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Catalysis Today
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / c a t t o d
Transformation of Z-thiacloprid by three advanced oxidation processes: Kinetics, intermediates and the role of reactive species
Georgina Rózsa
a,b, Zsuzsanna Kozmér
a,b, Tünde Alapi
a, Krisztina Schrantz
a,∗, Erzsébet Takács
b, László Wojnárovits
baDepartmentofInorganicandAnalyticalChemistry,UniversityofSzeged,H-6720Szeged,Dómtér7,Hungary
bRadiationChemistryDepartment,CentreforEnergyResearch,HungarianAcademyofSciences,H-1121Budapest,Konkoly-ThegeMiklósút29-33, Hungary
a r t i c l e i n f o
Articlehistory:
Received14July2016
Receivedinrevisedform5October2016 Accepted25November2016
Availableonlinexxx
Keywords:
Thiacloprid AOPs
Hydroxylradical Hydratedelectron Intermediates Electricenergyperorder
a b s t r a c t
Threeadvancedoxidationprocesses(AOPs),heterogeneousphotocatalysis,vacuumultraviolet(VUV) photolysisand␥radiolysiswereusedforthegenerationofreactiveprimaryfreeradicalstoinduce thetransformationofZ-thiaclopridinaqueoussolution.Theeffectsofdissolvedoxygenandtheinitial concentration(from10−6to10−4molL−1)wereinvestigated.Theinitialreactionratesincreasedwiththe initialconcentrationofthiacloprid,bothinoxygensaturatedandoxygenfreesolutions.Dissolvedoxygen hadsignificanteffectonthetransformationrateonlyincaseofheterogeneousphotocatalysis.Threemain intermediatesandtheE-thiachlopridweredetectedusingallthreemethods.Oneoftheseintermediates couldberelatedtothereactionwitheaq−,whiletheothertwocouldberelatedtothe•OH-initiated reactions.Heterogeneousphotocatalysisshowedthehighestefficiencyregardingthetransformationof intermediatesinpresenceofdissolvedoxygen,whilethiaclopridtransformedwiththehighestinitial reactionrateduringVUVphotolysis.However,accordingtotheElectricenergyperorder(EEO)data␥ radiolysiswasfoundtobetheeconomicallymostfeasiblemethod,requiringseveralordersofmagnitude lessenergythanVUVphotolysisandheterogeneousphotocatalysisforreductionofthetargetcompound concentrationbyoneorderofmagnitudeinaunitvolume.
©2016ElsevierB.V.Allrightsreserved.
1. Introduction
Largenumberofmethodshavebeensuggestedinthelitera- tureforthedegradationofharmfulorganicmolecules,forinstance pesticideresidues,inwaterandwastewater[1].Inthesestudies moreandmoreattentionisfocusedontheneonicotinoidfamily (imidacloprid,thiamethoxam,acetamiprid,clothianidin,dinotefu- ran,nitenpyramandthiacloprid),thefastestgrowinggroupamong theinsecticides.Thesecompoundshavelongtermimpactonthe ecosystem,especiallytheyhaveharmfuleffectsonbees,weakening theirimmunesystem,andincreasingtheirsensitivitytopathogenic viruses[2]. The name neonicotinoidssuggests similarityin the
∗Correspondingauthor.
E-mailaddresses:rozsa.georgina@chem.u-szeged.hu(G.Rózsa), kozmerzs@chem.u-szeged.hu(Z.Kozmér),alapi@chem.u-szeged.hu(T.Alapi), sranc@chem.u-szeged.hu(K.Schrantz),erzsebet.takacs@energia.mta.hu (E.Takács),wojnarovits.laszlo@energia.mta.hu(L.Wojnárovits).
chemical structurewithnicotineandacetylcholine,for exerting theireffectsontheacetylcholinereceptors[3].
One of the representative members of neonicotinoides is Z-thiacloprid({3-[(6-Chloropyridin-3-yl)methyl]-1,3-thiazolidin- 2-ylidene}cyanamide)(Fig.1).Ithasveryhighstabilityandgood solubilityinwater(184mgL−1 at20◦C)[4].Thiscompoundcan beaccumulatedintheenvironmentthroughthetrophicnetwork [5],andtherefore,itisdetectedinanincreasingamountinsurface waters.
Due to the three biologically active groups in its structure, chloropyridineandthiazolidinerings,andthecyanoiminogroup, thiaclopridisstronglytoxicwithlethaldoseofLD50=444mgkg−1 forrats.Inagricultureitisusedagainstsuckingandchewingpests [6]incrops,suchasrapeseed,sunflower,potatoes,apple,aswellas forcornseeddressing.Stabilitytestsshowedthatthiaclopridwas lesspersistentinacidicsolution,butinalkalinemedia,itwasstable forabout30days[7].
Thedecompositionofneonicotinoidshasbeenexaminedbysev- eraladvancedoxidationprocesses(AOP)[8].Zbilji ´cetal.showed, that the removalof acetamiprid using photo-Fenton process is http://dx.doi.org/10.1016/j.cattod.2016.11.055
0920-5861/©2016ElsevierB.V.Allrightsreserved.
Fig.1.Chemicalstructureofthiacloprid.
twiceaseffective(∼10min)asusingFentonprocessesonly[9].Imi- daclopridtransformationneeded∼60minusingbothFentonand photo-Fentonprocessescombinedwithhydrodynamiccavitation [10].Malatoetal.reportedthatthedegradationofimidacloprid usingtitaniumdioxide(TiO2)photocatalystunderUV-Airradia- tionis a relatively slowprocess (∼0.61mgL−1min−1)[11]. The photocatalytic degradation of imidacloprid, thiamethoxam and clothianidinonimmobilisedTiO2hasalsobeenstudied.Within2h ofphotocatalysis,allthreeneonicotinoidsweredegradedfollow- ingfirstorderkinetics[12].Thiamethoxams’transformationswere investigatedalsousingUVphotolysis,ozonationandtheircombi- nations[13].Theeffectofdissolvedozoneandferricionsonthe intermediatesofthiaclopridformedduringTiO2-basedheteroge- neousphotocatalysiswasinvestigatedinsuspensions[14–16]and usingimmobilizedcatalyst[17].
Toourbestknowledgenoresultshavebeenpublishedyetonthe vacuumultraviolet(VUV)photolysisand␥radiolysisofthiacloprid.
To understand themechanism of transformation of organic compounds,oneofthemostimportantstepsis gatheringinfor- mationaboutthenatureofreactionsofthetargetcompoundwith thereactivespeciesgeneratedbyAOPs.
Among the AOP methods VUV photolysis and ␥ radiolysis are good candidatesto investigatethe role of hydroxyl radical (•OH)andhydratedelectron(eaq−)inthedegradationoforganic molecules.IncontrasttotheVUVphotolysis,during␥radiolysis indissolvedoxygen(DO)freesolutionsbesidethe•OH,therole oftheeaq−isalsorelevant.Althoughthebasicmechanismofthe TiO2-basedheterogeneousphotocatalysishadbeeninvestigatedby agreatnumberofresearchgroups[18,19]theidentitiesofreactive speciesarestillunderintensivediscussion.Duringtheillumina- tionofphotocatalystsbyUVlight(max=365nmforTiO2)valence bandholes(h+,alocalizedoxidizingstate)andconductionband electrons(e−)form(Eq.(1)).Theoxidationoforganicsolutes(OS) isassumedtotakeplacedirectlybythesurfacehole,orthrougha
•OHonthesurface(Eq.(4))formedinthereactionoftheholewith ahydroxylanionorwatermolecule(Eq.(2)).Itisanopenquestion iftheOScanreactdirectlywiththee−atthesurfaceofthecatalyst.
InthepresenceofDOitsreactionwithe−(Eq.(3))producessuper- oxideradicalanion(O2•−).InfurtherreactionsO2•−yieldsH2O2, whichfinallytransformsinto•OH(Eq.(3),[19,20]).
TiO2+UVphoton=365nm→ e−+h+ (1) h++−OH(H2O) →•OHsurf(•OHsurf+H+) (2) e−+O2→ O2•−→HO2• → HO2−→H2O2→ 2•OH (3) h+(or•OHsurf)+OS →oxidizedproducts (4) DuringtheVUVphotolysisofaqueoussolutions,usingacom- mercialXe2*excimerlamp(=172nm),homolyticdissociationof watermolecules resultsin •OH andhydrogenradical(H•)with quantumyieldof0.42(Eq.(5))[21,22].Withlowyieldionization alsotakesplace.Theso-calleddryelectronreleasedinionization maystabilizeintheformofeaq−(Eq.(6))[22,23]:
H2O+VUVphoton=172nm→H•+•OH
˚172nm(•OH,H•)=0.42 (5)
H2O+VUVphoton=172 nm→H++eaq−+•OH
˚172nm(eaq−)< 0.05 (6)
InVUVphotolysisat172nmthephotonsareabsorbedinavery thinlayerofafewm.Inthisreactionzonethereisastrongcompe- titionbetweenthereactionsofreactivespecies(•OHandH•)with eachotherandwiththiaclopridanditsintermediates.
During␥radiolysisofaqueoussolutionsthedecompositionof watermoleculesresultsin•OH,eaq−and(inloweryield)H•,as primaryradicals(Eq.(7))withyields(so-calledG-values)of0.280, 0.280and0.062molJ−1,respectively[24,25].
H2O+␥photon→ •OH+eaq−(+•H) (7) InthepresenceofDOthereductiveprimaryspecies(H•/eaq−) transformtolessreactivehydroperoxylradical/superoxideradical anion(HO2•/O2•−)(Eqs.(8)and(9))[26].
•H+O2→HO2• k8=1.2×1010Lmol−1s−1 (8)
eaq−+O2→O2•− k9=1.9×1010Lmol−1s−1 (9) Inheterogeneousphotocatalysis, VUV photolysisand␥radi- olysis •OH are assumed to play a key role in the pollutants’
degradations,mainlyinthepresenceofDO.However,theirdistri- butionsinspacearesignificantlydifferent:i)inTiO2photocatalysis thereactivespeciesareonthecatalystsurface,ii)inVUVphotolysis theyareproducedclosetothewindow,withinhomogenousdistri- bution,andiii)in␥radiolysisthereactivespecies,withtheyields mentionedabove,aremore-or-lesshomogeneouslydistributedin thesolutionbulk.
Thiaclopridhasthreesensitivepartsforthe•OH-inducedoxi- dation: 2-cloropyridine, thiazolidine and the cyanoimino part.
Pyridine reacts with •OH, eaq− and •H with the rate con- stants of 3.0×109Lmol−1s−1, 7.7×109Lmol−1s−1 [27] and 6.0×108Lmol−1s−1 [28], respectively. •OH reacts with nico- tinicacidwithrateconstantof5.6×109Lmol−1s−1[29].Dueto theelectronwithdrawingCl substituenttherateconstantof 2- cloropyridinewith•OH(1.8×109Lmol−1s−1[29])issmallerthan thatofpyridine.•OHgenerallyattacksthethioethergroupsalso withrateconstantsofabout109Lmol−1s−1.Thedoublebondin thecyanoiminopartofthemoleculemayalsobeinvolvedinreac- tionwiththisradical.Incaseofthiazolidinenodataareavailable aboutitsreactionswiththeseprimaryradicals.Onlyonevalueof the•OHrateconstantwiththiaclopridwasdeterminedusingflash photolysis(7.5×1010Lmol−1s−1[30],howeverthisrateconstant isunrealisticsinceitishalfofanorderofmagnitudehigherthan thediffusionlimitedvalue).Thereisnoinformationavailableabout thereactionsofneonicotonoidinsecticideswitheaq−andH•.
Theaimofthisworkwasthecomparisonofthiaclopridtrans- formationusingheterogeneousphotocatalysis,VUVphotolysisand
␥radiolysisinthepresenceandabsenceofDO,thestudyofthe kineticpropertiesofthiaclopridreactionswiththeprimaryreac- tivespecies formedin theseprocesses,and theidentificationof theintermediatesformedduringtheappliedtreatments.Theeco- nomicfeasibilityofthethreemethodswascomparedbasedonthe Electricalenergyperorder(EEO).
0.0 0.2 0.4 0.6 0.8 1.0
0 50 100
c/c0
Irradiaon me (min) H - N H - O
a)
0.0 0.2 0.4 0.6 0.8 1.0
0 20 40
c/c0
Irradiaon me (min) V - N V - O
b)
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.5 1.0 1.5 2.0
c/c0
Doses (kGy) R - O R - N
c)
Fig.2. Kineticcurvesofthiacloprid(c0=1.0×10−4molL−1)degradationduringheterogeneousphotocatalysis(H,a),VUVphotolysis(V,b)and␥radiolysis(R,c)inthe presence()andabsence( )ofdissolvedO2.
0.0 0.2 0.4 0.6 0.8 1.0
0 50 100
c/c0
t/ttotal(%)
V - O R - O H - O
a)
0.0 0.2 0.4 0.6 0.8 1.0
0 50 100
c/c0
t/ttotal(%)
H - N R - N V - N
b)
Fig.3.Kineticcurvesofthiacloprid(c0=1.0×10−4molL−1)degradationduringheterogeneousphotocatalysis(H,䊉),VUVphotolysis(V, )and␥radiolysis(R, )inthe presence(a)andabsence(b)ofdissolvedO2.
2. Materialsandmethods 2.1. Materialsandequipment
In VUV photolytic and heterogeneous photocatalytic exper- iments 250mL thiacloprid (Sigma-Aldrich, 99.9%) solutions with initial concentrations (c0) of 1.0×10−4, 1.0×10−5 and 1.0×10−6molL−1,preparedin ultrapure MILLI-Qwater (MILLI- POREMilli-QDirect8/16),wereirradiated.Inthephotocatalytic experimentstheTiO2(DegussaP25,EvonikAeroxide)concentra- tionwas1.0gL−1;thesampleswerecentrifuged(DragonlabD2012, 2min,rpm=15000)andfilteredwithsyringefilter(SartoriusSte- dim,Ministart®-plus, 0.20m) after irradiation, to remove the photocatalystparticles.
During the VUV photolysis and heterogeneous photocataly- sisthesolutionsandsuspensionswerecirculated(375mLmin−1) betweenthetemperaturecontrolled(T=25±0.5◦C)reactorand thereservoir bya HeidolphPump drive5001 peristalticpump.
Thesolutionsandsuspensionsinthetankwereconstantlystirred withamagneticstirrer,whiletheywerebubbledwithO2 orN2
gas(Messer,>99.5%purity)at aflow rateof600mLmin−1.The injectionofthegaswasstarted30minbeforeeach experiment and was continued during the irradiation. For VUV photolysis a 20W Xe2* excimer lamp (Radium Xeradex TM, dimensions:
180mm×48mm) emitting at 172±14nm was used. The pho- tonflux,determinedbymethanolactinometry[31],wasfoundto be3.0×10−6molphotons−1.Theheterogeneousphotocatalysiswas performedwitha specificfluorescent UVlamp(GCL303T5/UVA, LighTech,Hungary,dimensions:360mm×30mm)emittinginthe rangeof300−400nmwithmax=365nm.Thephotonfluxofthe lightsourcewas1.2×10−5molphotons−1,determinedbyferriox- alateactinometry[32].
In␥radiolysis experimentsthe5mLampouleswiththiaclo- pridsolution,preparedinultrapureMILLI-Qwater(ELGAoption 4),wereplacedtoequaldistancefromthe60Co-␥sourceofanSSL-
01panoramictype irradiator,tohaveadoserateof0.7kGyh−1 (700Jkg−1h−1).Thesolutionswereirradiatedinopenampoulesor insealedampoulessaturatedwithN2.
MeasurementswererepeatedthreetimesandincasesofVUV photolysisand␥radiolysisthestandarddeviationoftheobtained valueswaslessthan±5%.Incaseofphotocatalyticmeasurements the sample preparation (filtration)slightly increasedthis value (5–8%).
2.2. Analyticalmethods
Theabsorptionspectraofthetreated samplesweredetected byspectrophotometry(Agilent8453orAgilent1200,0.5cmpath- way).Z-thiaclopridandtheformedintermediateswereseparated and detectedby Agilent 1100 type HPLC equipped withdiode array detector (DAD) and an Agilent G1956A quadrupolemass spectrometric(MS)detector.ALiChroCart® (250-4,RP-18,5m) reverse-phasecolumnwasutilizedfortheseparations.Amixtureof methanol(70%)andwater(30%)wasusedaseluent,at0.6mLmin−1 flowrateand25◦C.20Lsampleswasanalysedat242nm(absorp- tionmaximumofthiacloprid).TheMSanalysiswascarriedoutin positiveandnegativeionmodes,withanelectrosprayionization source(ESI)using70and90Vfragmentorvoltages.
The determination of Z-thiaclopridconcentration wasbased onthelinearregressionofcalibrationcurve(R2=0.9995)present- ingtheintegratedpeakareasofthechromatogramsmeasuredby HPLC-DAD.Kineticcurvesshowtheratiooftheactualandtheinitial Z-thiaclopridconcentrations(c/c0,c0=1.0×10−4molL−1).
ThetransformationofZ-thiaclopridwascharacterizedbythe initialrateof degradation(r0), whichwasobtainedfromlinear regressionfitsofthedecaycurvescorrespondingto10%oftrans- formation.
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
0 20 40 60 80
A/A0
c/c0
Irradiation time (min)
a)
0.0 0.5 1.0 1.5
0.0 0.2 0.4 0.6 0.8 1.0
0 20 40 60 80
A/A0
c/c0
Irradiation time (min)
b)
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
0 10 20 30 40 50
A/A0
c/c0
Irradiation time (min)
c)
0.0 0.4 0.8 1.2
0.0 0.2 0.4 0.6 0.8 1.0
0 10 20 30 40
A/A0
c/c0
Irradiation time (min)
d)
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.5 1.0 1.5 2.0
A/A0
c/c0
Doses (kGy)
e)
0.0 0.5 1.0 1.5
0.0 0.2 0.4 0.6 0.8 1.0
0.0 0.5 1.0 1.5 2.0
A/A0
c/c0
Doses (kGy)
f)
Fig.4. Changeofthetwomainabsorptions(242nm– ,270nm− )andthekineticcurvesofthethiacloprid(c0=1.0×10−4molL−1)degradation()duringheterogeneous photocatalysis(aandb),VUVphotolysis(candd)and␥radiolysis(eandf)inthepresence(a,cande)andintheabsence(b,dandf)ofdissolvedO2.
3. Resultsanddiscussion
3.1. DegradationfollowedbyUV-spectrophotometry
ThetransformationofthiaclopridwasfirstinvestigatedbyUV- spectrophotometrybetween200and350nm.IntheUVspectrum ofthiacloprid there is anabsorption maximum at242nm, and two shouldersaround 220and 270nm. In theUV spectrum of 2-chloropyridinethelattertwobandsalsoappear,beingcharacter- isticforthechloro-substitutedaromaticring.Thestrongabsorption bandwithcentreat242nm(εmax=18800molL−1cm−1)belongsto the2-thiazolidinecyanamidepartofthemolecule.Thisabsorbance maximumslightlyshiftstolowerwavelengthswiththeincrease ofthetreatmenttime.Therecordedspectraarepresentedinthe Supplementarymaterials,inFig.S1.
3.2. Degradationkineticsofthiacloprid
In Fig. 2 degradation kinetic curves of thiacloprid (c0=1.0×10−4molL−1) are shown in presence and absence ofDO.TheresultsshownosignificantdifferenceincaseofVUV photolysis,inspiteofthefactthatinpresenceofDOtherecombi- nationoftheprimaryradicals(•OHand•H)ishindered.Onlyslight increaseofthetransformationrateincaseofradiolysisinabsence of DO was observed, due to the presence of eaq−. The similar reactionrateofeaq−andthe•OHreactionsincaseofchloropyri-
dinesupportsthisobservation[33].Incaseoftheheterogeneous photocatalysiswithoutaneffectiveelectronscavenger,thereisno possibilityforanyredoxreactionatthesurface,duetothefavoured recombinationofthephotoinducedcharges.ThereforeinDO-free solutiontheroleoftheoxygencouldbetakenoverbythiacloprid, consequentlythiscreatesapossibilityforadsorbedthiaclopridto reactbothwiththeh+ande−.Thedifferenceinthetransformation rates in presence and absence of DO can be explained by the differencein their concentrationin solution (2.5×10−4molL−1 (DO) and 1.0×10−4molL−1 (thiacloprid)) and in the adsorbed amountofoxygenandthiacloprid.Thisissupportedbytheresults presentedinFig.2a.
Theinitialratesofdegradation(r0)areshown inTable1.As expected,allther0 decreased significantlywiththedecreaseof theinitialconcentrationofthiacloprid.However,thereactionrate constants (k) increased significantly. For heterogeneous photo- catalysis,inpresenceofDO,thisincreasewasobservedonlyfor smallestconcentration.InVUVphotolysisthekvaluesincreased stronglyandlinearlywiththedecreasingthiaclopridconcentration reflectingtheavailabilityofthe•OHfortheorganicmoleculesin theverythinreactionzone,bothinpresenceandabsenceofDO.In
␥radiolysisreactionsthekincreasedsignificantlyonlyinpresence ofDO,inthemostdilutedsolution.Thismaypartlybeduetothe higher•OHstationaryconcentrationatlowersoluteconcentration.
ThisincreasewasnotobservedinabsenceofDO.
Table1
Theinitialrates(r0),theapparentreactionrateconstants(k)andElectricenergyperorder(EEO)calculatedincaseofvariousinitialthiaclopridconcentrationsandmethods.
Heterogeneousphotocatalysis VUVphotolysis ␥radiolysis
c0(molL−1) r0(molL−1s−1)
N2 O2 N2 O2 N2 O2
1.0×10−4 8.3×10−9 6.7×10−8 2.5×10−7 2.4×10−7 4.3×10−8 3.1×10−8
1.0×10−5 1.5×10−8 6.4×10−9 1.0×10−7 䊐1.3×10−7 >1.8×10−9 䊐1.1×10−8
1.0×10−6 2.7×10−9 1.6×10−9 >4.0×10−8 䊐3.0×10−8 >1.8×10−9 䊐1.9×10−8
c0(molL−1) k(s−1)
N2 O2 N2 O2 N2 O2
1.0×10−4 8.3×10−5 6.7×10−4 2.5×10−-3 2.4×10−3 4.3×10−5 3.1×10−4
1.0×10−5 1.5×10−3 6.4×10−4 1.0×10−2 䊐1.3×10−2 >1.8×10−4 䊐1.1×10−3
1.0×10−6 2.7×10−3 1.6×10−3 >4.0×10−2 䊐3.0×10−2 >1.8×10−3 䊐1.9×10−2
EEO(kWm−3order-−1)
EEO1(10−4−10−5) 80.0 26.7 0.2
EEO2(10−5−10−6) 90.0 5.3 0.0015
EEO2/EEO1 1.1 0.2 0.008
InFig.3themethodsarecomparedinpresenceandabsenceof DO.Ontheabscissathetimescaleisnormalizedtothetimeneeded forcompleteremovalofthiacloprid(ttotal,duringheterogeneous photocatalysis,VUVphotolysisand␥radiolysisinthepresenceand absenceofDO:120and440min(estimatedbythelinearregression ofthelast3points);25and25min;1.5and1.0kGy,respectively).
Althoughthevalueofttotalisarbitrary,thiswayofplottinghelpsthe comparisonofthekineticcurves.Until∼20%t/ttotalsharpdecrease wasobservedinallthreemethods.InpresenceofDO(Fig.3a),dur- ing␥radiolysisthiaclopridconcentrationdecreasedlinearlywith thetime(dose,slope1.9×10−7molL−1J−1).Theamountofthi- aclopridtransformedislowerby∼30%thantheyieldof•OHG (•OH)=2.8×10−7molJ−1,sotheefficiencyisaround70%.Thiseffi- ciencyismuchlowerwhenitiscalculatedfromthedecreaseofthe 242nmabsorbanceobservedintheUVspectroscopicstudies(Fig.
S1e).Whenthesamplewasirradiatedwithanabsorbeddoseof0.1 kGy,theabsorbancedecreasedonlyby3%.
Itmeansthat∼10%ofthe•OHinducesthedegradationofthe chromofor.Sothemajorityofthefirsttransformationsleavethe chromoforintact.Thelineartime (dose)dependence, shouldbe duetothelowerreactionrateofthedegradationinitiatingrad- icalswiththeproductsthanwiththestarting molecules.Above
∼80%conversionthetransformationratebecomesslowerwhichis certainlyduetothelargeconcentrationoftheaccumulatedinter- mediates(Fig.4e).Athigherintermediateconcentrationthereisa realcompetitionbetweenthestartingcompoundandtheinterme- diatesforthereactiveradicals.InpresenceofDO,thiscompetition ismoreevidentinthecasesofphotocatalysisandVUVphotolysis thanin␥radiolysis(Fig.3a).
Fig.4showstherelativeabsorbancesat242and270nm(the characteristicabsorbanceof thearomaticsystems).In thepres- enceofDOtheabsorbanceat242nmdoesnotchangeparallelwith thethiaclopridconcentration(Fig.4a,cande),confirmingthefact thatthereactivespeciesdonotreactonlywiththechromophore ortheformedintermediatesalsoabsorbatthiswavelength.The absorbancesat270nmchangesimilarlytothoseat242nm,except incaseofheterogeneousphotocatalysis,whereitfollowsthethia- clopridconcentration.Inthiscasetheopeningofthearomaticring hasthehighestprobability,thusaccumulationofaromaticproducts canbeavoided.InabsenceofDOtheaccumulationoftheinterme- diatesobservedatboth242and270nmismuchhigher(Fig.4b,d andf).
3.3. Intermediates
Forthecharacterizationoftheintermediates,thesamplesthat havebeentakenatirradiationtimes(ordose)of50%conversionof thiacloprid,namely15and150min;5and10min;0.4and0.3kGy in case of heterogeneous photocatalysis, VUV photolysis and ␥
radiolysisinthepresenceandabsenceofDO,respectively,were analysedbyHPLC–MSinpositiveandnegativeionmodes(Tables S1–S4).
Inthechromatographicseparationsthiaclopridelutedwitha retentiontime(tr)of4.58min.Threemainintermediates(twowith tr=2.13−2.33min andone with3.65min,Fig.5a–c) and theE- isomerofthiaclopid(tr=4.40min,Fig.5d)weredetectedduring allthreemethods.TheMS-spectra(negativeionmode)and the UV–visspectraoftheZ-andE-isomers,havingdifferentretention timeswerefoundtobeidentical.
Theretentiontimerelatedtothechromatographicpeakofinter- mediatesA1andA2wasbetween2.13and2.33min.AstheUV–vis spectraofthesepeaks(Fig.5aandb)show,thisisthemixtureoftwo compounds,whichcannotbeseparatedbytheappliedchromato- graphicmethod.Accordingtothespectra,duringVUVphotolysis (bothinDO-freeandDO-containingsolutions)andheterogeneous photocatalysis(onlyinDO-containingsuspension)thesameinter- mediate(A1)forms.Theformationofthiscompoundismostlikely relatedtothe•OH-basedreaction,whichisthedominantreactive speciesundertheseexperimentalconditions.However,incaseof radiolysistheformationofeaq−isalsodominant.Regardingthat thereactivityofpyridine(partofthiacloprid)towardeaq−over- takesitsreactivitytoward•OH[27],inthiscasethetransformation ofthiachlopridcanbeinitiatedbyboththe•OHandeaq−,resulting inthepossibilityoftheformationofdifferentintermediates(A1 andA2).AccordingtoourresultsA2formationisdominantunder theusedexperimentalconditions.In thecaseof heterogeneous photocatalysis, inDO-freesuspension thedirect chargetransfer betweentheadsorbedthiaclopridandphotogeneratede−isalso possibleandresultstheformationofA2.Consequently,thesame intermediateformsasduring␥radiolysis.Thespectraofthiscom- pound(Fig.5b)arebackgroundcorrected,duetotheincomplete chromatographicseparationfromtheotherintermediateandtheir overlappingabsorptionsinthiswavelengthrange.Theresultsof MS(TableS1)supporttheformationoftwointermediateshaving retentiontimesclosetoeachother.A1andA2intermediatesdonot containchlorineatom.WeidentifyA1asanintermediateresulted bythesubstitutionofClbyOH,whichisfrequentlyobservedinthe
•OHreactionsofchlorinatedaromaticmolecules.
Theothermajorintermediatedetectedat3.65min(intermedi- ateB)formedineachcase,withdifferentconcentrations(Figs.5c and6c).Theconcentrationofthisintermediatedecreasedinthe followingorder:radiolysis(DO-free)∼=radiolysis(DO)>VUV(DO- free)>VUV(DO)»heterogeneousphotocatalysis(DO)anddetected onlyintraceinthecaseofheterogeneousphotocatalysis(DO-free).
MSresults(TableS1)supportthatthesameintermediateformed ineachcase.TheintermediateBmostprobablyresultedfromthi- aclopridviahydroxylation.In•OH-inducedreactionDell’Arciprete etal.suggestedtheformationofacompoundhydroxylatedonthe
0 5 10 15 20 25 30
210 260 310 360
mAU
Wavelength (nm) tr= 3.65 min
R - N R - O V - N V - O H - O
c) B
0 5 10 15 20 25
210 260 310 360
mAU
Wavelength (nm) tr= 4.40 min
R - O V - O H - O V - N H - N R - N
d) C
0 10 20 30 40 50
210 260 310 360
mAU
Wavelength (nm) tr~ 2.23 min
H - O V - O V - N
a)
A
10 10 20 30 40
210 260 310 360
mAU
Wavelength (nm) tr~ 2.17 min
R - O R - N H - N
b)
A
2Fig.5. Spectraofthethreemainintermediates(tr∼2.23min(a);tr∼2.17min(b);tr=3.65min(c))andtheE-isomerofthiaclopid(tr=4.40min,d)thathavebeentakenat 50%conversionofthiaclopridduringheterogeneousphotocatalysis(H, ),VUVphotolysis(V, )and␥radiolysis(R, )inthepresence(solidlines)andabsence(dashed lines)ofdissolvedO2.
CH2group[30],whereasCernigojetal.reportedtheformationof thesulfoxidecompoundasaresultofthermaloxidation[15].
3.4. Electricenergyperorder(EEO)
Thecomparisonofthemethodsisgenerallybasedonthetrans- formation rate of the target substance or total organic carbon concentration.In thepresent workvariousmethodsandexper- imentalparameters wereapplied.EEO,which is usuallyapplied forsituationswhereinitialconcentrationofpollutantislow,gives possibilityforeconomiccomparison.Itrepresentstheamountof electricenergyrequiredforreductionofthetargetcompoundcon- centrationinaunitvolume[e.g.,1m3]byoneorderofmagnitude [34].InbatchoperationEEOvalues[kWhm−3order−1]canbecal- culatedusingEq.10.
EEO= P×t×1000 V×lg
ci/cf
(10)wherePistheratedpower[kW]oftheAOPsystem,Visthevolume [L]ofwaterorairtreatedinthetimet[h],ci,cfaretheinitialand finalconcentrations[molL−1]ofthetargetcompound.Factor1000 convertsLtom3.HigherEEOvaluescorrespondtolowerremoval efficiencies.
Calculateddataare presented in Table1 and showthat the economicallymostfeasibleis␥radiolysis,followedbyVUVpho- tolysisandheterogeneousphotocatalysis.Anotherobservationcan bemade fromtheratioof theEEO calculatedat differentinitial concentrationsshowing,thatithasnoeffectontheefficiencyin caseofheterogeneousphotocatalysis, whileincase oftheother twomethodsitincreasesstronglywiththedecreaseoftheinitial concentration.
4. Conclusions
TransformationofthiaclopridwascomparedusingTiO2-based heterogeneousphotocatalysis,VUVphotolysisand␥radiolysis,in presenceandabsenceofDO.DOhassignificanteffectonlyincase ofheterogeneousphotocatalysis.Inaqueoussolutionsthetrans- formationofthiaclropidismostprobablyrelatedtoreactionswith both•OHandeaq−.InTiO2-containingsuspensionsreactionswith
•OHordirect chargetransferwithphotogenerated e− caniniti- atethetransformation.Threemainintermediatesweredetected ineachcase. TheformationofintermediatesA1 andBcouldbe relatedtothe•OH-initiatedtransformation,whiletheformationof A2ismostprobablyrelatedtothereactionofthiaclopridwitheaq−
orphotogeneratede−.Thiaclopridtransformedwiththehighest initialreactionrateduringVUVphotolysis,howeverregardingthe transformationoftheintermediateproductsheterogeneouspho- tocatalysis(inthepresentofDO)wasmoreefficient.
CalculatedEEOdatashow,that␥radiolysisisenergeticallymuch morefeasiblethanVUVphotolysisandheterogeneousphotocatal- ysis.
Acknowledgements
Theauthorswishtoexpresstheirdeepestandsincerestrecogni- tionofProf.AndrásDombiakeyfigureinthetopicofphotocatalytic materialsfor thedegradationofcontaminantsofenvironmental concern.
This researchwas supported by OTKA, NK 105802. T. Alapi acknowledgesthesupportoftheEuropeanUnionandtheStateof Hungary,co-financedbytheEuropeanSocialFundintheframe- workof TÁMOP-4.2.4. A/2-11/1-2012-0001‘NationalExcellence Program’.K.SchrantzacknowledgestheEuropeanUnionandthe StateofHungary,co-financedbytheEuropeanSocialFundinthe frameworkofTÁMOP4.2.4.A/1-11-1-2012-0001‘NationalExcel-
Fig.6.Relativeamountsofthemainintermediatesdetectedatretentiontimesof∼2.23min(a),∼2.17min(b),3.65min(c)and4.40min(d)at242nmversustheconversion ofthiacloprid(c0=1.00×10¬4molL−1)duringheterogeneousphotocatalysis(H,䊉and),VUVphotolysis(V, and )and␥radiolysis(R, and )inthepresence(filled symbols)andintheabsence(emptysymbols)ofdissolvedO2.
lenceProgram’.ThesupportoftheSwissContribution(SH7/2/20) isalsoacknowledged.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound, intheonlineversion,athttp://dx.doi.org/10.1016/j.cattod.2016.11.
055.
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