Mechanism of kaolinite dissolution at room temperature and pressure Part II: Kinetic study

Studies on the dissolution kinetics of kaolinite were performed using batch reactors at 25 degrees C and in the pH range from I to 13. A rapid initial dissolution step was first observed, followed by a linear kinetic stage reached after approximately 600 hr of reaction during which the kaolinite dissolves congruently at pH < 4 and pH > 11. The apparent incongruency between pH 5 and 10 was due to the precipitation of an Al-hydroxide phase. The true dissolution rates were computed from the amount of Si released into solution. The rate dependence on pH can be described by: r = 10(-12.19) a(H+)(0.55) + 10(-14.36) + 10(-10.71) a(OH-)(0.75) Between pH 5 and 10, the rate is approximately constant, although a smooth minimum was observed at pH close to 9. An attempt was made to obtain a general rate law based on the coordination theory, which was first applied to the mineral dissolution studies by Stumm and co-workers. The kinetic data were combined with the results obtained for the surface speciation by Huertas et al. (1998). It is possible to express the linear dissolution rate as a simple power function of the concentration of the surface sites active in various pH ranges: r = 10(-8.25) [>Al2OH2+] + 10(-10.82) [>AlOH2+](0.5) + 10(-9.1) [>Al2OH + >AlOH + >SiOH] + 10(3.78) [>Al2O- + >AlO-](3) This equation assumes that the dissolution mechanism is mainly controlled by the two Al surface sites (external and internal structural hydroxyls, and aluminol at the crystal edges) under both acidic and alkaline conditions. The model reflects well the important contribution of the crystal basal planes to the dissolution of kaolinite. Copyright (C) 1999 Elsevier Science Ltd.