Matsushita_2025_Water.Res_285_124068

The fates of organophosphorus insecticides during chlorination, a widely used disinfection step in drinking water treatment, have been extensively investigated. These compounds are known to partly transform into their corresponding "oxons", which retain the same mode of action-the inhibition of acetylcholinesterase (AChE)-as their parent compounds, but with markedly greater potency. In contrast, although carbamate insecticides share the same mode of action, their toxicological fates during chlorination remain entirely unexplored. Here, we investigated changes in anti-AChE activity during chlorination of a solution containing methomyl, a carbamate insecticide. The activity was assessed in the presence (indirect toxicity) and absence (direct toxicity) of metabolic activation. Methomyl reacted with free chlorine and almost completely disappeared at a chlorine-to-compound molar ratio of <=4. Even so, the chlorinated samples retained both direct and indirect toxicities. Indirect toxicity was attributable solely to residual methomyl. Direct toxicity originated from residual methomyl and the TPs methomyl-sulfoxide and a dioxygenated derivative of methomyl-sulfoxide. No other TPs contributed to it. Methomyl-sulfoxide was identified via MS/MS spectral matching, but the dioxygenated TP could not be structurally confirmed. From a toxicological perspective, drinking water treatment plants using free chlorine for disinfection should monitor at least methomyl-sulfoxide in addition to methomyl in finished water.