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Title: Glass dissolution as a function of pH and its implications for understanding mechanisms and future experiments

Abstract

Here, various rate equations for the dissolution of silicate glasses have been discussed in the literature. In this article, the published results from studies are discussed in which the dissolution rate data are collected under high flow conditions such that saturation with respect to alteration products is avoided. Additionally, the studies also covered broad ranges of temperature and pH. Starting with nuclear waste glass studies, a two-term rate expression is used to fit data with the result that the data point toward a three-term expression offered by Köhler et al. (2003). These rate expressions contain two or three pre-exponential or rate constants. However, it appears that a single rate constant, an intrinsic rate constant, is consistent with the data. Thus, a rate expression of the form R=k i [exp($$\frac{-EaH+}{RT})$$a$$ηH\atop{H}$$+exp ($$\frac{-EaH2O}{RT}$$) + exp ($$\frac{-EaOH-}{RT}$$) a$$ηOH\atop{OH}$$] appears to be applicable to a broad range of glasses. Here, R is the rate of dissolution, mol/(m 2·s) or similar; E is the activation energy associated with the acid, water, or hydroxide activated reactions, kJ/mol; a is the activity of H +, H 2O, or OH -, unitless; η is the order of the reaction with respect to H +, H 2O, or OH-; R is the gas constant, kJ/(mol·K); T is the temperature, Kelvin; and k i is the intrinsic rate constant, mol/(m 2·s) or similar. The contribution to the overall rate from the ‘water’ term is evident as a minor contribution in the middle pH range for some glass compositions and a major contributor for others. One nuclear waste glass (the Japanese P0798), a basalt glass (Köhler et al. (2003)), and a glass with a mineral composition (Bourcier (1998)) exhibit this contribution as a relatively flat response to changes in pH in the range of 5 to 8. However, to distinguish between the possible rate laws, more experiments and more carefully constrained experimentation are needed. Additionally, these may include experiments at pH values that differ by as little as 0.25. Lastly, experiments with glasses of different compositions are needed to determine the dependence of the intrinsic rate constant on the glass composition and structure, i.e. non-bridging oxygens, Si-O-Si and Si-O-X (X = a matrix-forming element, e.g. Al or Fe), and other glass structural properties, e.g. binding energies.

Authors:
 [1]
  1. Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE Office of Fossil Energy (FE)
OSTI Identifier:
1395269
Report Number(s):
PNNL-SA-82068
Journal ID: ISSN 0016-7037; PII: S001670371730563X
Grant/Contract Number:
AC05-76RL01830
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Geochimica et Cosmochimica Acta
Additional Journal Information:
Journal Volume: 219; Journal ID: ISSN 0016-7037
Publisher:
The Geochemical Society; The Meteoritical Society
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE; 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

Citation Formats

Strachan, Denis. Glass dissolution as a function of pH and its implications for understanding mechanisms and future experiments. United States: N. p., 2017. Web. doi:10.1016/j.gca.2017.09.008.
Strachan, Denis. Glass dissolution as a function of pH and its implications for understanding mechanisms and future experiments. United States. doi:10.1016/j.gca.2017.09.008.
Strachan, Denis. 2017. "Glass dissolution as a function of pH and its implications for understanding mechanisms and future experiments". United States. doi:10.1016/j.gca.2017.09.008.
@article{osti_1395269,
title = {Glass dissolution as a function of pH and its implications for understanding mechanisms and future experiments},
author = {Strachan, Denis},
abstractNote = {Here, various rate equations for the dissolution of silicate glasses have been discussed in the literature. In this article, the published results from studies are discussed in which the dissolution rate data are collected under high flow conditions such that saturation with respect to alteration products is avoided. Additionally, the studies also covered broad ranges of temperature and pH. Starting with nuclear waste glass studies, a two-term rate expression is used to fit data with the result that the data point toward a three-term expression offered by Köhler et al. (2003). These rate expressions contain two or three pre-exponential or rate constants. However, it appears that a single rate constant, an intrinsic rate constant, is consistent with the data. Thus, a rate expression of the form R=ki [exp($\frac{-EaH+}{RT})$a$ηH\atop{H}$+exp ($\frac{-EaH2O}{RT}$) + exp ($\frac{-EaOH-}{RT}$) a$ηOH\atop{OH}$] appears to be applicable to a broad range of glasses. Here, R is the rate of dissolution, mol/(m2·s) or similar; E is the activation energy associated with the acid, water, or hydroxide activated reactions, kJ/mol; a is the activity of H+, H2O, or OH-, unitless; η is the order of the reaction with respect to H+, H2O, or OH-; R is the gas constant, kJ/(mol·K); T is the temperature, Kelvin; and ki is the intrinsic rate constant, mol/(m2·s) or similar. The contribution to the overall rate from the ‘water’ term is evident as a minor contribution in the middle pH range for some glass compositions and a major contributor for others. One nuclear waste glass (the Japanese P0798), a basalt glass (Köhler et al. (2003)), and a glass with a mineral composition (Bourcier (1998)) exhibit this contribution as a relatively flat response to changes in pH in the range of 5 to 8. However, to distinguish between the possible rate laws, more experiments and more carefully constrained experimentation are needed. Additionally, these may include experiments at pH values that differ by as little as 0.25. Lastly, experiments with glasses of different compositions are needed to determine the dependence of the intrinsic rate constant on the glass composition and structure, i.e. non-bridging oxygens, Si-O-Si and Si-O-X (X = a matrix-forming element, e.g. Al or Fe), and other glass structural properties, e.g. binding energies.},
doi = {10.1016/j.gca.2017.09.008},
journal = {Geochimica et Cosmochimica Acta},
number = ,
volume = 219,
place = {United States},
year = 2017,
month = 9
}

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  • For years, we have been using a certain form of the glass dissolution rate equation. In this article, I examine the assumptions that have been made and suggest that the rate equation may be more complex than originally thought. Suggestions of experiments that are needed to correct or validate the exisiting form of the rate equation are made.
  • Here, we report and discuss results of atom probe tomography (APT) and energy-filtered transmission electron microscopy (EFTEM) applied to a borosilicate glass sample of nuclear interest altered for nearly 26 years at 90°C in a confined granitic medium in order to better understand the rate-limiting mechanisms under conditions representative of a deep geological repository for vitrified radioactive waste. The APT technique allows the 3D reconstruction of the elemental distribution at the reactive interphase with sub-nanometer precision. Profiles of the B distribution at pristine glass/hydrated glass interface obtained by different techniques are compared to show the challenge of accurate measurements ofmore » diffusion profiles at this buried interface on the nanometer length scale. Our results show that 1) Alkali from the glass and hydrogen from the solution exhibit anti-correlated 15 ± 3 nm wide gradients located between the pristine glass and the hydrated glass layer, 2) boron exhibits an unexpectedly sharp profile located just at the outside of the alkali/H interdiffusion layer; this sharp profile is more consistent with a dissolution front than a diffusion-controlled release of boron. The resulting apparent diffusion coefficients derived from the Li and H profiles are D Li = 1.5 × 10 -22 m 2.s -1 and D H = 6.8 × 10 -23 m 2.s -1. These values are around two orders of magnitude lower than those observed at the very beginning of the alteration process, which suggests that interdiffusion is slowed at high reaction progress by local conditions that could be related to the porous structure of the interphase. As a result, the accessibility of water to the pristine glass could be the rate-limiting step in these conditions. More generally, these findings strongly support the importance of interdiffusion coupled with hydrolysis reactions of the silicate network on the long-term dissolution rate, contrary to what has been suggested by recent interfacial dissolution-precipitation models for silicate minerals.« less
  • The objective of the EC funded GLAMOR project was to achieve a common understanding of the processes that control the decrease of the dissolution rate of high-level waste glass in water when silica becomes saturated. Is the affinity controlled concept, or the protective layer concept dominating? The following steps were taken: (1) review of the literature, (2) selection of an experimental dataset, and selection of the models r(t) and GM2003, and (3) application by the GLAMOR partners of the models to the datasets. The main focus has been on dissolution tests in pure water at different values of surface tomore » volume and pH. Some of the main conclusions were: (1) both affinity and protective layer concepts must be considered in the interpretation of the rate decreasing stage, (2) the residual dissolution rate observed beyond the silica saturation stage is far more important in view of the long-term performance of the glass, and deserves more attention in future R&D. GLAMOR also discussed in detail the modelling parameters such as the silica saturation concentration, the silica diffusion coefficient, the silica retention factor in the reaction layer, and the water diffusion coefficient.« less