Leaching of manganese using bio-solubilized iron reductant to prevent manganese carbonate formation

Conventional manganese bioleaching systems often suffer from manganese carbonate (MnCO3 ) precipitation due to the interaction of dissolved manganese with carbon dioxide generated during microbial metabolism. This precipitation lowers manganese recovery and limits process efficiency. The present study demonstrates a two-stage bioleaching strategy in which microorganisms first reduce iron minerals to produce dissolved Fe2+, which is subsequently used as a reductant for manganese dissolution in a separate stage. By separating microbial CO2 generation from manganese reduction, manganese carbonate formation was suppressed while improving manganese recovery. The two-stage system achieved dissolved manganese concentrations up to 1,792 mg/L, substantially higher than previously reported single-stage systems, and produced a manganese oxide product with 93 percent purity at pilot scale. These findings demonstrate a practical and scalable strategy for sustainable manganese extraction from oxide ores. Background Most manganese ores occur as manganese dioxide (MnO2 ), where manganese exists in the +4 oxidation state and must be reduced to the soluble +2 state before extraction can occur [1]. Conventional manganese extraction methods often rely on strong chemical reductants and energy-intensive processing routes, which can increase operating costs and environmental impacts. Bioleaching has emerged as an alternative approach for manganese extraction because microorganisms can facilitate reductive dissolution under relatively mild conditions. In previously reported single-stage bioleaching systems [2,3], fermentative organisms decomposed biomass to produce organic compounds such as acetic acid, while metal-reducing organisms used manganese dioxide as an electron acceptor. Although this approach successfully dissolved manganese, microbial metabolism simultaneously generated carbon dioxide (CO2), which reacted with dissolved Mn2+ to form manganese carbonate (MnCO3). The precipitation of manganese carbonate reduced overall manganese recovery by prematurely removing manganese from solution. Thermodynamic analysis using Pourbaix diagrams showed that manganese carbonate formation becomes favorable under moderately acidic to near-neutral conditions when dissolved carbon dioxide is present [4]. Maintaining sufficiently acidic conditions can suppress manganese carbonate precipitation, but highly acidic environments are less desirable for biological systems and may increase downstream processing challenges. To address this limitation, this study investigated the use of bio-solubilized Fe2+ as an intermediate reductant for manganese dissolution. In the proposed approach, microorganisms first reduce ferric iron minerals to generate dissolved ferrous iron in a separate stage. The Fe2+-rich solution is then introduced into the manganese leaching stage, where Fe2+ chemically reduces MnO2 to soluble Mn2+. This configuration separates microbial CO2 generation from manganese reduction, thereby minimizing manganese carbonate formation while also enabling iron precipitation under controlled pH conditions. Methods A two-stage bioleaching system (Fig. 1) was developed to evaluate the use of bio-solubilized iron as a reductant for manganese extraction. Pyrolusite-rich manganese ore was used as the manganese source, while iron ore tailings containing goethite and hematite served as the iron source. A mixed culture of metal-reducing microorganisms obtained from a local fen in Houghton County, MI, was used throughout the study. Organic-rich nutrient media were produced by fermenting Typha latifolia biomass in the presence of fermentative organisms. The resulting solution contained organic compounds including acetic acid, glucose, fructose, citric acid, lactic acid and formic acid, which supported microbial metal reduction. Flask-scale experiments were conducted using a two-stage, packed-bed configuration. In the first stage, iron ore tailings were contacted with nutrient media and metal-reducing organisms to generate dissolved Fe2+. The Fe2+-rich solution was then transferred into the second-stage manganese leaching flask, where dissolved ferrous iron reduced manganese oxides to soluble Mn2+. The system was operated under anaerobic conditions using fermentation locks to prevent oxygen intrusion. Fresh nutrient media were periodically supplied to sustain microbial activity during long-term operation. To evaluate scalability, the process was expanded into a 19-L pilot-scale system consisting of a biomass decomposition tank, iron leaching tank, manganese leaching tank, oxidation tank and settling tank. Organic-rich liquid generated in the biomass tank flowed into the iron leaching tank, where microbial iron reduction produced dissolved Fe2+. This solution was subsequently directed into the manganese leaching tank to dissolve manganese oxides. The manganese-rich solution was then transferred to an oxidation system where dissolved manganese was precipitated as manganese oxide. Fig. 1 Experimental setup for two-stage, iron-assistedmanganese bioleaching. The pH, dissolved iron concentration and dissolved manganese concentration were monitored during the leaching experiments. Mineralogical characterization of ores and residues was performed using X-ray diffraction (XRD), while the composition of the recovered manganese precipitates was analyzed using inductively coupled plasma atomic emission spectroscopy (ICP-OES). Results and discussion The two-stage bioleaching system successfully generated dissolved Fe2+ in the first-stage iron leaching flask while maintaining low iron concentrations in the manganese leaching stage. Over 123 days of operation, approximately 4.71 g of iron was solubilized in the iron leaching stage, whereas only 0.22 g of dissolved iron was detected in the manganese stage. This indicated that most dissolved iron was oxidized and precipitated after manganese reduction, thereby minimizing iron contamination in the manganese-rich solution. The operating pH remained near 4.4 to 4.7 throughout the experiments, which supported manganese dissolution while promoting iron precipitation. The two-stage system significantly improved manganese dissolution compared to previously reported single-stage systems. The maximum dissolved manganese concentration achieved in the two-stage biotic experiment reached 1,792 mg/L, compared to 928 mg/L in the single-stage biotic system and only 192 mg/L in the abiotic configuration. Over the 123-day operating period, the two-stage system dissolved approximately 5.92 g of manganese, substantially exceeding the manganese dissolution achieved in both single-stage systems. These results demonstrated that bio-solubilized Fe2+ enhanced manganese reduction kinetics and improved overall leaching efficiency. Fig. 2 Comparison of manganese ore residue mineralogy:Single-stage leaching versus two-stage leaching. XRD analysis further confirmed the effectiveness of the two-stage strategy in preventing manganese carbonate formation (Fig. 2). Residues obtained from previously reported single-stage leaching experiments showed clear MnCO3 peaks, indicating precipitation of manganese carbonate during microbial manganese reduction. In contrast, no MnCO3 peaks were detected in residues from the two-stage system. Separating microbial CO2 generation from manganese dissolution effectively suppressed MnCO3 formation while maintaining manganese in solution. Pilot-scale operation demonstrated the scalability of the process. The pilot system continuously generated manganese-rich solution containing approximately 767 mg/L manganese at a flow rate of 5 L/d. Following oxidation and precipitation, the recovered manganese product was washed, dried and roasted to produce a high-purity manganese oxide material. XRD analysis confirmed the formation of Mn3O4 without detectable manganese carbonate. The results of the ICP-OES analysis of the final product, with elemental compositions from the analysis converted to their respective oxides, were 93 weight percent Mn3O4, 2 weight percent Fe2O3, 4 weight percent CaO and 1 weight percent trace impurities (K, Mg, Si, S), showing high manganese oxide purity with only minor iron contamination and trace impurities. Conclusions This study demonstrated that bio-solubilized iron can effectively function as an intermediate reductant for manganese bioleaching within a two-stage process configuration. Separating microbial iron reduction from manganese dissolution minimized direct interaction between dissolved manganese and biologically generated carbon dioxide. This suppressed manganese carbonate formation, which commonly limits conventional single-stage bioleaching systems. The two-stage approach improved manganese dissolution while maintaining relatively low iron contamination in the manganese-rich solution. The process also enabled selective manganese recovery under moderately acidic conditions that supported manganese solubility while promoting iron precipitation. Mineralogical analysis confirmed the absence of manganese carbonate formation in the two-stage system, highlighting the effectiveness of the process design. Pilot-scale operation demonstrated the scalability of the process and showed that high-purity manganese oxide products could be recovered through downstream oxidation and precipitation. Overall, the results support the potential of this two-stage bioleaching strategy as a more sustainable and practical approach for manganese extraction using biologically generated reductants. Future work is expected to focus on process optimization, long-term operational stability, kinetic enhancement and techno-economic evaluation to further assess industrial applicability. Selected references Borkar PK, Eisele T (2025) A review of in situ manganese bioleaching and recovery techniques. Miner Process Extr Metall Rev 1–22 Borkar PK et al. (2025) Pilot plant study on manganese bioleaching using biomass decomposition products as a nutrient source and electrolysis for oxide precipitation. Hydrometallurgy 232:106430 Sharma N, Eisele TC (2021) Anaerobic reductive bioleaching of manganese ores. Miner Eng 173:107152 Bale CW et al. (2002) FactSage thermochemical software and databases. Calphad 26(2):189–228

Leaching of manganese using bio-solubilized iron reductant to prevent manganese carbonate formation

Authored by: Palas Kamlakar Borkar, Neha Sharma, Atharva Rane, Kwabena Boafo and Timothy Eisele

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