The reduction of Bi5+ to Bi3+ paire

The reduction of Bi5+ to Bi3+ paired with americium oxidation in the acidic conditions employed in nuclear fuel reprocessing is not well understood, and there is conflicting data in the literature. Bismuth was proposed to be dissolved in the pentavalent state (Bi5+), as determined by iodometry [9]. However, standard Pourbaix diagrams suggest Bi5+ cannot exist in aqueous solution at any pH, and Bi3+ persists well above the Eh stability window for water. Mincher et al. described a decrease in the concentration of [AmO2]2+ in solution if the solid NaBiO3 is removed [13].These observations and studies point towards a heterogeneous oxidation process, in which Bi5+ is reduced to Bi3+ at the surface of the particles, and it may or may not become a part of the evolving solid lattice. Ding and coworkers [7] suggest Bi3+ self-dopes the lattice along with co-doped ‘holes’, responsible for the increasingly dark color with acid treatment. Their X-ray diffraction (PXRD) and Xray photoelectron spectroscopy (XPS) analyses of sodium bismuthate respectively show that the material becomes X-ray amorphous with mild acid treatment, and up to 35% of surface bismuth is trivalent. They additionally noted that Bi3+ could be deposited as a hexameric phase (Bi6O4(NO3)5(OH)5·0.5H2O on the surface of the sodium bismuthate particles.