X-ray absorption studies of bismuth valence and local environments in borosilicate waste glasses

X-ray absorption spectra (XAS) were collected and analyzed to characterize bismuth (Bi) environments in borosilicate glass formulations developed for the immobilization of high level nuclear wastes (HLW), from the bismuth phosphate process. Therefore, the structural role of Bi in these glasses is of interest; in addition in the present study, more particular interest in Bi originated from unusual foaming that was observed during melt cooling, where it was initially suspected that Bi reduction to Bi may generate oxygen that caused the foaming. Observations from scanning electron microscopy of some HLW glass samples indicated a Bi-phosphate association. Bi LIII XAS of 13 Bi-containing waste glass formulations of various compositions were measured that exhibited varying degrees of melt foaming. The Bi XAS are similar for all glasses investigated, and indicate BiO3 nearest-neighbor environments with Bi–O distances near 2.13 Å. This environment is similar to the most localized Bi coordination characteristics in the crystalline Bi-silicates, eulytite (Bi4Si3O12) and bismutoferrite (BiFe2Si2O8OH). However, the Bi-environments in the glasses are distinctly different from the Bi-site in crystalline BiPO4; therefore, XAS indicates no evidence of Bi-phosphate domains in the glasses measured. No XAS evidence was observed in any of the glasses investigated for Bi clustering, such as metallic Bi, or Bi–O–Bi bonding. Since the local Bi environments look similar for all glasses investigated, Bi XAS data and analyses show no association of the melt foaming problems with changes of Bi environments in the corresponding glass. The foaming may be dependent on chromate or phosphate behavior in the glass structure. 2011 Elsevier B.V. All rights reserved.