938 PCBs AND ASSOCIATED AROMATICS
method shares the disadvantage of conventional distillation
that many components exhibit overlapping boiling ranges.
The addition to the PCB in oil system of an agent capable
of forming a low boiling azeotrope has been investigated.
In this case, separation can be accomplished by distilling a
small quantity of azeotrope rather than a large quantity of
transformer oil.
Solvent extraction of PCBs from transformer oil has
been investigated by Oak Ridge National Laboratory using
dimethyl formamide. A second stage solvent extraction
with water reduces the volume of fluid for disposal. The
process may be found particularly useful in the treatment
of PCB contaminated lubricating oils since fluids of this
type typically contain additives which tend to react with
alkali metal organometallic dechlorinating reagents thereby
making the process expensive in both reagent and additive
replacement cost.
The Franklin Research Center has reported (laconi-
anni et al.^110 ) a study in which measurements were made
of the partition coefficients of PCBs in a range of poten-
tial solvents while General Electric, under EPRI contract,
has developed a solvent extraction process for separating
PCB from mineral oil. Patents have been assigned to the
USA for the extraction of PCBs from oils using organic
solvents such as methanol or isopropanol. In these cases
the extracted mixture can be distilled to recover solvent
and the solvent recycled. The difficulties encountered with
these methods may out-weight their potential advantages
in the treatment of transformer oils. It does not seem likely
that the solutions offered by physical methods will com-
pete favourably with those offered by chemical destruction
systems.RADIATIONThe dehalogenation of askarels involves an initial addi-
tion of an electron to the aromatic molecule. There is little
chemical difference whether the electron is derived from
metallic alkali metals or organometallic compounds or
from electrons generated in solution by the absorption of
radiation. The UV-photolysis of PCBs in organic solutions
produces reductive dechlorination of the PCB as the main
photochemical reaction while in aqueous or alcoholic solu-
tion replacement of the chlorine by a hydroxyl group also
occurs (Plummer^111 ). A review on the photochemical degra-
dation of PCBs to yield less toxic and more toxic products
has been written by Safe et al.^112
The Atlantic Research Corporation in Alexandria,
Virginia experimented with a UV/H 2 system to dechlorinate
PCBs in 1983. More recently, G.A. Epling and coworkers at
the University of Connecticut^113 have reported on a hydride-
enhanced photoreaction of PCDDs in which the reaction
system was found not to be overwhelmingly influenced
by the presence of impurities. In general, impurities tend
to quench target molecule excited states and competitive
light absorption by impurities also causes a slowdown in the
rate of photoreaction. In the PCDD isomer systems tested,however, the presence of borohydride caused an enhance-
ment of photoreaction in the range of about 2 to 6 times the
rate observed without borohydride. Also, the photoreaction
in the presence of sodium borohydride proceeded to yield
predominantly the direct dehalogenated product. Ultimately,
the completely dechlorinated dibenzo-p-dioxin was formed.
BH 4 can be used as a strong nucleophile in the presence
of catalytic amounts of tris(triphenylphosphine)-nickel(0).
The reaction has been reported^159 by Shaw-Tao Lin and J.A.
Roth (1979). The reaction uses conditions of about 70°C, a
nitrogen blanket, and a reaction time of 16 hours to produce
an 80% benzene yield.
Roth et al. (1994)^160 describes the hydrodechlorination
of a series of PCBs. They found that NaBH 4 caused the
dechlorination of mono- and di-chlorobiphenyls in DMF
solution without catalysts, but the reaction was very
slow (15–20 hours) and the extent of dechlorination was
very small (1–5%). On the other hand, Ni 2 B reactions in
which a reactive form of hydrogen is on the surface of
the solid was said to produce dechlorination. Roth also
used a homogeneous dechlorination reaction with tetrakis-
(triphenylphosphine)-nickel (0), which is an air sensitive
complex soluble in dimethyl formamide. Small concen-
trations of around 2000 ppm PCB were dechlorinated
between 60–100 minutes at room temperature.
The Westgate Research Corporation in Los Angeles,
California has developed a process known as the Ultrox
system which involves the UV-catalyzed ozone oxidation
of chlorinated organics in industrial waters. Hydroxyl radi-
cals are frequently proposed as the reactive species in the
UV-ozonation of organic compounds. However, accord-
ing to the experimental data obtained by Leitis et al.^114
the major oxidizing species in the UV/O 3 Ultrox system
is ozone itself. Carbonhalogen bonds are broken before
other structures are oxidized and the halogen atoms appear
in solution as halide ions. A synergistic effect between
ozone and UV light has been observed. The magnitude of
the effect is greater at low (1 ppm) that at high (100 ppm)
concentrations of PCB.
It is postulated that the mechanism of oxidation of chlo-
robenzene in the Ultrox process is as follows:Chlorobenzene UV/O chlorophenol
mixture of [(chloro)
catec+→
→3hhol (chloro)
resorcinol
(chloro)hydroxyquinone trihydroxy+
+
→ ll(chloro)
benzenes
chloro muconic acids
maleic aldehyde→
→
+()gglyoxylic acid
oxalic acid
CO 22 H O→
→+C016_003_r03.indd 938C016_003_r03.indd 938 11/18/2005 1:12:44 PM11/18/2005 1:12:44 PM