| Plastics are widely used in the fields of household products,packaging,textiles,electronic products,automobiles,construction and agriculture due to their excellent properties of light weight,water resistance and abrasion resistance.With the rapid increase in the production and use of plastic products,a large amount of plastic waste is produced and discharged into the environment,which not only causes plastic pollution,but also causes an emergency pollutant—Microplastics.Microplastics refer to plastics with a size less than 5mm.Due to their small size and large quantity,microplastics are easy to be swallowed by organisms.Moreover,microplastics have been proved to be carriers of a variety of inorganic and organic pollutants,and their surface can colonize microorganisms such as pathogenic bacteria,and their potential ecological risks have aroused wide concern among researchers.Landfills are a major source of household waste and also contain large amounts of plastic wastes.Landfill is a long-term process that produces leachate,a secondary pollutant.However,there are few studies on the occurrence and characteristics of microplastics in landfill and leachate treatment.The landfill system also contains a variety of other emerging pollutants such as antibiotic resistance genes(ARGs),and whether the presence of microplastics will affect the spread of ARGs has not been studied.Based on this,this study firstly investigated microplastic occurrence characteristics and distribution in the refuses and leachate,on this basis,and the distribution and removal of microplastic by different process in the landfill leachate treatment system have been studied,and finally explores aging characteristics of microplastic in the leachate and microplastic and emerging contaminants ARGs interaction relationship and mechanism.The results of this study will contribute to a comprehensive understanding of the morphology,nature and potential environmental impacts of microplastics in landfill systems,and provide a certain research basis for the migration and transformation of microplastics in the environment.The main results of this study are as follows:1.The occurrence characteristics of microplastics in urban landfill and their trend with the age of landfill.This experiment studied the occurrence of leachate and microplastics in landfill at different landfill ages(3-20 years).The average abundance of leachate and microplastics in landfill was 8(±3)items/L and 62(±23)items/g,respectively.The main forms and polymer types of microplastics detected in leachate are fiber and cellophane respectively,while the main shape and polymer type of microplastics in landfill are sheet and PE(polyethylene)microplastics.The results show that the landfill is not only the main repository of plastic waste,but also an important source of microplastics.Infrared spectroscopy analysis shows that with the growth of landfill age,PE microplastics in landfill site undergo oxidative degradation.The characteristics of microplastics in leachate are different from those in landfill.The occurrence of microplastics in landfills is mainly affected by the increase of plastic wastes,the life of plastic products and the degradation of polymers.2.Distribution and removal characteristics of microplastics in different treatment processes of leachate treatment system.By studying the distribution characteristics and removal efficiency of microplastics in different processes in the leachate treatment system,the results show that PE microplastics play a dominant role in the whole leachate treatment process,and the maximum abundance of fibrous microplastics accounts for 42.2%.It was found that the removal efficiency of microplastics in the pretreatment stage(regulating tank)and MBR(membrane bioreactor),AO(Anoxic/Oxic)and ultrafiltration processes were16.67%,50%,20%and 75%respectively,but no removal of microplastics was observed in the nanofiltration and reverse osmosis processes.According to the results of this study,the total discharge of microplastics in the leachate treatment plant is estimated to be 1×10~6 items daily,and the average abundance of microplastics in the sludge tank is 893±252 items/kg(dry weight).In view of the large amount of leachate continuously discharged into the aquatic environment,the removal of microplastics should be considered in the leachate treatment process and applied to the leachate treatment system.3.Natural aging characteristics of microplastics in leachate and its effect on ARGs.About the research on the material property change of microplastics in leachate and the influence of microplastics on ARGs in leachate.The results of thermogravimetric analysis show that PE,PP and PS microplastics have higher thermal stability than pure microplastics after aging.The analysis of specific surface area and porosity showed that the specific surface area of the three kinds of microplastics increased after aging.X-ray photoelectron spectroscopy shows that C-O groups are formed on the surface of the microplastics.Infrared spectroscopy further verifies that the spectroscopic properties of the aged microplastics changed and new groups were formed.Scanning electron microscopy(SEM)showed that the surface of the microplastics was roughened and attached by microorganisms after aging.Through the study of relative abundance of ARGs on microplastics and leachate by Fluorescent quantitative PCR,it can be seen that sulfonamine-resistant genes have the highest abundance in the whole system,and all three kinds of microplastics have enrichment effects on ARGs in leachate.Through the analysis of microbial community structure in microplastics and leachate by high-throughput sequencing,it can be seen that Actinomycetes,Firmicutes and Proteobacteria are dominant.At the level of genus,Corynebacterium is the main pathogenic bacteria on microplastics,and the environmental risk of pathogenic bacteria on microplastics increases with time.By analyzing the correlation between ARGs and microorganisms on microplastics,it can be seen that microplastics enhance the enrichment effect of ARGs by enriching microorganisms with specific community structure,thus increasing the risk of transmission of ARGs in leachate.In summary,this study investigates the occurrence and distribution of microplastics in urban landfill systems,and discusses the interaction between microplastics and ARGs in leachate.The results of this study are conducive to a deeper understanding of the changing characteristics and long-term trend of emerging pollutant of microplastics in landfill systems,and are of great significance for a comprehensive assessment of the potential environmental risks of removing microplastics from leachate treatment systems. |
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