Aerobic and anaerobic biodegradation of 1,2,3-trichloropropane and 1,2-dichloropropane: implications for bioremediation

Abstract

1,2,3-Trichloropropane (1,2,3-TCP) is a suspected human carcinogen and a persistent emerging contaminant in groundwater and drinking water. 1,2,3-TCP was historically used as a solvent for cleaning and maintenance, paint and varnish removal, and degreasing, but its sources also include chemical manufacturing processes and application of soil fumigants. The California Department of Public Health (CDPH) has established a state maximum contaminant level (MCL) of 0.005 µg/L for 1,2,3-TCP in drinking water and a public health goal (PHG) of only 0.0007 µg/L. The primary research question addressed herein was whether aerobic or anaerobic cultures can potentially be applied for treatment of 1,2,3-TCP, and whether bacteria are capable of biodegrading 1,2,3-TCP to below the California MCL. During this study, we identified cultures capable of biodegrading 1,2,3-TCP via reductive dehalogenation as well as through aerobic cometabolic processes. Follow-on studies with organisms capable of aerobically degrading 1,2,3-TCP included kinetic modeling and assessment of concentrations of 1,2,3-TCP achievable via biodegradation. 1,2-Dichloropropane (1,2-DCP) is sometimes found co-mingled with 1,2,3-TCP, so studies also were conducted to quantify rates of 1,2-DCP biodegradation alone and when present together with 1,2,3-TCP. The dehalogenating consortium CPD-2, which was isolated from sewage sludge and includes Dehalococcoides, Dehalobacter and Dehalobium spp., biodegraded both 1,2,3-TCP and 1,2-DCP. Anaerobic 1,2,3-TCP degradation resulted in a transient production of 1,2-DCP followed by 1-chloropropane (1-CP), which accumulated nearly stoichiometrically and then slowly degraded, indicating complete dechlorination of 1,2,3-TCP by this mixed culture. Two different cometabolic pure cultures, Rhodococcus ruber ENV425 and Rhodococcus aetherivorans ENV493 degraded 1,2,3-TCP after growth on propane or isobutane. Importantly, both bacteria were capable of degrading 20 µg/L of 1,2,3-TCP to < 0.005 µg/L after growth on isobutane. Experiments conducted with ENV425 and ENV493 to quantify relevant kinetic parameters after growth on isobutane suggested that ENV425 facilitated more rapid 1,2,3-TCP degradation than ENV493. Both strains were observed to degrade 1,2-DCP much faster than 1,2,3-TCP when present individually or in mixtures. The data from this study suggest that cometabolic treatment of 1,2,3-TCP, or mixtures of 1,2-DCP and 1,2,3-TCP, is feasible and that relevant regulatory concentrations are achievable using this process. Similarly, anaerobic treatment may be possible at locations with higher concentrations or where 1,2,3-TCP occurs with other chlorinated solvents.