Overall,metal reactions with the monodentate sites were faster than with the bidentate and tridentate sites in most conditions, as characterized by both larger adsorption and desorption rate coefficients (Figure 4, Table 2). The bidentate and tridentate sites controlled the long-term metal slow release from soils. Among three metals, the desorption of Zn from the bidentate and tridentate sites was much faster compared with that of Cu and Pb, although the desorption rate coefficient of Zn (kd2) was close to that of Cu and even smaller than that of Pb for the bidentate and tridentate sites (Table 2). This is due to the much weaker binding ability of Zn to SOM, which results in smaller adsorption rate coefficients and faster metal release overall (eqs 1− 3). As also shown in Figure 4, the adsorption rate coefficients varied significantly during the kinetic experiments even at the constant pH, and they are highly dependent on the total metal concentrations in each group of SOM sites and the reactivity of the specific metal with SOM. The variations of adsorption rate coefficients are consistent with the nonlinear binding behavior of heavy metals. The wide range of adsorption rate coefficients observed in this study highlights that the nonlinearity of metal binding to SOM should be considered in kinetics models and the assumptions based on a linear adsorption isotherm are usually not appropriate in most reaction conditions.