Microbial Biogeochemical Impacts on Manganese Reduction in Abandoned Mine Drainage Passive Remediation Systems

Passive remediation systems ( PRSs ) treat abandoned legacy mines to remove the contaminants in abandoned mine drainage ( AMD ), using settling ponds, limestone beds, and wetlands to precipitate contaminants onsite. These PRSs remove high levels of metals in AMD, including manganese ( Mn ). The impact of microbes that naturally colonize the PRSs on Mn solubilization is poorly understood. We sought to determine the microbial mechanisms contributing to Mn solubilization. We determined in circumneutral AMD, Mn reduction can be microbially facilitated through acidogenesis and sulfidogenesis, whereas in acidic AMD systems it is primarily geochemically driven by low pH. Within the circumneutral Wingfield Pines AMD PRS, spikes of Mn occurred in the wetland at the end of the system. Culturable acidogenic and sulfidogenic bacteria were enumerated from the Wingfield Pines PRS in multiple locations with spikes at the end of the system. Bacterial isolates were identified via 16S rRNA gene sequencing for acidogenesis as Bacillus spp. and Corynebacterium spp., and sulfidogenesis as Citrobacter sp., Aeromonas spp., and Shewanella spp. Whole community surveys of Wingfield Pines PRS by 16S rRNA analysis showed the presence of potentially acidogenic or sulfidogenic bacteria throughout the system, with an increase in relative abundance in the wetlands. We demonstrated microbial Mn resolubilization as acidogenesis and sulfidogenesis in the laboratory to determine potential in-field impacts. This establishes a critical basis for considering microbial processes in future PRS design for effective Mn remediation and potential recovery of Mn as a critical mineral.