Removal of fluoride and hardness by layered double hydroxides: property and mechanism

Calcined layered double hydroxides are typical anionic adsorbents with excellent fluoride removal capabilities. However, in actual water bodies, cationic substances such as hardness are often widespread. This research explored the performance of simultaneous fluoride and hardness removal by the adsorbents. The results showed that adsorption kinetics of fluoride and hardness substances followed the quasi-second order. The adsorption isotherms for fluoride and hardness agents followed the Langmuir and Freundlich models, independently. The capability of calcined layered double hydroxides to remove magnesium ions is much higher than that of calcium ion. The removal of fluoride is mainly utilized the “memory effect”. The removal of magnesium occurred by forming metal oxide bond with magnesium and aluminum ions on the surface of the adsorbents and was embed in the gap on the surface of the material. To remove calcium, the -OH generated by fluoride entered into the interlayer of the material to form Ca (OH) 2 precipitate.