The effect of kaolinite on ferrihydrite colloid migration in soil: molecular-scale mechanism study

The widespread ferrihydrite colloids in soil can carry diverse environmental contaminants, while current knowledge still remains lacking regarding the effects of the soil surface properties on migration behavior. Kaolinite (KL), a significant component of the porous media in aquifers, was used as a model material in experiments designed to explore its effect on the ferrihydrite colloid migration behavior. The experiments determined that KL caused a nearly 10-fold reduction in the mobility of ferrihydrite colloids. The decrease in travel distance of the colloids was primarily caused by the adsorption of ferrihydrite colloids on KL. The maximum ferrihydrite colloid adsorption capacity of KL was 20.2 mg g⁻¹ at pH 5.5 and 5 mM NaNO₃ concentration. As confirmed by isothermal titration calorimetry, the inner-sphere complex formed between the ferrihydrite colloids and KL failed to cause the desorption of ferrihydrite colloids despite changes in pH or ionic strength. Density functional theory calculations and extended X-ray absorption fine structure spectroscopy analyses demonstrated that the ferrihydrite colloids coordinated to the KL surface by the formation of thermodynamically stable monodentate inner-sphere complexes with Al-O and Si-O active sites. To the best of our knowledge, this is the first investigation that describes the adsorption capacity of natural colloids by solid media, and studies their interaction mechanism in the molecular-scale. These new findings assist in the understanding of the migration behavior of ferrihydrite colloids in soil environments and provide a scientific basis for the prediction of contaminants' geochemical behavior in these environments.