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A Quantitative Raman and Density Functional Theory Investigation of Uranyl Sulfate Complexation and Liquid-Liquid Phase Separation under Hydrothermal Conditions

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Title: A Quantitative Raman and Density Functional Theory Investigation of Uranyl Sulfate Complexation and Liquid-Liquid Phase Separation under Hydrothermal Conditions
Author: Alcorn, Christopher Darrell
Department: Department of Chemistry
Program: Chemistry
Advisor: Tremaine, Peter
Abstract: State-of-the-art Raman spectroscopic techniques have been employed to quantitatively measure (i) the temperature-dependent Raman properties and thermal decomposition of several non-complexing anions in acidic and neutral solutions at temperatures up to 400 oC, (ii) the first and second formation constants of the uranyl sulfate system from 25 – 375 °C, and (iii) the speciation and concentrations of liquid-liquid phase separated mixtures of uranyl sulfate and sulfuric acid up to 425 °C. The results of the first study show the stability followed the order HSO4- (stable) > ReO4- > ClO4- > CF3SO3- in acidic solution, with half-lives over 7 h at 300 oC. In neutral solutions, the order was HSO4- (stable) > CF3SO3- > ReO4- > ClO4-, with half-lives over 8 h at 350 oC. Raman vibrational frequencies and relative Raman scattering coefficients of these anions were determined, providing temperature-dependent data for frequency calibration and for determining the efficiency of these anions to scatter Raman light. To our knowledge, this manuscript is the first of its kind to report quantitative values using this methodology. The first and second formation constants of the uranyl sulfate system were quantitatively determined for the first time at t > 75 °C up to supercritical conditions. A drastic increase in both constants is observed, compared to an extrapolation of literature values, owing to the formation of uranyl sulfate complexes at high temperature. At t > 375 °C, uranyl sulfate solutions separated into two immiscible liquids, rich and deficient in uranium, respectively. A solid, presumed to be UO3·H2O(s), formed in the rich phase of solutions without excess sulfuric acid. Analysis shows the uranyl ion is completely complexed in both phases, forming UO2SO40(aq) and minor amounts of UO2(SO4)22-(aq). The relative abundance of UO2SO40(aq) increases with temperature. At t ≥ 400 °C, excess HSO4-(aq) forms in the uranium rich phase. For the uranium deficient phase, no uranium or sulfur species are detectable at 400 °C. Concentrations in both phases were quantified by using the homogeneous phase spectra as an external standard, together with densities calculated using the Helgeson-Kirkham-Flowers equation of state, and a reasonable density estimation using apparent molar volumes.
URI: http://hdl.handle.net/10214/12965
Date: 2018-03
Rights: Attribution-NoDerivs 2.5 Canada
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Attribution-NoDerivs 2.5 Canada Except where otherwise noted, this item's license is described as Attribution-NoDerivs 2.5 Canada