Degradation of methiocarb by monochloramine in water treatment: Kinetics and pathways

The micropollution of drinking water sources with pesticides has become a global concern. This work investigated the degradation of methiocarb (MC), a most commonly-used carbamate pesticide, by monochloramine (NH₂Cl) under simulated water treatment conditions. Results indicate that the reaction was of first-order in MC and varied orders in NH₂Cl depending on water pH. The observed rate constant of MC degradation decreased quickly with either a decrease in the molar ratio of chlorine to ammonia (Cl₂:N) or an increase in water pH. The apparent activation energy of the reaction was determined to be 34kJmol^-1. The MC degradation pathways also exhibited a strong pH dependence: at pH 6.5, MC was first oxidized by NH₂Cl to methiocarb sulfoxide (MCX), and then hydrolyzed to methiocarb sulfoxide phenol (MCXP); while at pH 8.5, MCX, MCXP and methiocarb sulfone phenol (MCNP) were formed successively through either oxidation or hydrolysis reactions. Based on the identified byproducts and their concentrations evolution, the proposed pathways of MC degradation in the presence of NH₂Cl were further validated through kinetic model simulations.