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.

本研究提出了一种质子化桥氧原子（bridging O atoms, Obr）并溶解二氧化硅的全新机制，该机制与实验数据及α-石英(101)/水界面的量子力学模拟结果一致。
新假说认为，质子转移（H+-transfer）通过表面内氢键（即硅羟基-桥氧（SiOH–Obr））而非表面水氢键发生；且离子强度I的升高会促进这类表面内氢键的形成，进而使阿伦尼乌斯方程k = A exp(−ΔEa/RT)中的指前因子A增大，提升二氧化硅的溶解速率。
本研究针对α-石英(101)表面与纯水、氯离子（Cl–）、钠离子（Na+）及镁离子（Mg2+）体系，开展了投影缀加波平面波密度泛函理论（projector-augmented planewave density functional theory, DFT）分子动力学（molecular dynamics, MD）模拟与静态能量最小化计算；仅针对α-石英(101)表面与纯水体系开展了经典分子动力学模拟。
本研究对表面硅羟基（silanol, SiOH）基团与溶液及其他表面原子之间的氢键性质进行了分析，以验证上述假说。
模拟结果显示，硅羟基-桥氧内氢键的占比出现统计学上的显著提升，同时桥氧原子可通过与硅羟基形成氢键而发生质子化，这与新假说的预测相符。
本研究还结合硅酸盐溶解的相关实验数据，对该新假说展开了讨论。