Uranyl, actinide ions

The elucidation of actinide chemistry in solution is important for understanding actinide separation and for predicting actinide transport in the environment, particularly with respect to the safety of nuclear waste disposal.72,73 The uranyl CO + ion, for example, has received considerable interest because of its importance for environmental issues and its role as a computational benchmark system for higher actinides. Direct structural information on the coordination of uranyl in aqueous solution has been obtained mainly by extended X-ray absorption fine structure (EXAFS) measurements,74-76 whereas X-ray scattering studies of uranium and actinide solutions are more rare.77 Various ab initio studies of uranyl and related molecules, with a polarizable continuum model to mimic the solvent environment and/or a number of explicit water molecules, have been performed.78-82 We have performed a structural investigation of the carbonate system of dioxouranyl (VI) and (V), [U02(C03)3]4- and [U02(C03)3]5- in water.83 This study showed that only minor geometrical rearrangements occur upon the one-electron reduction of [U02(C03)3]4- to [U02(C03)3]5-, which supports the reversibility of this reduction. 

Uranyl carbonato complexes are important and much studied since they occur in Nature and are of environmental concern this is also the case for other actinide carbonate complexes.40 The C03 ion is exceptionally strongly bound to UO + and similar actinide ions. There are several naturally occurring minerals such as U02C03 while the anion [U02(C03)3]4 has importance in the extraction of U from ores and is responsible for the migration of U02+ in ground waters. Interaction of the 4-ion with HC104 proceeds as follows ... 

T-bonding in the uranyl, [U02] ion (a) d -px overlap (b) fx22-px overlap (c) cr-bonding in the uranyl ion (reproduced with permission from Figure 3.24 of S.A. Cotton, Lanthanides and Actinides, Macmillan, 1991). 

The majority of the chemistry that has been investigated for the actinide elements has been in aqueous solutions. For the light actinides in acidic solutions, four types of cations persist trivalent, tetravalent, pentavalent, and hexavalent. The later two ions are always found to have trans oxo ligands, making up a linear dioxo unit. Actinide ions of this type are typically referred to yls and have the names, uranyl (1, U02+/ +), neptunyl (2, Np02+ +), plutonyl (3, Pu02+ +), (4, Am02+/ ) and so on. 

Hexavalent. Nitrate complexation with actinide ions is very weak, and the determination of the formation constants for aqueous nitrate solution species is extremely difficult. Under aqueous conditions with high nitric acid concentrations, complexes of the form An02(N03)(H20)x+, An02(N03)2(H20)2, and An02(N03)3 (An = U, Np, Pu) are likely to be present. Solids of the anionic trisnitrato complex have been isolated for U and Np however, minimal structural data have been obtained. Solid uranyl nitrate, U02(N03)2-xH20, is obtained as the orthorhombic hexahydrate from dilute nitric acid solutions and as the trihydrate from concentrated acid. The Np analog can be precipitated from a mixed aqneons HNO3 and MeCN solntion by the addition of 18-crown-6. Multiple structural determinations have been made for the hexavalent uranium nitrate complexes, and all show the common formula unit of... 

The oxidation-reduction behavior of plutonium is described by the redox potentials shown in Table I. (For the purposes of this paper, the unstable and environmentally unimportant heptavalent oxidation state will be ignored.) These values are of a high degree of accuracy, but are valid only for the media in which they are measured. In more strongly complexing media, the potentials will change. In weakly complexing media such as 1 M HClOq, all of the couples have potentials very nearly the same as a result, ionic plutonium in such solutions tends to disproportionate. Plutonium is unique in its ability to exist in all four oxidation states simultaneously in the same solution. Its behavior is in contrast to that of uranium, which is commonly present in aqueous media as the uranyl(VI) ion, and the transplutonium actinide elements, which normally occur in solution as trlvalent... 

Polyfunctional phosphinopyridine VV,P-dioxides, (phosphinomethyl)pyridine A,P-dioxides and bis(phosphinomethyl)pyridine VV,P,P-trioxides have been prepared, and selected coordination chemistry with actinide ions has been explored. The phosphinopyridine A,P-dioxides form bidentate chelates with uranyl and Th , and in the solid state these complexes display six-membered chelate rings that appear to be relatively sterically congested." " The solvent extraction properties of these ligands are not unique since they resemble the performance of trialkylphosphine oxides." ... 

The solution photochemistry of the actinides begins with uranium none has been reported for actinium, thorium, and protactinium. Spectra have been obtained for most of the actinide ions through curium in solution (5). Most studies in actinide photochemistry have been done on uranyl compounds, largely to elucidate the nature of the excited electronic states of the uranyl ion and the details of the mechanisms of its photochemical reactions (5a). Some studies have also been done on the photochemistry of neptunium (6) and plutonium (7). Although not all of these studies are directed specifically toward separations, the chemistry they describe may be applicable. 

The present results show that the DV-DFS method is applicable to calculations of potential energy curves for such a heavy and complicated system as uranyl nitrate dihydrate. It may also be useful to derive first-principles potential energy curves for the MD simulations. The electronic structure and MD results will be valuable for understanding dynamical properties of actinide ions in solution and for molecular design of novel extractants for selective separation of actinides. 

The situation is similar in actinide compounds but here the 5f-orbitals play a more active role and contribute to the chemical bond. Most applications have so far been to systems with a high oxidation state of the actinide ion. A typical case is the uranyl ion, UO + where the uranium ion has a formal charge of - -6. As a result, three strongly covalent bonds are formed to each of the oxygen atoms. The resulting active space consists of 12 electrons in 12 orbitals [43]. This active space can also be used when the uranyl ion forms complexes with other ligands, such as carbonate [44]. Additional active orbitals are needed for the neutral UO2 molecule [45]. Wahlgren and co-workers have... 

Complicating the development of ISEs for higher actinide ions is their inherent radioactivity. They also have chemistry tiiat often differs from that of the uranyl cation. Actinides from americium to lawrencium display solution-phase chemical features that resemble those of the trivalent lanthanides. Conversely, in certain oxidation states, the early actinides (thorium through neptunium) often mimic transition metals. Also, as mentioned above, many of the actinides can exist in a large number of oxidation states. For instance, in the case of plutonium, four oxidation states can exist simultaneously in aqueous solution. Finally, as true for the lanthanides, complex salts with hydroxide, halogens, perchlorates, sulfates, carbonates, and phosphates are well known for most of the actinides. 

The majority of the photochemical studies with actinide ions have been carried out with the uranyl (UC ion. This ion is yellow in color both in the solid and solution states. The early photochemistry of this ion has been reviewed. " Excitation of this ion results in an LMCT absorption that involves a transition from an essentially nonbonding 7r-orbital on oxygen into an empty 5/orbital on uranium. This LMCT assignment is that given to the weak visible bands in the absorption spectrum at 500 nm and 360 nm. The absorption spectrum also shows a series of bands of increasing intensity to higher energy. The positions of the absorption bands of are very sensitive to both temperature and the chemical environment... 

The authors have concluded that hexaphyrin in its oxidized, aromatic form is capable of complexing actinide ions, specifically uranyl (U02 ) and nep-tunyl (Np02 ) neptunyl represents the first crystalographically characterized neptunium complex. The complex is very stable and the authors believe that the system could see application in remediation of radioactive waste. ... 

Conceptual Flowsheet for the Extraction of Actinides from HLLW. Figure 5 shows a conceptual flowsheet for the extraction of all the actinides (U, Np, Pu, Am, and Cm) from HLLW using 0.4 M 0< >D[IB]CMP0 in DEB. The CMPO compound was selected for this process because of the high D m values attainable with a small concentration of extractant and because of the absence of macro-concentrations of uranyl ion. Distribution ratios relevant to the flowsheet are shown in previous tables, IV, V, VI, and VII and figures 1 and 2. One of the key features of the flowsheet is that plutonium is extracted from the feed solution and stripped from the organic phase without the addition of any nitric acid or use of ferrous sulfamate. However, oxalic acid is added to complex Zr and Mo (see Table IV). The presence of oxalic acid reduces any Np(VI) to Np(IV) (15). The presence of ferrous ion, which is... 

As already mentioned, UV-Vis spectroscopy is an effective tool to study metal complexes. For different actinide(IV) compounds in ILs (Np(IV), Pu(IV), U(IV) as [C4CiIm]2[AnCy complexes in [C4CiIm][Tf2N]) a similarity with solid complexes having an octahedral An(IV) environment was estimated [14,15]. In other ILs, for example, uranyl ions dissolved in [C4QIm] [NfO], [U02] may be present as a bare cation [16]. 

Uranium and thorium are actinide elements. Their chemical behavior is similar under most conditions. Both are refractory elements, both occur in nature in the +4 oxidation state, and their ionic radii are very similar (U+4 = 1.05 A, Th+4 = l.lOA). However, uranium can also exist in the +6 state as the uranyl ion (U02 2), which forms compounds that are soluble in water. Thus, under oxidizing conditions, uranium can be separated from thorium through the action of water. 

Most commonly, metal ions M2+ and M3+ (M = a first transition series metal), Li+, Na+, Mg2+, Al3+, Ga3+, In3+, Tl3+, and Sn2+ form octahedral six-coordinate complexes. Linear two coordination is associated with univalent ions of the coinage metal (Cu, Ag, Au), as in Ag(NH3)2+ or AuCL Three and five coordination are not frequently encountered, since close-packing considerations tell us that tetrahedral or octahedral complex formation will normally be favored over five coordination, while three coordination requires an extraordinarily small radius ratio (Section 4.5). Coordination numbers higher than six are found among the larger transition metal ions [i.e., those at the left of the second and third transition series, as exemplified by TaFy2- and Mo(CN)g4 ] and in the lanthanides and actinides [e.g., Nd(H20)93+ as well as UC Fs3- which contains the linear uranyl unit 0=U=02+ and five fluoride ligands coordinated around the uranium(VI) in an equatorial plane]. For most of the metal complexes discussed in this book a coordination number of six may be assumed. 

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