The structural environments of cations adsorbed onto clays: A cesium-133 MAS NMR spectroscopic study
Weiss, Charles Arthur, Jr
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https://hdl.handle.net/2142/21845
Description
Title
The structural environments of cations adsorbed onto clays: A cesium-133 MAS NMR spectroscopic study
Author(s)
Weiss, Charles Arthur, Jr
Issue Date
1989
Doctoral Committee Chair(s)
Kirkpatrick, R. James
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Mineralogy
Geochemistry
Language
eng
Abstract
Chapter I investigates the local structural environment of adsorbed cations on the mineral hectorite using $\sp{133}$Cs Variable-Temperature Magic-Angle-Spinning Nuclear Magnetic Resonance (VT-MAS NMR) spectroscopy. The results show that Cs on hectorite occurs in several distinctly different chemical environments, and that motional averaging of Cs between some of these sites occurs above ${\sim}{-}40\sp\circ$C if water is present in the interlayer. Above ${\sim}{-}10\sp\circ$C, spectra for slurries of hectorite in CsCl solutions yield two peaks, one due to Cs in solution, and the other due to Cs motionally-averaged on the clay. Below ${\sim}{-}60\sp\circ$C, motional averaging of the adsorbed Cs slows sufficiently to allow resolution of two peaks representing different Cs-environments on the clay. The Stern-Gouy model is employed to explain these peaks and assign one to Cs in the Stern layer (relatively tightly bound to the basal oxygens), and the other to Cs in the Gouy diffuse layer. Between ${\sim}{-}60$ and $-10\sp\circ$C peaks for these two sites and a motionally-averaged peak are present. Cs-exchanged hectorite dehydrated at 500$\sp\circ$C yields peaks for two different sites on the clay, interpreted to be highly coordinated site (probably 12), and a less coordinated site (possibly 9), both in the interlayer.
Chapter II discusses $\sp{133}$Cs MAS NMR results for a number of other Cs-exchanged clays and the relationship of chemical and structural parameters to the $\sp{133}$Cs chemical shift. As for hectorite, Cs occupies a number of different sites depending on the hydration state of the clay. Increased rotational distortions of the basal oxygen sheet, total layer charge and tetrahedral Al$\sp{3+}$ for Si$\sp{4+}$ substitution correlate with increased deshielding of the $\sp{133}$Cs chemical shifts for both hydrated slurry and anhydrous samples. Correlations for the slurries are poorer because of the distances between the clay silicate and the Cs in solution. Al$\sp{3+}$ for Si$\sp{4+}$ substitution has a greater effect on the $\sp{133}$Cs chemical shift of the less (possibly 9) coordinated site of the 450$\sp\circ$C-dried samples than that for the more highly (possibly 12) coordinated site. Motional averaging between sites inferred to be the Stern and Gouy layers occur for other smectite minerals. Signal for adsorbed Cs in a 0.01M CsCl-kaolinite slurry is probably at defect sites or on a small amount of a second phase (possibly smectite).
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