Surface-Limited Chemical and Physical Alteration of Plastics during Composting: An In-Depth Field- and Lab-Scale Study

Abstract

Plastics entering composting systems are frequently found as fragments in finished compost, yet the mechanisms controlling their degradation and persistence remain unclear. In this study, food waste amended with virgin, 30-day UV-weathered, and 30-day sunlight-weathered polyethylene (PE) and poly(methyl methacrylate) (PMMA) microbeads was treated under aerobic and hyperthermophilic composting conditions. PE showed increased surface oxidation after both composting conditions, with the carbonyl index increasing from 3.6–16.6 in virgin to 12.2–25.1 in UV-weathered and 15.0–21.3 in sunlight-weathered beads. This oxidation corresponded with a decreased C/O ratio due to the formation of carbonyl and ester/carboxyl groups. Composting increased crack density on PE surfaces without altering molecular weight, indicating surface-limited degradation. In contrast, PMMA showed a 44–71% decrease in molecular weight after weathering, but no additional molecular weight reduction occurred during composting of weathered PMMA beads. After exposure to both composting conditions, PMMA showed increased thermal stability, whereas PE showed decreased thermal stability. Degradation rates were 6.2–19.5 μm yr–1 for PE and 5.9–33.9 μm yr–1 for PMMA, with estimated degradation times of 8.6–26.9 and 5.8–41.9 years, respectively. A field study showed that surface oxidation and crack density of PE, PP, PS, and PET correlated with compost maturity, likely due to prolonged exposure to dynamic composting conditions. These findings reveal plastic degradation pathways in compost and enable the prediction of their long-term environmental fate.