Neptunium aqueous chemistry

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. 

Aqueous coordination chemistry of neptunium. S. K. Patil, V, V. Ramakrishnan andM. V. Rama-niah, Coord. Chem. Rev., 1978, 25, 133-171 (216). 

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. 

Newton, T. W and F. B. Baker Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium. In if. F. Gould (Ed.), Lanthanide/Actinide Chemistry, Advances in Chemistry Series, Vol. 71, p. 268. Washington American Chemical Society 1967. 

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. 

No definite reason for these fluctuations could be identified, but it is known that neptunium, due to its complicated redox chemistry, reacts in a very sensitive way to even minor process variations (7,8). Based on these results the proposal was made (J5) to recover the "co-extracted" portion of the neptunium by running the second plutonium and uranium purification cycles under conditions where the Np is directed into the aqueous raffinates (2AW and 2DW streams). In the Pu purification cycle, this can be done by adding sufficient nitrous acid to keep the Np pentavalent, while in the U purification cycle (which is run under slightly reducing conditions) a low acidity and a high loading help to reject Np into the aqueous 2DW stream. The two raffinate streams are combined in WAK in the 3W evaporator, and the Np is thus collected in the concentrate from this unit (3WW stream). Consequently the proposal was made to recover the Np from this 3WW stream by use of the well-known anion exchange process (9,J ). 

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. 

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]. 

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