Nuclear magnetic resonance investigation of mineral surface reactions involving inner-sphere and outer-sphere complexes
This thesis investigates the local environments and dynamic behavior of anions and cations adsorbed onto mineral surfaces, using $\sp{31}$P and $\sp{133}$Cs magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. Phosphate and Cs are good examples of inner-sphere and outer-sphere complexes, respectively, and this study shows that NMR is in many ways a more useful tool to investigate surface adsorbed species than other spectroscopic methods. Chapter 1 reviews mineral surface reactions which involve inner-sphere and outer-sphere complexes. General applications of NMR spectroscopy to ion adsorption studies are also discussed. Chapter 2 discusses a $\sp{31}$P MAS NMR study of phosphate adsorption onto kaolinite, boehmite, and $\gamma$-$\rm Al\sb2O\sb3.$ The results in this chapter show that on these phases phosphate forms inner-sphere surface complexes and Al-phosphate precipitates that are pH and solution concentration dependent. $\sp{31}$P MAS NMR can distinguish between those two different types of sites, and the results also show that there is rapid proton exchange among phosphates adsorbed on the surfaces. Chapter 3 discusses $\sp{133}$Cs MAS NMR results for Cs adsorption on kaolinite and montmorillonite at different solution concentrations, humidities, and temperatures. The spectra for kaolinite are similar to those of montmorillonite, and the tightly bonded Cs in our sample appears to be in expandable interlayers within kaolinite and not as discrete smectite. The interlayer charge of the smectite-like layer is small compared to normal montmorillonite. Those conclusions were verified by the spectra for kaolinite collected at low temperature and after heating. Chapter 4 focuses on MAS NMR data of Cs exchanged onto the external surfaces of illite, kaotolinite, boehmite, and silica gel. These data indicate that Cs is adsorbed on these surfaces in primarily two ways: at sites relatively tightly bonded to the surface (Stern layer) and at more loosely bonded sites in the diffuse (Gouy) layer. The NMR peaks representing those two sites have different chemical shifts, intensities of side bands, and peak widths, and also show different features at different humidities and surface compositions. There is motional averaging between those two sites at 100% R.H., and the frequencies decrease with decreasing R.H. and temperature.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign