Sorption and Leaching of the Insecticide Thiamethoxam in Tropical Soils.

There is little information available in the literature on the behavior of thiamethoxam in soils in Brazil. On the other hand, there are many studies focusing on the effects of phosphate and vinasse amendments in those soils. The objectives of this study were to evaluate the sorption and desorption of thiamethoxam in samples of both A and B horizons of dystrophic Red-Yellow Latosol (LVAd), distroferric Red Latosol (LVdf), and Red-Yellow Argisol (PVA), and to measure the retention of thiamethoxam in vinasse- and phosphate-amended samples of LVAd and LVdf. The sorption and desorption isotherms were determined in laboratory, using the batch method. The study of the effect of phosphate and vinasse on the retention of thiamethoxam in samples of LVAd and LVdf was also carried out in the laboratory, using centrifuge tubes with filtration apparatus containing samples of the both soils. Data from sorption of thiamethoxam were well adjusted to the Freundlich equation, and showed low sorption, with Kf values below 1.4 (μmol kg^-1 L mol^-1). The LVAd presented higher sorption of thiamethoxam, followed by LVdf and PVA. Samples of the A horizon retained more thiamethoxam when compared to those of B horizon. The phosphate did not affect the sorption or retention of the compound in these soils, whereas the vinasse increased the interaction of the compound with the soil particles of LVdf and LVAd, thus reducing desorption to the soil solution. The results of micro-column study show that thiamethoxam has a greater leaching potential in LVAd than in LVdf.