Microplastics pollution is a pressing environmental problem of the 21st century. Composting is a dominant dissemination pathway of microplastics to the environment. To understand the degradation kinetics of microplastics in composting, we conducted food waste composting with polyethylene and polymethyl methacrylate microplastics of 1 mm size. We performed ultraviolet and sunlight weathering treatment of the microbeads for 30 days before composting according to ASTM methods. After aerobic and hyperthermophilic composting, microplastics were separated using density separation and characterized. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analysis demonstrated increased oxidation of carbon in both the microplastics after composting. For instance, the unoxidized to oxidized carbon ratio decreased from 24.8 to 12.4 (C/O ratio decreased from 38.6 to 11.1) after aerobic composting of the ultraviolet treated polyethylene beads compared to the virgin polyethylene beads. The molecular weight (MW) of the polymers decreased significantly (30-70%, p&lt;0.05) after weathering treatments and hyperthermophilic composting, which indicates the breakdown of polymer chains after treatment. We quantified 0.0 µm year-1 specific surface degradation rate for both microplastics after composting, indicating that the composting period and physicochemical conditions were not sufficient for mass degradation. We analyzed real-world yard waste compost for microplastics and the concentration ranged from 100,000 – 300,000 pieces kg-1 dry compost with the prevalence of polyethylene microplastics. To elucidate the co-occurrence of other contaminants in compost, we investigated 40 different per- and polyfluoroalkyl substances (PFAS) in the yard waste compost. We identified 21 different PFAS species with the predominance of Perfluorocarboxylic acids (26,770 ng kg-1), Perfluorosulfonic acids (3,443 ng kg-1), and Fluorotelomer sulfonates (334 ng kg-1). This work improved our understanding of the occurrence and degradation of microplastics in compost. The understanding will help develop future control and remediation measures to minimize microplastics proliferation in the environment.

Microplastics in organic waste compost: Occurrence and degradation

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ACS Fall 2022

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