- 1,4-dioxane and chlorinated aliphatic hydrocarbons (CAHs) such as trichloroethylene (TCE) and 1,1-dichloroethene (1,1-DCE) are hazardous compounds commonly found in soil and groundwater. Bioremediation through aerobic cometabolism is a potential option for the remediation of these contaminated sites. The purpose of this study was to examine the use of different primary substrates to stimulate bacteria that can potentially perform concurrent aerobic cometabolism of 1,4-dioxane and chlorinated solvents. Batch pure-culture rate tests were conducted utilizing isobutane, ethane, and propane-grown Rhodococcus rhodochrous ATCC® 21198™ and isobutene-grown Mycobacterium sp. ELW1. In support of a field demonstration, microcosms containing a groundwater and sediment slurry from the Naval Air Station North Island in San Diego, California, were also performed using isobutane and isobutene as cometabolic growth substrates. This site is contaminated with CAHs (predominately TCE and 1,1-DCE) and 1,4-dioxane due to historical improper storage of chemical waste.
The effectiveness of the cometabolic transformation of CAHs and 1,4-dioxane was evaluated through the determination of zero order degradation rates of TCE, 1,1-DCE, cis-1,2-dicholoroethylene (c-DCE), 1,2-dichloroethane (1,2-DCA), 1,1,1-trichloroethane (1,1,1-TCA), and 1,4-dioxane and transformation capacities of c-DCE and 1,1-DCE by microorganisms grown on different substrates. The general trend in rates indicated isobutane-grown 21198 > ethane-grown 21198 > propane-grown 21198. The rates for TCE were comparable among all growth substrates with 21198. The TCE degradation rate of isobutene-grown ELW1 was double the rates observed for 21198 grown on all substrates. 1,4-dioxane, 1,2-DCA, and 1,1,1-TCA were not readily transformed by ELW1. Transformation of mixtures of CAHs and 1,4-dioxane were consistent with the single compound tests.
Isobutane and isobutene were selected as primary growth substrates to promote cometabolism in microcosm studies. After 52 days of isobutene exposure, isobutene uptake was observed in a native microcosm system in the presence of TCE, 1,1-DCE, and 1,4-dioxane. Once nearly complete uptake of isobutene was achieved, rapid 1,1-DCE transformation and slow TCE degradation were observed. After 130 days of isobutane exposure, no primary substrate uptake and minimal TCE and 1,1-DCE degradation were observed. It is hypothesized that the presence and transformation of 1,1-DCE is inhibitory to the stimulation of native microbes. This is due to the formation of a toxic epoxide resulting from the activation of a monooxygenase by the primary substrate and the observation that 1,1-DCE was transformed prior to isobutane utilization. In order to circumvent the toxic effects of 1,1-DCE, a variety of strategies were implemented, including sparging to remove the chlorinated compounds through air stripping, bioaugmentation with isobutane and isobutene-utilizing cultures, and isobutanol addition to cultivate an active isobutane-utilizing population without the activation of the monooxygenase enzyme responsible for 1,1-DCE transformation. Sparging and bioaugmentation were effective strategies in the stimulation of the microcosms to transform 1,4-dioxane, 1,1-DCE, and TCE.