Plastic composition drives the toxicity of UV-aged melamine-polypropylene microplastics, nanoplastics, and leachates...

This study investigates how the plastic additive composition influences the toxicity of UV-aged polypropylene (PP) microplastics (MPs), nanoplastics (NPs) and leachates toward the green alga Raphidocelis subcapitata. Three materials were compared: an antioxidant-deprived additive-free PP (PPd), a base-stabilized PP containing Irgafos 168 (PPc), and a compounded melamine-polypropylene with Irgafos 168 (PPf). UV-irradiation induced surface oxidation and fragmentation across all samples, releasing NPs (< 1 µm) and ultrafiltrates (< 50 kDa), representing the dissolved and colloidal fraction. Characterization by ATR-FTIR, SEM, and GC/MS confirmed that the material deprived of the antioxidant (PPd) underwent more intense oxidation, displaying a higher abundance of oxygenated functional groups and smaller particle sizes compared to stabilized forms. Toxicity assays with R. subcapitata demonstrated a clear size-dependent trend, with ultrafiltrates > NPs > small MPs (1-50 µm) > large MPs (50-500 µm), yet the magnitude of this toxicity was strongly driven by the additive composition. Growth inhibition, reactive oxygen species (ROS) generation, and membrane depolarization were significantly increased in the absence of antioxidant additives across all size fractions, indicating higher bioavailability and reactivity of UV-generated particle surfaces or leached low-molecular-weight oligomers. But still, stabilized melamine-PP released additional leaching products and its ultrafiltrates enriched in oxidized additives and low-molecular-weight oligomers, caused high cytotoxic and oxidative responses. Overall, these results demonstrate that the additive composition not only modulates polymer stability during UV ageing but also governed the toxicity of plastics particles and leaching products toward aquatic primary producers, underscoring the ecological relevance of plastic ageing processes in aquatic ecosystems.