Removal of Sb(V) from aqueous solutions using Fe-Mn binary oxides: The influence of iron oxides forms and the role of manganese oxides

Fe-Mn binary oxides (FMBO) with different Fe/Mn molar ratios were synthesized to investigate the influence of the changed iron oxides forms and the manganese oxide on the removal efficacy of Sb(V) from water under neutral pH. The characterization tests by electron microscopy and X-ray diffraction spectroscopy showed that FMBO gradually changed from granular to aciculiform or fibrous structure and their crystalline degree increased with the decreasing Mn content. The ferric oxides in FMBO transformed from 2-L ferrihydrite-like substance with amorphous state to goethite or lepidocrocite with high crystalline as the amount of added Mn reduced during the preparation. The Sb(V) adsorption rates and capacities of FMBO both decreased and the influence of pH on the Sb(V) removal became greater with the decreasing Mn oxide content. All the results indicated that the addition of Mn could greatly improve the Sb(V) removal performance of FMBO through promoting the formation and preventing the transformation of amorphous ferric oxide in FMBO during the preparation, whose Sb(V) adsorption ability was confirmed to be stronger than that of goethite or lepidocrocite. Further exploration indicated the complexation reaction between Sb(V) and amorphous ferric oxide played a key role in Sb(V) adsorption by FMBO. Meanwhile, FMBO with a Fe/Mn molar ratio of 3:1 (FMBO3) exhibited excellent regeneration performance and the amorphous iron oxide in FMBO3 gradually transformed into crystalline iron oxide during the consecutive recycling.