Manganese Removal from Acid Mine Drainage by a Consortium of Mn-Oxidizing Bacteria in Continuous Stirred Tank Bioreactor: Long-Term Treatment and Reactive Mixture Characterization

Biological treatments based on Mn-oxidizing bacteria can effectively remove Mn from contaminated water. However, the mechanisms and key organisms of biological low-pH Mn(II) removal are largely unexplored. In this study, bioremediation based on an enriched consortium of acid-resistant Mn-oxidizing bacteria for Mn-rich acid mine drainage (AMD) was evaluated in a continuous stirred tank bioreactor (CSTB), and the removal mechanism of Mn(II) was further studied. The highest removal rate of 33.2 ± 1.8 mg/L/d and maximum removal efficiency of 82.4 ± 1.8% were achieved with a pH of 5.5, hydraulic retention time (HRT) of 48 h, chemical oxygen demand (COD) concentration of 2000.0 mg/L, and Mn concentration of 80.0 mg/L, respectively. The Mn oxidation rate was ∼4.1 μM/h in the continuous stirred tank bioreactor (CSTB) under stable conditions. Acinetobacter and Azospirillum were the main contributors for Mn oxidation in the bioreactor. X-ray diffraction (XRD) analysis suggested that Mn removal proceeds via the formation of biogenic precipitates consisting of MnO2, MnOOH, and MnCO3. The performance of the CSTB was further evaluated by actual Mn-rich acid mine drainage, and Mn, Cu, Zn, and Cd removal efficiencies were 86.5 ± 2.4%, 98.8 ± 2.2%, and 96.9 ± 1.0%, and 97.0 ± 1.7%, respectively. Results from this work have demonstrated that Mn-oxidizing bacteria can be effective at processing complex Mn-rich acid mine drainage