A biofilm reactor packed with aspen wood and polyurethane sponge for biological manganese(II) oxidation and minor metal recovery.

Biogenic manganese oxides (MnOx) are promising agents for the removal and recovery of minor metals from water. However, cultivating heterotrophic Mn(II)-oxidizing microorganisms in non-sterile bioreactors remains difficult, and few continuous-flow reactor designs have been evaluated for their ability to recover minor metals under such conditions. In this study, we operated a laboratory-scale biofilm reactor packed with aspen wood and a polyurethane sponge for 469 d to evaluate its feasibility for biogenic MnOx production. Aspen wood served as a solid-phase organic substrate, the biodegradation of which provided a continuous release of soluble organic substrates to sustain the growth of heterotrophic Mn(II)-oxidizing microorganisms. The maximum mean dissolved Mn(II) removal rate by the reactor fed with artificial freshwater reached 3.0 mg L-1 d-1, likely through oxidation to MnOx. The removal of dissolved nickel(II) and cobalt(II) by the reactor with in situ-produced biogenic MnOx was also confirmed. Amplicon sequencing of near-full-length bacterial 16S ribosomal ribonucleic acid (rRNA) genes revealed dominant bacterial operational taxonomic units (OTUs) in aspen wood biofilms (such as Acidibacter, Pseudorhodoplanes, and Terrimonas OTUs) and sponge media biofilms (such as Nitrospira, Reyranella soli, and Sediminibacterium goheungense OTUs). The dominance of a fungal 18S rRNA gene OTU related to Coniochaeta in both biofilm types was confirmed through amplicon sequencing of near-full-length eukaryotic 18S rRNA genes. These findings revealed the potential of the reactor for biogenic MnOx production and minor metal recovery, while providing insights into the associated microbial communities.