Visual and in situ Raman spectroscopic observations of the liquid-liquid immiscibility in aqueous uranyl sulfate solutions at temperatures up to 420 °c

Abstract

The phase behaviors of aqueous UO2SO4 solutions were investigated in situ with a microscope and a Raman spectrometer at temperatures from 25 to 420 °C. Results show that aqueous UO2SO4 solution separated into UO2SO4-rich (Urich) and UO2SO4-poor (Upoor) liquid phases coexisted with a vapor phase at =285.8 ± 0.5 °C. Both visual and Raman spectroscopic investigations suggest that a reversible strong UO2 2+-SO4 2- association was responsible for the liquid-liquid immiscibility in aqueous UO2SO4 solutions. Main evidences were summarized as: (1) the liquid-liquid phase separation temperature decreases with increasing UO2SO4 concentration up to 0.54 mol/kg, and then increased at greater concentrations, characterizing a lower critical solution temperature (LCST) at 285.8 °C ± 0.5 °C. LCST is commonly accepted as a diagnostic feature of polymer solutions; (2) analyses of the shapes of the Raman spectra of v1(UO2 2+) and v1(SO4 2-) bands show that the UO2 2+-SO4 2- association becomes stronger at elevated temperatures, especially in the immiscible Urich phase; and (3) with increasing temperature, the Urich phase becomes more concentrated, whereas the Upoor phase becomes more dilute, indicating that the hydration of UO2 2+ and SO4 2- cannot be maintained in the Urich phase. Destruction of the hydration spheres of UO2 2+ and SO4 2- further favors the ion association in the Urich phase. These results are important for describing similar sulfate solutions at elevated temperatures, especially under supercritical conditions.