Chemisorption of fluoride onto manganese-oxide-coated activated alumina in aqueous solution

This study investigated the adsorption mechanism of manganese-oxide-coated activated aluminum (MnAA) prepared with persulfate oxidation for fluoride removal from an aqueous solution. Pristine AA has a low adsorption capacity (q_e) of 0.6 mg/g compared to an optimal MnAA with q_e ∼2.1 mg/g, highlighting the superiority of MnAA in defluoridation. A Mn oxide coating effectively reduced aluminum dissolution from AA, when pure AA released ∼0.09 mg Al/g after 5h adsorption. Furthermore, Mn oxide coating could effectively reduce the pH-dependent drawback of conventional AA, but still sustain its high q_e values of ∼2 mg/g. With equilibrium, the final pH of the MnAA solution was barely changed (6.7 ± 0.2) relative to the rapid increase of pure AA from pH 6.8 to >10.1. MnAA adsorption favorably followed the Freundlich model (R² of 0.997) and was well fitted with the pseudo-2^nd model (R² >0.999), indicating the domination of multilayer chemisorption behavior of fluorides. While both Al2p and Mn2p XPS spectra of AA were comparable to before, these two in MnAA were apparently shifted to higher binding energies after adsorption. These results showed the formation of coordinate complexes between fluorides and Al and Mn, strongly confirming the control of chemisorption in MnAA defluoridation.