Alkaline precipitation occurs due to natural and anthropogenic causes. It happens when minerals, such as

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

aluminum Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It ha ...

, or

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

combine with other minerals to form alkaline residues that are emitted into the atmosphere, absorbed by water droplets in clouds, and eventually fall as rain. Aquatic environments are especially impacted by alkaline precipitation. Because alkaline precipitation can be harmful to the environment, it is important to utilize various methods such as air pollution control, solidification and stabilization, and remediation to manage it. Alkaline_precipitation

Natural causes

While most natural rains are weakly acidic, alkaline rain can also occur in natural conditions without the significant impact of pollutants. Natural alkaline rains from semiarid areas carry a substantial amount of

mineral dust Mineral dust is atmospheric aerosol originated from the suspension of minerals constituting the soil, composed of various oxides and carbonates. Human activities lead to 30% of the airborne dust load in the atmosphere. The Sahara Desert is th ...

lifted from desert soil

convection Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the conve ...

and transported by winds. After mixing with water vapor, they are carried by clouds and deposited on the ground in the form of

rain dust Rain dust or snow dust, traditionally known as muddy rain, red rain, or coloured rain, is a variety of rain (or any other form of precipitation) which contains enough desert dust for the dust to be visible without using a microscope. History ...

Anthropogenic causes

The principal cause of alkaline rain are emissions from factories and waste deposits.

Mineral dust Mineral dust is atmospheric aerosol originated from the suspension of minerals constituting the soil, composed of various oxides and carbonates. Human activities lead to 30% of the airborne dust load in the atmosphere. The Sahara Desert is th ...

containing large amounts of alkaline compounds such as calcium carbonate can also increase the pH of precipitation and contribute to basic rain. Alkaline rain can be viewed as opposite to acid rain. Industrial processes such as coal combustion,

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

, chromium ore, alumina extraction, iron, and steel manufacture can cause pollution by producing alkaline residue. These residues are significant and increasing in the global flux and are composed of

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

, or

oxides that are hydrated to produce soluble hydroxides. Other sources include the surfaces of unpaved roads and soils that are covered in major alkaline elements (e.g.

, and

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...

).{{Cite journal, last1=Gatz, first1=Donald F., last2=Barnard, first2=William R., last3=Stensland, first3=Gary J., date=1986, title=The role of alkaline materials in precipitation chemistry: A brief review of the issues, url=https://doi.org/10.1007/BF00305195, journal=Water, Air, and Soil Pollution, language=en, volume=30, issue=1, pages=245–251, doi=10.1007/BF00305195, issn=1573-2932

Impacts of alkaline precipitation

Alkaline precipitation increases the pH of rainwater to 8.5-10, causing disturbances in aquatic ecosystems. These disturbances can cause physiological changes to aquatic life, changing the rates at which

ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous wa ...

is dispelled, which leads to accumulation in organisms. The pH change in the water can cause precipitation of calcite from alkaline leachates that suffocate benthic and

littoral The littoral zone or nearshore is the part of a sea, lake, or river that is close to the shore. In coastal ecology, the littoral zone includes the intertidal zone extending from the high water mark (which is rarely inundated), to coastal a ...

aquatic habitats, along with reducing light penetration.

Management

Air pollution control yields two management practices of recycling

ceramic A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain ...

material or landfill after treatment. These practices are made possible by several treatment methods of solidification/stabilization, thermal, and combined. The most commonly used method for dealing with these types of waste such as

bauxite Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) and diaspore (α-AlO ...

is solidification / stabilization. Remediation for alkaline leachate requires active

aeration Aeration (also called aerification or aeriation) is the process by which air is circulated through, mixed with or dissolved in a liquid or other substances that act as a fluid (such as soil). Aeration processes create additional surface area in ...

in order to promote

carbonation Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids. In inorganic ch ...

, recirculation of drainage waters over stockpiled or

lagoon A lagoon is a shallow body of water separated from a larger body of water by a narrow landform, such as reefs, barrier islands, barrier peninsulas, or isthmuses. Lagoons are commonly divided into ''coastal lagoons'' (or ''barrier lagoons'') ...

ed residues, and acid dosing. Strong acids (e.g.

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

and sulfuric acid) are also used in neutralizing the pH; this is used at active processing plants but the liquid runoff can remain toxic to aquatic environments.

Wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (anoxic) processes prevailing, especially in the soils. The p ...

s are also a low-cost remedy for alkaline leachates.

References

Alkaline earth metals

Natural causes

Anthropogenic causes

Impacts of alkaline precipitation

Management

See also

References