Role of biofilm during groundwater biofiltration of manganese.

Manganese (Mn) contamination in groundwater poses significant challenges for drinking water treatment. This study explores the mechanisms of Mn removal in a long-term oxygenated groundwater biofilter. The filter media coating primarily consists of abiotic disordered birnessite (δ-MnO2) with a microglobular structure and an average oxidation state of approximately 3.45. This material plays a key role in the effective adsorption and oxidation of Mn(II) dissolved in groundwater. The results indicate that Mn removal is predominantly abiotic, with biofilm activity contributing to less than 10% of Mn(II) oxidation. Biological colonization is minimal, as evidenced by the low microbial activity and protein-to-polysaccharide ratio. However, Mn-oxidizing and Mn-reducing bacteria were identified under aerobic conditions, suggesting that they play facultative or complementary roles in Mn cycling. The unexpected coexistence of the two types of bacteria highlights the need for further investigation into their role in δ-MnO2 transformation and regeneration. The study provides foundational insights into the dynamics of Mn(II) removal in biofilters and proposes an initial framework for understanding the Mn(II) biogeochemical cycle within such common engineered systems.