Insights into Chlorination-Induced Degradation of Sulfonylurea Herbicides: Unraveling Kinetics and Intermediates during Water Treatment

Abstract

Chlorination is a common method for drinking water disinfection due to its efficiency and low cost. The strong oxidative properties of chlorine can lead to reactions with dissolved organic compounds, resulting in various transformation products. This study investigates the chlorination-induced degradation of the sulfonylurea herbicides metsulfuron-methyl and chlorimuron-ethyl, which are frequently found in surface and groundwater. The degradation of these herbicides follows a second-order kinetic model. The apparent second-order rate constants for metsulfuron-methyl range from 3.2 to 244 M⁻¹ s⁻¹, while those for chlorimuron-ethyl range from 2.2 to 287.7 M⁻¹ s⁻¹ within a pH range of 4 to 9. Reaction with HClO effectively reduced the concentration of pesticides. Under acidic conditions, the reaction was significantly enhanced, likely due to hydrolysis or changes in the speciation of the organic compounds. In fact, the rate constant under acidic conditions was approximately 35 and 27 times higher than the reaction rate at more neutral pH for chlorimuron-ethyl and metsulfuron-methyl, respectively. The reaction rate with ClO⁻ approached zero for both herbicides, suggesting a minor or negligible pathway involving the hypochlorite anion. Mass spectrometry identified six chlorination products for metsulfuron-methyl and five for chlorimuron-ethyl. Although the specific reaction mechanisms were not fully elucidated, these products provided valuable insights into the fate of sulfonylureas under chlorination. Under typical disinfection conditions (pH 7 and 4 mg L⁻¹ chlorine), the half-lives of 17.8 minutes for metsulfuron-methyl and 26.6 minutes for chlorimuron-ethyl demonstrate the potential for effective degradation in relatively short timeframes. This study underscores the potential for effective removal of these herbicides in drinking water treatment and highlights the importance of evaluating degradation products over time, as they remain detectable even after seven days.