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

Abstract 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 (00 1 ¯ ) 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 (00 1 ¯ ) 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 (00 1 ¯ ) 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.