The mechanism of dissolution of minerals in acidic and alkaline solutions: Part II Application of a new theory to silicates, aluminosilicates and quartz

• Proposedtheory describes kinetics of dissolution of silicates.
• Order of reaction wrt H+ described.
• Order of reaction wrt OH– described.
• Change in order of reaction wrt H+ for forsterite described.

The kinetics of the dissolution of silicates, aluminosilicates and quartz is described by a novel theory of dissolution. The experimental data for the rate of dissolution of these minerals shows a remarkable pattern: for many of these minerals, the order of reaction with respect to H+ is close to 0.5 in the acidic region, and the order of reaction with respect to OH− is close to 0.5 in the alkaline region. It is proposed that the site of bond breaking in the rate-determining step of dissolution is the weakest bond, and this is frequently the metal–oxygen bond because of the higher bond energy of silicon–oxygen bonds. Alternatively, the least number of silicon–oxygen bonds is broken. This means that silicate groups react intact as a unit. Both metal atoms and silicate groups react and are removed independently. The rates of these independent processes are coupled by the potential at the surface. In the acid region, H+ reacts with silicate groups at the surface. In the alkaline region, OH− ions react with the metal atom at the surface. The proposed theory of dissolution correctly predicts the observed orders of reaction with respect to H+ ions and OH− ions in solution. The order of reaction of forsterite with respect to H+ changes from 0.5 in the acidic region to 0.25 in the region above a value of pH of approximately 6. The proposed mechanism suggests that the reason for the change in order of reaction is that the H+ needs to be positioned at the inner Helmholtz plane to be effective in alkaline solutions.