Sorption of Cd-cysteine complexes by kaolinite

The effect of cysteine on the adsorption of Cd(II) by kaolinite was examined at 25 degreesC 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 degreesC and of H2O (m/z=18), CO2 (m/z=44) and SO2 (m/z=64) in the temperature range between 400 and 700 degreesC. The high-defect kaolinite also caused the evolution of molecular units with m/z=34 (H2S) and m/z=76 (N2O3). (C) 2002 Elsevier Science B.V All rights reserved.