Comparison of 1-propanol and propane as auxiliary substrates on 1,4-dioxane biodegradation via bioaugmentation with Azoarcus sp. DD4 at a landfill site†

1,4-Dioxane (dioxane) is a persistent contaminant of concern due to its high mobility and stability in aquatic environments, posing significant environmental and public health risks. Azoarcus sp. DD4, a novel propanotrophic bacterium, has demonstrated an enhanced ability to cometabolize and degrade dioxane using propane or 1-propanol as the carbon source. In this study, we evaluated DD4's efficiency in dioxane degradation in groundwater samples from a landfill site at site-specific concentrations (41.7 ± 0.5 μg L⁻¹ in MW-A and 12.3 ± 0.2 μg L⁻¹ in MW-B). Bioaugmentation with DD4 reduced dioxane concentrations below the detection limit (0.13 μg L⁻¹) in both propane and 1-propanol treatments. Dioxane degradation rates were 2.23 and 0.72 μg L⁻¹ per day in propane-amended treatments for MW-A and MW-B, respectively, and 1.94 and 0.57 μg L⁻¹ per day in 1-propanol-amended treatments. While propane showed slightly higher degradation rates, the differences were not statistically significant. Parallel treatments mimicking natural attenuation and biostimulation with propane or 1-propanol showed minimal dioxane removal compared to abiotic losses. Although total biomass remained constant before and after DD4 bioaugmentation, DD4 proliferation was evident during incubation with both propane and 1-propanol. Microbial community analysis revealed that Azoarcus was abundant in bioaugmented samples, accounting for 5.6% to 45.4% of the overall biomass, highlighting substantial shifts in response to the auxiliary substrate used. These findings were supported by tmoA biomarker analysis specific to DD4 via qPCR. Overall, our results demonstrated the feasibility of 1-propanol as an auxiliary substrate alternative for propane for supporting dioxane cometabolic biodegradation by DD4 in complex landfill environments with trace levels of dioxane.