Solvent extraction of thorium from mixed organic — aqueous nitric acid media by try-n-octylphosphine oxide

The extraction equilibria of Th in tri-n-octylphosphine oxide (TOPO)–HNO₃–H₂O systems have been investigated in the absence and presence of water-miscible alcohols and acetone. The influence of TOPO concentration in an inert diluent (toluene) and the concentration of nitric acid and the water-miscible additives have been systematically examined. The slopes of the log-log plots of D_Th vs. the TOPO concentration (up to 0.02M) are found to be close to 3 over a wide range of polar phase compositions leading to a coordination number of 11 for Th. The D_Th values were determined as a function of the concentration of uranyl nitrate in the aqueous phase, while the effect of additives was also examined in the absence and presence of uranyl nitrate in the polar phase. The results show that at TOPO concentrations above about 2.5 · 10^–3 M (about 1%) in toluene, polymerization of the solvent occurs.