorptionontoActivatedCarboninWaterandWastewaterTreatmentc028July2011;14:22:26planarsurfacesofthemicrocrystallitesandatthefunctionalgroupsontheedgesoftheplanes[3,6,7].AdsorptiononthemicrocrystalliteplanespredominantlyresultsfromvanderWaalsforces.Ontheotherhand,adsorptionattheedgesofthemicrocrystalliteoccursbecauseofchemicalbonding[7].Activatedcarbonsurfacesgenerallycontainvariousoxygencomplexes.Thesecomplexesarisefromrawmaterialsaswellasfromchemisorptionofoxygenduringtheactivationprocess.Oxygencomplexesontheactivatedcarbonsurfaceexistmainlyintheformoffourdifferentacidicsurfaceoxides,namelystrongcarboxylic,weakcarboxylic,phenolic,andcarbonylgroups.Also,therearebasicgroupssuchascyclicethers.Activationathighertemperaturesresultsinabasicsurface.Thepresenceofsurfaceoxidesaddsapolarnaturetoactivatedcarbons.Thermaltreatmentofcarbonsinaninertatmosphereoravacuumcanremovethesesurfaceoxidegroups[3].Surfacefunctionalgroupsplayanimportantroleintheadsorptionofvariousorganicmolecules.Forexample,aromaticcompoundscanbeadsorbedatthecarbonyloxygensonthecarbonsurfaceaccordingtoadonor–acceptorcomplexationmechanism.Thecarbonyloxygenactsastheelectrondonor,whilethearomaticringofthesoluteactsastheelectronacceptor.Adsorptionalsooccursbyhydrogenbondingofthephenolicprotonswithsurfacefunctionalgroupsandbycomplexationwiththeringsofthemicrocrystalliteplanes[7].Activatedcarbonmaycontainlargequantitiesofminerals,mostlycalcium,sulfate,andphosphateions.Thesegroupsaswellastheacidicorbasicorganicsurfacefunctionalgroupsinfluencetheactivatedcarbonsurfaceproperties[8].TheimportanceofsurfacechemistryonadsorptionanditsreversibilityinintegratedadsorptionandbiologicalprocessesarediscussedinChapter7.Activatedcarbonalsocontainssomeashderivedfromtherawmaterial,theamountofashrangingfrom1%to12%.Theashmainlyconsistsofsilica,alumina,ironoxides,andalkalineandalkalineearthmetals.Theashintheactivatedcarbonincreasesitshydrophilicity.ThisisadvantageouswhenPACisusedforwatertreatmentbecausePACdoesnotstickonthereactorwallsiftheashcontentishigh[3].2.1.3ActivatedCarbonTypes2.1.3.1PowderedActivatedCarbon(PAC)PACismadeupofcrushedorgroundcarbonparticlessuchthat95–100%ofitwillpassthroughadesignatedsieveof0.297mmaccordingtotheAmericanWaterWorksAssociationStandard,or0.177mmaccordingtoASTMD5158[5].PACisgenerallyproducedfromwoodintheformofsawdust,theaverageparticlesizeofPACbeingintherangeof15–25mm.PACfindswideapplicationinthetreatmentofbothdrinkingwaterandwastewater.Inwastewatertreatment,itiseitheraddedtoactivatedsludgeoriscontactedwithwastewaterinaseparateunit.PACmayalsoactasacoagulantforcolloidalfractionsintheliquidphase.TheregenerationofPACcanberatherdifficult,becausecolloidalparticleshavetobeseparatedfromwaterbeforeregeneration[3].2.1ActivatedCarbon|15c028July2011;14:22:262.1.3.2GranularActivatedCarbon(GAC)GACisusuallyintheformofcrushedgranulesofcoalorshell.GACmayalsobepreparedbygranulationofpulverizedpowdersusingbinderssuchascoaltarpitch[3].UnlikePACadsorptionfromtheliquidphase,theratelimitingstepinGACadsorptionfromtheliquidphaseoftenbecomestheintraparticlediffusion(Section2.2.3).GACparticleshavesizesrangingfrom0.2to5mm.GACisdesignatedbymeshsizessuchas820,2040,or830forliquidphaseapplicationsand46,48,or410forvaporphaseapplications[5].Particlesizesintherangeof1242meshareadvantageousforliquidphaseadsorption[3].GACfilters,asshowninSection2.3.2,arewidelyusedinpurificationprocessesfordrinkingwater,groundwaterandwastewaterasanadvancedtreatmentstep,particularlyfortheremovaloftoxicorganiccompounds.InsomeGACapplicationsindrinkingwaterandwastewatertreatment,amicrobiologicalfilmcanformontheparticles.Thereby,biologicalremovalofpollutantsiscombinedwithGACadsorption.Theresultingmaterial,calledbiologicalactivatedcarbon(BAC),isextensivelydiscussedinlaterchapters.2.2AdsorptionAdsorptionisconsideredtobeanimportantphenomenoninmostnaturalphysical,biological,andchemicalprocesses,andactivatedcarbonisthemostwidelyusedadsorbentmaterialinwaterandwastewatertreatment.Adsorptionistheaccumulationorconcentrationofsubstancesatasurfaceorinterface.Theadsorbingphaseistermedtheadsorbent,andthematerialbeingadsorbedtheadsorbate.Adsorptioncanoccurbetweentwophases,namelyliquid–liquid,gas–liquid,gas–solid,orliquid–solidinterfaces.Whenactivatedcarbonisused,theadsorbingphaseisasolid.Inactivatedcarbonadsorption,ahypotheticalinterfaciallayerexistsbetweenthesolidandfluidphases.Thislayerconsistsoftworegions:thatpartofthefluid(gasorliquid)residingintheforcefieldofthesolidsurfaceandthesurfacelayeroftheactivatedcarbon.Adsorptivesurfacereactionsoccurasaresultoftheactiveforceswithinthephaseorsurfaceboundaries.Theseforcesresultincharacteristicboundaryenergies.Thesurfacetensiondevelopedatthesurfaceoftheliquidphaseresultsfromtheattractiveforcesbetweenthemoleculesofliquid.Theliquid(solvent)moleculeshaveasmallerattractiveforceforthesolutemoleculesthanforeachother.Hence,asolutethatdecreasessurfacetensionisconcentratedatthesurface.Thephenomenonofincreasedconcentrationofthesolublematerialinaboundaryorsurfaceiscommonlyreferredtoasadsorption[9].Theterm‘adsorption’referstotheprocessinwhichmoleculesaccumulateintheinterfaciallayer,whereasdesorptiondenotesthereverse.2.2.1TypesofAdsorptionInmosttypesofwaterandwastewatertreatmentpractice,twoprimarydrivingforcesresultintheadsorptionofasolutefromasolutionontoasolidphase.16|2FundamentalsofAdsorptionontoActivatedCarboninWaterandWastewaterTreatmentc028July2011;14:22:26Thefirstdrivingforceisrelatedtothelyophobic(solventdisliking)characterofthesolute.Themostimportantfactorfortheintensityofadsorptionisthesolubilityofadissolvedsubstance.Ahydrophilicsubstancelikesthewatersystemandtendstostaythere.Hence,itislessadsorbableonasolidphase.Contrarily,ahydrophobicsubstancetendstobeadsorbedratherthanstayinginwater.Complexorganiccontaminants,suchashumicacidshavebothhydrophobicandhydrophilicgroups,sothatthehydrophobicpartofthemoleculeisadsorbed,whereasthehydrophilicparttendstostayinthesolution.Theseconddrivingforceforadsorptionistheaffinityofthesoluteforthesolidduetoelectricalattractionofthesolutetothesolid.ThistypeofadsorptioncanoccurasaresultofvanderWaalsattractionorchemicalinteractionwiththeadsorbent.Theseforcesresultduetothehighaffinityofthesolutefortheparticularsolid[9].TheadsorptioninducedbyvanderWaalsforcesisgenerallytermedphysicaladsorptionorphysisorption,inwhichintermolecularattractionstakeplacebetweenfavorableenergysites.Physisorptionisindependentoftheelectronicpropertiesoftheadsorbateandadsorbentmolecules.Exchangeofelectronsdoesnotoccurinphysisorption.TheadsorbateisattachedtothesurfacebyrelativelyweakvanderWaalsforces.Inphysisorption,multiplelayersmaybeformedwhichhavesimilarheatsofadsorption.Physicaladsorptionispredominantattemperaturesbelow1501C,andischaracterizedbyarelativelylowadsorptionenergy–atmostafewkcalmol�1.Asarule,itisareversibleprocessthatoccursatalowertemperatureclosetothecriticaltemperatureofanadsorbedsubstance.Theadsorbateislessstronglyattachedtoaspecificsiteinphysisorptioncomparedtochemicaladsorption[5].Therefore,physicallyadsorbedmoleculeisfreetomovewithintheinterface[9].Inchemicaladsorption,orchemisorption,theadsorbateundergoeschemicalinteractionwiththeadsorbent.Chemisorptioninvolvesanexchangeofelectronsbetweenspecificsurfacesitesandsolutemolecules,achemicalbondbeingformed.Chemicallyadsorbedadsorbatesarenotfreetomoveonthesurfaceorwithintheinterface.Chemicaladsorptionischaracterizedbyahighadsorptionenergy(tensofkcalmol�1),sincetheadsorbateformsstronglocalizedbondsatactivecentersontheadsorbent.Chemicaladsorptionismorepredominantathightemperaturescomparedtophysisorption,becausechemicalreactionsproceedmorerapidlyathighertemperatures.Generally,onlyasinglemolecularlayercanbechemicallyadsorbed[5,9].Table2.3givesacomparisonofphysicalandchemicaladsorption.Thesetwoformsofadsorptioninteracttogetherinadsorption.Adsorptionoforganicmoleculesexhibitalargerangeofbindingenergies.Lowerenergiesareusuallyassociatedwithphysisorption,whereashigherenergiesareassociatedwithchemisorption.However,itisgenerallyverydifficulttodistinguishbetweenphysisorptionandchemisorption[9].ThereversibilityofadsorptionandbioregenerationofactivatedcarboninintegratedadsorptionandbiologicaltreatmentisdiscussedinChapter7,andtheparticularimportanceofthetypeofadsorption(physicalorchemical)isemphasized.2.2Adsorption|17c028July2011;14:22:262.2.2FactorsInfluencingAdsorptionActivatedcarbonadsorptionisnotauniquelyhomogeneousprocess,butisratherdependentonthevariousfactorsoutlinedbelow.Thesefactorsalsoplayasignificantroleintheintegrationofadsorptionwithbiologicalprocesses(Chapters3–11).2.2.2.1SurfaceAreaofAdsorbentTheextentofadsorptionisgenerallyconsideredtobeproportionaltothespecificsurfacearea.Specificsurfaceareaisthatproportionofthetotalsurfaceareawhichisavailableforadsorption.Themorefinelydividedandmoreporousadsorbentswouldbeexpectedtoyieldmoreadsorptionperunitweightofadsorbent.Thesurfacecanbecharacterizedeitherasexternalwhenitinvolvesbulgesorcavitieswithwidthgreaterthandepthorinternalwhenitinvolvesporesandcavitiesthathavedepthgreaterthanwidth.2.2.2.2PhysicalandChemicalCharacteristicsoftheAdsorbateIngeneral,theadsorbabilityofacompoundincreaseswithincreasingmolecularweightandincreasingnumberoffunctionalgroupssuchasdoublebondsorhalogens[4].Largermoleculesareadsorbedontoactivatedcarbonbetterthansmallermolecules.Thedegreeofsolubilityofthesoluteisalsoofprimaryconcernforadsorption.Thereisaninverserelationshipbetweentheextentofadsorptionofaparticularsoluteanditssolubilityinthesolventfromwhichtheadsorptionoccurs.Table2.3Comparisonofphysicalandchemicaladsorption.PhysisorptionChemisorptionCoverageMonoormultilayerMonolayerNatureofadsorptionNondissociativeandreversibleOftendissociative,maybeirreversibleSpecifitytoadsorptionsitesNonspecificVeryspecificTemperaturerangeNearorbelowthecondensationpointofthegasunlimitedTemperaturedependenceofuptake(withincreasingT)DecreasesIncreasesAdsorptionenthalpy5–40kJmol�140–800kJmol�1KineticsofadsorptionFastVeryvariable,oftenslowDesorptionEasybyreducedpressureorincreasedtemperatureDifficult–hightemperatureisrequiredtobreakbondsDesorbedspeciesAdsorbateunchangedAdsorbatemaychange18|2FundamentalsofAdsorptionontoActivatedCarboninWaterandWastewaterTreatmentc028July2011;14:22:27Highsolubilitymeansthatthesolute–solventbondsarestrongerthantheattractiveforcesbetweenthesoluteandtheadsorbent.Polarityoftheadsorbateisanotherimportantfactor.Apolarsoluteispreferablyadsorbedbyapolaradsorbent,whereasanonpolarsoluteismoreeasilyadsorbedbyanonpolaradsorbent.Activatedcarbonadsorbsnonpolarmoleculesbetterthanpolarmolecules.Anincreaseinsolubilityreflectsagreateraffinitybetweenthesoluteandthesolvent,andactstoopposetheattractionexertedbythecarbon.Consequently,anychangethatincreasessolubilitymaybeassociatedwithreducedadsorbability.Thus,polargroups(characterizedbyanaffinityforwater)usuallydiminishadsorptionfromaqueoussolutions.Dissociationconstantsofweakacidsandbasesalsoinfluencetheextentoftheiradsorption.Thereasonisthattheextentofionizationisdeterminedbythedissociationconstant.Theadsorptionofionicandmolecularformsdiffersuchthatthelatteraremuchbetteradsorbedcomparedtotheformer.Greaterdissociationconstantresultsinhigheramountoftheionicformwhichisusuallynotadsorbableonactivatedcarbon.Thepresenceofsubstituentgroupsalsoaffectstheadsorbabilityoforganicchemicals(Table2.4).Theintroductionofasecondorthirdsubstituentoftenextendstheinfluenceofthefirstsubstituent.Theinfluenceofasubstituentgroupdependsonthepositionoccupied,forexample,ortho,meta,andpara[10].Branchedchainsareusuallymoreadsorbablethanstraightchains.Ontheotherhand,anincreasinglengthofchainresultsinanincreaseinadsorptioncapacity[11].Spatialarrangementsofatomsandgroupsinamoleculeinfluenceadsorbability.Aromaticcompoundsareingeneralmoreadsorbablethanaliphaticcompoundsofsimilarmolecularsize.2.2.2.3pHOrganicmoleculesformnegativeionsathighpHvalues,positiveionsatlowpHvalues,andneutralmoleculesatintermediatepHvalues.Adsorptionofmostorganicmaterialsishigheratneutralconditions.Ingeneral,liquidphaseadsorptionoforganicpollutantsbyactivatedcarbonisincreasedwithdecreasingTable2.4Influenceofsubstituentgroupsonadsorbabilityoforganiccompounds(adaptedfrom[10]).SubstituentgroupInfluenceHydroxylGenerallyreducesadsorbabilityAminoGenerallyreducesadsorbabilityCarbonylVariableeffectdependingonhostmoleculeDoublebondsVariableeffectdependingonhostmoleculeHalogensVariableeffectdependingonhostmoleculeSulfonicGenerallyreducesadsorbabilityNitroGenerallyincreasesadsorbability2.2Adsorption|19c028July2011;14:22:27pH.ThisresultsfromtheneutralizationofnegativechargesatthesurfaceofthecarbonatlowpHvalues.Neutralizationofnegativechargesreducesthehindrancetodiffusionandleadstomoreactiveadsorptionsites.Theextentofthiseffectvarieswiththetypeandactivationtechniqueofactivatedcarbon.ThedifferencesinpHvaluesmayalsoariseduetoacidicorbasicsurfacefunctionalgroupsonactivatedcarbon.Thesegroupscouldbefreedbysimplecontactwithdistilledwaterratherthanthefixedsurfacefunctionalgroups.Aninverserelationshiphaspreviouslybeenreportedbetweenadsorptioncapacityandsurfaceacidity[12,13].2.2.2.4TemperatureAdsorptioninvolvesspecificrelationsbetweenthepropertiesofactivatedcarbonandthesolute.Therefore,thequantitativeeffectsoftemperaturearenotthesamewithallcarbonsandsolutes.Theextentofadsorptionshouldincreasewithdecreasingtemperaturebecausetheadsorptionreactionsareexothermic.However,increasedtemperaturealsoincreasestherateofdiffusionofthesolutethroughtheliquidtotheadsorptionsites,whicheventuallyleadstoanincreasedadsorption.Animportantdifferenceintheadsorptionofsolutesversusgasesisfoundintheroleoftemperature.Anincreaseintemperatureincreasesthetendencyofagastoescapefromtheinterfaceandthusdiminishesadsorption.However,inadsorptionfromtheliquid,theinfluenceoftemperatureonsolventaffinitiesismoredominant[10].2.2.2.5PorosityoftheAdsorbentTheadsorptionperformanceisdependentontheconditionofinternalsurfaceaccessibility.Averyimportantanddecisivepropertyofadsorbentmaterialsistheporestructure.Thetotalnumberofporesandtheirshapeandsizedeterminetheadsorptioncapacityandeventherateofadsorption.Thesignificanceofporesinadsorptionprocesseslargelydependsontheirsizes.Mostofthesolidadsorbentspossessacomplexstructurethatconsistsofporesofdifferentsizesandshapes.Totalporosityisusuallyclassifiedintothreegroups.AccordingtotheIUPACrecommendation,microporesaredefinedasporesofawidthnotexceeding2nm,mesoporesareporesofawidthbetween2and50nm,andmacroporesareporesofawidthgreaterthan50nm.Theaboveclassificationiswidelyacceptedinadsorptionliterature.Afurtherclassificationinvolvesultramicropores,whichareporesofawidthlessthan0.7nm[5].Themechanismofadsorptiononthesurfaceofmacroporesdoesnotdifferfromthatonflatsurfaces.Thespecificsurfaceareaofmacroporousadsorbentsisverysmall.Therefore,adsorptiononthissurfaceisusuallyneglected.Capillaryadsorbatecondensationdoesnotoccurinmacropores[14].Macroporesdonotadsorbsmallmoleculesbyvolumebutbysurface.Forexample,phenoladsorptiononmacroporeswasreportedtobelesssignificantthanthatonmesoandmicropores[15].Inthecaseofmesopores,theadsorbentsurfaceareahasadistinctphysicalmeaning.Monoandmultilayeradsorptiontakesplacesuccessivelyonthesurfaceofmesopores,andadsorptionproceedsaccordingtothemechanismofcapillary20|2FundamentalsofAdsorptionontoActivatedCarboninWaterandWastewaterTreatmentc028July2011;14:22:27adsorbatecondensation.Specificsurfacearea,porevolume,andporesizedistributionarebasicparameterscharacterizingmesopores.Mesoporesandmacroporesplayanessentialroleinthetransportofadsorbatemoleculesthroughthemicropores[14].Thesizesofmicroporesareusuallycomparabletothoseofadsorbatemolecules.Allatomsormoleculesoftheadsorbentcaninteractwiththeadsorbatespecies.Thisisthefundamentaldifferencebetweenadsorptioninmicroporesandlargerporeslikemesoandmacropores.Consequently,adsorptioninmicroporesisessentiallyaporefillingprocess.Therefore,volumeisthemaincontrollingparameterforadsorptioninmicropores[14].Consideringalsothatmostofthetotalsurfaceareaisfoundinthemicroporesandthecontributionofmacroporestothetotalsurfaceareaisverylow,thecontributionofmicroporestoadsorptionwouldbeexpectedtobehigher.Also,thehighersorptionenergyinmicroporesmakesthemmoreavailableadsorptionsitesforrelativelylow