sonochemicaldestructionoffreeandmetalbindingEDTA

WaterResearch37(2003)3155–3163Sonochemicaldestructionoffreeandmetal-bindingethylenediaminetetraaceticacidJ.AaronFrima,b,JamesF.Rathmanc,LindaK.Weaversa,*aDepartmentofCivilandEnvironmentalEngineeringandGeodeticScience,TheOhioStateUniversity,470HitchcockHall2070NeilAve.,Columbus,OH43210,USAbDepartmentofGeologicalSciences,TheOhioStateUniversity,Columbus,OH43210,USAcDepartmentofChemicalEngineering,TheOhioStateUniversity,Columbus,OH43210,USAReceived4June2002;accepted20March2003AbstractThisstudyfocusedonthesonochemicaldegradationofethylenediaminetetraaceticacid(EDTA)andchromium-EDTAcomplexes.Degradationofthecopper(II)-EDTAcomplexwasalsoinvestigatedasacomparisonmetalcomplex.A90%degradationofa150-mMEDTAsolutionwithcontinuousO2-bubblingwasshownforthe20-kHzsysteminapproximately3h(kpseudo-firstorder=1.22
102min1)andlessthan1hforthe354-kHzsystem(kpseudo-firstorder=5.42
102min1).TheseresultsareconsistentwiththehigherconcentrationsofhydrogenperoxidefoundinthehigherfrequencysystemandanexpectedoxidationofEDTAinbulksolution.Thepresenceofachelatedmetaldecreasedtherateofdegradationatbothfrequencies.Cr(III)-EDTAdegradedtheslowest,supportingthetheorythattheextremelyslowligandexchangerateofchromiumisthedeterminingfactorinhowfastdegradationbyhydroxylradicalcanoccur.The354-kHzsystemshoweda17%decreaseintheoriginal150-mMCr(III)-EDTAcomplexafter3hofsonication.AllofthechromiumfromthedegradedEDTAcomplexexistedasacombinationofoxidizedCr(VI)andpossiblysmallamountsofanewCr(III)-organiccomplex(Cr(III)-Y).The20-kHzsystemshowedasimilarextentofdegradation(16%)after3hofsonication,despitelowerhydroxylradicalproduction.FiftypercentofthechromiumfromthedegradedEDTAcomplexwasfoundasfreeCr3+ion,withtheremaining50%existingasbothCr(III)-YandCr(VI).Varyingdegreesofbulkoxidation,near-bubblethermolysis,andperhapsdifferentdegradationpathwaysatthetwofrequenciesareresponsibleforthesedifferences.r2003ElsevierScienceLtd.Allrightsreserved.Keywords:EDTA;Sonolysis;Degradation;Chromium;Complexation1.IntroductionThetreatmentofwastestreamscontainingmetalsisaprocesscommontoindustriesfrommetalpartmachin-ingandrefurbishingtotanneries.Treatmentofmetal-ladenwastestreamsiscomplicatedbythepresenceofcomplexingagents.Ethylenediaminetetraaceticacid(EDTA)haslongbeenrecognizedforitschelationofmetals,buthasundergonerecentenvironmentalscrutinyduetoitswidespreaduse,limitedbiodegradation,anditspotentialtoincreasemetalmobility[1–3].EDTAisamemberofalargergroupofcompoundsknownasaminopolycarboxylicacids(APCAs),whosememberschelatemanydivalentandtrivalentmetalstoformstable,water-solublecomplexes[1].Whiletherearenaturallyoccurringsubstancesthatfitintothisclass,mostareanthropogenic.Thesecompoundsarewidelyusedinindustrialprocessesandmanycanbefoundasingredientsinhouseholditemsrangingfromdetergentstofooditems.Asaresult,largequantitiesaredisposed*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](L.K.Weavers).0043-1354/03/$-seefrontmatterr2003ElsevierScienceLtd.Allrightsreserved.doi:10.1016/S0043-1354(03)00169-6andsubsequentlyintroducedintowastewaterstreamsfortreatment[4].Commontreatmentprocessesarenotalwayseffectiveinremovingthesecompounds,allowingthemtoentertheenvironment.ThisdifficultyinremovingAPCAsusingstandardtreatmentmethodsnotonlyposestheproblemofmobilizingmetalionsalreadyintheecosystem,butalsoinfacilitatingthetransportofmetalsthroughthetreatmentprocess,substantiallyreducingeffectivemetalremovalfromthewastestream[3,5,6].Therefore,alternatetreatmentmethodstoreleasethemetalfromthechelatingspeciesarebeingsought.Interestinadvancedoxidationprocesses(AOPs)isduetothehighlyreactive,targetnon-specificnatureofOH,makingtheseprocessesidealcandidatesfortreatingwastewatercontainingorganiccomplexes[7].EDTAhasbeenshowntodegradeusingavarietyofAOPs,includingFenton’sreagent,titaniumdioxidephotocatalysis,hydrogenperoxidephotocatalysis,photolysisofH2O2,andpulseradiolysis[3,6–10].Thesedifferentmethodsincludebothhomogeneousandheterogeneoussystemsandtheresearchersproposevariousdegradationmechanisms.However,allofthemechanismsinvolvehydroxylradicalattack.SonolysisisanAOPwheresoundenergy,withinthefrequencyrangeof20kHztoseveralMHz[11],isappliedtoaqueoussolutions.Theacousticalenergyisconvertedtoheatviatheformationandcollapseofcavitationbubbles.Thehightemperaturesandpressuresthataccompanythecollapseofthesevaporbubblesdissociatewatertoproducereactivehydroxylandhydrogenradicals.Thelocationofthedegradationreactiondependsonthenatureoftheorganiccom-pound.Volatilecompoundsareexpectedtopartitionintothecavitationbubbleandundergothermolysisundertheextremebubble-collapseconditions[12,13].Non-volatilehydrophobiccompoundsandsurfactants,whilenotpassingintothevaporphase,partitiontotheinterface[13,14],aregionthatischaracterizedbyelevated,non-ambienttemperatures[15]andhighhydroxylradicalconcentrations[13].Degradationinthisregionmayresultfrombothhightemperaturethermolysisandhydroxylradicalattack[15].Water-solublesubstancesthatdonotpartitiontothevaporphaseortheinterfaceremainintheaqueouspha