This two-phase research project aims to improve the ability to treat commonly found mixed contaminants of concern in groundwater: trichloroethene (TCE), 1,1,1-trichloroethane (TCA), and 1,4-dioxane. Biological processes are ideal for removing groundwater contaminants due to their low cost, and each of the contaminants can be biodegraded. TCE and TCA can be reductively dechlorinated in an anaerobic process, and 1,4-dioxane can be aerobically biodegraded if a cometabolic substrate is present. However, applying biodegradation for this suite of contaminants has been thwarted for two reasons. First, daughter products of TCE and TCA reductive dechlorination may accumulate and are much more toxic than TCE and TCA. Second, cometabolic biodegradation of 1,4-dioxane requires a co-substrate, which normally is not present. To address both challenges, a novel synergistic system is being evaluated that involves complete catalytic reduction of TCE and TCA to ethane, followed by aerobic biodegradation of 1,4-dioxane utilizing the ethane as the primary substrate to drive co-metabolism. The overarching objectives of this project are to demonstrate proof-of-concept of this novel synergistic platform and devise strategies to optimize the synergy.

Phase 1 is complete and initial efforts successfully demonstrated the use of the novel synergistic platform. The results of Phase 1 studies can be found in the Phase I Final Report.

The concept begins with the hydrogen-based membrane palladium-film reactor (H₂-MPfR) to completely convert TCA and TCE to ethane by reductive dechlorination. ;A film of elemental-palladium (Pd) nanoparticles is deposited on the exterior surface of nonporous gas-transfer membranes by auto-catalytic reduction using H₂ gas delivered by diffusion through the walls of the membranes. Upon introduction of TCE and TCA in the water, the Pd nanoparticles catalyze reductive dechlorination (again using H₂ delivered from the membranes), which generates ethane. The ethane is then used in a downstream oxygen-based membrane biofilm reactor (O₂-MBfR) as the primary electron-donor substrate to stimulate 1,4-dioxane-degrading bacteria that mineralize dioxane through co-metabolism. ;The same gas-transfer membranes are used in the O₂-MBfR, and the biofilm naturally accumulates on the exterior walls of the membranes by oxidizing ethane and respiring O₂. Should some partially dechlorinated daughter products from TCE and TCA be in the effluent of the H₂-MPfR, they are aerobically biodegraded in the O₂-MBfR. Thus, the synergistic platform overcomes both challenges associated with biological removal of TCE, TCA, and 1,4-dioxane.

The project is being conducted in two phases that employ bench-scale reactors and mathematical modeling. In Phase 1, which is complete, the project team first demonstrated that both parts of the synergistic platform could accomplish their objectives. In particular, it was documented that the H₂-MPfR could reduce TCE and TCA all the way to ethane. In addition, it was proven that 1,4-dioxane could be mineralized in the O₂-MBfR when ethane was supplied as the primary substrate. Phase 1 was concluded by linking the H₂-MPfR to the O₂-MBfR. Over 130 days of continuous operation of the two systems in series, the H₂-MPfR gave nearly 100% reduction of TCE and TCA to ethane, the O₂-MBfR mineralized 1,4-dioxane when using the ethane as the primary substrate, and the trace amounts of dichloroethane and monochloroethane that were in the H₂-MPfR’s effluent were completely biodegraded in the O₂-MBfR.

In Phase 2, which is on-going now, the focus is on the O₂-MBfR. Using bench-scale experiments and mathematical modeling, two fundamental aspects are being evaluated that have profound practical impacts for the performance of the O₂-MBfR. The first fundamental issue concerns how much ethane must be supplied to the O₂-MBfR in order to accumulate enough active biofilm to fully mineralize the 1,4-dioxane in the influent. As a primary substrate, the ethane must fulfill two roles: (1) its oxidation must provide energy and electrons to support sufficient biofilm biomass to be able to biodegrade 1,4-dioxane and (2) its oxidation also must supply electrons to fuel the mono-oxygenation reactions that initiate biodegradation of 1,4-dioxane. The concentration of ethane needed to carry out both tasks is being systematically evaluated. The need for ethane will be compared with the amount of ethane provided by reductions of TCE and TCA in the H₂-MPfR.

The second fundamental issue concerns the kinetics of 1,4-dioxane biodegradation when the ethane supply is adequate. The project team is exploring systematically how the concentrations of 1,4-dioxane, ethane, and dissolved oxygen control the removal flux of 1,4-dioxane. The removal flux determines the amount of membrane surface area needed in the reactor, and this is the prime determinant of capital costs, which are also being evaluated.

This project is strongly driven by the pressing needs of many DoD sites that are contaminated by 1,4-dioxane plus TCE and/or TCA. It provides a means to completely detoxify the water without using hazardous materials or conditions. The synergistic platform can be used directly for ex situ treatment, but the principles of the synergy can be adapted for in situ treatment, such as in a permeable reactive barrier. This research also contributes to understanding the fundamental mechanisms controlling reductive dechlorination of TCE and TCA and the cometabolic biodegradation of 1,4-dioxane.

Wang, B., R. Krajmalnik-Brown, C. Zhou, Y. Luo, B.E. Rittmann, and Y. Tang. 2020. Modeling Trichloroethene Reduction, Methanogenesis, and Homoacetogenesis in an O₂-based Biofilm.  Journal of Environmental Engineering, 146(2):04019115.