Effective removal of trace 1,4-dioxane by biological treatments augmented with propanotrophic single culture versus synthetic consortium

Abstract

1,4-Dioxane (dioxane) is historically used as a stabilizer for chlorinated solvents and has emerged with increasing attention due to its frequent detection at numerous sites in the US. However, trace concentrations of dioxane and intricate environmental conditions hinder its effective treatment in the field. A number of propanotrophic microorganisms are reputed for their ability to degrade dioxane via metabolism (e.g., Pseudonocardia dioxanivorans CB1190 and Mycobacterium dioxanotrophicus PH-06) and cometabolism (e.g., Azoarcus sp. DD4). In this study, we assessed the feasibility of the novel dioxane cometabolizer, Azoarcus sp. DD4, and the mixture of CB1190, PH-06, and DD4 at the ratio of 1:1:2 to remove dioxane in various water samples from a landfill site in northern New Jersey. After being fed with propane for 6∼9 days, DD4 or the synthetic consortium can effectively degrade dioxane to below our method detection limit (i.e., 0.38 µg/L) in both influent and effluent samples from the existing pump-and-treat facility, as well as the contaminated groundwater from the monitoring well near the source zone of the plume. Relative abundances of Azoarcus and DD4’s toluene monooxygenase gene tmoA were both positively correlated with dioxane degradation rates observed in microcosms, suggesting DD4’s pivotal contribution. In light of both qPCR and 16S rRNA amplicon sequencing analyses, DD4 remained dominant (29∼52%) in these three types of groundwater, indicating DD4’s compatibility for both in situ and ex situ treatments. Interestingly, inoculation with the synthetic consortium did not elicit significant improvement in dioxane removal as compared with the treatments seeded with DD4 alone. Unlike DD4, CB1190 and PH-06 were absent after the complete course of the microcosm study, indicating the decay of both two exogenous species. This was probably due to outcompetion by DD4 and/or indigenous microbiomes and hindrance by field factors (e.g., inhibitors and nutrients). Native Rhodococcus species emerged and exhibited significant correlation with the prmA gene encoding the alpha subunit of propane monooxygenase of PH-06, suggesting them as possible recipients of this dioxane degradation gene via horizontal transfer. Collectively, this study demonstrated the DD4’s high viability and excellent compatibility with diverse field samples suited as an effective inoculum to mitigate trace levels of dioxane in the subsurface or in bioreactors. Amendment of Gram-positive propanotrophic metabolizers has the potential to spread dioxane degradation genes though their viability can be low at certain sites.