Abstract:
Dissolution and precipitation rates of brucite (Mg(OH)2) were measured at 25°C in a mixed-flow reactor as a function of pH (2.5 to 12), ionic strength (10-4 to 3 M), saturation index (-12 < log Ω < 0.4) and aqueous magnesium concentrations (10-6 to 5.10-4 M). Brucite surface charge and isoelectric point (pHIEP) were determined by surface titrations in a limited residence time reactor and electrophoretic measurements, respectively. The pH of zero charge and pHIEP were close to 11. A two-pK, one site surface speciation model which assumes a constant capacitance of the electric double layer (5 F/m2) and lack of dependence on ionic strength predicts the dominance of >MgOH2+ species at pH < 8 and their progressive replacement by >MgOH° and >MgO- as pH increases to 10-12. Rates are proportional to the square of >MgOH2+ surface concentration at pH from 2.5 to 12. In accord with surface speciation predictions, dissolution rates do not depend on ionic strength at pH 6.5 to 11. Brucite dissolution and precipitation rates at close to equilibrium conditions obeyed TST-derived rate laws. At constant saturation indices, brucite precipitation rates were proportional to the square of >MgOH2+ concentration. The following rate equation, consistent with transition state theory, describes brucite dissolution and precipitation kinetics over a wide range of solution composition and chemical affinity: R=kMg+ . {>MgOH2+}2 . (1-Ω2) where kMg+ is the dissolution rate constant, {>i} is surface species concentration (mol/m2), and Ω is the solution saturation index with respect to brucite.Measurements of nonsteady state brucite dissolution rates, in response to cycling the pH from 12 to 2 (pH-jump experiments), indicate the important role of surface hydroxylation - that leads to the formation of Mg oxo or -hydroxo complexes - in the formation of dissolution-active sites. Replacement of water molecules by these oxygen donor complexes in the Mg coordination sphere has a labilizing effect on the dynamics of the remaining water molecules and thus increases reaction rates.
