We report the use of $^{113}Cd$ NMR spectroscopy for the identification of cadmium species that are adsorbed when montmorillonite are in contact with aqueous $CdCl_2$ solution. This was done by (a) determining the change in the concentration od cadmium and chloride concentrations in the supernatant solutions from the peak intensity and chemical shift data and (b) $^{113}Cd$ MAS NMR on the sedimented montmorillonite that has been cadmium exchanged.
Montmorillonite is a 2:1 type layer, smectite group, dioctahedral clay and general approximate formula of montmorillonite is as follows ;
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Four series of the sample solutions $([CdCl_2] = 5.14\times10^{-2}, 4.11\times10^{-2}, 3.08\times10^{-2}, 2.06\times10^{-2}M)$ with 1%, 2%, and 3% (w/v) montmorillonites were prepared. Applying suitable NMR conditions(the pulse width of 30 ˚ the total delay of 25s), the concentrationof free aquoCd in the supernatant was determined by integration of the Cd peak intensities and the change of chloride in the supernatant. The concentration of free aquo Cd and the change of chloride concentration in the supernatants were determined by integration of the Cd peak intensities and the observed chemical shift data. The result shows that there is decrease of only cadmium concentration and no decrease of chloride concentration. This implies that $CdCl^+$ species are not adsorbed and only $Cd^{2+}$ species are adsorbed on the montmorillonite. To verify this result, the direct method of $^{113}Cd$ solid-state MAS NMR performed with the Cd exchanged montmorillonite. The $^{113}Cd$ chemical shift of -8 ppm obtained in the cadmium exchanged montmorillonite is similar to that observed in cadmium complexes, such as solid $[Cd(H_2O)_6](ClO_4)_2$ with -9 ppm. This result is another evidence to confirm the fact that only $Cd^{2+}$ is adsorbed on montmorillonite.
