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Sorption and desorption properties of cadmium (Cd) and copper (Cu) in 5 clay types (montmorillonite, sericite, kaolinite, halloysite and allophane), which had been selected as representatives of commonly distributed minerals in the world soil environment, were investigated, with special reference to the charge characteristics of clays. Sorption properties of both Cd and Cu in all the clays were highly pH-dependent, resembling those in oxides. When the pH value at which 50% metal sorption on clays occurred was defined as pH-sorb 50 , the pH-sorb 50 values of Cd in the five clay types ranged between pH 5 and 7. Sericite exhibited the highest affinity for Cd sorption. In montmorillonite, sorbed Cd in the high pH region was not desorbed at all by subsequent extraction. For Cu, the pH-sorb 50 values were significantly lower than those for Cd, revealing the strong adsorption ability of Cu for all clay types. Sorption behavior of heavy metals was not parallel to the overall charge characteristics of clays and could not be easily predicted from the total charge such as cation exchange capacity or zeta potential. The higher sensitivity of 2:1 type clay minerals for Cd contamination was revealed. The critical role of edge surfaces on the sorption behavior was suggested because of the high charge density. The importance of pH control for the alleviation of Cd contamination was stressed for all clay types.

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