Transformation of fenuron during sequential ozonation and chlorination

Ozonation before chlorination is widely used to reduce disinfection by-product formation, but it may also generate chlorinated transformation products (TPs) from ozone-derived intermediates. Here, we investigated the degradation of fenuron and the formation of its chlorinated TPs, monuron and diuron, during sequential ozonation-chlorination. The combined process enhanced TP formation compared with individual oxidation steps, yielding monuron (up to 11.2%) and diuron (up to 5.17%) during subsequent chlorination. TP formation depended strongly on pH and ozone dose, with higher yields observed under alkaline conditions (pH 11). Br⁻ ions promoted complete fenuron degradation and was associated with a brominated TP putatively assigned as metobromuron, whereas I⁻ ions scavenged ozone and suppressed chlorinated TP formation. HCO₃⁻ ions increased fenuron degradation and slightly increased TP yields, although the underlying mechanism was not directly verified. In matrices containing organic matter, oxidant competition reduced TP yields at higher concentrations, whereas post-chlorination luminescence inhibition reached or exceeded 30% in low-concentration Suwannee River humic acid and algal extracellular organic matter matrices. These findings highlight the need for further evaluation of halogenated TP formation under environmentally relevant water-treatment conditions.