Properties of TRPO-HNO3 complex used for direct dissolution of lanthanide and actinide oxides in supercritical fluid CO2

The mixed trialkylphosphine oxide-nitric acid (TRPO-HNO₃) complex prepared by contacting pure TRPO with concentrated HNO₃ may be used as additives for direct dissolution of lanthanide and actinide oxides in the supercritical fluid carbon dioxide (SCF-CO₂). Properties of the TRPO-HNO₃ complex have been studied. Experimental results show when the initial HNO₃/TRPO volume ratio is varied from 1:7 to 5:1, the concentration of HNO₃ in the TRPO-HNO₃ complex changes from 2.12 to 6.16 mol/L, the [HNO₃]/[TRPO] ratio of the TRPO-HNO₃ complex changes from 0.93 to 3.38, and the content of H₂O in the TRPO-HNO₃ complex changes from 0.97% to 2.70%. All of the density, viscosity and surface tension of the TRPO-HNO₃ complex change with the concentration of HNO₃ in the complex. The protons of HNO₃ and H₂O in the complex undergo rapid exchange to exhibit a singlet resonance peak in NMR spectra with D₂O insert. When the TRPO-HNO₃ complex dissolves in a low dielectric constant solvent, small droplets of HNO₃ appear which can be detected by NMR. Supported by the National Natural Science Foundation of China (Grant No. 20506014)