Preparation of hollow Fe-Al binary metal oxyhydroxide for efficient aqueous fluoride removal

Fluoride contamination of drinking water, which causes fluorosis and neurological damage, is an exigent worldwide problem. Adsorption by metal oxyhydroxides is an attractive technology for water defluorination, although the removal efficiency is restricted by protonation of the oxygen binding sites (M-OH and M-O-M) for F-. Herein, hollow Fe-Al binary metal oxyhydroxide micro-boxes (FeAl-OOH MB) were prepared by annealing derivatives of Prussian blue analogues (PBAs). XRD and Al-27 MAS NMR analyses showed that the hollow structures were composed of Al oxyhydroxides (Al(O)(6), Al(O)(4)) and Al incorporated ferrihydrite. The surface oxygen was easily protonated in the presence of Al oxyhydroxides; with the introduction of Al into the lattice, the extent of protonation on the surface of the ferrihydrite was intensified via weakening the electronegativity of Fe sites. Therefore the limitation on adsorption efficiency was alleviated through tuning the surface charge. Together with large specific surface area (242 m(2) g(-1)) and good mass diffusion, hollow FeAl-OOH MB exhibited outstanding maximum F-adsorption capacity (146.59 mg g(-1)) and adsorption rate (20 min was needed to reach equilibrium for initial F concentration of 10 mg L-1), outperforming comparable metal oxyhydroxide/oxide adsorbents. Meanwhile, except for PO43-, co-existing anions (NO3-, SO42- and HCO3-) showed negligible influence on the adsorption of F- on FeAl-OOH MB. Furthermore, the F- adsorbed on FeAl-OOH MB could be effectively desorbed using 0.1 M NaOH, and the desorption efficiency for reuse was 94.38%. The results illustrate the potential utility of hollow FeAl-OOH MB as an effective adsorbent for practical defluorination in drinking water treatment. (C) 2017 Elsevier B.V. All rights reserved.