Removal of fluoride from water using aluminum hydroxide-loaded zeolite synthesized from coal fly ash

The removal of fluoride from wastewater is essential as the excess accumulation of fluoride in environment is harmful to the health of humans. In this study, the defluorination of water by aluminum hydroxide-coated zeolite (AHZ), which was synthesized from coal fly ash, was investigated in batches. The Langmuir maximum adsorption capacity of fluoride by AHZ reached 18.12 mg/g. Aluminum hydroxide was shown to be the major component that adsorbed fluoride. More than 92% removal of fluoride was achieved within 2 h, and the fluoride adsorption kinetics were well fitted to a pseudo-second-order model. The point of zero charge (pH_pzc) of the AHZ was determined to be 5.52. Fluoride adsorption by AHZ depended greatly on pH, and maximum performance was obtained at pH 5.5-6.5. The AHZ showed good selectivity for the adsorption of fluoride in the presence of chloride, nitrate, sulfate, bicarbonate, and acetate ions, and the fluoride was nearly exhausted at a sufficiently high dose. The release of OH^- due to fluoride adsorption was confirmed. FTIR and XPS studies further illustrated that the adsorption mechanism of fluoride adsorption on AHZ was ligand exchange with hydroxyl groups and the formation of F-Al bonds.