Modeling Kinetic Retention of Atrazine and Metribuzin in Soil 1

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Knowledge of the retention mechanisms of herbicides in the soil environment is a prerequisite to minimization of their runoff and transport in the soil profile. In this study, we examined the adsorption characteristics of metribuzin and atrazine in a Commerce silt loam soil of Louisiana. Kinetic batch experiments were used for a wide range of initial concentrations and reaction times. Metribuzin exhibited limited kinetics, with Kd values ranging from 1.12 to 1.25 mL g−1 compared with 1.84 to 2.35 mL g−1 for atrazine. However, increased Kd with time indicated higher sorption, as well as stronger kinetic retention, for atrazine compared with metribuzin. We examined the capability of a nonlinear kinetic-equilibrium multireaction model (MRM) in describing metribuzin and atrazine retention by soil versus time. Several versions of the MRM model were evaluated. For atrazine, incorporation of irreversible retention along with a kinetic reversible mechanism was necessary to describe the time-dependent adsorption over the wide range of input concentrations used. For metribuzin, the use of MRM was not as successful, and several model versions failed to describe the retention results. Nevertheless, two versions that account for both kinetic and irreversible reactions were equally successful in describing atrazine and metribuzin retention versus time.