In-Situ Experimental Observation and Determination of Speciation in Phase-Separated Mixtures of Aqueous Uranyl Sulfate and Sulfuric Acid under Hydrothermal Conditions by Raman Spectroscopy

Liquid-liquid phase-separated aqueous solutions of [UO2SO4 + H2SO4] were investigated by quantitative Raman methods at temperatures from 300 to 425 °C and 25 MPa, using quartz-capillary optical cells. UO2SO4 concentrations ranged from 0.3 to 0.8 mol·kg-1, and the ratio of SO4/U was varied from 1 to 3.5 by the addition of sulfuric acid. Phase transition temperatures were determined by direct visual observation. The Raman spectra of the homogeneous phase solutions, at temperatures just below the lower critical solution temperature, were used as an external standard to quantify the concentrations of the species present in each of the phase-separated solutions. Deconvolution of the Raman spectra in both the dense and dilute aqueous phases showed that the uranyl ion is completely associated into the complexes UO2SO40(aq) and UO2(SO4)22−(aq). UO2SO40(aq) is the dominant species at 350 °C, and its relative concentration increases further upon heating. HSO4−(aq) was found to exist in appreciable concentrations in the dense phase only at t ≥ 400 °C. The concentration of HSO4−(aq) in the dilute phase is significant at 350 °C and increases above 375 °C. Raising the temperature to 400 °C resulted in no detectable concentration of uranium or sulfate species in the dilute phase. A solid phase formed inside the uranium-rich phase of solutions of UO2SO4 without added sulfuric acid, and is presumed to be UO3·H2O(s).