on the OH-initiated phototransformation of phenol New insights regarding the impact of radical transfer and scavengermaterials Journal of Photochemistry and Photobiology A:Chemistry

New insights regarding the impact of radical transfer and scavenger materials on the

OH-initiated phototransformation of phenol

Zsuzsanna Kozmér

^a,b,

*, Eszter Arany

^a

, Tünde Alapi

^a,b

, Georgina Rózsa

^a

, Klára Hernádi

^a,c

, András Dombi

^a

aResearchGroupofEnvironmentalChemistry,UniversityofSzeged,H-6720Szeged,RerrichBélatér1,Hungary

bDepartmentofInorganicandAnalyticalChemistry,UniversityofSzeged,H-6720Szeged,Dómtér7,Hungary

cDepartmentofAppliedandEnvironmentalChemistry,UniversityofSzeged,H-6720Szeged,RerrichBélatér1,Hungary

ARTICLE INFO

Articlehistory:

Received22May2015

Receivedinrevisedform19August2015 Accepted22August2015

Availableonline28August2015

Keywords:

Vacuumultravioletphotolysis tert-butanol

Formateanion Reactiveoxygenspecies Hydrogenperoxide pH

ABSTRACT

Fortheinvestigationsoftheoxidativetransformationsofvariousorganicsubstancesknowledgeofthe rolesand relativecontributions ofthereactivespeciesformedtothetransformationsof thetarget substancesisneeded.Thevacuumultraviolet(172nm)photolysisofaqueoussolutionofphenol(PhOH) (1.010⁴molL¹)asmodelcompoundwasthereforeinvestigatedinthisworkinthepresenceor absenceofvariousradicalscavenger(tert-butanol,t-BuOH)andtransfermaterials(dissolvedO2;formic acid,HCOOH;sodiumformate,HCOONa).

ItwasconcludedthattherateofdegradationofPhOHincreasedsignificantly(upto3-fold)inthe presenceofdissolvedO2mainlybecauseofthehinderedrecombinationoftheprimaryradicals(Hand

OH),theincreasedconcentrationofOH.

Alloftheappliedorganicradicalscavengerandradicaltransfermaterialsdecreasedtheinitialrateof degradationofPhOHmainlybyreducingtheconcentrationsofreactiveprimaryradicals.InO2-free solutions,theeffectsofHCOOHandformateanion(HCOO^–)werefoundtobemoresignificantthanthat oft-BuOH,whichcanbeexplainedbythedifferentreactivitiesofthecarbon-centredradicalsformed.In O2saturatedsolutions,therewasnosignificantdifferencebetweentheinitialratesoftransformation determinedinthepresenceofthevariousadditives.Inthesecases,thelessreactiveHO2orO2–werethe mostsignificantspeciesoftheradicalset,anditseemsthattheseradicalsmakeonlyminorcontributions tothetransformationofPhOH,theycontributemainlytotheformationofH2O2instead.

ã2015PublishedbyElsevierB.V.

1.Introduction

Variouspollutants, suchas medicinalagents and pesticides, may possibly enter the environment, accumulate in living organisms and cause serious environmental problems. These contaminantscannotusuallybecompletelyremovedbymeansof conventionalwatertreatmentmethods,whichthereforehavetobe supplemented, forexample, withadvanced oxidationprocesses (AOPs),whicharegenerallybasedonradical-initiatedreactions.

MostAOPsdependontheformationofhydroxylradicals(OH), whichareveryreactive,non-selectiveoxidizingspeciescapableto initiatetheoxidativetransformationofextensivevarietyoforganic pollutants [1]. One such AOP is the vacuum ultraviolet (VUV) process,whichinvolvesreactionsinitiatedbyhigh-energyphotons generatedbyVUVlampsemittingradiationatwavelengthsshorter than200nm.OnetypeofVUVlampistheexcimerlamp,which emitsquasi-monochromaticradiationwhosewavelengthdepends on the type of gas applied [2]. Although the efficiency and mechanisms of theVUV process have been studied byseveral researchers[3–7],furtherinvestigationsmaypresentchallenges duetothenatureofthesystem.Morethan30reactionsareknown tooccuronlyduringtheVUVphotolysisofpureliquidH2O[8], involving several radical species (OH; hydrogen radical, H;

hydratedelectrons,eaq–;hydroperoxylradical/superoxideradical

* Correspondingauthorat:H6720Szeged,Dómtér7,Hungary.

E-mailaddresses:kozmerzs@chem.u-szeged.hu(Zs.Kozmér),

arany.eszter@chem.u-szeged.hu(E.Arany),alapi@chem.u-szeged.hu(T.Alapi), rozsa.georgina@chem.u-szeged.hu(G.Rózsa),hernadi@chem.u-szeged.hu (K.Hernádi),dombia@chem.u-szeged.hu(A.Dombi).

http://dx.doi.org/10.1016/j.jphotochem.2015.08.023 1010-6030/ã2015PublishedbyElsevierB.V.

ContentslistsavailableatScienceDirect

Journal of Photochemistry and Photobiology A:

Chemistry

j o u r n a lh o m e p ag e : w w w . e l s e vi e r . c o m / l o c a t e / j p h o t o c h e m

anion,HO2/O2–etc.)which can reactwitheach otheror with otherspeciesinthesystem,suchasorganicmoleculesordissolved O2.Understandingoftherolesandrelativecontributionsofthese speciestothetransformationoforganicsubstancesisoneofthe keystooptimizationoftheVUVprocessandotherAOPs.

InthecourseoftheVUVphotolysisofaqueoussolutions,VUV photonsinducehomolyticdissociationofH2Omolecules[9].When axenonexcimerlampisused,whichemits172nmVUVlight,the initiationprocessesare[4,10]:

H2Oþhv172nm!HþOH

F

^172nmðOHÞ¼0:42 ½4 ð1Þ

H2Oþhv172nm!H^þþeaqþOH

F

^172nmðeaqÞ<0:05 ½10 ð2Þ

OH, Hand (in lower yield)eaq– are generated as primary radicalsduringtheVUVphotolysisofaqueoussolutions.Sinceeaq–

ispresent invery lowconcentrationduring VUVphotolysis, its reactionsaregenerallynottakenintoconsideration.

OH and Hcan dissociateat basic pH,regarding their acid dissociationconstants[11,12]:

HþH2OÐeaqþH3O^þ pKa¼9:6 ½11 ð3Þ

OHþH2OÐeaqþOþH3O^þ pKa¼11:9 ½12 ð4Þ

Theprimaryradicalsarepresumablyformedinasolventcage.

Inthiscase,H2Omoleculesactasthe‘cage’andpreventthespecies frombreakingthroughthefirstsolvationshell.Whenthesystem contains O₂ and/or organic molecules, competition generally occursbetweentherecombinationofprimaryradicalsandtheir reactionswiththesesubstances[7].

InthepresenceofO₂thepreviouslymentionedprimaryradicals areaccompaniedbyotherreactiveoxygenspecies(ROS),suchas HO2[12]:

HþH2OÐHO2

k5¼2:110¹⁰Lmol¹s¹ ½12 ð5Þ

ThespeciesHO2andO2–formaconjugateacid–basepair,the ratiooftheirconcentrationdependsonthecurrentpH[13].

HO2

þH2OÐO2

þH3O^þ pKa¼4:8 ½13 ð6Þ

AlthoughtherecombinationofOHtheoreticallyresultsinthe formationofhydrogenperoxide(H2O2)(Eq.(7)),thisreactiontakes placetoonlyaminorextentinconsequenceoftheothercompeting reactionsofOH(e.g.itsreactionswithHororganicsubstances), resulting in a minor or negligible H2O2 concentration under deoxygenatedconditions[12,14–16].

2OH!H2O2 k7¼5:510⁹Lmol¹s¹ ½12 ð7Þ H2O2ismainlyformedinthedisproportionationofHO2and O₂^–[13]:

2HO2

!H2O2þO2 k8¼8:310⁵Lmol¹s¹ ½13 ð8Þ 2O2

þ2H2O!H2O2þO2þ2OH

k9<310¹Lmol¹s¹ ½13 ð9Þ

HO2

þO2

þH2O!H2O2þO2þOH

k10¼9:710⁷Lmol¹s¹ ½13 ð10Þ Theexperimentallyobservedrateofthedisproportionationof HO2/O2–isdependentonthepH[13].TheconcentrationofH2O2

formedcanthereforegiveinformationabouttheconcentrationsof ROS(mainlyHO2/O2–).Ontheotherhand,theformationofboth HO2andO2–isalsopossible,duetothefurthertransformationof organicperoxylradicalsformedin thereactionsof theprimary radicalswithorganiccompoundsinthepresenceofmolecularO2

[17,18].

Radical-based reactions of the model compounds in VUV- irradiated aqueous solutions can be investigated by the use of variousradicalscavengerand/orradicaltransfermaterials.When suchadditionalcompoundsarealsopresent,theycompetewiththe modelmoleculesfortheprimaryradicals,whichgenerallyresultsin alowerrateoftransformationofthemodelmolecule.Thefurther transformationsoftheradicals or radical ionsformedinthereactions of theadditional compoundwith primaryradicalscan resultin specieswhichcanopenfurther,newreactionpathwaysorshiftthe ratios of theexisting ones for thetransformation of the model compound[17,19,20].Theseadditionalcompoundsarecalledradical transfer materials. The additional compoundis referredto as a radicalscavengerwhenitsfurthertransformationdoesnotresultin theformationofotherreactivespecies.

As mentioned previously, dissolved O2 affects the concen- trationsofprimaryradicals,sinceO₂reactswithHandconvertsit to HO2 (Eq. (5)).Additionally, it can form peroxyl radicals by additiontocarbon-centredradicals[17,21].Ontheotherhand,in theabsenceof dissolvedO₂,HcombinestoyieldmolecularH₂ which,duetoitslowsolubilityinaqueoussystems,isassumedto be of little importance within the manifold of reactions of oxygenatedintermediates[8].

2H!H2 k11¼1:010¹⁰Lmol¹s¹ ½8 ð11Þ In this study the phenol (PhOH) was chosen as model compound to investigate the role and contribution of various reactive species to the transformation in the VUV irradiated aqueoussolution.Bothoftheprimaryradicalsproduced during VUVphotolysisreactwithPhOHwithreactionrateconstantsofthe sameorderofmagnitude[12,22]:

HþPhOH!hydroxycyclohexadienylradical

k12¼1:710⁹Lmol¹s¹ ½12 ð12Þ

OHþPHOH!dihydroxycyclohexadienylradical

k13¼8:410⁹Lmol¹s¹ ½22 ð13Þ ThetransformationofPhOHcanbeinitiatedbytheadditionof

OHtothearomaticringintheortho(48%)orpara(36%)position.

Addition to the meta or ipso position is expected to be quite negligible [23]. In the presence of dissolved O2, furthertrans- formationsofdihydroxycyclohexadienylradicalsresultinmainly 1,2- (Eq. (14)) or 1,4-dihydroxyphenols (Eq. (15)) via HO₂ elimination[21,23].

InO2-freesolutionsthemostlikelyfurthertransformationof thecyclohexadienylradicalsistheirrecombinationanddismuta- tion[8]andvariousring-openingreactions[24].

Tert-butanol(t-BuOH)is aOH scavenger.Itreacts withOH withahighrateconstant(k₁₆)[12],andwithHwitha3ordersof magnitude lower rate constant (k17) [26]. Consequently, the concentrationofHmayremainsignificant,whereastheconcen- trationofreactiveOHmustbelowinsolutionscontainingt-BuOH.

H-abstractionfromt-BuOHyields2,2-dimethyl-2-hydroxyethyl radical (t-BuOH), which has low reactivity towards organic compounds[25].

Formicacid(HCOOH)andformateanion(HCOO^–)reactwith

OH(Eqs.(19)and(21))andH(Eqs.(20)and(22))andresultin carbon-centredradicalsoflowreactivity,thusthesecompounds behaveasradicaltransfermaterials[21,27,12].

HCOOHþH2OÐHCOOþH3O^þ pKa¼3:75 ½27 ð18Þ

HCOOHþOH! COOHþ H2O

k19¼1:310⁸Lmol¹s¹ ½12 ð19Þ

HCOOHþH !COOHþ H2

k20¼4:410⁵Lmol¹s¹ ½12 ð20Þ

HCOOþOH! CO2

þH2O

k21¼3:210⁵Lmol¹s¹ ½12 ð21Þ

HCOOþH!CO2

þH2

k22¼2:110⁸Lmol¹s¹ ½12 ð22Þ The carboxyl radicals (COOH) and carboxyl radical anions (CO₂^–)formaconjugateacid–basepair[27]:

COOHþ H2OÐCO2

þH3O^þ pKa¼1:4 ½27 ð23Þ InthepresenceofO2,COOHandCO2–undergotransformation toHO2andO2–,respectively[27,28]:

COOHþ O2!CO2þHO2

k24¼310⁹Lmol¹s¹ ½27 ð24Þ

CO2

þO2!CO2þO2

k25¼4:210⁹Lmol¹s¹ ½28 ð25Þ For optimization of the transformation pathways of organic substances, accurate knowledge of the mechanisms is needed.

Only limited knowledge is available concerning the effects of variousradicalscavengerandtransfermaterialsontheradicalset generatedduringAOPs.Theaimofthisworkwastoinvestigatethe effects of dissolved O2 and the influences of t-BuOH as OH

(14)

(15)

OH

OH OH

H

O

H OH

H

OO OH

OH

OH

OH H

H HO OO

OH H

H

OH

OH

•OH

− HO₂^• O₂

O₂

•

•

− HO₂^•

•

•

scavenger,andHCOOHandHCOO^–asOHtransfermaterialsonthe VUV degradation process of PhOH. Comparison of the results obtainedontheuseofHCOOH(pH1.9)andHCOO^–(pH8.0),the effectsofpHwerealsoinvestigated.

2.Materialandmethods

2.1.Theexperimentalsetup

250mL 1.010⁴molL¹ (c0)aqueous PhOH (VWR, 100.0%) solutionpreparedin ultrapureMILLI-QH2O(MILLIPORE Milli-Q Direct 8/16, permeate conductivity: 13.3

m

^Scm1, resistivity:

18.2M

V

^{cm, total organic carbon (TOC) content: 2ppb) was}

irradiatedwithVUV light produced bya Xeexcimerlamp.The solutionwas circulated between thethermostated (250.5C) reactorand reservoirbya HeidolphPumpdrive5001peristaltic pumpataflowrateof375mLmin¹.Duringirradiation,thepHand theconcentrationsofH2O2andPhOHweremeasured.Thekinetic measurementswerestartedbyswitchingonthelamp.

The Xe excimer lamp (Radium Xeradex^TM, length: 130mm, externaldiameter:40mm,20Welectricalinputpower)emitted quasi-monochromaticVUVphotonsat17214nm(7.21eV).The photonfluxofthelightsourcedeterminedbymeansofmethanol actinometry[3] was foundto be(3.00.1)10⁶molphotons¹. Thelampwasplacedintothecentreofatriple-walledtubularglass reactor(length:220mm,externaldiameter:70mm,theinnerwall being made of Suprasil¹ quartz). The irradiated solution was circulatedwithinthetwoinnerwallsofthereactorina2.0mm thicklayer.

2.2.Materials

To investigate the effect of dissolved O2, either N2 (Messer,

>99.99% purity) or O2 (Messer, >99.99% purity, resulting in a dissolved O2 concentration of 12.510⁴molL¹) was bubbled throughthesolutionsataflowrateof600mLmin¹.Theinjection ofthegaswasstarted30or15minbeforeeachexperimentinthe casesofN₂andO₂,respectively,andwascontinuedthroughoutthe irradiation.

The samples contained 0.50molL¹ t-BuOH (VWR, 100.0%), HCOOH(VWR,99.0%)orsodiumformate(HCOONa,FLUKA,99.0%).

Theconcentrationoftheadditiveswas5000timeshigherthanthe c0ofPhOH(1.010⁴molL¹)soastoensurethatthemajorityof theprimaryradicalsreactedwiththeorganicradicalscavengeror transfermaterials.

FortheinvestigationoftheeffectofthebasicpHontheVUV transformationofPhOH,NaOH(VWR,99%purity)wasaddedtothe solutionstoadjusttheinitialpHintherangeof7–11.

2.3.Analyticalmethods

TheH2O2concentrationwasmeasuredspectrophotometrically byusingtheHydrogenPeroxideTestbyMerck,validintherange 4.4110⁷–1.7610⁴molL¹. The method is based on the

reductionofCu^II-dimethylphenanthrolinebyH2O2 toresultina coloured Cu^I ion-containing complex (

e

454nm=14,300200L mol¹cm¹[29]).Theabsorbanceof thesamplewas measured at455nmincellswithapath-lengthof1.00cm,usinganAgilent 8453diodearrayspectrophotometer.ThepHofthesamplemust bebetween4and10formeasurementofitsH2O2concentration;

whennecessary,itwasthereforeadjustedwithHCl(VWR,diluted from36.0% solution)or NaOH(VWR, 99%purity). Analysiswas performedwithaConsortC835S/N74117pH-meter.

TheconcentrationofPhOHwasfollowed byanAgilent 1100 Serieshigh-performanceliquidchromatographwithUVdetection.

Aromatic compounds were separated on an RP-18 column (LiChroCART¹ 150-4.6,5

m

^{mparticlesize),using35% methanol}

(VWR,99.80%)and65%ultrapureMILLI-QH2Oaseluentataflow rateof0.8mLmin¹ at25C.In eachcase,20

m

^Lofsamplewas

analysed. The wavelength for UV detection was 210nm. The decomposition ofPhOHwas characterized bytheinitialrateof transformation,whichwasobtainedfromlinearregressionfitsto thecurvesoftheactualconcentrationofPhOHversusthetimeof irradiation,upto10%oftheconcentrationoftransformedPhOH.

Duringthedegradationprocesses,thepHusuallychanges,and itwas thereforemeasuredat5minintervalswithaninoLabpH 730ppH-meter.

The standard deviations of the measured PhOH and H2O2

concentrationsandpHvaluesarepresentedinthefigures.

3.Resultsanddiscussion

3.1.EffectsofdissolvedO2

O2 is one of the most important radical transfer materials, whichreactswithHandconvertsittolessreactiveHO2(Eq.(5)).

TherateofthetransformationofPhOHwassignificantlyhigher inO2-saturatedsolutions(Table1)thanunderO2-freeconditions (Fig. 1a), which can be explained mainly by the addition of molecularO2totheformedradicalinthefirst,reversiblestepof PhOH with OH (Eqs. (14) and (15)). With regard to the rate constants of H withO₂ and PhOH (k₅ and k₁₂), and since the concentrationofdissolved O2(cO2=1.2510³molL¹)wasone orderofmagnitudehigherthantheinitialconcentrationofPhOH (c0=1.010⁴molL¹),Hreacted mainlywithO2(Eq.(5)).The effect of the suppressed concentration of H on the rate of transformationofPhOHcouldbegreatlyovercompensatedbythe effect of the higher concentrations of OH. The hindered recombinationof theprimary radicals might alsocontributeto the higher rate of transformation of PhOH in oxygenated, as comparedwithdeoxygenatedsolutions.

In theabsence of O₂ thedihydroxy-cyclohexadienyl radicals mightdisproportionatetoyieldPhOHanddihydroxybenzene[8].

In oxygenated solutions, the addition of O2 to these radicals competes with thedismutation reaction, and thus hinders the regenerationofPhOH[30].Consequently,thisprocessmightalso contributetotheincreasedtransformationrateofPhOHmeasured inthepresenceofdissolvedO₂.

Table1

InitialratesoftransformationofPhOH(r0)andthemostsignificantspeciesofthepresumedradicalset.

Noadditive t-BuOH HCOOH HCOO^–

Injectedgas N2

r0(10⁸molL¹s¹) 121 5.70.3 3.20.1 3.10.3

Radicalset OHH H t-BuOH H COOH H CO2–

Injectedgas O2

r0(10⁸molL¹s¹) 331 6.20.5 7.50.4 6.10.6

Radicalset OH,HO2 O2– HO2 O2– HO2 O2–

The reactionrate constants of HO2and O2– with PhOH(k (PhOH+HO₂)=2.710³Lmol¹s¹ [31], k(PhOH+O₂^-

–)=5.8102Lmol¹s¹[32])aremuchlowerthantherateconstant of reaction of PhOH with OH (k13). This means that the contributionof these species tothe transformation of PhOHis negligible, and the reaction with OH must be the significant process.

DuringtheVUVphotolytictransformationofPhOH,thepHof thesolutionsdecreasedfrom7to4afterarelativelyshortperiodof irradiation (the conversion of PhOH being 80%) in solutions saturatedwithO2(Fig.1b).Theexplanationofthisacidificationis probablytheformationofvariousaliphaticorganicacidsformed bythering-openingreactionsfromPhOH,aromaticintermediates andthefurtherfragmentations[5].UnderO2-freeconditions,the pHdecreasedonlyslightly.

InpureH2O(intheabsenceofbothdissolvedO2andorganic substances),therecombinationoftheprimaryradicals(OHand H) is very favourable because of the ‘cage effect’ [33,34].

Consequently,therecombinationofOHradicals(k7)andtherefore theconcentrationofH2O2wasnegligible(Fig.2)intheabsenceof O2,inaccordancewiththeliterature.Thisconfirmedthatwithout dissolvedO2theconcentrationofprimaryradicalsinpureH2Ois verylow. On theotherhand,the concentrationof H2O2 in O2- saturated H2O was found tobesignificantly higher (it reached 2.510⁵molL¹).Inthiscase,dissolved O2reactswithHand convertsit intoHO2(Eq.(5)), which resultsin higher concen- trations of both OH and HO2. It may be the reason for the enhanced concentrationof H₂O₂, since the furtherreactions of

HO2andits deprotonatedformO2– (k8–k10)alsoresult inthe formationofH2O2,asdescribedpreviously.

In O2-saturated solutions, the presence of PhOH and other organic substances (t-BuOH, HCOOH or HCOONa) strongly increasedtheconcentrationofH2O2.The recombinationofOH was significantly suppressed also in these cases. However, the concentrationsofHO2/O2–werelikelytobehighsincetheycan beformedineliminationreactionsfromorganicperoxylradicals [21,28]andduetotheadditionofO₂toH(Eq.(5)).Asconcernsthe pH,themolarratioHO2/O2–waslessthan0.1untilamaximumof 10% PhOH was decomposed, but further acidification of the solutionincreasedthemolarratioHO₂/O₂^–.Sincethereaction rateconstantsofHO2andO2–withorganicsubstancesaremuch lowerthanthoseoftheirreactionswitheachother(Eqs.(8)and (10)), their further transformations result mainly in H2O2

formation. In O2-free solutions, the H2O2 concentration was negligiblebecauseofthelackofHO2/O2–.

3.2.Effectsoft-BuOH

t-BuOH as OH scavengerreacts with OH witha highrate constant(k16),andwithHwitha3ordersofmagnitudelowerrate constant(k17).Consequently,theconcentrationofHmayremain significantinthesolutionthatcontainst-BuOH.Thus,inO2-free solutionscontainingt-BuOH,thetransformationofPhOHcanbe inducedmainlybythereactionwithH[8],whichisatrelatively lowconcentrationduetothelargeexcessoft-BuOH.Ontheother hand,t-BuOHmayhaveminorcontributiontothetransformation ofPhOHinthiscase.

InsolutionssaturatedwithO2,t-BuOHundergoestransforma- tion totherespectiveperoxylradical(t-OOBuOH)(k(t-BuOH+ O2)=1.410⁹mol¹Ls¹ [35]) which also displays negligible reactivity towardsPhOH[21,36].Thepredominant decayroutes oft-OOBuOHdonotgiverisetoO2–[37].Thus,t-BuOHalsoacts asaneffectiveradicalscavengerinthepresenceofO2,thoughwith asmallradical-transferringcontribution.

Theadditionoft-BuOHreducedtherateoftransformationof PhOHsignificantly,toasimilarvalueinO₂-freeandinO₂-saturated solutions (Table 1) (Fig. 3a). One possible explanation of this phenomenonmightbethattheconcentrationofOHdecreasesto nearlythesamevalueinbothcases,becauseofthelargeexcessof t-BuOH.InO2-saturatedsolutions,HisconvertedtoHO2,whichis presentmainlyindeprotonatedform(O2–),inviewofthepHof thesolution.ThepHofthesolutionscontainingt-BuOHchanged similarlyasintheexperimentswithoutthisadditive,bothinO2- free and in O2-saturated solutions (Fig. 3b). Thus, the pH- dependentratioHO2/O2–shouldalsobesimilar,O2–beingthe Fig.1.PhOHconcentration(a)andpH(b)versusirradiationtimeintheabsenceandinthepresenceofO2.

Fig.2.H2O2 concentrationversus irradiationtime inthe absenceandinthe presenceof1.010⁴molL¹PhOH and5.010¹molL¹t-BuOH,HCOOHor HCOOinsolutionspurgedwithN2orO2.

predominantspeciesatthebeginningofirradiationinbothcases, andfurtherdecrease of thepHincreasing theconcentrationof HO2.ThetransformationofPhOHcanthereforebeinducedmainly by HO₂/O₂^–, with very low reactivity towards PhOH [17,18,31,32,38].

Ontheotherhand,theseresultssuggestthatthecontributionof the higher concentration of the less reactive O2– to the degradation of PhOH in solutions saturated with O2 is commensurablewiththerelativelylowconcentrationofHin O2-freesolutions.Consequently,O2–inelevatedconcentration maycontributetothedecompositionofPhOH.Additionally,it seemsthatthelow(butmeasurable)reactivityoft-BuOHand t-OOBuOHtowardsPhOHisnearlythesame.

3.3.EffectsofHCOOH

HCOOHisaweakacidanditsreactionswithOHandHresult inlessreactiveCOOH(Eqs.(19)and(20)).InthepresenceofO₂, this carbon-centred radical undergo transformation to HO2

(Eq.(24)).

Under O2-free conditions, HCOOH reduced the rate of degradation PhOH significantly (Table 1) (Fig. 4a) because it operatedas a OH scavengersimilarly tot-BuOH. Whereas the reactionrateconstantsofHCOOHandt-BuOHwithOH(k19and k16)andH(k20andk17)havesimilarvalues(thesameorderof magnitude),theeffectofHCOOHwasmoresignificantthanthatof t-BuOH,possiblybecausethereactivityoft-BuOHtowardsPhOH mightbehigherthanthatofCOOH.However,thecontributionof

thecarbon-centredradicalstothetransformationofPhOHshould beminor.

InO2-saturatedsolutions,bothprimaryradicalsareconverted tothelessreactiveHO₂/O₂^–.ThepHofthesolutionscontaining HCOOHwas2anddidnotchangeduringthephotolysis(Fig.4b);

inthiscase,thereforeonlyHO2waspresentinthesolutions.The totalradicalsetwasthereforeconvertedtoHO₂,meaningthatthis radical was the only one that could contribute in elevated concentrationtothetransformationprocess.Therelativelyhigh concentrationofthislessreactiveoxygenspeciesisthereforemost probablyresponsibleforthehigherinitialrateofPhOHtransfor- mationinO2-saturatedthaninO2-freesolutions.

3.4.EffectsofHCOO^–

TheadditionofHCOO^–inducesabasicpHduetothehydrolysis ofthisanionandconvertsthereactiveOHandHintolessreactive CO2– withrelativelyhighreactionrateconstants(Eqs.(21)and (22)). As described previously, in the presence of O2 CO2– is convertedtoO₂^–(Eq.(25)).

InO2-freesolutions,theadditionofHCOO^–reducedtheinitial rateoftransformationofPhOHtoasimilarvalueasforHCOOH (Figs.4aand5a).TheeffectsofHCOO^–andHCOOHweremore markedthanthatoft-BuOH.Thedifferencecanbeexplainedbythe differences in reactivity of the carbon-centred radicals formed, CO2–,COOHandt-BuOH,respectively.

AstheinsertinFig.5ashows,thekineticcurvesexhibiteda break-pointafterthedecomposition of6% ofPhOH.During the VUVirradiation,thepHincreasedfrom7.8toalmost11(Fig.5b) probablycausedbythereactionofCO2–withOHwhichleadsto hydroxide ions, and explains the increase in pH. This basic

Fig.4.PhOHconcentration(a)andpH(b)versusirradiationtimeintheabsenceandinthepresenceofHCOOHinsolutionspurgedwithN2orO2. Fig.3.PhOHconcentration(a)andpH(b)versusirradiationtimeintheabsenceandinthepresenceoft-BuOHinsolutionspurgedwithN2orO2.

conditionresultsthat PhOHwaspresent mainlyinitsdeproto- natedform(phenolateion,PhO^–;pKa=9.88[39])afterthebreak- point.Thismaybethereasonofthestrongdecreaseintherateof transformation of PhOH, since the reactivity of PhO^– might be lowerthanthatofPhOH.Fortheconfirmationofthisphenomenon, the effect of pH in the range of 7–11 on the initial rate of transformation of PhOH was also investigated in O2-saturated solutions.Theresultsshowedthattherate oftransformationof PhOHdecreaseswiththeincreaseofpH(Fig.6),butthiseffectis muchmoresignificantabovepH10,wherePhOHispresentmainly initsdeprotonatedform.

DissolvedO₂enhancedtherateoftransformationofPhOHin eachsysteminwhichtheorganicadditiveswerepresent.Ineach case,merelythelessreactiveradicals,suchasonlyHO2(inthe caseofHCOOHaddition),ormainlyO2–(inthecaseofHCOO^–ort- BuOHaddition),werepresentinthesesolutions,whichsuggests thatthese reactivespecies in elevatedconcentrationcanpartly contributetothetransformationofPhOH.Itshouldbenotedthat, forthedifferentadditives,thehighestinitialratewasobservedin thepresenceofHCOOH,which canbeexplainedbythe5times higherreactionrateconstantofHO2withPhOHthanthatofO2–. However,thecontributionofthesereactiveoxygenspeciestothe transformationofPhOHwerelikelytobenegligible,ascompared withthecontributionofOH,whichisthemostrelevantreactant.

Ontheotherhand,ineachcasetheconcentrationofH₂O₂was foundtobemuch higherinthepresenceoforganicsubstances thaninpureH2OsaturatedwithO2(Fig.2).Thissuggeststhatthe mainreactionsofHO2andO2–(formedduetotheadditionofO2

toHandtoeliminationfromorganicperoxylradicals)arethose thatresultinH2O2formationinsteadoftransformationofPhOH.

4.Conclusions

Aradicalscavenger(t-BuOH)andtworadicaltransfermaterials (HCOOHandHCOO^–)wereappliedinlargeexcesstoinvestigate theireffects on the rate of transformation of PhOH and the formationofH₂O₂inVUV-irradiated,O₂-freeandO₂-saturated aqueoussolutions.

TherateofdegradationofPhOHincreasedsignificantly(upto3- fold)inthepresenceofdissolvedO₂,mostprobablybecauseit hinderedtherecombinationoftheprimaryradicalsthroughits reactionwithH,consequentlygreatlyincreasingtheconcen- trationofOH.Atthesametime,theconcentrationsofHO2and O2–alsoincreased,asindicatedbythehighconcentrationof H2O2 formed,but theircontributionstothetransformation of PhOHwerelikelytobenegligible.

In O2-free solutions, each organic additive reduced the concentrationsofbothprimaryradicalsandhencetheinitialrate ofPhOHtransformation.TheeffectsofHCOOHandHCOO^–were morepronouncedthanthatoft-BuOH,whichcanbeexplainedby thedifferencesinreactivityofthecarbon-centredradicalsformed,

COOH,CO₂^–andt-BuOH,respectively.

In solutions saturated with O2, there was no significant differencebetweentheinitialratesoftransformationofPhOH determinedinthepresenceoft-BuOH,HCOOHorHCOONa.From thepresentedresultsitseemsthatHCOOHandHCOONacanbe consideredasradicalscavengermaterialsaswell.Inthesecases, thecontributionsoftheprimaryradical-initiatedreactionstothe transformation of PhOH were negligible, since the most significant species of the radical set were the less reactive HO2 orO2– (depending onthepH), which even in elevated concentrationmademerelyminorcontributionstothetransfor- mationofPhOH.Theslightlyhigherinitialrateoftransformation inthepresenceofHCOOHcanbeexplainedbythe5timeshigher reactionrateconstantofHO2withPhOHthanthatofO2–.Atthe sametime,therelativelyhighconcentrationsoftheH2O2inthe solutions containing organic substances proved that main reactionsof HO2 and O2– results in the formation of H2O2

insteadofthereactionwithPhOH.

Thekineticdatareportedcanbeexplainedbythevariationofthe concentrationofOHduetotheadditionofradicaltransferor scavengermaterialsandthetrappingofcarbon-centredradicals byO2.Thismeansthatinall8casesmentionedinTable1,OHis theonlyrelevantreactantforthetransformationofPhOH.

Fig.6.EffectofpHontheinitialrateoftransformationofPhOHinsolutionspurged withO2andtheratioPhO^–/PhOH.ThedashedlineshowsthepKaofPhOH.

Fig.5. PhOHconcentration(a)andpH(b)versusirradiationtimeintheabsenceandinthepresenceofHCOO^–insolutionspurgedwithN2orO2.

Acknowledgements

ThefinancialsupportoftheSwissContribution(SH7/2/20)is acknowledged and greatlyappreciated. Thisresearch was sup- ported by the European Union and the State of Hungary, co- financedbytheEuropeanSocialFundintheframeworkofTÁMOP- 4.2.4.A/2-11/1-2012-0001‘NationalExcellenceProgram’.

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