Neptunium species

B. Kuczewski, C.M. Marquardt, A. Seibert, H. Geckeis, J.V. Kratz, N. Trautmann, Separation of plutonium and neptunium species by capillary electrophoresis-inductively coupled plasma-mass spectrometry and application to natural groundwater samples, Anal. Chem. 75 (2003) 6769—6774. 

The new elements neptunium and plutonium have been produced in quantity by neutron bombardment of uranium. Subsequently many isotopes have been obtained by transmutation and synthetic isotopes of elements such as Ac and Pa are more easily obtained than the naturally occurring species. Synthetic species of lighter elements, e.g. Tc and Pm are also prepared. 

A rather more specific mechanism of microbial immobilization of metal ions is represented by the accumulation of uranium as an extracellular precipitate of hydrogen uranyl phosphate by a Citrobacter species (83). Staggering amounts of uranium can be precipitated more than 900% of the bacterial dry weight Recent work has shown that even elements that do not readily form insoluble phosphates, such as nickel and neptunium, may be incorporated into the uranyl phosphate crystallites (84). The precipitation is driven by the production of phosphate ions at the cell surface by an external phosphatase. 

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. 

Schreckhise RG, Cline JF. 1980. Comparative uptake and distribution of plutonium, americium, curium and neptunium in four plant species. Health Phys 38 817-824. 

Allard, B., Kipatsi, H. and Liljenzin, J. 0., "Calculated Species of Uranium, Neptunium and Plutonium in Neutral Aqueous Solution", subm. to Radiochem. Radioanal. Lett. 

In an analysis of the hazards of the alpha emitters from reactor operations it has been pointed out (25) that the most significant and hazardous species are plutonium, americium, curium, and neptunium. Plutonium is as hazardous as such fission products as ruthenium-106, cesium-137, cerium-144, and promethium-147, depending on the kind of fuel, the power of the reactor, the storage time of the waste, and whether it is released to the atmosphere or to water. If strontium-90 is removed... 

A radioactive element is an element that disintegrates spontaneously with the emission of various rays and particles. Most commonly, the term denotes radioactive elements such as radium, radon (emanation), thorium, promethium, uranium, which occupy a definite place in the periodic table because of their atomic number. The term radioactive element is also applied to the various other nuclear species, (which arc produced by the disintegration of radium, uranium, etc.) including (he members of the uranium, actinium, thorium, and neptunium families of radioactive elements, which differ markedly in their stability, and are isotopes of elements from thallium (atomic number 81) to uranium (atomic number... 

A single oxo bridge may subtend an angle between 140° and 180°, this angle being determined by steric or electronic factors (Table 3).95 103 Almost all these examples refer to the solid state, but there are also several homo- and hetero-nuclear M—O—M and M—O—M—O—M species known in solution. Often these are intermediates in, or products of, electron transfer reactions with oxide-bridging inner-sphere mechanisms. Examples include V—O—V in V(aq)2+ reduction of VO(aq)2+, and Act—O—Cr in Cr(aq)2+ reduction of UOj+ or PuOj+ a useful and extensive list of such species has been compiled. Tlie most recent examples are another V—O—V unit, this time from VO(aq)2+ and VOJ,105 and an all-actinide species containing neptunium(VI) and uranium-(VI).106 An example of a trinuclear anion of this type, with the metal in two oxidation states, is provided by (31).107... 

The oxidation-reduction behaviors of neptunium, plutonium and americium in basic solution have been determined via polarographic and coulometric studies (6-9). These studies, which showed that the more soluble (V), (VI), and (VII) oxidation states of these actinides are stable in alkaline solution under certain redox conditions, helped identify possible actinide species and oxidation states in our experiments. Actual identification of radioelement oxidation states was not done in the present experiments. 

Neptunium and plutonium sorption behaviors were remarkably similar, implying that they had similar sorption reactions and solution species. Both NaOH and NaA102 decreased neptunium and plutonium sorption. Several explanations can be offered to rationalize this behavior. First, NaOH and NaAlO, may have reacted with the sediment minerals to yield solids of lower sorptive capacity. Aluminate ion, as an anionic species, also may have competed with the similar neptunate and plutonate anions for sorption sites. Finally, sodium hydroxide may have stabilized the hydrolyzed Np02(0H) and Pu02(0H)2" species in solution, as was shown in the solubility tests, and prevented sorption. Explanation of the effect of NaOH and NaA102 on neptunium and plutonium sorption will require further investigation. 

In contrast, the solubilities of the neptunyl species were increased by HEDTA and unexpectedly decreased by EDTA. Plutonyl(V) solubility was not affected by HEDTA or EDTA. The component most significant to neptunium solubility was NaNOz. Sodium nitrite apparently decreased neptunium solubility by reducing the more soluble neptunyl(VI) to the less soluble (V) state. The existence of two oxidation states complicated interpretation of the neptunium data. Sodium carbonate, phosphate, aluminate, and sulfate all increased neptunium solubility, probably through complexation. Separation of the data... 

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. 

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... 

Figure 13.4 Stability fields (redox potential vs hydrogen ion concentration) of the more important neptunium oxidation-state species in perchlorate solution.

An alternative procedure for the study of neptunium oxidation states at trace concentrations has been described by Inoue and Tochiyama (1977). They showed that, in the pH range 6-7, Nplv may be quantitatively absorbed on silica gel whilst Npv remains in solution. In acid solution, however, a precipitate of barium sulfate selectively absorbs Nplv leaving the higher oxidation states in solution. The authors gave no environmental data for neptunium in their publication but Nelson and Orlandini (1979) subsequendy adapted the procedure to demonstrate that the dominant oxidised plutonium species in natural waters is Puv and not Puvl. 

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