QuinonesandKetonsandAldihydes对CDOM光学性质的影响

ContributionofQuinonesandKetones/AldehydestotheOpticalPropertiesofHumicSubstances(HS)andChromophoricDissolvedOrganicMatter(CDOM)RossanaDelVecchio,*,†TaraMarieSchendorf,‡andNeilV.Blough*,‡†EarthSystemScienceInterdisciplinaryCenter,UniversityofMaryland,CollegePark,Maryland20742,UnitedStates‡DepartmentofChemistryandBiochemistry,UniversityofMaryland,CollegePark,Maryland20742,UnitedStates*SSupportingInformationABSTRACT:Themolecularbasisoftheopticalpropertiesofchromophoricdissolvedorganicmatter(CDOM)andhumicsubstances(HS)remainspoorlyunderstoodandyettobeinvestigatedadequately.Thisstudyevaluatestherelativecontributionsoftwobroadclassesofcarbonyl-containingcompounds,ketones/aldehydesversusquinones,totheabsorptionandemissionpropertiesofarepresentativesuiteofHSaswellasaligninsample.Selectivereductionofquinonestohydroquinonesbyadditionofsmallmolarexcessesofdithionitetothesesamplesunderanoxicconditionsproducedsmallornegligiblechangesintheiropticalproperties;however,whenmeasurable,thesechangeswerelargelyreversibleuponexposuretoair,consistentwiththereoxidationofhydroquinonestoquinones.Withoneexception,estimatesofquinonecontentbasedondithioniteconsumptionbytheHSunderanoxicconditionswereingoodagreementwithpastelectrochemicalmeasurements.Incontrast,reductionofketones/aldehydestoalcoholsemployingexcesssodiumborohydrideproducedpronouncedandlargely,butnotcompletely,irreversiblechangesintheopticalproperties.Theresultsdemonstratethat(aromatic)ketones/aldehydes,asopposedtoquinones,playafarmoreprominentroleintheopticalabsorptionandemissionpropertiesoftheseHS,consistentwiththesemoietiesactingastheprimaryacceptorsincharge-transfertransitionswithinthesesamples.Asamethod,anoxicdithionitetitrationsmayfurtherallowadditionalinsightintothecontentandimpactofquinones/hydroquinonesontheopticalpropertiesofHSandCDOM.■INTRODUCTIONTheopticalabsorptionandemissionpropertiesofhumicsubstances(HS)andchromophoricdissolvedorganicmatter(CDOM)havebeenstudiedextensivelyformorethan50years,yetanunderstandingofthemolecularbasisofthesepropertiesremainsincomplete.RecentworkhasprovidedevidencethattheopticalpropertiesofHSandCDOMcannotarisesolelyfromasimplesuperpositionofnumerousindependentlyabsorbingandemittingchromophores.1−3Instead,electronicinteractionsbetweenchromophoreswithinHSandCDOMhavebeenproposedtoplayanimportantrole.2−5Accordingtothismodel,theopticalpropertiesresultinpartfromchargetransfer(CT)interactionsbetweenelectrondonormoieties(D)(e.g.,phenolicandpossiblysubstitutedaromaticcarboxylicacidgroups)6−8andelectronacceptormoieties(A)(e.g.,carbonyl-containinggroupssuchasaromaticketones/aldehydesandquinones)formedthroughthepartialoxidationoflignin,2,3,5andpossiblyotherhydroxy-aromaticprecursors.3,5Ourworkingstructuralmodelenvisionsthesedonor/acceptorpairstobeinclosecontact,helpingtostabilizetheformationofanensembleofhighermolecularweightsupramolecularaggregates9,10inafashionverysimilartothatdescribedbyDreyeretal.11forcloselyrelatedmaterials.12Further,similartothemodelsuggestedbyLatchandMcNeill,13outerchargedgroups(i.e.carboxylateanions)wouldbeexpectedtoencompassaninnercore14,15inwhichthecontactDAcomplexesformandarefurtherstabilized.Thelong-wavelengthvisibleabsorptionandemissionwouldthusarisefromCTtransitionsbetweenthesecontactDApairs,whereastheultravioletabsorptionandnear-visibleemissionwouldariseprincipallyfromthearomaticstructuresknowntobeapartoflignin(e.g.,localdonorandacceptorstates).2−5ThepresenceofelectrondonorandacceptormoietieswithinHSandCDOMisnowwellestablishedbasedonextensiveelectrochemicalmeasurements,16−18acid/basetitrimetricmeas-urements,7,19,20aswellascombinedoptical,(electro)chemical,photochemical,andstructuralmeasurements.4,21−34Thesestudieshaveprovidedevidenceofphenolicelectrondonorsaswellasquinoidandaromaticketone/aldehydeelectronacceptors.ThesedonorsandacceptorshavebeenlinkedinparttotheopticalpropertiesoftheHSthroughchangesinabsorptionandemissionproducedbyalteringtheabundanceorformoftheelectrondonors/acceptorsthroughchemical,Received:August14,2017Revised:November9,2017Accepted:November10,2017Published:November10,2017Articlepubs.acs.org/est©2017AmericanChemicalSociety13624DOI:10.1021/acs.est.7b04172Environ.Sci.Technol.2017,51,13624−13632DownloadedviaPEKINGUNIVonAugust24,2018at02:28:37(UTC).See−34How-ever,whilethesestudieshaveprovidedstrongevidencethatHSandCDOMcontainketone/aldehydeandquinoneacceptors,theirrelativecontributiontotheabsorptionandemissionpropertiesofHSaswellastheirimportanceaspossibleelectronacceptorsinCTtransitionshasnotbeenwellstudiedandhas,infact,beenthesubjectofsomecontroversy.35AlthoughsomestudieshavesuggestedthatreductionofquinonesinHScausesminorandmostlyreversiblechangesinfluorescenceemission,34,36,37othershaveclaimedsignificantchangesinthefluorescenceemission,38withfurtherclaimsthatthefluorescenceemissionarisesprimarilyfromthequinone/hydroquinonemoieties38,39andcanbeusedtodeducethere