Dark transformation from 17β-estradiol to estrone initiated by hydroxyl radical in dissolved organic matter.

The occurrence and fate of 17β-estradiol (E2) in natural water have gained extensive attention owing to its high ecotoxic risk to wildlife. Dissolved organic matter (DOM) is a ubiquitous water constituent and contributes significantly to E2 removal, although the reaction mechanism is rarely clarified. The present study aims to investigate E2 transformation in water containing fresh or aged DOM surrogates at environmentally relevant concentrations in the dark. Experiments along with radical probes of benzene and furfuryl alcohol reveal that reactive radicals, particularly hydroxyl radical (·OH), formed non-photochemically at higher concentrations in aged DOM than in fresh DOM. The contribution of ·OH in E2 removal is indicated by the decreases in the removal of radical probes in the presence of E2; moreover, E2 removal is inhibited in the presence of radical scavengers. The dose-dependent inhibitive effect of substrate concentrations, including E2 and coexistent propylparaben, shows that the radical concentration is a limiting factor for E2 removal, which could be enhanced by increasing DOM concentration, dissolved oxygen, and light supply. As the main byproduct, estrone (E1) is persistent in the current DOM water in the dark, but it can be easily photodegraded when exposed to light. Theoretical analysis reveals that the initial step is ·OH-initiated H- abstraction on the hydroxyl group in the cyclopentane ring of E2. The formed singlet excited state of E2 undergoes further intramolecular rearrangement and oxidative dehydrogenation to generate E1 and the hydroperoxy radical (·HO2). Considering the universal occurrence of E2 in DOM-rich aquatic matrices, the present findings have special implications for the biogeochemical cycle and risk assessment of this pollutant in natural aquatic environments, particularly those beyond the photic zone.