Recyclable Mg-Al layered double hydroxides for fluoride removal: Kinetic and equilibrium studies

Mg-Al layered double hydroxides (LDHs) intercalated with NO3− (NO3·Mg-Al LDH) and Cl− (Cl·Mg-Al LDH) were found to adsorb fluoride from aqueous solutions. Fluoride is removed by anion exchange in solution with NO3− and Cl− intercalated in the LDH interlayer. In both cases, the residual F concentration is lower than the effluent standards for F in Japan (8 mg/L). The rate-determining step in the removal of F using NO3·Mg-Al and Cl·Mg-Al LDH is chemical adsorption involving F− anion exchange with intercalated NO3− and Cl− ions. The removal of F is described by pseudo-second-order reaction kinetics, with Langmuir-type adsorption. The values obtained for the maximum adsorption and the equilibrium adsorption constant are respectively 3.3 mmol g−1 and 2.8 with NO3·Mg-Al LDH, and 3.2 mmol g−1 and 1.5 with Cl·Mg-Al LDH. The F in the F·Mg-Al LDH produced in these reactions was found to exchange with NO3− and Cl− ions in solution. The regenerated NO3·Mg-Al and Cl·Mg-Al LDHs thus obtained can be used once more to capture aqueous F. This suggests that NO3·Mg-Al and Cl·Mg-Al LDHs can be recycled and used repeatedly for F removal.