Roles of dissolved organic matter, Cl⁻, and NO₃⁻ in the photodegradation of β-estradiol in seawater

17β-Estradiol (E2), a typical endocrine-disrupting chemical, is widely detected in various aquatic environments, including rivers, lakes, groundwater, estuaries, and coastal waters, posing significant ecological and health risks. Photodegradation is a key pathway for the transformation and removal of E2 from aquatic systems. This study investigated the effects of dissolved organic matter (DOM) from different sources and common inorganic anions (Cl⁻ and NO₃⁻) on E2 photodegradation under simulated solar irradiation. The results indicated that the excited triplet state of DOM (³DOM*), generated upon light absorption, was the key reactive species driving E2 photodegradation. Among the DOM sources examined, humic acid exhibited the strongest promotive effect on E2 photodegradation. Correlation analysis confirmed that terrestrial humic-like components within seawater DOM were the primary contributors to ³DOM* formation. Seawater DOM from nearshore areas exhibited higher steady-state concentrations of ³DOM* owing to greater terrestrial input. Both Cl⁻ and NO₃⁻ effectively accelerated E2 photodegradation, and the E2 degradation rate increased with their concentrations. In contrast, the coexistence of Cl⁻, NO₃⁻, and DOM inhibited E2 photodegradation. In DOM-dominated systems, the main transformation pathways of E2 involved oxidation and hydrolysis reactions on the phenolic ring. This study elucidates the effects of DOM source and water chemical composition on E2 photodegradation, which is crucial for accurately predicting its environmental fate and ecological risks. Moreover, the findings provide valuable insights into the photochemical transformation of other emerging organic contaminants.