FLOCCULATION OF SILICA COLLOIDS WITH HYDROXY ALUMINUM POLYCATIONS - RELATION BETWEEN FLOC STRUCTURE AND AGGREGATION MECHANISMS

The aggregation of silica colloids by various aluminum hydroxide polycations (Al13, octahedral polycations) has been studied. Both physicochemical variables, such as f potential and settling velocity, and floe structure on a semilocal scale (10-1200 A, by means of SAXS) have been studied as a function of chemical nature, concentration and size of the flocculant, and pH of the silica suspension. Between pH 4.5-5.0 and 7.0-7.5, the floes have generally a fractal geometry with a fractal dimension varying from 1.7 to 2.2. Their effective density calculated from the distance distribution function obtained from SAXS was highest at low pH and with Al13 polycations as flocculants. When the pH increased, the density decreased but remained the highest for Al13 flocculants. The settling velocity and the effective density were conversely varied. The mechanisms of the flocculation depended on the reaction pH. At low pH, the charge neutralization was the main driving force for the flocculation. At neutral pH, both charge neutralization and bridging were responsible for the formation of less dense floes. © 1990, American Chemical Society. All rights reserved.