Fluoride removal using calcined Mg/Al layered double hydroxides at high fluoride concentrations

The aim of this study was to investigate the removal of high fluoride concentrations by Mg/Al layered double hydroxide (LDH). Batch experiments were conducted using calcined Mg/Al LDH at 105 °C (LDH-105), 300 °C (LDH-300), and 700 °C (LDH-700). The Brunauer-Emmett-Teller (BET) analysis showed that the specific surface area of LDH-700 was 1.7−1.8 times larger than those of LDH-105 and LDH-300. The X-ray diffractometry (XRD) analysis indicated that LDH-105 had a layered structure (Mg₄Al₂(OH)₁₂CO₃ · 3H₂O) while LDH-300 and LDH-700 were composed of mixed metal oxides (Al₈O₃N₆ + Mg_0.44Al_0.56) and magnesium oxide (MgO), respectively. Batch tests showed that the fluoride sorption capacity of LDH-700 was 1.7−2.9 times greater than those of LDH-105 and LDH-300 at initial fluoride concentrations of 100 and 200 mg/L. The kinetic data showed that fluoride sorption to LDH-700 arrived at equilibrium after 12 h. The equilibrium test demonstrated that the maximum sorption capacity of LDH-700 was 91.401 mg/g. Additional batch experiments indicated that the sorption capacity of LDH-700 decreased slightly when the solution pH was increased from 4.0 to 9.0. Moreover, fluoride sorption to LDH-700 decreased considerably in the presence of anions such as phosphate, sulfate, and carbonate in the following order: HPO₄²⁻ > SO₄²⁻ > CO₃²⁻ > Cl⁻ > NO₃⁻.