Borosilicate Glass Dissolution Research Articles

The corrosion behavior of borosilicate glass in the presence of cementitious waste forms.

Borosilicate glasses are widely used for radioactive waste disposal due to their ability to incorporate a variety of contaminants and radionuclides while exhibiting high durability in various disposal scenarios. This research evaluated the dissolution of borosilicate glass using both single-pass-flow-through (ASTM C1662-18) and product consistency test (ASTM C1285-21) methods with different solutions, including a cementitious-contacted water (called grout-contacted, GC, from this point) and solutions with varying levels of dissolved cementitious species such as Si, Ca, Al. The results indicated that the presence of Ca plays a crucial role in suppressing glass corrosion, as evidenced by the slower normalized dissolution rates, which were one order of magnitude lower for boron and two orders of magnitude lower for rhenium, observed in both Ca-amended and GC solutions compared to the pH 12 buffer solution. This effect is attributed to the formation of a dense, low-porosity, and strongly bonded calcium silicate hydrate (CSH) layer on the glass surface, which implies that a glass corrosion process is influenced by ion exchange involving alkali ions Na+, K+, Ca2+, and hydrogen-containing species. A small number of glass particles treated in the GC solution showed minor corrosion pits in the form of shallow craters with an average diameter of approximately 500 μm. This observation is correlated with a significant reduction, 2000 to 3000 times lower, in the cumulative volume of glass pores, indicating that smaller pore voids were "sealed" in the presence of Ca2+ ions, likely attributed to the formation of CSH precipitation or other corrosion products such as calcium carbonate saturated from the grout solution. These findings suggest that the presence of dissolved Ca in the GC solution can slow down the dissolution of borosilicate glass, contrary to the expected trend of higher dissolution rates resulting from exposure to high alkaline and thus higher pH solutions.

Dissolution kinetics of a sodium borosilicate glass in Tris buffer solutions: impact of Tris concentration and acid (HCl/HNO3) identity.

Understanding the corrosion behavior of glasses in near-neutral environments is crucial for many technologies including glasses for regenerative medicine and nuclear waste immobilization. To maintain consistent pH values throughout experiments in the pH = 7 to 9 regime, buffer solutions containing tris(hydroxymethyl)aminomethane ("Tris", or sometimes called THAM) are recommended in ISO standards 10993-14 and 23317 for evaluating biomaterial degradation and utilized throughout glass dissolution behavior literature-a key advantage being the absence of dissolved alkali/alkaline earth cations (i.e. Na+ or Ca2+) that can convolute experimental results due to solution feedback effects. Although Tris is effective at maintaining the solution pH, it has presented concerns due to the adverse artificial effects it produces while studying glass corrosion, especially in borosilicate glasses. Therefore, many open questions still remain on the topic of borosilicate glass interaction with Tris-based solutions. We have approached this topic by studying the dissolution behavior of a sodium borosilicate glass in a wide range of Tris-based solutions at 65 °C with varied acid identity (Tris-HCl vs. Tris-HNO3), buffer concentration (0.01 M to 0.5 M), and pH (7-9). The results have been discussed in reference to previous studies on this topic and the following conclusions have been made: (i) acid identity in Tris-based solutions does not exhibit a significant impact on the dissolution behavior of borosilicate glasses, (ii) ∼0.1 M Tris-based solutions are ideal for maintaining solution pH in the absence of obvious undesirable solution chemistry effects, and (iii) Tris-boron complexes can form in solution as a result of glass dissolution processes. The complex formation, however, exhibits a distinct temperature-dependence, and requires further study to uncover the precise mechanisms by which Tris-based solutions impact borosilicate glass dissolution behavior.

Monte Carlo simulation of borosilicate glass dissolution using molecular dynamics-generated glass structures

Atomic-scale insights into borosilicate glass dissolution are critical to the development of waste form corrosion models. While Monte Carlo (MC) simulations have been instrumental in advancing the state of knowledge, all simulations to date have relied on lattices to represent the glass structure, potentially introducing artifacts. A new approach (amorphous MC approach) was developed whereby glass structures generated from molecular dynamics simulations were used as starting points for MC simulations. Correlations between dissolution rate and structural features were revealed in MC simulations of sodium borosilicate glasses that covered a wide compositional range. Notably, the lattice MC approach predicted the fraction of tetrahedral boron to have a lesser influence on the dissolution rate than computed with the amorphous MC approach. The lattice MC simulations also overestimated the dissolution rate for a given glass composition. These findings highlight the importance of using truly amorphous structures in MC simulations of glass corrosion.

The dissolution behavior of borosilicate glasses in far-from equilibrium conditions

An area of agreement in the waste glass corrosion community is that, at far-from-equilibrium conditions, the dissolution of borosilicate glasses used to immobilize nuclear waste is known to be a function of both temperature and pH. The aim of this work is to study the effects of temperature and pH on the dissolution rate of three model nuclear waste glasses (SON68, ISG, AFCI). The dissolution rate data are then used to parameterize a kinetic rate model based on Transition State Theory that has been developed to model glass corrosion behavior in dilute conditions. To do this, experiments were conducted at temperatures of 23, 40, 70, and 90 °C and pH (22 °C) values of 9, 10, 11, and 12 with the single-pass flow-through (SPFT) test method. Both the absolute dissolution rates and the rate model parameters are compared with previous results. Rate model parameters for the three glasses studied here are nearly equivalent within error and in relative agreement with previous studies though quantifiable differences exist. The glass dissolution rates were analyzed with a linear multivariate regression (LMR) and a nonlinear multivariate regression performed with the use of the Glass Corrosion Modeling Tool (GCMT), with which a robust uncertainty analysis is performed. This robust analysis highlights the high degree of correlation of various parameters in the kinetic rate model. As more data are obtained on borosilicate glasses with varying compositions, a mathematical description of the effect of glass composition on the rate parameter values should be possible. This would allow for the possibility of calculating the forward dissolution rate of glass based solely on composition. In addition, the method of determination of parameter uncertainty and correlation provides a framework for other rate models that describe the dissolution rates of other amorphous and crystalline materials in a wide range of chemical conditions. The higher level of uncertainty analysis would provide a basis for comparison of different rate models and allow for a better means of quantifiably comparing the various models.

SON68 glass dissolution driven by magnesium silicate precipitation

Abstract Experimental results are reported on the effect of magnesium silicate precipitation on the mechanisms and rate of borosilicate glass dissolution. Leaching experiments with SON68 glass, a borosilicate containing no Mg, were carried out in initially deionized water at 50 °C with a glass-surface-area-to-solution-volume ratio of 20,000 m−1. After 29 days of alteration the experimental conditions were modified by the addition of Mg to trigger the precipitation of Mg-silicate. Additional experiments were conducted to investigate the importance of other parameters such as pH or dissolved silica on the mechanisms of precipitation of Mg-silicates and their consequences on the glass dissolution rate. Mg-silicates precipitate immediately after Mg is added. The amount of altered glass increases with the quantity of added Mg, and is smaller when silicon is added in solution. A time lag is observed between the addition of magnesium and the resumption of glass alteration because silicon is first provided by partial dissolution of the previously formed alteration gel. It is shown that nucleation does not limit Mg-silicate precipitation. A pH above 8 is necessary for the phase to precipitate under the investigated experimental conditions. On the other hand the glass alteration kinetics limits the precipitation if the magnesium is supplied in solution at a non-limiting rate.

소듐붕규산염 유리의 표면 구조에 대한 분자 동역학 시뮬레이션 연구

핵폐기물을 고화시키는 재료로 사용하는 붕규산염(borosilicate) 유리의 용해는 지층 처분장에 처리된 고준위 방사성 폐기물의 생태계 유출을 결정할 수 있는 중요한 화학반응이다. 습식 실험에서 유리의 용해속도(dissolution rate)는 유리 화학조성에 의해 크게 좌우되는 것이 관찰된다. 유리의 bulk 구조를 규명한 분광분석 실험에 의하면 유리의 화학조성과 분자수준(molecular-level) 구조(예: <TEX>$SiO_4$</TEX> 사면체의 연결구조와 B 원소의 배위구조) 사이의 상관관계가 존재한다. 따라서 화학조성에 따른 유리 용해도의 차이는 조성에 따른 bulk 내부구조의 변화로 이해되어 왔다. 그런데 유리 표면은 수용액과 계면을 이루면서 용해 과정에서 가장 직접적으로 반응하는 부분이기 때문에, 화학조성에 따른 표면구조 변화에 대한 지식 또한 필요하다. 본 논문에서는 분자 동역학(molecular dynamics, MD) 시뮬레이션을 사용하여 4가지의 다른 화학조성을 가지는 소듐붕규산염 유리(<TEX>$xNa_2O{\cdot}B_2O_3{\cdot}ySiO_2$</TEX> 화학조성)에 대하여 bulk 구조와 실험으로 얻기 어려운 표면(surface) 구조를 연구하였다. MD 시뮬레이션은 유리 표면의 화학조성과 분자수준 구조가 bulk의 것과 매우 상이한 결과를 보여준다. 본 연구의 MD 시뮬레이션 결과는 화학조성에 따른 유리 용해도(특히 초기 용해과정)는 bulk 구조의 변화보다 유리 표면구조의 변화에 의해 크게 좌우될 수 있다는 표면구조에 대한 이해의 중요성을 역설한다. Borosilicate glass dissolution is an important chemical process that impacts the glass durability as nuclear waste form that may be used for high-level radioactive waste disposal. Experiments reported that the glass dissolution rates are strongly dependent on the bulk composition. Because some relationship exists between glass composition and molecular-structure distribution (e.g., non-bridging oxygen content of <TEX>$SiO_4$</TEX> unit and averaged coordination number of B), the composition-dependent dissolution rates are attributed to the bulk structural changes corresponding to the compositional variation. We examined Na borosilicate glass structures by performing classical molecular dynamics (MD) simulations for four different chemical compositions (<TEX>$xNa_2O{\cdot}B_2O_3{\cdot}ySiO_2$</TEX>). Our MD simulations demonstrate that glass surfaces have significantly different chemical compositions and structures from the bulk glasses. Because glass surfaces forming an interface with solution are most likely the first dissolution-reaction occurring areas, the current MD result simply that composition-dependent glass dissolution behaviors should be understood by surface structural change upon the chemical composition change.

Monte Carlo simulations of the dissolution of borosilicate glasses in near-equilibrium conditions

Monte Carlo simulations were performed to investigate the mechanisms of glass dissolution as equilibrium conditions are approached in both static and flow-through conditions. The glasses studied are borosilicate glasses in the compositional range (80−x)% SiO2 (10+x/2)% B2O3 (10+x/2)% Na2O, where 5<x<30%. In static conditions, dissolution/condensation reactions lead to the formation, for all compositions studied, of a blocking layer composed of polymerized Si sites with principally 4 connections to nearest Si sites. This layer forms atop the altered glass layer and shows similar composition and density for all glass compositions considered. In flow-through conditions, three main dissolution regimes are observed: at high flow rates, the dissolving glass exhibits a thin alteration layer and congruent dissolution; at low flow rates, a blocking layer is formed as in static conditions but the simulations show that water can occasionally break through the blocking layer causing the corrosion process to resume; and, at intermediate flow rates, the glasses dissolve incongruently with an increasingly deepening altered layer. The simulation results suggest that, in geological disposal environments, small perturbations or slow flows could be enough to prevent the formation of a permanent blocking layer. Finally, a comparison between predictions of the linear rate law and the Monte Carlo simulation results indicates that, in flow-through conditions, the linear rate law is applicable at high flow rates and deviations from the linear rate law occur under low flow rates (e.g., at near-saturated conditions with respect to amorphous silica). This effect is associated with the complex dynamics of Si dissolution/condensation processes at the glass–water interface.

SON68 Glass Dissolution Kinetics at High Reaction Progress: Experimental Evidence of the Residual Rate

ABSTRACTSON68 borosilicate glass dissolution involves several parallel mechanisms: interdiffusion, hydrolysis of the silicate network, recondensation of dissolved species leading to the formation of an amorphous gel layer and precipitation of secondary crystalline phases. In a highly confined medium corresponding to geological disposal conditions, the gel formed during SON68 glass alteration quickly becomes protective and the hydrolysis rate drops by about 4 orders of magnitude compared with the initial dissolution rate. This study focuses on the state of reaction progress corresponding to apparent saturation of the solution. At 90°C in static mode, new experimental data show that the decreasing rate due to the protective gel formation is followed by a quasi-constant residual rate. Several hypotheses are discussed. We first demonstrate that silica dissociation related to a slowly rising pH is not a valid explanation. This hypothesis proposed 15 years ago was consistent with a rate controlled by affinity (equilibrium between the glass and solution). The hypothetical precipitation of secondary crystalline phases that consume elements from the protective gel layer is then considered in the light of data obtained with a simplified 8-oxide glass, but this mechanism is not sufficient to account for residual rate of the SON68 glass. Other hypotheses based on the gel evolution (dissolution, evolution of the porous texture) are also proposed. The proposed mechanisms imply the persistence of a protective gel with a constant thickness. Finally, this paper shows that the residual rate could be a decisive phenomenon affecting the long term behavior of glass in disposal conditions.

Compositional effects on long-term dissolution of borosilicate glass

Calculations have been performed to determine why, in static dissolution tests, the experimentally observed formation of a zeolite, such as analcime, causes a rapid increase in the dissolution rate for some glasses but not for others. The results of these calculations with a series of simple glasses show that there is a strong dependence on the Al content of the glass. For ratios of Si/(Si + Al) < 0.7, the amount of glass that has to dissolve for analcime to cause the dissolution rate to increase is 100–1000 times less than for glasses with higher ratios of Si/(Si + Al). Other glass components such as Na, Ca, B and Li are less effective at changing the rate of dissolution, although a minimum amount of Na needs to be present in the glass for analcime to form. These effects may be calculated for multi-component glasses by use of a simple six-component glass, although experimental verification is necessary to show that a six-component glass can be used. We also discuss the reasons why the effect of analcime precipitation on the silicon concentration in solution may be experimentally difficult to determine.

Simulating Radio Element Release and Water-Rock Interactions During Dissolution of Borosilicate Glass

ABSTRACTA model has been developed that can simulate in some detail the chemistry of the glass dissolution process. To test the performance of the model, calculations were carried out to predict the dissolution behaviour of a commercial borosilicate glass. The model could reproduce accurately the behaviour of major elements released from the glass, although in the case of silica, ‘sorption’ at the glass surface was required in order to achieve good agreement with experimental data. Secondary reaction products sepiolite, montmorillonite, analcime and goethite were predicted to form. Further calculations were carried out to simulate the release of radioelements (caesium, strontium, uranium, plutonium) from within the dissolving glass. The precipitation of insoluble solids was the only mechanism included in the model to retain the radioelements at the glass surface. For plutonium, there was good agreement between model predictions and available experimental data. For caesium and strontium, the model underestimated the amount of retention at the glass surface and additional retention mechanisms, such as coprecipitation or ion exchange reactions involving newly-formed clay-type minerals at the glass surface must be invoked to explain experimental observation.

Waste Glass Corrosion Modeling: Comparison with Experimental Results

ABSTRACTModels for borosilicate glass dissolution must account for the processes of (1) kinetically-controlled network dissolution, (2) precipitation of secondary phases, (3) ion exchange, (4) rate-limiting diffusive transport of silica through a hydrous surface reaction layer, and (5) specific glass surface interactions with dissolved cations and anions. Current long-term corrosion models for borosilicate glass employ a rate equation consistent with transition state theory embodied in a geochemical reaction-path modeling program that calculates aqueous phase speciation and mineral precipitation/dissolution. These models are currently under development. Future experimental and modeling work to better quantify the rate-controlling processes and validate these models are necessary before the models can be used in repository performance assessment calculations.

Modeling Surface Area to Volume Effects on Borosilicate Glass Dissolution

ABSTRACTWe simulated the reaction of SRL-131 glass with equilibrated J-13 water in order to investigate the effects of surface area to volume ratio (SAN/) on glass dissolution. We show that glass-fluid ion exchange causes solution pH to rise to progressively higher values as SA/V increases. Because the ion exchange is rapid relative to the duration of the glass dissolution experiment, the pH effect does not scale with (SA/V)*time. Experiments compared at the same (SA/V)*time value therefore have different pHs, with higher pHs at higher SA/V ratios. Both experimental data and our simulation results show similar trends of increasing reaction rate as a function of SAN ratio when scaled to (SA/V)*time. Glasses which react in systems of differing SA/V ratio therefore follow different reaction paths and high SAN ratios cannot be used to generate data which accurately scales to long time periods unless the ion exchange effect is taken into account. We suggest some simple test designs which enable more reliable high SAN accelerated tests.

Enhancement of Borosilicate Glass Dissolution by Silica Sorption and Diffusion in Compacted Bentonite: A Model Study.

ABSTRACTExperimental evidence indicates that glass corrosion rates decrease proportionally with the increase of silicic acid concentration in the solution contacting the glass surface. A minimum corrosion rate (Rsat) is reached when the solution becomes saturated with respect to an unidentified amorphous siliceous compound. In a repository where the vitrified waste form is surrounded by compacted bentonite, the silica dissolved from the glass will diffuse into the pore solution and concentration gradients will be established throughout the backfill material. The silicic acid concentration at the glass-bentonite interface, and thus the glass corrosion rate, will then be diffusion controlled. Moreover, experimental work suggests that significant sorption of silica by clay minerals in bentonite may accelerate glass corrosion.A model describing glass corrosion coupled with diffusive transport and sorption of silica in bentonite has been developed and incorporated in a FORTRAN computer code (GLADIS). The model assumes: (a) a linear isotherm for the sorption of silica (KD), (b) time and space invariant pH, temperature and ionic strength, (c) proportionality between the quantity of silica precipitated and the amount of glass dissolved and (d) cylindrical geometry. Preliminary calculations with a particular parameter set at 90°C, assuming no silica sorption on the bentonite, indicate for an unfractured glass block that a stationary state is rapidly reached in which the silica concentration at the glass-bentonite interface is lower than the saturation concentration. This implies that the glass corrodes at a more rapid rate than Rsat (RsS ∼ 8 Rsat) If moderate silica sorption is assumed (KD = 0.5 m3 kg−1), the attainment of stationary conditions is delayed by the removal of silicic acid from solution, and the average corrosion rate is further increased by a factor ∼ 2.

A Kinetic Model for Borosilicate Glass Dissolution Based on the Dissolution Affinity of a Surface Alteration Layer

ABSTRACTA kinetic model for the dissolution of borosilicate glass, incorporated into the EQ3/6 geochemical modeling code, is used to predict the dissolution rate of a nuclear waste glass. In the model, the glass dissolution rate is controlled by the rate of dissolution of an alkalidepleted amorphous surface (gel) layer. Assuming that the gel layer dissolution affinity controls glass dissolution rates is similar to the silica saturation concept of Grambow [1] except that our model predicts that all components concentrated in the surface layer, not just silica, affect glass dissolution rates. The good agreement between predicted and observed elemental dissolution rates suggests that the dissolution rate of the gel layer limits the overall rate of glass dissolution. The model predicts that the long-term rate of glass dissolution will depend mainly on ion concentrations in solution, and therefore on the secondary phases which precipitate and control ion concentrations.

The Effect of Groundwater Flow on Release Rate Behavior of Borosilicate Glass

AbstractA simple open-system model is used to evaluate the effect of groundwater flow on borosilicate glass dissolution. With appropriate assumptions, the mass balance equation is: dc/dt=R-kfc;R=k(ceq-c)where c is the concentration of dissolved species i, t is time, kf is the flushing frequency (i.e., volumetric flow rate divided by fixed pore volume), and R is the normalized rate of dissolution. A first-order dependence of R on departure of c from the equilibrium saturation concentration, Ceq, is also assumed. Results from steady-state (dc/dt = 0; c = css = constant) static and dynamic flow tests on a borosilicate glass at 90°C were used to calculate R and the dissolution rate constant, k, as a function of flow rate. The calculated results of the model are in good agreement with independent measurements.There are three important conclusions. First, even the slowest flow rate used (0.1 ml/hr) corresponds to a high flow rate with respect to the intrinsic dissolution rate of the glass and expected repository flow conditions. Second, previous release models that scale radionuclide release rate directly to solubility concentration may overestimate release at high flow rates. This is because previous models fail to account for the pronounced decrease in steady-state solution concentration at the solid interface with increasing flow rate. Third, increased flow rate accelerates the glass dissolution rate without changing the reaction mechanism. This effect is attributable to constant replenishment of undersaturated solution at the glass surface and an apparent increase in the dissolution rate constant with increasing flow rate.