HYDROLYSIS AND FLOCCULATION - A STRUCTURAL APPROACH THROUGH SMALL-ANGLE X-RAY-SCATTERING

The flocculation of silica by coagulation with hydrolyzed aluminum sal t and the flocculation of aluminum salt in the presence of organic aci ds at pH 6.5 have been studied by using Small-Angle X-ray Scattering. The flocculation of silica at pH 4.5 by isolated Al13 is very similar to its flocculation by non-hydrolyzable salt as Ca++; the fractal dime nsion D(f) of the flocs is 2.20. At pH 7.5, Al13 is hydrolyzed and str ongly aggregated through hydroxo bridges, and the structure of the flo cs is that of Al13 aggregates, i.e D(f) = 1.7. The flocculation of alu minum in the presence of organics such as acetate, salicylate, lactate and oxalate (organics/Al = 0.25, 0.5 and 1.0) occurs at lower pH valu es than in the absence of organics. The SAXS curves display a crossove r region in the Q-range 0.05 to 0.15 angstrom-1, due to the presence o f small, loose -Al-OH-Al-aggregates. The simulation of the outermost p art of the SAXS curves shows that the organic ligands limit the Al hyd rolysis at the stage of the monomers and small oligomers by competing with the OH- ions. This charge screening results in high coordination numbers and in high density of the flocs (2.3 less-than-or-equal-to D( f) less-than-or-equal-to 3).