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，简称Obr）并溶解二氧化硅的全新机制，该机制与α-石英(101)/水界面（α-quartz (101)/water interface）的实验数据及量子力学模拟结果相符。该全新假说认为，质子转移（H+-transfer）通过表面内部氢键（即SiOH–Obr氢键）而非表面水氢键发生；且离子强度（ionic strength，简称I）的升高会促进此类内部氢键的形成，进而使阿伦尼乌斯方程（Arrhenius equation）$k = A exp(-Delta E_a/RT)$ 中的指前因子A增大，最终提升溶解速率。研究针对α-石英(101)表面及纯水体系开展了投影缀加平面波密度泛函理论（Projector-augmented planewave density functional theory，简称DFT）分子动力学（molecular dynamics，简称MD）模拟与静态能量最小化计算，体系中包含Cl⁻、Na⁺与Mg²⁺离子。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。本研究通过考察表面硅羟基（SiOH，surface silanol groups）与溶液及其他表面原子间的氢键作用特性，对上述假说进行验证。模拟结果显示，SiOH–Obr内部氢键的占比出现了统计学意义上的显著提升，同时桥氧原子通过氢键与硅羟基相连进而被质子化的可能性也有所增加，这与本研究提出的全新假说相符。本研究还结合硅酸盐溶解的相关实验数据，对该全新假说展开了讨论。