The influence of silicic acid on aluminium hydroxide precipitation and flocculation by aluminium salts

Abstract

Silic acid is always present in natural waters and interactions between silicic acid and aluminium have been reported over wide concentration ranges. Since aluminium salts are commonly used as coagulants in water treatment, it is possible that their effects may be modified by the presence of silicic acid. In many cases flocculation of particles in water is dependent on the formation of an amorphous precipitate of aluminium hydroxide and silicic acid may influence the precipitation process. Using a simple continuous optical technique, the influence of silicic acid on the precipitation process and on the flocculation of clay suspensions by aluminium sulphate has been examined. It has been found that silicic acid can promote or prevent the aggregation of primary aluminium crystallites depending on solution pH and the concentration of silicic acid. These effects are closely matched by the effect of silicic acid on the flocculation process. At pH 7, with about 0.3 mM of silicic acid, the growth of precipitate and flocculation are enhanced; but with higher silicic acid concentrations these processes are hindered and eventually prevented altogether (at around 1.5 mM). The effects of silicic acid are more pronounced at pH 8, where less than 0.1 mM is sufficient to enhance precipitation and flocculation, and at 0.25 mM they are completely inhibited. These trends have been confirmed by determining the residual turbidity of suspensions after a standard stirring and sedimentation procedure and by electrophoretic mobility measurements on particles shortly after dosing of coagulant. It appears that the effects are closely connected with silicic acid–aluminium interactions, especially with regard to modifications in the charge of precipitated species.