TIME-DEPENDENT SORPTION OF PESTICIDES DURING TRANSPORT IN SOILS

Sorption nonequilibrium of pesticides in soil is relatively less understood compared with equilibrium sorption. Furthermore, batch techniques generally used in kinetic studies do not represent field conditions. Flow techniques, on the other hand, closely represent pesticide transport and have many advantages over batch techniques. A flow technique was used to study the sorption kinetics of three pesticides in four Western Australian soils. Sorption data of all pesticides showed an initial rapid rate followed by a slower rate of sorption. While a first-order equation failed to satisfactorily describe the sorption data, both the Freundlich two-site kinetic equation and the parabolic diffusion law fitted well to the data. The rapid component of sorption varied from 0 to 25% of total sorption (24 h sorption value) for various pesticides. A comparison of sorption data obtained by a batch technique and a flow technique revealed that sorption occurs at a much faster rate under batch conditions, presumably because of shaking and high solution to soil ratio. Under batch conditions, the instantaneous component of sorption was very high (up to 90% of 24-h sorption value). The characteristics of the soil also affected the rate of sorption of the pesticides. A well structured soil with high organic matter content showed slower rate of sorption compared with a dispersed soil with low organic matter content.