Neptunium solution chemistry

The sorption behaviors of neptunium and plutonium were similar, thus confirming their suspected similarities in solution chemistry. Both NaOH and NaA102 decreased neptunium and... 

The separation of the lanthanides from thorium, uranium, plutonium, and neptunium can fairly readily be achieved by exploiting the greater extractability of the higher oxidation states of the light-actinide elements. However, the transplutonium actinides do not have stable higher oxidation states. In this case, separation of the lanthanide fission products from the transplutonium actinides must exploit the small differences in the solution chemistry of the lanthanides and actinides in the trivalent oxidation state. It is the separation of the lanthanides from the trivalent actinide cations that is the focus of this chapter. 

Table 3. Formal reduction potentials (in volts) of uranium, neptunium, plutonium and americium for 1 M perchloric add solutions at 25 °C. (F. A. Cotton and G. Wilkinson Advanced Inorganic Chemistry. Interscience Publishers 1972)...

The chemistry of neptunium (jjNp) is somewhat similar to that of uranium (gjU) and plutonium (g4Pu), which immediately precede and follow it in the actinide series on the periodic table. The discovery of neptunium provided a solution to a puzzle as to the missing decay products of the thorium decay series, in which all the elements have mass numbers evenly divisible by four the elements in the uranium series have mass numbers divisible by four with a remainder of two. The actinium series elements have mass numbers divisible by four with a remainder of three. It was not until the neptunium series was discovered that a decay series with a mass number divisible by four and a remainder of one was found. The neptunium decay series proceeds as follows, starting with the isotope plutonium-241 Pu-24l—> Am-24l Np-237 Pa-233 U-233 Th-229 Ra-225 Ac-225 Fr-221 At-217 Bi-213 Ti-209 Pb-209 Bi-209. 

Cohen, D. Fried, S., "Some Observations on the Chemistry of Neptunium in Basic Solution. Inorg. Nucl. Chem. Letters 1969,5,653-663. 

Allard, B. Kipatsi, H. LiTjinzin, J. O. "Expected Species of Uranium, Neptunium, and Plutonium in Neutral Aqueous Solutions, J. Inorg. and Nuclear Chemistry 1982,42,1015-1027. 

LaChapette, T. J., L. B. Magnusson, and J. C. Hindman The Chemistry of Neptunium. First Preparation and Solubilities of some Neptunium Compounds in Aqueous Solution. In G. T. Seaborg, J. J. Katz, and W. M. Manning (Eds.), The Transuranium Elements, National Nuclear Energy Series, Div. IV, Vol. 14B, p. 1097. New York McGraw-Hill 1949. 

Investigations of the solid-state chemistry of the americium oxides have shown that americium has properties typical of the preceding elements uranium, neptunium, and plutonium as well as properties to be expected of a typical actinide element (preferred stability of the valence state 3-j-). As the production of ternary oxides of trivalent plutonium entails considerable difficulties, it may be justified to speak of a discontinuity in the solid-state chemical behavior in the transition from plutonium to americium. A similar discontinuous change in the solid-state chemical behavior is certainly expected in the transition Am Cm. Americium must be attributed an intermediate position among the neighboring elements which is much more pronounced in the reactions of the oxides than in those of the halides or the behavior in aqueous solution. 

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. 

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 chemical properties of the actinides are much less similar to each other than those of the lanthanides, because the additional electrons added to the 5/ and 6d are bound less t tly than those of the 4/and 5d shells of the lanthanides. As shown in Table 9.4, the lanthanides in aqueous solutions exist principally in a single, tiivalent oxidation state, whereas four or more oxidation states are observed in the aqueous chemistry of uranium, neptunium, and plutonium. The actinide ions normally formed in solution by the oxidation states II through VI are M, M, M, MO2, MOj , respectively. 

Nobelium is a member of the actinide series of elements. The ground state electron configuration is assumed to be (Rn)5fl47s2, by analogy with the equivalent lanthanide element ytterbium ([Kr]4fl46s2) there has never been enough nobelium made to experimentally verify the electronic configuration. Unlike the other actinide elements and the lanthanide elements, nobelium is most stable in solution as the dipositive cation No ". Consequently its chemistry resembles that of the much less chemically stable dipositive lanthanide cations or the common chemistry of the alkaline earth elements. When oxidized to No, nobelium follows the well-estabhshed chemistry of the stable, tripositive rare earth elements and of the other tripositive actinide elements (e.g., americium and curium), see also Actinium Berkelium Einsteinium Fermium Lawrencium Mendele-vium Neptunium Plutonium Protactinium Ruthereordium Thorium Uranium. 

The chemistry of uranium in the Purex process is simpler than that of plutonium or neptunium. In the dissolver solution, U is present exclusively. The lower oxidation states (IV) and (V) are both unstable in nitric acid media, U " disproportionating rapidly and U being formed only in... 

Initial studies of the chemistry of neptunium and plutonium actually preceded the official establishment of the Manhattan Project. But as soon as the project got underway, they became the subject of intensive investigation at several of the Manhattan Project laboratories (Seaborg and Katz 1954). Both elements turned out to have four major oxidation states -F3, -F4, -F5, -F6, similar to uranium, hut plutonium is unique in that these four states can all exist simultaneously in aqueous solution. Microchemical techniques were applied to prepare and study microgram quantities, such as the first weighahle sample of a man-made element, 2.77 ig Pu02, in September 1942 (Cunningham and Wemer 1949). At the Los Alamos Laboratory, chemists and metallurgists learned to produce metallic plutonium and studied its complex properties, which eventually turned out to involve no less than six allotropic phases, more than any other element. 

Pentavalent and hexavalent actinide elements take oxygenated actinyl ions AnOa and An02 (An = actinide elements). Stable pentavalent neptunium takes the Np02 type structure in solution. The relationship between oxidation states and structural variation is fundamental information for solution and separation chemistry of actinide elements. As these actinyl ions, An02 and An02, are less acidic than the An ions, the tendency of hydrolysis decreases in the order, An " > An02 > An > An02. ... 

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. 

MO2 ions are formed by the four elements from uranium to ameridum. For uranium, the hexavalent oxidation state is the most stable one. Though easily reduced, it is also prominent in the chemistries of neptunium and plutonium. Ameridum(vi) is a very strong oxidizing agent. As the ameridum isotopes available are quite radioactive, a steady radiation-induced reduction of AmO occurs in aqueous solution, as soon as a strongly oxidizing system is not present [19]. 

Neptunium. Np is in a class with Pa no efforts have been made to use it as a fuel solute, but consideration has been given to its formation in and removal from blanket solutions of [30a]. The chemistry of neptunium has been reviewed by Hindman et al. [30b], and the hydrolytic behavior has been reviewed by Kraus [30c]. Continuous separation of Np239 would provide a Pu product of high purity by radioactive decay, whereas plutonium recovered from long-term irradiation of usually contains appreciable amounts of Pu °. Spectrophotometric cells for use at elevated temperatures and pre.ssures in the study of the chemistry of neptunium (and other materials) have recently been developed by Wag-gener [30d] and have been used to measure the absorption spectra of dilute neptunium perchlorate in its six-, five-, four-, and three-valence states, using heavy w ater as the solvent. Dilute solutions of neptunyl nitrate in nitric acid have been so studied at temperatures up to 250°C the pentavalent state was found to be stable under the test conditions [30e]. 

Neptmiium chemistry in uranyl sulfate solution. Neptunium dissolved in 1.4 m UO2SO4 at 250°C under air, stoichiometric mixture hydrogen and oxygen, or oxygen is stable in an oxidized valence state, prob-... 

---