Production of birnessite-type manganese oxides by biofilms from oxygen-supplemented biological activated carbon (BAC) filters†

Biological oxidation of manganese (Mn) by bacteria results in the formation of biogenic Mn oxides (MnOx), which are known to be strong oxidants and effective catalysts. Manganese-oxidizing bacteria (MnOB) often develop in engineered systems for water treatment under oligotrophic conditions. In this study, we investigated the MnOB within biofilms sampled in two different seasons from full-scale oxygen-supplemented biological activated carbon (BAC) filters performing the complete removal of Mn from wastewater. By applying a novel batch enrichment approach ensuring the continuous presence of soluble Mn, after 42 days the start-up microbial community grew into thick, floccular biofilms efficiently oxidizing Mn²⁺ into numerous black nodules. The amount of Mn oxidized was quantified using inductively coupled plasma optical emission spectroscopy (ICP-OES). X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) revealed that the MnOx formed was a birnessite-type (δ-MnO₂) with a crystalline, nanoflower structure. Comparison of the microbial community composition before and after the enrichment by means of 16S rRNA gene amplicon sequencing showed increases of members of the orders Rhizobiales and Burkholderiales, and identified among the most abundant some bacterial groups which have rarely or never been associated with Mn oxidation before (Rhodococcus, Ellin6067, Planctomycetota Pir4 lineage, Rhizobiales A0839 and Amb-16S-1323). This study unravels the potential of production of crystalline MnOx by mixed-microbial communities which uniquely generate in a man-made biofilter. The new insights provided implement the knowledge in the field, with the perspective to design innovative biotechnologies to remove recalcitrant compounds where MnOB find optimal growth conditions to produce catalytic forms of MnOx.