Biological methods of metal extraction

In a nutshell

Biological methods of metal extraction are used as alternatives to traditional methods, such as mining. Biological extraction methods are advantageous as they do not require the use of high-grade copper ores.

Ores

Ores are rocks which contain useful minerals, such as metals. High-grade metal ores contain a higher concentration of metals, compared to low-grade ores. High-grade metal ores are running out. To address this shortage, scientists are exploring biological methods which exploit low-grade metal ores instead.

Low-grade metal ores are less profitable and they use more energy than high-grade ores. They also produce more waste.

Phytoextraction

Definition

Phytoextraction is a process in which useful metals are extracted from plants containing a metal compound of interest.

PROCEDURE

1.	Plants are grown in soil containing a metal compound of interest.
2.	The plants take up this metal compound via their roots.
3.	The plants are burned in the air, producing ash containing the metal compound of interest.
4.	The metal can be extracted from its metal compound by electrolysis or displacement reactions.
5.	The ions need to move freely in order for electrolysis or displacement to occur so they are dissolved in acid to form an aqueous solution of ions. This solution is called a leachate.

Example

The extraction of copper from copper sulfate using phytoextraction.

1.	Plants are grown in soil containing copper sulfate.
2.	The plants take up copper sulfate via their roots.
3.	The plants are then burned in the air, producing ash containing copper sulfate.
4.	Copper can be extracted from copper sulfate using displacement reactions or electrolysis.
5.	The copper ions ( $Cu^{2+}$ ) and sulfate ions ( $SO_4^{2-}$ ) need to move freely in order for electrolysis or displacement to occur so they are dissolved in acid to form an aqueous solution of ions. This solution is called a leachate.

Note: Phytoextraction is sometimes called phytomining.

Bioleaching

Definition

Bioleaching is the extraction of metals from low-grade metal ores using bacteria, which breaks down the ore to produce a leachate of aqueous metal ions. The metal ions can then be reduced to metal atoms using displacement reactions or electrolysis.

Pros and cons

	Phytoextraction	Bioleaching
Pros	Does not require mining (mining is harmful to the environment and costs a lot of money) Uses low-grade metal ores so high-grade metal ores can be conserved Cheaper than traditional methods	Does not require mining (mining is harmful to the environment and costs a lot of money) Uses low-grade metal ores so high-grade metal ores can be conserved Does not require high temperatures Cheaper than traditional methods
Cons	Carbon dioxide (a greenhouse gas) is emitted when the plants are burned It is much slower than traditional methods Produces less metal than traditional methods and is less profitable	Toxic substances such as sulfuric acid are produced Produces less metal than traditional methods and is less profitable It is much slower than traditional methods