Degradation of polylactide microplastics in the marine environment under low temperature and in fine-grained sediments - a laboratory scale evaluation

The degradation of microplastics (MPs) in marine environments, especially under low temperatures and in fine-grained sediments, remains poorly understood. In this study, laboratory experiments were conducted on MPs of polylactide (PLA), a bio-based and industrially compostable polymer, and polystyrene (PS), a conventional oil-based polymer, to investigate their degradation in marine water and sediments at low temperatures. After 3 months PLA incubation in marine water (6–8 °C), gel permeation chromatography/size exclusion chromatography (GPC/SEC) showed a 17 % reduction in molecular weight (M n ) and increased polydispersity index in samples under combined abiotic and biotic hydrolysis. Scanning electron microscopy indicated a local surface collapse of PLA. Degradation was inhibited in samples under abiotic hydrolysis alone. No molecular-level degradation occurred in samples buried in oxygen-deficient, fine-grained sediment. A limited (0.02–0.08 %) weight loss of PLA was observed across all experiments. Polystyrene microplastics showed no signs of degradation during the experiments. Our findings suggest that in bottom sediments, where temperatures are low and oxygen is scarce, PLA undergoes limited surface erosion rather than bulk degradation. This leads to long-term persistence and gradual reduction in particle size. Therefore, improperly managed PLA waste may contribute to micro- and nanoplastic pollution in deep-sea environments. • Polylactide microplastics limited degradation in cold marine water and sediment. • Microbial activity promotes polylactide degradation through enzymatic hydrolysis. • Anoxic fine sediments limit polylactide molecular degradation. • Polylactide degrades mainly via surface erosion in sediment. • Polylactide degrades more than polystyrene in marine water.