Characterization of flocs generated by preformed and in situ formed Al13 polymer

Abstract

During coagulation process, preformed and in situ formed Al13 polymer are the predominant aluminum hydrolysis species at the optimal pH condition for polyaluminum chloride (PACl) containing high content of Al13 and AlCl3 (AC), respectively. The structure and morphology of the flocs generated by preformed and in situ formed Al13 polymer were investigated from the viewpoint of the relationship between Al speciation and flocs surface structure. The results showed that at pH 6 the predominant composition of PACl and AC flocs were Al13 polymer aggregate and amorphous Al hydroxide, respectively. The flocs generated by preformed Al13 polymer had a better crystalline structure than those from in situ formed one, while they had smaller size and smoother surface structure than those formed by in situ formed Al13 polymer. During weak acidic pH condition and at high dosage of Al coagulant adsorption onto precipitated Al hydroxides was the main pathway for kaolin particle removal. Preformed Al13 polymer showed a stronger charge neutralization ability, while the flocs generated from in situ formed Al13 polymer exhibited a higher adsorption capacity for kaolin particles. Al species distribution during coagulation process governs the floc character and coagulation efficiency of Al coagulant. The present study further conduces to the understanding of coagulation mechanism and puts forward a basis for coagulant selection during water treatment process.