- Mixtures of 1,4-dioxane (1,4-D) and chlorinated aliphatic hydrocarbons (CAHs) are common and hazardous groundwater contaminants that can be challenging to address with traditional remediation strategies. Bioremediation through aerobic cometabolism has the potential to be an effective in-situ treatment strategy for these contaminants. Rhodococcus rhodochrous strain ATCC 21198 is capable of cometabolism of a variety of CAHs and 1,4-D utilizing short chain alkane monooxygenase enzymes.
The induction of cometabolism in ATCC 21198 was investigated after growth on 1-butanol, 2-butanol, and 2-ethyl-1-butanol and evaluated in resting cell kinetic tests of mixtures of 1,1,1-trichloroethane (1,1,1-TCA), cis-1,2-dichloroethene (cis-DCE), and 1,4-D. ATCC 21198 grown on all three substrates had the eventual ability to transform contaminants with growth on 2-butanol resulting in the fastest rates; however, a lag phase was observed in both 1-butanol and 2-butanol grown cells. Further efficacy of cometabolic transformation by 2-butanol grown ATCC 21198 was evaluated in single compound tests through determination of transformation rates and
capacities for additional CAHs; vinyl chloride (VC), 1,1-dichloroethane (1,1-DCA), 1,1-dichloroethene (1,1-DCE), and trichloroethylene (TCE). The trend in zero-order transformation rates was determined as follows; VC > cis-DCE > 1,1-DCA > 1,1-DCE > 1,4-D > 1,1,1-TCA > TCE, values ranged between 1 and 2 orders of magnitude lower than isobutane grown 21198. Transformation capacities followed the same general trend except for 1,1-DCE which had a capacity two orders of magnitude lower than other di-chlorinated compounds.
In microcosms constructed with aquifer material from Cape Canaveral Air Force Station (CCAFS) a site with 1,4-D contamination and traces of a variety of CAHs, native aerobic microorganisms were unable to be stimulated on isobutane or 2-butanol. Neither aerobic nor anaerobic native bacteria were able to be stimulated capable of 1,4-D transformation. Anaerobic conditions in microcosm and column studies stimulated methanogenesis and reductive dichlorination of PCE to TCE and TCE to cis-DCE and 1,1-DCE by native bacteria consuming site COD. In CCAFS microcosms ATCC 21198, suspended and coencapsulated in gellan gum with a slow-release substrate, showed initial capacity for cometabolic transformation of 1,4-D. Sustained transformation was inhibited in microcosm and column studies, potentially by the high COD of the groundwater despite partial predigestion and H2O2 additions up to 350 mg/L. Cometabolic transformation of cis-DCE was able to be restimulated in the column after a time of anaerobic conditions and periodically over 200 pore volumes. The high oxygen demand associated with CCAFS groundwater and aquifer material proved difficult conditions to achieve sustained aerobic cometabolism.