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 19F magic-angle spinning nuclear resonance (19F 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 (10mgg−1), while Al–F became the most abundant complex species with increasing F loads (80 and 138mgg−1). Three types of complex species (Al3–F, Cex–Fy and Fex–Fy) were identified by 19F MAS NMR analysis under 20 and 80mgg−1 F loads. AlF3 was suggested as the dominant specie at a high F load (138mgg−1) by both of 19F MAS NMR and XRD analysis.