Progressive exploration of the four municipal solid composting phases for microplastics pollution: Extraction, abundance, and distribution

Research on microplastic (MP) pollution in agriculture is limited, particularly concerning municipal solid waste (MSW) compost, which is commonly used to enhance soil quality but can introduce MPs into agricultural systems. This study evaluates the density separation efficiency of ZnCl₂ and CaCl₂ in both spiked and non-spiked compost. It also tracks MP abundance and size distribution throughout the composting stages-mesophilic, thermophilic, curing, and mature-at a Canadian facility. The research employs a combination of three extraction methods: wet-sieving, organic digestion, and density separation. Results indicate that ZnCl₂ yields an extraction recovery of ∼99.9 % in spiked tests, outperforming CaCl₂, which achieves about 95 %. In the non-spiked study, ZnCl₂ recovered 242 MP particles/kg of wet compost compared to CaCl₂, which extracted 138 MP particles. Moreover, MP abundance increased throughout the composting process, demonstrating a linear relationship between MP size and abundance. Characterization via ATR-FTIR spectroscopy revealed that polyethene, polypropylene, and polystyrene were the most common MPs, while polyurethane, polyvinyl chloride, and cigarette filters were less prevalent. These findings underscore the need to improve MP management in organic composting, given its potential impact on plant health. Additionally, it enables a more focused approach to MP studies in compost, with an effective, affordable extraction protocol.