Adsorption of propisochlor on soils and soil components equation for multi-step isotherms

Static equilibrium measurements were performed for the study of propisochlor on three different kinds of soils at pH = 7.0 at 25 degrees C. The concentration of herbicide was changed from 30 to 300 mumol/l. The obtained two-step adsorption isotherms cannot be evaluated by using the Freundlich or Langmuir equations. New equation has been derived by making use of the usual mass balance and equilibrium relationships of the adsorption and by considering the possibility of the formation of associates of the hydrophobic solute molecules. The characteristic model parameters of each step of the adsorption isotherm were estimated for the studied systems by a non-linear least square regression. The calculated curves fit well to the experimentally obtained two-step isotherms and the parameters of the model can be used for the characterization of the pesticide-soil interactions and consequently the mobility of the propisochlor in soil/water systems.