Predicting the movement and retention of pesticides in soils: Physical and chemical nonequilibrium modeling approaches

Atrazine retention and transport in a highly aggregated Sharkey clay soil were investigated through kinetic batch experiments, miscible displacement (column transport), and mathematical modeling. Batch results indicated that atrazine retention was kinetic and adsorption-desorption was hysteretic. Adsorption and desorption isotherms were well described using the Freundlich equation (S = KC;Atrazine adsorption kinetics was successfully described by a modified second-order two-site (SOTS) approach which was incorporated into the convective-dispersive transport equation. Atrazine was assumed to be present in four phases: a soil solution phase, a noncatalytic (equilibrium) adsorption phase with low binding energy, a catalytic (kinetic) adsorption phase with strong interactions with matrix surfaces, and irreversible sorbed hydroxyatrazine or other physical irreversible adsorbed atrazine. Retention reactions between different phases were based on second-order kinetics and the vacant sites were assumed to be accessible to both equilibrium and kinetic reactions. The total amount of retention sites was assumed constant for a specific soil. Based on one set of independently estimated parameters, the model was capable of predicting atrazine adsorption kinetics and desorption hysteresis. The SOTS model was rigorously validated by predicting fourteen atrazine column transport (breakthrough) results for different aggregate sizes, input concentrations, water flux, column lengths, and flow interruptions.;The SOTS model was further developed to a second-order two-site two-region model to examine the contribution of physical versus chemical nonequilibrium retention in this aggregated soil. Best model predictions were obtained by assuming that all the vacant adsorption sites of the dynamic and stagnant soil regions were considered accessible to solutes in the mobile and immobile phases. A major feature of the modified two-region model is that the fraction of sites, which is a difficult to measure parameter, need not be specified and the amount retained by each soil region is solely a function of reaction rates.