Document k9gxQdByqXGzJpL1eN2J157JO

Chemoaphera, Vol.14, No.10, pp 1403-1494, 1985 Printed In Great Britain 004S-6S3S/05 *3 OO + .OO 1985 Perqamon Press Ltd, high-temperature gas-phase formation and DESTRUCTION OF POLYCHLORINATED DIBENZOFURANS t Wayne A. Rubey , Barry Del linger*, Douglss L. Hall, and Sueann L. Maz6i University of Dayton Research Institute Environmental Sciences Group Dayton, Ohio 45469 U.S.A, ABSTRACT - The high-temperature gas-phase decomposition of a PCB Isomer was studied using a tubular flow reactor. The formation and destruction of various 7* PCDFs were investigated in atmospheres containing different concentrations of 02- \ The formation of polychlorinated dibenzofurans (PCDFs) as a result of the partial oxidation of polychlorinated biphenyls (PCBs) hes become an area of intense interest1 due to the reported toxicity of various PCDF isomers*. Situations of particular environmental concern ares PCB-containing trans formers in a fire environment, arcing within PCB-filled capacitors, and various modes of incineration of PCB-containing wastes. The thermal degradation of PCBs, whether accidental or planned, can take place under a wide range of conditions. Previous studies of pcdf formation from PCBs have been conducted primarily at temperatures less than 650*C and with exposure durations on the order of minutes*-. Other time- and temperature-dependent parameters, such as the composition and pressure of the reaction atmosphere, have generally been undefined. In this study, the forma tion of PCDFs from PCBs has been investigated using thermal instrumentation which incorporates an adjustable isothermal flow reactor system where reaction parameters are well characterized and controlled. In this investigation, the effects of oxygen concentration and temperature were studied relative to the formation and destruction of PCDFs and the thermal stability of the parent PCB compound. Additional objectives of this study were to identify other degradation products and to provide mechanistic informa tion for PCB destruction and PCDF formation in high-temperature gaseous environments. EXPERIMENTAL ihe thermal decomposition experiments were conducted using two different thermal instrumentation systems7 ' 9 . Both of these laboratory systems are of closed continuous configuration and contain narrow-bore quartz tubular thermal +A portion of the material contained in this article was presented at the Electric Power Research Institute PCDF Workshop in Palo Alto, California, December 6, 19 04 . 14S3 HONS 213735 )404 - reactor* capable of subjecting gaseous species to thermal exposures with precise control over gas-phase residence time, exposure temperature, reaction atmospheres, and other variables. Specifically, a Thermal Decomposition Unit- Gas Chromatographic System (TDU-GC) was used for generating thermal decomposi tion profiles and for determining the relative concentrations of the various reaction products. A Thermal Decomposition Analytical System (TDAS) identified the thermal reaction products using its in-line gas chromatograph-mass spectrometer (GC-MS). ' A pure PCS isomer, 2,3 *,4,4',5-pentachlorobipheny1 (2,3 ',4 ,4',S-PCB) was selected for the studies. A 40.0 microgram sample of 2,J',4.4',S-PCB isomer was transported to the reactor at an approximate feed rate of 0.16 ug/sec. Thia resulted m a gas-phase concentration of the sample in flowing carrier of approximately 0.4 ug/cm3. After introduction to the systems aid subsequent thermal exposures, the remaining parent material and the various thermal reaction products were cryogenically trapped. Residual air was purged from the system with the use of high-purity helium, and the trapped sample was then subjected to in-line gas chromatographic analysis using an open tubular column (15 m length by 0.25 mm ID) which possessed a 0.25 micron film of bondad phenyl methyl silicone. With the TDU-GC system a hydrogen flame ionisation detector was used for detection of the various solutes, while reaction products on the TDAS were identified using the electron impact (magnetic sector) LKB 2091 mass spectrometer. Structures of observed products were assigned based on inter pretation of the mass spectra and comparison with reference spectre. The thermal degradation of the PCB isomer was examined in four different oxygen/nitrogen reaction atmospheres while the gas-phase mean residence time at exposure temperature was held constant at 2.0 seconds. The oxygen concen tration in the reaction atmosphere is described using the equivalence ratio, +, herein defined as the oxygen in the reactor divided by the oxygen required for complete combustion. The values of used in this study wsre 3.0, 1.0, 0.2, and 0.05, which range from oxygen-starved conditions to a very oxygenrich condition. Using these different equivalence ratios, experiments were conducted at temperatures ranging from 500*C to 1000*C. When samples of 2,3',4,4 *,5-PCB were thermally decomposed in atmospheres containing different concentrations of oxygen, a variety of products were formed. Figure 1 shows a gas chromatogram that is typical of the complex mixture of effluent constituents resulting from the gas-phase thermal decomposition of the PCB isomer. Table 1 lists the stable compounds identified as thermal reaction products resulting from the PCB degradation. PCDF congeners represented a considerable portion of the products. Significant quantities of partially dechlormated biphenyl congeners were also found, along with dichlorobenzenes and HONS 213736 t \ 1485 2.3',,4'.wca V------- GC COLUMN I i5 by 0.2$ Mi ID, D8-5 (0.25 ifm| PROCJUUQCD FROM CC to 2*SC EXPOSURE CDHOITIONj: tr - 2,* * T - *WC t * 1,0 uLlJW LL. 0 10 20 10 40 sin. RETTNT ION TIW Figure 1. Typical chromatogram resulting from the thermal degradation of 2,3 *,4,4 *,5-PCB. TABLE 1 MAJOR THERMAL REACTION PRODUCTS OBSERVEO TROM THE THERMAL DEGRADATION OF 2,1'.4.A', S-PENTACHLOROBIPHENYL Product Class Tetrschlorodibensofurans Trlchlorodlbenlofursns Pentschlorodibvntofur*n TetrschlorobiphonyIs Trichloroblphony1 Trlchlorobemsne Dichlorobenzens Trlchloronaphthalene Tetrschloronsphth.lene Trichlorophanylathyne* Dichlorophcnyla thyns Tetrschloroblphenylenss C,HS0C1 Ci0H)Clj Number of Mslor Pssks 2 2 1 5 i i i i i 2 1 2 i i HONS 213737 1486 trichlorobenzenes, In addition, tetrachlorobiphenylene isomers were observed, These compounds are of particular interest due to their suspected toxicity2. At 1000*C and a 6 of 1.0, the reaction product identified as tetrachloronaphthalene exceeded the concentration of the residual parent PCB, Throughout these experiments no polychlorinated dibenso-p-dioxin (PCDD) isomers were observed (minimum detection limit wae 0,1 ng). The three-dimensional display presented in Figure 2 shows the variation in yields of chlorinated benzenes, PCBs, and PCDFs, relative to equivalence ratio 4. Figures 3 and 4 are semi-logarithmic plots illustrating PCB degrada tion and PCDF formation/destruction behavior. DISCUSSION Previous laboratory studies of PCDF formation from PCBs have suggested that the formation is a purely intramolecular eyeliration reaction3'*. This implies that only a limited number of PCDF congeners can be formed from a given PCB. Four global mechanisms have been suggested! ortho-Clj loss, ortho-HCl loss, HC1 loss involving a 2,3-chlorine shift, and ortho-Hj loss. In the case of 2,3*,4,4*,5-PCB, only ortho-HCl loss and ortho-H^ loss are possible intramoleculer mechanisms. The results presented in Tables 1 and 2 indicate that HC1 loss is the dominant mechanism with Hj loss making a smaller contribution to PCDF formation. At leaat two trichlorodibenzofuran (tri-CDF) isomers were observed indicating dechlorination of the parent PCB prior to PCDF formation or the occurrence of dechlorination of the product PCDF. The presence of lower chlorinated triand tetra-chlorinated biphenyls as products suggests that the former route is possible. The product yields at these reaction conditions are expected to be con trolled kinetically, as opposed to thermodynamically, However, thermodynamic calculations may be used to estimate the relative kinetic rates of the various reaction paths, and therefore, the relative yields, The reaction enthalpy for HCl elimination to form either 2,3,7,8-tetra-CDF or 2,3,6,7-tetra-CDF n\ very exothermic with a value of -29.7 kcal/mole. elimination of ortho hydrogens to form either 1,3,4,6,7-penta-CDF or 1,3,4,7.8-penta-CDF has an enthalpy of -14.2 kcal/mole. Both reactions are exothermic, indicating that both pathways are available (although HCl elimination may be favored). For PCBs with ortho chlorines, the elimination of Cl 2 is almost thermoneutral, in this case, the activation energy which controls the reaction rate may be large and the CI2 elimination pathway may play a minor role. Comparison of the results of this study to previously reported results3-'1"12 reveals some very interesting parallels and differences. already mentioned, this and previous studies agree that the majority of observed PCDFa may be attributed to intramolecular cyclization reactions. As This HONS 213738 1487 Figure 2. Three-dimensional skeletal graphing of products formed from the thermal decomposition of 2.3',4.4'.S-PCB. Figure 3. Pc!B degradation and PCDF formation/destruction p ro f11e s. MQNS 213739 1488 Figure 4. PCft degradation and PCDF formation/destruction profilea. TABLE 2 MAXIMUM WEIGHT PERCENT HELD OF PCDF* AS A FUNCTION OF REACTION ATMOSPHERE Temperature of MAViHUH Yield (9C) For Given Tri-CDF* Weight r*rcnt Yield* Tetra-crr* Pente-CDFe Total-PCDF* o.os 0.2 1.0 1. 0 750 900 900 950 o. O.J# 0.1} o.ots 4. } 1.7 1. ] 0.71 2.0 0.55 0 25 0.21 (9 2 ,f 1.7 0 99 'Cxprtiltd weight percent Of parent PCS (assume* equivalent FID reeponee for PCS end CDri HONS 213740 14 69 waa previously demonstrated in a atudy^ of PCDF formation from 16 different PCBs. Individual PCB isomers were sealed in quartz ampoules containing an air atmosphere and heated to a temperature of 600'C. Heating periods were on the order of one minute* but the actual time at a given temperature was unknown. POOF congener identities were consistent with those predicted by the four proposed intramolecular cyclization reactions. Yields were calculated to be in the 0.1 to several percent range. Subsequent studies involving PCDF a synthesis from PCBs have confirmed these results . Several other sealed tube studies*- of the formation of PCDFs from PCBs have been reported. Individual PCB isomers and Aroclor mixtures were heated to temparatures ranging from 55*C to 850*C. Molar ratios of PCB to oxygen were typically lr7.5 to 1:75. Total PCDF yields were consistently reported in the ra^ge of 0.1 to 2.0 percent with maximum yield in the 550*C to 600*C range for one-minute heating times. The 2,3,7,8-tetra-CDF isomer was observed as predicted by the four intramolecular formation routes. The degradation of Aroclor 1254 samples resulted in the formation of PCDFs with di-, tri-. and tetrs-CDFs being the dominant congeners. Degradation of both Aroclor 1254 and 1260 yielded total PCOFa of 2 percent or less, with 2,3,7.8-tetra-CDF present as a reaction product. Similar studies6 have been conducted on Aroclor 1241 in atmospheres of oxygen, nitrogen, and air. At the highest temperature studied (330*C), the air atmosphere tests produced the greatest yield of PCDF ("-15 ppm) illustrating that intermediate oxygen levels produced the greatest yield of PCDFs. In a very recent study1, the thermal degradation of three individual PCB congeners spiked into mineral oil was studied, using a flow reactor system. Three temperatures, three oxygen levels, and three residence times were tested. It was determined that a temperature of 675*C, 0.85 seconds mean residence time, and 6 percent excess oxygen produced the greatest yield of PCDFs. Other studies have been conducted with Aroclors doped into various trans former fluids. In a study11 of the thermal degradation of Aroclor 1260 mixed into polymethylsiloxane fluid, yields of penta-CDFs and hexa-CDFs were observed at the 0,1 and 0.03 percent levels, respectively. Yields of other PCDF iso-ners were below the detection limit. Maximum yields were at 600*C in an air atmosphere. Some PCDFs were observed at 400*C, but none could be detected at 800*C or 1000*C. In a somewhat different type of laboratory study1*, test mixtures of 1richlorobenzenes/Aroclor 1254 (50/50, w/wt) and three levels of Aroclor 1254 in mineral oil (10,000 ppm, 500 ppm, and 50 ppm) were subjected to electrical arcing. Yields of PCDF congeners were less than 0.001 percent in all cases. The most obvious difference in the results presented here and those of previous studies is the yield of product PCDFs (e.g., 6.9 percent versus 2 percent or less). The experimental sequence and number of data points obtained in this study enabled a more precise determination of the "maximum" in the HONS 213741 1490 formation/destruction profiles for the PCOFs. However, the higher temperatures at which thia study waa conducted may also have resulted in a different forma tion mechanism for PCDFs. As can be seen from the data in Figures 3 and 4, the degradation rates of 2,3',4,4',5-PCB rapidly increases above approximately 700*C. This is in the region where one would expect a transition from a peroxide-dominated reaction mechanism to a free radical mechanism1^. Pseudo-equilibrium calculations1* of the concentration of small reactive species indicate that the concentration of reactive radicala increases rapidly between 700*C and 900*C (see rigure 5). Hydroxyl radicals (OH), oxygen atoms (0), hydrogen atoms (H), and chlorine atoms (Cl) are the major radicals present in the system. Since incorporation of oxygen is necessary for the formation of PCDFs from PCBs, OH and 0 are implicated as the predominant reactive species responsible for PCOF formation. For values of 4<1.0 and temperatures between 700*C and 1000*C, the OH concentration is calculated to be roughly a factor of 10 greeter than the 0 concentration, which is in turn a factor of 1000 to 10,000 greater than the H concentration. For all but the most fuel rich gaseous environments, OH would appear to be the major reactive radical. Hhen the equivalence ratio increases, tha OH and 0 concentrations decrease as the H atom concentration increases. This shift in equilibrium to non-oxygen-conteining radicals results in a decreased yield of oxygenated products such as PCOF. Thus, for large equival ence ratios, larger yields of pyrolysis products (e.g., polychlorinated benzenes, PCB congeners, chlorinated naphthalenes, chlorinated biphenylenes, etc.), sre observed. Although H atoms are usually considered the dominant reactive radical in hydrocarbon systems under pyrolytic conditions, the large concentration of Cl atoms in PCB systems may result in Cl being the dominant reactive species. Additional research on the reactivity of Cl atoms is strongly suggested. For the range of oxygen levels studied, the PCOF yield increased with equivalence ratio. Although not addressed directly in this study, one^might expect the yields of PCOFs to evantually decrease with increasing oxygen concentration due to enhanced destruction of the PCOF product as it is oxidized to simplier products including carbon monoxide and carbon dioxide. The shift in the tsuiperature for maximum yield of PCOFs as a function of equivalence ratio is a reflection of the competition between oxidation of PCB to form PCOF and oxidation of the PCDF itself. The observation that the highest temperature of maximum PCOF yield is for 4*1.0 and decreases for 4*3-0 or 40.2, may well be due to the shifting concentrations of the species responsible for PCOF formation and destruction. HONS 213742 Figure 5. Pseudo-equilibrium calculations of concentration of major radical species for pentachlorobiphenyl for 0.2. Potentlally'important elementary reactions for PCDF formation by OK attack are shown in reactions 1 through 4. M-^k- C/. CA, * oh y Wx-i HC1 Cj/y HONS 213743 1492 <3) H (O A mechanism Involving reaction* 1 and 2 would correspond to the HC1 elimination mechanisms, while a mechanism involving reactions 1, 3,4 would correspond to a net loss of Hj, Reaction 1 is ahown as a substitution reaction but may be an addition followed by H atom elimination. - Similar reaction mechanisms may be drawn for 0 atom attack. Reaction 5 followed by reaction 4 would also result in H2 elimination. > a Reaction schemes involving Cl atom loss through addition or substitution reac tion* would be expected to be energetically less favorable with lower yields of PCDFs. This would account for the lower observed yields of PCDFs formed through the mechanism involving Cl2 elimination. ^ The changss in yields of various products as a function of oxygen level and temperature are very important with respect to a practical understanding of the Mechanisms of PCB degradation. For example, internal arcing in a sealed capacitor would result in degradation of PCBs in an oxygen-deficient environ ment. Under these conditions, one would expect the formation of pyrolysis products such as other FOBS, FCBzt, and PNAs rather than PCDF. On the other hand, open burning or incineration of the PCBs may provide an atmosphere with more oxygen available to participate in the formation of partially oxidized product* such as PCDFs. However, even under fire conditions oxygen may become depleted, resulting in oxygen-starved conditions and the formation of pyrolysis products. MONS 213744 1493 The relationship* between temperature* oxygen concentration (i.e,, equivalence ratio), and PCB degradation and product formation can be used to guide the environmentally safe incineration of PCB-containing wastes. To this effect, Figures 3 and 4 describe the thermal decomposition of the parent PCB isomer and the formation of the various PCDFs at 4 values of 0.05 and 1.0, respectivsly. It is interesting that the maximum formation of the chlorinated furans shifts approximately 125*C as the equivalence ratio changes from 0.05 to 1.0. Table 2 presents the yields of observed PCDFs at various equivalence ratios. As the oxygen concentration increased by a factor of 60, the yield of total PCDFs increased by a factor of 10. The percentage of total PCDFs identified ae tetra isomers ranged from 62 to 72 percent. ^ CONCLUSIONS This study has shown that the yields of PCDFs formed from the hightemperature gas-phase degradation of 2,3',4,4',5-PCB oan be on the order of several percent, and maximum yields increase as the oxygen concentration increases within the range studied (4 " 3.0 to 0.05). The predominant mechanism for PCDF formation is HCl elimination. For equivalent combustion conditions, it appears that PCDFs can be destroyed at temperatures near those required for the destruction of parent PCBs. However, evidence also indicates the formation of chlorinated polynuclear aromatica (such as polychlorinated naphthalenes) in potsntially significant yields which persist at higher exposure temperatures. REFERENCES 1. Lustenhouwer, J. W. A., Olio, K., and Hutxinger, 0., Chemosphere, 9, 501 (1980). 2. "Human and Environmental Risks of Chlorinated Dioxin* and Related Compounds," Tucker, R. E., Voung, A. L., and Gray, A. P., ed ., Plenum Press, New York, (1983). 3. Buser, H. R., and Rappe, C., Chenosphere, 8, 157 (1979). 4. Buser, H. R-, Bosshardt, H, P., Rappe, C., and Lindahl, R., ibid, 7, 419 (197*). 5. Buser, H. R., Bosshardt, H. P,, and Rappe, C., ibid, 7, 109 (1978). 6. MOrits, M., Nakagawa, J., and Rappe, C., Bull. Environ. Contam. Toxicol., 1*, 665 (1977) . 7. Rubey, W. A., "Design considerations for a Thermal Decomposition Analytical System," US-EPA Report EPA-600/2-80-098, August (1980). 8. Rubey, W. A., Fiscus, I. B., and Torres, J. L., "Description and Operation of a Thermal Dacompoaition Unit-Gas Chromatographic System," US-EPA Report for CR-807815, March (1984). 9. "Chlorinated Dioxins and Dibeuzofurans in the Total Environment," Choudhary, G., Keith, L. H., and Rappe, C., ed., Butterworth, Woburn, MA (1963). HONS 213745 1494 10 * Erickson , M. D, f Cola , C. J. f Flora, J. d ., Jr,, Goman, p , c., Haile, c I,, j Hinahaw, G, D., Hopkins, F, C,, Swanson, S, E., and lleggem, D. T,, *PCDF Formation from PCBs Under Fira Conditions.' submitted to Chemosphere. 11. Swanson, J. W., and Tiernan, T. 0., 'Formation of Polychlorinated Dibemop'Dioxins and Dibenxofurens from Pyrolysis of Silicone Dielectric Fluid Spiked with Polychlorinated Biphenyls,* Presented at the EPRI-PCDF Workshop Palo Alto, CA, December 6, 1984, 12. Deroos, F. L., Cooke, M., Addis, C., Xoami, B., and Cuertin, J., "Formation of PCDD and PCDF in Askarel and Contaminated Mineral Oil Equipment," Presented st the EPRI-PCDF Workshop, Palo Alto, CA, December 6, 1984. 13. Banaon, S. W., "Thermochemical Kinetics," Wiley, Mew York, (1976). 14. Samuelson, G. S., The Combustion Aspects of Air Pollution, in: "Advances in Environmental Science end Technology, Volume 5," Pitta, J. ,J., Jr., end Metcalf, R. L., ed., Wiley, New York, (1975). ACKNOWLEDGMENTS The authors would like to acknowledge the assistance provided by R. A. Grant and M. D. Graham during the conducting of this work. Although the research described in this article has been funded, in pert, by the U.S. Environmental Protection Agency through Cooperative Agreement CR-310793 with the University of Dayton, it has not been subjected to Agency peer review and therefore, does not necessarily reflect the views of the Agency; no official endorsement should be inferred. (Received in Germany 2 July 1985; accepted 14 August 1965) \ HONS 213746