Experimental study of the effect of pH and temperature on the kinetics of montmorillonite dissolution

The effect of pH on the kinetics of smectite (K-montmorillonite) dissolution was investigated at 50 and 70 °C in stirred flow-through reactors over the pH range of 1–13.5. Experiments done at very acidic and very basic pH were far from equilibrium. Near neutral pH experiments were closer to equilibrium. The Al/Si release ratio, while initially being incongruent, ultimately approached the stoichiometric value in most of the experiments. Temperature, extreme pH, and time favor congruency. Rates can be described by: R ( mol g - 1 s - 1 ) = 10 - 9.00 a H + 0.58 + 10 - 9.70 a OH - 0.38 + 10 - 12.15 at 50 ° C R ( mol g - 1 s - 1 ) = 10 - 8.25 a H + 0.75 + 10 - 8.70 a OH - 0.42 + 10 - 11.95 at 70 ° C. Dissolution rates decrease with increasing pH under acid conditions, minimize at near neutral pH and increase with increasing pH under basic conditions. Apparent activation energies are pH-dependent and have a maximum of 19.3 kcal mol −1 at pH 1, a minimum of 5.3 kcal mol −1 between pH 4–7 and another maximum of 20.3 kcal mol −1 at pH 11. Protons, water, and hydroxyls promote dissolution. Their relative contribution to the overall dissolution rate depends on pH.