<p>Manganese (Mn) is a common water contaminant in mining areas and one of the most challenging metals to treat. This study assessed Mn removal efficiencies of pilot-scale slag reactors using steelmaking slag mixed with limestone as the reactive material. Three different reactor configurations of baffle-type, weir-type, and vertical flow-type were installed and tested over a 316-day operational period with initial, middle, and final phases. Geochemical modeling indicated that most effluents were saturated with calcite (CaCO₃), rhodochrosite (MnCO₃), and manganite (MnOOH), suggesting that both carbonate and hydroxide precipitation contributed to Mn removal. As effluent pH increased, alkalinity decreased due to the consumption of carbonate ions (CO₃²⁻) during calcite precipitation. The potential formation of Mn carbonates may have contributed to the Mn removal efficiencies. X-ray photoelectron spectroscopy (XPS) analyses of the accumulated precipitates indicated that Mn(III, IV) oxides were the dominant phase. Although temperature, which influences Mn removal rate, decreased from middle to final phase, removal efficiencies and/or effluent concentrations of Mn exhibited increasing and decreasing trends, possibly due to the autocatalytic oxidation by the accumulated Mn(III, IV) oxides. These findings highlight the potential of slag reactors as a cost-effective and sustainable solution for treating Mn-contaminated mine drainage, groundwater, and industrial wastewater. Moreover, this approach contributes to reducing CO₂ emissions from lime production processes (e.g. calcination) while promoting the utilization of waste materials.</p>