Trivalent Actinide Ions Research Articles

Trends in Methanol-Solvated Actinide Ions and Actinide Expanded Porphyrin Complexes.

Trivalent actinide expanded porphyrin complexes have been of synthetic interest since the isolation of the series of trivalent lanthanide texaphyrin complexes in 1992, however, synthesis of these actinide-based complexes has not yet been achieved. In this work, a computational study with relativistic density functional theory was performed to determine how trivalent actinide ions (Ac3+ through Lr3+) interact with Schiff base expanded porphyrin macrocycles in a methanol solvent as an alternate pathway to stabilization. A thorough analysis of structural parameters, electronic structure, stability of microsolvation environments, and relative binding energies provided insight into the most stable structures. Trends in bonding and structure for the full actinide series are reported for methanol solvated ions, which reports actinide contraction along the series for early and mid actinides. Texaphyrin incorporates the actinide ions in a planar fashion, whereas for alaskphyrin a bent structure is more favorable; this bend results in stronger interaction energies than those calculated for the texaphyrin complexes. We also show that the redox-active expanded porphyrin ligands are able to accommodate a variety of actinides through charge transfer stabilization. Based on relative binding energy and energy decomposition analysis, it was found that texaphyrin and alaskaphyrin both exhibit a strong preference for binding to Th.

6-(6-Methyl-1,2,4,5-Tetrazine-3-yl)-2,2'-Bipyridine: A N-Donor Ligand for the Separation of Lanthanides(III) and Actinides(III).

Here, we report the synthesis of the 6-(6-methyl-1,2,4,5-tetrazine-3-yl)-2,2'-bipyridine (MTB) ligand that has been developed for lanthanide/actinide separation. A multimethod study of the complexation of MTB with trivalent actinide and lanthanide ions is presented. Single-crystal X-ray diffraction measurements reveal the formation of [Ce(MTB)2(NO3)3], [Pr(MTB)(NO3)3H2O], and [Ln(MTB)(NO3)3MeCN] (Ln = Nd, Sm, Eu, Gd). In addition, the complexation of Cm(III) with MTB in solution was studied by time-resolved laser fluorescence spectroscopy. The results show the formation of [Cm(MTB)1-3]3+ complexes, which occur in two different isomers. Quantum chemical calculations reveal an energy difference between these isomers of 12 kJ mol-1, clarifying the initial observations made by time-resolved laser fluorescence spectroscopy (TRLFS). Furthermore, quantum theory of atoms in molecules (QTAIM) analysis of the Cm(III) and Ln(III) complexes was performed, indicating a stronger covalent contribution in the Cm-N interaction compared to the respective Ln-N interaction. These findings align well with extraction data showing a preferred extraction of Am and Cm over lanthanides (e.g., max. SFAm/Eu = 8.3) at nitric acid concentrations <0.1 mol L-1 HNO3.

Newly synthesised diglycolamide functionalised silica gel with terminal 2-ethylhexyl alkyl chain, Silica-DGA (2EH) having high ligand density for adsorption of trivalent actinide and lanthanide from acidic medium

A novel diglycolamide resin with 2-ethylhexyl terminal alkyl chain functionalised to silica gel was synthesised and characterised by ATR-FTIR, SEM, TG/DSC, &13C (CP/MAS) solid state NMR techniques. Effective functionalization of the organic ligand upon silica gel was confirmed from the two carbonyl stretching frequencies at 1735 cm−1 and 1619 cm−1 in the FTIR spectrum and carbonyl carbon atom signal at 170 ppm in 13C (CP/MAS) solid state NMR spectrum. Thermo gravimetric measurement showed 17.3 ± 0.2 % (W/W) weight loss corresponding to the ligand loading value of 425 µmol/g. The resin showed very high uptake of trivalent lanthanide and actinide ions from 4.0 M HNO3 medium and very low distribution of Cs & Sr; Kd ∼ 1.1×103 mL/g for Am(III), ∼ 1.41×103 mL/g for Eu(III), ∼2.5 mL/g for Sr and 1.53 mL/g for Cs. FTIR spectroscopic study of the Eu3+ adsorbed resin showed single carbonyl stretching frequency at 1654 cm−1, instead of the two carbonyl stretching frequencies in fresh silica-DGA(2EH) resin indicating complexation through carbonyl oxygen. Very low Kd of ∼1.2–5.1 mL/g for lanthanides/actinides from 4.0 M HNO3 containing water soluble DGA namely N, N, N, N, Tetraethyldiglycolamide (TEDGA) in the concentration range 0.3–0.1 M indicates 99 % back adsorption feasibility.

Understanding the separation of trivalent lanthanides and actinides using multiple diglycolamide-containing ligands: a review

Abstract The complexation and separation behaviour of trivalent actinides and lanthanides with various multiple diglycolamide (DGA) group-containing ligands are compiled in the present review. The DGA arms are anchored on various scaffolds, viz., C- and N-pivot tripodal backbones, benzene-centred tripodal backbones, calix[4]arene, pillar[5]arene and azamacrocycle scaffolds. Dendrimers of different generations having DGA arms were also evaluated for the extraction and complexation studies of trivalent actinide and lanthanide ions. The extraction efficiency of the DGA units increases significantly when they are grouped together on a single backbone/scaffold. Furthermore, the selectivity was found to improve in many cases, particularly with respect to the UO2 2+ ion. This is very important for the development of suitable ligands for the extraction/separation of the actinide and lanthanide ions. Combined experimental, luminescence and DFT studies on the complexation of trivalent actinides and lanthanides with these DGA-based ligands are quite useful to understand their extraction and separation behaviour and this review article gives a critical analysis of the results available in the literature.

Spectroscopic Study on the Complexation of trivalent Actinide and Lanthanide ions with TEDGA in Solution

AbstractNMR spectroscopy studies on the complexation of La(III), Sm(III), Lu(III), Y(III) and Am(III) with N,N,N′,N′‐tetraethyldiglycolamide (TEDGA) have been performed. Initial studies concerning the stoichiometry of the complexes prove the formation of [M(TEDGA)3]3+ in D2O and [M(TEDGA)1‐3]3+ in CD3CN. Decreasing the solvent polarity translates to an increase of the complex stability as shown by complementary TRLFS studies with Cm(III). Due to delocalization of the lone electron pair of the amide nitrogen atom, the M(III)−O interaction has been studied indirectly by collecting 13C and 15N NMR data. The observed chemical shifts prove that the interaction of the trivalent ions, An(III) and Ln(III), and TEDGA is almost identical.

Mutual Separation of Ln and an Using TODGA and DTBA with High Organic Acid Concentrations

ABSTRACT Developing the conventional separation method by which minor actinides (MA) such as Am and Cm are separated from lanthanides is important with respect to their recovery and isolation from high-level radioactive waste (HLW). Owing to the chemical behavior of trivalent lanthanide and actinide ions with similar ionic radii, realizing this separation is still challenging. All lanthanides, Am, and Cm can be extracted using diglycolamide (DGA) extractants, and relatively high An/Ln separation efficiencies have been obtained using diethylenetriamine-N,N’,N”-triacetic acid-N,N”-bis(diethylacetamide) (DTBA). Referring to our previous study, where a good separation and a MA recovery of approximately 95% with the co-existence of 5% Nd in the aqueous phase were observed. To improve these results as well as the separation conditions, under modified conditions by adding lactic acid to stabilize pH, the recovery rates observed were as follows: 97.1% for average Ln metals with the co-existence of 1.59% Am in the organic phase and 98.4% for Am with the co-existence of 2.95% Ln in the aqueous phase.

Covalency of Trivalent Actinide Ions with Different Donor Ligands: Do Density Functional and Multiconfigurational Wavefunction Calculations Corroborate the Observed "Breaks"?

A comprehensive ab initio study of periodic actinide-ligand bonding trends for trivalent actinides is performed. Relativistic density functional theory (DFT) and complete active-space (CAS) self-consistent field wavefunction calculations are used to dissect the chemical bonding in the [AnCl6]3-, [An(CN)6]3-, [An(NCS)6]3-, [An(S2PMe2)3], [An(DPA)3]3-, and [An(HOPO)]- series of actinide (An = U-Es) complexes. Except for some differences for the early actinide complexes with DPA, bond orders and excess 5f-shell populations from donation bonding show qualitatively similar trends in 5f n active-space CAS vs DFT calculations. The influence of spin-orbit coupling on donation bonding is small for the tested systems. Along the actinide series, chemically soft vs chemically harder ligands exhibit clear differences in bonding trends. There are pronounced changes in the 5f populations when moving from Pu to Am or Cm, which correlate with previously noted "breaks" in chemical trends. Bonding involving 5f becomes very weak beyond Cm/Bk. We propose that Cm(III) is a borderline case among the trivalent actinides that can be meaningfully considered to be involved in ground-state 5f covalent bonding.

Aqueous soluble ‘N′ donor heterocyclic ligands for the mutual separation of Am3+ and Eu3+: Solvent extraction, flat sheet supported liquid membrane and hollow fiber microextraction studies

Preferential stripping of the trivalent actinide ions such as Am(III) from the organic extract containing a mixture of Am(III) and Eu(III) by a suitable separation method is one of the most challenging areas of research which can find application in the radioactive waste management scheme. In the present study, a comparative evaluation of aqueous soluble tetrasulphonated derivatives of bis-triazinyl pyridine (BTP, LI), bis-triazinyl bipyridine (BTBP, LII) and bis-triazinyl phenanthroline (BTPhen, LIII) was done for the preferential stripping of Am(III) from a mixture of Am(III)/Eu(III) in the N,N,N′,N′-tetra-n-octyl diglycolamide (TODGA) extract. The results obtained in the solvent extraction studies were used for setting up of flat sheet supported liquid membrane based separation systems where the feed phase contained 1 M HNO3 spiked with the 241Am and 152,154Eu radiotracers while the receiver phase contained 0.01 M solutions of the aqueous soluble complexing agents in the presence of 1 M HNO3. Analogous studies were carried out using polypropylene hollow fibers leading to micro extraction of both Am(III) and Eu(III) and preferential stripping of Am(III) where promising separation behavior was observed, especially in the cases of LI and LIII with the DF values of 66 and 72, respectively for Am3+ over Eu3+ along with >80% recovery of Am.

Recent Progress in Trivalent Actinide and Lanthanide Solvent Extraction and Coordination Chemistry with Triazinylpyridine N Donor Ligands

ABSTRACT A follow up to an earlier review on the coordination and extraction of trivalent actinide and lanthanide ions with triazinylpyridine N-donor ligands is presented. It reviews the recent development in the fields of ligand modifications and improvements, considering both hydrophobic compounds (to be used as extracting agents) and hydrophilic compounds (to be used as stripping agents), with a focus on fundamental studies.

On the Modeling of the S-Shaped Thermodynamic and Transport Behavior against the Atomic Number Z of Some Trivalent f-Element Ions in Aqueous Solutions at 298 K and Prediction for Completion of the Periodic Table of Chemical Elements

Ionic self-diffusion coefficients D of some trivalent lanthanide and actinide ions have been determined by the open-end capillary method (O.E.C.M.) in supporting lanthanide electrolyte 1 : 3 aqueous solutions at 25°C and optimal pH 2.50 in order to avoid hydrolysis, ion pairing and complexing of trivalent 4f ions. This study contributes to demonstrate similarities in transport and structure properties between 4f and 5f trivalent ions explained by a similar electronic configuration, ionic radius and same hydration number. These similarities are manifested in a strong causal correlation between certain properties by linear or S-shaped behaviors. In this context, we have suggested some expressions relating these properties and interesting to estimate or predict, by interpolation or extrapolation methods, some eventually not available experimental values, especially for the actinides for which the half-life is very short or the experimental conditions are very special and expensive. For this issue, we can contribute to the complement of periodic table of elements by some physicochemical properties, especially for the 5f-series, when we have some available data on the 4f‑series as well as the mathematical function of the causal correlation between these two nf-series.

Applications of Diglycolamide Based Solvent Extraction Processes in Spent Nuclear Fuel Reprocessing, Part 1: TODGA

ABSTRACTOver the last decade there has been much interest in the applications of diglycolamide (DGA) ligands for the extraction of the trivalent lanthanide and actinide ions from PUREX high active raffinates or dissolved spent nuclear fuel. Of the DGAs, the N,N,N’,N’-tetraoctyldiglycolamide (TODGA) is the best known and most widely studied. A number of new actinide separation processes have been proposed based on extraction with TODGA. This review covers TODGA-based processes and extraction data, specifically focusing on how phase modifiers have been used to increase metal loading and thus enhance the operating process envelopes. Effects of third phase formation and the organic phase speciation are reviewed in this context. Relevant aspects of the extraction chemistry of important solvents (TODGA-modifier-diluent combinations) are described and their performances demonstrated by a consideration of the published flowsheet tests. It is seen that modifiers are successfully enabling the use of TODGA in actinide separation processes but to date the identification and testing of suitable modifiers has been rather empirical. There is a growing understanding of the fundamental chemistry occurring in the organic phase and how that affects extractant speciation and metal loading capacity but studies are still needed if TODGA-based flowsheets are to become an industrially deployable option for minor actinide (MA) recovery processes.

On the isotropy of induced multipole moments in heavy ion complexes.

Polarization effects have been proven to play an important role in the theoretical description of chemical processes. With this respect, we report ab initio calculations describing multiconfigurational and relativistic effects as well as electron correlation to determine dipole- and quadrupole polarizabilities for all trivalent actinide and lanthanide ions. The results are used in an iterative point-multipole model to compute polarization interaction for different metal compounds. Significant differences between the usage of isotropic polarizabilities for the coordinating ligands compared to the full tensor representation is found. Quadrupole contributions are found to be negligible for symmetric geometries but can contribute up to 30 kJ/mol to the polarization energy in specific cases. The employed method allows a parameter free study of multipole interactions for a wide variety of systems. © 2017 Wiley Periodicals, Inc.

Europium binding to humic substances extracted from deep underground sedimentary groundwater studied by time-resolved laser fluorescence spectroscopy

ABSTRACTHumic substances (HSs) are ubiquitous in various environments including deep underground and play an important role in the speciation and mobility of radionuclides. The binding of Eu3+, a chemical homologue of trivalent actinide ions, to HSs isolated from sedimentary groundwater at −250 m below the surface was studied by time-resolved laser fluorescence spectroscopy combined with parallel factor analysis (PARAFAC) as a function of pH and salt concentration. PARAFAC modeling reveals the presence of multiple factors that correspond to different Eu3+ species. These factors resemble those observed for Eu3+ binding to HSs from surface environments; however, detailed comparison shows that there are some particularities in Eu3+ binding to the deep groundwater HSs. The distribution coefficients (Kd) of Eu3+ binding to the HSs calculated from the PARAFAC modeling exhibits a rather strong salt effect. At 0.01 M NaClO4, the Kd values are relatively large and comparable to those to the surface HSs; they are decreaed at 0.1 M NaClO4 by more than an order of the magnitude. The Kd values are larger for the humic acid fraction of the deep underground HSs than the fulvic acid fraction over the entire range of pH and salt concentration investigated in this study.

Electrochemical Nucleation and Growth of Uranium and Plutonium from Molten Salts

This work examines the nucleation and growth behavior of uranium and plutonium from molten LiCl-KCl eutectic on inert electrodes using electrochemical techniques. Current-time transients obtained from chronoamperometric experiments were compared with theoretical models to characterize the type of nucleation (progressive or instantaneous) for deposition of U and Pu, and co-deposition of U-Pu, from molten LiCl-KCl at inert electrodes. It was established that the nucleation mode of actinides present as chlorides in molten chloride salts changes from progressive to instantaneous with an increasing concentration of the trivalent actinide ions in the salt. The effect of the material of the working electrode was investigated, and it was found that changing the material from tungsten to silver improves resolvability of the nucleation peaks and allows more accurate analysis of the experimental measurements. Using the nucleation data, diffusion coefficients were obtained for U3+ and Pu3+, and were found to be in very good agreement with the values obtained from other studies. The density of nuclei produced during instantaneous nucleation, the rate of nucleation for progressive nucleation, and the radius of the deposited nuclei were evaluated and examined at different overpotentials.

Complexation of radionuclide 152+154Eu(III) with alumina-bound fulvic acid studied by batch and time-resolved laser fluorescence spectroscopy

To contribute to the understanding of Eu(III) interaction preperties on hydrous alumina particles in the absence and presence of fulvic acid (FA), the complexation properties of Eu(III) with hydrous alumina, FA and FA-alumina hybrids are studied by batch and time-resolved laser fluorescence spectroscopy (TRLFS) techniques. The continuous increase in the fluorescence lifetime of Eu-alumina and Eu-FA with increasing pH indicates that the complexation is accompanied by decreasing number of hydration water in the first coordination sphere of Eu(III). Eu(III) is adsorbed onto alumina particles as outer-sphere surface complexes of ≡(Al−O)−Eu· (OH)· 7H2O and ≡(Al−O)−Eu· 6H2O at low pH values, and as inner-sphere surface complexes as ≡(Al−O)2−Eu+· 4H2O at high pH. In FA solution, Eu(III) forms complexes with FA as (COO)2Eu+(H2O) x and the hydration water number in the first coordination sphere decreases with pH increasing. The formation of ≡COO−Eu−(O−Al≡)· 4H2O is observed on FA-alumina hybrids, suggesting the formation of strong inner-sphere surface complexes in the presence of FA. The surface complexes are also characterized by their emission spectra [the ratio of emission intensities of 5 D 0→7 F 1 ( λ =594 nm) and 5 D 0→7 F 2 ( λ =619 nm) transitions] and their fluorescence lifetime. The findings is important to understand the contribution of FA in the complexation properties of Eu(III) on FA-alumina hybrids that the clarification of the environmental behavior of humic substances is necessary to understand fully the behavior of Eu(III), or its analogue trivalent lanthanide and actinide ions in natural environment.

Sorption behavior of cesium, cobalt and europium radionuclides onto hydroxyl magnesium silicate

Abstract The radioactive wastes from different activities have to be safely disposed of and isolated from the human environment. The retardation of radioactive materials by designed barriers is originally controlled by the sorption ability of the mineral compositions. In this work, a naturally available mineral composite, a hydroxyl magnesium silicate (HMS) was investigated as potential natural inorganic sorbent for the retention of long-lived radionuclides (134Cs, 60Co and 152+154Eu) from aqueous solutions. The factors affecting the sorption process, such as contact time and pH were evaluated. Furthermore X-ray fluorescence (XRF), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), X-ray diffraction (XRD), differential thermal and thermogravimetry analyses (DTA/TGA) measurements were examined in order to assess the physicochemical properties of the magnesium silicate mineral. Langmuir and Freundlich isotherms fitted the result s substantially better than the Flory–Huggins isotherm and the sorption was found to follow pseudo-first order kinetic model. The proposed mineral has been successfully applied for the sorption of 134Cs, 60Co and 152+154Eu radionuclides from real radioactive waste. The results indicated that about 97.4–99% of 134Cs, 60Co and 152+154Eu radionuclides were efficiently retained onto the HMS mineral. Based on the results obtained, it can be concluded that the HMS mineral is an economic and efficient retaining material for environmental hazardous migration and/or leakage of some radionuclides such as 134Cs, 60Co and 152+154Eu and trivalent actinide (241Am, 242mAm and 243Am) ions. Therefore, this study could be used as a starting point to establish and consider that mineral as an engineered barrier around the disposal facilities at the nuclear activity centres.

Comparative NMR Study of nPrBTP and iPrBTP

Bistriazinyl-pyridine type ligands are important extracting agents for separating trivalent actinide ions from trivalent lanthanides. The alkyl substituents on the lateral triazine rings have a significant effect on the stability of the ligand against hydrolysis and radiolysis. Furthermore they influence solubility, extraction behaviour and selectivity. TRLFS and extraction studies suggest differences in complexation and extraction behaviour of BTP ligands bearing iso-propyl or n-propyl substituents, respectively. As NMR studies allow insight into the metal-ligand bonding, we conducted NMR studies on a range of 15N-labelled nPrBTP and iPrBTP Ln(III) and Am(III) complexes. Our results show that no strong change in the metal-ligand bonding occurs, thus excluding electronic reasons for differences in complexation behaviour, extraction kinetics and selectivity. This supports mechanistic reasons for the observed differences.

Effect of pH, humic acid and addition sequences on Eu(III) sorption onto γ-Al2O3 study by batch and time resolved laser fluorescence spectroscopy

To contribute to the comprehension of Eu(III) sorption properties in oxide-organic material systems, the influence of humic acid (HA) on Eu(III) sorption on bare and HA-bounded γ-Al2O3 is studied using batch experiments and time resolved laser fluorescence spectroscopy (TRLFS). The effect of pH and addition sequences of Eu(III)/HA/Al2O3 on Eu(III) sorption is carried out at different experimental conditions, and the results show that the sorption of Eu(III) is strongly dependent on pH and ionic strength. The sorption of Eu(III) is mainly dominated by ion exchange or outer-sphere surface complexation at low pH, and by inner-sphere surface complexation at high pH values. The presence of HA enhances Eu(III) sorption at low pH, but reduces Eu(III) sorption at high pH values. The addition sequences of Eu(III)/HA to alumina suspensions do not affect Eu(III) sorption, which is also evidenced by the TRLFS measurements. The surface complexes are characterized by their emission spectra [the ratio of emission intensities of 5D0→7F1 (λ=594nm) and 5D0→7F2 (λ=619nm) transitions] and their fluorescence lifetime. The TRLFS results show clearly that no effect of addition sequences on the sorption and speciation of Eu(III) in HA–Al2O3 hybrids after equilibration. These findings on the effect of humic substances in the complexation properties of Eu(III) indicate that the clarification of the environmental behavior of humic substances is necessary to understand the physicochemical properties of Eu(III), or its analog trivalent lanthanide and actinide ions in the natural environment.

Siderocalin-mediated recognition, sensitization, and cellular uptake of actinides

Synthetic radionuclides, such as the transuranic actinides plutonium, americium, and curium, present severe health threats as contaminants, and understanding the scope of the biochemical interactions involved in actinide transport is instrumental in managing human contamination. Here we show that siderocalin, a mammalian siderophore-binding protein from the lipocalin family, specifically binds lanthanide and actinide complexes through molecular recognition of the ligands chelating the metal ions. Using crystallography, we structurally characterized the resulting siderocalin-transuranic actinide complexes, providing unprecedented insights into the biological coordination of heavy radioelements. In controlled in vitro assays, we found that intracellular plutonium uptake can occur through siderocalin-mediated endocytosis. We also demonstrated that siderocalin can act as a synergistic antenna to sensitize the luminescence of trivalent lanthanide and actinide ions in ternary protein-ligand complexes, dramatically increasing the brightness and efficiency of intramolecular energy transfer processes that give rise to metal luminescence. Our results identify siderocalin as a potential player in the biological trafficking of f elements, but through a secondary ligand-based metal sequestration mechanism. Beyond elucidating contamination pathways, this work is a starting point for the design of two-stage biomimetic platforms for photoluminescence, separation, and transport applications.

Electron Transport in Graphene-Based Nanosensors for Eu(III) Detection

We performed quantum chemistry density functional theory calculations to provide insight and to calculate the response of nanographene (or molecular-graphene)-based sensors using Eu(III) complexes as a cold surrogate of trivalent actinide ions. We found that the electronic structure of graphene can be affected by its interaction with europium ions as well as with water molecules. We found an increase in electron transfer through the graphene-based device when exposed to a liquid sample of europium nitrate, Eu(NO3)3, and a decrease when Eu2O3 approaches a graphene oxide surface. Because the detection mechanism of these nanosensors is based on changes in electron transfer when a trivalent europium complex approaches the graphene-based nanosensors, fabricating arrays of these nanosensors is encouraged.