Degradation kinetics and DBP formation during chlorination of metribuzin

Abstract

Abstract A common nitrogen-containing herbicide, metribuzin, was studied for the effect of pH, bromide and ammonium concentrations as well as temperature on its degradation kinetics and disinfection by-product (DBP) formation during chlorination. Metribuzin chlorination can be well described by a second-order kinetic model, and the rate constants of the acid-catalyzed and HOCl oxidizing charged metribuzin (metribuzin + ) were calculated as 7.72 (±0.90) × 10 8 M −2 min −1 and 8.22 (±4.00) × 10 3 M −1 min −1 , respectively. Metribuzin degradation rate increased with increasing bromide concentration or decreasing ammonium concentration. A kinetic model of metribuzin chlorination in the presence of bromide and ammonium, respectively, was developed in this study, and the rate constant for the reaction of HOBr and metribuzin + was calculated as 2.44 (±2.30) × 10 5 M −1 min −1 . Moreover, the formation and distribution of chlorinated DBPs were analyzed. Chloroform was the major volatile degradation product with the molar yield of 0.86% under the circumneutral condition, and its formation increased gradually with the increase of reaction time and pH. However, the formation of dichloroacetonitrile, 1,1-dichloro-2-propanone, 1,1-trichloro-2-propanone and trichloronitromethane climbed up and then declined with increasing reaction time and pH. The formation of brominated DBPs increased with increasing bromide to chlorine molar ratio, especially for dibromochloromethane. The rate constants of metribuzin chlorination increased with increasing temperature, and the activation energy was estimated as 22.83 kJ mol −1 . In sum, special attention should be paid during metribuzin chlorination at alkaline conditions, especially in the presence of bromide, which would cause the formation of more toxic brmoninated DBPs than their chlorinated analogs.