Biodegradable Plastics Do not Form Chemically Persistent Microplastics

Abstract

Biodegradation of solid plastic materials is a heterogeneous reaction [1]. Only the surface is affected by biodegradation while the inner part should not be readily available for biodegradation. Current standard test methods for biodegradation are based on the exposure of a plastic sample to an environmental matrix (e.g. compost, soil, marine sediment, etc.) and on the measurement of the extra respiration possibly induced by the plastic sample in case of metabolization by microbes as an energy and carbon source [2]. Biodegradation is calculated as the mineralisation ratio i.e. the ratio between the carbon evolved as CO₂ (C-CO₂) and the total carbon present in the plastic sample (C-plastic) [3]. Under these conditions, the biodegradation rate refers to how the “C-CO₂/C-plastic” ratio changes over time. This is understood as the biodegradation rate of the polymer; however, most carbon (C-plastic) is not available but rather protected in the core of the plastic particles under testing. Thus, with the current test approach we can only determine an “apparent” biodegradation rate, most reactant being not available but nevertheless accounted in the denominator. Because of that, biodegradation of solid materials appears as a “slow” process, which suggests persistence of biodegradable plastics in the environment. In this paper, we show that the biodegradation rate happening at the surface of the plastic particles is very fast. We are currently proposing a different approach to measure the biodegradation rate of plastics, in order to determine the rate of the reaction effectively occurring at the surface of the plastics, i.e. at molecular level. To do so we suggest measuring biodegradation in samples with increasing surface areas and applying regression to determine the relevant parameters. This article is based on two previous publications where more details can be found [4, 5].