Formatted Title
What We Know Now and What We Need to Know to Establish Biodegradation of 1,4-Dioxane in the Environment
Background/Objectives
1,4-Dioxane, a probable human carcinogen, is an emerging contaminant of concern in water. Improper practices during manufacturing, use, and disposal of solvents have resulted in surface water and groundwater contamination by 1,4-dioxane. Its high water solubility and low Kow and Koc values limit high-resolution detection methods and remediation using air stripping, thermal desorption, and soil vapor extraction inefficient for sites with large, dilute plumes. While evidence for natural attenuation is growing, cleanup is challenging because 1,4-dioxane is often commingled with chlorinated volatile organic compounds (CVOCs), and most technologies targeting CVOCs are not effective for 1,4-dioxane removal. Aggressive technologies, such as advanced chemical oxidation, efficiently degrade 1,4-dioxane, but they are costly and energy intensive, especially for pumping and ex situ treatment. Biodegradation of 1,4-dioxane has been reported in previous laboratory and field studies, but reliable tools to assess, manage, and fully take advantage of natural attenuation and bioremediation in the field are currently lacking.
Approach/Activities
At the laboratory scale, we have identified the ability of specific bacteria to degrade 1,4-dioxane even at low, environmentally-relevant concentrations, as well as in the presence of commonly co-occurring CVOCs like cis-1,2-dichloroethene and vinyl chloride. 1,4-Dioxane degradation has also been shown to be enhanced in the presence of thiamine. Additionally, we have developed and validated molecular biological probes targeting monooxygenase genes to serve as monitoring tools for 1,4-dioxane bioremediation in contaminated groundwater. At the field scale, we have analyzed spatiotemporal and microbiological trends at various contaminated sites to understand the most critical biogeochemical parameters affecting 1,4-dioxane biodegradation, especially throughout inevitable fluctuations in natural environments. Mining of historical data has suggested the prevalence of natural attenuation during warmer months, and the close monitoring of 1,4-dioxane and CVOC concentrations upon bioaugmentation suggests that biodegradation is most effective when oxygen is present and/or restored. Isotope fractionation and 14C tracking have also been used in the field as direct evidence for 1,4-dioxane degradation and 1,4-dioxane-degrading microorganism presence. Employment of treatment trains (e.g., catalytic reduction of CVOCs to ethane in an H2-based membrane palladium-film reactor followed by aerobic biodegradation of ethane and 1,4-dioxane in an O2-based membrane biofilm reactor) has allowed for the mitigation of co-contaminant inhibition of 1,4-dioxane degradation. Microbial community analyses revealed that functional redundancy of genes served as a buffer to ensure a stable microbiome, guiding the implementation of ex situ bioreactors and in situ bioremediation.
Results/Lessons Learned
The accumulating mechanistic and quantitative data will be valuable in changing the landscape of 1,4-dioxane biological treatment by: 1) enhancing the industrial and regulatory perception of 1,4-dioxane biodegradability, 2) understanding treatment mechanisms, especially in contaminant mixtures and fluctuating redox conditions; and 3) improving tools to validate natural or enhanced bioremediation effectiveness.