Synthesis and Adsorption Properties of Ca-Al Layered Double Hydroxides for the Removal of Aqueous Fluoride

Ca-Al layered double hydroxides (LDHs) with different Ca/Al molar ratios were composited at pH ranges, using a co-precipitation method, and were experimented to remove fluoride from wastewater and studied in terms of isotherm models such as Langmuir and Freundlich reactions. The composite LDHs were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and Thermogravimetric analysis (TGA). The results showed that different synthesis conditions of Ca-Al LDHs had an influence on their morphology, layered structure, and particle size distribution, which substantially affected the uptake capacity for aqueous fluoride. LDHs with the Ca/Al molar ratio of 2 and synthesized at the pH of 12 had the highest capacity for the fluoride removal (e.g., 146.6 mg/g) and such reaction reached an equilibrium within 1 h. The Freundlich model was a better fit for this study. The high adsorption method of Ca-Al LDHs can be favorable to removing fluoride from wastewater streams.