In sorption experiments, Cd in the solution phase may be surface adsorbed or immobilized and precipitated into mineral phases. The reaction kinetics can be described by a two-site model combining a linear instantaneous model for the surface adsorption and a first-order reaction kinetic model with forward and backward reaction constants for the immobilization and precipitation of the mineral phase. A simplified sequential procedure was developed to study the contribution of these two processes. Results of batch Cd adsorption experiments with two California soils were used to illustrate the model validity. When the Cd sorption was obtained by varying the initial Cd solution concentration and maintaining constant equilibration time, the amounts of Cd in the solid phase as well as the adsorbed and mineral phases increase linearly with the initial Cd solution concentrations. As much as 90% of the sorbed Cd may be in the mineral phase. When the Cd sorption was examined at varying equilibration time and a constant Cd initial solution concentration, the sorbed Cd in the mineral phase increased exponentially to approach a maximum with time. The forward and backward reaction constants were obtained by fitting the sorption data to the model equations. It showed that the forward reaction was five times faster than the backward reaction for both soils and the reaction rates were two times faster in the heavier textured Holtville clay loam soil than the Arlington sandy loam soil.

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