Role of polystyrene microplastics in the photodegradation of steroidal estrogens: Influencing factors, mechanism and aquatic toxicity assessment

Steroid estrogens (SEs) and microplastics (MPs) often coexist in the aquatic environment, but their interactions in photochemical transformation are still unknown. This study explores how coexisting polystyrene microplastics (PS-MPs) modulate the UV photodegradation of SEs via changes in reactive oxygen species (ROS) under environmental-related concentration, aging state and pH conditions. Here, we studied the role of PS-MPs in the photodegradation of four SEs induced by UV irradiation - estrone (E1), 17β-estradiol (E2), estriol (E3) and 17α-ethinylestradiol (EE2). At the representative concentration of 0.015 g L − ¹ , PS-MPs increased the degradation rate of E2, E3 and EE2 by 17–44% through the ROS pathways involving singlet oxygen (¹O 2 ) and superoxide radicals (O₂ • ⁻). In contrast, the fastest degradation compound E1 under UV alone is slightly inhibited by a light-shading effect (about 5%). UV-aged PS-MPs further enhanced SEs degradation, while alkaline conditions promoted deprotonation and increased reactivity of SEs. Density functional theory (DFT) and mass spectrometry indicate the structure-dependent degradation routes dominated by molecular reactivity descriptors and ROS selectivity. The ECOSAR prediction shows that compared with parent compounds, the toxicity of degradation products of E3 and EE2 is reduced, while the toxicity of degradation products of E2 is predicted to increase. As a photochemical medium, PS-MPs regulate the indirect photolysis process, alter the kinetics of co-pollutants and the toxicity of transformation products, thereby affecting the fate and ecological risks of pollutants in water bodies affected by microplastics. • PS-MPs could modulate the photodegradation of SEs in aqueous solution. • ¹O₂ and O ₂ •⁻ were the major contributors during photodegradation. • Aging makes PS-MPs potent photosensitizers, amplifying indirect decay. • The degradation pathways of different SEs mediated by PS-MPs were identified. • Ecological toxicity of the degradation products of E2 increased.