Structural investigations of strontium in inorganic crystals, organic crystals, and phyllosilicate minerals with strontium-87 NMR .

摘要

The initial chapter of this thesis motivates the project by examining the political and chemical history of the main US plutonium production facility at Hanford, providing insight into the nature of the high-level waste and how it was generated. A brief introduction to soil mineralogy is also presented in this chapter, concluding with some comments on how the waste chemistry affects the natural soil minerals. The second chapter covers the theoretical basis of NMR, beginning with a framework for understanding the SIMPSON simulations used to extract NMR parameters in the remainder of the dissertation. It also presents the major internal interactions relevant to 87Sr NMR and the theory behind the experimental methods that will be used in subsequent chapters.;Chapter 3 details research into the effectiveness of magic angle spinning (MAS) at improving the sensitivity and resolution of 87Sr NMR spectra. MAS is used to examine the distribution of electric field gradients due to crystal defects in the simple systems SrO (octahedrally coordinated Sr), SrF2 (cubic Sr coordination), and SrCl2 (cubic Sr coordination). In situations where a moderate to large electric field gradient is present, experimental and theoretical results show that MAS is not effective at improving sensitivity or the interpretability of the resonances with quadrupolar couplings larger than 10 MHz.;Because MAS offers limited benefits with respect to 87Sr NMR, alternative methods of sensitivity enhancement are explored in Chapter 4. The first section summarizes 87Sr NMR studies of strontium-rich crystalline minerals with the quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) pulse sequence at a 21.14 T static field. Despite the benefits of QCPMG at 21.14 T, this technique proved incapable of efficiently detecting strontium in a saturated phyllosilicate mineral. The second section of Chapter 4 explores the additional enhancement produced by double-frequency sweep (DFS) preparation prior to QCPMG at 21.14 T. The use of DFS reduces the necessary spectral acquisition time for a SrCO3 sample by an additional factor of 10 to 40, permitting studies of strontium in systems that were not possible with QCPMG alone at 21.14 T.;In Chapter 5, DFS-QCPMG at 21.14 T is used to study the strontium environments in a number of strontium-saturated phyllosilicate minerals, including three synthetic micas designed as strontium remediation materials and two natural montmorillonites. The results of 87Sr NMR investigations are combined with 19F MAS NMR and X-ray diffraction (XRD) analyses to characterize the interlayer strontium binding environment in heated forms of the micas that are known to sequester strontium. These results show that the presence of a layer offset induces a steric hindrance, forcing strontium to bind deep in the di-trigonal cavities following dehydration.;Having successfully completed the objectives of the research project in Chapter 5, Chapter 6 reviews the key results and contains three preliminary studies that highlight the remaining challenges in 87Sr NMR. The first study applies 87Sr NMR to a pair of titanate strontium sorbants that are currently a part of the DOE waste remediation strategy at the Savannah River facility. The second study examines the precipitated material formed after a thirty-day incubation period in a strontium-rich simulated tank waste (Sr-STWL) solution seeded with colloidal silica. Both of these studies demonstrate that 87Sr NMR is highly sensitive to the formation of strontium carbonate that may be invisible to X-ray techniques. The final section recounts an attempt to study the strontium environment in apatite minerals with 87Sr NMR. Plans for future studies are then formulated based on these preliminary results. (Abstract shortened by UMI.).