BIOLOGY Volume 2 - A Guide to General Biology - 2004

12. MICROBIOLOGY AND BIOTECHNOLOGY

12.15. Biomining

Metals essential to human society—such as copper, iron, uranium, gold, lead, nickel, and cobalt—occur in nature as minerals, commonly referred to as ores. Where these ores are found in high concentrations, they are mined and the metals are extracted. Only recently has the vast potential of microorganisms begun to be harnessed in the metal purification process. To illustrate the core principles, let us examine the extraction of copper.

Copper was one of the first metals used by mankind. Bronze, an alloy of copper and tin, was first produced over 5,000 years ago; its widespread adoption—valued for its durability, cutting edge, and decorative appeal—gave the Bronze Age its name. In nature, copper typically occurs as copper sulfides. For example, over 50% of the world's production comes from chalcopyrite, CuFeS₂, which also contains iron and sulfur. Extracting copper from ore is a highly challenging process. However, it has long been known that copper can be recovered from Water seeping through copper-bearing rock formations. It is now understood that this leaching process is driven by Bacteria. These microorganisms convert insoluble metal compounds into soluble forms, such as copper sulfate, from which extracting the metal is considerably easier.

The bacterium playing the primary role in metal leaching was identified in 1947 as Thiobacillus ferrooxidans. Other key species—such as T. thiooxidans and Leptospirillum ferrooxidans—thrive in acidic environments and remain active at high temperatures. They derive their energy by oxidizing inorganic substrates. For instance, T. ferrooxidans obtains energy by oxidizing ore-bound Fe²⁺ to Fe³⁺ and reduced sulfur forms, such as sulfides, into sulfuric acid. T. ferrooxidans is an autotroph and, in biological Classification, is categorized as a chemoautotroph, or chemosynthetic bacterium (Table 2.3).

Bacterial leaching is now used worldwide as a complementary method for recovering metals from ores, primarily copper and uranium (Fig. 12.25). Several bacterial species participate in the leaching process, each making a unique contribution. In 1983, more than 10% of the copper produced in the United States, valued at over $300 million, was obtained using this technique. The advantages of bacterial leaching are as follows:

1. Low-grade ores can be utilized. Traditional metal extraction Methods are extremely costly, making it economically viable to process only high-grade ores. As a result, vast amounts of potential product were left behind in mining areas after conventional operations ceased.

2. Bacterial leaching eliminates The Need for deep underground mining. The rock is first fractured using explosive charges, after which a leaching solution is pumped into the deposit. Once leaching is complete, the solution containing dissolved metal salts is pumped out from boreholes drilled into the rock. This method is much more cost-effective and avoids the severe environmental disruption associated with deep mining, which brings massive amounts of rock to the surface and creates towering waste heaps.

3. Traditional Methods FOR extracting copper from ore require high temperatures. These processes are energy-intensive, rely heavily on fossil fuels, and consequently pollute the air, contributing to problems such as acid rain. (In the future, bacterial leaching may potentially be applied to purify fossil fuels by leaching out sulfur compounds.)

4. Uncontrolled leaching from mining waste has historically contaminated nearby water bodies with heavy metals. This can be prevented through controlled leaching and metal recovery. It is estimated that waste dumps in the western United States alone contain over 33 million tons of copper. Conventionally, waste dumps are located in valleys, allowing metals to be carried over long distances by river currents. Water containing dissolved leached metals can be collected behind a downstream dam and pumped to a Processing facility. If required, the purified water can be recycled back into the waste dump.

5. Ongoing research aims to improve bacterial strains, particularly T. ferrooxidans, using Introduction/32.html">Genetic Engineering techniques.

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Fig. 12.25. Open-pit mine in Rio Tinto (Spain), which yields 2.3 million tons of gold, silver, and copper annually.