tailieunhanh - ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - PCBs AND ASSOCIATED AROMATICS

Polychlorinated biphenyls (PCBs) are a family of chemical compounds which do not exist in nature but which are manmade. Commercial mixtures are clear, pale yellow liquids manufactured by the replacement of hydrogen atoms on the biphenyl molecule by chlorine. The reaction is controlled according to the percentage replacement of hydrogen and results in a product which contain chlorobiphenyls with one or, more usually, several chlorine atoms attached to each molecule. | PCBs AND ASSOCIATED AROMATICS INTRODUCTION Polychlorinated biphenyls PCBs are a family of chemical compounds which do not exist in nature but which are manmade. Commercial mixtures are clear pale yellow liquids manufactured by the replacement of hydrogen atoms on the biphenyl molecule by chlorine. The reaction is controlled according to the percentage replacement of hydrogen and results in a product which contain chlorobiphenyls with one or more usually several chlorine atoms attached to each molecule. The family of polychlorinated biphenyls can contain from 1 to 10 chlorine atoms arranged within the molecule in 209 different ways. The different molecular combinations are called congeners. The congeners which comprise the series of compounds with the same number of chlorine atoms are known as an homologous series. The congeners are named according to the homologous series to which they belong. For example a PCB containing 3 chlorine atoms would be called a trichlorobiphenyl or one with 5 chlorine atoms a pentachlorobiphenyl. The homologous series of PCBs and the number of isomers contained in each are listed in Table 1. The synthesis of biphenyl and its subsequent reaction with chlorine to yield polychlorinated biphenyls is shown in Figure 1. PCBs have been manufactured by various companies throughout the world and are often referred to by the manufacturer s trade name. Table 2 lists the main producers. The perceptions of risk associated with ownership and operation of PCB containing equipment is related to the risk of fires spills cleanup or noncompliance with regulations. The following time line focuses on the development of PCB risk perceptions as new knowledge grew from the time that PCBs were first used in industrial applications. 1881 PCBs were first described in the chemical literature in 1881 by Schmidt and Schultz. 1929-1931 PCBs were developed by the Federal Phosphorus Company and Swann Research Co. The latter business was bought by Monsanto in the Early