Coupled kinetic reactions in soil environments: mechanisms and quantitative models

Zhenqing Shi^a, S. W. Hu^a, Y. Lu^a, P. Wang^a and Y. Z. Liang^a

^a School of Environment and Energy, South China University of Technology, People’s Republic of China

zqshi@scut.edu.cn

Understanding the kinetics of coupled reactions of ions on soil particles (eg., adsorption/desorption and redox reactions) is crucial for predicting the dynamic behavior of contaminants in soil environments. Organic matter (OM) has abundant heterogeneous sites controlling the kinetic reactions of cations. Iron (Fe) oxides are among the most important mineral adsorbents to control the reactivity and fate of both cation and oxyanion contaminants, while manganese (Mn) oxides exhibit high reactivity with redox sensitive elements such as arsenic (As). Furthermore, the dynamic interactions between OM and minerals play a key role in controlling the fate of carbon (C) and metals/metalloids. Currently, there is still a lack of mechanistic and quantitative understanding on the role of soil binding sites and coupled reactions in controlling the kinetic behavior of ions in soil. In this talk, we will first describe the development of the unified kinetics model for both cation and oxyanion adsorption/desorption on OM and Fe oxides based on chemical speciation models including WHAM and CD-MUSIC model. The key idea of our model is to constrain the adsorption and desorption rate coefficients for each specific binding site and the variations of adsorption or desorption rate coefficients among different binding sites. We quantitatively demonstrated how the equilibrium binding of cations and oxyanions with various soil binding sites affected the adsorption and desorption rates. We will also introduce the development of the quantitative models for the coupled kinetics of oxyanions adsorption/desorption/oxidation on MnO₂. Then we will focus on our recent studies on the dynamic interactions between OM and minerals and how they affected the kinetic behavior of metals/metalloids. We studied the kinetic processes, at nano and even sub-nano scales, by time-resolved chemical imaging with spherical aberration corrected scanning transmission electron microscopy (Cs-STEM) equipped with energy-dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). Our experimental and modeling results demonstrated that the rates of metal/metalloid speciation changes within Fe minerals were coupled with Fe mineral transformation rates. The results help to elucidate the dynamic behavior of C and metals/metalloids during the Fe minerals transformation processes, and also shed lights on nano-scale mechanisms of OM interactions with Fe oxides, which is essential to accurately predict bioavailability and accessibility of contaminants and OM. Our research contributes to developing comprehensive models for predicting the kinetic behavior of metals/metalloids and OM in soil environments.