Effect of pH and shear force on flocs characteristics for humic acid removal using polyferric aluminum chloride–organic polymer dual-coagulants

The growth, breakage, regrowth and fractal nature of the aggregated flocs formed by a new composite coagulant, polyferric aluminum chloride–polydimethyldiallylammonium chloride, with humic acid were investigated. Different solution pH and various shear forces were investigated to evaluate the size, strength and structure of the flocs. The results showed that the flocs generated in acid conditions were more resistant to shear and more recoverable than those did in alkaline conditions, with the results suggesting that the order of the floc strength and recoverability was pH 5 > pH 6 > pH 8. The ability of flocs to resist breakage decreased as the shear rate increased and the reformation of flocs after high shear was limited. Fractal dimension analysis indicated that the flocs formed in acid pHs were more compact than those formed in alkaline pHs.

► A new composite coagulant was prepared for experiments.
► The breakage, regrowth and fractal nature of the flocs were investigated.
► The order of floc strength and recoverability was pH 5 > pH 6 > pH 8.