Quantifying hysteresis of atrazine desorption from a sandy loam soil.

Batch experiments were carried out to investigate the effects of initial atrazine concentrations and consecutive desorption steps on the desorption characteristics of atrazine from a sandy loam soil. As initial atrazine concentration increased, the average percentage of atrazine desorption on the sandy loam soil ranged gradually from 23.1% to 38.5% after five consecutive desorption steps. The values of the Freundlich capacity parameter, k(des), derived from the initial concentration and time-dependent desorption isotherm were consistently higher than those associated with sorption. The opposite trend was observed only for the values of nonlinear parameter, n(des), from the initial concentration-dependent desorption isotherms. Atrazine hydrolysis to hydroxyatrazine and bound residue formation were mainly responsible for the observed hysteresis in its sorption and desorption isotherms. For the initial concentration-dependent desorption isotherms, as initial atrazine concentration increased, the values of hysteretic coefficients omega and lamda decreased, and eta values increased. However, the relationships between initial atrazine concentration and hysteretic coefficients were not pronounced for omega, eta, or lamda. For the time-dependent desorption isotherms, lamda and eta values increased as the atrazine desorption step proceeded. The correlation between hysteretic coefficient and desorption step was highly significant for lamda (P < 0.0001), but not for eta.