Degradation and thermodynamic adsorption process of carbofuran and oxadicyl in a Colombian agricultural soil profile

Carbofuran and oxadixyl pesticides are used in Colombia to control pests and fungi, but their mobility through the soil profile is poorly understood. This study showed degradation and adsorption processes of these compounds in a Melanudands soil (0-100 cm) from Colombia using laboratory incubation and the batch equilibrium methods. First-order kinetic models indicated that the degradation rates of carbofuran (0.013-0.006 day^-1) and oxadixyl (0.013-0.008 day^-1) decreased at deeper soil layers, suggesting that the pesticides were more persistent in the sub-surface (60-100 cm) than in the surface layers (0-40 cm). The thermodynamic approach showed that the adsorption of both pesticides was similar, an exothermic and spontaneous process. The carbofuran and oxadixyl coefficient of distribution (5.8-0.3 L kg^-1) and the percentage of adsorption (71.2-11.3%) were very similar in the surface layers (0-40 cm) and decreased with the soil depth. The organic carbon (OC) and clay content showed a positive correlation with the pesticide adsorption throughout the soil profile; therefore, mathematical equations were developed from multiple linear regression models for these soil properties and initial concentration. The equations were important to the estimation of the mobility of the compounds using leaching models under laboratory and field conditions.