Metal Extraction By Biological Methods (HT Only) (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

Biological extraction methods

Higher Tier Only

• Copper ores are becoming increasingly scarce, making it necessary to find new ways to extract copper from low-grade ores

• Phytomining and bioleaching are two alternative methods that can extract metals from low-grade ores

• Both methods have environmental advantages and disadvantages compared to traditional mining

Advantages of biological extraction

• High-grade copper ores are becoming scarce

  • Biological methods allow extraction from low-grade ores, conserving finite supplies

• Avoid the need for large-scale mining, which means:

  • Less dust

  • Less noise

  • Less visual pollution

• Traditional mining burns fossil fuels:

  • This releases CO₂, which contributes to global warming

  • It also releases SO₂, which causes acid rain

  • Biological methods produce far fewer emissions

• Traditional mining destroys wildlife habitats and scars the landscape

  • Biological methods largely avoid this

• Can also be used to remove toxic metals from mining waste sites

Disadvantages of biological extraction

• Both methods are very slow

• Metal compounds produced by both methods require further processing to extract the actual metal

• Bioleaching produces toxic substances that must be treated to prevent contamination of the environment

Phytomining

• Some plants are able to absorb metal compounds through their roots when grown in soil containing metals of interest

• As the plants grow, metal compounds become concentrated in their shoots and leaves

• The plants are harvested, dried and burned

• The resulting ash contains metal compounds from which the metal can be extracted

Bioleaching

• Bioleaching uses bacteria to extract metals from ores

• Certain bacteria break down metal ores to produce an acidic solution containing metal compounds

  • This is called a leachate

• The leachate contains significant quantities of metal compounds which can then be processed to obtain the metal

  • This method is often used to extract metals from sulfides e.g. CuS or FeS

• Bioleaching is not only used for the primary extraction of metals, but it is also used in mining waste clean up operations

Processing

Obtaining the metal

• The metal compounds in solutions produced by phytomining and bioleaching must be processed to obtain the pure metal

• Copper can be obtained from solutions of copper compounds by two methods:

Displacement using scrap iron

• Iron is more reactive than copper, so it displaces copper from solution

• This is a displacement reaction (also called a redox reaction)

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

• A pink/brown solid (copper) is deposited, and the blue colour of the copper sulfate solution fades as iron sulfate forms

Electrolysis

• The solution can also be electrolysed to deposit copper

• At the cathode (negative electrode):

  • A pink/brown solid is deposited

  • This is the copper ions being reduced to copper metal

  • Copper deposits at the cathode rather than hydrogen because copper is less reactive than hydrogen

• At the anode (positive electrode):

  • Bubbles are produced

  • This is oxygen or chlorine gas being released, depending on the solution