Structural effects on dissolution of silica polymorphs in various solutions

Abstract

The dissolution and precipitation of silica minerals in rocks, soils, and sediments are essential processes of material transformations near the Earth’s surface. In this study, the dissolution of α-quartz and α-cristobalite are investigated at 25 °C in HNO3, NaOH, KCl, and MgCl2 solutions. The amounts of silicon release from α-quartz in HNO3 and electrolyte solutions are larger than those from α-cristobalite, and the circumstance is in consistence with the density of surface silanols. In NaOH solutions, the amounts of silicon release increase significantly and the maximum amount is about 30 times higher than that in acid and electrolyte solutions. Moreover, the maximum silicon release is inversely proportional to the density of surface silanols, resulting in a lower silicon release from α-quartz in comparison to that from α-cristobalite. It could be deduced that there is a possible correlation with structural defects, for instance, oxygen vacancies, demonstrated by the electron paramagnetic resonance (EPR) spectrum. By analyzing the surface composition of samples after dissolution, the result of X-ray photoelectron spectroscopy (XPS) confirms that no stable amorphous silica layer form on the surface of α-quartz and α-cristobalite under the current experimental conditions, along with the verification of high-resolution transmission electron microscope (TEM). Furthermore, the surface species of α-quartz and α-cristobalite after reacting with different solutions are also qualitatively determined by XPS.