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C catalyst in ethanol–water solution under mild conditions
Chemosphere 68 (2007) 1716–1722 www.elsevier.com/locate/chemosphere
The study on the dechlorination of OCDD with Pd/C catalyst in ethanol–water solution under mild conditions Feng Zhang, Jiping Chen *, Haijun Zhang, Yuwen Ni, Xinmiao Liang Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, China Received 11 July 2006; received in revised form 21 December 2006; accepted 26 March 2007 Available online 11 May 2007
Abstract Dechlorination of octachlorodibenzo-p-dioxin (OCDD) was carried out in ethanol–water (v/v = 1:1) solution of NaOH in the presence of Pd/C catalysts with the use of H2. The substrate was dechlorinated with Pd/C under mild conditions (atmospheric pressure and <100 C) to give a chlorine-free product, dibenzo-p-dioxin (DD), in high yields. After reaction of 3 h at 50 C, 95.9% OCDD was degraded to low dechlorinated congeners and the yield of DD was 77.4%. We have also studied the dechlorination selectivity of chlorine atoms on the different substituted positions and postulated the dechlorination pathway of OCDD. For OCDD, the 2-position has higher reactivity than 1-position, but the difference is very small. From the distribution statistics of the intermediates during the reaction, we postulate that the steric effect plays an important role during the reaction and affect the dechlorination pathway of OCDD. 2007 Elsevier Ltd. All rights reserved. Keywords: Hydrodechlorination; Pd/C catalyst; Dechlorination pathway; OCDD
1. Introduction Dioxins (PCDDs, PCDFs, and co-PCBs) are chlorinated organic compounds with high toxicity. Because of the high toxicity and stability, the degradation of dioxins attracts great attention of the researchers all over the world. Many methods for the decomposition of PCDD/Fs and PCBs have been consequently developed. For example, the techniques of using V2O5-based catalysts for decomposition of PCDD/Fs (Ide et al., 1996), oxidative treatment using supercritical water (Sako et al., 1997) and dehalogenation by hydroxide using KOH in DMI with heating (De Pava and Battistel, 2005) seemed to present some success for detoxification. However, these methods, which involve high temperature and/or high pressure conditions, have some disadvantages in recovering the vaporized dioxins, in driving up operating costs, and in incurring the high risk of de novo synthesis of dioxins. On the other hand, a few *
Corresponding author. Tel./fax: +86 411 84379562. E-mail address: [email protected] (J. Chen).
0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.03.056
methods, such as photolytic degradation (Mcpeters and Overcash, 1993; Hilarides et al., 1994), bioremediation method (Nam et al., 2005), electrochemical reduction (Miyoshi et al., 2004) and mechanochemical treatment (Nomura et al., 2005) were proposed. However, these methods also present some unfavorable aspects concerning the use UV-light, low decomposition rates, throughput, or efficiency. Noble-metal catalysis becomes an effective tool for the dechlorination of a variety of chlorinated organic compounds for its mild reaction conditions, high degradation reaction speed and promising conversion. In recent years, a number of noble-metal catalysts including Pd/C, Pt/C and Rh/C were used for the dechlorination of PCDD/Fs. In this field, Ukisu and co-workers have performed many significant studies including dechlorination of di-, tri- and tetrachlorinated congeners of PCDD/Fs in 2-propanol solution (Ukisu and Miyadera, 2002, 2003, 2004). This method used 2-propanol as a hydrogen source instead of molecular hydrogen. The dechlorination of PCDD/Fs in the fly ash form MSW incinerators has been studied in
F. Zhang et al. / Chemosphere 68 (2007) 1716–1722
the 2-propanol solution with the use of noble metal catalysts, which was suggested can be used as a practical disposal method for dioxins (Ukisu and Miyadera, 2004). Recently, in several methods, metallic calcium was used as a mild reducing reagent to degrade dioxin and dioxinlike compounds in the alcoholic solution under atmospheric pressure and at room temperature. Mitoma et al. (2004, 2006) used calcium topromote catalytic degradation of PCDD/Fs and PCBs in alcoholic solution and achieved good results. In the previous reports of dioxin detoxification, most investigators paid more attention on the efficiency of the reaction. Few of them attached importance to the pathway of the dechlorination of the PCDD/Fs, especially to the higher chlorinated congeners. Because of the complex reaction process and the multitudinous dechlorination products, it difficult to study the dechlorination routes of higher chlorinated PCDD/Fs. For higher chlorinated PCDD/Fs, it has been noted that the toxicity may increase if partially dechlorinated products are produced during the reaction. For example, OCDD may convert into the most toxic compound, 2,3,7,8-TCDD, when the dechlorination reaction is incomplete. Therefore, it is necessary to study the dechorination pathway of the higher chlorinated PCDD/Fs. In this work, we investigated the dechlorination of OCDD, whose dechlorination products is the most complex among the congeners of PCDDs. In the ethanol–water solution, with the flow of H2, OCDD was degraded on the surface of noble-metal catalyst. Pd/C catalyst can be kept stable under reaction conditions and can be used repeatedly for the reaction. Moreover, ethanol–water, which is one of the safest solvents for humans, acts not only as a solvent but also as an accelerator due to its ability to clean the surface of catalyst (Xia et al., 2003). This decomposition method is one of the most environmentally friendly detoxification methods. The conversion of substrate and the yield of chlorine-free product DD were promising. From the distribution statistics of PCDD congeners formed during the reaction, the potential dechlorination pathway was postulated. Compared with the results of theoretical study and the thermal dechlorination of PCDD/Fs in incinerator, the difference and similarities were discussed.
the pesticide residue solvents were obtained from Tedia (USA). The carbon-supported Pd-based catalyst in powdered form containing 5 wt% of Pd was obtained from Dalian Institute of Chemical Physics CAS. Prior to use, Pd/C was heated under the flow of N2 and H2, at 200 C for 1 h. All the chemicals and gases were high-purity grade and used without further purification. The water used in the experiment was deionig water. 2.2. Procedure for OCDD dechlorination OCDD and NaOH were dissolved in the ethanol–water solution (v/v = 1:1), and the final concentrations were 1 ppm and 400 ppm, respectively. Twenty-milliliter solution and 20 mg Pd/C catalyst were added into a 100 ml three-necked flask to start the dechlorination reaction. The reaction mixture was stirred vigorously with a magnetic stirrer under the flow of H2 and was kept at a desired temperature (30–80 C) using a water bath. In the course of the reaction, an aliquot of solution was taken out for product analysis. 2.3. Extraction and analysis After internal standard solution was added, 0.2 ml reaction mixture was centrifuged, lyophilized. The recovered catalyst was underwent Soxhlet extraction thoroughly by 250 ml toluene for 24 h. The toluene extraction was passed through a multilayer silica gel column then an alumina column for the pretreatment of PCDDs analysis. The incomplete degradation products, PCDDs, were analyzed using an Autospec Ultima high resolution mass spectrometer (Micromass, UK) interfaced with a Hewlett–Packard (Palo Alto, CA, USA) 6890 Plus gas chromatograph equipped with an Rtx-2330 capillary GC column (made by Restec) (60 m · 0.25 mm · 0.1 lm). Samples were injected in splitless mode at an injector temperature of 280 C and at an initial column temperature of 90 C. After 1.5 min, the temperature was programmed at 25 C min1 to 180 C, then at 3 C min1 up to 260 C and held for 25 min. All 2.5
conc (n mol/l)
The following 13C12-labeled compounds spiking solution (EDF-8999-4) and injection internal standard solution (13C12-1,2,3,4-TCDD) for EPA 1613 were purchased from Cambridge Isotope Laboratory (Andover, MA, USA). OCDD, DD were obtained form Accustandard. All the organic solvents were pesticide residue grade. Silica gel and basic alumina used in experiment were from ICN Medical, Germany. Anhydrous sodium sulfate (Aldrich, reagent grade) was rinsed with hexane and then dried. All
DD
2.0
2. Experimental 2.1. Materials
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OCDD
1.5 1.0 0.5 0.0
0
50
100
150
200
Time (min) Fig. 1. Time profile for substrate and chlorine free product DD. Reaction conditions: substrate, 1 ppm; water-ethanol, 20 ml; Pd/C, 20 mg; 50 C.
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F. Zhang et al. / Chemosphere 68 (2007) 1716–1722
the data were obtained in the selected ion monitoring (SIM) mode. When signal to noise ratio (S/N) for a given peak was lower than 3, it was recorded as non-detected in quantification. The PCDD congeners with three or less were not detected. Another 2 ml of reaction mixture was centrifuged, freezing dried and underwent Soxhlet extraction thoroughly by 250 ml toluene for the analysis of free dechlorination product DD. The concentration of chlorine-free product DD was determined using GC 6890-FID equipped with an DB-5 column (30 m · 0.25 mm · 0.25 lm). In this case, the oven temperature was held at 130 C for 5 min, then increased at a rate of 5 C min1 up to 150 C, held for 3 min, finally increased at a rate of 30 C min1 up to 280 C and held for 30 min. The flow rate of carrier gas (N2) was 2.60 ml/min. Biphenyl was used as the internal standard.
5min
10min
30min
3. Results and discussion 3.1. Dechlorination of OCDD Dechlorination of OCDD (1 ppm) was carried out in a solution of NaOH (400 ppm) in 20 ml ethanol–water solution with flow of H2 in the presence of Pd/C (20 mg) at 50 C. Fig. 1 shows the concentration changes of OCDD and DD during dechlorination reaction of OCDD. In the first five min, the conversion of OCDD reached 78%. In the followed stage, the conversion increased slowly with time and finally reached 95.9%. The yields of chlorine-free products, DD, increased gradually, reaching 77.4% in 180 min of reaction. The time profiles of different chlorinated degradation products are shown in Fig. 2. The concentrations of TeCDDs and PeCDDs increased to a maximum value in
60min
120min
180min
250
conc (ng/g)
200 150 100 50 0 TeCDD
PeCDD
HxCDD
HpCDD
OCDD
Fig. 2. Time profile for dechlorination products of OCDD. Reaction conditions are same with Fig. 1.
Table 1 Catalytic dechlorination of OCDD at different temperature Conc (ppm)
Temperature (C)
Time (min)
Conversion (%)
Yields TeCDD
PeCDD
HxCDD
HpCDD
DD
1.00 1.00 1.00
30 50 80
180 180 180
94.20 95.95 97.69
7.50 4.31 2.12
4.44 3.80 2.52
2.80 2.03 1.07
2.54 2.02 1.21
61.71 77.45 79.71
Table 2 The yield and TEQ of toxic congeners from dechlorination of OCDD at different temperature Toxic congers
2378-TCDD 12378-PeCDD 123478-HxCDD 123678-HxCDD 123789-HxCDD 1234678-HpCDD OCDD Total
TEF
1 0.5 0.1 0.1 0.1 0.01 0.001
TEQ (ng/kg)
Yield (%)
30 C
50 C
1448.10 1212.74 125.31 264.42 426.26 145.80 58.15
1061.15 1148.48 105.03 187.02 388.41 120.86 40.49
3680.79
3051.44
80 C
30 C
50 C
80 C
250.68 422.24 48.97 91.51 153.31 65.01 23.08
0.19 0.29 0.14 0.29 0.46 1.46
0.14 0.27 0.11 0.20 0.42 1.21
0.03 0.10 0.05 0.10 0.17 0.65
1054.79
2.82
2.36
1.10
F. Zhang et al. / Chemosphere 68 (2007) 1716–1722 5min
a
10min
30min
60min
120min
1719
180min
50000
(ng/l)
40000 30000 20000
b
1289-
1267-
1269-
1239-
1278/1469-
1236/1279-
1234-
1268-
1369/1247/ 1248-
1378-
1379-
1368-
0
2378-
10000
50000
(ng/l)
40000 30000 20000
c
12389-
12367-
12346-
12489-
12467-
12369-
12469/12347-
12379-
12478-
12368-
12479/12468-
0
12378-
10000
30000
(ng/l)
20000
123467-
123469-
123679/123689-
124679/124689/ 123468-
123789-
123678-
0
123478-
10000
d 50000
(ng/l)
40000 30000 20000 10000 0 1234678-
1234679-
Fig. 3. (a–d) Time profile of different chlorinated congeners during the dechlorination reaction.
30 min and then decreased gradually, which implied that the dechlorination proceeds stepwise. The concentrations of HxCDDs and HpCDDs decreased after the first five min of reaction. The dechlorination reaction were carried out at temperature of 30 C, 50 C, and 80 C separately. Table 1 shows the conversions of substrate, yields of chlo-
rine-free products, and the profiles of different chlorinated isomers from the OCDD dechlorination. With the temperature of reaction raised, the conversions and the yields increased. In the range of experimental temperatures, the conversion and yield were obviously affected by the temperature.
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F. Zhang et al. / Chemosphere 68 (2007) 1716–1722 Cl
Cl
Cl
O
Cl
Cl
O
Cl
Cl
Cl
Cl
Cl
Cl
O
Cl
Cl
O
Cl
Cl
Cl
Cl
Cl
O
Cl
O
Cl
Cl
Cl
45% Cl
55%
Cl
Cl
Cl
O
Cl
O
Cl
O
Cl
O
Cl
Cl
Cl
Cl
27%
Cl
22% Cl
Cl
O
Cl
O
Cl
O
Cl
O
Cl
Cl
Cl
Cl
Cl
23%
40% Cl
O
Cl
O
Cl
Cl
Cl
O
Cl
O
Cl
Cl
O
Cl
O
Cl
Cl
Cl
10%
16%
42%
Fig. 4. The postulated dechlorination pathway of OCDD.
Table 2 summarizes the yields and the TEQ of toxic congeners formed during the dechlorination reaction at temperature between 30 C and 80 C after reaction undergoing for 3 h. Among the dechlorination products, about 70% of TEQ was from 2,3,7,8-TCDD and 1,2,3,7,8PeCDD. Both yields and TEQ of toxic congeners decreased sharply when the temperature of reaction rise from 50 C to 80 C.
3.2. Dechlorination pathway of OCDD In order to study the dechlorination routes of OCDD in detail, the distribution of different chlorinated products formed during the reaction was analyzed with HR-GCMS. Fig. 3. shows the isomer profile during the OCDD dechlorination reaction. Examination of distribution in Fig. 3 shows that there are specific isomers in each set of homo-
F. Zhang et al. / Chemosphere 68 (2007) 1716–1722
logues, indicating that dechlorination of OCDD occurred selectively. As shown in Fig. 3d, in the isomers of HpCDDs, the yields of 1,2,3,4,6,7,9-HpCDD is a little higher than these of 1,2,3,4,6,7,8-HpCDD and both of them remained nearly in parallel proportion during the whole reaction process, which means that when a chlorine atom was abstracted from the OCDD molecule, the chlorine atom on the 2-position has higher dechlorination reactivity than that on 1-position, even though the difference is very small. The above experimental results were consistent with these of Fueno et al. (2002) who studied the pathway of OCDD in theoretical calculation method and obtained the same results. Fig. 3a–c shows the time profiles of TeCDDs, PeCDDs and HxCDDs during the reaction. Among the isomers of HxCDDs, 1,2,3,4,6,7- and 1,2,3,4,6,9HxCDD are the major species. During the reaction, the yields of two congeners are about 50% of total yield of HxCDDs and the yields of three toxic congeners, 1,2,3,4,7,8-, 1,2,3,6,7,8- and 1,2,3,7,8,9-PeCDDs are nearly 20% of whole yields. The more interesting phenomenon appeared when the HxCDDs abstracted a chlorine atom to turn into PeCDDs. From Fig. 3b, we can see that 1,2,3,4,6- and 1,2,3,8,9-PeCDD predominate among the PeCDD isomers. Their yields are about 65% of total yields of PeCDDs. The yield of toxic congener 1,2,3,7,8-PeCDD is not more than 5% of total PeCDDs. The profiles of TeCDDs are shown in Fig. 3a, 1,2,3,4-, 1,2,8,9- and 1,2,3,9TeCDD are the maximum species among the TeCDD isomers. Their yields are about 70% of the total yield of TeCDDs. The yield of the most toxic congener 2,3,7,8TeCDD is not more than 3% in the total yield TeCDDs. Based on the different distribution of isomers during the reaction, a dechlorination pathway of OCDD is postulated in detail, as shown in Fig. 4. Fig. 4 shows that in the whole process of dechlorination reaction, the steric effect works a lot. When the chlorine atom was abstracted from the OCDD molecule, the H atom will attack the C–Cl bond preferentially around which there are less substituted chlorine atoms. For example, when a chlorine atom on 1,2,3,4,6,9-HxCDD molecule is abstracted, 1,2,3,4,6PeCDD but not 1,2,3,6,9-PeCDD, or the other isomers, is the main product, as shown in Fig. 5. This result is similar to that of Ukisu and Miyadera (2003), who studied the dechlorination of 1,2,3,4-TCDD in 2-propanol and postulated the degradation pathway of 1,2,3,4-TCDD indicated by arrows: 1,2,3,4-TCDD ! 1,2,3-TrCDD ! 1,2DCDD !1-MCDD ! DD. The dechlorination pathway of PCDD/F in liquid phase seemed to have difference from the thermal dechlorination reactions that have been reported in recent years. No matter whether in theoretical prediction (Iino et al., 2001) or in practical study of thermal dechlorination in incinerators (Iino et al., 2000), the investigators considered that the selectivity of chlorine atom on the different substituted positions was the essential factor to affect the pathway of dechlorination. The steric effects of the other substituted atoms were not taken into account. According to the above comparisons, it is obvious that the
1721
Fig. 5. The action of steric effect in the reaction.
steric effect plays a more important role in the liquid phase dechlorination reaction than thermal dechlorination reaction. 4. Conclusions Catalytic dechlorination of OCDD has been achieved in ethanol–water solution of NaOH in the presence of Pd/C catalyst with the use of H2 under mild conditions. Pd/C catalyst exhibits a high dechlorination activity. The yields of toxic dechlorination products are low and the yield of free chlorine product, DD, is promising. From the analysis of the isomer distribution of dechlorination products, the dechlorination pathway of OCDD is postulated. The selectivities of chlorine atoms on the two kinds of substituted positions have some difference. The chlorine atom on the 2-position has higher reactivity than 1-position, but the difference is very small. The reaction selectivity of chlorine atom on the different substituted positions and the steric effect of the other substituted atoms are two important factors to control the dechlorination pathway. As the reactivity difference of chorine atoms on the two kinds of substituted positions is very small, the steric effect plays a crucial role in the course of the dechlorination reaction. When a chlorine atom is abstracted from the OCDD molecule, the molecular hydrogen will attack the C–Cl bond from the site of next carbon bearing with less chlorine atoms. Base on the dechlorination pathway of OCDD, we are currently studying the degradation pathway of OCDF. References De Pava, E.V., Battistel, E., 2005. Polychlorinated dibenzo-dioxins and -furans detoxification of soil promoted by K-polyethylene glycol technology. Chemosphere 59, 1333–1342. Fueno, H., Tanaka, K., Sugawa, S., 2002. Theoretical study of the dechlorination reaction pathways of octachlorodibenzo-p-dioxin. Chemosphere 48, 771–778. Hilarides, R.J., Gray, K.A., Guzzetta, J., Cortellucci, N., Sommer, C., 1994. Radiolytic degradation of 2,3,7,8-Tcdd in artificially contaminated soils. Environ. Sci. Technol. 28, 2249–2258. Ide, Y., Kashiwabara, K., Okada, S., Mori, T., Hara, M., 1996. Catalytic decomposition of dioxin from MSW incinerator flue gas. Chemosphere 32, 189–198.
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Iino, F., Imagawa, T., Gullett, B.K., 2000. Dechlorination-controlled polychlorinated dibenzofuran isomer patterns from municipal waste incinerators. Environ. Sci. Technol. 34, 3143–3147. Iino, F., Tsuchiya, K., Imagawa, T., Gullett, B.K., 2001. An isomer prediction model for PCNs, PCDD/Fs, and PCBs from municipal waste incinerators. Environ. Sci. Technol. 35, 3175–3181. Mcpeters, A.L., Overcash, M.R., 1993. Demonstration of photodegradation by sunlight of 2,3,7,8-tetrachlorodibenzo-P-dioxin in 6 cm soil columns. Chemosphere 27, 1221–1234. Mitoma, Y., Uda, T., Egashira, N., Simion, C., Tashiro, H., Tashiro, M., Fan, X.B., 2004. Approach to highly efficient dechlorination of PCDDs, PCDFs, and coplanar PCBs using metallic calcium in ethanol under atmospheric pressure at room temperature. Environ. Sci. Technol. 38, 1216–1220. Mitoma, Y., Tasaka, N., Takase, M., Masuda, T., Tashiro, H., Egashira, N., Oki, T., 2006. Calcium-promoted catalytic degradation of PCDDs, PCDFs, and coplanar PCBs under a mild wet process. Environ. Sci. Technol. 40, 1849–1854. Miyoshi, K., Kamegaya, Y., Matsumura, M., 2004. Electrochemical reduction of organohalogen compound by noble metal sintered electrode. Chemosphere 56, 187–193. Nam, I.H., Hong, H.B., Kim, Y.M., Kim, B.H., Murugesan, K., Chang, Y.S., 2005. Biological removal of polychlorinated dibenzo-p-dioxins
from incinerator fly ash by Sphingomonas wittichii RW1. Water Res. 39, 4651–4660. Nomura, Y., Nakai, S., Hosomi, M., 2005. Elucidation of degradation mechanism of dioxins during mechanochemical treatment. Environ. Sci. Technol. 39, 3799–3804. Sako, T., Sugeta, T., Otake, K., Sato, M., Tsugumi, M., Hiaki, T., Hongo, M., 1997. Decomposition of dioxins in fly ash with supercritical water oxidation. Journal of Chemical Engineering of Japan 30, 744–747. Ukisu, Y., Miyadera, T., 2002. Dechlorination of polychlorinated dibenzo-p-dioxins catalyzed by noble metal catalysts under mild conditions. Chemosphere 46, 507–510. Ukisu, Y., Miyadera, T., 2003. Hydrogen-transfer hydrodechlorination of polychlorinated dibenzo-p-dioxins and dibenzofurans catalyzed by supported palladium catalysts. Appl. Catal. B – Environ. 40, 141–149. Ukisu, Y., Miyadera, T., 2004. Dechlorination of dioxins with supported palladium catalysts in 2-propanol solution. Appl. Catal. A – General 271, 165–170. Xia, C., Xu, J., Wu, W., Luo, Q., Chen, J., Zhang, Q., Liang, X., 2003. Catalytic hydrodechlorination of 2,4,4 0 -trichloro-2 0 -hydroxydiphenylether under mild conditions. Appl. Catal. B – Environ. 45, 281– 292.
The study on the dechlorination of OCDD with Pd/C catalyst in ethanol–water solution under mild conditions Feng Zhang, Jiping Chen *, Haijun Zhang, Yuwen Ni, Xinmiao Liang Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, China Received 11 July 2006; received in revised form 21 December 2006; accepted 26 March 2007 Available online 11 May 2007
Abstract Dechlorination of octachlorodibenzo-p-dioxin (OCDD) was carried out in ethanol–water (v/v = 1:1) solution of NaOH in the presence of Pd/C catalysts with the use of H2. The substrate was dechlorinated with Pd/C under mild conditions (atmospheric pressure and <100 C) to give a chlorine-free product, dibenzo-p-dioxin (DD), in high yields. After reaction of 3 h at 50 C, 95.9% OCDD was degraded to low dechlorinated congeners and the yield of DD was 77.4%. We have also studied the dechlorination selectivity of chlorine atoms on the different substituted positions and postulated the dechlorination pathway of OCDD. For OCDD, the 2-position has higher reactivity than 1-position, but the difference is very small. From the distribution statistics of the intermediates during the reaction, we postulate that the steric effect plays an important role during the reaction and affect the dechlorination pathway of OCDD. 2007 Elsevier Ltd. All rights reserved. Keywords: Hydrodechlorination; Pd/C catalyst; Dechlorination pathway; OCDD
1. Introduction Dioxins (PCDDs, PCDFs, and co-PCBs) are chlorinated organic compounds with high toxicity. Because of the high toxicity and stability, the degradation of dioxins attracts great attention of the researchers all over the world. Many methods for the decomposition of PCDD/Fs and PCBs have been consequently developed. For example, the techniques of using V2O5-based catalysts for decomposition of PCDD/Fs (Ide et al., 1996), oxidative treatment using supercritical water (Sako et al., 1997) and dehalogenation by hydroxide using KOH in DMI with heating (De Pava and Battistel, 2005) seemed to present some success for detoxification. However, these methods, which involve high temperature and/or high pressure conditions, have some disadvantages in recovering the vaporized dioxins, in driving up operating costs, and in incurring the high risk of de novo synthesis of dioxins. On the other hand, a few *
Corresponding author. Tel./fax: +86 411 84379562. E-mail address: [email protected] (J. Chen).
0045-6535/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2007.03.056
methods, such as photolytic degradation (Mcpeters and Overcash, 1993; Hilarides et al., 1994), bioremediation method (Nam et al., 2005), electrochemical reduction (Miyoshi et al., 2004) and mechanochemical treatment (Nomura et al., 2005) were proposed. However, these methods also present some unfavorable aspects concerning the use UV-light, low decomposition rates, throughput, or efficiency. Noble-metal catalysis becomes an effective tool for the dechlorination of a variety of chlorinated organic compounds for its mild reaction conditions, high degradation reaction speed and promising conversion. In recent years, a number of noble-metal catalysts including Pd/C, Pt/C and Rh/C were used for the dechlorination of PCDD/Fs. In this field, Ukisu and co-workers have performed many significant studies including dechlorination of di-, tri- and tetrachlorinated congeners of PCDD/Fs in 2-propanol solution (Ukisu and Miyadera, 2002, 2003, 2004). This method used 2-propanol as a hydrogen source instead of molecular hydrogen. The dechlorination of PCDD/Fs in the fly ash form MSW incinerators has been studied in
F. Zhang et al. / Chemosphere 68 (2007) 1716–1722
the 2-propanol solution with the use of noble metal catalysts, which was suggested can be used as a practical disposal method for dioxins (Ukisu and Miyadera, 2004). Recently, in several methods, metallic calcium was used as a mild reducing reagent to degrade dioxin and dioxinlike compounds in the alcoholic solution under atmospheric pressure and at room temperature. Mitoma et al. (2004, 2006) used calcium topromote catalytic degradation of PCDD/Fs and PCBs in alcoholic solution and achieved good results. In the previous reports of dioxin detoxification, most investigators paid more attention on the efficiency of the reaction. Few of them attached importance to the pathway of the dechlorination of the PCDD/Fs, especially to the higher chlorinated congeners. Because of the complex reaction process and the multitudinous dechlorination products, it difficult to study the dechlorination routes of higher chlorinated PCDD/Fs. For higher chlorinated PCDD/Fs, it has been noted that the toxicity may increase if partially dechlorinated products are produced during the reaction. For example, OCDD may convert into the most toxic compound, 2,3,7,8-TCDD, when the dechlorination reaction is incomplete. Therefore, it is necessary to study the dechorination pathway of the higher chlorinated PCDD/Fs. In this work, we investigated the dechlorination of OCDD, whose dechlorination products is the most complex among the congeners of PCDDs. In the ethanol–water solution, with the flow of H2, OCDD was degraded on the surface of noble-metal catalyst. Pd/C catalyst can be kept stable under reaction conditions and can be used repeatedly for the reaction. Moreover, ethanol–water, which is one of the safest solvents for humans, acts not only as a solvent but also as an accelerator due to its ability to clean the surface of catalyst (Xia et al., 2003). This decomposition method is one of the most environmentally friendly detoxification methods. The conversion of substrate and the yield of chlorine-free product DD were promising. From the distribution statistics of PCDD congeners formed during the reaction, the potential dechlorination pathway was postulated. Compared with the results of theoretical study and the thermal dechlorination of PCDD/Fs in incinerator, the difference and similarities were discussed.
the pesticide residue solvents were obtained from Tedia (USA). The carbon-supported Pd-based catalyst in powdered form containing 5 wt% of Pd was obtained from Dalian Institute of Chemical Physics CAS. Prior to use, Pd/C was heated under the flow of N2 and H2, at 200 C for 1 h. All the chemicals and gases were high-purity grade and used without further purification. The water used in the experiment was deionig water. 2.2. Procedure for OCDD dechlorination OCDD and NaOH were dissolved in the ethanol–water solution (v/v = 1:1), and the final concentrations were 1 ppm and 400 ppm, respectively. Twenty-milliliter solution and 20 mg Pd/C catalyst were added into a 100 ml three-necked flask to start the dechlorination reaction. The reaction mixture was stirred vigorously with a magnetic stirrer under the flow of H2 and was kept at a desired temperature (30–80 C) using a water bath. In the course of the reaction, an aliquot of solution was taken out for product analysis. 2.3. Extraction and analysis After internal standard solution was added, 0.2 ml reaction mixture was centrifuged, lyophilized. The recovered catalyst was underwent Soxhlet extraction thoroughly by 250 ml toluene for 24 h. The toluene extraction was passed through a multilayer silica gel column then an alumina column for the pretreatment of PCDDs analysis. The incomplete degradation products, PCDDs, were analyzed using an Autospec Ultima high resolution mass spectrometer (Micromass, UK) interfaced with a Hewlett–Packard (Palo Alto, CA, USA) 6890 Plus gas chromatograph equipped with an Rtx-2330 capillary GC column (made by Restec) (60 m · 0.25 mm · 0.1 lm). Samples were injected in splitless mode at an injector temperature of 280 C and at an initial column temperature of 90 C. After 1.5 min, the temperature was programmed at 25 C min1 to 180 C, then at 3 C min1 up to 260 C and held for 25 min. All 2.5
conc (n mol/l)
The following 13C12-labeled compounds spiking solution (EDF-8999-4) and injection internal standard solution (13C12-1,2,3,4-TCDD) for EPA 1613 were purchased from Cambridge Isotope Laboratory (Andover, MA, USA). OCDD, DD were obtained form Accustandard. All the organic solvents were pesticide residue grade. Silica gel and basic alumina used in experiment were from ICN Medical, Germany. Anhydrous sodium sulfate (Aldrich, reagent grade) was rinsed with hexane and then dried. All
DD
2.0
2. Experimental 2.1. Materials
1717
OCDD
1.5 1.0 0.5 0.0
0
50
100
150
200
Time (min) Fig. 1. Time profile for substrate and chlorine free product DD. Reaction conditions: substrate, 1 ppm; water-ethanol, 20 ml; Pd/C, 20 mg; 50 C.
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F. Zhang et al. / Chemosphere 68 (2007) 1716–1722
the data were obtained in the selected ion monitoring (SIM) mode. When signal to noise ratio (S/N) for a given peak was lower than 3, it was recorded as non-detected in quantification. The PCDD congeners with three or less were not detected. Another 2 ml of reaction mixture was centrifuged, freezing dried and underwent Soxhlet extraction thoroughly by 250 ml toluene for the analysis of free dechlorination product DD. The concentration of chlorine-free product DD was determined using GC 6890-FID equipped with an DB-5 column (30 m · 0.25 mm · 0.25 lm). In this case, the oven temperature was held at 130 C for 5 min, then increased at a rate of 5 C min1 up to 150 C, held for 3 min, finally increased at a rate of 30 C min1 up to 280 C and held for 30 min. The flow rate of carrier gas (N2) was 2.60 ml/min. Biphenyl was used as the internal standard.
5min
10min
30min
3. Results and discussion 3.1. Dechlorination of OCDD Dechlorination of OCDD (1 ppm) was carried out in a solution of NaOH (400 ppm) in 20 ml ethanol–water solution with flow of H2 in the presence of Pd/C (20 mg) at 50 C. Fig. 1 shows the concentration changes of OCDD and DD during dechlorination reaction of OCDD. In the first five min, the conversion of OCDD reached 78%. In the followed stage, the conversion increased slowly with time and finally reached 95.9%. The yields of chlorine-free products, DD, increased gradually, reaching 77.4% in 180 min of reaction. The time profiles of different chlorinated degradation products are shown in Fig. 2. The concentrations of TeCDDs and PeCDDs increased to a maximum value in
60min
120min
180min
250
conc (ng/g)
200 150 100 50 0 TeCDD
PeCDD
HxCDD
HpCDD
OCDD
Fig. 2. Time profile for dechlorination products of OCDD. Reaction conditions are same with Fig. 1.
Table 1 Catalytic dechlorination of OCDD at different temperature Conc (ppm)
Temperature (C)
Time (min)
Conversion (%)
Yields TeCDD
PeCDD
HxCDD
HpCDD
DD
1.00 1.00 1.00
30 50 80
180 180 180
94.20 95.95 97.69
7.50 4.31 2.12
4.44 3.80 2.52
2.80 2.03 1.07
2.54 2.02 1.21
61.71 77.45 79.71
Table 2 The yield and TEQ of toxic congeners from dechlorination of OCDD at different temperature Toxic congers
2378-TCDD 12378-PeCDD 123478-HxCDD 123678-HxCDD 123789-HxCDD 1234678-HpCDD OCDD Total
TEF
1 0.5 0.1 0.1 0.1 0.01 0.001
TEQ (ng/kg)
Yield (%)
30 C
50 C
1448.10 1212.74 125.31 264.42 426.26 145.80 58.15
1061.15 1148.48 105.03 187.02 388.41 120.86 40.49
3680.79
3051.44
80 C
30 C
50 C
80 C
250.68 422.24 48.97 91.51 153.31 65.01 23.08
0.19 0.29 0.14 0.29 0.46 1.46
0.14 0.27 0.11 0.20 0.42 1.21
0.03 0.10 0.05 0.10 0.17 0.65
1054.79
2.82
2.36
1.10
F. Zhang et al. / Chemosphere 68 (2007) 1716–1722 5min
a
10min
30min
60min
120min
1719
180min
50000
(ng/l)
40000 30000 20000
b
1289-
1267-
1269-
1239-
1278/1469-
1236/1279-
1234-
1268-
1369/1247/ 1248-
1378-
1379-
1368-
0
2378-
10000
50000
(ng/l)
40000 30000 20000
c
12389-
12367-
12346-
12489-
12467-
12369-
12469/12347-
12379-
12478-
12368-
12479/12468-
0
12378-
10000
30000
(ng/l)
20000
123467-
123469-
123679/123689-
124679/124689/ 123468-
123789-
123678-
0
123478-
10000
d 50000
(ng/l)
40000 30000 20000 10000 0 1234678-
1234679-
Fig. 3. (a–d) Time profile of different chlorinated congeners during the dechlorination reaction.
30 min and then decreased gradually, which implied that the dechlorination proceeds stepwise. The concentrations of HxCDDs and HpCDDs decreased after the first five min of reaction. The dechlorination reaction were carried out at temperature of 30 C, 50 C, and 80 C separately. Table 1 shows the conversions of substrate, yields of chlo-
rine-free products, and the profiles of different chlorinated isomers from the OCDD dechlorination. With the temperature of reaction raised, the conversions and the yields increased. In the range of experimental temperatures, the conversion and yield were obviously affected by the temperature.
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F. Zhang et al. / Chemosphere 68 (2007) 1716–1722 Cl
Cl
Cl
O
Cl
Cl
O
Cl
Cl
Cl
Cl
Cl
Cl
O
Cl
Cl
O
Cl
Cl
Cl
Cl
Cl
O
Cl
O
Cl
Cl
Cl
45% Cl
55%
Cl
Cl
Cl
O
Cl
O
Cl
O
Cl
O
Cl
Cl
Cl
Cl
27%
Cl
22% Cl
Cl
O
Cl
O
Cl
O
Cl
O
Cl
Cl
Cl
Cl
Cl
23%
40% Cl
O
Cl
O
Cl
Cl
Cl
O
Cl
O
Cl
Cl
O
Cl
O
Cl
Cl
Cl
10%
16%
42%
Fig. 4. The postulated dechlorination pathway of OCDD.
Table 2 summarizes the yields and the TEQ of toxic congeners formed during the dechlorination reaction at temperature between 30 C and 80 C after reaction undergoing for 3 h. Among the dechlorination products, about 70% of TEQ was from 2,3,7,8-TCDD and 1,2,3,7,8PeCDD. Both yields and TEQ of toxic congeners decreased sharply when the temperature of reaction rise from 50 C to 80 C.
3.2. Dechlorination pathway of OCDD In order to study the dechlorination routes of OCDD in detail, the distribution of different chlorinated products formed during the reaction was analyzed with HR-GCMS. Fig. 3. shows the isomer profile during the OCDD dechlorination reaction. Examination of distribution in Fig. 3 shows that there are specific isomers in each set of homo-
F. Zhang et al. / Chemosphere 68 (2007) 1716–1722
logues, indicating that dechlorination of OCDD occurred selectively. As shown in Fig. 3d, in the isomers of HpCDDs, the yields of 1,2,3,4,6,7,9-HpCDD is a little higher than these of 1,2,3,4,6,7,8-HpCDD and both of them remained nearly in parallel proportion during the whole reaction process, which means that when a chlorine atom was abstracted from the OCDD molecule, the chlorine atom on the 2-position has higher dechlorination reactivity than that on 1-position, even though the difference is very small. The above experimental results were consistent with these of Fueno et al. (2002) who studied the pathway of OCDD in theoretical calculation method and obtained the same results. Fig. 3a–c shows the time profiles of TeCDDs, PeCDDs and HxCDDs during the reaction. Among the isomers of HxCDDs, 1,2,3,4,6,7- and 1,2,3,4,6,9HxCDD are the major species. During the reaction, the yields of two congeners are about 50% of total yield of HxCDDs and the yields of three toxic congeners, 1,2,3,4,7,8-, 1,2,3,6,7,8- and 1,2,3,7,8,9-PeCDDs are nearly 20% of whole yields. The more interesting phenomenon appeared when the HxCDDs abstracted a chlorine atom to turn into PeCDDs. From Fig. 3b, we can see that 1,2,3,4,6- and 1,2,3,8,9-PeCDD predominate among the PeCDD isomers. Their yields are about 65% of total yields of PeCDDs. The yield of toxic congener 1,2,3,7,8-PeCDD is not more than 5% of total PeCDDs. The profiles of TeCDDs are shown in Fig. 3a, 1,2,3,4-, 1,2,8,9- and 1,2,3,9TeCDD are the maximum species among the TeCDD isomers. Their yields are about 70% of the total yield of TeCDDs. The yield of the most toxic congener 2,3,7,8TeCDD is not more than 3% in the total yield TeCDDs. Based on the different distribution of isomers during the reaction, a dechlorination pathway of OCDD is postulated in detail, as shown in Fig. 4. Fig. 4 shows that in the whole process of dechlorination reaction, the steric effect works a lot. When the chlorine atom was abstracted from the OCDD molecule, the H atom will attack the C–Cl bond preferentially around which there are less substituted chlorine atoms. For example, when a chlorine atom on 1,2,3,4,6,9-HxCDD molecule is abstracted, 1,2,3,4,6PeCDD but not 1,2,3,6,9-PeCDD, or the other isomers, is the main product, as shown in Fig. 5. This result is similar to that of Ukisu and Miyadera (2003), who studied the dechlorination of 1,2,3,4-TCDD in 2-propanol and postulated the degradation pathway of 1,2,3,4-TCDD indicated by arrows: 1,2,3,4-TCDD ! 1,2,3-TrCDD ! 1,2DCDD !1-MCDD ! DD. The dechlorination pathway of PCDD/F in liquid phase seemed to have difference from the thermal dechlorination reactions that have been reported in recent years. No matter whether in theoretical prediction (Iino et al., 2001) or in practical study of thermal dechlorination in incinerators (Iino et al., 2000), the investigators considered that the selectivity of chlorine atom on the different substituted positions was the essential factor to affect the pathway of dechlorination. The steric effects of the other substituted atoms were not taken into account. According to the above comparisons, it is obvious that the
1721
Fig. 5. The action of steric effect in the reaction.
steric effect plays a more important role in the liquid phase dechlorination reaction than thermal dechlorination reaction. 4. Conclusions Catalytic dechlorination of OCDD has been achieved in ethanol–water solution of NaOH in the presence of Pd/C catalyst with the use of H2 under mild conditions. Pd/C catalyst exhibits a high dechlorination activity. The yields of toxic dechlorination products are low and the yield of free chlorine product, DD, is promising. From the analysis of the isomer distribution of dechlorination products, the dechlorination pathway of OCDD is postulated. The selectivities of chlorine atoms on the two kinds of substituted positions have some difference. The chlorine atom on the 2-position has higher reactivity than 1-position, but the difference is very small. The reaction selectivity of chlorine atom on the different substituted positions and the steric effect of the other substituted atoms are two important factors to control the dechlorination pathway. As the reactivity difference of chorine atoms on the two kinds of substituted positions is very small, the steric effect plays a crucial role in the course of the dechlorination reaction. When a chlorine atom is abstracted from the OCDD molecule, the molecular hydrogen will attack the C–Cl bond from the site of next carbon bearing with less chlorine atoms. Base on the dechlorination pathway of OCDD, we are currently studying the degradation pathway of OCDF. References De Pava, E.V., Battistel, E., 2005. Polychlorinated dibenzo-dioxins and -furans detoxification of soil promoted by K-polyethylene glycol technology. Chemosphere 59, 1333–1342. Fueno, H., Tanaka, K., Sugawa, S., 2002. Theoretical study of the dechlorination reaction pathways of octachlorodibenzo-p-dioxin. Chemosphere 48, 771–778. Hilarides, R.J., Gray, K.A., Guzzetta, J., Cortellucci, N., Sommer, C., 1994. Radiolytic degradation of 2,3,7,8-Tcdd in artificially contaminated soils. Environ. Sci. Technol. 28, 2249–2258. Ide, Y., Kashiwabara, K., Okada, S., Mori, T., Hara, M., 1996. Catalytic decomposition of dioxin from MSW incinerator flue gas. Chemosphere 32, 189–198.
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