Degradation of chlorpyrifos in aqueous chlorine solutions: Pathways, kinetics, and modeling

Chlorpyrifos (CP) was used as a model compound to develop experimental methods and prototype modeling tools to forecast the fate of organophosphate (OP) pesticides under drinking water treatment conditions. CP was found to rapidly oxidize to chlorpyrifos oxon (CPO) in the presence of free chlorine. The primary oxidant is hypochlorous acid (HOCl), k(r) = 1.72 (+/- 0.68) x 10(6) M(-1)h(-1). Thus, oxidation is more rapid at lower pH (i.e., below the pK(a) of HOCl at 7.5). At elevated pH, both CP and CPO are susceptible to alkaline hydrolysis and degrade to 3,5,6-trichloro-2-pyridinol (TCP), a stable end product. Furthermore, hydrolysis of both CP and CPO to TCP was shown to be accelerated in the presence of free chlorine by OCl-, k(OCl,CP) = 990 (+/- 200) M(-1)h(-1) and k(OCl,CPO) = 1340 (+/- 110) M(-1)h(-1). These observations regarding oxidation and hydrolysis are relevant to common drinking water disinfection processes. In this work, intrinsic rate coefficients for these processes were determined, and a simple mechanistic model was developed that accurately predicts the temporal concentrations of CP, CPO, and TCP as a function of pH, chlorine dose, and CP concentration.