Neptunium solid compounds

This study is concerned with the chemistry of the actinides in saturated KF solution. The areas examined are solubilities, absorption spectra, oxidation-reduction reactions, and solid compounds that can be produced in this medium. This paper reports work with neptunium which is essentially complete, and also includes work with uranium and americium. 

Solid Compounds. The literature (2, 5, 8, 9, 12) on solid actinide fluoride complexes is extensive, and many new compounds are being prepared. For example, Thoma, Friedman, and Penneman (12) reported the first preparation of complex fluorides formed between UF3 and the alkali fluorides KF, RbF, and CsF. This section summarizes the work which was performed concerning the solid compounds resulting from neptunium in saturated KF solution. 

Np(IV). Two solid compounds containing neptunium in the IV state have been prepared. If the purple Np(III) compound is oxidized... 

Neptunium and plutonium complexes [AnL4] with 8-hydroxyquinoline and its derivatives were obtained in the form of solid compounds (L=Ox, MOx, COx, DCOx) [62], In all cases the coordination number of metal atom is equal to 8, while Th(IV) and U(IV) are known to form complexes H[M(Ox)5] [114], Stability constants were determined by solvent extraction and are equal to logy04=45.28 and logy04=46.O5 for Np complexes with Ox and DCOx, respectively [62],... 

Pentavalent neptunium forms with monopyridinecarboxylic acids (picolininc, nicotinic, isonicotinic) solid complexes of different compositions. For picolinic acid, solid compounds with Np Pic ratio from 1 1 to 1 3 were synthesized and characterized using X-ray single crystal crystallography [101]. For nicotinic and isonicotinic acids only 1 1 complexes were obtained. In all cases heterocyclic nitrogen atom participates in the formation of coordination bonds with metal atom. 

The NIR spectra of solid compounds 6-10 were measured [101]. The positions of absorption bands of solid [Np02(Pic)(H20)2] and complex particle [Np02(Pic)] in solution were found similar. It may be concluded that the coordination spheres of Np atom are close in both cases, that is, in solution the picolinate ion coordinates to Np with the formation of chelate ring. Alternatively, significant shifts of absorption band to lower energies were observed in spectra of neptunium nicotinate and isonicotinate as the result of the formation of cation-cation structures in solid 9 and 10. 

A huge amount of research has been done on the solid compounds of transuranium elements. In this section, fundamental properties of oxide and some compounds of the transuranium elements are briefly overviewed. The oxygen to metal ratio (O/M ratio) is one of the important parameters to understand the solid-state behavior of these oxides. The fluorite-structure dioxide is well studied in the research of mixed oxide (MOX) nuclear fuels. O Table 18.14 gives fundamental properties of transuranium oxides (Morss 1991). Neptunium... 

Although PUCI4 has been well characterized in the gas phase (51) in the temperature range 670-1025 K, all attempts to obtain tTiTs compound in the solid state have failed. Use of a plot of the difference AHf(MCl4,c) - AHf(M , aq) (, 8) as a function of the actinide ionic radii ( ) (as done above for PuFa) in the case of thorium, protactinium, uranium and neptunium yields a first path leading to aH (PuC14,c). A second path involves the extrapolation to the plutonium system of the difference AH5o n( C 4 ) ... 

Np(III). a purple solid was produced when Np(III) in dilute DCl (which had been prepared by hydrogen reduction) was added to a saturated KF solution. This solid was very sensitive to oxidation and changed to a green solid after two water washes. The absorption spectrum of the purple compound showed that the neptunium is in the III state. The x-ray powder pattern for this compound, given in Table III, has not yet been indexed. It is not isomorphous with the recently prepared 3KF UF3 (12). 

A variety of methods have been used to characterize the solubility-limiting radionuclide solids and the nature of sorbed species at the solid/water interface in experimental studies. Electron microscopy and standard X-ray diffraction techniques can be used to identify some of the solids from precipitation experiments. X-ray absorption spectroscopy (XAS) can be used to obtain structural information on solids and is particularly useful for investigating noncrystalline and polymeric actinide compounds that cannot be characterized by X-ray diffraction analysis (Silva and Nitsche, 1995). X-ray absorption near edge spectroscopy (XANES) can provide information about the oxidation state and local structure of actinides in solution, solids, or at the solution/ solid interface. For example, Bertsch et al. (1994) used this technique to investigate uranium speciation in soils and sediments at uranium processing facilities. Many of the surface spectroscopic techniques have been reviewed recently by Bertsch and Hunter (2001) and Brown et al. (1999). Specihc recent applications of the spectroscopic techniques to radionuclides are described by Runde et al. (2002b). Rai and co-workers have carried out a number of experimental studies of the solubility and speciation of plutonium, neptunium, americium, and uranium that illustrate combinations of various solution and spectroscopic techniques (Rai et al, 1980, 1997, 1998 Felmy et al, 1989, 1990 Xia et al., 2001). 

The ni3(P04)4 and U3(P04)4 phosphates were first characterized in 1957-1963 [15-18] as isostructural compounds with monoclinic unit cells, space group P2, or Pm, or P2 m. More recently, the data were critically revised in a number of papers [8,19-24], where the authors used new methods of studying solids, and carried out detailed experiments on the phase formation in the systems Th02 -P2O5 and UO2 - P2O5. In the authors opinion [8,19-24], attempts to synthesize single phase products such as "M3(P04)4" for both thorium and uranium [15-18] failed. This was also the case for the compounds of similar stoichiometry with neptunium (IV) and plutonimn (IV) [19, 24],... 

Purification of the desired product is accomplished by sublimation where only the unreacted A1(BHi+)3 and AnCBH ) are volatile. The large difference in volatilities of these compounds permit easy separation. ThCBHit) and PaCBHit) are obtained on a 0° cold finger by heating the solid reaction mixture to 120° and 55°, respectively. Uranium, neptunium, and plutonium borohydrides sublime at room temperature and are collected in a dry ice trap through which the A1(BHi+)3 passes into a liquid nitrogen trap. 

A tabulation of some of these results shows that only the + 3 state of neptunium is observed in frozen solutions, whereas in the pure solid a range of values from +3 to +5 is observed, which is independent of the charge state of the americium in the parent compound. An interesting comparison for K5[Am 02(C03)s]3 in the pure solid and in frozen solution indicates the existence of Np + alone in the pure solid and only Np in the frozen solution. This suggests that the presence of ice at low temperatures provides a reducing medium for neptunium. 

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