Microplastic Generation and Persistence of Biodegradable Plastics under Anaerobic Conditions

Biodegradable plastics (BPs) are increasingly replacing conventional plastics, yet their degradation behaviors and potential to generate microplastics in anaerobic environments remain unclear, partly due to the lack of reliable methods for separating BP-derived microplastics (BPMs) from complex sludge matrices. This study developed and confirmed high-accuracy, polymer-specific separation and identification methods tailored to eight commonly used BPs, achieving recovery rates above 94.9% for most BPMs. Using these methods, we systematically investigated microplastic generation, intermediates and final products transformation, and molecular and structural evolution of BPs during mesophilic anaerobic degradation. Distinct transformation pathways were observed depending on polymer structure. PHB, PHBV, and CDA degraded efficiently into methane with transient microplastics and high biodegradability (79.1-87.1%). PBSA and PLA exhibited incomplete hydrolysis, producing abundant and persistent microplastics and showing low biodegradability (27.3% and 19.5%, respectively). PBS fragmented extensively, whereas PBAT and PCL generated limited quantities of predominantly small microplastics (<300 μm), despite apparent molecular weight reduction and chain scission, with no substantial methanogenesis. The results demonstrate that polymer chemistry governs whether BPs undergo complete anaerobic transformation or fragment into persistent microplastics, revealing that several certified BPs can act as significant microplastic sources in anaerobic environments common to waste management systems and natural settings.