²³Na and ¹³³Cs NMR study of cation adsorption on mineral surfaces: Local environments, dynamics, and effects of mixed cations

²³Na and ¹³³Cs MAS NMR methods were used to study the adsorption sites and atomic-scale dynamics of Cs, Na, and their mixtures on the surfaces of illite, kaolinite, boehmite, and silica gel reacted with 0.1 M NaCl and mixed NaCl and CsCl solutions. The ²³Na spectra were collected at relative humidities (R.H.) from ≈0 to 100% and temperatures from room temperature to -80°C and compared to previously published ¹³³Cs spectra collected under the same conditions. Surface Cs⁺ and Na⁺ behave differently due mainly to their different hydration energies, smaller for Cs⁺ than for Na⁺ . Cs⁺ is adsorbed in two distinguishable environments, (1) inner-sphere complexes and (2) outer-sphere complexes and in the diffuse layer. Na⁺ is adsorbed only as outer-sphere complexes and in the diffuse layer. Na⁺ undergoes dynamical averaging at frequencies ≫10 kHz at room temperature and high R.H., as indicated by low temperature data. For Na⁺ the increasing peak width with decreasing temperature indicates a decreasing frequency of motional averaging among Na⁺ sites with similar nearest-neighbor environment but a range of next nearest neighbor (NNN) environment. In contrast, for Cs⁺ there is exchange between two distinguishable sites. Experiments with mixed solutions support our peak assignments and interpretations. Surface Na⁺ successfully competes with Cs₊ for outer-sphere sites and the diffuse layer, whereas Cs⁺ prefers inner-sphere complexes. This differences results in a surface Cs/(Cs + Na) ratio greater than the bulk solution. Chemical analysis shows that Cs⁺ is preferentially adsorbed by illite relative to silica gel, and this is due to the large permanent surface charge of illite caused by Al substitution for Si in the tetrahedral sites.