Evaluation of TCDD biodegradability under different redox conditions

Polychlorinated dibenzo-p-dioxins have been generated as unwanted by-produc ts in many industrial processes. Although their widespread distribution in different environmental compartments has been recognized, little is known a bout their fate in the ultimate environment sinks. The highly stable dioxin isomer 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been called the most toxic compound known to man. In this laboratory microcosm study, TCDD bioa vailability was evaluated under five reduction/oxidation (redox) conditions including aerobic biodegradation, aerobic cometabolism, methanogenesis, ir on reduction, and reductive dechlorination. Activated sludge and aquifer se diments from a TCDD and a pentachlorophenol (PCP) contaminated site were us ed as the inocula. Acetate, sludge cake, and cane molasses were used as the primary substrates (carbon sources) in cometabolism and reductive dechlori nation microcosms. After a 90-day incubation period, microcosms constructed under reductive dechlorination conditions were the only treatment showing promising remediation results. The highest TCDD degradation rate [up to 86% of TCDD removal (with an initial concentration of 96 mug/kg of soil)] was observed in the microcosms with anaerobic activated sludge as the microbial inocula and sludge cakes as the primary substrates. Except for reductive d echlorination microcosms, no significant TCDD removal was observed in the m icrocosms prepared under other conditions. Thus, application of an effectiv e primary substrate to enhance the reductive dechlorination process is a fe asible method for TCDD bioremediation. Bioremediation expense can be signif icantly reduced by the supplement of some less expensive alternative substr ates (e.g., sludge cakes, cane molasses). Results would be useful in design ing a scale-up in situ or on-site bioremediation system such as bioslurry r eactor for field application. (C) 2001 Elsevier Science Ltd. All rights res erved.