Supercritical fluid extraction (SFE) of plutonium in its nitrate form from actual waste, i.e. plutonium bearing cellulose matrix was demonstrated using 0.1 litre capacity extraction vessel. Complete recovery of plutonium was demonstrated using modified supercritical carbon dioxide (Sc-CO2), i.e. Sc-CO2 containing octylphenyl-N, N-diisobutyl-carbamoylmethylphosphine oxide (fCMPO). Near complete recovery of uranium was demonstrated from simulated waste matrices, i.e. uranium bearing teflon, glass and cellulose matrices using preparative scale SFE, i.e. from 1 litre capacity extraction vessel. The recovery of uranium was established using Sc-CO2 modified with acetyl acetone.
In recent years, Supercritical fluid extraction (SFE) has emerged as a powerful technique in the extraction of various metal ions from diverse waste matrices and as an alternative technique to solvent extraction. Liquid like density and gas like transport properties of supercritical fluids make them unique to act as special solvents. Supercritical fluids offer faster, efficient and cleaner extraction of compounds of interest with minimum generation of secondary liquid waste as CO2 escapes as gas after the extraction, leaving the solute in its pure form [1-3]. CO2 has a low critical temperature (31.2˚C) and a moderate critical pressure of 73.8 bar. It is non-flammable, non-toxic and environmentally friendly. CO2 being a non polar molecule, can be mainly employed for the extraction of non-polar and moderately polar compounds. Direct extraction of metal ions with supercritical carbon dioxide (Sc-CO2) is not possible because of weak solute-solvent interactions and charge neutralization requirement . The extraction of metal ions is achieved by modifying the properties of carbon dioxide, i.e. mixing carbon dioxide with appropriate extractants or chelating agents. A major requirement for the use of a ligand as chelating agent in SFE is that the ligand and resulting metal chelate should have sufficient solubility in Sc-CO2 medium [4,5].
Removal of various long lived radionuclides from waste is of major concern in nuclear industry. Various organic materials such as polymers, rubbers and cellulose, widely used in nuclear industry and research facilities are prone to be contaminated with various radioactive materials. Several techniques such as solid phase extraction, solvent cleaning etc., are used for decontamination of radionuclide from waste matrices. However, these techniques result in the generation of secondary liquid waste, which requires further treatment. SFE is an alternative and attractive technique for recovery of actinides from such waste matrices since it reduces the generation of secondary liquid waste.
Supercritical carbon dioxide modified with ligands was employed for the extraction of various metal ions such as transition metal ions, lanthanides and actinides from different media [4-20]. The extraction of lanthanides and actinides was carried out using Sc-CO2 modified with b-diketones . Supercritical fluid extraction of uranium and plutonium from nitric acid medium was examined using Sc-CO2 containing tri-n-butyl phosphate (TBP) . Quantitative dissolution of uranium dioxide in Sc-CO2 containing TBP with nitric acid was reported [9-10]. Uranium dioxide and its solid solutions with neptunium, plutonium and americium dioxides were dissolved using Sc-CO2 modified with TBP/nitric acid . Supercritical fluid extraction of plutonium and americium from surrogate soil was investigated using Sc-CO2 modified with theonyltrifluoroacetone (TTA) and TBP ; the extraction efficiencies of Pu and Am were found to be 69% and 88%, respectively. Sc-CO2 containing hexafluoroacetylacetone and pyridine was employed for the removal of actinides from stainless steel surface .
Complete removal of uranium from simulated solid waste matrices, e.g. tissue paper matrix was demonstrated in our laboratory using modified Sc-CO2 . In our earlier studies, complete recovery of plutonium from simulated waste matrices such as tissue, glass, teflon and stainless steel was demonstrated using Sc-CO2 modified with octylphenyl-N, N-diisobutyl-carbamoylmethylphosphine oxide (fCMPO) in methanol . In all these investigations, an analytical scale SFE was carried out, i.e. extraction from 1 mL capacity vessel. The actual waste generated in the laboratory, when subjected to SFE for the recovery of actinides resulted in an incomplete extraction. It was observed that the extraction efficiency of actinides from actual waste matrices (e.g. cellulose matrix) was highly influenced by the parameters such as storage period, nature of actinide species, moisture content etc.
In the present study, SFE of plutonium was carried out from the actual waste i.e. cellulose based waste matrix using a 0.1 litre capacity extraction vessel. The extraction efficiency was investigated as a function of ligand and its content in modifier, temperature, HNO3 content in the Sc-CO2 phase and extraction time. Initially the recovery of plutonium was carried out using a 1 mL extraction vessel and methods were evolved for the complete removal of plutonium from actual cellulose matrix. Subsequently, the plutonium bearing waste was processed using a 0.1 litre capacity extraction vessel. Similarly, the recovery of uranium was investigated in a preparative scale SFE facility, extraction vessel of 1 litre capacity. The recovery of uranium was investigated from different simulated waste matrices viz., cellulose, teflon and glass, which are generally encountered in various process steps in a radioactive laboratory. The results on the recovery of actinides from these waste matrices are discussed.