treatmentofwastecontainingEDTAbychemicaloxidation

TreatmentofwastecontainingEDTAbychemicaloxidationM.D.Tuckera,L.L.Bartonb,*,B.M.Thomsonc,B.M.Wagenerb,A.AragoncaSandiaNationalLaboratories,Albuquerque,NM88185,USAbDepartmentofBiology,UniversityofNewMexico,Albuquerque,NM87131,USAcDepartmentofCivilEngineering,UniversityofNewMexico,Albuquerque,NM87131,USAAccepted21July1999AbstractEthylenediaminetetraaceticacid(EDTA)isachelatingagentthathasbeenextensivelyusedtoenhancethesolubilizationofheavymetalcationsandreleaseofEDTAcontributestoenvironmentalproblems.EDTAisrecalcitranttomicrobialmetabolismandchemicaloxidationisconsideredapossiblemethodofremedialtreatment.TheuseofthecommerciallyavailableprocessofMIOXCorporationgeneratesmixedoxidantsonsiteandthissolutionismarkedlye€ectiveinthedestructionofthechelatingcharacteristicandthedecarboxylationofEDTA.WhenmeasuringthereleaseofC-14fromcarboxyllabeledEDTA,themixedoxidantsolutionwascomparabletotheFenton'sreactionoverabroadpHrange.ThepresenceofMn2+,Cr3+,orFe3+atlevelsequaltothatofEDTAstimulatedtherateofEDTAdecomposition;however,therateofEDTAbreakdownwasinhibitedwhentheconcentrationofCr3+orMn2+exceededtheconcentrationofEDTA.ThetreatmentofCo2+±EDTAorCu2+±EDTAwithmixedoxidantsinthepresenceofultravioletlightresultedinthelossofchelationabilityofEDTA.Intheabsenceofchelatedmetals,over75%ofthechelationpropertyofa70mMEDTAsolutionwasdestroyedin45min.ThereactionproductsresultingfromtheuseofmixedoxidantsaddedtoEDTAwerenon-toxictobacteriaandshouldnotcontributetoadditionalenvironmentalproblems.#1999ElsevierScienceLtd.Allrightsreserved.Keywords:EDTA;Fenton'sreaction;Mixedwaste1.IntroductionContaminationduetomixedwasteisaproblemthatmaybeattributedtothepresenceoftwoormoretypesoftoxicorhazardouschemicals.Insomesitesmixedwastesmayconsistofheavymetalionsandorganicchelatingagents[i.e.ethylenediaminetetraaceticacid(EDTA);nitrilotriaceticacid(NTA);ordiethylene-triaminepentaaceticacid,(DTPA)]ororganiccomplex-ingagents(i.e.oxalicacid,formicacidorcitricacid).Theseorganiccompoundshavetheabilitytoformstrongcomplexeswithradionuclidesresultinginmetalcomplexesthatarehighlymobileintheenvironment[1].Insomeinstances,industrymayemploychelatingagentsintheirprocessingandthereleaseofchelatorsintotheenvironmentsequestersmetalions.Forexample,thepresenceofEDTAinmunicipalwastewatercanincreasethelevelofzincandleadinthesecondaryclari®ere‚uentbyasmuchas200%[2].Ithasbeenestimatedthat65,000tonsofEDTAandDTPAwouldbereleasedayearbythepulpandpaperindustry[3].Thus,thereisalongstandingconcernthatchelatorssuchasEDTAmaynotonlycarrytoxicmetalsintoanenvironmentbutthattheymayalsomobilizetoxicheavymetalsfromcontaminatedsediments[2,4,5].Itisapparentthattoprotecttheenvironment,proce-duressuitablefordestructionofEDTAareneeded.DecompositionofEDTAhasbeenreportedunderirra-diationwithahighintensityxenonarclampoverapHrangeof4.5to6.9[6].OzonehasbeenfoundtodegradeEDTAandtherateofdestructionisin¯uencedbycoordinationsincethelossofstructuralintegritywasgreaterinthepresenceofCa2+thanFe3+[7].ItwasdeterminedthatthermaldegradationofEDTAishighlye€ectiveandoccursatpH10whentemperaturesare0956-053X/99/$-seefrontmatter#1999ElsevierScienceLtd.Allrightsreserved.PII:S0956-053X(99)00235-4WasteManagement19(1999)477±482*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](L.L.Barton)greaterthan230C[8];however,incinerationbasedproceduresarefacedwithconsiderablesocialresistance.TheoxidationofEDTAbygrowingbacteriaisarelativelyslowreaction[9,10]andnotlikelytobeapplicableincommercialprocess.Therecalcitrantnat-ureofEDTAisevidentinthatitpassesthroughindus-trialormunicipaltreatmentplants[11±13]andthedegradationofEDTAinmunicipalsludgeproceedsataslowrate[5].PureculturesofAgrobacteriumhavebeenreportedtogrowonEDTA[9]withabout30%ofche-latormetabolized/day.Eventheconstructionofanimmobilizedenzymesystemobtainedfrombacteriawouldnotbecommerciallyfeasibleduetotheinstabil-ityofenzymesandtherequirementforacontinuoussupplyofbiologicalenergy.Ontheotherhand,chemi-calprocessesforthedecompositionofEDTAappeartohavethecapabilitytodestroychelatoractivityofcertainmoleculesandinsomeinstances,thecompletedecom-positionofEDTAtocarbondioxideandammoniawilloccur.However,withpartialoxidationofEDTAbychemicalprocessesanarrayoforganicacidsmaybeproducedandthesecouldserveasexcellentsubstratesforbacteria.Thus,weproposetheutilizationofatwostageprocessforthecompletedetoxi®cationofmetal±EDTAcompoundsanditwouldinclude:chemicaloxi-dationasa®rststageandtheremovaloftoxicmetalsthroughtheuseofimmobilizedanaerobicbacteriainthesecondstage.Appropriateforthesecondstageistheremovalofheavymetalsfromawastestreamthroughtheuseofimmobilizedsulfate-reducingbacteria[14,15]however,thechemicaltreatmentforthe®rststageisunresolved.PreviousstudieshaveexaminedthedestructionofEDTAbyozoneandhydrogenperoxideinthepresenceofCu2+andFe3+[16].TherateofhydrogenperoxideoxidationofEDTAwasdependentontheironcon-centrationandtherateofozonedestructionofEDTAwasdependentontheconcentrationofcopperpresent.Thus,thepresenceofchelatedme