Abstract:
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.