Composting treatment increases the risk of microplastics pollution in process and compost products

Organic compost is a significant carrier of microplastics (MPs) entering agricultural soil. However, the extent of MPs pollution during composting, a widely employed organic waste treatment technology, remains unknown. This study investigated MPs dynamic pollution characteristics during composting and compost products using agricultural wastes as raw materials and quantitatively evaluated ecological risks of MPs pollution. Sorting/screening before and after composting significantly reduced MPs abundance by 29.4% and 32.1%, respectively, but substantial quantities of MPs persisted in final compost products. Microplastics were mainly 0.5–1 mm (30.3%), colored fibers/fragments composed of PP/PE, PE, or PP. Regardless of whether plastics in raw materials were sorted, composting increased MPs abundance from 5133–7867 items kg⁻¹ in raw materials to 5667–11200 items kg⁻¹ in compost products and smaller MPs (<1 mm) proportion from 48.5% to 54.4%, while significantly decreased size from 1328 μm to 1093 μm. After composting, MPs exhibited rougher morphologies (folds, cracks, and grooves) and adhered to more mineral colloids, and the pollution level increased. Collectively, composting treatment enhances MPs pollution risk both during composting process and in compost products. These findings present a theoretical reference for the establishment of limit standards for MPs in organic compost.