Glass Leaching Research Articles

A new method for studying leached glasses: Analytical electron microscopy on ultramicrotomic thin sections

A method of ultramicrotomic thin sections has been developed for the study of the alteration rim of solids. Chemical and structural analysis of layers a few microns thick are possible using the Scanning Transmission Electron Microscope (STEM). The examples presented enable us to distinguish between two formation mechanisms of the alteration rims: (1) During interaction of basaltic glass with seawater, the alteration rim is formed by precipitation of the least soluble phases on the surface after a complete breakdown of the glass network. (2) Diffusion appears to be the dominant phenomenon in the case of a SiO 2Na 2OCaOP 2O 5 glass altered by tris-hydroxymethyl-aminomethane. The first nuclei of hydroxy-apatite are formed in a hydrated residual glass.

Interactions of SRP waste glass with potential canister and overpack metals

The leachability of Savannah River Plant (SRP) simulated waste glass was evaluated in the presence of potential canister and overpack metals. Modified MCC-1 static leach tests were used to leach waste glass in the presence of the following metals: A516 low-carbon steel, 304L stainless steel, 409 and 430 ferritic steels, E-brite, Ticode 12, Inconel 600 and lead. Results indicated that only two of the metals studied, A516 low-carbon steel and lead, exhibited a significant effect on glass leaching. In an oxic environment, A516 low-carbon steel had a detrimental effect on glass durability, while leaching in the presence of lead significantly improved the performance of the waste glass. For the rest of the metals studied, including the SRP reference canister material, 304L stainless steel, no significant effect on glass leachability was observed.

Preparation of glass capillary columns coated with immobilized OV‐240‐OH (33% cyanopropyl substituted, OH‐terminated). An approach to improve the inertness and thermal stability of cyanoalkyl polysiloxane coatings

AbstractCoating intensely leached glass capillaries with OV‐240‐OH, the most polar commercially available OH‐terminated polysiloxane phase at the present time, is described. Beside chemical bonding of the phase via condensation with silanol groups of the support, additional immobilization using dimethyltetramethoxysiloxane as a crosslinker is carried out. After in situ methylation of the immobilized coating, and transformation of acidic impurities of the cyanopropyl substituted phase into the respective methyl esters, increased inertness of the column was observed. Working instructions are given, and processes involved are discussed.

Long-Term Release Rates of Borosilicate Glass Waste Forms

Evaluating the durability of nuclear waste glass material in terms of leach test results requires that one make reasonable extrapolations from laboratory experiments performed over a few years’ duration to repository behavior over time scales ranging up to tens of thousands of years. These require an understanding of the mechanisms that govern the leaching of glass as well as an accompanying predictive capability. By comparing the measured behavior with the predictions of mechanistic models, it can be concluded that at high flow rates, kinetic factors are predominant, while at low flow rates, saturation of the aqueous medium with respect to major matrix elements, particularly with respect to silica present in the glass and in its alteration products, becomes a controlling factor. A mathematical framework in which this synthesized picture can be expressed is presented. A careful analysis of the situation indicates that under likely repository conditions the fractional annual release rates can be expected to fall below the U.S. Nuclear Regulatory Commission criterion of 10−5 yr−1.

Nuclear waste solids

Glass and polycrystalline materials for high-level radioactive waste immobilization are discussed. Borosilicate glass has been selected as the waste form for defence high-level radwaste in the US. Since release of the radionuclides to the biosphere is the major concern, this paper focuses on the potential interactions between the waste form and its surroundings. In addition to laboratory data, results from field testing are also presented in order to provide a comprehensive overview of glass leaching under near repository-like conditions.

Structural Changes Around Uranium at the Surface of Waste Glasses Caused by Leaching

AbstractStructural changes occurring at the surface of leached borosilicate glasses containing 3%wt of uranium have been investigated using fluores- cence EXAFS spectroscopy at grazing incidence. X-ray penetration depths between 30 Å and above 1000 Å are possible by varying the incidence angle of the X-ray monochromatic radiation in the vicinity of the critical angle for total external reflection, øc. This enables the surface and bulk structure to be compared. It is found that a local increase in uranium concentration takes place near the surface of the glass during the first 30 minutes of leaching time. Uranium is coordinated with oxygen and other uranium atoms. The local distribution can be modelled by a square planar island structure comprised of uranyl-type units. This structure changes with corrosion time. After 30 minutes of leaching there is evidence of partial removal of the surface layer.

Waste Glass Leaching: Chemistry and Kinetics

AbstractNuclear waste glass leaching has been studied extensively during the past ten years. Although much has been learned concerning the kinetics and mechanisms of glass dissolution, it does not appear that accurate predictions can yet be made concerning the release kinetics for specific elements from a given glass as a function of environmental conditions. In order to reliably predict elemental release rates, one needs to know: 1) how a given element is incorporated into the glass structure, 2) how specific sites in the glass react with water, 3) how the composition and reactivity of the leachate influence glass reactivity, 4) how the structure and reactivity of the glass changes in surface alteration layers, and 5) how glass dissolution modifies the chemistry of the leachate. At our current level of understanding, we are only able to make qualitative predictions concerning each of the above factors which allow us to make “order of magnitude” or “upper limit” predictions for radionuclide release rates.

Raman Spectra of Rings in Silicate Materials

ABSTRACTRaman spectroscopic studies on gel-derived silicates have confirmed that narrow bands near 607 cm-1 and 492 cm-1, first observed in the Raman spectrum of fused silica, are associated with three- and four-fold siloxane rings. Using these results, we have identified three- and four-fold siloxane rings in other high-surface-area silica materials, including leached glasses and Cab-OSil. This Raman spectroscopic evidence not only shows that small siloxane rings are a common characteristic of a number of silica materials but also suggests that they form preferentially at silica surfaces. This paper reviews the Raman spectroscopic evidence that led to the identification of the vibrational frequencies of the small siloxane rings and presents the results of Raman experiments on high-surface-area silica materials in which the concentration of small siloxane rings is enhanced compared to fused silica.

Preparation of porous glass-ceramics of the TiO2-SiO2 system

Porous glass-ceramics of the TiO2-SiO2 system of high titania content have been prepared by heat treatment and subsequent acid leaching of phase-separated glasses of the TiO2-SiO2-Al2O3-B2O3-CaO-MgO (or -Na2O) system. The porous glass-ceramics obtained in the study had a surface area of 100 to 300 m2g−1, with an average pore radius of 3 to 9 nm. The ceramics which contained a large amount of anatase and rutile were expected to be applied in the field of photocatalysts.

Role of Acid Concentration in Leaching of Cordierite and Alkali Borosilicate Glass

The leaching rates of both cordierite and heat‐treated alkali borosilicate glass were studied in hot nitric acid solutions ranging from ∼0.1 to ∼16N. Weight loss and penetration measurements of the specimens showed that the reaction rates go through a maximum with acid concentration. The data indicate that the rate of leaching, which involves ion‐exchange and hydration reactions, is controlled not only by acid strength, but also by water activity.

Applications of ion-beam analysis to solid-state reactions

Solid-state reactions in the near-surface region of glasses and metals are studied by measuring the changes in the concentration profiles of the elements involved in the reactions. The experimental procedure is to analyse a series of samples at different stages of the reaction under study, or to follow the progress of the reaction continuously in a given sample. Results on the following topics will be discussed; (i) Leaching of glasses due to contact with aqueous media, (ii) Absorption of hydrogen from the gas phase in Ta and Ti. (iii) Gettering of hydrogen in Fe and Ni caused by ion implantation of hydride-forming metals.

An ion exchange model of glass leaching

An ion exchange model of glass leaching

The absence of a mixed-alkali effect in the leaching of corrosion-resistant glasses

Alkali depth profiles in water-leached glasses with molar composition 16A2O·8CaO· 2MgO·1Al2O3·73SiO2 (A=Na, K and Na+K) were determined using secondary ion mass spectroscopy (SIMS). In the glass containing only sodium an apparent correlation exists between the alkali diffusion coefficient derived from the leaching kinetics, assuming this process is governed by ion exchange, and that determined via electrical conductivity measurements in the dry glass. In the other two glasses the diffusion coefficients derived on the basis of ion exchange are much higher than the ones found in the dry glasses. These data can be interpreted in terms of a model for the leaching of glass where the rate-determining step is the diffusion of molecular water.

Development of the Code Melodie for Long Term Risk Assessment of Nuclear Waste Repositories

AbstractMELODIE is a computer code developed at the CEA/IPSN for risk assessment of nuclear waste repositories in geological formations. The interactive evolution of the source, geosphere and biosphere is studied for periods of time longer than 100 000 years. In its first version, the code can describe a repository in granite rock located at a specific site.The code is built in a modular form which allows to use different versions of the sub-system models.The basic model for radionuclide migration and hydrpgeology is a subroutine version of the METIS code developed by ENSMP. METIS is a 2D finite element code which solves the advection-dispersion equation for porous media with explicit fracture representation. Linear adsorption kinetics is included as well as matrix diffusion and radionuclide decay chains.The source model developed at CEA/DRDD**** is derived from CONDIMENT which is a 1D finite difference code describing the behaviour of high level waste packages. Four axisymetric layers are individualized: glass matrix, container, bentonite and granite. The glass leaching is modelled as a dissolution and diffusion process of the individual chemical components.The biosphere model ABRICOT developed at the CEA/DPT** is based on a detailed description of agricultural activities defined in individual systems.MELODIE is tested by participation to international exercices such as Pagis [1], Intracoin [2] and Hydrocoin [3]. Future developments will include introduction of scenarios constructed from geoprospective studies and algorithms for sensitivity studies.

Effects of Rocks and Backfill Materials on Waste Glass Leaching

AbstractExtensive studies have been made on the interactions between a waste glass and repository materials under static conditions. One of the PNC reference glasses was leached in the solution prepared from water in contact with crushed granite, tuff, diabase and backfill materials such as bentonite and zeolite. The leachant solutions except for some bentonite solutions reduced the glass leach rate compared with that measured in distilled water. The extent of the reduction was a function of silicon concentration in solution. The bentonite solutions enhanced the glass dissolution rate by a factor of 2 to 3 at low bentonite/water ratios but the effect was found to be less important at high bentonite/water ratios and in the long-term experiment. Addition of granite and zeolite to the bentonite solutions decreased the leach rate below the value measured in distilled water.

Variables affecting leaching of Swedish nuclear waste glass

Borosilicate glass is a candidate material for the disposal of nuclear wastes in Sweden. Current plans involve the burial of the waste package containing the glass in granite boreholes several hundred meters deep. It is expected that the waste package, and eventually the glass containing the wastes, will come into contact and interact with the groundwater. The interaction of glass with water will almost certainly result in a leaching of radionuclides from the glass. Although the leaching processes in a repository environment will be complex, they must be understood in order to make reliable long-term predictions of glass performance. Some of the important variables affecting the leaching processes of Swedish nuclear waste glasses are discussed in the current paper.

Investigations on the Role of Surface Layers in HLW Glass Leaching

AbstractA corrosion test series was performed to clarify the role of reaction product layers on the corrosion of a simulated HLW borosilicate glass in a salt brine under hydrothermal conditions. The layers were unprotective at 200°C. At 120°C, slight protective effects ocurred when the leachant contained dissolved reaction products. The consequence for the long term behaviour between 120 and 200°C is a constant glass dissolution rate.

Noneffect of electronegative gases on pulsed laser-induced optical surface damage thresholds

The pulsed laser-induced damage threshold of several optical surfaces has been measured as a function of the pressure and type of ambient gas. Measurements were made in vacuum and various pressures of nitrogen, Freon 14, (CF4), and SF6 in an attempt to decrease the growth rate of the avalanche which accompanies pulsed laser-induced damage. Test samples included conventional dielectric coatings of ZrO2, diamond-turned copper, bare surfaces of fused silica and ARG-2 glass, and porous surface layers of both leached ARG-2 glass and some solgel coatings. From this preliminary study, it is apparent that the ambient gas has no observable effect on 1-on-1 surface damage thresholds (although some improvement in N-on-1 damage levels had been reported). The present finding is in agreement with previous observations that damage sites appear to be initiated on or just below the surface of optical elements. For uncoated surfaces, both conventional and porous initiation sites seem to be isolated impurities embedded near the surface resulting in the formation of pits. In either case, such damage initiation levels are unmodified by the presence of electronegative gases, and no change in threshold is observed.

Precipitation of colloidal silica and pore size distribution in high silica porous glass

Abstract Porous glass was prepared by acid leaching of a phase separated soda borosilicate glass. Fine pores with radius smaller than 15 A and coarse pores with radius larger than 50 A were observed in the glass by nitrogen adsorption and mercury penetration methods, respectively. Effects of the time of heat treatment and the acid/glass ratio, that is, the ratio of volume of leaching acid to the weight of phase separated glass on the characteristics of fine and coarse pores were studied. The size of coarse pores which were ascribed to the phase separated structure was proportional to the cube root of the time of heat treatment. The extraction of silica from specimens by acid solution during leaching increased with increasing acid/glass ratio. The volume of coarse pores was proportional to the amount of extracted silica. These facts are discussed on the basis of the formation of colloidal silica particles from the soluble borate phase during leaching.

Rutherford backscattering for measuring corrosion layers on glasses for long-term storage of radioactive waste

The method considered safest for isolation of nuclear waste is vitrification (solidification in a glass or glassceramic matrix) with long-term storage in stable geological repositories. Borosilicate glasses are candidates for this purpose on a world-wide scale. Though it is very unlikely that water will ever have access to these repositories, the leaching (corrosion in aqueous solutions) of glasses is studied. Rutherford backscattering of beams of charged particles (e.g. He-ions of 2–3 MeV energy) is shown to be a versatile technique to study surface layers and to obtain information on their thickness and composition. The method is non-destructive and can be applied to layers with thicknesses in the range of about 0.1 to 2 μm. Results for nuclear waste glasses leached in H2O or in quinary salt solution (Q-brine) with and without presence of material from the metal container are presented. Presence of steel is shown to greatly enhance leaching rates by preventing glass components to become saturated in the leachant.