Adsorption of different valence metal cations on kaolinite: Results from experiments and molecular dynamics simulations

The hydration and ion adsorption of kaolinite, which is the most common clay mineral, play crucial roles in engineering geology and environmental applications. To investigate the effects of different valence metal cation solutions on the hydration of kaolinite, we used microcalorimetry, quartz crystal microbalance with dissipation monitoring (QCM-D) and molecular dynamics (MD) simulations to measure the hydration heat and adsorption behaviors between kaolinite and different concentrations of NaCl, MgCl2, and AlCl3 solutions. Based on the results obtained from microcalorimetry, the heat generated by the mixing of kaolinite with different ionic solutions increases with increasing metal cation valence and ion concentration. The QCM-D shows that the increase in the ion concentration leads to an increase in the adsorption volume of metal ions on the kaolinite surface. The adsorption priority of the metal ions on the kaolinite surface can be ranked as Al3+ > Mg2+ > Na+. The results of MD simulations show that the ability of hydrated ions to destroy the hydrated films on the kaolinite surface increases with the increase of the metal cation valence, while a new thicker and more dilute ion hydration structure can be easily formed. The adsorption of ions on the kaolinite surface is controlled by the crystal structure of kaolinite.