(630d) Methodology Advancement of Life Cycle Impact Assessment for Biodegradable Microplastics

Biodegradable plastics have emerged as a promising solution to mitigating plastic pollution by offering superior biodegradability compared to conventional plastics. However, their end-of-life (EOL) fate may introduce environmental trade-offs due to greenhouse gas (GHG) emissions from biodegradation. While life cycle assessment (LCA) has been applied to evaluate the engineered EoL pathways of biodegradable plastics, such as industrial composting and anaerobic digestion [1], their fate in and impacts on natural ecosystems remains largely unexplored. A key challenge is the absence of standardized approaches to assess microplastics in LCA, particularly concerning their biodegradation performance.

To bridge this gap, we developed a novel life cycle impact assessment (LCIA) method to evaluate biodegradable microplastics in freshwater ecosystems [2]. Our approach integrates the fate, exposure, and effect factors of microplastics using USEtox [3], considering them as toxic particles. The fate modeling includes key removal mechanisms, such as biodegradation and sedimentation, by linking microplastic size, density, and specific surface degradation rate (SSDR) to their respective rate constants. Unlike previous research [4], which simplifies the interactions between biodegradation and sedimentation, our method captures their dynamic interplay. We applied the proposed method to five biodegradable plastics: bio-based poly(lactic acid) (PLA), poly(3-hydroxybutyrate) (PHB), and thermoplastic starch (TPS), as well as fossil-based poly(ε-caprolactone) (PCL) and poly(butylene succinate) (PBS). Our study examines the five microplastics across a series of particle sizes, from micrometers (1000, 100, and 10 µm) to nanometers (1 and 0.1 µm), quantifying their aquatic ecotoxicity. By combining fate modeling with biodegradation performance, we further assessed the time-varying GHG emissions resulting from microplastic biodegradation in freshwater environments, utilizing both static and dynamic methods assessment methods.

Our findings reveal that microplastics with low SSDR, such as PLA, exhibit the highest aquatic ecotoxicity but the lowest GHG at the sizes of 1000, 100, and 10 µm. This suggests a potential environmental trade-off, where substituting PLA with more biodegradable plastics (for example, PHB, PCL, TPS, and PBS) reduces ecotoxicity but increases GHG emissions in freshwater EoL. However, at nanometer sizes (1 and 0.1 µm), no such burden shifting occurs, as PLA demonstrates the highest impact in both categories. Additionally, our results indicate that biodegradable plastic degradation in freshwater ecosystems can lead to higher GHG emissions than in engineered EoL pathways, substantially contributing to their life cycle carbon footprint. Our sensitivity analysis identifies critical SSDR that maximizes GHG emissions at different microplastic sizes. These insights enhance our understanding in the environmental impacts of biodegradable microplastics, providing a robust method for designing next-generation biodegradable plastics with optimized environmental performance.

Reference

[1]. Cazaudehore et al., Biotechnology Advances 2022, 56, 107916.

[2]. Piao et al., Nature Chemical Engineering 2024, 1(10), 661-669

[3]. Rosenbaum et al., The International Journal of Life Cycle Assessment 2008, 13, 532.

[4]. Corella-Puertas et al., Journal of Cleaner Production 2023, 418, 138197