A comparative study of the acidity toward the aqueous phase and adsorptiveproperties of Al-13-pillared montmorillonite and Al-13-pillared saponite

The selectivity of an Al-13-pillared saponite and an Al-13-pillared montmor illonite for Cd2+ and Cu2+ adsorption was studied. The quantity of metal ad sorbed on both pillared clays depends on the pH of the solution and the pil lars density. Adsorption equilibria are regulated by the protonation equili bria of the amphoteric sites on the pillars. Pillared clays adsorb more cad mium and copper than classic aluminum hydroxides which is simply attributab le to a higher density of surface aluminum groups. Significant differences in behavior are observed between pillared montmorillonite and pillared sapo nite. Pillared montmorillonite appears to be more acidic, which is correlat ed with a more advanced degree of structural modification of the pillars on calcination. We propose a tentative, partial structural model of pillar tr ansformation compatible with these differences. At the same time, both pill ared clays have similar affinities for cadmium II at low pH (5-6), but pill ared montmorillonite seems to be a more efficient cadmium trap at pH = 8 wh en 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.