Adsorption of cadmium and copper by modified kaolinites

Novel amorphous derivatives of kaolinites were prepared by thermal modification followed by acid activation which improved the exchangeability of kaolinites. The crystalline structure of kaolinite was thermally transformed to amorphous metakaolin which on acid activation released preferentially the octahedral aluminium ions from the clay lattice, without disturbing its structure and revealing these sites for substitution with other metal ions. When the substituted metal ion has a lower charge the net negative charge at the site enables adsorption of cations. Additional –Al–OH and –Si–OH bonds formed also will act as cation exchange centres. Adsorption of cadmium and copper on these modified kaolinites was studied as a function of equilibration time and temperature and it was found that the initial 1 h was sufficient to exchange most of the metal ions. Two kaolinitic clay samples obtained from Thonnakkal, south Kerala (TK) and Madai, north Kerala (MK) were modified to study their exchange behaviour. The exchangeability was found to decrease with an increase in the calcination temperature (to 600°C) of kaolinite samples; i.e., from 6.34 to 3.08 (Cd) and 6.06 to 3.06 (Cu) mmolc kg−1 for TK and 6.55 to 3.15 (Cd) and 6.21 to 4.16 (Cu) mmolc kg−1 for MK. An improved exchange kinetics applies for their modified (calcined and acid activated) counterparts, i.e., 13.36(Cd) and 18.80(Cu) mmolc kg−1 for TK and 13.64(Cd) and 18.09(Cu) mmolc kg−1 for MK. The amount of metal ions adsorbed showed a direct correlation with the surface area and CEC values. The uptake of Cd and Cu at 30, 40, 50 and 60°C displayed similar kinetics with maximum uptake at 40°C for both Cd and Cu.