Environ. Sci. Technol. 2006, 40, 4765-4770
Semiconductor-Mediated Photocatalyzed Degradation of a Herbicide Derivative, Chlorotoluron, in Aqueous Suspensions MALIK M. HAQUE,† M O H D . M U N E E R , * ,† A N D DETLEF W. BAHNEMANN‡ Department of Chemistry, Aligarh Muslim University, Aligarh-202002, India, and Institut fuer Technische Chemie, Universitaet Hannover, Callinstrasse- 3, D-30167 Hannover, Germany
The photocatalytic degradation of a herbicide derivative, chlorotoluron [3-(3-chloro-4-methylphenyl)-1,1-dimethylurea, 1], has been investigated in aqueous suspensions of titanium dioxide (TiO2) under a variety of conditions. The degradation was studied by monitoring the change in substrate concentration employing UV spectroscopic analysis technique and depletion in total organic carbon (TOC) content as a function of irradiation time. The degradation kinetics of the model compound was investigated under different conditions such as type of TiO2, pH, catalyst concentration, substrate concentration, temperature, and in the presence of different electron acceptors such as hydrogen peroxide (H2O2), potassium persulfate (K2S2O8), and potassium bromate (KBrO3) besides molecular oxygen. TiO2 Degussa P25 was found to be a more efficient photocatalyst for the degradation of the model compound as compared with other photocatalysts. The degradation products were analyzed using GC/MS analysis technique and probable pathways for the formation of different products have been proposed.
Introduction The control of organic pollutants in water is an important measure in environmental protection. Large amounts of chemicals are used in agricultural field for better quality and quantity of the crop. Hence, a wide variety of herbicides is applied on agricultural sites, which, due to their chemical stability, resistance to biodegradation, and sufficient water solubility, penetrate deep into the groundwater (1, 2). Their toxicity, stability to natural decomposition, and persistence in the environment has been the cause of much concern to the societies and regulation authorities around the world (3). Development of appropriate methods for the degradation of contaminated drinking, ground, surface waters, and wastewaters containing toxic or nonbiodegradable compounds is necessary. Among many processes proposed and/ or being developed for the destruction of organic contaminants, biodegradation has received the greatest attention. However, many organic chemicals, especially those that are toxic or refractory, are not amendable to microbial degrada-
tion. Recently, considerable attention has been focused on the use of semiconductors as a means to oxidize toxic organic chemicals (4-11). The photocatalyzed degradation of various organic systems employing irradiated TiO2 is well documented in the literature (4). The initial step in the TiO2 mediated photocatalyzed degradation is proposed to involve the generation of (e-/h+) pair leading to the formation of hydroxyl radical and superoxide radical anion (eqs 1-3):
TiO2 + h*ν f TiO2 + h+vb + e-cb
(1)
O2 + e-cb f O•-2
(2)
H2O + h+vb f •OH + H+
(3)
It has been suggested that the hydroxyl radicals and superoxide radical anions are the primary oxidizing species in the photocatalytic oxidation processes. These oxidative reactions would result in the degradation of the pollutant and the efficiency of the degradation will depend on the oxygen concentration, which determines the efficiency with which the conduction band electrons are scavenged and the (e-/h+) recombination is prevented. The model compound chlorotoluron (1) is a selective urea derivative used as herbicide throughout the world acting by inhibition of photosynthesis to regulate plant growth (12). It is used for pre- and post-emergence control of annual grass, wild oats, annual meadow grass, and many broadleaf weeds in winter wheat, spring and winter barley, and winter rye (13). The solubility of chlorotoluron (1) reported in the pesticide manual is 74 mg L-1 (12), which is sufficient to allow the drainage of this herbicide from soil to surface water (14). The presence of herbicides in groundwater, surface water, effluents of wastewater treatment plants, and other sources of drinking water indicates that many of these compounds are recalcitrant and/or nonbiodegradable (15). It can also persist for long periods in the environment. Photodegradation of chlorotoluron under different conditions such as direct photolysis (16, 17) in the presence of Fe(III) aquacomplexes (18), TiO2 (19, 20), and using a biological reactor (21) has been reported earlier. Despite of these studies, no major efforts have been made to study the detailed degradation kinetics, which is essential for any practical application. Therefore, we have made a detailed degradation kinetic study of the model compound 1 in aqueous suspensions of TiO2 under a variety of conditions such as types of TiO2, pH, catalyst concentration, substrate concentration, temperature, and in the presence of different electron acceptors such as hydrogen peroxide (H2O2), potassium persulfate (K2S2O8), and potassium bromate (KBrO3) besides molecular oxygen. An attempt has also been made to identify the intermediate products formed during the photooxidation process through GC/MS analysis technique.
* Corresponding author phone: +91-571-2703515; fax: +91-5712702758; e-mail:
[email protected]. † Aligarh Muslim University. ‡ Universitaet Hannover. 10.1021/es060051h CCC: $33.50 Published on Web 07/01/2006
2006 American Chemical Society
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Experimental Section Materials. Analytical grade chlorotoluron (1) was obtained from Ciba Geigy and Fluka and used as such without any further purification. Double-distilled water was employed in this study to make a solution for the irradiation experiments. The photocatalyst titanium dioxide Degussa P25 was used in most of the experiments. Other catalyst powders, namely Hombikat UV100 (Sachtleben Chemie GmbH) and PC500 (Millennium Inorganic Chemicals), were used for comparative studies. Degussa P25 contains 75% anatase and 25% rutile with a specific BET surface area of 50 m2g-1 and a primary particle size of 20 nm (22). Hombikat UV100 consists of 100% pure anatase with a specific BET surface area of 250 m2g-1 and a primary particle size of 5 nm (23). The photocatalyst PC500 has a BET surface area of 287 m2g-1 with 100% anatase and primary particle size of 5-10 nm (24). The other chemicals used in this study, such as sodium hydroxide, nitric acid, hydrogen peroxide, potassium persulfate, and potassium bromate are of reagent grade and were obtained from Merck. Methods. For the irradiation experiment for kinetic studies of the model compound 1, the solution of desired concentration was prepared in double-distilled water. An immersion well photochemical reactor made of Pyrex glass was used in this study. Aqueous solution (250 mL) of the model compound 1 was poured into the reactor and the required amount of photocatalyst was added. The solution was stirred for at least 15 min in the dark to allow equilibration of the system. The zero time reading was obtained from a blank solution kept in the dark but otherwise treated similarly to the irradiated solution. The suspensions were continuously purged with molecular oxygen throughout each experiment. Irradiations were carried out using a 125 W medium-pressure mercury lamp (Philips). The light intensity was measured using UV-light intensity detector (Lutron UV-340) and was found to be 4.86 mW/cm2. IR radiation and short wavelength UV radiation were eliminated by a water-circulating Pyrex jacket. Samples (10 mL) were collected before, and at regular intervals during, the irradiation and centrifuged before analysis. The pH of the reaction mixture was adjusted by adding dilute aqueous solution of NaOH or HNO3. For the characterization of intermediate products, an aqueous solution (250 mL) of the pesticide containing TiO2 (P25, 1.5 g L-1) was taken in a photochemical reactor made of Duran glass with a plain quartz window (through which the parallel light beam entered) equipped with a magnetic stirring bar, a water-circulating jacket, and an opening for electrodes and gas supplies. Irradiations were carried out using a high-pressure mercury lamp (Osram HBO 450 W). The light intensity was found to be 20 mW/cm2. IR radiation and short-wavelength UV radiation were eliminated by a 10 cm water filter. A 320 nm cutoff filter was used to avoid any direct excitation of the compound under investigation. Samples were collected at different time intervals during the irradiation, centrifuged to remove TiO2, and extracted with methylene chloride, which was subsequently dried over anhydrous sodium sulfate and analyzed by GC/MS. Analysis. The degradation of chlorotoluron and its intermediate products was followed by measuring the change in absorption intensity at their λmax (242 nm) using a Shimadzu UV-Vis spectrophotometer (model 1601). The mineralization was monitored by measuring the total organic carbon (TOC) content with a Shimadzu TOC 5000A analyzer by directly injecting the aqueous solution after centrifugation. For GC/MS analysis a Shimadzu gas chromatograph and mass spectrometer (GCMS-QP 5050) equipped with a 25 m CP SIL 19 CB (d ) 0.25 mm) capillary column, operating temperature programmed (220 °C for 40 min at the rate of 10 °C min-1) in splitless mode injection volume (1.0 µL) with helium as a carrier gas was used. 4766
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FIGURE 1. Photodegradation of chlorotoluron (0.25 mM) as a function of time in the presence of TiO2 (1 g L-1) with continuous stirring and O2 purging.
Results and Discussion Photocatalysis of TiO2 Suspensions Containing Chlorotoluron (1). Irradiation of an aqueous solution of the herbicide derivative chlorotoluron (1, 0.25 mM, 250 mL, pH 5.4) in the presence of TiO2 (Degussa P25, 1 g L-1) led to a decrease in absorption intensity and depletion in TOC content as a function of time. The logarithm of percent degradation of the decomposition and depletion in TOC content as a function of irradiation time in the presence of photocatalyst is shown in Figure 1. It can be seen from the figure that 99.6% degradation of chlorotoluron and its intermediate products and 69.1% mineralization of the model compound take place after 120 min of irradiation in the presence of TiO2, whereas no observable loss of compound could be seen when the reaction was carried out in the absence of TiO2. For each experiment the degradation rate constant of the model pollutant was calculated from the linear regression of a plot of the natural logarithm of the absorbance as a function of irradiation time, i.e., first-order degradation kinetics. For the total mineralization the rate was determined from the initial slope obtained by the linear regression of the TOC content versus irradiation time, i.e., zeroth order degradation kinetics. Unfortunately, by this procedure one often obtains higher degradation rates for the TOC removal than for the decomposition of the respective model compound itself, which of course is impossible from the logical point of view. However, considering that the TOC content consists of undegraded substrate and intermediates containing the same, less, or in the case of dimerization reactions even a higher number of carbon atoms, a TOC degradation rate in terms of [mol “molecules” min-1] would result in significantly lower rates than those of the removal of the model compound. The degradation rate for the decomposition of the pesticide and its intermediate products was calculated using the formula given below
-d[A] ) kcn dt
(4)
where A ) absorbance, k ) rate constant, c ) concentration of the pollutant, and n ) order of reaction. Blank experiments were carried out by irradiating the aqueous solution of the model compound 1 in the absence of TiO2, where no observable loss of the compound was observed. The zero irradiation time reading was obtained from blank solution kept in the dark in the presence of TiO2, but otherwise treated similarly to the irradiated solutions.
TABLE 1. Photocatalytic Degradation of Chlorotoluron in Aqueous Suspensions under Different Parameters
parameter type of photocatalyst
P25 UV100 PC500 pH 3.0 5.4 7.3 9.1 substrate 0.15 concentration (mM) 0.25 0.35 0.45 catalyst 0.5 concentration (g/L) 1.0 2.0 3.0 electron acceptors P25 P25/H2O2 P25/KBrO3 P25/K2S2O8
degradation mineralrate ization (mol L-1 rate (min-1) min-1 × 10-5) 0.190 0.171 0.109 0.225 0.190 0.143 0.130 0.121 0.190 0.201 0.211 0.169 0.190 0.202 0.211 0.190 0.232 0.204 0.213
1.299 0.389 0.188 1.585 1.299 1.221 1.132 0.946 1.299 1.416 1.555 0.676 1.299 1.377 1.472 1.299 1.333 1.642 1.414
The degradation rate for the decomposition of the compound for the first-order reaction was calculated in terms of mol L-1 min-1. Comparison of Different TiO2 Photocatalysts. We have tested the photocatalytic activity of three different commercially available TiO2 powders (namely Degussa P25, Hombikat UV100, and Millennium Inorganic PC500) on the degradation kinetics of the herbicide derivative 1 under investigation. The degradation rate obtained for the decomposition and for the mineralization of compound 1 (0.25 mM, 250 mL) in the presence of different types of TiO2 powders (1 g L-1) is shown in Table 1. It has been observed that the degradation of the model compound under investigation proceeds much more rapidly in the presence of Degussa P25 as compared with other TiO2 samples. The reason for the better photocatalytic activity of Degussa P25 could be attributed to the fact that P25 being composed of small nanocrystallites of rutile being dispersed within an anatase matrix. The band gap energy of anatase is slightly higher than that of the rutile. Hence, when a light intensity of 4.86 mW/cm2 is irradiated on the TiO2 surface, the smaller band gap of rutile “catches” the photons, generating an electron-hole pair. The electron transfer from the rutile conduction band to electron traps in anatase phase takes place. Recombination is thus inhibited allowing the hole to move to the surface of the particle and react (25). In all following experiments, Degussa P25 was used as the photocatalyst since this material exhibited the highest overall activity for the degradation of the pesticide. Effect of pH. An important parameter in the photocatalytic reactions taking place on the particulate surfaces is the pH of the solution, since it dictates the surface charge properties of the photocatalyst and size of aggregates it forms. Employing Degussa P25 (1 g L-1) as photocatalyst the decomposition and mineralization of chlorotoluron (1, 0.25 mM, 250 mL) in aqueous suspensions was studied in the pH range between 1 and 12. Table 1 shows the degradation rate for the TOC depletion and for the decomposition of the herbicide derivative 1 as a function of pH. Slightly higher degradation rates were observed under acidic pH; the rates slowly decrease with increase in pH. Similar rates were obtained when the catalyst concentration was used as 2 g L-1 under analogous conditions as shown in Table 2.
TABLE 2. Photocatalytic Degradation of Chlorotoluron in Aqueous Suspensions under Different Parametersa degradation rate (mol L-1 min-1 × 10-5)
parameter pH
substrate concentration (mM)
a
1.2 3.0 5.4 7.3 9.1 11.2 0.15 0.25 0.35 0.45
1.653 1.589 1.377 1.359 1.263 1.159 0.940 1.377 1.724 1.926
Experimental condition: photocatalyst, TiO2 Degussa P25 (2 g L-1).
The interpretation of pH effect on the photocatalytic process is a difficult task because of its multiple role, such as electrostatic interactions between the semiconductor surface, solvent molecules, substrate, and charged radicals formed during the reaction process. The ionization state of the surface of the photocatalyst can be protonated and deprotonated under acidic and alkaline conditions, respectively, as shown in following equations:
TiOH + H+ f TiOH2+
(5)
TiOH + OH- f TiO- + H2O
(6)
The point of zero charge (pzc) of the TiO2 (Degussa P25) is widely reported at pH ∼6.25 (26). Thus, the TiO2 surface will remain positively charged in acidic medium (pH < 6.25) and negatively charged in alkaline medium (pH > 6.25). The amino and carbonyl groups present in the model pollutant can be protonated and deprotonated depending on the pH of the reaction mixture. The better degradation rate in acidic pH may be attributed on the basis of the fact that the structural orientation of the molecule is favored for the attack of the reactive species under this condition. The adsorption of the herbicide on the surface of the photocatalyst was investigated by stirring the aqueous solution in the dark for 24 h in a round-bottomed flask at different pH values such as 1.2, 3.0, 5.4, 7.3, 9.1, and 11.2. Analysis of the samples after centrifugation indicate slight loss of the compound due to adsorption only at pH 11.2. Effect of Substrate Concentration. It is important both from a mechanistic and an application point of view to study the dependence of substrate concentration on the photocatalytic reaction rate. Effect of substrate concentration on the degradation rate of the herbicide derivative 1 was studied at different substrate concentrations from 0.15 to 0.45 mM. The degradation rate for the TOC depletion and for the decomposition of compound 1 as a function of substrate concentration employing Degussa P25 (1 g L-1) as photocatalyst is shown in Table 1. The degradation rate for the decomposition and for the mineralization was found to increase gradually with the increase in substrate concentration from 0.15 to 0.45 mM. Similar results were obtained when the photocatalyst concentration was raised from 1 to 2 g L-1. As oxidation proceeds, less and less of the surface of the TiO2 particle is covered as the pollutant is decomposed. Evidently, at total decomposition the rate of degradation is zero and a decreased photocatalytic rate is to be expected with increasing irradiation time. It has been agreed with minor variation that the expression for the rate of photomineralization of organic substrates with irradiated TiO2 VOL. 40, NO. 15, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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follows the Langmui-Hinshelwood (L-H) law for the four possible situations i.e., when the reaction (a) takes place between two adsorbed substances; (b) occurs between a radical in solution and an adsorbed substrate molecule; (c) takes place between a radical linked to the surface and a substrate molecule in solution; and (d) occurs with both the species being in solution (27). In all cases the expression for the rate equation is similar to that derived from the L-H model, which has been useful in modeling the process although it is not possible to find out whether the process takes place on the surface, in the solution, or at the interface. Earlier studies (28-30) on the photocatalytic degradation of colorless organic pollutants also indicate that the degradation rate increases with the increase in substrate concentration. Effect of Catalyst Concentration. Whether in static, slurry, or dynamic flow reactors the initial reaction rates were found to be directly proportional to catalyst concentration indicating a heterogeneous regime. However, it was observed that above a certain concentration the reaction rate decreases and becomes independent of the catalyst concentration. This limit depends on the geometry and working conditions of the photoreactor and for a definite amount of TiO2 in which all the particles, i.e., surface exposed, are totally illuminated. When the catalyst concentration is very high, after traveling a certain distance on an optical path, turbidity impedes further penetration of light in the reactor. In any given application this optimum catalyst concentration [(TiO2)OPT] has to be found in order to avoid excess catalyst and ensure total absorption of efficient photons. The effect of photocatalyst concentration on the degradation kinetics of chlorotoluron (1) was investigated employing different concentrations of Degussa P25, varying from 0.5 to 3 g L-1. The degradation rate for the TOC depletion and for the decomposition of the herbicide derivative 1 (0.25 mM, 250 mL) as function of catalyst loading is shown in Table 1. As expected, the degradation rate was found to increase with the increase in catalyst concentration, which is the characteristic of heterogeneous photocatalysis, and results are in agreement with number of studies reported earlier (31, 32). The adsorption of the herbicide derivative on the surface of the photocatalyst was also investigated at its original pH (5.4) by stirring the aqueous solution in the dark for 24 h in a round-bottomed flask containing varying amounts of photocatalyst such as 0.5, 1, 2, and 3 g L-1. Analysis of the samples after centrifugation indicated no observable loss of the compound. Effect of Electron Acceptors. One practical problem in using TiO2 as a photocatalyst is the undesired electron/hole recombination which, in the absence of proper electron acceptor or donor, is extremely efficient and represents the major energy-wasting step thus limiting the achievable quantum yield. One strategy to inhibit electron-hole pair recombination is to add other (irreversible) electron acceptors to the reaction. They could have several different effects such as to (1) increase the number of trapped electrons and, consequently, avoid recombination; (2) generate more radicals and other oxidizing species; (3) increase the oxidation rate of intermediate compounds; and (4) avoid problems caused by low oxygen concentration. In highly toxic wastewater where degradation of organic pollutants is the major concern, the addition of electron acceptors to enhance the degradation rate may often be justified. With this view, we have studied the effect of electron acceptors such as potassium persulfate (5 mM), potassium bromate (5 mM), and hydrogen peroxide (10 mM) in the presence of TiO2 (Degussa P25, 1 g L-1) on the photocatalytic degradation of the model compound 1 (0.25 mM, 250 mL) under investigation, and the results are depicted in Table 1. 4768
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Among all the additives, potassium bromate showed a pronounced effect for the decomposition of the compound 1, while H2O2 shows better mineralization rate as compared to other additives. The electron acceptors such as hydrogen peroxide, bromate, and persulfate ions are known to generate hydroxyl radicals by the mechanisms shown in eqs 7-11
H2O2 + e-cb f OH• + OH-
(7)
BrO3- + 2H+ + e-cb f BrO2• + H2O
(8)
BrO3- + 6H+ + 6e-cb f [BrO2-, HOBr] f
Br- + 3H2O (9)
S2O82- + e-cb f SO42- + SO4-
(10)
SO4•- + H2O f SO42- OH• + H+
(11)
The respective one-electron reduction potentials of different species are as follows: E (O2/O2•-) ) -155 or -146 mV; E (H2O2/OH•) ) 800 or 1776 mV; E (BrO3-/BrO2•) ) 1150 mV; and E (S2O82- /SO4-) ) 1100 or 2123 mV (33, 34). From the thermodynamic point of view all employed additives should therefore be more efficient electron acceptors than molecular oxygen. The effective electron acceptor ability of KBrO3 has been observed in number of studies reported before (32, 35). The reason can be attributed to the number of electrons it reacts with as shown in eq 9. Another possible explanation might be a change in the reaction mechanism of the photocatalytic degradation. Since the reduction of bromate ions by electrons does not lead directly to the formation of hydroxyl radicals, but rather to the formation of other reactive radicals or oxidizing agents, eg. BrO2- and HOBr. Linder has proposed a mechanism for the photocatalytic degradation of 4-chlorophenol in the presence of bromate ions considering direct oxidation of the substrate by bromate ions (36). A similar mechanism might be operative in the model compound 1 under investigation. The effect of H2O2 has been investigated in numerous studies and it was observed that it increases the photodegradation rates of organic pollutants. The enhancement of the degradation rate on addition of H2O2 can be rationalized in terms of several reasons. First, it increases the rate by removing the surface-trapped electrons, thereby lowering the electron-hole recombination rate and increasing the efficiency of hole utilization for reactions such as (OH- + h+ f OH•). Second, H2O2 may split photolytically to produce hydroxyl radicals directly, as cited in studies of homogeneous photooxidation using UV/(H2O2 + O2) (37). During the photocatalytic degradation the free radicals formed serve a dual function. They are not only the strong oxidants but also at the same time their formation and subsequent rapid oxidation reactions inhibit the electronhole pair recombination. The observations of these investigations clearly demonstrate the importance of choosing the optimum degradation parameters to obtain high degradation rate, which is essential for any practical application of photocatalytic oxidation processes. Effect of Temperature. The effect of temperature on the photocatalytic degradation of chlorotoluron (1, 0.25 mM, 250 mL) in the presence of TiO2 (Degussa P25, 2 g L-1) was followed at different temperatures such as 20, 30, 40, and 50 °C as a function of time, and the degradation rate for the decomposition of the compound 1 is shown in Figure 2. The
10.310, 10.255, and 5.520 min, respectively, along with some unchanged starting material (1) appearing at (tR) 5.097 min) were identified. It is pertinent to mention here that the model compound 1 did not show the molecular ion peak, instead it showed the peak equivalent to the loss of N(CH3)2 group. The products were characterized by comparing the molecular ion and mass fragmentation pattern with those reported in the GC/MS library, as follows: compound 7, 194 (M+), 183, 165, 148, 132, 109, 84, 72, 68, 44, and 40; compound 8, 184 (M+), 155, 148, 127, 112, 102, 92, 77, 65, 45, and 40; compound 9, 141 (M+), 132, 113, 106, 84, 77, 63, 49, and 41; compound 1, 169 [M+, -N(CH3)2], 167, 140, 132, 125, 104, 85, 77, 63, 51, and 40.
FIGURE 2. Influence of temperature on the degradation rate for the decomposition of chlorotoluron (1). Experimental conditions: 0.25 mM chlorotoluron, V ) 250 mL, photocatalyst: TiO2 Degussa P25 (2 g L-1).
When the reaction of the model compound 1 was carried out in the presence of Hombikat UV 100 (100% anatase with BET surface area of 250 m2 g-1), a difference in the yield of the above products was observed with that of the Degussa P25 (75% anatase and 25% rutile with BET surface area of 50 m2 g-1).
degradation rate was found to be more or less similar in the temperature range studied within the experimental error limits. Intermediate Products. An attempt was made to identify the intermediate products formed in the photocatalytic degradation of the herbicide derivative 1 in aqueous suspensions of titanium dioxide through GC/MS analysis. An aqueous suspension of chlorotoluron (1, 0.35 mM, 250 mL) in the presence of TiO2 (Degussa P25, 1.5 g L-1) under constant bubbling of atmospheric oxygen was irradiated for 2 h. The catalyst was removed through centrifugation and irradiated aqueous solution was extracted with methylene chloride, which was dried over anhydrous sodium sulfate. The GC/MS analysis of the organic extract showed the formation of several intermediate product out of which three products (3-(3-hydroxy-4-methylphenyl)-1,1-dimethylurea (7), (3-chloro-4-methylphenyl) urea (8), and 3-chloro4-methylphenylamine (9) appearing at retention times (tR)
A possible mechanism for the formation of products 7, 8, and 9 from chlorotoluron (1) involving electron-transfer reaction and reaction with hydroxyl radicals could be understood in terms of pathways shown in Scheme 1. The model compound 1, upon the transfer of an electron followed by the loss of methyl radical and abstraction of a proton to give species 5. This species on subsequent similar reaction can give the observed product 8. The observed product 9 from 1 may be arising through the transfer of an electron to give radical anionic species 3, which may undergo addition of a hydroxyl radical to give 6. This species may lose N(CH3)2COOH followed by abstraction of a proton to give the observed product 9 as shown in the Scheme 1. The product 7 could be formed upon the transfer of an electron followed by the loss of chloride ion with subsequent addition of hydroxyl radical. The analysis of the intermediate products formed during the photodegradation process could be a useful source of information on the degradation pathways.
SCHEME 1
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Acknowledgments Financial support by the Alexander von Humboldt Foundation, Bonn, Germany (for providing three months Fellowship to M.M. under follow-up program) and The Muslim Association for the Advancement of Science (MAAS), India (for providing Senior Research Fellowship to M.M.H) is gratefully acknowledged. The total organic carbon (TOC) analyzer used in this study was a gift from the Alexander von Humboldt Foundation, Bonn, Germany. Thanks to Prof. P. Boule, Laboratoire de Photochimie Mole´culaire et Macromoleculaire (UMR 6505), Universite´ Blaise Pascal (Clermont-Ferrand), F-63177 Aubie´re, cedex France, for providing chlorotoluron as a gift sample.
(18)
(19)
(20)
(21)
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(22) (23) (24)
(25)
(26) (27) (28)
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Received for review January 10, 2006. Revised manuscript received May 12, 2006. Accepted May 31, 2006. ES060051H
