| The impact of non-degradable impurities in composting is one of the main challenges of aerobic composting technology for rural domestic waste(RDW)at this stage,however,the distribution of environmentally emerging pollutants among impurities,plastic fragments smaller than 5 mm – microplastics(MPs)-and their potential pollution problems have not been fully studied.Therefore,in this study,we investigated the distribution characteristics of MPs in aerobic composting of RDW,and the effect of weathering and fragmentation of macroplastics in composting on the change of MPs distribution was studied,and finally,the effects of polypropylene MPs on the physicochemical properties,humification degree,enzyme activity,and microbial community structure and diversity of aerobic compost were investigated,and the microbial communities colonized on the surface of MPs were also characterized.The results of this study can help to correctly understand the characteristics of MPs in composting,provide reference data for quantifying the sources of soil MPs,and provide a theoretical basis for controlling MPs pollution in composting process.The main findings of this study are as follows.(1)The distribution characteristics of MPs changed during the aerobic composting of RDW.MPs of 1100 ± 141 items/kg and 800 ± 200 items/kg were detected in the RDW feedstock at the two stations,while the abundance of MPs increased to 2533±457 items/kg and 2267±115 items/kg in the output fertilizer.The depletion of organic matter and the production of secondary MPs were responsible for the increased abundance of MPs.The main shapes of MPs in compost were fibers and films,and the most common types were Polyester,PP and PE.The MPs detected in the final compost product were generally characterized by small particle size and showed a typical weathering morphology.(2)Weathering and fragmentation of different types of macroplastics was an important cause of changes in the distribution of MPs in composting.The composting process leads to a yellowing effect of macroplastics with cracking,flaking,embrittlement,colonization by microorganisms,and surface oxidation of microscopic surfaces,and secondary MPs can be released from the weathered surfaces of macroplastics.The weathering surface of a piece of macroplastic can release 4-63 items of MPs,of which the percentage of MPs smaller than 0.5 mm in size was 73.5%.And the number of MPs produced from high to low was EPS>PE>PP.In terms of influencing factors,the variation in the distribution of MPs in composting was determined by both the physical composition of the RDW and the treatment mode of the composting process.(3)Polypropylene(PP)MPs influenced the RDW composting effect.The addition of 1% and 3%(w/w)of MPs had a negative effect on the duration of the high temperature period of composting,the amount of water evaporation and the degradation efficiency of the organic matter,with 3% PP MPs causing the GI value of the compost product to fall below 50% that were no longer suitable for application to soils.PP MPs inhibited the formation of large molecules and complex humic acid in compost.The presence of MPs had no significant effect on the activity of compost catalase,3% MPs significantly increased compost urease activity,while the addition of 1% and 3% MPs significantly inhibited the activity of cellulase.(4)PP MPs changed the structure of the compost microbial community.In the early stage of composting,MPs reduced the relative abundance of Firmicutes by27.20%-40.47%,in which the abundance of bacterial genera highly related to lignocellulose degradation was significantly reduced,while the relative abundance of Proteobacteria increased by 13.91%-34.23%.In the late stage of composting,MPs increased the relative abundance of Actinomyces and Bacteroidetes.The microbial community colonized on the surface of the PP MPs was significantly different from that in the composting environment.Bacteroidetes was the dominant phylum on the surface of MPs at the end of composting. |
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