Aging of polyester microplastics in sewer systems: Crucial role of sulfides

Sewers served as primary gateways for microplastics (MPs) into urban drainage systems, yet MP transformation under dark, anaerobic, and sulfide-rich sewer conditions remained poorly understood. This study elucidated the aging characteristics of polyester MPs (polyethylene terephthalate (PET) and polybutylene adipate terephthalate (PBAT)) in sewers and identified the key mechanisms governing sulfide-induced MP aging. Sewer exposure reduced the number-average molecular weight and increased the surface hydrophilicity, carbonyl index, and O/C ratio of PET/PBAT MPs, indicating pronounced oxidative modification. Among representative sewer constituents, sulfide was identified as the crucial driver of MP aging. Hydroxyl radicals and superoxide anions generated via sulfide autoxidation induced oxidation and sulfide-addition reactions in both PET and PBAT MPs, while PBAT MPs likely underwent concurrent reduction and more pronounced physical damage. Sulfide also promoted the leaching of dissolved organic matter (DOM) from polyester MPs, particularly PBAT (up to 12.86 mg/L), yielding more humified, predominantly endogenous-like DOM signatures and generating up to 10⁷ nanoplastics/mL. These findings provided mechanistic evidence of sulfide-induced MP aging in sewers, offering essential insights into the ecological risks of MPs in urban drainage systems.