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.
The human body needs 20 different amino acids to maintain good health and functioning. We can synthesise 11 of these acids, but we must get the other 9 from food. The acids that we do not synthesise ourselves are known as 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 is the extraction of minerals and other geological deposits from Earth.
Mining has existed since the prehistoric era with Flint being first mined by prehistoric people as it splits into shards with sharp edges, make it perfect for tools and weapons. These days Coal, iron and bauxite are among the most mined materials.
Minerals are the name given to these valuable materials that are extracted from the ground. Often, the pure form of the mineral is mingled with other undesirable rocks and minerals in the soil so the desired mineral needs to be isolated from this mixture by processing and refinement after they have been removed from the mine.
Surface mining and underground mining are the two main subcategories of contemporary mining.
In surface mining, the ground is blasted to make it possible to take ores that are close to the Earth's surface. These are then taken to refineries where the minerals can be extracted. Ores are obtained from deep within the earth by underground mining. To get to the mineral deposits, miners carve tunnels through the rock.
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.
Acid mine drainage is the formation and movement of acidic water rich in heavy metals.
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
If you're a chemistry buff, you may be interested to know the chemical equation for the breakdown of pyrite into sulfuric acid and dissolved iron. But if you're not, don't worry, you won't need to know it for your exams.
2FeS2 + 7O2 + 2H2O → 2FeSO4 + 2H2SO4
iron disulfide + oxygen + water → iron sulfate + sulfuric acid
Acid Mine Runoff
The sulfuric acid and dissolved iron act as a leachate.
A leachate is a liquid that has percolated through a solid and leached out some of the constituents.
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.
Tailings are mine waste, left over after the target mineral has been extracted from the ore.
What are tailings made of?
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
The effects of acid mine drainage are long-lived. It can occur for decades after mining activity has ceased.
Acid mine drainage is still leaching from European mines worked by the Romans over 1500 years ago.
Reduced pH
When pyrite is exposed to the air, it forms sulfuric acid, a chemical with a low pH.
pH is an expression of hydrogen ion concentration in a solution.
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.
Some of the most acidic water ever recorded was found in the Richmond Mine, California. It was so acidic that the pH was reported as -3.6, far more acidic than battery acid, and had been known to catch fire! The bacteria and archaea living in the mine are of scientific interest because they can survive in such an extreme, hostile environment. Three novel species of archaea were discovered at the mine in 2006, found nowhere else on Earth!
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?
| Metal | Problems |
| Arsenic | Arsenic contamination of groundwater can pollute human drinking water, leading to cardiovascular disease if ingested. Long-term exposure can cause cancer and skin lesions. Arsenic contamination of water is estimated to affect 500 million people worldwide. |
| Cadmium | Exposure to toxic cadmium affects organ systems and causes cancer. It can also interact with calcium, leading to osteoporosis and other bone conditions. |
| Copper | Ingesting large amounts of copper, usually through drinking water, leads to nausea, vomiting, and abdominal pain. Continuous exposure can cause liver and kidney damage. |
| Chromium | Chromium affects plants; it reduces growth, yield, and quality of crops. Continuous exposure by humans leads to respiratory tract conditions. |
| Iron | Mild symptoms of iron toxicity include nausea, vomiting, and drowsiness. Severe symptoms include tachypnea (rapid shallow breathing), seizures, and coma. |
| Lead | Lead pollution disrupts plant growth and soil communities. In humans, it can cause organ damage. Young children are especially at risk, often suffering intellectual disabilities as a result of lead poisoning. |
| Manganese | High levels of manganese in the soil impact plant health. In humans, poisoning can lead to a permanent neurological disorder called manganism. Symptoms include tremors, motor difficulties, and hallucinations. |
| Zinc | When ingested in excess, zinc can cause various health problems, including organ damage, neurological problems, and fungal sepsis. |
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.
Acid Mines - Key takeaways
- Mining harms the environment, causing water pollution. The most prevalent kind of water pollution is acid mine drainage, the formation and movement of acidic water rich in heavy metals.
- Acid mine drainage occurs when pyrite is exposed to water and air, where it forms sulfuric acid and dissolved iron. The iron precipitates to form colourful sediments. Acid mine drainage acts as a leachate, dissolving heavy metals.
- Tailing ponds are a major source of acid mine drainage.
- Acid mine drainage causes reduced pH in bodies of water, harming aquatic life. It also causes the solubilisation and mobilisation of toxic metals, which can harm plants and humans.
- Techniques for controlling acid mine drainage include minimising surface area of waste rock, preventing permeation into the soil, compacting waste, covering finished waste heaps, and treating runoff with lime to neutralise an acidic material.
1. Alaskans for Responsible Mining, Mining: Environmental Impacts, 2004
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
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