Pyrite reactions
AMD is not limited to coal mining activities in Pennsylvania. Colorado has a long history of mining and has been the victim of problems similar to those found in PA. The Colorado School of Mines web site describes AMD Chemistry and Microbial Influences.
The Mineral Gallery has general information about pyrite at http://mineral.galleries.com/minerals/sulfides/pyrite/pyrite.htm
The mineral pyrite (FeS2) and other sulfur containing minerals are common. Also common is the occurrence of these minerals in the geologic strata in the vicinity of coal seams. Mining coal inevitably involves exposing these pyritic materials to oxygen and water. In deep mines, these materials are exposed in the voids created by the mining process. They also are brought to the surface as an unwanted waste product along with the coal, where they and other unwanted materials were separated from the coal and historically put in huge refuge piles known variously as gob piles, boney piles, slate dumps, and culm banks. Here too the pyritic materials were exposed to oxygen and water. Strip mining also exposes the pyritic materials as the overburden is striped away. With abandoned strip mines, it was common to simply leave these materials open to the weathering process.
Wherever pyrite can come into contact with oxygen and water a potential problem exists. This trio of pyrite, oxygen and water gets the ball rolling in the formation of AMD. Let's examine how. When pyrite is initially exposed to oxygen and water the following reaction can occur
4FeS2(s) + 14O2(g) + 4H2O(l) ---> 4Fe2+(aq) + 8SO42-(aq) + 8H+(aq) (1)
This can be stated as pyrite, oxygen, and water react to form dissolved ferrous ions (a.k.a. iron II), dissolved sulfate ions and acidity. Thus ferrous ions and acidic hydrogen ions are released into the waters that runoff through the mine tunnels or refuge piles. The pH of the water will likely go down depending on just how often this reaction occurs. While dissolved, ferrous iron (Fe2+(aq) ) and sulfate ions (SO42-(aq) ) are colorless and the water may actually look crystal clear. In some AMD discharges, this is the condition of the water as it makes its way to the surface. Also note that this is not a particularly fast reaction just as the formation of rust takes a while to happen.
The next step in the process is for the ferrous iron to be oxidized to ferric iron as shown in the following reaction
4Fe2+(aq) + O2(g) + 4H+(aq) ---> 4Fe3+(aq) + 2H2O(l) (2)
Aqueous ferrous (Fe2+) ions react with oxygen and acidic hydrogen ions to form ferric (Fe3+) ions and water. Note that oxygen needs to be present for this reaction to happen. Often this reaction doesn't happen to any great extent underground because of limited available oxygen. Also note that acidity is consumed in this process. This reaction rate is pH dependant with the reaction proceeding slowly under acidic conditions (pH 2-3) with no bacteria present and several orders of magnitude faster at pH values near 5. This reaction is referred to as the "rate determining step" in the overall acid-generating sequence.
Equation 3 describes the next reaction where the ferric ions now hydrolyze in water to form ferric hydroxide. (Hydrolysis is a reaction in which water reacts with another reactant and which a hydroxyl group and a hydrogen ion are formed.)
4Fe3+(aq) + 12 H2O(l) ---> 4Fe(OH)3(s) + 12H+(aq) (3)
This process releases even more hydrogen ions into the aquatic environment and further reducing the pH. The ferric hydroxide formed in this reaction is also called "yellow boy", a yellowish-orange precipitate that turns the acidic runoff in the streams to an orange or red color and covers the stream bed with a slimy coating. Aquatic life dwelling on the bottom channel of the stream is soon killed off. If the pH is greater than 3.5 this precipitation reaction will occur.
Often the net effect of equations 1 through 3 is summarized in the following equation
4FeS2(s) + 15O2(g) + 14H2O(l) ---> 4Fe(OH)3(s) + 8SO42-(aq) + 16H+(aq) (4)
Overall pyrite is oxidized releasing acidic hydrogen ions into the water and coating the stream bed with "yellow boy".
These reactions give a fair representation of how pyrite reacts to form pollution. However, a number of other reactions are also possible, mostly leading to the same kind of products. For example
FeS2(aq) + 14Fe3+(aq) + 8H2O(l) ---> 15Fe2+(aq) + 2SO42-(aq) + 16H+(aq) (5)
involves pyrite reacting with ferric ions and water producing ferrous ions, sulfate ions and acidity. What happens in any particular environment is largely dependant on the conditions existing in that environment. One such factor is the presence of a bacteria known as Thiobacillus ferroxidans
which likes acidic conditions and can greatly enhance the rate of oxidation of iron and sulfur containing compounds.See Colorado School of Mines page on Microbial Influences on AMD
Other links for Thiobacillus ferroxidans
From an environmental standpoint, the pyrite reactions detailed above cause an increase in the acidity, i.e. the amount of H+ produced, and the heavy metal iron that ultimately precipitates as yellow boy. To make matters worse, the acidity produced may further react with other minerals that may liberate heavy metals such as aluminum and manganese into the water. Treating AMD polluted water generally has as its particular goals: (1) neutralizing the acidity by adding alkalinity, and (2) precipitating the heavy metals out before they are allowed to enter streams and waterways.
The Science of Acid Mine Drainage and Passive Treatment from PA DEP
Environmental Chemistry in Colorado Toxic Mine Drainage Chemistry and Treatment