ABSOLUTE AGGREGATION RATE CONSTANTS OF HEMATITE PARTICLES IN AQUEOUS SUSPENSIONS - A COMPARISON OF 2 DIFFERENT SURFACE MORPHOLOGIES

Measurements of absolute aggregation rate constants were performed for two samples of well-characterized spheroidal hematite particles with rather different surface morphology. At high ionic strength, the syste m is in the fast aggregation regime with pH independent rate constants around (1-4) x 10(-18) m(3)/s. At low ionic strengths, the rate const ant is a function of pH and goes through a flat maximum around the poi nt of zero charge (PZC) where fast aggregation conditions are reached. With increasing pH the rate constants increase gradually below the PZ C while they decrease very rapidly above the PZC. Above the PZC the ra te constants are rather well predicted by the classical DLVO theory. B elow the PZC, however, it is necessary to assume a distribution in the surface potentials with a coefficient of variation around 30% to acco unt for the observed behavior. In spite of the pronounced differences in the surface roughness of both hematite samples, their aggregation r ate constants are rather similar. (C) 1997 Academic Press.