X-ray absorption and Raman spectroscopy studies of molybdenum environments in borosilicate waste glasses (original) (raw)
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High molybdenum concentration in glass compositions may lead to alkali and alkaline-earth molybdates crystallization during melt cooling that must be controlled particularly during the preparation of highly radioactive nuclear glassy waste forms. To understand the effect of molybdenum addition on the structure of a simplified nuclear glass and to know how composition changes can affect molybdates crystallization tendency, the structure of two glass series belonging to the SiO2–B2O3–Na2O–CaO–MoO3 system was studied by 29Si, 11B, 23Na MAS NMR and Raman spectroscopies by increasing MoO3 or B2O3 concentrations. Increasing MoO3 amount induced an increase of the silicate network reticulation but no significant effect was observed on the proportion of BO4 units and on the distribution of Na+ cat-ions in glass structure. By increasing B2O3 concentration, a strong evolution of the distribution of Na+ cations was observed that could explain the evolution of the nature of molybdate crystals (CaMoO4 or Na2MoO4) formed during melt cooling.
The incorporation of molybdenum is one of the major challenges in the nuclear glass formulation issues, since its relatively low incorporation rate could induce the crystallization of Mo-rich soluble phases during the cooling down of the glass. In the scope of the reprocessing of increased burn up rates of UO 2 spent fuels, a new glassy material with high level waste (HLW) content is in development. The aim of this paper is to present the molybdenum oxide behavior in a simplified soda-lime borosilicate composition derived from that of what could be this HLW glass. Several studies performed on glass series with 5.6 wt% MoO 3 and with variable contents of boron and calcium oxides showed the strong impact of these oxides on the molybdates crystallization tendency (XRD experiments). Moreover, addition of calcium and boron oxides promoted the incorporation molybdenum in the glass network (microprobe analysis).
Physics and Chemistry of Glasses: European Journal of Glass Science and Technology Part B, 2018
Understanding the roles of vanadium and molybdenum in borosilicate glasses has attracted much attention in the development of high performance nuclear waste glasses with high waste loads. In such applications, the Na + charge compensation mechanism is central in order to control poorly durable Mo-rich crystalline phases. Therefore, structural analyses of sodium borate and silicate glasses with the coexistence of vanadium and molybdenum have been performed. 11 B and 29 Si solid state NMR and Raman spectroscopies were applied to study the resulting structural changes of the glasses as a function of the V and Mo composition. For the borate glasses, the Na + charge compensation mechanism was not influenced by the presence of V 2 O 5 and MoO 3. For the silicate glasses, vanadium was found to be an Na + charge compensator in the absence of MoO 3. For silicate glass containing MoO 3 , polymerisation of the glass containing 5 mol% V 2 O 5 was lower than that of the glass containing 2 mol% V 2 O 5. Apparently, the presence of vanadium in the glass containing 5 mol% V 2 O 5 and 2 mol% MoO 3 led to the formation of a sufficient number of positively charged units to compensate for MoO 4 2- .
Journal of Non-Crystalline Solids, 2002
X-ray absorption spectroscopic data were collected and analyzed to characterize vanadium in borosilicate glasses used for immobilization of sulfur-containing nuclear wastes. Data are presented for borosilicate glasses, some with and some without sulfur, that have V 2 O 5 concentrations as high as 12 wt%, and for the sulfides: sulvanite and patronite, the silicates: cavansite, hadaraite, and roscoelite, and the oxide: vanadinite. X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data for the glasses have no sulfur dependent features, but do show changes that parallel various redox conditions for the corresponding melts. EXAFS data for the glasses indicate V-O distances near 1.70 A A that are considerably shorter than typical V-S distances found in the sulfides. Both XANES and EXAFS indicate that most or all vanadium in these glasses is in the form of V 5þ O 4 tetrahedra; glasses synthesized under reducing conditions can have penta-coordinated V 4þ populations up to approximately 20-25% of all vanadium present. Ó (D.A. McKeown). 0022-3093/02/$ -see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 -3 0 9 3 ( 0 2 ) 0 0 9 4 5 -6
Vienna/Environmental, 2012
The immobilisation of molybdenum in model UK high level nuclear waste glasses was investigated by X-ray Absorption Spectroscopy (XAS). Molybdenum K-edge XAS data were acquired from several inactive simulant high level nuclear waste glasses, including specimens of Magnox and Magnox / UO 2 waste glasses prepared during commissioning of the BNFL Sellafield Waste Vitrification Plant. These data demonstrate that molybdenum is immobilised in the Mo (VI) oxidation state as the tetrahedral MoO 4 2species, with a MoO contact distance of ca 1.76 Å. The MoO 4 2species are not immobilised within the polymeric borosilicate network, instead they are likely to be located in extra-framework cavities, together with network modifier cations. Molybdenum K-edge XAS data acquired from "yellow phase" material, removed from a sample of simulant Magnox waste glass, show that this substance incorporates Mo (VI), also present as tetrahedral MoO 4 2species. The presence of isolated MoO 4 2tetrahedra within the glass matrix provides an explanation for the initial rapid release of molybdenum from simulant HLW waste glasses in static dissolution experiments and the strong correlation of the initial Mo leach rate with that of Na and B.
2010
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In nuclear borosilicate glasses, when molybdenum is in too high concentration and when it combines with other elements such as alkali and alkaline-earth elements it may form crystalline molybdates, including sodium molybdate, Na2MoO4, during melt cooling. In a nuclear vitrification context, the origin of this phenomenon must be understood to control and to avoid the appearance of this water-soluble crystalline phase. The solubility limit of MoO3 was found to be 2.5 mol% in a simplified SiO2–B2O3–Na2O–CaO nuclear glass at about 1300°C. Higher MoO3 concentrations induced liquid phase separation followed by crystallization of Na2MoO4 and CaMoO4. This study assessed the impact of increasing the CaO and B2O3 content on the tendency of the melts to crystallize and the impact on the glass network structure. Structural analysis (95Mo MAS NMR and 11B MAS NMR) of several glass series and standard SiO2–Na2O–MoO3 or SiO2–CaO–MoO3 glass showed that the nature of the crystallized phases that may appear during cooling of the melt can be controlled by correlation of the proportion of Na+ cations remaining free in the glass network with the soda/lime environment of tetrahedral MoO42- entities.
Journal of Nuclear Materials, 2010
Raman spectroscopy and X-ray absorption spectroscopy (XAS) are used to characterize structural changes that took place in hydrothermally altered (Na,K)-alumina-borosilicate glasses with different Na/K ratios, formulated as part of a durability study to investigate the behavior of glasses for nuclear waste storage. The hydrothermal experiments, or vapor hydration tests (VHT), were performed on each glass for 3 and 20 days at 200°C to accelerate and approximate long-term alteration processes that may occur in a nuclear waste repository. Results found for both glasses and their VHT altered counterparts show little, if any, structural influence from the different starting Na/K ratios. X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and Raman spectroscopy indicate that the altered samples are mostly amorphous with small amounts of analcime-like and leucite-like crystals within 200 lm of the sample surface and contain up to 9.7 wt.% water or OH. The Raman data are nearly identical for the amorphous portions of all altered VHT samples investigated, and indicate that two glass structural changes took place during alteration: one, partial depolymerization of the alumina-borosilicate network, and two, introduction of water or OH. Al and Si XAS data indicate tetrahedral AlO 4 and SiO 4 environments in the original glasses as well as in the altered samples. Small energy shifts of the Si K-edge also show that the altered VHT samples have less polymerized networks than the original glass. Na XAS data indicate expanded Na environments in the VHT samples with longer Na-O distances and more nearestneighbor oxygen atoms, compared with the original glasses, which may be due to hydrous species introduced into the expanding Na-sites.
X-ray absorption studies of manganese valence and local environment in borosilicate waste glasses
Journal of Non-Crystalline Solids, 2003
X-ray absorption data were collected and analyzed to characterize the manganese environments in borosilicate glass formulations to be used for immobilization of nuclear wastes. Mn can become a significant constituent in some radioactive wastes, because of the use of Mn-compounds in waste pretreatment processes. Sixteen borosilicate glasses were investigated, which were synthesized to simulate the Mn environments in the anticipated waste glasses, where MnO concentrations range from 0.4 to 13.6 wt%. The X-ray absorption near edge structure (XANES) for all glasses investigated indicate that most of the manganese within these samples is divalent. The extended X-ray absorption fine structure (EXAFS) analysis results for the glasses show average Mn-O distances near 2.07 A A, coordination numbers between 4.3 and 5.2, and large first-shell Debye-Waller factors. The EXAFS findings indicate that Mn 2þ in borosilicate glass is most likely within a distribution of environments that include 4-and 5-coordinated sites. EXAFS data and fitting results also show that the average manganese environments in these glasses are statistically invariant with respect to composition as well as to synthesis conditions.