What is PCBs?

    Polychlorinated biphenyls (PCBs) are aromatic, synthetic chemicals which do not occur naturally in the environment. They consist of the biphenyl structure with two linked benzene rings in which some or all of the hydrogen atoms have been substituted by chlorine atoms. The basic molecular structure, including the conventional numbering of the substituent positions, is shown in Figure 1.

Figure 1. The chemical structure of PCBs compounds 

    The chemical formula of PCBs is C12H10-nCln, where n ranges from 1 to 10. Theoretically, 209 different congeners are possible, but only about 130 of these have been identified in commercial products.

Properties

    PCBs are hydrophobic and thus they have very low water solubility. Congeners with a lower degree of chlorination are more volatile than those with a higher degree. As hydrophobic and very stable compounds, PCBs may volatilize from water despite their low vapor pressure. PCBs also easily adsorb onto organic particles in soils, sediments, biological systems, or water. (Panero et al. 2005) These organic particles can be transported long distances and has been shown as one of the reasons PCBs are distributed throughout the planet including remote areas.

    By contrast, they are slightly soluble in oil and highly soluble in most organic solvents. PCBs are unaffected by light and they have remarkable heat stability – which increases with their chlorine content – and only break down at very high temperature (> 1,000 °C). PCBs have a high level of chemical inertia and are highly resistant to such chemical agents as acids, bases and oxidizers. While not affecting base metals, they dissolve or soften certain rubbers and plastics.

    PCBs are practically fire resistant because of their high flash points (170–380 °C). They form vapours which are heavier than air, but are not explosive. They have low electrical conductivity, high thermal conductivity and high resistance to thermal degradation. On the basis of these properties they have been used in hundreds of industrial and commercial applications including electrical, heat transfer and hydraulic equipment; as plasticizers in paints, plastics, and rubber products; in pigments, dyes, and carbonless copy paper; and many other industrial applications (EPA 2013a). PCBs were attractive in many applications because they resist breakdown at high temperatures or from aging, or oxidation.

    PCBs have another feature that contributes greatly to their stability. Carbon and chlorine form a very strong bond and the amount of energy needed to break apart the bond is higher than most other covalent bonds. The strength of this bond greatly increases the ability of PCBs to persist in the environment. Persistence is also related to the number of chlorine atoms with increasing degree of persistence with increasing chlorine mass. Comparatively, the mono- and di-chlorobiphenyls are less persistent than the larger congeners; however, all PCBs meet the definition of persistence.