Vanadium and chromium redox behavior in borosilicate nuclear waste glasses

Abstract

X-ray absorption spectroscopy (XAS) was used to characterize vanadium (V) and chromium (Cr) environments in low activity nuclear waste (LAW) glasses synthesized under a variety of redox conditions. V 2 O 5 was added to the melt to improve sulfur incorporation from the waste; however, at sufficiently high concentrations, V increased melt foaming, which lowered melt processing rates. Foaming may be reduced by varying the redox conditions of the melt, while small amounts of Cr are added to reduce melter refractory corrosion. Three parent glasses were studied, where CO-CO 2 mixtures were bubbled through the corresponding melt for increasing time intervals so that a series of redox-adjusted-glasses was synthesized from each parent glass. XAS data indicated that V and Cr behaviors are significantly different in these glasses with respect to the cumulative gas bubbling times: V 4+ /V total ranges from 8 to 35%, while Cr 3+ /Cr total can range from 15 to 100% and even to population distributions including Cr 2+. As Nacontent decreased, V, and especially, Cr became more reduced, when comparing equivalent glasses within a series. The Na-poor glass series show possible redox coupling between V and Cr, where V 4+ populations increase after initial bubbling, but as bubbling time increases, V 4+ populations drop to near the level of the parent glass, while Cr becomes more reduced to the point of having increasing Cr 2+ populations.

X-ray absorption and Raman spectroscopy studies of molybdenum environments in borosilicate waste glasses

“…The XANES data for all seven glasses (Fig.S1, red spectra) are nearly identical with respect to each other and are most similar to that for vanadinite. 5+ O 4 and V 4+ O 5 sites in each glass, as done earlier[25]. The fitting results indicate V 5+ O 4 tetrahedra in all glasses measured, with possible minor amounts (5 to 8 %) of V 4+ O 5 groups.…”

“…XANES data collected for the standards vary significantly from tetrahedral V 5+ O 4 in vanadinite to penta-coordinated V 4+ O 5 in cavansite (Fig. S1), and are nearly identical to the V XANES spectra of these compounds presented earlier [24][25][26][27]. The narrow edge feature near 5468 eV is most prominent for tetrahedral V 5+ in vanadinite, while the main edge shifts to lower energies for the more reduced penta-coordinated V 4+ in cavansite.…”

A crucible salt saturation method for determining sulfur solubility in glass melt

“…Chromium changes were very similar for all 3 LAW glasses; therefore, we only present the results of LAWB99 as an example (Figure C). It should be noted that the Cr in the glass may have Cr(III) and Cr(VI) coexisting, and the extracted Cr in the sodium sulfate salt is likely in the form of . Without measurable volatile loss, Cr is extracted into the salt after the first mixing and melting and remains stable in both phases.…”

XAS and XRF investigation of an actual HAWC glass fragment obtained from the Karlsruhe vitrification plant (VEK)

“…XAS (X-ray Absorption Spectroscopy) studies of glass materials are numerously reported in the material sciences literature, covering such disparate applications as the investigation of historic artifacts (e.g., [6]) or geological specimen (e.g., [7]), characterization of optical glasses for solid state lasers (e.g., [8]) or the encapsulation of radiotoxic residues from nuclear fuel reprocessing (notably the work of McKeown et al [9][10][11][12][13][14][15][16]). These studies benefit from the ability of X-ray Absorption Fine Structure (XAFS) measurements to provide direct chemical speciation (e.g., determination of oxidation states and coordination geometries) through analysis of the Xray Absorption Near Edge Structure (XANES) -without the need for any sample pretreatment -while the Extended X-ray Absorption Fine Structure (EXAFS) reveals structural details on the local coordination environments (bond lengths, neighboring atom types) of glass matrix constituents -unlike standard X-ray Diffraction (XRD) analysis, which is severely hampered by the amorphous nature of elemental networks in glassy materials lacking any long range order.…”