The inhibitory effects of synthetic polyacrylic acid and humic substances on the initial stage of colloidal flocculation induced by polycationic flocculant with low charge density

Abstract

Humic substances (HS) coexisting with colloidal particles are inevitable in water bodies. Since the cationic flocculant used for flocculation treatment in such water bodies interacts electrostatically with HS and colloidal particles, the efficiency of the flocculant tends to be lower than in water bodies containing only colloidal particles. The colloidal flocculation mechanism by polycation with low charge density is dominated by bridging effects rather than charge neutralization effects. Furthermore, the polyionic complex formation of low charge density polycations with HS is not stoichiometric, based on the oppositely charged groups. Therefore, it was unclear how HS affects the flocculation of colloidal particles by polycation with low charge density. In this study, the effects of two naturally occurring HS and two synthetic polyanions on the flocculation behavior of negatively charged polystyrene latex (PSL) particles induced by polycationic flocculant with low charge density (4 %) were investigated under standardized mixed fluid conditions. Considering the flocculation inhibition effect in terms of the molar concentration ratio of flocculant-derived cations to polyanion-derived anions, it was found that polyanions less inhibit the flocculation of PSL particles by flocculant with low charge density compared to flocculant with high charge density. The relationship between the degree of inhibition of flocculation and the affinity of polyanions for polycations obtained from electrophoretic mobility experiments was verified, and the effectiveness of using low charge density polycations as particle flocculants in water bodies containing particles and HS was discussed.