Inhibitory mechanisms of humic substances and polyacrylic acid during the initial stage of polycation-induced flocculation

Abstract

Although natural organic matter (NOM) is known to reduce the efficiency of polycationic flocculants used for solid-liquid separation in water treatment processes, mechanisms underlying this inhibitory effect are not yet fully understood. In this study, the inhibitory effects of naturally occurring humic substances and a synthetic polyelectrolyte (as a comparison), polyacrylic acid (PAA), on the initial stage of colloidal flocculation induced by the addition of a cationic linear polyelectrolyte are investigated under a standardized mixing flow condition. Flocculation enhancement is hindered by all the tested polyanions, while electrophoretic mobility experiments reveal that the polyanions affect the isoelectric point of particles. At the same polycation to polyanion charge ratio, the degree of inhibition decreases in the order PAA > Suwannee River humic acid > Suwannee River fulvic acid, corresponding to their affinity toward the polycations and particles. We propose that the inhibitory mechanism is mainly driven by the electrostatic attractions between the flocculant and polyanions, and this mechanism involves three stages. The influence of polyanion affinity on the complexation with polycations is also discussed. The insights gained in this study can be useful in predicting the flocculation behavior of suspended solids co-existing with NOM in natural environments when flocculants are added.