The removal efficiency and reaction mechanism of aluminum coagulant on organic functional groups-carboxyl and hydroxyl

In order to remove the synthetic organic matter in drinking water and wastewater efficiently, aluminium chloride (AlCl3) and polyaluminum chloride (PAC) with different B values (B = 0.5, 1.5, 2.3) were used to coagulated polyacrylic acids (PAAs) and polyvinyl alcohols (PAs) in this study. The effect of molecular weight and functional groups on the coagulation behavior was investigated by the experiments of the removal efficiency, floc size and fourier transform infrared spectrometer (FTIR) analysis of the formed flocs. The results indicated that aluminum salt coagulants had a fairly high removal efficiency for the carboxyl containing organic matter water but almost no removal efficiency for hydroxyl containing organic matter water. When organic matter interacts with aluminum salt coagulants, the molecular weight (MW) of organic matter affects the coagulation performance. For MW = 450 000 of PAA, the dissolved organic carbon (DOC) removal efficiency can reach 94% at the dose of 8 mg/L. However, for MW = 1800, no DOC removal occurred for all tested coagulants. FTIR analysis indicated that during the coagulation process of PAA, the COOH existed in PAA molecule was transformed into COO−.

► SOM with the carboxyl functional group could be coagulated effectively.
► SOM with the hydroxyl functional group could not be coagulated.
► Pre-hydroxide degree of coagulant has an important effect on SOM of MW = 3000.
► Pre-hydroxide degree of coagulant has no effect on SOM of MW higher than 450 000.