Exploring the multiple photochemical behaviors of ionizable EDCs in aquatic environments: Kinetics, intermediates,...

Endocrine-disrupting chemicals (EDCs), as emerging pollutants, require mechanistic insights into their diverse photochemical behaviors, especially those of their molecular and dissociated forms (i.e., HEDCs⁰ and EDCs^-). This study investigates the photo-transformation of five ionizable EDCs (estrone, α-estradiol, β-estradiol, estriol, and ethinyl estradiol) during their apparent photolysis, and photooxidation dominated by reactive oxygen species (ROS, e.g., •OH and ¹O₂). Under simulated sunlight conditions (λ > 290 nm), only estrone degraded significantly (p < 0.05), whereas all the compounds degraded under UV-Vis light (λ > 200 nm). Notably, the photodegradation kinetics were pH-dependent, with deprotonated species (EDCs^-) exhibiting higher photoreactivity due to greater electron density, which led to faster reaction rates with singlet oxygen (¹O₂) and hydroxyl radicals (•OH). Their photolytic reactivities were governed by cumulative light absorption Σ(L_λε_i,λ) rather than quantum-yields (Φ_i). Based on the lab-data extrapolation to sunlit surface waters, the calculated half-lives were influenced by aqueous pHs, with ¹O₂ playing a more critical role than apparent photolysis and •OH photooxidation. Through HPLC-QQQ analysis, different photo-transformation pathways aligned with the three photochemical reactions of estriol, which was ascribed to the reaction capacities of triggering factors, and reactivities of reaction sites. Crucially, certain intermediates exhibited higher toxicity to Vibrio fischeri than the parent EDC, as partially supported by ECOSAR predictions. These findings elucidate the multivariate phototransformation mechanisms of EDCs, which is of great significance for assessing the environmental persistence and potential risk of ionizable EDCs in surface-water systems.