Effect of aluminum fluoride complexation on fluoride removal by coagulation

Effect of aluminum fluoride complexation on fluoride removal by Al salts coagulation was investigated by a series of batch experiments. Transformation of fluoride species in coagulation was studied by simultaneous determination of free fluoride and total fluoride at different pH and fluoride concentrations. Results showed that aluminum fluoride complexes dominated at low pH and high fluoride concentrations but more complexes did not represent higher fluoride removal. Comparison between coagulation with complexation and adsorption by Al(OH)3 flocs was conducted and coagulation showed higher fluoride removal efficiency than adsorption in the pH range of 6.0–9.0. Titration test showed that AlF complexes were coprecipitated with less OH− than Al3+. Characterization of FTIR and XPS also indicated that flocs after complexation was different from Al(OH)3 flocs with or without fluoride. It was concluded that a new AlFOH coprecipitate formed in coagulation and fluoride was effectively removed by the coprecipitation.