Understanding Acid Mine Drainage Formation
Acid mine drainage (AMD) is a significant environmental problem often associated with mining activities, particularly coal mining. It results from a specific chemical and biological process.
Key Factors in AMD Occurrence
The formation of AMD requires the interaction of several key elements:
- Sulfur Compounds: Primarily present as pyrite (iron sulfide minerals, FeS$_2$) or other sulfur-bearing materials within coal seams or surrounding rock.
- Oxygen (O$_2$): Atmospheric oxygen is crucial for oxidizing the sulfur compounds.
- Water (H$_2$O): Acts as a solvent and facilitates the chemical reactions.
- Bacteria: Specific types of bacteria, like Thiobacillus ferrooxidans, accelerate the oxidation process significantly.
The Chemical Process
The process can be summarized as follows:
- Oxidation of Pyrite: In the presence of oxygen and water, sulfur compounds like pyrite are oxidized. This initial step can be slow without bacterial help.
Example reaction: $2 \text{FeS}_2 + 2\text{H}_2\text{O} + 7\text{O}_2 \rightarrow 2\text{Fe}^{2+} + 4\text{SO}_4^{2-} + 4\text{H}^+$ - Bacterial Acceleration: Certain bacteria utilize the energy released from oxidizing iron and sulfur compounds, greatly speeding up the reaction rate. They further oxidize ferrous iron (Fe$^{2+}$) to ferric iron (Fe$^{3+}$) and sulfide to sulfate.
Example reaction: $4\text{Fe}^{2+} + \text{O}_2 + 4\text{H}^+ \rightarrow 4\text{Fe}^{3+} + 2\text{H}_2\text{O}$
And: $ \text{FeS}_2 + 14\text{Fe}^{3+} + 8\text{H}_2\text{O} \rightarrow 15\text{Fe}^{2+} + 2\text{SO}_4^{2-} + 16\text{H}^+$ - Sulfuric Acid Formation: The overall process releases hydrogen ions (H$^+$), leading to the formation of sulfuric acid (H$_2$SO$_4$). This drastically lowers the pH of the water.
Simplified overall reaction: $2 \text{FeS}_2 + 2\text{H}_2\text{O} + 7\text{O}_2 \rightarrow 2\text{FeSO}_4 + 2\text{H}_2\text{SO}_4 + 2\text{H}_2\text{SO}_4$ (Simplified representation showing acid formation)
Therefore, the combined action of oxygen, water, and specific bacteria is necessary for sulfur in coal (or rock) to efficiently form sulfuric acid, causing acid mine drainage.
Important Questions from Environmental Pollution and Control
A 500 MW coal based power station is operating at an efficiency of 30%. If the coal has 1% of sulphur content and 1 tonne of coal produces 8000 kWh energy, how much SO2 will be emitted daily by the plant?
Name the low heat thermal process that destroys the pathogen in biomedical waste by heating that occurs inside the waste material.
Arrange the following functional elements of solid waste management in the order in which they appear :
(i) On-site handling, storage and processing
(ii) Collection
(iii) Process recovery
(iv) Transfer and transport
Choose the correct answer from the code given below :
The dominant mechanism(s) of deposition of aerosol particles in the size range 5 -10 μm in the respiratory tract are
(i) Sedimentation
(ii) Impaction
(iii) Diffusion
Choose the correct answer from the code given below :
A tributary flowing at a rate of 4 m3/s converges into a river flowing at a rate of 8.0 m3/s. The concentration of a pollutant 'X' at the upstream of the tributary before convergence was 12 mg/L and that of the river was 30 mg/L. If the pollutant X is completely mixed in the downstream, what would be its concentration?
