Abstract
This study uses empirical experimental evidence and Material Studio simulations to explain the interaction of sodium hydroxide (NaOH) with quartz. Density functional theory (DFT) calculations were carried out using the Cambridge Serial Total Energy Package. In addition, quartz grains subjected to dissolution in NaOH were characterized using scanning electron microscopy. The so-called O-middle termination in the quartz tetrahedron structure, typified by a solitary exposed oxygen atom at the surface, is the most susceptible SiO2 terminations to NaOH attack, as it is associated with the lowest surface energy. The adsorption energy values are − 1.44 kcal/mol and − 5.90 kcal/mol for a single atom layer and five-layered atomic structure, respectively. The DFT calculation reveals intramolecular energy is the dominant adsorption energy, followed by a weak van der Waals energy. The NaOH adsorbed on quartz (001) surface constitutes a lower band gap of 0.138 eV compared to cleaved quartz (001) surface (0.157 eV). In addition, the energy range of NaOH adsorbed on quartz is wider (− 50 to 10 eV), compared to (001) quartz (− 20 to 11 eV). The dissolved quartz showed the precipitation of sorbed silicate phases due to incongruent reactions, which indicates new voids and etch pits can be created through the cleaving of the sodium silicates sorbed into the quartz surface. The adsorption energy for NaOH interactions with reservoir sandstone was significantly higher compared to the solitary crystal grains, which can be attributed to the isotropic deformation of a single crystal, and non-uniform deformations of adjacent grains in granular quartz of sandstone reservoir. It can be inferred that exposure to NaOH will affect the structure and reactivity of quartz. The quartz surface textural study indicates that dissolution of crystalline (granite) and clastic rocks (sandstone) is critical to the development of voids, which will improve permeability by providing channels and routes for the passage of hydrothermal and reservoir fluids.
Highlights
Quartz is an abundant silicate mineral that plays a significant part in geological studies, as oxygen and silicon are the key elements of Earth’s crust
Despite the several experimental studies (Rimstidt 2015; Dove 1999) on quartz dissolution in NaOH, few studies have researched molecular-level elucidation of the dissolution mechanism with regard to singular SiO2 grains (Wang et al 2018; Yanina et al 2006). Deciphering this interaction using density functional theory (DFT) modelling is vital since traditional microscopic and spectroscopic analyses have not been completely successful in providing comprehensive structural properties of the quartz surface at atomic/molecular level due to their restricted resolutions or precisions
Cambridge Serial Total Energy Package (CASTEP) simulation was used to elucidate the action of NaOH molecules adsorbed on the optimized SiO2 surfaces, by acquiring data on orbital overlapping, electron density, band structures, band gap, orbital and spin density of states (DOS)
Summary
Quartz is an abundant silicate mineral that plays a significant part in geological studies, as oxygen and silicon are the key elements of Earth’s crust. Despite the several experimental studies (Rimstidt 2015; Dove 1999) on quartz dissolution in NaOH, few studies have researched molecular-level elucidation of the dissolution mechanism with regard to singular SiO2 grains (Wang et al 2018; Yanina et al 2006). Deciphering this interaction using density functional theory (DFT) modelling is vital since traditional microscopic and spectroscopic analyses have not been completely successful in providing comprehensive structural properties of the quartz surface at atomic/molecular level due to their restricted resolutions or precisions
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