In situ degradation of biodegradable bio-based plastics in urban soil: Pilot study for PLA, PHB, PHBH, and Bio-PBS in...

Biodegradable bio-based plastics (BBPs) are increasingly replacing conventional plastics for reducing negative environmental impacts, while few studies tested BBPs' biodegradability in urban soil, although cities can be major sources of BBP waste. In this study, polylactic acid (PLA), polyhydroxybutyrate (PHB), poly (3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), and bio-based polybutylene succinate (Bio-PBS) were buried in soil with or without food waste for 120 days in central Tokyo, Japan, evaluating BBPs degradation based on weight loss measurements, thermogravimetric/differential thermal analysis (TG-DTA), and Fourier-transform infrared spectroscopy (FTIR), while comparing weight loss data with tea bag index (TBI), indicating soil microbial activity and organic matter degradation. We found that the addition of food waste reduced the relative mass remaining for PHB and PHBH from 91.1-99.5% to 76.9-91.1%. Although much of PLA and Bio-PBS remained (98.3-100%), TG-DTA and FTIR analyses indicated the progression of structural changes necessary for biodegradation in soil. Low stabilisation factor (S) and high decomposition rate (k) values from the TBI were associated with greater degradation rates for PHB and PHBH. Our findings suggest that, if the average degradation rates observed for PHB and PHBH in this study can be applied, the complete degradation of BBPs in urban soil in Tokyo may take at least several years, which could be reduced by composting BBPs with food waste in soil. Future research should test BBPs' multi-year degradation in urban soils across different climate zones and soil types, while comparing TBI and BBPs' weight loss.