Slow degradation of compostable plastic carrier bags in a stream and its riparian area

Maxime Artru,Antoine Lecerf

doi:10.1051/limn/2019017

Abstract

There is no place on Earth where plastic debris could not be found. Impacts of plastics on aesthetics, biota and ecosystems are dependent on how long plastic items last, and what degradation products are released, in recipient environments. As bio-based plastics tend to replace petroleum-based plastics in everyday life, it is important to upgrade knowledge on the degradation of new polymers in natural environments. Single-use plastic carrier bags are nowadays made of bio-plastics certified as biodegradable and compostable. It is unclear, however, whether claims of biodegradability and compostability can be taken as evidence of rapid degradation of plastic bags outside recycling/composting facilities. This study sought to provide quantified information about the degradation of compostable plastic carrier bags in streams and riparian zones. We found that plastic samples enclosed in different types of mesh bags lost weight at extremely slow rates, albeit significant when submerged in a stream. 95% of initial plastic mass remained after 77 days spent in water whereas alder leaf litter allowed to decompose under the same condition had completely disappeared before the end of the study. Determination of respiration rate and invertebrate abundance in plastic samples showed a greater decomposer activity in the stream than in the riparian environment. However, biotically-mediated degradation by decomposers was probably overridden by dissolution processes in mediating plastic mass loss. Our findings suggest that mismanaged plastic carrier bags could impact recipient ecosystems even when they are claimed as biodegradable or compostable.

Highlights

Plastic pollution has raised as a major driver of global environmental change (Thompson et al, 2009a)
After 77 days spent in natural environments, samples of biodegradable plastic bags had lost at most 5% of their initial mass (Tab. 1)
Plastic degradation did not occur in coarse mesh bags in riparian area whereas mass loss was detected for plastic samples tested in other conditions (Tab. 1)

Summary

Introduction

Plastic pollution has raised as a major driver of global environmental change (Thompson et al, 2009a). Discharge of plastic debris into natural environments have been largely sustained by the ever-growing importance of single-use plastic products in everyday life (Plastics Europe, 2017). Quantitative assessment of plastic degradation in natural environments is an important prerequisite if we are to gain a robust understanding of how plastic pollution impacts biota and ecosystems. Policies and efforts to reduce plastic pollution increasingly rely on banning conventional single-use plastics (Xanthos and Walker, 2017) while ensuring the design and commercialization of new biodegradable plastics (European Commission, 2000). Biodegradability claim is unlikely to tell us whether mismanaged plastics are efficiently degraded in natural environments. Latest biodegradability standards (e.g. AFNOR, 2015) and conformity marks (e.g. Vinçotte/TÜVAUSTRIA, 2013) developed for single-use plastic bags may provide further guarantees as to their shorter residence time in natural environments than banned conventional plastic bags

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