Limestone-Based Constructed Wetlands for High-Strength Mn-Rich Mine Drainage: Effects of Reed Vegetation on Mn and Zn Removal

Mine drainage containing manganese (Mn) and zinc (Zn) poses a major environmental challenge. In Japan, Mn-rich mine drainage frequently exceeds the discharge standards of 10 mg/L for Mn and 2 mg/L for Zn. This study evaluated laboratory-scale continuous-flow constructed wetlands (CWs) for the treatment of both synthetic and actual mine drainage without addition of external organic carbon. The actual mine drainage was collected from an anonymous former Cu–Pb–Zn mine in Japan. Two limestone-filled CWs (2 L), one planted with Phragmites australis and the other unplanted, were operated at hydraulic retention times of 1–2 d. The influent mine drainage contained approximately 66 mg/L Mn and 10–15 mg/L Zn together with Fe, Cu, Cd, and Pb. During 170 days of continuous operation, the planted CW consistently achieved lower effluent Mn and Zn concentrations than the unplanted CW under both synthetic and actual mine drainage conditions. Effluent Mn concentrations in the planted CW ranged from 5.5 to 20.0 mg/L, compared with 46.1–54.2 mg/L in the unplanted CW. Likewise, effluent Zn concentrations ranged from 0.21 to 3.0 mg/L in the planted CW and from 2.4 to 10.1 mg/L in the unplanted CW. The superior performance of the planted CW was associated with enhanced limestone dissolution, elevated pH, and favorable rhizosphere conditions that likely promoted biologically mediated Mn(II) oxidation and the formation of Mn oxides, while Mn carbonate precipitation may also have contributed to Mn removal. These findings demonstrate that limestone-based planted CWs can effectively treat Mn-rich mine drainage without addition of external organic carbon and highlight their potential as sustainable, low-energy passive treatment systems for the long-term management of abandoned mine drainage.