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

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. (C) 2014 Elsevier B.V. All rights reserved.