Abstract:
133Cs MAS NMR of Cs-exchanged illite, kaolinite, boehmite, and silica gel is shown to be a powerful tool to investigate the adsorption sites and atomic dynamics of Cs on mineral surfaces. Cesium is adsorbed on these mineral surfaces in primarily two ways: at sites relatively tightly bonded to the surface (Stern layer, Cs1) and at more loosely bonded sites in the diffuse (Gouy) layer (Cs2). For illite, both edge sites and crystallite basal surfaces are important adsorption sites. For kaolinite, edge sites, expandable layers, and probably crystallite basal surfaces are important. The 133Cs NMR chemical shifts for the Cs1 site become more shielded (more negative) as the Si/Al ratio of the substrate phase increases, paralleling the chemical shift variations of other cations and consistent with this site being relatively tightly bonded to the surface. The 133Cs NMR chemical shifts of Cs2 do not vary systematically with solid composition due to the larger distance of these sites from the surface and weaker electrostatic attraction to the surface compared to Cs1. Rather, the Cs2 chemical shifts are significantly influenced by relative humidity (R. H.) and Cs population (Cs/H2O ratio) on the surface. The Cs1 chemical shifts vary less with these parameters. Cs2 is removed by washing with 1-5 mL of deionized water due to its weak attraction to the surface. The Cs1 chemical shifts become less shielded after washing and with decreasing solution concentration due to a decrease in the Cs surface density. At 100% R. H., Cs in the two sites undergoes motional averaging at frequencies > 100 kHz. With decreasing R. H., peaks for Cs on the two sites are resolved due to decreasing exchange frequencies related to a decreasing number of adsorbed water layers. Motional averaging at 100% R. H. is verified by low temperature experiments with illite.