Fluoride adsorption on an Fe–Al–Ce trimetal hydrous oxide: Characterization of adsorption sites and adsorbed fluorine complex species

Abstract To better understand fluoride (F) adsorption mechanisms of metal oxide-based adsorbent, a previously developed iron–aluminum–cerium hydroxide (Fe–Al–Ce) adsorbent with a high adsorption capacity for F was investigated using various spectroscopic methods including X-ray photoelectron spectroscopy (XPS) and 19 F magic-angle spinning nuclear resonance ( 19 F MAS NMR). XRD and BET results showed that the Fe–Al–Ce adsorbent maintained an amorphous structure with high specific surface areas and large pore volumes. Deconvolution of XPS O1s peaks showed a quantitative ligand exchange relationship between the metal–hydroxyl (−OH) groups and F ions. Deconvolution of XPS F1s peaks showed that the F ions were able to substitute for all three metal–OH groups. Ce–OH was the preferential adsorption site at low F load (10 mg g −1 ), while Al–F became the most abundant complex species with increasing F loads (80 and 138 mg g −1 ). Three types of complex species (Al 3 –F, Ce x –F y and Fe x –F y ) were identified by 19 F MAS NMR analysis under 20 and 80 mg g −1 F loads. AlF 3 was suggested as the dominant specie at a high F load (138 mg g −1 ) by both of 19 F MAS NMR and XRD analysis.