Dissolution and hydration kinetics of MgO

The dissolution and hydration kinetics of MgO single crystals and powder samples were investigated with regard to the H⁺ and Mg²⁺ concentrations and the temperature. The rate of dissolution of rotating MgO discs in buffered solutions was determined from measurements of [Mg²⁺] and those of the crystals and powder fractions were determined by pH and conductivity analysis. The degree of hydration was analysed by means of a thermogravimetric method. Several rate-controlling processes depending on pH were present at room temperature.

(1) At pH < 5 the rate-controlling step was proton attack followed by desorption of Mg²⁺ of OH^- depending on the value of [Mg²⁺]. The rate was proportional to either -pH or pMg-pH. These processes are part of the overall neutralization reaction. MgO + 2H⁺→Mg²⁺ + H₂O

(2) At pH ≈ 5 the rate-controlling step was a diffusion-limitation process due to protons. The rate was proportional to the proton concentration.

(3) At pH > 7 the rate-controlling step was OH^- adsorption followed by Mg²⁺ and OH^- desorption leading to a rate maximum. These processes are part of the overall dissolution reaction. MgO + H₂O→Mg²⁺ + 2OH^- The neutralization processes are interpreted in terms of irreversible thermodynamics yielding a linear dependence of the rate on pH or pMg-pH. It is concluded from conductivity and scanning electron microscopy measurements during and after hydration experiments that the hydration rate is controlled by the dissolution rate under given conditions. After a supersaturation period Mg(OH)₂ precipitates preferentially at the MgO surface, so that an MgO lattice reaction can be excluded. All processes undergo an Arrhenius acceleration with increasing temperature (activation energy, 70 kJ mol^-1) and the overall reactions are then limited by proton and OH^- diffusion.