Relation between rates of aluminosilicate mineral dissolution, pH, temperature, and surface charge

At 25 degrees C surface charge on Al2O3 and SiO2 is directly proportional to the rates at which these oxides dissolve with the minimum dissolution rate per unit surface area occurring at their pH of zero point of charge, 8.4 and 2.5, respectively. As charged surface complexes polarize and therefore weaken the Al-O and Si-O surface bonds, the first order relation between surface charge and dissolution rate for aluminosilicates can be predicted by combining the effects on Al2O3 and SiO2. This analysis explains the pH dependence of the initial non-stoichiometric dissolution of aluminosilicates such as that documented for kaolinite and albite as well as the dynamics of stoichiometric and steady-state Al and Si detachment at both low and high PH where rates increase relative to near neutral pH solutions. Aluminosilicates do not dissolve with a minimum rate at their pH of zero point of charge but where the sum of the absolute value of charge from Al surface sites plus the absolute value of charge at Si surface sites is a minimum. It Is argued that the ratio of Al and Si surface charge sites changes during the initial nonstoiciometric period of detachment converging to the ratio needed for stoichiometric dissolution of the aluminosilicate. The temperature dependence of dissolution rate appears to be directly correlated with the increase in surface charge with temperature at constant pH. In alkaline solutions the log of the concentration of negative Si surface complexes on silicates, and; therefore, log dissolution rate is a linear function of pH whose slope increases with increasing temperature from 0.3 degrees at 25 degrees to 0.6 degrees at 80 degrees C. These slopes display Arrhenius behavior; decreasing linearly with the inverse of temperature. However, it is postulated that at supercritical temperatures the neutral surface complexes of Al and Si become dominant over an increasing range of pH at near neutral pH with increasing temperature. Under these conditions the rates of mineral dissolution become nearly independent of pH except at extreme PH.