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

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 F-19 magic-angle spinning nuclear resonance (F-19 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 F-19 MAS NMR analysis under 20 and 80 mg g(-1), F loads. AlF3 was suggested as the dominant specie at a high F load (138 mg g(-1)) by both of F-19 MAS NMR and XRD analysis. (C) 2013 Elsevier B.V. All rights reserved.