title: Degradation of methiocarb by monochloramine in water treatment: kinetics and pathways.
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.
authors.affiliation: State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuang-qing Road, Beijing 100085, China. Electronic address: qiangz@rcees.ac.cn.
authors.forename: Zhimin
authors.initials: Z
authors.lastname: Qiang
authors.affiliation: State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuang-qing Road, Beijing 100085, China; School of Environment, Tsinghua University, Beijing 100084, China.
authors.forename: Fang
authors.initials: F
authors.lastname: Tian
authors.affiliation: School of Environment, Tsinghua University, Beijing 100084, China.
authors.forename: Wenjun
authors.initials: W
authors.lastname: Liu
authors.affiliation: State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuang-qing Road, Beijing 100085, China.
authors.forename: Chao
authors.initials: C
authors.lastname: Liu
mesh_terms: D002384:Catalysis; D002700:Chloramines / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008714:Methiocarb / Q000737:chemistry*; D013696:Temperature; D013997:Time Factors; D014867:Water; D018508:Water Purification
chemical_list: D002700:Chloramines; D014867:Water; D008714:Methiocarb; C030816:chloramine
keywords: Degradation pathways; Kinetic modeling; Methiocarb; Monochloramine; Water treatment