Role of manganese in cycling metals in mining-impacted environments, The

Abstract

Manganese oxides are well-known scavengers of metals in the environment and are also chemically active, with changes in pH or redox leading to dissolution or precipitation. These properties can be both beneficial, allowing for treatment of toxic metals through control of manganese, and harmful, as changing geochemical conditions can lead to unintended release of manganese and associated metals. This research focuses on three aspects of manganese in mining-impacted environments. First, manganese-rich sludge from a mine water treatment plant was evaluated for use in removing molybdenum from mine water. Batch tests showed that over 90% of molybdenum was removed from solution by sorption onto the sludge at pH 6 and below. Sorption was found to be sensitive to mine water composition and pH. Impurities in the sludge sorbent did not reduce sorption relative to pure-phase manganese oxide; a finding which broadens the range of applicable sorbents and potential for reuse of manganese-rich materials onsite. Second, the role of manganese oxides in controlling metals mobility was evaluated for waste rock from a 100+ year old pile. Although manganese was found to be a small fraction of the waste rock (1.3% by mass), sequential extraction tests demonstrated that approximately one-third of the extractable copper, lead, and zinc is associated with manganese oxides. During reclamation, metals associated with the oxides can be released if organic carbon amendments cause geochemically reducing conditions. Lastly, electrolytic methods were evaluated for manganese removal from mine water at low pH. Manganese concentrations in pH 3 mine water were reduced from approximately 40 mg/L to less than 2 mg/L in batch and flow-through tests with applied voltages of 3.5 V. Electrolytic treatment of manganese has potential to reduce chemical addition and sludge mass compared to conventional treatment methods, which precipitate manganese hydroxides at pH 10, followed by neutralization prior to discharge. Collectively, these studies demonstrate the wide range of properties of manganese oxides and their ability to act as a sink for metals in the environment. Manganese oxides have potential to be used in water treatment applications; however, due to the wide range of characteristics, site-specific studies are necessary.