Environmental degradation and fragmentation of microplastics: dependence on polymer type, humidity, UV dose and temperature

Depending on the environmental compartment, plastics are subjected to various stressors, including UV light, water, microbial exudates (enzymes), and temperature. Among these, stress on plastics from photo-chemical processes was identified as a leading exposure pathway of plastics, e.g., in the atmosphere or on the water surface. While the focus of earlier studies mainly was on deterioration of the chemical and mechanical properties, more recent studies demonstrate how photo-oxidation leads to fragmentation and release of secondary micro- and nanoplastic fragments, as well as low-molecular weight species. These studies tend to focus on a single exposure condition and a limited number of polymer types. Therefore, this study focuses on systematically evaluating the influence of temperature and relative humidity during simulated UV exposure on the fragmentation and degradation of five types of pristine microplastic powders: polypropylene, low density polyethylene, polyamide 6, high impact polystyrene and thermoplastic polyurethane. We quantified the dose-dependent release of water-soluble organics, as well as secondary micro- and nanoplastics (including their particle size distributions) and found that the polymer identity dictated the type and quantity of species released rather than the aging protocol. With this systematic assessment the generated data can be used in mechanistic microplastic fragmentation models to determine fragmentation rates and fragment size distributions.