Comparison of cationic flocculants with different branching structure for the flocculation of negatively charged particles coexisting with humic substances

Abstract

Cationic flocculants with varying branching structure sometimes exert dramatically different flocculating abilities on oppositely charged particles. However, their flocculation mechanisms and performances under aqueous conditions in the presence of oppositely charged colloidal particles and polyanions, which commonly exist in natural systems, have not yet been fully elucidated. In this work, three cationic flocculants with different branches per molecule and similar charge densities were used to flocculate negatively charged colloidal particles coexisting with synthetic and naturally occurring polyanions. The electrophoretic mobility of the polymer-coated particles revealed that the branched flocculants required a higher optimum dosage than the linear ones. Under all tested conditions, flocculation was remarkably enhanced by increasing the number of branches in the flocculant. Electrophoretic titrations showed that the interactions between the flocculants and the polyanions to form a polyion complex (PIC) were mainly driven by electrostatic attractions. Hence, the number of branches had no significant effect on PIC formation. These results suggest that unlike the PIC formed by flexible linear flocculants that adopt a flat conformation upon adsorption onto the particle surface, the rigid branched-polyanion PIC tends to retain its shape. This effect is due to the presence of branches that are less prone to undergo relaxation. As a result, a thick adsorbed layer was formed on the particle surface, which improved the effective collision between particles and facilitated bridging flocculation.