Isabelle S. Muller

Raman studies of sulfur in borosilicate waste glasses: sulfate environments

Abstract Raman spectroscopy has been used to characterize sulfur environments in a variety of borosilicate glass formulations developed for long-term radioactive waste storage. The spectra of these glasses all have S–O symmetrical stretch modes (ν1) near 1000 cm −1 from tetrahedral SO4 (sulfate) environments. The Raman data indicate that the sulfate environments are independent of the borosilicate network; in particular, isolated SO4 tetrahedra in the glass are surrounded by network modifying cations, such as Na. By changing the type of network modifying cations in the borosilicate glass, the ν1 peak shifts, in such a way that larger cation charge densities correlate to higher ν1 frequencies. The ν1 peak for Li, Ca, Na, K, and Cs borosilicate glasses is broadened and shifted to lower frequencies with respect to ν1 for Li, Ca, Na, K, and Cs sulfate crystals, respectively; this indicates that sulfate tetrahedra in borosilicate glass are more disordered and more weakly bonded to their surrounding environments than sulfate tetrahedra in the corresponding sulfate crystals.

Synthesis and characterization of a neutral, low spin iron(III) complex of a hexadentate tripodal ligand containing three imidazolate arms. Use as a dinucleating agent

Abstract The iron(III) complex of the Schiff base formed from the condensation of 1 equiv. of trist(2-aminoethyl)amine (tren) and 3 equiv. of 4-methyl-5-imidazolecar☐aldehyde has been synthesized by the reaction of iron(III) methoxide, tren and 4-methyl-5-imidazolecar☐aldehyde in methanol. Fetren(meim)3 is low spin as indicated by magnetic moment, Mo¨ssbauer and ESR measurements. The crystal structure of Fetren(meim)3 was determined (space group Pna21, R1 = 0.0331 and wR2 = 0.0857). The complex is six-coordinate although the ligand is potentially heptadentate. Basicity of the three ‘backside’ imidazolate nitrogens is demonstrated in the crystal structure by H-bonding interactions and in solution by reactions with acidic metal complexes. ZnTPP and M(hfa)2 (M = Cu(II), Ni(II)), to give imidazolate bridged adducts. Products of the reaction of Fetren(meim)3 with M(hfa)2 were isolated and identified as the 1:1 adducts.

Structural characterization of high-zirconia borosilicate glasses using Raman spectroscopy

Polarized Raman spectra were obtained for a collection of borosilicate glasses that have been developed as candidate compositions for the immobilization of wastes generated from the reprocessing of Zircaloy-clad spent nuclear fuel. Raman spectra were obtained for borosilicate glasses with zirconia compositions as high as 21 wt%, as well as for crystalline ZrO2 (baddeleyite) and crystalline ZrSiO4 (zircon). As zirconia content in the glass is increased, two trends in the spectra indicate that the partially polymerized silicate tetrahedral network becomes more depolymerized: one, the polarized mid frequency envelope near 450 cm ˇ1 , assigned to Si‐O‐Si symmetrical bend modes, decreases in area; and two, the higher frequency band assigned to Si‐O stretch in Q 2 units (silicate chains) increases in area, while band areas decrease for modes assigned to Si‐O stretch in more polymerized Q 3 and Q 4 units (silicate sheets and cages). These trends take place whether the glass composition is relatively simple or considerably more complex. As zirconia concentrations in the glass increase beyond 15 wt%, a series of sharp lines are observed in the spectra from baddeleyite crystals, and to a minor extent Zn‐Cr spinel phases, superimposed on broad features from the glass matrix. A low intensity, broad band near 1400 cm ˇ1 in the glass spectra is probably due to B‐O stretch modes within BO3 units. ” 2000 Elsevier Science B.V. All rights reserved.

X-ray absorption studies of vanadium valence and local environment in borosilicate waste glasses using vanadium sulfide, silicate, and oxide standards

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 V2O5 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 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 V5+O4 tetrahedra; glasses synthesized under reducing conditions can have penta-coordinated V4+ populations up to approximately 20–25% of all vanadium present.

X-ray absorption studies of the local environment of Zr in high-zirconia borosilicate glasses

Zirconium X-ray absorption spectroscopic (XAS) data were collected and analyzed for a series of zirconia borosilicate glasses developed as potential formulations for immobilization of nuclear fuel reprocessing wastes. For the first time, XAS are presented for silicate glasses with zirconia concentrations as high as 21 wt% and for zektzerite (LiNaZrSi6O15), a crystalline standard that is chemically similar to the glasses. Two other glass series were investigated where the CaO and ZnO contents were varied individually, while the Zr content was held constant. The analysis results indicate that Zr4+ is predominantly in octahedral sites in the glasses. Trends in the data also show that the Zr environments in the glasses are sensitive to changes in ZrO2 concentration, but are less sensitive to changes in Zn or Ca content. With addition of zirconia, the Zr site population shifts from mostly six-coordinated with minor amounts of seven-coordinated Zr, to having a larger fraction of seven-coordinated Zr sites, while maintaining a majority of octahedral Zr. The seven-coordinated Zr atoms probably act as nucleation sites in the glass; as the zirconia content increases to 15 wt% and higher, the numbers of these sites become large enough to promote ZrO2 baddeleyite crystallization. There is no evidence of Zr–Zr pair correlations, similar to those observed in baddeleyite, in any of the crystal-free glasses.

Local environment of Zn in zirconium borosilicate glasses determined by X-ray absorption spectroscopy

The Zn environments in six zirconium borosilicate glasses were characterized using X-ray absorption spectroscopy (XAS). The Zn environment is of interest because the addition of ZnO to borosilicate glass produces beneficial eAects on certain bulk physical properties that are important in the production and long-term storage of radioactive waste glasses. Zn XAS data and analyses show that all glass samples investigated contain Zn aa surrounded by four oxygens in tetrahedral coordination, with most likely, Si second nearest neighbors. The Zn environments are essentially invariant in all of the glass samples investigated, despite significant diAerences in glass chemistry and sample homogeneity. The results indicate that tetrahedral Zn in these glasses takes on the role of a weakly bonded network former. As ZnO is added to a silicate glass, Zn tetrahedra most likely link to silicate tetrahedra, decrease the volume of voids between linked tetrahedra, and restrict diAusion of large ions. These inferences concur with observations that leach rates are reduced for zirconium borosilicate glass as ZnO is added. ” 2000 Elsevier Science B.V. All rights reserved.

X-ray absorption fine structure studies of speciation of technetium in borosilicate glasses

A series of glass samples were prepared analogously to high level waste glass using either glass frit or glass precursors combined with a high level waste surrogate containing NaTcO4. Three different technetium species were observed in these glasses depending upon the synthesis conditions. If the glasses were prepared by reducing NaTcO4 to TcO2•2H2O with hydrazine or if a large amount of organic material was present, inclusions of TcO2 were observed. If no organic material was present, technetium was incorporated as TcO4 - . If only a small amount of organic material was present, isolated Tc(IV) sites were observed in the glass. The relative technetium retention of these glasses was estimated from the Tc K-edge height, and had no correlation with the oxidation state of the technetium. Pertechnetate was well retained in these glasses.

Neptunium in borosilicate glasses. A study of oxidation states and coordination sites by optical spectroscopy

Abstract Optical spectroscopic methods were used to investigate the electronic structure of neptunium in a sodium borosilicate glass matrix. Various doped glasses were obtained by melting together Na 2 CO 3 , SiO 2 , Na 2 B 4 O 7 :10H 2 O and NpO 2 powders. Neptunium was found to be present in equilibrium between the tetravalent oxidation state and trivalent state, a result of reduction in a graphite crucible, as determined in previously published Mossbauer spectra. New optical spectra are discussed, as well as the dependence of the redox equilibria on the composition of the glass and the melting conditions (time, crucible material, gaseous environment above the liquid, and temperature). Based on absorption bands corresponding to transition in the first J groups and their variation with glass composition, the coordination environment of Np ions is characterized.

Formulation, Testing, and Structural Characterization of High-Zirconium High-Level Waste Glasses

A range of compositions of high-zirconia borosilicate glasses were formulated and their structures investigated by a combination of techniques. These compositions have potential applications for high-level nuclear waste storage in combination with advanced reprocessing methods. Raman and Zr EXAFS data were collected for a series of glasses spanning a range of zirconia concentrations. The Raman spectra indicate that Zr acts as a silicate network modifier, where the silicate tetrahedral network depolymerizes as the zirconia content increases. Zr EXAFS analysis indicates that Zr is found in octahedral sites, and to a minor extent, sevencoordinated sites. As the zirconia content increases, the fraction of seven-coordinated Zr-sites increases; this may be the cause of ZrO 2 baddeleyite crystallization that was observed in some Zr-rich glasses investigated.

Optimization of Savannah River M-Area Mixed Waste for Vitrification

Vitrification studies of actual Savannah River M-Area mixed wastes have shown that the limiting factor for high waste loading of this waste stream is its chemical durability as defined by the toxicity characteristics leaching procedure (TCLP). As part of the optimization study of Savannah River M-Area wastes, a number of additives were examined including Na{sub 2}O, Li{sub 2}O, B{sub 2}O{sub 3}, ZrO{sub 2}, and TiO{sub 2}. This paper reports on the effect of varying the boron to total alkali ratio and on the effect of substitutions such as ZrO{sub 2} for waste and TiO{sub 2} for SiO{sub 2} on the chemical durability and processability of M-Area waste glasses.