942 PCBs AND ASSOCIATED AROMATICS
Further work by Pilgrim and Webber^61 on an accelerated
PCB destruction process by metallic sodium in the pres-
ence of small quantities of isopropanol as a hydrogen donor
showed that abstractable hydrogen altered the reaction kinet-
ics and mechanism to such an extent that decontamination
was completed in about 20 minutes at room temperature.
Part of the accelerative effect is probably due to the regen-
eration of active sodium surface by direct reaction with iso-
propanol. The reaction with alcohol would tend to renew the
sodium surface and avoid the passivating effect caused by the
formation of sodium phenolate derived from 2,6-di-tertiary-
butyl-p-cresol (DBPC) used as oxidation inhibitor. Evidence
which suggests that an alteration of mechanism may have
taken place is that relatively little polyphenyl sludge is pro-
duced in the accelerated reaction.
The inhibition of dechlorination by DBPC in the reac-
tion of dispersed sodium with PCBs was investigated by
Webber and Wilson^62. The oxidation inhibitor interacts with
peroxy radicals in solution to produce a phenoxy radical
stabilized by resonance. The phenoxy radical undergoes
second-order disproportionation to yield an unstable qui-
none methide. Subsequent dimerization occurs to produce
an intensely yellow colored compound 3,3,5,5,-tetra-
tert-butyl-4,4-stilbenequinone and 1,2bis(3,5-di-t-butyl-4-
hydroxyphenylethane).
To determine whether the stilbenequinone affected
the PCB dechlorination reaction, a 1% Aroclor 1242 in
oil solution was prepared with 0.3% of the stilbenequi-
none and heated with sodium dispersion. No reaction was
observed in 10 hours where, in the absence of the stilben-
equinone, complete dechlorination was observed in about
15 minutes. Exclusion of oxygen from the reaction, and
the use of a dispersed form of sodium, was found to pre-
vent the formation of the stilbenequinone and to overcome
the inhibition of the PCB dechlorination reaction at con-
centrations of DBPC typical of transformer oil. When the
sodium was dispersed into particles of about 10 μ m and
reacted under a nitrogen blanket, a 10,000 ppm Aroclor
1242 solution was completely dechlorinated in less than
15 min. at 130°C in the absence of DBPC. When the max-
imum recommended inhibitor concentration in dielectric
oils of 0.3% DBPC was used, the reaction was complete in
less than 30 min. under the same conditions. Exclusion of
oxygen from the reaction mixture prevented the formation
of stilbenequinone and thereby avoided the inhibition of
dechlorination.
The interaction of metallic sodium with the oil produces
a reducing environment which tends to react with both natu-
ral and added alcohol oxidation inhibitors to yield sodium
chloride and, upon complete reaction, a dechlorinated
polyphenyl sludge. When the reaction is conducted under
a blanket of nitrogen there is little likelihood of producing
toxic oxidation products such as PCDFs or PCDDs. Also
the formation of the sodium salts of oxidation inhibitors and
acids tend to precipitate and can be easily separated from the
oil by filtration or washing. The product can then be easily
reclaimed as a decontaminated dielectric oil suitable for fur-
ther use by adding oxidation inhibitor.Ontario Hydro has developed the laboratory scale reac-
tions performed by the author at B.C. Hydro and has had
success in cost effectively treating large quantities of oil to
yield a reusable product. A disadvantage of the system is
that excess alkali metal is very reactive and needs to be used
with caution. Ontario Hydro has had built (1989) mobile, 40
foot trailers, containing processing equipment to apply the
sodium technology.
ENSR’s (SunOhio’s) “PCBX” process is contained in a
forty foot trailer accompanied by an auxiliary trailer contain-
ing reclamation and oil test equipment. The process has been
engineered to be continuous, as opposed to a batch opera-
tion, and runs at about 10 gal./min. Oil is pumped from the
transformer or storage tank, through a heater to a reaction
chamber where dispersed sodium is metered into the pro-
cess stream. The slurry is passed through a recovery system
containing heat exchangers, filters, a centrifuge and vacuum
degassing apparatus. PCBs are dechlorinated to less than
2 ppm PCB in oil.
Oxidation inhibitor is reported to be added to the product
oil. No alteration of the oxidation stability of the product was
observed when measured by ASTM D2440 and a low tem-
perature modified version of ASTM D2112. The presence of
sodium compounds in the oil returned to the transformer was
shown to be less than 1 ppm. The cost of be “PCBX” trailers
is high at about $500,000, but, like the B.C. Hydro process,
it has been shown to be cost-effective in the treatment of a
wide range of PCB concentrations.
Vertac Chemical Corporation in Memphis, Tennessee
has developed and patented (Howard et al.^147 ) a method for
the destruction of PCDDs in agricultural chemicals. The
PCDDs are formed in unwanted side reactions during the
product synthesis. The process uses alkali metal alcoholates
of short chain alkanols or polyalkoxyalkane glycols or alkali
metal hydroxides to react with PCDDs. The extension of the
process to the dechlorination of PCBs is obvious. The reac-
tion takes place at about 140°–220°C at atmospheric pres-
sure when sodium ethylene glycolate is used.
The alkali metal alcoholate can be produced by reacting
alkali metal with a polyol. Alternatively, the reagent can be
formed from an aqueous solution of sodium or potassium
hydroxide with an alkane-polyol such as ethylene glycol in
the presence of an azeotropic solvent under azeotropic distil-
lation conditions.
The mechanism of reaction with TCDD is believed to
occur in two steps:TCDD 4NaOC H OH (OC H OH) dibenzo-
p-dioxin 4NaCl
(NaO) di24 244+→
+
→——bbenzo-p-
dioxin+4(HOC H 24 ) 2 OThe polyhydroxy aromatic species formed in the process are
more readily decomposed by aerobic bacteriological actionC016_003_r03.indd 942C016_003_r03.indd 942 11/18/2005 1:12:45 PM11/18/2005 1:12:45 PM