Acid mine drainage (AMD) is the drainage produced from mines containing sulfide minerals which oxidize, causing the production of acid waters. Sulfide mines and coal mines are often sources for acid mine drainage. Over 11000 miles of streams in this country are contaminated by acid mine drainage (Kentucky, Illinois, Pennsylvania, Wyoming, Texas, Missouri, Kansas, Oklahoma, West Virginia, Maryland, Ohio, Colorado). AMD can be either naturally caused or caused by mining operations. Natural AMD is found around volcanoes and coal-rich areas. 78% of the AMD problems in the US today are due to abandoned mines.
AMD is characterized by low pH (1-4), high dissolved iron, high dissolved sulfate, and high total hardness (Ca + Mg), and high trace metals. When AMD is neutralized by flowing over limestone (a base), or by mixing with neutral water, iron oxides precipitate. These red and orange and yellow solids are called yellow boy. AMD ruins the aesthetic quality of surface waters, makes water unfit to drink, causes precipitation in tanks and boilers and washing machines, etc., and may release toxic metals into solution where the metals may enter the food chain.
Coal mining is often the site of creation of AMD because almost all coal contains pyrite (iron disulfide, FeS2). In general, coal from the western US has lower sulfur content (pyrite content) than coal in the east. In the west however, there are many sulfide mines which have produced great volumes of AMD.
There are many steps in the reaction of pyrite causing AMD. First, the sulfide is oxidized by interacting with oxygen to form sulfate, and then the reduced iron is oxidized to form oxidized iron. The oxidized iron will usually precipitate to form yellow boy, if the pH rises above 3 or so. Oxidized iron in solution can also back-react with the pyrite to oxidize the sulfate, causing the reaction to speed up. So, if there is oxidized iron present in the water, the reaction occurs more rapidly. Because one of the products of the reaction, oxidized iron, can back-react with the reactant pyrite, this is called an autocatalytic reaction. Under acid conditions, the oxidation of iron is slow, however, so the reaction would not proceed very fast, except for one interesting fact: oxidation of iron is catalyzed by bacteria such as Thiobacillus ferrooxidans. Oxidation of the iron, speeded up (catalyzed) by the bacteria, causes more oxidized iron and more autocatalysis of the reaction. The presence of the bacteria can cause the reaction to proceed at a rate 5 to 6 orders of magnitude faster than in the absence of bacteria. This bacteria is a chemolithotroph, which means that it gets all of its carbon from air, and it uses minerals as electron acceptors. This bacterium thrives at very low pH, and it causes a lot of the worst AMD.
Many techniques have been attempted to stop AMD. Early workers tried to stop the access of oxygen into a mine (by filling with water or nitrogen). AMD can be neutralized with limestone or with alkaline water -- however this causes very hard water. The problem bacteria can be inhibited with detergents, or bactericides. The mine can be backfilled with mine spoil to limit access of oxygen and water to the pyrite. Wetlands can be made which allow the growth of plants which are acid and metal tolerant, and which sequester metals as they grow. For example, sphagnum moss is effective in improving water quality because it sequesters iron, magnesium, sulfate, and manganese and helps to raise the pH. Wetlands are engineered with long flow paths so that AMD flows through compartments where the metals precipitate and the acid water is neutralized. These engineered wetlands are some of the most exciting new techniques developed for containing AMD.