Effect of natural organic coatings on the polymer-induced coagulation of colloidal particles

In this research, we investigate the effect of natural organic coatings of humic acid on the flocculation of well-characterized amidine polystyrene latex microspheres by a polymeric flocculent (polyvinyl alcohol, or ‘PVA’). Our results demonstrate that in the absence of humic acid, PVA effectively coagulates the particles as a result of polymer bridging interactions. When the particle surface is saturated with humic acid, however, the latex particles are stable at all dosages of PVA tested, presumably as a result of repulsive electrostatic interactions between primary particles and a decrease in the number of active sites available for bridging. At low humic acid surface coverage, on the other hand, an increase in PVA-induced coagulation is observed compared with when PVA is the sole polymer in the system. Here, both polymer bridging and electrostatic interactions appear to play a role in controlling the coagulation rate of these particles. Our results suggest that the predominance of polymer bridging in natural systems may depend on the preferential displacement of low molecular humic and fulvic acids and/or favorable interactions between large macromolecules and particle surfaces coated with humic materials.