Determining the impact of common microplastic extraction methods from soil matrices on the biodegradable polymers polylactic acid and polyhydroxybutyrate

The use of conventional non-biodegradable plastic polymers in agricultural applications has raised concerns regarding their degradation into micro- and nano-plastics and accumulation in soils. As a result, biodegradable polymers are increasingly used in agricultural applications. Complete environmental biodegradation is expected to prevent the formation of persistent micro- and nano-plastics. However, incomplete biodegradation may result in the presence of fragments from biodegradable polymers. Understanding the environmental fate of biodegradable polymers is essential, and reliable extraction and analytical methods are needed to detect and quantify them in soil matrices that do not induce further polymer degradation. In this study, biodegradable polylactic acid (PLA) and polyhydroxybutyrate (PHB) polymer films were exposed to a commonly used protocol for the analysis of microplastics in soil samples. Specific steps were density separation with zinc chloride solution, oxidation with Fenton’s reagent and enzymatic digestion. Polymer degradation was assessed by comparison of polymer properties before and after exposure to the complete soil extraction protocol, and to each individual reagent. Degradation of both PLA and PHB films was observed after exposure to the complete soil extraction protocol. PLA showed significant degradation following exposure to protease, highlighting its vulnerability to this specific treatment step. This study highlights the need for appropriate polymer specific sample extraction methods to minimise extraction-induced degradation and ensure accurate measurements of biodegradable polymers. This is critical to allow future assessment of the environmental fate of biodegradable polymers.