Sorption kinetics and chemical forms of Cd(II) sorbed by thiol-functionalized 2:1 clay minerals

The interaction between Cd(II) in aqueous solution and two 2:1 expandable clay minerals (i,e., montmorillonite and vermiculite), showing different layer charge, was addressed via batch sorption experiments on powdered clay minerals both untreated and amino-acid (cysteine) treated. Reaction products were characterized via X-ray diffraction, chemical analysis (Elemental Analysis and Atomic Absorption Spectrophotometry), thermal analysis, mass spectrometry and synchrotron-based X-ray absorption spectroscopy (XAS) techniques. Namely, the uptake of Cd(II) from each adsorbent was measured by analyzing, via atomic absorption spectroscopy, the liquid portion separated after centrifugation following a controlled period of exposure. The main outcome was thus the definition of the sorption behavior of Cd(II) as a function of the features of the substrates (e.g. different layer charge, presence or not of the amino acid cysteine). Thermal decomposition of Cd-cysteine treated clay minerals evidences the evolution of H2O, H2S, NO2, SO2, and N2O3. EXAFS studies suggest that Cd(II) coordinates with oxygen atoms, to give monomer complexes or CdO molecules, either on the mineral surface and/or in the interlayer. For Cd-cysteine complexes XAS data agree with the existence of Cd-S clusters, thus suggesting a predominant role of the thiol group in the bonding of Cd with the amino acid.