Performance and Mechanism for the Selective Separation of Trivalent Americium from Lanthanides by a Tetradentate Phenanthroline Ligand in Ionic Liquid

Abstract

The selective separation of trivalent americium from lanthanides in a nitric acid medium by a tetradentate ligand, N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen), in an ionic liquid (IL), C 4 mimNTf 2 , was studied by batch solvent extraction and spectroscopic approaches. The effect of various parameters such as the contact time, temperature, extractant concentration, and acidity on the extraction of Am 3+ and Eu 3+ have been evaluated. A significant enhancement in the extraction ability of Et-Tol-DAPhen dissolved in IL was observed as compared to that in molecular diluents under low-acid conditions. The chemical stoichiometry of Am 3+ and Eu 3+ complexes during extraction was determined to be 1:2 (metal/ ligand) by slope analysis of the extraction data. The extraction mechanism of Am 3+ and Eu 3+ by Et-Tol-DAPhen in IL was determined to be cation exchange on the basis of the effect of nitrate, NTf 2 − , and C 4 mim + ions on extraction. The coordination chemistry of Ln 3+ with the ligand in C 4 mimNTf 2 was studied by spectroscopic titrations, which helped to further identify and confirm the extracted species as well as the extraction mechanism. Results from the present study emphasize the unique role of IL in altering the extraction behavior and suggest that the Et-Tol-DAPhen/IL system has potential applications in trivalent actinide/ lanthanide separation under low-acid conditions.

Ionic Liquids as Components of Systems for Metal Extraction

“…A similar enhancement of the extraction capacity was observed when the [bmim][NTf 2 ] IL was used as the diluent in comparison with molecular diluents such as N,N -diethyland N,N -ditolyl-2,9-diamide-1,10-phenanthrolines in the recovery of americium(III) from nitric acid solutions [92]. The distribution ratio of americium was approximately 10 4 when the extraction was carried out from a nitric acid solution using [bmim][NTf 2 ], whereas in the case of a molecular diluent, cyclohexanone, the distribution ratio was reported [93] to be about 10.…”

Substituent Effect on the Selective Separation and Complexation of Trivalent Americium and Lanthanides by N,O-Hybrid 2,9-Diamide-1,10-phenanthroline Ligands in Ionic Liquid

“…23 Nevertheless, this is a common observation in an extraction system using an IL as the diluent. 42,51,52 According to the above extraction behavior and the results from previous studies, 45 the extraction of Am(III) and Eu(III) by the DAPhen ligands in the present IL system proceeds mainly through a cation exchange mechanism that can be expressed by eq 1…”

“…Such a stoichiometry is in agreement with that of Et-Tol-DAPhen in an IL system in our previous study. 45 As such, the extraction reaction can be defined as eq 4:…”

“…Moreover, the starting absorption spectra of Nd(III) in the IL resembles that of free Nd(III) in aqueous solution well, indicating that Nd(III) in the IL is initially in a hydrated form, and this is consistent with our previous observations. 45,53 With the addition of the ligands, the absorption spectra of Nd(III) change in approximately three stages depending on the molar ratio of the ligand to the metal (c L /c M ). For c L /c M = 0−1.0, a new absorption band appears at 584 nm and gradually intensifies.…”

“…42 In a previous work, we also reported the excellent separation ability of Et-Tol-DAPhen for Am(III) over Eu(III) in an ionic liquid and unfolded the extraction mechanism in the system through spectroscopic characterizations. 45 To further explore the possibility of using DAPhen ligands in ionic liquids for An(III)/Ln(III) separation and help illustrate the relationship between the extraction behavior and the structures of the substituent groups in DAPhen ligands, herein we investigate the extraction of Am(III) and Eu(III) by four DAPhen ligands (Et-Et-DAPhen, But-But-DAPhen, Hex-Hex-DAPhen, and Oct-Oct-DAPhen; the structures are shown in Figure 1) with different alkyl groups on the amide moieties in the common ionic liquid C 4 mimNTf 2 (Figure 1). The effects of contact time, HNO 3 concentration, and extractant concentration on the extraction efficiency were systematically investigated.…”