Mechanistic Consideration of Fluoride Removal Using Aluminum Sulfate

Aluminum salt-based coagulants are widely used for fluoride removal in the world. However, there is little information about the mechanism of fluoride removal by aluminum salt. The present study aimed to elucidate the mechanism of fluoride removal using aluminum sulfate. The coagulation mechanism is generally composed of three types, clathration by aluminum hydroxide, chemisorption and physical adsorption by the Van der Waals force. Jar tests were performed under two conditions, with or without clathration, and then the residual fluoride concentration in treated water was analyzed. If fluoride ion formed clathrates with aluminum hydroxides, a difference of fluoride concentration in treated water under two conditions would be expected. However, no significant difference was observed in the experiments. Therefore, clathration was thought to be insignificant in fluoride removal by aluminum sulfate. In addition, X-ray diffraction analysis (XRD) was applied to the precipitates obtained from the jar tests, and revealed that the precipitates were amorphous. When chemisorption occurred, the generation of crystalline precipitates was expected. Accordingly, chemisorption was also inferred to be insignificant in fluoride removal. These results suggested that physical adsorption of fluoride onto aluminum hydroxide surface was responsible for fluoride removal using aluminum sulfate.