Adsorption of metal ions at kaolinite surfaces: Ion-specific effects, and impacts of charge source and hydroxide formation

Adsorption of metal ions at clay surfaces plays a critical role in a variety of geochemical, environmental and engineered processes, and p-DFT calculations with dispersion corrections were employed with aim to comprehensively understand the adsorption of metal ions at hydrated kaolinite surfaces, considering both (001) and (001) surfaces and different metal ions (Mn+ = Li+, Na+, K+, Mg2+, Ca2+, Al3+, Fe3+). Ion-specific effects resulting from adsorption also became the focus of this study. For adsorption of alkali ions, inner-sphere complexes dominated for (001) surface while owing to hydroxide formation, outer-sphere complexes dominated for (001) surface. Both charge sources and surface structures can significantly alter adsorption structures and their relative stabilities, and surface structures can further perturb ion-specific sequences. Hydroxide formation at (001) surface was favored for metal ions (Mn+) of higher oxidation states: Non-existent for M+, geometrically stable but disfavored for M2+ while dominant for M3+. Al3+ adsorption was likely to form AlOH2+, Al(OH)(2)(+) and Al(OH)(3) and released more H+ than base cations, which further caused soil acidification. Hydroxide formation affected significantly the extents of ion-specific effects and may further alter their sequences. Results were also helpful to rationalize and guide experimental studies.