COI Funded Project: Microbial Degradation of Polychlorinated Biphenyls in Marine Coastal Sediments: An Isotopic Investigation


Project Duration: 6/1/99-12/31/00
Key Words: Polychlorinated biphenyls (PCBs), microbial degradation, stable isotopes, coastal sediments

Proposed Research

Polychlorinated biphenyls (PCBs) are a class of toxic and persistent organic compounds that contaminate coastal sediments. In some instances, microbes can degrade these compounds by a process called reductive dehalogenation. The mechanisms and removal rates of reductive dehalogenation are not fully constrained and are difficult to understand due to complex distributions of these compounds in environmental matrices and the variability of PCBs sources. In order to expand our understanding of these reactions, we propose to follow the stable isotopic fractionation effects in individual PCBs during laboratory incubation experiments. We postulate that during reductive dehalogenation bacteria will fractionate the stable isotopes of carbon, hydrogen, and chlorine, and that these fractionation effects will aid in determining mechanisms, rates, and the overall significance of this poorly understood but potentially important process.

Final Report

Stable Carbon Isotopic Fractionation during the Reductive Dechlorination of Polychlorinated Biphenyls

PCBs are a family of chlorinated compounds widely used for many decades in industrial applications such as retardants, heat transfer fluids, printing inks, paints, and pesticides. Their utility was mostly due to their chemical inertness and favorable physical properties, which include resistance to oxidation, low vapor pressures, and high flash points. Ironically, these attributes have resulted in the persistence and accumulation of PCBs in the environment, where they have the potential to endanger humans and perturb ecosystems. Production of PCBs in most industrial countries ceased in the 1970s.

There is substantial evidence that under reducing conditions some PCBs in contaminated sediments are dechlorinated. Laboratory evidence has shown that natural populations of bacteria are responsible. This process has been observed in sediments from several sites, including the Hudson River (NY), New Bedford Harbor (MA), and the St. Lawrence River (Canada). The results of this laboratory study suggest that microbial reductive dechlorination of PCBs in contaminated sediments will create congeners with more depleted ã?â? 13 C values than native PCBs of similar chlorination. Such information may help to confirm this process and aid in further understanding the biogeochemistry of these compounds.