Photochemistry of microplastics-derived dissolved organic matter: Reactive species generation and organic pollutant degradation

Dissolved organic matter (DOM) originating from microplastics (MPs-DOM) is increasingly recognized as a substantial component of aquatic DOM. The photochemistry of MPs-DOM, essential for understanding its environmental fate and impacts, remains largely unexplored. This study investigates the photochemical behaviors of MPs-DOM derived from two common plastics: polystyrene (PS) and polyvinyl chloride (PVC), which represent aromatic and aliphatic plastics, respectively. Spectral and high-resolution mass spectrometry analyses demonstrated that photoreactions preferentially targeted poly-aromatic compounds within the MPs-DOM, leading to degradation products that predominantly form N-aliphatic/lipid-like substances. This transformation is characterized by decreased aromaticity and unsaturation. Additionally, irradiation of MPs-DOM generated reactive species (RS), including triplet intermediates (³DOM*) and singlet oxygen (¹O₂), with apparent quantum yields of 0.06–0.16% and 0.16–0.35%, respectively—values considerably lower than those for conventional DOM (1.19–1.56% for ³DOM* and 1.34–1.90% for ¹O₂). Despite this, the RS generated from MPs-DOM significantly enhance the degradation of coexisting organic pollutants, such as antibiotic resistance genes (ARGs). The findings shed light on the photoinduced transformation of MPs-DOM and suggest that MPs-DOM functions as a natural photocatalyst, mediating redox reactions of pollutants in sunlit aquatic settings. This highlights its previously underestimated role in natural attenuation and aquatic photochemistry.