Outpacing sol-gel-prepared magnesium fluoride for rare earth element separation by mixed mechanisms over many second-group fluorides

Abstract

The synthesis, characterization, and testing of new inorganic compounds as perspective adsorbents or ion exchangers are of significant importance. Magnesium fluoride is a well-known inorganic compound that is affordable and widely used in various scientific and industrial applications. A series of adsorption experiments have been conducted, including investigations of pH range, magnesium fluoride dose weights, initial concentration of the Rare earth elements (REEs) and adsorption time. An outstanding finding regarding magnesium fluoride as an adsorbent is that REEs were separated at pH 1 through ion exchange and at pH 4 through adsorption mechanisms, showing nearly identical uptake of 191 mg/g. Under optimal adsorption conditions, the REEs were adsorbed on magnesium fluoride with an uptake of 302.4 mg. The morphological traits of magnesium fluoride were characterized using energy dispersive spectroscopy (EDS), scanning electron microscope (SEM) and Fourier-transform infrared spectroscopy (FTIR). Kinetically, the adsorption of REEs on magnesium fluoride aligns with the pseudo-second-order model and exhibits a theoretical loading capacity of 310 mg/g. What’s more, isothermally, the adsorption of REEs corresponds to the Langmuir model and predicts a theoretical uptake of 305 mg/g. Interference studies indicate that magnesium fluoride is a promising adsorbent and exhibits notable selectivity. The study suggested that magnesium fluoride has shown a higher saturation capacity compared to other Group IIA fluorides. From a sustainability perspective, magnesium fluoride has been sourced from the Mediterranean Sea. This approach achieved a REEs separation capacity of 308.7 mg/g after implementing all the optimum sol-gel magnesium fluoride adsorption conditions.

Graphical Abstract