Adsorption of anionic polymers on hematite - a study of zeta potential distributions

The effect of adsorption of two carboxymethyl celluloses and sodium lignosulfonate on the zeta potential distributions and the aggregation/dispersion behavior of hematite (isoelectric point at pH 6.7) was investigated at pH 5.5 and 10.5. At pH 5.5, all three polymers caused hematite aggregation at lower dosages followed by steric re-dispersion at higher concentrations. It was found that the width of the zeta potential distribution at pH 5.5 gradually increased with polymer adsorption and reached a maximum value at polymer concentrations that led to strongest aggregation of hematite. The results indicated that the main role of the polymers in hematite aggregation at pH 5.5 was to generate large variations in the zeta potential characteristics between individual hematite particles, producing populations of oppositely charged particles that readily underwent aggregation. In the absence of such polymer-induced variations (e.g., at pH 10.5), adsorption of the polymers resulted in strong dispersion of hematite. The width of the zeta potential distribution, rather than the adsorption density of the polymers, was found to be a common factor determining the onset of steric re-dispersion by the tested polymers regardless of the polymer type. Using the width of the zeta potential distribution as a measure of zeta potential variations among the particles, it was shown that steric dispersion at pH 5.5 started to occur when the hematite particles acquired very similar zeta potential characteristics as a result of polymer adsorption. The onset of steric dispersion coincided with a well-defined transition from broad to narrow zeta potential distributions. It was also shown that zeta potential distributions offered additional insights into the mechanism of the aggregating/dispersing action of the tested polymers, compared to the average zeta potential value alone.