Occurrence and cycle of dissolved iron mediated by humic acids resulting in continuous natural photodegradation of 17α-ethinylestradiol

17α-ethinylestradiol (EE2), a synthetic endocrine-disrupting chemical, can degrade in natural waters where humic acids (HA) and dissolved iron (DFe) are present. The iron is mostly bound in Fe(III)-HA complexes, the formation process of Fe(III)-HA complexes and their effect on EE2 degradation were explored in laboratory experiments. The mechanism of ferrihydrite facilitated by HA was explored with results indicating that HA facilitated the dissolution of ferrihydrite and the generation of Fe(III)-HA complexes with the stable chemical bonds such as C-O, C=O in neutral, alkaline media with a suitable Fe/C ratio. 1O2, •OH, and 3HA⁎ were all found to be important in the photodegradation of EE2 mediated by Fe(III)-HA complexes. Fe(III)-HA complexes could produce Fe(II) and hydrogen peroxide (H2O2) to create conditions suitable for photo-Fenton reactions at neutral pH. HA helped to maintain higher dissolved iron concentrations and alter the Fe(III)/Fe(II) cycling. The natural EE2 photodegradation pathway elucidated here provides a theoretical foundation for investigating the natural transformation of other trace organic contaminants in aquatic environments. 17-ethinylestradiol (EE2) is a synthetic endocrine disruptor with significant biological toxicity and environmental risk. Because the cumulative level of EE2 in plateau lake water is much lower than its intake, it is hypothesized that there is a natural transformation pathway of EE2 in plateau lake water. However, research on the natural transformation mechanism of EE2 is incomplete. As a result, our study explored the effect of humic acids (HA) on iron cycling and discovered a natural photodegradation pathway of EE2 mediated by Fe(III)-HA complexes, establishing a theoretical foundation for the management and control of trace emerging contaminants.