Preferential binding properties of carboxyl and hydroxyl groups with aluminium salts for humic acid removal

To systematically elucidate the removal characteristics of humic acid (HA), which are highly dependent on the molecular structure of HA, a series of representative HA model compounds containing different numbers and positions of carboxyl and hydroxyl groups were selected, and the chemical reaction behaviour between HA and aluminium coagulants was investigated. The results indicated that the number of carboxyl groups in the benzene ring and binding environment had great effects on the Al-binding properties of HA molecules. Under weakly acidic conditions, coordination occurred between carboxyl and Al ions, and the complexing capacity was restricted by the substituted position of hydroxyl groups. Under neutral conditions or at higher coagulant dosages, sweep coagulation occurred by surface complexation of aluminium hydroxide by carboxyl groups and hydrogen bonding between the hydroxyl group and aluminium hydroxide; this process was dependent on the substitutive pattern of the functional groups. Moreover, increased aliphatic chain length and benzene ring size could enhance hydrophobicity, and hence resulted in higher coagulation efficiency. This study provided new insight into the mechanism of the interaction between HA and aluminium coagulants.