SPECIFIC CADMIUM SORPTION IN RELATION TO THE CRYSTAL-CHEMISTRY OF CLAY-MINERALS

Three possible locations on clay minerals exist where Cd can be sorbed by ion exchange. These are the external planar, interlayer, and edge surfaces. In this study, montmorillonite, kaolinite, vermiculite, and biotite minerals were used to separate these sorption sites. The vermiculite and biotite were fractionated into five size fractions ranging from less than 2 to 1000 mu m. The relative effect of one surface type on the others was enhanced by further structural manipulations by K** plus or Li** plus treatments on the vermiculite and montmorillonite. Equilibrium adsorption and desorption of Cd was measured using **1**0**9Cd at 0. 02 mu Ci/mL for isotopic dilution to a concentration range of 10** minus **9 to 10** minus **5 M in a 0. 005 M CaCl//2 background. The mineral concentrations were defined for a constant cation exchange capacity (CEC) or constant external surface area. The results using these different phyllosilicates suggest that the edge and high charge density planar sites (as in biotite) contribute the greatest influence.