EFFECT OF ORGANIC COSOLVENT ON ADSORPTION AND DESORPTION OF LINURON AND SIMAZINE IN SOIL

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 decreasedwith increasing methanol content of the solution. An inverse log-linear relationship betweenthe Freundlich adsorption coefficients for the two herbicides and the concentration ofmethanol was found. Adsorption of the herbicides in the presence of methanol followedthe solvophobic theory, which describes the adsorption of hydrophobic organic compoundsin soils. Values of the Freundlich adsorption coefficient for aqueous solutions, of the twoherbicides extrapolated from adsorption data for the methanol/water mixtures, showed closeagreement with those obtained experimentally. Hence, adsorption in aqueous solutions forpesticides of low aqueous solubility can readily be predicted on this basis. Hysteresis, observed after desorption in CaCh solution, decreased with increasing methanol contents for both herbicides. The decrease in hysteresis was ascribed to the swelling of theorganic matter and the accompanying increased accessibility to solutes. The study showedthat the presence of an organic solvent significantly affected not only the adsorption butalso the desorption of herbicides in soils. © 1990 CSIRO. All rights reserved.