Environmental Engineering Reference
In-Depth Information
•
The slow rate of reductive dechlorination and the fact that it is usually
incomplete
•
The low solubility of PCBs, and hence their poor bioavailability
•
Practical barriers, including a microbial delivery technology that en-
sures high survivability of introduced microorganisms in soils
•
Appropriate field-scale remediation technologies.
Polychlorinated biphenyls represent a class of chlorinated compounds,
the general structure for which is given in Figure 6.1. Each of the numbered
positions may or may not be chlorinated, resulting in 209 different congeners.
For industrial purposes, PCBs were manufactured as complex mixtures con-
taining from 60 to 90 congeners by the catalytic chlorination of biphenyl
(Shulz et al., 1989). Depending on the amount of chlorine added, these
mixtures were mobile oils, viscous liquids, or sticky resins, but all were
nonflammable, thermally stabile, chemically inert, and excellent electrical
insulators. Because of these properties, they were widely used as dielectric
fluids in electrical capacitors and transformers and as plasticizers. Smaller
but still significant amounts were used as lubricants, hydraulic fluids, heat
transfer fluids, cutting oils, extenders in waxes, pesticides, and inks, and in
carbonless copy paper (Hutzinger et al., 1974).
In the United States and Great Britain, nearly all PCBs were manufac-
tured by Monsanto under the trade name Aroclor and given a four-digit
numerical designation. The first two digits in the numerical designation
indicate either PCBs (12), polychlorinated terphenyls (PCTs) (54), or mixtures
of PCBs and PCTs (25 or 44), and the last two digits indicated the percent
chlorine by weight. Thus, Aroclor 1242, for example, is a PCB mixture that
is 42% chlorine by weight and averages 3.1 chlorines per molecule. Aroclor
1260 contains 60% chlorine and averages about six chlorines per molecule.
Aroclor 1016 is an exception to this scheme; it contains 41% chlorine by
weight and appears to be a fractional distillation product from Aroclor 1242,
with a marked reduction in the amount of congeners with five or more
chlorines. In other countries, PCB mixtures were manufactured under the
trade names Fenclor, Pheneclor, Pyralene, Clophen, and Kanechlor, to name
a few (Hutzinger et al., 1974).
3
2
6′
5′
1
1
′
4
4
′
5
6
2
′
3
′
Biphenyl
Figure 6.1
The general structure of polychlorinated biphenyl compounds.
