Book Description

Bioventing under anaerobic reductive conditions has been presented as a potential bioremediation technology for dealing with highly chlorinated compounds present in the capillary fringe of a contaminated soil. Anaerobic reductive bioventing is similar to aerobic bioventing, providing the gas injected is free of oxygen and contains an electron donor, here hydrogen. The concept of bioventing is reported to be cost effective because of the in-situ solution it provides. Tetrachloroethylene (PCE) typically degrades under anaerobic, reductive conditions to Vinyl Chloride (VC), which can then be readily oxidized to environmentally benign products. In this study, enhanced anaerobic degradation of PCE in the unsaturated zone of the soil was simulated in soil microcosms. Four 15 cm high and 8 cm in diameter reactors were filled with a contaminated soil sampled at an Air Force base site. Each reactor allowed for a different set of conditions in components concentration in the gas feed. The work focused on optimizing the injection gas composition, monitoring electron donor delivery and utilization, and evaluating the treatment performance of PCE. Only one microcosm, fed with 0.5 % of hydrogen and 0.5 % of carbon dioxide, showed dechlorination of PCE into Trichloroethylene (TCE) and Cis-Dichloroethylene (Cis-DCE). This microcosm completely consumed hydrogen and produced little methane, thus proving that the feeding conditions favored dechlorinators over methanogenes. PCE sequential dehalogenation did not proceed further than Cis-DCE but the total quantity of intermediates produced accounted for more than 40 % of the PCE removed in that dechlorinating microcosm. In the other microcosms, PCE removal was mainly due to adsorption of PCE onto the soil organic matter. Adding biomass to the microcosm or increasing the gas retention time did not result in significant improvements of the microcosm dechlorinating activity. Pre-existing soil contaminants, mainly hydrocarbons, were not responsible for the lack of dechlorination. Gas feeding concentrations were the only parameters proved to trigger PCE sequential dechlorination since they helped maintaining reducing conditions. This study was useful to assess the potential for dechlorination of highly chlorinated compounds in a specific soil sample and to optimize the feed conditions to be applied when testing anaerobic bioventing in-situ.