Recent Advances in the Study of Trivalent Lanthanides and Actinides by Phosphinic and Thiophosphinic Ligands in Condensed Phases

The separation of trivalent actinides and lanthanides is a key step in the sustainable development of nuclear energy, and it is currently mainly realized via liquid-liquid extraction techniques. The underlying mechanism is complicated and remains ambiguous, which hinders the further development of extraction. Herein, to better understand the mechanism of the extraction, the contributing factors for the extraction are discussed (specifically, the sulfur-donating ligand, Cyanex301) by combing molecular dynamics simulations and experiments. This work is expected to contribute to improve our systematic understanding on a molecular scale of the extraction of lanthanides and actinides, and to assist in the extensive studies on the design and optimization of novel ligands with improved performance.

Assessment of the Equilibrium Constants of Mixed Complexes of Rare Earth Elements with Acidic (Chelating) and Organophosphorus Ligands

A survey of the experimental equilibrium constants in solution for the mixed complexes of 4f ions with acidic (chelating) and O-donor organophosphorus ligands published in the period between 1954 and 2022 is presented. These data are widely used in both analytical and solvent extraction chemistry. Important data evaluation criteria involved the specification of the essential reactions, process conditions and the correctness of techniques and calculations used, as well as appropriate equilibrium analysis of experimental data. Higher-quality data have been evaluated, compiled and presented herein, providing a synoptic view of the unifying theme in this area of research, i.e., synergism.

Separation of trivalent actinides and lanthanides using various ‘N’, ‘S’ and mixed ‘N,O’ donor ligands: a review

Separation of trivalent actinide (An) and lanthanide (Ln) elements is one of the burning topics in the back end of the nuclear fuel cycle due to the similarity in their chemical behaviour. A significant amount of research is being carried out worldwide to develop suitable ligands for the separation of the trivalent actinides and lanthanides. Some of the research groups are engaged in continuous improvement of the di-ethylene-triamine-penta acetic acid (DTPA) based Ln/An separation method, whereas extensive research is going on for the development of the lipophilic and hydrophilic ‘N’ donor heteropolycyclic ligands as the actinide selective ligand. A number of ‘S’ donor ligands are also explored for the Ln/An separation. In the present review, we made an attempt to highlight various separation processes based on soft donor ligands developed for Ln/An separations. Beside the conventional solvent extraction processes, separation possibilities membrane based and solid phase extraction techniques are evaluated for the Ln/An separation and are compiled in the present review.

Separation of Protactinium Employing Sulfur-Based Extraction Chromatographic Resins

Protactinium-230 ( t_1/2 = 17.4 d) is the parent isotope of ²³⁰U ( t_1/2 = 20.8 d), a radionuclide of interest for targeted alpha therapy (TAT). Column chromatographic methods have been developed to separate no-carrier-added ²³⁰Pa from proton irradiated thorium targets and accompanying fission products. Results reported within demonstrate the use of novel sulfur bearing chromatographic extraction resins for the selective separation of protactinium. The recovery yield of ²³⁰Pa was 93 ± 4% employing a R₃P═S type commercially available resin and 88 ± 4% employing a DGTA (diglycothioamide) containing custom synthesized extraction chromatographic resin. The radiochemical purity of the recovered ²³⁰Pa was measured via high purity germanium γ-ray spectroscopy to be >99.5% with the remaining radioactive contaminant being ⁹⁵Nb due to its similar chemistry to protactinium. Measured equilibrium distribution coefficients for protactinium, thorium, uranium, niobium, radium, and actinium on both the R₃P═S type and the DGTA resin in hydrochloric acid media are reported, to the best of our knowledge, for the first time.

Aminophosphine Oxides: A Platform for Diversified Functions

This review summarizes significant contributions reported on aminophosphine oxides (AmPOs), specifically those containing at least one amino group present as amino substituents on α- and β-carbons including direct P-N bond containing molecules. AmPOs have additional 'N' site(s), including highly basic 'P=O' groups, and these features make favor smooth and unexpected behavior. The most striking manifestations of flexibility of AmPOs are that they are exciting ligand systems for the coordination chemistry of actinides, and their involvement in catalytic organic reactions including enantioselective opening of meso-epoxides, addition of silyl enol ethers, allylation with allyltributylstannane, etc. The diverse properties of the AmPOs and their metal complexes demonstrate both the scope and complexity of these systems, depending on the basicity of phosphoryl group, and nature of the substituents on the pentavalent tetracoordinate phosphorus atom and metal. Two components key to understanding the challenges of actinide separations are detailed here, namely, previously described separation methods, and recent investigations into the fundamental coordination chemistry of actinides. Both are aimed at probing the critical features necessary for improved selectivity of separations. This review leads to the conclusion that, although many AmPOs have already been discovered and developed over the past century, many opportunities nevertheless exist for further developments towards new extraction processes and new catalytic materials by fine tuning the electronic and steric properties of substituents on the central phosphorus atom.

Cyanex based uranyl sensitive polymeric membrane electrodes

Novel uranyl selective polymeric membrane electrodes were prepared using three different low-cost and commercially available Cyanex extractants namely, bis(2,4,4-trimethylpentyl) phosphinic acid [L1], bis(2,4,4-trimethylpentyl) monothiophosphinic acid [L2] and bis(2,4,4-trimethylpentyl) dithiophosphinic acid [L3]. Optimization and performance characteristics of the developed Cyanex based polymer membrane electrodes were determined. The influence of membrane composition (e.g., amount and type of ionic sites, as well as type of plasticizer) on potentiometric responses of the prepared membrane electrodes was studied. Optimized Cyanex-based membrane electrodes exhibited Nernstian responses for UO₂(2+) ion over wide concentration ranges with fast response times. The optimized membrane electrodes based on L1, L2 and L3 exhibited Nernstian responses towards uranyl ion with slopes of 29.4, 28.0 and 29.3 mV decade(-1), respectively. The optimized membrane electrodes based on L1-L3 showed detection limits of 8.3 × 10(-5), 3.0 × 10(-5) and 3.3 × 10(-6) mol L(-1), respectively. The selectivity studies showed that the optimized membrane electrodes exhibited high selectivity towards UO₂(2+) ion over large number of other cations. Membrane electrodes based on L3 exhibited superior potentiometric response characteristics compared to those based on L1 and L2 (e.g., widest linear range and lowest detection limit). The analytical utility of uranyl membrane electrodes formulated with Cyanex extractant L3 was demonstrated by the analysis of uranyl ion in different real samples for nuclear safeguards verification purposes. The results obtained using direct potentiometry and flow-injection methods were compared with those measured using the standard UV-visible and inductively coupled plasma spectroscopic methods.

Conjugates of magnetic nanoparticle-actinide specific chelator for radioactive waste separation

A novel nanotechnology for the separation of radioactive waste that uses magnetic nanoparticles (MNPs) conjugated with actinide specific chelators (MNP-Che) is reviewed with a focus on design and process development. The MNP-Che separation process is an effective way of separating heat generating minor actinides (Np, Am, Cm) from spent nuclear fuel solution to reduce the radiological hazard. It utilizes coated MNPs to selectively adsorb the contaminants onto their surfaces, after which the loaded particles are collected using a magnetic field. The MNP-Che conjugates can be recycled by stripping contaminates into a separate, smaller volume of solution, and then become the final waste form for disposal after reusing number of times. Due to the highly selective chelators, this remediation method could be both simple and versatile while allowing the valuable actinides to be recovered and recycled. Key issues standing in the way of large-scale application are stability of the conjugates and their dispersion in solution to maintain their unique properties, especially large surface area, of MNPs. With substantial research progress made on MNPs and their surface functionalization, as well as development of environmentally benign chelators, this method could become very flexible and cost-effective for recycling used fuel. Finally, the development of this nanotechnology is summarized and its future direction is discussed.

Oxidative degradation of bis(2,4,4-trimethylpentyl)dithiophosphinic acid in nitric acid studied by electrospray ionization mass spectrometry

RATIONALE

The selective separation of the minor actinides (Am, Cm) from the lanthanides is a topic of ongoing nuclear fuel cycle research, and dithiophosphinic acids are candidate ligands in these processes. Ligand instability has been noted under radiolytic and harsh acid conditions but explicit degradation pathways for ligands such as bis(2,4,4-trimethylpentyl)-dithiophosphinic acid (CyxH), the major compound in the commercial product Cyanex 301, have been elusive.

METHODS

Organic solutions of CyxH were contacted with aqueous solutions of HNO(3), and their degradation was studied by analyzing samples from these experiments by direct infusion electrospray ionization mass spectrometry. Ions were identified using accurate mass measurement and collision-induced dissociation.

RESULTS

The positive ion spectra contained cationized CyxH cluster ions, and oxidatively coupled species (designated Cyx(2)) cationized by either H or Na. The Cyx(2)-derived ions increased with acid contact time. The negative ion spectra consisted almost entirely of the CyxH conjugate base. The negative ion spectra of the HNO(3)-contacted samples also contained conjugate bases corresponding to the dioxo and perthio derivatives of CyxH.

CONCLUSIONS

CyxH is oxidized by acid contact to form the coupled species Cyx(2), and the dioxo species arise from subsequent oxidation of Cyx(2). Oxidative coupling increases with contact time, and with higher HNO(3) concentrations. The direct infusion measurements provided a simple approach for assessing degradation pathways and kinetics.

Thermodynamics of extraction of Am(III) and Eu(III) from different anionic media with Tri-n-octyl phosphine oxide

Extraction of Am(III) and Eu(III) from NO3 -, ClO4 -, SCN- and NO3 - + ClO4 - media with tri-n-octyl phosphine oxide (TOPO) in xylene has been carried out at 15, 25, 30 and 35°C. Under the extraction conditions the species M(NO3)3·3TOPO, M(SCN)3·4TOPO, M(ClO4)3·4TOPO and MNO3(ClO4)2·4TOPO are predominantly extracted at all the temperatures (M=Am or Eu). The two phase equilibrium constant (log K ex) follows an order SCN- > NO3 - + ClO4 - > ClO4 -> NO3 -. Extraction enthalpies have been calculated in each system from the temperature dependence of the K ex. The thermodynamic parameters ΔG, ΔH and ΔS have been discussed and compared with other mono- and bidentate organophosphorus extractants for Am(III) and Eu(III) from same aqueous medium. The -ΔH values for the two-phase reaction follow an order NO3 - + ClO4 - > ClO4 - > SCN- > NO3 - for both metal ions. This has been explained on the basis of heat energies involved with the formation of ML3 complex (L being the anion used), the transfer of ML3 to the organic phase and the attachment of TOPO molecules, and also on the limiting partial molal volume of the anions.

Enhanced extraction of thorium(IV) and uranium(VI) with 1-phenyl-3-methyl-4-pivaloyl-5-pyrazolone in the presence of various neutral organophosphorus extractants

This paper describes the results of the investigations carried out on the extraction of thorium(IV) and uranium(VI) from dilute nitric acid solutions into chloroform with 1-phenyl-3-methyl-4-pivaloyl-5-pyrazolone (HPMPP) in the presence and absence of various neutral organophosphorus extractants, tributylphosphate (TBP), octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO) and trioctylphosphine oxide (TOPO). The results demonstrated that these metal ions were extracted into chloroform as Th(PMPP)4 and UO2(PMPP)2 with HPMPP alone and as Th(PMPP)4·S and UO2(PMPP)2·S in the presence of neutral organophosphorus extractants (S). High selectivity (S.F.=2.8×105) has been observed between thorium(IV) and uranium(VI) when extracted with HPMPP alone. The equilibrium constants of the extracted complexes have been deduced by non-linear regression analysis. The equilibrium constants of the synergistically extracted complexes have been correlated with the donor ability of the phosphoryl oxygen of the neutral organophosphorus extractants in terms of their 31P NMR chemical shifts and basicity values (K H = nitric acid uptake constant). The addition of a neutral organophosphorus extractant to the metal chelate system significantly enhances the extraction efficiency of these metal ions. Thorium(IV) and uranium(VI) complexes of HPMPP and mixtures of HPMPP and neutral organophosphorus extractants were synthesized and characterized by IR spectral data to further clarify the nature of the extracted complexes.

Synergistic extraction of Ce(III), Eu(III) and Tm(III) with a mixture of picrolonic acid and tributylphosphine oxide in chloroform

Synergistic extraction of Ce(III), Eu(III) and Tm(III) as a representative of lanthanides(III) with a mixture of picrolonic acid {1-p-nitrophenyl-3-methyl-4-nitro-5-pyrazolon (HPA)} acting as an acidic chelating agent and tributylphosphineoxide (TBPO) as a neutral ligand in chloroform from pH 2 buffer solution has been studied. The composition of the synergistic adduct has been determined to be M(PA)3·2TBPO (M=Ce(III), Eu(III), Tm(III)). The effect of various anions and cations on the extraction of these metal ions has also been studied. Among the anions, fluoride, oxalate, citrate and cyanide ions masked the extraction, whereas Fe(III) and Cu(II) reduced the extraction to ∼90%. The formation constant, and separation factor from various metal ions has also been determined and discussed.

Systematic Behavior of Charge-Transfer Transitions and Energy Level Variation in Soft Donor Complexes of the Trivalent Lanthanides

The systematic behavior of the charge-transfer (CT) energies in mixed 2,2'-bipyridyl (bipy), N,N-diethyldithiocarbamate (Et2dtc-) complexes of the trivalent lanthanides, Ln(Et2dtc)3(bipy), is investigated to understand the electronic structure of f-element complexes containing soft donor ligands. The energies of ligand to Ln3+ CT are extremely low in this system, an effect attributed to the presence of the soft donor ligands. The lowest CT energy level for the Sm3+, Eu3+, and Yb3+ complexes falls into the visible range. In Eu(Et2dtc)(bipy), the Eu3+ ion becomes nonluminescent because the CT energy stretches below the metastable 5D0 electronic state, whereas luminescence from the CT state and the 4f13 (2)F(5/2) state are observed in the Yb compound. The variation in the energy of the lowest level CT transition for the entire Ln(Et2dtc)3(bipy) series has been evaluated using the experimentally determined CT levels of the Sm3+, Eu3+, and Yb3+ compounds based on the systematic behavior of the lanthanides, which is invariant with respect to the type of ligand. The energy difference between the ground electronic states of the lanthanide ions and the ligand-centered valence band may also be calculated from these results.