CALCIUM-ZINC AND CALCIUM-CADMIUM EXCHANGE IN SUSPENSIONS OF VARIOUS TYPES OF CLAYS

Calcium-cadmium and calcium-zinc exchange equilibria were studied at 20-degrees-C and constant ionic strength (0.015) on four clay minerals, viz. montmorillonite, bentonite, illite and vermiculite. Vermiculite and, to a lesser extent, illite and bentonite showed the strongest affinity for Cd2+ or Zn2+ over Ca2+ ions, whereas on the Camp Berteau montmorillonite nearly non-preferential exchange isotherms were observed. With this one exception, selectivity for the heavy metals was greatly enhanced in the trace Cd or Zn regions, suggesting the presence of specific adsorption sites in these clays. Generally, the adsorption increased with the polarizing power of the exchangeable cation. Thermodynamic equilibrium constants and standard free energy changes for the complete exchange of Ca-clay to Cd- or Zn-clay were calculated.