Reductive bioleaching of goethite-rich and hematite-rich iron tailingsby anaerobic organisms

Iron extraction from low-grade ores and tailings is increasingly constrained by high energy consumption and production costs. This study demonstrates that reductive bioleaching, driven by anaerobic iron-reducing microorganisms and biomass-derived organic substrates, can effectively dissolve iron from both goethite-rich and hematite-rich iron tailings under mild conditions. While goethite dissolves more rapidly, prolonged microbial activity enables sustained dissolution of hematite, a mineral traditionally considered refractory to bioleaching. These findings highlight the potential of biologically mediated iron recovery as a low-energy, environmentally responsible alternative for upgrading iron tailings and low-grade resources. Background Iron is the most widely used metal globally, with the vast majority of production dedicated to steelmaking. Conventional iron extraction relies on energy-intensive mining, beneficiation, and high-temperature reduction processes that contribute significantly to greenhouse-gas emissions and environmental degradation. As high-grade iron ores are depleted, attention is increasingly shifting toward low-grade ores, tailings and secondary resources. However, conventional processing of these materials is often economically and environmentally unattractive. Reductive bioleaching offers an alternative approach in which anaerobic microorganisms reduce ferric iron (Fe3+) to the more soluble ferrous form (Fe2+), enabling iron dissolution under relatively mild conditions [1]. In this process, iron-bearing minerals serve as terminal electron acceptors during microbial respiration, while organic compounds act as electron donors. Hydrated iron oxides such as goethite (FeOOH) are generally more amenable to microbial reduction, whereas crystalline hematite (Fe2O3) is widely regarded as resistant to biological dissolution [2]. As a result, most prior studies report limited or negligible hematite leaching, particularly over short experimental durations. This work investigates the long-term reductive bioleaching behavior of two iron tailings streams with contrasting mineralogy: one dominated by goethite and the other dominated by hematite. The study aims to clarify the role of mineralogy, particle size and prolonged microbial activity on iron dissolution, with a focus on implications for tailings reprocessing and sustainable iron recovery. Methods Two iron ore tailings samples were evaluated. Source A consisted primarily of quartz and goethite with minor hematite, while source B was composed mainly of quartz and hematite. Mineralogical composition was determined by X-ray diffraction (Table 1a), and particle size distributions were measured to assess surface area differences between the two materials (Table 1b). Fig. 1 Experimental setup for iron leaching by iron-reducing organisms and T. latifolia. A mixed culture of iron-reducing anaerobic microorganisms was collected from a natural fen environment exhibiting active iron reduction [3]. These organisms were enriched and used as inoculum for all bioleaching experiments. Typha latifolia (a common wetland plant) biomass was fermented using local fermentative microorganisms to generate an organic-rich liquid containing simple organic acids such as acetic, citric, lactic and formic acids. This liquid served as both a carbon source and an electron donor for iron-reducing organisms. Bioleaching experiments were conducted in 250-mL flasks containing iron tailings, microbial inoculum and nutrient media under anaerobic conditions (Fig. 1). After an initial incubation period, fresh organic-rich media was supplied regularly to sustain microbial activity. Experiments were carried out for up to 368 days, significantly longer than typical bioleaching studies. Dissolved iron concentrations and pH were monitored throughout the experiments, and leached residues were characterized using X-ray diffraction and scanning electron microscopy with energy-dispersive spectroscopy to assess mineralogical changes and iron liberation. Results The two tailings types exhibited distinct bioleaching behaviors. Cumulative dissolved iron over 368 days is shown in Fig. 2, with a summary of bioleaching behavior provided in Table 2. In goethite-rich source A, iron dissolution occurred rapidly following the start of nutrient addition. Dissolved iron concentrations reached approximately 640 mg/L early in the experiment and increased steadily, peaking at 1,177 mg/L. After reaching maximum levels, iron concentrations remained relatively stable, indicating sustained microbial reduction and dissolution. After 368 days, approximately 49 percent of the total iron in source A had been dissolved, with solution pH maintained near mildly acidic conditions. In contrast, hematite-rich source B exhibited slower leaching kinetics and lower overall iron dissolution. Initial dissolved iron concentrations were modest, reaching a maximum of 619 mg/L over the course of the experiment. Total iron dissolution after 368 days was approximately 29 percent. Unlike source A, dissolved iron concentrations in source B fluctuated over time, reflecting the greater resistance of hematite to microbial reduction and the sensitivity of microbial activity to organic substrate availability. When fresh biomass-derived media was introduced later in the experiment, microbial activity and iron dissolution increased, demonstrating that hematite dissolution could be sustained under favorable conditions. Mineralogical and microscopic analyses supported the solution chemistry results. X-ray diffraction analysis showed a decrease in the intensity of iron oxide peaks after bioleaching, accompanied by an increase in quartz peak intensity, indicating preferential removal of iron-bearing phases and greater exposure of silica-rich minerals. Scanning electron microscopy coupled with EDS mapping further confirmed these trends, showing surface roughening, reduced iron signal intensity and increased silica exposure after leaching. Discussion The results confirm that reductive bioleaching is more effective for hydrated iron oxides such as goethite than for crystalline hematite, consistent with prior studies. The higher dissolution rates observed for source A can be attributed to both mineralogical factors and finer particle size, which increases surface area and microbial accessibility. Importantly, this study demonstrates that hematite dissolution is possible when microbial activity is sustained over extended periods. Although hematite is commonly described as refractory, prolonged exposure to iron-reducing microorganisms and continuous supply of organic electron donors enabled measurable and sustained iron release. This finding challenges the assumption that hematite-rich tailings are unsuitable for biological processing and suggests that time, microbial adaptation and nutrient management are critical variables. From a practical standpoint, the long-duration nature of the experiments reflects conditions more relevant to low-energy, field-scale tailings treatment systems rather than short-term reactor-based processes. The sustained iron dissolution in goethite-rich tailings and the ability to restore microbial activity in hematite-rich tailings demonstrate the durability and adaptability of the biological system. Fig. 2 Comparison of cumulative iron dissolved in bioleaching flasksin 368 days. Conclusions This study demonstrates that anaerobic reductive bioleaching can effectively dissolve iron from both goethite-rich and hematite-rich iron tailings using biomass-derived organic substrates. Goethite-rich tailings exhibit faster kinetics and higher overall iron recovery, while hematite-rich tailings require longer leaching durations but can still achieve meaningful dissolution. These findings indicate that reductive bioleaching represents a low-energy, environmentally favorable strategy for recovering iron from tailings and low-grade materials. Future work will focus on scaling the process and integrating downstream oxidation and precipitation steps to recover iron products from solution, advancing the technology toward practical implementation. Selected references Eisele TC, Gabby KL (2014) Review of reductive leaching of iron by anaerobic bacteria. Miner Process Extr Metall Rev 35(2):75–105 Papassiopi N, Vaxevanidou K, Paspaliaris I (2010) Effectiveness of iron reducing bacteria for the removal of iron from bauxite ores. Miner Eng 23(1):25–31 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

Reductive bioleaching of goethite-rich and hematite-rich iron tailingsby anaerobic organisms

Authored by: Palas Kamlakar Borkar, Neha Sharma, Kwabena Boafo, Daniel Amponsah-Berko and Timothy Eisele

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