Opposite pH-dependent roles of hydroxyl radicals in ozonation and UV photolysis of genistein

Phytoestrogens were frequently detected in municipal or industrial wastewater, and raised great attentions due to potential risks to humans or organisms. Until now, transformation mechanisms of phytoestrogens in advanced wastewater treatments were largely unknown. Here, pH influence mechanisms on transformations of phytoestrogens during two typical advanced wastewater treatments (ozonation and photolysis) were investigated, employing genistein (Gs) as a case. Removal efficiencies of Gs decreased significantly with increases of pH during ozonation, while photolytic rates increased by 44 or 200 times from pH 4.9 to 11.6 under irradiations without or with UVC. pH increases caused both dissociation of Gs and formation of hydroxyl radicals (center dot OH) in ozonation or photolysis, however, led to opposite changes to degradation rates. This was because that center dot OH played negatively as a competitor for O-3 in ozonation, but acted as an accelerating species inducing self-sensitized photooxidation of Gs under UV light. Ozonation and photolytic products of Gs were similar at pH 4.9 or 8.6, butwere totally different atpH 11.6. Most of the transformation productsmaintained isoflavone structures, and might possess phytoestrogenic effects. This study provided a deep insight into the pH influencing mechanism on typical advanced wastewater treatment processes of phytoestrogens. Main finding of the work: Opposite pH-dependent degradation mechanisms caused by hydroxyl radicals (center dot OH) were elucidated for ozonation and UV photolysis of phytoestrogens, taking genistein as a case. (C) 2019 Elsevier B.V. All rights reserved.