Study On Migration And Degradation Mechanism Of Chlorinated Alkanes In Landfill Cover

Anaerobic decomposition of organic matter in refuse produces landfill gas,commonly consisting of CH₄ and minor concentrations of chlorinated aliphatic hydrocarbons.Our previous study found that methane oxidation capacity changed with the depth of the landfill cover and the microorganisms can degrade chlorinated aliphatic hydrocarbons.Therefore,the landfill cover soil plays an important part in controlling chlorinated aliphatic hydrocarbons into the atmosphere.However,the biodegradation mechanism of chlorinated aliphatic hydrocarbons in landfill cover remains unclear,which limits the construction of chlorinated aliphatic hydrocarbon bioremediation methods.In-depth analysis of the biodegradation mechanism along the landfill cover has important guiding value for in-situ remediation of chlorinated aliphatic hydrocarbon contaminated sites.Chlorinated alkane is a high detection rate of chlorinated aliphatic hydrocarbons in landfill gas.Chloroalkanes have the characteristics of high toxicity,high enrichment,high environmental residue and carcinogenic,teratogenic and mutagenic effects,which cause serious harm to human health and ecological environment.Based on this,this study constructed a simulated landfill cover system with the actual landfill cover soil as the biological medium.Combined with diverse sequencing technology,the migration and degradation mechanism of three typical chlorinated alkanes of dichloromethane（DCM）,chloroform（CF）and carbon tetrachloride（CT）in landfill cover were systematically analyzed for the first time.The results showed that:1)Methane oxidation has a significant impact on the distribution of landfill gas,so that the landfill cover layer has three layers:aerobic layer（0²0 cm）,anoxic layer（20⁴0cm）and anaerobic layer（>40 cm）.Combined with the principal component analysis of soil physical and chemical properties and microbial community structure,it is known that the distribution of landfill gas,physical and chemical properties of soil and microbial community structure in the feature layer is significantly different.The landfill gas passes through the anaerobic layer,the anoxic layer and the aerobic layer of the landfill cover layer in the middle of the migration process.Methane oxidation end-product carbon dioxide makes the aerobic layer pH lower than the anoxic layer and the anaerobic layer.2)DCM has a low degree of chlorine substitution,and it is difficult for DCM to perform anaerobic reductive dechlorination degradation in the anaerobic layer.In the aerobic and anoxic layers,DCM can be catalytically oxidized to carbon dioxide and produce inorganic chloride ions under the action of methane monooxygenase produced by methanotrophs.The highest Cl^-content was found at 15 cm below the surface of the aerobic layer,which is 214.6 mg/kg_soil.The relative abundance of methanotrophs in the aerobic layer and the anoxic layer is significantly higher than that in the anaerobic layer.The dominant methanotrophs in this system is unclassified_f<sub>Methylocystaceae.3)Degree of CF chlorine substitution is relatively high,with different degradation mechanisms and functional microbial communities along the landfill cover.The dominant bacteria in the aerobic layer was methanotrophs,of which type I methanotrophs Methylobacter and type II methanotrophs Methylosinus were mostly.Anaeromyxobacter in anoxic and anaerobic layers became the dominant CF anaerobic degrading bacteria.CF was effectively degraded under aerobic,anoxic and anaerobic conditions.CF was transformed into DCM by anaerobic reduction dechlorination in the anoxic and anaerobic layers.And some DCM was converted to acetate,H₂ and CO₂ by Dehalobacter.The remaining CF and DCM were co-metabolized by methanotrophs in the aerobic layer.The relative abundance of methanotrophs at 15 cm below the surface of the landfill cover soil was the highest（>20%）,the most methane monooxygenase was produced,and CF was degraded mostly,so the landfill cover soil has the highest Cl^-content,which is 293mg/kg_soil.4)CT is a perchloric acid substitute,and the degradation mechanism and microbial community along the landfill cover also specific.CT can be dechlorinated under the action of CT anaerobic degrading bacteria to produce CF and DCM in the anaerobic layer.Dominant CT anaerobic degrading bacteria was Pseudomonas.CT dechlorination products CF and DCM can be metabolized by methanotrophs in the aerobic layer.The relative abundance of CT anaerobic degrading bacteria and methanotrophs was relatively high in the anoxic layer,so the reactions in the anaerobic layer and the aerobic layer can occur simultaneously in the anoxic layer.The CT degradation product Cl^-gradually increased,and the Cl^-content of the anaerobic layer and the anoxic layer was higher than that of the aerobic layer,and the highest content was 186.7 mg/kg_soil.5)Due to the different degree of chlorine substitution,DCM was not detected in the surface layer of the covered soil,and the removal rate of DCM was 100%.So DCM is degraded easily by aerobic co-metabolism.The aerobic co-metabolism degradation efficiency of CF in the landfill cover was 36%,anaerobic reductive dechlorination degradation efficiency was 9.6%,and the aerobic co-metabolism plays a major role in CF degradation.The biodegradation of CT occurs only in the anoxic layer and the anaerobic layer,and CT was degraded by anaerobic reductive dechlorination with a degradation efficiency of 34.9%,the results were obtained at gas flux 0.612 m³·m^-2·d^-1.Different degrees of chlorine substitution also lead to different dominant degradation microorganisms of chlorinated alkanes in the landfill cover.The dominant degrading microorganism of DCM in the landfill cover is unclassified_f<sub>Methylocystaceae,the dominant degrading microorganism of CF is Methylosinus,and dominant degrading microorganism of CT is Pseudomonas.In this study,batch experiment and simulation system experiment were performed to deeply understand the migration and degradation mechanism of chlorinated aliphatic hydrocarbons in the landfill cover,and landfill gas analysis,soil physical and chemical properties analysis,microbial community structure and relative abundance analysis were used.The research conclusions can enrich the microbial information of the contaminated sites,provide theoretical support and practical basis for the chlorinated hydrocarbon pollution control process,and provide optimization strategies for in-situ remediation of pollutants.