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

The main goal of this paper is to propose a new rate law describing the combined effect of pH (1 to 4.5) and temperature (25 to 70 °C) on smectite dissolution rate, under far from equilibrium conditions, as a step towards establishing the full rate law of smectite 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. Smectite 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 smectite dissolution rate. Low Al/Si ratios were obtained in experiments that were conducted at pH ≥4. These low Al/Si ratios are explained by precipitation of gibbsite and/or diaspore. Dissolution rate increases with temperature and decreases with increasing pH. Dissolution rates of experiments in which ΔG_r ≤ -21 kcal mol^-1, are not affected by deviation from equilibrium. Dissolution rates in most experiments are not affected by the addition of up to 0.3 M NaNO₃ to the input solution. A simple model is used to describe the combined effect of pH and temperature on smectite dissolution rate. According to this model, dissolution rate is linearly proportional to the concentration of adsorbed protons on the mineral surface, and proton adsorption is described using a Langmuir adsorption isotherm. All experimental results at pH >4 were fitted to the model using a multiple non-linear regression. The resulting rate law is: Rate = 220 · e^-17460RT · 3·10^-6 · e¹⁰⁷⁰⁰/RT · a_H⁺/1 + 3.10^-6 · e^10700/RT · a_H⁺ where R is the gas constant, T is the temperature (K) and a H⁺ is the activity of protons in solution. According to the model, the dependence of dissolution rate on temperature is affected by the activation energy and the adsorption enthalpy. The activation energy obtained from the fitting (17 ± 2 kcal-mol) is within error equal to the average value of 15 kcal-mol of apparent activation energies for silicates dissolution rate (Lasaga et al., 1994). The obtained net enthalpy of adsorption (-11 ± 2 kcal-mol) is within the range of -7.9 to -23.1 kcal-mol, experimentally obtained for oxides (Sverjensky and Sahai, 1998).