Highly efficient extraction separation of uranium(VI) and thorium(IV) from nitric acid solution with di(1-methyl-heptyl) methyl phosphonate

The extraction behaviors of U(VI) and Th(IV), as well as the extraction of HNO3 were investigated by using di(1-methyl-heptyl) methyl phosphonate (DMHMP) as an extractant and kerosene as a diluent. Effects of contact time, acidity, concentration of extractant and temperature on the distribution ratios of U(VI) and Th(IV) were evaluated. HNO3 was extracted as mono-solvated species. In the investigation of acidity range from 0.01 to 6.0 mol/L, the distribution ratios of U(VI) and Th(IV) increased at first with the rise of HNO3 concentration up to 3.0 mol/L and then decreased. DMHMP exhibited higher selectivity for U(VI) than for Th(IV) with the separation factor SFU/Th values of 25-120, whereas under the same conditions, tri-n-butyl-phosphate (TBP) only had SFU/Th, values of 1.4-26. Slope analyses showed that U(VI) and Th(IV) were extracted as di-solvated and tri-solvated species respectively. The thermodynamic parameters (Delta H, Delta S and Delta G) were presented and revealed that the extraction was exothermic. For the extraction of Th(VI) from various HNO3 by 30 vol.% DMHMP or TBP, the limiting organic concentration (LOC) was also given. FT-IR spectra indicated that it was P=O bond, not P-O bond that took part in the coordination with U(VI) and Th(IV). In addition, the solubility of DMHMP and TBP in various concentration HNO3 was determined using ICP-AES, and their thermal stability in air was also examined through TGA. DMHMP has much lower water solubility and higher thermal stability than TBP. (C) 2015 Elsevier B.V. All rights reserved.