Acid Rain: Environmental Damage & Solutions

Understanding Acid Rain: Causes & Impacts

Global Impact of Acid Rain

Each year, acid rain causes hundreds of millions of dollars in damages to buildings and monuments worldwide. Some chemists use the term "leprosy of the stones" to describe the severe corrosion of rocks caused by acid rain. Beyond infrastructure, acid rain is also profoundly harmful to vegetation and aquatic life. There are many well-documented cases illustrating how acid rain has destroyed farmland and forests, leading to the death of aquatic organisms.

Chemical Causes of Acid Rain

The average pH of rainfall in the northwestern United States is 4.3. Given that atmospheric carbon dioxide (CO2) is in equilibrium with rainwater, one would not expect its pH to be less than 5.5. The primary culprits for making rainwater more acidic are sulfur dioxide (SO2) and, to a lesser degree, nitrogen oxides (NOx) emissions, often originating from vehicles and industrial processes. Acidic oxides like SO2 react with water to form corresponding acids.

Sources of Sulfur Dioxide (SO2)

Atmospheric SO2 originates from several sources. Nature contributes to SO2 emissions through volcanic eruptions. Additionally, many metals naturally occur combined with sulfur. To extract these metals, it is often necessary to melt or burn the minerals. For instance, metal sulfide is heated in air to form metal oxide and SO2. The resulting metal oxide is then more easily reduced than the sulfide (using a more reactive metal or, in some cases, coal) to release the pure metal.

Atmospheric Transformation & Erosion

In the troposphere, SO2 is almost completely oxidized to sulfuric acid (H2SO4) within aerosols, which then precipitates as acid rain. SO2 can also be oxidized to sulfur trioxide (SO3), and subsequently, through heterogeneous catalysis on solid particles, transformed into H2SO4. Over time, this acidic precipitation severely erodes limestone buildings and marble (calcium carbonate, CaCO3).

Mitigating Acid Rain Effects

Remedying the effects of SO2 pollution can be approached in two primary ways:

• Pre-combustion Sulfur Removal: The most direct method involves removing sulfur from fossil fuels before they are burned. However, this approach presents significant technological challenges.
• Post-combustion SO2 Removal: A more common, though often less effective, method is to remove SO2 as it forms during combustion. For example, in some processes, limestone powder is injected into the burner or furnace of coal-fired power plants. To capture remaining SO2, an aqueous suspension of lime is injected into a purification chamber before gases are released through the chimney.

Liming: A Remedial Approach

Beyond industrial controls, lime is also added to lakes and soils to reduce their acidity, a procedure known as liming. This helps to counteract the environmental damage caused by acid rain.