Abstract
Understanding the dissolution of silicate glasses and minerals from atomic to macroscopic levels is a challenge with major implications in geoscience and industry. One of the main uncertainties limiting the development of predictive models lies in the formation of an amorphous surface layer––called gel––that can in some circumstances control the reactivity of the buried interface. Here, we report experimental and simulation results deciphering the mechanisms by which the gel becomes passivating. The study conducted on a six-oxide borosilicate glass shows that gel reorganization involving high exchange rate of oxygen and low exchange rate of silicon is the key mechanism accounting for extremely low apparent water diffusivity (∼10−21 m2 s−1), which could be rate-limiting for the overall reaction. These findings could be used to improve kinetic models, and inspire the development of new molecular sieve materials with tailored properties as well as highly durable glass for application in extreme environments.
Highlights
Understanding the dissolution of silicate glasses and minerals from atomic to macroscopic levels is a challenge with major implications in geoscience and industry
In the case of borosilicate glass used to confine nuclear wastes, several hypotheses are still under debate to account for the 3–5 orders-of-magnitude decrease of the dissolution rate measured in confined environments: (i) saturation of the solution in silica thereby decreasing the chemical driving force for the dissolution reaction[12], (ii) pore closure within the gel layer limiting the transport of aqueous species[9], (iii) water diffusion in the pristine solid[13], (iv) diffusion of dissolved silica through the gel layer leading to higher concentrations at the reaction front[2], and more recently (v) slow mobility of water molecules in the gel due to their confinement in constricted micropores[14,15]
We propose an approach based on three experiments with isotopically tagged water molecules to study their mobility and their reactivity in a passivating gel formed on International Simple Glass (ISG), a six-oxide borosilicate reference glass[17], as well as molecular dynamics (MD) simulations and continuumscale modeling to support experimental findings
Summary
Understanding the dissolution of silicate glasses and minerals from atomic to macroscopic levels is a challenge with major implications in geoscience and industry. We propose an approach based on three experiments with isotopically tagged water molecules to study their mobility and their reactivity in a passivating gel formed on International Simple Glass (ISG), a six-oxide borosilicate reference glass[17], as well as molecular dynamics (MD) simulations and continuumscale modeling to support experimental findings.
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