Fig. 1.  Percent kaolinite and smectite dissolved as Si in solution
Fig. 1. Percent kaolinite and smectite dissolved as Si in solution

Experiments measuring kaolinite and smectite dissolution rates were carried out using batch reactors at 35° and 80°C. No potential catalysts or inhibitors were present in solution. Each reactor was charged with 1 g of clay of the ≤2 μm fraction and 80, 160 or 240 ml of 0.1–4 M KOH solution. An untreated but sized kaolinite from St. Austell and two treated industrial smectites were used in the experiments. One smectite is a nearly pure montmorillonite, while the second has a significant component of beidellitic charge (35%). The change in solution composition and mineralogy was monitored as a function of time. Initially, the 3 clays dissolved congruently. No new formed phases were observed by XRD and SEM during the pure dissolution stage. The kaolinite dissolution is characterized by a linear release of silica and Al as a function of the log of time. This relationship can be explained by a reaction affinity effect which is controlled by the octahedral layer dissolution. Far from equilibrium, dissolution rates are proportional toa0.56±0.12OH at 35°C and to a0.81±0.12OH at 80°C. The activation energy of kaolinite dissolution increases from 33±8 kJ/mol in 0.1 M KOH solutions to 51±8 kJ/mol in 3 M KOH solutions. In contrast to kaolinite, the smectites dissolve at much lower rates and independently of the aqueous silica or Al concentrations. The proportionality of the smectite dissolution rate constant at 35 and 80°C was a0.15±0.06OH. The activation energy of dissolution appears to be independent of pH for smectite and is found to be 52±4 kJ/mol. The differences in behavior between the two kinds of minerals can be explained by structural differences. The hydrolysis of the tetrahedral and the octahedral layer appears as parallel reactions for kaolinite dissolution and as serial reactions for smectite dissolution. The rate limiting step is the dissolution of the octahedral layer in the case of kaolinite, and the tetrahedral layer in the case of smectite.

This article originally appeared in Applied Geochemistry 13(7), 1998, Pages 905-916