Phytomining and Bioleaching: GCSE Chemistry

Why are alternative extraction methods needed?

Traditional mining and smelting of metals like copper requires high-grade ores — rock containing a high percentage of the metal compound. As high-grade deposits are exhausted, miners must process increasingly lower-grade ores, which requires moving and processing far more rock per tonne of metal. This raises costs and environmental impact.

Copper, in particular, is facing this challenge. Global copper demand is rising (electric vehicles, wind turbines, electronics all require copper), yet many of the richest ore bodies have been mined for centuries. Phytomining and bioleaching make it economically and environmentally viable to extract metals from:

• Low-grade ores (too little metal to justify traditional smelting costs)
• Mining spoil heaps and tailings (waste rock left behind from earlier mining)
• Metal-contaminated soils (land polluted by industrial activity)

How does phytomining work?

Phytomining exploits plants called hyperaccumulators — species that naturally absorb unusually high concentrations of specific metal ions from the soil through their roots.

The process:

1. Hyperaccumulator plants (e.g. Thlaspi caerulescens for zinc and cadmium; Berkheya coddii for nickel) are grown on the metal-containing soil.
2. The plants absorb metal ions through their roots and concentrate them in their leaves and shoots. Some hyperaccumulators can accumulate thousands of parts per million of a metal — concentrations far higher than in the surrounding soil.
3. The plants are harvested and dried to reduce water content.
4. The dried plant material is burned (incinerated). The metals cannot burn, so they remain in the ash (called bio-ore).
5. The metals are extracted from the ash using conventional smelting or chemical methods, but from a far richer starting material than the original soil.

For copper, species such as Haumaniastrum robertii accumulate significant copper concentrations and have been trialled in the Democratic Republic of Congo and Zambia.

How does bioleaching work?

Bioleaching uses bacteria (typically acidophilic bacteria such as Acidithiobacillus ferrooxidans) to dissolve metal compounds in ores.

The process:

1. Low-grade ore is crushed and heaped into a pile (heap leaching) or placed in a bioreactor.

2. Acidic water containing bacteria is sprinkled over or through the ore.

3. The bacteria oxidise metal sulfide compounds in the ore. For copper sulfide (CuS or Cu₂S):

   • Bacteria catalyse: Cu₂S + O₂ + H₂SO₄ → 2Cu²⁺(aq) + S + H₂O (simplified)
   • The copper ions (Cu²⁺) dissolve into the acidic leachate solution.

4. The leachate solution (containing Cu²⁺ ions) is collected.

5. Copper metal is recovered from the leachate, typically by:

   • Displacement with scrap iron: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) — copper deposits on the iron.
   • Electrolysis of the copper sulfate solution (preferred for high purity).

Comparison table: extraction methods

What are the advantages and limitations of these methods?

Advantages:

• Use less energy than smelting (no high-temperature furnaces required).
• Allow previously uneconomic deposits to be worked.
• Phytomining can clean up metal-contaminated land (phytoremediation), turning a liability into a resource.
• Bioleaching produces less sulfur dioxide than roasting sulfide ores in a furnace.

Limitations:

• Both methods are slow compared to conventional extraction (months to years vs days for smelting).
• Bioleaching produces acidic leachate that must be managed carefully to prevent contamination of groundwater.
• Phytomining is weather-dependent and restricted to climates where the hyperaccumulator plants grow.
• Yields are lower per unit time than traditional methods — not suitable for high-demand, urgent supply situations.

Frequently asked questions

Which metals can be extracted by bioleaching?

Bioleaching has been demonstrated commercially for copper (the most common application), gold (from low-grade gold sulfide ores), cobalt, nickel, zinc, and uranium. Copper bioleaching accounts for approximately 10–20% of global copper production. Gold bioleaching pre-treats refractory ores (where the gold is locked inside sulfide minerals) before cyanide extraction. Research continues into expanding the range of metals and improving bacterial strains.

How is the copper actually separated from the leachate solution?

Two methods are used: displacement with scrap iron (iron is higher in the reactivity series than copper, so iron displaces copper from solution: Fe + Cu²⁺ → Fe²⁺ + Cu), which is cheap but produces lower-purity copper. Electrolysis of the copper sulfate leachate (using a copper anode and stainless steel cathode) deposits pure copper at the cathode and is the preferred method when high purity is needed for electrical uses. The combined solvent extraction–electrowinning (SX-EW) method is standard in commercial bioleach operations.

Is phytomining the same as phytoremediation?

Not exactly. Phytoremediation is the use of plants to remove pollutants from contaminated soil — the goal is clean-up of the land. Phytomining additionally involves harvesting and burning the plants to recover the metal as a product. Phytomining is therefore phytoremediation with an economic return. The same hyperaccumulator plants and techniques are used, but in phytomining the metal-rich bio-ore (ash) is the valuable output rather than just the cleaner soil.

Why are the bacteria in bioleaching acid-tolerant?

The bioleaching process requires an acidic environment (pH 1–3) because the bacteria produce sulfuric acid as a by-product of oxidising sulfide minerals. Ordinary bacteria cannot survive at this pH, but acidophilic (acid-loving) bacteria such as Acidithiobacillus ferrooxidans have evolved to thrive in it. The acid also helps dissolve the metal ions into the leachate. This is why bioleach heaps are inherently acidic environments, and why controlling acid drainage to prevent it entering waterways is a key engineering challenge.

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Professor Curie at aitutors.me can take you through the particle model of metal extraction, test you on displacement reactions, and build your six-mark answer on alternative extraction methods.