Biological methods of metal extraction - Higher
The Earth's supply of metal oreA rock containing enough quantities of a mineral for extraction to be possible. is limited. For example, high-grade copper ores, which contain a high percentage of copper, are becoming harder to find and mine. There are some alternative methods to extract metalShiny element that is a good conductor of electricity and heat, and which forms basic oxides. from low-grade copper ores that use living organismLiving entity, eg animals, plants or microorganisms.. These have advantages and disadvantages compared to the usual extractionThe process of obtaining a metal from a mineral, usually by reduction or electrolysis. methods.
Phytoextraction (phytomining)
Plants absorb mineral ionElectrically charged particle, formed when an atom or molecule gains or loses electrons. through their roots. phytoextraction/phytominingUsing plants to absorb metal compounds from the ground through their roots. The plants are then burned to produce an ash containing a high concentration of the metal compounds. makes use of this:
- plants are grown in soil that contains low grade ore
- the plants absorb metal ions through their roots and concentrate these ions in their cells
- the plants are harvested and burnt
- the ash left behind contains metal compoundA substance formed by the chemical union of two or more elements.
Phytoextraction is slow but it:
- reduces the need to obtain new ore by miningWhen natural resources are extracted from the ground.
- conserves limited supplies of high-grade ores
- reduces the amount of rock waste that must be disposed of after traditional mining
Worked example
Suggest reasons why phytoextraction reduces damage to the environment.
Mining for metal ores involves quarryAn area of land that is used to remove rocks for use in industries. which are large holes in the ground. These create noise, dust and traffic. They also destroy natural habitatA place where plants, animals and microorganisms live.. Phytoextraction reduces the need for mining and reduces this damage.
Question
Give one disadvantage of phytoextraction.
It is a slow process.
Bioleaching
Certain bacteriaSingle-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. can break down low-grade ores to produce an acidicHaving a pH lower than 7.solutionMixture formed by a solute and a solvent. containing copper ions. The solution is called a leachate and the process is called bioleachingUsing bacteria to extract metals from their ores..
Bioleaching does not need high temperatures but it:
- produces toxicPoisonous. substances, including sulfuric acid, which damage the environment
Processing the metal compounds
Iron is more reactive than copper. It can displaceTake the place of another substance in a chemical reaction. For example, a metal can displace a less reactive metal from its oxide, removing oxide ions from the less reactive metal and becoming an oxide itself. copper from the leachate. For example:
iron + copper sulfate → iron(II) sulfate + copper
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
Since iron is cheaper than copper, the use of scrap iron is a cost-effective way to produce copper from the leachate.
Alternatively, the copper compounds can be dissolved and the solution electrolysisThe decomposition (breakdown) of a compound using an electric current. to produce copper metal.
