In the early 1900's, Western Pennsylvania was a great coal mining region. The remnants of this industry include abandoned mines that underlay a great portion of the land on which we live. An unfortunate side effect of this industry is the drainage of acidic waters from these abandoned mines into our streams and rivers. Studies have shown a drastic reduction in fish counts in polluted streams as the runoff from these mines is causing the pH of the water to fall, and heavy metals from the drainage are precipitating out of solution and entering our rivers as toxic materials.
The main culprit in the formation of acidic mine runoff is pyrite, FeS2. Natural pyrite exists underground, and is harmless because it is not exposed to air or groundwater. When mines are dug to remove coal, the pyrite is exposed to air and water. As shown in the reaction below, the pyrite reacts with oxygen in air and water to form iron(III) hydroxide, Fe(OH)3, and sulfuric acid, H2SO4.
4 FeS2(s) + 14 H2O(l) + 15 O2 -> 4 Fe(OH)3(s) + 8 H2SO4(aq)
The solubility of Fe(OH)3 is both temperature and pH dependent. In the highly acidic waters of the mine, the Fe(OH)3 will be very soluble. As the mine runoff is mixed with the waters of streams and rivers, it is diluted and the Fe(OH)3 that was in solution precipitates out, and pollutes the riverbed. This is shown in the accompanying picture. Notice the red solid, Fe(OH)3 the has been deposited in the stream just outside the mine exit.
In this activity, we'll consider a mine that puts out 10 liters of effluent every hour. The stockroom of the Virtual Lab contains a sample of such an effluent. This sample is a solution of H2SO4 that is saturated with Fe(OH)3. The pH of this effluent is 1.0. The river itself flows by the mouth of the mine at a rate of 10,000 liters/hour. A sample of the river water is also included in the stockroom of the Virtual Lab. For this problem, the river water will be modeled as pure H2O at 25°C.