Coupled abiotic-biotic processes driving manganese removal in a pozzolona biofilter of a full-scale passive mine water treatment plant

• Full-scale passive mine water treatment achieved high Mn removal within the biofilter. • Fe(III) precipitates act as catalytic surfaces promoting Mn oxidation. • Putative Mn-associated bacterial taxa were enriched in specific treatment units. • Temporal geochemical shifts regulate Mn removal efficiency. • Abiotic processes and microbial community patterns were consistent with coupled Mn attenuation mechanisms. Passive mine water treatment is widely used to remove metals from mine water, but manganese (Mn) remains a challenge due to its slow oxidation kinetics at neutral pH. This study investigated the mechanisms of Mn removal and factors influencing its efficiency at the passive treatment plant of Messeix, central France, which integrates a settling pond, wetlands and pozzolana biofilters. Temporal monitoring over one year encompassed water chemistry, Mn precipitate mineralogy, and microbial community analysis. Results showed that the settling pond and wetlands achieved 28-64% Mn removal efficiency depending on season which was insufficient to consistently meet regulatory discharge limits, whereas the pozzolana biofilters achieved 98-100% Mn removal efficiency in all seasons. Mn attenuation was primarily explained by abiotic processes controlled by physicochemical parameters such as pH, dissolved oxygen, redox potential, and iron content. The biofilter provided surfaces for catalytic Mn removal mechanisms on pre-existing Mn oxides and Fe oxyhydroxides. 16S rRNA gene profiles and PICRUSt2 predictions revealed bacterial genera previously reported in Mn-rich environments ( Hydrogenophaga, Sideroxydans, Flavobacterium, Aquabacterium, Bacteriovorax, and Leptothrix ) together with predicted Mn-oxidase-related genes. Although no direct evidence of bacterial Mn oxidation is provided, the spatial and temporal organisation of microbial communities in relation to physicochemical gradients within the treatment plant is consistent with a potential contribution to Mn transformations. This study highlights the interplay between geochemical factors and microbial community structure during the passive treatment of Mn in mine water and offers insights for optimising biofilter design and operation.