Kinetics and hydrolysis of fenamiphos, fipronil, and trifluralin in aqueous buffer solutions

Abstract

Hydrolyses of fenamiphos, fipronil, and trifluralin were studied in aqueous buffer solutions of pH 4.1, 7.1, and 9.1 at different temperatures, 5, 22 ± 1, 32 ± 1, and 50 ± 1 °C. Fenamiphos, fipronil, and trifluralin were found to be more stable in acidic and neutral buffer solutions at temperatures of 5 and 22 ± 1, and dissipation is rapid at 50 ± 1 °C. In basic buffer and at higher temperature, degradation of fenamiphos was found to be very rapid when compared with fipronil and trifluralin. The rate constants calculated at 32 °C for fenamiphos were 2349.4 x 10-8 (pH 4.1), 225.2 x 10-8 (pH 7.1), and 30476.0 x 10-8 (pH 9.1); for fipronil 1750.0 x 10-8 (pH 4.1), 3103.0 x 10-8 (pH 7.1), and 3883.0 x 10-8 (pH 9.1); and for trifluralin 2331.0 x 10-8 (pH 4.1), 2360.0 x 10-8 (pH 7.1), and 3188.0 x 10-8 (pH 9.1). On the basis of rate constant values, these pesticides appeared to be more susceptible to hydrolysis than synthetic organophosphorus compounds such as chlorpyriphos, diazinon, malathion, and ronnel. DT50 values calculated at 32 °C were 228 (pH 4.1), 5310.24 (pH 7.1), and 37.68 (pH 9.1) h for fenamiphos; 608.6 (pH 4.1), 373.9 (pH 7.1), and 270.2 (pH 9.1) h for fipronil; and 502.1 (pH 4.1), 496.8 (pH 7.1), and 355.7 (pH 9.1) h for trifluralin.