Acid mine drainage has become a serious problem and a major water and environmental contaminator in South Africa and, in fact, the world.
What is acid mine drainage (AMD)?
Acid rock drainage may occur naturally in certain areas due to rock weathering but is exacerbated by large-scale earth disturbances associated with mining activities. Acid mine drainage is caused when water flows over sulphur-bearing rocks forming solutions of net acidity. Aquatic life in many dams is totally destroyed and various rivers seriously polluted as the result of AMD.
Millions of years ago, when minerals were deposited in rock, they were formed by high temperatures and pressures. This caused these mineral deposits and gangue material to be unstable under surface conditions when exposed to oxygen and water.
AMD comes from abandoned coal mines and current mining activities. The excavation of minerals and coal takes place beneath the water table. The result is the continuous seepage of water into shafts. The seepage water has to be pumped out regularly to ensure normal and safe mining activities. Mine drainage is metal-rich water formed from chemical reactions between water and rocks containing sulphur-bearing minerals. The runoff formed this way is usually acidic and frequently comes from areas where ore or coal mining activities have exposed rocks containing pyrite which is a sulphur-bearing mineral. Pyrite is iron sulphide (FeS2) and is also known as Fool’s Gold.
How does acid mine drainage occur?
Acid mine drainage is formed when pyrite (iron sulphide) is exposed to air and reacts with oxygen and water to form sulphuric acid (H2SO4) and dissolved iron – iron(II) sulphate (ferrous state):
FeS2 + H2O + 3.5O2 = H2SO4 + FeSO4
iron sulphide + water + oxygen = sulphuric acid + iron sulphate
Soluble iron, or iron(II) sulphate is then subsequently oxidised to form insoluble iron of iron(III) sulphate (Fe2(SO4)3 – ferric state. Heavy metals such as copper, lead, cadmium, zinc, nickel, arsenic and mercury are dissolved by this acidic runoff into the ground or surface water. The rate by which acid mine drainage proceeds is often increased by the action of certain strains of bacteria such as acidophiles. The activity of these bacteria is pH dependent with optimal conditions in the range of pH 2 to 3. These microbes are known as extremophiles because they can survive in such harsh conditions. As the acidic water flows away from the mine, the pH changes and cause the heavy metals to precipitate out of solution. The change in pH also causes iron to precipitate out to form the red, orange, or yellow sediment often found in the bottom of streams containing mine drainage. Soluble ferrous iron or iron(II) sulphate [Fe++] forms insoluble ferric iron [Fe+++] or iron(III)hydroxide [Fe(OH)3).
Problems associated with acid mine drainage.
Acid mine drainage causes:
- Contaminated drinking water.
- Disrupted growth and reproduction of aquatic life.
- Corroding effects of acid on parts of infrastructure such as bridges, etc.
- This cause serious economic and ecological problems.
Consumers do not always realise what adverse effects heavy metals can have on the human body. The consumption of certain heavy metals at very low dosages, measured in micrograms per litre, can cause cancer and are strictly regulated by water governing bodies. Informed and uninformed people simply don’t know if heavy metals are removed effectively from drinking water, as in minute quantities, they have no taste, odour or colour. We are experiencing serious water treatment challenges in many towns in South Africa. Plants are not maintained appropriately or unskilled and negligent employees operate them. The best way to safeguard yourself and your family from ingesting toxins, heavy metals or endocrine disruptive chemicals is to install a reverse osmosis system at home for drinking, cooking and making ice.