Anion assisted extraction of U(VI) in alkylammonium ionic liquid: Experimental and DFT studies

• Published in: Separation and purification technology Vol. 261; p. 118275
• Main Authors: Venkateswara Rao, Ch, Rout, Alok, Boda, Anil, Musharaf. Ali, Sk, Venkatesan, K.A.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.04.2021
• Subjects: DFT calculations; Extraction; FTIR spectroscopy; Raman spectroscopy; Room temperature Ionic liquid; Uranium; Room temperature Ionic liquid; Extraction; Uranium; Raman spectroscopy; DFT calculations; FTIR spectroscopy
• ISSN: 1383-5866; 1873-3794
• DOI: 10.1016/j.seppur.2020.118275

[Display omitted] •Conventional imidazolium ionic liquids undergo undesirable cation exchange mechanism.•Strongly hydrophobic alkyl ammonium ionic liquids as an alternative to imidazolium ionic liquids.•Extraction behavior of U(VI) in hydroxyacetamide in alkyl ammonium ionic liquid.•Raman and FTIR spectroscopic and DFT investigations of the extracted phase.•Excellent extraction in alkylammonium ionic liquid phase without any cation exchange mechanism. The extraordinary extraction of metal ions in imidazolium ionic liquid phase containing neutral extractants was most often attributed to the cation exchange mechanism in addition to the usual solvation-type mechanism. The cation exchange mechanism results in an irreversible loss of the ionic liquid cation, which eventually leads to the pollution of aqueous phase also. To minimize the limitations of imidazolium based ionic liquid, strongly hydrophobic ionic liquids containing tetra-alkyl ammonium ion have been introduced and studied for actinide separation. Even though these tetra-alkyl ammonium ionic liquids did not undergo cation exchange, but showed exceptional extraction of actinides for the reason, which was not established so far. To unravel the unique role of these hydrophobic ionic liquids, the extraction behavior of U(VI) was studied in a solution of tri-n-octylmethylammonium nitrate ([N1888 ][NO3]) in conjunction with a neutral ligand, 2-hydroxy-N,N-dioctyl acetamide (DOHyA). The extraction of U(VI) was studied as a function of various extraction parameters and the organic phase obtained after extraction was probed by FTIR and Raman spectroscopy to elucidate the mechanism of U(VI) extraction in ionic liquid phase. The result revealed that U(VI) was extracted into ionic liquid phase by a couple of extraction modes, namely through metal–ligand coordination and by loss-less anion exchange mode and these pathways did not cause any undue burden to the organic and aqueous phases, unlike the traditional imidazolium ionic liquids. Further, DFT studies were performed to elucidate the underlying mechanism of U(VI) complexation in ionic liquid phase. The calculated values of binding energy (ΔE) and Gibbs free energy (ΔG) for the complexation of U(VI) with DOHyA in dodecane and tetra-alkyl ammonium ionic liquids medium were found in good agreement with the experimentally determined complexation trends of U(VI) ion with DoHyA. The DFT calculations further established that the cation exchange mechanism by both the ionic liquids, [N1888][NO3] and [N1888][NTf2] was not feasible, but anion exchange mechanism could be possible as observed in the experimental studies.