Advanced SorbentMaterials for Treatment of Wastewaters.docx
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Advanced SorbentMaterials for Treatment of Wastewaters.docx
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AdvancedSorbentMaterialsforTreatmentofWastewaters
AdvancedSorbentMaterialsforTreatmentofWastewaters
AbstractDespitetheexistenceofawiderangeofwastewatertreatmenttechnologies,sorptionisstillcommonlyappliedforthepurificationofindustrialeffluents.Thisisparticularlyvalidfortextileindustryeffluents,wheredifferenttechniquesmustbecombinedtoachievetheoptimumeffectofpurification.Thecombinationofbiologicalandphysicochemicalprocessesresultsintheremovalofmostorganicandinorganicpollutantsfromtextilewastewaterbuttheresultingeffluentisstillfairlycoloredandneedstobeadditionallytreated.Therefore,anoverviewofconventionalandadvancedsorbentsforthetreatmentofwastewatersisreported.Theuseofactivatedcarbonandzeolitesincolorremovalandremovalofheavymetalionsfromtextileeffluentsisdiscussedindetail.Thepotentialoflow-costorcost-effectivesorbentsasanalternativetotheconventionallyusedsorbentsisalsounderlined.Finally,theopportunitiesandchallengesofusingnanomaterialsinthetreatmentofindustrialwastewatersarehighlighted.
KeywordsColorremoval·Heavymetalremoval·Low-costsorbents·Sorbents·Textileeffluents
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1.Introduction
Anytreatmentofindustrialormunicipalwastewatersisaprocesswhichresultsinthetransformationofamixtureofsubstancesintotwoormoreproductsthatdifferfromeachotherincomposition.Thisprocess,alsoknownastheseparation,maybedifficulttoachieveinducingthehigheffluenttreatmentcostsinchemical,petrochemical,andpharmaceuticalindustries.Formanyseparationprocesses,theseparationiscausedbyamassseparatingagent[1].Themassseparatingagentfortheprocessofadsorption(adsorptiveseparation)istheadsorbentorsorbent.Consequently,theperformanceofanyadsorptiveseparationorpurificationprocessisdirectlydeterminedbythequalityofthesorbent.Becauseoftheprogressmadeinsorbentandcyclicprocessdevelopments,adsorptionhasalreadybecomeakeyseparationtoolthatisusedintextileandmanyotherindustries.Adsorptionisusuallyperformedincolumnspackedwithsorbentparticles,orfixed-bedadsorbers.Thehighseparatingpowerofchromatographyachievedinacolumnisauniqueadvantageofadsorptioncomparedtootherseparationprocesses.Thehighseparatingpotentialisprovidedbythecontinuouscontactandequilibrationbetweentheliquidandsorbentphases.Adsorptionisideallysuitedforpurificationapplicationssuchasindustrialwastewatertreatmentaswellasdifficultseparations[2].
Inspiteofmanyresearcheffortsandpatentsonadsorption,thereareonlyafewcommerciallyavailablesorbentsthatareusedinthecurrentadsorptionprocesses(activatedcarbon,zeolites,silicagel,andactivatedalumina).Futureapplicationsofadsorptionarelimitedbytheavailabilityofnewandmoreefficientsorbents.Ideally,thesorbentshouldbetailoredinsuchawaytomeettherequirementsofeachspecificapplication.Developmentofbettersorbentscanalsoimprovetheperformanceofthecurrentindustrialprocesses.Thelasttwodecadesshowedanexplosioninthedevelopmentofnewformsofcarbon(carbonmolecularsieves,super-activatedcarbon,activatedcarbonfibers,carbonnanotubes,andgraphitenanofibers)andnanoporousmaterialsi.e.nanomaterials(metal-containingnanoparticles,dendrimersetc.).However,thepotentialuseoftheadsorptionpropertiesofthesenewmaterialsisinaprocessofintensiveexploration.Advancesinnano-scalescienceandengineeringareprovidinggreatopportunitiestodevelopmorecosteffectiveandenvironmentallyacceptablewaterpurificationprocesses[3].
Activatedcarbon(AC)hasbeenusedasanall-purposesorbent.Itsprecedent,charcoal,wasfirstusedinthesugarindustryinEnglandin1794todecolorizesugarsyrup.ThemajordevelopmentofactivatedcarbontookplaceduringWorldWarI,foruseinfilterstoremovechemicalagentsfromair.Thecommercialactivatedcarbonhastakenitspresentformsincethe1930s[4].Silicagelandactivatedaluminaareusedmainlyasdesiccants,althoughmanymodifiedformsareavailableforspecialpurificationapplications.Syntheticzeolites,theyoungesttypeamongthefour,wereinventedbyMiltonin1959[5,6].ThezeolitesthatareincommercialusetodayaremainlythetypesinMilton’sinvention,i.e.,typesA,X,andY.Zeolitesexhibitparticularlygoodadsorptionpropertiesduetotheiruniquesurfacechemistriesandcrystallineporestructures.Itshouldbenoted,however,thatasizableportionofthecommercialzeolitesisusedforionexchangeprocessesandascatalysts.Polymericresinsareincreasinglyusedinpotablewaterpurification,becausetheycanreducesomeorganicstolowerconcentrationlevelsthanactivatedcarbondoes.Acid-treatedclaysandpillaredclaysareusedfortreatmentsofedibleandmineraloils.
Theadsorptiveseparationisachievedbyoneofthreemechanisms:
thesteric,kinetic,orequilibriumeffect[2].Inthecaseofthestericeffectonlysmallandproperlyshapedmoleculescandiffuseintothesorbent,whereasothermoleculesaretotallyexcluded.Kineticseparationisachievedbyvirtueofthedifferencesindiffusionratesofdifferentmolecules.Alargemajorityofprocessesoperatethroughtheequilibriumadsorptionofamixtureandhencearecalledequilibriumseparationprocesses.Stericseparationischaracteristicforzeolitesandmolecularsievesbecauseoftheuniformaperturesizeinthecrystallinestructure.
Forequilibriumseparation,thestartingpointforsorbentdesign/selectionistoexaminethefundamentalpropertiesofthetargetedmoleculethatistobeadsorbed(comparedwiththeothermoleculesinthemixture):
polarizability,magneticsusceptibility,permanentdipolemoment,andquadrupolemoment.Ifthetargetedmoleculehashighpolarizabilityandmagneticsusceptibility,butnopolarity,carbonwithahighsurfaceareawouldbeagoodcandidate.Sorbentswithhighlypolarsurfaces(e.g.,activatedalumina,silicagel,andzeolites)wouldbedesirableforatargetedmoleculethathasahighdipolemoment(andhighpolarizability).Ifthetargetedmoleculehasahighquadrupolemoment,sorbentswithsurfacesthathavehighelectricfieldgradientsareneeded.Zeolitesaretheonlysuchsorbents,asthecationsaredispersedabovethenegativelychargedoxidesontheirsurfaces.Cationswithhighvalences(i.e.,charges)andsmallionicradiiwouldresultinstronginteractions.
Sorbentdesign/selectionisacomplexproblem,becausetheprocessforwhichthesorbentisusedneedstobeconsideredatthesametime.Forpurificationandparticularlyultrapurification,strongadsorptionbondsareneeded.Sorbentsthatformweakchemicalbondswiththetargetedmoleculecanbeparticularlyuseful.Forkineticseparation,theporesizeneedstobepreciselytailoredtoliebetweenthekineticdiametersofthetwomoleculesthataretobeseparated.Manymicroporousmolecularsieveswithvariousporedimensionshavebeensynthesizedwhichcouldbeusedassorbents[7].
2.TextileWastewaters
Themajorpartofthewastegeneratedbythetextileindustrycorrespondstowastewaterderivingfromthewetprocessingstages.Textileplants,particularlythoseinvolvedindyeing,printing,andfinishingprocessesaremajorwaterconsumersandthesourceofconsiderablepollution.Theaveragewaterconsumptionintextileprocessesis160kgperkgoffinishedproduct[8].
Inadditiontothedirectenvironmentalimpactofthewastewater,thelargeconsumptionofwaterresourceisbecomingintolerableincountriessubjectedtorealorpotentialwatershortages(asthesouthernpartofEuropeis).Arationalpolicyinwatermanagementwouldallocatethepurestfreshwatersourcesforpotableusewhileencouragingneweffluentrecyclingsolutionsforindustrialconsumption.Afinalimportantenvironmentalimpactcanbeidentifiedinthelargeconsumptionofenergymainlyduetotheheatingofchemicalbathsandtothedryingoffabricsandyarns,whichcouldbesubstantiallyreducedbyefficientwaterrecycling[9].
Waterandchemicalconsumptionisratherimportantandconsequentlylargevolumesofwastewateraregenerated.Thesestreamscontainawiderangeofcontaminantswhichmustberemovedfromthetextileeffluentsbeforetheirdisposal.Organicandinorganiccompoundsusedinthetextileprocessesaredischargedinthewastewaterataveragelevelsof80%and90%,respectively.Table1presentsasummaryofcompoundspotentiallyreleasedduringthevarioustextileoperations.Scouring,dyeing,printing,andfinishinggeneratethemajorityoftextilewastewater,astheyrequiremanyrinsingsequencesaftereachstep.Asanaverage,60to90%ofthetotalwaterconsumptionisforrinsingpurposes[10].
Desizing,insomecasescombinedwithscouring,isoneoftheindustry’slargestsourcesofwastewaterpollutants,contributingtoarelevantpartofthetotalorganicpollutioninwastewater[9].
Thespecificpollutingloadofsizingagentsis1–2gramsofCODpergramofsize.Naturalsizes,suchasthosebasedonstarchorproteins,arealsocharacterizedbyahighBODandaBOD/CODratioof0.6–0.7.Syntheticsizingagentssuchaspolyvinylalcoholorcarboxymethylcellulose,onthecontrary,havealmostnoBOD.Theorganicloadduetodesizingcanberoughlycalculatedonthebasisoftheamountofsizeappliedinthepreviousphases(thesizeappliedisusually5to20%onaweightbasisoftheyarntreated).
Thecontentofpollutantsinscouringeffluentsdependsonthenatureandquantityoftheimpuritiespresentonthefibersandontheintensityoftheprocessitself.HighTSSandhighorganicloadsarecommonineffluentsfromscouringofnaturalfibers,duetotheremovalofdirt,waxes,vegetablematter,etc.Soa
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