The effect of pH and temperature on kaolinite dissolution rate under acidic conditions

The main goal of this paper is to demonstrate a new rate law describing the combined effect of pH (0.5 to 4.5) and temperature (25°C to 70°C) on kaolinite dissolution rate, under far from equilibrium conditions, as a step towards establishing the full rate law of kaolinite dissolution under acidic conditions. Dissolution experiments were carried out using non-stirred flow-through reactors fully immersed in a thermostatic water bath held at a constant temperature of 25.0°C, 50.0°C or 70.0°C ± 0.1°C. Kaolinite dissolution rates were obtained based on the release of silicon and aluminum at steady state. The results show good agreement between these two estimates of kaolinite dissolution rate. Kaolinite dissolution rates range as a function of temperature and fluid composition from 8 ± 1 × 10^-15 mol m^-2 s^-1 (at 25°C and pH 4.5) to 1.5 ± 0.2 × 10^-11 mol m^-2 s^-1 (at 70°C and pH 0.5. In general, dissolution rate increases with temperature and decreases with pH. The combined effect of pH and temperature is modeled by two independent proton promoted reaction paths. The first reaction path controls the overall dissolution rate at pH ≥ 2.5, whereas the second path controls it below pH 0.5. Between pH 0.5 and 2.5 the two reaction paths influence the rate. Using this model the effects of pH and temperature on the overall dissolution rate of kaolinite under acidic condition can be described by: Rate = 2· 10²·e^-22/RT· 2· 10¹⁰ · e 19/RT · a_H² = 5 · 10⁷ · e^-2⁸/RT · 1.4 · 10^-7 · e^10/RT · a_H⁺/1 + 2 · 10^-10 · e^19/RT · a_H⁺ 1+ 1.4 · 10^-7 · e^10/RT · a_H⁺ 1 where R is the gas constant, T is the temperature (K) and a_H⁺ is the activity of protons in solution.