Adsorption of copper and zinc ions on various montmorillonites : an XRD study

The adsorption of Cu- and Zn ions on four Na-montmorillonites with rather different mineralogical features and charge characteristics was investigated by XRD in order to observe structural changes during the adsorption. The adsorption capacity of the samples is 0.33-0.38 mol/kg for Cu, and 0.32-0.47 mol/kg for Zn. Zinc adsorption capacity increase with layer charge and CEC, while copper adsorption seems to be independent from these parameters. The basal spacing of metal-adsorbed montmorillonites depends on the type of metal ion, on the starting metal ion concentrations, and on the equilibrium pH-s. Cu-adsorbed montmorillonites have about 13-14 Á basal spacing (two water layer arrangement) in case of small concentration and high pH, while in case of high metal ion concentration and low pH Cu-adsorbed montmorillonites have 12.5 Á basal spacing (one layer water arrangement). Zn-adsorbed montmorillonites tend to have 14-15 Á basal spacing (two water layer).Sharpening of the basal peaks with increasing metal ion concentration indicates increase of domain size, suggesting structural re-building of montmorillonite crystallites during the adsorption. The expansion capacity in ethylene-glycol of the metal-adsorbed montmorillonites did not change, but in case of glycerol solvation loss of expansion capacity can be observed for both metals, but only for the montmorillonites with low layer charge. Upon 250°C heat treatment Cu enter into the silicate sheet of 2+ 2+ montmorillonites resulting total loss of ability for expansion (Hofmann-Klemen effect).Although ionic radius of Cu and Zn are similar, in case of Zn-adsorbed montmorillonites the Hofmann-Klemen effect can not be observed