Effect of freeze drying on characteristics of Mg–Al layered double hydroxides and bimetallic oxide synthesis and implications for fluoride sorption

In this study, freeze-drying was applied in the synthesis of nanocrystalline layered double hydroxides (LDHs), and the properties of resulting LDHs along with their efficacies for the removal of fluoride were evaluated. Nanocrystalline NO₃- and Cl-type LDHs were produced by freeze-drying using liquid N₂. The solid properties of the freeze-dried LDHs were compared with those of LDHs dried at 100 °C. Both NO₃- and Cl-type LDHs were analyzed via X-ray diffraction (XRD), Fourier transform infrared spectroscopy, Raman spectroscopy, and scanning electron microscopy (SEM). XRD analysis confirmed that the LDHs dried by liquid N₂ were hydrotalcite-like compounds with lower degrees of crystallinity compared with those dried at 100°C. SEM images showed agglomerated nanoparticles in the freeze-dried LDHs. No significant difference was observed between the NO₃- and Cl-type LDHs. However, all characterization results suggested that the freeze-drying method resulted in small nanoparticle sizes. Moreover, bimetallic oxides produced by the calcination of LDHs were affected by the crystallinities of starting materials. Fluoride (F⁻) sorption experiments on the synthesized LDHs indicated that the freeze-dried LDHs and resulting bimetallic oxides were better sorbents compared with the LDHs prepared at 100 °C. The sorption capacities of F⁻ on the LDHs and bimetallic oxides from freeze drying were 0.78–0.87 mM and 4.34 mM, respectively.