11.6
Acid Mine Drainage
Mining exposes pyrite to oxygen in the air and water from rain or groundwater, initiating a series of oxidation reactions that release ferrous iron.
This reaction also produces sulfuric acid, which leaches into the surrounding ecosystems and damages them.
Certain acid-loving bacteria grow well on mine rocks and speed up the pyrite oxidation.
For example, Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans oxidize the ferrous iron into ferric iron.
The resulting ferric iron then accelerates the oxidation of more pyrite, creating a self-sustaining cycle that continues to generate sulfuric acid.
As the acidic runoff flows into oxygen-rich streams, more ferrous iron is oxidized into ferric iron.
Ferric iron reacts with water to form insoluble iron hydroxide, often seen as an orange or reddish sludge.
When this acidic water reaches rivers, it further dissolves toxic metals, such as aluminum and cadmium, harming aquatic life.
Mining activities that disturb sulfide-rich rocks, particularly those containing pyrite (FeS₂), initiate a cascade of geochemical and microbiological processes with serious environmental implications. When exposed to air and water, pyrite undergoes oxidation, releasing sulfate, ultimately forming sulfuric acid and mobilizing heavy metals into surrounding water systems. This phenomenon, known as acid mine drainage (AMD), results in low pH waters laden with toxic elements that threaten aquatic ecosystems and water quality.
Pyrite Oxidation and Microbial Acceleration
The abiotic oxidation of pyrite initiates with the exposure of FeS₂ to molecular oxygen and water, yielding ferrous iron (Fe²⁺) and eventually sulfuric acid (H₂SO₄). However, this process is significantly accelerated by acidophilic bacteria, such as Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans, which thrive in the acidic conditions typical of mine sites. These microbes oxidize ferrous iron into ferric iron (Fe³⁺), which acts as a potent oxidizing agent of pyrite, perpetuating the cycle of acid generation and metal release.
The simplified microbial-mediated reaction can be expressed as:
4Fe2+ + O2 + 4H+ → 4Fe3+ + 2H2O
Ferric iron continues to oxidize pyrite even in the absence of oxygen:
FeS2 + 14Fe3++ 8H2O → 15Fe2+ + 2SO42− + 16H+
Formation of Iron Hydroxide Sludge and Role of Archaea
As AMD flows into oxygen-rich aquatic environments, ferrous iron is further oxidized abiotically to ferric iron, which hydrolyzes to form ferric hydroxide:
Fe3+ +3H2O → Fe(OH)3↓ + 3H+
This reaction produces a yellowish-red precipitate known as “yellow boy,” which settles in streams and smothers benthic habitats. Extreme AMD environments also harbor acidophilic archaea such as Ferroplasma and Thermoplasma, which maintain and amplify metal oxidation under high-temperature and ultra-acidic conditions.
Environmental Impact
When acid-laden mine waters reach downstream ecosystems, they significantly lower pH levels and enhance the solubility and mobility of toxic metals like aluminum and cadmium. This leads to bioaccumulation and acute toxicity in aquatic organisms, disrupting food webs and reducing biodiversity. The persistent nature of AMD and its self-sustaining feedback loops pose long-term challenges for mine remediation and environmental protection.
Mining exposes pyrite to oxygen in the air and water from rain or groundwater, initiating a series of oxidation reactions that release ferrous iron.
This reaction also produces sulfuric acid, which leaches into the surrounding ecosystems and damages them.
Certain acid-loving bacteria grow well on mine rocks and speed up the pyrite oxidation.
For example, Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans oxidize the ferrous iron into ferric iron.
The resulting ferric iron then accelerates the oxidation of more pyrite, creating a self-sustaining cycle that continues to generate sulfuric acid.
As the acidic runoff flows into oxygen-rich streams, more ferrous iron is oxidized into ferric iron.
Ferric iron reacts with water to form insoluble iron hydroxide, often seen as an orange or reddish sludge.
When this acidic water reaches rivers, it further dissolves toxic metals, such as aluminum and cadmium, harming aquatic life.
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