Biological Methods of Metal Extraction

Most metals are typically found bonded to other elements, such as oxygen. As the global demand for metals increases and metal ores become scarcer, alternative methods for extracting metals from low-grade ores are necessary.

For example, copper is an important metal, but its extraction has become increasingly challenging as copper ores grow scarcer. Therefore, extracting copper from low-grade ores has become more common.

Low-grade ores have a low concentration of metals, which makes extraction more difficult. Two alternative methods to extract metals from low-grade ores are:

  • Phytoextraction
  • Bioleaching

Both phytoextraction and bioleaching are relatively new methods that use living organisms to extract metals.

Phytoextraction

Some plants absorb metals through their roots, and phytoextraction takes advantage of this. This process involves:

1. Growing plants on soil containing the desired metal.

2. Allowing the plants to absorb the metal, which becomes concentrated in their tissues.

3. Harvesting, drying and burning the plants in a furnace.

4. Extracting the metal from the plant ash using displacement reactions or electrolysis.

  • The plant ash will contain a relatively high concentration of the metal compound

Advantages of using phytoextraction

Although phytoextraction is a slow process, it offers several benefits:

  • It can be used to extract metals that contaminate soil.
  • It can help conserve high-grade ore supplies, preventing over-mining.
  • It reduces environmental damage by not requiring the disposal of large rocks.

Bioleaching

Some bacteria are capable of breaking down ores. When combined with low-grade ore, the bacteria carry out chemical reactions that produce an acidic solution called leachate. This solution contains the desired metal ions (e.g., copper ions).

Bioleaching doesn’t require high temperatures; however, it produces toxic substances like sulfuric acid. These toxic substances need to be treated to avoid causing environmental harm.

The metal ions can be reduced to solid metal using displacement reactions or electrolysis. For example, iron is more reactive than copper, so it can displace copper from the leachate:

Iron + Copper sulfateIron(II) sulfate + Copper

Fe (s) + CuSO4 (aq)FeSO4 (aq) + Cu (s)

  • Scrap iron is used because it is cheaper than copper, therefore reducing costs.

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