Acidithiobacillus ferrooxidans is only one of the amazing bacteria used in biometallurgy. In an age of environmental concerns, innovative methods for metal extraction today include using the natural talents of busy microbes like bacteria and fungi.
Some fungi and bacteria are happily at home in toxic environments. Known as extremophiles, they can survive severe acidic or toxic conditions. Sometimes, they miraculously produce more than their weight in gold, like Cupriavidus metallidurans bacterium does.
Biometallurgy: Bioleaching and Biomining
Bioleaching specifically refers to the process of using microorganisms to extract metals from their ores or concentrates. The bacteria oxidize ferrous iron or sulfide minerals, which solubilizes the metals in solution. Bioleaching solubilizes metals using microbes.
It specifically involves microorganisms who dissolve metals from ores. Bacteria and archaea are important in breaking down minerals containing valuable metals such as copper, gold, and uranium. Bioleaching recovers more than 90% of copper from low-grade ores.
There are two main types of bioleaching. The first involves utilizing microorganisms to oxidize refractory minerals in order to extract valuable metals like gold and silver. Typically, the minerals targeted for oxidation are pyrite and arsenopyrite.
The second type involves leaching sulfide minerals to extract the corresponding metal, such as extracting nickel by leaching pentlandite, or extracting copper by leaching chalcocite, covellite, or chalcopyrite.
Biomining covers bioleaching and broader application of biological processes in mining, including introduction of microorganisms for ore beneficiation and metal recovery. While prokaryotes are commonly used, fungi and plants work in phytoextraction or phytomining.
Biomining methods are applied in ore processing, recovery of metals and environmental cleanup. Current application of biomining is in treating mining waste containing iron, copper, zinc, and gold to recover valuable metals.
Acidithiobacillus ferrooxidans
Acidithiobacillus ferrooxidans is a chemolithoautotrophic bacterium, or microbe which can get energy by oxidizing inorganic substrates. It's known primarily for its role in the bioleaching of metals, especially copper and gold.
Acidithiobacillus ferrooxidans is the first biometallurgical bacteria discovered, in 1951. This bacterium is an extremophile, thriving in highly acidic environments. Its ability to oxidize iron and sulfur compounds enables extraction of metals from ores.
Characteristics
Morphology: A. ferrooxidans is rod-shaped and can be found in varied cellular morphologies, from single cells to more filamentous forms. It can form colonies and biofilms on mineral surfaces.
Acidity: It enjoys acidic conditions, with an optimum pH range of 1.5 to 2.5, making it adept at surviving in environments too harsh for many other organisms.
Oxygen Requirement: As an aerobic organism, it requires oxygen for its metabolic processes, contributing to its efficiency in oxidizing metal sulfides.
Autotrophy is a unique form of metabolism found only in bacteria. Inorganic compounds are oxidized directly (without using sunlight) to yield energy. The bacterium derives energy from oxidation of iron and sulfur compounds.
A. ferrooxidans mainly feeds on ferrous iron (Fe2+) and reduced sulfur compounds, such as thiosulfate. Acidithiobacillus ferrooxidans is commonly found in extreme environments, including:
Sulfidic mineral deposits.
Acidic mine drainage sites (AMD).
Hot springs with acidic waters.
Natural mineral deposits.
Reproduction
This bacterium reproduces asexually through binary fission, allowing it to multiply rapidly in favorable conditions. Under optimal circumstances, A. ferrooxidans can double its population every 12 to 24 hours, making it highly effective for ongoing bioleaching processes.
Acidithiobacillus ferrooxidans in Biometallurgy
A. ferrooxidans catalyzes oxidation of pyrite (FeS₂) and other metal sulfides. It extracts metals like copper, gold, and uranium from low-grade or waste ores.
Here is a typical process:
Crushed Ores: The ore is crushed and then mixed with water to create a slurry.
Microbial Activity: A. ferrooxidans is introduced to oxidize the ferrous iron and sulfur from the ore.
Solution Extraction: As the microorganism works, it generates ferric iron, which further helps extraction of metals into the solution.
Metal Recovery: The metals can then be recovered from the solution through various methods, including precipitation and solvent extraction.
In biometallurgy, Acidithiobacillus ferrooxidans is crucial for biotechnological extraction of valuable metals. Through its unique metabolic pathways, this bacterium transforms insoluble metal compounds into soluble forms.
When applied in mining operations, A. ferrooxidans is often introduced to heap leach piles, where mineralized rock is stacked and sprayed with nutrient solutions. The bacterium colonizes the metal-containing minerals.
It oxidizes ferrous iron and promotes solubilization of metals like copper and gold. Using A. ferrooxidans increases gold recovery rates by up to 80%. This process extracts valuable materials and also mitigates environmental impact compared to traditional mining techniques.
Examples of Biometallurgy or Biomining Using Microbes
Microbial biometallurgy is not solely confined to Acidithiobacillus ferrooxidans. Other bacteria and fungi include:
1. Fungi in Gold Recovery
Certain fungi, such as Fusarium oxysporum, can effectively recover gold from ores. They use biolomic processes to selectively dissolve gold using organic acids, which can boost recovery rates from low-grade ores significantly. Aspergillus niger can solubilize metals like gold and uranium through acid production and biosorption.
2. Cyanobacteria and Uranium Extraction
Cyanobacteria, often called blue-green algae, can bioleach uranium from contaminated sites. These organisms produce byproducts that help solubilize uranium, aiding cleaner extraction technologies.
Leptospirillum ferrooxidans is another iron-oxidizing bacterium utilized in copper bioleaching, similarly to A. ferrooxidans. Rhodococcus sp. are known for their ability to recover valuable metals from electronic waste through biosorption.
3. Leptospirillum and Copper Biomining
Leptospirillum species are bacteria thriving in acidic conditions. They contribute to copper extraction via bioleaching. By oxidizing ferrous iron, they enhance the solubilization of copper minerals, for effective metal recovery.
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