Acid Rain Effects Acid Rain  Elmhurst College
Acid Lake Effects Effects on Forests Acid Rain Solutions  Chemistry Department
Acid Soils and Effects Effects on Buildings    Virtual ChemBook

 Limestone Neutralization:

The sulfuric acid reacts with the limestone in a neutralization reaction.

Limestone: CaCO3 + H2SO4 --> CaSO4 + H2CO3

The calcium sulfate is soluble in water and hence the limestone dissolves and crumbles.

H2CO3 --> CO2 gas + H2O

The original acid (hydrogen ions) have been converted to water in these reactions.

Acid Rain - Soil Interactions

Limestone Neutralization:

When acid rain falls to the earth, it undergoes a new round of physical and chemical changes. These changes may reduce the acidity and change the chemical characteristics of the water reaching the streams and lakes.

Alkaline or basic soils, such as those rich in limestone, calcium carbonate, can neutralize the acid directly. These types of soils are found in the Midwest, Great Plains, and most of the Western states.

Neutralization Effectiveness

The extent to which soils can neutralize acid rain depends on several factors: type of soil, thickness, weather, and water flow patterns. If the ground is frozen, as in the winter, soil process cannot work, the acid is not neutralized. If the soil is mainly quartz, such as sand, it is resistant to weathering and no bases are present to neutralize acid. If the soil has very little base such as limestone, the acid is neutralized only slightly or with the passage of time, not at all.

In the slightly acidic soils in typical evergreen forests in the Northeastern U.S., Canada, and Europe, two other effects can neutralize the acid rain. The acid can be immobilized as the soil or vegetation retains the sulfate and nitrate ions (from sulfuric and nitric acids). Very deep soils have a large capacity to retain sulfate and nitrate ions.

 

Cation-Exchange:

In cation exchange, the ions of calcium, magnesium, potassium, and other metals are attached to the clay and humus particles in the soil. The attractive forces of positive metal ions to the negatively charged clay particles is strong enough to hold the metal ions in the soil despite the passage of water through the soil.

Effect of Acid: The hydrogen ions in the sulfuric acid trade places with the metal ions. The hydrogen ions are retained and neutralized by the soil. The calcium, potassium, and magnesium ions are leached or washed out of the top soil into lower inaccessible subsoil. These ions are then not available as nutrients or fertilizers needed for tree and plant growth.

Effects on Soils - Related to Tree Growth Leaching Nutrients:

Trees derive their nutrition primarily from element ions such as calcium, Ca, magnesium, Mg, and potassium, K that have dissolved from rocks into the soil.

Acid deposition adds hydrogen ions, which displace these important nutrients in a process called leaching. Leaching means that the ions are washed deeper into the subsoil or washed out of the top soil. If ions are leached from the soil, they are no longer available to the roots of the plants.

Calcium ion is used in the cells of a tree for cell formation and in the processes that transport sugars, water, and other nutrients from the roots to the leaves.

Magnesium ion is a vital element in photosynthesis and as a carrier of phosphorus which is important in the production of DNA. These ions may be unavailable to the tree roots because they have been leached away.

 Aluminum Hydroxide Neutralization:

The sulfuric acid reacts with the aluminum hydroxide in a neutralization reaction.

Limestone: Al(OH)3 + H2SO4 --> Al2(SO4 )3 + H2CO3

The aluminum sulfate is soluble in water and are thus released from the soil particles.

Effects on Soils - Mobilize Aluminum:

Increasing amounts of acids can "mobilize" aluminum ions which are normally present in an insoluble nontoxic form of aluminum hydroxide. It appears that when the soil pH dips to 5 or lower, aluminum ions are dissolved into the water and become toxic to plants. Aluminum ions cause a stunting of the root growth and prevent the roots from taking up calcium. The result may be the overall slowing of the growth of the entire tree.

Lower soil pH and aluminum mobilization can reduce populations of soil microorganisms. Soil bacteria have the job of breaking down the dead and decaying leaves and other debris on the forest floor. The effect of this action is to release nutrients such as calcium, magnesium, phosphate, nitrate, and others. Low pH and high aluminum ion concentrations inhibit this process.

Higher amounts of acids can mobilize other toxic metals from the insoluble to the soluble ion forms in the same fashion as aluminum.
The toxic metals include lead, mercury, zinc, copper, cadmium, chromium, manganese, and vanadium.

These may all contribute to slow the growth of a tree. In addition, this combination of toxic metals may also adversely effect the growth of soil bacteria, mosses, algae, fungi, and earthworms.