How Do Specific Surface Area And Particle Size Distribution Change When Granular Media Dissolve?

The measured dissolution rate of a granular medium depends on its surface area and how the surface area changes during the course of the measurement. Moreover, the assumption that the specific surface area either remains constant or initially increases during dissolution is not always valid. This paper demonstrates that when the particle size distribution has sufficient variance, the instantaneous change in surface area during dissolution can be negative, even before the smallest particles dissolve away. The concept is explained using spherical particles, extended for use with prismatic particles, and demonstrated experimentally with gypsum powder. For the commercial gypsum powder used, the specific surface area decreases by about 50% during the first 10% of mass loss in water, so this effect may have practical importance and have a significant impact on the uncertainty in reported dissolution rates measured with batch reactors.