Neptunium oxidation-reduction potentials

Pentaralent neptunium is the most stable state in solution. It hydrolyzes only in basic solutions, disproportionates only at high acidity, and forms no polynuclear complexes. As shown by the oxidation-reduction potentials of Table 9.6, hexavalent neptunium is much less stable in solution than is hexavalent plutonium in fact, hexavalent neptunium is a strong oxidizing agent and is easily reduced in the presence of oxidizable substances, such as those present in ion-exchange and solvent extraction separations [K2]. 

Table 10.21 Oxidation-reduction potentials at 2S°C in neptunium recovery processes (Chap. 8)...

Oxidation-Reduction Reactions. The various oxidation states of neptunium in the KF solution were obtained by adding the neptunium, which had been adjusted to the desired valence state in dilute acid, to the KF solution. Observations were made of the eflFect of a number of potentially useful oxidizing and reducing agents upon the oxidation state of the neptunium. The valence states of the neptunium ions, IV, V, and VI, were determined by absorption spectra. During the reduction to the III state, the solution color disappeared, and a purple solid was produced. 

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. 

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