Acid Mines

If you've ever been on holiday to a volcanic island, you might remember a smell of rotting eggs. That unpleasant odour is caused by sulfur compounds.

In its pure form, sulfur is a bright yellow solid at room temperature. Sulfur is essential to human life – it's used in two essential amino acids.

But when sulfur compounds are exposed by mining, they can harm human life. Exposure to air and water leads to the production of sulfuric acid, a dangerous and corrosive liquid. In this article, we will look at how sulfuric acid is produced through mining and the effects this can have.

What is Acid Mine Drainage?

First, let's recap mining.

Mining harms the environment. It causes soil erosion, sinkholes, loss of biodiversity, and Pollution (especially water pollution).

Figure 1 – Mining for resources like coal generates large amounts of waste rock. Unsplash

There are three main types of water Pollution caused by mining.

• Alkaline mine drainage, which occurs in the presence of calcite or dolomite.

• Metal mine drainage, which occurs when Lead and other heavy metals leak into waterways.

• Acid mine drainage, which occurs in the presence of sulfur.

The final type, acid mine drainage, is by far the most prevalent. So, we're going to focus on it now.

Causes of Acid Mine Drainage

Acid mine drainage occurs when pyrite (iron disulfide) is exposed during the mining process. The pyrite reacts with air and water to form sulfuric acid and dissolved iron. The iron precipitates to form red, orange, or yellow sediments. These colourful sediments are telltale signs of a polluted waterway.

Figure 2 – Acid mine drainage polluting a lake in Bolivia. Unsplash

Acid Mine Runoff

The sulfuric acid and dissolved iron act as a leachate.

As the acid runoff passes through rocks, it dissolves heavy metals such as copper, Lead, and mercury.

The heavy metal ions are moved into groundwater or surface water, where they cause further pollution.

Acid Mine Tailings

Tailing ponds are a major source of acid mine drainage.

What are tailings made of?

• Crushed rock

• Water

• Trace amounts of metal

• Additives used in the mining process

Tailing takes the form of a wet slurry. It's typically stored in tailing ponds – engineered dam and dyke facilities. Storing it in this way enables the water to separate from the solid matter. With the water left to drain, any sulfide compounds will become exposed to the natural elements.

Effects of Acid Mine Drainage

Reduced pH

When pyrite is exposed to the air, it forms sulfuric acid, a chemical with a low pH.

The pH of acid mine drainage varies from a relatively neutral 6, to a highly acidic 2.

Waterways polluted by acid mine drainage will have a lower pH than usual, affecting chemical and biological processes in the water. Acid mine drainage can be up to 300 more acidic than acid rain, killing fish and aquatic organisms:

• Acidic water causes gill mucus to thicken, restricting the amount of oxygen that fish can take in

• A low pH disrupts animal spawning and reproduction

• Amphibian's special skin, adapted for gas exchange, cannot tolerate acidic conditions

• Acidic water dissolves nutrients that aquatic plants need to survive

It's not just the ecosystem that's impacted. Acid mine drainage erodes artificial structures, such as bridges.

Solubility and Mobilisation of Toxic Metals

Acid mine runoff leaches toxic metals (and metalloids) from ore and waste rock, causing additional water pollution and harm to life. What toxic metals are commonly leached, and what problems do they cause?

Figure 3 – A major cause of heavy metal poisoning is drinking water contaminated with acid mine drainage. Unsplash

Controlling Acid Mine Drainage

Once acid mine drainage has polluted waterways, there's little that can be done. So, most control techniques focus on stopping acid mine drainage at its source.

• Minimise the surface area of waste rock that is exposed to the air.

• Stockpile waste heaps on surfaces with a low permeability, such as clay-rich soils. Compact them as much as possible to reduce exposure to oxygen and minimise rainwater seeping through.

• Cover finished waste heaps with impermeable materials to reduce rainwater leaching.

• Collect and treat runoff from the mine in lined settlement ponds. Use an alkali, such as limestone chips, to neutralise acidic material.

I hope that this article has explained acid mine drainage for you. Remember that mining exposes iron disulfide, which reacts with air and water to form sulfuric acid. This highly acidic liquid leaches toxic metals from ore and waste rock, worsening water pollution.

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^{2. Brett Baker, Lineages of Acidophilic Archaea Revealed by Community Genomic Analysis, Science, 2006}

^{3. E. Shaji, Arsenic contamination of groundwater: A global synopsis with focus on the Indian Peninsula, Geoscience Frontiers, 2021}

^{4. Jerry M. Bigham, Encyclopedia of Soil Science, Third Edition, 2016}