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April 26, 2019 @ 3:10 pm - 4:00 pm
AJ Krzysko (Sue Clark Group)
Isotope effects in aluminum hydroxide and aluminum oxyhydroxide slurries: Linking solubilities, solvent structure, and thermodynamic properties to rheology
The dissolution and precipitation reactions of gibbsite (α-Al(OH)3, eq 1) and boehmite (γ-AlO(OH), eq 2) in caustic solutions are not well understood, despite over a century of research from multiple industries.
Al(OH)3(s)+OH−(aq) ⇌ Al(OH)4 −(aq) (1)
AlO(OH)(s)+OH−(aq)+H2O ⇌ Al(OH)4 −(aq) (2)
One specific industrial application that is particularly interesting is the treatment and removal of over 56 million gallons of mixed radioactive and chemical waste from processing irradiated fuels to produce plutonium at the Hanford Site in Washington, USA. The tank waste slurries comprise particles and aggregates that vary greatly in size and surrounding chemistry. Two major components of the waste are solids and sludges composed of boehmite and gibbsite. One proposed method for waste retrieval is caustic leaching to dissolve these solids and sludges prior to vitrification. However, the current empirical models used to develop these methods have not been successful in predicting process performance, specifically in terms of particle aggregation and rheological responses.
A key scientific challenge that must be overcome in order to address this problem is understanding how solution structure and speciation affect the macroscopic properties such as aggregation, precipitation, and fluid response to flow, especially in complex environments far from our ideal solution approximations. Isotopic substitution will be used in three systems to better understand the effects of ion pairing, hydrogen bonding, and solvation on the gibbsite and boehmite solubility reactions. The first system will measure the thermodynamic solubility of boehmite and gibbsite as a function of NaOH and NaOD concentration. The solubility of gibbsite and boehmite in alkaline deuterated systems should be different due to hydrogen/deuterium bonding and hydrogen/deuterium activity differences. This analysis will probe these differences to obtain insights into which aspects of the solution structure are important in solubility. It will also lay the foundation for other analytical methods that require use of isotopic substitution (e.g. quasi elastic neutron scattering (QUENS) and nuclear magnetic resonance (NMR)).
The second system will measure gibbsite and boehmite solubility in NaOH and NaOD as a function of temperature to obtain the enthalpy and entropy of the reactions using the van ‘t Hoff analysis. It is predicted that the solubility differences of the aluminum minerals in NaOD vs NaOH will become more pronounced at higher temperatures. The differences in the solubility’s dependence on temperature will provide a relative measure of the importance of bonding strength and solution organization. Furthermore, temperature is an important parameter for large scale processing. Quantifying the solubility dependence on temperature will be useful to chemical engineers who are designing processing flowsheets.
The third system will use the combination of small angle X-ray scattering (SAXS) and isotopic substitution to measure how changes in aggregation lead to evolutions in rheology. The differences in solution structure and organization that lead to the solubility differences observed for the other systems described above will lead to measurable differences in properties important to engineers, such as viscosity. It is the goal of this work to use isotopic substitution to measure how subtle differences in chemistry lead to emergent macroscopic phenomena. This not only has the potential to greatly impact nuclear waste reprocessing, but also may help our understanding of chemical systems that are far from ideal.