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Evaluation of Conceptual Models for Describing Nonequilibrium Adsorption-Desorption of Pesticides During Steady-flow in Soils¹

ABSTRACT

Breakthrough curves (BTC) from miscible displacement of two pesticides through three soils were measured for two input concentrations of each pesticide. These BTC data were used to evaluate two conceptual models for describing the nonequilibrium adsorption-desorption of pesticides in soils under steady-state water flow conditions. In both models, adsorption on one group of sites was assumed to be instantaneous, while the rate of adsorption on the second group of sites followed either nonlinear reversible kinetics (Model I) or was a diffusion-controlled process (Model II). Parameters in both models were estimated by curve-fitting model predictions to one set of measured BTC data using a nonlinear least-squares optimization procedure. These parameter values were then used to verify the conceptual models by comparing simulated and measured BTC for a different input concentration. A different set of model parameters were required to describe the BTC data for each input concentration for the same soil-pesticide combination. The measured ³H₂O BTC for all three soils were symmetrical in shape with no apparent tailing. Evaluation of Model I and II using these BTC suggested that the mobile-immobile water concept may not be applicable to the three soils used in this study.

¹ Contribution from Soil Sci. Dept., Univ. of Florida. Florida Agric. Exp. Stn. J. Series No. 1115.

² Assistant Research Scientist, Professor, and Scientific Programmer, Soil Sci. Dept., Univ. of Florida, Gainesville, FL 32611; and Assistant Professor, Agronomy Dept., Louisiana State Univ., Baton Rouge, LA 70803, respectively.

Received for publication March 27, 1978. Accepted for publication August 28, 1979.

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