(419g) Assessing the Environmental Impacts of Microplastics Considering Biodegradation

Biodegradation is considered a potential solution to the microplastic problem, which primarily arises from improper disposal of plastics at their end-of-life (EoL), such as littering into aquatic environments. This has driven the development of biodegradable plastics with significantly enhanced biodegradability compared to conventional plastics. Understanding the potential environmental impacts of biodegradable plastics compared to conventional plastics is essential for sustainability-informed decision making, which is related to material selection and technology development. Many studies have used life cycle assessment (LCA) to evaluate plastics. However, none of them have addressed the potential impacts of biodegradable microplastics due to the lack of assessment methods. This refers to the challenges to consider both biodegradation and microplastics in LCA.

To address these gaps, our study integrates experimental measurements of plastic biodegradation into the fate modeling of microplastics in freshwater ecosystems [1]. Furthermore, we incorporated ecotoxicological measurements of microplastics, assessing their aquatic ecotoxicity based on the fate modeling. Additionally, we accounted for biodegradation performance in carbon balances, quantifying the time-dependent GHG emissions resulting from microplastic degradation in water and sediment. This improved LCA approach was applied to five biodegradable plastics, including 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). It should be noted that although PLA is compostable with engineered EoL treatments (for example, industrial composting), its degradation rate in freshwater is as low as that of conventional plastics. We considered five microplastic size classes, ranging from micrometers (1000, 100, and 10 µm) to nanometers (1 and 0.1 µm).

Our findings indicate that at the micrometer scale, biodegradable microplastics exhibit lower ecotoxicity but higher GHG emissions compared to PLA or conventional microplastics. Biodegradation plays a key role, which simultaneously shortens the residence time of microplastics in water while converting their carbon content into GHG emissions. Within a 100-year period, microplastics derived from PHB, PCL, TPS, and PBS can fully degrade, whereas the degradation of PLA or conventional microplastics remains below 7% at the micrometer scale. This highlights a trade-off between reducing ecotoxicity and mitigating climate change impacts when using biodegradable plastics.

At the nanometer sale, our results show that biodegradable microplastics are lower in both ecotoxicity and GHG emissions. This is primarily due to their extremely small size, which allows not only biodegradable microplastics but also PLA or conventional microplastics to fully degrade within 100 years. Moreover, biodegradable microplastics predominantly undergo aerobic degradation in water before settling into sediment, resulting in carbon dioxide emissions. In contrast, the low degradability of PLA or conventional microplastics causes them to degrade mostly under anaerobic conditions in sediment, leading to methane emissions. Since the global warming impact of methane is 27 – 29 times that of carbon dioxide, PLA or conventional microplastics show higher GHG emissions at the nanometers scale.

In addition to biodegradation and size, our findings reveal that density and carbon content are also crucial factors. Microplastics with high density tend to have shorter residence time in water, leading to lower ecotoxicity. Additionally, plastics with inherently high carbon content can contribute to elevated GHG emissions if their microplastics fully degrade under anaerobic conditions. Our study introduces a flexible and practical approach to integrating various microplastic and biodegradation experiments into LCA, enhancing the understanding of biodegradable plastics from environmental perspectives.

Reference

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