Fate of hydrolyzed Al species in humic acid coagulation

The hydrolysis of Al-based coagulants in acidic conditions is necessary for the removal of organic matter by the coagulation/sedimentation process. However, interactions between hydrolyzed Al species and organic matter are complicated and this makes it difficult to optimize coagulant dosing for organics removal. The goal of this study was to investigate the reactions of hydrolyzed Al species in the coagulation of organic matter. Two polyaluminum chloride (PACl) coagulants, a commercial product with sulfate (PACl-C) and lab-prepared material (PACl-Al13) containing 7% and 96% of total Al as Al13, respectively, have been applied to investigate the coagulation of humic acid (HA). At pH 6, a lower dosage of PACl-Al13 than of PACl-C was required for optimized HA removal through coagulation/sedimentation due to the strong complexation and charge neutralization by Al13. Observation of the coagulation process using wet scanning electron microscopy showed that PACl-C produced both clustered flocs and linear precipitates in the presence of sulfate while PACl-Al13 produced curled precipitates due to the formation of intermolecular complex, when both coagulants were added at the optimum doses. Investigation of Al–HA floc by 27Al-NMR and Al 2p XPS suggested that monomeric Al (Alm) was hydrolyzed into Al(OH)3 with tetrahedron for PACl-C coagulation while a half of Al13 slowly decomposed into octahedral Al–HA precipitates for PACl-Al13 coagulation. Meanwhile, C ls XPS indicated that aromatic CC of HA was preferentially removed from solution to Al–HA flocs for both PACl-C and PACl-Al13 coagulation. It was concluded that Al–HA complexation strongly affects the reaction pathways for Al hydrolysis and the final nature of the precipitates during PACl coagulation of HA and that the hydrolysis products are also strongly affected by the characteristics of the PACl coagulant. [Display omitted] •A lower dosage of PACl-Al13 than of PACl-C was required for optimized HA removal.•PACl-C produced clustered and linear flocs while PACl-Al13 produced curled flocs.•Monomeric Al in PACl-C substantially hydrolyzed into Al(OH)3 with tetrahedron.•A half of Al13 in PACl-Al13 slowly decomposed into octahedral Al–HA precipitates.•Aromatic CC of HA was preferentially removed from solution to Al–HA flocs.