Extraction of thorium employing N,N-dialkyl amide into room temperature ionic liquid followed by supercritical carbon dioxide stripping

• First report on solvent extraction of Th(IV) employing amides into RTIL.
• Novel strategy of stripping with SC CO2 modified with DBOA·HNO3.
• Kinetics, acid molarity, stoichiometry, thermodynamic parameters were studied.
• Compared with molecular diluent (n-hexane) and supercritical fluid extraction.
• Mechanism of extraction was investigated.

N,N-dialkyl aliphatic amides with varying alkyl groups viz. N,N-dibutyl-2-ethyl hexanamide (DBEHA), N,N-dibutyl-3,3-dimethyl butanamide (DBDMBA), N,N-dihexyl octanamide (DHOA), N,N-di-sec-butyl pentanamide (DBPA), N,N-dibutyl octanamide (DBOA), have been evaluated for solvent extraction of thorium from nitric acid medium into the hydrophobic ionic liquid phase, 1-butyl-3-methyl imidazolium hexafluorophosphate. Distribution ratio values were markedly higher for room temperature ionic liquids (RTIL) as compared to molecular diluent, n-hexane. DBOA yielded the highest extraction. The extraction behaviour of Th(IV) into RTIL was found to be different from n-hexane with respect to extraction kinetics, effect of molarity of nitric acid and stoichiometry of Th(IV)-amide complex. Thermodynamic studies revealed that the extraction was enthalpy driven. Mechanism of the extraction was found to be dependent on the nitric acid molarity of the aqueous phase. High stripping efficiency was obtained with supercritical carbon dioxide which also provided insight into the mechanism of extraction into RTIL phase. Extraction of Th(IV) from aqueous phase (having low acid molarity, 0.2 M) into RTIL employing DBOA followed by stripping with supercritical carbon dioxide modified with DBOA·HNO3 adduct yielded the highest overall efficiency.