Kinetic Modeling of pH‐Dependent Molybdenum(VI) Adsorption and Desorption on Iron Oxide‐Coated Sand

Abstract

Core Ideas Molybdenum(VI) sorption and kinetic rate are significantly reduced by increasing pH. A Diffuse Layer Model simulated protonated monodentate complex was dominant at low pH. A kinetic two‐site model successfully described Mo(VI) adsorption–desorption behavior. Quantitative understanding of retention kinetics controlling molybdenum [Mo(VI)] on soil constituents such as iron oxide binding sites is crucial for accurately evaluating environmental risk of ground and surface water contamination. In this study, kinetic batch experiments were performed to investigate Mo(VI) adsorption–desorption behavior on iron oxide‐coated sand (IOCS) at different reaction conditions and time scale. Batch results indicated that adsorption of Mo(VI) on IOCS was highly nonlinear and kinetic, where both sorption and the extent of kinetics decreased with increasing solution pH. A two‐site model based on the heterogeneity of binding sites and variations of solution chemistry was developed to describe Mo(VI) adsorption and desorption on IOCS. The approach integrates kinetic nonlinear sorption into the surface complexation Diffuse Layer Model (DLM). Our results indicated that kinetic rate of non‐protonated monodentate complexes was much higher than that of protonated monodentate complexes. The kinetic model was capable of describing the experimental data with limited number model fitting parameters.