Oxidation and adsorption of Sb(III) in the presence of iron (hydr)oxides and dissolved Mn(II)

Abstract

Iron (Fe) (hydr)oxides are effective antimony (Sb) adsorbents with limited individual Sb(III) oxidation capacity in natural waters. The coexistence of Mn(II) and Fe (hydr)oxides may significantly promote the oxidation of Sb(III). However, the mechanism of Sb(III) oxidation and adsorption in the presence of Mn(II) and Fe (hydr)oxides remains unclear. Therefore, in the present study, the effect of dissolved Mn(II) on the oxidation and adsorption of Sb(III) in the ferrihydrite/hematite-Mn(II) system under aerobic conditions and the mechanism were comprehensively investigated. The results of kinetic experiments showed that Sb(III) was efficiently removed in the presence of ferrihydrite/hematite and Mn(II). Further characterization confirmed the formation of Mn(III/IV) oxides on the surface of ferrihydrite and hematite, which were the main oxidants for Sb(III) oxidation. However, removal mechanisms were different between the ferrihydrite-Mn(II) system and the hematite-Mn(II) system. The adsorption of Sb was the main cause of Sb(III) removal in the ferrihydrite-Mn(II) system. For the hematite-Mn(II) system, Sb(III) oxidation played a more important role in Sb(III) removal; however, Sb adsorption was inhibited as a result of Mn oxide formation on the hematite surface and the limited adsorption capacity of hematite. Therefore, the present study revealed that Mn(II) and ferrihydrite/hematite were both critical to Sb(III) oxidation and adsorption under aerobic conditions, which may be helpful for a better understanding of Sb migration and transformation in natural waters.