Study On Cometabolic Bioremediation Of 1,4-Dioxane-Contaminated Groundwater And Its Mechanism

1,4-Dioxane was widely used as solvents for paints,cosmetics,deodorants and detergents,as well as as stabilizers for chlorinated solvents in industrial activities.Historically,improper storage and management have resulted in large-scale leakage of dioxane into groundwater,surface water and landfill leachate.Due to its high water-solubility and chemical stability,dioxane has been persistent in the aquatic environment.Dioxane has been identified by the U.S.Environmental Protection Agency(USEPA)as a possible human carcinogen and was listed as a "high priority" pollutant in the 2016 Toxic Substances Control Act Amendment.Due to its non-reliance on the concentration of pollutants,high removal efficiency of dioxane,and can ultimately achieve sufficient dioxane cleaning goals,cometabolic bioremediation technology may play an important role in the treatment of dioxane.Therefore,it is of great significance to develop treatment technologies based on microorganisms capable of co-metabolic degradation of dioxane.Thetrahydrofuran(THF)is low toxicity,biodegradable,and common contaminated with dioxane.Therefore,it may be advantageous to develop the treatment technology of co-metabolic biodegradation of THF and dioxane to remediate their co-contamination.Because of its safety,abundant carbon source and inexpensive,molasses can also be widely used as an excellent auxiliary substrate for the treatment of groundwater polluted by dioxane.In this study,we attempted to isolate strains capable of degrading dioxane with THF or molasses as auxiliary carbon sources.The their biodegradation performance and mechanism of dioxane was further explored,to provide valuable reference strains for in situ bioremediation of sites co-contaminated by THF and dioxane and groundwater polluted by dioxane respectively.The main conclusions were as follows:(1)Efficient cometabolic degradation of THF and dioxane by a novel Arthrobacter sp.WN18:(1)A new short rod-shaped bacterium Arthrobacter sp.WN18 was isolated to co-metabolic degradation of dioxane and THF.Under the optimized ratio of THF to dioxane(3:1),WN18 has a higher tolerance potential for dioxane-degrading activity.In addition,WN18 can cometabolic degradation of THF and dioxane over a wide range of p H,temperature and salinity.Like other Arthrobacter species,WN18 exhibited the capability of fixing nitrogen in the atmosphere.In addition,WN18 exhibits strong biological activity under the inhibitory effect of trichloroethylene(TCE).(2)A complete thm ADBC gene cluster in WN18 was identified,encoding the THF monooxygenase with high sequence identities previously reported in Pseudonocardia and Rhodococcus.Moreover,the abundance of thm A transcripts induced by THF was positively correlated with the dioxane degradation rate.In combination with derivation and acidification procedures,mass spectrometry analyses revealed ?-butyrolactone and HEAA as the key metabolites of THF and dioxane,respectively.Microcosm assays showed that THF could effectively stimulate the indigenous microorganisms to remove dioxane in groundwater,and the addition of bioenhanced strain WN18 further enhanced the removal efficiency.Overall,WN18 exhibited the effectiveness of co-metabolism bioremediation of dioxane contaminated groundwater,especially when co-contaminated with THF.(2)Cometabolic degradation of dioxane by a molasses-growing Acinetobacter sp.M21:(1)In this study,a strain of Acinetobacter sp.M21 that can rapidly and efficiently degrade dioxane with molasses as an auxiliary substrate was isolated without any apparent lag phase.With the optimized 0.3% molasses as the auxiliary substrate,M21 can tolerate dioxane concentration as high as 1,000 mg/L.Under a wide range of environmental conditions(p H,temperature and salinity),strain M21 was capable of effective degradation of dioxane.The main inhibitors to the removal of dioxane by continuously cultured M21 are the absence of molasses as the inducer and cell senescence.(2)The molasses-inducible thm ADBC gene cluster in M21 responsible for the initial ring cleavage of dioxane was identified,and the dioxane-metabolizing enzyme encoded by the cluster was mainly secreted into the cell to degrade dioxane.The degradation pathway of dioxane was also proposed,which was supported by the detection of 2-hydroxyethoxyacetic acid(HEAA)as the key metabolite of dioxane.Microcosm assays showed that biostimulation with molasses enhanced the removal of dioxane by indigenous microorganisms,and the injection of M21 further enhanced the removal efficiency of dioxane in the microcosm samples.These results indicate that the coupling of molasses and inorganic nutrients may be an excellent stimulant for dioxane bioremediation,and Acinetobacter sp.M21 is a promising and reliable inoculum for the in situ bioremediation of groundwater contaminated by dioxane with molasses as an auxiliary substrate.