Polychlorinated Biphenyls (PCBs)
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The ATSDR Toxicological Profile for Polychlorinated Biphenyls (PCBs) describes PCBs as follows:
PCBs are a class of chemical compounds in which 2-10 chlorine atoms are attached to the biphenyl molecule. Monochlorinated biphenyls (i.e., one chlorine atom attached to the biphenyl molecule) are often included when describing PCBs. The general chemical structure of chlorinated biphenyls is shown below.
It can be seen from the structure that a large number of chlorinated compounds are possible. The 209 possible compounds are called congeners. PCBs can also be categorized by degree of chlorination. The term "homolog" is used to refer to all PCBs with the same number of chlorines (e.g., trichlorobiphenyls). Homologs with different substitution patterns are referred to as isomers. For example, the dichlorophenyl homolog contains 12 isomers.
The numbering system for the PCBs is also shown above. Positions 2, 2', 6, and 6' are called ortho positions, positions 3, 3', 5, and 5' are called meta positions, and positions 4 and 4' are called para positions. The benzene rings can rotate around the bond connecting them; the two extreme configurations are planar (the two benzene rings in the same plane) and the nonplanar in which the benzene rings are at a 90o angle to each other. The degree of planarity is largely determined by the number of substitutions in the ortho positions. The replacement of hydrogen atoms in the ortho positions with larger chlorine atoms forces the benzene rings to rotate out of the planar configuration. The benzene rings of non-ortho substituted PCBs, as well as mono-ortho substituted PCBs, may assume a planar configuration and are referred to as planar or coplanar congeners; the benzene rings of other congeners cannot assume a planar or coplanar configuration and are referred to as non-planar congeners.
In the United States mixtures of PCBs were sold as Aroclors. The numerical designation of the Aroclor product is generally associated with its percent chlorine by weight. Hence 1221 would correspond to about 21% chlorine and 1248 to 48 % chlorine. In general, the fewer chlorine atoms a biphenyl has the more soluble, more volatile, and more likely it is to biodegrade. Conversely, more chlorine atoms reduce both volatility and solubility. The biodegradation rates of higher chlorinated PCBs are congener specific. Because of the differing degradation rates the chemical makeup of a given Aroclor will change over time. PCBs also have relatively high octanol to water ratios, which indicate that they would favor partitioning to organic matter in soils and the water column. A table showing various chemical and physical properties of Aroclors is available.
Analytical Chemistry of PCBs, 2nd ed.
Lewis Publishers, Boca Raton, FL. ISBN: 0-87371-923-9, 667 pp, 1997.
Chemical Specific Summary Tables: Bioaccumulation Testing and Interpretation for the Purpose of Sediment Quality Assessment, Status and Needs
U.S. EPA, Office of Water, Office of Solid Waste.
EPA 823-R-00-002, 828 pp, 2000.
Handbook of Environmental Fate and Exposure Data for Organic Chemicals, Volume II: Solvents
Lewis Publishers, Boca Raton, FL. ISBN: 0-87371-204-8, 546 pp, 1991.
Environmental Transport and Transformation of Polychlorinated Biphenyls
U.S. EPA, Office of Toxic Substances.
EPA 560-5-83-025, 206 pp, 1983.
Evaluating Potential Exposures to Ecological Receptors Due to Transport of Hydrophobic Organic Contaminants in Subsurface Systems
Ford, R.G., M.C. Brooks, C.G. Enfield, and M. Kravitz.
EPA 600-R-10-015, 69 pp, 2014
Detailed discussion of enhanced transport mechanisms is the focus of this technical paper. It recommends several types of screening assessments to evaluate site conditions for the potential to enhance transport of HOCs—PCBs, dioxins, fuels (including the influence of MTBE), and creosote and tar DNAPL—as well as site artifacts that result from inadequate well installation and sampling procedures within a groundwater monitoring network. These assessments are incorporated into a suggested three-tiered decision analysis process that provides a summarized view of the upland contaminant-source characteristics that need evaluation to establish whether facilitated transport of HOCs might occur at a given site.
Illustrated Handbook of Physical-Chemical Properties and Environmental Fate for Organic Chemicals, Volume I: Monoaromatic Hydrocarbons, Chlorobenzenes, and PCBs
D. Mackay, W.Y. Shiu, and K. Ching Ma.
Lewis Publishers, Boca Raton, FL. 697 pp, 1992.
Technical Factsheet on Polychlorinated Biphenyls (PCBs)
U.S. EPA, Office of Ground Water and Drinking Water.