Definition of Acid Mine Drainage

Acid Mine Drainage is caused when water flows over or through sulfur-bearing materials forming solutions of net acidity. Acid Mine Drainage comes mainly from abandoned coal mines and currently active mining.

For e.g. AMD degrades more than 4,500 stream miles in the mid-Atlantic region with the loss of aquatic life, and restricts stream use for recreation, public drinking water and industrial water supplies.

(Source: U.S. Environmental Agency)

Chemistry of Acid Mine Drainage

The chemistry of oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options (Blodau, 2006).

Although a host of chemical processes contribute to acid mine drainage, pyrite oxidation is by far the greatest contributor. A general equation for this process is:

2FeS_2(s) + 7O_2(g) + 2H₂O_(l) = 2Fe2+(aq) + 4SO₄²⁻_(aq) + 4H⁺_(aq)

The oxidation of the sulfide to sulfate solubilizes the ferrous iron (iron(II)), which is subsequently oxidized to ferric iron (iron(III)):

4Fe²⁺_(aq) + O_2(g) + 4H⁺_(aq) = 4Fe³⁺_(aq) + 2H₂O_(l)

Either of these reactions can occur spontaneously or can be catalyzed by microorganisms that derive energy from the oxidation reaction. The ferric cations produced can also oxidize additional pyrite and reduce into ferrous ions:

FeS_2(s) + 14Fe³⁺_(aq) + 8H₂O_(l) = 15Fe²⁺_(aq) + 2SO₄²⁻_(aq) + 16H⁺_(aq)

The net effect of these reactions is to release H⁺, which lowers the pH and maintains the solubility of the ferric ion.

(Source: Blodau, C. (2006). A review of acidity generation and consumption in acidic coal mine lakes and their watersheds. Science of the Total Environment. 369(1–3):307–332. doi:10.1016/j.scitotenv.2006.05.004. PMID 16806405).

Causes of Acid Mine Drainage

1. Mines built as early as the 1800’s were developed in a manner which utilized gravity drainage, to avoid excessive water accumulation in the mines.
2. As a result, water polluted by acid, iron, sulfur and aluminum drained away from the mines and into streams
3. Mine drainage is formed when pyrite, an iron sulfide, is exposed and reacts with air and water to form sulfuric acid and dissolved iron.
4. Some or all of this iron can precipitate to form the red, orange, or yellow sediments in the bottom of streams containing mine drainage.
5. The acid runoff further dissolves heavy metals such as copper, lead, mercury into ground or surface water.
6. The rate and degree by which acid-mine drainage proceeds can be increased by the action of certain bacteria.

Effect of Acid Mine Drainage

1. Acid mine drainage is one of Region 3 most serious water pollution problems.
2. It is not only an ecological concern to the states but an economic concern as well.
3. Contaminated drinking water.
4. Disrupted growth and reproduction of aquatic plants and animals.
5. A region impacted by acid mine drainage often has a decline in valued recreational fish species such as trout as well as a general decline in outdoor recreation and tourism along with contamination of groundwater drinking supplies.
6. Corroding effects of acid on parts of infrastructure such as bridges.

(Source: U.S. Environmental Agency)

Control of Acid Mine Drainage

Water is the basic transport medium for contaminants and consequently, all measures aimed at AMD migration control are concerned with the control of water flow. Control of water exit is of little value as long-term storage is negligible. Water entry into the site of acid formation may be controlled by:

1. Diversion of surface water flowing towards the site of pollution;
2. Prevention of groundwater infiltration into the pollution site;
3. Prevention of hydrological water seepage into the affected areas; and
4. Controlled placement of acid-generating waste.

(Source: Akcil, A. and Koldas, S. 2006. Acid Mine Drainage (AMD): causes, treatment and case studies. Journal of Cleaner Production. 14(12–13):1139–1145)