1,4-Dioxane (dioxane) is a probable carcinogen and persistent groundwater pollutant often found comingled with chlorinated solvents (e.g., trichloroethene, dichloroethene, and trichloroethane). Because of 1,4-dioxane’s high mobility in groundwater, their plumes tend to be large and dilute. State-issued clean-up guidelines for 1,4-dioxane are on the order of 1 microgram per liter (μg/L) or less. Reaching these low clean-up guidelines through remediation has proven to be particularly difficult and costly. Utilizing aggressive pump-and-treat and ex situ technologies such as advanced oxidation (AO) on dilute 1,4-dioxane plumes is often prohibitively expensive. During this project, the team evaluated bioaugmented phytoremediation, a promising, cost-effective clean-up strategy for 1,4-dioxane-impacted groundwater. The objective of this research was to discover microbial strains that can degrade 1,4-dioxane to health advisory levels. In addition, the project team evaluated the performance of candidate organisms when bioaugmented into the poplar rhizosphere.

During this work, the project team conducted bench-scale experiments to compare 1,4-dioxane degradation rates of poplar bioaugmented with Pseudonocardia dioxanivorans CB1190 to that of metabolic 1,4-dioxane-degrader Mycobacterium dioxanotrophicus PH-06 (PH-06). The project team completed experiments testing whether these bacterial strains can utilize root exudates as an auxiliary substrate. In addition, the project team evaluated the use of inexpensive B-vitamin supplements to accelerate 1,4-dioxane metabolism by Rhodococcus ruber (R. ruber) 219. The team also tested R. ruber 219’s ability to withstand chlorinated solvents. Finally, the team conducted simulated aquifer experiments to evaluate whether phytoremediation and bioaugmentation can treat dilute plumes impacted with 1,4-dioxane to below health advisory levels over long periods.

In the findings, the team reports the phytoremediation of 1,4-dioxane by hybrid poplar to health advisory levels (~1 μg/L) in bench-scale experiments. Bioaugmentation with dioxane-degrading bacteria significantly increased the rate of removal by hybrid poplar. In addition, PH-06-bioaugmented poplar significantly outperformed all other treatments. However, growth curve experiments found that PH-06 could not utilize root extract as an auxiliary carbon source for growth. Despite this limitation, the findings suggest that PH-06 is a strong bioaugmentation candidate to enhance the treatment of dioxane by phytoremediation.

The project team also identified R. ruber 219 as a very strong candidate for field bioaugmentation. With the addition of B-vitamins, the strain is able to sustain growth in dilute dioxane concentrations (<100 μg/L) and degrade 1,4-dioxane to below health advisory levels (<0.35 μg/L). The team did observe 1,1-dichloroethene as inhibitory for 1,4-dioxane degradation by R. ruber 219. Ongoing work aims to explore if bioaugmentation with R. ruber 219 in tandem with phytoremediation can overcome inhibition by 1,1-dichloroethene. Furthermore, poplar trees may release sufficient B-vitamins in root exudates, reducing the need for vitamin amendments.

This project demonstrated that combining phytoremediation with bioaugmentation is a promising treatment alternative for 1,4-dioxane-impacted groundwater to achieve low concentrations (<0.35 μg/L) as recommended by health advisories. 1,4-Dioxane-metabolizing microbes have been utilized, eliminating the need for auxiliary substrates required by cometabolic microorganisms. While challenges remain, the successful implementation of this strategy offers a green and cost-effective solution to a widespread problem of national and international importance. The results of this SERDP effort are now being extended to the field under an ESTCP project.

Simmer, R., J. Mathieu, M.L. da Silva, P. Lashmit, S. Gopishetty, P.J. Alvarez, and J.L. Schnoor. 2020. Bioaugmenting the Poplar Rhizosphere to Enhance Treatment of 1, 4-dioxane. Science of the Total Environment, 744: 140823.