Pesticide adsorption on a calcareous soil modified with sewage sludge and quaternary alkyl-ammonium cationic surfactants.

Pesticides are often found in soil as a result of their application to control pests. They can be transported on soil particles to surface waters or they can lixiviate and reach other environmental compartments. Soil modification with amendments, such as sewage sludge, and with surfactants, h been proposed to reduce pesticide environmental fate. The sorption of atrazine, methidathion and diazinon using the batch technique has been studied on non-modified soil and soil modified with sewage sludge and cationic surfactants, as well as the effect of their addition on soil properties such as organic carbon (OC) content and exchange cations. The OC content of the surfactant modified soils was the highest with the surfactant with the longest hydrocarbon chain (hexadecyltrimethyl ammonium bromide, HDTMA). The results of the OC content run in parallel with the increase in pesticide retention. When the sorption was n malized to soil OC content, the retention induced by addition of HDTMA was still the highest, which is an indication that the organic matter derived from the organic cations is a more effective medium to retain dissolved contaminants, than organic matter from native soil. The addition of sewage sludge to the soil did only result in a slight increase of the soil CEC and, hence, moderately affected the ability of the cationic surfactant to retain the pesticides. The addition of cationic surfactants to soil would possibly reduce the movement to groundwater of atrazine, methidathion and diazinon. In the case of HDTMA, the decrease in sorption at high surfactant loadings was very slow, being that the surfactant was able to retain the pesticides at concentration values which clearly exceeded the monolayer coverage. Contamination by pesticides, which are present in the soil due to their direct input in this medium or to spills or illegal tipping, may be hindered from migration to groundwater by application of a cationic surfactant.