Kinetics and Mechanisms of Abiotic Degradation of Fipronil (Hydrolysis and Photolysis)

The abiotic degradation of fipronil (compound I), a phenylpyrazole insecticide, was studied in aqueous solution and on the surface of two soils from Niger (Saguia and Banizoumbou) and one Mediterranean soil (Montpellier). The rate of hydrolysis of fipronil in solution was measured at different values of pH and temperature. The pH was an influencing factor: hydrolysis kinetics were pseudo-first-order, the half-life being 770 h at pH 9.0, 114 h at pH 10.0, 11 h at pH 11.0, and 2.4 h at pH 12.0. Fipronil was stable under acid (pH 5.5) and neutral conditions. The Arrhenius relation was verified over the temperature range 22−45 °C: the activation energy was 62 kJ mol-1 and the calculated entropy change −32 J mol-1. Compound II [5-amino-3-carbamoyl-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-4-[(trifluoromethyl)sulfinyl]pyrazole] was the only hydrolysis product detected. The hydrolysis reaction mechanism involves nucleophilic addition of OH- to the polar nitrile bond, producing a hydroxyimine which then undergoes tautomerization into an amide. Fipronil in acidic (pH 5.5) aqueous solution exposed to light from a xenon lamp degraded with first-order kinetics (K obs = 1.7 h-1) with concomitant appearance of 5-amino-3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-4-(trifluoromethyl)pyrazole (compound III) and 5-amino-3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)pyrazole-4-sulfonic acid (compound IV). The photolyte formation rate constants were 0.11 h-1 (III) and 0.05 h-1 (IV). The observed degradation process corresponded to a desulfinylation and an oxidation. The reaction mechanisms were not elucidated. The irradiation (xenon lamp) of fipronil adsorbed on three natural soils when dry led to the formation of 4-(trifluoromethyl)pyrazole (III). The K obs values were 0.0047 h-1 (Saguia), 0.0039 h-1 (Banizoumbou), and 0.0032 h-1 (Montpellier). The degree of photodegradation was inversely proportional to fipronil adsorption: the Freundlich adsorption coefficients were, respectively, 4.3 (Saguia), 7.3 (Banizoumbou), and 45.5 (Montpellier). Keywords: Fipronil; hydrolysis; photodegradation; kinetics; soils; degradation products