A comparative study of the acidity toward the aqueous phase and adsorptive properties of Al13-pillared montmorillonite and Al13-pillared saponite

The selectivity of an Al-13-pillared saponite and an Al-13-pillared montmorillonite for Cd2+ and Cu2+ adsorption was studied. The quantity of metal adsorbed on both pillared clays depends on the pH of the solution and the pillars density. Adsorption equilibria are regulated by the protonation equilibria of the amphoteric sites on the pillars. Pillared clays adsorb more cadmium and copper than classic aluminum hydroxides which is simply attributable to a higher density of surface aluminum groups. Significant differences in behavior are observed between pillared montmorillonite and pillared saponite. Pillared montmorillonite appears to be more acidic, which is correlated with a more advanced degree of structural modification of the pillars on calcination. We propose a tentative, partial structural model of pillar transformation compatible with these differences. At the same time, both pillared clays have similar affinities for cadmium II at low pH (5-6), but pillared montmorillonite seems to be a more efficient cadmium trap at pH = 8 when its surface groups are negatively ionized. Thus, the nature of the clay layers conditions the structural modifications of the intercalated [Al-13] polycations, which in turn determine adsorptive behavior.