Neptunium pentavalent

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

The plutonium is oxidized to Pu(IV) and Pu(VI), while the neptunium ends up in the pentavalent or hexavalent states. Small amounts of plutonium and fission products... 

Neptunium - Neptunium is known to be present in oxic seawater predominantly as the highly soluble pentavalent species NpOj but, as would seem to be suggested... 

The pentavalent oxidation state is accessible for the early actinides uranium, protactinium, neptunium, and plutonium. Pentavalent species with neutral Group 16 bases can include either adducts of AnXg or complexes incorporating oxo-containing cations, AnO " or An02". ... 

Binary halides. A number of homoleptic halides of pentavalent protactinium, uranium, and neptunium have been reported. In particular, the fluoride complexes AnFs are prepared by high... 

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. 

Compounds 6-10 represent complexes of pentavalent neptunium with all isomers of pyridinemonocarboxylic acids. In all compounds, Np atom was found in seven coordinated pentagonal bipyramidal geometry where axial... 

The complexation of pentavalent neptunium with pyridinecarboxylic acids was investigated by different methods including spectrophotometry, potentiometry and solvent extraction [18,42,101,127-130]. Depending on the technique, complexation model used in calculations and experimental conditions, different values of stability constants were obtained (Table 8). 

Two complexes of pentavalent neptunium with imidazole were synthesized and structurally characterized [133]. In the sructure of neptunyl acetate [(Np02)(CH3C00)(Im)2(H20)] (21) coordination surrounding of Np atom is formed by two imidazole molecules, bidentate acetate anion and water molecule (Fig. 21). Nitrogen atoms deviate significantly from the equatorial plane (by 0.430 A). Water molecule also displays significant deviation (0.380 A). Equatorial distances are equal to 2.551(5) A (Np-N), 2.532(5) A (Np-Oac) and 2.398(5) A (Np-Ow). 

G. B. Andreev, Interaction of pentavalent neptunium with N-containing ligands, Ph. D. Dissertation, Institute of Physical Chemistry RAS, Moscow, 2001. 

Since 1958, more than 20 nuclides of actinides ranging from neptunium to einsteinium were identified and prepared for tracer studies. From neutron-irradiated uranium samples 2 9Np was adjusted to the pentavalent state and separated by TBP extraction from perchloric acid media. Plutonium-239 was separated by TBP extraction from nitric acid solution followed by anion exchange in a system of Dowex-1 resin and nitric acid. Neptunium-237 was separated from a spent fuel solution of JRR-1 (Japan Research Reactor -1) using anion exchange and TBP extraction. The TBP extraction in the hydrochloric acid medium is a simple and effective technique to purify neptunium from plutonium contamination. On the other hand, both anion exchange and solvent extraction with HDEHP could be used to separate tracer scale plutonium from irradiated neptunium targets. 

A laboratory study was undertaken to determine the behaviour of neptunium in the WAK flowsheet, and to devise a procedure for its recovery. Based on static ( ) and counter-current experiments (J5), the conclusion was reached that about half of the Np is co-extracted with the U and Pu in the HA-HS mixer-settlers of WAK while the other half is rejected to the HAW, see Fig.1. It could also be shown that an increase of the aqueous acidity, or the addition of pentavalent vanadium as an oxidant into the lower stages of the HA mixer-settler (6), would increase the Np yield in the organic solvent. In the 1BX-1BS mixer-settlers where the partitioning of U and Pu is carried out by use of uranium (IV)nitrate - hydrazine nitrate, a splitting of the coextracted Np between the two product streams was observed the proportions of the (co-extracted) Np which ended up in the 1CU (uranium product) stream fluctuated from 30 to 93 % while the difference amount (from 7 to 70 %) ended up in the 1 BP (plutonium product) stream. 

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 ). 

NH4, Rb, and Cs, are isostructural with KgPaF (37, 39), but the analogous uranium(V) complexes possess diiferent structures. The rubidium salts of pentavalent uranium, neptunium, and plutonium are, in fact, isostructural with K2NbF7, being therefore 7-coordinate. 

Plutonium in oxide fuel dissolves as a mixture of tetravalent and hexavalent plutonyl nitrates, both of which are extractable with TBP. Neptunium dissolves as a mixture of inextractable pentavalent and extractable hexavalent nitrates. 

Because the distribution coefficient in TBP of hexavalent neptunium is higher than tetravalent, the hexavalent form is preferred for the first extraction from fission products. The first part of Table 10.22 gives equations for the concentration ratio of hexavalent to pentavalent neptunium calculated for the three oxidants listed there, with the coefficient evaluated from exp(-38.93 A °). 

Distribution coefficients of neptunium in 30 v/o TBP depend on neptunium valence, temperature, and concentrations of uranyl nitrate, nitric acid, and other nitrates. At the nitric acid concentraticHis below 4 M usually used in Purex processes, the distribution coefficient of hexavalent neptunium is hi er than that of tetravalent neptunium at the same nitric acid and uranyl nitrate concentrations. Both are much higher than that of pentavalent neptunium. Both tetravalent and hexavalent neptunium are extracted as the complexes with two molecules of TBP, Np (N03)4 2TBP and Np 02(N03)2-2TBP. 

In the HA extracting and HS scrubbing sections of the Purex process, pentavalent neptunium is partially oxidized to the hexavalent state by nitrate ion,... 

HNOj concentration of the aqueous phase must be over 0.00004 M. At equilibrium, neptunium in the aqueous phase is then divided between the hexavalent and pentavalent states. The ratio of hexavalent to pentavalent neptunium is given by Eq, (10.27), obtained from the equilibrium ratio /lnp defined by Eq. (10.24), and plotted in Fig. 10.30. 

Because pentavalent neptunium is essentially inextractable, the neptunium concentration in the organic phase Np is related to the distribution coefficient of hexavalent neptunium Np(vi) by... 

Oxidation of pentavalent neptunium by nitric acid. Oxidation of pentavalent neptunium to hexavalent by nitric acid requires catalysis by nitrous acid. The kinetics of this reaction have been studied by Siddall and Dukes [S16], Swanson [S24] and Mouline [M9]. Siddall and Dukes reported that the reaction was first order in neptunium concentration, independent of nitrous acid concentration if greater than 5 X 10" M, and depended on temperature T (K) and nitric acid molarity Xh as can be represented by Eq. (10.33) ... 

Oxidation of pentavalent neptunium by pentavalent vanadium. Oxidation of pentavalent neptunium by pentavalent vanadium proceeds at a practical rate without catalyst. Dukes [D4] found that the rate of reaction could be represented by... 

Table 10.24 Rate of oxidation of pentavalent neptunium by pentavalent vanadium...

Reduction with tetravalent uranium. Newton [N3] found the rate of reduction of hexavalent neptunium to pentavalent to be rapid and given at 25°C by... 

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