In this video, an environmental technologist visits an abandoned coal mine in Kentucky to talk about how a remediation system (a series of settling ...
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The Chemical Processes Involved in Acid Mine Drainage
In a society heavily dependent on electronic devices, we need to understand how electricity is produced and the environmental impact of different production techniques. As of 2007, over 70 percent of electricity produced in the United States came from fossil-fuel-powered generating facilities. Coal and natural gas or methane are the two most common fuels in the U.S., with approximately 20 percent of production facilities using natural gas and almost 50 percent of electricity coming from coal-fired power plants. These plants have direct environmental effects, such as greenhouse gas emissions. However, the immediate and long-term environmental dangers of mining fossil fuels are also significant. A subtle, yet wide-reaching issue related to mining is acid mine drainage or AMD. AMD is acidified water runoff containing metal ions that escapes from older abandoned or unmonitored mines. AMD results from several chemical reactions that take place after the mining process exposes minerals beneath the surface of the earth, such pyrite, to air. These minerals are fairly inert when sealed underground, but react to the oxygen in the air and/or the oxygen dissolved in water. Oxygen oxidizes the sulfur present in the pyrite. Oxidation refers to a positive change in the oxidation state of an atom or ion, due to the loss of one or more electrons.
In the presence of water, the oxidation of sulfur also produces hydrogen ions and releases iron ions. The acidic nature of acid mine drainage water is the result of the excess hydrogen ions released in this process. The mechanisms for this reaction may be biotic (caused by living organisms) or abiotic (not caused by living organisms).
The acidity of the water is measured using the pH scale. Any solution with pH value below seven is considered “acidic.” The pH scale is a logarithmic measure of the amount of available hydrogen ions in a solution. “Logarithmic” refers to the fact that the concentration of hydrogen ions in each measurement on the scale below seven (neutral) is 10 times greater than the concentration of hydrogen in the measurement above it. The lower the pH value, the higher the concentration of hydrogen ions. For example, if the number of hydrogen ions in a solution with a pH of five were 100, the number of hydrogen ions in a solution with a pH of 4 would be 1000.
Acidic runoff from mines is inherently an environmental hazard, making natural waterways unsuitable for plant and animal life due to low pH values. Geographical regions rich in limestone (calcium carbonate) have some degree of natural protection against the acidity of AMD. The carbonate ions will react with hydrogen ions, making the water less acidic (i.e., increasing the pH) by decreasing the hydrogen ion concentration. If the water travels through significant limestone deposits, it will be safer and more hospitable for plant and animal life. However, if there is some limestone, but not enough to neutralize the acidity, the reaction between the carbonate ions and the hydrogen ions in the water will produce carbon dioxide (CO2), contributing an unwanted greenhouse gas to the atmosphere.
Limestone may restore an acceptable pH to the stream water, but the increased pH initiates a secondary reaction with ferric ions (Fe3+) also present in acid mine drainage. As the pH increases, the concentration of hydroxide ions (OH-) also increases, and iron (III) hydroxide [Fe(OH)3, ferric hydroxide] begins to form.This compound is fairly insoluble, especially at higher pH values. As iron hydroxide forms, it precipitates out of the water, creating an orange/brown crust on the banks and bottom of the stream. This metallic crust makes it difficult for plants and bottom-dwelling organisms to grow and live in and around the stream.To avoid continued problems from acid mine drainage, government agencies have attempted remediation to repair existing damage and prevent further damage. This remediation requires steps to both correct the water pH and eliminate the ferric ions or the iron hydroxide before the mine drainage mixes with other waterways. This goal is often accomplished by controlled neutralization using limestone, sodium hydroxide (NaOH), or other bases, then collection of the iron hydroxide precipitate in settling ponds (areas where the water moves slowly enough that the solids may settle to the bottom). Both biotic and abiotic approaches may be applied in remediation. The treated AMD water may still have a high sulfate ion concentration, but surrounding waterways will not be damaged severely enough to make them unsuitable for animal and plant life.
- How does coal mining cause acid mine drainage (AMD)?
- Discuss the common characteristics of solutions with a pH of less than 7.
- Discuss the common characteristics of solutions with a pH greater than 7.
- Explain why solutions with low pHs have few visible solids in solution.
- If you encountered a stream in the forest, what, if any, visual evidence would indicate a stream with a low pH? What visual evidence would indicate that a stream has an acceptable pH, but still suffers from AMD contamination? Are these visual indications sufficient to determine if the water is “safe”? If not, how would you make this determination?
- What pH does the water that comes out of your tap most likely have? Why? What factors in a municipal water system and the plumbing system of buildings might affect the water’s pH?