THE INFLUENCE OF SILICIC-ACID ON ALUMINUM HYDROXIDE PRECIPITATION ANDFLOCCULATION BY ALUMINUM SALTS

Silic acid is always present in natural waters and interactions betwee n silicic acid and aluminium have been reported over wide concentratio n ranges. Since aluminium sails are commonly used as coagulants in wat er 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 alu minium hydroxide and silicic acid may influence the precipitation proc ess. Using a simple continuous optical technique, the influence of sil icic acid on the precipitation process and on the flocculation of clay suspensions by aluminium sulphate has been examined. It has been foun d that silicic acic: 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 s ilicic 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 hind ered and eventually prevented altogether (at around 1.5 mM). The effec ts 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 sti rring and sedimentation procedure and by electrophoretic mobility meas urements on particles shortly after dosing of coagulant. It appears th at the effects are closely connected with silicic acid-aluminium inter actions, especially with regard to modifications in the charge of prec ipitated species. (C) 1998 Elsevier Science Inc. All rights reserved.