A novel multi-reaction model for kinetics of Zn release from soils: Roles of soil binding sites

The kinetics of Zn release from soil is a key process affecting the fate and bioavailability of Zn in soil, which may be complicated by multiple soil binding sites and reactions during the Zn release process. In this study, we developed a novel quantitative model for the dynamic Zn speciation in soil during the Zn release process, by integrating the mechanistic equilibrium model WHAM 7 and molecular level investigation of Zn speciation with X-ray absorption spectroscopy (XAS). The kinetics model specifically considered Zn reactions with soil organic matter (SOM), iron (hydr)oxides, and clay minerals, and dissolution of Zn precipitates. Batch and replenishment experiments were conducted to study kinetics of Zn release from laboratory-spiked soils and XAS was employed to identify soil Zn speciation. Overall, the model described the experimental data reasonably well with minimal model fitting parameters. We quantitatively assessed the dynamic changes of Zn speciation in different soil components and SOM binding sites at various time scales and reaction conditions. Our kinetics model provides a quantitative framework for predicting the dynamic behavior of Zn in soil environments.