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 (BioMnO_x) can effectively remove Mn(II) from drinking water. In the gravity-driven membrane (GDM) system for Mn(II)-containing water treatment, both BioMnO_x 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 BioMnO_x and MnOB using the GDM process and identified the different roles of BioMnO_x and MnOB in the removal of Mn(II). The experimental results showed that the BioMnO_x 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 BioMnO_x with MnOB reached 64.1% and 35.8%, respectively, while BioMnO_x without MnOB did not contain Mn(IV). These findings suggest that BioMnO_x 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 BioMnO_x and 2) sustaining the high valence state of MnO_x.