Wollastonite: Incongruent dissolution and leached layer formation

Abstract

Measurements of the dissolution rates of wollastonite in solutions with pH ranging from 2 to 6 in an externally recycled mixed flow reactor show that the concentrations of Ca and Si in the reactor effluent decline with time following a power law behavior. After 24 h of reaction, the release rate of Si was 1.90 × 10 −9 mol/m 2 s and the release rate of Ca was 9.09 × 10 −9 mol/m 2 s and these rates were effectively independent of pH. The more rapid release of Ca relative to Si produced a leached layer with an average thickness, x, that is a function of both hydrogen ion activity and time: x = (1.19 × 10 −9)( a H + ) 0.121 t 0.412. The rate of Si release from the hydrated silica leached layer was thousands of times faster than the dissolution rate of vitreous silica. We believe that this is best explained by the release of large silica polymers from the leached layer. The rate of silica release declined as a power law function of time. This behavior is consistent with the idea that the silica in the leached layer undergoes reconstruction reactions that produce regions that are more polymerized and therefore dissolve at a slower rate. In addition, we found that the specific surface of the reacted grains increases as the leached layer grows. This additional surface area appears to come from crazing of the surface and the development of internal porosity. These results are not consistent with the idea that the dissolving mineral will eventually display a steady state behavior.