Acid Mine Drainage Problem

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. Heavy metals also precipitate out of solution and enter our rivers as toxic materials.

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. So some of the Fe(OH)3 that is in solution in the acidic conditions of the mine will precipitate out when the effluent (runoff) mixes with river water. This is shown in the accompanying picture, where the red solid is Fe(OH)3 that has been deposited by the river just outside the mine exit.

In this activity, we will model acid mine drainage at various levels of sophistication. 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 these problems.

We will consider three models of this system, with increasing sophistication. We will consider both the change in pH of the river due to the mine effluent and the amount of Fe(OH)3 that precipitates outside of the mine.

Virtual Lab Problem 1

Assuming that the river is pure water at 25C,
a) . 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+]?
b) . Use the Virtual Lab to explore the chemical system and measure the change in pH resulting from the dilution.
c) . 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 behaviour.
d) . 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?

Virtual Lab Problem 2

Assume that the river is still pure water, but consider the changes in solubility of Fe(OH)3 due to temperature changes throughout the year.
a) . Perform experiments to determine if the amount of Fe(OH)3 that precipitates outside the mine is greater in the winter or in the summer.
b) . Based on your measurements from part (a), what can you say about the thermodynamic properties of the following reaction:

Fe(OH)3 => Fe+3 + 3 OH-

Virtual Lab Problem 3

Our previous assumption that a river consists of pure water was obviously a simplification. Water arrives at the river after flowing over land and through the ground, and it carries some dissolved chemical species it picks up along the way.
a) . Predict how the buffering capacity of the river will affect both the pH of the river and the amount of Fe(OH)3 precipitated in the river. Will the pH of the river be larger or smaller than you obtained in problem 1? Will more or less Fe(OH)3 precipitate? Please explain your reasoning.
b) . Use the Virtual Lab to explore the chemical system and measure the pH and amount of Fe(OH)3 that precipitates when the effluent is mixed with the sample of river water.
c) . Did your measurements agree with your predictions from part (a)? If not, can you adjust your reasoning, for instance by including additional chemical processes, to account for the observed behaviour?
d) . Based on the pH you measured in part (b), calculate how much Fe(OH)3 precipitates when the acidic mine effluent is discharged into the buffered model of the river. Check your answer against the amount of precipitate you measured in part (b). How much Fe(OH)3 will be deposited outside the river each hour? How does this result compare with that obtained from the simpler, unbuffered, model in part (d) of problem 1.