Study On Biodegradation Mechanism Of Chloroalkene And Associated Microbial Communities In Landfill Cover

Anaerobic decomposition of organic matter in refuse produces landfill gas,commonly consisting of CH₄ and minor concentrations?up to 1%by vol.?of CAHs.The cover soil plays an important role in controlling CAHs into the atmosphere.Our previous study found that methane oxidation capacity changed with the depth of landfill cover and the microorganisms can degrade CAHs by anaerobic dechlorination and aerobic co-metabolism.Therefore,landfill cover was one of the typical vapour zone.Comprehensive analysis of degradation mechanisms and the related microorganisms in vapour zone is of great value to strengthening CAHs biodegradation.Thus,in this study,landfill covered soil was selected as the biological medium,and the adsorption and bio-degradation characteristics of typical CAHs in landfill cover soil were first studied through static batch experiments.Limiting factor of chlorinated hydrocarbon pollution control were identified by comparing the capacity of adsorption and degradation in landfill cover soil.Then,the simulated landfill cover system was set up,and according to the concentration distribution of biogas,CAHs and physicochemical property and the distribution of microbial community structure in landfill cover,the formation mechanism and change rule of functional zone were analyzed.Finally,the functional microbial were isolated from landfill cover soil.Tolerance characteristics of SWA1 for CAHs were investigated based on typical inhibition models.The results showed that:?1?Landfill cover soil has high biodegradability and strong adsorption capacity for CAHs under static conditions.?1?The degradation rate of chloroalkene in aerobic co-metabolism degradation is influenced by the concentration of growth substrate and co-metabolizing substrate.Compared with strains,landfill cover soil has a higher capacity for adsorption of CAHs and can continuously degrade pollutants within a certain range,and during the aerobic co-metabolism degradation of TCE,the dominant methane oxidizing bacteria was Methylobacter.?2?Under anaerobic co-metabolism conditions,methane and CAHs can be degraded by covering soil.The anaerobic methane oxidation rate is 0.209?mol·g^-1_soil·h^-10.257?mol·g^-1_soil·h^-1,and anaerobic co-metabolism of CAHs can be achieved on 16 days.The apparent degradation rate of CAHs accords with the first-order reaction kinetics.?3?Based on the fitting results,adsorbing capacity of CAHs in cover soil was predicted,and the results showed that the adsorption rate of CAHs increased with the number of chlorine substitution.With the same number of chlorine substitution,the absorption capacity of chlorinated alkanes was greater than chloroalkene and chlorinated aromatic hydrocarbons.Therefore,chlorinated alkanes which has higher concentration in landfill gas should be the focus pollutants in landfill operation management.CAHs adsorption rate was range of26?g·g^-1_soil·h^-1250?g·g^-1_soil·h^-1,which was far higher than degradation rate of CAHs in landfill cover soil.So,the bioaugmentation soil potential of should be paid more attention for CAHs pollution better reduction in landfill.?2?Typical chloroalkene can be degraded in simulated landfill cover system,with different degradation mechanisms and functional microbial communities.?1?t-1,2-DCE can be completely biodegraded in simulated landfill cover?The degradation efficiency was 100%?.There is obvious competitive inhibition between methane oxidation and t-1,2-DCE biodegradation.During t-1,2-DCE biodegradation,according to the distribution characteristics of biogas and microbial community structure,the whole landfill cover can be divided into aerobic zone and anoxic zone.The removal efficiency of t-1,2-DCE in aerobic zone was 60%,in anoxic zone was 40%.Major degradation mechanism of t-1,2-DCE was co-metabolism.The dominant methanotroph in anoxic zone was Methylobacer,while in aerobic zone were Methylocaldum and Microbacterium.Due to lack of assimilation-bacterial and anaerobic dechlorinating microorganisms in environment,strengthening aerobic co-metabolism was the effective measure for t-1,2-DCE removal.?2?The biodegradation rate of TCE was 0.66g·m^-2·d^-1-2.45 g·m^-2·d^-1.TCE degradation efficiency decreased with biogas flux and TCE flux and the maximum degradation efficiency was 72%.During TCE biodegradation,stable anaerobic zone,anoxic zone and aerobic zone exist in landfill cover due to persistent methane oxidation.In anaerobic zone and anoxic zone,TCE can be biodegraded by reductive dechlorination to t-1,2-DCE and c-1,2-DCE,the concentration of which reach the highest in depth of 30¹5 cm.Co-metabolic biodegradation of TCE mainly occurred in anoxic zone and aerobic zone.The results of PICRUSt showed that the landfill cover soil contained various gene for methane oxidation and CAHs biodegradation.The TCE biodegradation pathways revealed that unknown degradation pathways of TCE may exist in landfill cover soil.In contaminated site,enlarging the scope of anoxic zone by effective measures was a good choice for TCE removal.?3?PCE biodegradation efficiency was 15.50%⁶0.18%and the biodegradation rate was 0.45 g?m^-2·d^-11.45 g?m^-2·d^-1.According to the distribution characteristics of biogas and microbial community structure,the mainly zone for PCE biodegradation were anaerobic zone and anoxic zone.In anaerobic and anoxic zone,PCE was degraded to TCE and then to DCE by reductive dechlorinating.In anoxic zone,the dechlorination products were completely biodegraded by cometabolic degradation.So,enlarging the scope of anoxic zone by effective measures was a good choice for PCE removal.?4?The biodegradation mechanism of chloroalkenes is affected by its molecular structure and the environment of landfill cover.Rates of CAHs oxidation are inversely different depending on the number of Cl^-.Polychlorohydrocarbon is more likely to undergo anaerobic dechlorination,the perchloro-hydrocarbon can hardly undergo aerobic co-metabolism degradation,low chlorinated hydrocarbons are more likely to undergo aerobic oxidation,while extremely difficult to undergo anaerobic dechlorination.The degradation mechanism of t-1,2-DCE,TCE and PCE along the landfill cover is very different and the dominant genus in different depth are closely related to the types of CAHs.These results suggest that the types of CAHs and microbial community structure in contaminated site are the first important information to be identified.Combination of bio-stimulation and bioaugmentation will be the development direction of the in-situ CAHs.?3?The tolerance of microbial consortium SWA1 which was screened from landfill cover soil and good at degrading trichloroethylene?TCE?to chloroalkenes was evaluated.?1?Inhibition form of cover soil microbe methane oxidation for various chlorinated hydrocarbon was investigated.The maximum tolerated concentration of SWA1 for t-1,2-dichloroethene?t-1,2-DCE?,TCE and tetrachloroethylene?PCE?were580,250 and 500 mg·L^-1,respectively,which was significantly higher than that published.The biodegradation of chloroalkenes by SWA1 was further studied.The results showed that chloroalkenes could be efficiently removed by SWA1 and over 90%of t-1,2-DCE were removed in microbial consortium SWA1 at mid-stationary phase.?2?The dynamic composition structure and diversity of SWA1 were determined by MiSeq sequencing.The results indicated that there were significant differences in community structure of SWA1 acclimatized by different chloroalkenes.Correlation analysis showed that the dominant bacteria Methylophilus with abundance of17.4%²6.6%and Methylomonas with abundance of 31.7%⁶2.2%were closely related to methane oxidation and co-metabolic biodegradation of t-1,2-DCE,respectively.Similarly,the dominant bacteria Methylophilus with abundance of 26.9%⁴6.3%and Methylocystaceae with abundance of 1.7%³3.4%were responsible for methane oxidation and co-metabolic biodegradation of TCE.Overall,Network analysis revealed that quorum sensing improved the oxidative activity of SWA1.In this study,batch experiment and simulation system experiment were performed to deeply understand the degradation mechanism of CAHs and microbial community structure along the depth of landfill cover by analyzing degradation products,Diversity analysis and PICRUSt.Research outcomes can solve basic science problems of remediation and restoration of contaminated environment by chlorinated hydrocarbons and provide the theoretical foundation for setting up the pollution precise control strategy in contaminated site.