Atrazine sorption on surface soils: time-dependent phase distribution and apparent desorption hysteresis

Non-equilibrium sorption-desorption behavior of atrazine was studied on two surface soils. Impact of sorption contact time was evaluated by interpreting temporal variations in Freundlich sorption isotherm parameters n(t) and K-F(t) obtained from the phase distribution relationships. The extent of sorption linearity was very similar (n similar to 0.90) for the two soils at all sorption contact times. K-F(t) increased with contact time and stabilized upon reaching apparent equilibrium. K-F for woodland soil was significantly higher than that for agricultural soil. The Apparent Hysteresis Index (AHI) parameter was used to quantify sorption-desorption hysteresis arising from non-equilibrium sorption. AHI was a function of the sorption contact time and correlated well with KF(t). The woodland soil sorbed more herbicide due to its higher organic matter content. However, a larger fraction of the herbicide sorbed to this soil was released rapidly (within 24 h) following sorptive uptake. The differences in sorption-desorption behavior of atrazine in the two soils appear to be related to variations in the type and location of organic matter in the two soils. The parameters K-F(t) and AHI(t) consistently demonstrated the effects that arise when batch systems are not brought to equilibrium during sorption studies. (C) 2003 Elsevier Science Ltd. All rights reserved.