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-firstorder, 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).

A comparative study of the adsorption of fipronil, a phenylpyrazole acridicide, on two Sahelian soils (Saguia and Banizoumbou in Niger) and a Mediterranean soil (Montpellier) has shown that this phenomenon is dependent on the level of organic matter (OM):  the adsorption coefficients (K f) were, respectively, 4.3 (Saguia, 0.1% OM), 7.3 (Banizoumbou, 0.3% OM), and 45.5 (Montpellier, 6.5% OM). The partial destruction of the OM of the Montpellier soil sample by oxidation with hydrogen peroxide confirmed the fundamental role of this component:  the values of log K OM were practically constant (2.8−2.9) for OM levels ranging from 6.5 to 1.1%. Following the Giles classification, the adsorption isotherms of the two Sahelian soils seemed to be type S, characteristic of soils with low OM content. For natural Montpellier soil, the shape of the isotherm began close to type S but changed rapidly to type C. The temperature and the methanol cosolvent level were factors influencing the adsorption. For the Banizoumbou soil, increasing the temperature (from 22 to 35 °C) caused an increase in K f (from 7.3 to 9.3) and modification of the mechanism of the fipronil−soil interaction. K f decreased exponentially as the methanol fraction of the binary mixture increased. The quantitative study of the adsorption of fipronil on the soil demonstrated the influence of the soil/water ratio:  K f increased proportionately as the soil/water ratio decreased. Keywords: Fipronil; adsorption; soil; temperature; cosolvent; soil/water ratio

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