Effect of pH on the sedimentation, zeta potential, and rheology of anatase suspensions

The sedimentation behavior and the resulting consolidation of concentrated anatase suspensions have been studied in a stable and a flocculated state, the latter being induced by a pH change. X-ray absorption scanning was used to follow the time dependency of sedimentation under gravity. The solids content as a function of time and height was calculated from the intensity of the transmitted X-rays. The average final solids content of the sediment cake was related to the effective particle charge represented by the ζ-potential. Very dense sediments with high solids content were observed for suspensions with a high ζ-potential, whereas a low ζ-potential produced porous cakes. The consolidation properties can, therefore, be predicted by the ζ-potential. The shift between stable and flocculated suspensions occurred, however, in a narrow pH and ζ-potential range. The gel point was found to be φg=0.03, which was obtained from measurements with different initial solids contents. The particle size with size distribution and the compressive stress were calculated from the X-ray data. The rheological behavior of the suspensions was characterized by the shear yield stress. Rheology was found to follow the net interaction between the particles. From the sedimentation data it was possible to determine the regions of flocculating and stabilizing properties of the suspensions.