Neutralization

Neutralization of acidic water is an important step in reclaiming land that was once mined. Mine run-off renders water around the mine site acidic, and the acid must be neutralized with lime, or calcium oxide (CaO) acting as a base, before the area can be reclaimed. However, this procedure is expensive. Recent studies have shown that constructed wetlands—human-made shallow ponds containing water plants such as cattails—can be an effective and less costly method of neutralizing acid mine drainage.

Neutralization of soil is sometimes necessary in order to promote plant growth. The ability of plants to take nutrients from the soil into their roots is affected by the pH content of the surrounding soil particles. Acid rain can cause soil to become acidic. Some plants benefit from liming the soil. In the liming process, agricultural limestone-calcium carbonate (CaCO₃) that may also contain magnesium carbonate (MgCO₃) neutralizes acid in the soil and provides nutrients to promote plant growth. Since liming can stimulate plant growth, it is important to also fertilize limed soil. This insures that all of the nutrients used by the plants for growth are maintained in the soil for the following year's growth. Soils may also become too basic, or alkaline, especially in areas where there is little precipitation. Substances that can act as acids such as calcium sulfate, also called gypsum (CaSO₄), and sulfur (S₂) can be applied to the soil for neutralization.

In areas where acid precipitation is a problem, neutralization reactions can damage limestone, marble, and plaster buildings and statues. These structures all are made of calcium carbonate (CaCO₃), a basic substance that is neutralized by acidic precipitation. These structures decay and become coated with a black substance that contains gypsum. Gypsum is the salt calcium sulfate (CaSO₄) that is formed when the sulfuric acid (H₂S) in acid rain reacts with the calcium carbonate (CaCO₃) in the building materials.

Acidic and basic hazardous wastes can often be safely disposed of using neutralization because the salts produced are usually non-hazardous and take up less space as solids than the liquid acids and bases.