Progress in actinide extraction chemistry

Progress in actinide extraction chemistry for period 1985–1989 is discussed.     

Radiochemistry and actinide chemistry

The paper deals with radiochemistry of tracer amounts of radionucli des with N=1 to 100 or several hundreds of species/cm³.     

Recent developments in computational actinide chemistry

This review describes recent computational investigations into the electronic and geometric structures of molecular actinide compounds. Following brief introductions to (i) the effects of relativity in chemistry and (ii) ab initio and density functional quantum chemical methods, four areas of contemporary research are discussed. These are pi backbonding in uranium complexes, the geometric structures of bis benzene actinide compounds, the valence electronic structure of the uranyl ion, and the inverse trans influence in pseudo-octahedral [AnOX5]n-. Comparisons are made with experimental studies, and similarities and differences between d- and f-block chemistry are highlighted.     

Recent progress in actinide borate chemistry

The use of molten boric acid as a reactive flux for synthesizing actinide borates has been developed in the past two years providing access to a remarkable array of exotic materials with both unusual structures and unprecedented properties. [ThB(5)O(6)(OH)(6)][BO(OH)(2)]·2.5H(2)O possesses a cationic supertetrahedral structure and displays remarkable anion exchange properties with high selectivity for TcO(4)(-). Uranyl borates form noncentrosymmetric structures with extraordinarily rich topological relationships. Neptunium borates are often mixed-valent and yield rare examples of compounds with one metal in three different oxidation states. Plutonium borates display new coordination chemistry for trivalent actinides. Finally, americium borates show a dramatic departure from plutonium borates, and there are scant examples of families of actinides compounds that extend past plutonium to examine the bonding of later actinides. There are several grand challenges that this work addresses. The foremost of these challenges is the development of structure-property relationships in transuranium materials. A deep understanding of the materials chemistry of actinides will likely lead to the development of advanced waste forms for radionuclides present in nuclear waste that prevent their transport in the environment. This work may have also uncovered the solubility-limiting phases of actinides in some repositories, and allows for measurements on the stability of these materials.     

Mathematical modelling of actinide extraction equilibria and extraction processes

Progress is reviewed in the authors' works on actinide extraction equilibria (including activity coefficient evaluation from extraction data) and their mathematical modelling (including Tc influence on Pu/U separation).     

Development of a standardized sequential extraction protocol for simultaneous extraction of multiple actinide elements

Sequential extraction is a useful technique for assessing the potential to leach actinides from soils; however, current literature lacks uniformity in experimental details, making direct comparison of results impossible. This work continued development toward a standardized five-step sequential extraction protocol by analyzing extraction behaviors of ²³²Th, ²³⁸U, ^239,240Pu and ²⁴¹Am from lake and ocean sediment reference materials. Results produced a standardized procedure after creating more defined reaction conditions to improve method repeatability. A NaOH fusion procedure is recommended following sequential leaching for the complete dissolution of insoluble species.     

Gas-phase chemistry of bare and oxo-ligated protactinium ions: a contribution to a systematic understanding of actinide chemistry

Gas-phase chemistry of bare and oxo-ligated protactinium ions has been studied for the first time. Comparisons were made with thorium, uranium, and neptunium ion chemistry to further the systematic understanding of 5f elements. The rates of oxidation of Pa(+) and PaO(+) by ethylene oxide compared with those of the homologous uranium ions indicate that the first and second bond dissociation energies, BDE[Pa(+)-O] and BDE[OPa(+)-O], are approximately 800 kJ mol(-1). The relatively facile fluorination of Pa(+) to PaF(4)(+) by SF(6) is consistent with the high stability of the pentavalent oxidation state of Pa. Reactions with ethene, propene, 1-butene, and iso-butene revealed that Pa(+) is a very reactive metal ion. In analogy with U(+) chemistry, ethene was trimerized by Pa(+) to give PaC(6)H(6)(+). Reactions of Pa(+) with larger alkenes resulted in secondary and tertiary products not observed for U(+) or Np(+). The bare protactinium ion is significantly more reactive with organic substrates than are heavier actinide ions. The greatest difference between Pa and heavier actinide congeners was the exceptional dehydrogenation activity of PaO(+) with alkenes; UO(+) and NpO(+) were comparatively inert. The striking reactivity of PaO(+) is attributed to the distinctive electronic structure at the metal center in this oxide, which is considered to reflect the greater availability of the 5f electrons for participation in bonding, either directly or by promotion/hybridization with higher-energy valence orbitals.     

Di-2-pyridyl ketone in actinide chemistry: Dinuclear uranyl complexes

Treatment of UO₂X₂ (X = OAc, Cl, NO₃) with 1 mol equiv of (py)₂CO in THF afforded the adducts [UO₂X₂{(py)₂CO}] in almost quantitative yields. The same reactions in MeOH, in the presence of NEt₃ for X = Cl and NO₃, gave yellow crystals of [(UO₂X)₂{μ-(py)₂C(OMe)O}₂]·MeOH (X = OAc, 1·MeOH and X = Cl, 2·MeOH) and [{UO₂(NO₃)}₂{μ-(py)₂C(OMe)O}₂] (3). Reactions of UO₂X₂ (X = OAc, Cl) with 2 mol equiv of (py)₂CO and NEt₃ in MeOH or further treatment of 1 and 2 with 1 mol equiv of (py)₂CO and NEt₃ afforded the methoxide derivative [{UO₂(OMe)}₂{μ-(py)₂C(OMe)O}₂] (4), while UO₂(NO₃)₂ was transformed into [{UO₂(NO₃)}{UO₂(OH)}{μ-(py)₂C(OMe)O}₂] (5). In these first structurally characterized actinide compounds with a (py)₂CO-based ligand, the uranium atoms are located at the center of pentagonal (X = Cl and OMe) or hexagonal (X = OAc and NO₃) bipyramids sharing one edge defined by the μ-alkoxo oxygen atoms. Crystals of [{UO₂(OMe)}₂{μ-(py)₂C(OMe)O}₂]·[(UO₂)₄{μ₂-(py)₂C(OMe)O}₂(μ₂-OAc)₂(μ₃-O)₂(MeOH)₂]·H₂O (6·H₂O) were serendipitously obtained in one experiment with [UO₂(OAc)₂(H₂O)₂] and (py)₂CO.     

Selective actinide solvent extraction used in conjunction with liquid scintillation

New commercial liquid scintillation counters allow rapid / measurements. Associated with liquid-liquid extraction techniques, rapid and selective actinide analyses are possible. Uranium, thorium and americium extractions with tri-n-octylphosphine oxide (TOPO) in toluene have been investigated. Detection limits of 40 mBq·l^–1 for -emitters are currently obtained with a Packard 2550 TR/AB^TM liquid scintillation analyzer.     

Trivalent actinide and lanthanide separations by tetradentate nitrogen ligands: a quantum chemistry study

Although a variety of tetradentate ligands, 6,6'-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2'-bipyridines (BTBPs), have been proved as effective ligands for selective extraction of Am(III) over Eu(III) experimentally, the origin of their selectivity is still an open question. To elucidate this question, the geometric and electronic structures of the actinide and lanthanide complexes with the BTBPs have been investigated systematically by using relativistic quantum chemistry calculations. We show herein that in 1:1 (metal:ligand) type complexes substitution of electron-donating groups to the BTBP molecule can enhance its coordination ability and thus the energetic stability of the formed Am(III) and Eu(III) complexes in the gas phase. According to our results, Eu(III) can coordinate to the BTBPs with higher stability in energy than Am(III), no matter whether there are nitrate ions in the inner-sphere complexes. The presence of nitrate ions leads to formation of the probable Am(III) and Eu(III) complexes, M(NO(3))(3)(H(2)O)(n) (M = Am, Eu), in nitric acid solutions. It has been found that the changes of Gibbs free energy play an important role for Am(III)/Eu(III) separation. In fact, the weaker complexing ability of Am(III) with nitrate ions and water molecules makes the decomposition of Am(NO(3))(3)(H(2)O)(4) more favorable in energy, which may thus increase the possibility of formation of Am(BTBPs)(NO(3))(3). Our work may shed light on the design of novel extractants for Am(III)/Eu(III) separation.     

Probing Ni[S2PR2]2 electronic structure to generate insight relevant to minor actinide extraction chemistry

A method to evaluate the electronic structure of minor actinide extractants is described. A series of compounds containing effective and ineffective actinide extractants (dithiophosphinates, S(2)PR(2)(-)) bound to a common transition metal ion (Ni(2+)) was analyzed by structural, spectroscopic, and theoretical methods. By using a single transition metal that provides structurally similar compounds, the metal contributions to bonding are essentially held constant so that subtle electronic variations associated with the extracting ligand can be probed using UV-vis spectroscopy. By comparison, it is difficult to obtain similar information using analogous techniques with minor actinide and lanthanide complexes. Here, we demonstrate that this approach, supplemented with ground state and time-dependent density functional theory, provides insight for understanding why high separation factors are reported for the extractant HS(2)P(o-CF(3)C(6)H(4))(2), while lower values are reported and anticipated for other HS(2)PR(2) derivatives (R = C(6)H(5), p-CF(3)C(6)H(4), m-CF(3)C(6)H(4)). The implications of these results for correlating electronic structure with the selectivity of HS(2)PR(2) extractants are discussed.     

Shape-persistent macrocycles: efficient extraction towards lanthanide and actinide elements

The extraction of three shape-persistent aromatic oligoamide macrocycles (cycloaramides) bearing either apolar or polar side chains at the periphery of the rings has been investigated towards some representative lanthanide and actinide ions, and alkali metal ions. The results from the liquid–liquid extraction of lanthanide and thorium ions from aqueous solutions into dichloromethane revealed remarkably high extractability of up to 99% and selectivity over alkali metal cations. The stoichiometry of the complex formed between the macrocycle and Eu³⁺ or Th⁴⁺ was determined to be 1:1.     

Tricyclic chemistry. Synthesis and chemistry of a novel polyheterocycle

The photochemical reaction of 1 with acrylonitrile gave the cyclobutane adducts II and III, which on treatment with base afforded a novel polycyclic structure IV. Reduction of IV with LAH followed by Eschweiler-Clarke methylation yielded the polyheterocyclic structure VI.     

Main group supramolecular chemistry

Metal directed self-assembly has yielded a wide array of two- and three-dimensional structures with fascinating new chemical properties. These structures have typically been prepared utilizing transition metals as directing units, owing to the well-defined coordination preferences these metals exhibit. An area of growing research interest involves the preparation of structures containing main group elements as directing units. This tutorial review surveys the wide range of structure types available through this approach, specifically covering unique structure types accessible from the unusual coordination geometries often exhibited by the elements in Groups 12-17 of the periodic table. This review should be of interest to supramolecular and main group chemists, and researchers in the fields of crystal engineering, host-guest chemistry, and molecular recognition.     

Coordination chemistry of trivalent lanthanide and actinide ions in dilute and concentrated chloride solutions

We have used EXAFS spectroscopy to investigate the inner sphere coordination of trivalent lanthanide (Ln) and actinide (An) ions in aqueous solutions as a function of increasing chloride concentration. At low chloride concentration, the hydration numbers and corresponding Ln,An-O bond lengths are as follows: La3+, N = 9.2, R = 2.54 A; Ce3+, N = 9.3, R = 2.52 A; Nd3+, N = 9.5, R = 2.49 A; Eu3+, N = 9.3, R = 2.43 A; Yb3+, N = 8.7, R = 2.32 A; Y3+, N = 9.7, R = 2.36 A; Am3+, N = 10.3, R = 2.48 A; Cm3+, N = 10.2, R = 2.45 A. In ca. 14 M LiCl, the early Ln3+ ions (La, Ce, Nd, and Eu) show inner sphere Cl- complexation along with a loss of H2O. The average chloride coordination numbers and Ln-Cl bond lengths are as follows: La3+, N = 2.1, R = 2.92 A; Ce3+, N = 1.8, R = 2.89 A; Nd3+, N = 1.9, R = 2.85 A; Eu3+, N = 1.1, R = 2.81 A. The extent of Cl- ion complexation decreases going across the Ln3+ series to the point where Yb3+ shows no Cl- complexation and no loss of coordinated water molecules. The actinide ions, Am3+ and Cm3+, show the same structural effects as the early Ln3+ ions, i.e., Cl- ion replacement of the H2O at high chloride thermodynamic activities. The Clion coordination numbers and An-Cl bond lengths are: Am3+, N = 1.8, R = 2.81 A; Cm3+, N = 2.4, R = 2.76 A. When combined with results reported previously for Pu3+ which showed no significant chloride complexation in 12 M LiCl, these results suggest that the extent of chloride complexation is increasing across the An3+ series. The origin of the differences in chloride complex formation between the Ln3+ and An3+ ions and the relevance to earlier work is discussed.