Fate of fipronil in soils under sugar cane cultivation from the northeast of Brazil: Sorption and degradation

Fipronil(TM) has been used for termite control in sugar cane plantations in Brazil in recent years. However, there is a lack of information regarding its adsorption-desorption and degradation in the humid tropics. The objectives of this research were to: study the adsorption-desorption behavior of fipronil and its sulfide derivative, and evaluate fipronil biodegradation in soils from the sugar cane industry in the Northeast of Brazil. Surface soils from summit (Ustox) and footslope (Aquept) landscapes were collected and characterized. Samples were exposed to fipronil and its sulfide derivative using the batch equilibrium technique. Experiments to measure the biological degradation of fipronil were conducted in both sterilized and non-sterilized soils. Footslope soils had the highest clay, OC, CEC, exchangeable Al3+ and oxalate-Fe values; however, it had a lower dithionite-citrate extractable Fe content, compared to summit soils. Fipronil adsorption on soils and goethite showed C and L type isotherms, whereas sulfide derivative adsorption on soils exhibited L type isotherms. Freundlich isotherms fitted the data better than the Langmuir equation. Freundlich adsorption coefficients (K f) for fipronil ranged from 14.68 to 30.63 while the sulfide derivative ranged from 7.13 to 10.34 for the Ustox and Aquept samples. Kf value in goethite was 38.70, due to its high particle surface area. Fipronil and sulfide derivative desorption Kf was higher than their adsorption Kf for both soils and goethite. Fipronil underwent biodegradation in the Ustox samples, with levels decreasing from 0.689 to 0.399 mug g-1 after 120 days of incubation in non-sterile conditions. Fipronil half-lives, based on zero-order kinetics, were 83 and 200 days, with the half-life increasing with decreasing fipronil initial concentration. Fipronil degradation rate appeared to be biphasic with an initial slower rate followed by a faster rate after 90 days of incubation, which may lead to shorter half-life than that calculated with the zero-order model. The sulfone derivative was the dominant metabolite, whereas the sulfide and amide derivatives were also formed under non-sterile condition. Metabolites also underwent biodegradation, particularly the sulfone derivative. Fipronil displayed affinity and effective binding to soil particles in the humid tropics, and underwent biodegradation, which reduced the residence time of the insecticide in the soil.