113Cd, 1H NMR and FTIR analysis of Cd2+ adsorption on dioctahedral and trioctahedral smectites

Dioctahedral and trioctahedral smectites of very low Fe content were exchanged with Ca (0.8 N CaCl2) and then with Cd (0.004-0.2 N CdCl2), resulting in the approximate Ca/Cd interlayer compositions 100/0, 75/25, 50/50, 0/100. The samples were analysed using 113Cd and 1H MAS NMR spectroscopy, at different hydration states, and by FTIR using films oriented at 90 and 45° to the infrared beam. When adsorbed from low-concentration CdCl2 solutions, as in this study, Cd is in the smectite interlayer as Cd2+, surrounded by water molecules. In smectite specimens with no tetrahedral charge, Cd occupies one single site in the centre of the interlayer along the c axis but in smectite with tetrahedral charge, the basal oxygen atoms with excess negative charge interact with some of the Cd atoms more strongly, generating a second site in which Cd is displaced from the centre of the interlayer and closer to those oxygen atoms. The 1H NMR study showed protons from hydration water and smectite hydroxyls and that both have more than one component. The FTIR and 1H NMR experiments indicate that Cd interacts with hydroxyl groups through one of the solvating water molecules in the first coordination sphere. Cadmium retains water molecules bridging Cd and OH groups even at low hydration states. Comparison with previous studies shows that, when adsorbed from high-concentration CdCl2 solutions, Cd species other than Cd2+ are present in sufficient quantities to enter the interlayer and create a different Cd-water complex, seemingly displacing Cd2+ from the centre of the interlayer.