Differently charged polyacrylamides (PAMs) significantly affect adsorption affinity and associated floc growth behaviors during ballasted flocculation: Performance and mechanism

Abstract

Various organic polymers have been widely used to provide inter-particle bridging between microsand and microflocs for producing ballasted floc aggregates during ballasted flocculation. However, it is not yet clear how differently charged polymers will determine predominant mechanisms of ballasted floc growth at diverse coagulant dosages. In this study, three representative aluminum sulfate (AS) coagulant dosages (where microfloc aggregates formed after coagulant addition possessed negative, zero and positive zeta potentials, respectively) were prearranged, and then the role of cationic, anionic and nonionic polyacrylamides (PAMs) in ballasted flocculation of clay suspensions was comparatively evaluated at each AS dosage. As expected, the temporal evolution of floc average size at stages of ballasted floc maturation and regrowth (following breakage) was dependent upon both AS coagulant dosage and PAM selection. Also, the selection of differently charged PAMs produced maturated floc aggregates with different shear-resistant and recovery abilities at a fixed AS dosage, presumably owing to the predominant mechanism of ballasted floc growth. Differently from the removal efficiency of water turbidity after sedimentation, the involvement of nonionic PAM (rather than the case of “without adding PAM”) gave rise to the lowest removal percentage of UV254 at the same AS dosages. This finding was probably attributed to the role of possible flocculation mechanism(s) in the transformation of dissolved humic acid (HA) complexes into insoluble HA composites at the maturation stage. Based on the experimental results, a conceptual model of ballasted floc growth process induced by differently charged polymers was proposed to vividly explore the possible floc-growth mechanism (such as charge neutralization, polymer bridging, and/or electrostatic patch) occurring for each combination of coagulant dosage and PAM type. This research may contribute to gaining a deep insight into the combined effects of coagulant dosage and differently charged polymer selection on the interaction between polymer-enhanced floc cohesion and ballasted floc formation.