Unraveling the co-release dynamics of micro/nanoplastics and toxic transformation products from weathered single-use plastics.

The extensive accumulation of single-use plastics (SUPs) in aquatic environments necessitates a mechanistic understanding of their transformation products. This study characterizes the release dynamics of micro/nanoplastics (MNPs) and plastic associated organic compounds (PAOCs) from polystyrene (PS), polypropylene (PP), and polylactic acid (PLA) during environmental weathering. Results demonstrate that photo-aging drives a dual trajectory of degradation. While PS exhibited the highest physical susceptibility to fragmentation, particle abundance alone did not explain the polymer-dependent acute toxicity. High-resolution mass spectrometry and in silico profiling identified over 110 annotated candidate structures, including additive-related constituents and transformation products with predicted persistence, bioaccumulation potential, or aquatic toxicity. Crucially, toxicological assays on Daphnia magna indicated that most of the measured acute toxicity was retained in the dissolved fractions of weathered PP and PLA, whereas the predominant toxicity-associated fraction for PS remained unresolved. Unexpectedly, the biodegradable polymer PLA exhibited the highest acute lethality and neurotoxicity-related response, revealing a critical misalignment between material degradability and biological safety. These findings indicate that weathered SUPs function as chemically active sources of pollution, underscoring the necessity of incorporating additive mixture toxicity into ecological risk assessments.