Retention of organophosphorous insecticides on a calcareous soil modified by organic amendments and a surfactant.

Pesticides may affect soil quality since they are applied either directly to the soil or transported from the treated crops. Although the soil is able to partially retain environmental contaminants, the use of organic amendments, such as sewage sludge, peat or surfactants, may increase the retention in the upper soil layers, where the contaminants can be degraded and thus diminish their environmental fate. The effect of adding sewage sludge, peat and humic acids, together with a cationic surfactant to the soil, on the adsorption and desorption of organophosphorous insecticides has been studied. The results indicate that humic acids induce an adsorption increment of the pesticides, while peat and sewage sludge do not significantly affect pesticide adsorption at the dosage applied. The use of a cationic surfactant considerably enhances the insecticide retention. The increase was highest for the combined application of the surfactant and the humic acids. Desorption isotherms are inversely related to the adsorption behaviour, being higher for only soil, lower for soil added with carbon-rich amendments, and drastically reduced when the cationic surfactant is present. Concerning the insecticides, adsorption and desorption are related to their physicochemical properties.