Effect of heat treatment on the synthesis of Mg/Al-NO3 hydrotalcite-type compounds for fluoride removal

Hydrotalcite-type compounds (HTs) are crystalline ion-exchange materials formed by positively charged sheets of metal hydroxides stabilized with interlayer anions. The synthesis methods significantly influence their crystallographic and morphological properties. Alternative methods, such as microwave or ultrasound irradiation, can reduce the synthesis time and obtain materials with specific characteristics. HT with the interlayer anion of NO3− was synthesized via conventional aging (hydrothermal), microwave, ultrasound, and combined (microwave/ultrasound) methods. The obtained compounds were characterized via X-ray diffraction (XRD), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM–EDS), N2 adsorption-desorption, Fourier-transform infrared spectroscopy (FTIR), and point of zero charge. A larger crystal size (� = 5.9 nm) and a higher surface area (� = 6.70 m2 g−1) were obtained when HT was irradiated by microwaves/ultrasound compared to the synthesized compound via hydrothermal aging (� = 5.5 nm, � = 1.83 m2 g−1). Although the removal capacity of the combined-mode irradiated compound was slightly lower (�max= 41.7 ± 4.8 mg g−1) than the conventionally aged sample (�max= 44.4 ± 6.8 mg g−1), the synthesis time was drastically decreased from 24 h to 5 min, and the combined-mode irradiated compound could remove fluoride as efficiently as the compounds obtained conventionally.

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