Sono-assisted preparation of magnetic magnesium–aluminum layered double hydroxides and their application for removing fluoride

A simple ultrasound-assisted co-precipitation method in combination with a calcination treatment was developed to prepare magnetic Mg–Al layered double hydroxides composite as an adsorbent material to remove fluoride ions from aqueous solutions. The application of ultrasound in the preparation process promoted the formation of the hydrotalcite-like phase and drastically shortened the time being required for preparation of the crystalline composite. It was found that the ultrasound irradiation assistance decreased the size of the composite particles and increased the specific surface area, being favorable to the improvement of the adsorption capacity. The composite prepared under the ultrasound irradiation exhibited fairly high maximum adsorption capacity of fluoride (47.7 mg g−1), which was 60% higher than that of the composite prepared without the ultrasound irradiation assistance with the same aging time. The thermodynamic and kinetic studies demonstrated that the adsorption of fluoride ions involved the reconstruction of the layered structure in the composite. In addition, the magnetic composite can be effectively and simply separated by using an external magnetic field, and then regenerated by desorption and calcination.

Research highlights
► Magnetic Mg–Al layered double hydroxides were prepared by a sono-assisted method.
► The newly developed composites exhibit high efficiency for fluoride removal.
► The adsorption of fluoride involves the reconstruction of the layered structure.
► The used adsorbents can be recovered and reused for removing fluoride ions.