A New Hypothesis for the Dissolution Mechanism of Silicates

A novel mechanism for protonating bridging O atoms (Obr) and dissolving silica is proposed that is consistent with
experimental
data and quantum mechanical simulations of the α-quartz (101)/water
interface. The new hypothesis is that H+-transfer occurs
through internal surface H-bonds (i.e., SiOH–Obr) rather than surface water H-bonds and that increasing ionic strength, I, favors formation of these internal H-bonds, leading to
a larger pre-exponential factor, A, in the Arrhenius
equation, k = A exp­(−ΔEa/RT), and higher
rates of dissolution. Projector-augmented planewave density functional
theory (DFT) molecular dynamics (MD) simulations and static energy
minimizations were performed on the α-quartz (101) surface and
with pure water, with Cl–, Na+, and Mg2+. Classical molecular dynamics were performed on α-quartz
(101) surface and pure water only. The nature of the H-bonding of
the surface silanol (SiOH) groups with the solution and with other
surface atoms is examined as a test of the above hypothesis. Statistically
significant increases in the percentages of internal SiOH–Obr H-bonds, as well as the possibility of Obr protonation
with H-bond linkage to silanol group, are predicted by these simulations,
which is consistent with the new hypothesis. This new hypothesis is
discussed in relation to experimental data on silicate dissolution.