Effect of organic cosolvent on adsorption and desorption of linuron and simazine in soil

Abstract

The adsorption and desorption of two herbicides, Linuron and Simazine, on a clay soil have been studied in mixtures of methanol and water. Adsorption of the herbicides decreased with increasing methanol content of the solution. An inverse log-linear relationship between the Freundlich adsorption coefficients for the two herbicides and the concentration of methanol was found. Adsorption of the herbicides in the presence of methanol followed the solvophobic theory, which describes the adsorption of hydrophobic organic compounds in soils. Values of the Freundlich adsorption coefficient for aqueous solutions, of the two herbicides extrapolated from adsorption data for the methanol/water mixtures, showed close agreement with those obtained experimentally. Hence, adsorption in aqueous solutions for pesticides of low aqueous solubility can readily be predicted on this basis. Hysteresis, observed after desorption in CaCl2 solution, decreased with increasing methanol contents for both herbicides. The decrease in hysteresis was ascribed to the swelling of the organic matter and the accompanying increased accessibility to solutes. The study showed that the presence of an organic solvent significantly affected not only the adsorption but also the desorption of herbicides in soils.