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 .
The release of Mn(II) occurs in the degradation of organic matters by manganese ore (MnO 2 ), resulting in a reduced efficiency. During the degradation of ciprofloxacin (CIP), in a biofilter, this paper put forward a novel method that similar to the geo-cycle of Mn (MnCS) on the Earth to regenerate MnO 2 . The freshly prepared MnO 2 was suitable for the use in the MnCS. It indicated that the mutual conversion between Mn(II), Mn(III), and Mn(IV) in the MnCS, which was driven by CIP and manganese oxidizing bacteria (MnOB), could maintain the activity of MnO 2 . The MnCS showed feasibility in the coexistence of ammonia or humic acid , and provided a kinetic degradation. The physicochemical features of MnO 2 before and after bio-regeneration were characterized by TEM , XRD, BET, and XPS. It was found that the morphological structure of MnO 2 became loose and the maximum peak of pore size distribution became smaller, but the increase of surface area, the change of Mn(III/IV) content, and the decrease of crystallinity favored the bio-regeneration process. Moreover, as a mediator in the MnCS, the group of MnOB was dramatically inhibited by CIP, and the bacterial community had changed significantly. The typical MnOB shared low abundance in the biofilter, while the rarely reported genera (e.g. Sphingomonas ) that related to the formation of Mn deposits appeared to be involved in the MnCS.
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