Biodegradation and electro-assisted biodegradation of 1,4-dioxane under different electron accepting conditions

Abstract

1,4-Dioxane (DX) is an emerging contaminant of concern in water resources. In the past, it was commonly utilized to stabilize chlorinated solvents, and it has now been discovered as a contaminant present in various personal care and food products. DX is also used in many industries including plastics and polymers, adhesiveness and sealants, and pharmaceuticals. Therefore, discharge from industrial processes is the major route of DX release to the water bodies. Once discharged to the environment, DX mostly ends up in groundwaters and is persistent (half-life of 3-5 years) and causes ecological and human health impacts if exposed (a probable carcinogen). This thesis focuses on developing novel, green and effective processes for removal of 1,4-dioxnae from water. The main objective was to fill the knowledge gaps in the literature about (1) DX biodegradation in low dissolved oxygen concentrations, (2) effect of carrier’s properties on growth of DX-degrading biofilm, and (3) mechanisms of electrochemical stimulation of DX biodegradation. In order to answer the related research questions to each gap, experiments were designed systematically and performed in lab-scale batch or continuous reactors using pure culture or microbial community as the inoculation source. Experiments conducted under low dissolved oxygen concentrations (between 1-3 mg/L) demonstrated that the biodegradation of DX occurred at a very slow rate (0.21 mgDX/L/day) when external sources of oxygen were not provided. As time passed, the overall diversity and richness of the microbial community decreased, though certain genera, such as Pseudonocardiaceae, Xanthobacteraceae, and Chitinophagaceae, were able to thrive by using DX as a carbon source. These findings suggest that the microbial community has the ability to adapt to different electron-accepting conditions, which could be beneficial for in-situ bioremediation or natural attenuation of DX in water resources. Experiments conducted on delignified porous wood as biofilm carriers demonstrated an improvement in biofilm formation in terms of growth rate and hydrophilicity compared to natural untreated wood as biofilm carriers. This improvement was attributed to the superior physiochemical properties of the treated wood, which exhibited higher porosity, formation of macropores, and an increase in surface roughness and hydrophilicity compared to untreated wood. Moreover, the rate of DX biodegradation increased by 5.33-fold when treated woods were used as carriers compared to untreated woods. These findings shed light on the physiochemical properties of biofilm carriers that contribute to biofilm formation and can be valuable for ongoing research on DX bioremediation. The experiments on electrochemical stimulation of DX biodegradation in a continuous flow bioelectrochemical reactor revealed that two different mechanisms could be responsible for the observed stimulation at low applied voltages between 1.0-1.2 V. One mechanism was attributed to the production of oxygen as a result of the water electrolysis reaction occurring at 1.2 V. This oxygen could then serve as a source of electron acceptor for aerobic microorganisms in the medium. The other mechanism was linked to an increase in microbial population activity due to electrochemical stimulation at 1.0 V. The analysis of changes in microbial composition revealed the enrichment of Alistipes and Lutispora at 1.0 V, which was attributed to their ability to directly transfer electrons with a conductive surface. The results can be helpful to issue the challenge of lack of electron acceptors in DX bioremediation.