EXCHANGE IN SUSPENSIONS OF VARIOUS TYPES OF CLAYS

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 Cd 2+ or Zn 2+ over Ca 2 § ions, whereas on the Camp Berteau montmorillonite n arly 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 i creased with the polarizing power of the exchangeable cation. Thermodynamic equilibrium constants and standard free energy changes for the complete xchange ofCa-clay to Cd- or Zn-clay were calculated. The increasing dispersion of heavy metals in the environment as the result of municipal, industrial and agricultural activities has received considerable attention in recent years. Zinc and Cd--of ten found in geochemical ssociat ion--have b en shown to accumulate to a considerable extent in plant tissue produced on sludge treated soils (Dowdy et al., 1978). Therefore, the bioaccumulated C and Zn might pose a health risk to man or animal species consuming the plant material, and there is need to improve knowledge of the basic chemistry of these metals in soil-water systems. Indeed, little is known about how low concentrations of Zn or Cd are distributed in the soil, considering that, for minute concentrations of heavy metals in solution, highly specific sorption mechanisms mus