dimethyl-N,N'-dioctylhexylethoxymalonamide Research Articles

Exploring the Aggregation Behavior of Extractant Molecules in Ionic Liquids: A Coupled Polarizable Molecular Dynamics and SAXS Study.

This study presents a comprehensive investigation of the aggregation behavior of a malonamide extractant molecule (N,N'-dimethyl,N,N'-dioctylhexylethoxymalonamide (DMDOHEMA)) in three different solvents, including two piperidinium- and (trifluoromethylsulfonyl)imide-based ionic liquids (1-ethyl-1-butylpiperidinium bis(trifluoromethylsulfonyl)imide ([EBPip+][NTf2-]) and 1-ethyl-1-octylpiperidinium bis(trifluoromethylsulfonyl)imide ([EOPip+][NTf2-])) and n-dodecane. By combining polarizable molecular dynamics simulations and small-angle X-ray scattering experiments, we extensively investigated the arrangement of supramolecular assemblies of the extractant molecules. Our results showed that the insertion of the alkyl chains of the extractant molecules into the apolar domain of [EOPip+][NTf2-] has a significant impact on the aggregation behavior of the extractant molecules, leading to the formation of smaller aggregates having a higher dispersion compared to other solvents. These findings provide new insights into the physicochemical properties of this type of system and are crucial in designing more effective solvents for rare earth metal extraction.

How Acidity Rules Synergism and Antagonism in Liquid–Liquid Extraction by Lipophilic Extractants—Part I: Determination of Nanostructures and Free Energies of Transfer

ABSTRACT Efficient formulations used in hydrometallurgy are often based on synergistic action of two oil-soluble extractants at an optimized mole fraction. In this series of two papers, we describe the N,N’-dimethyl-N,N’-dioctylhexylethoxymalonamide (DMDOHEMA)/di(2-ethylexyl) phosphoric acid (HDEHP) system, and how expression of the synergistic effect varies with acidity, using the three common representations: distribution ratios, yields, and free energies of transfer. The experimental results are detailed in this first part (Part I) with the so-called supramolecular approach, which considers the reverse micelle-like aggregates formed by the lipophilic extractant molecules. It shows that no specific aggregation state is observed at the maximum of synergism and antagonism. Besides, the critical aggregation concentration that characterizes the micellization free energy appears to follow some synergistic trends but cannot explain how acid tunes synergism to antagonism of this mixed system. Based on these results, Part II proposes a theoretical model based on the ienaic approach to interpret acid effect on synergism and antagonism.

Amphiphile Organization in Organic Solutions: An Alternative Explanation for Small-Angle X-ray Scattering Features in Malonamide/Alkane Mixtures.

The role of different intermolecular interactions in the aggregation of amphiphiles in an organic solvent is studied for systems of relevance to liquid-liquid extraction (LLE), a chemical process used to selectively recover metals from complex mixtures. Of specific interest is the role, or lack thereof, of hydrogen bonding, which is often assumed to be a main driver of the organic phase structural organization that has been linked to separation efficacy. Toward that end, a series of malonamide extractants in n-dodecane have been studied in the absence of any extracted aqueous solutes, including water. The series of extractants includes N,N'-dimethyl-N,N'-dibutyltetradecylmalonamide (DMDBTDMA), two of its homologs, and N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide (DMDOHEMA). This simplified model LLE system enables systematic investigation of the role of dipole-dipole and alkyl tail steric interactions in amphiphile aggregation. Small-angle X-ray scattering (SAXS) profiles computed from molecular dynamics trajectories are in good agreement with experimental SAXS data. Molecular dynamics simulations show that malonamide aggregation results from dipole-driven self-association and lacks characteristic aggregate sizes. Mid-q correlation peaks in the SAXS profiles emerge at high concentration for each malonamide. In those densely packed solutions, the correlation peaks are observed to result from alkyl tail-induced spacing between electron-rich polar head groups, with peak positions determined by the different alkyl tail lengths present in the malonamide molecule. This explanation of the SAXS correlation peaks contrasts with the prevailing literature, which attributes mesoscale features observed in small-angle scattering to the formation of microemulsions. Instead, this work finds that these features are present in the absence of water or any reverse micellar organization of the malonamides. As such, molecular-scale malonamide self-association and packing, rather than microemulsion-based colloidal-scale descriptions, is a more appropriate framework for these LLE systems.

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

ABSTRACTN,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]

Stoichiometry of An(iii)-DMDOHEMA complexes formed during solvent extraction.

N,N'-Dimethyl,N,N'-dioctylhexylethoxymalonamide (DMDOHEMA) is used to separate An(iii) and Ln(iii) from fission products in several liquid-liquid extraction processes that aim at recycling actinides. The stoichiometry of the extracted complexes is important for a complete understanding of the processes. The presented work focuses on the complexation of Cm(iii) with DMDOHEMA studied by TRLFS in mono- and biphasic (solvent extraction) systems. The formation of [Cm(DMDOHEMA)n]3+ (n = 1-3) in 1-octanol containing 1.7 mol L-1 of water with log β'1 = 2.6 ± 0.3, log β'2 = 4.0 ± 0.5, log β'3 = 4.3 ± 0.5 was confirmed. In addition, fluorescence lifetime measurements indicated the formation of a 1 : 4 complex. Furthermore, solvent extraction experiments were performed, varying the proton and nitrate concentrations. TRLFS measurements of organic phases confirmed the existence of two species, [Cm(DMDOHEMA)3(NO3)(H2O)1-2]2+ (dominant at high proton and nitrate concentrations) and [Cm(DMDOHEMA)4(H2O)]3+ (dominant at low proton and nitrate concentrations). To support the proposed stoichiometries, vibronic side-band spectroscopy (VSBS) was employed, allowing the observation of vibrations of functional groups coordinated to the probed metal ion. Clear differences between the vibronic side bands of the 1 : 3 and 1 : 4 complex in the range of 900-1300 cm-1 were observed. Vibrational spectra calculated by DFT complimented the experimental data and confirmed the proposed stoichiometries. They revealed a monodentate coordination mode of the nitrate and two water molecules in the 1 : 3 complex.

Understanding the synergistic effect on lanthanides(III) solvent extraction by systems combining a malonamide and a dialkyl phosphoric acid

To separate minor actinides from lanthanides for used nuclear fuel reprocessing, solvent extraction processes combining neutral and acidic extractants are considered. Mixed systems present singular extraction properties where synergistic effects can be observed but information on chemical equilibria is lacking to properly model such behavior. Speciation of Eu(III) and Nd(III) complexes formed in an organic phase combining N,N′-dimethyl-N,N′-dioctylhexylethoxy malonamide (DMDOHEMA) and di(2-ethylhexyl)phosphoric acid (HDEHP) was investigated through electrospray ionization mass spectrometry (ESI-MS), time-resolved laser-induced fluorescence spectroscopy (TRLIFS), UV–Visible, nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and density functional theory (DFT) calculations. A method consists of adding one of the ligands to a solution containing Ln(III) complexes formed with the other ligand while following changes by each technique was used. In both cases, significant modifications in the metal coordination sphere were observed due to formation of mixed complexes containing both ligands: addition of DMDOHEMA to a Ln-HDEHP solution led to the formation of Ln(DEHP)3(HDEHP)2(DMDOHEMA) complexes while addition of HDEHP to a Ln-DMDOHEMA solution gave Ln(NO3)3-n(DEHP)n(HDEHP)m(DMDOHEMA)x′ (0≤n≤3; n+m≥1 and 1≤x′≤3) complexes.

Thermodynamic Description of Synergy in Solvent Extraction: I. Enthalpy of Mixing at the Origin of Synergistic Aggregation.

Revisiting aggregation of extractant molecules into water-poor mixed reverse micelles, we propose in this paper to identify the thermodynamic origins of synergy in solvent extraction. Considering that synergistic extraction properties of a mixture of extractants is related to synergistic aggregation of this mixture, we identify here the elements at the origin of synergy by independently investigating the effect of water, acid, and extracted cations. Thermodynamic equations are proposed to describe synergistic aggregation in the peculiar case of synergistic solvent extraction by evaluating critical aggregation concentration (CAC) as well as specific interactions between extractants due to the presence of water, acid and cations. Distribution of two extractant molecules in the free extractants and in reverse micelles was assessed, leading to an estimation of the in-plane interaction parameter between extractants in the aggregates as introduced by Bergström and Eriksson ( Bergström, M.; Eriksson, J. C. A Theoretical Analysis of Synergistic Effects in Mixed Surfactant Systems . Langmuir 2000 , 16 , 7173 - 7181 ). Based on this model, we study the N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide (DMDOHEMA) and di(2-ethylexyl) phosphoric acid (HDEHP) mixture and show that adding nitric acid enhances synergistic aggregation at the equimolar ratio of the two extractants and that this configuration can be related to a favored enthalpy of mixing.

Behavior of Molybdenum (VI) in {DMDOHEMA–HDEHP/nitric acid} Liquid–Liquid Extraction Systems

ABSTRACTThe EXAm (extraction of americium) process was developed for americium recycling in future nuclear fuel cycles. In this solvent extraction system, a combination of two extractants, N,N′-dimethyl-N,N′-dioctyl-hexylethoxy malonamide (DMDOHEMA) and di-2-ethylhexyl phosphoric acid (HDEHP), in TPH (hydrogenated tetrapropylene) is used to extract americium in the first step of the process at high acidity (HNO3 5–6 M). Americium is co-extracted with light lanthanides and other fission products like molybdenum, iron, ruthenium, etc.. Molybdenum is selectively scrubbed during the second step at low acidity using citric or glycolic acid as a buffer and complexing agent. The speciation of Mo(VI) in aqueous solutions is highly dependent on acidity and Mo concentration. In this article, a simple thermodynamical model is proposed for Mo(VI) scrubbing based on batch extraction experiments (with pH and cation concentration variations) and stoichiometries of complexes formed in the organic phase according to electrospray ionization mass spectrometry (ESI-MS) experiments and published data on Mo(VI) speciation. At high acidity ([HNO3] > 1 M), the MoO22+ species is strongly extracted by the solvent DMDOHEMA–HDEHP according to a solvate mechanism. At lower acidity ([HNO3] < 1 M), cation exchange mechanisms become predominant and DMDOHEMA does not participate to the extraction Mo(VI) anymore. During Mo scrubbing at pH higher than 1, the extraction of Mo as neutral species (like MoO3) and anionic species (like MoO42–) has to be taken into account in the model to predict the “bell-shape” of Mo distribution ratio evolution as a function of pH. This model was then implemented in the PAREX simulation code developed by the CEA to build the flowsheet for the “Mo scrubbing” section of EXAm process and predict Mo concentrations profiles in batteries of mixer-settlers during pilot-scale tests.

Liquid-Liquid Extraction of Acids and Water by a Malonamide: I-Anion Specific Effects on the Polar Core Microstructure of the Aggregated Malonamide

In a solvent extraction process, the compositions of the phases in thermodynamic equilibrium (described as a Winsor-II regime) must be determined to obtain the extraction isotherms of ions as well as co-extracted water. By comparing the extractions of a series of acids by the malonamide DMDOHEMA (N,N’-dimethyl-N,N’-dioctyl hexylethoxy malonamide) in n-heptane, the specific anion effects regarding third phase formation and the strength of the acid-extractant interaction were investigated. It is shown that third phase formation is driven by hydration enthalpy of acid, while the polar core microstructure is controlled by the pKa of the acids. Upon acid extraction, the promotion of third phase formation follows the series H2SO4 ≈ H3PO4 ≈ HClO4 > HNO3 > HCl > HCOOH, which correlates to hydration enthalpy of acid in the case of monoacids. The combination of IR spectroscopy and DFT calculations revealed two different modes of acid extraction, either by hydrogen bonding (extraction of non-dissociated acid: HA) or by protonation of the extractant (extraction of dissociated acid: H+A−). The strength of the amide-acid interaction (protonation vs. hydrogen bonding) is correlated to the pKa of the acid and is responsible for the microstructure of the solution.

Palladium Extraction by a Malonamide Derivative (DMDOHEMA) from Nitrate Media: Extraction Behavior and Third Phase Characterization

Liquid-liquid extraction of palladium(II) from nitric media was carried out using, N,N’–dimethyl,N,N’-dioctylhexylethoxymalonamide (DMDOHEMA) in n-heptane. To this purpose, various experimental parameters such as reaction time, extractant concentration, pH, and nitrate concentration were investigated in detail. Efficient extraction of palladium can then be achieved, with good distribution coefficients (D up to 10) and performing kinetics (equilibration time ca. 30 min). In some cases, a solid phase appears at the interface between aqueous and organic layers. It was characterized as a palladium(II) complex with DMDOHEMA with appropriate techniques, and the conditions of its formation are discussed.

Development of a New Flowsheet for Co-Separating the Transuranic Actinides: The “EURO-GANEX” Process

A flowsheet for a novel GANEX (Grouped ActiNide EXtraction) process has been tested in a spiked flowsheet trial in a 32 stage plutonium-active centrifugal contactor rig with a simulant feed that contained 10 g/L plutonium as well as some fission products and other transuranic actinides. The solvent system used was a combination of 0.2 mol/L N,N,N’,N’-tetraoctyl diglycolamide (TODGA) and 0.5 mol/L N,N’-(dimethyl-N,N’-dioctylhexylethoxy-malonamide (DMDOHEMA) in a kerosene diluent that co-extracted actinides and lanthanides. Actinides were subsequently selectively co-stripped away from the lanthanides using a sulphonated and, therefore, hydrophilic bis-triazinyl pyridine (BTP) complexant in conjunction with acetohydroxamic acid (AHA). Plutonium and americium recoveries were high with decontamination factors across the strip contactors of ˜14,000 and ˜390, respectively. However, approximately 30% of neptunium was lost to the aqueous raffinate which was due to recycling within the first extract-scrub section causing a large build-up of neptunium. Some accumulation of strontium was also observed but in this case it was fully directed to the raffinate stream. In the stripping section, a small fraction of europium (taken as a model lanthanide ion), ca. 7%, was found in the actinide product stream. Modelling of selected data using the PAREX code has shown that even with a relatively simplistic treatment, reasonable agreement between modelling and experiment can be obtained, giving confidence in the use of modelling to refine the GANEX flowsheet design prior to further testing with irradiated fast reactor fuel.

Periodic Behavior of Lanthanide Coordination within Reverse Micelles

Trends in lanthanide(III) (Ln(III)) coordination were investigated within nanoconfined solvation environments. Ln(III) ions were incorporated into the cores of reverse micelles (RMs) formed with malonamide amphiphiles in n-heptane by contact with aqueous phases containing nitrate and Ln(III); both insert into pre-organized RM units built up of DMDOHEMA (N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide) that are either relatively large and hydrated or small and dry, depending on whether the organic phase is acidic or neutral, respectively. Structural aspects of the Ln(III) complex formation and the RM morphology were obtained by use of XAS (X-ray absorption spectroscopy) and SAXS (small-angle X-ray scattering). The Ln(III) coordination environments were determined through use of L(3)-edge XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure), which provide metrical insights into the chemistry across the period. Hydration numbers for the Eu species were measured using TRLIFS (time-resolved laser-induced fluorescence spectroscopy). The picture that emerges from a system-wide perspective of the Ln-O interatomic distances and number of coordinating oxygen atoms for the extracted complexes of Ln(III) in the first half of the series (i.e., Nd, Eu) is that they are different from those in the second half of the series (i.e., Tb, Yb): the number of coordinating oxygen atoms decrease from 9O for early lanthanides to 8O for the late ones--a trend that is consistent with the effect of the lanthanide contraction. The environment within the RM, altered by either the presence or absence of acid, also had a pronounced influence on the nitrate coordination mode; for example, the larger, more hydrated, acidic RM core favors monodentate coordination, whereas the small, dry, neutral core favors bidentate coordination to Ln(III). These findings show that the coordination chemistry of lanthanides within nanoconfined environments is neither equivalent to the solid nor bulk solution behaviors. Herein we address atomic- and mesoscale phenomena in the under-explored field of lanthanide coordination and periodic behavior within RMs, providing a consilience of fundamental insights into the chemistry of growing importance in technologies as diverse as nanosynthesis and separations science.

Solid-phase extractants based on taunit carbon nanotubes for actinide and REE preconcentration from nitric acid solutions

The sorption properties of solid-phase extractants (SPEs) prepared by impregnation of Taunit carbon nanotubes with adducts of diphenyl(dibutylcarbamoylmethyl)phosphine oxide (CMPO) and tri-n-octylphosphine oxide (TOPO) in HNO3 solutions were studied. The SPEs exhibit high ability to sorb U(VI), Pu(IV), Np(V), Am(III), and Eu(III) from nitric acid solutions, with good kinetic properties. The impregnation conditions and distribution coefficients of the radionuclides in their recovery from 3 M HNO3 were determined. The possibility of preparing SPEs by Taunit impregnation in HNO3 solutions with adducts of tributyl phosphate (TBP) and N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA) and with Cyphos IL-101 phosphonium ionic liquid was demonstrated.

Aqueous Partitioning of Minor Actinides by Different Processes

Actinide partitioning is a proposed strategy for effective mitigation of the long-term hazards associated with high-level waste (HLW). Octyl-(phenyl)–N,N-diisobutyl carbamoyl methyl phosphine oxide (CMPO) and diphenyl–N,N-diisobutyl carbamoyl methyl phosphine oxide (DφCMPO) are amongst the promising extractants extensively studied since the 1980s for actinide partitioning from wastes of different origin. During the last two decades, substituted malonamide extractants such as N,N'-dimethyl-N,N'-dibutyl tetradecyl malonamide (DMDBTDMA) and N,N'-dimethyl-N,N'-dioctyl hexylethoxy malonamide (DMDOHEMA) have emerged as viable green alternatives to phosphine oxides. During the last decade, diglycolamide-based extractants such as N,N,N′,N′-tetraoctyl diglycolamide (TODGA) and N,N,N′,N′-tetra-2-ethylhexyl diglycolamide (TEHDGA) have received considerable attention due to overwhelmingly favourable extraction and stripping efficiencies of minor actinides from different types of transuranium (TRU) wastes. The focus of the present review is to carry out comparative evaluation of the key physical and chemical properties of these extractants for hydrometallurgical applications. Merits of flow sheets proposed for the separation and recovery of minor actinides from high-level wastes have also been discussed.

Aggregation in Organic Solutions of Malonamides: Consequences for Water Extraction

The molecular organization of N,N′‐dimethyl‐N,N′‐dioctylhexylethoxymalonamide (DMDOHEMA), the current reference extractant for the DIAMEX (DIAMide EXtraction) process, is correlated with its water extraction properties from neutral media. The aggregation of DMDOHEMA in n‐heptane was investigated by vapor pressure osmometry (VPO) and the aggregate speciation characterized by combined small‐angle neutron and X‐ray scattering (SANS and SAXS, respectively). Two approaches were taken to model the aggregation of the diamide and the water extraction as a function of the diamide concentration by taking into account a single aggregation equilibrium with an average aggregation number N equal to 4.28 ± 0.05; and a competition between two types of aggregates in the organic phase, namely, aggregates of the reverse micelle type with 4 diamides per aggregate, and an oligomeric structure composed of about 10 diamide molecules which appears at high extractant concentration (>1 mol/L). In both cases, the supramolecular speciation representing the monomers/aggregates distribution was determined, and for each supramolecular organization, a solubilization parameter was calculated using the Sergievskii‐Dannus relationship. Thus, the correlation between the two types of micellization of the diamide and the extraction of water into the organic phase was demonstrated. The larger aggregates can extract about five times more water than monomers.

Aggregation in Solvent Extraction Systems Containing a Malonamide, a Dialkylphosphoric Acid and their Mixtures

Aggregation phenomena in n-alkane solutions of di-n-hexylphosphoric acid (HDHP), N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA), and their mixtures, were investigated by electrospray ionization–mass spectrometry (ESI-MS), vapor pressure osmometry (VPO), and small-angle X-ray and neutron scattering (SAXS and SANS). The objective of the study was to probe the formation of mixed HDHP-DMDOHEMA species before and after extraction of trivalent lanthanide and actinide (M3+) nitrates. The most important species formed by HDHP upon metal extraction has the composition M(DHP)3(HDHP)3(H2O). These species exist as spherical aggregates of the reverse micelle type with a polar core diameter of ∼ 7 Å and total diameter of ∼ 11 to ∼ 15 Å. The aggregation of DMDOHEMA is a progressive phenomenon, with an average aggregation number of ∼ 2 in the 0.2 to 0.6 M range and larger aggregates forming at higher concentrations. The metal loaded DMDOHEMA aggregates can be considered as interacting spheres with a polar core diameter between ∼ 11 and ∼ 16 Å, depending on composition, a total diameter of up to ∼ 25 Å, and a weight-average aggregation number of ∼ 9. The results obtained in this work provide strong evidence for the formation of mixed aggregates when mixtures of HDHP and DMDOHEMA are used for the extraction of trivalent Ln and An cations. These mixed reverse micelles have a diameter between 19 and 24 Å with a polar core diameter of 10 and to 14 Å. The most recurrent micellar composition is 2 HDHP and either 4 or 5 DMDOHEMA molecules.

DIAMEX Counter‐Current Extraction Process for Recovery of Trivalent Actinides from Simulated High Active Concentrate

The partitioning of trivalent actinides was demonstrated with a new version of the French DIAMEX (DIAMide EXtraction) process. A continuous counter‐current experiment using a 16‐stage centrifugal extractor battery was tested using 1 mol/L N,N′-dimethyl‐N,N′-dioctyl‐hexylethoxy‐malonamide (DMDOHEMA) in TPH as the extractant. A high active concentrate (HAC), obtained after concentration and denitration of a high active raffinate (HAR) with a concentration factor of 10, was used as a feed. Based on results from cold and hot batch extraction experiments and computer code calculations, a flowsheet was developed and a full test was carried out using a simulated HAC solution spiked with radionuclides (241Am, 244Cm, 152Eu, and 134Cs). In the DIAMEX process, five extraction stages were sufficient to obtain Am and Cm (feed/raffinate) greater than 5000 and for the coextracted lanthanides decontamination factors between 1100 and 4500. Co‐extraction of zirconium, molybdenum, and palladium was prevented by using oxalic acid and HEDTA. The back extraction comprising 4 stages was also efficient and the recoveries of actinides were greater than 99.8%, which can be further improved by a minor process flowsheet optimisation. The experimental steady‐state concentration profiles of important solutes were determined and compared with model calculations and good agreement was generally obtained.

Structural study of trivalent lanthanide and actinide complexes formed upon solvent extraction

The coordination of the trivalent 4f ions, Ln = Nd3+, Eu3+ and Yb3+, as well as the trivalent 5f ion, Am3+, with diamide and dialkylphosphoric acid extractants, individually and in combination, was studied by use of X-ray absorption spectroscopy. These studies provide metrical information about the interatomic interactions between the f-ions (M3+) and the ligands, dihexylphosphoric acid (HDHP) and N,N'-dimethyl-N,N'-dioctylhexylethoxymalonamide (DMDOHEMA), that is of practical relevance to the control of metal-ligand binding in liquid-liquid extraction systems for the separation of trivalent actinide ions, An3+, from trivalent lanthanide ions, Ln3+. Through systematic variations of extraction conditions and extractant combinations, we have found that the HDHP complexes with M3+ involve MO6 coordination and distant M...P interactions, whereas the DMDOHEMA complexes with M3+ involve MO8 coordination. The combination of the EXAFS results with ancillary extraction data and IR results facilitates descriptions of the stoichiometries and structures of the molecular species formed in solution upon liquid-liquid extraction and leads to a new understanding of the binary extraction systems in terms of the strength and selectivity of An3+- vs. Ln3+-ligand interactions. This fundamental structure information affords insight into solvent extraction processes that are of contemporary and practical importance in heavy element chemistry and to environmentally related issues arising from the separation and disposal of radioactive materials, particularly actinides and selected fission products, in the field of nuclear waste reprocessing research.

Recovery of minor actinides from HLLW using the DIAMEX process

Summary In this work a hot demonstration of the DIAMEX process using the new reference molecule DMDOHEMA (N,N′-DiMethyl-N,N′-DiOctylHexylEthoxyMalonAmide) and genuine high-level PUREX raffinate as feed is reported. The continuous counter-current experiment was carried out in a 16-stage centrifugal extractor battery, installed in a hot cell. In order to produce a representative High Level Liquid Waste (HLLW) a PUREX process was applied on dissolved fuel using the same equipment. It was demonstrated that 5 extraction stages were sufficient to achieve feed decontamination factors above 2200 and 320 for Am and Cm, respectively. Co-extraction of molybdenum and zirconium, as well as of palladium, were efficiently prevented using oxalic acid and HEDTA scrubbing, respectively. However, technetium, yttrium, and to some extent ruthenium, were co-extracted. The back-extraction proved to be very efficient, yielding in 4 stages more than 99.9% recovery of Am and Cm. The extraction profiles were modeled with computer code simulations and the results compared with data obtained from an experiment using the former reference molecule DMDBTDMA (DiMethylDiButylTetraDecylMalonAmide).

Americium(III)/curium(III) separation by countercurrent chromatography using malonamide extractants

Abstract The separation of Am(III) and Cm(III) by countercurrent chromatography (CCC) was achieved using the liquid phase systems "diamide–hydrogenated tetrapropylene (TPH)–HNO3" following the optimisation of: (i) the compositions of the mobile (HNO3 concentration) and the stationary (nature of the diamide and its concentration in TPH) phases, (ii) column length and (iii) operating parameters of the CCC (flow rate of the mobile phase and rotation speed of the coil column). The following diamide extractants have been studied: (i) N,N´-dimethyl-N,N´-dibutyltetradecylmalonamide (DMDBTDMA), (ii) N,N´-dimethyl-N,N´-dioctylhexyl-ethoxymalonamide (DMDOHEMA) and (iii) N,N´-dimethyl-N,N´-dibutyldodecylethoxymalonamide (DMDBDDEMA). It is shown that these diamides can be used for the separation of Am(III) and Cm(III) by CCC. Increasing the column length leads to an increase of the stationary phase retention on the column while improving the Am/Cm separation. Increasing the speed of rotation of the centrifuge from 660 to 950 rpm also results in increasing the stationary phase retention but does not influence the resolution of the Am/Cm separation. Decreasing the flow rate of the mobile phase from 1.0 to 0.5 mL/min leads to a better resolution of Am and Cm separation. The best Am/Cm separation was achieved with systems based on DMDBDDEMA and DMDOHEMA in TPH using a two-layer coil column and an isocratic elution mode. The application of CCC makes it possible to separate the elements within 100 min: the Cm fraction contains 99.5% of Cm(III) and 0.6% of Am(III) inventories and the Am fraction contains 99.4% of Am(III) and 0.5% of Cm(III).