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Electron-beam decomposition of vaporized VOCs in air
Radiation Physics and Chemistry 65 (2002) 415–421
Electron-beam decomposition of vaporized VOCs in air Koichi Hirota*, Teruyuki Hakoda, Hidehiko Arai, Shoji Hashimoto Department of Radiation Research for Environment and Resources, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, 370-1292 Gunma, Japan
Abstract A study on electron-beam treatment was carried out to find out the effective decomposition conditions of vaporized volatile organic compounds. Air streams containing aromatic and aliphatic compounds were irradiated with electron beams in batch and flow systems. The research showed that chloroethene was readily decomposed through a chain reaction with one of the fragmentation products, Cl radical. A thermal electron and negative oxygen ion were important active species for decomposing carbon tetrachloride. The formation of particles was observed only from the irradiation of aromatics, like benzene, xylene, and chlorobenzene. Dechlorination of chlorobenzene was enhanced in the presence of ammonia. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electron-beam; VOC; Decomposition; Dechlorination; Products; Mass balance
1. Introduction To achieve the standard of living we enjoy requires a considerable expenditure of energy and manufacturing of many products. However, this often occurs at the expense of a healthy environment. Many nations are very concerned with the deterioration of the environment, which is caused by the pollution of air, water, and soil. Volatile organic compounds (VOCs) are one of the sources causing that deterioration. They are produced and emitted in many industrial processes. The Environmental Agency of Japan has initiated regulations for emissions of various kinds of toxic VOCs. Also, a new regulation, Pollution Release and Transfer Register (PRTR) was established in 1999. This regulation requires VOC users to report the release and transfer of VOCs. Many manufacturing industries have to reduce the emission of VOCs. It is known that ionizing radiation is a very effective and efficient way of destroying organic pollutants in not *Corresponding author. Department of Material Development, Laboratory of Environmental Conservation Process, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, 370-1292 Gunma, Japan. Tel.: +81-27-346-9688. E-mail address: [email protected] (K. Hirota).
only waters, but also in gas streams. We have been studying the decomposition of various kinds of VOCs. In the course of study, the decomposition of VOCs by EB irradiation was shown and their decomposition products and mechanisms examined.
2. Experimental 2.1. Batch system Vaporized aromatic and aliphatic VOCs in moist air were irradiated using electron accelerators. A schematic of the batch system is shown in Fig. 1. Firstly, benzene, chlorobenzene, o-xylene, and chloroethene were sealed hermetically in a glass reactor of 50 mm 200 mm 50 mm. Chloroethene involved di-chloroethylene (trans- and cis-DCE), tri-chloroethylene (TCE), and tetra-chloroethylene (PCE). Then, the glass reactor was conveyed to the irradiation zone of an electron accelerator which supplied 3 MeV with a current of up to 25 mA. The number of passes to the zone adjusted the absorbed dose of the sample gases. The concentrations of VOCs in the reactors were measured with a gas chromatograph (GC8A,
0969-806X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 2 ) 0 0 3 5 3 - 5
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Shimadzu). A total organic carbon analyzer (TOC5000A, Shimadzu) was used to measure the total hydrocarbon and carbon dioxide. Carbon monoxide was detected with a CO analyzer (TH-D4 and TA-470 Komyo). Detailed information is available elsewhere for the analyses of the products formed in these irradiations (Hakoda et al., 2000). 2.2. Flow system In the case of the flow system, two accelerators were used. The low energy of electrons were generated by an accelerator operated on 175 keV with a current of up to 10 mA. This accelerator was used to decompose vaporized chlorobenzene and carbon tetrachloride (CCl4). A schematic of the flow system is shown in Fig. 2. Electron beams penetrated through a sheet of titanium foil attached to the top of a 70 mm 200 mm 25 mm reactor, through which the sample gases flowed. The concentration of chlorobenzene was measured using the FTIR equipped with a glass cell of 20 m pathlength. Also, this device was used to analyze the gaseous products formed in the irradiation of the aromatics and aliphatics. In the case of CCl4, a gas chromatograph (GC14B, Shimadzu) was installed downstream of the reactor to measure its concentration. The particles formed when aromatic compounds were
irradiated, were detected with a scanning mobility particle sizer (SMPS 3025A, TSI). Water concentration in the sample gases was monitored with a moisture detector (MAH-50D, Shimadzu) during irradiation. The mass balances of carbon and chlorine in the chlorobenzene irradiation were determined for the gaseous products, particles, and residues deposited on the inner surface of the reactor. They were firstly dissolved in alkaline solution, and then analyzed with an ion chromatograph (LC-10A, Shimadzu), a total chlorine analyzer (TOX-100, Mitsubishi chemical), and the total organic carbon analyzer. Carboxylate ions in the alkaline solution were identified with an organic acids analyzer (S-3000, Elyla). Another accelerator used in the flow system supplied 550 keV electrons with a current of up to 30 mA. Gaseous xylene in a 1000 m3/hN air was irradiated with this accelerator. The separation and detection of xyleneisomers was obtained using a gas chromatograph (GC 6000 Vega Series 2). The gaseous products were trapped into alkaline solution, and were followed by ion chromatographic analysis. The particle mass was measured using pre-weighed filters.
3. Results and discussion 3.1. Decomposition of aromatics
Fig. 1. Apparatus in batch system.
The aromatics in the batch system were irradiated at their concentrations of approximately 100 ppm. Their decomposition rates are shown in Fig. 3. The decomposition G-values of benzene, chlorobenzene and oxylene were 1.0, 1.1, and 2.1, respectively. They were calculated from the slope of the line fitted with all data by the least-squares method. The EB irradiation in the flow system was conducted for xylene and chlorobenzene at their initial concentrations of approximately 20 ppm. The results are shown in Fig. 4. The decomposition rate of xylene was higher than that of
Fig. 2. Apparatus in flow system.
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EB irradiation have a low probability to collide with less VOC molecules, namely low concentration. The radicals effectively reacted with the VOCs in the case of the batch system. 3.1.1. Xylene Since the decomposition of aromatics initiates through reactions with OH radicals formed by the irradiation of moist air, their OH rate constants should be in proportion to the obtained G-values. This was consistent with the results for the o-xylene irradiation, as shown in Fig. 3. Namely, the G-value of o-xylene was the highest of the three and its rate constant (14.7 10 12 cm3 molecule 1 s 1) is more than one order of magnitude higher than the others (Finlayson-Pitts and Pitts, 1986). The decomposition rates of isomeric xylenes in the flow system were roughly in proportion to their OH rate constants (Hirota et al., 1995). Fig. 3. Decomposition of aromatics in batch system: xylene (93 ppm), chlorobenzene (102 ppm), benzene (87 ppm), H2O (300 ppm).
Fig. 4. Decomposition of aromatics in flow system: xylene (22 ppm), chlorobenzene (20 ppm), H2O (1.5%).
chlorobenzene, which was similar to the results obtained in the batch system. The decomposition G-values of 0.5 and 1.8 are lower than those for chlorobenzene and xylene in the batch system, respectively. An aromatic irradiation causes the ring cleavage and forms the fragmentation products. These fragmentation products in the flow system consumed OH radicals, which led to a lower G-value. This consumption did not occur in the batch reactor, since the products were readily deposited on the surface of the glass reactor. Also, a lower initial concentration is probably another reason for the lower G-values in the flow system. The radicals formed in an
3.1.2. Chlorobenzene Although the OH rate constant for chlorobenzene is lower than that of benzene, its decomposition rate was slightly higher than that of benzene, as shown in Fig. 3. This is probably due to the formation of particles. The G-value ratios of the particle formation to the VOC decomposition (Gparticles =Gdecomposition ) were found to be 0.51 and 0.30 from the irradiation of chlorobenzene and benzene, respectively (Hakoda et al., 1998). This indicates that half of the carbon atoms in the decomposed chlorobenzene were converted to the particles. The irradiation of chlorobenzene was more likely to form the particles compared with that of benzene. There are several steps for the formation of particles, like the condensation of low-vapor pressure products, and the coagulation of nuclei. Some fragmentation products in an aromatic irradiation have low-vapor pressure, which readily condense to form the nuclei. Then, the nuclei coagulate to form the particles. In the processes of the particle formation, gaseous VOCs including their fragmentation products are caught by the nuclei and fine particles. 3.2. Products from the irradiation of aromatics The EB irradiation of aromatics produced gaseous products and particles. The analyses of the irradiated aromatics revealed that carboxylic acids and esters were gaseous fragmentation products produced from the ring cleavage. The particles contained the low-vapor pressure products and unreacted aromatics. 3.2.1. Xylene The gaseous products were identified to be formic, acetic, propionic, and butyric acids and/or the corresponding esters with CO and CO2. Approximately 30% of the reacted xylene was the gaseous products at a dose
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of 8 kGy (Hirota et al., 1995). This value was lower than that of chlorobenzene, as described below. The majority of the products in xylene irradiation were the particles, which accounted for approximately 50% of the reacted xylene on the basis of carbon-balance (mg C/Nm3) at all investigated doses. The molecular formula was determined to be C34H50NO20. 3.2.2. Chlorobenzene At least 40% of the gaseous products were found to be carboxylic acids or esters on the basis of carbonbalance (mg C/Nm3) (Hirota et al., 2000b). Formate ions (HCOO ) accounted for approximately 60% of the detected carboxylate ions in the alkaline solution trapping the gaseous products. Other ions, like a-ketoglutarate (HOOCCO(CH2)2COO ), acetate (CH3COO ), and glycolate (HOCH2COO ) were also found in the alkaline solution. In addition, approximately 25% of reacted chlorobenzene was oxidized completely to carbon dioxide at doses of 4 and 8 kGy. Carbon monoxide was also formed as a minor product. Totally, approximately 65% of the reacted chlorobenzene was the gaseous products like carboxylic acids, esters, CO, and CO2 on the basis of carbon-balance (mg C/Nm3) at doses of 4 and 8 kGy. The particles contained organic nitrate (R–O–NO2), esters, aliphatic oxygenates, and aromatics. Only 3% of the particles were found to be carboxylic acids or esters when the solution dissolving the particles was analyzed with the organic acid analyzer. In the detected carboxylate ions in the solution, a-ketoglutarate was a dominant product and no formate was detected. This result suggests that low-vapor pressure products including organic nitrate contributed to the formation of particles. The particles formed at a dose of 4 kGy had a chemical formula of C25H27NO24Cl by elemental analysis. Approximately 35% of the reacted chlorobenzene was converted to the particles at a dose of 8 kGy.
obtained in all runs. Approximately 15% of the reacted chlorine was found in the particles at doses of 4 and 8 kGy. The addition of NH3 doubled the amount of chlorine in the particles. The measurement of inorganic and organic chlorine in the products gave us more interesting results. Fig. 5a and b show inorganic and organic chlorine found in chlorobenzene irradiation. More than 60% of the reacted chlorine was found to be inorganic in the gaseous products, particles, and residues. Double energy input to the sample did not enhance the dechlorination. The presence of NH3 enhanced the dechlorination in the particles, but not in the gaseous products. As a whole, 65% of the reacted chlorine was dechlorinated at doses of 4 and 8 kGy. Approximately 80% of the dechlorination was obtained in the presence of NH3 with a 4 kGy dose.
3.3. Dechlorination of aromatics Dechlorination of chlorobenzene was investigated at doses of 4, 8, and 4 kGy in the presence of NH3. The investigation was conducted to measure chlorine in the gaseous products, particles and residues. The results are shown in Table 1. Good recoveries of chlorine were
Fig. 5. Inorganic and organic chlorine found in products from chlorobenzene irradiation.
Table 1 Chlorine balance in chlorobenzene irradiation Dose (kGy)
Input (mg Cl)
Output (mg Cl)
Particles (mg Cl)
Gas products (mg Cl)
Residues (mg Cl)
Recovery (%)
4.0 8.0 4.0 with NH3
12.5 10.6 15.2
7.5 4.5 10.1
0.8 1.0 1.7
4.0 4.3 3.2
0.2 0.7 0.5
100.0 99.1 102.0
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3.4. Decomposition of aliphatics In the case of aliphatics, the decomposition profile was greatly dependent on the chemical structure. An H abstraction occurred in the treatment of butylacetate (Hirota et al., 1995). Chloroethene initiated the decomposition through reactions with the OH radicals, followed by a chain reaction. A dissociative electron attachment is the main route for decomposing chloromethane. Thermal electrons are generated upon the irradiation of air. 3.4.1. Chloroethene The irradiation of chloroethene (DCE, TCE, and PCE) in the batch system was carried out at their concentrations of approximately 300 ppm. The results are shown in Fig. 6. A higher chlorinated ethylene was readily decomposed. A 90% decomposition of PCE, TCE, trans-DCE, and cis-DCE required the energies of 2.2, 3.5, 5.7, and 8.0 kGy, respectively. Steric hindrance probably caused a higher energy input for cis-DCE decomposition. In processing electron beams, chloroethene was decomposed first to produce Cl radicals through reactions with OH radicals (Hakoda et al., 1999). The chain reactions of the Cl radicals with PCE, TCE, trans-DCE, and cis-DCE gave higher decomposition G-values of 45, 36, 25, and 14, respectively. Chloroethene was readily decomposed compared with the aromatics.
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since it does not have a double bond in the structure. Table 2 provides the chloromethane rate constants for reactions with the OH radicals and thermal electrons (Atkinson, 1986; Schultes et al., 1975; Evans et al., 1993). The thermal electron is the main contributor for the oxidation of chloromethane. Negative oxygen ions (O2 ) also play an important role in the oxidation of CCl4 (Hirota et al., 2000a). Fig. 7 shows the decomposition profile of CCl4 at concentrations of approximately 10, 50, and 100 ppm. The highest decomposition G-value of 3.3 was obtained from the irradiation of 100 ppm CCl4, which gave the lowest decomposition rate. This indicates that the reactants, like thermal electron and O2 , were effectively consumed to decompose CCl4 at the highest concentration. The G-value of thermal-electron generation is 3.06 in air (Willis et al., 1970). A dose of 1 kGy produces 9.3 ppm of thermal electrons in air. At lower doses, the thermal electrons had higher probability to collide with CCl4 molecules since the concentration of CCl4 was higher than that of the thermal electrons. The lower the CCl4 concentration,
3.4.2. Carbon tetrachloride Chloromethane has a lower rate constant for the reactions with OH radicals compared with chloroethene,
Fig. 7. Decomposition of carbon tetrachloride in flow system: H2O (1.5%).
Table 2 Chloromethane rate constants for reactions with thermal electron and OH radical Chloromethane
Fig. 6. Decomposition of chloroethene in batch system: transDCE (321 ppm), cis-DCE (316 ppm), TCE (316 ppm), PCE (316 ppm), H2O (300 ppm).
CCl4 CHCl3 CH2Cl2 CH3Cl
e 109k (cm3 molecule 330 4.9 0.16 0.061
1
s 1)
OH 1014k (cm3 molecule 0.042 11 12 5.5
1
s 1)
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Table 3 Mass balance in trichloroethylene irradiation Dose (kGy)
Input (ppm)
Output (ppm)
CHCl2COCl (ppm)
COCl2 (ppm)
CO (ppm)
CO2 (ppm)
Recovery (%)
1.2 4.2 7.9 12.9
650 650 650 650
136 0.7 0 0
300 400 368 354
57 70 141 97
87 120 159 179
135 154 189 183
85.5 88.1 94.2 89.8
The main products were CO and CO2 in the irradiation of chloromethane (Wahyuni et al., 2000).
4. Conclusion
Fig. 8. Formation of CO and CO2 against reacted chlorobenzene as a function of decomposition rate.
the lower is the collision probability. The decomposition G-values of 1.3 and 2.7 were obtained from the EB irradiation of 10 and 50 ppm CCl4, respectively. 3.5. Products from the irradiation of aliphatics 3.5.1. Chloroethene A product study on the TCE irradiation showed that dichloroacetyl chloride (CHCl2COCl) was a primary intermediate, following by oxidation to produce phosgene and inorganic carbon (Hakoda et al., 2000). Table 3 provides the mass balance in TCE irradiation. A higher energy input led to the oxidation of the primary and even secondary products. The products listed in Table 3 were also identified in the irradiation of DCE and PCE. With an alkaline solution, the products except CO were completely removed from the gas phase. 3.5.2. Carbon tetrachloride Fig. 8 shows the concentration ratio of inorganic carbons to the reacted CCl4 as a function of the decomposition rate of CCl4. The ratio increased with the decomposition rate. The reacted CCl4 was completely dechlorinated beyond a 60% decomposition rate.
Vaporized aromatic VOCs in moist air were irradiated with electron beams. The decomposition of the aromatics initiated though reactions with OH radicals. The irradiation produced particles and aliphatic oxygenates, like carboxylic acids, and esters. Approximately 50% of the reacted xylene converted to the particles on the basis of carbon-balance (mg C/Nm3), which contributed to its higher decomposition. The presence of NH3 enhanced the dechlorination of chlorobenzene. On the other hand, a chain reaction occurred through reactions with Cl radicals in the irradiation of chloroethene. A thermal electron was an important oxidant in the case of chloromethane. Inorganic carbons are the main products in the irradiation of chloroethene and chloromethane.
References Atkinson, R., 1986. Kinetics and mechanisms of the gas-phase reactions of hydroxyl radical with organic compounds under atmospheric conditions. Chem. Rev. 86, 69. Evans, D., Rosocha, L.A., Anderson, G.K., Coogan, J.J., 1993. Plasma remediation of trichloroethylene in silent discharge plasmas. J. Appl. Phys. 74, 5378. Finlayson-Pitts, B.J., Pitts Jr., J.N., 1986. Atmospheric Chemistry. Wiley Interscience, New York. Hakoda, T., Yang, M., Hirota, K., Hashimoto, S., 1998. Decomposition of volatile organic compounds in air by electron beam and gamma ray irradiation. J. Adv. Oxid. Technol. 3, 79. Hakoda, T., Zhang, G., Hashimoto, S., 1999. Decomposition of chloroethenes in electron beam irradiation. Radiat. Phys. Chem. 54, 541. Hakoda, T., Hashimoto, S., Fujiyama, Y., Mizuno, A., 2000. Decomposition mechanism for electron beam irradiation of vaporized trichloroethylene-air mixtures. J. Phys. Chem. A 104, 59. Hirota, K., M.atzing, H., Paur, H.-R., Woletz, K., 1995. Analyses of products formed by electron beam treatment of VOC/air mixtures. Radiat. Phys. Chem. 45, 649.
K. Hirota et al. / Radiation Physics and Chemistry 65 (2002) 415–421 Hirota, K., Arai, H., Hashimoto, S., 2000a. Electron-beam decomposition of cabon tetrachloride in air/nitrogen. Bull. Chem. Soc. Jpn. 73, 2719. Hirota, K., Hakoda, T., Arai, H., Hashimoto, S., 2000b. Dechlorination of chlorobenzene in air with electron beam. Radiat. Phys. Chem. 57, 63. Schultes, E., Christodoulides, A.A., Schindler, R.N., 1975. Studies by the electron cyclotron resonance (ECR) technique. Chem. Phys. 8, 354.
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Wahyuni, S., Hirota, K., Hakoda, T., Arai, H., Hashimoto, S., Kawamoto, F., Mukunoki, Y., 2000. Enhanced decomposition of dichloromethane in air by multi-pass electron beam irradiation. Bull. Chem. Soc. Jpn. 73, 1939. Willis, C., Boyd, A.W., Young, M.J., 1970. Radiolysis of air and nitrogen–oxygen mixtures with intense electron pulses: determination of a mechanism by comparison of measured and computed yields. Can. J. Chem. 48, 1515.
Electron-beam decomposition of vaporized VOCs in air Koichi Hirota*, Teruyuki Hakoda, Hidehiko Arai, Shoji Hashimoto Department of Radiation Research for Environment and Resources, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, 370-1292 Gunma, Japan
Abstract A study on electron-beam treatment was carried out to find out the effective decomposition conditions of vaporized volatile organic compounds. Air streams containing aromatic and aliphatic compounds were irradiated with electron beams in batch and flow systems. The research showed that chloroethene was readily decomposed through a chain reaction with one of the fragmentation products, Cl radical. A thermal electron and negative oxygen ion were important active species for decomposing carbon tetrachloride. The formation of particles was observed only from the irradiation of aromatics, like benzene, xylene, and chlorobenzene. Dechlorination of chlorobenzene was enhanced in the presence of ammonia. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Electron-beam; VOC; Decomposition; Dechlorination; Products; Mass balance
1. Introduction To achieve the standard of living we enjoy requires a considerable expenditure of energy and manufacturing of many products. However, this often occurs at the expense of a healthy environment. Many nations are very concerned with the deterioration of the environment, which is caused by the pollution of air, water, and soil. Volatile organic compounds (VOCs) are one of the sources causing that deterioration. They are produced and emitted in many industrial processes. The Environmental Agency of Japan has initiated regulations for emissions of various kinds of toxic VOCs. Also, a new regulation, Pollution Release and Transfer Register (PRTR) was established in 1999. This regulation requires VOC users to report the release and transfer of VOCs. Many manufacturing industries have to reduce the emission of VOCs. It is known that ionizing radiation is a very effective and efficient way of destroying organic pollutants in not *Corresponding author. Department of Material Development, Laboratory of Environmental Conservation Process, Japan Atomic Energy Research Institute, 1233 Watanuki, Takasaki, 370-1292 Gunma, Japan. Tel.: +81-27-346-9688. E-mail address: [email protected] (K. Hirota).
only waters, but also in gas streams. We have been studying the decomposition of various kinds of VOCs. In the course of study, the decomposition of VOCs by EB irradiation was shown and their decomposition products and mechanisms examined.
2. Experimental 2.1. Batch system Vaporized aromatic and aliphatic VOCs in moist air were irradiated using electron accelerators. A schematic of the batch system is shown in Fig. 1. Firstly, benzene, chlorobenzene, o-xylene, and chloroethene were sealed hermetically in a glass reactor of 50 mm 200 mm 50 mm. Chloroethene involved di-chloroethylene (trans- and cis-DCE), tri-chloroethylene (TCE), and tetra-chloroethylene (PCE). Then, the glass reactor was conveyed to the irradiation zone of an electron accelerator which supplied 3 MeV with a current of up to 25 mA. The number of passes to the zone adjusted the absorbed dose of the sample gases. The concentrations of VOCs in the reactors were measured with a gas chromatograph (GC8A,
0969-806X/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 9 - 8 0 6 X ( 0 2 ) 0 0 3 5 3 - 5
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Shimadzu). A total organic carbon analyzer (TOC5000A, Shimadzu) was used to measure the total hydrocarbon and carbon dioxide. Carbon monoxide was detected with a CO analyzer (TH-D4 and TA-470 Komyo). Detailed information is available elsewhere for the analyses of the products formed in these irradiations (Hakoda et al., 2000). 2.2. Flow system In the case of the flow system, two accelerators were used. The low energy of electrons were generated by an accelerator operated on 175 keV with a current of up to 10 mA. This accelerator was used to decompose vaporized chlorobenzene and carbon tetrachloride (CCl4). A schematic of the flow system is shown in Fig. 2. Electron beams penetrated through a sheet of titanium foil attached to the top of a 70 mm 200 mm 25 mm reactor, through which the sample gases flowed. The concentration of chlorobenzene was measured using the FTIR equipped with a glass cell of 20 m pathlength. Also, this device was used to analyze the gaseous products formed in the irradiation of the aromatics and aliphatics. In the case of CCl4, a gas chromatograph (GC14B, Shimadzu) was installed downstream of the reactor to measure its concentration. The particles formed when aromatic compounds were
irradiated, were detected with a scanning mobility particle sizer (SMPS 3025A, TSI). Water concentration in the sample gases was monitored with a moisture detector (MAH-50D, Shimadzu) during irradiation. The mass balances of carbon and chlorine in the chlorobenzene irradiation were determined for the gaseous products, particles, and residues deposited on the inner surface of the reactor. They were firstly dissolved in alkaline solution, and then analyzed with an ion chromatograph (LC-10A, Shimadzu), a total chlorine analyzer (TOX-100, Mitsubishi chemical), and the total organic carbon analyzer. Carboxylate ions in the alkaline solution were identified with an organic acids analyzer (S-3000, Elyla). Another accelerator used in the flow system supplied 550 keV electrons with a current of up to 30 mA. Gaseous xylene in a 1000 m3/hN air was irradiated with this accelerator. The separation and detection of xyleneisomers was obtained using a gas chromatograph (GC 6000 Vega Series 2). The gaseous products were trapped into alkaline solution, and were followed by ion chromatographic analysis. The particle mass was measured using pre-weighed filters.
3. Results and discussion 3.1. Decomposition of aromatics
Fig. 1. Apparatus in batch system.
The aromatics in the batch system were irradiated at their concentrations of approximately 100 ppm. Their decomposition rates are shown in Fig. 3. The decomposition G-values of benzene, chlorobenzene and oxylene were 1.0, 1.1, and 2.1, respectively. They were calculated from the slope of the line fitted with all data by the least-squares method. The EB irradiation in the flow system was conducted for xylene and chlorobenzene at their initial concentrations of approximately 20 ppm. The results are shown in Fig. 4. The decomposition rate of xylene was higher than that of
Fig. 2. Apparatus in flow system.
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EB irradiation have a low probability to collide with less VOC molecules, namely low concentration. The radicals effectively reacted with the VOCs in the case of the batch system. 3.1.1. Xylene Since the decomposition of aromatics initiates through reactions with OH radicals formed by the irradiation of moist air, their OH rate constants should be in proportion to the obtained G-values. This was consistent with the results for the o-xylene irradiation, as shown in Fig. 3. Namely, the G-value of o-xylene was the highest of the three and its rate constant (14.7 10 12 cm3 molecule 1 s 1) is more than one order of magnitude higher than the others (Finlayson-Pitts and Pitts, 1986). The decomposition rates of isomeric xylenes in the flow system were roughly in proportion to their OH rate constants (Hirota et al., 1995). Fig. 3. Decomposition of aromatics in batch system: xylene (93 ppm), chlorobenzene (102 ppm), benzene (87 ppm), H2O (300 ppm).
Fig. 4. Decomposition of aromatics in flow system: xylene (22 ppm), chlorobenzene (20 ppm), H2O (1.5%).
chlorobenzene, which was similar to the results obtained in the batch system. The decomposition G-values of 0.5 and 1.8 are lower than those for chlorobenzene and xylene in the batch system, respectively. An aromatic irradiation causes the ring cleavage and forms the fragmentation products. These fragmentation products in the flow system consumed OH radicals, which led to a lower G-value. This consumption did not occur in the batch reactor, since the products were readily deposited on the surface of the glass reactor. Also, a lower initial concentration is probably another reason for the lower G-values in the flow system. The radicals formed in an
3.1.2. Chlorobenzene Although the OH rate constant for chlorobenzene is lower than that of benzene, its decomposition rate was slightly higher than that of benzene, as shown in Fig. 3. This is probably due to the formation of particles. The G-value ratios of the particle formation to the VOC decomposition (Gparticles =Gdecomposition ) were found to be 0.51 and 0.30 from the irradiation of chlorobenzene and benzene, respectively (Hakoda et al., 1998). This indicates that half of the carbon atoms in the decomposed chlorobenzene were converted to the particles. The irradiation of chlorobenzene was more likely to form the particles compared with that of benzene. There are several steps for the formation of particles, like the condensation of low-vapor pressure products, and the coagulation of nuclei. Some fragmentation products in an aromatic irradiation have low-vapor pressure, which readily condense to form the nuclei. Then, the nuclei coagulate to form the particles. In the processes of the particle formation, gaseous VOCs including their fragmentation products are caught by the nuclei and fine particles. 3.2. Products from the irradiation of aromatics The EB irradiation of aromatics produced gaseous products and particles. The analyses of the irradiated aromatics revealed that carboxylic acids and esters were gaseous fragmentation products produced from the ring cleavage. The particles contained the low-vapor pressure products and unreacted aromatics. 3.2.1. Xylene The gaseous products were identified to be formic, acetic, propionic, and butyric acids and/or the corresponding esters with CO and CO2. Approximately 30% of the reacted xylene was the gaseous products at a dose
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of 8 kGy (Hirota et al., 1995). This value was lower than that of chlorobenzene, as described below. The majority of the products in xylene irradiation were the particles, which accounted for approximately 50% of the reacted xylene on the basis of carbon-balance (mg C/Nm3) at all investigated doses. The molecular formula was determined to be C34H50NO20. 3.2.2. Chlorobenzene At least 40% of the gaseous products were found to be carboxylic acids or esters on the basis of carbonbalance (mg C/Nm3) (Hirota et al., 2000b). Formate ions (HCOO ) accounted for approximately 60% of the detected carboxylate ions in the alkaline solution trapping the gaseous products. Other ions, like a-ketoglutarate (HOOCCO(CH2)2COO ), acetate (CH3COO ), and glycolate (HOCH2COO ) were also found in the alkaline solution. In addition, approximately 25% of reacted chlorobenzene was oxidized completely to carbon dioxide at doses of 4 and 8 kGy. Carbon monoxide was also formed as a minor product. Totally, approximately 65% of the reacted chlorobenzene was the gaseous products like carboxylic acids, esters, CO, and CO2 on the basis of carbon-balance (mg C/Nm3) at doses of 4 and 8 kGy. The particles contained organic nitrate (R–O–NO2), esters, aliphatic oxygenates, and aromatics. Only 3% of the particles were found to be carboxylic acids or esters when the solution dissolving the particles was analyzed with the organic acid analyzer. In the detected carboxylate ions in the solution, a-ketoglutarate was a dominant product and no formate was detected. This result suggests that low-vapor pressure products including organic nitrate contributed to the formation of particles. The particles formed at a dose of 4 kGy had a chemical formula of C25H27NO24Cl by elemental analysis. Approximately 35% of the reacted chlorobenzene was converted to the particles at a dose of 8 kGy.
obtained in all runs. Approximately 15% of the reacted chlorine was found in the particles at doses of 4 and 8 kGy. The addition of NH3 doubled the amount of chlorine in the particles. The measurement of inorganic and organic chlorine in the products gave us more interesting results. Fig. 5a and b show inorganic and organic chlorine found in chlorobenzene irradiation. More than 60% of the reacted chlorine was found to be inorganic in the gaseous products, particles, and residues. Double energy input to the sample did not enhance the dechlorination. The presence of NH3 enhanced the dechlorination in the particles, but not in the gaseous products. As a whole, 65% of the reacted chlorine was dechlorinated at doses of 4 and 8 kGy. Approximately 80% of the dechlorination was obtained in the presence of NH3 with a 4 kGy dose.
3.3. Dechlorination of aromatics Dechlorination of chlorobenzene was investigated at doses of 4, 8, and 4 kGy in the presence of NH3. The investigation was conducted to measure chlorine in the gaseous products, particles and residues. The results are shown in Table 1. Good recoveries of chlorine were
Fig. 5. Inorganic and organic chlorine found in products from chlorobenzene irradiation.
Table 1 Chlorine balance in chlorobenzene irradiation Dose (kGy)
Input (mg Cl)
Output (mg Cl)
Particles (mg Cl)
Gas products (mg Cl)
Residues (mg Cl)
Recovery (%)
4.0 8.0 4.0 with NH3
12.5 10.6 15.2
7.5 4.5 10.1
0.8 1.0 1.7
4.0 4.3 3.2
0.2 0.7 0.5
100.0 99.1 102.0
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3.4. Decomposition of aliphatics In the case of aliphatics, the decomposition profile was greatly dependent on the chemical structure. An H abstraction occurred in the treatment of butylacetate (Hirota et al., 1995). Chloroethene initiated the decomposition through reactions with the OH radicals, followed by a chain reaction. A dissociative electron attachment is the main route for decomposing chloromethane. Thermal electrons are generated upon the irradiation of air. 3.4.1. Chloroethene The irradiation of chloroethene (DCE, TCE, and PCE) in the batch system was carried out at their concentrations of approximately 300 ppm. The results are shown in Fig. 6. A higher chlorinated ethylene was readily decomposed. A 90% decomposition of PCE, TCE, trans-DCE, and cis-DCE required the energies of 2.2, 3.5, 5.7, and 8.0 kGy, respectively. Steric hindrance probably caused a higher energy input for cis-DCE decomposition. In processing electron beams, chloroethene was decomposed first to produce Cl radicals through reactions with OH radicals (Hakoda et al., 1999). The chain reactions of the Cl radicals with PCE, TCE, trans-DCE, and cis-DCE gave higher decomposition G-values of 45, 36, 25, and 14, respectively. Chloroethene was readily decomposed compared with the aromatics.
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since it does not have a double bond in the structure. Table 2 provides the chloromethane rate constants for reactions with the OH radicals and thermal electrons (Atkinson, 1986; Schultes et al., 1975; Evans et al., 1993). The thermal electron is the main contributor for the oxidation of chloromethane. Negative oxygen ions (O2 ) also play an important role in the oxidation of CCl4 (Hirota et al., 2000a). Fig. 7 shows the decomposition profile of CCl4 at concentrations of approximately 10, 50, and 100 ppm. The highest decomposition G-value of 3.3 was obtained from the irradiation of 100 ppm CCl4, which gave the lowest decomposition rate. This indicates that the reactants, like thermal electron and O2 , were effectively consumed to decompose CCl4 at the highest concentration. The G-value of thermal-electron generation is 3.06 in air (Willis et al., 1970). A dose of 1 kGy produces 9.3 ppm of thermal electrons in air. At lower doses, the thermal electrons had higher probability to collide with CCl4 molecules since the concentration of CCl4 was higher than that of the thermal electrons. The lower the CCl4 concentration,
3.4.2. Carbon tetrachloride Chloromethane has a lower rate constant for the reactions with OH radicals compared with chloroethene,
Fig. 7. Decomposition of carbon tetrachloride in flow system: H2O (1.5%).
Table 2 Chloromethane rate constants for reactions with thermal electron and OH radical Chloromethane
Fig. 6. Decomposition of chloroethene in batch system: transDCE (321 ppm), cis-DCE (316 ppm), TCE (316 ppm), PCE (316 ppm), H2O (300 ppm).
CCl4 CHCl3 CH2Cl2 CH3Cl
e 109k (cm3 molecule 330 4.9 0.16 0.061
1
s 1)
OH 1014k (cm3 molecule 0.042 11 12 5.5
1
s 1)
K. Hirota et al. / Radiation Physics and Chemistry 65 (2002) 415–421
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Table 3 Mass balance in trichloroethylene irradiation Dose (kGy)
Input (ppm)
Output (ppm)
CHCl2COCl (ppm)
COCl2 (ppm)
CO (ppm)
CO2 (ppm)
Recovery (%)
1.2 4.2 7.9 12.9
650 650 650 650
136 0.7 0 0
300 400 368 354
57 70 141 97
87 120 159 179
135 154 189 183
85.5 88.1 94.2 89.8
The main products were CO and CO2 in the irradiation of chloromethane (Wahyuni et al., 2000).
4. Conclusion
Fig. 8. Formation of CO and CO2 against reacted chlorobenzene as a function of decomposition rate.
the lower is the collision probability. The decomposition G-values of 1.3 and 2.7 were obtained from the EB irradiation of 10 and 50 ppm CCl4, respectively. 3.5. Products from the irradiation of aliphatics 3.5.1. Chloroethene A product study on the TCE irradiation showed that dichloroacetyl chloride (CHCl2COCl) was a primary intermediate, following by oxidation to produce phosgene and inorganic carbon (Hakoda et al., 2000). Table 3 provides the mass balance in TCE irradiation. A higher energy input led to the oxidation of the primary and even secondary products. The products listed in Table 3 were also identified in the irradiation of DCE and PCE. With an alkaline solution, the products except CO were completely removed from the gas phase. 3.5.2. Carbon tetrachloride Fig. 8 shows the concentration ratio of inorganic carbons to the reacted CCl4 as a function of the decomposition rate of CCl4. The ratio increased with the decomposition rate. The reacted CCl4 was completely dechlorinated beyond a 60% decomposition rate.
Vaporized aromatic VOCs in moist air were irradiated with electron beams. The decomposition of the aromatics initiated though reactions with OH radicals. The irradiation produced particles and aliphatic oxygenates, like carboxylic acids, and esters. Approximately 50% of the reacted xylene converted to the particles on the basis of carbon-balance (mg C/Nm3), which contributed to its higher decomposition. The presence of NH3 enhanced the dechlorination of chlorobenzene. On the other hand, a chain reaction occurred through reactions with Cl radicals in the irradiation of chloroethene. A thermal electron was an important oxidant in the case of chloromethane. Inorganic carbons are the main products in the irradiation of chloroethene and chloromethane.
References Atkinson, R., 1986. Kinetics and mechanisms of the gas-phase reactions of hydroxyl radical with organic compounds under atmospheric conditions. Chem. Rev. 86, 69. Evans, D., Rosocha, L.A., Anderson, G.K., Coogan, J.J., 1993. Plasma remediation of trichloroethylene in silent discharge plasmas. J. Appl. Phys. 74, 5378. Finlayson-Pitts, B.J., Pitts Jr., J.N., 1986. Atmospheric Chemistry. Wiley Interscience, New York. Hakoda, T., Yang, M., Hirota, K., Hashimoto, S., 1998. Decomposition of volatile organic compounds in air by electron beam and gamma ray irradiation. J. Adv. Oxid. Technol. 3, 79. Hakoda, T., Zhang, G., Hashimoto, S., 1999. Decomposition of chloroethenes in electron beam irradiation. Radiat. Phys. Chem. 54, 541. Hakoda, T., Hashimoto, S., Fujiyama, Y., Mizuno, A., 2000. Decomposition mechanism for electron beam irradiation of vaporized trichloroethylene-air mixtures. J. Phys. Chem. A 104, 59. Hirota, K., M.atzing, H., Paur, H.-R., Woletz, K., 1995. Analyses of products formed by electron beam treatment of VOC/air mixtures. Radiat. Phys. Chem. 45, 649.
K. Hirota et al. / Radiation Physics and Chemistry 65 (2002) 415–421 Hirota, K., Arai, H., Hashimoto, S., 2000a. Electron-beam decomposition of cabon tetrachloride in air/nitrogen. Bull. Chem. Soc. Jpn. 73, 2719. Hirota, K., Hakoda, T., Arai, H., Hashimoto, S., 2000b. Dechlorination of chlorobenzene in air with electron beam. Radiat. Phys. Chem. 57, 63. Schultes, E., Christodoulides, A.A., Schindler, R.N., 1975. Studies by the electron cyclotron resonance (ECR) technique. Chem. Phys. 8, 354.
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Wahyuni, S., Hirota, K., Hakoda, T., Arai, H., Hashimoto, S., Kawamoto, F., Mukunoki, Y., 2000. Enhanced decomposition of dichloromethane in air by multi-pass electron beam irradiation. Bull. Chem. Soc. Jpn. 73, 1939. Willis, C., Boyd, A.W., Young, M.J., 1970. Radiolysis of air and nitrogen–oxygen mixtures with intense electron pulses: determination of a mechanism by comparison of measured and computed yields. Can. J. Chem. 48, 1515.