Iron-induced nanoscale restructuring of birnessite drives functional shifts in groundwater biofilters for iron and manganese removal

Iron and manganese frequently co-occur in groundwater and are commonly removed by biological filtration. While long-term Mn removal is predominantly driven by abiotic oxidation on birnessite-coated media, the impact of iron on this catalytic system remains poorly understood. This study investigated the structural, mineralogical, and microbiological organization of a full-scale groundwater biofilter treating Fe– and Mn-bearing water seven years after start-up. Scanning electron microscopy revealed a Fe-rich surface overlayer at the top of the filter, overlying a MnOx coating present throughout the bed. Fe K-edge EXAFS analysis demonstrated that this poorly crystalline Fe phase is ferrihydrite-like and exhibits short Fe–Mn distances (∼2.88 Å), consistent with close structural association with underlying birnessite rather than isolated ferrihydrite precipitation. Despite substantial Fe loading, MnOx remained predominantly abiotic turbostratic birnessite (AOS 3.61–3.67). Biofilm analyses showed increased metabolic activity and dominance of filamentous bacteria affiliated with the genus Crenothrix compared to a Mn-only filter media. Biokinetic assays performed on the extracted biofilm revealed similar Mn(II) oxidation rates ( k = 1.2 h -1 ) but a 25-fold reduction in biological Mn oxidation capacity relative to a Mn-only biofilter, indicating a functional shift toward Fe removal rather than Mn accumulation. These findings demonstrate that iron does not merely compete with manganese but restructures the mineral and microbial architecture of the biofilter, altering its long-term functional trajectory. Spatial separation of Fe and Mn removal may help preserve Mn polishing capacity and biofilter stability in groundwater treatment systems.