The reaction between monosilicic acid and aluminium hydroxide. I. Kinetics of adsorption of silicic acid by aluminium hydroxide

Abstract

Studies of the reaction between monosilicic acid and crystalline aluminium hydroxide showed that a number of layers of silicic acid could be formed on the surface of the hydroxide. Silicate is considered to be adsorbed as silicic acid rather than as silicate ions. The first layer was produced by rapid reaction of silicic acid with the surface of aluminium hydroxide. The isotherm for this initial reaction was not affected by varying the temperature from 10 to 35°C or by increasing the ionic strength of the supporting electrolyte solution. Adsorption of silicic acid resulted in increased KOH uptake by (or H2SO4 displacement from) the solid phase, which corresponded to a decrease in pH of the suspension. Subsequent layer formation was slower; the rate increased both with increasing temperature and with the ionic strength of the supporting electrolyte solution. Study of the kinetics of the reaction showed that these layers could have formed by polymerization of silicic acid on the hydroxide surface. The activation energy for the reaction increased with increasing surface coverage from 15 to 24 kcal/mole for the second layer and was about 24 kcal/mole for the third layer.