VOLATILIZATION OF POLYCHLORINATED BIPHENYLS: IMPLICATIONS FOR THEIR DISTRIBUTION, FORENSICS AND TOXICITY IN URBAN ENVIRONMENTS

Polychlorinated biphenyls (PCBs) are a group of man-made organic chemicals found virtually everywhere in the environment due to their widespread use historically, with higher concentrations around urban and industrial areas. There are 209 possible PCB congeners each with a unique chlorine substitution pattern, they tend to be quite stable under environmental conditions, and 12 have been assigned a Toxic Equivalency Factor (TEF) to relate their concentrations to a highly toxic dioxin compound. The volatilization of PCBs has been shown through previous experiments to have a potentially large effect on congener composition due to differences in vapor pressures among the congeners. In addition to the expected overall loss of lower molecular weight congeners, it was observed that the less toxic ortho-substituted congeners were more volatile, potentially leaving the residual relatively more toxic. The original investigators did not report concentrations of the congeners with TEFs, but recent work on congener-specific vapor pressure estimates allows us to model the changes in the relative toxicity of a congener mixture during volatilization. Here we test whether the observations can be reproduced using three vapor pressure estimating methods. We found that all although all methods captured the general compositional changes, but that a quantitative structure-property relationship method best matched the observed preferential loss of ortho-substituted PCBs. In order to see these changes, a large portion of the starting material has to be lost. For example, in order for Aroclor 1242 (42 wt% chlorine) to approach the 48 wt% chlorine composition of Aroclor 1248, at least 70% of the original Aroclor 1242 mass must be lost. The rate of this loss is highly variable, depending on temperature, surface area and interactions with water or other solvents. However, if this degree of fractional mass loss is achieved, the residual Aroclor 1242 reaches approximately 46% chlorine, has a congener composition very similar to Aroclor 1248, and its relative toxicity increases. The weighted mean TEF approximately doubles, but is still an order of magnitude lower than the weighted mean TEF of Aroclor 1248. These changes have been shown experimentally with very small amounts of PCBs, and their practical importance remains to be demonstrated.