Hydrolysis of Chlorpyrifos in Aqueous and Colloidal Systems

Hydrolysis of chlorpyrifos [O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate] to TCP (3,5,6-trichloro-2-pyridinol) is an important degradation process influencing the fate of chlorpyrifos in aquatic environments. The effects of water chemistry and suspended colloids (smectites, humic acid, and river sediment) on the hydrolysis of chlorpyrifos were investigated in this study. Chlorpyrifos was incubated in various aqueous and colloidal systems at 23 ± 2 °C for up to 100 days, and aqueous phase concentrations of chlorpyrifos and TCP were determined using solid phase extraction–high performance liquid chromatography (SPE–HPLC). Chlorpyrifos degraded in the aqueous solutions with half-lives ranging from 27 to 158 days, depending on the initial concentration of chlorpyrifos and the chemistry of the aqueous solutions. The rate of degradation was slower in systems containing low concentrations of suspended colloids (1 g L in 0.01 M CaCl 2 ) than in the control (0.01 M CaCl 2 , with no colloids), and negligible hydrolysis of chlorpyrifos was found for concentrated colloidal systems (20 g L in 0.01 M CaCl 2 ). Total recoveries of chlorpyrifos for the concentrated colloidal systems ranged from 78% to 97%, with a tendency to decrease with incubation time. The study indicates that the formation of chlorpyrifos–colloid complexes inhibits the hydrolysis of chlorpyrifos in aqueous systems and that chlorpyrifos–colloid complexes may act as a buffer by slowly releasing chlorpyrifos to the aqueous phase.