Advanced Uranium Removal with Pure, Nonfluorinated Ionic Liquids: Fine-Tuning Hydrophilic and Hydrophobic Properties for Enhanced Selectivity

Ionic liquids (ILs) have significant potential for eco-friendly extraction of uranium from aqueous solutions, which is critical for nuclear technology, fuel cycle management, and environmental protection. This study examines the impact of the adjustable hydrophobic/hydrophilic properties of ILs on the removal of uranium(VI) (UO22+) from aqueous solutions utilizing both a novel hydrophilic IL (1-butoxyethyl-1-methylmorpholinium butoxyethylphosphite - Mor1-2O4-BOEP) and 1-heptyl-1-methylmorpholinium heptylphosphite (Mor1-7-HP) as an example of a hydrophobic IL with a similar structure. The transfer mechanism of uranyl ions from water to organic or solid phases closely depends on the physicochemical properties of ILs, especially their hydrophobicity. The hydrophobic Mor1-7-HP extracts uranyl via neutral complex formation as UO2(NO3)2-(Mor1-7-HP)2. Conversely, hydrophilic Mor1-2O4-BOEP induced selective precipitation as UO2(NO3)-(BOEP), transferring uranyl to the solid phase. Optimization of the working parameters, in terms of acidity of the aqueous solution and amount of ILs used, allowed the extraction of over 98% of U(VI). The stoichiometry of the organic complex and the precipitate was determined using physicochemical techniques. These tunable H-phosphonate-based ILs have advantages over traditional solvent extraction and conventional ILs, allowing easier handling, improved selectivity, and lower environmental impact. This work advances uranium separation techniques with applications in hydrometallurgy, particularly in the treatment of wastewater and radioactive waste for sustainable uranium recovery.

Optimization of Iron Removal in the Recovery of Rare-Earth Elements from Coal Fly Ash Using a Recyclable Ionic Liquid

Rare-earth elements (REEs) are essential for modern technologies, and the United States currently lacks a secure domestic supply. Coal combustion residuals, specifically coal fly ash (CFA), can be a potential source. Our previous work demonstrated that REEs could be preferentially extracted from CFA using the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf₂N]), and the process yielded a mildly acidic REE-rich solution with coextracted Fe and regenerated IL. In this study, we investigated three strategies to limit Fe coextraction: magnetic separation, complexing salts, and ascorbic acid (AA) reduction. Magnetic separation of CFA was ineffective in significantly lowering the Fe content in the IL phase. When NaCl was used instead of NaNO₃ during extraction, chloride complexation lowered iron distribution to the IL phase over the aqueous phase (D_Fe) by five folds, from ∼75 to ∼14, while REE leaching (L_REEs) and recovery (R_REEs) both increased. Using AA for iron reduction lowered the overall amount of Fe extracted and further decreased D_Fe to ∼0.16, effectively shifting Fe preference from the IL phase to the aqueous phase. Combining the strategies of NaCl, AA, and supplemental betaine addition, leaching and extraction of REEs from CFA by [Hbet][Tf₂N] were achieved in higher efficiency for REE recovery with minimized Fe concentration.

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.

Indium and thallium extraction into betainium bis(trifluoromethylsulfonyl)imide ionic liquid from aqueous hydrochloric acid media

Carrier-free In and Tl radionuclides were extracted into a protic ionic liquid [Hbet][Tf₂N] from HCl media with and without the presence of zwitterionic betaine in the aqueous phase.

Highly efficient extraction and selective separation of uranium (VI) from transition metals using new class of undiluted ionic liquids based on H-phosphonate anions

In this paper, we report the development of an environmental friendly process to decontaminate uranium-containing ores and nuclear wastes by using non-fluorinated ionic liquids (ILs). The main advantages of this extraction process are the absence of any organic diluent and extra extraction agents added to the organic phase. Moreover, the process is cost-effective and maybe applied as a sustainable hydrometallurgical method to recover uranium. The distribution ratio (D_U) and the extraction efficiency (%E) of uranium(VI) (UO₂²⁺) were found to be dependent on the acidity of the aqueous phase, the extraction time, the alkyl chain length in the ILs, the concentration of the aqueous feed and molar quantity of ILs. The D_U value is higher than 600 and the %E is equal to 98.6% when [HNO₃]=7M. The extraction reactions follows a neutral partition or ionic exchange mechanism depending on nitric acid concentration. The nature of bonding in the extracted complexes was investigated by spectroscopic techniques. The potential use of Mor_1-8-OP for the separation of UO₂²⁺ from a mixture containing transition metal ions Mⁿ⁺ was also examined. The UO₂²⁺ ions were separated and extracted efficiently. These ILs are promising candidates for the recovery and separation of uranium.

Significant enhanced uranyl ions extraction efficiency with phosphoramidate-functionalized ionic liquids via synergistic effect of coordination and hydrogen bond

The influence of the linking group between the phosphoryl and bridging moieties in phosphoryl-containing task-specific ionic liquids (TSILs) on the extraction of uranyl ions was experimentally and theoretically investigated. A novel phosphoramidate-based TSIL with an amine group as the linking moiety resulted in a higher uranyl ion extraction efficiency compared with that of other phosphoryl-based TSILs. A distribution ratio of 4999 ± 51 can be achieved for uranyl ions. The uranyl ions (76.7 ± 1.5%) were stripped from the loaded ionic liquid phase in a single stage using 0.05 M diethylenetriamine pentaacetic acid in a 1.0 M guanidine carbonate solution. The extraction stoichiometry of the uranyl ions was determined by a slope analysis of the extraction data. Furthermore, the fundamental nature of the interaction between the phosphoramidate-based TSIL and uranyl ions was theoretically studied for the first time. The theoretical calculations demonstrated that the synergistic effect of the complexation interaction and H-bond formation between the phosphoramidate-functional ionic liquid and uranyl nitrate led to the higher extraction efficiency. These results provide a basis for rational design, synthesis and potential applications of novel TSILs for uranyl extraction.

Fast selective homogeneous extraction of UO22+ with carboxyl-functionalised task-specific ionic liquids

The carboxyl-functionalised task-specific ionic liquid of 1-carboxymethyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl)imide ([HOOCmim][NTf₂]) was used as solvent and extractant for UO₂²⁺ extraction from aqueous solution. A homogeneous phase of [HOOCmim][NTf₂]-H₂O system could be achieved at 75 °C, and 86.8 ± 4.8% of UO₂²⁺ was separated from the aqueous solution after vibrating for only 1 min. Furthermore, nearly 97.3 ± 2.9% of UO₂²⁺ was stripped from [HOOCmim][NTf₂] phase by 1 M HNO₃ solution. K⁺, Na⁺, Mg²⁺, Dy³⁺, La³⁺, and Eu³⁺ have little influence on the homogeneous extraction of UO₂²⁺, and the extraction efficiency of UO₂²⁺ still remained at ca. 80%. Experimental and theoretical study on the selectivity of [HOOCmim][NTf₂]-H₂O system were performed for the first time. Density functional theory calculation indicates that the solvent effect plays a significant role on the selectivity of [HOOCmim][NTf₂]-H₂O.

Radiation Effect of Carboxyl-Functionalized Task-Specific Ionic Liquids on UO22+ Removal: Experimental Study with DFT Validation

Experimental study with DFT validation on the effect of radiation on 1-carboxymethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([HOOCCH₂MIM][NTf₂]) as a solvent and extractant in rapid homogeneous extraction of UO₂²⁺ was performed for the first time. The radiolytic products of the anions and cations of [HOOCCH₂MIM][NTf₂] were identified by ¹⁹F NMR and high-resolution ESI-MS, respectively, and they were attributed to a decrease in UO₂²⁺ partitioning. Experimental study with DFT validation for complexing reactions between [HOOCCH₂MIM][NTf₂] and radiolytic products proved that F^- competition was one of the main reasons for the decrease in UO₂²⁺ partitioning. However, UO₂²⁺ partitioning in irradiated [HOOCCH₂MIM][NTf₂] can largely be recovered after thorough water washing because of the removal of radiolytic products of [NTf₂]^-.

U(VI) Extraction by 8-hydroxyquinoline: a comparison study in ionic liquid and in dichloromethane

Room temperature ionic liquids (RTILs) represent a recent new class of solvents with potential application in liquid/liquid extraction based nuclear fuel reprocessing due to their unique physical and chemical properties. The work herein provides a comparison of U(VI) extraction by 8-hydroxyquinoline (HOX) in a commonly used RTIL, i.e. 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim][PF6]) and in conventional solvent, i.e. dichloromethane (CH2Cl2). The effect of HOX concentration, solution acidity and nitrate ions on the extraction were discussed in detail, and the speciation analyses of the extracted U(VI) were performed. One of the main emphasis of this work is the extraction mechanism of U(VI) extracted from aqueous phase into RTILs and conventional solvent. In CH2Cl2, the extraction occurs through a combination of ion change and neutral complexation, and the extracted complex is proposed as UO2(OX)2HOX. In [C4mim][PF6], although a cation-change mechanism as previously reported for RTILs-based system was involved, the extracted complex of UO2(OX)1.5(HOX)1.5(PF6)0.5 gave a clear indication that the usage of HOX as an acidic extractant markedly inhibited the solubility loss of [C4mim][PF6] during the extraction by leaching H+ to aqueous phase. Moreover, the extracted U(VI) in [C4mim][PF6] can be easily stripped by using 0.01 M nitric acid, which provides a simple way of the ionic liquid recycling.

Actinide ion extraction using room temperature ionic liquids: opportunities and challenges for nuclear fuel cycle applications

Studies on the extraction of actinide ions from radioactive wastes have great relevance in nuclear fuel cycle activities, mainly in the back end processes focused on reprocessing and waste management.

Ligands for f-element extraction used in the nuclear fuel cycle

This review describes the latest advances regarding the development, modification and application of suitable ligands for the liquid–liquid extraction of actinides and lanthanides from nuclear waste.

Overview of the effect of salts on biphasic ionic liquid/water solvent extraction systems: anion exchange, mutual solubility, and thermomorphic properties

Hydrophobic (water-immiscible) ionic liquids (ILs) are frequently used as organic phase in solvent extraction studies. These biphasic IL/water extraction systems often also contain metal salts or mineral acids, which can significantly affect the IL trough (un)wanted anion exchange and changes in the solubility of IL in the aqueous phase. In the case of thermomorphic systems, variations in the cloud point temperature are also observed. All these effects have important repercussions on the choice of IL, suitable for a certain extraction system. In this paper, a complete overview of the implications of metal salts on biphasic IL/water systems is given. Using the Hofmeister series as a starting point, a range of intuitive prediction models are introduced, supported by experimental evidence for several hydrophobic ILs, relevant to solvent extraction. Particular emphasis is placed on the IL betainium bis(trifluoromethylsulfonyl)imide [Hbet][Tf2N]. The aim of this work is to provide a comprehensive interpretation of the observed effects of metal salts, so that it can be used to predict the effect on any given biphasic IL/water system instead of relying on case-by-case reports. These prediction tools for the impact of metal salts can be useful to optimize IL synthesis procedures, extraction systems and thermomorphic properties. Some new insights are also provided for the rational design of ILs with UCST or LCST behavior based on the choice of IL anion.