PCBs AND ASSOCIATED AROMATICS 933
HYDRODECHLORINATIONAhonkhai et al.^96 have investigated the thermal hydrodechlo-
rination of hexachlorobenzene by passing chlorobenzene
vapor into a quartz tubular flow reactor at different tempera-
tures. Above about 900°C a 99% conversion to HCl was
observed. When H 2 was replaced by CH 4 the same reaction
took place but it occurred with the formation of copious
quantities of soot.
The catalytic hydrodechlorination of PCBs was investi-
gated by Lapiere and co-workers^97 using either 61% nickel
on Kieselguhr or 10% palladium on charcoal. The method
requires the handling of high pressure hydrogen at 30–50
atmospheres and the HCl produced is a source of catalyst
poisoning and corrosion problems. Approximately 90%
dechlorination is achieved in five hours at 100–200°C.
A nickel catalyzed zinc reduction of PCB has been
reported by N.S. Chu and S.C. Vick^98. The reaction is best
carried out in an aprotic solvent, such as dimethylformamide
(DMF), rather than a protic solvent such as water or alcohol.
The major reaction product of PCBs is biphenyl. The reaction
of trichlorobenzene in a mixture of MDF and methanol was
found to be essentially complete after four hours at 50°C.
The mechanism of reaction was suggested to parallel
the Pd(OAc) 2 catalyzed dehalogenation reaction reported
by Okamoto and Oka^99. These authors explain their obser-
vations by proposing that a low valent metal complex was
generated in situ and reacts with aromatic halides to give
oxidative addition products which are then reduced by the
alkoxide ion. The nickel catalyzed reaction of Chu and Vick
was therefore suggested to involve similar steps as follows.- the reduction of Ni(II) to Ni(0) by Zn;
- oxidative addition of aryl halide to the lower
valent Ni and; - reaction of the aryl-Ni species with alcohol sol-
vent to form a reduced product or reaction with
another aryl halide to give a coupling product.
A similar mechanism may exist for similar catalytic reac-
tions involving the Pd or Ru catalyzed reduction of aromatic
halides by hydrogen sources, such as hydrazine,^100 amine,^101
formate,^102 alcohol or alkoxide.^99
A uniquely effective catalytic dechlorination method cur-
rently under investigation by the author and which will be the
subject of patents, has been shown to completely dechlorinate
Arochlor 1260, a transformer dielectric fluid, in 4 minutes at
room temperature. The reagent, packed into a column con-
figuration in a mobile system, allows the reaction to be used
on-site for the destruction of askarels under mild chemical
conditions without the liability of transportation and interim
storage at a high temperature incinerator facility.MATRIX ISOLATIONSolid solutions which are held together by weak Van der
Waals forces are probably inapplicable to the fixation ofPCBs. A chemically fixed product, on the other hand, is able
to hold the contaminant even though it may fragment under
service conditions. A third type of fixation product is one
which contains microencapsulates. Microencapsulates con-
sist of microscopic and macroscopic contaminant particles
encased by the fixative to yield a product which maintains its
integrity under environmental stress (Lubowik et al.^103 ). PCB
fluid has been disposed of as a nonleachable solid waste by
microencapsulating it in concrete. The effectiveness of the
method is limited for large volumes of PCBs because of the
amount of concrete which is generated and the possible risk
associated with the long term leachability of the product.
Organic wastes are not usually amenable to fixation but
Envirosafe Services Inc., in Horsham, Pennsylvania has used
pozzolanic chemistry to chemically fix PCBs in a rock-like
product which has good load-bearing properties and which
is hydrolytically stable. Three PCDD contaminated sites in
Missouri have been used to assess the feasibility of cementi-
tious and asphaltic stabilization techniques. Asphalt alone
was found not to be satisfactory but a combination of lime and
asphalt was found to solubilize the 2,3,7,8-TCDD sufficiently
to avoid its detection in leachate studies.VITRIFICATIONIt situ soil vitrification is being proposed by Battelle (des
Rosiers et al.^104 ) in which heat from the electric current
passed into electrodes placed in the ground converts the soil
and its contaminants into a non-leaching fused glasslike
material. Temperatures exceeding 20,000°C are produced.
Gases and volatile materials are collected and treated on the
surface. The product obtained is a grasslike material which
is more or less homogeneous depending upon the duration
of heating and the extent of convective mixing which has
occurred while the zone is molten.TRANSFORMER RETROFILLThe retrofilling of askarel transformers to a safe and cost-
effective level would practically eliminate the 99.8% of PCBs
located in transformers. Work done by M. Erickson et al.^23 at
Midwest Research Institute on the formation of toxic pyroly-
sis products in PCB contaminated transformer oils has shown
that there is a narrow range of temperature and oxygen con-
ditions which permit the formation of PCDFs and PCDDs
from PCBs and chlorobenzenes from dilute solutions. The
probability of formation of toxic products from low concen-
tration askarel residuals in retrofill fluids at concentrations
of concern under real world conditions, however, seems to
be extremely small. For example, a fire which occurred in
the basement transformer vault of the Electrical Engineering
Laboratory of the University of Manitoba during March,
1982 involved six mineral oil transformers contaminated
with 250 ppm PCB. An electrical failure had caused one of
the transformers to arc and catch fire. All six transformers
were damaged by the fire. During fire-fighting attempts, aC016_003_r03.indd 933C016_003_r03.indd 933 11/18/2005 1:12:44 PM11/18/2005 1:12:44 PM