Acid Mine Drainage Problem - Part 1
Western Pennsylvania was once a great coal mining region. The remnants of this industry are still with us in the form of abandoned mines that underlay a great portion of the land on which we live, and in the drainage of acidic waters from the mines into our streams and rivers. Studies have shown a drastic reduction in fish counts when the pH of a stream goes below 5.5.
The main culprit in the formation of acidic mine runoff is pyrite, FeS2. When exposed to air and water, FeS2 forms iron(III) hydroxide, Fe(OH)3, which precipitates out of solution, and sulfuric acid, H2SO4. The solubility of Fe(OH)3 is both temperature and pH dependent.
Problem 1 starts with a simple model of the river as pure H2O at 25°C. We'll consider a mine that puts out 10 liters of effluent every hour, with the river flowing at a rate of 10,000 liters/hour. The stockroom contains a sample of such an effluent - a solution of H2SO4 that is saturated with Fe(OH)3 with a pH of 1.0.
Assuming that the river is pure water at 25°C:
- As the mine effluent enters the river, it is diluted. What would you expect the pH of the river to be if the only factor you considered was the effects of dilution on the concentration of [H+]?
- Use the Virtual Lab to explore the chemical system and measure the change in pH resulting from the dilution.
- Compare the result you predicted in part (a) against pH you measured in part (b). Explain qualitatively the chemical processes that account for the observed behavior.
- Based on the measured pH in part (b) calculate the amount of Fe(OH)3 that should precipitate as a result of the dilution. Check your answer using the Virtual Lab. How much Fe(OH)3 will be deposited outside the river each hour?
Continue to: Part 2 - Temperature Effects | Part 3 - Buffer Effects