Influence of Co-Solvent on the Extraction Behaviour of Uranium and Thorium Nitrates with Organophosphorous Compounds

Abstract

Supercritical Fluid Extraction (SFE) is emerging as a powerful technique in the extraction of metalions. In the present study, the extraction of nitrates of uranium and thorium in their solid form was carried out using supercritical carbon dioxide (Sc-CO 2 ) modified with various organophosphorous compounds such as dialkylalkyl phosphonates, trialkyl phosphates and trialkyl phosphine oxides in the presence of co-solvents such as methanol, dichlormethane and n-hexane. The influence of ligand and co-solvent on the extraction of the metal nitrates was studied in detail. These studies have established that co-solvent plays an important role in the extraction as well as fractionation of uranium and thorium nitrates. Polar co-solvent, methanol provided faster extraction without fractionation whereas the non-polar solvent, e.g. n-hexane provided some fractionation of metal nitrates though the extraction kinetics was slower. Keywords Co-Solvent, Uranium and Thorium, Organophosphorous Compounds* Corresponding author. K. C. Pitchaiah et al. 34sults in interesting analytical separations and provides a medium for various organic reactions. CO 2 is the most widely used supercritical fluid due to its moderate critical temperature and critical pressure; moreover, it is non-flammable, non-toxic, inexpensive and environmental friendly [1]. The most distinctive properties of the supercritical fluids are their enhanced mass transfer and variable density. The density of supercritical fluid can be fine tuned by varying the pressure and temperature [1]; hence solvating power of carbon dioxide can be easily altered. Supercritical fluids offer faster, efficient and cleaner extraction as CO 2 escapes as gas after extraction leaving the solute in its pure form. SFE is attractive technique to the nuclear industry, where the replacement of organic solvents with Sc-CO 2 has the potential to minimize the generation of secondary liquid waste.Supercritical carbon dioxide is an excellent candidate for the extraction of non-polar and moderately polar compounds. The extraction of polar compounds is generally carried out with Sc-CO 2 containing small quantities of organic solvent e.g. methanol as the co-solvent. The extraction of metal ions is accomplished by the addition of suitable ligand into Sc-CO 2 medium [2]- [4]. Addition of organic ligands results in the conversion of charged metal species into neutral metal complexes, which become quite soluble in Sc-CO 2 . Supercritical carbon dioxide (Sc-CO 2 ) containing chelating agents was employed for the extraction of metal ions such as transition metal ions, lanthanides and actinides [5]- [17]. Sc-CO 2 containing ligands such as diketones, fluorinated diketones and trin-butyl phosphate (TBP) was used for the recovery of actinides such as thorium, uranium, plutonium and americium. SFE technique was developed and demonstrated in our laboratory for the recovery of actinides from various matrices such as cellulose and polymer [12]-[17].In the present study, we have examined for the first time the use o...