Specificity and affinity of multivalent ions adsorption to kaolinite surface

Understanding the specific adsorption mechanism(s) of multivalent ions onto clay mineral surfaces in a high salinity environment is theoretically and practically important in many areas including mineral separation and tailings disposals. This paper focuses on both qualitative and quantitative studies to describe the effect of multivalent-ion specificity on the kaolinite surface. The measured zeta potential of kaolinite particles treated with di- and trivalent chloride salts (up to 100 mM) was used to quantify the surface binding constant of metal ions to the kaolinite surface by the surface complexation model. The model results show that ion-pairing and hydration energy are two main factors that control the adsorption of di- and trivalent ions onto the kaolinite surface. The established affinity series of adsorption to kaolinite surface, Al3+ > Fe2+ > Fe3+ > Mg2+ > Ca2+, matches the hydration energy and calculated ion-pairing concentration. Further XPS analysis and molecular dynamics simulation revealed that Al3+ and Fe2+ could directly form inner-sphere complexes due to their high electronegativity and weak pairing ability with Cl- ion, while Mg2+ and Ca2+ ions adsorb onto the outer-sphere of kaolinite surface because of their strong pairing ability with Cl- ion and low electronegativity. However, higher electronegativity with large negative hydration energy leads to weaker affinity to kaolinite surface, as in the case of Fe3+. A linear relationship between surface binding constant and first hydrolysis constant of metal and kaolinite surface was also established. The outcomes of this work shed a new insight into the multivalent ion adsorption to kaolinite in controlling the selectivity and efficiency of mineral separation and dewatering processes.