Fig. 4

XPS spectra of the adsorbent before and after fluoride adsorption: (a) Ce 3d, (B) O 1 s, (c) F 1 s, (d) Total survey spectrum. The spectra labelled “Deionized water” and “Fluoride solution” represent the samples before and after fluoride adsorption, respectively. The purple line in Ce 3d corresponds to the oxidation state of Ce4+, and the blue line represents Ce3+