Polychlorinated Biphenyl - Environmental Transport and Transformations

Environmental Transport and Transformations

Due to their low vapour pressure, PCBs accumulate primarily in the hydrosphere, in the organic fraction of soil, and in organisms. Despite their hydrophobicity, the immense volume of water in the oceans is still capable of dissolving a significant quantity of PCBs.

However, a small volume of PCBs has been detected throughout the atmosphere, from the most urbanized areas that are the centers for PCB pollution, to regions north of the Arctic Circle. While the hydrosphere is the main reservoir, the atmosphere serves as the primary route for global transport of PCBs, particularly for those congeners with one to four chlorine atoms.

Atmospheric concentrations of PCBs tend to be lowest in rural areas, where they are typically in the picogram per cubic meter range, higher in suburban and urban areas, and highest in city centres, where they can reach 1 ng/m³ or more. In Milwaukee, an atmospheric concentration of 1.9 ng/m³ has been measured, and this source alone was estimated to account for 120 kg/year of PCBs entering Lake Michigan. Concentrations as high as 35 ng/m³, 10 times higher than the EPA guideline limit of 3.4 ng/m³, have been found inside some houses in the U.S.

Volatilization of PCBs in soil was thought to be the primary source of PCBs in the atmosphere, but recent research suggests ventilation of PCB-contaminated indoor air from buildings is the primary source of PCB contamination in the atmosphere.

In the atmosphere, PCBs may be degraded by hydroxyl radicals, or directly by photolysis of carbon-chlorine bonds (even if this is a less important process).

In biosphere, PCBs can be degraded by either bacteria or eukariotes, but the speed of the reaction depends on both the number and the disposition of chlorine atoms in the molecule: less substituted, meta- or para- substituted PCBs undergo biodegradation faster than more substituted congeners.

In bacteria, PCBs may be dechlorinated through reductive dechlorination, or oxidized by dioxygenase enzyme.

In eukariotes, PCBs may be oxidized by the cytochrome P450 enzyme.