Hydrolysis and flocculation : a structural approach through small-angle X-ray scattering

The flocculation of silica by coagulation with hydrolyzed aluminum salt and the flocculation of aluminum salt in the presence of organic acids at pH 6.5 have been studied by using Small-Angle X-ray Scattering. The flocculation of silica at pH 4.5 by isolated All3 is very similar to its flocculation by nonhydrolyzable salt as Ca++; the fractal dimension Df of the flocs is 2.20. At pH 7.5, Al13 is hydrolyzed and strongly aggregated through hydroxo bridges, and the structure of the flocs is that of All3 aggregates, i.e Df = 1.7. The flocculation of aluminum in the presence of organics such as acetate, salicylate, lactate and oxalate (organicsIA1 = 0.25, 0.5 and 1.0) occurs at lower pH values than in the absence of organics. The SAXS curves display a crossover region in the Q-range 0.05 to 0.15 A-l, due to the presence of small, loose -Al-OH-Alaggregates. The simulation of the outermost part of the SAXS curves shows that the organic ligands limit the A1 hydrolysis at the stage of the monomers and small oligomers by competing with the OHions. This charge screening results in high coordination numbers and in high density of the flocs (2.3 S Df S 3).