Effect of addition of iron oxide nanomaterials on the degradation of polyethylene microplastics during aerobic composting of pig manure

Composting is widely recognized as an economical, environmentally friendly, and potentially sustainable method for treating various types of organic waste. Microplastics, as common contaminants in organic solid waste, are known to pose significant ecological risks to ecosystems and threaten the health of animals and humans through bioaccumulation. The presence of polyethylene microplastics (PE MPs) in livestock and poultry manure is undeniably significant, therefore, microplastics present in aerobic composting not only adversely affect compost quality but also enter the soil environment via compost as a carrier. It is necessary to consider optimizing composting methods to promote the degradation of microplastics in aerobic composting. This study aims to investigate the impact of adding different iron oxide nanomaterials (nanoscale ferric oxide and nanoscale magnetite) on the degradation of polyethylene microplastics in a composting environment, as well as the potential microbiological mechanisms underlying this degradation. Polyethylene microplastics were selected as a representative microplastic type. By comparing the surface morphology, elemental analysis, and functional group changes of polyethylene microplastics before and after composting, along with the microbial composting during the composting period, the study explores the effects of adding various iron oxide nanomaterials on the degradation of polyethylene microplastics. By adding 250 mg/kg concentrations of Fe₂O₃ NPs and Fe₃O₄ NPs through a 36-day aerobic composting period, we aim to explore the influencing mechanisms. The results show that Fe₂O₃ NPs and Fe₃O₄ NPs increase humic acid (HA) content while promoting the degradation of PE MPs, as evidenced by surface cracks, carbon chain breaks, and oxygen loading, and causes cracks and holes to appear on the surface of polyethylene microplastic, with the addition of Fe₂O₃ NPs being the most pronounced. Compared with the control treatment (CKM) which only added 0.5% PE MPs, the relative abundance of Bacillus cereus increased by 53.39% and 4.96% when added 0.5% PE MPs and 250 mg/kg Fe₂O₃ NPs (FM2) or 0.5% PE MPs and 250 mg/kg Fe₃O₄ NPs (FM3). Similarly, the relative abundance of spore-forming bacteria increased by 68.36% and 63.03%, extending the thermophilic phase of composting, and the relative abundance of iron-reducing bacteria was also significantly higher in FM2 and FM3 treatments compared to CKM. Microorganisms attach to plastics surfaces. As biological cells grow, they degrade the plastic into fragments by altering surface tension, surface structure, or secreting enzymes, thereby causing plastic degradation. Redundancy analysis results indicate that Fe₂O₃ NPs and Fe₃O₄ NPs promote the degradation of PE MPs by increasing the relative abundance of dominant bacteria during the high-temperature phase, which also increases the occurrence of iron oxidation-reduction reactions in the compost environment, resulting in more hydroxyl free radicals (·OH) acting on the degradation of PE MPs. Free radicals are ubiquitous in the environment and play a significant role in the global carbon cycle. As the most potent reactive radicals, the hydroxyl free radicals can promote oxidation on the surface of microplastics, accelerating their degradation. This study has practical significance for the harmless treatment of aerobic composting processes.