Assessment of plastics’ biodegradability under soil conditions based on modified biodegradability testing methods

Plastics are vital but pose environmental challenges, particularly in soil ecosystems due to agricultural use and improper disposal. Biodegradable plastics (BPs) offer a potential solution, yet their real-world degradation remains uncertain. Laboratory tests such as ISO 17556 are widely applied, but they leave ambiguities regarding reactor setup, soil preparation, and operational conditions, resulting in inconsistent interpretations and limited reproducibility. This study modified the ISO 17556 framework by establishing a decision framework and a validated standard soil protocol. Preliminary respirometry tests identified Arakida-soil as a suitable base medium and demonstrated that compost addition elevates background respiration, obscuring sample degradation signals. Therefore, compost-free standard soil (sand:clay:soil AK = 70:10:20) was selected. Reactor operation was standardized using a 500 mL vessel, an initial 3–5 day aeration interval followed by 10-day intervals. Continuous respirometry (2-h logging) was integrated with time-resolved material characterization (ATR-FTIR, DSC, SEC), enabling direct comparison of mineralization dynamics with structural changes. Three BP films with different polymer compositions were evaluated for 196 days. Results satisfied ISO 17556 validity criteria and revealed distinct mechanisms: Sample A (PHB–PBAT) degraded rapidly via bulk erosion, Sample C (PLA–PBAT) showed delayed but comparable mineralization, while Sample B (PLA–PBAT, higher PLA fraction) exhibited limited degradation. The modified methodology provides clarified operational guidance and reproducible metrics (including kinetic rate constants), improving the comparability and mechanistic understanding of BP biodegradation in soil.