Phytomining: extracting copper with plants, and bioleaching with bacteria

Phytomining uses plants to extract metals, especially copper, from low-grade ores. The plants absorb metal compounds from the soil as they grow; they are then harvested and burned, and the ash contains the metal compounds. The metal is then obtained from a solution of the ash by displacement with scrap iron or by electrolysis. It is one of two biological methods in GCSE Chemistry, alongside bioleaching.

Why we need new ways to extract copper

AQA's GCSE Chemistry specification (4.10.1.4, higher tier) explains that the Earth's resources of metal ores are limited and copper ores are becoming scarce. New ways of extracting copper from low-grade ores, which contain only small amounts of copper, include phytomining and bioleaching. These methods avoid traditional mining, with its digging, moving and disposing of large amounts of rock.

How phytomining works

• Grow plants on land containing low-grade copper ore. The plants absorb copper compounds through their roots.
• Harvest and burn the plants. The ash produced contains the copper compounds in a concentrated form.
• Dissolve the ash in acid to make a solution of a copper compound, such as copper sulfate.
• Obtain the copper by displacement with scrap iron or by electrolysis.

How bioleaching works

Bioleaching uses bacteria. The bacteria produce leachate solutions that contain metal compounds, and the metal is then obtained from the solution in the same ways: displacement or electrolysis.

Getting copper from the solution

Displacement with scrap iron

Iron is more reactive than copper, so it displaces copper from a solution of its compound. Scrap iron is cheap and readily available.

As an ionic equation: Fe + Cu²⁺ → Fe²⁺ + Cu. Iron atoms lose electrons (oxidation) and copper ions gain them (reduction).

Electrolysis

Passing a current through the copper compound solution deposits pure copper at the negative electrode (cathode): Cu²⁺ + 2e⁻ → Cu.

Advantages and disadvantages

AQA asks you to evaluate these biological methods given information, so use the data in the question: compare the time, the amount of copper, the energy needed and the effect on the environment.

Exam questions with answers

Describe how phytomining is used to obtain copper. (4 marks)

Plants are grown on land containing low-grade copper ore and absorb copper compounds. The plants are harvested and burned, and the ash contains copper compounds. The ash is dissolved to make a solution. Copper is then obtained by displacement using scrap iron, or by electrolysis.

Why is iron used to obtain copper from copper sulfate solution? (2 marks)

Iron is more reactive than copper, so it displaces copper from the solution. Scrap iron is cheap.

Common mistakes

• Mixing up the two methods. Phytomining uses plants; bioleaching uses bacteria.
• Missing the burning step. The plants are burned to ash before the copper is extracted.
• Saying the plants produce pure copper. They absorb copper compounds; the metal comes later.
• Saying copper displaces iron. The more reactive metal, iron, displaces copper.

Practise with a short timed paper

Phytomining and bioleaching are in GCSE Chemistry (higher tier). The daily GCSE Chemistry Higher paper is a short timed paper of 8 questions, with a new one every day.

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Chemistry · Reactions and electrolysisBioleaching: how bacteria extract copper from oresWhat bioleaching is, how bacteria extract copper from low-grade ores, how the copper is recovered, and its advantages and disadvantages for GCSE Chemistry.