Manganese removal in a full-scale constructed wetland for passive mine water treatment: Environmental factors and microbial communities

Manganese (Mn) removal in passive mine water treatment remains a challenge due to its slow oxidation kinetics, requiring specific biogeochemical conditions. Constructed wetlands are often the key functional units enabling Mn removal in full-scale passive treatment plants. This study examines the key biogeochemical factors influencing Mn removal in a full-scale passive mine water treatment plant located in Alès (South-East France). Over one year, monitoring of physicochemical parameters, microbial communities, and Mn speciation in solid phases was conducted every two months. Results highlight temporal variations in Mn removal efficiency, with two main mechanisms identified: (1) Mn carbonate (MnCO₃) precipitation, likely influenced by high carbonate concentrations in mine water, and (2) Mn oxide (δ-MnO₂) formation, mainly associated with reed rhizosphere, where it accumulates as mineral plaque. In mine water, Mn removal correlates with Fe particle concentrations, suggesting a catalytic effect, as well as with alkalinity and the abundance of microorganisms affiliated to Alteromonadaceae, suggesting a microbial influence. Mn removal appears to be primarily abiotic, driven by favourable pH and alkaline conditions that promote Mn carbonate precipitation, by autocatalytic oxidation reactions occurring on rhizosphere surfaces and by plant's design including surface area and hydrological conditions. Microbial communities may facilitate certain Mn removal processes depending on environmental conditions.