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 <2 to 1000 μm. The relative effect of one surface type on the others was enhanced by further structural manipulations by K⁺ or Li⁺ treatments on the vermiculite and montmorillonite. Equilibrium adsorption and desorption of Cd was measured using ¹⁰⁹Cd at 0.02 μCi/mL for isotopic dilution to a concentration range of 10^-9 to 10^-5 M in a 0.005 M CaCl₂ 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. Generally an increase in the edge to planar surface area ratio with decreasing particle size, or the increase in edge to interlayer surface area ratio by either K⁺ or Li⁺ treatments, resulted in an enhanced sorption of Cd. These results suggest that CEC alone is not an accurate measurement to predict Cd sorption behavior for layer silicates. The concentration range used in this study allowed a full inspection of specific and nonspecific Cd-clay interaction. Equilibrium desorption was used to infer the specific or nonspecific Cd-clay interaction type. The structural manipulations such as K and Li fixation in vermiculite and montmorillonite, respectively, showed that edge sites retained Cd specifically. The high charge density edge and planar sites of biotite also desorbed Cd the least, suggesting the role of high charge density in specific Cd retention.