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Photoreactivity of the fungicide chlorothalonil in aqueous medium†
Cite this: Environ. Sci.: Processes Impacts, 2014, 16, 839
Samira Bouchama,ab Pascal de Sainte-Claire,ac Emmanuel Arzoumanian,d Esther Oliveros,d Abdelaziz Boulkamhb and Claire Richard*ae The photoreactivity of chlorothalonil was studied by means of steady-state irradiation and laser-flash photolysis. Experiments were conducted in water containing acetonitrile as a co-solvent. This fungicide undergoes very slow phototransformation in the first stages of the reaction, but the consumption profile is auto-accelerated. To understand the reaction mechanism, we undertook a detailed study of the rates, products and transient species. The rates and photoproduct distribution vary greatly with the oxygen concentration. Concerning the transient species, we measured the absorption of the triplet, its yield of formation, and its reactivity with oxygen in various water–acetonitrile mixtures and with isopropanol. The reduced radical, CTHc, could be produced and its transient spectrum was recorded. Combining all the experimental data, it is hypothesized that in the first step of the reaction CT is excited to the triplet state. The triplet has several possible fates including reduction by organic constituents to form the radical which gives photoproducts. Another characteristic of the CT triplet is its capacity to generate singlet oxygen. The production of this species was measured by phosphorescence and compared to the
Received 14th October 2013 Accepted 28th November 2013
percentage of the triplet trapped by oxygen in air-saturated solutions. The yield varies from 0.88 in pure acetonitrile to 0.48 in water–acetonitrile (95 : 5, v/v). Therefore, in surface waters, chlorothalonil is
DOI: 10.1039/c3em00537b
expected to sensitize the photooxidation of micropollutants, and to be competitively phototransformed
rsc.li/process-impacts
through reaction with any H donor or electron donor water constituents.
Environmental impact Chlorothalonil is a widely used and highly toxic fungicide that can be released into the aquatic environment. The detailed study presented here helps to understand its phototransformation mechanism. Chlorothalonil is quite photostable but its photodegradation is greatly enhanced by the water constituents able to reduce its triplet excited state. This means that the rates of chlorothalonil photodegradation will be impacted by the surface water composition. Chlorothalonil produces singlet oxygen in water with a quantum yield near 0.5 and can thus sensitize the photooxidation of other water micropollutants. Finally, this work shows that although considered as an inert cosolvent, acetonitrile seems able to react with the triplet excited state of chlorothalonil and to promote its phototransformation.
1. Introduction Chlorothalonil (CT, 1,3-dicyano-2,4,5,6-tetrachlorobenzene) is a widely used non-systemic fungicide, classied as a potential human carcinogen (group B), highly toxic and persistent.1,2
a Clermont Universit´e, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France. E-mail: [email protected]; Tel: +33 (0)4 73 40 71 42 b Laboratoire des Techniques Innovantes de Protection de l'Environnement, Universit´e de Constantine 1, Route de A¨ın-el-Bey, 25000 Constantine, Algeria c
Clermont Universit´e, ENSCCF, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France
d Laboratoire des IMRCP, UMR CNRS 5623, Universit´e Toulouse III (Paul Sabatier, UPS), 31062 Toulouse Cedex 9, France e
Equipe Photochimie CNRS, UMR 6296, ICCF, F-63171 Aubi`ere, France
† Electronic supplementary 10.1039/c3em00537b
information
(ESI)
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available.
See
DOI:
Relevant literature shows that CT photolysis is far from being understood. In water, CT photochemically degrades slowly with quantum yields ranging from 103 to 104.3,4 This means that one absorbed photon in 103 or 104 only is able to induce a chemical modication in the molecule. However, the phototransformation is faster in natural than in pure water.4–6 Photolysis in organic solvents or in a solid form is also very slow.7,8 Interestingly, an auto-accelerated photodegradation was reported in benzene.7 Concerning photoproducts, different types of reactions are reported. Photoreductive dechlorination is observed in water,4,5 in organic solvents8,9 and in solid.8 Formation of 4-hydroxyCT is also mentioned in water, probably due to both hydrolysis and photolysis.10 Finally, photoadditions occur in organic media. Adducts between dechlorinated CT and alcohols are described11 and in plant cuticle cutin bound residues of CT are detected.12
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The recent ndings that CT populates efficiently its triplet excited state in pure acetonitrile and generates high amounts of singlet oxygen (quantum yields close to 1)8 help to understand the relative photostability of CT in oxygenated media: the deactivation of the CT triplet by oxygen competes with the other triplet reaction pathways. The photoreductive dechlorination in organic solvents and the photoadditions on solvents or cuticle constituents were suggested to involve radicalar reactions but were not studied in detail. If the replacement of a chloride atom by an H atom is actually expected to occur in H-donor organic solvents, such a reaction in water is more intriguing. This may be linked to the use of co-solvents to increase the CT solubility in water. Acetonitrile is considered to be a suitable unreactive co-solvent and it is used in many studies.6 The aim of the paper was to understand the mechanism of CT phototransformation in aqueous medium using steady state and laser-ash photolysis. To clarify the potential effect of acetonitrile as a co-solvent, we dissolved CT in water–acetonitrile mixtures, and measured the evolution of some properties (triplet excited state population, singlet oxygen and photoproduct formations) as a function of the acetonitrile percentage. Moreover, we studied in detail the effect of the oxygen concentration and acetonitrile percentage on the photoproduct distribution to clarify their formation mechanism. Finally, as previous experiments showed that CT generates singlet oxygen with a quantum yield close to 1 in acetonitrile,8 we investigated the singlet oxygen formation in various water–acetonitrile mixtures by phosphorescence.
2.
Experimental part
2.1. Materials CT (99.3%) was supplied by Sigma-Aldrich and 4-hydroxychorothalonil (4-OHCT) was from Dr Ehrenstorfer company. Phenalenone (PN, 1H-phenalene-1-one, 97%) and acetonitrile (Chromasolv gradient grade, >99.9%) were provided by SigmaAldrich. Water was puried using a reverse osmosis RIOS 5 and Synergy (Millipore) (resistivity 18 MU cm, DOC < 0.1 mg L1). Other reagents were of the best purity available. Argon used for de-oxygenation was of ultra-pure quality (Air Products). All the solvents and chemicals were used as received. To solubilize CT at 5 106 M, a minimum of 5% acetonitrile (v/v) was added. For solutions at 2 105 M, 20% acetonitrile (v/v) was added. Under these conditions, CT solutions were stable. The absence of CT volatilization, especially under argon bubbling, was checked by monitoring the UV absorption spectrum before and aer bubbling. CT solutions were buffered by phosphate buffers made with sodium hydrogenophosphate (>99%, Sigma-Aldrich) and disodium phosphate (>99%, Sigma-Aldrich). The buffer concentration was 5 104 M. Phosphate buffers at this concentration were shown not to affect the photolysis. 2.2. Irradiation protocols Polychromatic irradiation was conducted in a device equipped with six uorescent tubes (TLAD 15W05 Philips) emitting between 300 and 450 nm (see the spectrum in Fig. SI-1†). The
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Pyrex reactor (15 cm height and 1.4 cm internal diameter) was lled with 15 mL of solution and le in contact with air. Experiments in the absence of oxygen were conducted in a cylindrical cell of 1 cm path which was purged with argon for 20 min prior to irradiation, then sealed. Irradiation was performed in a parallel beam at 313 nm using a mercury arc lamp (200 W) equipped with an Oriel monochromator. A radiometer QE65000 from Ocean Optics was employed to measure the photon uence rate, which was (6.4 0.5) 1014 photons cm2 s1 at 313 nm. For the polychromatic light, the radiometer was used to measure the photon distribution while a reference compound with a known quantum yield of photolysis (metamitron, F ¼ 0.025 (ref. 13)) was used to evaluate the amount of light received by the 15 mL of solutions (1.8 105 Einstein L1 s1). Then Ia, the rate of CT photon absorption, was obtained using the relationship: 380 X li Ia ¼ I0li 1 10ACT 290
I0li
is the amount of light intensity received by the soluwhere i tion between li1 and li1 + Dl and AlCT is the averaged absorbance of CT within the same wavelength range. The wavelength interval, Dl, was set at 5 nm. The absorption spectrum of CT is shown in Fig. SI-2.† The use of two different irradiation devices was necessary to optimize irradiation times, these being much longer for the experiments conducted in the presence of oxygen than in its absence. In both devices, the same long-wavelength absorption bands of CT are excited to limit the risk of any wavelength effects on the reaction. Laser ash photolysis experiments were performed on an apparatus consisting of a Nd:YAG laser (GCR 130-1, pulse width 9 ns, 266 nm), a 150 W pulsed xenon arc lamp, a R928 photomultiplier and a 05-109 Spectra Kinetics Applied Photophysics monochromator.14 Benzophenone was used as a chemical actinometer. The formation of triplet benzophenone was monitored at 525 nm. The product of the quantum yield of benzophenone triplet formation and its molar absorption coefficient at 525 nm was assigned 6500 M1 cm1.15 Experiments in the absence of oxygen were conducted in a cell equipped with a valve. Solutions were deoxygenated in the cell and the valve was closed just before excitation. For experiments in argon-saturated medium, solutions were renewed aer 3 ashes. A cooled (80 C) near infra-red (NIR) photomultiplier (Hamamatsu R5509 PMT) was used as a 1O2 detector. Irradiation was carried out at 313 nm and 367 nm with a xenon/ mercury arc (1 kW) through a water lter, focusing optics, and a monochromator. The 1O2 luminescence was collected with a mirror, chopped (at 11 Hz), and, aer passing through a focusing lens, a cut-off lter (1000 nm) and an interference lter (1271 nm), it was detected at 90 with respect to the incident beam using the NIR photomultiplier and a lock-in amplier (Stanford Research Systems, Model SR830 DSP). Singlet oxygen luminescence signals were recorded as a function of irradiation time for a minimum of 3 min. Square signals were obtained showing that the sensitizers were stable under continuous irradiation. No sensitizer bleaching was observed during
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irradiation. The incident radiant powers (W0, mW) at the wavelengths of irradiation (l) were measured using a thermopile (Laser Instrumentation, model 154), and the relative incident photon uxes were calculated as P0/P0,R ¼ W0l/W0,RlR. Detailed calculations are given in the ESI.†
that included the implicit contribution of the solvent were also performed with the Polarizable Continuum Model (PCM) and acetonitrile as solvent. In that case, molecules were fully optimized within the solvent eld. However, these calculations did not improve the results signicantly, and the solvent was not included in this work.
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2.3. Analyses UV-visible spectra were recorded on a Cary 3 (Varian) spectrophotometer. The monitoring of CT and its photoproducts was performed at room temperature using an HPLC-UV system equipped with an autosampler (Waters 717plus), a degasser (Agilent 1100 series), a pump (Waters 515) and a photodiode array detector (Waters 996) and a conventional reversed-phase column. The eluent was a mixture containing water with formic acid 0.1% (v/v) and acetonitrile. The gradient elution started with 30% of acetonitrile that was linearly increased until 70% in 4 min. This percentage was maintained for 15 min. The ow rate was set at 1 cm3 min1. The injection volume was 90 mL. Mass spectrometry was performed on a LC/QTOF apparatus equipped with an orthogonal geometry Z-spray ion source (Waters/Micromass, Manchester, UK). Analyses were made in negative mode. UV detection under MS heading was a Waters Alliance 2695 photodiode array detector system. The column was a Kinetex™ C18, 100 mm 2.1 mm, with a particle size of ˚ The binary solvent system used 2.6 mm, and a size pore of 100 A. was composed of acetonitrile and water acidied with 0.5& v/v formic acid. The gradient elution started with 5% of acetonitrile and reached 95% in 15 min. The ow rate was set at 0.2 cm3 min1. Tandem mass spectrometric (MS/MS) experiments were conducted in a collision cell with an argon pressure of 1 bar. A collision energy of 20 eV was used. MS analyses were conducted on CT solutions (5 105 M) that had undergone conversion up to 30%. The data recorded were processed using MassLynx (version 4.1).
3.
Results
3.1. Steady-state irradiation Aerated solutions of CT were irradiated with polychromatic light. CT was dissolved in various acetonitrile–pH 7 water mixtures. The decay of CT with increasing irradiation times is shown in Fig. 1A. The consumption prole of CT is rather independent of the acetonitrile percentage between 5 and 50%. Aer 2 h the CT loss is hardly measurable while aer 4 and 6 h, 9 and 21% of CT have disappeared, respectively. The decay rate of
2.4. Theoretical calculations Density Functional Theory (DFT) calculations were performed with the Gaussian series of programs16 to investigate the optical properties of three hydroxychlorothalonil (OHCT) species (see Scheme 1). The molecules were optimized at the PBEPBE/ 6-31G(d,p) level and the singlet excited state energies were obtained from Time-Dependent DFT calculations (TD-DFT) with the above functional and basis set. The PBEPBE functional was chosen because experimental electronic absorption spectra are well reproduced with this method. This good agreement has been reached consistently for several species of environmental interest in our laboratory for the past few years. Calculations
Decay profile of CT in acetonitrile–pH 7 buffered water mixtures irradiated (A) in air-saturated medium in polychromatic light (Ia ¼ 1.7 107 E L1 s1) and (B) in argon-saturated medium at 313 nm (Ia ¼ 8.8 108 E L1 s1). O ACN 5%–water 95%, [CT] ¼ 0.5 105 M; B ACN 20%–water 80%, [CT] ¼ 2 105 M; ; ACN 50%–water 50%, [CT] ¼ 2 105 M. Fig. 1
Scheme 1
The species investigated in this work from DFT calculations.
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CT has therefore a tendency to increase with the irradiation time. Such an auto-accelerating effect was never mentioned in the literature except by Kawamura et al. who irradiated CT in benzene.7 The decay prole obtained by irradiating CT (2 105 M, acetonitrile–water, 20/80) in deoxygenated solution is shown in Fig. 1B. The reaction is much faster than in aerated medium. CT is converted by 18% aer 10 min of irradiation in deoxygenated medium while by 21% aer 360 min in air-saturated medium. Taking into account that Ia is twice as high in the former case, the photolysis is 60-fold faster in deoxygenated medium than in an air-saturated acetonitrile–water 20/80 mixture. As observed in the presence of oxygen, the rate of CT photolysis increases with increasing irradiation time. Similar results are obtained when CT is irradiated in the mixtures containing 5 and 50% acetonitrile. Table 1 gives the time necessary to convert 20% of CT (t20) under different experimental conditions. This time is between 6 and 7 h in polychromatic light and aerated medium whatever the percentage of acetonitrile, and between 8 and 20 min at 313 nm and argon-saturated medium. In this latter case, increasing the acetonitrile percentage slightly decreases t20, and thus increases the photolysis rate. From these rst results one can draw the following conclusions. The drastic inhibiting effect of oxygen is in line with the involvement of the triplet excited state in the reaction. The auto-accelerated decay prole shows that new processes leading to CT consumption occur in the course of the reaction. Finally, acetonitrile seems to be involved in the photolytic process. Several photoproducts were detected whose nature and distribution depend on the experimental conditions. In airsaturated medium, we observed the formation of three photoproducts: one resulting from substitution of Cl by H, CT-Cl, one resulting from substitution of Cl by OH, 4-OHCT, and one bearing an additional oxygen atom compared to 4-OHCT, P1. The formation of CT-Cl was previously mentioned by several authors.4,5,10 This product was identied based on the UV spectrum and retention time.8 The two other photoproducts were detected by HPLC and HPLC/MS in negative mode (see Table 2). Based on the MS pattern, they have both lost a Cl atom and gained OH or O2H. 4-OHCT shows a UV spectrum very different from that of CT, with four maxima at 238, 297, 345 and 360 nm. P1 also has a very different UV spectrum with maxima
Table 1
at 238, 266, 300 and 310 nm. By MS/MS, 4-OHCT and P1 give different fragmentations, losing Cl, Cl + CO and 2Cl and Cl, Cl + CO and Cl + 2CO, respectively. The structure of P1 is not rmly established, but it may be the one given in Table 2 corresponding to dichloro-hydroxy-hypochlorite-1,3-benzenedicarbonitrile. By monitoring the evolution of the UV-visible spectrum of the solution during irradiation, one conrms that the main absorbance changes are between 250 and 300 nm and 330 and 350 nm (see Fig. SI-3†). In argon-saturated medium, CTCl is observed again, but neither 4-OHCT nor P1. Instead, a photoproduct with the same MS data than 4-OHCT and a UV spectrum very similar to maxima at 231, 288 and 346 nm is formed. This compound is proposed to be 2-OHCT based on spectrum calculation (see Section 3.4). The formation proles of CT-Cl, P1 and 4-OHCT in air-saturated medium are shown in Fig. 2. They show the same autoaccelerated trend as CT consumption. The chemical yields of these compounds vary with the irradiation conditions (oxygen concentration and acetonitrile percentage) (see Table 1). Chemical yields are given for a conversion extent of CT of 20%. CT-Cl is produced in higher yields in the absence of oxygen (hCT-Cl lying between 0.25 and 0.33) than in aerated medium where hCT-Cl drastically decreases from 0.20 to 0 when the percentage of acetonitrile is increased from 5 to 50%. 4-OHCT and P1 are only formed in air-saturated medium. Their respective chemical yields increase as hCT-Cl decreases. Lastly, 2-OHCT is only produced in argon-saturated medium. Its chemical yield decreases from 0.15 to 0.02 when the percentage of acetonitrile is increased from 5 to 50%. Thus one concludes that medium oxygenation changes the nature of photoproducts and that CT-Cl formation is in competition with 2-OHCT formation in deoxygenated medium and with P1 and 4-OHCT in aerated medium. Solutions were generally buffered at pH 7 to maintain stable the pH conditions. Experiments were also conducted with unbuffered solutions. In the absence of any buffer, one observes an acidication of the solution upon irradiation. In aerated solutions containing 20% acetonitrile and CT 2 105 M, the initial pH is equal to 6.8. It falls to 6.2 aer 8 h of irradiation, corresponding to a CT conversion extent of 30%, and to 3.9 aer 23 h and a total CT conversion. Therefore, 4.7 107 and 1.3 104 M of protons are released when CT losses are 6 106 and 2 105 M respectively. It can be concluded that H+ release is
Irradiation times necessary to transform 20% of CT, t20, under different conditions and chemical yields of photoproducts t20/min at
Conditions
[CT]
5% ACN, aerated 20% ACN aerated 50% ACN aerated 5% ACN argon 10% ACN argon 20% ACN argon 50% ACN argon
5 106 2 105 2 105 5 106 5 106 2 105 2 105
313 nm M M M M M M M
20 20 10 8
2 2 2 2
300–450 nm
hCT-Cla
390 40 360 40 360 40
0.20 0.07 nd 0.24 0.33 0.33 0.25
0.02 0.02 0.02 0.02 0.02 0.02
h2-OHCTb
h4-OHCTb
Area of P1c
ndd nd nd 0.15 0.02 0.11 0.02 0.06 0.01 0.02 0.005
nd 0.01 0.005 0.10 0.02 nd nd nd nd
10 000 16 000 50 000 nd nd nd nd
Measured at 232 nm using the same epsilon as for CT (66 000 M1 cm1). b Measured at 290 nm using the same epsilon as for 4-OHCT (22 500 M1 cm1). c Measured at 266 nm. d Not detected. a
842 | Environ. Sci.: Processes Impacts, 2014, 16, 839–847
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Environmental Science: Processes & Impacts List of detected photoproducts, elemental compositions and proposed structure
Compound
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4-OHCT
Measured masses in ES in u and MS/MS fragments (percentage)
245/247/249/251 M-Cl (20) M-Cl-CO (10) M-2Cl (60)
Proposed elemental composition
Structure
lmax/nm
C8Cl3N2OH
238, 297, 345 and 360
P1
261/263/265/267 M-Cl (15) M-Cl-CO (95) M-Cl-2CO (30)
C8Cl3N2O2H
238, 266, 300 and 310
2-OHCT
245/247/249/251
C8Cl3N2OH
231, 288 and 341
3.2. Transient spectroscopy The laser ash photolysis of air-saturated solutions of CT (105 M, acetonitrile–water 10/90) yields the transient spectrum at the pulse end shown in Fig. 3. This spectrum is similar to the one measured in pure acetonitrile and assigned to the triplet– triplet absorption,8 even though a red-shi of 25 nm is observed. The same species is observed in argon-saturated solution. The triplet decays by rst-order kinetics with a rate constant that depends on the oxygen concentration. In argon-saturated medium the rate constant, kargon , is equal to (1.1 0.3) 105 s1, d
Photoproduct formation upon irradiation of CT (2 105 M) in acetonitrile–pH 7 buffered water (20 : 80, v/v) mixtures in air-saturated medium in polychromatic light. CT-Cl (C), P1 (O), 4-OHCT (+). Fig. 2
minor in the rst stages of the reaction. The important medium acidication at the end of the reaction is likely due to successive dechlorination of photoproducts with HCl formation and possible organic acid formation arising from CT and acetonitrile. The rate of CT photolysis was measured in the presence of several additives. Most of them accelerate the CT phototransformation. For example, bisphenol A (5 105 M) reduces t20 to 15 min, MON (5 mg dm3) to 4 h, isopropanol (0.017 M) to 3 h and benzoic acid (104 M) to 4 h. These additives also enhance the rate of CT-Cl, 4-OHCT and P1 formation. Addition of Cl and ClO at 5 105 M did not show signicant effect on the CT photolysis.
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Triplet–triplet absorption measured at the end of the pulse upon excitation of CT in acetonitrile–pH 7 water (10/90, v/v). In the inset: variation of the 3320 FT product with the percentage of acetonitrile. Fig. 3
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while in air-saturated medium, the rate constant, kaerated , is equal d to (2.5 0.3) 105 s1. Assuming that kaerated ¼ kargon + kO2[O2], d d kO2 being the bimolecular rate constant of the reaction of 3CT* with O2, and that in acetonitrile–water 10/90, [O2] ¼ 4.2 104 M (see ESI†), one gets kO2 ¼ 3.3 108 M1 s1. When the percentage of acetonitrile is increased, kaerated increases (see Fig. SI-4†). This is mainly due to the increase d of [O2] with the percentage of acetonitrile, but also to an increase of kO2. In a 50/50 acetonitrile–water mixture, a value of 7.6 108 M1 s1 is obtained. Using benzophenone as an actinometer, we measured the product 3320 FT, where 3320 is the molar extinction coefficient of 3CT* at 320 nm and FT the quantum yield of inter-system crossing. Data are given in inset of Fig. 3. The value varies from 6300 M1 cm1 at 10% acetonitrile to 7300 M1 cm1 at 80% acetonitrile. Knowing that the quantum yield of 3CT* formation is close to 1 in acetonitrile,8 one can deduce that (i) the molar extinction coefficient of 3CT* is close to 7300 M1 cm1 and (ii) the quantum yield of 3CT* formation is higher than 0.85 in media enriched with water. To determine whether 3CT* can be reduced and to measure the transient spectrum of the reduced species, we irradiated CT in pure isopropanol. As shown in Fig. SI-5,† 3CT* is readily trapped by isopropanol to give the new species shown in Fig. 4. It displays an intense band with a maximum around 320 nm, and a weaker one within the wavelength range 400–600 nm. This transient that is likely the reduced radical CTHc disappears with a rst order rate constant of 1.0 106 s1 in argon-saturated medium and 2.5 106 s1 in air-saturated medium. Thus its decay is enhanced by oxygen in accordance with the oxidation of CTHc by oxygen to yield CT back and the hydroperoxyl radical. 3.3. Singlet oxygen production The aim of this set of experiments was to determine the quantum yield of singlet oxygen (1O2) production by photoexcitation of CT in acetonitrile–D2O solutions. The 1O2 luminescence signals were
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measured at various CT concentrations (104–4.5 104 M) and different percentages of acetonitrile (100, 80, 50 and 30). Detailed calculations are given in the ESI.† Using PN as a reference (yield of singlet oxygen close to unity in most solvents17,18) the quantum yields of 1O2 production by CT, FCT D , under our experimental conditions are determined and presented in Table 3. They vary between 0.85 for 100% acetonitrile and 0.69 for 30% acetonitrile. As singlet oxygen is produced by energy transfer between 3CT* and ground state oxygen, it is interesting to try to connect the singlet oxygen formation with the ability of oxygen to scavenge 3 CT*. The percentage of the triplet trapped by oxygen is given by the ratio 100 (kaerated kargon )/kaerated where kaerated and d d d d argon 3 kd are the rate constants of CT* decay measured by laser ash photolysis. Data in Table 3 show that the percentage of the triplet trapped by oxygen divided by 100 is quite similar to the quantum of singlet oxygen measured independently by phosphorescence. This means that the reaction of oxygen with 3CT* yields almost exclusively singlet oxygen.
3.4. Quantum calculations The experimental UV spectrum of 4-OHCT is shown in Fig. 5a, while the theoretical spectrum of 4-OHCT is shown in Fig. 5b. It was found that the energy of the more hindered conformational isomer of 4-OHCT (form b, Scheme 1) was only 2.0 kcal mol1 above that of 4-OHCT (form a). Thus, the two species might be in equilibrium at room temperature. The respective spectra were computed and are shown in Fig. 5b. The three absorption bands of 4-OHCT are reproduced and the positions of the maxima lay within an interval of 20 nm of the experimental maxima. The spectrum of 4-OHCT (b) shows absorption features that are blue shied with respect to those of 4-OHCT (a). This may result in the broadening of the 4-OHCT band at 290 nm (shoulder at 285 nm) and the double dip feature observed between 340 and 360 nm in the experimental spectrum. Next, the theoretical absorption spectra of 2-OHCT and 5-OHCT were computed in order to identify the photoproduct detected by HPLC/MS in deoxygenated medium. The stoichiometry is that of an OHCT species and its absorption spectrum shows three bands at 231 nm (shoulder at 240 nm) 287 nm, and 341 nm (see Fig. 6). Both 2-OHCT and 5-OHCT show an absorption band at 350 nm. However, the theoretical spectra of these species are signicantly different below 300 nm. The
Table 3 Values of FD in various acetonitrile–D2O mixtures and percentage of 3CT* trapped by oxygen calculated from kargon and d measured by laser flash photolysis kaerated d
Acetonitrile content in %
Transient spectrum of the reduced radical measured at the end of the pulse by irradiating CT in pure isopropanol.
Fig. 4
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100 80 50 30 20 5
[CT]/M
FD
(1.3 4.3) 104 (1.3 4.9) 104 (1.3 4.7) 104 (0.73 4.7) 104
0.85 0.06 0.85 0.04 0.85 0.02 0.69 0.08
Percentage of 3 CT* trapped by O2 93 10 91 10 87 9 78 9 72 8 48 7
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Fig. 5 Experimental (a) and theoretical (b) electronic absorption spectra of 4-OHCT. Spectra in (b) were obtained at the TD-BPEBPE/6-31G(d,p) level. The UV peak Half-Width at Half Maximum was set to 0.044 eV (2 nm).
Fig. 6 Experimental absorption spectrum of the photoproduct formed in deoxygenated solution (a) and theoretical spectra (b) of 2-OHCT (solid) and 5-OHCT (dashed). Spectra in (b) were obtained at the TD-BPEBPE/6-31G(d,p) level. The UV peak Half-Width at Half Maximum (HWHM) was set to 0.044 eV (2 nm). Inset: theoretical spectrum of 2-OHCT with HWHM ¼ 0.11.
positions of the theoretical absorption features of 2-OHCT (245 nm, 258 nm, 284 nm and 348 nm) are similar to those of the experimental data. Moreover, when HWHM ¼ 0.11 eV (see the inset in Fig. 6b), the rst absorption band develops a
shoulder, which is similar to that observed in the experimental spectrum. In addition, the ratio of the respective molar extinction coefficients is also in good agreement with the experimental data for 2-OHCT. Thus, the above theoretical investigation provides evidence that the photoproduct is indeed 2-OHCT.
4. Discussion All these observations let us propose that the CT phototransformation takes place through the mechanism shown in Scheme 2. It is hypothesized that in the rst step of the reaction CT populates its triplet excited state. 3CT* has several possible fates including deactivation, quenching by water to form products, reaction with oxygen to form singlet oxygen, reduction by organic constituents to form the CTHc radical from which arise the main photoproducts. 4.1. Evidence for 3CT* involvement
Scheme 2
Proposed reaction mechanism.
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The formation of 3CT*, on the one hand, is clearly demonstrated by the laser ash photolysis experiments. The quantum yield of 3CT* formation is high (>0.85) whatever the percentage of acetonitrile. The involvement of 3CT* in the CT phototransformation, on the other hand, is evidenced by the double observation that oxygen inhibits the CT decay and traps 3CT*. Based on the competing effects of oxygen, water and acetonitrile
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on the photoproduct formation, one can also conclude that all the photoproducts arise from the triplet.
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4.2. Evidence for the CTHc radical formation To conrm the hypothesis that 3CT* can be reduced, one tried to generate the CTHc reduced radical by irradiating CT in an H-donor solvent. The experiment was successful enabling the direct detection of CTHc. The transient spectrum of this species was recorded and its reactivity toward oxygen is determined. 4.2.1. Reactions of the CTHc radical and product formed. When the oxygen concentration in the medium is increased by increasing the acetonitrile percentage, the formation of CT-Cl is reduced while those of P1 and 4-OHCT are favored. This opposite oxygen effect on CT-Cl on the one hand and P1 and 4-OHCT on the other hand suggests that all these photoproducts have the same precursor produced from 3CT*. The formation of CT-Cl from CT implies the replacement of a Cl atom by an H atom. CTHc is therefore a likely precursor of this reaction, as CT-Cl is directly formed subsequently to chloride atom elimination. In aerated medium CTHc is expected to be involved in several other competitive processes: (i) reaction with oxygen yielding back CT and hydroperoxyl radicals, this reaction was quantied and (ii) reaction with the newly created hydroperoxyl radicals, yielding hydroperoxide and HCl. We hypothesize that P1 and 4-OHCT arise from this hydroperoxide. The formation of the byproduct HCl also explains in part the medium acidication in the course of the reaction. The hydroperoxide has not been experimentally observed likely because of the relative instability of the peroxide bond, which does not allow its accumulation in high enough concentrations to be detectable by HPLC analyses. Quantitative analysis of data shows that 3CT* is not the only intermediate trapped by oxygen. In 20% acetonitrile aer a CT conversion of 20%, CT decay is reduced by a factor of 60. Yet, data of laser ash photolysis experiments indicate that at this acetonitrile percentage the stationary concentration of 3CT* is reduced by a factor 5. Therefore, the strong inhibiting effect of oxygen on the CT phototransformation is not only due to the triplet scavenging by oxygen. Oxygen must be involved in other reactions. Oxidation of CTHc giving HO2c and CT is a likely candidate as it is supported by experimental data. 4.2.2. Regeneration of the CTHc radical (autoacceleration). The way of formation of the CTHc radical is worth investigating. In the rst stages of the reaction, the only H-donor present in the medium is acetonitrile. Moreover, the decrease of t20, i.e., the increase of the CT photolysis rate, when the percentage of acetonitrile is increased, is observed in deoxygenated medium. This accelerating effect of acetonitrile on CT decay and conversely the inhibiting effect of acetonitrile on the chemical yield of 2-OHCT in deoxygenated medium suggest that it is directly involved in the CTHc radical formation. This implies that acetonitrile is oxidized.19 The intrinsic reactivity of acetonitrile with 3CT* is expected to be very low, but the huge concentration of acetonitrile under our experimental conditions (molar range) can counterbalance the low rate constant and make this reaction non-negligible. The auto-accelerated effect should be due to the occurrence of new reactions in the
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course of the reaction. Photoproducts arising from CT, 2-OHCT for example, or any acetonitrile photoproducts in turn may participate in the reduction of 3CT* and induce the autoacceleration.
4.3. Other reactions of triplet 3CT* 4.3.1. Quenching by water. Due to the fast trapping of 3CT* by oxygen, quenching of 3CT* by water is only detectable in deoxygenated medium. Interestingly this reaction yields 2-OHCT but not 4-OHCT. This latter is only detected in oxygenated medium. 2-OHCT which corresponds to the substitution of Cl by OH probably arises from the direct reaction of 3CT* with water, as found for other chlorinated compounds.20 In a good agreement, its chemical yield increases when the percentage of water is increased. 4.3.2. Quenching by oxygen and singlet oxygen production. The quantum yields of singlet oxygen measured by phosphorescence for 100 to 30% of acetonitrile t well with the fraction of the triplet trapped by oxygen. This means that the reaction between 3CT* and oxygen leads almost exclusively to energy transfer. Using this and the percentage of the triplet trapped by oxygen at 5% acetonitrile, one computes that the yield of singlet oxygen in 95% water should be close to 0.5 which is quite high.
5.
Conclusion
This work provides a detailed study of the rates, products, and chemical mechanisms of the photochemical decomposition of chlorothalonil and helps to understand the literature data. The phototransformation of chlorothalonil involves its triplet excited state as a rst intermediate and the reduced radical as a second intermediate. The poor photodegradability of chlorothalonil is explained by the oxygen reactivity with both species, through deactivation and reoxidation, respectively. We also show that the triplet is able to be reduced by acetonitrile, a co-solvent oen used to enhance the water solubility of poorly water soluble micropollutants. Finally, the reduction of the chlorothalonil triplet by photoproducts arising from acetonitrile or chlorothalonil is proposed to explain the enhancement of the phototransformation rate with increasing irradiation time. This work demonstrates that dissipation of chlorothalonil is actually essentially governed by the other chemicals present in the medium and able to reduce the triplet. In surface waters, one may expect chlorothalonil phototransformation thanks to the numerous chemicals contained in water. It must also be outlined that chlorothalonil generates high amounts of singlet oxygen, an important surface water oxidant.
Acknowledgements Authors thank Ang´ elique Abila (ICCF, France) for HPLC/MS analyses.
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References 1 P. Y. Caux, R. A. Kent, G. T. Fan and G. L. Stephenson, Crit. Rev. Environ. Sci. Technol., 1996, 26, 45–93. 2 P. E. Davies, Bull. Environ. Contam. Toxicol., 1988, 40, 405– 409. 3 M. Millet, W. U. Palm and C. Zetzsch, Ecotoxicol. Environ. Saf., 1988, 41, 44–50. 4 G. A. Penuela and D. Barcelo, J. Chromatogr. A, 1998, 823, 81– 91. 5 V. A. Sakkas, D. A. Lambropoulou and T. A. Albanis, Chemosphere, 2002, 48, 939–945. 6 D. F. Wallace, L. H. Hand and R. G. Oliver, Environ. Toxicol. Chem., 2009, 29, 575–581. 7 Y. Kawamura, M. Takeda and M. Uchiyama, J. Pestic. Sci., 1978, 3, 397–400. 8 S. Monadjemi, M. el Roz, C. Richard and A. ter Halle, Environ. Sci. Technol., 2011, 45, 9582–9589. 9 A. G. Giumanini, G. Verardo and P. Strazzolini, J. Photochem. Photobiol., A, 1989, 48, 129–153. 10 J. W. Kwon and K. L. Armbrust, J. Agric. Food Chem., 2006, 54, 3651–3657. 11 S. Samanta, R. K. Kole, L. K. Ganguly and A. Chowdhury, Bull. Environ. Contam. Toxicol., 1977, 59, 367–374. 12 C. Jahn and W. Schwack, J. Agric. Food Chem., 2001, 49, 1233– 1238. 13 S. Kouras, P. de Sainte-Claire, A. ter Halle, A. AmineKhodja and C. Richard, J. Phys. Chem. A, 2011, 115, 14397–14406. 14 F. Bonnichon and C. Richard, J. Photochem. Photobiol., A, 1998, 119, 25–32.
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15 H. Kuramochi, T. Kobayashi, T. Suzuki and T. Ichimura, J. Phys. Chem. B, 2010, 114, 8782–8789. 16 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford CT, 2010. 17 E. Oliveros, P. Suardi-Murasecco, T. Aminian-Sagha, A. M. Braun and H.-J. Hansen, Helv. Chim. Acta, 1991, 74, 79–90. 18 C. Mart´ı, O. J¨ urgens, O. Cuenca, M. Casals and S. Nonell, J. Photochem. Photobiol., A, 1996, 97, 11–18. 19 V. Augugliaro, C. Salvatore, E. Garcia-Lopez, V. Loddo, G. Marci, G. Martra, L. Palmisano and M. Schiavello, Top. Catal., 2005, 35, 237–244. 20 P. Boule, K. Othmen, C. Richard, B. Szczepanik and G. Grabner, Int. J. Photoenergy, 1999, 1, 49–54.
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PAPER
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Photoreactivity of the fungicide chlorothalonil in aqueous medium†
Cite this: Environ. Sci.: Processes Impacts, 2014, 16, 839
Samira Bouchama,ab Pascal de Sainte-Claire,ac Emmanuel Arzoumanian,d Esther Oliveros,d Abdelaziz Boulkamhb and Claire Richard*ae The photoreactivity of chlorothalonil was studied by means of steady-state irradiation and laser-flash photolysis. Experiments were conducted in water containing acetonitrile as a co-solvent. This fungicide undergoes very slow phototransformation in the first stages of the reaction, but the consumption profile is auto-accelerated. To understand the reaction mechanism, we undertook a detailed study of the rates, products and transient species. The rates and photoproduct distribution vary greatly with the oxygen concentration. Concerning the transient species, we measured the absorption of the triplet, its yield of formation, and its reactivity with oxygen in various water–acetonitrile mixtures and with isopropanol. The reduced radical, CTHc, could be produced and its transient spectrum was recorded. Combining all the experimental data, it is hypothesized that in the first step of the reaction CT is excited to the triplet state. The triplet has several possible fates including reduction by organic constituents to form the radical which gives photoproducts. Another characteristic of the CT triplet is its capacity to generate singlet oxygen. The production of this species was measured by phosphorescence and compared to the
Received 14th October 2013 Accepted 28th November 2013
percentage of the triplet trapped by oxygen in air-saturated solutions. The yield varies from 0.88 in pure acetonitrile to 0.48 in water–acetonitrile (95 : 5, v/v). Therefore, in surface waters, chlorothalonil is
DOI: 10.1039/c3em00537b
expected to sensitize the photooxidation of micropollutants, and to be competitively phototransformed
rsc.li/process-impacts
through reaction with any H donor or electron donor water constituents.
Environmental impact Chlorothalonil is a widely used and highly toxic fungicide that can be released into the aquatic environment. The detailed study presented here helps to understand its phototransformation mechanism. Chlorothalonil is quite photostable but its photodegradation is greatly enhanced by the water constituents able to reduce its triplet excited state. This means that the rates of chlorothalonil photodegradation will be impacted by the surface water composition. Chlorothalonil produces singlet oxygen in water with a quantum yield near 0.5 and can thus sensitize the photooxidation of other water micropollutants. Finally, this work shows that although considered as an inert cosolvent, acetonitrile seems able to react with the triplet excited state of chlorothalonil and to promote its phototransformation.
1. Introduction Chlorothalonil (CT, 1,3-dicyano-2,4,5,6-tetrachlorobenzene) is a widely used non-systemic fungicide, classied as a potential human carcinogen (group B), highly toxic and persistent.1,2
a Clermont Universit´e, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France. E-mail: [email protected]; Tel: +33 (0)4 73 40 71 42 b Laboratoire des Techniques Innovantes de Protection de l'Environnement, Universit´e de Constantine 1, Route de A¨ın-el-Bey, 25000 Constantine, Algeria c
Clermont Universit´e, ENSCCF, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France
d Laboratoire des IMRCP, UMR CNRS 5623, Universit´e Toulouse III (Paul Sabatier, UPS), 31062 Toulouse Cedex 9, France e
Equipe Photochimie CNRS, UMR 6296, ICCF, F-63171 Aubi`ere, France
† Electronic supplementary 10.1039/c3em00537b
information
(ESI)
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available.
See
DOI:
Relevant literature shows that CT photolysis is far from being understood. In water, CT photochemically degrades slowly with quantum yields ranging from 103 to 104.3,4 This means that one absorbed photon in 103 or 104 only is able to induce a chemical modication in the molecule. However, the phototransformation is faster in natural than in pure water.4–6 Photolysis in organic solvents or in a solid form is also very slow.7,8 Interestingly, an auto-accelerated photodegradation was reported in benzene.7 Concerning photoproducts, different types of reactions are reported. Photoreductive dechlorination is observed in water,4,5 in organic solvents8,9 and in solid.8 Formation of 4-hydroxyCT is also mentioned in water, probably due to both hydrolysis and photolysis.10 Finally, photoadditions occur in organic media. Adducts between dechlorinated CT and alcohols are described11 and in plant cuticle cutin bound residues of CT are detected.12
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The recent ndings that CT populates efficiently its triplet excited state in pure acetonitrile and generates high amounts of singlet oxygen (quantum yields close to 1)8 help to understand the relative photostability of CT in oxygenated media: the deactivation of the CT triplet by oxygen competes with the other triplet reaction pathways. The photoreductive dechlorination in organic solvents and the photoadditions on solvents or cuticle constituents were suggested to involve radicalar reactions but were not studied in detail. If the replacement of a chloride atom by an H atom is actually expected to occur in H-donor organic solvents, such a reaction in water is more intriguing. This may be linked to the use of co-solvents to increase the CT solubility in water. Acetonitrile is considered to be a suitable unreactive co-solvent and it is used in many studies.6 The aim of the paper was to understand the mechanism of CT phototransformation in aqueous medium using steady state and laser-ash photolysis. To clarify the potential effect of acetonitrile as a co-solvent, we dissolved CT in water–acetonitrile mixtures, and measured the evolution of some properties (triplet excited state population, singlet oxygen and photoproduct formations) as a function of the acetonitrile percentage. Moreover, we studied in detail the effect of the oxygen concentration and acetonitrile percentage on the photoproduct distribution to clarify their formation mechanism. Finally, as previous experiments showed that CT generates singlet oxygen with a quantum yield close to 1 in acetonitrile,8 we investigated the singlet oxygen formation in various water–acetonitrile mixtures by phosphorescence.
2.
Experimental part
2.1. Materials CT (99.3%) was supplied by Sigma-Aldrich and 4-hydroxychorothalonil (4-OHCT) was from Dr Ehrenstorfer company. Phenalenone (PN, 1H-phenalene-1-one, 97%) and acetonitrile (Chromasolv gradient grade, >99.9%) were provided by SigmaAldrich. Water was puried using a reverse osmosis RIOS 5 and Synergy (Millipore) (resistivity 18 MU cm, DOC < 0.1 mg L1). Other reagents were of the best purity available. Argon used for de-oxygenation was of ultra-pure quality (Air Products). All the solvents and chemicals were used as received. To solubilize CT at 5 106 M, a minimum of 5% acetonitrile (v/v) was added. For solutions at 2 105 M, 20% acetonitrile (v/v) was added. Under these conditions, CT solutions were stable. The absence of CT volatilization, especially under argon bubbling, was checked by monitoring the UV absorption spectrum before and aer bubbling. CT solutions were buffered by phosphate buffers made with sodium hydrogenophosphate (>99%, Sigma-Aldrich) and disodium phosphate (>99%, Sigma-Aldrich). The buffer concentration was 5 104 M. Phosphate buffers at this concentration were shown not to affect the photolysis. 2.2. Irradiation protocols Polychromatic irradiation was conducted in a device equipped with six uorescent tubes (TLAD 15W05 Philips) emitting between 300 and 450 nm (see the spectrum in Fig. SI-1†). The
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Pyrex reactor (15 cm height and 1.4 cm internal diameter) was lled with 15 mL of solution and le in contact with air. Experiments in the absence of oxygen were conducted in a cylindrical cell of 1 cm path which was purged with argon for 20 min prior to irradiation, then sealed. Irradiation was performed in a parallel beam at 313 nm using a mercury arc lamp (200 W) equipped with an Oriel monochromator. A radiometer QE65000 from Ocean Optics was employed to measure the photon uence rate, which was (6.4 0.5) 1014 photons cm2 s1 at 313 nm. For the polychromatic light, the radiometer was used to measure the photon distribution while a reference compound with a known quantum yield of photolysis (metamitron, F ¼ 0.025 (ref. 13)) was used to evaluate the amount of light received by the 15 mL of solutions (1.8 105 Einstein L1 s1). Then Ia, the rate of CT photon absorption, was obtained using the relationship: 380 X li Ia ¼ I0li 1 10ACT 290
I0li
is the amount of light intensity received by the soluwhere i tion between li1 and li1 + Dl and AlCT is the averaged absorbance of CT within the same wavelength range. The wavelength interval, Dl, was set at 5 nm. The absorption spectrum of CT is shown in Fig. SI-2.† The use of two different irradiation devices was necessary to optimize irradiation times, these being much longer for the experiments conducted in the presence of oxygen than in its absence. In both devices, the same long-wavelength absorption bands of CT are excited to limit the risk of any wavelength effects on the reaction. Laser ash photolysis experiments were performed on an apparatus consisting of a Nd:YAG laser (GCR 130-1, pulse width 9 ns, 266 nm), a 150 W pulsed xenon arc lamp, a R928 photomultiplier and a 05-109 Spectra Kinetics Applied Photophysics monochromator.14 Benzophenone was used as a chemical actinometer. The formation of triplet benzophenone was monitored at 525 nm. The product of the quantum yield of benzophenone triplet formation and its molar absorption coefficient at 525 nm was assigned 6500 M1 cm1.15 Experiments in the absence of oxygen were conducted in a cell equipped with a valve. Solutions were deoxygenated in the cell and the valve was closed just before excitation. For experiments in argon-saturated medium, solutions were renewed aer 3 ashes. A cooled (80 C) near infra-red (NIR) photomultiplier (Hamamatsu R5509 PMT) was used as a 1O2 detector. Irradiation was carried out at 313 nm and 367 nm with a xenon/ mercury arc (1 kW) through a water lter, focusing optics, and a monochromator. The 1O2 luminescence was collected with a mirror, chopped (at 11 Hz), and, aer passing through a focusing lens, a cut-off lter (1000 nm) and an interference lter (1271 nm), it was detected at 90 with respect to the incident beam using the NIR photomultiplier and a lock-in amplier (Stanford Research Systems, Model SR830 DSP). Singlet oxygen luminescence signals were recorded as a function of irradiation time for a minimum of 3 min. Square signals were obtained showing that the sensitizers were stable under continuous irradiation. No sensitizer bleaching was observed during
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irradiation. The incident radiant powers (W0, mW) at the wavelengths of irradiation (l) were measured using a thermopile (Laser Instrumentation, model 154), and the relative incident photon uxes were calculated as P0/P0,R ¼ W0l/W0,RlR. Detailed calculations are given in the ESI.†
that included the implicit contribution of the solvent were also performed with the Polarizable Continuum Model (PCM) and acetonitrile as solvent. In that case, molecules were fully optimized within the solvent eld. However, these calculations did not improve the results signicantly, and the solvent was not included in this work.
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2.3. Analyses UV-visible spectra were recorded on a Cary 3 (Varian) spectrophotometer. The monitoring of CT and its photoproducts was performed at room temperature using an HPLC-UV system equipped with an autosampler (Waters 717plus), a degasser (Agilent 1100 series), a pump (Waters 515) and a photodiode array detector (Waters 996) and a conventional reversed-phase column. The eluent was a mixture containing water with formic acid 0.1% (v/v) and acetonitrile. The gradient elution started with 30% of acetonitrile that was linearly increased until 70% in 4 min. This percentage was maintained for 15 min. The ow rate was set at 1 cm3 min1. The injection volume was 90 mL. Mass spectrometry was performed on a LC/QTOF apparatus equipped with an orthogonal geometry Z-spray ion source (Waters/Micromass, Manchester, UK). Analyses were made in negative mode. UV detection under MS heading was a Waters Alliance 2695 photodiode array detector system. The column was a Kinetex™ C18, 100 mm 2.1 mm, with a particle size of ˚ The binary solvent system used 2.6 mm, and a size pore of 100 A. was composed of acetonitrile and water acidied with 0.5& v/v formic acid. The gradient elution started with 5% of acetonitrile and reached 95% in 15 min. The ow rate was set at 0.2 cm3 min1. Tandem mass spectrometric (MS/MS) experiments were conducted in a collision cell with an argon pressure of 1 bar. A collision energy of 20 eV was used. MS analyses were conducted on CT solutions (5 105 M) that had undergone conversion up to 30%. The data recorded were processed using MassLynx (version 4.1).
3.
Results
3.1. Steady-state irradiation Aerated solutions of CT were irradiated with polychromatic light. CT was dissolved in various acetonitrile–pH 7 water mixtures. The decay of CT with increasing irradiation times is shown in Fig. 1A. The consumption prole of CT is rather independent of the acetonitrile percentage between 5 and 50%. Aer 2 h the CT loss is hardly measurable while aer 4 and 6 h, 9 and 21% of CT have disappeared, respectively. The decay rate of
2.4. Theoretical calculations Density Functional Theory (DFT) calculations were performed with the Gaussian series of programs16 to investigate the optical properties of three hydroxychlorothalonil (OHCT) species (see Scheme 1). The molecules were optimized at the PBEPBE/ 6-31G(d,p) level and the singlet excited state energies were obtained from Time-Dependent DFT calculations (TD-DFT) with the above functional and basis set. The PBEPBE functional was chosen because experimental electronic absorption spectra are well reproduced with this method. This good agreement has been reached consistently for several species of environmental interest in our laboratory for the past few years. Calculations
Decay profile of CT in acetonitrile–pH 7 buffered water mixtures irradiated (A) in air-saturated medium in polychromatic light (Ia ¼ 1.7 107 E L1 s1) and (B) in argon-saturated medium at 313 nm (Ia ¼ 8.8 108 E L1 s1). O ACN 5%–water 95%, [CT] ¼ 0.5 105 M; B ACN 20%–water 80%, [CT] ¼ 2 105 M; ; ACN 50%–water 50%, [CT] ¼ 2 105 M. Fig. 1
Scheme 1
The species investigated in this work from DFT calculations.
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CT has therefore a tendency to increase with the irradiation time. Such an auto-accelerating effect was never mentioned in the literature except by Kawamura et al. who irradiated CT in benzene.7 The decay prole obtained by irradiating CT (2 105 M, acetonitrile–water, 20/80) in deoxygenated solution is shown in Fig. 1B. The reaction is much faster than in aerated medium. CT is converted by 18% aer 10 min of irradiation in deoxygenated medium while by 21% aer 360 min in air-saturated medium. Taking into account that Ia is twice as high in the former case, the photolysis is 60-fold faster in deoxygenated medium than in an air-saturated acetonitrile–water 20/80 mixture. As observed in the presence of oxygen, the rate of CT photolysis increases with increasing irradiation time. Similar results are obtained when CT is irradiated in the mixtures containing 5 and 50% acetonitrile. Table 1 gives the time necessary to convert 20% of CT (t20) under different experimental conditions. This time is between 6 and 7 h in polychromatic light and aerated medium whatever the percentage of acetonitrile, and between 8 and 20 min at 313 nm and argon-saturated medium. In this latter case, increasing the acetonitrile percentage slightly decreases t20, and thus increases the photolysis rate. From these rst results one can draw the following conclusions. The drastic inhibiting effect of oxygen is in line with the involvement of the triplet excited state in the reaction. The auto-accelerated decay prole shows that new processes leading to CT consumption occur in the course of the reaction. Finally, acetonitrile seems to be involved in the photolytic process. Several photoproducts were detected whose nature and distribution depend on the experimental conditions. In airsaturated medium, we observed the formation of three photoproducts: one resulting from substitution of Cl by H, CT-Cl, one resulting from substitution of Cl by OH, 4-OHCT, and one bearing an additional oxygen atom compared to 4-OHCT, P1. The formation of CT-Cl was previously mentioned by several authors.4,5,10 This product was identied based on the UV spectrum and retention time.8 The two other photoproducts were detected by HPLC and HPLC/MS in negative mode (see Table 2). Based on the MS pattern, they have both lost a Cl atom and gained OH or O2H. 4-OHCT shows a UV spectrum very different from that of CT, with four maxima at 238, 297, 345 and 360 nm. P1 also has a very different UV spectrum with maxima
Table 1
at 238, 266, 300 and 310 nm. By MS/MS, 4-OHCT and P1 give different fragmentations, losing Cl, Cl + CO and 2Cl and Cl, Cl + CO and Cl + 2CO, respectively. The structure of P1 is not rmly established, but it may be the one given in Table 2 corresponding to dichloro-hydroxy-hypochlorite-1,3-benzenedicarbonitrile. By monitoring the evolution of the UV-visible spectrum of the solution during irradiation, one conrms that the main absorbance changes are between 250 and 300 nm and 330 and 350 nm (see Fig. SI-3†). In argon-saturated medium, CTCl is observed again, but neither 4-OHCT nor P1. Instead, a photoproduct with the same MS data than 4-OHCT and a UV spectrum very similar to maxima at 231, 288 and 346 nm is formed. This compound is proposed to be 2-OHCT based on spectrum calculation (see Section 3.4). The formation proles of CT-Cl, P1 and 4-OHCT in air-saturated medium are shown in Fig. 2. They show the same autoaccelerated trend as CT consumption. The chemical yields of these compounds vary with the irradiation conditions (oxygen concentration and acetonitrile percentage) (see Table 1). Chemical yields are given for a conversion extent of CT of 20%. CT-Cl is produced in higher yields in the absence of oxygen (hCT-Cl lying between 0.25 and 0.33) than in aerated medium where hCT-Cl drastically decreases from 0.20 to 0 when the percentage of acetonitrile is increased from 5 to 50%. 4-OHCT and P1 are only formed in air-saturated medium. Their respective chemical yields increase as hCT-Cl decreases. Lastly, 2-OHCT is only produced in argon-saturated medium. Its chemical yield decreases from 0.15 to 0.02 when the percentage of acetonitrile is increased from 5 to 50%. Thus one concludes that medium oxygenation changes the nature of photoproducts and that CT-Cl formation is in competition with 2-OHCT formation in deoxygenated medium and with P1 and 4-OHCT in aerated medium. Solutions were generally buffered at pH 7 to maintain stable the pH conditions. Experiments were also conducted with unbuffered solutions. In the absence of any buffer, one observes an acidication of the solution upon irradiation. In aerated solutions containing 20% acetonitrile and CT 2 105 M, the initial pH is equal to 6.8. It falls to 6.2 aer 8 h of irradiation, corresponding to a CT conversion extent of 30%, and to 3.9 aer 23 h and a total CT conversion. Therefore, 4.7 107 and 1.3 104 M of protons are released when CT losses are 6 106 and 2 105 M respectively. It can be concluded that H+ release is
Irradiation times necessary to transform 20% of CT, t20, under different conditions and chemical yields of photoproducts t20/min at
Conditions
[CT]
5% ACN, aerated 20% ACN aerated 50% ACN aerated 5% ACN argon 10% ACN argon 20% ACN argon 50% ACN argon
5 106 2 105 2 105 5 106 5 106 2 105 2 105
313 nm M M M M M M M
20 20 10 8
2 2 2 2
300–450 nm
hCT-Cla
390 40 360 40 360 40
0.20 0.07 nd 0.24 0.33 0.33 0.25
0.02 0.02 0.02 0.02 0.02 0.02
h2-OHCTb
h4-OHCTb
Area of P1c
ndd nd nd 0.15 0.02 0.11 0.02 0.06 0.01 0.02 0.005
nd 0.01 0.005 0.10 0.02 nd nd nd nd
10 000 16 000 50 000 nd nd nd nd
Measured at 232 nm using the same epsilon as for CT (66 000 M1 cm1). b Measured at 290 nm using the same epsilon as for 4-OHCT (22 500 M1 cm1). c Measured at 266 nm. d Not detected. a
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Environmental Science: Processes & Impacts List of detected photoproducts, elemental compositions and proposed structure
Compound
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4-OHCT
Measured masses in ES in u and MS/MS fragments (percentage)
245/247/249/251 M-Cl (20) M-Cl-CO (10) M-2Cl (60)
Proposed elemental composition
Structure
lmax/nm
C8Cl3N2OH
238, 297, 345 and 360
P1
261/263/265/267 M-Cl (15) M-Cl-CO (95) M-Cl-2CO (30)
C8Cl3N2O2H
238, 266, 300 and 310
2-OHCT
245/247/249/251
C8Cl3N2OH
231, 288 and 341
3.2. Transient spectroscopy The laser ash photolysis of air-saturated solutions of CT (105 M, acetonitrile–water 10/90) yields the transient spectrum at the pulse end shown in Fig. 3. This spectrum is similar to the one measured in pure acetonitrile and assigned to the triplet– triplet absorption,8 even though a red-shi of 25 nm is observed. The same species is observed in argon-saturated solution. The triplet decays by rst-order kinetics with a rate constant that depends on the oxygen concentration. In argon-saturated medium the rate constant, kargon , is equal to (1.1 0.3) 105 s1, d
Photoproduct formation upon irradiation of CT (2 105 M) in acetonitrile–pH 7 buffered water (20 : 80, v/v) mixtures in air-saturated medium in polychromatic light. CT-Cl (C), P1 (O), 4-OHCT (+). Fig. 2
minor in the rst stages of the reaction. The important medium acidication at the end of the reaction is likely due to successive dechlorination of photoproducts with HCl formation and possible organic acid formation arising from CT and acetonitrile. The rate of CT photolysis was measured in the presence of several additives. Most of them accelerate the CT phototransformation. For example, bisphenol A (5 105 M) reduces t20 to 15 min, MON (5 mg dm3) to 4 h, isopropanol (0.017 M) to 3 h and benzoic acid (104 M) to 4 h. These additives also enhance the rate of CT-Cl, 4-OHCT and P1 formation. Addition of Cl and ClO at 5 105 M did not show signicant effect on the CT photolysis.
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Triplet–triplet absorption measured at the end of the pulse upon excitation of CT in acetonitrile–pH 7 water (10/90, v/v). In the inset: variation of the 3320 FT product with the percentage of acetonitrile. Fig. 3
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while in air-saturated medium, the rate constant, kaerated , is equal d to (2.5 0.3) 105 s1. Assuming that kaerated ¼ kargon + kO2[O2], d d kO2 being the bimolecular rate constant of the reaction of 3CT* with O2, and that in acetonitrile–water 10/90, [O2] ¼ 4.2 104 M (see ESI†), one gets kO2 ¼ 3.3 108 M1 s1. When the percentage of acetonitrile is increased, kaerated increases (see Fig. SI-4†). This is mainly due to the increase d of [O2] with the percentage of acetonitrile, but also to an increase of kO2. In a 50/50 acetonitrile–water mixture, a value of 7.6 108 M1 s1 is obtained. Using benzophenone as an actinometer, we measured the product 3320 FT, where 3320 is the molar extinction coefficient of 3CT* at 320 nm and FT the quantum yield of inter-system crossing. Data are given in inset of Fig. 3. The value varies from 6300 M1 cm1 at 10% acetonitrile to 7300 M1 cm1 at 80% acetonitrile. Knowing that the quantum yield of 3CT* formation is close to 1 in acetonitrile,8 one can deduce that (i) the molar extinction coefficient of 3CT* is close to 7300 M1 cm1 and (ii) the quantum yield of 3CT* formation is higher than 0.85 in media enriched with water. To determine whether 3CT* can be reduced and to measure the transient spectrum of the reduced species, we irradiated CT in pure isopropanol. As shown in Fig. SI-5,† 3CT* is readily trapped by isopropanol to give the new species shown in Fig. 4. It displays an intense band with a maximum around 320 nm, and a weaker one within the wavelength range 400–600 nm. This transient that is likely the reduced radical CTHc disappears with a rst order rate constant of 1.0 106 s1 in argon-saturated medium and 2.5 106 s1 in air-saturated medium. Thus its decay is enhanced by oxygen in accordance with the oxidation of CTHc by oxygen to yield CT back and the hydroperoxyl radical. 3.3. Singlet oxygen production The aim of this set of experiments was to determine the quantum yield of singlet oxygen (1O2) production by photoexcitation of CT in acetonitrile–D2O solutions. The 1O2 luminescence signals were
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measured at various CT concentrations (104–4.5 104 M) and different percentages of acetonitrile (100, 80, 50 and 30). Detailed calculations are given in the ESI.† Using PN as a reference (yield of singlet oxygen close to unity in most solvents17,18) the quantum yields of 1O2 production by CT, FCT D , under our experimental conditions are determined and presented in Table 3. They vary between 0.85 for 100% acetonitrile and 0.69 for 30% acetonitrile. As singlet oxygen is produced by energy transfer between 3CT* and ground state oxygen, it is interesting to try to connect the singlet oxygen formation with the ability of oxygen to scavenge 3 CT*. The percentage of the triplet trapped by oxygen is given by the ratio 100 (kaerated kargon )/kaerated where kaerated and d d d d argon 3 kd are the rate constants of CT* decay measured by laser ash photolysis. Data in Table 3 show that the percentage of the triplet trapped by oxygen divided by 100 is quite similar to the quantum of singlet oxygen measured independently by phosphorescence. This means that the reaction of oxygen with 3CT* yields almost exclusively singlet oxygen.
3.4. Quantum calculations The experimental UV spectrum of 4-OHCT is shown in Fig. 5a, while the theoretical spectrum of 4-OHCT is shown in Fig. 5b. It was found that the energy of the more hindered conformational isomer of 4-OHCT (form b, Scheme 1) was only 2.0 kcal mol1 above that of 4-OHCT (form a). Thus, the two species might be in equilibrium at room temperature. The respective spectra were computed and are shown in Fig. 5b. The three absorption bands of 4-OHCT are reproduced and the positions of the maxima lay within an interval of 20 nm of the experimental maxima. The spectrum of 4-OHCT (b) shows absorption features that are blue shied with respect to those of 4-OHCT (a). This may result in the broadening of the 4-OHCT band at 290 nm (shoulder at 285 nm) and the double dip feature observed between 340 and 360 nm in the experimental spectrum. Next, the theoretical absorption spectra of 2-OHCT and 5-OHCT were computed in order to identify the photoproduct detected by HPLC/MS in deoxygenated medium. The stoichiometry is that of an OHCT species and its absorption spectrum shows three bands at 231 nm (shoulder at 240 nm) 287 nm, and 341 nm (see Fig. 6). Both 2-OHCT and 5-OHCT show an absorption band at 350 nm. However, the theoretical spectra of these species are signicantly different below 300 nm. The
Table 3 Values of FD in various acetonitrile–D2O mixtures and percentage of 3CT* trapped by oxygen calculated from kargon and d measured by laser flash photolysis kaerated d
Acetonitrile content in %
Transient spectrum of the reduced radical measured at the end of the pulse by irradiating CT in pure isopropanol.
Fig. 4
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100 80 50 30 20 5
[CT]/M
FD
(1.3 4.3) 104 (1.3 4.9) 104 (1.3 4.7) 104 (0.73 4.7) 104
0.85 0.06 0.85 0.04 0.85 0.02 0.69 0.08
Percentage of 3 CT* trapped by O2 93 10 91 10 87 9 78 9 72 8 48 7
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Fig. 5 Experimental (a) and theoretical (b) electronic absorption spectra of 4-OHCT. Spectra in (b) were obtained at the TD-BPEBPE/6-31G(d,p) level. The UV peak Half-Width at Half Maximum was set to 0.044 eV (2 nm).
Fig. 6 Experimental absorption spectrum of the photoproduct formed in deoxygenated solution (a) and theoretical spectra (b) of 2-OHCT (solid) and 5-OHCT (dashed). Spectra in (b) were obtained at the TD-BPEBPE/6-31G(d,p) level. The UV peak Half-Width at Half Maximum (HWHM) was set to 0.044 eV (2 nm). Inset: theoretical spectrum of 2-OHCT with HWHM ¼ 0.11.
positions of the theoretical absorption features of 2-OHCT (245 nm, 258 nm, 284 nm and 348 nm) are similar to those of the experimental data. Moreover, when HWHM ¼ 0.11 eV (see the inset in Fig. 6b), the rst absorption band develops a
shoulder, which is similar to that observed in the experimental spectrum. In addition, the ratio of the respective molar extinction coefficients is also in good agreement with the experimental data for 2-OHCT. Thus, the above theoretical investigation provides evidence that the photoproduct is indeed 2-OHCT.
4. Discussion All these observations let us propose that the CT phototransformation takes place through the mechanism shown in Scheme 2. It is hypothesized that in the rst step of the reaction CT populates its triplet excited state. 3CT* has several possible fates including deactivation, quenching by water to form products, reaction with oxygen to form singlet oxygen, reduction by organic constituents to form the CTHc radical from which arise the main photoproducts. 4.1. Evidence for 3CT* involvement
Scheme 2
Proposed reaction mechanism.
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The formation of 3CT*, on the one hand, is clearly demonstrated by the laser ash photolysis experiments. The quantum yield of 3CT* formation is high (>0.85) whatever the percentage of acetonitrile. The involvement of 3CT* in the CT phototransformation, on the other hand, is evidenced by the double observation that oxygen inhibits the CT decay and traps 3CT*. Based on the competing effects of oxygen, water and acetonitrile
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on the photoproduct formation, one can also conclude that all the photoproducts arise from the triplet.
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4.2. Evidence for the CTHc radical formation To conrm the hypothesis that 3CT* can be reduced, one tried to generate the CTHc reduced radical by irradiating CT in an H-donor solvent. The experiment was successful enabling the direct detection of CTHc. The transient spectrum of this species was recorded and its reactivity toward oxygen is determined. 4.2.1. Reactions of the CTHc radical and product formed. When the oxygen concentration in the medium is increased by increasing the acetonitrile percentage, the formation of CT-Cl is reduced while those of P1 and 4-OHCT are favored. This opposite oxygen effect on CT-Cl on the one hand and P1 and 4-OHCT on the other hand suggests that all these photoproducts have the same precursor produced from 3CT*. The formation of CT-Cl from CT implies the replacement of a Cl atom by an H atom. CTHc is therefore a likely precursor of this reaction, as CT-Cl is directly formed subsequently to chloride atom elimination. In aerated medium CTHc is expected to be involved in several other competitive processes: (i) reaction with oxygen yielding back CT and hydroperoxyl radicals, this reaction was quantied and (ii) reaction with the newly created hydroperoxyl radicals, yielding hydroperoxide and HCl. We hypothesize that P1 and 4-OHCT arise from this hydroperoxide. The formation of the byproduct HCl also explains in part the medium acidication in the course of the reaction. The hydroperoxide has not been experimentally observed likely because of the relative instability of the peroxide bond, which does not allow its accumulation in high enough concentrations to be detectable by HPLC analyses. Quantitative analysis of data shows that 3CT* is not the only intermediate trapped by oxygen. In 20% acetonitrile aer a CT conversion of 20%, CT decay is reduced by a factor of 60. Yet, data of laser ash photolysis experiments indicate that at this acetonitrile percentage the stationary concentration of 3CT* is reduced by a factor 5. Therefore, the strong inhibiting effect of oxygen on the CT phototransformation is not only due to the triplet scavenging by oxygen. Oxygen must be involved in other reactions. Oxidation of CTHc giving HO2c and CT is a likely candidate as it is supported by experimental data. 4.2.2. Regeneration of the CTHc radical (autoacceleration). The way of formation of the CTHc radical is worth investigating. In the rst stages of the reaction, the only H-donor present in the medium is acetonitrile. Moreover, the decrease of t20, i.e., the increase of the CT photolysis rate, when the percentage of acetonitrile is increased, is observed in deoxygenated medium. This accelerating effect of acetonitrile on CT decay and conversely the inhibiting effect of acetonitrile on the chemical yield of 2-OHCT in deoxygenated medium suggest that it is directly involved in the CTHc radical formation. This implies that acetonitrile is oxidized.19 The intrinsic reactivity of acetonitrile with 3CT* is expected to be very low, but the huge concentration of acetonitrile under our experimental conditions (molar range) can counterbalance the low rate constant and make this reaction non-negligible. The auto-accelerated effect should be due to the occurrence of new reactions in the
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course of the reaction. Photoproducts arising from CT, 2-OHCT for example, or any acetonitrile photoproducts in turn may participate in the reduction of 3CT* and induce the autoacceleration.
4.3. Other reactions of triplet 3CT* 4.3.1. Quenching by water. Due to the fast trapping of 3CT* by oxygen, quenching of 3CT* by water is only detectable in deoxygenated medium. Interestingly this reaction yields 2-OHCT but not 4-OHCT. This latter is only detected in oxygenated medium. 2-OHCT which corresponds to the substitution of Cl by OH probably arises from the direct reaction of 3CT* with water, as found for other chlorinated compounds.20 In a good agreement, its chemical yield increases when the percentage of water is increased. 4.3.2. Quenching by oxygen and singlet oxygen production. The quantum yields of singlet oxygen measured by phosphorescence for 100 to 30% of acetonitrile t well with the fraction of the triplet trapped by oxygen. This means that the reaction between 3CT* and oxygen leads almost exclusively to energy transfer. Using this and the percentage of the triplet trapped by oxygen at 5% acetonitrile, one computes that the yield of singlet oxygen in 95% water should be close to 0.5 which is quite high.
5.
Conclusion
This work provides a detailed study of the rates, products, and chemical mechanisms of the photochemical decomposition of chlorothalonil and helps to understand the literature data. The phototransformation of chlorothalonil involves its triplet excited state as a rst intermediate and the reduced radical as a second intermediate. The poor photodegradability of chlorothalonil is explained by the oxygen reactivity with both species, through deactivation and reoxidation, respectively. We also show that the triplet is able to be reduced by acetonitrile, a co-solvent oen used to enhance the water solubility of poorly water soluble micropollutants. Finally, the reduction of the chlorothalonil triplet by photoproducts arising from acetonitrile or chlorothalonil is proposed to explain the enhancement of the phototransformation rate with increasing irradiation time. This work demonstrates that dissipation of chlorothalonil is actually essentially governed by the other chemicals present in the medium and able to reduce the triplet. In surface waters, one may expect chlorothalonil phototransformation thanks to the numerous chemicals contained in water. It must also be outlined that chlorothalonil generates high amounts of singlet oxygen, an important surface water oxidant.
Acknowledgements Authors thank Ang´ elique Abila (ICCF, France) for HPLC/MS analyses.
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References 1 P. Y. Caux, R. A. Kent, G. T. Fan and G. L. Stephenson, Crit. Rev. Environ. Sci. Technol., 1996, 26, 45–93. 2 P. E. Davies, Bull. Environ. Contam. Toxicol., 1988, 40, 405– 409. 3 M. Millet, W. U. Palm and C. Zetzsch, Ecotoxicol. Environ. Saf., 1988, 41, 44–50. 4 G. A. Penuela and D. Barcelo, J. Chromatogr. A, 1998, 823, 81– 91. 5 V. A. Sakkas, D. A. Lambropoulou and T. A. Albanis, Chemosphere, 2002, 48, 939–945. 6 D. F. Wallace, L. H. Hand and R. G. Oliver, Environ. Toxicol. Chem., 2009, 29, 575–581. 7 Y. Kawamura, M. Takeda and M. Uchiyama, J. Pestic. Sci., 1978, 3, 397–400. 8 S. Monadjemi, M. el Roz, C. Richard and A. ter Halle, Environ. Sci. Technol., 2011, 45, 9582–9589. 9 A. G. Giumanini, G. Verardo and P. Strazzolini, J. Photochem. Photobiol., A, 1989, 48, 129–153. 10 J. W. Kwon and K. L. Armbrust, J. Agric. Food Chem., 2006, 54, 3651–3657. 11 S. Samanta, R. K. Kole, L. K. Ganguly and A. Chowdhury, Bull. Environ. Contam. Toxicol., 1977, 59, 367–374. 12 C. Jahn and W. Schwack, J. Agric. Food Chem., 2001, 49, 1233– 1238. 13 S. Kouras, P. de Sainte-Claire, A. ter Halle, A. AmineKhodja and C. Richard, J. Phys. Chem. A, 2011, 115, 14397–14406. 14 F. Bonnichon and C. Richard, J. Photochem. Photobiol., A, 1998, 119, 25–32.
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15 H. Kuramochi, T. Kobayashi, T. Suzuki and T. Ichimura, J. Phys. Chem. B, 2010, 114, 8782–8789. 16 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford CT, 2010. 17 E. Oliveros, P. Suardi-Murasecco, T. Aminian-Sagha, A. M. Braun and H.-J. Hansen, Helv. Chim. Acta, 1991, 74, 79–90. 18 C. Mart´ı, O. J¨ urgens, O. Cuenca, M. Casals and S. Nonell, J. Photochem. Photobiol., A, 1996, 97, 11–18. 19 V. Augugliaro, C. Salvatore, E. Garcia-Lopez, V. Loddo, G. Marci, G. Martra, L. Palmisano and M. Schiavello, Top. Catal., 2005, 35, 237–244. 20 P. Boule, K. Othmen, C. Richard, B. Szczepanik and G. Grabner, Int. J. Photoenergy, 1999, 1, 49–54.
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