Solid-state 113Cd Nuclear Magnetic Resonance Investigation of the Nature of Cation Binding Sites on Clay Minerals

Cation exchange by clay minerals impacts many aspects of the chemistry of soils and sediments, and affects the catalytic properties of clays used widely in industry. For these reasons, cation exchange by clay minerals has been the subject of intense research for several centuries. However, due the inherent disorder present in these materials, a complete characterization of the nature of cation exchange has not yet been achieved. A lack of direct evidence linking the cations to the clay structure is the primary reason for many ambiguities regarding the location and structure of the binding sites. This dissertation is an attempt to obtain such evidence through the observation of direct nuclear interactions between adsorbed cations (1 13Cd2+) and nuclei present in the lattice structure of the clay (1H and 27 Al) using solid-state NMR. Relaxation measurements and two dimensional exchange experiments were first used to identify samples in which the cations were rigidly bound to the surface of the clay. Samples which were dried at 0% relative humidity (RH) or vacuum-dried were found to have cadmium cations bound directly to the clay. The cations on samples rehydrated at 100% RH were coordinated to a sphere of water molecules, and were not bound directly to the clay. 1H-113Cd cross-polarization and 27 Al-113Cd spin-echo double resonance experiments were then successfully performed on the dried samples, showing for the first time a direct interaction between the adsorbed cations and the clay. The observation of coupling between the cadmium cations and either H or Al in the minerals requires a close proximity of the coupled species, within ~4-6A, placing the cations in the cavities of the tetrahedral sheet