Article pubs.acs.org/est
Is Chlorination One of the Major Pathways in the Formation of Polychlorinated Naphthalenes (PCNs) in Municipal Solid Waste Combustion? Jae-Yong Ryu,*,† Do-Hyong Kim,‡ and Seong-Ho Jang§ †
Division for Industrial and Environmental Research, Korea Atomic Energy Research Institute (KAERI), 1266, Sinjeong-dong, Jeongeup-si, Jeollabuk-do, 580-185, Korea ‡ Land Protection Branch, Georgia Environmental Protection Division, Atlanta, Georgia, United States § Department of Bioenvironmental Energy, Pusan National University, Miryang 627-706, Korea ABSTRACT: The chlorination patterns of unsubstituted naphthalene were studied using a laminar flow reactor with a 1 cm particle bed of 0.5% (mass) copper(II) chloride (CuCl2) mixed with silicon dioxide (SiO2), operated over a temperature range of 100 to 400 °C and at gas velocities of 2.7 and 0.32 cm/s. The polychlorinated naphthalene (PCN) yield increased until a temperature reached at 250 °C, where a peak yield of 3.07% (percent of naphthalene input, carbon basis) was observed. All PCN homologue groups, mono- through octa-chlorinated naphthalenes, were observed. To test the hypothesis that PCNs in combustion processes are formed via chlorination pathways, the PCN homologue and isomer patterns from the experiments were compared with those observed in municipal solid waste combustion (MSW) incinerators. PCN congeners with 1,4-substituents dominated formation in the naphthalene chlorination experiments, whereas 2,3-substituents were major congeners in both MSW combustion flue gas and fly ash samples. These results suggest that contrary to the hypothesis, chlorination is not a primary PCN formation route in either the flue gas or fly ash from MSW combustion. Even so, naphthalene chlorination pathways presented in this paper provide an improved means for evaluating PCN formation mechanisms in combustion processes.
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were proposed.9,10 Recently, other researchers proposed dibenzofuran and PCDFs can be formed from conversion of chlorinated benzenes in copper oxide-mediated pyrolysis and oxidation process.13 Chlorination of unsubstituted naphthalene (N) is also considered to be an important route of PCN formation in combustion processes, due to the high concentration of unsubstituted N in the flue gas.14 Because of naphthalene’s chemical similarity to DD/DF, chlorination pathways may be important in the formation of PCNs as well as PCDDs/PCDFs. If so, the distribution of PCN congeners observed in field might be similar to the distributions obtained by chlorination of naphthalene. Chlorination can occur by metal catalysis, and, in particular, by copper(II) chloride (CuCl2). The Deacon process can convert HCl to Cl2, which then can lead to gas-phase N chlorination. The overall Deacon reaction is as follows.
INTRODUCTION Today polychlorinated naphthalenes (PCNs) are mainly formed and released through industrial processes such as municipal solid waste incineration as combustion byproducts along with polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs), and polychlorinated dibenzofurans (PCDFs).1−3 PCNs are structurally similar to PCDDs and PCDFs, and display similar physicochemical properties, biological effects, and toxic responses. The amount of PCNs formed from a solid waste incinerator has been reported to be the same order of magnitude as PCDD/PCDF yields.4 Compared to substantial improvements in the understanding of thermal PCDD/PCDF formation processes in the past, the formation mechanism of PCNs in combustion has been investigated far less than other halogenated compounds.5 Although formation of naphthalene at high temperatures in flames has largely been attributed to the hydrogen-abstraction/ acetylene-addition (HACA) mechanism,6−8 PCNs in the postcombustion zone are formed, along with PCDDs and PCDFs, directly from coupling of chlorinated phenoxy radicals.9,10 In our previous studies, the formation of PCNs with PCDDs/PCDFs was observed in gas-phase pyrolysis11 and oxidation12 of chlorinated phenols. Building on published mechanisms, PCN formation pathways from chlorophenols © 2013 American Chemical Society
4HCl + O2 → 2Cl 2 + 2H 2O Received: Revised: Accepted: Published: 2394
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Table 1. Composition of Halowax Standards Halowax
Cl-content (%)
chlorinated degree ClxN
concentration (μg/mL in MeOH)
1000 1001 1014 1051
26 50 62 70
x = 1,2 x = (1,2)3,4,(5) x = (3) 4,5,6,(7) x = 7,8
100 100 100 100
chlorination experiments, a 1 g particle bed of 1 cm height, consisting of silicon dioxide (SiO2: 99.6% purity, 325 mesh, Aldrich) and 0.5% (Cu mass) copper(II) chloride (CuCl2: anhydrous, 99.999% purity, Aldrich) was prepared by mechanical mixing and was located at the center of the reactor. The experimental apparatus is shown elsewhere.22 Sample Collection. The entire product gas stream was rapidly quenched at the bottom of the reactor by blowing room temperature air to facilitate product collection. The samples were collected in a dual ice-cooled dichloromethane (DCM) trap. After each experiment, the quartz tube (reactor), all fittings, and collection trap were also rinsed with DCM to remove any products deposited on the surface. The particles were also thoroughly vigorously rinsed after each experiment. Rinsed DCM was combined with the DCM in the collection trap. Sample and rinsed solutions were filtered with a polytetrafluorothylene (PTFE) membrane filter of 0.25-μL pore diameter by a vacuum filtration to remove soot, defined as the DCM-insoluble fraction. In all experiments, the quartz tube reactor was replaced with a new one. PCN Analysis. Analysis of PCN congeners was performed using GC/MS (HP 6890 series gas chromatograph with a model 5973 mass-selective detector, EI type) equipped with a HP-5MS capillary column with length 30 m, i.d. 0.25 mm, and phase 0.25-μm film thickness of cross-linked 5% phenylmethylsiloxane (J&W Scientific, California). PCN congeners were identified based on the published relative retention time and elution order of PCNs in Halowax 1001, 1014, and 1051.23−26 Due to the lack of standards of individual PCN congeners, Halowax 1001, 1014, and 1051, commercially manufactured PCN mixtures were used as PCN standards. Halowaxes used for this study were analyzed, and the results are summarized in Table 1. For quantification, the mass spectrometer was operated in selective ion mode at the two intensive and characteristic ion masses.26 Unsubstituted naphthalene was used as a universal response factor to estimate yield of each PCN product. Municipal Solid Waste Incineration Data. To evaluate whether chlorination pathways govern in PCN distributions in combustion processes, the experimentally observed PCN homologue and isomer distributions are compared with those from municipal solid waste (MSW) incinerator. Since different analytical methods (different columns) were employed to identify the PCN distributions from MSW, the PCN congeners from MSW data were regrouped to be consistent with the PCN product groups presented in the chlorination experiments. PCN Data in MSW Combustion .27 MSW combustion data used for the comparison were obtained from the flue gas of a laboratory-scale fluidized-bed reactor. Jansson and co-workers represented PCN homologue distributions and isomer patterns, ranging from mono- to octa chlorinated naphthalenes, at three different temperatures in postcombustion zone. PCN Data in Fly Ash.24 MWI data were obtained for fly ash samples produced from waste incineration. PCN isomer
Alternatively, direct chlorination of an aromatic molecule such as naphthalene by CuCl2 can occur by the following transfer mechanism.15,16 ArH + CuCl 2 → ArHCl* + CuCl
(2)
ArHCl* + CuCl 2 → ArCl + CuCl + HCl
(3)
Abundant unsubstituted phenol and monochlorinated phenols are typically present in municipal waste incinerators. The molar concentration of unsubstituted phenol was 100 times higher than the sum of other chlorinated phenols.17 Unsubstituted phenol is known to be an important precursor in the gas phase for the formation of dibenzofuran (DF) and naphthalene (N).14 It is important to understand the role of PCN congener distributions because it is present in the highest amount in municipal waste incinerator (MWI) exhaust gas. Recently, we reported distributions of the PCN products formed by CuCl2-catalyzed chlorination of naphthalene;18,19 however, no attempt was made to elucidate the PCN formation pathways leading to those observed PCN products. Building on our previous work, this study was conducted to further investigate whether the observed PCN homologue and isomer patterns from the experiments could explain those found in municipal waste incinerators. Also, the dependence of naphthalene chlorination on temperature and gas velocity is presented. The naphthalene chlorination patterns presented in this paper contribute to a comprehensive understanding of PCN formation during thermal processes.
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MATERIALS AND METHODS Experimental Details. Experiments were conducted in an electrically heated, quartz tube flow reactor, 40 cm long and 1.7 cm in diameter. High purity naphthalene reactant (40 mg, nominal) was placed in a glass vessel and heated. Reactant vapor was transported to the reactor by 92% nitrogen and 8% oxygen gas stream. The resulting gas stream, containing 0.1% reactant vapor, was introduced to the isothermal reactor. The temperature of the reactor was controlled using an electric heater. Temperature profiles inside the quartz tube reactor were measured using a thermocouple for different temperature settings and a gas flow rate of 200 standard mL/min.20 Measurements of the reactor temperature profiles indicate that temperatures at the top 8 cm and bottom 9 cm of the quartz tube are lower than the set value. The gas temperatures in the remaining 31 cm of the reactor, defined as a reaction zone, are approximately constant within ±10 °C of the set value. Radial variation of temperature was assumed to be negligible due to small tube diameter. Naphthalene chlorination experiments were conducted at temperatures ranging from 100 to 400 °C, in 50 °C increments because previous work shows that PCDD/F yields from DD/ DF chlorination are greatest in this temperature range.21 The gas velocities were 2.7 cm/s (10 s of naphthalene vapor and particle contact time) and 0.32 cm/s (0.3 s of naphthalene vapor and particle contact time). For particle-mediated 2395
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concentration peaked at 200 °C and then decreased as temperature increased. At all temperatures, MCN yields were greatest (not shown). Similar results were obtained in previous studies.21,22 PCDD/F yields from dibenzo-p-dioxin (DD) and dibenzofuran (DF) chlorination by copper(II) chloride were found to be the greatest at the range of 225−250 °C. A comparison with MSW data shows that the temperature range of PCN formation is more like that of the naphthalene chlorination experiments. PCN Homologue Patterns. Figure 2 shows the homologue fraction of PCNs formed from the naphthalene chlorination experiments (bottom) and measured from flue gas in MSW combustion (top). Since the degree of chlorination was greater and isomer patterns were broader at 300 °C with a gas velocity of 0.32 cm/sec in the previous DD/DF chlorination experiments,21,22 experiments were performed to investigate PCN homologue and isomer distributions at two different gas velocities of 2.7 and 0.32 cm/sec. The average and ± one standard deviation are shown from replicated experiments. Although complete PCN homologue profiles were observed only at a gas velocity of 0.32 cm/sec, the relative yields of PCN homologues are similar at both gas velocities. In both cases, the homologue patterns were clearly dominated by the lower chlorinated homologues. MCN fraction was approximately 0.6 and 0.55 of the PCNs at a gas velocity of 2.7 and 0.32 cm/sec. DCN fraction was approximately 0.27 and 0.28 of the PCNs at a gas velocity of 2.7 and 0.32 cm/sec, respectively. The PCN homologue fraction decreased with increasing number of chlorine substituents. H6CN through O8CN were not detected or close to the quantification limit at a gas velocity of 2.7 cm/ sec, and 0.0005% of H6CN through O8CN were formed at a gas velocity of 0.32 cm/sec. These PCN homologue distributions are consistent with the results by Jansson et al.,27 who performed experiments to investigate PCN formation and chlorination in the postcombustion zone during municipal solid waste (MSW) combustion. They found that lower chlorinated PCNs were clearly dominant, with MCN accounting for about 50% of PCNs at all experimental conditions and the O8CN below detection limits. The PCN homologue fraction decreased with increasing number of chlorine substituents in both naphthalene chlorination experiments and MSW combustion
patterns from fly ash were measured and compared with those of PCN technical mixtures (Halowax).
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RESULTS AND DISCUSSION PCN Yields. The average yields of PCNs over the temperature range of 100−400 °C at a gas velocity of 2.7
Figure 1. Comparison of average PCNs concentration from chlorination experiments and MSW.26 Error bars represent ± one standard deviation.
cm/sec (gas−particle contact time of 0.3 s) from naphthalene chlorination experiments are presented in Figure 1, expressed in unit of total naphthalene (N) conversion to PCNs. The average and ± one standard deviation are shown from replicated chlorination experiments. The total PCN concentration measured in municipal solid waste combustion flue gas sample is also shown. The average yields of PCNs from naphthalene chlorination experiments increased with increasing temperatures. A peak yield of 1.31 mmol/m3 was observed at 250 °C, and then, PCN yields decreased, similar to the results of others who published PCN concentration measured in municipal solid waste combustion flue gas sample at three different temperatures of 200, 300, and 450 °C.27 PCN
Figure 2. PCN homologue patterns formed naphthalene chlorination experiments (bottom) and measured from flue gas in MSW combustion (top). 26 Error bars represent ± one standard deviation. 2396
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Figure 3. Comparison of PCN isomer patterns. Error bars represent ± one standard deviation.
formation route involving successive chlorination of the naphthalene backbone. This is in agreement with the previous studies.22,28 PCDF and PCN homologues were closely related, and the main PCDF and PCN formation mechanisms were likely related to chlorination and/or dechlorination. Schneider et al. also found that PCN homologue fraction decreased with increasing number of chlorine substituents in the municipal waste incinerator fly samples.24 At 300 °C, the broad homologue distributions were obtained as shown in Figure 2. The homologue distributions for gas velocities of 2.7 and 0.32 cm/sec, corresponding to gas−particle contact times of 0.3 and 2.5 s, were similar, although the total PCN yields was higher for the latter. However, homologue patterns are sensitive to combustion conditions, such as a chlorine-to-hydrogen ratio and temperature.21,22,29
Figure 4. The numbering system of chlorinated naphthalene.
measurements. This result is also similar to those of previous studies.21,22 In particle-mediated dibenzofuran (DF) chlorination experiments, lower chlorinated dibenzofurans were clearly dominant, with MCDF most abundant and homologue fraction decreasing with increasing number of chlorine substituents. Jansson et al.27 have speculated that PCN patterns, favoring the least-chlorinated PCN homologue, may be attributed to a PCN 2397
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Figure 5. Observed chlorination pattern from the naphthalene chlorination experiments.
Table 2. Correlation R Values for PCN Isomer Patterns with MWI and MSW Data MCN
DCN
T3CN
T4CN
P5CN
H6CN
H7CN
1 1 −1 −1
1 0.18 −0.27 −0.17
1 0.47 −0.08 −0.17
1 0.86 0.48 0.07
1 0.94 0.12 −0.22
1 0.92 −0.04 −0.36
1 1 −1 −1
flue gas in MSW combustion fly-ash in MWI chlorination experiment Halowax
Table 3. Correlation R Values for PCN Isomer Patterns with Halowax chlorination experiment Halowax
MCN
DCN
T3CN
T4CN
P5CN
H6CN
H7CN
1 1
1 0.98
1 0.93
1 0.58
1 0.61
1 0.88
1 1
°C. As shown in Figure 3, little variation in the PCN isomer distributions was observed, even though the total PCN yield did vary significantly. This is also evidenced by the small standard deviation in isomer fractions for the three experiments. From the naphthalene chlorination experiments, complete isomer distributions, mono- through octa-chlorinated congeners, were observed. The isomer patterns from the chlorination experiments are very similar to those in Halowax; however, the observed PCN isomer pattern in the experiments were different from the isomer patterns in MWI fly ash samples and flue gas samples in MSW combustion. The MWI fly ash samples and flue gas samples have similar PCN isomer distributions in all seven isomer sets (mono through hepta). Features of the PCN isomer distributions in flue gas samples in MSW combustion are as follows:27 1- and 2-MCNs were more or less equally abundant. Among DCNs, 2,6-/1,7-DCNs were most abundant, followed by 1,3-DCN and 1,2-DCN. The
PCN Isomer Patterns. PCN isomer distributions were much less variable than homologue distributions with varying combustion conditions. To further assess the influence of naphthalene chlorination pathways in PCN formation, the PCN isomer distributions were investigated. Broader isomer distributions were obtained from the experiment at a gas velocity of 0.32 cm/sec. As shown in Figure 3, these isomer distributions were compared to those obtained from flue gas sample in MSW combustion, fly ash sample in municipal waste incinerator (MWI), and PCN technical mixture (Halowax). PCN, with genetic chemical formula of C10H8-xClx or ClxN, is a class of 75 possible congeners similar to the PCDD congener class. Isomer groupings are developed on the basis of combinations of coeluting PCN isomers. Means and standard deviations are given for the three sets of measurements from particle-mediated naphthalene chlorination experiments, flue gas samples in MSW combustion, and fly ash samples in MWI. All major PCN products yields peaked between 200 and 300 2398
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T3CN pattern was dominated by 1,2,7- T3CN, followed by 1,2,4-, 1,3,6-, 1,2,3-, and 1,3,7- T3CN. For T4CN isomers, the 1,2,3,7-/1,2,3,4-/1,2,6,7- T4CN and 1,2,5,7-/1,2,4,6-/1,2,4,7T4CN were most abundant isomers. The major P5CN isomer was dominated by 1,2,3,5,7-/1,2,4,6,7- P5CN, followed by 1,2,3,6,7- P5CN and 1,2,3,5,6- P5CN. The major H6CN isomers were 1,2,3,4,6,7- H6CN and 1,2,3,5,6,7- H6CN. For H7CN isomers, 1,2,3,4,5,6,7- H7CN was in much higher abundance than 1,2,3,4,5,6,8- H7CN. The isomer pattern of PCNs from the chlorination experiments shows a distinct difference compared to the patterns measured from the flue gas of MSW combustion and the fly ash of MWI. The numbering system of chlorinated naphthalene is shown in Figure 4. For MCN isomers, 1-MCN is more abundant than 2-MCN in both the chlorination experiments and Halowax, whereas 2MCN dominates in both MSW combustion flue gas and fly ash samples. For DCN isomers, 1,4-/1,6-DCN isomers are the major isomers in both chlorination experiments and Halowax mixture, whereas 1,2-/2,3-/2,6- isomers are dominant in both MSW combustion flue gas and fly ash samples. 1,2,7-/1,6,7-/ 2,3,6-T3CN isomers were most abundant in both MSW combustion flue gas and fly ash samples, whereas 1,4,6-T3CN isomer was the largest in both chlorination experiments and Halowax mixture. The most abundant T4CN, P5CN, H6CN, and H7CN isomers produced in both chlorination experiments and halowax are as follows: 1,2,4,6-/1,2,4,7-/1,2,5,7- T4CN, 1,2,4,6,8- P 5 CN, 1,2,4,5,6,8-/1,2,4,5,7,8- H 6 CN, and 1,2,3,4,5,6,8- H7CN. On the other hand, isomer patterns from both MSW combustion flue gas and fly ash samples do not show a dominance of these isomers. The results clearly demonstrate a highly selective chlorination pattern at 1, 4 positions. That is, the predominant naphthalene chlorination pathways leading to the PCN congeners with chlorinated at 1,4 positions are shown below. These are 1-MCN, 1,4-DCN, 1,4,6-T3CN, 1,2,5,7-/1,2,4,6-/ 1,2,4,7-T4CN, 1,2,4,6,8-P5CN, 1,2,4,5,6,8-/1,2,4,5,7,8-H6CN, 1,2,3,5,7,8-H6CN, and 12,3,4,5,6,8-H7CN as shown in Figure 5. A correlation analysis was performed to provide a qualitative assessment of PCN isomer distributions between chlorination experiments and the MSW data. For data shown in Tables 2 and 3, correlation R-values indicate that chlorination patterns on unsubstituted naphthalene are not related to the PCN isomer patterns observed in MSW combustion. The study by Jansson et al.27 suggested that chlorination on naphthalene seemed to be somewhat favored at 2,3,6,7substitution sites, whereas chlorination in the formation of PCDD and PCDF favored in the lateral 2,3,7,8-positions. Although 2,3,7,8-congeners were also seen as predominant in PCDD and PCDF formation in previous study,21,22 naphthalene chlorination appears to be favored at 1,4,6,8-positions under our experimental conditions. The experimental results and the comparison to the field data in this study suggest that PCN formation is not mainly affected by chlorination of unsubstitued naphthalene. However, results presented here on PCN congener patterns from chlorination are limited to the copper chloride system studied. To be used more effectively, more information is needed on the PCN congener patterns from other mechanisms.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +82-63-570-3345; fax: +82-63-570-3347; e-mail:
[email protected],
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.-Y.R.’s work was supported by the U.S. National Science Foundation (NSF) under grant CTS-0210089 and by the Nuclear Research and Development Program of Ministry of Education, Science and Technology and the Eco-Innovation Project through the Korea Environmental Industry & Technology Institute funded by the Ministry of Environment. We also thank the anonymous reviewers for time and comments provided that improved this paper.
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