Hydrogels that Co-Encapsulate Slow Release Compounds and Rhodococcus rhodochrous ATCC 21198 for the Aerobic Cometabolic Treatment of 1,4-Dioxane and CAHs

A novel aerobic process has been developed for the passive cometabolic treatment of contaminant mixtures in groundwater. Our studies have focused on Rhodococcus rhodochrous ATCC 21198 that can concurrently oxidize 1,4-dioxane (1,4-D) and diverse mixtures of chlorinated aliphatic hydrocarbons (CAHs). We found that the short chain alkane monoxygenase (SCAM) responsible for the cometabolism of the contaminants is induced after grown on 1-butanol and 2-butanol, permitting the use of Slow Release Compounds (SRCs) that slowly hydrolyze to produce these alcohols. Methods were developed to co-encapsulate the SRCs and ATCC 21198 in gellan-gum hydrogel beads. ATCC 21198 grows within the gellan-gum beads and upon diffusion into the beads the contaminants are transformed. In batch reactors containing the gellan-gum beads successive additions of a mixture of 1,1,1-trichloroethane (1,1,1-TCA_, cis-dichloroethene (cis-DCE), and 1,4-D were transformed for over 300 days, with the rates of cometabolism correlated with the rates of alcohol release, oxygen consumption and CO2 production. Continuous flow tests have been performed with columns packed with the gellan-gum beads. The columns mimic passive treatment that might be achieved using an in-situ permeable reactive barrier (PBR) constructed with the gellan-gum beads. Over 99% removal of a mixture of 1,1,1-TCA, cis-DCE, and 1,4-D, each at influent concentration of 250 µg/L, was achieved with a hydraulic residence time of approximately 12 hours The columns effectively transformed the contaminant mixture for over 600 pore volumes (300 days). The columns performance was negatively affected when cis-DCE was replaced by 1,1-dichloroethene (1,1-DCE), due to 1,1-DCE transformation product toxicity. The column containing the SRC that produced 2-butanol was more negatively impacted by 1,1-DCE due to the lower biomass that developed in the gellan-gum beads. Studies are currently being performed in a 3-D physical aquifer model using a funnel-and-gate system with the co-encapsulated gellan-gum beads used to create a cometabolic permeable reactive barrier.