FLOCCULATION OF COLLOIDAL SILICA WITH HYDROLYZED ALUMINUM - AN AL-27 SOLID-STATE NMR INVESTIGATION

Flocculation of colloidal silica with aluminum fractal polymers was in vestigated by Al-27 magic angle spinning and Si-29 cross polarization magic angle spinning NMR as a function of aluminum concentration and p H. Aluminum flocculant species were prepared by dilution of a commerci ally available flocculant, WAC HE, the hydrolysis of which yields Al-1 3 polymers. The results showed that destabilization with hydrolyzed al uminum has many features in common with flocculation by addition of co nventional organic polymers. Interaction of aluminum polymers with sil ica leads to the formation of four coordinated aluminum retained at th e silica surface as negatively charged aluminosilicate sites. These si tes, similar to those found in clay minerals and zeolitic materials, r epresent potential anchors to aluminum polycations. Hence, aggregation of silica particles proceeds with either charge neutralization or bri dging. Tetrahedral aluminum in contact with silica may be assimilated to polymer segments bound to the surface. Study of the effect of alumi num concentration and pH suggests that aluminum partition within silic a flocs may be ascribed to a competition between structural rearrangem ent of individual aluminum polymers, which tend to adopt a flat confor mation on the silica surface, and excluded area effects originating fr om neighboring flocculant species.