A WHAM-based kinetics model for Zn adsorption and desorption to soils

A novel model has been developed to describe the kinetics of Zn adsorption and desorption to soils. The model incorporates the mechanistic-based equilibrium model WHAM (Windermere humic aqueous model) to account for the chemical variation during the reaction (e.g., pH and Zn(2+) concentration), the heterogeneity of binding sites of soil organic matter (SOM), and the nonlinear binding of Zn to SOM. To test the model, kinetic experiments were conducted using a stirred-flow method. Six soils, with low clay fractions and covering a wide range in SOM concentrations, and various Zn concentrations and pHs were studied. Under these experimental conditions, SOM is found to be the major adsorbent for Zn binding. The fast and slow Zn reactions with soils were associated, respectively, with the monodentate and bidentate binding sites of humic substances in WHAM. The model has only three fitting parameters, the two desorption rate coefficients for the fast (monodentate) and slow (bidentate) reaction sites which are constant and independent of soil type, and the reactive organic matter fraction of the total SOM in each soil. All other parameters are derived from WHAM. The model is able to predict Zn release from spiked soils including the effects of Ca competition.