A New Hypothesis for the Dissolution Mechanism of Silicates

A novel mechanism for protonating bridging O atoms (O-br) and dissolving silica is proposed that is consistent with experimental data and quantum mechanical simulations of the alpha-quartz (101)/water interface. The new hypothesis is that H+-transfer occurs through internal surface H-bonds (i.e., SiOH-O-bt) 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(-Delta E-a/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 alpha-quartz (101) surface and with pure water, with Cl-, Na+, and Mg2+. Classical molecular dynamics were performed on alpha-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-O-br H-bonds, as well as the possibility of O-br 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.