Bioleaching and Oxalic Acid for REE Recovery

By Dr Silpaja Chandrasekar, PhDReviewed by Laura ThomsonFeb 14 2025

A recent scientific article in Scientific Reports discussed an innovative method for extracting rare earth elements (REEs) from gold mine tailings (GMT) using Acidithiobacillus thiooxidans with a pretreatment step. Researchers treated GMT with 2 M oxalic acid at 90 °C for 6 hours to remove iron, enhancing bioavailability. Bioleaching improved bacterial activity and acid production during the logarithmic growth phase, with structural analysis confirming surface modifications. This approach significantly increased the recovery of praseodymium (Pr), cerium (Ce), and europium (Eu), demonstrating its potential for resource recovery.

Bioleaching Research

Past work on bioleaching primarily focused on extracting heavy metals from mine tailings, with limited studies on rare earth element mobilization.

Traditional extraction methods like pyrometallurgy and hydrometallurgy were costly and environmentally harmful, prompting interest in microbial-based approaches.

Acidithiobacillus thiooxidans has been widely studied for its sulfur oxidation capabilities, aiding in mineral dissolution. Various bioleaching techniques, including single-step, two-step, and spent culture medium methods, have been explored to enhance metal recovery using acidophilic microorganisms.

Metal Recovery Process

The GMT used in this study were sourced from the Mouteh gold factory’s tailings dam. Before testing, the sample was air-dried and sieved through a 200-mesh screen to obtain particles smaller than 75 μm.

The GMT powder was digested according to the American Society for Testing and Materials standard D4698-92—Standard Practice for Total Digestion of Sediment and Soil Samples for Chemical Analysis (ASTM D4698-92), and elemental concentrations were determined using inductively coupled plasma emission spectrometry (ICP-OES).

The primary elements detected in GMT powder included iron (322,743 mg/kg), aluminum (527 mg/kg), praseodymium (100 mg/kg), europium (213 mg/kg), and cerium (112 mg/kg). The mineralogical composition was analyzed using X-ray diffraction, chemical bonding using Fourier transform infrared spectroscopy, surface morphology using field emission scanning electron microscopy, and elemental distribution using energy dispersive X-ray spectroscopy.

The bacterial strain Acidithiobacillus thiooxidans was obtained from the Iranian Research Organization for Science and Technology (IROST). This strain facilitates metal dissolution by oxidizing sulfur compounds to produce sulfuric acid. The culture medium contained sulfur, ammonium sulfate, potassium phosphate, magnesium sulfate, and potassium chloride. Each Erlenmeyer flask was inoculated with 10% (v/v) bacterial culture and incubated at 30°C with a shaking speed of 140 rpm to promote bacterial growth.

Oxalic acid pretreatment was conducted to enhance metal extraction. Key parameters influencing the leaching process included temperature, duration, liquid-to-solid ratio, and oxalic acid concentration. The GMT powder was mixed with an oxalic acid solution in an Erlenmeyer flask, sealed with foil, and subjected to leaching at 500 rpm for six hours. After the reaction, vacuum filtration separated the leachate from solid residues.

A two-step bioleaching process was employed, wherein GMT powder was introduced after the bacterial culture reached the end of the logarithmic phase. The bioleaching experiment lasted seven days at 30 °C with a stirring speed of 140 rpm. Metal ion recovery was assessed using inductively coupled plasma optical emission spectrometry (ICP-OES), while sulfate ion concentration, bacterial cell count, pH, and oxidation-reduction potential were measured.

Cell counting was performed using a hemocytometer under a phase-contrast microscope at 400× magnification. Metal recovery efficiency was calculated based on the concentrations of metals in the leachate and GMT powder.

Enhanced Bioleaching Efficiency

Oxalic acid pretreatment led to only 20% iron dissolution due to its stability in silicate minerals and oxalate precipitation. X-ray diffraction showed iron was tightly bound, making release difficult under mild acidity. Silica gelatinization further hindered solubilization while REEs remained in the residue. The dissolution process relied on surface interactions, where oxalic acid dissociation facilitated iron release. However, limited leaching efficiency was observed due to mineral complexity.

Acidithiobacillus thiooxidans showed significant growth during bioleaching, entering the logarithmic phase after 12 days. The bacteria oxidized sulfur to sulfate, lowering the pH from 3.5 to 0.7 and enhancing metal solubilization. Introducing GMT powder caused temporary bacterial adhesion, reducing cell count and slightly increasing pH due to acid consumption. Over time, pH stabilized, and oxidation-reduction potential rose, indicating metal release. After 7 days, Pr, Ce, and Eu recovery rates reached 76%, 46%, and 14%, with higher extraction from pretreated samples.

Structural analysis confirmed that pretreatment and bioleaching altered the GMT powder composition. X-ray diffraction showed reduced clinochlore and gypsum phases, exposing mineral surfaces for bioleaching. Fourier-transform infrared spectroscopy detected functional group changes, indicating mineral modifications. Scanning electron microscopy revealed a shift from a smooth to a fragmented structure, confirming iron removal. Iron content decreased from 15.65% to 5.12% after pretreatment and further to 1.71% post-bioleaching, enhancing rare earth element recovery.

Conclusion

Acidithiobacillus thiooxidans was also used alongside oxalic acid pretreatment to enhance rare earth element extraction from GMT powder. Pretreatment at 90 °C with 2 M oxalic acid effectively reduced iron interference, improving bioleaching efficiency—the metabolic activity generated biogenic sulfuric acid, facilitating metal dissolution under acidic conditions. Structural analysis confirmed significant mineralogical changes, validating the combined approach's effectiveness. Further optimization using advanced techniques can enhance bioleaching for sustainable rare earth element recovery.

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