An Evaluation of Kinetic and Equilibrium Equations for the Prediction of Pesticide Movement Through Porous Media

Abstract

Abstract Miscible displacement techniques were used to study the movement of picloram (4‐amino‐3,5,6‐trichloropicolinic acid) through a water‐saturated Norge loam soil. The equilibrium adsorption and desorption isotherms for picloram and Norge loam soil were not single‐valued relations. Picloram mobility was reduced significantly when the average pore‐water velocity was decreased from 145 to 14.2 cm/day. Observed and predicted effluent concentration distributions were compared. Predictions were made with a S/360 CSMP simulation model, using two kinetic rate equations and an equilibrium Freundlich equation. The two kinetic models and the equilibrium model each satisfactorily described the observed effluent concentration distributions at low pore‐water velocities provided the nonsingle‐valued character of the adsorption‐desorption process was included in the calculations. At high pore‐water velocities, the kinetic adsorption models were found inadequate to predict the picloram movement. An empirical model was then developed, based on the assumption that equilibrium existed during displacement and that only a fraction of the soil participated in the adsorption process. This fraction was found to be a function of the average pore‐water velocity. With the empirical model, a reasonable fit between data and calculated effluent curves was obtained for all pore‐water velocities.