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Systematic Development of Alkaline-Earth Borosilicate Glasses for Caesium Loaded Ion Exchange Resin Vitrification
Abstract
Caesium loaded ion exchange resin wastes are problematic for vitrification due to their organic nature; the presence of problematic anionic species which lead to phase separation in glasses; and the volatility of caesium at melting temperatures. The presence of a large inventory of radiologically-contaminated ion exchange resins from past, current and future civil nuclear power generation means that the development of a suitable route of vitrification is essential. This paper explores the development of a glass-forming system intended for the purpose of ion exchange resin vitrification, covering systematic studies of compositional variation and the structural and physical effects of these changes, resulting in the development of novel glass compositions optimised for ion exchange resin vitrification.
During the operation of nuclear power plants, spent ion-exchange resins are formed, which are heterogeneous radioactive low-level waste in the form of particles from a cross-linked organic polymer. Such resins may not always be regenerated. Therefore, the disposal of spent ion exchange resins is currently one of the primary problems at nuclear power plants. Conventional technologies for the processing of waste resins are relatively expensive. In addition, there are difficulties with transportation and storage of waste, and the disposal of spent ion exchange resins is a complex process. In the present study, an attempt has been made to solve the problem of spent ion-exchange resins utilization on example of the sulfonic acid cation exchanger’s KU-2×8 oxidative degradation with the Fenton reaction. The decomposition of the cation exchanger was carried out with 20% hydrogen peroxide in the temperature range 323-353 K in the presence of a catalyst – low concentration copper(II) sulfate (0.001-0.009 mmol/l). The influence of process temperature and catalyst concentration on the reaction rate was estimated. When determining the rate of the cation exchanger KU-2×8 heterogeneous oxidation reaction with hydrogen peroxide in the presence of a catalytic additive, the spherical shape of the sorbent granules, the surface area of which changed during reaction, was taken into account. It was shown that with a reaction temperature increasing from 323 to 353 K, the rate constant of cation exchanger's oxidative decomposition have increased by a factor of 20-37. The activation energy values of the sulfonic acid cation exchanger's KU-2×8 decomposition with hydrogen peroxide in the presence of copper(II) sulfate are 89.7-115.2 kJ/mol, which indicates that the process is in the kinetic mode. It was established with electron-microscopic studies that the beads of the cation exchanger KU-2×8, when decomposed in H2O2 solution in the presence of a catalyst can stick together, change their shape and volume, and their surface becomes covered with cracks. The studies performed showed almost complete catalytic decomposition of cation exchanger KU-2×8 in a hydrogen peroxide solution at 323-353 K after 420-220 minutes, which allows accelerating the oxidation at relatively low temperatures.
Vitrification was considered as a potential treatment option for four types of radioactive wet intermediate level waste (ILW) arising from decommissioning of a UK Magnox nuclear power station. Some of the ILWs under consideration were eminently suitable for vitrification as they consisted largely of SiO2, MgO, Fe2O3, Na2O, Al2O3 and CaO. However, the high organic and sulphur contents of some of the ILWs, particularly spent ion exchange (IEX) resins, presented a number of challenges. Here we discuss the ILWs concerned and the property and processing requirements for suitable glass host materials. A total of 80 glass compositions was surveyed and a combination of measured and modelled properties and processing requirements was used, on the basis of the property selection criteria, to select suitable candidate glass compositions for detailed study in laboratory-based trials.
IR and RS spectroscopy are used to study the effect of substituting barium and calcium for the sodium cation on the structure of sodium borosilicate glass with the composition 0.25Na2O; 0.25B2O3 ; 0.5SiO2 . Analysis of the recorded spectra showed that the substitution of the alkali-metal cations Ba2+ and Ca2+ for the Na+ cation results in breaking of B–O–B and Si–O–Si bridges, as a result of which the fraction of oxygen end atoms increases and the degree of polymerization of the glass decreases.
The concentration and structural role of non-bridging oxygen (NBO), defined as oxygen bonded to one framework cation, play a major role in the physical properties of glasses and liquids. In this study, the distribution of NBO connected to either silicon and boron was investigated in a borosilicate glass system using 17O magic-angle spinning (MAS) and triple quantum (3QMAS) nuclear magnetic resonance. In order to obtain better separation of different 17O NBO resonances, barium borosilicate glasses were chosen and measured by MAS and 3QMAS. The two-dimensional 3QMAS spectra show clear resolution between peaks for Si–NBO and B–NBO, allowing their relative proportions to be at least roughly quantified for the first time. The NMR parameters for these sites were also determined and are consistent with previous studies of binary barium borate and silicate systems. Site populations are significantly different from predictions of conventional models based on alkali borosilicates, which suggests a more disordered nature for these barium borosilicate systems. In order to explain the anomalously high NBO concentrations, we hypothesize that a few percent of the oxygens may have three network cation neighbors, forming ‘triclusters’.
The sodium environment in oxide glasses was investigated by 23Na multiple-quantum magic-angle spinning (MQ-MAS) NMR spectroscopy and compared with molecular dynamics simulations. In the experimental approach, a spectrum-inversion was employed taking into account the transfer efficiency involved in the MQ-MAS experiment. This allowed the reconstruction of the underlying two-dimensional distribution of the isotropic chemical shift correlated with the quadrupolar interaction. The isotropic chemical shift distributions were extracted from the MQ-MAS spectra to infer Na–O distance distributions. First, a Na2O–2SiO2 glass and its crystal analogue were characterized by this method to observe the disorder effect in the glass through the Na–O distance distribution. Thereafter, in order to study the influence of the chemical composition on the Na–O distance and distribution, additional glasses were investigated with NMR and simulation: Na2O–5SiO2, Na2O–2CaO–3SiO2, Na2O–Al2O3–3SiO2 and Na2O–B2O3–3SiO2. The molecular dynamics results are in good agreement with the experimental findings. The mean Na–O distance is higher when network formers are added to the sodium silicate glass. The effects on the Na–O distance distribution are also discussed. The simulation relates these results to the existence of several types of Na: near the non-bridging oxygen of the silicon, or as aluminum or boron charge compensator. This can be explained through charge and geometric effects.
Barium borosilicate (BBS) and sodium borosilicate (SBS) glass samples, prepared by the conventional melt-quench method, were used for the uptake of Rhodamine 6G dye from aqueous solution. The experimental conditions were optimized to get maximum uptake and was found to be 0.4 mg of dye per gram of BBS glass sample. For the same network former to modifier ratio, barium borosilicate glasses are found to have improved extent of uptake for the dye molecules from aqueous solutions compared to sodium borosilicate glasses. Based on 29Si MAS NMR studies on these glasses, it is inferred that significantly higher number of non-bridging oxygen atoms present in barium borosilicate glasses compared to sodium borosilicate glasses is responsible for its improved uptake of Rhodamine 6G dye. 11B MAS NMR studies have confirmed the simultaneous existence of boron in BO3 and BO4 configurations in both barium borosilicate and sodium borosilicate glasses. The luminescence studies have established that the dye molecule is incorporated into the glass matrix through ion exchange mechanism by replacing the exchangeable ions like Na+/Ba2+ attached with the non-bridging oxygen atoms present in the glass.
The state of ordering among network-modifier cations in molten silicates has a potentially large effect on their overall configurational
entropy, on free energies of mixing, and on viscosity and diffusivity. Most models of thermodynamic and transport properties
assume random mixing, but there is relatively little direct microstructural information to constrain the real extent of this
disorder. Two-dimensional 17 O NMR can produce spectra that are free of quadrupolar broadening and thus in which peak widths for non-bridging oxygen sites
directly reflect the extent of disorder in the local structural environment. In this report, we describe new data from triple-quantum
magic-angle-spinning (3QMAS) NMR for a series of barium and calcium silicate glasses. Results are best explained by completely
random mixing of Ba and Ca. confirming conventional modeling assumptions. Other recent data show, however, that significant
ordering may be present (at least at the temperature of the glass transition) for modifier cations with greater differences
in size or charge, and among network-forming cations.
Mossbauer and Raman spectroscopy have been used to relate redox equilibria of iron to melt structure in MO-SiO2 systems (M = Ba2+, Sr2+, Ca2+, Mg2+).-J.A.Z.
The experimental work and published data were used to calculate the anionic constitution of common igneous melts. Apparently all magmas have an (NBO/T) < 1. Basalts have (NBO/T) = 0.9 to 0.6. Andesites are about 0.3 and granite melts are between 0.2 and 0.05. The anionic structure is actually a mixture of 2, 1, and 0. -K.A.R.
Approximately 60 g of an iron-enriched borosilicate glass was made in the radiochemical labs of the Savannah River Technology Center (SRTC). The glass was made to demonstrate the immobilization of the radioisotopes contained on representative Argentine ion exchange resins (similar to those used at the Embalse plant). The product was approximately 90% amorphous and was quite durable as measured by the release rates from the Product Consistency Test (PCT). The release rates were considerably better than those of the U. S. High Level Waste (HLW) benchmark DWPF EA glass. The release rate of the Cs-137 was predictably similar to that of Na and Li. No Co-60 or Sr-90 was measured in the PCT leachate. The mass balances for the inactive additives were quite good. Of the radioisotopes, approximately 71% of Cs-137 was accounted for in the glass product. This was similar to the Na mass balance. Approximately 89% of the Co-60 was accounted for in the glass product.
In the nuclear industry, ion exchange resins are used for purification of aqueous streams. The major contaminants of the resins are usually the radioactive materials that are removed from the aqueous streams. The use of the ion exchange resins creates a waste stream that can be very high in both organic and radioactive constituents. Therefore, disposal of the spent resin often becomes an economic problem because of the large volumes of resin produced and the relatively few technologies that are capable of economically stabilizing this waste. Vitrification of this waste stream presents a reasonable disposal alternative because of its inherent destruction capabilities, the volume reductions obtainable, and the durable product that it produces.
We report multi-nuclear (Na-23 and O-17) solid-state NMR [MAS and triple quantum (3Q) MAS] spectra for sodium tetrasilicate glasses (NS4) quenched from melts at high pressure up to 8 GPa. The results show clear evidence for the pressure-induced structural changes in the glasses, forming oxygen linking [4]Si and [5,6]Si ([4]Si–O–[5,6]Si) with increasing pressure. Whereas the general trend in the effect of pressure is consistent with that of sodium trisilicate glasses (NS3), detailed pressure-induced structural changes for NS4 are largely different from NS3. These differences include the larger fraction of [4]Si–O–[5,6]Si and smaller fraction of Na–O–[5,6]Si for NS4 than NS3 at isobaric conditions. Topological disorder due to Si–O bond length distribution in [4]Si–O–[4]Si is also larger for more polymerized NS4 than that for NS3, demonstrating the complexity in structural rearrangement with pressure in silicate glasses and melts with composition at elevated pressure.
Summary 1.An investigation established the congruence of melting of mullite, and the appropriate corrections were introduced into the phase diagram of the system Al2O2-SiO2.2.The phase diagram of the system BaO-Al2O2-SiO2 was studied; it comprises 13 fields of stability of the various phases.3.In the system is formed a ternary solid solution between the silicates 2BaO·3SiO2 and BaO·2SiO2 and the aluminosilicate BaO · Al2O3 · 2SiO2.4.Formation of a previously unknown aluminosilicate of the composition 3BaO·2Al2O3·2SiO2 was observed. This compound melts with decomposition into the monoaluminate BaO·Al2O3 and a liquid.5.A new high-temperature micro-furnace was designed.
Infrared reflection spectra from alkali silicate glasses exposed to water for various times are analyzed using several causal methods, based on Kramers–Kronig (KK) relations between the real and imaginary parts of the dielectric functions. The results show that use of causal Gaussian dispersion equations works well in producing peaks in the imaginary component of the dielectric function that remain fixed in wavelength position and width during a surface leaching process. This allows accurate definition of the behavior of associated composition and structural changes in the glass surface. This method is suggested for non-destructive surface analysis of materials.
The combined effect of alkali and alkaline-earth ions on the redox, distribution, co-ordination and environment of Fe ions in alkali–alkaline-earth–silica glasses has been studied using a multi-technique approach. Wet chemical analysis and Mössbauer, electron spin resonance (ESR), optical absorption and photoluminescence spectroscopies were utilised. Behaviour generally falls into two categories, which we have termed ‘collective’ and ‘selective’. Collective behaviour occurs when alkali and alkaline-earth ions have similar effects on a property and the overall effect is cumulative. This is characterised by a linear relationship with optical basicity of the glass. Some parameters associated with the environment of Fe2+ ions fall into this category. Selective behaviour occurs when alkali and alkaline-earth ions have opposing effects on a property, suggesting competition or selectivity. This is characterised by a linear relationship with the alkali/alkaline-earth ionic radius ratio, cation field-strength ratio or oxide-basicity ratio. The Fe2+/ΣFe ratio and several parameters associated with the distribution, coordination and environment of Fe3+ ions fall into this category. These results have implications for the local structure surrounding Fe species. A relationship has been suggested linking coordination and distribution of Fe3+ ions.
Bench-scale studies were performed to determine the feasibility of vitrification treatment of six resins representative of those used in the commercial nuclear industry. Each resin was successfully immobilized using the same proprietary borosilicate glass formulation. Waste loadings varied from 38 to 70 g of resin/100 g of glass produced depending on the particular resin, with volume reductions of 28 percent to 68 percent. The bench-scale results were used to perform a melter demonstration with one of the resins at the Clemson Environmental Technologies Laboratory (CETL). The resin used was a weakly acidic meth acrylic cation exchange resin. The vitrification process utilized represented a approximately 64 percent volume reduction. Glass characterization, radionuclide retention, offgas analyses, and system compatibility results will be discussed in this paper.
A bstract
Relationships previously deduced and tested by the authors for the calculation of the density, refractive index, and. dispersion of a glass from its composition, expressed in terms of the “atomic fractions” of the components, are transformed into equivalent equations for the calculation of these quantities from the weight percentages or weight fractions. The necessary constants for use with these equations are tabulated.
Borosilicate glass formulations adopted worldwide for immobilization of high-level radioactive liquid waste (HLW) is not suitable for sulphate bearing HLW, because of its low solubility in such glass. A suitable glass matrix based on barium borosilicate has been developed for immobilization of sulphate bearing HLW. Various compositions based on different glass formulations were made to examine compatibility with waste oxide with around 10 wt% sulfate content. The vitrified waste product obtained from barium borosilicate glass matrix was extensively evaluated for its characteristic properties like homogeneity, chemical durability, glass transition temperature, thermal conductivity, impact strength, etc. using appropriate techniques. Process parameters like melt viscosity and pour temperature were also determined. It is found that SB-44 glass composition (SiO2: 30.5 wt%, B2O3: 20.0 wt%, Na2O: 9.5 wt% and BaO: 19.0 wt%) can be safely loaded with 21 wt% waste oxide without any phase separation. The other product qualities of SB-44 waste glass are also found to be on a par with internationally adopted waste glass matrices. This formulation has been successfully implemented in plant scale.
The effect of morphology on the thermal expansion behavior, helium permeability, density, and refractive index of alkaline earth silicate glasses has been determined for the calcium, strontium, and barium silicate systems. These data have been used to determine the miscibility limits and the limits of connectivity of the high silica phase for each of these systems. The miscibility limits occur at approximately 46, 42, and 40 mol% CaO, SrO, and BaO, respectively. The limit for a connected high silica phase occurs at approximately 33, 25, and 22 mol% CaO, SrO, and BaO, respectively. This paper demonstrates the use of these techniques in the determination of the morphology of simple glass‐forming systems.
We present enhanced-precision viscosity determinations for Na2OAl22O3SiO2 liquids which challenge the commonly held view that melts of tectosilicate stoichiometry in this system are fully polymerised. The maximum in melt viscosity produced by the substitution of Al for Na does not generally occur at molar Na/Al = 1; rather, it deviates systematically into the peraluminous field for SiO2 > 50 mol%. This deviation is greatest at 67 mol% SiO2 and increases with decreasing temperature. These features may be most simply explained by the presence of “triclusters” consisting of a threefold coordinated oxygen to which one aluminate and two silicate tetrahedra are attached. The presence of such triclusters has important implications for phase equilibria, mechanisms of viscous flow, and the solubilities of elements in natural and synthetic highly polymerised aluminosilicate melts.
Diffusion coefficients of sodium in barium borosilicate glasses having varying concentration of barium were determined by heterogeneous isotopic exchange method using (24)Na as the radiotracer for sodium. The measurements were carried out at various temperatures (748-798 K) to obtain the activation energy (E(a)) of diffusion. The E(a) values were found to increase with increasing barium content of the glass, indicating that introduction of barium in the borosilicate glass hinders the diffusion of alkali metal ions from the glass matrix. The results have been explained in terms of the electrostatic and structural factors, with the increasing barium concentration resulting in population of low energy sites by Na(+) ions and, plausibly, formation of more tight glass network. The leach rate measurements on the glass samples show similar trend.
The Diagram of the State of the Ternary System BaO-Al2O3-SiO2
- N. Toropov
- F. Galakhov
- I. Bondar
The Diagram of the State of the Ternary System BaO-Al2O3-SiO2
- Toropov