Extraction separation of rare-earth ions via competitive ligand complexations between aqueous and ionic-liquid phases

The extraction separation of rare earth elements is one of the most challenging separation processes in hydrometallurgy and advanced nuclear fuel cycles. The TALSPEAK process (trivalent actinide lanthanide separations by phosphorus-reagent extraction from aqueous komplexes) is a prime example of these separation processes. The objective of this paper is to explore the use of ionic liquids (ILs) for the TALSPEAK-like process, to further enhance its extraction efficiencies for lanthanides, and to investigate the potential of using this modified TALSPEAK process for separation of lanthanides among themselves. Eight imidazolium ILs ([C(n)mim][NTf(2)] and [C(n)mim][BETI], n = 4,6,8,10) and one pyrrolidinium IL ([C(4)mPy][NTf(2)]) were investigated as diluents using di(2-ethylhexyl)phosphoric acid (HDEHP) as an extractant for the separation of lanthanide ions from aqueous solutions of 50 mM glycolic acid or citric acid and 5 mM diethylenetriamine pentaacetic acid (DTPA). The extraction efficiencies were studied in comparison with diisopropylbenzene (DIPB), an organic solvent used as a diluent for the conventional TALSPEAK extraction system. Excellent extraction efficiencies and selectivities were found for a number of lanthanide ions using HDEHP as an extractant in these ILs. The effects of different alkyl chain lengths in the cations of ILs and of different anions on extraction efficiencies and selectivities of lanthanide ions are also presented in this paper.     

PVT properties of PUREX/UREX solvent tri-n-butyl phosphate

Tri-n-butyl phosphate (TBP) is the workhorse of solvent extraction operations in nuclear fuel cycle as well as an important industrial chemical. Its PVT properties are not available in literature. In this study, PVT properties of TBP, estimated using group contribution method, are reported. Wagner constants were also reported in the range of 273.15?K to critical temperature.     

Theoretical studies on the complexation of Eu(III) and Am(III) with HDEHP: structure,bonding nature and stability

Separation of trivalent lanthanides (Ln(III)) and actinides (An(III)) is a key issue in the advanced spent nuclear fuel reprocessing. In the well-known trivalent actinide lanthanide separation by phosphorus reagent extraction from aqueous komplexes (TALSPEAK) process, the organophosphorus ligand HDEHP (di-(2-ethylhexyl) phosphoric acid) has been used as an efficient reagent for the partitioning of Ln(III) from An(III) with the combination of a holdback reagent in aqueous lactate buffer solution. In this work, the structural and electronic properties of Eu³⁺ and Am³⁺ complexes with HDEHP in nitric acid solution have been systematically explored by using scalar-relativistic density functional theory (DFT). It was found that HDEHP can coordinate with M(III) (M=Eu, Am) cations in the form of hydrogen-bonded dimers HL₂^- (L=DEHP), and the metal ions prefer to coordinate with the phosphoryl oxygen atom of the ligand. For all the extraction complexes, the metal-ligand bonds are mainly ionic in nature. Although Eu(III) complexes have higher interaction energies, the HL₂^- dimer shows comparable affinity for Eu(III) and Am(III) according to thermodynamic analysis, which may be attributed to the higher stabilities of Eu(III) nonahydrate. It is expected that this work could provide insightful information on the complexation of An(III) and Ln(III) with HDEHP at the molecular level.     

Thermal decomposition of reversed TALSPEAK solvent

Reversed TALSPEAK (RT) Solvent [0.3 M di-(2-ethyl hexyl) phosphoric acid or D2EPHA in 0.2 M TBP in normal paraffinic diluent] is used as extractant in partitioning of lanthanides and actinides in post-UREX process flow sheets. There is no experimental data available in literature describing pressurization either due to the thermal decomposition or the reaction of RT solvent with nitric acid at elevated temperatures. Experiments were conducted for heating of RT solvent and acid equilibrated RT solvent under closed-vent adiabatic conditions. During these experiments, considerable pressurization was observed. In this paper, results of these experiments are discussed in detail.     

Thermophysical properties of reversed TALSPEAK (RT) solvent

Thermophysical properties of reversed TALSPEAK extractant (0.3 M D2EPHA/0.2 M TBP/n-dodecane) were not available in literature. Authors have experimentally measured and correlated several thermophysical properties of RT solvent like density, viscosity, refractive index, acid uptake and flash point. In this paper, results of these studies will be discussed in detail.     

Activity Coefficients of di-(2-ethylhexyl) Phosphoric Acid in Select Diluents

This work characterizes the non-ideal behavior of the solvent extraction agent di-(2-ethylhexyl) phosphoric acid (HDEHP), constituting one piece of an effort to develop increasingly accurate models of advanced fuel separation processes such as TALSPEAK. Robust models are particularly important for processing high-level radioactive material in order to minimize the generation of secondary waste and to ensure reliable process control. Here, vapor pressure osmometry (VPO) data on binary solutions of HDEHP in toluene, dodecane, or cyclooctane yields the activity coefficients for each component after analysis. Initially, diluent activity data is obtained using the VPO results and then modeled using Scatchard-Hildebrand theory to provide the activity coefficients for HDEHP.     

Estimation of PVT properties and vapour pressure of alternate PUREX/UREX extractant TRI-iso amyl phosphate

Tri-iso-amyl phosphate is an alternate solvent, proposed in literature as an alternate to the PUREX/UREX solvent tri-n-butyl phosphate for better physical properties. Its PVT properties and accurate expression for estimation of its vapour pressure are not available in the literature. Recently PVT properties of TiAP were estimated by authors and its vapour pressure was measured in a ASTM certified vapor pressure measurement system at temperatures ranging from 273.15 to 373.15 K. In this paper, results of these studies are presented.     

Extraction of plutonium from sulphuric acid by TOA in toluene

The extraction of plutonium(VI) and plutonium(III) from sulphuric acid by TOA in toluene has been studied as a function of the acid and tri-octyl amine concentration. A comparison of the extraction properties of plutonium with those of uranium(VI) and uranium(IV) has been made. It was found that the extraction properties of plutonium(VI) are very similar to those of uranium(VI) and that TOA is a relatively poor extractant for plutonium(III). Uranium(IV) shows better extraction properties than plutonium(III). The results obtained are considered in the light of the stabilities of the complexes formed by these elements in the organic and aqueous phase. A method of separation of both elements by solvent extraction based on changing their oxidation states is suggested.     

Radiochemical solvent extraction of Cr(III) using51Cr tracer by 8-hydroxyquinoline and acetylacetone as mixed ligands in chloroform

A radiochemical solvent extraction method has been developed for the determination of Cr(III) using⁵¹Cr tracer. It is based on the complexation of Cr(III) with 8-hydroxyquinoline (oxine) and acetylacetone as mixed ligands at pH 3.8 and extraction in chloroform. Effect of various parameters such as pH, time of equilibration, nature of solvent, quantitative nature, effect of diverse ions has been studied. The method can be used up to 200 ng of Cr.     

Elucidating the radical kinetics involved in the radiolytic destruction of lanthanide-complexed DTPA

The reprocessing of nuclear fuels to extract the remaining actinides is one of the most important strategies for viable nuclear power in the future, as geologic waste disposal of high-level radioactive waste could have considerable negative impacts on the environment. As many of these strategies are based on solvent extraction processes, our aim is to establish the radiolytic stability of the component extraction ligands. The oxidative stability of the lanthanide metal-complexed extraction ligand DTPA (diethylenetriaminepentaacetic acid) has been investigated through its reaction rate constant with the hydroxyl radical under TALSPEAK acidic pH conditions, both for the free ligand and complexed with Eu³⁺, Lu³⁺ and Gd³⁺. Specific rate constants were obtained over a range of pH conditions using thiocyanate competition kinetics. The rate constants for the complexed metals are seen to be greater than the corresponding values determined for only the DTPA protonated ligands at these pHs.     

Acid Dissociation Constants and Rare Earth Stability Constants for DTPA

Diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) is an octadentate aminopolycarboxylate complexing agent whose f-element complexes find important practical applications in nuclear medicine and in advanced nuclear fuel reprocessing. This investigation focuses primarily on the latter application, specifically on characterization of lanthanide–DTPA complexes of relevance to the Trivalent Actinide–Lanthanide Separations by Phosphorus reagent Extraction and Aqueous Komplexants (TALSPEAK) process. To function acceptably, the TALSPEAK process requires the presence of moderate concentrations (0.5–2.0 mol·L^?1) of a (Na⁺/H⁺) lactate (or citrate) buffer. Competition between DTPA, lactate, and the extractant bis(2-ethylhexyl)phosphoric acid (HDEHP) for the lanthanides and trivalent actinides governs the course of the extraction process. To facilitate modeling and to support process improvements, the acid dissociation constants and stability constants for rare earth complexes with DTPA have been determined in 2.0 mol·L^?1 ionic strength (NaClO₄) media. The acid dissociation constants for DTPA and the stability constant for [Eu(DTPA)]^2? also were determined in sodium trifluoromethanesulfonate at 2.0 mol·L^?1 ionic strength to evaluate the potential impact of changing the nature of the electrolyte. The thermodynamic data are compared with earlier reports of similar data at lower ionic strength and used to complete calculations exploring the relative stability of lanthanide–DTPA and lactate complexes under TALSPEAK extraction conditions. Lanthanide–DTPA stability trends are discussed in comparison with literature data on a variety of other metal ions.     

Solvent extraction of Au(III) for preparation of a carrier-free multitracer and an Au tracer from an Au target

Separation of Au(III) and various carrier-free radionuclides by solvent extraction was investigated using an Au target irradiated by an energetic heavy-ion beam. Percentage extraction of Au(III) and coextraction of the radionuclides were determined with varying parameters such as kinds of solvent, molarity of HCl or pH, and Au concentration. Under the conditions where Au(III) was effectively extracted, namely extraction with ethyl acetate or isobutyl methyl ketone from 3 mol·dm^–3 HCl, carrier-free radionuclides of many elements were found to be more or less coextracted. Coextraction of radionuclides of some elements was found to increase with an increase in the concentration of Au(III). This finding is ascribed to the formation of strong association of the complex of these elements with chloroauric acid. In order to avoid serious loss of these elements by the extraction, lowering of the Au(III) concentration or the use of a masking agent such as sodium citrate is necessary. Gold(III) was shown to be effectively back extracted with a 0.1 mol·dm^–3 aqueous solution of 2-amino-2-hydroxymethyl-1,3-propanediol. Thus, a radiochemical procedure has been established for preparing a carrier-free multitracer and an Au tracer with carrier form from an Au target irradiated with a heavy-ion beam. Both tracers are now used individually for chemical and biological experiments.     

Separation of Rare-Earth Elements from Nitrate Solutions by Solvent Extraction Using Mixtures of Methyltri-n-octylammonium Nitrate and Tri-n-butyl Phosphate

The article presents data on the solvent extraction separation of rare-earth elements (REEs), such as La(III), Ce(III), Pr(III), and Nd(III), using synergic mixtures of methyltrioctylammonium nitrate (TOMANO₃) with tri-n-butyl phosphate (TBP) from weakly acidic nitrate solutions. Specifically, experimental results on separation of REEs, for the pair Ce(III)/Pr(III) for quaternary mixtures of REEs (La(III), Ce(III), Pr(III), Nd(III)) and for the pair La(III)/Pr(III) for solutions containing La(III), Pr(III), and Nd(III), are presented. It was shown that effective separation for the pair Ce(III)/Pr(III) from a solution containing 219 g Ce(III)/L, 106 g La(III)/L, 20 g Pr(III)/L, 55 g Nd(III)/L, and 0.1 mol/L HNO₃, was achieved using 56 steps of a multistage, counter-current solvent extraction cascade with scrubbing, at an organic-to-aqueous phase volume ratio (O/A) equal to 2/1 on the extraction section and O/A equal to 4/1 on the scrubbing section, using 3.3 mol/L solutions of the mixture TOMANO₃-TBP with molar ratio 0.15:0.85 in dodecane. Separation for the pair La(III)/Pr(III) could be achieved using a solvent extraction cascade with scrubbing in 32 steps at O/A equal to 2/1 on the extraction section and O/A equal to 2.8/1 on the scrubbing section of the solvent extraction cascade from a solution containing 258 g La(III)/L, 58 g Pr(III)/L, 141 g Nd(III)/L, and 0.1 mol/L HNO₃ with 3.0 mol/L solution of the mixture TOMANO₃-TBP with molar ratio 0.2:0.8 in dodecane.     

Solvent Extraction–Fluorimetric Determination of Thallium(III) in Manganese Oxide Ores with a Neutral Basic Red Dye

The solvent extraction of neutral red chlorothallate(III) from aqueous HCl solutions was studied. The compound formed is readily extracted with amyl acetate. The optimum conditions for the formation and extraction of the ion pair were found. The ratio of the reacting components in the compound formed was determined. The extraction selectivity in the presence of several interfering elements was studied. A procedure for the solvent extraction–fluorimetric determination of trace thallium(III) was developed. The procedure was used for the analysis of manganese oxide ores.     

Radiation stability of benzyldibutylamine and benzyldimethyldodecylammonium nitrate in the extraction of lanthanides and americium

The radiation stability was investigated of organic phases containing tertiary benzyldialkylamines and quaternary benzyltrialkylammonium salts which are sultable for the separation of lanthanides and americium from irradiated nuclear fuel. Attention was paid to changes of the extraction properties in Eu(III) and Am(III) extraction. The influence of the individual components forming the organic phase (extractant, solvent, solubilizer and nitric acid) on the decrease of the extraction capacity of the organic phase after irradiation is discussed. The greatest changes in the distribution coefficients D_Eu and D_Am after irradiation were shown for extraction in the presence of nitric acid. As regards the absorbed dose, these systems can be considered as stable in comparison with organophosphorus extractans.     

Separation of AM(III) by extraction from nitrate solutions using some quaternary benzylammonium salts

Benzyldimethyldodecylammonium nitrate and benzyltrioctylammonium nitrate were used for the extraction of Am(III) from aqueous nitrate solutions. The dependence of the extraction performance for Am(III) on the concentration of nitric acid, the kind and concentration of salting-out agents in the aqueous phase, and the kind of solvent was investigated. Americium is extracted by the above quarternary salts as a R₄NAm(NO₃)₄ associate. The extraction of Am(III) is compared with the extraction of lanthanides. The high differences in the distribution coefficients for lanthanides and americium can be utilized for the separation of lanthanides and americium.     

From BTBPs to BTPhens: The Effect of Ligand Pre-Organization on the Extraction Properties of Quadridentate Bis-Triazine Ligands

The synthesis, lanthanide complexation and solvent extraction of An(III) and Ln(III) radiotracers from nitric acid solutions by a pre-organized, phenanthroline-derived bis-triazine ligand CyMe4-BTPhen are described. It was found that the ligand separated Am(III) and Cm(III) from the lanthanides with remarkably high efficiency, high selectivity, and faster extraction kinetics compared to its 2,2’-bipyridine counterpart CyMe4-BTBP. The origins of the ligands extraction properties were established by a combination of solvent extraction experiments, X-ray crystallography, kinetics and surface tension measurements and lanthanide NMR spectroscopy.     

A Short Review of the Separation of Iridium and Rhodium from Hydrochloric Acid Solutions by Solvent Extraction

Iridium and rhodium are among the platinum group metals. The properties, production processes, and aqueous chemistry of both metals are reviewed. The separation of Ir(IV) and Rh(III) from hydrochloric acid solution is dependent on the characteristics of the solvent extraction systems. In most of the extraction conditions, Ir(IV) is selectively extracted over Rh(III) by either amines or neutral extractants. Rh(I) can be selectively extracted over Ir(III) by neutral extractants after Rh(III) is reduced in the presence of a reducing agent. The separation of these two metals using cationic extractants has also been reported. Although selective extraction of one metal over the other is possible, more efficient solvent extraction systems need to be developed.     

核燃料循环中镅的氧化分离研究进展

镅离子在溶液中主要以三价形式(Am(III))存在,因其离子半径与三价镧系离子Ln(III)接近,化学特性相似,使得Am(III)与Ln(III)的有效分离被认为是核燃料循环领域最具挑战性的课题之一。利用镅的多价态特性,采用不同氧化方法可将Am(III)氧化成高价态的AmO_2~+和AmO_2~(2+)形式,再通过溶剂萃取、沉淀等方法进行分离,是实现Am与Ln分离的一种新思路。本文综述了不同氧化方法对水溶液环境中Am(III)的氧化分离研究进展,描述了相关机理,指出了不同氧化方法的优劣并展望了未来发展趋势,以期为发展新型镧系与锕系元素分离技术提供参考。