Relationships Between Sorption of Pesticides in Tropical Soils and Their Physicochemical Properties.

Pesticide sorption is important in controlling both product effectiveness and its fate in the environment. Yet, pesticide sorption in Brazilian soils is not as well understood as that for soils from temperate regions. We used batch sorption experiments to measure sorption of 12 non-ionized pesticides (herbicides, insecticides and fungicides), chosen to span a wide range of physicochemical properties (-0,13 < log K_ow < 4,70), in a dystrophic Red-Yellow Latosol (LVAd), a distroferric Red Latosol (LVdf), and a Red-Yellow Argisol (PVA). Quantification was done by high-performance liquid chromatography. Linear free energy relationships (LFER) between the lipophilicity of the compounds (as measured by the logarithm of the l-octanol/water partition coefficient, log K_ow) and their soil sorption parameters (as measured by their log K_om) were calculated. There were no differences among the K_om values for the three soils, which had about the same organic matter content (2.5%); these values followed a linear relationship with the lipophilicity of the compounds [logK_om=0.60 logK_ow+0.59 (r²=0.77)]. Lipophilic compounds with log K_ow > 4 were concentrated more than 1700-fold. However, the sorption of thiamethoxan and imidacloprid was up to twelve fold more than expected from their K_ow values, which can be attributed to specific interactions of these compounds with the soil matrix. On a second experiment, Freundlich isotherms for the sorption of atrazine (0.125 to 30 mg L^-1) in another dystrophic Red-Yellow Argisol (PVA) were built with soil samples collected in seven different depths, ranging from 0 to 100 cm deep. The isotherms were linear for all tested depths (0.97 < 1/n < 1.09) and the average K_om for this compound was 125 ± 25.8 L kg^-1. The calculated K_om for atrazine based on the above LFER is 123 L kg^-1, which shows the robustness of this equation in predicting soil sorption for pesticides under equilibrium conditions.