Adsorption of calcium hydroxide by montmorillonite and kaolinite

Abstract

Rapid removal of calcium hydroxide from aqueous solution by previously calcium-saturated montmorillonite and kaolinite was observed. It was shown that initial adsorption of calcium hydroxide by kaolinite is completed within about ten minutes at room temperature, and that further removal of lime from solution does not take place for several hours. The time dependence of adsorption by montmorillonite was shown to be more complex, with chemical reaction appearing to occur relatively soon after initial uptake. Most of the adsorbed calcium hydroxide was readily removable subsequently by simple dispersion in distilled water. Adsorption isotherms were shown to be of the BET type II variaty, indicating probable multimolecular adsorption. Adsorption seemed to be confined to the external surfaces of the clay, and estimates of coverage areas per molecule were about 25 A. 2 for the montmorillonite but were significantly higher for the kaolinite. The temperature dependence of the adsorption by montmorillonite was examined and shown to be small at or below room temperature; however, enhanced adsorption was observed at 50°C. Similar adsorption phenomena occur in systems of highly restricted water content as shown by studies of the electrical conductivity of thick clay-water pastes to which crystalline calcium hydroxide was added.