Experimental study of uranyl(VI) chloride complex formation in acidic LiCl aqueous solutions under hydrothermal conditions (T=21 °C-350 °C, Psat) using Raman spectroscopy

The chemistry of mineralizing fluids associated with several types of uranium deposits are chloride brines. To understand and model the formation of uranium deposits, knowledge of the behavior of U(VI) in chloride brines is necessary. The speciation of U(VI) in chloride aqueous solutions is studied here along the vapor saturation curve using Raman spectroscopy. Chemical composition of solutions is the following: UO2Cl2 (0.01 M), HCl (0.1 M), and LiCl concentrations (0.3 up to 12 M). These solutions have been loaded in silica glass capillary and heated from 21 degrees C up to 350 degrees C at saturated vapor pressure. Raman spectra show an evolution of the band profile of the symmetric stretching (nu(1)) of UO22+ with increasing temperature and chlorinity. This band profile evolution results from the variation of the contribution of each chloride complex UO2Cln2-n (n = 0 to 5) and an unidentified complex at 841 cm(-1) which could be a polyuranyl chloride complex. U(VI) is transported by a mixture of uranyl chloride complexes in acidic brines conditions. From fitted Raman spectra, equilibrium constants Kn+1 (UO2Cln2-n ((aq)) + Cl-(aq)(-) = UO2Cln+11-n ((aq))) have been calculated as a function of temperature and chlorinity. Comparison of the value of the stepwise equilibrium constant (beta(0)) at room temperature for the first chloride complex (n = 1) agrees with literature data. The stability of the presumed polyuranyl complex (nu(1) approximate to 841 cm(-1)) has to be unraveled for lower uranyl concentration.