DISSOLUTION RATE OF QUARTZ IN LEAD AND SODIUM ELECTROLYTE-SOLUTIONS BETWEEN 25-DEGREES-C AND 300-DEGREES-C - EFFECT OF THE NATURE OF SURFACE COMPLEXES AND REACTION AFFINITY

The dissolution rate of quartz has been measured at 25 degrees C in batch reactors and at 200 and 300 degrees C in mixed flow reactors. These experiments have been carried out in both pure H2O and solutions containing Na or Pb at various ionic strengths and pH. The measured rates were found to increase significantly with the addition of either Na or Pb. In an attempt to determine the mechanism of these effects, the degree of adsorption of Na and Pb were measured on amorphous silica at 25 and 150 degrees C. At 25 degrees C, Na is found to adsorb on the quartz surface as an outer-sphere complex, and the corresponding dissolution rate increase is explained by an increase of the ionic strength. By contrast, at 25 degrees C, lead, which forms inner-sphere complexes, increases the quartz dissolution rate specifically. At high temperature, quartz dissolution is promoted in the presence of both Na and Pb by a pH-dependent formation of surface inner-sphere complexes. This effect tends to vanish when the degree of saturation of the solution increases, as a result of the competition between electrolyte and aqueous silica adsorption on quartz surface. These results show that the electrolytes which adsorb as inner-sphere complexes dominate the overall reaction at conditions far from equilibrium only. Consequently, for a large range of chemical affinity quartz dissolution in Na and Pb electrolyte solutions can be modeled within the framework of the Transition State Theory by simply taking into account the protonated surface species and the ionic strength of the solution.