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

Composting has been widely recognized as the treatment for the various types of organic wastes, particularly in an economical, environmentally friendly, and potentially sustainable manner. One of the most common contaminants in the organic solid wastes, the microplastics (MPs) have posed serious ecological risks to the ecosystems in recent years. Polyethylene (PE) MPs can be bioaccumulated in the livestock and poultry manure. The MPs can also enter the soil environment via compost as a carrier. However, these MPs can cause adverse effects on the compost quality during aerobic composting. Therefore, it is very necessary to optimize the aerobic composting for the better degradation of the MPs. This study aims to investigate the impact of the different iron oxide nanomaterials (nanoscale ferric oxide and nanoscale magnetite) on the degradation of the PE MPs in a composting environment. The microbiological mechanisms of the degradation were also determined. The PE MPs were selected as a representative type of microplastics. A comparison was then made on the surface morphology, elemental analysis, and functional group of the PE MPs before and after composting. The microbial composting was also utilized to explore the effects of the various iron oxide nanomaterials on the degradation of PE MPs in the composting period. Among them, the 250 mg/kg concentrations of Fe₂O₃ nanoparticles (NPs) and Fe₃O₄ NPs were added in a 36 day aerobic composting period. The results show that the Fe₂O₃ and Fe₃O₄ NPs increased the content of the humic acid (HA), while the better degradation of the PE MPs was achieved in the surface cracks, carbon chain breaks, and oxygen loading. The cracks and holes caused to appear on the surface of the PE microplastics. The better performance was observed in the addition of Fe₂O₃ NPs. 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), respectively. Similarly, the relative abundance of the spore-forming bacteria increased by 68.36% and 63.03%, respectively, thus extending the thermophilic phase during composting. The relative abundance of iron-reducing bacteria was also significantly higher in the FM2 and FM3 treatments, compared with the CKM. Microorganisms were also attached to the plastics surfaces. The increasing biological cells were used to degrade the plastics into the fragments. Surface tension, surface structure, or secreting enzymes were also altered to cause the plastic degradation. Redundancy analysis indicate that the Fe₂O₃ NPs and Fe₃O₄ NPs were promoted the degradation of the PE MPs. There was the increase in the relative abundance of the dominant bacteria during the high-temperature phase. The iron oxidation-reduction reactions also increased in the compost environment, thus resulting in more hydroxyl free radicals (·OH) acting on the degradation of PE MPs. Free radicals in the environment often played a significant role in the global carbon cycle. One of the most potent reactive radicals, the hydroxyl free radicals were promoted the oxidation on the surface of MPs, thereby accelerating their degradation. This finding can provide the practical significance for the sustainable treatment on aerobic composting.