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

Abstract

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 (8mg/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 3.3mmolg−1 and 2.8 with NO3·Mg–Al LDH, and 3.2mmolg−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.