HOW TO ACCESS TO THE PURE DISSOLUTION RATE OF RAPIDLY DISSOLVING MINERALS?

The knowledge of the dissolution rates of rapidly dissolving minerals, like sulfates or carbonates, in aqueous solutions is of foremost importance in the study of the weathering of rocks, of karst formation, of the quality of drinking water, of the spreading of pollutants, … In the case of minerals containing the calcium ion, like gypsum or calcite, their dissolution rate is also crucial in the investigation of the geological storage of CO₂.

In standard dissolution experiments (batch, rotating disk, …), the mineral is dissolving in stirred water. So the dissolution kinetics is mixed with diffusive and convective contributions. For hard minerals, dissolution is so slow that it drives the whole kinetics. But for softer minerals, dissolution, diffusion and convection timescales are of the same order of magnitude and their respective contributions can be difficult to disciminate.

As an example, we have collected dissolution rates of gypsum in water measured by various methods found in the literature. The deduced dissolution rate constants span over several decades. Therefore we have analysed the hydrodynamics of the experimental setups, eliminated the diffusive and convective contributions and deduced from them the pure surface reaction rate constant of gypsum in water. It appears to be much smaller than expected from the literature results. An holographic interferometry experiment, performed in still water, is carried out and enables to measure directly this rate constant. Both values agree within experimental uncertainty, which confirms the unexpected small value of the dissolution rate constant of gypsum, and give clues to understand the discrepancy between the reported values of calcite dissolution rate.