Modeling Cd(II) adsorption to heterogeneous subsurface soils in the presence of citric acid using a semi-empirical surface complexation approach

Laboratory batch sorption experiments were conducted to understand the effect of citrate on cadmium sorption to heterogeneous subsurface soils. Our results indicate that citrate may have a pronounced effect on Cd(II) sorption depending on system conditions (e.g. pH, ligand concentration). While the presence of citrate had no effect on Cd(II) sorption at pH < 4, it led to a decrease in Cd sorption under slightly acidic to alkaline pH range depending on the concentration of citrate used. Maximum effect of citrate on Cd(II) sorption was observed at pHs between 5 and 7. This coincides with the observed range of maximum citrate adsorption and the formation of Cd–citrate complexes.A two-site non-electrostatic surface chemical model (SCM) based on the Generalized Composite (GC) approach was able to describe the experimental data well over a wide range of conditions, with only six different surface reactions including two ternary (Cd/citrate/soil) surface complexes. Although the semi-empirical surface model used in the simulations does not accurately represent the actual mechanisms at the molecular level, it is relatively simple, and can be effectively used in transport calculations as an alternative to the Kd approach.