940 PCBs AND ASSOCIATED AROMATICS
substitution on the biphenyl ring. Neely^134 has developed a
mathematical model to predict the lifetime of selected chlo-
robiphenyl congeners in the environment. His conclusions
predict a half-life of about 3 days for monochlorobiphenyl,
150 days for di-, 579 days for tri-, 1044 days for tetra- and
3445 days for penta-.
Camoni et al.^135 evaluated the microbial degradation
of TCDD in Seveso soil. Samples were analyzed in the
presence and absence of organic compost. The results
indicated a 25 No degradation of any of the% reduction
in TCDD concentration in 480 days. Studies have also
indicated that the average TCDD remaining in the soil
after 1 year of weathering was approximately 50% at all
the concentrations tested (1 to 10 ppm). The results are
complicated by the uncertain contribution of photo-
degradative effects. On the other hand, research by Huetter
et al.^156 and also by Young et al.^137 indicates that micro-
bial degeneration of TCDD in soil is very slow under the
most optimum conditions and therefore tends to support a
conclusion that PCDDs are essentially non-biodegradable.
Several strains of bacteria have been discovered which can
metabolize PCBs. These represent five genera, Acetobacter,
Acinetobacter, Alcaligenes, Klebsiella and Pseudomonas.
The bacteria are found in river sediments.
A combination of photodegradation and bacterial deg-
radation has been investigated by R.A. Clyde in Asheville,
N.C.A. porous, high area, fiber webbing impregnated with
bacteria was mounted inside a tubular reactor so that the
webbing could be rotated on an axle and drawn through the
solution flowing through the lower half of the tube. A sub-
sequent chamber admitted UV light to degrade any reactive
material remaining after the bacterial treatment. The process
has been reported to be useful for the treatment of waste-
waters containing metal ions and may be found useful for
large scale application to PCB contaminated water. A patent
was issued on the process (#4,446,236 May 1, 1984).
It is well known that biological degradation of oil
occurs at an accelerated rate in the presence of emulsifi-
ers. Petroleum Fermentations Inc. has several patents con-
cerned with the production and use of such emulsifiers.
Emulsification clearly plays an important role in promoting
contact between PCB contaminated oil and bacteria in the
aqueous phase. Researchers at the University of Georgia
have found, for example, that Pseudomonas aeruginosa
and Serratia liquefaciens, incubated in a medium contain-
ing Tween 80, were able to destroy 97% of the PCB present
within 90 to 130 days.
The Polybac Corporation in Allentown, PA, have dem-
onstrated the in-situ degradation of PCBs using a proprietary
blend of bacteria. The higher chlorinated PCB isomers were
found to be unaffected during the 5–6 month treatment pro-
gram and were therefore chemically reduced prior to bacte-
rial treatment.
Fundamental biochemical degradation mechanisms are
not well understood. Chemical pathways are therefore dif-
ficult to predict and the potential exists for the release of haz-
ardous compounds into the environment through incomplete
degradation or system failure. Degradation rates in the fieldtend to be much slower than laboratory rates where pure cul-
tures are tested on pure compounds. Also, the problem of
PCB identification and quantitation can be very difficult in
partially degraded mixtures and process streams.
The identification of chlorobiphenyl congeners has been
the subject of extensive research. Safe et al.^138 at Texas A&M
University has synthesized most of the 209 PCB congeners
and has used these to identify the constituents of Aroclor
mixtures using high resolution gas chromatography (HRGC).
Similar work has been done by Pellizzari et al.^139 who identi-
fied 73 PCB congeners in a mixture of Aroclor 1016, 1254
and 1260 by HRGC/negative chemical ionization mass spec-
trometry (HRGC/NCI-MS). Publications by Stalling and his
co-workers 140,141 describe a data base for the isomer specific
determination of PCBs and a pattern recognition method for
the classification and determination of PCBs in environmen-
tal samples. Current research is focusing on the application
of “white rot” fungus, Phanerochacte Chrysosporium, to
degrade toxic halogenated organics. The fungus enzyme is
one of the strongest, nonspecific, oxidizing enzymes known.
In labscale tests, pentachlorophenol in water was reduced
from 250 mg/L to 5 mg/L in 24h (des Rosiers^142 ).
The mechanism of action of P. chrysosporium involves
an initial one electron oxidation to produce an aryl cation
radical which then undergoes cleavage and further oxidation
to produce quinones. The quinones are degraded further by a
quinone reducing enzyme until finally all of the organically
bound chlorine becomes chloride.ELECTROCHEMICALRussling et al.,^143 at the University of Connecticut, has
investigated the application of controlled potential electrol-
ysis as a method to dechlorinate PCBs. Radical anions of
such compounds as anthracene, 9,10-diphenylanthracene and
phthalonitrile in dimethylformamide solution form radical
anions at a Hg electrode surface. The radical anions react with
PCBs in solution to produce chloride in much the same way as
the mechanism of action of organometallic reagents. Stepwise
reduction is observed with biphenyl as an end product.
Kinetics of the reactions were characterized by cyclic
voltammetry. It was observed that the rate constant for the
rate determining electron exchange between radical anion
and chlorobiphenyl varied inversely with the difference in
reduction potential between the radical anion and the chlo-
robiphenyl.
Hydrogen peroxide generated by an alternating current
field has been used as an oxidant for the PCB decontamina-
tion of aqueous wastes. Westinghouse Electric Corporation
in Pittsburgh, Pennsylvania, has patented an apparatus for
the electrolysis of water which consists of a series of packed
beds of alternating high and low electrical conductivity.
Each bed has separately controlled pairs of electrodes. The
low conductivity bed contains conductive particles such as
carbon or nonconductive particles coated with oxides such
as MnO 2 or PbO 2. The proportion of particles in the more
conductive bed is about 30% oxide coated particles with theC016_003_r03.indd 940C016_003_r03.indd 940 11/18/2005 1:12:45 PM11/18/2005 1:12:45 PM