Factors Influencing the Adsorption of Fipronil on Soils

Abstract

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 (Kf) 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 KOM 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 Kf (from 7.3 to 9.3) and modification of the mechanism of the fipronil−soil interaction. Kf 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: Kf increased proportionately as the soil/water ratio decreased. Keywords: Fipronil; adsorption; soil; temperature; cosolvent; soil/water ratio