Extraction of Th(IV) from aqueous solution by room-temperature ionic liquids and coupled with supercritical carbon dioxide stripping

• Th4+ was extracted efficiently from aqueous phase by HTTA in CnmimNTf2.
• Th4+ with four TTA− anions is predominant during the extraction.
• Optimization of conditions for supercritical CO2 stripping of Th(IV).
• Optimized conditions resulted in more than 94% recovery of Th(IV).

The extraction of Th(IV) from aqueous phase into the ionic liquids (ILs) 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imides (CnmimNTf2, n = 2, 4, 6, 8) by the β-diketone extractant 2-thenoyltrifluoroacetone (HTTA), and the stripping of Th(IV) from the IL phase using supercritical carbon dioxide (sc-CO2) were studied. Th(IV) was extracted efficiently by HTTA-CnmimNTf2 systems. A neutral complex extraction mechanism was proposed via log–log plot analysis method and ion chromatograph (IC) method. The formed neutral complex partitioning in all four IL systems was Th(TTA)4, and the structure was further confirmed by Extended X-ray Absorption Fine Structure Spectroscopy (EXAFS). Supercritical CO2 with ethanol showed good stripping of Th(IV) from the IL phase. The pressure and temperature of CO2, the volume and polarity of modifiers, and the dynamic extraction time influencing on the stripping efficiency were examined.