Temperature effects on alkaline earth metal ions adsorption on gibbsite: approaches from macroscopic sorption experiments and molecular dynamics simulations.

Two approaches, macroscopic adsorption experiments and molecular dynamics simulations, were employed to study the effect of temperature on alkaline earth metals adsorption on gibbsite surfaces. Increased reaction temperature enhanced the extent of metal ion adsorption for all of the alkaline earth metals studied. Whereas Mg(2+) and Sr(2+) adsorption displayed dependence on ionic strength, Sr(2+) adsorption exhibited less dependence on background ionic strength regardless of temperature. The ionic strength dependence was attributed to outer-sphere complexation reactions. The ionic strength effect on metal ion removal decreased with increasing temperature for both metals. Ba(2+) removal by gibbsite, on the other hand, was not affected by ionic strength. Results from molecular dynamics simulations were in agreement with the findings of the experimental study. The amount of thermal energy required to remove waters of hydration from the metal cation and the ratio of outer-sphere to inner-sphere complexation decreased with increasing ionic radii. It was observed from both macroscopic and molecular approaches that the tendency to form inner-sphere complexes on gibbsite decreased in the order: Ba(2+)>Sr(2+)>Mg(2+) and that the common assumption that alkaline earth metal ions form outer-sphere complexes appears to be dependent on ionic radius and temperature.