Effects Of Microplastics On Aerobic Composting Of Cow Manure And Its Degradation

Microplastics are plastic particles with particle size less than 5 mm.The study found that a large number of microplastics were frequently detected in different types of organic waste,such as sewage sludge（1.0×10³-1.8×10⁵items/kg dry sludge）,municipal organic waste,human and animal feces（0-3.78×10³items/kg）,etc.Improper disposal of organic waste（e.g.,direct application or landfill）will significantly contribute to microplastic pollution in aquatic and terrestrial ecosystems.Aerobic composting is a biotechnology that is recognized as an effective resource-based treatment for organic waste and plastics.However,recent studies have found that amounts of microplastics also remain in compost.However,the effects of microplastics on composting and their degradation in compost are still unclear.Therefore,in this study,aerobic composting experiments were conducted with cow manure and sawdust as composting raw materials by adding different types of microplastics（PE,PVC and PHA）in the same proportion（0.5%,wt/wt）and using compost mixtures without microplastics as controls to explore the interaction between different microplastics and aerobic composting.The effects of different microplastics on the physicochemical properties and greenhouse gas emissions of aerobic composting and the degradation characteristics of different microplastics in the process of aerobic composting and their effects on microbial community were investigated.Secondly,biochar（10%wt/wt）and PHA-MPs（0.5%wt/wt）were added to a mixture of pristine cow manure and sawdust for composting,whereas a mixture without biochar served as the control（CK）to explore the effect of biochar on PHA microplastic degradation and its microbial mechanism during composting.The main results of this study are as follows:（1）Compared to the control,the addition of PE and PVC microplastics had a negative effect on organic matter（OM）degradation,while the addition of PHA microplastics promoted the increase of composting temperature and OM degradation.The addition of PE and PHA microplastics increased the cumulative emissions of CH₄and NH₃compared to the control,while the addition of PVC microplastics decreased the cumulative emissions of CH₄and NH₃.In addition,the addition of PE,PVC and PHA microplastics reduced the NO₃^--N content and compost quality.（2）In addition,the degradation efficiency of PE,PVC and PHA microplastics were significant different after aerobic composting.The abundance of PE and PHA microplastics significantly decreased（p<0.05）by 12.88%and 29.22%,respectively,while the abundance of PVC decreased by only 3.24%.The surface of all microplastics showed obvious cracks and grooves,and the carbon content decreased significantly,and the peak areas of oxygen-containing functional groups O-H,C=O and C-O increased.In addition,the addition of PE,PVC and PHA microplastics reduced the richness and diversity of bacterial communities during the thermophilic phase of composting,especially in the treatment with the addition of PVC.Compared to the control,microplastics had no significant effect on bacterial community composition at the phylum level,but significantly changed their relative abundance.（3）Compared with the control,biochar significantly enhanced the degradation rate of PHA microplastics during composting（49.87%）and promoted the carbon loss（50.80%）and oxygen loading（50.80%）.The highest degradation rate of PHA microplastics（22.42-31.23%）in both control and biochar treatments during aerobic composting was found in the thermophilic phase of compost.The addition of biochar increased the roughness of PHA microplastics surface,accelerated the breakage of C-H bonds of PHA microplastics,and improved the hydrophilicity of microplastics by increasing the oxygen-containing functional groups（O-H,C=O and C-O）on the microplastics surface.Meanwhile,the addition of biochar changed the structure of the microbial community,increased the relative abundance of Firmicutes,Proteobacteria,Deinococcus-Thermus,Bacteroidetes,and Ascomycota,and promoted the enrichment of dominant microorganisms（Bacillus,Thermobacillus,Luteimonas,Chryseolinea,Aspergillus and Mycothermus）that degrade PHA microplastics,thus improving the degradation rate of PHA microplastics.Co-occurrence network analysis showed that the addition of biochar enhanced the commensalism and mutualism relationship between the dominant microorganisms and the degradation characteristics of PHA microplastics,thus accelerating the degradation of PHA microplastics.