Transcript Document
Polychlorinated Biphenyls (PCBs) PCBs are a family of chemical compounds formed by the addition of chlorine to biphenyl (C12H10). There are 10 substitution positions where chlorine may be added, leading to a possible 209 unique chemical compounds termed congeners. 3 2 2’ 3’ 4’ 4 5 6 6’ 5’ ClnH(10-n) Congeners have been assigned numbers (1209) and are also classified by the positions occupied by chlorine. Referencing the substitution positions in the figure above, three examples are: Congener 1: 2-Chlorobiphenyl Congener 101: 2,2’,4,5,5’-Pentachlorobiphenyl Congener 209: Decachlorobiphenyl Another classification, homologs, refers to a subcategory of PCBs having equal numbers of chlorine substituents, e.g. pentachlorobiphenyls, all having 5 chlorines. Polychlorinated Biphenyls (PCBs) PCBs were typically manufactured as a mixture of congeners, created through the progressive chlorination of biphenyl until a target chlorine content (percentage by weight) was obtained and the mixture has characteristic physical-chemical properties. One of the most common mixtures was Arochlor 1254, which contained 54% chlorine by weight. Once released to the environment, mixtures undergo diagenesis, changing the congener balance and making it difficult to determine their origin. PCBs have physical properties ranging from oily liquids to waxy solids. Due to their non-flammability, chemical stability, high boiling point and electrical insulating properties, they were used in a variety of industrial and commercial applications including electrical, heat transfer, and hydraulic equipment; as plasticizers in paints, plastics and rubber products and in pigments, dyes and carbonless copy paper. More than 1.5 billion pounds of PCBs were manufactured in the U.S. before they were banned under the Toxic Substances Control Act of 1976. Polychlorinated Biphenyls (PCBs) World production of PCBs since 1930 is estimated at one million tons2. Although the manufacture of PCB ceased in 1979 under TSCA, some 750 million pounds remained in use as of the mid1980s3, largely in electrical equipment. PCBs enter the environment through effluent discharge, incineration and leakage. General Electric Corporation, a manufacturer of capacitors, discharged PCBs to the Hudson River under a federal permit for over 20 years. Over a halfmillion pounds of PCBs remain in the Hudson River, with thousands of pounds migrating downstream each year3. Other noteworthy PCB contamination sites include the Fox River at Green Bay (pulping and paper de-inking discharges4) and the Manistique River/Harbor (paper and electric industries5) in the Upper Peninsula. All of these locations have been the focus of intensive investigation and remediation. Manistique River Site6 Polychlorinated Biphenyls (PCBs) PCBs have been demonstrated to cause a variety of adverse health effects, including cancer and disruption of the endocrine, nervous, and reproductive systems1. It has a potency factor for toxicity more than 4 times that of arsenic7. PCBs have a very high bioconcentration factor and are regularly found present in fish tissue. mg chemical BCF C fish Cwater L kg fish mg chemical kg L water The BCF has been empirically correlated with Kow, and thus a tendency to partition into the fatty tissue of fish or humans. PCBs have a BCF of 100,000 L/kg, compared with 4 for chloroform and 44 for arsenic7. Polychlorinated Biphenyls (PCBs) As a result, fish consumption advisories are common in Michigan and other states8. The environmental fate and transport of PCBs is largely governed by their physical-chemical characteristics, properties which vary considerable across the spectrum of species included in this family of chemicals. Chief among these properties are the octanol-water partition coefficient, a measure of the potential to associate with particles, and the Henry’s Law constant, a reflection of the partitioning of the chemical between air and water. In general, high MW PCBs are strongly associated with particles and low MW PCBs are more strongly partitioned to the atmosphere. Polychlorinated Biphenyls (PCBs) References 1. http://www.epa.gov/pcb 2. Kupchella and Hyland, Environmental Science 3. Revelle and Revelle, The Environment, 2nd Edition 4. http://wi.water.usgs.gov/pubs/FS-116-96/index.html 5. http://www.epa.gov/region5/sites/manistique/pdfs/manistiq.pdf 6. http://www.epa.gov/region5fields/htm/projects/manistique/ 7. Masters, Introduction to Environmental Science, 2nd Edition 8. http://www.michigan.gov/documents/FishAdvisory03_67354_7.pdf