Microbial and chemical reductive dechlorination of polychlorinated biphenyls and chlorinated benzenes.

The environmental and biological conditions under which chlorinated aromatic compounds, such as PCBs and chlorinated benzenes, undergo microbially-mediated reductive dechlorination are not well understood. The problem is due to our lack of understanding about how microorganisms mediate electron transfer to oxidized chlorinated pollutants in the environment. Since reductive dechlorination requires a source of electrons, organic substrates and nutrients were tested for their ability to enhance the reductive dechlorination of PCBs in anaerobic sediments. In a biomimetic model, with vitamin B$\sb{12}$ as electron transfer mediator, reductive dechlorination of PCBs and chlorinated benzenes was studied. Organic substrates are limiting in some sediments. Reductive dechlorination of PCBs was accelerated by the addition of acetone, methanol or glucose, and to a lesser extent by acetate and butanol, in methanogenic sediment. The rate of meta and para chlorine removal was proportional to the concentration of organic substrate that was added to sediment. Long-term sorption of PCBs to sediments inhibits their reductive dechlorination. Reductive dechlorination of sorbed PCBs was induced by the addition of bioavailable PCBs. In addition, anaerobic degradation of mono and dichlorobiphenyls was observed relative to autoclaved controls. In experiments with deuterated water, the deuterium atom that replaced the chlorine that was lost during dechlorination was derived from water. Corrinoids such as vitamin B$\sb{12}$ may have a role in microbially-mediated reductive dechlorination in the environment. In the biomimetic model system, reductive dechlorination rates of PCB congeners and chlorinated benzenes were dependent on the concentration of the electron donor, electron mediator, and the number and substitution structure of chlorines on the dechlorinating ring. Meta and para chlorines were removed during vitamin B$\sb{12}$ mediated reductive dechlorination of PCBs in a similar pattern to that of microbially-mediated reductive dechlorination of PCBs in anaerobic sediments. Ortho chlorine removal occurred from unique PCB congeners that are not generally found in commercial PCB mixtures. A quantitative correlation was made between chlorine substituent effects and PCB reductive dechlorination rates using the Hammett equation.PhDEnvironmental EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/103652/1/9332143.pdfDescription of 9332143.pdf : Restricted to UM users only