PCBs AND ASSOCIATED AROMATICS 881
of PCBs: PCDFs in both Yusho and Yu-Cheng poisoning
incidents were about 200:1 the greater toxicity of PCDFs
seem to be more than sufficient to offset the difference. No
single line of evidence is conclusive but, taken together,
they do provide a compelling argument for the conclusion
that PCDFs were the primary causative agents.THE FORMATION OF COMPOUNDS OF CONCERNCompounds of concern can be contained in PCB fluids for
a variety of reasons. For example, they may be formed from
impurities in the feedstock used to manufacture PCBs or
from the cyclization of PCBs induced by heat, or even from
phenolic or ether precursors.
Polychlorinated terphenyls, quaterphenyls and naptha-
lenes could result from feedstock contamination by traces
of the aromatic hydrocarbons before the mixture was chlo-
rinated. PCNs have been identified as a pyrolysis product
from an askarel transformer fire. Their formation has been
explained (Buser^54 ) in a laboratory study by invoking the
formation of benzyne intermediates or the re-arrangement of
intermediates formed between an ortho-chloro phenyl radi-
cal with a chlorobenzen (Figure 19).
The overall effect of radical reactions on the product
distribution during askarel degradation will be effected by
both temperature and the availability of oxygen. Chittim
et al.^43 have analyzed unused North American askarels and
found that they contained less than 0.05 ppm of PCDFs. It is
therefore of concern that these workers found a correlation
between the concentration of TCDFs in used askarel with
the length of time the fluid had been in service. The degra-
dation of PCBs in electrical equipment is discussed above in
the section on the perception of PCB health effects.
The Power Transformer Guide ANSI-C57.92-1981 lists
a maximum top oil limit of 110°C which may occur when
the maximum hot spot conductor temperature is at 180°C
due to short term loading. The Distribution Transformer
Guide ANSI C57.91-1974 lists a maximum top oil tempera-
ture of 120°C which may occur at maximum conductor hot
spot temperature of 200°C. Maximum fluid temperatures in
a transformer could exceed values in the overload guide if
units are subjected to more severe loads. Units with extreme
overloads would be expected to overheat and fail. The most
probable cause of eventful transformer failure is violent rup-
ture (Doble 1982) and the quantities of any compounds of
concern which may be produced is determined by the avail-
ability of oxygen in an oxygen depleted, high temperature
reaction zone of short duration (Figure 20).
Buser^54 has shown that pyrolysis of chlorobenzenes at
600°C in the presence of excess air yielded about 1% of
tetra- to octa-CDFs and tetra- to octa-CDDs. It has also been
shown (Buser and Rappe^55 ) that the pyrolysis of PCBs occurs
intramolecularly by four alternative reaction routes to yield
different isomeric PCDF products.
The thermal degradation of PCBs can result in a com-
plex set of reactions which may produce compounds of
concern under uncontrolled conditions. The mechanism of theformation of dioxins under pyrolysis conditions is discussed
later in this section. Temperature and residence time rela-
tionships have been extensively studied to establish the
conditions necessary for satisfactory destruction (Mescher
et al.^56 ). It was found, for example, that with a Is residence
time, most PCB decomposition occurred in a temperature
range between 640°C and 740°C. Commercial incineration
equipment for the destruction of PCBs must therefore be
designed so that the energy input to disrupt the molecule is
made available either by supplying a very high temperature
or a satisfactorily long residence time. Several types of incin-
erator are found useful for this purpose and include rotary
kilns, high temperature fluid wall reactors, plasma pyrolysis
units, circulating bed combustors and so on.
Uncontrolled reactions which lead to the formation of
chlorophenols as a side reaction product have the potential to
generate PCDFs and/or PCDDs in the following ways:1) The dimerization of chlorophenates;
2) The cyclization of polychlorinated diphenyl ethers
(PCDPEs);
3) The cyclization of polychlorinated phenoxy
phenols.The Seveso accident occurred in 1976 and was one of
six similar occurrences in which uncontrolled reaction
conditions caused the dimerization of trichlorphenol produced
from tetrachlorobenzene and sodium hydroxide in ethyl-
ene glycol to yield PCDDs. The reactions are shown in
an early section in explanation of the chemistry observed in- In the case of liquid systems, reactants are retained
in the reaction zone for periods of time which are long com-
pared to the time required for the formation of product. In a
gaseous reaction, on the other hand, such as in a flame, the
reaction zone is relatively short-lived and the yield of product
is therefore less. The importance of even statistically unlikely
reactions lies in the toxicity of the products. The pyrolysis
of PCDPEs follows two competitive reaction pathways viz.,
reductive dechlorination or ring closure to PCDFs.
The cyclization of polychlorinated phenoxy phenois
is also a bimolecular reaction which yields PCDDs under
the influence of heat. The above discussion has considered
which compounds are of concern and how they may be
generated in uncontrolled reactions. There have been many
methods developed for the PCB decontamination of materi-
als and these will be discussed more fully later. Incineration
methods have already been mentioned. In general, dechlo-
rination reactions of PCBs involve an initial addition of an
electron to the aromatic molecule. The end result of the reac-
tion should be that there is no remaining organically bound
chlorine. In that case none of the chlorinated toxic compound
classes discussed above can be present in the reaction resi-
due. However, depending upon the chemistry involved and
the stoichiometric excess of reagent, toxic intermediates can
be produced which, while satisfying the requirements for
the removal of PCBs, can produce a severe problem where
only a relatively slight hazard existed before the process was
applied.
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