Byproducts of aqueous chlorination of equol and their estrogenic potencies

While the phytoestrogen metabolite equol has been reported to exist in surface water, its behavior in drinking water treatment plants remains unrevealed. In this study, eight products including four chlorinated equols (monochloro-equol, dichloro-equol, trichloro-equol, and tetrachloro-equol) were identified in an aqueous chlorinated equol solution by UHPLC-quadrupole-orbitrap-HRMS. Two main pathways of chlorination reaction are proposed: (1) chlorine-substitution reactions on the aromatic ring and subsequent dehydration to form the chlorine-substituted equols, and (2) break-up of the heterocyclic ring with oxygen followed by oxidation of aldehyde to carboxyl. The human estrogen receptor (hER) activating activity for monochloro-equol (EC50 = 3456 nM) and dichloro-equol (EC50 = 2456 nM) were slightly stronger than that of equol (EC50 = 3889 nM). This is the first report on the behavior of equol in drinking water chlorination, which provided an important information on the risk assessment of equol in drinking water.