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Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis
Polymer Degradation and Stability 62 (1998) 145-155 0 1998 Elsevier Science Limited. All rights reserved
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Printed in Great Britain front matter
0141-3910/98/%see
SO141-3910(97)00272-3
ELSEVIER
Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis Ian C. McNeill,“* Livia Memetea,” Musarrat H. Mohammed,” Alwyn R. Fernande# & Peter Ambidgebt “Chemistry Department, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ. UK bAEA Technology, 551 Harwell, Didcot, Oxfordrhire OX11 ORA, UK
(Received 23 September 1997; accepted 10 October 1997) The tars produced by PVC pyrolysis under different conditions of temperature and oxygen concentration were analysed for PCDDs and PCDFs content. The tar collected after low temperature pyrolysis (SOODC)and in reduced oxygen atmosphere (11.6% 02) had the highest PCDDs/PCDFs content. The toxicity of the tars is mainly due to the high PCDFs concentration. The most toxic compound, 2,3,7,8-T4CDD, had a contribution of only 054.4% of the total toxicity of the tar. Several hypotheses for PCDDs/PCDFs formation during PVC pyrolysis, based on the analysis of the tar composition, temperature interval in which is formed and the mechanism of PVC breakdown are nronosed. 0 1998 Elsevier Science Limited, All rights reserved
1 INTRODUCTION
(CM 1
1.1 Background to this investigation
/ \ 6-??
n=4-8
I
II
:RD
/
3
4
The combustion and incineration of organic wastes such as wood, coal, garbage and chlorine-containing polymers is accompanied by the formation of trace compounds of very high toxicity-the polychlorinated dibenzo-p-dioxins (I), dibenzofurans (II) and biphenyls (III) designated as PCDDs, PCDFs and PCBs. Formulae I-III list the groups in which the highly toxic isomers can be found. These groups are designated by the degree of chlorination n, with n = 4-8 for the toxic isomers of PCDD/PCDF and n = 4-10 for PCBs, respectively. Each group is made up of a number of positional isomers called congeners. Structures IV-VI show the numbering of the positions in the PCDD, PCDF and PCB structures.
~4-8
\
2
n=4-10
IV
III
1 2
:m
3 4
6 V
40-04 5’
6
6’
5
VI
The most toxic of the above trace pollutants is 2,3,7,8tetrachlorinated dibenzo-p-dioxin (2,3,7,8T4CDD). Any PCDD/PCDF isomers having these positions occupied are toxic but with various degrees of toxicity as compared to 2,3,7,8-T4CDD. The international toxic equivalent factor (I-TEQ) has been defined as the relative potency of the dioxin-like compound to produce the same effect when compared with 2,3,7,8-T4CDD. This means that the toxic isomer can produce the same toxic effect as 2,3,7,8-T4CDD in a concentration l/ITEQ times higher than that of 2,3,7,8-T4CDD. PCBs vary in toxicity according to the type of congener and Cl content. The isomers with Cl
*To whom correspondence should be addressed. ‘Present address: CBD, Porton Down, Salisbury, Wiltshire, UK, SP4 OJQ. 145
146
I. C. McNeil1
atoms in positions 4,4’ or 3,4 or 3,4,5 on one or both rings are more toxic, The most toxic isomers include 3,3’4,4’,5-P5CB, 3,3’,4,4’-T4CB, 3,3’,4,4’, 5,5’-H6CB. All the toxic PCDDs, PCDFs and PCBs tend to assume a relatively coplanar conformation, generally similar to that of 2,3,7,8-T4CD. All these pollutants are environmentally persistent resisting bacterial and chemical breakdown. They accumulate in the environment in lake sediments, fish, birds, mammals. Humans exposed to wastes containing 60 mg 2,3,7,8-T4CDD/kg wastes presented a 39% higher cancer mortality than the average population. 1 Other serious conditions following PCDD poisoning are: chloracne, thymic atrophy (a wasting syndrome), immunotoxic impairment effects, reproductive and liver damage. The development of the analytical means for the isolation, detection and quantification of these micropollutants in the 1980s coincided with the extended use of PVC which became the second most used polymer (after polyethylene). The presence of PVC waste in the municipal garbage acquired the blame. 2 Even though research work from the plastics industry claimed that garbage with or without PVC produces the same level of PCDDs/ PCDFS,~ PVC was not exonerated since it was proved that industrial operations such as the burning of the PVC insulated scrap wire or the hot iron soldering of the same wire produced copious amounts of dioxins. The present work is part of a project supported by EPSRC for evidencing and quantifying the environmentally toxic products formed by PVC pyrolysis. The first paper of the series analysed the products formed through pyrolysis of PVC alone and in some industrial formulations with dioctyl phthalate.4 The second paper presented a quantitative evaluation of the pyrolysis fractions and an investigation into the mechanism of PVC pyrolysis.5 This paper reports on work with the purpose of analysing the PCDDs and PCDFs produced by PVC pyrolysis in laboratory conditions which approximate to those used in incineration. The decision to analyse the PCDDs/PCDFs content of the PVC pyrolysis products was taken because, in spite of the abundant work done on PVC mixed with other waste in municipal waste incinerators (MWI), there is limited information on the PCDDs/PCDFs isomer specific determination and total toxicity of pyrolysis products.2 Since PCBs are of lower toxicity than PCDDs/PCDFs, they
et al.
were not analysed in this study. However they will be considered as potential precursors of PCDFs. 1.2 Survey of main reactions for PCDDs/PCDFs formation An impressive number of authors have investigated experimental routes for the synthesis of PCDDs and PCDFs, starting with different reactants. An excellent book reviews the possible routes for PCDDs, PCDFs and PCBs formation from various monomers, polymers, organic and inorganic materials.6 We shall limit our discussion here to routes which are relevant for PVC. Potentially, the dioxins can be formed by the oxidative chlorination of organic compounds both in the gas phase and in condensed matter. A de nmvo synthesis of PCDDs/PCDFs from elemental carbon in the presence of C4, O2 and H2 and catalysed by fly ash has been identified7 and can take place at temperatures as low as 300°C. A mixture of propene and HCl produces at 500580°C and in the presence of fly ash a mixture of polychlorinated benzenes, PCDDs, chlorinated mono- and dibenzofurans.8 Interestingly, ethene plus HCl in the presence of fly ash does not generate any of the above compounds.8 The catalytic effect of Cu, Fe, A1C13in the fly ash in the process of dioxin formation from precursors is well documented but we shall not proceed along the line of catalysed formation of PCDDs/PCDFs since the present paper studies the pyrolysis of pure PVC which has no inorganic content and forms no ashes upon complete pyrolysis. The main precursors in PCDDs/PCDFs formation are considered to be the chlorinated benzenes, phenols, diphenyl ethers and PCBs as follows. Polychlorinated benzenes can form PCBs in the absence of air or can be oxidized to polychlorinated phenols and further to PCDDs/PCDFs if oxygen is present. In the absence of air the yield of PCBs produced from polychlorinated benzenes depends very much on temperature. At 690°C in N2 with a residence time of 24s monochlorbenzene can form dichlorobiphenyl as a major product but at higher temperature, 770°C the ring is ruptured and gaseous products are mainly formed. lo The pyrolysis of tri-, tetra- and penta-chlorobenzene at 600°C in sealed miniampoules in the presence of air has produced chlorobenzenes with a higher degree of chlorination than the original chlorobenzenes and also PCDDs, PCDFs, chlorophenols and PCBs.” The
147
Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis
of the PCDFs was 10-50 times higher than PCDDs and the lower chlorinated PCDDs and PCDFs were formed in higher concentrations, i.e the pattern was not that encountered in pyrolysis or combustion generated PCDDs/PCDFs. Biphenyls can generate PCDFs by oxidation with the loss of one to two chlorine atoms or two H atoms from the ortho-position (Scheme 1, reactions (l)-(3)).12,‘3 During oxidation by reaction (1) a shift of the 2,3 Cl atoms at the benzene ring is also possible producing another isomer. The PCDF yield of reactions (l)--(3) is very low, in the
formation of polychlorinated benzenes with a higher degree of chlorination than the original materials indicates that a thermal isomerization is occurring. For example, trichlorobenzenes gave rise to tetra- and pentachlorobenzenes, tetrachlorobenzenes gave rise to penta- and hexachlorobenzene and pentachlorobenzene gave rise to hexachlorobenzene. Certain isomers are formed preferentially, for among triexample chlorobenzenes the 1,2,4- isomer is formed predominantly and so is 1,2,3,5-tetrachlorobenzene among the tetrachloroisomers. The concentration
l
@+l,2o2 A
c133-(=$ +112o2
Ci
@
L
kl
c,w, Cl
Scheme 1
f ZHCI
0
Cl
(2)
+H20
(3)
148
I. C. McNeil1
et al.
range of 0. l-several %. The reaction was conducted in closed ampoules by heating from 20 to 600°C in 1 min with 5 s residence time at 600”C.13 Both PCDDs and PCDFs can also be formed from polychlorinated phenols as precursors. Heating pentachlorophenol in a tube 1 m long at 300°C for 24 h produced hexachlorobenzene and OCDD in a ratio of 8.5:1 l4 (reaction (4)). The reaction proceeds through an intermediate-the chlorinated phenoxyphenol species VII, formed through an equilibrium (5).i5 The chlorinated phenoxyphenol species VII is present as an impurity in pesticides based on chlorinated phenols and is responsible for the formation of PCDDs associated with these pesticides.i6 In another experiment the combustion of 2,4,6tri- and 2,3,4,6-tetra- and pentachloro-phenol mixed with wood chips indicated the formation of PCDDs of all degrees of chlorination.17 The pyrolysis of eight isomers of polychlorinated diphenyl ethers containing 3-6 and 8 Cl atoms has shown the formation of mainly PCDFs as pyrolysis products but also some PCDDs.‘* The pyrolysis took place in closed ampoules and the highest yield was found at 600°C 0.7-4.5% depending on the isomer. At 700°C most of the PCDF/PCDDs have decomposed and the yields were about 20 times lower. Finally, PCDDs can be formed through the pyrolysis of chlorophenoxyacids or better, their Na salts6 but compounds with such a high oxygen content were not identified in PVC pyrolysis.
effluent gases from municipal waste incineration (MWI)3,20-28 in which PVC is present at a level of 0.5-5% mixed with a wide range of materials. The influence of the PVC content in the waste on the PCDDs/PCDFs level in the gaseous effluents from incineration is still a debated matter due to the publication of many conflicting results. A clear correlation between PCDDs/PCDFs level and PVC concentration in the waste could not be demonstrated. Some authors have found no differences in the PCDD/PCDF level in emissions whether the source of chlorine in the waste was NaCl or PVC3~2oor when fuel of different origin was mixed with variable amounts of PVC up to a chlorine level of 5.6% (10% PVC).21,22 The emission of’ chlorobenzenes20 or chlorophenols23q24 as an indicator of the risk of PCDD/ PCDF formation was monitored in various conditions and it was shown that it does not change significantly upon replacing PVC with NaCl as a source of chlorine. Other authors have found increased PCDDs/PCDFs or precursors emissions with increasing PVC concentration.25,26 The presence of PVC in MWI or in wood burning facilities continues to generate concern as larger facilities could release as much dioxins as 7.2 pg ITEQ/h with the effluent gas.27 Medical waste incinerators in hospitals are also a major source of concern since PVC is widely used for disposable medical items.28
1.3 PCDDs/PCDFs formation in PVC pyrolysis and combustion
2.1 Pyrolysis conditions
PCDDs/PCDFs formation in the incineration of PVC was first observed in a copper recycling plant where PVC insulated copper wire was burnt2 and also in building fires.” Only one paper attempted a clear account of the PCDD/PCDF congener composition produced in PVC pyrolysis or combustion,2 to our knowledge. This, the only systematic investigation of the pyrolysis and combustion of pure PVC and copper wire insulation, failed to detect many of the toxic isomers of T4CDD-H6CDD. However, it demonstrated the influence of PVC molecular weight on the level of dioxin formation and the role of copper catalysis. Most of the published material refers to the formation of PCDDs/PCDFs in the soot and the
2 EXPERIMENTAL
Suspension grade PVC from European Vinyls Corporation with a MW of 40000 as determined by GPC was pyrolysed as described below. One hundred milligrams of PVC powder was spread as a uniform thin layer in a silica boat which was introduced in a silica tube which was placed in an oven. The silica tube ended with a thinner tube spiralling in the oven through which a flow of gas was introduced over the sample. The role of the spiral tube was to allow the heating of the gas before it reached the sample. Outside the oven, a water cooler of about 25cm was inserted in the outer end of the silica tube to allow for the condensation of the pyrolysis products with low volatility (high boiling point). The more volatile pyrolysis products which escaped the water cooler condensed further on in a trap cooled with
149
Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis
methylated spirit and liquid nitrogen at a temperature of -70°C. Noncondensable gases, C& hydrocarbons, HCl and CO*, were able to pass over the traps and leave the system through the gas exhaust which contained filters which were later destroyed as toxic material. Air or a 1:1 mixture of air and nitrogen was introduced into the system at a rate of 80mlmin-*. The temperature rise of the oven was programmed at lO”Crni~--~ up to either 500°C or 1000°C where a period of isothermal heating of 1Omin was allowed. The residence time of the pyrolysis products in the oven was calculated as 0.9s. The residence time is defined as the time taken by 1 ml of gaseous product to pass through the bulk volume of the pyrolysis vessel in the oven. Preliminary determinations have shown that PCDDs/PCDFs can be found in the tar fraction. At the end of the pyrolysis experiment, the tar was collected from the condenser zone by washing with dichoromethane. As an additional precaution, dioxins possibly entrained in the liquid fraction were also recovered. In the trap cooled at -70°C there was a liquid fraction consisting mainly of water, benzene, toluene, xylene and ethylbenzene. This liquid fraction was evaporated under a gentle stream of nitrogen, the trap was washed with dichloromethane and the washings were added to the tar fraction. The tar contained both a major fraction of polyaromatic hydrocarbons (PAH) and a significant fraction of oxygen-containing derivatives of PAH (ketones, aldehydes, ethers, alcohols, lactones) constituting the matrix from which the PCDDs/PCDFs had to be recovered. After the evaporation of the dichlormethane, the tar was subjected to matrix purification and PCDD/PCDF determination as follows.
123789 HCDD, 123678/234678 HCDF, 1234678 HpCDD/F, OCDD, OCDF), solvent exchanged to hexane to exclude polar interfering compounds and then treated with concentrated sulphuric acid followed by AR water. The purification and analysis stages have been described before2g and are summarised below. This method is NAMAS accredited. The hexane extract thus derived from the tar was chromatographically purified using a multilayer column containing acid-modified, base-modified and unmodified silica. The column was eluted with hexane directly onto a conditioned Florisil@ column where the PCDD/Fs were fractionated from similar compounds by elution with successively polar solvents-different compositions of chloroform-modified hexane, followed by dichloromethane. The PCDD/Fs were collected in the last dichloromethane fraction and concentrated using a gentle stream of dry air. This fraction was then solvent exchanged to 100 ~1 of the keeper solvent which contains the syringe standard mixture. This standard is used to calculate the efficiency of the extraction and purification procedures. The purified extract derived from this procedure was analysed using HRGC-HRMS. The chromatographic separation was carried out using a 60mx0.22 mm id, DBS-MS GC column. The mass spectrometer was operated in selected ion recording mode using voltage scanning, at a resolution of approximately 10000. The PCDD/Fs were monitored in five groups from tetra- to octachlorinated homologues, determined by chromatographic retention.
2.2 Matrix purification and PCDD/PCDF analysis
The amount of tar collected under various pyrolysis conditions is shown in Table 1. A comparison between samples Al and A5 shows that most of the tar is formed up to 500°C. Little tar if any is formed at temperatures between 500 and 1000°C. On the contrary, above 500°C a small fraction of the tar seems to be lost (burnt). Another observation based on Table 1 is that the pyrolysis in an oxygen-deficient atmosphere leads to a higher
The tar residue was washed out of the container with hexane followed by acetone. The insoluble remaining residue was dissolved using dichloromethane. A portion of the combined washings was taken for analysis. This portion was spiked with a i3C-labelled internal standard mixture (2378 TCDD/F, 12378 PCDD, 23478PCDF, 123478/
3 RESULTS
Table 1. The amount of tar collected per 100 mg pyrolysed PVC under different pyrolysis conditions Sample Al A5 LM
Maximum
temperature 1000 500 500
(“C)
mg tar (100 mg)-’ 5.50 5.67 18
PVC
Gas Air Air Air:Nz 1:l
O2 Concentration 23.25 23.25 11.6
(%)
150
I. C. McNeil1 et
fraction of tar than the pyrolysis in dynamic air as can be seen from comparing samples A5 (5.67% tar) with LM (18% tar). Table 2 presents the concentration of the toxic isomers of PCDD/PCDF expressed in ng isomer g -i tar for the samples pyrolysed under the conditions presented in Table 1. It also lists the values of I-TEQ for each congener used to calculate the total toxicity of the tars expressed in ITEQ, which was obtained by multiplying the concentration of a specific isomer by its I-TEQ and summing up for all the isomers to give the results of Table 2. Several observations can be made regarding the results in Table 2. When the pyrolysis is conducted in air to 1000°C (sample Al) the concentrations of PCDDs are 1.55 4 times lower than when the pyrolysis stops at 500°C (sample A5). On the contrary, more PCDFs are formed when the pyrolysis is conducted up to 1000°C. Pyrolysis conducted to 500°C and in an oxygen-deficient atmosphere (sample LM) produces the highest levels of PCDDs and PCDFs. The most toxic isomer, 2,3,7,8-T4CDD, is now detected as 3.94ngg-‘. The rest of PCDDs are 5-6 times as much and the PCDFs are 3-8 times as much as at 1000°C. In general, the higher chlorinated isomers (hepta, octa) for both PCDDs and PCDFs are formed in concentrations one or two orders of magnitude higher than their lower chlorinated isomers with the exception of sample A5, for which the PCDFs
Table 2. The concentration in ngg-’
Isomer 2,3,7,8-T4CDD 1,2,3,7,8-P5CDD 12 7 73 94 97 ,8-H6CDD 1,2,3,6,7,8-H6CDD 12 > 93 97 38 99-H6CDD 1,2,3,4,6,7,8-H7CDD OCDD Toxicity of PCDDs (I- TEQs) 2,3,7,8-T4CDF 172 ,3 97 78-P5CDF 2,3,4,7,8-P5CDF 1,2,3,4,7,8-H6CDF 1,2,3,6,7,8-H6CDF 1,2,3,7,8,9-H6CDF 2,3,4,6,7,8-H6CDF 1234678-H7CDF >,,11> 1,2,3,4,7,8,9-H7CDF OCDF Toxicity of PCDFs (I-TEQs) Total toxicity Average analytical recovery
al.
are surprisingly low. Also, in general, the levels of PCDFs are considerably higher than that of PCDDs, again with the exception of sample A5. Our results are in general consistent with former dioxin analysis for PVC pyrolysis2 with the exception of the fact that the cited paper has not identified many of the lower chlorinated isomers, especially 2,3,7,8-T4CDD. The total toxicity of our tar obtained through pyrolysis up to 500°C and in an oxygen deficient atmosphere is 178 I-TEQs, ng g-i, well above the toxicity reported previously for PVC pyrolysis (1941 I-TEQ, ngg-1).2 Although the dioxins are generally more toxic, the highest contribution to the overall toxicity of our tar is due to the PCDFs which are formed in greater amounts. The results presented in Table 2 suggest that the PCDD/PCDF isomers have their own temperature range and oxygen concentration range which favour formation over decomposition and the isomer pattern in Table 2 is a resultant of these reactions. Table 3 presents the total PCDD and PCDF content of congeners of tetra- to octachlorodibenzodioxins and dibenzofurans. Table 3 shows that the most toxic congeners represent only a small fraction of the total concentration of all the congeners within the group. For example 2,3,7,8-T4CDD represents less than 1% of the total T4CDD for samples Al, about 1% for sample A5 and 5% for sample LM.
tar of the toxic PCDD and PCDF congeners pyrolysis conditions
and the toxicity
of the tars formed under various
Al (lOOoOC, air)
A5 (SOO’C, air)
LM (SOO’C, 11.6% 02)
co.12 1.22 1.29 1.88 1.68 28 23 1s2 3.68 15.2 10.9 54 48 12.9 24 310 100 300 24.9 26.4 85%
0.47 4.63 2.64 6.04 5.02 70 35 4.89 3.37 11.3 7.2 4.02 3.72 1.82 3.26 7.72 2.11 3.16 5.89 10.8 71%
3.94 0.27 6.36 10.5 8.09 180 140 12.0 11.8 90 69 380 330 100 160 2100 610 2200 166 178 86%
TEF I 0.5 0.1
0.1 0.1 0.01 0.001 0.1
0.05 0.5 0.1 0.1 0.1 0.1 0.01 0.01 0.001
Polychlorinateddibenzodioxinsand dibenzofurans in PVC pyrolysis Table 3. Total PCDD md PCDF content (ng g-l tar)
Isomer
Al
A5
LM
T4CDD PSCDD H6CDD H7CDD OCDD T4CDF PSCDF H6CDF H7CDF OCDF
12.1 17.4 53 52 23
43 89 170 130 35 120 80 33 18 3.2
75 110 240 320 140 480 1100 2600 4300 2200
110
150 360 650 300
4 DISCUSSION 4.1 Product formation and temperature intervals in PVC pyrolysis During pyrolysis in vacuum PVC generates several product fractions which can be distinguished and quantified.4 4.1 .I Hydrogen chloride This formed with a yield of 53% relative to PVC. If all HCl was eliminated the value would be 58.4%. This means that 10% of the Cl atoms remain in the charring polymer to form organic chlorinated products. HCl emission takes place in the temperature interval of 200-350°C with a maximum around 25&300”C.4~30 After 300°C there is little HCl emitted although the emission does not cease completely. 4.1.2 Other gases This gaseous fraction is made up of Cl-C4 alkanes and alkenes and some hydrogen. MS ion monitoring has shown that the major part of this fraction is formed at temperatures higher than 350°C. This fraction accounts for 5.6% of the initial polymer by weight. 4.1.3 Liquid fraction The liquid fraction consists of aromatic hydrocarbons with MW up to 150 and accounts for 7% of the original PVC. In this fraction, chlorinated compounds like chlorobenzene (0.15% of the liquid fraction), benzyl chloride plus ochlorotoluene (0.2%), o-tolylchloromethane plus 2-chloropropylbenzene (0.35%), Cl-hexane, Clhexene, 1-Cl-2- hexene (0.2%), 2-Cl-butane, 2-Cl1-butene, 2-Cl-2-butene, 2-Cl-3-butene were found. Dichlorobenzene was identified in concentrations one order of magnitude lower than the above
151
chlorinated hydrocarbons and trichlorobenzene, tentatively. No higher chlorinated benzenes were identified by the present authors and trace analysis was not attempted for their identification. However, there are reports that pyrolysis of PVC in the temperature range of 200-800°C has shown the formation of monochlorbenzene, o-, p-dichloro1,3,5-trichlorobenzene and 1,2,4-&ibenzene, chlorobenzene in trace amounts.31 Combustion experiments between 570-l 130°C at different oxygen concentrations of 0.5-16% in the effluent gases have shown the formation of chlorihexanated benzenes from dichloto chlorobenzenes and PCBs with a total amount of bound organic chlorine of 2.2-60mg g-’ PVC32 which represents 0.4-l% of the total amount of Cl in PVC. Increased temperature and decreased transit time seemed to favour their formation. 4.1.4 Tar/wax fraction A tar/wax fraction called hereafter ‘the tar’ consists of polynuclear aromatic hydrocarbons (PAH) with 2-6 rings, their methyl substituted derivatives and some Cio-Cl3 alkenes and alkanes (waxes). The tar is mainly formed after 350°C upon the breakdown of the crosslinked polyene system remaining after HCl emission. The tar is the major fraction after HCl accounting for 24% of the initial PVC. 4.1.5 Char residue The char is what remains after the polymer pyrolysis at 500°C. It represents 9.5 % of the weight of the initial PVC sample. The pyrolysis in air showed an increased gas fraction, decreased liquid, tar and char fractions and the presence of 0 in the charring polymer.5 Recent work33 has identified a fraction of oxygencontaining PAH in the tar next in amount to the main fraction consisting of PAH. Polyaromatic ketones such as benzanthracen-7-one, 9-anthracenone, 9, lo-anthracenedione, 1 ,Cchrysenequinone and polyaromatic aldehydes such as 9, lo-anthracenedicarboxaldehyde, 1-pyrene-carboxaldehyde and 1, I’-biphenyl-4carboxaldehyde are main constituents. Important amounts of dibenzofuran, benzonaphthofuran, phenanthrofuran, 1,3-diphenyl isobenzofuran, biphenyl ether and o-hydroxy biphenyl were also detected. Interestingly, 3,3’4,4’-tetrachloro-, 1 , l’-biphenyl- and 2chlorodibenzodioxin were present in concentrations high enough to be detected with low resolution GC-MS and without matrix purification. Our investigations have shown that PCDDs/PCDFs are formed together with the
I. C. McNeil1 et al.
152
above compounds 35&5Oo”C.
in the temperature
range from
thought to lead to PCDDs/PCDFs during the breakdown of the crosslinked polyene system. Structure VIII (Scheme 2) represents a fragment of the PVC chain containing a head-to-head structure. These structures do not dehydrochlorinate more readily than normal structures as shown by studies on model compounds prepared by chlorinating cis- 1,4-polybutadiene.34 Hence dehydrochlorination as in reaction (7), Scheme 2, has a higher probability and produces structure IX. Chlorine addition to some of the double bonds, either molecular or as free Cl radicals, creates structure (X) in which Cl substituted segments alternate with double bonds. The free radical addition of chlorine is a random phenomenon and does not have to be regularly alternating as in reaction (8) which represents a simplification for ease of illustration. The double bonds can take part in intermolecular condensation as in Scheme 3, reaction (9) creating structure (XI) in which the secondary free radicals formed in the vicinity of the hexane rings can backbite a double bond in the same chain and break the bond with the substituted cyclohexane rings in order to form an aromatic species XII. This is accompanied by a Cl atom transfer to form species XIII and the expulsion of another Cl as a free radical (reaction 10). The newly formed structure XIII is oxidized to a PCDF (PCDD) by reaction (11). In practice the
4.2 Hypotheses for PCDDs/PCDFs formations in PVC pyrolysis We shall first present a picture of the crosslinked polyene breakdown from which stems a hypothesis for PCDDs/PCDFs formation. The zip dehydrochlorination of PVC leads to small polyenes with an average number of double bonds of 3.5, except for the initial stages when the polyenes are longer. It is believed that the zip dehydrochlorination stops at defects of chemical structure which are head-to-head structures. There is also the possibility of a small scale readdition of HCl or possibly of Cl2 to polyenes. Chlorine is formed in the system through the Deacon equilibrium (6). K 2 HCI + l/2 O2 _
H20 + Cl2
(6)
Cl radicals are also present in the system, formed through Cl2 dissociation which becomes important after 400°C. The chlorine readdition to polyene in the vicinity of head-to-head defects already containing chlorine atoms creates moieties of high Cl concentration in these zones of the polyene system. Schemes 2 and 3 present the reactions which are
-
CH-CH2-CH-CH2-CH-CH2-CH-CH-CH2-CH-CH2-CH-CH2I I I I I
dl
kl
dr
(21 6
I
I
Cl
Cl VIII
dehydrochlorination
(7)
-CH=CH-CH=CH-CH=CH_CH_CH-CH=CH_CH=CH-CH=CH_ I I Cl Cl
(8)
-
CH=CH-
CH-CHI
I
Cl
Cl
Ix
J
chlorination
CH=CH-CH-CH_CH L,
Ll
=CH_CH
-CH_CH=CH
I Cl
-
I Cl
x Scheme 2
Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis
(vl) ,FH=CH
I’ “w”\
CH-CH I Cl
‘\ *\
I’
,’
CH=CH
‘.
153
‘\ r
/CH=C~cH_c~cH=c~CH4~cH=c~ CH-CH I Cl
I Cl
I I Cl Cl
I Cl
r 77 CH-CH
(
)
(
)
// CH
/
I
I
Cl
Cl
1
\c H-CH
~ZH-CH ‘CH-C<
’ CH-CH ’
c\ CH-CH
CH-CH
CH-CH
I
I
Cl
Cl
\
‘CH-C/H
!
\H
CH--dH
’ CH-CH ’ Ll
I
I
Cl
Cl
I (=j+cl#cl +l (-J (10)
+
Cl
Cl
Cl
XI
Cl
+ Cl
Cl
XII
XIII
XII
Cl
ci
61
Cl
+
water, chain fragments
Cl Scheme 3
aromatization of structure XIII and its oxidation to PCDF/PCDD probably occur simultaneously since oxygen is known to help the aromatization process. On the other hand, the aromatization of substituted structures is difficult for reasons of orbital symmetry and scission of substituents often accompanies aromatization. Hence, it appears possible that with the thermal oxidative breakdown of the crosslinked polyene system forming PAH and O-containing PAH, the chlorine concentrating moieties also break down producing PCDDs/ PCDFs and other chloroaromatics. The presence
of head-to-head structures is not necessary to explain the PCDDs/PCDFs formation through this route, but it is useful in explaining the preferential formation of highly substituted isomers. The cracking pattern of the crosslinked polyene system in the parts concentrating Cl atoms resembles to some extent the cracking pattern of highly Cl-substituted aliphatic hydrocarbons and perchlorocarbons (C/Cl ratio 1:4 to l:l, respectively). This pattern involves C-C scission rather than CCl scission35 and aromatization between 400 and 600°C.
154
I. C. McNeil1
A crude estimate of the concentration of the PCDDs/PCDFs formed from head-to-head structures can be compared with the total concentration of PCDDs/PCDFs found in the present experiments. The head-to head structures are found in a maximum concentration of 0.2/ 1000 vinyl chloride units.36 Considering that 20% of the initial polymer forms the tar where PCDDs/PCDFs are found, considering an average molecular weight of 400 for PCDDs/PCDFs and a yield of 1% for PCDDs/PCDFs formation from head-to-head structures, a total concentration of 60 ng PCDDs/ PCDFsg-’ tar is calculated. Table 3 shows a total concentration of 102-lo4 ng PCDDs + PCDFs g-r tar which suggests that either the yield is underestimated or an additional source of dioxins exists. Let us examine another possibility for PCDDs/ PCDFs formation-benzene oxychlorination with Cl2 formed through the oxidative decomposition of HCl (equilibrium 6) which is well established in the literature.4,‘1 In PVC pyrolysis, HCl and benzene are emitted simultaneously.4,30 The main temperature interval for HCl and benzene emission is between 200 and 350°C. However, HCl and benzene emission never ceases completely for they are emitted in lower amounts over the whole temperature interval up to 500°C. The thermal equilibrium (6) through which Cl2 is formed in the system has a high equilibrium constant K = [H20][Clz]/[HC1]*[02]“* as calculated from thermodynamic data.6 The calculated K values are 4.17x lo9 at 300°C and 3.3 1 x 10’ at 500°C and they decrease considerably above 500”C,6 which could explain why PCDDs/ PCDFs formation is favoured in pyrolyses up to 500°C. Since the free radical chlorination of benzene is known to favour the formation of the highly substituted hexachlorobenzene, the preferential formation of the highly chlorine substituted PCDD/PCDF isomers can also be explained by this possibility. A study of the PCDDs/PCDFs levels as a function of temperature starting at 300°C would clarify the participation of equilibrium (6) in PCDDs/PCDFs formation. Also, the measurement of the levels of these materials in experiments in which the benzene and HCl emissions have been suppressed by mixing PVC with metal oxides could establish whether benzene oxychlorination is the major source of dioxins in PVC pyrolysis. This idea is based on the observation that the presence of metal oxides in
et al. PVC suppresses the formation of benzene while increasing the char formation.30
5 CONCLUSIONS PCDDs/PCDFs formed in PVC pyrolysis in a dynamic experiment conducted up to either 500 or 1000°C in air and in reduced oxygen concentration were analysed from the point of view of the toxic congeners level and isomer group level. The PCDDs/PCDFs formed in PVC pyrolysis were detected in a fraction of pyrolysis products which is formed above 350°C. The PCDDs/PCDFs are found in a matrix of PAH and oxygen containing polyaromatic compounds constituting the tar-a fraction of low volatility compounds. Pyrolysis at high temperature (1OOOC) and air produced the least amounts of PCDDs. Lower temperature and oxygen deficient atmosphere produced the highest amounts of PCDDs/PCDFs. This pattern confirms previous results reported on PVC combustion and incineration.2y3 Under any conditions, the amount of the most toxic congeners is only a small fraction of the total amount of congeners. For example the amount of the most toxic 2,3,7,8-T4CDD is only l-5% of the total amount of the tetrachlorinated congeners. Although of lower toxicity, PCDFs are formed generally in much higher amounts than PCDDs and for this reason their contribution to the total toxicity of the tar is much higher than that of dioxins. The higher chlorinated isomers (hexa-, hepta-, octa-) of both PCDDs and PCDFs are generally formed in higher amounts than the lower chlorinated isomers (tetra-, penta-). None of the proven ways of synthesis existing in the literature can explain this fact. Two hypotheses on PCDDs/PCDFs formation during PVC pyrolysis were examined and retained as most probable. The presence of zones of high chlorine substitution in the crosslinked polyene system can produce PCDDs/PCDFs upon the oxidative breakdown of the crosslinked polyene system in a moderate oxygen atmosphere. The second possibility for the PCDDs/PCDFs formation in PVC pyrolysis is from the benzene and HCl emissions of this polymer. Though it is already an accepted fact in the literature that the oxidative chlorination of benzene at temperatures between 300-500°C produces PCDDS/PCDFS,~>” this might not be the only mechanism of PCDDs/ PCDFs formation in PVC pyrolysis.
Polychlorinated
dibenzodioxins
and dibenzofurans
155
in PVC pyrolysis
The total toxicity (I-TEQs) of the tar obtained through pyrolysis at low temperature (500%) and in reduced oxygen concentration found in our experiments is the highest reported in the literature in connection with PVC, as revealed by a comparison between the total I-TEQs of our tar and the values we calculated from the data of other references dealing with PVC. It confirms that under certain conditions the pyrolysis of PVC is indeed an important source of PCDDs/ PCDFs.
10. Fields, E. K. and Meyerson, S., J. Am. Chem. Sot., 1966,
ACKNOWLEDGEMENTS
19. Theissen J., Funcke W., Balfanz E. and Konig J., Chemo-
88, 3388.
11. Buser, H. R., Chemosphere, 1979,415. 12. Morita, M., Nakagawa, J. and Rappe, C., Bull. Environm. Contam. Toxicol., 1978, 19, 665.
13. Buser, H. R. and Rappe, C., Chemosphere, 1979, 157. 14. Sandermann, W., Stockmann, H. and Casten, R., Chem. Ber., 1957,90,
690.
15. Langer, H. G. Brady, T. P, Dallon, L. A., Shannon T. W. and Briggs, P. R., Advances in Chem. Series, 1973,120,26. 16. Nilsson, C. A., Andersson, K., Rappe, C. and Westmark, S. 0, J. Chromatogr., 1974, 96. 17. Jansson B., Sundstrom, G. and Ahling, B., Sci. Tot. Environ., 1978, 10, 209. 18. Lindahl, R., Rappe, C. and Buser, H. R., Chemosphere, 1980, 351.
The authors gratefully acknowledge financial support from EPSRC for the work described in this paper, and are indebted to European Vinyls Corporation for the supply of a PVC sample.
1. Manahan, S.E., Toxicological Chemistry, 2nd edn. Lewis Publishers, Boca Raton, Florida, 1992. 2. Christmann, W., Kasiske, W., Kloppel, K. D., Partscht, H. and Rotard, W., Chemosphere, 1989, 19, 387. 3. Gottesman, R. T., Carol& W. F. and Fishbein, L., Energy 148.
4. McNeill, I. C. and Memetea, L., Polym Degrad. Stab., 1994, 43, 9.
the Thermal Formation of Halogenated Organic Componds.
Gordon & Breach, New York, 1983. 7. Stieglitz, L., Zwick, G., Beck, G., Roth, W. and Vogg, H., Chemosphere, 1987, 16, 1917.
W. and Mulder, P., Chemosphere, 1994, 29,
1911.
9. Voncina, 1994,29,
23. Kanters,
M. J., Vannispen
R., Low R. and Mulder P.,
27. Kolenda J., Gass H., Wilken M., Jager J. and Zeschmarlahl, B., Chemosphere, 1994, 29, 1927. 28. Thornton, J., McCally, M., Orris, P. and Weinberg, J., Public Health Reports, 1996, 111,4. 29. Femandes, A. R., Bushby, B. and Wallace, D., in Measurements of Airborne Pollutants, ed. S. Couling. Butterworth Heinemann, Oxford, 1973, Chapter 15, pp. 260-272. 30. Montaudo, G. and Puglisi, C., Polym. Degrad. Stab., 1991, 33, 229.
I. C., Memetea, L. and Cole, W. J., Polym Degrad. Stab., 1995, 49, 181. 6. Choudry, G. G. and Hutzinger, O., Mechanistic Aspects of 5. McNeill,
8. Jarmohamed,
S.,
Environmental Sci. Technol., 1996, 30, 1637. 21. Hassel G. R., Proceedings of the 11th Incineration Conference, 1992, p. 483. 22. Frankenhauser M., Chemosphere, 1992, 25, 1599. Environmental Sci. Technol., 1996,30, 2 12 1. 24. Kanters, M. J. and Low R., Chemosphere, 1994, 29, 1919. 25. Ruuskanen, J., Chemosphere, 1994,28, 1989. 26. Leonir, D., Chemosphere, 1991,23, 1491.
REFERENCES
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sphere, 1989, 19, 423.
20. Wilkstrom E., Lofvenius G., Rappe C. and Marklund
E., Medved, M. and Zerjal, E., Chemosphere, 2029.
31. Iida, T., Nakanishi,
M. and Goto, K., J. Polym. Sci.,
1974, 12, 737.
32. Ahling, B., Bjorseth, A. and Lunde, G., Chemosphere, 1978, 799.
33. McNeill, I., Memetea, L. and Cole, W. J., unpublished. 34. Crawley, S. and McNeill, I. C., J. Polym. Sci., 1978, 16, 2593.
35 Hjertberg, T. and Sorvik, E. M., in Degradation and Stabilisation of PVC, ed. E. D. Owen. Elsevier, London, 1984, p. 21. 36. Aubrey, N. E. and Van Wazer, J. R., J. Am. Chem. Sot., 1964,86,4380.
PII:
Printed in Great Britain front matter
0141-3910/98/%see
SO141-3910(97)00272-3
ELSEVIER
Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis Ian C. McNeill,“* Livia Memetea,” Musarrat H. Mohammed,” Alwyn R. Fernande# & Peter Ambidgebt “Chemistry Department, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ. UK bAEA Technology, 551 Harwell, Didcot, Oxfordrhire OX11 ORA, UK
(Received 23 September 1997; accepted 10 October 1997) The tars produced by PVC pyrolysis under different conditions of temperature and oxygen concentration were analysed for PCDDs and PCDFs content. The tar collected after low temperature pyrolysis (SOODC)and in reduced oxygen atmosphere (11.6% 02) had the highest PCDDs/PCDFs content. The toxicity of the tars is mainly due to the high PCDFs concentration. The most toxic compound, 2,3,7,8-T4CDD, had a contribution of only 054.4% of the total toxicity of the tar. Several hypotheses for PCDDs/PCDFs formation during PVC pyrolysis, based on the analysis of the tar composition, temperature interval in which is formed and the mechanism of PVC breakdown are nronosed. 0 1998 Elsevier Science Limited, All rights reserved
1 INTRODUCTION
(CM 1
1.1 Background to this investigation
/ \ 6-??
n=4-8
I
II
:RD
/
3
4
The combustion and incineration of organic wastes such as wood, coal, garbage and chlorine-containing polymers is accompanied by the formation of trace compounds of very high toxicity-the polychlorinated dibenzo-p-dioxins (I), dibenzofurans (II) and biphenyls (III) designated as PCDDs, PCDFs and PCBs. Formulae I-III list the groups in which the highly toxic isomers can be found. These groups are designated by the degree of chlorination n, with n = 4-8 for the toxic isomers of PCDD/PCDF and n = 4-10 for PCBs, respectively. Each group is made up of a number of positional isomers called congeners. Structures IV-VI show the numbering of the positions in the PCDD, PCDF and PCB structures.
~4-8
\
2
n=4-10
IV
III
1 2
:m
3 4
6 V
40-04 5’
6
6’
5
VI
The most toxic of the above trace pollutants is 2,3,7,8tetrachlorinated dibenzo-p-dioxin (2,3,7,8T4CDD). Any PCDD/PCDF isomers having these positions occupied are toxic but with various degrees of toxicity as compared to 2,3,7,8-T4CDD. The international toxic equivalent factor (I-TEQ) has been defined as the relative potency of the dioxin-like compound to produce the same effect when compared with 2,3,7,8-T4CDD. This means that the toxic isomer can produce the same toxic effect as 2,3,7,8-T4CDD in a concentration l/ITEQ times higher than that of 2,3,7,8-T4CDD. PCBs vary in toxicity according to the type of congener and Cl content. The isomers with Cl
*To whom correspondence should be addressed. ‘Present address: CBD, Porton Down, Salisbury, Wiltshire, UK, SP4 OJQ. 145
146
I. C. McNeil1
atoms in positions 4,4’ or 3,4 or 3,4,5 on one or both rings are more toxic, The most toxic isomers include 3,3’4,4’,5-P5CB, 3,3’,4,4’-T4CB, 3,3’,4,4’, 5,5’-H6CB. All the toxic PCDDs, PCDFs and PCBs tend to assume a relatively coplanar conformation, generally similar to that of 2,3,7,8-T4CD. All these pollutants are environmentally persistent resisting bacterial and chemical breakdown. They accumulate in the environment in lake sediments, fish, birds, mammals. Humans exposed to wastes containing 60 mg 2,3,7,8-T4CDD/kg wastes presented a 39% higher cancer mortality than the average population. 1 Other serious conditions following PCDD poisoning are: chloracne, thymic atrophy (a wasting syndrome), immunotoxic impairment effects, reproductive and liver damage. The development of the analytical means for the isolation, detection and quantification of these micropollutants in the 1980s coincided with the extended use of PVC which became the second most used polymer (after polyethylene). The presence of PVC waste in the municipal garbage acquired the blame. 2 Even though research work from the plastics industry claimed that garbage with or without PVC produces the same level of PCDDs/ PCDFS,~ PVC was not exonerated since it was proved that industrial operations such as the burning of the PVC insulated scrap wire or the hot iron soldering of the same wire produced copious amounts of dioxins. The present work is part of a project supported by EPSRC for evidencing and quantifying the environmentally toxic products formed by PVC pyrolysis. The first paper of the series analysed the products formed through pyrolysis of PVC alone and in some industrial formulations with dioctyl phthalate.4 The second paper presented a quantitative evaluation of the pyrolysis fractions and an investigation into the mechanism of PVC pyrolysis.5 This paper reports on work with the purpose of analysing the PCDDs and PCDFs produced by PVC pyrolysis in laboratory conditions which approximate to those used in incineration. The decision to analyse the PCDDs/PCDFs content of the PVC pyrolysis products was taken because, in spite of the abundant work done on PVC mixed with other waste in municipal waste incinerators (MWI), there is limited information on the PCDDs/PCDFs isomer specific determination and total toxicity of pyrolysis products.2 Since PCBs are of lower toxicity than PCDDs/PCDFs, they
et al.
were not analysed in this study. However they will be considered as potential precursors of PCDFs. 1.2 Survey of main reactions for PCDDs/PCDFs formation An impressive number of authors have investigated experimental routes for the synthesis of PCDDs and PCDFs, starting with different reactants. An excellent book reviews the possible routes for PCDDs, PCDFs and PCBs formation from various monomers, polymers, organic and inorganic materials.6 We shall limit our discussion here to routes which are relevant for PVC. Potentially, the dioxins can be formed by the oxidative chlorination of organic compounds both in the gas phase and in condensed matter. A de nmvo synthesis of PCDDs/PCDFs from elemental carbon in the presence of C4, O2 and H2 and catalysed by fly ash has been identified7 and can take place at temperatures as low as 300°C. A mixture of propene and HCl produces at 500580°C and in the presence of fly ash a mixture of polychlorinated benzenes, PCDDs, chlorinated mono- and dibenzofurans.8 Interestingly, ethene plus HCl in the presence of fly ash does not generate any of the above compounds.8 The catalytic effect of Cu, Fe, A1C13in the fly ash in the process of dioxin formation from precursors is well documented but we shall not proceed along the line of catalysed formation of PCDDs/PCDFs since the present paper studies the pyrolysis of pure PVC which has no inorganic content and forms no ashes upon complete pyrolysis. The main precursors in PCDDs/PCDFs formation are considered to be the chlorinated benzenes, phenols, diphenyl ethers and PCBs as follows. Polychlorinated benzenes can form PCBs in the absence of air or can be oxidized to polychlorinated phenols and further to PCDDs/PCDFs if oxygen is present. In the absence of air the yield of PCBs produced from polychlorinated benzenes depends very much on temperature. At 690°C in N2 with a residence time of 24s monochlorbenzene can form dichlorobiphenyl as a major product but at higher temperature, 770°C the ring is ruptured and gaseous products are mainly formed. lo The pyrolysis of tri-, tetra- and penta-chlorobenzene at 600°C in sealed miniampoules in the presence of air has produced chlorobenzenes with a higher degree of chlorination than the original chlorobenzenes and also PCDDs, PCDFs, chlorophenols and PCBs.” The
147
Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis
of the PCDFs was 10-50 times higher than PCDDs and the lower chlorinated PCDDs and PCDFs were formed in higher concentrations, i.e the pattern was not that encountered in pyrolysis or combustion generated PCDDs/PCDFs. Biphenyls can generate PCDFs by oxidation with the loss of one to two chlorine atoms or two H atoms from the ortho-position (Scheme 1, reactions (l)-(3)).12,‘3 During oxidation by reaction (1) a shift of the 2,3 Cl atoms at the benzene ring is also possible producing another isomer. The PCDF yield of reactions (l)--(3) is very low, in the
formation of polychlorinated benzenes with a higher degree of chlorination than the original materials indicates that a thermal isomerization is occurring. For example, trichlorobenzenes gave rise to tetra- and pentachlorobenzenes, tetrachlorobenzenes gave rise to penta- and hexachlorobenzene and pentachlorobenzene gave rise to hexachlorobenzene. Certain isomers are formed preferentially, for among triexample chlorobenzenes the 1,2,4- isomer is formed predominantly and so is 1,2,3,5-tetrachlorobenzene among the tetrachloroisomers. The concentration
l
@+l,2o2 A
c133-(=$ +112o2
Ci
@
L
kl
c,w, Cl
Scheme 1
f ZHCI
0
Cl
(2)
+H20
(3)
148
I. C. McNeil1
et al.
range of 0. l-several %. The reaction was conducted in closed ampoules by heating from 20 to 600°C in 1 min with 5 s residence time at 600”C.13 Both PCDDs and PCDFs can also be formed from polychlorinated phenols as precursors. Heating pentachlorophenol in a tube 1 m long at 300°C for 24 h produced hexachlorobenzene and OCDD in a ratio of 8.5:1 l4 (reaction (4)). The reaction proceeds through an intermediate-the chlorinated phenoxyphenol species VII, formed through an equilibrium (5).i5 The chlorinated phenoxyphenol species VII is present as an impurity in pesticides based on chlorinated phenols and is responsible for the formation of PCDDs associated with these pesticides.i6 In another experiment the combustion of 2,4,6tri- and 2,3,4,6-tetra- and pentachloro-phenol mixed with wood chips indicated the formation of PCDDs of all degrees of chlorination.17 The pyrolysis of eight isomers of polychlorinated diphenyl ethers containing 3-6 and 8 Cl atoms has shown the formation of mainly PCDFs as pyrolysis products but also some PCDDs.‘* The pyrolysis took place in closed ampoules and the highest yield was found at 600°C 0.7-4.5% depending on the isomer. At 700°C most of the PCDF/PCDDs have decomposed and the yields were about 20 times lower. Finally, PCDDs can be formed through the pyrolysis of chlorophenoxyacids or better, their Na salts6 but compounds with such a high oxygen content were not identified in PVC pyrolysis.
effluent gases from municipal waste incineration (MWI)3,20-28 in which PVC is present at a level of 0.5-5% mixed with a wide range of materials. The influence of the PVC content in the waste on the PCDDs/PCDFs level in the gaseous effluents from incineration is still a debated matter due to the publication of many conflicting results. A clear correlation between PCDDs/PCDFs level and PVC concentration in the waste could not be demonstrated. Some authors have found no differences in the PCDD/PCDF level in emissions whether the source of chlorine in the waste was NaCl or PVC3~2oor when fuel of different origin was mixed with variable amounts of PVC up to a chlorine level of 5.6% (10% PVC).21,22 The emission of’ chlorobenzenes20 or chlorophenols23q24 as an indicator of the risk of PCDD/ PCDF formation was monitored in various conditions and it was shown that it does not change significantly upon replacing PVC with NaCl as a source of chlorine. Other authors have found increased PCDDs/PCDFs or precursors emissions with increasing PVC concentration.25,26 The presence of PVC in MWI or in wood burning facilities continues to generate concern as larger facilities could release as much dioxins as 7.2 pg ITEQ/h with the effluent gas.27 Medical waste incinerators in hospitals are also a major source of concern since PVC is widely used for disposable medical items.28
1.3 PCDDs/PCDFs formation in PVC pyrolysis and combustion
2.1 Pyrolysis conditions
PCDDs/PCDFs formation in the incineration of PVC was first observed in a copper recycling plant where PVC insulated copper wire was burnt2 and also in building fires.” Only one paper attempted a clear account of the PCDD/PCDF congener composition produced in PVC pyrolysis or combustion,2 to our knowledge. This, the only systematic investigation of the pyrolysis and combustion of pure PVC and copper wire insulation, failed to detect many of the toxic isomers of T4CDD-H6CDD. However, it demonstrated the influence of PVC molecular weight on the level of dioxin formation and the role of copper catalysis. Most of the published material refers to the formation of PCDDs/PCDFs in the soot and the
2 EXPERIMENTAL
Suspension grade PVC from European Vinyls Corporation with a MW of 40000 as determined by GPC was pyrolysed as described below. One hundred milligrams of PVC powder was spread as a uniform thin layer in a silica boat which was introduced in a silica tube which was placed in an oven. The silica tube ended with a thinner tube spiralling in the oven through which a flow of gas was introduced over the sample. The role of the spiral tube was to allow the heating of the gas before it reached the sample. Outside the oven, a water cooler of about 25cm was inserted in the outer end of the silica tube to allow for the condensation of the pyrolysis products with low volatility (high boiling point). The more volatile pyrolysis products which escaped the water cooler condensed further on in a trap cooled with
149
Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis
methylated spirit and liquid nitrogen at a temperature of -70°C. Noncondensable gases, C& hydrocarbons, HCl and CO*, were able to pass over the traps and leave the system through the gas exhaust which contained filters which were later destroyed as toxic material. Air or a 1:1 mixture of air and nitrogen was introduced into the system at a rate of 80mlmin-*. The temperature rise of the oven was programmed at lO”Crni~--~ up to either 500°C or 1000°C where a period of isothermal heating of 1Omin was allowed. The residence time of the pyrolysis products in the oven was calculated as 0.9s. The residence time is defined as the time taken by 1 ml of gaseous product to pass through the bulk volume of the pyrolysis vessel in the oven. Preliminary determinations have shown that PCDDs/PCDFs can be found in the tar fraction. At the end of the pyrolysis experiment, the tar was collected from the condenser zone by washing with dichoromethane. As an additional precaution, dioxins possibly entrained in the liquid fraction were also recovered. In the trap cooled at -70°C there was a liquid fraction consisting mainly of water, benzene, toluene, xylene and ethylbenzene. This liquid fraction was evaporated under a gentle stream of nitrogen, the trap was washed with dichloromethane and the washings were added to the tar fraction. The tar contained both a major fraction of polyaromatic hydrocarbons (PAH) and a significant fraction of oxygen-containing derivatives of PAH (ketones, aldehydes, ethers, alcohols, lactones) constituting the matrix from which the PCDDs/PCDFs had to be recovered. After the evaporation of the dichlormethane, the tar was subjected to matrix purification and PCDD/PCDF determination as follows.
123789 HCDD, 123678/234678 HCDF, 1234678 HpCDD/F, OCDD, OCDF), solvent exchanged to hexane to exclude polar interfering compounds and then treated with concentrated sulphuric acid followed by AR water. The purification and analysis stages have been described before2g and are summarised below. This method is NAMAS accredited. The hexane extract thus derived from the tar was chromatographically purified using a multilayer column containing acid-modified, base-modified and unmodified silica. The column was eluted with hexane directly onto a conditioned Florisil@ column where the PCDD/Fs were fractionated from similar compounds by elution with successively polar solvents-different compositions of chloroform-modified hexane, followed by dichloromethane. The PCDD/Fs were collected in the last dichloromethane fraction and concentrated using a gentle stream of dry air. This fraction was then solvent exchanged to 100 ~1 of the keeper solvent which contains the syringe standard mixture. This standard is used to calculate the efficiency of the extraction and purification procedures. The purified extract derived from this procedure was analysed using HRGC-HRMS. The chromatographic separation was carried out using a 60mx0.22 mm id, DBS-MS GC column. The mass spectrometer was operated in selected ion recording mode using voltage scanning, at a resolution of approximately 10000. The PCDD/Fs were monitored in five groups from tetra- to octachlorinated homologues, determined by chromatographic retention.
2.2 Matrix purification and PCDD/PCDF analysis
The amount of tar collected under various pyrolysis conditions is shown in Table 1. A comparison between samples Al and A5 shows that most of the tar is formed up to 500°C. Little tar if any is formed at temperatures between 500 and 1000°C. On the contrary, above 500°C a small fraction of the tar seems to be lost (burnt). Another observation based on Table 1 is that the pyrolysis in an oxygen-deficient atmosphere leads to a higher
The tar residue was washed out of the container with hexane followed by acetone. The insoluble remaining residue was dissolved using dichloromethane. A portion of the combined washings was taken for analysis. This portion was spiked with a i3C-labelled internal standard mixture (2378 TCDD/F, 12378 PCDD, 23478PCDF, 123478/
3 RESULTS
Table 1. The amount of tar collected per 100 mg pyrolysed PVC under different pyrolysis conditions Sample Al A5 LM
Maximum
temperature 1000 500 500
(“C)
mg tar (100 mg)-’ 5.50 5.67 18
PVC
Gas Air Air Air:Nz 1:l
O2 Concentration 23.25 23.25 11.6
(%)
150
I. C. McNeil1 et
fraction of tar than the pyrolysis in dynamic air as can be seen from comparing samples A5 (5.67% tar) with LM (18% tar). Table 2 presents the concentration of the toxic isomers of PCDD/PCDF expressed in ng isomer g -i tar for the samples pyrolysed under the conditions presented in Table 1. It also lists the values of I-TEQ for each congener used to calculate the total toxicity of the tars expressed in ITEQ, which was obtained by multiplying the concentration of a specific isomer by its I-TEQ and summing up for all the isomers to give the results of Table 2. Several observations can be made regarding the results in Table 2. When the pyrolysis is conducted in air to 1000°C (sample Al) the concentrations of PCDDs are 1.55 4 times lower than when the pyrolysis stops at 500°C (sample A5). On the contrary, more PCDFs are formed when the pyrolysis is conducted up to 1000°C. Pyrolysis conducted to 500°C and in an oxygen-deficient atmosphere (sample LM) produces the highest levels of PCDDs and PCDFs. The most toxic isomer, 2,3,7,8-T4CDD, is now detected as 3.94ngg-‘. The rest of PCDDs are 5-6 times as much and the PCDFs are 3-8 times as much as at 1000°C. In general, the higher chlorinated isomers (hepta, octa) for both PCDDs and PCDFs are formed in concentrations one or two orders of magnitude higher than their lower chlorinated isomers with the exception of sample A5, for which the PCDFs
Table 2. The concentration in ngg-’
Isomer 2,3,7,8-T4CDD 1,2,3,7,8-P5CDD 12 7 73 94 97 ,8-H6CDD 1,2,3,6,7,8-H6CDD 12 > 93 97 38 99-H6CDD 1,2,3,4,6,7,8-H7CDD OCDD Toxicity of PCDDs (I- TEQs) 2,3,7,8-T4CDF 172 ,3 97 78-P5CDF 2,3,4,7,8-P5CDF 1,2,3,4,7,8-H6CDF 1,2,3,6,7,8-H6CDF 1,2,3,7,8,9-H6CDF 2,3,4,6,7,8-H6CDF 1234678-H7CDF >,,11> 1,2,3,4,7,8,9-H7CDF OCDF Toxicity of PCDFs (I-TEQs) Total toxicity Average analytical recovery
al.
are surprisingly low. Also, in general, the levels of PCDFs are considerably higher than that of PCDDs, again with the exception of sample A5. Our results are in general consistent with former dioxin analysis for PVC pyrolysis2 with the exception of the fact that the cited paper has not identified many of the lower chlorinated isomers, especially 2,3,7,8-T4CDD. The total toxicity of our tar obtained through pyrolysis up to 500°C and in an oxygen deficient atmosphere is 178 I-TEQs, ng g-i, well above the toxicity reported previously for PVC pyrolysis (1941 I-TEQ, ngg-1).2 Although the dioxins are generally more toxic, the highest contribution to the overall toxicity of our tar is due to the PCDFs which are formed in greater amounts. The results presented in Table 2 suggest that the PCDD/PCDF isomers have their own temperature range and oxygen concentration range which favour formation over decomposition and the isomer pattern in Table 2 is a resultant of these reactions. Table 3 presents the total PCDD and PCDF content of congeners of tetra- to octachlorodibenzodioxins and dibenzofurans. Table 3 shows that the most toxic congeners represent only a small fraction of the total concentration of all the congeners within the group. For example 2,3,7,8-T4CDD represents less than 1% of the total T4CDD for samples Al, about 1% for sample A5 and 5% for sample LM.
tar of the toxic PCDD and PCDF congeners pyrolysis conditions
and the toxicity
of the tars formed under various
Al (lOOoOC, air)
A5 (SOO’C, air)
LM (SOO’C, 11.6% 02)
co.12 1.22 1.29 1.88 1.68 28 23 1s2 3.68 15.2 10.9 54 48 12.9 24 310 100 300 24.9 26.4 85%
0.47 4.63 2.64 6.04 5.02 70 35 4.89 3.37 11.3 7.2 4.02 3.72 1.82 3.26 7.72 2.11 3.16 5.89 10.8 71%
3.94 0.27 6.36 10.5 8.09 180 140 12.0 11.8 90 69 380 330 100 160 2100 610 2200 166 178 86%
TEF I 0.5 0.1
0.1 0.1 0.01 0.001 0.1
0.05 0.5 0.1 0.1 0.1 0.1 0.01 0.01 0.001
Polychlorinateddibenzodioxinsand dibenzofurans in PVC pyrolysis Table 3. Total PCDD md PCDF content (ng g-l tar)
Isomer
Al
A5
LM
T4CDD PSCDD H6CDD H7CDD OCDD T4CDF PSCDF H6CDF H7CDF OCDF
12.1 17.4 53 52 23
43 89 170 130 35 120 80 33 18 3.2
75 110 240 320 140 480 1100 2600 4300 2200
110
150 360 650 300
4 DISCUSSION 4.1 Product formation and temperature intervals in PVC pyrolysis During pyrolysis in vacuum PVC generates several product fractions which can be distinguished and quantified.4 4.1 .I Hydrogen chloride This formed with a yield of 53% relative to PVC. If all HCl was eliminated the value would be 58.4%. This means that 10% of the Cl atoms remain in the charring polymer to form organic chlorinated products. HCl emission takes place in the temperature interval of 200-350°C with a maximum around 25&300”C.4~30 After 300°C there is little HCl emitted although the emission does not cease completely. 4.1.2 Other gases This gaseous fraction is made up of Cl-C4 alkanes and alkenes and some hydrogen. MS ion monitoring has shown that the major part of this fraction is formed at temperatures higher than 350°C. This fraction accounts for 5.6% of the initial polymer by weight. 4.1.3 Liquid fraction The liquid fraction consists of aromatic hydrocarbons with MW up to 150 and accounts for 7% of the original PVC. In this fraction, chlorinated compounds like chlorobenzene (0.15% of the liquid fraction), benzyl chloride plus ochlorotoluene (0.2%), o-tolylchloromethane plus 2-chloropropylbenzene (0.35%), Cl-hexane, Clhexene, 1-Cl-2- hexene (0.2%), 2-Cl-butane, 2-Cl1-butene, 2-Cl-2-butene, 2-Cl-3-butene were found. Dichlorobenzene was identified in concentrations one order of magnitude lower than the above
151
chlorinated hydrocarbons and trichlorobenzene, tentatively. No higher chlorinated benzenes were identified by the present authors and trace analysis was not attempted for their identification. However, there are reports that pyrolysis of PVC in the temperature range of 200-800°C has shown the formation of monochlorbenzene, o-, p-dichloro1,3,5-trichlorobenzene and 1,2,4-&ibenzene, chlorobenzene in trace amounts.31 Combustion experiments between 570-l 130°C at different oxygen concentrations of 0.5-16% in the effluent gases have shown the formation of chlorihexanated benzenes from dichloto chlorobenzenes and PCBs with a total amount of bound organic chlorine of 2.2-60mg g-’ PVC32 which represents 0.4-l% of the total amount of Cl in PVC. Increased temperature and decreased transit time seemed to favour their formation. 4.1.4 Tar/wax fraction A tar/wax fraction called hereafter ‘the tar’ consists of polynuclear aromatic hydrocarbons (PAH) with 2-6 rings, their methyl substituted derivatives and some Cio-Cl3 alkenes and alkanes (waxes). The tar is mainly formed after 350°C upon the breakdown of the crosslinked polyene system remaining after HCl emission. The tar is the major fraction after HCl accounting for 24% of the initial PVC. 4.1.5 Char residue The char is what remains after the polymer pyrolysis at 500°C. It represents 9.5 % of the weight of the initial PVC sample. The pyrolysis in air showed an increased gas fraction, decreased liquid, tar and char fractions and the presence of 0 in the charring polymer.5 Recent work33 has identified a fraction of oxygencontaining PAH in the tar next in amount to the main fraction consisting of PAH. Polyaromatic ketones such as benzanthracen-7-one, 9-anthracenone, 9, lo-anthracenedione, 1 ,Cchrysenequinone and polyaromatic aldehydes such as 9, lo-anthracenedicarboxaldehyde, 1-pyrene-carboxaldehyde and 1, I’-biphenyl-4carboxaldehyde are main constituents. Important amounts of dibenzofuran, benzonaphthofuran, phenanthrofuran, 1,3-diphenyl isobenzofuran, biphenyl ether and o-hydroxy biphenyl were also detected. Interestingly, 3,3’4,4’-tetrachloro-, 1 , l’-biphenyl- and 2chlorodibenzodioxin were present in concentrations high enough to be detected with low resolution GC-MS and without matrix purification. Our investigations have shown that PCDDs/PCDFs are formed together with the
I. C. McNeil1 et al.
152
above compounds 35&5Oo”C.
in the temperature
range from
thought to lead to PCDDs/PCDFs during the breakdown of the crosslinked polyene system. Structure VIII (Scheme 2) represents a fragment of the PVC chain containing a head-to-head structure. These structures do not dehydrochlorinate more readily than normal structures as shown by studies on model compounds prepared by chlorinating cis- 1,4-polybutadiene.34 Hence dehydrochlorination as in reaction (7), Scheme 2, has a higher probability and produces structure IX. Chlorine addition to some of the double bonds, either molecular or as free Cl radicals, creates structure (X) in which Cl substituted segments alternate with double bonds. The free radical addition of chlorine is a random phenomenon and does not have to be regularly alternating as in reaction (8) which represents a simplification for ease of illustration. The double bonds can take part in intermolecular condensation as in Scheme 3, reaction (9) creating structure (XI) in which the secondary free radicals formed in the vicinity of the hexane rings can backbite a double bond in the same chain and break the bond with the substituted cyclohexane rings in order to form an aromatic species XII. This is accompanied by a Cl atom transfer to form species XIII and the expulsion of another Cl as a free radical (reaction 10). The newly formed structure XIII is oxidized to a PCDF (PCDD) by reaction (11). In practice the
4.2 Hypotheses for PCDDs/PCDFs formations in PVC pyrolysis We shall first present a picture of the crosslinked polyene breakdown from which stems a hypothesis for PCDDs/PCDFs formation. The zip dehydrochlorination of PVC leads to small polyenes with an average number of double bonds of 3.5, except for the initial stages when the polyenes are longer. It is believed that the zip dehydrochlorination stops at defects of chemical structure which are head-to-head structures. There is also the possibility of a small scale readdition of HCl or possibly of Cl2 to polyenes. Chlorine is formed in the system through the Deacon equilibrium (6). K 2 HCI + l/2 O2 _
H20 + Cl2
(6)
Cl radicals are also present in the system, formed through Cl2 dissociation which becomes important after 400°C. The chlorine readdition to polyene in the vicinity of head-to-head defects already containing chlorine atoms creates moieties of high Cl concentration in these zones of the polyene system. Schemes 2 and 3 present the reactions which are
-
CH-CH2-CH-CH2-CH-CH2-CH-CH-CH2-CH-CH2-CH-CH2I I I I I
dl
kl
dr
(21 6
I
I
Cl
Cl VIII
dehydrochlorination
(7)
-CH=CH-CH=CH-CH=CH_CH_CH-CH=CH_CH=CH-CH=CH_ I I Cl Cl
(8)
-
CH=CH-
CH-CHI
I
Cl
Cl
Ix
J
chlorination
CH=CH-CH-CH_CH L,
Ll
=CH_CH
-CH_CH=CH
I Cl
-
I Cl
x Scheme 2
Polychlorinated dibenzodioxins and dibenzofurans in PVC pyrolysis
(vl) ,FH=CH
I’ “w”\
CH-CH I Cl
‘\ *\
I’
,’
CH=CH
‘.
153
‘\ r
/CH=C~cH_c~cH=c~CH4~cH=c~ CH-CH I Cl
I Cl
I I Cl Cl
I Cl
r 77 CH-CH
(
)
(
)
// CH
/
I
I
Cl
Cl
1
\c H-CH
~ZH-CH ‘CH-C<
’ CH-CH ’
c\ CH-CH
CH-CH
CH-CH
I
I
Cl
Cl
\
‘CH-C/H
!
\H
CH--dH
’ CH-CH ’ Ll
I
I
Cl
Cl
I (=j+cl#cl +l (-J (10)
+
Cl
Cl
Cl
XI
Cl
+ Cl
Cl
XII
XIII
XII
Cl
ci
61
Cl
+
water, chain fragments
Cl Scheme 3
aromatization of structure XIII and its oxidation to PCDF/PCDD probably occur simultaneously since oxygen is known to help the aromatization process. On the other hand, the aromatization of substituted structures is difficult for reasons of orbital symmetry and scission of substituents often accompanies aromatization. Hence, it appears possible that with the thermal oxidative breakdown of the crosslinked polyene system forming PAH and O-containing PAH, the chlorine concentrating moieties also break down producing PCDDs/ PCDFs and other chloroaromatics. The presence
of head-to-head structures is not necessary to explain the PCDDs/PCDFs formation through this route, but it is useful in explaining the preferential formation of highly substituted isomers. The cracking pattern of the crosslinked polyene system in the parts concentrating Cl atoms resembles to some extent the cracking pattern of highly Cl-substituted aliphatic hydrocarbons and perchlorocarbons (C/Cl ratio 1:4 to l:l, respectively). This pattern involves C-C scission rather than CCl scission35 and aromatization between 400 and 600°C.
154
I. C. McNeil1
A crude estimate of the concentration of the PCDDs/PCDFs formed from head-to-head structures can be compared with the total concentration of PCDDs/PCDFs found in the present experiments. The head-to head structures are found in a maximum concentration of 0.2/ 1000 vinyl chloride units.36 Considering that 20% of the initial polymer forms the tar where PCDDs/PCDFs are found, considering an average molecular weight of 400 for PCDDs/PCDFs and a yield of 1% for PCDDs/PCDFs formation from head-to-head structures, a total concentration of 60 ng PCDDs/ PCDFsg-’ tar is calculated. Table 3 shows a total concentration of 102-lo4 ng PCDDs + PCDFs g-r tar which suggests that either the yield is underestimated or an additional source of dioxins exists. Let us examine another possibility for PCDDs/ PCDFs formation-benzene oxychlorination with Cl2 formed through the oxidative decomposition of HCl (equilibrium 6) which is well established in the literature.4,‘1 In PVC pyrolysis, HCl and benzene are emitted simultaneously.4,30 The main temperature interval for HCl and benzene emission is between 200 and 350°C. However, HCl and benzene emission never ceases completely for they are emitted in lower amounts over the whole temperature interval up to 500°C. The thermal equilibrium (6) through which Cl2 is formed in the system has a high equilibrium constant K = [H20][Clz]/[HC1]*[02]“* as calculated from thermodynamic data.6 The calculated K values are 4.17x lo9 at 300°C and 3.3 1 x 10’ at 500°C and they decrease considerably above 500”C,6 which could explain why PCDDs/ PCDFs formation is favoured in pyrolyses up to 500°C. Since the free radical chlorination of benzene is known to favour the formation of the highly substituted hexachlorobenzene, the preferential formation of the highly chlorine substituted PCDD/PCDF isomers can also be explained by this possibility. A study of the PCDDs/PCDFs levels as a function of temperature starting at 300°C would clarify the participation of equilibrium (6) in PCDDs/PCDFs formation. Also, the measurement of the levels of these materials in experiments in which the benzene and HCl emissions have been suppressed by mixing PVC with metal oxides could establish whether benzene oxychlorination is the major source of dioxins in PVC pyrolysis. This idea is based on the observation that the presence of metal oxides in
et al. PVC suppresses the formation of benzene while increasing the char formation.30
5 CONCLUSIONS PCDDs/PCDFs formed in PVC pyrolysis in a dynamic experiment conducted up to either 500 or 1000°C in air and in reduced oxygen concentration were analysed from the point of view of the toxic congeners level and isomer group level. The PCDDs/PCDFs formed in PVC pyrolysis were detected in a fraction of pyrolysis products which is formed above 350°C. The PCDDs/PCDFs are found in a matrix of PAH and oxygen containing polyaromatic compounds constituting the tar-a fraction of low volatility compounds. Pyrolysis at high temperature (1OOOC) and air produced the least amounts of PCDDs. Lower temperature and oxygen deficient atmosphere produced the highest amounts of PCDDs/PCDFs. This pattern confirms previous results reported on PVC combustion and incineration.2y3 Under any conditions, the amount of the most toxic congeners is only a small fraction of the total amount of congeners. For example the amount of the most toxic 2,3,7,8-T4CDD is only l-5% of the total amount of the tetrachlorinated congeners. Although of lower toxicity, PCDFs are formed generally in much higher amounts than PCDDs and for this reason their contribution to the total toxicity of the tar is much higher than that of dioxins. The higher chlorinated isomers (hexa-, hepta-, octa-) of both PCDDs and PCDFs are generally formed in higher amounts than the lower chlorinated isomers (tetra-, penta-). None of the proven ways of synthesis existing in the literature can explain this fact. Two hypotheses on PCDDs/PCDFs formation during PVC pyrolysis were examined and retained as most probable. The presence of zones of high chlorine substitution in the crosslinked polyene system can produce PCDDs/PCDFs upon the oxidative breakdown of the crosslinked polyene system in a moderate oxygen atmosphere. The second possibility for the PCDDs/PCDFs formation in PVC pyrolysis is from the benzene and HCl emissions of this polymer. Though it is already an accepted fact in the literature that the oxidative chlorination of benzene at temperatures between 300-500°C produces PCDDS/PCDFS,~>” this might not be the only mechanism of PCDDs/ PCDFs formation in PVC pyrolysis.
Polychlorinated
dibenzodioxins
and dibenzofurans
155
in PVC pyrolysis
The total toxicity (I-TEQs) of the tar obtained through pyrolysis at low temperature (500%) and in reduced oxygen concentration found in our experiments is the highest reported in the literature in connection with PVC, as revealed by a comparison between the total I-TEQs of our tar and the values we calculated from the data of other references dealing with PVC. It confirms that under certain conditions the pyrolysis of PVC is indeed an important source of PCDDs/ PCDFs.
10. Fields, E. K. and Meyerson, S., J. Am. Chem. Sot., 1966,
ACKNOWLEDGEMENTS
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88, 3388.
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690.
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The authors gratefully acknowledge financial support from EPSRC for the work described in this paper, and are indebted to European Vinyls Corporation for the supply of a PVC sample.
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