Biodegradability of 2,4-Dichlorophenol under Different Redox Conditions

Abstract

Chlorophenols are generated from a number of industrial producing processes and are extensively used as insecticides, herbicides, and fungicides. Their discharge into the environment is of great concern due to their toxicity. Bioremediation is one attractive method for managing hazardous waste contaminated sites because of its economic benefit and complete mineralization in most cases. In this study, a chlorophenol spill site was selected to evaluate the feasibility of using bioremediation technology for the remediation of chlorophenol-contaminated groundwater. Because 2,4-dichlorophenol (2,4-DCP) was one of the major contaminants appeared at this site, it was used as the target compound in this study. Microcosm experiments were constructed to: (1) determine the feasibility of applying different microbial consortia [e.g., aquifer sediments, extracted supernatant of aquifer sediments, aerobic activated sludge collected from an industrial wastewater treatment plant, and anaerobic sludge collected from an upflow anaerobic sludge bed (UASB) system] for the biodegradation of 2,4-DCP; (2) evaluate the biodegradability of 2,4-DCP under aerobic, aerobic cometabolic, iron reducing, methanogenic, and reductive dechlorinating conditions; and (3) determine the potential of enhancing 2,4-DCP biodegradation using cane molasses, biological sludge cake, and sodium acetate as the substitute primary substrates under aerobic cometabolic and reductive dechlorinating conditions. The inocula used in the microcosm study indicate that aquifer sediments, extracted supernatant of aquifer sediments, and activated sludge could metabolize 2,4-DCP directly without the addition of any extra carbon sources. In addition, indigenous microorganisms were capable of using 2,4-DCP as the sole carbon and energy source. Enhanced 2,4-DCP biodegradation was not observed under cometabolic conditions due to the substrate competition. The preferential removal of added substrates caused the decrease in 2,4-DCP degradation rates. Under anaerobic conditions, the UASB sludge could biodegrade 2,4-DCP only with the addition of sludge cake. Thus, a suitable carbon source for UASB sludge might play an important role in the removal of 2,4-DCP. Moreover, 2,4-DCP biodegradation was not detected under any anaerobic conditions (e.g., iron reduction, reductive dechlorination, methanogenesis). This indicates that anaerobic biodegradation processes might not be the dominant biodegradation patterns at this site. Results will also aid in designing a remedial system for field application.