Abstract
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 34 kJ mol−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.
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•The reaction of MC with NH2Cl was of first-order in MC and varied orders in NH2Cl.•The observed rate constant of MC degradation was strongly pH dependent.•Both oxidation and hydrolysis reactions accounted for MC degradation.•MCX, MCXP and MCNP were three major byproducts identified during MC degradation.•The proposed pathways were substantiated through kinetic model simulations.