Document nmQLK2OYYrmOOb71gpnNZ48Z2

Cheircephsrs , Vol.14, No.10, pp 1483-1494, 1985 Printed In Greet Briteln 0045-6535/85 $3.00 .00 01985 Psroamon Preee Ltd. high-temperature gas-phase formation and DESTRUCTION OF POLYCHLORINATED DIBCNIOFURAHS* Wayne A. Rubc-y , Berry Dellinger*, Douglas L. Hell, end Sueenn L. Merer University of Deyton Reeeerch Institute Environments! Sciences Group Deyton, Ohio 45469 U.S.A, ABSTRACT - The high-temperature ges-phess decomposition of s PCB isomer was ._ studied using e tubuler flow reector. The formation end destruction of verious i < PCDFs were lnvsstigeted in etmospheree conteining different concentrations of Oj. \ The formation of polychlorinated dibenzofursne (PCDFs) as a result of the partiei oxidstlon of polychlorinated biphenyls (PCBe) has become an arse of intense interest1 due to the reported toxicity of verious PCDF isomers^. Situations of psrtlcular environmental concern are: PCB-containing trans formers in s fire environment, arcing within PCB-fllled capacitors, end various modes of incineration of PCB-containlng wastes. The thermal degradation of PCBe, whether eccidental 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 6S0*C and with exposure durstione on the order of minutes1-*. Other time- snd temperature-dependent parameters, such se the composition and pressure of the reaction atmosphere, have generally been undefined, in this study, the forma tion of PCDFs from PCBe has been investigated using thermal instrumentation which incorporates an adjustable isothermal flow reactor system where reaction parameters are well characterized snd controlled. In this investigation, the effects of oxygen concentration and temperature were studied relative to the formation snd destruction of PCDFs and the thermal stebillty 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 Tha thermal decomposition experiments were conducted using two different thermal instrumcntetlon systems7'*. Both of these laboratory systems are of closed continuous configuration and contain narrow-bore quartz tubular thermal tA 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 . 1483 HONS 214123 1464 - raactor* capable of subjecting gaseous species to thermal exposures with precis* control over gas-phase residence time, exposure tenperatura. reaction atmospheres, and othar variables. Specifically, s Thermal Decomposition UnitGas Chromatographic System (TOU-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 laomer, 2,3',4,4',5-pentachlorobiphenyl (2,3*,4,4',5-PCB) was selected for the studies. A 40.0 mlcrogram sample of 2,3',4,4' ,5-PCB isomer was transported to the reactor at an approximate feed rate of 0.16 ug/sec. This resulted in a gas-phase concentration of the samp la in flowing carrier of approximately 0.4 ug/cml. After Introduction to the systons and subsequent thermal exposures, the remaining parent material and the vanout thermal reaction products were cryogenically trapped. Residual air waa purged from the system with the use of high-purity helium, and the trapped sample waa then subjected to in-line gas chromatographic analysis using an open tubular column (1& m length by 0.2$ mm ID) which possessed a 0.25 micron film of bonded phenyl methyl silicone. Kith 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 tha alectron impact (magnetic sector) LK8 2091 mass spectrometer. Structures of observed product* were assigned based on Inter pretation of the mast spectra and comparison with referance spectra. The thermal degradation of the PCB isomer was examined in four different oxygen/nitrogen reaction atmospheres while the gae-phase mean residence time at axposure temperature was held constant at 2.0 seconds. The oxygen concen tration In the reaction atmosphere la described using the equivalence ratio, 0, herain defined as the oxygen in the reactor divided by the oxygen required for complete combustion. The values of g used In this study were 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 sample* of 2,3 ' ,4 , 4' ,5-PCB were thermally decomposed in atmospheres containing different concentrations of oxygen, a variety of p-oducts were formed. Figure 1 shows a ges chromatogram that is typicsl 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 a* thermal reaction products resulting from the PCB degradation. PCDF congeners represented e considerable portion of the products. Significant quantities of partially dechlormated biphenyl congener* were also found, along with dichlorobenzenes and MONS 1485 2,>\4.4`,srcb T oc colimii li - by a.u 10, Di-i 10.ii tmi mOSMMCD thoh ob to i*& HPOfuac condition,. tf - i.s t* wc - 1.0 lLJLajlUjJM ' JL-------- JO KTtirTjOH Tin 40 ala. Figure 1. Typical chromatogram rcaulting from the thermal degradation of 2,3',4,4',5-PCB. table 1 MAJOR THERMAL REACTION PRODUCTS OBSERVED FROM THE THERMAL DEGRADATION Or J.r.O.J'.S-PENTACHLOAOSIPHENTL Humbr of i Product Claom Mt^or Puki r2 Trichlorodlbeniofurana 2 Pontbchlorodlbontofurn 1 tetrackloroblphenyl. i Tr iehloroblphony1 i Tr tchlorabenrene l Dictilorotn<onb i Trichloronaphthaleno i Ttrichloron*phthaian l Tr ichlorophony lth/nM i Diehlorophtnylcthyno l TeLrachlorobiphenyltnea 2 C,HtOCl 1 C10KJC1> 1 HONS I486 trichlorobenzenea. In addition, tetrachlorobipheny lene isomers ware observed. These compound* arc of particular intar oat due to their autpected toxicity3. At 1000'C and t of 1,0, the reaction product Identified as tetrachloronaphthalene exceeded the concentration of the residual parent PCB. Throughout theae experiment* no polychlorinated dibenzo-p-dioxin (KDD) isomers ware observed (minimum detection limit vsa 0.1 ng) . The three-dimensional diaplay preaented in Figure 2 shows the variation in yields of chlorinated benzenes, KBs, and PCDFe, relative to equivalence ratio $. Figures 3 and 4 are a ami-logarithmic plots illustrating KB degrada tion and PCDF formotion/destruction behavior. DISCUSSION Previous laboratory studies of PCDT formation froai KBs have suggtsted that the formation it a purely intramolecular cyclizetion reaction3'. This implies that only a limited number of PCDF congenere can be formed from a given PCB. Four global mechanisms have been suggested! ortho-Cl^ lose, ortho-HCi lose, HC1 lose Involving e 2,1-chlorine shift, and ortho-H^ lose. In the csaa of 2, 3 *, 4,4 ' ,5-PCJ, only orttio-HCl loas and ortho-Hj loss are possible intramolecular mechanisms. The results presented in Tables 1 and 2 indicate that HC1 lose is the dominant mechanism with Hj lots making s smeller contribution to PCDF formation. At least two trlchlorodlbenzofuren (tri-CDF) isomers were observed indicating dechlorination of the parent KB prior to PCDF formation or the occurrence of dechlorination of the product KDF. The presence of lower chlorinated triand tetre-chlorlnated biphenyls as products suggests that the former route is possible - The product yields at theaa reaction conditions are expected to be con trolled kinetlcally, as opposed to thermodynamically. However, thermodynamic calculation* may be used to estimate the relative kinetic rates of the various reaction paths, and therefore, th* relative yield*. The reaction enthalpy for HCl elimination to form aithar 2,2,7,8-tetra-CDF or 2,3,6,7-tetra-CDF ia'wery exothermic with a value of -29.7 kcal/mole. Elimination of ortho hydrogens to form aithar 1, 3,4,(,7-pents-CDF or 1,3,4,7,8-panta-CDF has an enthalpy of -14.2 kcal/mole. Both reaction* are exothermic, indicating that both pathways are available (although HCl elimination may be favored). For PCBe with ortho chlorine*, the elimination of C12 almost tharmonautral. In this case, the activation energy which controls the reaction rate may be large and the Clj elimination pathway may play a minor role. Comparison of the results of this study to previously reported results3'*1*'13 raveals acme vary interesting parallel* and differences. already mentioned, this and previous studies agree that the majority of observed PCDFs may be attributed to Intramolecular cycllzttion reactions. As This HONS 2L4L26 148 7 Figure ?. Three-dimensional skeletal graphing of products formed from the thermal decomposition of 2, V ,4,4* ,5-PC8. Figure 3. PC8 degradation and PCDF formation/destruction prof1 lea . MONS 21412? USB too r I 'Or ot oot 500 800 700 800 900 EXPOSURE TEMPERATURE. *C Figure 4. PCR degradation end FCDP formation/de*truetlon profi let. TABLE J MAXIMUM WEIGHT PERCENT YIELD OF PCDFa AS A FUNCTION OF REACTION ATMOSPHERE Tmporaturo of * MaxiiaiM Yield (*C For Given a Tri-CDF* Haight Percent Yield* Tctra-CDfa Panta-CDr. Total-PCDFi o.et O.i i. 1.0 no too 00 ISO O.iS 0.34 0. 13 0 0(1 4J 1.7 1J 0.71 3.0 0*56 0.2% 0.31 .9 2( 1.7 0 * *Eiprsitd wtifht percent of parent PCB (asiurati equiva Lent FID reifemt for PCR and CDF) MONS 214128 HB9 was previously demonstrated In a study3 of PCDF formation from 18 different PCBa. individual PCB isomere wars salad In quarts ampoule* containing an air atmosphere and heated to a temperature of SOO'c. Heating periods were on the order of one minute, but the actual time at a given temperature was unknown. PCDF congener identities wars consistent with those predicted by the four proposed lntramoleculer cyclliation reactlone. 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 tha formation of PCDPa from PCBa have been reported. Individual PCB isomer* and Aroclot mixture# wars heated to temperatures ranging from 55'C to S50*C. Holer ratios of PCB to oxygen were typically li7.S to 1>75. Total PCDF yields were consistently reported In the raAge of 0.1 to 2.0 percent with maximum yield in the 550'C to 600*C range for one-minute heating time*. The 2,3,7,8-tetre-CDF isomer wa* obaarvad es pradioted by the four intramolecular formation routes. The degradation of Aroclor 1254 samples resulted in the formation of PCDFe with dl-, tri-. an) tstre-CDFe being the dominant congener*. Degradation of both Aroclor 1254 and 12(0 yielded totel PCDFe of 2 percent or less, with 2,3,7,8-tetre-CDF present es a reaction product. Similar studies* have been conducted on Aroclor 1241 in atmospheres of oxygen, nitrogen, end sir. At the highest temperature studied <330*C>, the air atmosphere testa produced the greatest yield of PCDF (vlS ppm) illustrating that intermediate oxygen levels produced the greatest yield of PCDFe. In a very recent atudy*, the thermal degradation of three individual PCB congeners spiked into mineral oil was atudied, using a flow reactor ayatam. Three temperatures, three oxygen levels, and threo residence times were tasted. It was determined that e temperature of 675*C, 0.85 seconds mean residence time, end 8 percent excess oxygen produced the greatest yield of PCDFt. Other studies have been conducted with Aroclore doped into various trans former fluids. In a study** of the thermal degradation of Aroclor 12(0 mixed into polymethylailoxene fluid, yielda of penta-CDFa and hexa-CDFa were observed at the 0,1 end 0,03 percent levels, respectively. Yields of other PCDF isomers were betar the detection limit. Maximum yielda were at 600*C in an air atmosphere. Some PCDFe were observed at 400*C, but none could be detected at B00*C or 1000*C. In a aomewhet different type of laboratory study*2, test mixtures of trichlorobenzenea/Aroclor 1254 (50/50, v/vl) and three levels of Aroclor 1254 in mineral oil (10,000 ppm, 500 ppm, and 50 ppm) were subjected to electrical arcing. Yielda of PCDF congeners were less than 0.001 percent in ell cases. The moat obvious difference in the results presented here end those of previous studies is the yield of product PCDFs (e.g., (.9 percent versus 2 percent or lees). The experimental sequence and number of data points obtained in thia study enabled a more precise determination of the "maximim* in the HONS 214129 1490 format ion/daatruct ion profiles for the PCDFa. However, the higher temperature* t which thia study waa conducted may also have resulted m a different forma tion mechanism for PCDFa. Aa can be aeen from the data in figures 1 and 4, the degradation rates of 2,3`, 4,4`,S-PCB rapidly increeaea above approximately 700*C. Thie la in the region where one would expect a transition from a peroxide-dominated reaction mechanlam to e free radical mechanism13. Pseudo-equilibrium calculations14 of the concentration of small raactive species indicate that the concentration of reactive radicala increases rapidly between 700*C and 900*C (see Figure S). Hydroxyl radicals (OH), oxygen atoms (0), hydrogen etoais (H), and chlorine atoms (Cl) are the major radicals preeent in the syatam. Since incorporation of oxygen is necaaaary for the formation of PCDFa from PCBa, OH and 0 are implicated ee the predominant raactive species reeponaibla for PCDF formation. For valuea of 4<1.0 and tamperaturaa between 700*C and 1000'C, the OH concentration is calculated to be roughly a factor of 10 greater then the 0 concentration, which ia in turn a factor of 1000 to 10,000 greater than the H concentration, for all but the moat fuel rich gaaeoua environmenta, OH would appear to be tha major raactive radical. When the equivalence ratio increeaea, the OH and 0 concentrationa decraaea aa tha H atom concentration increeaea. Thia ehift in equilibrium to non-oxygen-containing radicala raaulta in a decraaead yield of oxygenated product! auch aa PCOF. Thus, for larga equival ence ratios, larger yield* of pyrolyaia product* (e.g., polychlorinated benzenes, PCS congeners, chlorinated naphthalanee, chlorinated biphenylanes, ate.), are obearvad. Although H atoms are usually considered the dominant reactive radical in hydrocarbon aystams under pyrolytic conditions, the large concentration of Cl atoms in PCS systems may result in Cl being the dominant reactive speciea. Additional research on the reactivity of Cl atoms la strongly auggeatad. Tor tha range of oxygan levels atudiad, the PCDF yield increased with equivalence ratio. Although not addressed dirsctly in this study, one^might expect the yields of PCDFa to eventually daersasa with increasing oxygsn concentration due to enhanced destruction of the PCDF product ae It is oxidized to aimplier products including carbon monoxide and carbon dioxida. The shift in the toapsrature for maximum yield of PCDFs as a function of equivalence ratio ia a reflection of the competition between oxidation of PCB to fora PCDF and omidation of the PCDF itself. The obsarvation that tha highest temperature of maximum PCDT yield ia for 41.0 and decreases for 40.0 or 40.2, may well be due to the shifting concentration* of the specie* responsible for PCDF formation and destruction. M0NS 214130 1491 Figure 5. Paeudo-equi libri um calculatlona of concentration of major radical apeclea for pentachlorobiphanyl for 4*0.2. Potentially'important elementary reaction* for PCDF formation by OH attack are ahown in reactione 1 through 4. (1) oc^M^r c/. -1 1C1 C/v (21 MONS 214131 1492 *M A mechanism involving reactions 1 and 2 would corraapond to tha HCl a1Inination mechanisms, while a neehanian involving raaetiona 1, 3, 4 would corraapond to a net loaa of Hj. Reaction 1 ie ahown ai a substitution reaction but nay be an addition followed by H aton alinination. similar reaction nechaniama nay ba drawn for 0 aton attack. Reaction S followed by reaction 4 would also result in H2 elimination. H Reaction schemes involving Cl atom loss through addition or substitution reac tions would be axpscted to be energetically lesa favorable with lower yields of Peers. Thie would account for tha lowsr observed yielda of PCDPe formed through the mechanism involving Clj elimination. ^ The chengee In yielde of various products aa a function of oxygon lavel and temperature are very important vith respect to a practical understanding of the mechanisms of PCS degradation. For example) internal arcing in a sailed capacitor would reeult in degradation of PCBs m an oxygen-deficient environ ment) Under theae conditione, one would expect the formation of pyrolysis producte such as other PCBa, PCBxa, and PNAs rather than PCDF. On the other hand, open burning or incinsrstion of the PCBs stay provide an stmosphare with more oxygen available to participate in the formation of partially acidised products such as PCbFa. However, even under fire conditions oxygen may become depleted, resulting in oxygen-starved conditions and the formation of pyrolysis producte. HONS 214132 1493 The relationships between temperature, oxygen concentration (i.e,, equivalence ratio), and PCB degradation and product formation can be ueed to guide the environmentally aafe incineration of PCB-containlng waetea. To thla effect. Figure* 3 and 4 deacribe the thermal decompoaition of the parent PCB iaomer and the formation of the varioua PCDF* at 4 valuea of 0.05 end 1.0, reapectively. It i* interacting that the maximum formation of the chlorinated furana ahlfta approximately 125'C ae the equivalence ratio chengee from 0.05 to 1.0. Table 2 preaente the yield* of obaerved PCDF* at varioua equivalance ratioa. A* the oxygen concentration incraeaed by a factor of CO, the yield of total PCOFe increaaed by a factor of 10. The percentage of total PCOFe identified ae tatra iaomer* ranged from C2 to 72 percent. ^ CONCLUSIONS Thla etudy haa a hewn that the yield* of PCDFa formed from the hightemperature gaa-phaae degradation of 2,3',4,4'.5-PCB can be on the order of aeveral percent, and maximum yielda incraaae aa the oxygen concentretion increaaee within the range etudied (4 3.0 to 4 0.05). The predoadnent mechaniam for PCDF formation it HC1 elimination. For equivalent combustion condition*, it appear* that PCOFe can be deatroyed at temperature* near thoae required for the destruction of parent PCB*. However, evidence alao indicate* tha formation of chlorinated polynuclear aromatic* (auch aa polychlorinated naphthalene*) in potentially aignificant yialda which peraiat at higher expoaure temperature*. REFERENCES 1. Luatenhouwer, J. W. A., Olie, K., and Hutzinger, O., chemosphere, 9, 501 (1980). 2. "Human and Environmental Rlakt of Chlorinated Dioxin* and Related Compounds,* Tucker, R. E., Young, A. L. , and Gray, A. P., ed.. plenum Frees, New York, (1983). 3. Buser, H. R., and Rappe, C., chenoaphere, 8, 157 <1979). 4. Buser, H. R., Boashardt, H. P., Rappe, C,, and Lindahl, R., ibid, 7, 419 <1978). 5. Buaer, H. R., Boaahardt, H. P., and Rappe. C., ibid, 7 , 109 (1978). 6. Morita, H., Nakagawa, J., and Rappe, C., Bull. Environ. Contam. Toxicol., 19, 5 (1977). 7. Rubey, W. A., "Oealgn Considerations for a Thermal Decomposition Analytical System,* us-EPA Report EPA-600/2-80-098, August <1980). 8. Rubey, w. A., riscus, I. B., and Torres, J. L., "Description and Operation of a Thermal Decomposition Unit-Gas Chromatographic System," US-EPA Report for CR-B0781S, March (1984). 9. 'Chlorinated Dioxina and Dibenzofurana in the Total Environment,* Choudhary, G., Kaith. L. H., and Rappe, C., ad., Butterworth, Woburn, MA (1983). MOWS 214133 1494 10. Erickaon, H. D., Cole, C. J., Flora, J. D., Jr., Gorman, p c.r Haile, c. L., Hindiaw, S. D., Hopkina, F. C., Swanaon, . E., and Heggem, D. T,, *PCDF Formation from PCBa Under Fire Condition*,* aubmitted to Chamotphefe. 11. Swanaon, J, H., and Tiernan, T. 0., 'Formation of Polychlorinated Dibeniop-Dioxina and Oibemofurana from Pyrolyaia of Silicone Dielectric Fluid Spiked with Polychlorinated Biphenyl*,* Presented at the EPRl-PCDF Workshop Palo Alto, CA, Decembar 4, 19*4. li. Derooa, F. L., Cooke, H.( Addia, C., Xoami. R.. and Guertin, J., 'Formation of PCDD and PCDF in Aakarel end Contaminated Mineral Oil Equipment,' Preaantad et the EPRI-PCDF Workahop, Palo Alto, CA. December *. 1)84. 13. Benaon, S. W., "Thormochamical Kinetic*Wiley, Hew fork, <1934). 14. Samualaon, G. S-. The Combuetion Aapects of Air Pollution, in: "Advance* in Environmental Science and Technology, volume 5," Pitta, J. Jr., and Metcalf, R. L., ed., Wiley, Hew York, (1975). ACKNOWLEDGMENTS The author* would like to acknowledge the aaaiatance provided by R. A. Grant and M. D. Graham during the conducting of thi* work. Although the raaearch described in this articls haa bean funded, in pert, by the t).S. Environmental Protection Agency through Cooperative Agreement Cft-3 1 07 93 with the Univeraity of Dayton, it haa not been aubjactad to Agency peer review and therefore, doe* not nsceassrily reflect the views of the Agency: no official endoraement should be inferred. (Received In Germany 2 July ltlSi accepted 14 Auguat 1915) \ HONS 214134