Modeling kinetics of heavy metal release from field-contaminated soils: Roles of soil adsorbents and binding sites

Release of heavy metals from soils affects the dynamic behavior of metals in the field, but quantitative models for predicting the rates of heavy metal release from field-contaminated soils are lacking. We studied heavy metal release from several field-contaminated soils with a stirred-flow method, and developed a multiple component kinetics model for the kinetic behavior of metal adsorption/desorption on field-contaminated soil. The fraction of heavy metals extracted with 0.43M HNO3 accounted for the heavy metal desorption from the field-contaminated soils and WHAM 7 reasonably predicted the initial metal distribution among various soil adsorbents and soil organic matter (SOM) binding sites. In our kinetics model, the nonlinear binding of metals to multiple heterogeneous binding sites of soil adsorbents and dissolved organic matter was simultaneously considered. Overall, our model fit experimental data well under varying reaction conditions. Model calculations showed that SOM was dominant in controlling the release behavior of Cd, Cu, and Zn, but iron oxides were significant for controlling Pb release. The bidentate sites were the most important SOM binding sites controlling metal release from soils for all four metals. The release rates of all four metals from the field-contaminated soils were smaller than the release rates from the laboratory metal-spiked soils as reported previously. Our model is able to accurately describe the adsorption/desorption kinetics of heavy metals in field-contaminated soils and contributes to predicting the fate of metals in soil environments.