Environmental Transport and Abiotic Degradation

The properties which made PCBs useful industrial chemicals (particularly their chemical inertness) make them persistent within the environment. PCBs are volatile enough and soluble enough to be transported effectively in the air and in water bodies, but because they partition strongly to organic material they are readily concentrated into biota and other organic material (particularly the organic matter present in soils and sediments). PCBs are found in all regions of the globe, including the Poles, which implies that air transport is the most important mode of global movement, but at the local and regional scale transport in water bodies is likely to be important, and indeed transport in migratory animals may have an impact in certain (particularly relatively uninhabited, nonindustrial) regions.

PCBs partition between the vapor and particulate phases in air, with generally increasing partitioning to particles with increasing level of chlorination (the octanol-air partition coefficient, K0A, has been shown to be a good descriptor of the partitioning of PCBs between the vapor and particulate phases in air). At most environmentally relevant temperatures and particulate loadings PCBs are found predominantly in the vapor phase. However, at high atmospheric particulate loadings and low temperatures the particulate bound fraction will be appreciable. Transport of PCBs from source regions in the air, followed by partitioning to vegetation and soils and deposition with precipitation, is likely to be the primary source of PCBs to the Poles and other cold environments (including high mountains) - the low temperatures in these environments strongly favoring partitioning to the solid (especially organic) phase. This process has been proposed for a range of organic chemicals with similar properties, and has been called 'cold-condensation'.

The relatively high concentrations of suspended organic matter in water bodies, coupled with the Kow of PCBs, generally leads to higher concentrations ofPCBs in sediments and suspended matter than in the dissolved phase. It has been estimated that most of the PCBs that have been released into the environment are now held in aquatic sediments. These sediments act predominantly as sinks, but can release PCBs to organisms or back to the water column.

PCBs partition strongly to soils, which act as long-term, high-capacity reservoirs. Leaching from soils may be a source of PCBs to groundwater. Volatilization from soils can also be a source of PCBs to the atmosphere, as can the atmospheric entrainment of soil particles by the wind. Much higher concentrations of PCBs have been measured in soils from the temperate Northern Hemisphere, where they were predominantly used, than in other regions, implying that the bulk ofPCBs have not, on the whole, been transported very far from their primary sources.

Differences in the K0a and K0W of individual PCBs cause changes in the patterns of PCBs seen in the environment after the 'weathering' processes which control environmental transport have taken place. Since both the water solubility and vapor pressure of PCBs become lower with increasing chlorination level (and vary to an extent with other structural differences), in general, the more-chlorinated PCBs are less mobile in the environment than the less-chlorinated PCBs. Thus, a PCB mixture that has been in the environment for a time will become deficient in the less-chlorinated PCBs compared to the original mixture. Equally, PCBs transported to nonsource regions are likely to be enriched in the less-chlorinated PCBs compared to the original mixtures released into the environment.

In general, the persistency of PCBs increases with the level ofchlorination. The vapor phase reaction with hydro-xyl radicals in the atmosphere is probably the major abiotic degradation pathway. Direct photolysis ofPCBs is likely to be a less important mode of degradation in the atmosphere, but is likely to be the major mode in water. Degradation rates are dependent on the structure of individual PCB congeners, and therefore degradation causes changes in the patterns seen in PCB mixtures over time.

Concentrations of PCBs in many environmental media in many regions (notably in human foodstuffs and air in industrial countries) have fallen dramatically since the introduction of controls and legislation. However, in some regions temporal trends cannot be established because few measurements have been made, and in others (including some equatorial areas) PCB concentrations do not yet show significant reductions.

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