Fundamental study on the removal of Mn²⁺ in acid mine drainage using sulfate reducing bacteria

The optimum conditions for Mn²⁺ removal from acid mine drainage was studied by a SRB (sulfate reducing bacteria) bioreactor. Chemical experiments with Na₂S as a S^2- source were conducted to investigate the effects of pH, coexisting metal ions, and the components in a growth medium for SRB on MnS formation from Mn²⁺ solutions. The amount of Mn removed from the Mn²⁺ solutions decreased with decreasing pH. The Zn²⁺ or Fe²⁺ coexisting in the solutions consumed S^2- by forming ZnS or FeS, and this inhibited Mn removal. Sodium citrate, a component of the growth medium for SRB, formed a complex with Mn²⁺ and suppressed MnS formation. Biological experiments using the SRB reactor were carried out at 37°C and it was confirmed that the Mn²⁺ concentration decreased to less than 10 g.m^-3 from 100g.m^-3 at neutral pHs (pH 5-7) after 100 hours when other metal ions and sodium citrate were absent. The formed precipitate was identified to be metastable γ-MnS with a band gap of about 3.8 eV by XRD, XRF, and UV-VIS.