New Insight into the Americium/Curium Separation by Solvent Extraction using Diglycolamides

The liquid–liquid extraction process called EXAm was developed by the CEA to allow the recovery of Americium alone from a PUREX raffinate. Americium is extracted from a highly acidic feed solution (HNO₃ 4–6 M) by a mixture of two extractants: DMDOHEMA and HDEHP. The Am/Cm selectivity is improved using a specific diglycolamide (TEDGA) as a selective aqueous complexing agent which retains preferentially Cm and heavier lanthanides in the aqueous phase. In this study, the impact of the lipophilicity and steric hindrance of several diglycolamides on the Am/Cm selectivity was investigated in order to understand the enhancement brought by TEDGA. For this purpose, liquid–liquid extraction and partitioning experiments were performed under various conditions.     

Modified Diglycolamides for the An(III) + Ln(III) Co-separation: Evaluation by Solvent Extraction and Time-Resolved Laser Fluorescence Spectroscopy

The use of two recently developed diglycolamide-based extractants for the co-separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) is described and compared to the well-known extractant TODGA (N,N,N’,N’-tetraoctyl diglycolamide). The addition of one or two methyl groups to the central methylene carbon atoms of the TODGA molecule leads to a reduction of the extraction efficiency for An(III) and Ln(III). This is attributed to a lower complex formation constant, which was proven by Time-Resolved Laser Fluorescence Spectroscopy (TRLFS). Conditional stability constants were determined by solvent extraction and TRLFS. The reduction in extraction efficiency leads to overall reduced distribution ratios of all tested metal ions, including Sr(II). The reduced Sr(II) extraction is beneficial as a co-extraction in a solvent extraction process could be avoided, while an efficient extraction of the desired An(III) and Ln(III) is still achieved. Furthermore, this might be a benefit, as the stripping behavior might be improved, even at moderate nitric acid concentrations. The slightly higher affinity of the diglycolamides towards Eu(III) over Am(III) is represented by all ligands, although the selectivity is rather low. This results in promising extraction properties of the modified diglycolamides towards the development of continuous solvent extraction processes.     

Studies on the Use of N,N,N´,N´-Tetra(2-ethylhexyl) Diglycolamide (TEHDGA) for Actinide Partitioning. I: Investigation on Third-Phase Formation and Extraction Behavior

Abstract

Diglycolamides have emerged as an interesting class of extractants for actinide partitioning from high-level waste (HLW). N,N,N´,N´-tetraoctyl diglycolamide (TODGA) has been extensively studied for lanthanide-actinide co-extraction behavior. The present work deals with a branched isomer of TODGA, that is, N,N,N´,N´-tetra(2-ethylhexyl) diglycolamide (TEHDGA). TEHDGA was studied for the extraction of ²⁴¹Am and third-phase formation. The effect of using different phase modifiers on the prevention of the formation of a third phase during nitric acid extraction by TEHDGA along with the acid uptake behavior by TEHDGA in the presence of the modifiers was studied. The modifiers used for this purpose were di(n-hexyl)octanamide (DHOA), isodecanol, and n-decanol. The effect of the modifiers on the uptake of ²⁴¹Am as a function of acid concentration and as a function of modifier concentration was also examined. DHOA was found to be a suitable modifier, in spite of its high acid uptake. The uptake of lanthanides Ce, La, Eu, Gd, and Nd and elements such as Fe, Ni, Mn, Mo, Ru, Sr, and Cs with DHOA-modified TEHDGA–n-dodecane solvent systems were investigated. The results obtained indicated that, while DHOA-modified TEHDGA/n-dodecane extracted lanthanides and actinides, it did not show any significant uptake of other elements. Thus, the TEHDGA-DHOA/n-dodecane solvent system can be used effectively for the partitioning of lanthanides and actinides from HLW.     

Extraction of some Hexavalent Actinide Ions from Nitric Acid Medium using Several Substituted Diglycolamides

Several substituted diglycolamides, namely TPDGA, THDGA, TODGA, and TDDGA, were evaluated in a comparative study on the extraction of hexavalent actinide ions such as UO₂²⁺, NpO₂²⁺, and PuO₂²⁺ from nitric acid medium. The acid extraction constants (K_H) for the diglycolamides were determined to be 3.8 ± 0.6, 1.6 ± 0.1, 4.1 ± 0.4, and 1.4 ± 0.2 for TPDGA, THDGA, TODGA, and TDDGA, respectively. Though metal ion extraction generally increased with increasing the feed acid concentration, the nature of the extracted species changed with aqueous-phase acidity. While complexes of the type MO₂(NO₃)₂·nL (where L is the diglycolamide extractant and n is 1 and 2) were found to be extracted at 1 M HNO₃, the average number of ligand molecules associated with the complex decreased to ?1 when the nitric acid concentration increased to 3 M. These results have great significance from the actinide separation point of view, as the actinides ions can be made virtually inextractable by adjusting their oxidation state. The thermodynamic parameters were also calculated, which indicated spontaneous reactions with large exothermicities.     

Radiolytic and hydrolytic degradation of the hydrophilic diglycolamides

The stability of different hydrophilic diglycolamides against acid degradation and radiolysis was studied. Tetraethyldiglycolamide (TEDGA) was found to undergo degradation in nitric acid at high reaction rates at elevated temperatures with a maximum of a ~8% decrease per hour at 65°C in 4 mol L^–1 HNO₃. The radiolysis was studied for tetramethyldiglycolamide (TMDGA), TEDGA, methyl-tetraethyldiglycolamide (Me-TEDGA), and dimethyl-tetraethyldiglycolamide (Me₂-TEDGA). The degradation rates decreased with increasing molecular weight, following the trend TMDGA > TEDGA > Me-TEDGA ≥ Me₂-TEDGA. Degradation products were identified by mass spectrometric techniques and were found to be comparable to those previously reported for the radiolysis of lipophilic diglycolamides in dodecane. Significant insight into the degradation mechanism in water was gained using pulse radiolysis experiments. The ^?OH radical was identified as the most important reactive species and predominant mechanism of radical reaction is one of electron transfer rather than H-atom abstraction.     

The Partitioning of Americium and the Lanthanides Using Tetrabutyldiglycolamide (TBDGA) in Octanol and in Ionic Liquid Solution

Separations among the lanthanides and the separation of Am from the lanthanides remain challenging, and research in this area continues to expand. The separation of adjacent lanthanides is of interest to high-tech industries because individual lanthanides have specialized uses and are in short supply. In nuclear fuel cycle applications Am would be incorporated into fast-reactor fuels, yet the lanthanides are not desired. In this work, the diglycolamide N,N,N′,N′-tetrabutyldiglycolamide (TBDGA) was investigated as a ligand for lanthanide and Am solvent extraction in both molecular and room-temperature-ionic-liquid (RTIL) diluents.The RTIL [C₄MIM][Tf₂N] showed very high extraction efficiency for these trivalent metals from low nitric acid concentrations, while the molecular diluent 1-octanol showed high extraction efficiency at high acid concentrations. This was attributed to the extraction of ionic nitrate complexes by the RTIL, whereas 1-octanol extracted neutral nitrate complexes. TBDGA in RTIL did not provide adequate separation factors for Am/lanthanide partitioning, but 1-octanol did show reasonable separation possibilities. Lanthanide intergroup separations appeared to be feasible in both diluents, but with higher separation factors from 1-octanol.     

Extraction behavior of selected rare earth metals from acidic chloride media using tetrabutyl diglycolamide

Rare earth elements (REEs) are vital to modern, high-tech devices. Recycling REEs from post-consumer electronics can potentially diminish supply chain risks. Toward that end, liquid–liquid solvent extraction of various REEs was investigated with tetrabutyl diglycolamide (TBDGA) in 1-octanol from hydrochloric acid media. Metal partitioning to the organic phase was shown to increase as [Cl^?] increased. In contrast, increasing [H⁺] did not improve extraction. The use of the polar diluent 1-octanol provided high extraction efficiency, especially for the partition of heavy lanthanides from solutions of high chloride concentration. Although the polar diluent also extracted molar amounts of water and acid, it was concluded that a neutral metal/TBDGA complex as mainly the di-solvate was extracted, and that complexation was observed to be exothermic. These results indicate that REE extraction from aqueous chloride solutions can be efficient without the use of high acid concentrations.     

Actinide–lanthanide co-extraction by rigidified diglycolamides

Within the actinide and lanthanide co-extraction strategy, three rigidified diglycolamides, namely 2,6-bis (N-dodecyl-carboxamide)-4-oxo-4H-pyran (1), 2,6-bis-[N-(4-tert-butylphenyl)carboxamide]-4-oxo-4H-pyran (2), 2,6-?bis[(N-docecyl-N-methyl)carboxamide]-?4-methoxy-?tetrahydro-pyran (3), were synthesized. Moreover, the effect of structural rigidification on Am(III) and Eu(III) extraction under different conditions was investigated. The carboxamide extractant 3 resembles the extracting behavior of N,N,N′,N′‐tetraoctyl diglycolamide (TODGA) in terms of efficiency and affinity within the lanthanide family, together with fast kinetics and satisfactory cation back-extraction. The presence of 1-octanol in the diluent mixture strongly affects the ligand stability. Moreover, despite the low extraction efficiency showed by 1 and 2, all the three ligands exhibit a higher affinity for Am with respect to TODGA, resulting in a lower lanthanide/Americium separation factor, of around 4 for ligand 3 and close to 1 for ligands 1 and 2.     

四乙基双三嗪吡啶与二酰胺荚醚在硝酸介质中对Am(Ⅲ)/Eu(Ⅲ)的协同萃取

以四乙基双三嗪吡啶(C2-BTP)和四种不同链长的N,N,N′,N′-四烷基-3-氧-戊二酰胺(酰胺荚醚:DGA)作为萃取剂、1,2-二氯乙烷作为稀释剂,在硝酸体系中研究了对Am(Ⅲ)和Eu(Ⅲ)的协同萃取行为。结果表明,长链DGA与C2-BTP具有良好的协萃效应,在水相酸度为1.0 mol/L、盐析剂浓度为1.0 mol/L时,C2-BTP与六个碳链DGA(C6-DGA)的混合萃取剂摩尔比为3∶1时,Am(Ⅲ)和Eu(Ⅲ)的分配比(D)比单独使用C2-BTP提高一倍以上,Am(Ⅲ)与Eu(Ⅲ)的分离因子(SF)最高值约为21。利用斜率法分别确定了各个DGA作为单一萃取剂萃取Am(Ⅲ)和Eu(Ⅲ)的平均配位数约为1.5,表明氯代溶剂可能与DGA的配位氧原子发生了相互作用,影响了DGA对Am(Ⅲ)和Eu(Ⅲ)的配位性能。     

Evaluation of the Hydrophilic Complexant N,N,N’,N’-tetraethyldiglycolamide (TEDGA) and its Methyl-substituted Analogues in the Selective Am(III) Separation

ABSTRACT

N,N,N’,N’-tetraethyldiglycolamide (TEDGA) is used in the French EXAm (extraction of americium) process to separate Am(III) from Cm(III) and Ln(III). In this study, the complexation behavior of TEDGA towards actinides(III) and lanthanides(III) was compared to its methyl-substituted derivatives Me-TEDGA and Me₂-TEDGA under experimental conditions applying to the EXAm process. Using the EXAm solvent, 0.6 mol/L N,N’-dimethyl-N,N’-dioctyl-hexylethoxymalonamide (DMDOHEMA) and 0.45 mol/L bis(2-ethylhexyl)-phosphoric acid (HDEHP), An(III) and Ln(III) distribution ratios increase in the order TEDGA < Me-TEDGA < Me₂-TEDGA. This is explained by differences in the strength of complexation in the aqueous phase: Conditional stability constants for the formation of [Cm(DGA)_x]³⁺ complexes decrease in the order TEDGA > Me-TEDGA > Me₂-TEDGA, as shown by time-resolved laser fluorescence spectroscopy (TRLFS). TRLFS measurements verified the exclusive existence of [Cm(DGA)₃]³⁺ complexes in the aqueous phase. Both the homoleptic [Cm(DMDOHEMA)_n]³⁺ and the heteroleptic [Cm(DGA)_x(DMDOHEMA)_y]³⁺ complexes were detected in the organic phase, as postulated in the literature.^[14]