Acid Mine Drainage Problem - Part 4
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 4 considers remediation strategies for treating acid mine drainage. We'll consider the same 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 mine effluent - a solution of H2SO4 that is saturated with Fe(OH)3 with a pH of 1.0.
For treatment purposes, the stockroom also contains strong base (19M NaOH) and concentrated phosphoric acid (14.6M H3PO4) solutions:
- Design a treatment strategy to neutralize the mine effluent before it enters the river. Calculate the amount of base needed to raise the pH to an environmentally safe level (pH > 5.5).
- Investigate how neutralization affects the precipitation of Fe(OH)3. Does neutralizing the acid increase or decrease the amount of iron hydroxide that precipitates?
- Evaluate the cost-effectiveness of different treatment approaches. Consider both the amount of chemicals needed and the environmental impact.
- Propose an optimal remediation strategy that balances chemical costs, environmental safety, and practical implementation considerations.
Previous: Part 1 - Dilution Effects | Part 2 - Temperature Effects | Part 3 - Buffer Effects