廉价重金属吸附材料综述

Low-costadsorbentsforheavymetalsuptakefrom

contaminatedwater:areview

SandhyaBabel∗,TonniAgustionoKurniawan

EnvironmentalTechnologyProgram,SirindhornInternationalInstituteofTechnology(SIIT),

ThammasatUniversity,P.O.Box22,Pathumthani12121,ThailandReceived28June2002;receivedinrevisedform19September2002;accepted23September2002

Abstract

Inthisarticle,thetechnicalfeasibilityofvariouslow-costadsorbentsforheavymetalremovalfromcontaminatedwaterhasbeenreviewed.Insteadofusingcommercialactivatedcarbon,re-searchershaveworkedoninexpensivematerials,suchaschitosan,zeolites,andotheradsorbents,whichhavehighadsorptioncapacityandarelocallyavailable.Theresultsoftheirremovalperfor-mancearecomparedtothatofactivatedcarbonandarepresentedinthisstudy.Itisevidentfromourliteraturesurveyofabout100papersthatlow-costadsorbentshavedemonstratedoutstandingremovalcapabilitiesforcertainmetalionsascomparedtoactivatedcarbon.Adsorbentsthatstandoutforhighadsorptioncapacitiesarechitosan(815,273,250mg/gofHg2+,Cr6+,andCd2+,re-spectively),zeolites(175and137mg/gofPb2+andCd2+,respectively),wasteslurry(1030,560,540mg/gofPb2+,Hg2+,andCr6+,respectively),andlignin(1865mg/gofPb2+).Theseadsor-bentsaresuitableforinorganiceffluenttreatmentcontainingthemetalionsmentionedpreviously.Itisimportanttonotethattheadsorptioncapacitiesoftheadsorbentspresentedinthispapervary,dependingonthecharacteristicsoftheindividualadsorbent,theextentofchemicalmodifications,andtheconcentrationofadsorbate.

©2002ElsevierScienceB.V.Allrightsreserved.

Keywords:Low-costadsorbents;Activatedcarbon;Heavymetalremoval;Contaminatedwater;Wastewatertreatment

1.Introduction

Sinceitsfirstintroductionforheavymetalremoval,activatedcarbonhasundoubtedlybeenthemostpopularandwidelyusedadsorbentinwastewatertreatmentapplicationsthroughouttheworld.Inspiteofitsprolificuse,activatedcarbonremainsanexpensive

Correspondingauthor.Tel.:+66-2986-9009x2307;fax:+66-2986-9112-3.E-mailaddress:sandhya@siit.tu.ac.th(S.Babel).

0304-34/02/$–seefrontmatter©2002ElsevierScienceB.V.Allrightsreserved.PII:S0304-34(02)00263-7

∗

220S.Babel,T.A.Kurniawan/JournalofHazardousMaterialsB97(2003)219–243

materialsincehigherthequalityofactivatedcarbon,thegreateritscost.Activatedcarbonalsorequirescomplexingagentstoimproveitsremovalperformanceforinorganicmatters.Therefore,thissituationmakesitnolongerattractivetobewidelyusedinsmall-scaleindustriesbecauseofcostinefficiency.

Duetotheproblemsmentionedpreviously,researchinterestintotheproductionofal-ternativeadsorbentstoreplacethecostlyactivatedcarbonhasintensifiedinrecentyears.Attentionhasbeenfocusedonthevariousadsorbents,whichhavemetal-bindingcapacitiesandareabletoremoveunwantedheavymetalsfromcontaminatedwateratlowcost.Be-causeoftheirlowcostandlocalavailability,naturalmaterialssuchaschitosan,zeolites,clay,orcertainwasteproductsfromindustrialoperationssuchasflyash,coal,andoxidesareclassifiedaslow-costadsorbents.

Chitosanhasreceivedconsiderableinterestforheavymetalsremovalduetoitsexcellentmetal-bindingcapacitiesandlowcostascomparedtoactivatedcarbon.InAsiancountriessuchasThailand,Japan,andChina,fisherywastessuchasshrimp,lobster,andcrabshellshavebeendevelopedintooneofthepromisingoptionstoproducechitosan.Thesewastescouldbeobtainedforfreefromlocalfisheryindustries.Sincesuchwastesareabundantlyavailable,chitosanmaybeproducedatlowcost.Consequently,chitosanoffersalotofpromisingbenefitsforwastewatertreatmentapplicationstoday.

Naturalzeolitesalsogainedasignificantinterestamongscientist,mainlyduetotheirvaluablepropertiessuchasionexchangecapability.LargedepositsofnaturalzeolitesinmanycountriessuchasGreece,UK,Italy,Mexico,Iran,andJordan,providelocalindustriessomepromisingbenefitssuchascostefficiencysincetheyareabletotreatwastewatercontaminatedwithheavymetalatlowcost.

Clayisoneofpotentialalternativestoactivatedcarbonaswell.Similartozeolites,claymineralsarealsoimportantinorganiccomponentsinsoil.Theirsorptioncapabilitiescomefromtheirhighsurfaceareaandexchangecapacities.Thenegativechargeonthestructureofclaymineralsgivesclaythecapabilitytoattractmetalions.TheUSAandtheformerRepublicsofSovietUnionsuchasLithuania,Georgia,andKazakhstanarewellknownfortheirlargedepositsofnaturalclayminerals.

Industrialwasteisalsooneofthepotentiallylow-costadsorbentforheavymetalremoval.Itrequireslittleprocessingtoincreaseitssorptivecapacity.Generallyindustrialwastesaregeneratedasby-products.Sincethesematerialsarelocallyavailableinlargequantities,theyareinexpensive.InIndia,varioustypesofindustrialwastessuchaswasteslurry,lignin,iron(III)hydroxide,andredmud,havebeenexploredfortheirtechnicalfeasibilitytoremoveheavymetalsfromcontaminatedwater.

Lowrankcoal,suchaslignite,iscapableofhavingionexchangewithheavymetalsduetoitscarboxylicacidandphenolichydroxylfunctionalgroups.Thesematerialsexistaslargedepositsinmanycountries,notablyAustraliaandIndia.

Otherlow-costadsorbents,suchasagriculturalwastes,havebeenstudiedlessextensivelyduetotheirlocalavailability.Althoughmanyresearchworkshavebeendonerecentlytofindthepotentialofusingvariousalternativeadsorbents,sofarnoeffortshavebeenmadetoobtainacomparativeoverviewofalladsorbentsmentionedpreviouslyintermsoftheirremovalperformance,adsorptioncapacity,andcosteffectiveness.

Anoverviewofsomelow-costadsorbentsbasedonrecentpublicationsispresentedinthispaperandtheirremovalperformanceiscompared.Adsorbentsthatstandoutfor

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highremovalefficienciesandadsorptioncapacitiesarecomparedwiththeactivatedcarbon.

2.Low-costadsorbentsandremovalofheavymetals2.1.Chitosan

Amongvariousbiosorbents,chitinisthesecondmostabundantnaturalbiopolymersaftercellulose.However,moreimportantthanchitinischitosan,whichhasamolecularstruc-turesimilartocellulose.Presently,chitosanisattractinganincreasingamountofresearchinterest,asitisaneffectivescavengerforheavymetals.

ChitosanisproducedbyalkalineN-deacetylationofchitin,whichiswidelyfoundintheexoskeletonofshellfishandcrustaceans.ItwasestimatedthatchitosancouldbeproducedfromfishandcrustaceansatamarketpriceofUS$15.43/kg[1].Thegrowingneedfornewsourcesoflow-costadsorbent,theincreasedproblemsofwastedisposal,theincreasingcostofsyntheticresinsundoubtedlymakechitosanoneofthemostattractivematerialsforwastewatertreatment.

Variousresearchesonchitosanhavebeendoneinrecentyears.In1988,theutilizationofchitosanforcadmiumremovalwasintensivelyinvestigated[2].Itwasdemonstratedthatanadsorptioncapacityof5.93mgofCd2+/gofchitosanwasachievedatapHrangeof4.0–8.3andthatthepresenceofethylenediaminetetraaceticacid(EDTA)significantlydecreasedthecadmiumremovalbychitosansinceEDTA,astrongerchelatingagentthanchitosan,suppressedthemetaluptakebychitosan.ItwasalsoreportedthatinthepresenceofEDTA,theaffinityofCd2+fortheaminogroupswasdrasticallyreducedsincetheEDTAmaskedthepresenceofCd2+inaqueoussolution,causingtheirremovalfromthesolutiontobecomedifficult.

Anothersimilarresearchevaluatedthesorptionofsomemetalionsontochitosan[3].ItwasfoundthatthemaximumadsorptioncapacitiesofchitosanforHg2+,Cu2+,Ni2+,andZn2+were815,222,1,and75mg/g,respectively.However,theresultofmercuryremovalwasdifferentfromthatobtainedinthelatterstudy[4],whichindicatedthatanadsorptioncapacityof430mgofHg2+/gwasachievedbychitosan.Thisdifferenceoccursduetothefactthatthelatterstudyusedchitosan,withparticlesizerangingfrom1.25to2.5mm(against0.21–1mmintheformerstudy[3]).Suchreductioninparticlesizeofchitosan,ofcourse,increasesitssurfaceareatobeadsorbedbythemetalionsanditresultsinhigherremovalefficiencyofheavymetal.Therefore,theextentofsurfaceoccupancybyadsorbatehasadecisiveinfluenceupontheremovalefficiencyofchitosan.

Theinteractionbetweenchitosanandhexavalentchromiumwasintensivelyinvestigated[5].Itwasobservedthatanadsorptioncapacityof273mgofCr6+/gchitosanwasachievedatpHof4.0.In1996,acomparativestudyontheadsorptioncapacityofchitosanforvariousmetalionssuchCu2+,Cd2+,Ni2+,Pb2+,andHg2+wasconducted[6].ItwasfoundthatchitosanexhibitsthehighestbindingcapacityforHg2+(Table1).

Theadsorptionofcopperonchitosanwasalsostudiedanditwasfoundthat1gchitosancouldadsorb4.7mgofCu2+atpHof6.2[7].Thisresultissignificantlydifferentfromthatobtainedinapreviousstudy[8].ItwasreportedthatpHof5.5wasfoundtobe

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Table1

Adsorptioncapacities(mg/g)ofchitosanforvariousheavymetalsMaterialChitosan

Sources[2][3][4][5][6][7][8][11][12][12][10][13][13][13]

Cr6+

Ni2+

Pb2+

Hg2+815430

273

2.40

16.36

51.55

16.804.7013.008586

280

606246

8.54

Zn2+75

Cu2+222

Cd2+5.93

Pt6+

Chitosanbeads

Non-crosslinkedchitosanCrosslinkedchitosanCrosslinkedchitosanwithGDECEGDE

250.00

8050

optimumforcopperremovalandabout13mgofCu2+couldbeadsorbedby1gchitosanatequilibriumcondition.Thedifferenceinchitosanadsorptioncapacitybetweenthetwostudiescouldbeduetothefactthatabiggerparticlesizeofchitosan(200mesh)wasusedintheformerstudy[7](against50meshinthelatterstudy[8]).Suchincreaseinparticlesizeofchitosan,ofcourse,decreasessurfaceareaavailableforadsorption,resultinginloweradsorption.

Chemicalmodificationsofchitosanwerealsoconductedtoimproveitsremovalperfor-manceandadsorptioncapacityformetalions.In1994,itwasevaluatedwhetherchemicalmodificationofchitosanpromotesselectivityinvanadiumsorptionornot[9].Itwasreportedthatchitosananditsoxo-2-glutaricacidsubstituteformareeffectivetoadsorb450mgofvanadium/gandthatthesorptionpreferentiallyfollowtheFreundlichisotherm.ItwasalsofoundthatpHof3.0wasfoundtobeoptimumforvanadiumuptake.

Theremovalofplatinumusingchitosan-derivedsorbentswasalsocarriedout[10].Itwasfoundthatglutaraldehyde-crosslinkedchitosanwasveryeffectiveforremovingPt6+andthatitsadsorptioncapacitywasabout280mgofPt6+/g.ItwasalsoreportedthattheoptimumpHforPt6+sorptionisaround2.0.

In1998,thecrosslinkingeffectofglutaricaldehydeontheremovalofCd2+usingchitosanbeadswasdemonstrated[11].Itwasreportedthattheadsorptioncapacityofthecrosslinkedgelbeadsexponentiallydecreased60%from250mgofCd2+/g.

Thecrosslinkingeffectsofchitosanwerealsoinvestigated[12].Itwasfoundthatnon-crosslinkedchitosanhaspotentialtoadsorb30mgmoreofCr6+/gofchitosan.Thisisconsistentwiththefactthatcross-linkingreducestheadsorptioncapacitiesofchitosan,butthislossofcapacitymaybenecessarytoensurethestabilityofchitosan.

ThelatestsimilarexperimentalworkdemonstratedtheadsorptionofCu2+ontochitosancrosslinkedwithglutaraldehyde(GD),epichlorohydrin(EC),andethyleneglycoldiglycidylether(EGDE)[13].ItwaspointedoutthatpHof6.0wasfoundtobeoptimumforCu2+

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removalandthattheuptakeofCu2+onchitosancrosslinkedwithGD,EC,andEGDEbeadswere59.67,62.47,and45.94mg/g,respectively.

Overall,theresultsmentionedpreviouslyindicatethatchitosanisagoodadsorbentforallheavymetals.Itiswidelyknownthattheexcellentadsorptionbehaviorsofchitosanforheavymetalremovalisattributedto:(1)highhydrophilicityofchitosanduetolargenumberofhydroxylgroups,(2)largenumberofprimaryaminogroupswithhighactivity,and(3)flexiblestructureofpolymerchainofchitosanmakingsuitableconfigurationforadsorptionofmetalions.2.2.Zeolites

Basicallyzeolitesareanaturallyoccurringcrystallinealuminosilicatesconsistingofaframeworkoftetrahedralmolecules,linkedwitheachotherbysharedoxygenatoms.During1970s,naturalzeolitesgainedasignificantinterestamongscientistsduetotheirion-exchangecapabilitytopreferentiallyremoveunwantedheavymetalssuchasstrontiumandcesium[14].Thisuniquepropertymakeszeolitesfavorableforwastewatertreatment.ThepriceofzeolitesitselfisconsideredverycheapaboutUS$0.03–0.12/kg,dependingonthequalityofthezeolitesitself[15].

Zeolitesconsistofawidevarietyofspeciessuchasclinoptiloliteandchabazite.Clinop-tiloliteismostabundantinnatureandisreadilyavailablefrommorethan40naturalze-olitesspecies[16].Amongthemostfrequentlystudiednaturalzeolites,clinoptilolitewasshowntohavehighselectivityforcertainheavymetalionssuchasPb2+,Cd2+,Zn2+,andCu2+.

In1990,theremovalofheavymetalsfromwastewaterusingclinoptilolitewasstudied[17].Theresultsindicatedthattheionexchangeloadingvaluescouldrangefrom1.6mg/gforPb2+to0mg/gforCr3+.Theselectivityoftheseriesoftheheavymetalsstudiedwasdeterminedtobeasfollows:

Pb2+>Cd2+>Cu2+>Co2+>Cr3+>Zn2+>Ni2+>Hg2+

Researchhasbeenconductedtoshowtheeffectivenessofclinoptilolitetoremoveleadandcadmiumaswell[18].ItwasindicatedthatclinoptiloliteismoreselectiveforPb2+,butCd2+isalsoexchangedatsatisfactorylevel.Approximately1.4mg/gofPb2+and1.2mg/gofCd2+wereremoved.Concerningtheeffectoftemperatureontheadsorptionprocess,itwasfurthermentionedthatthemetalsuptakeisfavoredathighertemperature[19]sinceahighertemperatureactivatesthemetalionsforenhancingadsorptionatthecoordinationsiteofzeolites(Table2).

In1992,afurtherinvestigationontheuseofcarbonizedzeolitesforremovingleadfromwastewaterwasalsoconducted[20].Thestrategybehindcreatingcarbonizedzeolitesistocombinelyophilicandlyophobicsurfacesthatcanbindwithorganicandinorganicsubstances,whicharefoundinwastestreams.Itwasreportedthatthecarbonizedzeolitesremovedabout99%of260ppmleadsolution.

Theinfluenceofpretreatmentuponthecationexchangecapacity(CEC)andselectivityofzeolitesformetalionswasinvestigatedbyanumberofresearchers.ItwasdemonstratedthattheCECofclinoptilolitedependsonthepretreatmentmethodandthatcondition-ingimprovesitsionexchangeabilityandremovalefficiency[21].Theirfindingswerein

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Table2

Metaluptakebyclinoptiloliteatdifferenttemperaturesandparticlesize[19]Particlesizeofclinoptilolite(␮m)

Pb2+a25◦C

<600

160–600600–10001000–2000

a

Cd2+a

50◦C1.41–1.311.29

25◦C1.061.021.010.98

50◦C1.20–1.121.08

1.311.291.281.27

Pb2+andCd2+takenatequilibrium(meq/g).

agreementwith[22–24].ItwasalsoreportedthatconditioningofzeoliteswithNaOHsolutionimprovedremovalefficiency.Chabaziteandclinoptilolitetreatedwithsodiumhy-droxideperformedbestwithPb2+andCd2+exchangecapacityexceeding100mg/gandtheperformanceofchabazite’sCECwasindicatedtobesuperiortothatofclinoptiloliteforboththeions.

Inafurtherstudy,theremovalperformanceofclinoptiloliteandchabazitewascompared[25].ThetwozeoliteswereevaluatedwithrespecttotheirperformancefortreatingeffluentscontaminatedwithPb2+,Cd2+,Cu2+,Zn2+,Ni2+,andCo2+.Itwasreportedthatboththezeolitesexhibited100%removalefficiencyatthemetalconcentrationof10mg/l.ItwasalsofoundthatclinoptiloliteandchabaziteexhibiteddifferentselectivityforallmetalsstudiedexceptPb2+,forwhichbothperformedexceptionallywell.Finally,itwasconcludedthatthesuperiorityofchabazite’sCECwasmainlyduetothefactthatchabazitehasahigherAlsubstitutionofSithanclinoptilolite.Thisprovideschabaziteanegativeframeworkfavorableforhigherexchangecapability.

TheinteractionsofPb2+,Cd2+,andCr6+competingforionexchangesitesinclinop-tilolitewasalsoinvestigated[26].ItwasreportedthatdissolvedPbandCdwereeffectivelyremovedinacidicpHrange.ItwasalsofoundthatthepresenceofCr6+diminishesthere-movalefficiencyofPb2+andCd2+.Itwassuggestedthatdecreasedremovalperformanceisduetothepresenceofligandsthatformcomplexeswithreducedaccessibilityand/oraffinityforionexchange.

In1991,theremovalofCr3+fromindustrialwastewaterusingItaliannaturalzeolitetuffscontainingphillipsiteandchabazitewasevaluatedincolumnoperation[27].Table3describestheoperatingconditionssetforcolumnexperiments.Itwasreportedthatalower

Table3

Operationalparametersforcolumnexperiments[27]Operationalparameters(units)Columndiameter(cm)Beddepth(cm)

Doseofadsorbent(g)

FeedcompositionofCr3+(mg/l)Flowrate(ml/min)Contacttime(min)

Value1.4338–48

47.10–50.9052–561.83–7.008.7–33.3

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Table4

SummaryofcolumnrunsforCr3+uptakebyItalianzeolites[27]TypeoftuffsampleGSNA–GSNPNP

CEC(meq/g)2.112.112.662.66

Flowrate(ml/min)7.007.007.001.83

WorkingCEC(meq/l)56525252

Efficiency(%)9.015.210.215.4

225

Adsorptioncapacity(mgofCr3+/g)3.35.54.77.1

Table5

Columnoperationconditions[28]Operationalparameters(units)Columninternaldiameter(cm)Beddepth(cm)Bedweight(g)

Averagebeddensity(g/cm3)Bedvolume(cm3)Flowrate(ml/min)

Value4.525400140015

flowrate(1.83ml/min)ismorefavoredbytheNaionsofzeolitesforahighercationexchangecapacity(CEC)withCr3+(Table4).Itcanbeexplainedduetothefactthatmorephysicochemicalinteractionsoccurredbetweenzeolitesandmetalspeciesduringcolumnoperation.Theresultsindicatedthatflowrateisthemostcrucialcharacteristicinevaluatingtheeffectivenessofanadsorbentforchromiumadsorption.

Theeffectofflowrateontheadsorptioncapacitywasalsodemonstratedinthelatterstudy[28].ThespecificconditionsofcolumnoperationarelistedinTable5.FromtheTable6,itcanbeseenthattheCr3+uptakebyzeolitesincolumnoperationissignificantlyhigherinthepreviousstudy[27]thanthatinthelatterstudy[28]duetotheeffectofflowrate.Duringcolumnoperation,aflowrateof15ml/minwasusedinthelatterstudy[28],butalowerflowrateof1.83ml/minwasappliedintheformerstudy[27].

Itwasalsoreportedthatatthesameflowrate,zeoliteshadahighercationexchangecapacitywithNi2+andCu2+thanthatwithZn2+,Cr3+,andFe2+(Table7)[28].Itcan

Table6

Adsorptioncapacities(mg/g)ofzeolitesforsomeheavymetalsMaterialClinoptilolite

Source[17][18][21][25][21][25][27][28]

Cd2+2.401.2070.003.701376.70

7.100.25Cr3+0

Cr6+

Co2+1.42

Ni2+0.48

Zn2+0.50

Cu2+1.

Pb2+1.601.4062.006.001756.00

2.403.60

1.505.8

0.904.500.56

2.705.500.04

3.805.100.37

Chabazite

Chabazite–phillipsite

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Table7

SummaryofcolumnrunsforNi2+,Zn2+,Cu2+,Cr3+,andFe2+uptakebyJordanianzeolites[28]ParametersVb(BV)Vt(BV)MTZ(cm)CEC(meq/g)WEC(meq/g)Efficiency(%)Selectivity(%)

Ni2+23.7052.5021.801.120.5329.6063.60

Zn2+27.5060.0018.500.080.042.204.60

Cu2+25.0047.5015.500.760.4122.7041.10

Cr3+27.5062.5019.400.740.3519.7042.40

Fe2+27.5065.0020.200.130.063.107.10

Remarks

Vb:volumeatbreakthroughpointinbedvolume(BV)Vt:totalvolumeatexhaustionpointexpressedinBVMTZ:masstransferzoneCEC:CECforindividualcationattheexhaustionpointWEC:CECforindividualcationatbreakthroughpointEfficiency(%)=(WEC/totalCEC)×100

Selectivity(%)=(CEC/totalCEC)×100

beexplainedduetothefactthattheNi2+andCu2+weremorepreferableforzeolitesduetotheirhigherconcentrations.Therefore,bothionshadgreateropportunitytohaveionexchangewiththeNaionsofzeolitesthanothers.

Overall,theresultspresentedaboveshowthatinsteadofusingcostlyactivatedcarbon,zeolitesholdgreatpotentialtoremoveheavymetalspeciesfromindustrialwastewatereffluents.However,lowpermeabilityofzeolitesrequiresanartificialsupportwhenusedincolumnoperations.2.3.Clay

Itiswidelyknownthattherearethreebasicspeciesofclay:smectites(suchasmont-morillonite),kaolinite,andmicas;outofwhichmontmorillonitehasthehighestcationexchangecapacityandthatitscurrentmarketprice(aboutUS$0.04–0.12/kg)isconsid-eredtobe20timescheaperthanthatofactivatedcarbon[29].Therefore,anumberofstudieshavebeenconductedusingclays,mainlymontmorillonite,toshowtheireffec-tivenessforremovingmetalionssuchasZn2+,Pb2+,andAl3+fromaqueoussolutions[30–32].

In19,theremovalperformanceofmontmorilloniteandkaoliniteforleadandcadmiumwascompared[33].ItwasfoundthattheadsorptioncapacityofPb2+andCd2+isgreateronmontmorillonite(Pb:0.68,Cd:0.72mg/g)thanonkaolinite(Pb:0.12,Cd:0.32mg/g).Itwasalsoindicatedthatthepresenceofcationicsurfactantreducestheuptakeofbothions,whiletheanionicsurfactantsenhancetheirremoval.

TheadsorptionofmontmorilloniteonCd2+andZn2+wasalsoevaluated[34].ItwasfoundthattheZn2+isadsorbedinlargeramountsthanCd2+duetothefactthatzinchashigherionicpotentialthancadmium.Therefore,Zn2+adsorptionwasfavoredoverCd2+bycationicinterchange.

TheremovalofzincfromwastewaterusingChinaclay,whichmainlyconsistsofalumi-nosilicates,wasstudied[35].Theessentialcharacteristicofkaolinitegroupisthattheydonotswellwiththeadditionofwater.Itwasreportedthatanadsorptioncapacityof1.25mg

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Table8

AdsorptioncapacityofChinaclayandwollastoniteforPb2+atdifferenttemperatures[37]TypeofadsorbentChinaclay

Temperature(K)293303313293303313

Adsorptioncapacity(mg/g)0.4110.3950.3461.6801.2901.100

󰀁G◦(kJ/mol)−8.08−4.53−3.07−2.36−1.58−1.05

Remarks

227

󰀁H◦=−77.95kJ/mol󰀁S◦=−238.46J/Kmol󰀁H◦=−16.40kJ/mol󰀁S◦=−47.92J/Kmol

Wollastonite

ofZn2+/gwasachievedbykaoliniteandthatthemaximumremovalefficiencywasfoundtobeatpHof8.0.

Usingahomogenousmixtureofadsorbents,whichconsistsofChinaclay–flyash,wollastonite–flyash,andChinaclay–wollastonite,theremovalofcopperfromaqueoussolutionwasalsoevaluated[36].Itwasdemonstratedthatthehighestadsorptioncapacityofabout1.18mgofCu2+/gwasachievedbyflyash–wollastonite.

TheeffectoftemperatureontheremovalofleadusingChinaclayandwollastonitewasinvestigated[37].ItwasfoundthattheamountofPb2+removedishighlydependentonthetemperatureandthatthesorptionfollowsLangmuirisotherm.ItwasalsoreportedthatthemaximumadsorptioncapacityofChinaclayandwollastonitewasfoundtobe0.411and1.680mgofPb2+/g,respectively(Table8).Itwasindicatedthatahighermetalremovalisobservedatlowertemperature.Thenegativevalueofenthalpychange(󰀁H◦)forbothadsorbentsindicatedthattheadsorptionprocesswasexothermic.

In2001,theadsorptionofleadonThaikaolinandclayconsistingmainlykaoliniteandillitewasstudied[38].Itwasreportedthatthemaximumadsorptioncapacityofbothmaterialswasfoundtobe1.41and4.29mgofPb2+/g,respectively,andthattheiradsorptionfollowedbothLangmuirandFreundlichmodelsofisotherm.ItwasalsoobservedthatleadadsorptionincreasedwithanincreaseinpH.However,thepresenceofco-ionssuchasCd2+,Cr6+,Cu2+,Ni2+,andZn2+,reducestheleaduptakefromaqueoussolutionduetothefactthattheco-ionsbindstronglywithorganicmatterpresentinclaytoformacomplex.

Anothermaterialfromclaymineralstoadsorbmetalisbentonite,whichmainlyconsistsofclay,silt,andsand.Thismaterialishighlyvaluedforitstendencytoabsorbwaterintheinterlayersites.TheadsorptionofCd2+andZn2+fromaqueoussolutionwasinvestigatedusingnaturalbentonite[39].ItwasreportedthattheCd2+areadsorbedtwotimesmorethanZn2+(Table9)duetothefactthatCd2+havelesspolarizingeffectstothesurfacechargeofbentonitethanzincions.

In1995,theadsorptionofCr6+onbentoniteatdifferenttemperaturesrangingfrom20to40◦Cwasinvestigated[40].ItwasreportedthatthesorptionisfavoredathighertemperaturesincethesorptionofCr(VI)isanendothermicprocessasindicatedbypositiveenthalpychange(󰀁H◦)valuelistedinTable10.Itwasalsoobservedthatthemaximumadsorptioncapacityof0.572mgofCr6+/gbentonitewasachievedatpHof2.0.

Thesorptionofstrontiumwasalsostudiedusingbentonite[41].Itwasfoundthatanadsorptioncapacityof32.94mgofSr2+/gbentonitewasachievedatpHof8.5andthatthesorptionprocessfollowedLangmuirisotherm.Itwasalsoreportedthatthesorption

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Table9

Adsorptioncapacities(mg/g)ofclayfordifferentheavymetalsMaterialMontmorilloniteKaolinite

Sources[33][34][35][33][38][38][36][40][41][42][43][36]

1.18Cu2+

Pb2+0.68

Cd2+0.724.780.32

Zn2+4.981.25

0.121.414.29

11.41

4.54

0.57

32.94

52.91

20

Cr6+

Sr2+

IlliteBentonite

Flyash–wollastonite

processofstrontiumonbentoniteisendothermicashighermetalremovalismorefavorableathighertemperature(Table11).

In1997,theuseofbentoniteforzincremovalwasevaluated[42].Itwasfoundtobe52.91mgofZn2+/gbentoniteasgivenbyLangmuirmodel.Inthelatterstudy,outstandingremovalcapabilityofbentoniteclaytouptakePb2+wasdemonstrated[43].Itwasreportedthatadsorptioncapacitiesof20mgofPb2+/gwereachievedbybentoniteatpHof3.4.Theusageofbentonitewasalsocarriedoutforremovalofradioactivewaste[44]andcesium[45].

Whenclaymineralsareusedforindustrialapplication,theswellingfactorshouldbetakenintoaccountsinceitmaycauseremarkablepressuredropduetotheirdifferentstructuralcharacteristicsandion-exchangemechanism.Thisisdifferentfromzeolites,whichdonot

Table10

Adsorptioncapacityofbentoniteforhexavalentchromiumatdifferenttemperatures[40]Temperature(K)293303313

Adsorptioncapacity(mg/g)0.330.450.57

󰀁G◦(kJ/mol)−0.24−0.44−0.

Remarks

󰀁H◦=+5.62kJ/mol󰀁S◦=+0.02kJ/Kmol

Table11

AdsorptioncapacityofbentoniteforSr2+atdifferenttemperatures[41]Temperature(K)298303308

Adsorptioncapacity(mgofSr2+/g)28.6530.4932.94

󰀁G◦(kJ/mol)−10.69−11.37−12.08

Remarks

󰀁H◦=+30.62kJ/mol󰀁S◦=+0.14kJ/Kmol

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Table12

Adsorptioncapacities(mg/g)ofpeatmossforsomemetalsMaterialEutrophicpeatOligotrophicpeatSphagnumpeatmoss

Sources[47][48][47][48][49][50]

Cu2+12.0719.5612.076.41

13243.9Cr6+

Cd2+20.2322.48

Zn2+11.1213.08

229

Ni2+11.1511.74

showanyswellingwhenembeddedinliquidmedium.Althoughtheremovalefficiencyofclaysforheavymetalsmaynotbeasgoodasthatofzeolites,theireasyavailabilityandlowcostmaycompensatefortheassociateddrawbacks.2.4.Peatmoss

Peatmoss,acomplexsoilmaterialcontainingligninandcelluloseasmajorconstituents,isanaturalsubstancewidelyavailableandabundant,notonlyinEurope(BritishandIreland),butalsointheUS.Peatmosshasalargesurfacearea(>200m2/g)andishighlyporoussothatitcanbeusedtobindheavymetals.PeatmossisarelativelyinexpensivematerialandcommerciallysoldatUS$0.023/kgintheUS[46].

In1986,theuseofpeattoremoveheavymetalswasinvestigated[47].Itwasobservedthatpeatmossplaysanimportantroleintreatmentofmetal-bearingindustrialeffluentssuchasCu2+,Cd2+,Zn2+,andNi2+usingeutrophicandoligotrophicpeat.Eutrophicpeatispoorincellulose,butrichinhumicsubstances.Oligotropicpeatisasphagnumone,whichismoreacidicthaneutrophicpeatandcontainsmoreorganicmatter.Bothpeatscontainabout85%ofhumicacidand15%offulvicacid.TheiradsorptioncapacityfordifferentmetalsislistedinTable12.

Usingbothpeatsmentionedpreviously,theexchangepropertiesofpeatforcopperre-movalwereinvestigated[48].Itwasreportedthatanadsorptioncapacityof19.56mgofCu2+/geutrophicpeatwasexhibitedandthateutrophicpeatshowedhigheradsorp-tioncapacitythanoligotrophicpeat(6.41mgofCu2+/g),althoughthelatterisricherincellulose.

TheremovalofCr6+usingsphagnumpeatmosswasexplored[49].Theadsorptioncapacityofsphagnumpeatmosswasfoundtobe132mgofCr6+/gatapHrangeof1.5–3.0.Themostattractiveadvantageofthisadsorbentintreatmentisthesimplicityofthesystem,lowcost,andtheabilitytoacceptawidevariationofeffluentcomposition.

Inanextendedstudy,theadsorptionofCr6+wasalsoinvestigatedusingsphagnumpeatmoss[50].ItwasfoundthattheadsorptioncapacityishigheratlowerpH.AtpHof2.0,peatisabletoadsorb20%Cr6+higher(about43.9mgofCr6+/g)thanthatatpHof2.5.Overall,theresultsmentionedpreviouslyindicatethatpeatmossisagoodadsorbentforallmetals.ItiswidelyknownthatpeatmossexhibitedahighCECandcomplexitiestowardsmetalsduetothepresenceofcarboxylic,phenolic,andhydroxylicfunctionalgroups.

230S.Babel,T.A.Kurniawan/JournalofHazardousMaterialsB97(2003)219–243

Table13

Adsorptioncapacities(mg/g)offlyashforsomemetalsMaterialFlyash

Flyash–wollastoniteFlyash–Chinaclay

Sources[51][53][52][52]

Cu2+1.39

2.82

1.18

2.920.31Cr6+

Hg2+

2.5.Flyash

Flyash,anindustrialsolidwasteofthermalpowerplantslocatedinIndia,isoneofthecheapestadsorbentshavingexcellentremovalcapabilitiesforheavymetalssuchascopperions[51].Itwasreportedthatanadsorptioncapacityof1.39mgofCu2+/gwasachievedbyflyashatpHof8.0(Table13).Itwasalsofoundthattheadsorptioncapacityincreaseswithanincreaseintemperature.

Otherstudieshavebeenconductedtoshowtheeffectivenessofflyashontheremovalof6Cr+fromaqueoussolutionusingahomogenousmixtureofflyashandwollastonite(1:1)[52].Itwasreportedthatanadsorptioncapacityof2.92mgofCr6+/gcouldbeachievedatpHof2.0andthattheadsorptionprocessfollowedLangmuirmodelofisotherm.Thismixedadsorbentperformedbetterthanflyash–Chinaclay,wherethemaximumadsorptioncapacitywasfoundtobe0.31mgofCr6+/gatpHof2.0.Thisdifferencecouldbeduetothefactthattheadsorptiveforcebetweenadsorbateandmixture(flyash–wollastonite)isstrongerthanthatoftheotherone(flyash–Chinaclayandadsorbate)sothatCr6+isadsorbedmoreeffectively.

In1987,researchontheadsorptionofmercuryusingflyashwascarriedout[53].Itwasreportedthatthemaximumadsorptioncapacityof2.82mgofHg2+/gtookplaceatapHrangeof3.5–4.5andthatadsorptionfollowedtheFreundlichmodel.

Itisalsoknownfromvariousstudiesthatflyashcouldbeeasilysolidifiedaftertheheavymetalsareadsorbed.However,sinceitalsocontainsheavymetals,thepossibilityofleachingshouldbeconsideredandevaluated.2.6.Coal

In1984,theremovalofCd2+usingGiridihcoal(GC)wasintensivelyinvestigated[54].Itwasreportedthatanadsorptioncapacityof0.91mgofCd2+/gGCwasachieved.ItwasalsofoundthatsorptionfollowedFreundlichisothermandthatitdecreasedbeyondpH10duetotheformationofsolublehydroxycomplexes.

AsimilarstudyforHg2+sorptionusingGiridihbituminouscoal(GBC)wasalsocon-ducted[55].Itwasnotedthatchemicalpretreatmentforbituminouscoalwithnitricacidsignificantlyenhancedmercurysorptiontoalevelhigherthanthatexhibitedbyactivatedcarbonintermsofinitialrateofsorptionandadsorptioncapacity.Itwasalsoreportedthatanadsorptioncapacityof10mgofHg2+/gwasobservedatapHrangeof7.0–8.5.ItwasfoundthatthesorptionreactioninvolvesarapidinteractionbetweenHg2+andactivesitesontheexternalsurfaceofcoal.

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Table14

Comparativestudyofchromedyeremovalusingmixedflyashandcoal(1:1)andactivatedcarbon[56]AdsorbentsFlyash+coal(1:1)

Temperature(◦C)304050304050

Removal

efficiency(%)92.7082.4870.8098.4184.1371.43

Adsorptioncapacity(mg/g)0.760.690.612.091.720.98

Optimumcontacttime(min)100

231

Unitprice(US$/kg)0.03

Activatedcarbon

(commercialgrade)

600.82

Guptaetal.studiedtheremovalofchromedyefromaqueoussolutionsusingahomoge-nousmixedadsorbentconsistingofflyashandcoal[56].Itwasreportedthatadsorptioncapacityof0.76mgofchromedyepergramwasobservedatpHof2.0andthatitwasnearlythreetimeslessthanthatbyactivatedcarbon(Table14).ItwasalsonotedthatthesorptionprocessfollowsLangmuirisothermandthatlowertemperature,rangingfrom30to50◦C,favoredhigherremovalefficiencyofchromedyeasthesorptionprocessisexothermic.2.7.Naturaloxide

In1985,astudyontheuseofaluminiumoxidetoremoveCr6+fromaqueouswastewasconducted[57].Itwasreportedthattheultimateadsorptioncapacityof11.7mgofCr6+/galuminawasobservedatpHof4.0.ItisimportanttonotethattheadsorptivecapacityofaluminasignificantlyreducedinthepresenceofCN−anions.Itcanbeexplainedduetothefactthatcyanidehasastronganionicinfluenceuponthesorptioncharacteristicsofalumina.Therefore,CN−anionsarecompetitivelyadsorbedcoveringthesurfacesitesofalumina,whichinturnpreventtheCr6+tobeadsorbedontheinternalsurfaceofadsorbent.

TheremovalofPb2+andCd2+fromaqueoussolutionsusingaluminiumoxideandgoethite,anironoxidewasalsoexplored[58].ItwasfoundthatgoethiteexhibitsabettersorptioncapacityforbothionsthanalumuniumoxideandthattheuptakeofPb2+ishigherthanthatofCd2+(Table15).

AfurtherstudywasconductedusingironoxidecoatedwithsandtoremoveCr6+[59,60].Itwasreportedthatthisadsorbentwasfoundtobeeffectiveforremovingmetalfromsolutionandthatabout99%of0.038mMCr6+removalwasachievedconsistently[59].Incolumnoperation,itwasnotedthatthisadsorbentshowedgoodperformanceintreating

Table15

Adsorptioncapacities(mg/g)ofnaturaloxidesforsomemetalsMaterialAluminiumoxideFerricoxide

Sources[57][58][58]

Cd2+3172

Pb2+33230

Cr6+11.7

232S.Babel,T.A.Kurniawan/JournalofHazardousMaterialsB97(2003)219–243

cadmium-platingwastes,butpoorintreatingchromium-platingwaste[60].Itwasalsoreportedthatcadmium-platingwasteisadsorbed28%higherthanthatofchromium(about2.70mg/l).

In1996,theremovalofarsenicionsfromgroundwaterinhometreatmentunitsusingironoxidecoatedwithsandwasinvestigated[61].ItwasreportedthatthisadsorbentwasapromisingmediumtoremoveAs3+andAs5+fromgroundwatersinceitcouldremoveabout80–85%of1.0ppmarsenicsolution.TheprocessitselfcostsUS$8toproduce700–800lwaterfreefromanyarseniccompound.

Theuseofmanganeseoxide,(costaboutUS$0.05/kg),forremovingarseniccompoundsfromgroundwaterwasdemonstrated[62].Removalefficiencyofalmost100%wasreportedforbothAs3+andAs5+atapHrangeof2.0–8.0.ItwasalsofoundthatthepresenceofbivalentcationssuchasNi2+,Co2+,andMg2+ingroundwaterenhancestheadsorptioncapacityofmanganeseoxideduetothefactthattheseco-ionsprovideaframeworktowhichthemetalionscanbeaffixedbythesurfaceofadsorbent.2.8.Industrialwaste

Iron(III)hydroxidewasteisoneofwastematerialfromfertilizerindustries.IthasbeenextensivelyinvestigatedforremovingCr6+fromwastewater[63].Itwasreportedthatthemaximumadsorptioncapacityofiron(III)hydroxidewasfoundtobe0.47mgofCr6+/gatpHof5.6.Thisresultisnotinagreementwiththepreviousstudy[],whichobservedthatHCrO4−iseffectivelyadsorbedatpHof8.5.ThisdifferencecanbeexplainedduetothefactthatadsorptionofCr6+inthelatterstudyissuppressedbythepresenceofbothSO42−andSCN−anions,whichcompeteforadsorptionsites.

Wasteslurryisalsooneoftheindustrialby-productsgeneratedinfertilizerplantshowinggoodsorptivecapacities.In19,theuseofwasteslurrytoremoveCu2+,Cr6+,Hg2+,andPb2+fromaqueoussolutionwasinvestigated[65].ItwasreportedthatthisproductexhibitsoutstandingadsorptioncapacityforCr6+,Hg2+,andPb2+(Table16).Asimi-larstudyevaluatedalsotheremovalofCu2+andCd2+usingwasteslurrygeneratedinseafoodprocessing[66].Theadsorptioncapacityofwasteslurrywasfoundtobe20.97and15.73mg/gforCu2+andCd2+,respectively.

Table16

Adsorptioncapacitiesofindustrialwaste(mg/g)MaterialWasteslurryIron(III)hydroxideLignin

Blast-furnaceslagSawdust

ActivatedredmudBagasseflyash

Sources[65][66][63][67][69][70][73][72][71]

160

1.6260

Ni2+

Cu2+20.97

0.47

186540

13.80

95

7.5

Pb2+1030

Hg2+560

Cr6+0

15.73

Zn2+

Cd2+

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Table17

Metalsuptakebyligninatdifferenttemperatures[67]Typeofmetal

Temperature(K)303

Pb2+Zn2+

158673.24

313186594.83

󰀁G◦(kJ/mol)30−26.17−32.49

40−28.47−36.79

Remarks

233

󰀁H◦=+43.6kJ/mol,󰀁S◦=0.23kJ/Kmol󰀁H◦=+97.8kJ/mol,󰀁S◦=0.43kJ/Kmol

In1994,researchontheadsorptionofPb2+andZn2+ontoligninextractedfromblackliquorwascarriedout[67].Blackliquor,awasteproductoriginatedfrompaperindustry,couldbepurchasedatUS$1.00/tonandthelignincouldbeprocessedforUS$60/t,com-parabletoactivatedcarbonsoldatUS$100/t.ItwasreportedthattheadsorptioncapacityofligninforbothPb2+andZn2+wasfoundtobe1865and95mg/g,respectively,at40◦C.Resultsindicatedthatthehighadsorptioncapacityofligninisduetothepresenceofpoly-hydricphenolgroupsonthesurfaceoflignin.Itwasalsoindicatedthattheadsorptionisanendothermicprocesssinceahigherremovalwasfavoredathighertemperature(Table17).Anotherlow-costadsorbentshowingcapabilitytoadsorbheavymetalsisblast-furnaceslag,anindustrialby-productgeneratedinsteelplants.In1996,thesorptionofCu2+,Ni2+,andZn2+usingblast-furnaceslagwasstudied[68].Itwasfoundthatmetalionssorptiontakesplaceintheformofhydro–oxocomplexesandthatthehighsorptioncapacityisrelatedtotheformationofsolublecompoundsontheinternalsurfaceofadsorbent.

AfurtherstudywasalsoconductedtoinvestigatetheremovalofPb2+andCr6+usingblast-furnaceslagsoldatUS$38/t[69].Itwasfoundthatthemaximummetalsuptakeobservedunderidenticalconditionsis40and7.5mg/gofPb2+andCr6+,respectively.Itcanbeconcludedthatitsremovalperformanceiscomparabletothecheapestcommercialactivatedcarbon(costaboutUS$1000/t),as1gofactivatedcarboncanadsorb32.4mgofPb2+.

Theroleofsawdust,collectedfromatimberworkingshop,forcopperremovalwasevaluated[70].Itwasreportedthatanadsorptioncapacityof13.80mgofCu2+/gsawdustwasachieved.Itwassuggestedthatsawdustisagoodadsorbentforcopperremoval.

Theuseofbagasseflyash,anindustrialwastegeneratedinthesugarindustry,toremoveCr6+fromelectroplatingwastewaterwasinvestigated[71].FromtheTable18,itwasfoundthatthesorptioncapacitydecreaseswithanincreaseintemperatureastheadsorptionprocessisexothermic.ItwasalsoreportedthattheadsorptionofCr6+onbagasseflyashfollowedbothFreundlichandLangmuirisothermandthatthesorptioncapacityofbagasseflyashwasfoundtobe260mgofCr6+/gasgivenbytheLangmuirmodel.

Table18

Hexavalentchromiumuptakebybagasseflyashatdifferenttemperatures[71]Temperature(K)303313

Adsorptioncapacity(mg/g)260.00123.76

󰀁G◦(kJ/mol)−16.26−15.14

Remarks

󰀁H◦=+50.43kJ/mol󰀁S◦=−112.76kJ/Kmol

234S.Babel,T.A.Kurniawan/JournalofHazardousMaterialsB97(2003)219–243

In1999,researchontheusageofactivatedredmudtoadsorbhexavalentchromiumfromaqueoussolutionwascarriedout[72].Redmud,aby-productofthealuminumindustry,iscomposedmainlyoftheparticlesofsilica,aluminium,iron,andtitaniumoxide.Itwasfoundthatanadsorptioncapacityof1.6mgofCr6+/gredmudwasachievedatpHof5.2.However,theadsorptioncapacityofredmudwassignificantlydifferentfromthatobtainedinapreviousstudy[73],whichinvestigatedtheuseofredmudforNi2+removal.Itwasreportedthatanadsorptioncapacityof160mgofNi2+/gredmudwasaccomplished.Itcanbeexplainedduetothefactthattherewaspretreatmentforredmud.Suchpretreatment,ofcourse,couldincreaseitseffectivenesstoadsorbthetargetedmetalandconsequently,redmudhasahighercationexchangecapacitywithNi2+.2.9.Miscellaneousadsorbents

Otherlow-costadsorbentshavebeenstudiedlessextensivelysuchasxanthate,ricehuskcarbon(RHC),andcoconutshell.Xanthateisoneoftheeffectivelow-costadsorbents.Itisagroupofsulfur-bearingcompounds,whichhaveahighaffinityforheavymetalsandformedwhenanorganichydroxyl-containingsubstratereactswithcarbondisulfide.AstudyconductedbyTareetal.comparedtheremovalperformanceofsolubleandinsolublestarchxanthateforheavymetalsuchasCd2+andCr3+[74].Itwasfoundthattheperformanceofinsolublestarchxanthateisbetterintermsofadsorptioncapacityandeaseofoperation.ItwasalsoreportedthatsolublestarchxanthatescostonlyUS$1.0/kg.

TheuseofRHCforremovingCr6+wasalsodemonstrated[75].Theactivatedcarbonwasderivedfromricehuskusingcarbonizationwithsulfuricacid.ThemaximumadsorptioncapacityofRHCwasfoundtobe45.6mgofCr6+/gatpH2.5.

Alaertsetal.[76]reportedthatcoconutshell-basedactivatedcarboncouldbeusedfor6Cr+removal.ItwasshownthatoptimumchromiumremovalwasachievedatpH<7.0.ItwasalsofoundthatremovalefficiencyincreaseswithadecreaseinchromiumconcentrationandthattheadsorptionfollowstheFreundlichisotherm.

In2000,theuseofactivatedaluminatoremovearseniteandarsenatefromgroundwaterwasstudied[77].ItwasreportedthattheuptakeofarseniteismuchlessthanthatofarsenateforaluminagrainatoptimumpH.Theadsorptioncapacityofaluminagrainforarseniteandarsenatewerefoundtobe3.48and12.34mg/gatpH6.9and2.6,respectively.ThedifferencemaybeduetothefactthatundermostpHconditionsfornaturalwater,arsenateispresentinnegativelyionicformandarseniteisinnon-ionicform.

Ajmaletal.[78]carriedoutanadsorptionstudyonCitrusreticulata,anagriculturalwasteoriginatedfromthefruitpeeloforange,fortheremovalofNi2+fromelectroplatingwastewater.ItwasreportedthatmaximumremovalofNi2+occurredatpHof6.0andthattheadsorptionfollowedtheLangmuirisotherm.Itwasalsofoundthatanadsorptioncapacityof158mgofNi2+/gwasachievedbyCitrusreticulataat50◦CandthatthesorptionofNi2+wasendothermic,asshownbythenegativevalueoffreeenergy(󰀁G◦)(Table19).Itcanbeconcludedthattheadsorptioncapacityofthewasteincreaseswithanincreaseintemperature.In2002,thepotentialofParthenium,anIndianagriculturalwaste,forremovingNi2+fromaqueoussolutionwasalsoinvestigated[79].ItwasreportedthattheadsorptioncapacityofPartheniumwasfoundtobesignificantlylower(about54.35mgofNi2+/g)thanthepreviousstudy[78],althoughbothareagriculturalwastes.

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Table19

AdsorptioncapacityofCitrusreticulataforNi2+atdifferenttemperatures[78]Temperature(K)303313323

Adsorptioncapacity(mg/g)80119158

󰀁G◦(kJ/mol)−8.24−8.95−9.49

Remarks

235

󰀁S◦=−0.06kJ/Kmol󰀁H◦=10.37kJ/mol

AcomparativestudyontheremovalofCr6+fromaqueoussolutionwasalsocarriedoutusinglow-costadsorbentsderivedfromusedtyres(TAC),sawdust(SPC),andgranularactivatedcarbon(GAC)typeFiltrasorb400[80].ItwasfoundthattheadsorptioncapacitiesofTACarecomparabletothatofGACatoptimumpHof2.0(Table20).However,theadsorptioncapacityofSPCissignificantlylowerthanthatofbothTACandGAC.ItcanbeexplainedduetothefactthatbothTACandGAChavesmallerparticlesize(0.2mm)thanSPC(0.65mm).SuchreductioninparticlesizeofadsorbentsincreasesitssurfaceareaformetaladsorptionanditresultsinhigherremovalefficiencyonCr6+.ItwasalsoindicatedthattheadsorptionofCr6+wasmorefavorableathighertemperature.

TheuseofdiatomitetoremoveCr6+fromaqueoussolutionwasdemonstratedaswell[81].Microemulsionstreateddiatomitearequiteefficientinremovalprocessofmetallicions.FromTable21,itwasreportedthatanadsorptioncapacityof1.68mgofCr6+/gdiatomitewasachievedatpHof2.95.

Aninvestigationontheuseofspheroidalcellulosetoremovechromiumwasalsocon-ductedinChina[82].Celluloseisthemostabundantamongrenewableandnaturalpolymersandithasthreereactivehydroxylgroups.Theadsorptioncapacityofspheroidalcellulosewasfoundtobe73.46mgofCr6+/gatpHof6.0.Thisinvestigationisusefultodevelopadvancedtechnologyforwastewatertreatmentfacilitysincetheprocessiseconomicallyfeasibleandeasytocarryout.PresentlyitsmarketpriceisaboutUS$1.07/kg.2.10.Activatedcarbon

Basedonitssizeandshape,activatedcarbonisclassifiedintofourtypes:powder(PAC),granular(GAC),fibrous(ACF),andclothe(ACC).Duetothedifferentsourcesofraw

Table20

Comparisonoftheadsorptioncapacitiesbetweenlow-costadsorbents(TACandSPC)andGACatdifferenttemperatures[80]Temperature(◦C)22

TypeofadsorbentTACSPCGACTACSPCGACTACSPCGAC

Adsorptioncapacity(mg/g)48.081.9344.4455.252.18.5458.482.2953.19

󰀁G◦(kJ/mol)−12.33−1.17−2.37−12.92−3.30−4.35−13.86−5.06−6.94

30

38

236S.Babel,T.A.Kurniawan/JournalofHazardousMaterialsB97(2003)219–243

Table21

Adsorptioncapacities(mg/g)ofmiscellaneousadsorbentsMaterial

Cellulosexanthate

SolublestarchxanthatesRicehuskcarbonActivatedaluminaDiatomite

SpheroidalcelluloseAgriculturalwaste(Citrusreticulata)PartheniumWastetyreSawdust

Source[74][74][75][77][81][82][78][79][80][80]

Cd2+19.8833.27

Cr3+17.57

45.6

3.48

1.6873.46

15854.35

58.482.29

12.34

Cr6+

As3+

As5+

Ni2+

materials,theextentofchemicalactivation,andthephysicochemicalcharacteristics;eachtypeofactivatedcarbonhasitsspecificapplicationaswellasinherentadvantagesanddisadvantagesinwastewatertreatment.

Althoughasignificantnumberoflow-costadsorbentsfromvariousmaterialshavebeenfound,commercialactivatedcarbon(CAC)hasstillbeenusedintensivelytoday.Alargenumberofresearchersarestillstudyingtheuseofactivatedcarbonforremovingheavymetalssuchasmercury[83],copper[84],lead[85],chromium[86–91],cadmium[92,93],Ni[94,95],zinc[96–98],andlithium[99].Recentlythemarketpriceofactivatedcarbonforindustrialgradeisconsideredtobeveryexpensive(aboutUS$20–22.00/kg),dependingonthequalityofactivatedcarbonitself[100].

VarioustypesofCACforheavymetalsremovalhavebeenreported.In1984,theremovalefficiencyofmercuryusingdifferentbrandsofPACsuchasNucharSAandSNwasevalu-ated[83].Itwasreportedthatabout99–100%oftotal0.2mMHg2+removalwasattainedbyboththetypesatpHof4.0–5.0.

AnothersimilarresearchwascarriedouttocomparethephenomenonofcopperandcobaltadsorptionbyGACfromaqueoussolution[84].ItwasreportedthattheremovalcapabilityofCACforbothmetalswasfoundtobesignificantlydifferent.ItwasalsodemonstratedthatatpH4.0,GACcouldremove99%of10ppmofcobaltsolution,butonly93%ofcoppersolutionatthesameconcentration.

TheuseofGACforleadremovalwasalsoevaluated[85].Itwasdemonstratedthattheadsorptioncapacityofactivatedcarbonwasfoundtobe30mgofPb2+/gandthattheamountofPb2+notremovedcorrespondedtotheamountthatwascomplexedbyEDTA.

TheuseofactivatedcarbonforCr6+removalwasalsointensivelyinvestigatedinrecentyears.In1995,theremovalefficiencyofdifferenttypesofPACpreparedfromdifferentrawmaterialssuchasleather,olivestone,andalmondshellwasevaluated[86].Itwasindicatedthattheextentoftheadsorptionprocessdependsonthepretreatmentofactivatedcarbonandthatthehighestremovalperformancewasobtainedwiththosepreparedbyphysicalactivation.ItwasalsoreportedthatatpHof1.0,theretentionofCr6+wasaffectedbyitsreductiontoCr3+.

S.Babel,T.A.Kurniawan/JournalofHazardousMaterialsB97(2003)219–243

Table22

Adsorptioncapacities(mg/g)foractivatedcarbonTypeofactivatedcarbonPAC-NucharSAGAC-HD400GAC-CGAC

GAC-Filtrasorb400

Source[83][99][85][97][98][87][80][][91][92][][93][95][94][96]

30

6.8414553.190.18

30

8

0.13

146

40

210

2

3.75

930

1.90

38

Cr6+

Cr3+

Pb2+

Zn2+

Cd2+

Ni2+

Cu2+

Li+0.45

237

Hg2+40.12

GAC-LB830As-receivedACF

OxidizedACFACC

65

In1996,theremovalofCr6+fromaqueoussolutionusingGACtypeFiltrasorb400wasalsostudied[87].Itwasreportedthatanadsorptioncapacityof145mgofCr6+/gwasachievedatapHrangeof2.5–3.0.Thisresultisnotinagreementwiththatobtainedinthelatterstudy[]conductingasimilarcomparativestudyusingactivatedcarbonLB830andFiltrasorb400.ItwasreportedthatthemaximumadsorptioncapacityofFiltrasorb400inthelatterstudyisonly0.18mgofCr6+/g(Table22).

ChemicalmodificationonthesurfaceofGACwithoxidizingagentsuchasnitricacidwasalsoconductedtoimproveitsadsorptioncapacity[90].Itwasfoundthattheamountofchemisorbedoxygenonthecarbonsurfaceincreasedafteroxidativetreatment.ItisinterestingtonotethattheadsorptioncapacityofCr3+ontheoxidizedcarbonisenhancedabout300%of30mgofCr3+/g.ThismaybeattributedtothefactthatthesurfaceofoxidizedGAChasalargernegativechargethanthatofnon-oxidizedone.Duetotheelectrostaticattractiveinteractionsbetweenthem,thechangeinthenegativechargeonthecarbonsurface,andthatofthepositivechargeofCr3+inthesolutionfavormoreadsorptionofmetalions.

AnothersimilarresearchwasalsoconductedtoevaluatetheremovalofCr6+byACFsplatedwithcoppermetal[91].ItwasreportedthattheintroductionofCu2+onACFssignificantlyleadtoanincreaseinthesurfacebasicity,resultingintheadsorptioncapacityofCr6+fromanaqueoussolutionregardlessofadecreaseinsurfacearea.Itwaspointedoutthattheadsorptionofchromiumionswasessentiallydependentonsurfaceproperties,ratherthanbysurfaceareaandporosityofACFs.

TheadsorptionofCd2+fromtheaqueoussolutionusingGACwasalsostudied[92].ItwasreportedthatmaximumadsorptioncapacityofGACwasfoundtobe8mgofCd2+/gatpH8.0.ItwasalsoobservedthattheamountofCd2+adsorbedwasreducedaboutthree

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timesbyincreasingthetemperaturefrom10to40◦C.Thisindicatedthattheadsorptionprocesswasexothermicinnature.

TheoxidativeeffectsofnitricacidforACFswerealsoinvestigatedforCd2+removal[93].DuetomoreelectrostaticattractionsbetweenthepositivechargeofCd2+andthenegativechargeofACFs,itwasfoundthatasignificantincreaseinion-exchangecapacitywasachievedafteroxidativetreatment.ThemaximumadsorptioncapacityofACFswasfoundtobe146mgofCd2+/gatpH5.0–6.0.

TheuseofoxidizedACFsforNi2+andCu2+adsorptionwasalsoevaluatedandcomparedtothatofas-receivedACFs[94,95].ItwasdemonstratedthattheadsorptioncapacityofoxidizedACFsonboththemetalswasindicatedtobesuperiortothatofas-receivedACFsandthattheadsorptioncapacityofACFswashigherforCu2+thanthatofNi2+(Table22).ThisindicatedthatoxidativetreatmentincreasestheacidityofthesurfacefunctionalgroupofACFssothatmoreelectrostaticattractionsoccurbetweenthenegativechargeofACFsandthepositivechargeofcations.

Inthelatestresearch[96]investigatingtheadsorptionofvariousheavymetalssuchasZn2+,Cd2+,andHg2+ontoACC,itwasreportedthattheadsorptioncapacityofACCwasfoundtobesignificantlyhigherforHg2+thanthatforZn2+and/orCd2+(Table22).ItwasalsoindicatedthattheamountofadsorbedmetalsincreaseswithanincreaseinpH.

OthersimilarworkforZn2+removalwasalsoconductedusingdifferenttypesofGACsuchasC,F-300,F-400,andCentaurHSL[97].ItwasfoundthatChashigheradsorptioncapacityforZn2+thanothertypesofGAC.ItwasalsoreportedthattheadsorptioncapacityofGACtypeCisabout18mgofZn2+/gatpH7.0.

ChemicalmodificationsofCACusingtetrabutylammoniumiodide(TBAI)andsodiumdiethyldithiocarbamate(SDDC)werealsocarriedout[98].ItwasreportedthattheTBA-carbonadsorbentwasfoundtohaveaneffectiveadsorptioncapacityofapproximatelyfivetimesthanthatofas-receivedcarbonandthatusingSDDC–carboncolumn,heavymetalssuchasCu2+,Zn2+,andCr6+couldbeeliminatedwitharemovalcapacityof38,9.9,and6.84mg/g,respectively.ItwasalsosuggestedthatthetechniqueofTBAandSDDCmodificationsoptimizetheexistingpropertiesofactivatedcarbon,givinggreaterremovalcapacitytotheas-receivedactivatedcarbon.

In1996,theuseofPACforLi+removalwasevaluatedusinganelectriccurrent[99].ItwasreportedthattheadsorptioncapacityofPACisverylow(about0.45mgofLi+/g).ThiscanbeexplainedduetothefactthatelectrochemicalpolarizationofthecarbonmaterialmodifiesthesurfacefunctionalgroupsandexertsasignificantinfluenceonthesorptionofLi+.Thus,indicatingthatthesurfacechemicaleffectsappeartodominate,althoughthespecificsurfaceareamaybeimportant.

3.Comparisonofcommercialactivatedcarbonwithlow-costadsorbents

Fig.1illustratestheadsorptioncapacityofsomeoutstandingadsorbentsmentionedintheabovestudy.ItisevidentfromourliteraturesurveyandFig.1thatsomelow-costadsorbentssuchaschitosan,zeolites,wasteslurry,andligninhavedemonstratedoutstandingremovalcapabilitiesforheavymetals,whichisfarbetterthancommercialactivatedcarbon.Mosttoxicheavymetalssuchascopper,zinc,cadmium,andmercuryions,forinstance,have

Fig.1.Summaryofsomeadsorbentswithhighadsorptioncapacities(mg/g).

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beeneffectivelyremovedfromhighlydilutedsolutionsusingchitosan.EvenforCr6+,Hg2+,Cu2+,andCd2+removal,chitosanperformssignificantlybetterthananytypesofcommercialactivatedcarbon(CAC)intermsofmetal-loadingcapacity.However,chitosanisconsideredasthemostexpensivealternativeadsorbentsinceitsmarketpriceisnearlycomparabletothatofCAC.RecentlymarketpriceofchitosanisUS$16/kgandthatofCAC(dependingonitsgradeandquality)isaboutUS$20.0–22.0/kg.

Clinoptilolite,oneofabundantnaturalzeolitesspecies,isagoodlow-costadsorbentforcadmiumandleadremoval.Nevertheless,chabaziteshasbettercationexchangecapacitythanclinoptilolite.BothPb2+andCd2+areadsorbedbychabazitesnearlytwotimesmorethanthosebyclinoptilolite.Althoughthezeolites’currentcommercialprice(lessthanUS$1.0/kg)isnowconsiderednearly20timescheaper,theadsorptioncapacityofchabazitesforCd2+iscomparabletothatofCAC.Infact,itsadsorptioncapacityforPb2+isfourtimeshigherthanthatofCACtypeHD400.

WasteslurryisanotheralternativeadsorbentthatisabletoremoveCr6+,Pb2+,andHg2+effectivelyfromaqueoussolutionsatlowcost.AsforCr6+removal,theadsorptioncapacityofwasteslurryisthehighestamongotherlow-costadsorbentsandisnearlyfourtimeshigherthanthatofCACtypeFiltrasorb400.Infact,itsadsorptioncapacityforPb2+andHg2+ismorethan20and10timeshigherthanthatofanytypeofCAC,respectively.However,thereisnoavailableinformationforthecommercialpriceofwasteslurryasitcanbeobtainedforfreeoratinexpensivecostfromfertilizerplants.

Lastbutnottheleast,ligninisconsideredasthebestlow-costadsorbentforleadandzincremoval.Ligninisabletoremoveboththeionseffectivelyundertemperaturesrang-ingfrom30to40◦C.ItadsorbsPb2+nearlytwotimeshigherthanwasteslurryanditsremovalcapabilityforZn2+iscomparabletothatofchitosan.EvenitsadsorptioncapacityforPb2+issignificantlymorethan30timeshigherthanthatofCAC.Inspiteofitsinex-pensivecommercialprice(aboutUS$0.06/kgin1994),ligninundoubtedlyhasexcellentmetal-adsorbentbindingcapacities,whicharecomparabletothatofchitosan.Moreover,theadsorptioncapacityofligninforPb2+isthemostoutstandingcomparedtoCACandotherlow-costadsorbentssuchaschitosan,zeolites,orwasteslurry.Duetothereasonsmentionedpreviously,acost–benefitanalysisofusingligninforleadandzincremovalinwastewatertreatmentapplicationsneedstobeconductedtojudgetheeconomicfeasibilityofitspracticaluseandthepotentialforitscommercialapplicationsinthefuture.4.Conclusionsandrecommendations

Awiderangeoflow-costadsorbentshasbeenstudiedworldwideforheavymetalremoval.Itisevidentfromourliteraturesurveythatinexpensiveandlocallyavailablematerialscouldbeusedinsteadofcommercialactivatedcarbon.

Afewadsorbentsthatstandoutforhighadsorptioncapacitiesarechitosan(815,273,250,222,75mg/gofHg2+,Cr6+,Cd2+,Cu2+,andZn2+,respectively),zeolites(175and137mg/gofPb2+andCd2+,respectively),wasteslurry(1030,560,0mg/gofPb2+,Hg2+,andCr6+,respectively),andlignin(1865and95mg/gofPb2+andZn2+,respec-tively).Theseadsorbentsareefficientandcanbeeffectivelyusedforinorganiceffluenttreatmentcontainingmetalions.

S.Babel,T.A.Kurniawan/JournalofHazardousMaterialsB97(2003)219–243241

Amongtheircurrentcommercialprices,zeolitesareundoubtedlythemostinexpensivealternativeadsorbentscomparedtoothers.Itisconsidered15timescheaperthanchitosan(US$15.43/kg).Itisexpectedthatthepriceofchitosanwillslowlygodownsincemoreindustriesworldwidemayconsiderusingitinwastewatertreatmentduetoitshigherremovalefficiencies.

Toimproveremovalefficienciesandadsorptioncapacities,chemicalmodificationsoflow-costadsorbentssuchascoconutshellcharcoalneedstobeconductedusingcoatingprocess.Coconutshellcharcoalhaslowremovalefficienciesandadsorptioncapacitiesformetalsremoval.Itisexpectedthatcoatingcoconutshellcharcoalwithchitosanmaysignificantlyimproveitsremovalperformance.

Sofar,costinformationofotheradsorbentssuchasflyash,coal,ferricoxide,andwasteslurryisseldomreportedinanyofthepublicationssincetheexpenseofindividualadsorbentsvaries,dependingontheprocessingrequiredandlocalavailability.Thissituationmakesacomprehensivecomparisonamongalternativeadsorbentsdifficulttomaterializeduetoinconsistenciesindatapresentation.

Inspiteofthescarcityofconsistentcostinformation,thewidespreadusesoflow-costadsorbentsinindustriesforwastewatertreatmentapplicationstodayarestronglyrecom-mendedduetotheirlocalavailability,technicalfeasibility,engineeringapplicability,andcosteffectiveness.Iflow-costadsorbentssuchaschitosan,zeolites,wasteslurry,andligninperformwellinremovingheavymetalsatlowcost,theycanbeadoptedandwidelyusedinindustriesnotonlytominimizecostinefficiency,butalsoimproveprofitability.Duetothereasonsmentionedpreviously,acost–benefitanalysisofusinglow-costadsorbentsforheavymetalremovalneedstobeconductedtojudgetheeconomicfeasibilityofitspracticaluseinwastewatertreatmentapplicationstoday.

Lastbutnottheleast,ifthealternativeadsorbentsmentionedpreviouslyarefoundhighlyefficientforheavymetalremoval,notonlytheindustries,butthelivingorganismsandthesurroundingenvironmentwillbealsobenefitedfromthepotentialtoxicityduetoheavymetal.Thus,theuseoflow-costadsorbentsmaycontributetothesustainabilityofthesurroundingenvironment.Undoubtedlylow-costadsorbentsofferalotofpromisingbenefitsforcommercialpurposeinthefuture.

Acknowledgements

Theauthorsarepersonallythankfultothereviewersfortheircriticalcommentsandusefulsuggestions,whichsignificantlyimprovedthequalityoftheirmanuscript.References

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