Microbial Leaching of Ores
Pyrite is not the only mineral oxidized by A. ferrooxidans. Metals such as copper are often present in ores as sulfides. A. ferrooxidans can convert the sulfide chalcolite (Cu2S) to covellite (CuS) to obtain energy. Copper miners take advantage of this metabolic step during the microbial leaching of low-grade ores. Cu2S is insoluble but can be converted by a series of steps (some of which involve the bacteria) to soluble Cu2+ ions. Copper metal (C0) is then recovered when water, rich in copper ions, is passed over metallic iron in a long flume (Fe0 + Cu2+ --> Cu0 + Fe2+).
In heap leaching, a dilute sulfuric acid solution is percolated through crushed low-grade ore that has been stacked on an impervious pad. The liquid coming out of the bottom of the pile, rich in copper ions, is collected and the metal is precipitated by contact with iron (as above). The liquid is then recycled by pumping it back over the pile. Three different oxidation reactions take place within the ore pile:
- Cu2S + O2 --> CuS + Cu2+ + H20 is accomplished by bacteria
- CuS + O2 --> Cu2+ + SO42- is accomplished by both chemical and biological processes
- CuS + 8Fe3+ + 4H2O -->
Cu2+ + 8Fe2+ + SO42- + 8H+ is a chemical reaction The resultant Cu2+ is recovered from the solution when it reacts with iron and the Fe2+, which enters the solution, is oxidized (again by A. ferrooxidans) to Fe3+. After oxidation this solution is delivered once again to the ore heap. The last oxidation, dependent on the bacteria, provides the Fe3+ that drives step 3.
The mining industry has increased biological leaching techniques for various reasons, including environmental concerns related to smelting, the decline in the quality of ore reserves, and difficulties in processing. This new interest has motivated increased research. We now know that ore heaps contain a much wider range of organisms than previously thought. In fact, a succession of microbial populations occurs during the leaching of sulfide minerals. Heterotrophic acidophiles belonging to the genera Acidiphilium and Acidocella are found frequently, often in close association with A. ferrooxidans. These heterotrophic species probably scavenge organic molecules that are metabolic byproducts of the chemolithotrophs. Perhaps this association is detrimental, or perhaps it helps A. ferrooxidans thrive by removing wastes.
Research continues into the composition of bacterial communities that occur naturally in bioleaching activities. Because ore heaps get quite hot during bioleaching, scientists are also asking whether novel bacteria - perhaps thermophiles from Yellowstone or deep-sea vents - might be seeded onto heaps to provide more efficient biomining.
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