Efficiency and degradation products elucidation of the photodegradation of mefenpyrdiethyl in water interface using TiO2 P-25 and Hombikat UV100

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Journal of Environmental Sciences 2012, 24(9) 1686–1693

JOURNAL OF ENVIRONMENTAL SCIENCES ISSN 1001-0742 CN 11-2629/X

www.jesc.ac.cn

Efficiency and degradation products elucidation of the photodegradation of mefenpyrdiethyl in water interface using TiO2 P-25 and Hombikat UV100 Amina Chnirheb1,3 , Mourad Harir1,∗, Basem Kanawati1 , Mohammed El Azzouzi3 , Istvan Gebef¨ugi1 , Philippe Schmitt-Kopplin1,2 1. Department of BioGeoChemistry and Analytik, German Research Center for Environmental Health, Helmholtz Center Munich, Ingoldst¨adter Landstraβe 1, D-85764 Neuherberg, Germany. E-mail: [email protected] 2. Lehrstuhl f¨ur Chemische-Technische Analyse und Chemische Lebensmitteltechnologie, Technische Universit¨at Muenchen, Weihenstephaner Steig 23, 85354 Freising-Weihenstephan, Germany 3. University Mohammed V-Agdal, Faculty of Sciences, Avenue Ibn Batouta, BP 1014 Rabat, Morocco. Received 11 November 2011; revised 21 February 2012; accepted 21 March 2012

Abstract The photodegradation of mefenpyrdiethyl (MFD), an herbicide safener, was investigated in aqueous suspensions by using Degussa P-25 and Hombikat UV100 titanium oxide under simulated sunlight irradiation. The effects of initial concentration of the herbicide, pH, catalysts and hydrogen peroxide doses as well as their combinations were studied and optimized. Accordingly, the kinetic parameters were determined and the effectiveness of the processes was assessed by calculating the rate constants. A pseudo first-order kinetics was observed. Under experimental conditions, the degradation rate constants were strongly influenced using P-25 and no noticeable effect was observed for Hombikat UV100. DFT calculations with B3LYP/6-311+G(2d,p)//B3LYP/6-31+G(d,p) level of theory were performed to check whether significant conformational changes occur when the charge state of the MFD substrate changes and whether these changes could play a role in the dependency of photodegradation rate constant on the studied pH. High resolution mass spectrometry (FT-ICR/MS) was implemented to identify the main degradation products. Key words: photodegradation; mefenpyrdiethyl; P-25/Hombikat UV100; kinetic; degradation products DOI: 10.1016/S1001-0742(11)60990-X

Introduction The photochemical degradation of pesticides under sunlight irradiation is considered among the most destructive ways to the release of more hydrophilic degradation organic compounds in the environment. Mainly, the degradation pathways of pesticides can be described in the form of oxidation/reduction, dechlorination, hydroxylation, dealkylation, rearrangement, and dimerization processes (Crosby, 1976; Cessna and Vitaliano, 1991; Sanlaville et al., 1996; Tusnelda and Fritz, 2004; Rahman and Muneer 2005; Harir et al., 2007a, 2007b, 2008). It has been shown that the degradation of pesticides is largely dependent on their chemical structure as well as the nature of photocatalysts used (Bockelmann et al., 1996). Thus, because of it’s stability and biochemical inactivity; the photocatalytic mechanism of TiO2 has been widely studied (Turchi and Ollis, 1990; Mathews and McEvoy, 1992; Saquib et al., 2008). The photodegradation of pesticides by using titanium dioxide as semiconductor has been confirmed to be an effective way for reducing the amounts of pesticides contaminants in water (Campanella and Vitaliano, 2007; * Corresponding author. E-mail: [email protected]

Wu et al., 2009; Faisal et al., 2010, Lagunas-Allue et al., 2010; Song et al., 2010; Paul et al., 2010; El Madani et al., 2010). Generally, absorption of energy exceeding 3.2 eV (in the case of anatase TiO2 ) excites electrons (e− ) from the valence band to the conduction band and holes (h+ ) are produced in the valence band (Reaction (1)). The generated holes react with H2 O or OH− adsorbed on the catalyst surface to generate .OH radicals which are strong oxidants (Reactions (2) and (3)). The formed hydroxyl radicals react with organic molecules (OM) adsorbed on the surface of TiO2 by electron abstraction leading to formation of organic radical cations (Reaction (4)). Furthermore, abstraction of hydrogen atom by .OH radicals can also occur and leads to the formation of reactive radical forms of the precursor organic molecule. The excess of the photogenerated electrons attach to the adsorbed O2 molecules on the catalyst surface to form strong oxidative radicals, namely superoxide ions (.O2 − ) (Reaction (5)). TiO2 + hν −→ e− + h+ H2 O + h+ −→ H+ + .OH OH− + h+ −→ .OH

(1) (2) (3)

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Efficiency and degradation products elucidation of the photodegradation of mefenpyrdiethyl······

OM + .OH −→ by-products O2 + e− −→ .O−2

(4) (5)

To date, little is known about the photochemical degradation of herbicide safeners and only safener dichloromid as well as naphtalic anhydre have been reported (Grabtchev et al., 1997; Abu-Qare and Duncan, 2002). Mefenpyrdiethyl (IUPAC: diethyl (RS)-1-(2,4-dichlorophenyl)-5-methyl-2-pyrazoline-3,5dicarboxylate) is mainly applied in combination with other herbicides over cereal grain crops to improve herbicide selectivity between crops and weed specie (King, 2007; Hacker et al., 2000; Chnirheb et al., 2010). This study reports the photocatalytic efficiency of TiO2 P-25 and Hombikat UV100 to degrade mefenpyrdiethyl (MFD) herbicide under simulated sunlight irradiation. In parallel, the kinetic degradation of the herbicide has been evaluated at its different initial concentrations of the herbicide. pH changes, H2 O2 effect and the catalyst type were also investigated. In parallel, high resolution mass spectrometry (FT-ICR/MS) was applied to identify the main degradation products of MFD.

1 Materials and methods 1.1 Chemicals Mefenpyrdiethyl (MFD) (Fig. 1), purity > 97%, was purchased from Sigma Aldrich (Augsburg, Germany). UPLC solvents were of chromatography grade and all other chemicals were of analytical grade. All chemicals were used as received without any further purification. Ultra pure water was produced with a MilliQ system (Millipore, Billerica, USA). Titanium dioxide degussa P25 (mainly anatase ca. 50 m2 /g) and hydrogen peroxide (30%) were purchased from (Aldrich, Germany). Hombikat UV100 (100% anatase ca. 250 m2 /g) was purchased from Sachtleben Chemie (Duisburg, Germany). Cl

Cl N

N

O O

O Fig. 1

O

Chemical structure of mefenpyrdiethyl.

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400 and 800 nm similar to the solar spectrum was used. The UV radiation is limited to the wavelength 280 nm and the illuminance is approximately 150 kLux. For all kinetic experiments, the desired solutions were magnetically stirred for one hour in darkness using a magnetic stirrer to homogenize the solution before starting the irradiation. Samples were collected at different irradiation times, centrifuged and then the supernatants were collected and analyzed with UPLC equipped with photo diode-array detector. All investigations were done at pH (< 6.0) since the hydrolysis of MFD take place at pH  7.0 (Chnirheb et al., 2010). 1.3 Analytical tool 1.3.1 UPLC The analysis was carried out using an UPLC System (Acquity, Waters, Eschborn, Germany) equipped with a 2996 PDA (photo diode-array) detector. The chromatographic separations were performed by the use of an ACQUITY BEH C18 column (1.7 μm, 2.1 × 100 mm) using a gradient of methanol/water (A: 10% methanol, 0.1% formic acid in water, B: methanol). The gradient used was increased from 50% (B) to 100% (B) in 1 min, and then was kept stable to 100% of B until 2.5 min. A total time of 3.5 min was reached for each measurement. The column oven temperature, the injection volume at partial loop with needle overfills as well as the flow rate were 40°C, 5 μL and 0.4 mL/min, respectively. The detection wavelength was set to 308 nm with a scan rate of 20 Hz. 1.3.2 Mass spectrometry High-resolution mass spectra for sum formula assignment were acquired on a Bruker (Bremen, Germany) APEX Qe Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR/MS) equipped with a 12 Tesla superconducting magnet and an APOLLO II electrospray ionization source. Samples were concentrated and purified by solid phase extraction with single-use of SPE cartridges (JT Baker) of 1 mL, 100 mg of reversed phase endcapped octadecylsilane (C18) bonded to silica gel (40 μm ADP, 60 Å). After preconditioning of the cartridge with 1 mL of methanol and 1 mL of acidified water, 1 mL of the analytes were passed through the cartridge and then eluted with 0.5 mL of methanol. Samples were introduced into the electrospray source at a flow rate of 120 μL/hr with a nebulizer gas pressure of 20 psi and a drying gas pressure of 15 psi and the measurements were acquired in positive mode. Spectra were acquired with a time domain of 1 MW over a mass range of 100–600 m/z and one hundred scans were accumulated for each spectrum.

1.2 Reactor and light source

1.3.3 Density Functinnal Theory calculation (DFT)

The photoreactor used for all experiments was made of a Pyrex glass vessel 500 mL, connected to a cooling and refrigerated systems to control the temperature. For simulated sunlight irradiation, a Suntest apparatus from Heraeus (Hanau, Germany) equipped with a Xenon lamp; UV-B (280 to 320 nm), 2.71 W/m2 and UV-A (320 to 400 nm), 58.0 W/m2 with an emission spectrum between

Gaussian 03W program (Frisch et al., 2004). The hybrid DFT method B3LYP was implemented. All geometry optimizations were performed using 6-31+G(d) basis set. Frequency calculations were also done for each optimized geometry using the same basis set 6-31+G(d) to confirm that each optimized geometry does represent an energy minimum structure (not a transition state). The use of

Journal of Environmental Sciences 2012, 24(9) 1686–1693 / Amina Chnirheb et al.

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diffuse functions was important to represent the correct geometry of ionic species. Stability tests on all calculated structures were performed to ensure that the used wave function does represent the lowest energy solution of the self consistent field SCF equations. For geometry optimization, the Berny (Schlegel, 1982) analytical gradient optimization routines were used in combination with the GDIIS algorithm (Cs´asz´ar and Pulay, 1984; Farkas and Schlegel, 2002, 1999). The requested convergence in the density matrix was 10−8 , the threshold value for maximum displacement was 0.0018 Å, and that for the maximum force was 0.00045 Hartree/Bohr. All geometries of electronic structures calculated were viewed by Gauss View program (Dennington et al., 2003).

2 Results and discussion 2.1 Effect of TiO2 doses Experiments were carried out using solutions at initial MFD concentration of 1.6 × 10−5 mol/L and various doses of P-25 and Hombikat UV100 ranging from 0.03 to 2.33 g/L. The effect of photocatalysts on the constant rate of degradation of MFD is shown in Table 1. The degradation rates were found to increase as a function of the catalysts doses and the obtained optimum amount of photocatalysts necessary for the degradation of MFD was 1.33 g/L. Beyond this value a decrease in the degradation rates of MFD was observed for both photocatalysts. It is well known that the increase in the number of TiO2 particles increases the degree of TiO2 surface which is effectively exposured to the photons. The amount of adsorbed MFD molecules on the surface of TiO2 (up 1.33 g/L) also increases. A further increase of the photocatalyst concentration cause light scattering and screening effects (beyond 1.33 g/L). The photocatalytic efficiency was more obvious for P-25 as compared with Hombikat UV100. 2.2 Effect of MFD initial concentration The effect of the initial concentration on the constant rates of degradation of MFD was investigated in the presence of 1.33 g/L of P-25 and Hombikat UV100 in a concentration range of MFD from 1.61 × 10−5 to 4.02 × 10−5 mol/L (Table 2). The degradation rate decreases by increasing

the initial concentration of MFD and a highest value was observed at 1.61 × 10−5 mol/L for both catalysts P-25 and Hombikat UV100 with constant rates of degradation of 56 × 10−3 and 26 × 10−3 min−1 , respectively. The rate constant of degradation of MFD at 1.61 × 10−5 mol/L is twice higher for P-25 than for Hombikat UV100. Generally, the photocatalytic degradation of pesticides by means of illuminated TiO2 can be formally described by the Langmuir-Hinshelwood kinetics model Eq. (6). −

dC kC = kr dt 1 + kC0

dC = kobsC dt k kobs = kr 1 + kC0 C0 ln( ) = kobs t C 1 1 C0 = + kobs kr k kr −

Hombikat UV100 0.33 1.00 1.33 2.33 3.33 P-25 0.33 1.00 1.33 2.33 3.33

kobs (min−1 )

t1/2 (min)

R2

0.011 0.018 0.019 0.014 0.016

63.58 38.72 36.48 48.47 39.83

0.9975 0.9991 0.9896 0.9979 0.9993

0.014 0.022 0.045 0.027 0.032

49.5 31.2 15.27 25.76 21.45

0.9965 0.9877 0.9996 0.9967 0.9598

Conditions: MFD conc. = 1.61 × 10−2 mmol/L, pH = 5.

(7) (8) (9) (10)

where, kobs is the observed pseudo first-order rate constant. The values of the adsorption equilibrium constant (k) as well as the second-order rate constant (kr ) are calculated from the plot in Fig. 2 (Eq. (10)) and the results obtained are presented in Table 3. 2.3 Effect of H2 O2 concentration The photooxidation of MFD at different initial concentrations of H2 O2 ranged from 0.00147 to 1.17 mol/L was investigated. Figure 3 shows the rate constant of degradation of MDF versus the different initial concentration of H2 O2 . The rate constant of degradation starts to reach its saturation level at H2 O2 concentration of 0.29 mol/L. This result was expected due to the strong UV absorption by H2 O2 . Thus, the photooxidation in the presence of H2 O2 60

Degradation rate constant of MFD at different catalysts doses

Catalysts dose (g/L)

(6)

where, dC/dt is the rate of degradation, C is the MFD concentration at time t, kr is the second-order rate constant, k is the equilibrium adsorption constants of MFD onto TiO2 , and C 0 is the initial concentration of MFD. The photoreaction of MFD in the presence of TiO2 exhibits pseudo first-order kinetic as described in Eq. (7).

45 1/kobs (min)

Table 1

Vol. 24

30

15

P-25 Hombikat UV100

0

0

0.01

0.02

0.03

0.04

0.05

Initial concentration of MFD (mmol/L) Fig. 2 Plot of the initial MFD concentration versus the reciprocal of the observed rate constant (1/kobs ) of P-25 and Hombikat UV100.

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Efficiency and degradation products elucidation of the photodegradation of mefenpyrdiethyl······

Table 2 Degradation rate constant of MFD at different concentration values kobs (min−1 )

Concentration (×10−5 mol/L) Hombikat UV100 1.61 2.14 2.68 4.02 P-25 1.61 2.14 2.68 4.02

0.026 0.023 0.022 0.020

26.860 30.800 31.935 34.307

0.9877 0.9956 0.9976 0.9898

0.056 0.046 0.044 0.031

12.331 15.131 15.822 22.141

0.9978 0.9896 0.9988 0.9923

Conditions: catalysts = 1.33 g/L, pH = 5.0. Table 3 Values of second-order rate constant and the adsorption equilibrium constant of MFD in the presence of photocatalysts P-25 and Hombikat UV100 Catalyst

1/kr

kr (mmol/(L·min))

1/(kr ·kMFD )

kMFD (mmol/L)

P-25 Hombikat UV100

571192 402690

0.00175 0.00248

8.6765 34.176

65.832 11.782

0.020

Rate constant (min-1)

0.016 0.012 0.008 0.004 0.000 0.0

0.2

0.4

0.6 0.8 1.0 C H2O2 (mol/L)

1.2

Table 4 Degradation rate constant of MFD at different pH values pH

R2

t1/2 (min)

1.4

Fig. 3 Degradation rate constant of MFD at different H2 O2 concentrations. Conditions: MDF conc. = 1.61 × 10−2 mmol/L, pH = 5.0, no catalyst was used in this experiment.

is mainly due to the formation of (.OH) which have high reactivity to organic compounds. The mechanisms of free radical attack (.OH) on the organic compounds reactions are either to replace a hydrogen atom on a chain carbon or addition reactions of unsaturated bonds, or electron transfer group of organic radicals (.OH) (Calvert and Pitts, 1966a, 1966b). Furthermore, the constancy observed from the concentration 0.29 mol/L is probably due the saturation of the medium by (.OH) radicals, since H2 O2 in the presence of UV light degrades much faster (Mansour, 1985). 2.4 pH Effect Well known that the heterogeneous photocatalysis reactions are pH dependent, since they determine the surface charge properties of the photocatalysts as well as their adsorption/desorption properties in regard to organic compounds. Thus, the point of zero charge (pzc) of P-25 has been reported to be 6.25 (Bahnemann et al., 2007; Ahmed

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Hombikat UV100 2 3 4 5 P-25 2 3 4 5

kobs (min−1 )

t1/2 (min)

R2

0.0037 0.0077 0.0194 0.0271

187.34 90.02 35.73 25.58

0.9886 0.9947 0.9900 0.9930

0.0019 0.0107 0.0438 0.0597

364.81 64.78 15.83 11.61

0.9935 0.9921 0.8991 0.9861

Conditions: MDF conc. = 1.61 × 10−2 mmol/L, catalysts = 1.33 g/L.

et al., 2010; Augusynski, 1988). The particle surface is positively charged below pHpzc value, while, they are negatively charged above pHpsc value. The degradation rate of MFD was investigated under simulated sunlight irradiation at different pH values ranging from pH 2 to 5. Results obtained are shown in Table 4. The degradation rate either for P-25 or Hombikat UV100 increase by increasing pH values and a maximum was observed at pH 5, indicating that the pH can significantly affect the corresponding rate of degradation of MFD. To better understand this behaviour, DFT calculations were investigated. The experimental results that were obtained for the photocatalytic degradation of MFD in the pH range (2–5) indicated that the rate constant is the lowest at pH = 2. In an attempt to investigate whether this observation is related to protonation of the pesticide and therefore cation-cation coulombic repulsion between the protonated substate the the positive charges on the catalyst surface, we optimized the geometries of the neutral pesticide together with its two protonated forms (Fig. 4) and calculated the proton affinity of each nitrogen in the heterocycle. The calculated gas phase proton affinities of the sp2-N and sp3N parts of MFD are 0.9 and 0.2 kcal/mol, respectively. These two significantly low values indicate that neither the sp2 hypridized or the sp3 hybridized nitrogen atoms can be protonated. Therefore the MFD cannot bear an effective positive charge even at pH = 2. No pK a values could be obtained for MFD in solution even by the use of high levels of DFT calculations. The low rate constant for the photodegradation of MFD at pH = 2 is probably due to the inactivity of the catalyst (titanium oxide) under this condition. Effective coulumbic repulsion between protonated MFD and positive charges which are localized on the catalyst surface are less likely due to difficulties of the pesticide to be protonated. No noticeable conformational change could be observed, when a proton is added to any nitrogen atom inside the five membered ring of the MFD. Thus, steric hindrance between the substrate and the catalyst do not exist in this case. 2.5 Degradation products Molecular structure information regarding the photodegradation of MFD could be obtained by comparing the extracted elemental composition of the degradation products and the elemental composition of MFD. Thus, the

Journal of Environmental Sciences 2012, 24(9) 1686–1693 / Amina Chnirheb et al.

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a

b

Vol. 24

c

Fig. 4 Optimized geometries of MFD (a), sp2-N protonated MFD (b) and sp3-N protonated MFD (c).

plausible elemental composition for the accurate mass of each peak was calculated only when the isotopes of chlorine atoms exist in extracted sum formulas of the degradation products. This approach was successfully applied for the photodegradation of MFD. Sum formulas, nominal masses as well as the calculated double bond equivalent (DBE) of MDF and its main degradation products are listed in Table 5. Furthermore, Fig. 5 shows the mass spectra of the degradation of MFD in absence and presence of H2 O2 , Hombikat UV100 and P-25, respectively. The main obtained degradation products mostly arise from the pathways cited below: Rearrangement inside the pyrazole ring: Degradation product with mass 272 amu which corresponds to the sum formula C11 H10 Cl2 N2 O2 (IUPAC: ethyl 2-[(E)-(2,4dichlorophenyl)azo]prop-2-enoate) is obtained by shifting C16H19Cl2N2O4 373.0716

Intensity (%)

MFD alone 100 80 60 40 20 0

C14H13Cl2N2O3 327.0298

Intensity (%) Intensity (%)

MFD/H2O2 100 80 60 40 20 0 100 80 60 40 20 0

373.0716 . 327.0299

284.0202 MFD/UV100

389.0666

247.1290

327.0302

Intensity (%)

MFD/P-25 247.1287 100 80 60 40 327.0296 373.0716 20 0 150 200 250 300 350 400 m/z Fig. 5 FT-ICR mass spectrum of the degradation of MFD in absence and presence of H2 O2 , Hombikat UV100 and P-25.

the N–C3 binding electrons to form an N=N bond concerted with C2–C3 bond breakage. This forms also the stable ketene CH3 –CH=C=O and produce the terminal ethanol CH3 CH2 OH in water. The formation of the degradation product with nominal mass 270 amu which corresponds to the sum formula C11 H8 Cl2 N2 O2 (IUPAC: vinyl 5-[(E)-(2,4dichlorophenyl)azo]-2,3-dipydropyran-6-one) is due to a reduction step of the primary degradation product C11 H10 Cl2 N2 O2 (m/z = 272) since the DBE (double bond equivalent) difference between the two degradation products (C11 H10 Cl2 N2 O2 ; mass = 272) and (C11 H8 Cl2 N2 O2 ; mass = 270) equals to one unit (DBE “C11 H10 Cl2 N2 O2 ” – DBE “C11 H8 Cl2 N2 O2 ” = 1). It is less likely that the reductive event is due to an extra double bond formation in the terminal ethyl group. Figure 6 shows a plausible proposed mechanism for this reduction to form an additional stable six membered ring (lactone) which is resistant to hydrolysis in acidic aqueous conditions. The product mass = 286 which corresponds to the sum formula C11 H8 Cl2 N2 O3 (IUPAC: 5-[(E)-(2,4-dichloro-3,5 or 6-hydroxy-phenyl)azo]-2,3-dihydropyran-6-one) can be formed from the degradation product C11 H8 Cl2 N2 O2 with nominal mass of 270 (discussed above) as a result of a possible oxidation of the aromatic benzene ring (caused by a known OH radical attack). Hydroxylation: Formation of the product C16 H18 Cl2 N2 O5 (mass = 388; IUPAC: diethyl 1-(2,4-dichlorophenyl)-5methyl-4H-pyrazole-3,5-dicarboxylate; methanol) can be due to oxidation of MFD by OH radicals in solution as shown in Fig. 5. Once formed, this oxidized form C16 H18 Cl2 N2 O5 (mass = 388) can undergo a stepwise degradation by OH radical attacks to the terminal ester groups to finally form the degradation product (C10 H8 Cl2 N2 O; IUPAC: 2,4-dichloro-5-(5-methylene-4Hpyrazol-1-yl)phenol) with nominal mass 242 (Fig. 6). Formation of the product C14 H14 Cl2 N2 O4 (mass = 344): As described previously, the degradation product C14 H14 Cl2 N2 O4 (IUPAC: 1-(2,4-dichlorophenyl)-5ethoxycarbonyl-5-methyl-4H-pyrazole-3-carboxylic acid) with nominal mass = 344 and DBE = 9 is identified as a result of mono deestrification of the free ester attached to the pyrazole ring of MFD (Chnirheb et al., 2010). This is possible because OH− exist also in solution due to the capability of OH radicals to attach electrons

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Efficiency and degradation products elucidation of the photodegradation of mefenpyrdiethyl······

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MFD and its degradation products in absence and presence of H2 O2 , Hombikat UV100 and P-25 (errors < 1 ppm)

Table 5 Neutral mass

Sum formula

DBE

IUPAC-name

242.0014 269.9963 272.0119 285.9912 326.0225 330.0174

C10 H8 Cl2 N2 O C11 H8 Cl2 N2 O2 C11 H10 Cl2 N2 O2 C11 H8 Cl2 N2 O2 C14 H12 Cl2 N2 O3 C13 H12 Cl2 N2 O4

7 8 7 8 9 8

344.0331

C14 H14 Cl2 N2 O4

8

358.0487

C15 H16 Cl2 N2 O4

8

372.0644 388.0593

C16 H18 Cl2 N2 O4 C16 H18 Cl2 N2 O5

8 8

Dichloro-2,3 or 5-(5-methylene-4H-pyrazol-1-yl)phenol 5-[(E)-(2,4-dichlorophenyl)azo]-2,3-dihydropyran-6-one Ethyl 2-[(E)-(2,4-dichlorophenyl)azo]prop-2-enoate 5-[(E)-(2,4-dichloro-3,5 or 6-hydroxy-phenyl)azo]-2,3-dihydropyran-6-one Ethyl 4-[(E)-(2,4-dichlorophenyl)azo]-2-methylene-5-oxo-pent-4-enoate 2-(2,4-dichlorophenyl)-5-methoxycarbonyl-3-methyl-4H-pyrazole-3-carboxylic acid or 1-(2,4-dichlorophenyl)-5-methoxycarbonyl-5-methyl-4H-pyrazole-3carboxylic acid 1-(2,4-dichlorophenyl)-5-ethoxycarbonyl-5-methyl-4H-pyrazole-3carboxylic acid O5 -ethyl O3 -methyl 1-(2,4-dichlorophenyl)-5-methyl-4H-pyrazole-3,5dicarboxylate or O3 -ethyl O5 -methyl 1-(2,4-dichlorophenyl)-5-methyl-4Hpyrazole-3,5-dicarboxylate MFD: Diethyl 1-(2,4-dichlorophenyl)-5-methyl-4H-pyrazole-3,5-dicarboxylate Diethyl 1-(2,4-dichloro-2,5 or 6-hydroxy-phenyl)-5-methyl-4H-pyrazole-3,5dicarboxylate Cl

Cl O

Cl

Cl Cl

7 O

6

5

N

4

N

Cl

O

O

N

1 3

N

C 8

O 10

1

N

N

2

8 O 9

Cl

Cl

2

O

Exact mass : 272.0119 Formula: C 11H10Cl2N2O2

O

Exact mass : 326.0225 Formula: C 14H12Cl2N2O3

N

N

O

Exact mass: 269.9963 Formula: C 11H8Cl2N2O2 Cl

Cl

Cl

Cl OH

N

N

7

O O

N

N

O

6 Exact mass: 344.0331 Formula: C 14H14Cl2N2O4

5

Cl

O 9

Cl O

1 4

O

8

3

N

N

O 10

2

O

HO

O

Exact mass: 285.9912 Formula: C11H8Cl2N2O3

Exact mass: 372.0644 Formula: C 16H18Cl2N2O4

Cl

Cl

or N

N

O

HO Cl

Cl

Cl

N

N

Cl

O

O

N

N

or

O

O

O

O

Stepwise

Exact mass: 358.0487 Formula: C 15H16Cl2N2O4 Cl Cl

Cl

Cl

N

N

HO

N

N

OH

O

or O

O

Cl

Cl

O

O

O

Exact mass: 388.0593 Formula: C 16H18Cl2N2O5

O O

O

O

O

Exact mass : 330.0174 Formula: C 13H12Cl2N2O4

Fig. 6 Scheme of the main pathways of the degradation of MFD.

N

N

HO

Exact mass: 242.0014 Formula: C 10H8Cl2N2O

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Journal of Environmental Sciences 2012, 24(9) 1686–1693 / Amina Chnirheb et al.

and produce OH− anions. This reaction is similar to known deerterification of esters in alkalic solutions. Accordingly, Degradation product (mass = 326 amu) which corresponds to the sum formula C14 H12 Cl2 N2 O3 (IUPAC: ethyl 4-[(E)-(2,4-dichlorophenyl)azo]-2-methylene-5-oxopent-4-enoate) is obtained by shifting the N–C3 binding electrons to form an N=N bond concerted with C8–O9 bond breakage. This product is formed either by loss of one molecule ethanol or one molecule water from MFD or the degradation product (mass = 344) (Fig. 6). Formation of C15 H16 Cl2 N2 O4 (mass 358): Concerted attacks of H and OH radicals on one of the terminal methyl groups in MFD leads to degradation products with nominal mass of 358 (C15 H16 Cl2 N2 O4 ) as shown in Fig. 6 and methanol is produced in these reactions. These products with nominal mass of 358 can be attacked by H radicals in solution to form the final degradation product C13 H12 Cl2 N2 O3 (mass 314). Furthermore, loss of one terminal ethyl groups from the degradation product O5 -ethyl O3 -methyl 1-(2,4-dichlorophenyl)-5-methyl-4H-pyrazole3,5-dicarboxylate or O3 -ethyl O5 -methyl 1-(2,4dichlorophenyl)-5-methyl-4H-pyrazole-3,5-dicarboxylate with mass 358 (C15 H16 Cl2 N2 O4 ) leads to the degradation product 2-(2,4-dichlorophenyl)-5methoxycarbonyl-3-methyl-4H-pyrazole-3-carboxylic acid or 1-(2,4-dichlorophenyl)-5-methoxycarbonyl-5methyl-4H-pyrazole-3-carboxylic acid with mass 330 (C13 H12 Cl2 N2 O4 ) as shown in Fig. 6.

3 Conclusions Photodegradation of MFD was studied in the presence of H2 O2 , P-25 and Hombikat UV100 as photocatalysts. The results obtained are as follows. The degradation rates were always higher for heterogeneous photocatalysis as compared to non-catalytic reactions. The degradation of MFD followed a pseudo first-order kinetic model. The constant rates of photodegradation increased with increasing photocatalyst doses; however, an overdose caused rate retardation because the effect of light scattering becomes significant. The highest obtained value of the constant rate of degradation of MFD either for P-25 and Hombikat UV100 was 1.33 g/L. In the absence of photocatalysts, the degradation rate increases with an increase in H2 O2 concentration and the degradation rates were constant over catalyst concentration of 29.41 × 10−3 mol/L. Linear correlation between pH and DFT calculations in explaining the photochemical behaviour of MFD at studied pH. The main degradation products were found to be as results of rearrangements of pyrazole ring, hydroxylation of benzene ring as well as breakage of carbon-carbon (C–C) and carbon-oxygen (C–O) bonds. Acknowledgments This work was supported by the German Academic Exchange Service (DAAD) within the Morrocan/German exchange program. The authors are thankful for Look

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Brigitte and Jenny Westphal for all their skilful technical assistance.

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