HYSTERETIC BEHAVIOR OF METOLACHLOR ADSORPTION-DESORPTION IN SOILS

Adsorption and desorption of herbicides are important processes that influence the amount of herbicide retained by the soil and that which is susceptible to runoff or movement in the soil profile. In this study, we examined the adsorption and desorption characteristics of metolachlor in two Louisiana soils: Sharkey clay and Commerce loam. Kinetic batch experiments were used for a wide range of initial concentrations and reaction times (up to 504 h). After adsorption of the metolachlor, desorption took place using successive dilution. Six consecutive desorption steps (6 day) were carried out. Metolachlor adsorption and desorption results were strongly kinetic for both soils. After the six desorption steps, the average amount of metolachlor desorbed was 30.5% for the Sharkey clay and 43.9% for the Commerce loam. The amount of metolachlor desorbed was also dependent on the initial concentration. Families of desorption isotherms were represented in two ways. The first family is based on the traditional approach, in which successive desorption isotherms are presented. The second family is time-dependent desorption isotherms, which are presented for each given reaction (adsorption plus desorption) time in a manner similar to those for adsorption isotherms. Two sets of Freundlich parameters were thus derived for the desorption isotherms. Moreover, a hysteresis coefficient, based on the difference in the area between the adsorption and desorption isotherms, was proposed. This coefficient was capable of quantifying hysteresis for both types of desorption isotherms.