Coagulation behaviors of aluminum salts towards humic acid: Detailed analysis of aluminum speciation and transformation

Abstract

The hydrolysis of Al-based coagulants, especially aluminum speciation and transformation, is necessary for organic matter removal by the coagulation process. The goal of this study was to investigate the efficiency and mechanism of hydrolyzed Al species in the coagulation of humic acid (HA) removal. Three aluminum coagulants have been applied to investigate HA coagulation, which are aluminum chloride AlCl3 dominated by monomeric aluminum species Ala, polyaluminum chloride PACl1 dominated by medium polymer species Alb, and commercial polyaluminum chloride PACl2 dominated by larger polymer species and/or colloidal aluminum species Alc. Results showed that the best coagulation effect on HA removal (higher than 90%) occurred at pH 6 for three aluminum coagulants and their effect decreased in the order: PACl1 > AlCl3 > PACl2. Except that, the in situ hydrolyzed aluminum species produced by AlCl3 had synergistic effects on various sites of HA, and showed good adaptability to raw water and coagulant dosage. Hydrolyzed larger polymeric aluminum species of PACl1 could selectively bind to the HA site with strong binding, especially Al13 under the weak acidic condition. While for PACl2, complexation of small and median polymeric aluminum species, charge neutralization effect of Al13, and the effect of sweep coagulation of large polymeric aluminum species were involved. Moreover, diversity of hydrolyzed aluminum species in the coagulation process presented a significantly impact on flocs characteristics. This study provided a new insight into the relationship between HA and aluminum speciation and transformation for Al-based coagulants, which can be indicative to select suitable coagulants according to the water quality in the coagulation process.