Insights into the transformation of alloxydim during chlorination process; by-product identification, DFT calculations and ecotoxicological assessment

Sodium hypochlorite is one of the most widely used disinfectants in water treatment, effectively eliminating pathogenic microorganisms and ensuring the safety of drinking water. However, its strong oxidizing power can generate potentially hazardous disinfection by-products (DBPs) when reacting with xenobiotic compounds, such as pesticides, present in natural waters. Within the framework of EU Regulation 1107/2009, identifying these by-products and assessing their potential risks is critical. In this study, we investigated reactions between sodium hypochlorite and the herbicide alloxydim under varying pH conditions and disinfectant-to-herbicide ratios. Degradation kinetics, including half-lives and degradation constants, revealed that alloxydim degradation is extremely rapid and strongly dependent on both pH and disinfectant concentration, with a half-life of less than 30 s under excess chlorine. The major chlorinated by-product was isolated and its chemical structure rigorously elucidated using QTOF mass spectrometry, ¹H and ¹³C NMR. Toxicity assessments-combining experimental assays with bacteria, plants, and fish and theoretical QSAR predictions-demonstrated that this by-product exhibits significantly higher toxicity than the parent compound toward bacteria and plants. Complementary theoretical calculations at the B3LYP level were performed to predict the reaction mechanism and to identify the most stable chlorination site in the alloxydim molecule. These findings underscore the substantial environmental and human health risks posed by chlorinated by-product formed during water disinfection and highlight the urgent need to evaluate their formation and toxicity.