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¹¹³Cd, ¹H MAS NMR AND FTIR ANALYSIS OF Cd²⁺ ADSORPTION ON DIOCTAHEDRAL AND TRIOCTAHEDRAL SMECTITE

¹ Institute of Methodologies for Environmental Analysis, NRC, C. da S. Loja, Zona Industriale, 85050 Tito Scalo (PZ), Italy
² Department of Mineralogy, The Natural History Museum, Cromwell Road, London SW7 5BD, UK

^* E-mail address of corresponding author: pdileo{at}imaa.cnr.it

Dioctahedral and trioctahedral smectites of very low Fe contents 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 and 0/100. The samples were analyzed 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 center 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 center of the interlayer and closer to those oxygen atoms. The ¹H NMR study showed protons from hydration water and smectite hydroxyls and showed 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 center of the interlayer.

Key Words: Cation Exchange • FTIR • MAS NMR • Smectite

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