Modeling Zn Adsorption and Desorption to Soils

The rate of reaction of trace metals between soils and solutions is important in predicting trace metal behavior as equilibrium may not be attained. Although chemical equilibrium models are commonly used for soil systems, there has been less progress in developing predictive kinetic models. Kinetics of Zn adsorption/desorption have been modeled based on the linear adsorption isotherm assumption (1, 2), where adsorption and desorption rate coefficients remain constant at constant pH and are constrained by the equilibrium partition coefficient. This approach is in agreement with Zn adsorption/desorption behavior only within a narrow range of Zn concentrations. However, a nonlinear adsorption isotherm is often observed over wide range of Zn concentrations, indicating that a general kinetics model needs to account for the nonlinearity of Zn binding to soils. In the nonlinear adsorption process, the adsorption rate could vary with the Zn loading in soils. Moreover, Zn adsorption and desorption reactions with soil particles are controlled by both soil composition, such as soil organic matter (SOM) content, and solution chemistry, e.g. pH and Zn concentration.