ALTERNATIVE METHODS use biological organisms (bacteria or plants) to concentrate metals from low-grade ores or contaminated soils — more sustainable and less damaging.
Two main alternatives:
1. PHYTOMINING — using plants.
2. BIOLEACHING — using bacteria.
Phytomining
PHYTOMINING uses PLANTS to absorb metal ions from low-grade ores or contaminated land.
PROCESS:
1. Hyperaccumulator plants are grown on soil containing low concentrations of metal compounds.
2. The plants absorb metal ions through their roots and CONCENTRATE them in their shoots and leaves.
3. The plants are HARVESTED and BURNED (combustion/incineration).
4. The ash (called BIO-ORE or phytomass) contains a concentrated form of the metal compound.
5. The metal is then extracted from the ash by traditional methods (smelting or electrolysis).
EXAMPLES:
Plants like Thlaspi caerulescens and Alyssum species accumulate nickel and zinc.
Used at former mining sites with contaminated soil.
ADVANTAGES:
Uses low-grade ores that would otherwise be uneconomical.
Remediation — cleans up contaminated land as a by-product.
Lower environmental impact than open-cast mining.
Carbon-neutral in principle (plants absorb CO₂ while growing).
DISADVANTAGES:
VERY SLOW — takes growing seasons (months to years).
Limited to certain metals that plants can hyperaccumulate.
Yield is low compared to traditional mining of rich ores.
Bioleaching
BIOLEACHING uses BACTERIA to extract metal ions from low-grade sulfide ores.
PROCESS:
1. Bacteria are applied to crushed low-grade ore in large heaps or tanks.
2. Bacteria oxidise the metal sulfide compounds — using them as an energy source.
3. The metal ions are LEACHED (dissolved) into the acidic solution produced.
4. The LEACHATE (metal-rich acidic solution) is collected.
5. Metal is recovered from the leachate, e.g. by adding scrap iron to DISPLACE copper:
Copper sulfate leachate → displacement with iron → copper metal deposited.
ADVANTAGES:
Extracts copper (and other metals) from low-grade ores uneconomical to smelt.
Lower energy than smelting.
Lower environmental impact than open-cast mining for low-grade ores.
DISADVANTAGES:
VERY SLOW — bacteria work slowly.
Produces ACIDIC WASTE that must be contained and treated.
Risk of acid leaking into groundwater.
Bacteria sensitive to conditions (temperature, pH).
⚠️ Common Mistake
In phytomining, plants are BURNED after harvesting — the metal is in the ASH, not extracted directly from the plants. In bioleaching, bacteria produce a SOLUTION (leachate) containing dissolved metal ions — the metal must then be recovered from this solution, e.g. by displacement or electrolysis.
CuS + O₂ + H₂SO₄ → CuSO₄ + S + H₂O (bacterial oxidation in bioleaching)
📌 Key Note
Phytomining: hyperaccumulator plants absorb metal ions → harvest and burn → metal in ash. Bioleaching: bacteria oxidise metal sulfide ores → metal ions leach into acidic solution → recover by displacement or electrolysis. Both: slow but lower impact, work on low-grade ores. Bioleaching produces acidic waste.
🎯 Matching Activity — Phytomining vs Bioleaching
Match each feature to phytomining or bioleaching. — drag the symbols on the right to match the component names on the left.
Phytomining
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Bioleaching
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Both methods
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Bioleaching
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Phytomining
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Plants absorb metal ions from soil → harvested and burned → metal recovered from ash
Also remediates contaminated land — cleans up old mining sites as a by-product
Used for low-grade ores that are uneconomical to smelt by traditional methods
Produces acidic leachate that must be treated — risk of groundwater contamination
Bacteria oxidise metal sulfide ores → metal ions dissolve into leachate solution
🎯 Test Yourself
Question 1 of 2
1. In phytomining, where is the metal found after the plants are harvested?
2. After bioleaching, copper ions are in solution. How can copper metal be recovered from this leachate cheaply?
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