Chemistry 4.8 - Biological metal extraction
Bacteria and plants can help recover metals from low-grade materials. Follow the metal into a solution or plant ash, distinguish these intermediates from pure metal, and compare the methods for a particular site. This lesson is Higher tier only.
Biological extraction
Higher tier only
Chemistry 4.8 is Higher-only. You should already know that metal compounds must be processed to recover the element.
Traditional high-temperature extraction may be uneconomic when an ore is low grade, meaning that it contains only a small proportion of the wanted metal. Biological methods can concentrate or release metal compounds from low-grade ores, contaminated soils or mining waste. They do not usually deliver pure metal as soon as the living process ends.
Bacterial extraction, often called bioleaching, uses bacteria whose activity helps release metal ions from an ore into solution. The metal-containing solution is called a leachate. The bacteria help move the metal from the rock into a recoverable liquid; they do not turn the ions directly into pieces of metal. The dissolved metal ions must then be processed to obtain the metal, for example by a suitable displacement reaction or by electrolysis.
Phytoextraction, also called phytomining in some resources, uses plants that take up metal ions through their roots. The ions become concentrated in plant tissues. The plants are harvested and usually dried and burned; the ash contains concentrated metal compounds, which then need further chemical processing to obtain the metal. For example, copper compounds can be dissolved from the ash, then copper recovered from the resulting solution. Burning removes much of the organic plant material; it does not make the ash pure copper.
Choosing a biological route
| Criterion | Potential advantage | Limitation or trade-off |
|---|---|---|
| Ore grade | Can recover useful material from low-grade ores or waste that may not justify conventional extraction. | The final leachate or ash may still contain a low concentration and need further processing. |
| Energy and temperature | Biological stages can operate at lower temperatures, reducing direct heating demand. | Recovering the metal from solution or ash still uses chemicals or electricity. |
| Speed | Slow treatment may be acceptable when there is no urgent production deadline. | Bacterial action and plant growth are much slower than high-temperature processing. |
| Land and containment | Phytoextraction can treat metal-contaminated soil where it lies, reducing further excavation. | Plants need suitable land and growing conditions; leachates and contaminated biomass must be contained and handled safely. |
| Scale and cost | Lower-grade resources may become usable and energy costs may fall. | Long processing times, monitoring, harvesting and final recovery all add cost. |
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