Neptunium oxidation

In solution, neptunium oxidizes to Np + and Np + ions, the salts of which are pink and greenish-yellow, respectively. Unlike its rare earth analog promethium, neptunium also forms oxoions, such as, NpO" " (blue green) and NpQ2+ (light pink). 

The neptunium oxide system exhibits complexity similar to that found in the uranium oxide system. Thus, the important oxide is NpCL and there exists a range of compositions, depending upon conditions, up to Np30g. 

In research at the Institute of Radiochemistry, Karlsruhe, West Germany, during the early 1970s, investigators prepared alloys of neptunium with indium, palladium, platinum, and rhodium. These alloys were prepared by hydrogen reduction of the neptunium oxide in (he presence of finely divided noble metals. The reaction is called a coupled reaction because the reduction of the metal oxide can be done only m the presence of noble metals. The hydrogen must be extremely pure, with an oxygen content of less than 10 25 torr. 

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. 

Ba2CaNpOe (s) Barium Calcium Neptunium Oxide BajCaNpOg (s) 2BaO CaO NpOs (s) 2BaO CaO NpOs (s) ... 

Bo.M2(P04)3. The thorium, uranium, and neptunium phosphates of general formula Bo 5M2(P04)3 with B = Be, Mg, Ca, Sr, Ba are characterized by extensive polymorphism [5,53,60,61,62,63,64]. As a result of thermal treatment of Mg, Ca, and Sr neptunium phosphates with unknown structures in an Ar + 5% H2 atmosphere, these compounds were established [5,53] as substances crystallizing with the monazite structure type. The authors of [35] have obtained a series of neptunium phosphates with an analogous composition by interaction between solutions of salts of divalent metals (Mg, or Ca, or Sr) with neptunium oxide Np02 and phosphoric acid followed by thermal stage-by-stage treatment... 

Duyckaerts, Landresse and Lysy reported investigations of the solubility of various uranium and neptunium oxides in the molten KCl-LiCl eutectic at 660 °C [231, 252], The solubility product of Np02 in the chloride melt was estimated to be pPNp02 = 20.3 0.5 [252]. 

Neptunium oxides. Keller [K2] reports five binary oxides or oxide hydrates of neptunium NpOj, NpjOs, NpsOg, Np03 2H20, and NpOs-HjO. Anhydrous Np(VI) oxide has not been prepared. 

Neptunium dioxide, Np03, is the most stable of the neptunium oxides. It crystallizes with the fluorite structure of all the actinide dioxides, with a crystalline density of 11.14 g/cm . It can be formed from the thermal decomposition of other neptunium compounds, such as the hydroxide, the nitrate, or the oxalate, in the temperature range of 600 to 1000°C. High-fired NpOj can be dissolved in hot concentrated nitric acid containing small amounts of fluoride. 

These reaction rates are too low to explain the appreciable extraction of neptunium obtained in the short-residence-time HA contactors used at Hanford and elsewhere. Swanson [S24] reported that radiolysis reaction products of TBP and nitric acid present in Purex solutions increased the neptunium oxidation rate and provided a possible explanation. He found that the oxidation rate could be increased several orders of magnitude by adding a synthetic rate-accelerating material (RAM) produced by reacting the aciform of nitropropane, CjHs(CH)(NO)(OH), with nitric and nitrous acids, and recommended addition of such a catalyst to Purex feed if increased neptunium extraction were desired. 

Fig. 18.4 The spectra of four neptunium oxidation states showing quadrupole...

Preliminary electrolysis This results in codeposition of uranium and neptunium oxides on a cathode. 

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