Polychlorinated Dibenzo-p-Dioxins in Anaerobic Soils and Sediments

Significant differences have been observed in the 2,3,7,8-substituted residue patterns of polychlorinated dibenzo-p-dioxins (PCDD) in freshwater, estuarine and marine sediments. Whereas these patterns can, to some degree, be explained by source identification, PCDD at environmental concentrations were recently found to be dechlorinated via microbial and chemical processes. Both peri- (1,4,6,9-substituted chlorines) and lateral (2,3,7,8-substituted chlorines) dechlorination patterns, as well as differences in extent of dechlorination were found to be correlated to specific abiotic and biotic catalytic activities. Qualitative relationships were based on isomer-specific analysis and the appearance of selective congeners under different conditions. The relevance of these processes to sediment biogeochemistry indicates that microbial dechlorination contributes significantly to the natural weathering of these types of pollutants. Whereas the lack of knowledge on the catalytic nature of the dechlorination reaction precludes the establishment of QSARs, characterization of microbial activities in combination with geochemical indicators may eventually present a means to describe the potential for microbial PCDD dechlorination in a given sediment environment and allow for a scientifically justified interpretation of patterns observed.