Fluoride removal by Al, Ti, and Fe hydroxides and coexisting ion effect

Batch experiments were conducted to evaluate fluoride removal by Al, Fe, and Ti-based coagulants and adsorbents, as well as the effects of coexisting ions and formation of aluminum–fluoride complexes on fluoride removal by co-precipitation with alum (Al₂(SO₄)₃·18H₂O). Aluminum sulfate was more efficient than the other coagulants for fluoride removal in the pH range between 6 and 8. Nano-crystalline TiO₂ was more effective for fluoride removal than Al and Fe hydroxides in a pH range of 3–5. Coexisting anions in water decreased the removal of fluoride in the order: phosphate (2.5 mg/L) > arsenate (0.1 mg/L) > bicarbonate (200 mg/L) > sulfate (100 mg/L) = nitrate (100 mg/L) > silicate (10 mg/L) at a pH of 6.0. The effect of silicate became more significant at pH > 7.0. Calcium and magnesium improved the removal of fluoride. Zeta-potential measurements determined that the adsorption of fluoride shifted the PZC of Al(OH)₃ precipitates from 8.9 to 8.4, indicating the chemical adsorption of fluoride at the surface. The presence of fluoride in solution significantly increased the soluble aluminum concentration at pH < 6.5. A Visual MINTEQ modeling study indicated that the increased aluminum solubility was caused by the formation of AlF^2 +, AlF₂⁺, and AlF₃ complexes. The AlFx complexes decreased the removal of fluoride during co-precipitation with aluminum sulfate.