Sorption of Cd-cysteine complexes by kaolinite

The effect of cysteine on the adsorption of Cd(II) by kaolinite was examined at 25°C using a low-defect kaolinite from Minas Gerais (Brazil) and a high-defect kaolinite from Berici Hills (Italy). The chemical data demonstrated that the amount of Cd–cysteine detected on kaolinite increases rapidly during the first few hours with a first-order reaction, thereafter adsorption becomes linear with time for low-defect kaolinite and slows rapidly for high-defect kaolinite. In low-ordered kaolinite, the Cd–cysteine is intercalated in the structure. Evidence of these changes was also observed by differential thermal analysis (DTA). The thermal decomposition of the Cd–cysteine adsorbed on the kaolinites was studied by analysing the evolved gases. The mass spectra of both samples show the evolution of NO (or CH3CH3, m/z = 30), CO2 (m/z = 44), and SO2 (m/z = 64) between 150 and 300°C and of H2O (m/z = 18), CO2 (m/z = 44) and SO2 (m/z = 64) in the temperature range between 400 and 700°C. The high-defect kaolinite also caused the evolution of molecular units with m/z=34 (H2S) and m/z=76 (N2O3).