CU-II ON AL-13-PILLARED SAPONITES - MACROSCOPIC ADSORPTION MEASUREMENTS AND EPR-SPECTRA

We studied the interaction of Cu-II ions in the aqueous phase with two well-characterized pillared clays, obtained from one natural and one synthetic saponite. Adsorption isotherms can be fitted to a Freundlich-type equation and are strongly pH-dependent: this suggests an adsorption mechanism involving the formation of inner-sphere complexes between copper ions and surface ligands, rather than a simple ion exchange. EPR spectroscopy allows easy discrimination between mobile and immobile Cu-II species; the comparison of EPR spectra of wet and dry samples of Cu-II/pillared saponites suggests the presence of a grafted species with formula [Cu-II(AlO)(n)(H2O)(6-n)](x+) (wet samples) or [Cu-II(AlO)(n)(H2O)(4-n)](x+) (dry samples; n = 1 or 2; (AlO) denotes either a neutral or deprotonated hydroxide group bound to Al), even for low Cu loadings. At higher loadings, it seems that a bulk Cu-containing phase (undetectable by EPR) is present in the dry samples and partly redissolves in the wet samples, leading to [Cu(H2O)(6)](2+) outside the layers. There is no evidence for the presence of ion-exchanged [Cu(H2O)(6)](2+) in the interlayer region. Thus, the adsorption mechanism on pillared clays is more reminiscent of that on aluminum hydroxides or oxyhydroxides, but pillared clays are more efficient due to the high internal surface developed by the pillars.