Reaction mechanism in fluoride adsorption to aluminum oxide
Besides the presence of Al and O atoms on an Aluminum Oxide surface, there are also H atoms, which are bounded to the solid by OH bonds, - O --- H+ . Direct evidence for the presence of H atoms, and OH bonds, come from XPS or FTIR (grazing incidence) data. Indirect evidence comes from several sources, but especially from the reactivity of the surface to some compounds, like Silanes, where Si --- O --- R groups react readily with OH. Based on what is said above, I´d say the best explanation for the F– anion interaction with the Aluminum Oxide surface is a chemisorption process, through an indirect process,  bonding of the F– anion to the  H+ of the surface, forming an -O --- H+ --- F– , anion (F–) – to – dipole (+H  --- O– ) complex structure. A real solid – state Hydrogen bonding.