Trivalent Actinides Research Articles

Solvent structural effects on the solubility of bis(2-ethylhexyl)phosphoric acid (HDEHP) in room-temperature ionic liquids

ABSTRACT Efforts to improve the TALSPEAK (Trivalent Actinide Lanthanide Separation with Phosphorus-Reagent Extraction from Aqueous Komplexes) Process have considered, among other options, the replacement of the organic solvent employed with a room-temperature ionic liquid (RTIL). The solubility of certain extractants in RTILs has been shown to be limited, however, calling into question their potential as replacement solvents. A systematic investigation of the solubility of bis(2-ethylhexyl)phosphoric acid (HDEHP) in a series of pyridinium- and imidazolium-based ionic liquids has therefore been undertaken, with the objective of relating the extractant solubility to structural characteristics of the RTIL. HDEHP solubility is shown to increase with solvent molar volume, with both larger IL cations and anions favoring dissolution.

Theoretical Insights into Modification of Nitrogen-Donor Ligands to Improve Performance on Am(III)/Eu(III) Separation.

Nitrogen-donor ligands have been considered to be promising agents for separating trivalent actinides (An(III)) from lanthanides (Ln(III)). Thereinto, how to decorate these ligands for better extraction performance is urgent to design "perfect" separating extractants. In this work, we systematically explored a series of heterocyclic N-donor ligands (L1 = dipyridazino[4,3-c:3',4'-h]acridine, L2 = dipyridazino[3,4-a:4',3'-j]phenazine, L3 = 2,6-di(cinnolin-3-yl)pyridine)), as well as their substituted derivatives, and compared their extraction and complexation ability toward An(III) and Ln(III) ions by using quasi-relativistic density functional theory (DFT). We found that the pyridazine N atoms probably play a notable role in electron donation to metal cations by molecular orbital (MO) and bond order analyses. Besides, the calculated results clearly verified that these N-donor ligands possess higher coordination affinity toward Am(III) over Eu(III). The rigid ligands (L1 and L2) exhibit higher selective abilities for the Am(III)/Eu(III) separation compared with that of the flexible ligand (L3). For each ligand, the 1:2 (metal/ligand) extraction reaction is predicted to be most probable in the separation process. The introduction of an alkyl group on the lateral chain or an electron-donating group on the main chain gives rise to a better extraction performance of the ligands, and the CyMe4 or MeO substituted ligands show higher extraction and separation ability. Simultaneous introduction of CyMe4 and MeO groups can enhance the extraction ability of the ligand to metal ions, but the separating ability depends on the differences of the extraction capacity of An(III) and Ln(III). This work can help to gain a more in-depth understanding the selectivity differences of similar N-donor ligands and provide more theoretical insights into the design of novel extractants for An(III)/Ln(III) separation.

Gamma Radiolysis of TODGA and CyMe4BTPhen in the Ionic Liquid Tri-n-Octylmethylammonium Nitrate

ABSTRACTRadiolytic stability of the extractants N,N,N’,N’-tetraoctyl diglycolamide (TODGA) and 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4BTPhen) in the ionic liquid tri-n-octylmethylammonium nitrate ([N1888][NO3]) was studied via the combination of a steady-state irradiation by cobalt-60 gamma-radiation, followed by solvent extraction and identification of radiolysis products by High-Performance Liquid Chromatography coupled to Electro Spray Ionisation Mass Spectrometry (HPLC-ESI-MS). The observed trend for the distribution ratios of trivalent minor actinides and Eu(III) showed a decrease as a function of absorbed dose in the case of the TODGA solvent. The gamma irradiations were conducted in the presence or absence of an acidic aqueous phase. The determined dose constants for TODGA under neutral or acidic conditions were up to four times lower than the values reported in aliphatic diluents under comparable conditions. The degradation patterns were similar to those found by former studies on TODGA diluted in n-dodecane; only de-alkylation and bond ruptures within the diglycolamide core were observed. The presence of an acidic aqueous phase during irradiation resulted in a different relative abundance of certain radiolysis products of both the diluent and TODGA and a higher dose constant compared to the neutral irradiation conditions. A quantification of CyMe4BTPhen in irradiated samples with the [N1888][NO3] diluent was not possible, but the effect on its extraction properties was evaluated. The distribution ratios slightly increased under the effect of absorbed dose, indicating the formation of radiolysis products that are efficient extractants as well.

Interference of Zr(IV) during the extraction of trivalent Nd(III) from the aqueous waste generated from metallic fuel reprocessing

Abstract A metallic alloy of uranium–zirconium and uranium–plutonium–zirconium has been proposed as a fuel for fast reactors, owing to the possibility of achieving high breeding ratio in a short span of time. About 6–10 wt.% of zirconium has been added to these actinide fuels to increase the melting temperature and thermal-mechanical stability. Aqueous reprocessing of the spent metallic fuel generates the high-level liquid waste (HLLW) that contains about 60 % of the total zirconium from the fuel. In view of this, the extraction behavior of a trivalent representative ion, Nd(III) in the presence of Zr(IV) was studied from nitric acid medium using the candidate ligands proposed for trivalent actinide separation from HLLW, such as N,N,N′N′-tetraoctyldiglycolamide (TODGA), and N,N-di-octyl-2-hydroxyacetamide (DOHyA). The extraction was studied as a function of nitric acid concentration, zirconium and neodymium concentration and Nd(III) to Zr(IV) ratio. The findings of dynamic light scattering (DLS) and ATR-FTIR spectral techniques were used for understanding the complex chemistry of Zr(IV) extraction under different conditions. Poor extraction of nitric acid, smaller aggregate size, no third phase formation during the extraction of Zr(IV) and Nd(III) and other unique solvent properties favor the DOHyA molecule in n-dodecane as a solvent for partitioning of trivalent actinides from HLLW generated from metallic fuel reprocessing.

Solvent extraction of An(III) and Ln(III) using TODGA in aromatic diluents to suppress third phase formation

The extraction of nitric acid, trivalent actinides and lanthanides with N,N,N′,N′-tetra-n-octyl-3-oxapentanediamide (TODGA) dissolved in the aromatic diluents 1,4-diisopropylbenezene and tert-butylbenzene was studied. Nitric acid extraction proceeds along the formation of 1:1 and 2:1 HNO3-TODGA adducts. An(III) and Ln(III) distribution ratios were determined as a function of nitric acid, TODGA and Ln(III) concentrations. Distribution ratios are approximately an order of magnitude lower in aromatic diluents compared to aliphatic diluents. No third phase formation was observed when contacting the solvent with an aqueous phase containing 1.5 mol/L Er(NO3)3 in 2–4.3 mol/L HNO3. Slope analysis indicates the formation of 1:3 and 1:4 M(III):TODGA complexes. Using TRLFS, the formation of a previously unknown 1:2 complex in organic phases loaded with Ln(III) was observed for the first time.

Highly Efficient Extraction of Trivalent f‐Cations Using Several N‐Pivot Tripodal Diglycolamide Ligands in an Ionic Liquid: The Role of Ligand Structure on Metal Ion Complexation

Complexation of trivalent lanthanides and actinides was studied in a room temperature ionic liquid (RTIL) using five N‐pivot tripodal diglycolamide ligands (LI–LV) having different structural features, viz. spacer lengths and substituents. The nature of the metal/Ligand complex changed with the substitution of N atom near the tripodal platform from ML with LIII and LIV to ML2 with LI, LII and LV (where M = Eu3+/Nd3+/Am3+). With 0.2 mmol/L ligands, the distribution ratio of Am3+ at 1 m HNO3 followed the order: LI (210) > LII (42) > LIII (2.1) > LV (0.7) > LIV (0.2). The formation constants between Nd3+ and the ligands (log βML,ML2) followed an identical order, viz. LI > LII > LIII > LV > LIV. The extraction of Am3+ or Eu3+ with all the ligands followed a cation‐exchange extraction mechanism, which is in sharp contrast to the results obtained in a molecular solvent (n‐dodecane), where a solvation extraction mechanism prevalied. Luminescence studies on the Eu3+ extract in RTIL confimed the absence of water molecules in the inner coordination sphere of the complex. EXAFS studies indicated that all three DGA arms were binding to the Eu3+ cation.

The structure and composition of solid complexes comprising of Nd(III), Ca(II) and D-gluconate isolated from solutions relevant to radioactive waste disposal

Abstract Certain complexing agents (such as D-gluconate, D-isosaccharinate, etc.) as well as actinides and lanthanides are simultaneously present in cementitious radioactive waste repositories and (in the presence of water) are capable of forming complex compounds. Such processes may immobilize radionuclides and are of importance in the thermodynamic modelling of the aqueous chemistry of waste repositories. Nd(III) is considered to be a suitable model for trivalent lanthanides and actinides, due to the similarity of their ionic radii. In the current work, solid complexes isolated from aqueous solution containing Nd(III), Ca(II) and D-gluconate (Gluc−) were investigated. In an aqueous solution containing Nd(III) and Gluc−, the formation of a precipitate was observed at pH ≥ 8. This precipitate was found to redissolve around pH ~ 11, but reprecipitated when Ca(II) ions were added to the solution. In order to gain an insight in binary and ternary aqueous systems, in the present work we report the structure of these solid complexes obtained from XRD, FT-IR, Raman, SEM-EDAX and UV-DRS measurements. The structure of these solids, where possible, was compared with those identified in solution. The compositions of these complexes are suggested to be NdGlucH−1(OH) · 2H2O and CaNdGlucH−1(OH)3 · 2H2O, respectively. In these, the chemical environment of the Nd(III) was found to be the same as that in the NdGlucH−1(OH)0(aq) solution species.

Theoretical insights into selective separation of trivalent actinide and lanthanide by ester and amide ligands based on phenanthroline skeleton.

Phenanthroline based ligands have shown potential performance for partitioning trivalent actinides from lanthanides. In this work, we have explored four ester and amide ligands based on the phenanthroline skeleton and elucidated the separation mechanism between Am(iii) and Eu(iii) ions. The molecular geometries and extraction reactions of the metal-ligand complexes were modeled by using scalar-relativistic density functional theory. The results show that the amide based ligands have stronger coordination ability with the metal ions than the corresponding ester based ligands. According to the thermodynamic results, ligands N,N'-diethyl-N,N'-ditolyl-2,9-diamide-1,10-phenanthroline (L2) and N,N'-(1,10-phenanthroline-2,9-diyl)bis(N-ethyl-P-methyl-N-(p-tolyl)phosphinic amide) (L4) appear to have the strongest complexing ability, which is supported by the result of electrostatic potential (ESP) and the M-OL bond orders. Moreover, ligand L2 has excellent selectivity for Am(iii)/Eu(iii) among the four ligands. Additionally, the bonding properties between the metal ions and the ligands reveal that the Am(iii)/Eu(iii) selectivity stems from the Am-N bonds with more covalent character, which is supported by the analysis of the hardness of the ligands and the bond orders. This work provides useful information for understanding the Am(iii)/Eu(iii) selectivity of phenanthroline derived ligands bearing ester and amide groups.

Thiocyanate complexes of the lanthanides, Am and Cm.

The synthesis and single crystal structures of Am(iii) and Cm(iii) thiocyanate complexes are reported along with an isostructural series of lanthanide thiocyanate complexes. Because the complexes are isostructural, a comparative study of the metal-thiocyanate bonds was conducted using Raman spectroscopy indicating very similar and electrostatic metal-ligand interactions.

Structure and chemical durability studies of powellite ceramics Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) for radioactive waste storage

Powellite ceramic represents a waste form matrix material to immobilize minor actinides and Mo from reprocessed UMo nuclear fuel. In this paper, the Ca1−xLix/2Gdx/2MoO4 (0 ≤ x ≤ 1) series is prepared by solid-state reaction using Gd3+ as trivalent minor actinide (Cm3+) surrogate, and the structure/microstructure is characterized by XRD, HRTEM, Raman spectroscopy and SEM. Rietveld refinements show that the couple (Gd3+ and Li+) enters into the eightfold coordinated Ca site of the powellite structure. With the increase in the contents of Gd and Li, Raman bands broaden due to the distortion of MoO4 tetrahedra and disordered arrangements of Gd3+ and Li+. The chemical durability analyzed by the PCT-B indicates that the leaching behaviors of Gd and Mo are related to the interfacial dissolution–reprecipitation mechanism. For the Ca0.5Li0.25Gd0.25MoO4 ceramic, 7-day NLGd and NLMo are shown in the order of ~ 10−4 and ~ 10−4 g m−2, respectively. Thus, our initial results of the structure and chemical durability will provide insights to design new single- or multiphase waste forms for the Mo-rich HLW conditioning.

Complexation of trivalent lanthanides and actinides with diethylenetriaminepentaacetic acid: Theoretical unraveling of bond covalency

Separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) is a pivotal step with respect to the treatment of nuclear waste based on the partitioning and transmutation (P&T) strategy, which is also extraordinarily challenging due to their similar coordination chemistry. The diethylenetriaminepentaacetate (DTPA) ligand has been demonstrated to possess selective separation ability toward An(III) over Ln(III) in aqueous media. Nevertheless, the extracted complexes of An(III)/Ln(III) with DTPA, and the origin of selectivity for An(III) are still not well deciphered. In this work, from theoretical perspective and at the molecular level, the geometrical structures, bonding nature as well as thermodynamic behaviors of possible complexes of An(III) (An = Am, Cm, Cf) and Ln(III) (Ln = Nd, Eu) with DTPA in the aqueous phase have been systematically studied using scalar relativistic density functional theory (DFT). All bonding analyses indicate that the metal-ligand bonds possess weak but non-negligible covalent interactions, and An(III)-DTPA species demonstrate more covalency compared to Ln(III) analogues. In addition, the covalency of the metal-ligand bonding of An(III)-DTPA increases across the actinide series from Am to Cf, which stems from the increased orbital degeneracy of the 5f orbitals of actinides and the 2p orbitals of the ligands. According to thermodynamic analysis, the anhydrous species [Cf(HDTPA)]− is the most likely species for Cf(III) in acidic solution, whereas [M(HDTPA)(H2O)]− are more favorable for Am(III) and Cm(III), probably due to the different bonding nature of the transplutonium series. This work affords new theoretical insights into the coordination chemistry of An(III)/Ln(III)-DTPA complexes, and paves the way to further design efficient DTPA-based ligands for An(III)/Ln(III) separation. Moreover, the DTPA ligand may also be applied for in-group separation of trivalent actinides, which is quite meaningful for the extraction of Cf(III).

Role of Phase Modifiers in Controlling the Third-phase Formation During the Solvent Extraction of Trivalent Actinides

ABSTRACTDiglycolamides have been proposed for partitioning of trivalent actinides from high-level liquid waste (HLLW). Third-phase formation is an undesirable event during the course of solvent extraction of trivalent actinides from HLLW into the solution of N,N,N’,N’-tetra(2-ethylhexyl)diglycolamide (TEHDGA) in n-dodecane (n-DD). Polar reagents such as tri-n-butyl phosphate (TBP), and N,N-dihexyloctanamide (DHOA) have been added to the TEHDGA phase, as phase modifiers in significant concentration, to overcome the third-phase formation. To understand the role of these phase modifiers in controlling the third-phase formation, the extraction behaviour of nitric acid and the trivalent representative metal ion Nd(III) was studied in a binary solution containing TEHDGA and phase modifier in n-dodecane. The organic phase obtained after extraction was subjected to dynamic light-scattering studies to examine the aggregation behaviour of the reverse micelles formed upon extraction and to unravel the unique role of TBP and DHOA in controlling the third-phase formation. The study revealed that the addition of these modifiers brought down the average size of aggregates and their distribution in organic phase below the limiting aggregate size for third-phase formation and increased the dispersion of aggregates in the n-dodecane phase. Among the two phase modifiers proposed for trivalent actinide separation from HLLW, TBP has been identified as a promising reagent for minimizing the third-phase formation.

Unraveling the role of phase modifiers in the extraction of Nd(III) from nitric acid medium in tetra-bis(2-ethylhexyl)diglycolamide in n-dodecane containing long chain aliphatic alcohols

Tetra-bis(2-ethylhexyl)diglycolamide (TEHDGA) alone in n-dodecane (n-DD) is unsuitable for the solvent extraction of trivalent actinides from high-level liquid waste due to the occurrence of third phase formation during the course of solvent extraction. Significant concentration of long chain aliphatic alcohols ranging from 5% to 15% (V/V) have been added to the solvent phase, TEHDGA/n-DD, to control the undesirable third phase formation. The alcohols investigated were n-octanol, n-decanol, and isodecanol. To understand the role of alcohols in controlling the third phase formation, the extraction behaviour of the trivalent metal ion, Nd(III), from nitric acid medium was studied in a solution of 0.2 M TEHDGA + 1 M alcohol in n-DD. The equilibrium concentration of Nd(III) and nitric acid present in organic and aqueous phases were determined. The organic phase obtained after extraction was subjected to dynamic light scattering studies to unravel the role of alcohol phase modifiers in organic phase. The aggregate size and their distribution in organic phase was determined as a function of various parameters such as concentrations of nitric acid, Nd(NO3)3 and the nature of alcohol. The results revealed that the presence of long chain alkyl aliphatic alcohols interact with the polar acid-solvates and metal-solvates by solvation and prevents these solvates undergoing aggregation in organic phase. In view of this, the aggregate size was controlled much below the limiting aggregate size required for third phase formation by these alcohol phase modifiers. Even though all the alcohols investigated in the present study could bring down the aggregate size much below the limiting value, n-decanol was found to be superior to other alcohols.

Improvement in flow-sheet of extraction chromatography for trivalent minor actinides recovery

Extraction chromatography flow-sheet employing octyl(phenyl)-N,N-diisobutylcarbonoylmethylphosphine oxide and bis(2-ethylhexyl) hydrogen phosphate extractants for trivalent minor actinide recovery was modified to improve column separation performance. Excellent trivalent minor actinides recovery performance was obtained by column separation experiments on nitric acid solution containing the trivalent minor actinides and representative fission product elements, i.e. recovery yields > 93% with sufficient decontamination factors against the fission products. Those are the best performance which we have ever obtained by experiments inside hot cell.

Fabrication of a novel porous silica-based extraction resin and application in the adsorption of trivalent actinides

Separating long-lived minor actinides is an important subject in the field of radioactive waste disposal and radioactive analysis. The simple, convenient, effective separation of adjacent trivalent actinides with very similar chemical properties, Am(Ⅲ) and Cm(Ⅲ), represents a very difficult challenge. Here we report a novel porous silica-based extraction resin embedded with bis(chlorophenyl)dithiophosphinic acid (BCPDTPA) as extractants, BCPDTPA/SiO2- polymer. The multiporous silica particles with a high adsorption capacity were particularly prepared. The physicochemical properties of the BCPDTPA/SiO2-P resin materials (i.e., morphology, porous characteristics, and chemical structure) were comparatively investigated with pure SiO2 particles. The stability experiments in acidic and irradiation environment were also performed, respectively. The adsorption characteristics of BCPDTPA/SiO2-P resins for Am(Ⅲ) and Cm(Ⅲ) were mainly discussed. The results demonstrated that the BCPDTPA/SiO2-P resins fabricated in this work showed a high adsorption capacity, structural stability, and excellent adsorption characteristics for Am(Ⅲ) and Cm(Ⅲ). It may thus have great potential to develop the as-assembled silica-based materials as a novel, highly effective extraction resins for the separation of actinides(Ⅲ)/lanthanides(Ⅲ), especially for those elements with very similar chemical properties, such as Am(Ⅲ) and Cm(Ⅲ).

A Mathematical Model of Reduction Potentials of Lanthanide Alloys Correlation with Atomic Radius and Electronegativity Based on the Hume-Rothery Rules

To achieve closed nuclear fuel cycle and sustainable development of nuclear energy, efficient pyroprocessing of spent fuel becomes one of the major concerns in Generation IV nuclear energy [1]. Molten salt based electrolysis is a promising method in future pyroprocessing technology. As neutron poison, lanthanides are the typical detrimental elements that encumber the realization of the so-called partitioning & transmutation (P&T) strategy, and thus must be separated from actinides [2]. Therefore, investigating the electrochemical properties of actinides and lanthanides is of pivotal importance. In fact, in light of the chemical similarity of trivalent actinides and lanthanides, the chemistry of actinides can be revealed somewhat through exploring the lanthanide elements which are more readily obtained [3, 4]. In this work, it was found that the reduction potentials of Cu(Ni, Al, Zn)-rich Cu(Ni, Al, Zn)-Ln alloys manifest an evident “Bimodal Effect”. Hume-Rothery rules pointed out that the main factors affecting the formation of alloy solid solution of two metal elements are: 1) the difference of the atomic radius of the solvent and the solute metals, and 2) the electronegativity difference between the solvent and the solute metals. By fitting the data of reduction potentials, differences of the atomic radii and electronegativity differences, a formula was built up, which provides a useful and reliable approach for predicting related important reduction potentials of actinide alloys. The result of multiple linear regression analysis (Fig. 1) indicated that the good reliability of forecast and goodness of fit. Acknowledgments This paper is funded by the International Exchange Program of Harbin Engineering University for Innovation-oriented Talents Cultivation.

Dynamic and Solvation Behaviors of ALSEP Organic Ligands.

The Actinide-Lanthanide Separation Process (ALSEP) is a solvent extraction approach for separating relevant trivalent minor actinides (e.g., americium and curium) from used nuclear fuel. However, relatively slow kinetics in the stripping step of the process restricts process throughput when scaled for industrial implementation. To assist in identifying specific kinetic barriers associated with the separation, the solvation and dynamic behaviors of the two organic extractants in the current ALSEP implementation, N,N,N',N'-tetra(2-ethylhexyl)diglycolamide (T2EHDGA) and 2-ethylhexylphosphonic acid mono-2-ethylhexyl ester (HEHEHP), were probed through molecular dynamics (MD) simulations. The simulations examined the effects of extractant and nitric acid concentration on the interfacial behavior of the extractants in three solvent systems (n-dodecane, water, and n-dodecane + water). Solvation analyses of T2EHDGA revealed expected amphiphilic behavior in pure solvent systems. In a nitric-acid-free biphasic solvent, it was found that T2EHDGA expressed similar interfacial conformations as HEHEHP, suggesting that a parallel-like configuration, relative to the interface, is adopted at low concentrations. When HNO3 was introduced to biphasic systems containing a single molecule of extractant, HEHEHP was observed to retain a relatively parallel orientation while the T2EHDGA orientation was no longer affected by the presence of the interface. At bulk extractant concentrations, representative of the ALSEP process, the presence of nitric acid had minimal impact on the ligand orientation. Calculated diffusion constants showed that only some systems involving T2EHDGA were affected by the presence of acid.

Preparation and characterization of powellite ceramics Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) for Mo-rich HLW condition

Powellite ceramic represent one candidate to immobilize minor actinides and Mo from reprocessed UMo nuclear fuel. In this work, the Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) series is prepared via a solid-state reaction using Ce3+ as trivalent minor actinide (Am3+) surrogate, with structure/microstructure characterized by XRD, XPS, HRTEM, and SEM as well. The Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) compositions crystallize in tetragonal and are isostructural with scheelite. Rietveld refinements show that Ce3+ and Li+ simultaneously enter into the eight-fold coordinated Ca site of powellite crystal. The chemical durability of powellite phases is evaluated by the ASTM C1285-14 product consistency test method B. The leaching behaviours of Ce and Mo are accordance with the interfacial dissolution-reprecipitation mechanism. For all the Ca1-xLix/2Cex/2MoO4 (0 ≤ x ≤ 1) ceramics, 7-days NLCe and NLMo are found to be in the order of 10−3-10−5 g·m−2 and 10−2-10−4 g·m−2 respectively, which exhibit the great retention of Ce and Mo. Interestingly, the values of 7-days NLCe and NLMo are predominantly controlled by the distortion of MoO4 tetrahedra and disordered arrangements of Ce3+ and Li+. Thus, our initial understanding on the structure and chemical durability relation will provide insight to design new waste forms for the Mo-rich HLW condition.

Exploring Soft Donor Character of the N-2-Pyrazinylmethyl Group by Coordinating Trivalent Actinides and Lanthanides Using Aminopolycarboxylates.

The trivalent f-element coordination chemistry of a novel aminopolycarboxylate complexant was investigated. The novel reagent is an octadentate complexant that resembles diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA), but a single N-acetate pendant arm was substituted with a N-2-pyrazinylmethyl functional group. Thermodynamic studies of ligand protonation and trivalent lanthanide, americium and curium, complexation by N-2-pyrazinylmethyldiethylenetriamine-N,N',N″,N″-tetraacetic acid (DTTA-PzM) emphasize the strong electron withdrawing influence of the N-2-pyrazinylmethyl group. Specifically, DTTA-PzM is more acidic compared to a N-2-pyridinylmethyl-substituted structural equivalent, DTTA-PyM, with a substantial lowering of pK7, corresponding to the protonation of a second aliphatic amine site. The participation of the pyrizyl nitrogen in the metal ion coordination sphere is evident from the fluorescence lifetime decay measurements of metal hydration and the interpretation of the stability constants for ML- and MHL(aq) complexes. The overall conditional stability constants for the trivalent f-element complexation by DTTA-PzM complexes decrease, relative to DTTA-PyM, as expected based on the lower basicity of pyrazine in water relative to pyridine. Replacement of the N-2-pyridinylmethyl group with N-2-pyrazinylmethyl, while enhancing the total acidity of DTTA-PzM, also reduces its softness, as manifested by a small lowering of β101Am/Nd and liquid-liquid separation of trivalent lanthanides from trivalent americium. Despite this, the 4f/5f separation is doubled when DTTA-PzM replaces DTPA as an aqueous complexant in solvent extraction.

Insights into the retention behaviour of europium(III) and uranium(VI) onto Opalinus Clay influenced by pore water composition, temperature, pH and organic compounds

In the scientific community, a consensus exists to store high-level radioactive waste (HLW) in deep geological formations for several hundred thousand years to protect humanity and the environment. After a container damage followed by the release of HLW components, the retention of the radionuclides can be affected by diverse chemical reactions in the geosphere. In this study, geochemical effects possibly influencing the retention of the radionuclides were investigated. Therefore, the retention behaviour of U(VI) and Eu(III) (as homologue for trivalent actinides) onto Opalinus Clay (OPA) as a potential host rock for a HLW disposal was analysed at different geochemical conditions. For these experiments, a wide range of metal concentrations in presence and absence of natural clay organic matter (NOM) (lactate or humic acid) in different types of background electrolytes (0.01 mol L−1 sodium perchlorate and 0.4 mol L−1 synthetic OPA pore water) were investigated. Inductively coupled plasma mass spectrometry (ICP-MS) was used as a high specific and very sensitive method for elemental trace analysis. For the speciation of Eu3+-NOM-complexes, a method with high separation performance, capillary electrophoresis, was hyphenated with ICP-MS. The performed batch experiments simulate a potential water intrusion into the waste disposal. The retention (determined as log Kd values) of the metal ions strongly depends on the pH value, the presence of competing cations, temperature and NOM. At a pH value of pH 7.6 in synthetic OPA pore water, which is relevant for a disposal in OPA formations, the sorption of Eu(III) onto OPA (4 g L−1) is significantly higher (log Kd = 3.35 ± 0.04 at 25 °C and 4.85 ± 0.08 at 60 °C) than the retention of uranyl (log Kd = 2.17 ± 0.21 at 25 °C and 2.86 ± 0.01 at 60 °C) due to the formation of an uncharged aqueous calcium-uranyl-carbonate-complex (Ca2UO2(CO3)3(aq)) which does not sorb onto OPA. Modelling metal retention data onto OPA with PHREEQC reveals a very good agreement with most measurement results. At elevated temperatures, PHREEQC modelling could not be performed due to lack of data, especially for possible secondary minerals. The observed increased Kd values at elevated temperature revealed in this study might be assumed advantageous for the safe storage of HLW in a future repository.