差分光谱的NOM氯化反应的显示

ExaminationofNOMChlorinationReactionsbyConventionalandStop-FlowDifferentialAbsorbanceSpectroscopyGREGORYV.KORSHIN,*,†MARKM.BENJAMIN,†HYUN-SHIKCHANG,†ANDHERVEÄGALLARD†DepartmentofCivilandEnvironmentalEngineering,UniversityofWashington,Box352700Seattle,Washington98195-2700,andLaboratoiredeChimiedel’Eauetdel’Environnement,Universite´dePoitiers,40AvenueduRecteurPineau,Poitiers,France86022Mechanismsofchlorinationofnaturalorganicmatter(NOM)insurfacewater(LakeWashington)wereexploredviadifferentialspectroscopy.Twotypesofdifferentialspectra(overallandincremental)weregeneratedforthiswaterchlorinatedatpH7usingvaryingchlorinedosesandreactiontimes.Thedifferentialspectracontaintwokineticallyandspectroscopicallydistinctcomponents.Oneofthesecomponentsisattributabletofunctionalgroupsthatreactrapidlywithchlorine,whiletheotherreflectstransformationsofslowlyreactingchromophoresthatarisefollowingthedepletionofthefastchromophores.Smallconcentrationsofdisinfectionbyproducts(DBPs),exemplifiedinthisstudybydichloroaceticacidandchloralhydrate,wereproducedduringtheinitialphaseofchlorination,whenthefast-reactingchromophoreswerebeingconsumed.Rather,thereleaseofthoseDBPswascorrelatedwiththebreakdownoftheslowlyreactingchromophores.IntroductionSincetherecognitionofchloroformasaubiquitousdisin-fectionbyproduct(DBP)inthe1970s,numerouschlorine-andbromine-containingDBPspecieshavebeenfoundindrinkingwater(1-3).Trihalomethanes(THMs)andhalo-aceticacids(HAAs)normallyaccountforca.50%ofthetotalorganichalogen(TOX)indrinkingwater,withtherestoftheTOXbeingmainlyincorporatedintolargermoleculesofnaturalorganicmatter(NOM)(1,4,5).ChlorinationsitesinNOMarethoughttobepredominantlypolyhydroxyaromatic(PHA)moieties(6-8)and,secondarily,estersandketones(9,10).DespiteprogressinexploringthereactivityofthePHAgroups(8,11,12),mechanismsoftheirreactionswithhalogenspecieshavenotbeenascertained.Evenformodelprecursorcompoundssuchasresorcinolanddihydroxy-benzoicacid,theincorporationofchlorineinvolvesmultistep,branchingreactions(13-16);thereactionsofNOMandchlorineareundoubtedlymorecomplex.Thesedifficultiesnotwithstanding,numerousmechanisticandphenomenologicalmodelsofDBPformationhavebeenproposed.Themechanisticmodelsassumethatthechlo-rinationofNOMisamultistepreactionthatinvolveschlorineincorporationintothepredominantlyaromaticreactivecentersofNOM,followedbybreakdownofthesegroupstoreleaseindividualDBPs(10,17-20).SuchmodelscanbeemployedtodevelopakineticdescriptionofNOMhaloge-nation,buttheutilityofthisapproachislimitedbyuncertaintiesregardingthechemicalandkineticpropertiesofthereactiveNOMfunctionalgroups.Ontheotherhand,phenomenologicalmodels(e.g.,basedonregressionsrelatingDBPformationtoDOC,chlorinedose,pH,etc.)canachieveatolerablegoodness-of-fit,butonlybyinclusionofseveral,site-specificempiricalfittingparameters(10,19,21,22).ThecomplexityofNOMpropertiesandreactionshasnecessitatedtheuseofsurrogateparameterssuchasab-sorbanceoflightat254nm(A254)andthespecificabsorbanceatthatwavelength(SUVA254)toestimateitsreactivitytowardchlorination(8,23-26).Wepreviouslyproposedanalterna-tiveapproach,inwhichtheintensityofthedifferentialabsorbanceat272nm(¢A272)ismeasuredduringthecourseofchlorinationandcorrelatedwiththeindividualDBPsand/orTOXconcentrationsgeneratedinthesolution(27-29).Thevalueof¢Aìiscomputedasthechangeinabsorbanceinducedbychlorination,measuredatanytimeafterchlorinehasbeendosed(eq1)whereAì(t)andAì(0)aretheabsorbancesofNOMatwavelengthìafterreactiontimetandpriortochlorination,respectively.BecauseNOMchlorinationleadstoadeclineinUVabsorbance,¢Aìisnegative.ThedifferentialspectraofchlorinatedNOMfromallsurfacewatersinvestigatedtodate(27-29)exhibitabroadbandwithamaximumatawavelengthbetween265and275nm,andamonotonicincreaseinthemagnitudeof¢Aìwithincreasingchlorinedoseorreactiontime.Thedifferentialspectraundergosubtlechangesinshapeasafunctionofchlorinedoseandreactiontime(30),butthesechangeshavenotbeensystematicallystudied.Despitethestrengthofthecorrelationsbetween¢AìandtheconcentrationsofindividualDBPs,onlyafewattemptshavebeenmadetodeterminehowthepresenceofkineticallydistinctfunctionalitiesinNOMismanifestedinthedifferentialspectra,andhowthecorrespondingfeaturesofthedifferentialspectrarelatetoDBPrelease(30).Inpriorpublicationsdealingwithdifferentialabsorbance(27-30),wefocusedonthechangeintheabsorbancespectrumfromtheinitiationofchlorination.Thatspectrumisreferredtohereastheoveralldifferentialabsorbancespectrum.Inthiswork,wealsoconsiderincrementaldifferentialspectra;i.e.,thevalueof¢Aì(t1,t2)ineq2Theoveralldifferentialabsorbancespectrumisthustheincrementaldifferentialspectrumevaluatedatt1)0,i.e.,itis¢Aì(0,t2).Inthispaper,weanalyzebothoverallandincrementaldifferentialabsorbancespectraofchlorinatedNOMtogaininsightsintothemechanismofNOMchlorina-tionandDBPrelease.ExperimentalSectionWaterfromLakeWashington(LW)inSeattle,WA,wasusedinallexperiments.TheDOCofthewaterwas3.0mg/L.Chlorinationwascarriedoutat20°C.ThepHwasbufferedat7.0with0.03Mphosphate.Allexperimentalprocedures*Corrrespondingauthorphone:(206)543-23