Cd-113 double-resonance NMR as a probe of clay mineral cation exchange sites

A lack of direct spectroscopic evidence linking cations to the clay mineral structure is a primary reason for many ambiguities regarding the location and structure of the binding sites. This study attempts to obtain such evid ence through the observation of nuclear-spin interactions between adsorbed cations (Cd-113(2+)) and nuclei present in the lattice structure of the cla y (H-1 and Al-27) using solid-state NMR. H-1-Cd-113 variable-amplitude cros s-polarization (VACP) and Al-27-Cd-113 spin-echo double-reso nance (SEDOR) experiments were successfully performed on dried, Cd2+-exchanged beidellite , montmorillonite, and vermiculite samples, demonstrating for the first tim e a direct interaction between the adsorbed cations and the clay. VACP prov ides much greater cross-polarization (CP) efficiency which enhances signal intensity necessary for these experiments. Signal intensity varies between octahedral and tetrahedral binding sites because hydroxyls in the ditrigona l cavities of tetrahedral sites produce greater CP efficiency presumably du e to shorter H-1-Cd-113 distances in this environment. CP efficiency decrea sed in the order vermiculite>beidellite>montmorillonite. The observation of nuclear-spin coupling between cadmium and either H-1 or Al-27 in the miner als is consistent with the presence of the cations in the ditrigonal caviti es of the tetrahedral sheet. Inter-sample comparison of the strengths of co upling interactions follow expected trends based on charge location in the crystal structure and Al speciation.