Complexation of Actinyl Ions with DHOA and TBP in an Ionic Liquid and a Molecular Solvent: How Similar or Different Are They?

Complexation thermodynamics of uranyl ions with well-known reprocessing ligands like tributyl phosphate (TBP) and dihexyl octanamide (DHOA) was studied in an ionic liquid (IL) versus a molecular solvent. Whereas 1-butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)imide (Bumim·Tf2N) was used as an IL due to its favorable viscosity, acetonitrile was the choice of molecular solvent due to its poor coordinating nature. Optical spectroscopy studies revealed that UO22+ ions formed species of the types ML1 and ML2 with both TBP and DHOA, in a stepwise manner. The formation constants (log β) of UO22+ with DHOA in the IL were determined as 3.58 ± 0.03 and 6.54 ± 0.10, while those with TBP were obtained as 3.42 ± 0.05 and 6.38 ± 0.08, respectively. In the case of acetonitrile, on the other hand, these β values were significantly lower than those observed in the IL medium, which was supported by the DFT calculations. Calorimetric titrations of the UO22+ ions with DHOA and TBP confirmed that the complex formation reactions were thermodynamically more favored in the ionic liquid medium than the molecular solvent. The nature of binding through FTIR investigations and DFT calculations suggested that the complexes formed in the IL medium were cationic in nature of the types [UO2(H2O)3(TBP)2]2+ and [UO2(H2O)3(DHOA)2]2+, but neutral complexes were formed in the molecular solvent of the types [UO2(NO3)2(TBP)2] and [UO2(NO3)2(DHOA)2].

Mutual Solubilities between Ethylene Glycol and Organic Diluents: Gas Chromatography and NMR

In this work, the mutual solubilities of sets of organic diluents (CHCl₃, C₆H₆, C₂H₄Cl₂, CCl₄, C₆H₁₂, and n-hexane) with the organic compound ethylene glycol are investigated via gas chromatography (GC). The experimental data measured for these binary organic systems are used to adjust the future nonaqueous systems for the solvent extraction of various metals with ligands. The obtained results showed that the solubility of ethylene glycol decreased in the order CHCl₃ > C₆H₆ > C₂H₄Cl₂ > CCl₄(0%) ≈ C₆H₁₂ ≈ n-hexane. On the other hand, the solubility of the tested traditional organic diluents in ethylene glycol decreased in the following order: C₆H₆ > CHCl₃ > C₂H₄Cl₂ > n-hexane > C₆H₁₂ > CCl₄. ¹H NMR was also used as an analytic method in order to compare the obtained results for the samples showing significant solubility only, including an additional study with 1,2- or 1,3-propanediol. The enhanced solubility of the C₆H₆ compound in ethylene glycol was identified here as critical due to the GC technique, which will be without future consequences in chemical technology. Therefore, it was found that the best molecular diluent for the recovery of metals among the tested ones is C₆H₁₂, with a green protocol as the new paradigm, replacing the aqueous phase with another nonaqueous phase, i.e., a second organic diluent.

Thenoyltrifluoroacetone: Preferable Molecule for Solvent Extraction of Metals—Ancient Twists to New Approaches

A review of the investigations devoted to the solvent extraction processes of metal ions with a chelating ligand thenoyltrifluoroacetone (HTTA) is presented herein. It seems that this molecule has been preferred in the field for more than half a century, and that it is used very often as an extractant for almost all metals. The main objective of the present review is also to provide an overview of the synergistic solvent extraction of lanthanoids, particularly with the use of a β-diketone−neutral mixture. Based on the previous published results in the open literature, the extraction efficiency has been examined in detail and discussed further mainly in terms of the corresponding equilibrium constants among other outlined, so-important parameters. Major conclusions on the role of ligating groups of extractants towards the mechanism, an improved extraction enhancement, and selectivity are additionally provided. The fact that ionic liquids (ILs) appear to be replacing volatile diluents in the field of the liquid–liquid extraction of metals, again with the participation of this β-diketone, is not surprising. As is well known, a very efficient and simple way to determine the stoichiometry of the extracted species in the organic phase is by the simple use of the slope analysis method; however, it is sometimes difficult to perform, either because it somehow requests good solubility of the ligand or because obtained slopes are quite often far from integer values in ILs.

Solvent extraction of rare earths elements from nitrate media in DMDOHEMA/ionic liquid systems: performance and mechanism studies

Extraction of La(iii), Eu(iii) and Fe(iii) was compared in n-dodecane and two ionic liquids (ILs) (1-ethyl-1-butylpiperidinium bis (trifluoromethylsulfonyl)imide [EBPip⁺] [NTf₂⁻] and 1-ethyl-1-octylpiperidinium bis (trifluoromethylsulfonyl)imide [EOPip⁺] [NTf₂⁻]). Using the extractant N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA), the effect of pH was investigated in detail to recover extraction mechanisms. The use of ILs as the organic solvent instead of n-dodecane, greatly enhances extraction efficiency, and an ionic liquid with a shorter alkyl chain [EBPip⁺] [NTf₂⁻] provides higher extraction than [EOPip⁺] [NTf₂⁻]. The mechanistic study points out that for low nitric acid concentrations ([HNO₃] ≤ 0.01 M), metal is extracted via a cation of the ionic liquids, while for higher nitric acid concentrations ([HNO₃] ≥ 1.0 M), extraction occurs through pure solvation mechanism of DMDOHEMA as in conventional diluents. This latter case is of high interest for applications, as higher extraction can be obtained without any loss of ILs by ion exchange mechanisms.

Extraction of La(iii), Eu(iii) and Fe(iii) was compared in n-dodecane and in two ionic liquids (ILs) [EBPip⁺] [NTf₂⁻] and [EOPip⁺] [NTf₂⁻]. Extraction mechanisms have been investigated as a function of pH.

Utilizing Recyclable Task-Specific Ionic Liquid for Selective Leaching and Refining of Scandium from Bauxite Residue

Ionic liquids (ILs) have attracted great interest in the field of extractive metallurgy mainly because they can be utilized in low temperature leaching processes where they exhibit selectivity and recyclability. A major drawback in mixed aqueous-IL systems, is IL dissolution in the aqueous phase, which leads to IL losses, increasing the overall processing cost. This study advances the method for recovering scandium (Sc) from bauxite residue (BR) using as leaching agent the IL betainium bistriflimide, [Hbet][Tf2N] mixed with water, which has been reported in previous publications. Ionic liquid leachate (IL-PLS) was prepared by leaching BR with a mixture of [Hbet][Tf2N]-H2O and subjected to different stripping experiments using hydrochloric acid. The advancement, presented in this work, is related with the optimization of the metal extraction (stripping) from the IL-PLS, where an aqueous solution with high Sc concentration and minimum metal impurities and minimum IL co-extraction is produced. It is further proven that the metal cation extraction is defined by the stoichiometry of the acidic solution and the dissolution (losses) of the IL in the aqueous phase can be minimized by adjusting the volume ratio and the acid concentration. A two-step stripping process described, achieves the selective increase of Sc concentration by 8 times in the aqueous solution, while maintaining cumulative IL losses to similar levels as the optimum 1 step non-Sc selective stripping process.

One-pot synthesis of symmetric imidazolium ionic liquids N,N-disubstituted with long alkyl chains

The Debus–Radziszewski imidazole synthesis was adapted to directly yield long-chain imidazolium ionic liquids. Imidazolium acetate ionic liquids with side-chains up to sixteen carbon atoms were synthesised in excellent yields via an on-water, one-pot reaction. The imidazolium acetate ILs acted as surfactants when dissolved in various solvents. The imidazolium acetate ionic liquids were also derivatised via an acid metathesis to the chloride, nitrate, and hydrogen oxalate derivatives. The thermal behaviour of all the ionic liquids was determined via thermogravimetric and calorimetric analysis.

The modified Debus–Radziszewski reaction was used as a one-pot on-water reaction to allow a greener synthesis of long-chain 1,3-dialkylimidazolium acetate ionic liquids in high yield from long-chain linear amines.

Interfacial Forces across Ionic Liquid Solutions: Effects of Ion Concentration and Water Domains †

Using the surface force apparatus (SFA), the interaction forces between mica surfaces across ionic liquid (IL) solutions are studied. The IL solution, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide in propylene carbonate solvent, is used at different concentrations to elucidate the ions' conformation at the interface from the analysis of short-range structural forces. A direct correlation between the ion layer thickness at the interface and the IL molar fraction in the solution is observed, suggesting conformational changes relative to the ion packing density. In addition, effects of large microscopic and macroscopic water domains at the interface are investigated. The microscopic water domains induced significant adhesion at contact because of the long-range capillary forces, which are found to depend on solvent concentration. The macroscopic water domains entirely cover the interaction area, ensuring that the long-range interfacial interactions occur entirely across the aqueous electrolyte solution with dissolved IL ions as the electrolyte. These results help elucidate the interfacial interactions in IL-charged solid interfaces with practical importance in green energy storage, catalysis, and lubrication.

Hydrophobic polymerized ionic liquids for trace metal solid phase extraction: thallium transfer from hydrochloric acid media

To our knowledge, there are a few articles on the application of polymerized ionic liquids for metal extraction from aqueous solutions.

Complexation of 2-thenoyltrifluoroacetone (HTTA) with trivalent f-cations in an ionic liquid: solvent extraction and spectroscopy studies

HTTA forms M(TTA)²⁺, M(TTA)₂⁺, M(TTA)₃° and M(TTA)₄⁻ complexes with lanthanides. Complexation is strongly favoured when the salt of TTA⁻ is used.

The role of association of ions in ionic liquid/molecular solvent mixtures on metal extraction

Platinum extraction from aqueous phase into ionic liquid can be correlated to ionic association.

Complexation of tetraalkyl diglycolamides with trivalent f-cations in a room temperature ionic liquid: extraction and spectroscopic investigations

This paper reports the complexation of a series of tetraalkyl diglycolamides (TRDGA) with trivalent f-cations in a room temperature ionic liquid,viz. 3-octyl-1-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C₈mim][Tf₂N]).

Thallium Transfer from Hydrochloric Acid Media into Pure Ionic Liquids

Pure hydrophobic ionic liquids are known to extract metallic species from aqueous solutions. In this work we have systematically investigated thallium (Tl) extraction from aqueous hydrochloric acid (HCl) solutions into six pure fluorinated ionic liquids, namely imidazolium- and pyrrolidinium-based ionic liquids with bis(trifluoromethanesulfonyl)imide and bis(fluorosulfonyl)-imide anions. The dependence of the Tl extraction efficiency on the structure and composition of the ionic liquid ions, metal oxidation state, and initial metal and aqueous acid concentrations have been studied. Tl concentrations were on the order of picomolar (analyzed using radioactive tracers) and millimolar (analyzed using inductively coupled plasma mass spectrometry). The extraction of the cationic thallium species Tl(+) is higher for ionic liquids with more hydrophilic cations, while for the TlX(z)(3-z) anionic species (where X = Cl(-) and/or Br(-)), the extraction efficiency is greater for ionic liquids with more hydrophobic cations. The highest distribution value of Tl(III) was approximately 2000. An improved mathematical model based on ion exchange and ion pair formation mechanisms has been developed to describe the coextraction of two different anionic species, and the relative contributions of each mechanism have been determined.

Mutual solubility of water and hydrophobic ionic liquids in the presence of hydrochloric acid

Possible mechanism for ionic liquid dissolution in HCl solution.

Ionic liquid-based uranium(vi) extraction with malonamide extractant: cation exchange vs. neutral extraction

The U(VI) extraction with DMDBMA is performed in the [C₄mim][Tf₂N] ionic liquid (IL) either as a cationic or a neutral species, without the help of IL ions. However, this does not prevent a noticeable pollution of the aqueous phase with the IL.

Binding of pyrazine-functionalized calix[4]arene ligands with lanthanides in an ionic liquid: thermodynamics and coordination modes

Complexation thermodynamics of Eu³⁺ with pyrazine-functionalized calix[4]arene in ionic liquid was investigated by absorption spectroscopy, luminescence spectroscopy and microcalorimetry.

Extraction of actinides by tertiary amines in room temperature ionic liquids: evidence for anion exchange as a major process at high acidity and impact of acid nature

The extraction of U(vi) and Pu(iv) by tri-alkyl amines in room temperature ionic liquids, [C_nmim][Tf₂N] (where, n = 4, 6 or 8) follow anion exchange mechanism.

Binary lanthanide(iii)/nitrate and ternary lanthanide(iii)/nitrate/chloride complexes in an ionic liquid containing water: optical absorption and luminescence studies

Thermodynamic and spectroscopic data indicate that nitrate and chloride are stronger ligands than water for lanthanides in water saturated BumimTf₂N.

Possibilities and limitations in separating Pt(iv) from Pd(ii) combining imidazolium and phosphonium ionic liquids

The extraction of Pt(iv) using [OMIM][NTf₂] and of Pd(ii) using (Cyphos 102) is reported. Pd(ii) was extracted quantitatively using Cyphos 102. Separation of Pt(iv) from Pd(ii) was carried out in a simple two step process.

Unusual complexation of nitrate with lanthanides in a wet ionic liquid: a new approach for aqueous separation of trivalent f-elements using an ionic liquid as solvent

The energetics of lanthanide nitrate complexation in a wet ionic liquid (IL; saturated with water as seen in the aqueous separation process) differs entirely from that in dry IL.

Extraction of iridium(iv) from aqueous solutions using hydrophilic/hydrophobic ionic liquids

Removal of iridium(iv) using hydrophilic or hydrophobic ionic liquids was studied. IrCl₆²⁻ anionic metal complexes were precipitated out from water using bromide-based ionic liquids, yielding water-insoluble salts with low K_s values. Liquid–liquid extraction of Ir(iv) using [NTf₂]-based ionic liquids yielded distribution coefficients up to 70 with [OMIM][NTf₂] at pH 2.