Sorption of fipronil and its sulfide derivative by soils and goethite

Fipronil, (±)-5-amino-1-(2,6-dichloro-α,α,α-trifluoro-p-tolyl)-4-[(trifluoro-methyl) sulfinyl] pyrazole-3-carbonitrile, has been applied to sugar cane plantations at stand establishment in Brazil for the past 7 years. However, there is a lack of information on fipronil dynamics in humid tropical soils, particularly its behavior at the soil–solution interface. The objectives of this study were to determine the adsorption and desorption behavior of fipronil and its sulfide derivative in the soil. This study was conducted on two selected tropical soils (Ustox and Aquept) and synthetic goethite to establish adsorption and desorption isotherms of fipronil and the sulfide derivative. Freundlich adsorption–desorption isotherms fitted the fipronil and sulfide derivative data (R2 > 0.94). Freundlich adsorption coefficients (Kf) for fipronil ranged between 14.68 and 30.63 while for the sulfide derivative they ranged from 7.13 to 10.34 for the Ustox and Aquept, respectively. The Aquept sample had a higher amount of poorly crystalline Fe-oxyhydroxides. Adsorption was higher on goethite (Kf = 38.70) than on soils, due to mainly its high adsorptive surface. Fipronil Kfd values for both soils (85.76–194.36) and goethite (3805.11) were higher than its Kf, indicating that its adsorption was hysteretic, despite the reversibility. The sulfide derivative Kfd values (12.24–17.65) were also higher than its Kf. Desorption of the sulfide derivative was slightly lower than fipronil desorption, especially for the Aquept sample, despite the presence of methanol, confirming the sulfide derivative has higher affinity for soil surfaces, relative to the parent compound.