Speciation and ammonia-induced precipitation of neptunium and uranium ions

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The pH evolution and corresponding changes in the UV-Vis-NIR absorption spectra of oxygenated neptunium (NpO₂⁺ and NpO₂²⁺) and uranyl ions (UO₂²⁺) in nitric acid are investigated during titration with aqueous NH₃ solution. The speciation and precipitation regimes between acidic (pH 1.5) and alkaline (pH 10) conditions at room temperature are discussed to assess the suitability of Np(V) or Np(VI) in sol-gel conversion processes for fuel target fabrication. Under the applied experimental conditions, Np(V) hydrolyzes and precipitates into the insoluble hydroxide NpO₂OH only above pH values 7.5 and an increase up to pH 10.0 is required to precipitate quantitatively. Np(VI) displays changes in the coordination environment of NpO₂²⁺ ions in the pH interval 1.6 to 4.0, similar to what is observed for U(VI). Precipitation into NpO₃ . H₂O or other hydroxide compounds takes place between pH 4.0 – 5.9, which overlaps largely with precipitation of ammonium diuranate species from U(VI) solution. The use of concentrated NH₃ aqueous solution, as commonly used in the external gelation process, allows to quantitatively precipitate both Np(V) and Np(VI) species. Internal gelation process conditions, on the other hand, seem incompatible with the high pH required to precipitate Np(V) completely. For fabricating mixed-oxide (U,Np) targets using sol-gel conversion, a feed broth containing Np(VI) and U(VI) will be required to achieve homogenous gelation.