We focused on the use of abiotic MnO2 to develop reactors for enriching manganese-oxidizing bacteria (MnOB), which may then be used to treat harmful heavy metal-containing wastewater and in the recovery of useful minor metals. Downflow hanging sponge (DHS) reactors were used under aerobic and open conditions to investigate the potential for MnOB enrichment. The results of an experiment that required a continuous supply of organic feed solution containing Mn(II) demonstrated that MnOB enrichment and Mn(II) removal were unsuccessful in the DHS reactor when plain sponge cubes were used. However, MnOB enrichment was successful within a very short operational period when sponge cubes initially containing abiotic MnO2 were installed. The results of a microbial community analysis and MnOB isolation revealed that MnOB belonging to Comamonadaceae or Pseudomonas played a major role in Mn(II) oxidation. Successful MnOB enrichment was attributed to several unidentified species of Chitinophagaceae and Gemmataceae, which were estimated to be intolerant of MnO2, being unable to grow on sponge cubes containing MnO2. The present results show that MnO2 exerted anti-bacterial effects and inhibited the growth of certain non-MnOB groups that were intolerant of MnO2, thereby enabling enriched MnOB to competitively consume more substrate than MnO2-intolerant bacteria.
Keywords: anti-bacterial effects; biological manganese oxidation; biological manganese oxide; downflow hanging sponge reactor; metal oxide.