Synthesis of Li-Al Layered Double Hydroxides (LDHs) for Efficient Fluoride Removal

A novel adsorbent of Li-Al layered double hydroxides (LDHs) was prepared through the precipitation of metal nitrates and further applied to remove excessive fluoride ions from water. The physical and chemical properties of synthesized materials were examined by powder transmission electron microscopy (TEM), X-ray diffraction (XRD) analysis, N-2 adsorption/desorption analysis, and thermogravimetric analysis (TGA). The TEM results indicated that the materials synthesized via coprecipitation present a preferential orientation of the nanoscale LDH platelets. The XRD analysis confirmed that the synthesized products have a highly crystalline nature and a well-ordered layer structure. The high specific surface areas (37.24-51.27 m(2)/g) of calcined products were demonstrated to be beneficial for the adsorption of fluoride. For the adsorption experiment, the effects of the adsorption conditions including pH, coanions and adsorbent dose were investigated at initial fluoride concentration of 20 mg/L. The kinetics and isotherms of fluoride adsorption by calcined Li-Al LDHs were studied. The results indicated that the Li-Al LDHs can be effectively used to remove fluoride from water, where the maximum percentage removal (97.36%) could be reached and the adsorption equilibrium could be attained within 1 h. The kinetic data were well-fitted to the pseudo-second-order model, while the Freundlich isotherm model provided the better correlation of the equilibrium data. Based on FT-IR and kinetic analysis, the "memory effect' may play an important role in the early adsorption stage, while the ion-exchange process may control the adsorption rate at the second adsorption stage. Since the Li-Al LDHs show excellent fluoride removal efficiency, they are expected to separate fluoride from water in pollution control.