Investigating aquatic biodegradation and changes in the properties of pristine and UV-irradiated microplastics from conventional and biodegradable agricultural plastics

Abstract

There is an increasing tendency to replace conventional agricultural plastic mulching films with biodegradable alternatives. However, while the latter biodegrade well under controlled conditions (e.g. industrial compost), their biodegradation in non-target environments (e.g. aquatic environments) is questioned and poorly understood. Therefore, in this study, microplastics derived from conventional polyethylene (PE) and biodegradable polybutylene adipate terephthalate starch blend (PBAT) mulching films were exposed to UV irradiation and subsequently tested for their ready biodegradability in an aqueous medium where changes in their characteristics were evaluated. The results showed limited biodegradation for pristine and UV-aged PE: no morphological, surface chemical or internal changes were observed. Pristine PBAT showed signs of initial biodegradation, while UV-aged PBAT biodegraded by up to 57%. New functional groups appeared on the PBAT surface after UV irradiation according to FTIR analysis and crystallinity increased after biodegradation. Elemental analysis revealed a range of metals in PE and PBAT microplastics. No changes in metal distribution analysed in micro-plastic after UV-aging or biodegradation were found, except that less titanium was present in PBAT after biodegradation indicating potential leaching. None of the PBAT microplastics had ecotoxic effects towards the aquatic plant Lemna minor. Pristine and UV-aged PE showed negative effects on roots, but these were not observed after biodegradation. Low biodegradation of pristine PBAT and possible leaching of metals demonstrated here raise questions about the sustainable use of biodegradable alternatives, especially when they enter non-target environments.There is an increasing tendency to replace conventional agricultural plastic mulching films with biodegradable alternatives. However, while the latter biodegrade well under controlled conditions (e.g. industrial compost), their biodegradation in non-target environments (e.g. aquatic environments) is questioned and poorly understood. Therefore, in this study, microplastics derived from conventional polyethylene (PE) and biodegradable polybutylene adipate terephthalate starch blend (PBAT) mulching films were exposed to UV irradiation and subsequently tested for their ready biodegradability in an aqueous medium where changes in their characteristics were evaluated. The results showed limited biodegradation for pristine and UV-aged PE: no morphological, surface chemical or internal changes were observed. Pristine PBAT showed signs of initial biodegradation, while UV-aged PBAT biodegraded by up to 57%. New functional groups appeared on the PBAT surface after UV irradiation according to FTIR analysis and crystallinity increased after biodegradation. Elemental analysis revealed a range of metals in PE and PBAT microplastics. No changes in metal distribution analysed in micro-plastic after UV-aging or biodegradation were found, except that less titanium was present in PBAT after biodegradation indicating potential leaching. None of the PBAT microplastics had ecotoxic effects towards the aquatic plant Lemna minor. Pristine and UV-aged PE showed negative effects on roots, but these were not observed after biodegradation. Low biodegradation of pristine PBAT and possible leaching of metals demonstrated here raise questions about the sustainable use of biodegradable alternatives, especially when they enter non-target environments.