Formatted Title
Column Study of Aerobic Cometabolism of Chlorinated Solvents and 1,4-Dioxane with Co-Encapsulated Hydrogel Beads
Background/Objectives
Chlorinated aliphatic hydrocarbons (CAHs) are a class of organic compounds which includes several commonly used industrial solvents. Commonly referred to as chlorinated solvents, these chemicals have known toxic effects and are common groundwater contaminants across the United States. Groundwater is often contaminated with mixtures of CAHs and other organic contaminants.
Rhodococcus rhodochrous ATCC 21198 (ATCC 21198) is a bacteria species which can dechlorinate CAHs and degrade the common groundwater CAH co-contaminant 1,4-dioxane. Due to these desirable properties, ATCC 21198 is being studied in an engineered permeable reactive barrier system for eventual application at contaminated groundwater sites. In this novel remediation approach, ATCC 21198 is encapsulated in hydrogel beads along with the slow-release substrate tetrabutyl-s-orthosilicate (T2BOS). T2BOS hydrolyzes to produce 2-butanol, a growth substrate for ATCC 21198. As ATCC 21198 metabolizes 2-butanol, a co-metabolic process occurs with the short-chained aliphatic monooxygenase (SCAM) enzyme to dechlorinate certain CAHs.
Approach/Activities
A packed column of hydrogel-encapsulated ATCC 21198 is being laboratory-tested with a dilute groundwater media containing the CAHs 1,2-cis-dichloroethylene (cis-DCE) and 1,1,1-trichlorethane (1,1,1-TCA), as well as 1,4-dioxane (1,4-D), which is used as a stabilizer in many industrial chlorinated solvent mixtures. The contaminant concentrations in both the column influent and effluent are tracked through a combination of gas chromatography and gas chromatography-mass spectrometry analysis. Through this analysis, treatment percentages and first-order cometabolism rates can be calculated for the contaminants. With column operation changes, the effects of flow rate, residence time, contaminant concentration, and oxygen supply on these treatment parameters can be quantified.
In different phases of column operation, hydrogen peroxide and oxygen gas have alternately been dissolved into the groundwater media to serve as oxygen sources for ATCC 21198 growth and metabolism. Ongoing experiments are being conducted to determine the system’s oxygen requirements via in-situ dissolved oxygen (DO) measurements with a fluorescent probe. Through the usage of fluorescent dots placed on the inside of the glass packed column, the external probe can determine in situ dissolved oxygen concentrations without disturbance of the column microbiome or exposure to laboratory atmospheric oxygen levels. With this method, the dependency of treatment efficiency on DO concentrations can be analyzed in real time.
Results/Lessons Learned
With a hydraulic residence time of 26 hours and a supply of excess oxygen, the packed column system has provided over 99% remediation of 66 ppb cis-DCE, over 90% remediation of 200 ppb 1,1,1-TCA, and over 60% remediation of 1 ppm 1,4-D. Upon doubling the flow rate to decrease the hydraulic residence time (HRT) to 13 hours while maintaining excess oxygen, the CAH cometabolism rates decreased more than was predicted by a simple first order kinetic model. With an HRT of 13 hours and the same influent concentrations as above, the transformation rates dropped to 87% remediation of cis-DCE, 63% remediation of 1,1,1-TCA, and 36% remediation of 1,4-D. Ongoing experiments will allow for the quantification of oxygen consumption rates under different groundwater media flow conditions. In addition, the synthetic groundwater media currently in use will be replaced by groundwater from a contaminated site with the associated levels of chlorinated solvents and 1,4-dioxane. This change in media will allow for a more thorough representation of ATCC 21198 cometabolism of chlorinated solvents in a system with natural pH levels, redox conditions, and mineral content.