Sorption and Desorption of Fipronil in Midwestern Soils

Abstract

Fipronil, {5-amino-1-[2,6-dichloro-4-(trifluoromethyl)phenyl]-4-[(trifluoromethyl)sulfinyl]-1H-pyrazole-3-carbonitrile} is commonly applied to soil to protect structures against termite infestations. The fate and bioavailability of fipronil in soil is dependent upon the variability of sorption processes and will differ from soil to soil. Adsorption of fipronil to three Nebraska soils with varying organic matter (OM) content was determined. At the concentrations tested (0.025, 0.1, 0.25, 0.5, 1.0, 1.5, and 2.0 mg L(-1)), adsorption curves showed constant partitioning of fipronil to the soil matrices (r (2) = 0.998 - 0.999). Calculated organic carbon partitioning coefficients (K (oc)) ranged from 244 to 628 with an average K(oc) of 396. Reported K(d) and K(f) values increased with increasing organic matter content. Desorption hysteresis was observed as fipronil has a propensity to stay in the adsorbed state. After five soil washes with 0.003 M CaCl2, approximately 30% of adsorbed fipronil residues were desorbed. Reported K (oc) values for fipronil suggests that it has intermediate mobility in the field collected soils utilized in this study.