Induced transformation of antimony trioxide by Mn(II) oxidation and their co-transformed mechanism.

Antimony (Sb) is a toxic and carcinogenic element that often enters soil in the form of antimony trioxide (SbO) and coexists with manganese (Mn) in weakly alkaline conditions. Mn oxides such as birnessite have been found to promote the oxidative dissolution of SbO, but few researches concerned the co-transformations of SbO and Mn(II) in environment. This study investigated the mutual effect of abiotic oxidation of Mn(II) and the coupled oxidative dissolution of SbO. The influencing factors, such as Mn(II) concentrations, pH and oxygen were also discussed. Furthermore, their co-transformed mechanism was also explored based on the analysis of Mn(II) oxidation products with or without SbO using XRD, SEM and XPS. The results showed that the oxidative dissolution of SbO was enhanced under higher pH and higher Mn(II) loadings. With a lower Mn(II) concentration such as 0.01 mmol/L Mn(II) at pH 9.0, the improved dissolution of SbO was attributed to the generation of dissolved intermediate Mn(III) species with strong oxidation capacity. However, under higher Mn(II) concentrations, both amorphous Mn(III) oxides and intermediate Mn(III) species were responsible for promoting the oxidative dissolution of SbO. Most released Sb (∼72%) was immobilized by Mn oxides and Sb(V) was dominant in the adsorbed and dissolved total Sb. Meanwhile, the presence of SbO not only inhibited the removal of Mn(II) by reducing Mn(III) to Mn(II) but also affected the final products of Mn oxides. For example, amorphous Mn oxides were formed instead of crystalline Mn(III) oxides, such as MnOOH. Furthermore, rhodochrosite (MnCO) was formed with the high Mn(II)/SbO ratio but without being observed in the low Mn(II)/SbO ratio. The results of study could help provide more understanding about the fate of Sb in the environment and the redox transformation of Mn.