Mechanism of Mn(II) removal by the cake layer containing biogenic manganese oxides in a flow-through mode: Biological or chemical catalysis?

Biogenic manganese oxides (BioMnOx) can effectively remove Mn(II) from drinking water. In the gravity-driven membrane (GDM) system for Mn(II)-containing water treatment, both BioMnOx and Mn(II)-oxidizing bacteria (MnOB) present in the cake layer, and their role in Mn(II) oxidation is still controversial. To address this issue, in this study, we naturally formed a cake layer containing BioMnOx and MnOB using the GDM process and identified the different roles of BioMnOx and MnOB in the removal of Mn(II). The experimental results showed that the BioMnOx accounted for a major proportion of the cake layer of GDM, and it played a principal role in the removal of Mn(II) through the combination of adsorption and catalytic oxidation. However, this removal efficiency would be continuously reduced in the absence of MnOB, while stabilized in the presence of MnOB. XPS analysis revealed that the proportion of Mn(III) and Mn(IV) in BioMnOx with MnOB reached 64.1% and 35.8%, respectively, while BioMnOx without MnOB did not contain Mn(IV). These findings suggest that BioMnOx may be the primary catalyst for Mn(II) oxidation, but its long-term activity is dependent on MnOB, which can maintain the catalytic activity by 1) producing new BioMnOx and 2) sustaining the high valence state of MnOx.