Reactions and Interfacial Behaviors of the Water–Amorphous Silica System from Classical and Ab Initio Molecular Dynamics Simulations Page: 28
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The first stage of dissolution is based on the inter-diffusion between the network
modifiers and the water, as well as hydration of the interfacial layer by breaking the
weaker oxygen-cations bonds [179,180]. This results in the reorganization of the
surface due to the removal of sodium and calcium with protons from the water being
absorbed into the glass to balance the negative charge of the surface [154]. The
composition of the surface varies with the time it has been in contact with the water due
to the time dependent removal of Ca2+, Na+, and other network modifiers [178].
The second stages is the hydrolysis of the silica surface, which includes the
breaking of the highest energy siloxane bonds and the removal of silica into solution
[154]. Theoretically, this process would result in all elements being removed from the
surface at an equal rate, termed congruent dissolution, but this does not occur in
nuclear waste glasses based on the differences in the stability of the network forming
elements, specifically silicon and boron [154]. The hydrolysis process has been
investigated in detail for pure silica glasses using computational methods and is
discussed in Section 4.4.2, with few investigations of the stability of mixed network
former systems [167]. The initial hydrolysis rate depends on the temperature, pH, glass
composition, and dissolved species in solution where the time required to reach the next
stage of dissolution varies from seconds to days [154].
During the next step, there is a drop in the dissolution rate due to the formation of
a protective surface layer which is an amorphous porous hydrated silica gel phase
[154,178]. The drop in dissolution rate typically occurs which silica reaches a
concentration of 1 mg/L in solution at ~350K [154] with the thickness of the gel varying
with pH [181]. Though the exact process responsible for its generation is still discussed28
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Rimsza, Jessica M. Reactions and Interfacial Behaviors of the Water–Amorphous Silica System from Classical and Ab Initio Molecular Dynamics Simulations, dissertation, May 2016; Denton, Texas. (https://digital.library.unt.edu/ark:/67531/metadc849660/m1/39/?rotate=90: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; .