Chemical Engineering Journal 231 (2013) 314–325

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Chemical Engineering Journal

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Kinetic and mechanistic investigation of photocatalytic degradation ofthe N,N-diethyl-m-toluamide

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.06.123

⇑ Corresponding author. Tel.: +30 2641074186; fax: +30 2641074166.E-mail address: iokonst@cc.uoi.gr (I. Konstantinou).

M. Antonopoulou, A. Giannakas, Y. Deligiannakis, I. Konstantinou ⇑Department of Environmental and Natural Resources Management, University of Patras, 30100 Agrinio, Greece

h i g h l i g h t s

� Photocatalytic degradation andmineralization kinetics of DEET werestudied.� Degradation products were

elucidated using HR-LC–MS andGC–MS techniques.� Hydroxy-methylcyclohexadienyl

radicals were detected as initial DPsby EPR.� The acute toxicity of DEET and its TPs

was assessed along photocatalytictreatment.

g r a p h i c a l a b s t r a c t

3340 3360 3380 3400 3420 3440 3460

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(iv)

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(ii) Theoretical

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Experimental

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0.0E+002.0E+074.0E+076.0E+078.0E+071.0E+081.2E+081.4E+081.6E+081.8E+082.0E+08

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a r t i c l e i n f o

Article history:Received 14 March 2013Received in revised form 7 June 2013Accepted 30 June 2013Available online 9 July 2013

Keywords:DEETPhotocatalysisHigh resolution mass spectroscopyEPRScavengersDegradation mechanisms

a b s t r a c t

Analytical and electron paramagnetic resonance (EPR) spectroscopic methods were systematically usedfor the kinetic and mechanistic investigation of the photocatalytic degradation of N,N-diethyl-m-tolua-mide (DEET), in aqueous TiO2 suspensions under simulated solar light. The degradation of DEET followedfirst-order kinetics while enhanced reduction (>85%) of total organic carbon (TOC) and stoichiometrictransformation of nitrogen to nitrate and ammonium ions took place after 240 min of irradiation.Numerous different structures of transformation products (TPs), with at least one isomer for the majorityof them, were identified with high resolution accurate mass liquid chromatography (HR-LC–MS) and gaschromatography mass spectrometry (GC–MS). Low temperature EPR spectroscopy was used to study thephotoinduced radicals created during the initial events of the photocatalytic oxidation. Two kinds ofaromatic ring carbon-centered radicals i.e. hydroxy-methylcyclohexadienyl radicals have been resolvedat 77 K. The second-transient conformation of the radicals is maximized after 5 min. of irradiation andthen slowly decays. On the basis of identified products and radicals, a proposed pathway of photocata-lytic degradation of DEET is presented, involving mono- and polyhydroxylation and/or oxidation,dealkylation and continuously the opening of the aromatic ring. Scavenging experiments indicated thecontribution of �OH as the main species in the DEET oxidation while O��2 contributes also to thedegradation in a lesser extend after the initial steps of the reaction. Finally, toxicity studies based onluminescence of Vibrio fischeri bacteria before and after the photocatalytic treatment were alsoperformed.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

N,N-diethyl-meta-toluamide, commonly known as DEET, hasbeen used as the active ingredient in the majority of the

commercial insect repellents for decades [1] at concentrations of4–100% in various formulations such as lotions, creams, gels, aero-sols, and sticks [2]. Its extensive usage has been related to the totalprotection it provides against a broad spectrum of biting insects[3]. In the last few years, contamination of DEET has been widelyreported and it has been detected in various aquatic environments,including rivers, groundwater, seawater, wastewaters and even in

M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325 315

drinking water treated by conventional water-treatment systems[1,4–6]. DEET can enter the environment mainly through munici-pal wastewater and is considered to be persistent in hydrolysis[1]. As a result of both extensive use and unintentional discharges,DEET has become an emerging contaminant and its removal fromwater is of high priority.

Recently, the treatment with advanced oxidation processes(AOPs) has been established as one of the most promising methodsfor the removal of organic contaminants from the water, avoidingthe formation of toxic end products from the water. Heterogeneousphotocatalysis using TiO2 as catalyst represents an example of ad-vanced oxidation processes, capable of achieving complete miner-alization of various organic pollutants [7–9]. The detailedmechanisms for the formation of both reducing and highly oxidiz-ing species on the surface of TiO2 particles via a series of redoxreactions are well documented in the literature [8,10,11]. Due tothe complexity of the redox reactions, including also radical-in-duced reactions, various TPs are formed before complete mineral-ization is achieved. Some of these TPs can be toxic and, in somecases, more persistent than the parent compound [7,12]. Thus, amechanistic investigation based on the identification of the TPsand the photoinduced radicals is essential for maximizing theoverall process efficiency. Gas chromatography–mass spectrome-try (GC–MS), liquid chromatography–mass spectrometry (LC–MS), 1H NMR, FT-IR and EPR techniques have been widely usedfor the determination of organic intermediates [8].

To the best of our knowledge there is only one study dealingwith the identification of photocatalytic TPs of DEET [13] usingLC–MS that focused mainly on the interpretation of TPs mass spec-tra. The present study focuses on the detailed kinetic and mecha-nistic investigation of DEET photocatalytic degradation in thepresence of TiO2 under simulated solar light. Thus, the aims ofthe study were: (i) to evaluate the kinetics of DEET disappearanceand mineralization; (ii) to identify the TPs by using a combinationof powerful analytical techniques such as accurate mass HR-LC–MS, GC–MS and EPR spectroscopy; (iii) to probe the role of thereactive species in the reaction mechanism using different scav-engers; (iv) to study the degradation pathways followed and (v)to evaluate the toxicity along the treatment.

2. Materials and methods

2.1. Chemicals

DEET, analytical grade (97.6%), was purchased from Sigma–Al-drich (USA). TiO2 P25 (Degussa, Germany) was used as photocata-lyst. HPLC-grade solvents (acetonitrile, isopropanol, methanol andLC-grade water) were supplied by Merck (Darmstadt, Germany).Sodium azide (NaN3), potassium iodide (KI) and p-benzoquinone(BQ) were obtained from Sigma–Aldrich. Cellulose acetate (HA0.45 lm) filters were supplied by Millipore (Bedford, USA). OasisHLB (hydrophilic–lipophilic balanced copolymer of divinylben-zene/N-vinylpyrrolidone) cartridges (60 mg, 3 mL) from Waters(Mildford, MA, USA) were used for the by-products extraction.Ultrapure water was obtained from a Millipore Waters Milli-Qwater purification system. Primary stock standard solution of DEETwas prepared in water at a concentration of 200 mg L�1 andprotected from light. The working solutions were further dilutedfrom the stock solution. Calibration standards were renewed everyweek.

2.2. Irradiation procedures

Irradiation experiments were carried out in a SuntestXLS + apparatus (Heraeus Germany) simulating natural sunlight

irradiation. The light source was an air-cooled xenon lamp(2.2 kW) jacked with special glass filters restricting the transmis-sion of wavelengths below 290 nm. Irradiation experiments wereperformed using a cylindrical 250 mL Duran� glass UV-reactorwith a flat flange lid with three necks (Lenz, Germany; 9.7 cm innerdiameter, 12.8 cm external diameter, 9.5 cm reaction vessel heightand 17.8 cm total height, light absorbance k < 300 nm) with ther-mostatic jacket containing 250 mL of DEET aqueous solutions atnear-neutral pH (pH = 6.5). Tap water cooling circuit was used toprevent any heating of the suspension, keeping the temperatureat 23 ± 1 �C. In addition, Suntest apparatus is equipped with a blackstandard thermometer (BST) to regulate air temperature condi-tions in the exposure chamber by air-cooling circuit. The irradia-tion was measured by a calibrated internal radiometer of theSuntest apparatus. A uniform repartition of the light within theirradiation cell is ensured by the reflectors of the instrument. Thesolution was mixed with the appropriate amount of catalyst andthe suspension was magnetically stirred before and during the illu-mination. The suspension was kept in the dark for 30 min, prior toillumination to reach adsorption equilibrium onto semiconductorsurface. Irradiation experiments (I = 750 W m�2) were performedusing DEET aqueous solution (10 mg L�1) and 100 mg L�1 of TiO2.Initial DEET concentration of 10 mg L�1 was selected, in order tobe high enough to facilitate identification of byproducts. Theexperiments were carried out using a relatively low amount ofTiO2 that enabled us to obtain slower kinetics and provided favor-able conditions for the determination of reaction intermediates.Aliquots (15 mL) were withdrawn from the reactor at specific timeintervals and filtered through a 0.45 lm filter to remove the TiO2

particles.

2.3. Analytical procedures

2.3.1. Kinetic study – high performance liquid chromatography (HPLC)analysis

DEET concentrations were determined by a Dionex P680 HPLCequipped with a Dionex-100 photodiode array detector using aDiscovery C18, (250 mm � 4.6 mm ID � 5 lm) column (Supelco,Bellefonte, PA, USA). The mobile phase was a mixture of LC-gradewater H2O (50%) pH 3 (adjusted with formic acid) and acetonitrile(50%) with a flow rate of 1 mL min�1. 20 lL of standard solutionsand samples were injected into HPLC system. Column temperaturewas set at 40 �C and DEET was detected at 210 nm. The determina-tion of DEET concentrations was performed by a calibration curve(peak area against concentration) with seven points in the range0.25–20 mg L�1. The linear regression equation was y = 2.45x + 0.03 and the coefficient of determination (R2) was 0.9999. Thedetection limit was 30 lg L�1 using a signal-to-noise ratio (S/N)equal to 3. Finally, the relative standard error (RSE) in the deter-mined DEET concentrations was lower than 3.5%.

2.3.2. Mineralization studiesTo determine the extent of mineralization during photocataly-

sis, total organic carbon (TOC) were measured, by direct injectionof filtered samples into a Shimadzu, TOC V-CSH Analyzer using com-bustion catalytic oxidation non dispersive infrared method. Theinstrument was calibrated using total carbon standard solutionsof potassium hydrogen phthalate. The RSE in the determined TOCconcentrations was lower than 4.6%. The evolution of NO�3 andNO�2 ions, released during the oxidation of DEET, was followed bya Dionex ICS-1500 Ion Chromatography (IC) system equipped withASRS Ultra II self-regenerating suppressor. The column was an Ion-Pac AS9-HC and the mobile phase consist of 9 mM aqueous sodiumcarbonate at a flow rate of 1 ml/min. 25 lL of the samples were in-jected into the column. The determination of NO�3 and NO�2 con-centrations was performed using calibration curves (peak area

316 M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325

against concentration) with five points in the range 0.5–10 mg L�1.The coefficients of determination (R2) were 0.9998 and 0.9999 forNO�2 and NO�3 , respectively. The RSE in the determined NO�3 con-centrations was lower than 3.2%.

Ammonium ions (NHþ4 ) were analyzed spectrophotometricallyusing the colometric method based on indophenol blue formation[14]. The absorbance at 640 nm was measured with a UV–Vis spec-trophotometer (Hitachi, model U-2000). A calibration curve in therange of 0.075–2 mg L�1 was used. The coefficient of determination(R2) was 0.9999 and the RSE in the determined NHþ4 concentrationswas lower than 5.5%.

2.3.3. Scavenging experiments for reactive speciesTo determine the role of radical reactive species such as hydro-

xyl (�OH) and superoxide anion radicals (O��2 ), isopropanol(1000 mg L�1) and p-benzoquinone (20 mg L�1) were added tothe solutions containing 10 mg L�1 DEET. Moreover, the participa-tion of oxidative holes (h+) and the role of the oxygen singlet (1O2)in the reaction mechanism were evaluated by adding iodine anions(100 mg L�1) and azide (10 mg L�1) to the reaction system, respec-tively. Finally, photocatalytic experiments using CH3CN solutionswere performed for discrimination between the oxidation by �OHand h+.

2.4. Determination of photocatalytic TPs and toxicity

2.4.1. Sample extractionFor the GC–MS analysis, the irradiated solutions (20 mL) were

extracted by means of solid-phase extraction (SPE), using OasisHLB cartridges (60 mg, 3 mL). SPE was performed using a 12-foldvacuum extraction box (Visiprep, Supelco, USA). The SPE cartridgeswere conditioned with 3 mL of acetonitrile and 3 mL LC-gradewater. Then, irradiated samples were added at a flow rate of10 mL min�1. Subsequently, the cartridges were dried both bynitrogen stream and vacuum for 30 min. The compounds trappedin the cartridge were collected by using 2 � 2 mL of acetonitrileas eluting solvent. Small quantities of anhydrous Na2SO4 wereadded to remove any water traces in the sample. Finally, the eluatewas evaporated to a final volume of 0.5 mL under nitrogen flowprior to GC–MS analysis.

2.4.2. LC–MS and GC–MS analysisAn ultra performance liquid chromatography–electrospray ion-

ization–mass spectrometry (UPLC–ESI–MS) system including anAccela Autosampler, an Accela LC pump and a LTQ Orbitrap massspectrometer (Thermo Fisher Scientific, Germany) was used forthe identification of DEET degradation products. A C18 HypersilGold column, (100 mm � 2.1 mm i.d., 1.9 lm particle size, ThermoFisher Scientific), was used at 40 �C. Injection volume was 10 lLand flow rate 300 lL min�1. Gradient mobile phase compositionwas adopted using water/5 mM ammonium formate as solvent Aand methanol/5 mM ammonium formate as solvent B with the fol-lowing program: 90/10 to 0/100 in 14 min, and 90/10 in 20 min. AllESI–MS experiments were acquired in the positive mode and theESI-source parameters were as follows: sheath and auxiliary gasflow rate 30 and 8 (nitrogen, arbitrary units), respectively; sourcevoltage at 3.70 kV; capillary temperature at 320 �C. For fragmenta-tion study, the voltage of the HCD collision cell was set at 35 eV.Prior to analysis, the orbitrap mass analyzer was externallycalibrated, in the scan range m/z 70–650, to obtain mass accuracywith ±5 ppm. The tuning parameters adopted for the ESI sourcewere: capillary voltage 40.00 V, tube lens 110 V. The analysis wasperformed using a resolving power of 60.000. UPLC–ESI/MS systemwas controlled with Xcalibur software version 2.1 that includedchemical formula calculator used to provide chemical formulaand mass accuracy values.

GC–MS analysis were performed using a gas chromatograph–mass spectrometer QP-2010 (Shimadzu, Kyoto, Japan), equippedwith a split/splitless auto-injector model AOC-20i. The GC was fit-ted with a capillary SLB-5 ms column (30 m � 0.25 mm � 0.25 lm)(Supelco, Bellefonte, USA). Separation of the by-products was con-ducted using the following conditions: injector temperature220 �C, column temperature program 60 �C (2 min) to 280 �C(2 min) at 6 �C min�1 to 300 �C (2 min) at 10 �C min�1. Heliumwas used as the carrier gas at a flow rate of 1.7 mL min�1. Theion source and transfer line were kept both at 250 �C. Electron im-pact (EI) ionization mode at 70 eV was used in the scan range ofm/z 50–550. The splitless mode was used for injection of 1 lL volume.

2.4.3. EPR spectroscopyEPR spectra were recorded with a Bruker ER200D spectrometer

at liquid nitrogen temperature (77 K), equipped with an Agilent5310A frequency counter. Samples were illuminated in situ insidethe EPR cavity using a 450 W white-light lamp (Oriel model66929) in the cavity while spectra were recorded. EPR sampleswere prepared by mixing 4 mg and 7 mg of TiO2 with 200 mg L�1

aqueous DEET solution into a 5 mm suprasil (Willmad Co.) quartztubes. Spectral simulation of the EPR signals has been carried outusing the Sinfonia software.

2.5. Toxicity measurements

The toxicity of the samples was evaluated by using the marineluminescence bacteria Vibrio fischeri and the Microtox Analyzer(Model 500, Azur Enviromental, Carlsbad, CA, USA). A brieflydescription of the procedure has been reported in our previousstudy [15].

3. Results and discussion

3.1. Preliminary experiments – adsorption in the dark and directphotolysis

Prior to TiO2 photocatalytic degradation experiments, adsorp-tion (DEET aquatic solution plus TiO2 in the dark) and directphotolysis (UV–Vis irradiated DEET solution without TiO2) experi-ments were carried out and the kinetic profiles are depicted inFig. 1. In the absence of UV–Vis light, negligible adsorption of DEETon TiO2 surface was exhibited while direct photolysis resulted inlow degradation of DEET. In contrast, the addition of TiO2

promoted significantly the degradation of DEET and intermediates.The degradation kinetics for both DEET and TOC (Fig. 1) followedapparent first-order degradation curve according to the simplifiedLangmuir–Hinshelwood model [7] with k = 10.3 10�2 min�1 and0.81 10�2 min�1, respectively. The corresponding half-lives (t1/2)of DEET and TOC were 6.7 min and 85.6 min, respectively.

3.2. Identification of intermediates by mass spectrometric and EPRtechniques

3.2.1. Identification of TPs by mass spectrometric techniquesThe TPs were characterized by accurate mass determination

and MS2 fragmentation using HR-LC-LTQ Orbitrap MS and elec-tron-impact mass spectra using GC–MS. Up to 17 structures ofTPs with multiple isomeric compounds (equal m/z molecular ions)in most cases were identified during the photocatalytic degrada-tion of DEET. The mass spectra characteristics of the detected TPsare presented in Tables 1 and 2 and Tables S1 and S2 (Supportinginformation, S.I.) for HR-LC–MS and Table S3 for GC–MS analysis.The formation and degradation kinetics of TPs were also monitoredin order to distinguish the major compounds. Figs. 2 and 3 show

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Fig. 1. Degradation kinetics of DEET (C0 = 10 mg L�1, 100 mg L�1, I = 750 W m�2) (a)in the dark with TiO2 (adsorption); (b) UV–Vis irradiation (photolysis); (c) UV–Visirradiation with TiO2; (d) TOC evolution and (e) evolution of NO�3 and NHþ4 ions; C0

and TOC0 are the initial concentrations of DEET and TOC; NO�3 s , NHþ4 s and Ns are thestoichiometric concentrations of the inorganic ions and nitrogen, respectively.

M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325 317

the evolution profiles of the TPs as a function of the irradiationtime.

3.2.1.1. Hydroxylated compounds. Using HR-LC–MS, six species with[M + H]+ ions at m/z 208.1332 (208-A,B,C,D,E,F), were detected inthe initial steps of DEET degradation as shown in the extractedchromatographic profiles of selected TPs (S.I., Fig. S1). On the basisof their exact mass that differed 15.995 units from the DEET mol-ecule ([M + H]+ = 192.1382) they were attributed to oxygenatedTPs and more specifically to hydroxy-DEET derivatives, as a conse-quence of the �OH radicals addition. More than one derivative wassimply the consequence of the non-selectivity of the �OH radical at-tack [12,16]. The exact position of the �OH radical attack can beproposed from the analysis of the MS2 spectra (S.I., Table S1). Theproduct ions at m/z 100.0753 (loss of hydroxy-toluene) and135.0439 (loss of diethylamine) were common for C,D,E,F isomersand evidenced the location of OH group in the aromatic ring. Inaddition, other structurally diagnostic losses occurred; in 208-A,the product ion at 178.1128 (attributed to loss of formaldehyde)pointed the OH group’s location on the methyl group attached tothe aromatic ring. The formation of the product ion at m/z163.0750 for the isomer 208-B, originating from CH2CH2OH group

Table 1High resolution accurate mass data (retention time (Rt), quasimolecular [M + H]+ m/z ionderivatives and oxidized derivatives.

Mono-, di-, tri-hydroxylated derivatives

Rt m/z [M + H]+ Molecular formula D (ppm)

7.506 208.1332 (208-A) C12H18NO2 3.51088.097 208.1339 (208-B) C12H18NO2 3.42528.784 208.1337 (208-C) C12H18NO2 2.27298.886 208.1336 (208-D) C12H18NO2 2.08459.375 208.1334 (208-E) C12H18NO2 0.97599.577 208.1333 (208-F) C12H18NO2 0.63515.864 224.1282 (224-A) C12H18NO3 0.39296.469 224.1282 (224-B) C12H18NO3 0.33976.795 224.1284 (224-C) C12H18NO3 1.16718.318 224.1285 (224-D) C12H18NO3 1.55118.574 224.1283 (224-E) C12H18NO3 0.64795.735 240.1225 (240-A) C12H18NO4 �2.07066.034 240.1229 (240-B) C12H18NO4 �0.72456.469 240.1231 (240-C) C12H18NO4 0.45466.640 240.1230 (240-D) C12H18NO4 �0.29517.209 240.1235 (240-E) C12H18NO4 1.9742

loss, well matched with OH group addition on the ethyl group. Incomparison with the previous study focused on the photocatalyticTPs of DEET [13], two more isomers with m/z 208 were identifiedin the present study.

The formation and disappearance kinetics of the hydroxylatedTPs are shown in Fig. 2A. Based on the kinetic profiles the isomersbearing the –OH group in the alkyl groups of DEET were the minorTPs while the isomers bearing the –OH group in the aromatic ringwere the major TPs. Six isomers with [M+] = 207 were also detectedusing GC–MS (S.I., Table S3). The presence of fragments at m/z = 135 and m/z = 72 (loss of diethyl-amine group) in the EI–MSspectrum of the five from the six isomeric species, is well matchedwith the hydroxylation on the toluene moiety.

Five species with [M + H]+ ions at m/z 224.1282–224.1284 (224-A,B,C,D,E) were also formed in the initial steps of DEET degradation(S.I., Fig. S1). They were attributed to di-hydroxylated DEET on thebasis of the 31.99 amu mass difference comparing to DEET. The iso-mers 224-A,B,D,E exhibited a product ion at 151.0388 (loss ofdiethylamine), allowing the exclusion of ethyl chain hydroxylation.Concerning the isomer 224-C, its MS2 spectrum (S.I., Table S1)showed the formation of a product ion at m/z 206.1174 as basepeak resulting from the loss of H2O. MS3 fragmentation (data notshown) exhibited a product ion at m/z = 133.0281, suggestinghydroxylation of the ethyl chain. Taking into account the kineticprofiles in Fig. 2B, the isomers 224-A,D,E bearing the two hydroxylgroups in the aromatic ring were the major products. Three of thedi-hydroxylated isomers were also detected using GC–MS. Thenon-detection for the other isomers identified by LC–MS could beattributed to their lower formation as explained previously, as wellas to the low volatility and increased polarity.

Tri-hydroxylation of DEET leads to the formation of five isomerswith [M + H]+ ions at m/z 240.1225–240.1235 (240-A,B,C,D,E).Based on their exact mass that differ 47.9853–47.9895 units fromthat of DEET, their formation was linked to the insertion of threeOH groups. These TPs were readily transformed and completelyabated after 180 min (Fig. 2C). MS2 spectra (Table S1) of the isomer240-B was only recorded at the sampled irradiation times. The pres-ence of product ions at m/z 74.0953 (protonated diethylamine),222.1120 (loss of H2O) and 194.1173 (loss of H2O and CO) indicatedthat hydroxylation take place on the aromatic ring.

3.2.1.2. Oxidized compounds. Oxidized TPs (ketones or aldehydes)were formed simultaneously or consecutively with the formationof mono-, di and tri-hydroxylated derivates. The formation anddisappearance kinetics of the oxidized TPs are shown in Fig. 2.

s, molecular formulas and relative error, D (ppm)) for mono-, di-, tri-hydroxylated

Oxidized derivatives

Rt m/z [M + H]+ Molecular formula D (ppm)

7.633 206.1176 (206-A) C12H16NO2 0.28859.962 206.1177 (206-B) C12H16NO2 0.51066.012 222.1127 (222-A) C12H16NO3 0.28517.238 222.1129 (222-B) C12H16NO3 1.75787.980 222.1126 (222-C) C12H16NO3 0.38308.117 222.1128 (222-D) C12H16NO3 1.60366.209 238.1074 (238-A) C12H16NO4 0.02896.640 238.1073 (238-B) C12H16NO4 �0.41976.985 238.1076 (238-C) C12H16NO4 0.73387.121 238.1074 (238-D) C12H16NO4 0.22408.018 238.1075 (238-E) C12H16NO4 0.54168.189 238.1072 (238-F) C12H16NO4 �2.3730

Table 2High resolution accurate mass data (retention time (Rt), quasimolecular [M + H]+ m/z ions, molecular formulas and relative error, D (ppm)) for demethylated-hydroxylated andring opening derivatives.

Demethylated-hydroxylated derivatives Ring opening derivatives

Rt m/z [M + H]+ Molecular formula D (ppm) Rt m/z [M + H]+ Molecular formula D (ppm)

8.951 178.1229 C11H16NO 1.3256 4.907 256.1180 (256-A) C12H18NO5 0.22487.633 194.1180 (194-A) C11H16NO2 2.1334 5.543 256.1183 (256-B) C12H18NO5 1.41247.848 194.1179 (194-B) C11H16NO2 1.5754 5.770 256.1182 (256-C) C12H18NO5 1.16208.276 194.1181 (194-C) C11H16NO2 2.8203 6.269 256.1185 (256-D) C12H18NO5 2.03074.750 210.1127 (210-A) C11H16NO3 1.1746 6.950 256.1179 (256-E) C12H18NO5 �0.35246.057 210.1126 (210-B) C11H16NO3 0.4484 4.182 244.1184 (244-A) C11H18NO5 1.88056.441 210.1126 (210-C) C11H16NO3 0.5936 5.876 244.1181 (244-B) C11H18NO5 0.63046.830 210.1127 (210-D) C11H16NO3 0.9567 5.981 244.1181 (244-C) C11H18NO5 0.75547.087 210.1127 (210-E) C11H16NO3 1.1746 6.405 244.1181 (244-D) C11H18NO5 0.75547.430 210.1128 (210-F) C11H16NO3 1.6103 4.000 228.1234 (228-A) C11H18NO4 1.52568.238 210.1127 (210-G) C11H16NO3 0.9567 4.182 228.1233 (228-B) C11H18NO4 1.25814.836 226.1071 (226-A) C11H16NO4 �1.0493 4.877 228.1233 (228-C) C11H18NO4 1.25815.218 226.1070 (226-B) C11H16NO4 �1.7242 5.165 228.1234 (228-D) C11H18NO4 1.39185.603 226.1068 (226-C) C11H16NO4 �2.4665 5.633 228.1233 (228-E) C11H18NO4 1.25817.121 226.1070 (226-D) C11H16NO4 �1.8592 6.602 228.1234 (228-F) C11H18NO4 1.45877.761 164.1072 C10H14NO 1.3079 4.654 242.1389 (242-A) C12H20NO4 0.89275.815 180.1017 (180-A) C10H14NO2 �1.2251 5.346 242.1385 (242-B) C12H20NO4 �0.87196.284 180.1018 (180-B) C10H14NO2 �0.8015 6.075 242.1381 (242-C) C12H20NO4 �2.32127.056 180.1018 (180-C) C10H14NO2 �0.6321 8.844 182.1174 C10H16NO2 �0.76277.177 180.1017 (180-D) C10H14NO2 �1.3946 5.346 198.1124 (198-A) C10H16NO3 �0.37178.799 180.1017 (180-E) C10H14NO2 �1.4793 5.825 198.1124 (198-B) C10H16NO3 �0.60289.557 180.1017 (180-F) C10H14NO2 �2.4960 9.831 198.1124 (198-C) C10H16NO3 �0.6028

318 M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325

Two TPs with [M + H]+ at m/z 206.1176 and 206.1177 (206-A,B)were detected. The elimination of diethylamine fragment (m/z133.0281) in MS2 spectra for both isomers suggests that oxidationtakes place in the toluene group. Four oxidized TPs were detectedby GC–MS (S.I., Table S3). Three of them presented fragment ions atm/z = 133–134 and m/z = 106–105 through the loss of diethyl-amine and N,N-diethylformamide, respectively, indicating theexclusion of N-alkyl chains oxidation. The fourth isomer shows astructurally diagnostic ion at m/z 119 suggesting the oxidationon the N-ethyl chain. The determined peak area revealed that thisoxidation is a minor route.

Four derivatives with [M + H]+ ions at m/z 222.1126–222.1129(222-A,B,C,D) were detected as oxidation TPs after a second oxida-tion step. Similarly the fragment elimination of diethylamine (m/z149.0230) in MS2 spectra (S.I., Table S1) allows the conclusion thatoxidation takes place in the aromatic ring moiety. In addition, thefragment ion at m/z = 149 (loss of diethylamine) was observed inthe GC–MS spectra of three TPs, in agreement with the proposedexclusion of N-alkyl chains oxidation.

Finally, further steps of oxidation lead to the detection of sevenTPs with [M + H]+ ions at m/z 238.1072–238.1076 (238-A,B,C,D,E,F)derived by the oxidation of the tri-hydroxylated TPs(240-A,B,C,D,E). The presence of MS2 product ion at m/z165.0179–165.0181 for the three of the detected isomers suggeststhat oxidation takes place in the aromatic moiety. The isomer 238-F exhibited a product ion at m/z 220.0968 (loss of H2O) and permitsto allocate a hydroxyl-group in ortho-position to CH3 group. Thetri-hydroxylated TPs were not detected by GC–MS because of theirincreased polarity.

3.2.1.3. Dealkylated TPs. The TPs with [M + H]+ at m/z 178.2229,194.1179 (isomers 194-A,B,C), 210.1126 (isomers 210-A,B,C,D,E,F,G) and 226.1071 (isomers 226 A,B,C,D) were derivedall by the elimination of methyl group attached to the aromaticring from DEET, mono-hydroxylated, di-hydroxylated and tri-hydroxylated TPs, respectively. This assignment is supported: a)by the mass difference of 13.9153–14.0152 from the parent com-pounds; and b) the formation of MS2 product ions (S.I., Table S1)at m/z 105.0333, 121.0282, 137.0236 and 153.0182 respectively,

which is consistent with the loss of –C4H11N group. The demethy-lated-DEET ([M+] = 177) was also identified by GC–MS showing thecharacteristic fragment at m/z = 105 (loss of diethylamine) as abase peak. The TPs with [M + H]+ at m/z 164.1072 and 180.1018were both formed through the elimination of one ethyl group fromDEET and mono-hydroxylated TPs, respectively. Confirmationcomes from: (a) the mass difference of 28.031–28.0318 comparedwith the parent compounds and (b) the formation of MS2 productions at m/z 119.0487 for the mono-hydroxylated, 135.0434 and109.0646 for the di-hydroxylated corresponding to the loss ofethylamine and ethylisocyanate, respectively. The N-deethylatedderivative of DEET was also detected by GC–MS (S.I., Table S3),showing [M+] = 163 and characteristic fragments at m/z = 119 (lossof N-ethyl group) and m/z = 91 (toluene group). Using GC–MS sys-tem, three dealkylated-monohydroxylated isomers with[M+] = 179 were detected as well, and the formation of fragmentsat m/z = 134 (loss of ethylamine) ruled out the rejection of alkylchain hydroxylation. One isomer presented characteristic frag-ments at m/z = 125, m/z = 72 and m/z = 58 which corresponded topropionitrile loss, N-ethylformamide and N-methylformamiderespectively, suggesting also non-hydroxylation of ethyl chain.The kinetic profiles of the dealkylated TPs using HR-LC–MS areshown in Fig. 3.

3.2.1.4. Ring opening TPs. The TPs with [M + H]+ at m/z 256.1179–256.1185 were formed via the rupture of the aromatic ring. Fiveisomers (256-A,B,C,D,E) were formed corresponding to tetra-oxy-genated ring opening TPs in which oxygenation could be in theform of hydroxyl, aldehyde, ketone or carboxylic acid group. Inter-estingly, water elimination is observed for all isomers showing MS2

product ions at m/z 238.1071. In addition, 256-A,B isomers showedMS2 product ion at 214.1073 by the loss of –C2H2O group. Isomers256-B,D exhibited a MS2 product fragment at m/z 210.1127through the elimination of formic acid (loss of 46 Da). In the caseof isomer 256-C, the formation of an MS2 product fragment at m/z 212.1280 was observed, resulting from CO2 elimination, a com-mon fragmentation pathway for carboxylic acids.

Four isomers with [M + H]+ at m/z 244.1181–244.1184 (244-A,B,C,D) were assigned as tetra-oxygenated and demethylated ring

0.0E+002.0E+074.0E+076.0E+078.0E+071.0E+081.2E+081.4E+081.6E+081.8E+082.0E+08

Are

a208-A

208-B

208-C

208-D

208-E

208-F

0.0E+002.0E+064.0E+066.0E+068.0E+061.0E+071.2E+071.4E+071.6E+071.8E+072.0E+07

Are

a

224-A

224-B

224-C

224-D

224-E

0.0E+002.0E+054.0E+056.0E+058.0E+051.0E+061.2E+061.4E+061.6E+061.8E+062.0E+06

Are

a

240-A

240-B

240-C

240-D

240-E

0.0E+00

2.0E+07

4.0E+07

6.0E+07

8.0E+07

1.0E+08

1.2E+08

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

Are

a

206-A

206-B

0.0E+00

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2.0E+07

3.0E+07

4.0E+07

5.0E+07

6.0E+07

7.0E+07

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a

222-A

222-B

222-C

222-D

0.0E+005.0E+051.0E+061.5E+062.0E+062.5E+063.0E+063.5E+064.0E+064.5E+065.0E+06

Are

a

238-A

238-B

238-C

238-D

238-E

238-F

A

B

D

E

CF

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

Fig. 2. Time evolution profiles of mono-, di- and tri-hydroxylated and oxidized TPs.

M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325 319

opening TPs. However their MS2 product ions were not obtainedfor the sampled irradiation times, thus a more precise structurecannot be given. The TPs with [M + H]+ at m/z 228.1234(228-A,B,C,D,E,F) and m/z 242.1389 (242-A,B) were also detectedand assigned to tri-oxygenated ring opening products. Analysis ofthe MS2 spectrum of the major isomers (228-C,D), displayed MS2

product ions at m/z 210.1124 and 198.1122, which are consistentwith the loss of water and –CHOH group respectively, allowingto propose hydroxylation of the carbon atoms involved in the ringcleavage. A loss of a water molecule from the TP 242-A is also evi-dent, with the consequent formation of a product MS2 ion at m/z224.1282. The TPs with [M + H]+ at m/z 182.1174 and m/z198.1124 (198-A,B,C) were assigned to mono- and di-oxygenatedring opening TPs, respectively. Regarding, the TP with m/z182.1174, the MS2 fragments at m/z 109.0282 (loss of amino-diethyl group) and at m/z 100.0755 (diethylamide) were compati-ble with the presence of unmodified amino diethyl chains. MS2

product ions for 198-A,B,C species were not obtained for the sam-pled irradiation times and further structural characterization wasnot possible. The kinetic profiles of all ring-opening TPs based onHR-LC–MS data are shown in Fig. 3. Only one ring-opened product

with [M+] = 181 was detected by GC–MS and mass pattern (m/z = 138, 109, 72) allowed once more to exclude modification ofthe N-ethyl chains. Finally, the TPs with [M+] = 143 (with frag-ments at m/z 128 (–CH3), 99 (–CH3CHO) and 58 (–CH3 loss fromdiethylamine)) and [M+] = 101 (m/z fragments at = 86 (–CH3), 72(–CH2CH3) and 59 (–CH3, –CO) were assigned to N,N-diethyl-2-oxo-propanamide and N,N-diethyl-formamide, respectively.

3.2.2. EPR spectroscopyLow-temperature EPR spectroscopy was used to trap radical

intermediates in the initial reaction stages. Upon irradiation at77 K a multiplet-signal EPR was developed in the g � 2 region, asshown in Fig. 4. No other signals were resolved e.g. the well knownEPR signals due to holes [h+] and electrons [e�] of TiO2, [17], as aconsequence of the low-concentration of TiO2 powder. Photoirradi-ation of the DEET alone resulted in no-photoinduced EPR signals.This is in agreement with the analytical-HPLC data in Fig. 1 wherethe photolysis yield of DEET was negligible after 45 min ofirradiation. The main features of the EPR signals (Fig. 4) consistedof a four-line hyperfine pattern, marked as signal-I (gx, gy, gz valuesat 2.0024, 2.0024, 2.0023, respectively) in Fig. 4A, while a weaker

0.0E+00

1.0E+07

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3.0E+07

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rea

178

194-A

194-B

194-C

164

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210-B

210-C

210-D

210-E

210-F

210-G

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226-A

226-B

226-C

226-D

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Are

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180-A

180-B

180-C

180-D

180-E

180-F

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0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

Are

a

256-A

256-B

256-C

256-D

256-E

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4.0E+05

6.0E+05

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1.0E+06

1.2E+06

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1.6E+06

Are

a

244-A

244-B

244-C

244-D

0.0E+002.0E+054.0E+056.0E+058.0E+051.0E+061.2E+061.4E+061.6E+061.8E+062.0E+06

Are

a

228-A

228-B

228-C

228-D

228-E

228-F

A

B

C

D

E

F

G

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0 30 60 90 120 150 180Irradiation Time (min)

Are

a

0.0E+00

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2.5E+06

Area

182

198-A

198-B

198-C

242-A

242-B

242-C

H

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

0 15 30 45 60 75 90 105 120 135 150 165 180Irradiation Time (min)

Fig. 3. Time evolution profiles of demethylated, demethyalted-hydroxylated and ring-opened TPs.

320 M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325

signal, singal-II (gx, gy, gz values at 2.0025, 2.0024, 2.0023,respectively) depicted also in Fig. 4A, could be detected at earlyirradiation times <5 min. The time-evolution of the EPR signals(Fig. 4B) showed that signal-I followed a pseudo-linear increasewith irradiation time, approaching its maximum after 25 min. Sig-nal-II can be discerned at early irradiation times, while its intensitydecayed after 5 min of irradiation.

Close inspection of the EPR signal-I (Fig. 4A), shows a four-linepattern with a 1:3:3:1 intensity ratio and �20 Gauss splitting. Thispatttern is strongly resembling the EPR signals typically detectedfor R-CH3 radicals in solid matrices [18–20]. A computer simulationof the EPR signal-I and signal-II (dashed lines in Fig. 4A) shows thatboth originate from S = 1/2 radicals coupled with three 1H(I = 1/2)nuclei with each one. The difference between the two radicals

3340 3360 3380 3400 3420 3440 3460

* **

(iv)

(iii)(ii) Theoretical

Signal-I

dX"/

dH [a

rb. u

n.]

Magnetic Field [Gauss]

Experimental

Signal-II

(i)*

A

0

2

4

6

8

10

B

Fig. 4. (A) EPR spectra recorded for TiO2/DEET in H2O under illumination with awhite-light Xe-lamp [450 W]. (i) Experimental spectrum recorded after 6 min ofillumination; (ii) total theoretical spectrum calculated by adding the two theoret-ical spectra (iii) and (iv) using the Spin-Hamiltonian parameters listed in Table S4.EPR conditions: microwave frequency = 9.53 GHz, T = 77 K, modulation ampli-tude = 4.0 G, Microwave Power = 2 mW. (B) Time evolution of the spectra labeledsignal-I and signal-II.

Table 3Kinetic parameters (rate constants, half-lives (t1/2), correlation coefficients (R2) andpercent inhibition of rate constant,% Dk) of DEET in the presence of isopropanol(1000 mg L�1), CH3CN, azide ions (10 mg L�1), iodine ions (100 mg L�1) and p-benzoquinone (20 mg L�1), in TiO2 suspensions (100 mg L�1) using simulated irradi-ation. [DEET]0 = 10 mg L�1, [TiO2] = 100 mg L�1, I = 750 W m�2.

Treatment system k (10�2 min�1) t1/2 (min) R2 % Dk

Control (no scavenger) 10.28 6.7 0.9978 –TOC 0.81 85.6 0.9822 –Isopropanol 0.44 157.5 0.9748 95.7CH3CN 0.56 123.8 0.9689 95.6Azide ions 0.81 85.6 0.9753 92.1Iodine ions 0.64 108.3 0.9483 93.8p-Benzoquinone 1.51 45.9 0.9745 85.1

M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325 321

originates from 1H(I = 1/2) hyperfine-tensors (S.I., Table S4). Signal-II is characterized by a strong anisotropy in the Az-component e.g.Az = 22.1 G vs. Az = 28.1 G in signal-I. The two hyperfine couplingtensors (S.I., Table S4) for signal-I and signal-II can be attributedto two different conformations of the same or a similar moleculediffering in their local geometry and/or spin distribution.

3.2.2.1. Molecular assignment of radicals. The spectral pattern andhyperfine splittings for signal-I and -II are comparable to those re-ported for 4-methyl-2,6-di-tert-iarybutylphenol radical in singlecrystals [18]. As shown in a previous study [18], the unpaired elec-tron spin is distributed throughout the orbitals of the benzene ring,resulting in a high spin fraction of density (�0.5) on the carbonatom to which the CH3 group is attached. As a result, the threemethyl protons give rise to the characteristic four-line [1:3:3:1]hyperfine structure. Accordingly, taking into account the detectedring-hydroxylated TPs of DEET (Table 1), we assign signal-I to thehydroxy-methylcyclohexadienyl (HMCHD) radical. In a similarmanner, signal-II is assigned to a second conformation of the sameradical. The EPR data exclude the assignment of the detected rad-icals to other hydroxylated TPs such as 208A or 208B.

3.2.2.2. Information on the dynamics of the radical in the vicinity ofTiO2. The EPR proton hyperfine structure for a methyl groupcoupled to an unpaired electron was discussed by Freed [21]who predicted a seven-line hyperfine structure that prevails at

liquid-He temperatures. Rapid classical rotation of the methylgroup leads to a 1:3:3:1 quartet at high temperatures as observedin our case (Fig. 4A). The subtle features marked by (�) corre-sponded to traces of the 7-line hyperfine structure according toFreed’s model. Thus, in the DEET/TiO2 system the HMCHD radicalshave a non-negligible degree of hindrance, therefore the A-tensorof the CH3 moiety of HMCHD radical is not averaged out e.g. Ax -– Ay = Az, as would be expected for fast-rotating CH3 radicals[18–20]. In the DEET/TiO2 system the hindered mobility of theCH3 can be attributed to hindered mobility of DEET moleculeslocalized at the TiO2 interface. Signal-II with a more pronouncedhyperfine anisotropy, corresponds to a more hindered conforma-tion than signal-I. Thus, the EPR data show that early degradationsteps of DEET, result in the formation of HMCHD radicals at theintimate proximity of the TiO2 particles. The intimate proximityjustifies the fast photogeneration of the radicals at low tempera-tures. Two HMCHD radical conformations are resolved, howeverthe more hindered one (signal-II) decays within 5 min, while theless hindered one (signal-I) dominates after 25 min.

3.3. Kinetics, mineralization and effect of scavengers

3.3.1. Degradation kinetics and mineralization by TiO2-photocatalyticprocess

As shown in Fig. 1 complete degradation of DEET is achievedwithin 45 min with a half time of 6.7 min whereas almost com-plete TOC (85%) removal was succeeded after 240 min with a halftime of 85.5 min indicating the formation of intermediates whichneed longer irradiation times to achieve complete removal. Inaddition, as already shown in Figs. 2 and 3, the maximum con-centrations of the TPs have been attained in time periods overthe end of the sharp decrease section of DEET exponential decaycurve. Comparing Figs. 2 and 3 and 1, it is obvious that NO�3 andNHþ4 concentrations peak after 120 min, the time period that cor-responded to the nearly total degradation of the TPs. Similarly,about 70% of TOC removal coincides to this time framework.Stoichiometric release of nitrogen is achieved within 240 min ofirradiation as nitrates (almost 47%) and ammonium ions (50%)while nitrites were not detected. Based on the above resultsand that of the identification of TPs, only short-chain aliphaticacids remained in the solution after 240 min before completemineralization to CO2.

3.3.2. Contribution of reactive species to photodegradation of DEETVarious reactive species including h+, e�, �OH, O��2 , 1O2, etc. are

generated in UV–Vis TiO2 photocatalysis [22]. Several scavengerswere employed in order to elucidate the role of different reactivespecies [23–25]. In the present study, isopropanol and CH3CN,p-benzoquinone (p-BQ), iodine and azide ions were used as hydro-xyl, superoxide radicals, holes and singlet oxygen inhibitors,

Fig. 5. Hydroxylation-oxygenation pathways of DEET photocatalytic degradation in the presence of TiO2 and simulated solar light.

Fig. 6. Demethylation-hydroxylation pathways of DEET photocatalytic degradation in the presence of TiO2 and simulated solar light.

322 M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325

respectively. The degradation kinetics of DEET in the presence ofscavengers (Fig. S2) followed also apparent first-order degradationcurve according to the simplified Langmuir–Hinshelwood model.Table 3 lists the apparent first-order kinetic constants and the cor-relation coefficients of DEET degradation curves. The inhibitory ef-fect was evaluated by the percent reduction of kinetic constants (%Dk, Table 3).

Isopropanol is a well known hydroxyl radical quencher due toits high-rate constant reaction with the radical (1.9 � 109 M�1 s�1)[26] and has been widely used to discriminate the direct oxidationof substrates by hydroxyl radicals [23,27]. The addition of isopro-panol leads to almost completely inhibition on TiO2 photocatalytic

degradation of DEET. The first order rate constant for photocata-lytic degradation of DEET decreased by 96%, from 10.28 � 10�2

min�1 to 0.44 � 10�2 min�1. Strong inhibition of the process byisopropanol indicates the significant role of �OH in the oxidationof DEET. This is also consistent with the detected by-products (ringhydroxylated and demethylated) on the initial stages of the degra-dation. Hydroxylation and demethylation reactions are attributedto hydroxyl-type chemistry [28]. �OH has an electrophilic characterand it usually attacks the position with higher electron density, i.e.the aromatic moiety of DEET. Moreover, the presence of two elec-tron donating substituents i.e. methyl group and –NHCOR groupactivates the aromatic ring towards electrophilic �OH attack.

Fig. 7. Ring-opening pathways of DEET photocatalytic degradation in the presence of TiO2 and simulated solar light.

M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325 323

The use of a non-aqueous solution as reaction media could alsohelp to elucidate the role of �OH radicals in the reaction mechanism[29]. Acetonitrile has been widely used to discern the participationof holes and �OH [26]. A similar inhibition (�95%) was observedwhen photocatalysis was performed in acetonitrile solution, veri-fying the participation of �OH radicals and the limited contributionof direct oxidation by holes.

Azide has been used as both a singlet oxygen and �OH radicalscavenger with rate constants of k = 1.0 � 1010 M�1 s�1 andk = 2.0 � 109 M�1 s�1 respectively [22]. The addition of azide dur-ing photocatalytic treatment decreases the degradation rate ofDEET to a rate of 0.81 � 10�2 min�1 (around 92% reduction). Stronginhibition of the process by the azide verifies once more the partic-ipation of �OH radicals in the oxidation of DEET. If singlet oxygenplayed a significant role in the degradation of DEET under ourexperimental conditions the level of inhibition by azide is expectedto be greater than that observed for isopropanol which effectivelytrap only �OH radicals.

Iodide ion was used to investigate the participation of the holesin the photocatalytic mechanism due to its efficient iodine forma-tion [26]. When iodine anions were added to the DEET solution, aninhibition of 93% at 60 min reaction was observed with a calcu-lated value of k = 0.64 � 10�2 min�1 (Table 3). This suggests thatholes play a significant role on DEET oxidation, either indirectlyas oxidants of TiO2 surface OH-groups and water to produce �OH,or with the direct oxidation of the DEET molecule. Slightly lowerinhibition of the degradation was observed compared to the inhi-bition in the presence of isopropanol suggesting that reactions oc-cur mainly at the surface of the TiO2 via surface-bound �OH radicalsand/or holes rather than by free diffusion into the homogeneousphase. Considering the similar inhibition in the presence of CH3CNit can be concluded that holes contribute mainly to the formationof �OH radicals.

Superoxide anion is able to react directly by oxidative pathwaysas well as to produce singlet oxygen or decompose to H2O2 and fur-ther production of hydroxyl radical. The addition of p-BQ to thesolution, which has been used to quench O��2 by a simple electrontransfer mechanism [30] provokes 85.1% inhibition of DEET degra-dation (k = 1.53 � 10�2 min�1). It has been proposed that ring-opening pathways are initiated by electron transfer [28] (Li et al.2001) thus, O��2 scavengers can inhibit this pathway. Previously

published studies using superoxide dismutase in related ring-opening reactions concluding that O��2 strongly implicates the reac-tion [31]. The ring-opening reaction is initiated by electron transferfrom the aromatic compound to the TiO2 (h+ holes). Contrary to�OH, O��2 are nucleophilic species, thus they can attack structureswith lower electron density as the radical cation formed after elec-tron transfer [31–34]. As observed in the kinetic profiles of the TPs(Figs. 2 and 3), the ring opening follows the hydroxylation reaction.It is considered that the added hydroxyl groups lead to an in-creased binding of the TPs to the TiO2 surface presumably by thedeprotonated phenolic groups, thus, facilitating the electron trans-fer process. It is reported that aromatic ring opening after the �OHradical attack is favoured when the aromatic ring substitutinggroups are electron donors as in the case of DEET [35]. In conclu-sion, the participation of O��2 in the degradation is consistent withthe detected ring-opening TPs.

3.4. DEET transformation pathways

Based on the mass spectrometric and EPR results on the de-tected intermediates as well as the mineralization and scavengingstudies, the main pathways (Figs. 5–7) that can be proposed for thephotocatalytic degradation of DEET are: (a) hydroxylation that oc-curred mainly in the aromatic ring and to a lesser extend in methyland alkyl groups; (b) oxidation of the aromatic ring and alkyl-groups to corresponding aldehyde or keto structures; (c) demeth-ylation of the aromatic ring and the N-alkyl chain followed bythe ring hydroxylation and (d) ring opening products after succes-sive ring hydroxylation.

Based on the kinetic profiles (Fig. 2A–C) of the hydroxylatedTPs, the mono-hydroxylated derivatives attained maximum con-centration within 10–20 min irradiation time whereas di-hydrox-ylated and tri-hydroxylated derivatives attained maximumconcentration within 20–30 min proving the consecutive steps ofhydroxylation. It is known that hydroxylation of an aromatic ringproceeds easier after the first insertion of a hydroxyl group sinceOH activates more the aromatic ring towards electrophilic attackby other �OH radicals. Our data show that all oxidized products at-tained their maximum concentrations within the same time scale(10–30 min) showing that oxidation and hydroxylation took placesimultaneously. Based also on the EPR results the formation of the

324 M. Antonopoulou et al. / Chemical Engineering Journal 231 (2013) 314–325

oxidized products can be explained by the following process; car-bon-centered radicals generated by hydroxyl radical attacks mayreact with O2 to give organoperoxy radicals (ROO�) which maydecompose to form H2O2 or, ultimately, non-radical oxygenatedproducts [36]. Oxidation of one hydroxyl-group of the hydroxyl-ated derivatives could be also followed for the generation of theoxidized products. The demethylated TPs showed fast formationwithin 5 min of irradiation and could be formed either by attackof hydroxyl radicals at the methyl group or at the ipso aromaticcarbon. N-dealkylation attained the maximum concentration after10 min of irradiation showing a less favorite oxidation site. This isalso confirmed by the identification of all TPs the most of whichbear the ethyl chains unchanged and the late release nitrogen ions(Fig. 1). Subsequently, hydroxylation of the demethylated andde-ethylated TPS took place and peak up after 10 min and 20–30 min., respectively. Ring opening products were formed quicklyafter successive hydroxylation of the aromatic ring. Thus, ring-opened oxidation products presented their maximum concentra-tion within the same time irradiation period of 20–30 min withthat of tri-hydroxylated TPs.

3.5. Toxicity evaluation

Toxicity was evaluated by monitoring changes in the biolumi-nescence of Vibrio fischeri at different irradiation times. The initialDEET solution showed an inhibition value of 20%. After an initialdecrease to 13% at 10 min of irradiation, the toxicity increasedand reached a maximum of 25% inhibition at 20 min. After thattime, the toxicity is smoothly decreased and reached less that 5%at 60 min of irradiation (S.I., Fig. S3). It is worth mentioning thatthe highest toxicity is observed at irradiation times where all theidentified TPs are present and the majority of them attain theirmaximum concentration. This slight increase can be related inthe formation of slight more toxic compounds compared to DEETand/or synergistic effects between the intermediates.

4. Conclusions

The photocatalytic degradation mechanism of DEET in aqueousmedia has been studied by a battery of powerful techniques suchas HR-LC–MS/MS, EPR, GC–MS and by scavenging experiments.Accurate mass measurements and tandem mass spectra fragmen-tation pattern were employed to identify numerous TPs and dis-criminate the structure of the diverse isomers formed duringDEET degradation. The degradation of DEET followed first-orderkinetics and proceeds through the �OH radicals attack mainly onthe aromatic ring to form the hydroxy-methylcyclohexadienylradicals as intermediates and subsequently the mono and poly-hydroxylated TPs. Other transformation routes proceed throughthe addition of superoxide radicals and finally ring opening reac-tions took place before mineralization that was accomplished atlonger irradiation times. Toxicity assessment revealed the effi-ciency of the photocatalytic treatment to achieve almost completedetoxification of the solution.

Acknowledgments

This research has been co-financed by the European Union(European Social Fund – ESF) and Greek national funds throughthe Operational Program ‘‘Education and Lifelong Learning’’ ofthe National Strategic Reference Framework (NSRF) – ResearchFunding Program: Heracleitus II. Investing in knowledge societythrough the European Social Fund. The authors would like to thankalso the unit of environmental, organic and biochemical highresolution-Orbitrap-LC–MS analysis of the University of Ioanninafor providing access to the instrumentation facilities.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2013.06.123.

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