Neptunium behavior

Neptunium behavior in Purex systems is discussed further in Sec. 7. 

Gil. Gourisse, D. L oratory Studies of Nitrous Acid and Neptunium Behavior in TBP Extraction Processes, Proceedings of the International Solvent Extraction Confererwe, vol. 1, 1971, p. 781. 

The chemistry of plutonium is unique in the periodic table. This theme is exemplified throughout much of the research work that is described in this volume. Many of the properties of plutonium cannot be estimated accurately based on experiments with lighter elements, such as uranium and neptunium. Because massive amounts of plutonium have been and are being produced throughout the world, the need to define precisely its chemical and physical properties and to predict its chemical behavior under widely varying conditions will persist. In addition to these needs, there is an intrinsic fundamental interest in an element with so many unusual properties and with so many different oxidation states, each with its own chemistry. 

Figures 10, 11 and 12 show the results of the contact experiments for the basalt, shale and quartz monzonite samples. The rate of adsorption was rapid during the first two weeks and changed slowly thereafter. In these experiments Pu, Am and Cm exhibited behavior similar to the results obtained in the blank experiments. Uranium showed moderate adsorption ( 50 percent) on the basalt but only slight adsorption (10-20 percent) on the shale and quartz monzonite wafers. Neptunium showed strong adsorption (70-80 percent) on the shale and slight adsorption ( v 10 percent) on the basalt and quartz monzonite.

Taylor, R.J., Sinkov, S.I., Choppin, G.R., May, I. 2008. Solvent extraction behavior of neptunium(rV) ions between nitric acid and diluted 30 % tri-butyl phosphate in the presence of simple hydroxamic acids. Solvent Extr. Ion Exch. 26 (1) 41-61. 

The sorption and desorption behavior of uranium is similar to neptunium. Figure 3 shows that hysteresis is more important for uranium sorption under reducing conditions than under oxidizing conditions. Values of Ng/Nd are 10 and approximately 200 for oxidizing and reducing conditions, respectively. 

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. 

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

Another unsuccessful attempt has been made to prepare AmFe, by fluorinating Am203 in the presence of PtFe (100). Complexes of UFe such as Na2UF8 (d3), NH4UF7 (102) and NOUF7 (56) are now known, but the complexing behavior of neptunium and plutonium hexafluorides and uranium hexachloride has scarcely been investigated. 

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. 

Oxidation state. Differences among the potentials of the redox couples of the actinides account for much of the differences in their speciation and environmental transport. Detailed information about the redox potentials for these couples can be found in numerous references (e.g., Hobart, 1990 Silva and Nitsche, 1995 Runde, 2002). This information is not repeated here, but a few general points should be made. Important oxidation states for the actinides under environmental conditions are described in Table 4. Depending on the actinide, the potentials of the III/IV, IV/V, V/VI, and/or IV/VI redox couples can be important under near-surface environmental conditions. When the redox potentials between oxidation states are sufficiently different, then one or two redox states will predominate this is the case for uranium, neptunium, and americium (Runde, 2002). The behavior of uranium is controlled by the predominance of U(VI) species under... 

Americium. The low solubilities and high sorption affinity of thorium and americium severely limit their mobility under environmental conditions. However, because each exists in a single oxidation state—Th(IV) and Am(III)— under environmentally relevant conditions, they are relatively easy to study. In addition, their chemical behaviors provide valuable information about the thermodynamic properties of trivalent and tetravalent species of uranium, neptunium, and plutonium. 

Lemire R. J. (1984) An Assessment of the Thermodynamic Behavior of Neptunium in Water and Model Groundwaters from 25 to 150°C. AECL-7817, Atomic Energy of Canada Limited, Pinawa, Manitoba, Canada. 

McCubbin D. and Leonard K. S. (1997) Laboratory studies to investigate short-term oxidation and sorption behavior of neptunium in artificial and natural seawater solutions. Mar. Chem. 56, 107-121. 

The purpose of this study was to investigate the anionic exchange behavior of neptunium(V) in sulfate-sulfuric acid, because neptunium is often present as a contaminant during the separation of other actinides (l ). Sulfuric acid systems are seldom utilized in industrial processes, but are often used as part of a laboratory analytical procedure. Literature on neptunium in HC101, HC1, HC1-HF, and HNO3 is quite complete, but the information on the H2S0l system is sketchy at best. There is one report 2) that neptunium(V) is adsorbed strongly on Dowex 2 resin from 0.1 IT to 1 IT H SOip Our measurements indicate that there is very little adsorption of Np(V) on Dowex 1 resin even at low concentrations of sulfate-sulfuric acid. We believe the differences in chemical structure of the two resins are not sufficient to explain the disparity in adsorption. 

The sucessful experiments for the retention of plutonium onto alumina from TTN0 -HF solution gave enough confidence to recomend the proposed method to separate traces of plutonium from waste solutions in the presence of macroamounts of uranium (VI). Of course, only macroamounts of thorium, uranium (IV) and rare earths are serious interfering ions, since they precipitate with HF. The behavior expected for neptunium in the same system should be similar to plutonium, thorium and rare earths. The retention of neptunium from HNO - HF solutions is in progress. The sorption yield for Pu was around 95%. The sorption mechanism is not well established. Figure 3 shows the proposed flowsheet for recovery of Pu traces from reprocessing waste solutions. 

Fujita, T, M. TsuKAMoro, T. Ohe, S, Nakaya.ma, and Y. Sakamoto. 1995, Modeling of neptunium(V) sorption behavior onto iron-containing minerals. Mat. Res Soc. symp. proc. 353, pp. 965-72. 

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