Journal of Environmental Quality 30:2062-2070 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
TECHNICAL REPORT
Organic Compounds in the Environment
Kinetics and Mechanism of Chlorobenzene Degradation in Aqueous Samples Using Advanced Oxidation Processes
Mine Dilmeghani and
K. Omar Zahir*
Department of Chemistry, California State Univ., Northridge, CA 91330-8262
* Corresponding author (omar.zahir{at}csun.edu)
Received for publication July 5, 2000.
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ABSTRACT
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Degradation of chlorobenzene using various photoinduced oxidation processes such as direct ultraviolet light–induced photolysis (UV), UV–H2O2, UV–O3, and UV–H2O2–O3 was investigated under aerobic and anaerobic conditions. Kinetics and mechanisms of the degradation process were studied using high performance liquid chromatorgraphy (HPLC) and gas chromatorgraphy–mass spectrometry (GC–MS). In all cases, loss of chlorobenzene followed first-order kinetics. For UV-induced degradation of chlorobenzene, the observed pseudo first-order rate constant, kobs, ranged from 1.8 x 10-4 s-1 under anaerobic conditions to 6.4 x 10-4 s-1 for oxygen-saturated solution. Among the four systems studied, under identical conditions, the degradation rates for UV–H2O2 and UV–H2O2–O3 were very similar and were an order of magnitude higher than the one observed for UV. For the UV–H2O2 system, the observed pseudo first-order rate constant varied linearly with [H2O2] and followed the rate expression kobs = kOH
, where kOH is the observed second-order rate constant for the reaction of .OH radical with cholorbenzene. A plot of kobs vs. [H2O2] gave a value of 0.17 ± 0.02 M-1 s-1 for kOH. Both HPLC and GC–MS studies showed that depending upon the time of photolysis and the advanced oxidation processes (AOP) method employed, various intermediates were formed during the degradation process. For the UV process, these intermediates were identified as phenol, biphenyl, chlorobiphenyl isomers, and benzaldehyde. For the other three systems, chlorophenol, and various isomers of chlorobiphenyl and dichlorobiphenyl, were observed as the intermediates. The initial pH of the solution decreased from 5.8 to 3.5, showing the release of chlorine from cholobenzene. The HPLC results also showed that at longer times, the subsequent degradation of the intermediates also took place. Carbon dioxide and water are suspected to be the likely end products. Mechanistic schemes for the formation of such intermediates are proposed.
Abbreviations: AOP, advanced oxidation processes • GC–MS, gas chromatography–mass spectrometry • HPLC, high performance liquid chromatography • UV, direct ultraviolet light–induced photolysis
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INTRODUCTION
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OXIDATION of organic pollutants present in water is an attractive method of treatment because, if carried to completion, it results in the conversion of organic compounds into innocuous materials such as carbon dioxide and water. Unfortunately, the direct oxidation of organic compounds by molecular oxygen at ambient conditions is, in most cases, too slow to be of any potential use. On the other hand, AOP offers the most effective way of oxidizing organic contaminants to less harmful compounds (Glaze et al., 1987; Carey, 1990; Ollis et al., 1991).
Some of the most commonly studied homogeneous AOP are ultraviolet light–hydrogen peroxide (UV–H2O2) and ultraviolet light–ozone (UV–O3) systems. In addition, direct UV-induced photolysis (UV) can also lead to degradation either as a result of photodecomposition of the excited organic contaminant or due to the interaction of the organic excited state with ground or excited state molecular oxygen. Numerous studies have been carried out to investigate the effectiveness of these technologies toward the degradation of a wide variety of organic contaminants (Glaze et al., 1992; Bolton and Lipczynska-Kochany, 1992). Some of these studies include degradation of nitrobenzene and nitrophenol by UV and UV–H2O2 (Lipczynska-Kochany, 1992), treatment of the aqueous phase of polysaccharide alginic acid by UV–O3 (Von Sonntag et al., 1990), and degradation of various aromatic pollutants (Sundstrom et al., 1989; Froelich, 1992), acetone (Bolton and Stefan, 1996), and dioxane (Bolton et al., 1998) using UV–H2O2. Oxidation of chlorinated hydrocarbon using UV–O3 and UV–H2O2 has also been studied (Hoigne and Masten, 1992). In certain cases, investigation of the intermediates formed have also been carried out (Bolton and Lipczynska-Kochany, 1992; Andreozzi et al., 1992a,b) and low molecular weight oxygenated compounds have been reported as the oxidation products (Bolton and Lipczynska-Kochany, 1990). In another study, the efficiency of oxidation processes involving a combination of ozone, ultrasound, and Fe2+ was carried out for the degradation of substituted phenols (Munter et al., 1998).
However, no systematic studies have been carried out to investigate the kinetics and mechanisms of the degradation process involving each of these technologies, and to identify the intermediate formed in various stages of photolysis. Such intermediates make the degradation process multicomponent even when starting with a single component. Therefore, a demonstration of the formation and degradation of the intermediates is very important. Here, we wish to describe the relative effectiveness of various advanced oxidation processes, namely UV alone (AOP I), UV–H2O2 (AOP II), UV–O3 (AOP III), and UV–H2O2–O3 (AOP IV) toward the degradation of a common contaminant, chlorobenzene, under identical conditions, and provide details of the kinetics and mechanism of the degradation processes and the identification of the various intermediates formed.
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MATERIALS AND METHODS
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Chlorobenzene was obtained in high purity (Fisher Scientific, Pittsburgh, PA) and was used without further purification. A 30% aqueous solution of hydrogen peroxide (Fisher Scientific) was used as received. Synthetic water samples were prepared using Millipore (Bedford, MA) deionized water, which was further purified through double distillation. Photolysis of an 850-mL synthetic air–saturated water sample was carried out at ambient temperature in a recirculating stainless steel flow reactor (7.5-cm diameter, 27-cm length). Figure 1
shows the schematics for the reactor configuration and associated analytical techniques. The source of ultraviolet radiation was a 14-watt low pressure mercury lamp with a maximum output of 10.2 mW cm-2 at 254 nm. The lamp was placed in the center of the reactor and separated from the solution using a quartz tube. The intensity of the lamp was continuously monitored throughout the work for any fluctuation, and was found to be less than 2%. Ozone was generated from dioxygen using a Model T-408 ozone generator built by Welsback Ozone System (Sunnyvale, CA) that was operated at 3.5 amperes and 60 Hz. The synthetic solution was bubbled with ozone at a flow rate of 1.0 L min-1 for 20 min first in the flask, and then for 10 min in the reactor. During those studies, an ozone atmosphere was maintained throughout the photolysis. The concentration of ozone in the solution was determined using the indigo method, and is estimated to be approximately 100 µM (Hoigne and Bader, 1978, 1983). In certain cases, when N2 or O2 atmosphere was needed, the solution was purged with the respective gas for 30 min outside the reactor and then for another 20 min inside the reactor before photolysis. A gas-tight syringe was used to collect 2- to 3-mL samples from the samples port of the reactor, which were then analyzed for the loss of starting material and the intermediates formed using a Shimadzu (Kyoto, Japan) Model LC10AS/SPD10A HPLC system. The disappearance of starting contaminants was monitored at the wavelength of their maximum absorption. A C18 reverse phase column was used with 70:30 methanol and water as the mobil phase at a flow rate of 0.6 mL min-1. A Hewlett–Packard (Palo Alto, CA) 5889 GC–MS system with HP-5 capillary column was used for the identification of the intermediates formed at various stages of the photolysis. The GC–MS analysis of pure chlorobenzene showed no traces of any of the intermediates observed during AOP. For GC–MS analyses of the samples, the organic species present in the total sample were extracted using 25 mL of dichloromethane. Our control experiments showed that when an aqueous sample containing 10 µM sample of chlorobenzene was subject to the same extraction procedure, greater than 90% of the chlorobenzene could be quantitatively determined. Therefore, we set our detection limit for chlorobenzene at 10 µM. The extracted samples were normally concentrated, and 5.0 µL of the sample was injected into GC–MS. A suitable temperature programming scheme was developed for each GC–MS analysis. The identification of the intermediates was made by searching a match (usually greater than 90%) with the National Institute of Standards and Technology (NIST) library available on the GC–MS and also by examining the fragmentation pattern. Further confirmation was made by comparing the retention time and the on-line UV absorption spectra of various intermediates with the UV spectra of the known compound using a HP 1050 HPLC system that was equipped with a diode array detector. The pH of the solutions was measured using a Beckman (Fullerton, CA) 
II equipped with a glass electrode.
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Fig. 1. Schematic of the experimental set up used in advanced oxidation processes (AOP) of chlorobenzene.
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RESULTS AND DISCUSSION
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Kinetics and Mechanism of the Photoinduced Processes
The sequence of events that could occur during a particular AOP employed are summarized below.
AOP I (UV only)
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 | [2] |
 | [3] |
 | [4] |
 | [5] |
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 | [8] |
Here Eq. [1] involves the excitation of chlorobenzene. The rate of excitation will depend upon the concentration and the molar absorptivity of the chlorobenzene at 254 nm, and the intensity of UV light. The excited chlorobenzene can then either undergo thermal deexcitation [2] or photodecomposition to give intermediate Iuv as described in [3]. The intermediate Iuv can also absorb UV light to produce excited state intermediate *Iuv as shown in [4] and undergo further decomposition [5]. Both Iuv and *Iuv could also react with ground state dioxygen, if present, to produce oxidized products [6]. The loss of chlorobenzene was found to follow first-order kinetics. The observed first-order rate constants are tabulated in Table 1. The data in Table 1 show that the degradation of chlorobenzene was independent of the initial concentration of chlorobenzene. Under aerobic conditions, the degradation of chlorobenzene was found to be three to four times faster than the degradation under anaerobic conditions. This observed dependance of the rate upon oxygen suggests that molecular oxygen may be involved in the rate-determining step. Interaction of excited state aromatic compounds with ground state molecular oxygen to give excited state singlet oxygen via energy transfer is well known (Wasserman and Murray, 1979) and the reactivity of singlet oxygen toward organic molecules (especially olefines, dienes, and aromatic hydrocarbons) via the ene type reactions has been extensively reviewed. However, considering that the lifetime of singlet dioxygen in aqueous media is only a few microseconds (Rodger and Snowden, 1982), photosensitized degradation alone cannot be the sole pathway for chlorobenzene degradation. We suspect that under these conditions, both photosensitized oxygenation ([7] followed by [8]) as well as the direct decomposition of chlorobenzene [3] could be operative simultaneously, leading to an enhanced rate of degradation.
In the case of air-saturated solution or when the solution was saturated with oxygen, a variety of intermediates were formed after 80 min of photolysis. These intermediates were identified using GC–MS and HPLC and were found to be phenol, 4,4' biphenyl, benzyldehyde, and chlorobiphenyl. Scheme 1 (Fig. 2)
shows the possible pathways by which these intermediates can be formed. The mechanism for the formation of benzaldehyde, which was consistently observed both by HPLC and GC–MS, is not shown in Scheme 1. Such formation of benzaldehyde would require addition of both a carbon and an oxygen atom to the aromatic ring. We speculate that it could happen as a result of insertion of some fragment of an aromatic ring that had already been opened upon photooxidation. When photolysis was carried out under nitrogen atmosphere, no intermediates could be detected after 100 min of photolysis. As the rate and product distributions are completely different between the aerobic and anaerobic cases, two different mechanisms may be operative. We propose that under anaerobic conditions, products such as biphenyl and chlorobiphenyl undergo further photooxidation at a rate higher than Eq. [3] and no accumulation of the intermediates occurs. On the other hand, under aerobic conditions, benzyldehyde, biphenyl, and halo substituted biphenyl are formed, and all of these species along with H2O2 and chlorobenzene compete for UV light. In that case, the emitted photons have to be distributed among all these species, leading to an appreciable concentration of the intermediates.
AOP II (UV–H2O2)
In the presence of H2O2, in addition to Eq. [1–8], the following pathways will also be operative:
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When the UV–H2O2 system is employed, there will be competition for UV light between H2O2 and chlorobenzene, and, depending upon the concentrations and molar absorptivities of the two compounds at the wavelength of lamp emission, a relative distribution of light will take place. Absorption of light by H2O2 will lead to its decomposition, giving ·OH radicals with a quantum yield of two (Bolton and Kochany, 1991). The ·OH radical then attacks the aromatic ring (Atkinson et al., 1990) leading to the degradation of chlorobenzene. At the same time, absorption of light by chlorobenzene could also lead to its decomposition via AOP I. When a solution containing 1.8 mM chlorobenzene and 0.030 M H2O2 was irradiated with UV light, HPLC results showed that the peak corresponding to chlorobenzene decreased with time and new peaks were observed (Fig. 3)
. The plots of ln(peak area of chlorobenzene) versus time were linear and gave kobs, the observed first-order rate constant for the loss of chlorobenzene. The values of kobs varied linearly with [H2O2] and a plot of kobs against [H2O2] is shown in Fig. 4
. The slope of this plot will give the composite second-order rate constant, kOH, for the reaction of ·OH with chlorobenzene. A value of 0.17 ± 0.02 M-1 s-1 was obtained for this reaction. Considering that the intercept of this plot is significantly different than the rate constant for the loss of chlorobenzene when no H2O2 was present (AOP I), we propose that the loss of chlorobenzene follows the kinetic equation given below:
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Fig. 3. Absorption spectra of 1.8 mM chlorobenzene and 0.030 M hydrogen peroxide. Dotted line = H2O2, solid line = chlorobenzene.
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Fig. 4. Plot of the observed pseudo first-order rate constant (kobs) vs. H2O2 for chlorobenzene using UV–H2O2 system.
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This kinetic behavior is different from the one observed earlier for studies on the degradation of substituted benzene and toluene (K.O. Zahir, J. Guo, and M.J. Libardoni, unpublished data, 2001), which gave the kinetic equation of the type kobs = ko + kOH. In that case, the intercept of the plots of kobs versus [H2O2] was the same as the experimentally determined rate constant for the AOP I system. Therefore, we suggest that for chlorobenzene, in the presence of both UV and H2O2, under our experimental conditions, only H2O2 mediated degradation takes place. Figure 5
shows that at 254 nm, which is the wavelength of maxima for Hg lamp emission, the absorbance of 1.8 mM chlorobenzene is about 10 times less than the absorbance of 0.030 M H2O2. Thus, almost all of the light is being absorbed by H2O2 and the contribution from Eq. [1–8] toward cholorobenzene degradation is negligible. Kinetic results show that for chlorobenzene, the addition of hydrogen peroxide significantly enhances the degradation rate and the UV–H2O2 system is a much more efficient system than the direct UV-induced degradation.
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Fig. 5. High performance liquid chromatography (HPLC) chromatograms for chlorobenzene using UV–H2O2 system at two different times.
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The photodecomposition of H2O2 leads to the formation of ·OH along with small amounts of ·HO2 radicals (Neta et al., 1966). The reaction of hydroxyl radicals with various aromatic compounds has been studied both in the gas phase and in aqueous media (Alif et al., 1991; Anbar et al., 1973). As shown in Fig. 5, the degradation of chlorobenzene under AOP II would lead to the formation of various intermediates. These intermediates formed are usually hydroxlated aromatic products. During photolysis of chlorobenzene, these intermediates will also absorb light and undergo further photooxidation. As shown in Fig. 6a,b
, the GC–MS analysis showed that for the UV–H2O2 system, the major intermediates formed after 6 min were chlorophenol, chlorobiphenyl, and various isomers of dichlorobiphenyl. Scheme 2 (Fig. 7)
describes the possible pathways for the formation of such intermediates. It shows that one of the first steps in the degradation process may be the attack of ·OH on chlorobenzene to produce chlorophenol. The formation of both biphenyl and dichlorobiphenyl could be due to the attack of the ·OH on the aromatic ring, leading to the abstraction of hydrogen atom to produce chlorophenyl radical, which then undergoes dimerization to give dichlorobiphenyl. Based on the various intermediates formed, and the pH of the solution after 10 min of photolysis, we suggest that one of the steps involved in the degradation process is the breaking of the C–Cl bond, which ultimately leads to the formation of acidic products. A quantitative analysis of 1.8 mM chlorobenzene solution after allowing it to undergo 90% degradation using AOP II gave the concentration of the intermediates as chlorophenol (180 µM), chlorobiphenyl (110 µM), biphenyl (80 µM), and dichlorobiphenyl (190 µM). The balance of the starting 1.8 mM chlorobenzene is suspected to be already converted into carbon dioxide and water. It has been demonstrated earlier that photooxidation of such intermediates leads to ring opening (Von Sonntag et al., 1993) and eventually at longer times into carbon dioxide and water (Lipczynska-Kochany, 1992; Ollis et al., 1991).
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Fig. 6a. Gas chromatography–mass spectrometry (GC–MS) analyses of intermediates formed during UV–H2O2 induced degradation of chlorobenzene.
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Fig. 6b. Gas chromatography–mass spectrometry (GC–MS) analyses of intermediates formed during UV–H2O2 induced degradation of chlorobenzene.
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AOP III (UV–O3)
Photolytic ozonation has been known to effectively degrade organic compounds that are refractory to ozonation (Glaze and Peyton, 1988; Carey, 1992). The chemistry of ozone in solution involves a variety of reactions and is complicated when other factors such as UV and H2O2 are included. It can lead to the formations of superoxide, O-2, and HO3 and ultimately gives ·OH radical (Carey, 1992). Ozone photolysis in aqueous solution yields hydrogen peroxide [16] that along with ozone participates in secondary reactions to produce hydroxyl radical:
 | [16] |
The kinetic data in Table 1 show that the values of kobs for UV–H2O2 and UV–O3 were very similar. Considering that the hydroxyl radical is the principal active species in photolytic ozonation (Glaze and Peyton, 1988) and is also the active species formed due to UV-induced decomposition of H2O2, we propose that both processes (AOP II and III) proceed via the attack of ·OH on chlorobenzene as the rate determining step. This conclusion is further supported by the fact that the intermediates formed for the UV–O3 system were the same as the intermediate observed for the UV–H2O2 system, and kobs was found to be independent of the initial concentration of chlorobenzene.
AOP IV (UV–H2O2–O3)
As shown in Table 1, under identical conditions of chlorobenzene, O3, and H2O2 concentrations, the value of kobs obtained for the UV–H2O2–O3 system was very similar to the value obtained for the UV–H2O2 system but somewhat larger than the value obtained for the UV–O3 system. These results show a pattern similar to the one obtained for the degradation of methylene chloride for the three systems (Zeff and Barich, 1992). A rather similar value of kobs for UV–H2O2 and UV–H2O2–O3 systems may be due to the fact that the oxidizing species in both cases is the hydroxyl radical, which can be formed either as a result of direct photodecomposition of H2O2 [9] or via O3 mediated formation of H2O2 [16] followed by its photodecomposition [9]. Using the molar extinction coefficient of 19.6 M-1 cm-1 for hydrogen peroxide and 3300 M-1 cm-1 for ozone at 254 nm (Glaze et al., 1987) we conclude that under our experimental conditions, about 70% of the light is absorbed by hydrogen peroxide. Nevertheless, the combination of H2O2 and O3 would also generate sufficient levels of ·OH radicals. The GC–MS analyses of the intermediates showed that chlorophenol, chlorobiphenyl, and various isomers of dichlorbiphenyl were observed as the intermediates. These intermediates are the same as the ones observed for the UV–H2O2 and UV–O3 systems and are assumed to undergo subsequent degradation. The HPLC results also showed that after a longer period of time, there was no species present in the system that could absorb light in the 200- to 450-nm region. We suspect the final products of the degradations to be carbon dioxide and water.
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CONCLUSION
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Present studies show that among the four processes studied, the UV-induced degradation of chlorobenzene is the slowest. For the other three processes, UV–H2O2, UV–O3, and UV–H2O2–O3, the degradation was quite effective and well over 90% of chlorobenzene was degraded within 5 min. Therefore, these processes can provide an efficient and clean method for degrading chlorobenzene. However, for economical reasons and from the feasibility point of view, the UV–H2O2 system is proposed as the method of choice. The pH of the solution decreased upon photolysis and indicates the formation of acidic by-products. The most common intermediates observed were chlorophenol, biphenyl, different isomers of chlorobiphenyl, and dichlorobiphenyl. These intermediates undergo subsequent degradation leading to complete mineralization of chlorobenzene to give carbon dioxide and water.
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ACKNOWLEDGMENTS
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This work was supported in part by the financial support of Research and Sponsored Projects of the California State University, Northridge.
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REFERENCES
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ABSTRACT
INTRODUCTION
