Desorption of tefluthrin insecticide from soil in simulated rainfall runoff systems - kinetic studies and modelling

The sorption and subsequent desorption from soil particles, of tefluthrin, a widely used soil-active pyrethroid insecticide, were studied in a simulated rainfall runoff system. Experimentation was facilitated by use of the ¹⁴C-labelled compound. The desorption of such highly-hydrophobic organic compounds from soils has rarely been reliably determined previously. Sorption results showed that the partition coefficients (K(p) and K(oc)) were determined with good precision, although the resultant coefficients were shown to be underestimates of true adsorption to soil due to non-attainment of true equilibrium and the presence of colloidal material in the aqueous phase supernatants analysed. Once the adsorption slurries were diluted (30 ml of aqueous phase to 3.51) to simulate transport of soil to a larger water body after, heavy rainfall, kinetic studies, using a centrifugation method, indicated that the soil phase concentration decreased once more, whilst at the same time a biphasic re-equilibration process occurred in the aqueous phase. A direct particle counting method was used to rapidly and reproducibly measure desorption of ¹⁴C-tefluthrin from soil under contrasting experimental conditions. Initial re-equilibration was shown to be rapid when soil-to-water ratios were changed significantly, temperature having a pronounced effect on this desorption. The results are important for the accurate modelling of tefluthrin behaviour in the environment.