Writer: Lava Naz Bagdu
While the miners extract most of the ores underground, some might get overlooked or left in the excavated soil. In this case, nature helps us. These overlooked or excavated ores are being broken down and mined by some kinds of bacteria in a process called bioleaching. In this article, we are going to talk about how bioleaching works, and the characteristics of the bacteria that are used for it.
Bioleaching
Bioleaching is a process used in mining and biohydrometallurgy. It involves the use of microorganisms such as fungi and bacteria to produce various organic acids for the extraction of valuable metals like gold, copper, uranium, and zinc from low-grade ores. Specific bacteria, such as Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans, Leptospirillum ferrooxidans, Acidithiobacillus caldus, and Sulfolobus metallicus, play a key role in this process. These bacteria help the bioleaching process by oxidizing metal sulfides, creating an acidic environment that leads the metal to leach out from the ore (Anglo American, n.d.; Science Direct, 2023; Dresher, W. H., 2004).
Bioleaching Process
In the bioleaching process, these bacteria release sulfuric acid which is extremely bad for the environment. To control the process and prevent the sulfuric acid, which is produced by bacteria, from mixing with the soil, the excavated soil (mineral processing waste containing minerals and bacteria) is taken to a laboratory. It is then crushed down to prepare the waste powder. This waste powder is then placed into a bioreactor (“a device, vessel, or system for cultivating products from plants, animals, and bacteria on the desired scale”) with water and nutrients for the bacteria to thrive. As a result, a solution called leach liquor is created, which contains the metals that were originally in the mineral processing waste. Then this leach liquor is chemically purified by some substrates like Hydroxyoximes (used in the extraction of copper) and ion exchange resins (used to capture specific metal ions, like copper and cobalt ions). Hence, the metals are retrieved in solid form, and the sulfuric acid that is produced by the bacteria can be kept in a lab or a factory. (Science Direct, 2011)
Flow sheet of bioleaching process of solid waste for metals recovery (researchgate.net)
Some of the Bacteria Used in Bioleaching
Acidithiobacillus ferrooxidans:
“Acidithiobacillus ferrooxidans is by far the most widely studied of all extremely acidophilic prokaryotes.” (Quatrini, et. al., 2019) It is a major participant in the industrial recovery of copper (bioleaching), it uses the energy from the oxidization of sulfur and iron-containing minerals for growth, thrives in low-pH environments like pH 1-2, and fixes both carbon and nitrogen from the atmosphere (Valdés, et. al., 2008).
Acidithiobacillus thiooxidans:
Acidithiobacillus thiooxidans (A. thiooxidans) is a widespread, mesophilic, obligately aerobic, extremely acidophilic, rod-shaped, and chemolithoautotrophic gram-negative gammaproteobacterium. It can obtain energy and electrons from the oxidation of reducible sulfur, and it can fix carbon dioxide and assimilate nitrate, nitrite, and ammonium to satisfy carbon and nitrogen requirements. (Yang, et. al., 2019)
Acidithiobacillus caldus:
Acidithiobacillus caldus is a rod-shaped, widely used bacterium in bioleaching. For carbon dioxide fixation and growth, it gains energy from reduced inorganic sulfur compounds (RICs), and oxidization of elemental sulfur this way, adequate acidity for bioleaching to occur is reached. A. Calbus is typically found in mineral-rich environments (Linxu, et. al., 2012; Jonah, et. al., 2020).
Sulfolobus metallicus:
Sulfolobus metallicus is a coccoid-shaped archeon that gains energy by oxidating sulfur and sulfuric ores into sulfuric acid (Scholarly Community Encyclopedia, n.d.).
Leptospirillum ferrooxidans:
“Leptospirillum ferriphilum has been identified as the dominant, moderately thermophilic, bioleaching microorganism in bioleaching processes. It is an acidic and chemolithotrophic bacterium that gains electrons from ferrous iron oxidation for energy production and cell growth.” (Shuang, et. al., 2011)
As a result, bioleaching provides a more environmentally friendly alternative to modern mining techniques, helping the recovery of valuable metals from low-grade ores. However, this process must be carried out in a laboratory environment, taking into account the damage that can be caused by sulfuric acid, which occurs when bacteria oxidize metal sulfides. Bioleaching not only improves existing mining processes but is also considered an important step in sustainable resource management. In the future, further research is needed to increase the applicability of these microorganisms to a wider range and improve process efficiency.
References
Bioleaching Definition & Process, Anglo American, (2024.08.29)
Biohydrometallurgy, (2023), Science Direct
Dresher, W., (2004), Producing Copper Nature's Way: Bioleaching, Copper Development Association Inc. https://www.copper.org/publications/newsletters/innovations/2004/05/producing_copper_natures_way_bioleaching.html
Bioreactor, (2011), Science Direct
Flow sheet of bioleaching process of solid waste for metals recovery, ResearchGate, (2024.08.30)
Quatrini, R., Johnson, D. B., (2019), Acidithiobacillus ferrooxidans, 50 Trends in Microbiology
Valdés, J., Pedroso, I., Quatrini, R., Dodson, R. J., Tettelin, H., Blake, R., Eisen, J. A., Holmes, D. S., (2011), Acidithiobacillus ferrooxidans metabolism: from genome sequence to industrial applications, National Library of Medicine
Yang, L., Zhao, D., Yang, J., Wang, W., Chen, P., Zhang, S., Yan, L., (2019), Acidithiobacillus thiooxidans and its potential application, National Library of Medicine
Linxu, Y., Yilin, R., Jianqun, L., Xianmei, L., Xin, P., Jianqiang, L., (2012), Acidithiobacillus caldus Sulfur Oxidation Model Based on Transcriptome Analysis between the Wild Type and Sulfur Oxygenase Reductase Defective Mutant, National Library of Medicine
Sulfolobus Metallicus, Scholarly Community Encyclopedia, (2024.08.30)
Jonah, N., Hom, E., (2020), Acidithiobacillus caldus, Mikrobe Wiki
Shuang, M., Jian, S., Jianqun, L., Yuanyuan, C., Huajun, Z., Jianqiang, L., (2011), Complete genome of Leptospirillum ferriphilum ML-04 provides insight into its physiology and environmental adaptation, Springer Link
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