Silica dissolution kinetics in the presence of NaOH

Abstract

Silica dissolution was simulated using Gaussian 03 software at B3LYP/3-21G* and the B3LYP/6-311G(2d, p) levels, applied to a modeled silica Si4O6(OH)4 in the presence of an NaOH molecule and up to five water molecules. The atomic positions of the sodium hydroxide, the water molecules and the surface SiO3(OH) moiety were varied to mimick the surface reaction whereas the positions of the remaining Si3O3(OH)3 atoms were frozen throughout the geometry optimization. The energy barrier was 88 kJ/mol (85 kJ/mol after the zero-point correction), which is still in the range of experimental activation energies of 46 - 96 kJ/mol. The sodium plays a role in stabilizing the surface silicon in five-fold coordination, which induces the elongation of Si-O distances and the weakening of the bonded interactions. This makes the Si-O rupture easier and faster, which is a clue to how presence of alkali enhances silica dissolution.