Case Study: Coal Mine Run Off

The prevalence of coal mining in Western Pennsylvania has left a legacy of polluted water systems. Run off from old mine sites is very acidic and often contains heavy metals. The main culprit is FeS2 which, when exposed to air and water forms Fe(OH)3 and sulfuric acid (H2SO4). The sulfuric acid lowers the pH of the watershed which can result in the death of fish for pH’s lower than 6.0. The Fe(OH)3 has a solubility that is dependent on pH and can precipitate leaving a reddish toxic sludge at the edges of rivers and wetlands.

Stream impacted by acid mine drainage from an abandoned coal mine (photo credit: website)
You have been asked by the EPA of Western PA to calculate the pH of the run off of a particular mine and what volume should be allowed into a river each hour. A safe runoff rate (in L/hr) is defined as one that does not allow the river water to reach pH below 6.0. In addition, you have been asked to determine whether or not Fe(OH)3 will precipitate under these “safe” conditions.
The river that runs by the particular mine flows at a rate of 10,000 L/hr and can be assumed to be pure water for the purposes of these calculations. This means that if one added 0.01 moles of H3O+ per hour the pH would decrease from 7 to 6 and still be “safe”. The mine effluent (run off) contains 0.0885 M sulfuric acid and has an iron (III) concentration of 1.0×10-9 M (For Fe(OH)3 the Ksp = 1.6×10-39). Sulfuric acid (H2SO4) has two acidic protons. In aqueous solution 100% of the first proton is donated to water to make H3O+ and HSO4-. The resulting HSO4- (aq) is in equilibrium with SO42- and H3O+ with a Ka = 0.013. Determine the pH of the effluent solution (note that it will be the result of the sum of the concentration of H3O+ due to the (100%) loss of the first proton and the partial loss of the second proton in the sulfuric acid) and what volume of that solution could be allowed into the river each hour. In addition, determine whether or not Fe(OH)3 will precipitate at this pH (6.0) along the river bank.
Your task is to write a report to your supervisor detailing your findings, your conclusions and how you reached your conclusions. See the assignment requirements for specific things to include in your report.
Note that this assignment will be scored based on the rubric under these questions in the Course Overview page:
Does the work demonstrate thorough understanding of the concepts? Does the work meet the assignment requirements?

Sample Solution