Environmental pollution is one of the major problems for humans and animals. Radioactive wastes and heavy metals are important sources of pollution. Recent studies suggest that soil and geomedia have a high potential for the disposal of radioactive wastes and heavy metals. The clay minerals in geomedia can act as a barrier against radioactive wastes and heavy metals. However, enough information and experimental evidence about the mechanisms involved in the fixation of metals by clays are not available. This study aimed to show the effect of clay type on the fixation of cobalt and cadmium as affected by thermal energy. Increasing the temperature from 25 to 400°C decreased the CEC of clays in Ahwaz and Darab soils saturated with cobalt from 35.6 and 50.1 cmol_c+/kg to 28.8 and 4.5, respectively. In cadmium clays, however, reductions in CEC were minimal (from 34.3 and 51.2 at 25°C to 32.5 and 47.7 cmol_c+/kg at 400°C, respectively) compared to cobalt saturated clays. Consequently, the bonding characteristics change according to the size of the cations upon thermal treatment. The small cation, cobalt with an ionic radius of 0.68 Å, apparently migrates to the octahedral vacant sites and forms covalent bonds after heating at 400°C to become non-exchangable (fixed). The larger cation, Cd with an ionic radius of 0.97Å, apparently does not migrate into lattice sites on thermal treatment and remains exchangeable.

Keywords: Pollutant metals, Layer charge, Cation fixation, Clay mineralogy, X-ray diffractometry.

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