HYSTERETIC CHARACTERISTICS OF ATRAZINE ADSORPTION-DESORPTION BY A SHARKEY SOIL

The purpose of this study was to quantify hysteresis during adsorption and desorption of atrazine as a function of incubation time for a Sharkey clay soil. Adsorption was carried out using one day batch equilibration and was followed by incubation periods ranging from 1 to 24 d. Incubation was subsequently followed by six consecutive desorption steps where each step represented 1 d. The Freundlich equation (S = K C-N where S is the amount of atrazine retained, mu g g(-l); C is concentration, mu g ml(-l); K is the distribution coefficient, cm(3)g(-1); and N is a dimensionless parameter) was used to describe batch results. Both adsorption and desorption isotherms were well described by the Freundlich model. Fitted K parameter values for desorption isotherms were consistently higher than those associated with adsorption. The opposite trend was observed for the exponential parameter N. The results revealed that desorption deviated significantly from adsorption data. The deviation, which is commonly referred to as hysteresis, was more pronounced as incubation time increased. Batch equilibration results also indicated that the extent of hysteresis was not influenced by soil sterilization. Attempts to quantify the extent of hysteresis using a simplified approach are presented. We found that, for a given batch data set, hysteresis can be quantified provided that Freundlich N from adsorption and desorption isotherms is known.