Acid mine drainage — AQA A-Level Environmental Science

Acid mine drainage (AMD) is a major chemical pollutant stream resulting from extractive industries, specifically metal ore and coal mining. When previously deep, anoxic deposits containing sulphide minerals—most notably iron pyrite (FeS2)—are fractured and exposed to water and atmospheric oxygen, chemical and biological oxidation generates dilute sulphuric acid.

This low-pH discharge leaches hazardous heavy metals from surrounding geology, creating a toxic, highly acidic effluent that causes severe, long-term degradation of freshwater aquatic habitats. Within AQA A-Level Environmental Science, this topic synthesises concepts from mineral extraction, aquatic pollution, biotic indices, and chemical/biological remediation strategies.

• →Pyrite Oxidation Chemistry: 2FeS2 + 7O2 + 2H2O -> 2Fe2+ + 4SO4(2-) + 4H+. The reaction is autotrophic and heavily catalysed by extremophilic acidophilic bacteria such as Acidithiobacillus ferrooxidans.
• →Heavy Metal Mobilisation: Acidic water shifts metal equilibria, solubilising harmful cations (e.g. Al3+, Cd2+, Pb2+, Cu2+) that are normally locked within insoluble mineral matrices at neutral pH.
• →Benthic Smothering ('Yellow Boy'): As acidic effluent enters higher-pH river systems downstream, iron precipitates out as insoluble iron(III) hydroxide (Fe(OH)3), coating riverbeds in thick orange flocculant and smothering benthic biota.
• →Remediation Methods: Active treatment relies on chemical dosing (e.g. crushed limestone/slaked lime addition to raise pH), whereas passive treatment utilizes constructed anaerobic reed beds and limestone drains.

• Explains that low pH increases the solubility of metal ions such as Fe²⁺, Cu²⁺, Zn²⁺, Pb²⁺ and Al³⁺.
• States that increased solubility allows toxic metals to move from solid rock or sediment into the water.
• Describes mobilisation as the downstream transport of dissolved metals away from the mine site.
• Links mobilised toxic metals to contamination of water supplies, sediments and harm to organisms.

• 💡Trace the chain: low pH, then increased solubility, then mobilisation, then toxic impact.
• 💡Use correct ion notation such as Cu²⁺ and Al³⁺ when naming mobilised metals.
• 💡Reject options that describe metals remaining immobile or being removed from solution by the acid.
• 💡Always reference both physical and chemical consequences: do not just state that the water becomes acidic, explicitly explain the precipitation of ferric hydroxide ('yellow boy') and metal ion mobilisation.
• 💡Use proper chemical nomenclature when discussing remediation; distinguish between active additions (calcium carbonate, calcium hydroxide) and passive biological systems (constructed wetlands containing sulphate-reducing bacteria).

• Assumes metals stay locked in rock regardless of pH; state that low pH increases their solubility and release.
• Confuses mobilisation with deposition at the source; describe mobilisation as transport of dissolved metals away from the site.
• Names only one metal as the sole toxic species; recognise that several metal ions become more soluble at low pH.
• Misconception: Acid mine drainage only occurs while a mine is actively working. Correction: AMD is frequently worse in abandoned, flooded underground mines where unmanaged spoil heaps and exposed adits continuously leach acid for decades or centuries after operations cease.
• Misconception: Crushed limestone completely cleans AMD effluents. Correction: While limestone neutralises sulphuric acid and raises pH, toxic heavy metal precipitates remain as contaminated sludge that must be physically settled, filtered, and safely disposed of.

1. 1Step 1: Memorise the reactants and conditions of pyrite oxidation, including the bacterial action of Acidithiobacillus ferrooxidans.
2. 2Step 2: Diagram the chain of ecological effects: low pH -> metal mobilisation -> gill damage -> benthic sedimentation ('yellow boy') -> loss of sensitive macroinvertebrates.
3. 3Step 3: Compare and contrast active remediation (limestone dosing) vs passive remediation (wetlands, anoxic limestone drains) in terms of cost, maintenance, and long-term viability.
4. 4Step 4: Practice quantitative exam questions involving water treatment volumes, neutralization stoichiometries, and interpretation of biotic index data.

• 📋Short chemical explanations (2-3 marks): Outlining how pyrite exposure and bacterial catalysts form sulphuric acid.
• 📋Ecological impact evaluations (4-6 marks): Assessing the trophic impacts of low pH, metal toxicity, and ferric hydroxide deposits on stream communities.
• 📋Remediation comparison questions (6-9 marks): Evaluating active chemical treatment versus passive constructed wetland strategies.