Defluoridation of synthetic and natural waters by polyaluminum chloride-chitosan (PACl-Ch) composite coagulant

Removal of fluoride from drinking water by polyaluminum chloride-chitosan (PACl-Ch) composite coagulant was studied in a batch system. Two types of PACl-Ch coagulant were synthetized as PACl-Ch I and PACl-Ch II with chitosan to aluminum (Ch:Al) weight ratios of 0.5 and 1.0, respectively, and were used for defluoridation under different operating conditions. The composite coagulants were found to be more efficient than both PACl and chitosan. By an increase in the Ch:Al ratio from 0.5 to 1.0, the composite coagulant provided a little more efficiency of fluoride removal and lower residual level of Al. The optimum conditions of pH for fluoride removal by PACl-Ch I and PACl-Ch II were 8.0 and 7.5, respectively. Fluoride removal by the coagulants was not significantly affected by natural organic matter and turbidity, but was declined by high concentrations of common ions. To achieve desirable fluoride levels (lower than 1.0 mg/L) for natural water samples with fluoride levels of 2.0–2.9 mg/L, the required dosage of the composite coagulants were determined to be as low as 3–5 mgAl/L. The PACl-Ch coagulants demonstrated high efficiency for fluoride removal as well as low Al residual level (mainly lower than 0.2 mg/L) in a wide range of operating conditions.