Oxidative Precipitation of Manganese from Dilute Waters

Various metals may be removed from mine water by precipitation of their hydroxides. However, over the pH range of 7–9, it is more efficient to oxidize Mn II to Mn IV than to precipitate Mn II as its hydroxide. Chlorine (or hypochlorite) can be used to do this, but its use may not be appropriate for mine waters that will be recycled as process solutions or discharged into a receiving body. Consequently, there has been interest in reducing chlorine use or its total replacement by non-chlorinated oxidants, such as oxygen and peroxygens, for treatment of mine water. In this work, we report the results of a comparative investigation of the following oxidants: NaClO (as reference), O₂; H₂O₂; Caro’s acid (H₂SO₅), and the combination of H₂O₂ with NaClO, with initial Mn] = 10 mg/L, at 25 °C, with 100 and 300 % excess oxidant above the stoichiometric requirement. It was found that the reaction pH has to be greater than 8 to obtain effective precipitation. It is possible to reach a final Mn] below 1.0 mg/L in 60 min of batch reaction time, using either NaClO, Caro’s Acid, or a combination of NaClO + H₂O₂. Using only O₂ or H₂O₂ was ineffective.