Compound bioflocculant and polyaluminum chloride in kaolin-humic acid coagulation: Factors influencing coagulation performance and floc characteristics

•Coagulant dose and pH influenced coagulation performance and floc properties.•Coagulation mechanisms dominating floc characteristics were analyzed.•Compound bioflocculant offered absorption bridging effect.•Solution pH affected coagulation mechanisms.•Fractal characteristic of flocs was related to coagulation mechanisms. The objective of this study was to investigate the influence of coagulant dosage and pH on coagulation performance and floc properties using polyaluminum chloride (PAC) and compound bioflocculant (CBF) dual-coagulant in kaolin-humic acid (HA) treatment. Results showed that as PAC dosage rose, comparatively better coagulation efficiencies and floc characteristics were achieved due to stronger charge neutralization and sweeping effect. Addition of CBF could enhance coagulation performance and floc properties, including size, strength and recoverability, except fractal dimension. Solution pH had a significant effect on coagulation efficiencies and flocs formation. Under acidic condition, flocs showed higher strength and recoverability but lower fractal dimension, where charge neutralization was the foremost mechanism. More compact flocs were generated under alkaline condition due to the sweeping effect of hydrolyzed Al species.