Alkaline precipitation occurs due to natural and anthropogenic causes. It happens when minerals, such ascalcium,aluminum, ormagnesium 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 asair pollution control, solidification and stabilization, and remediation to manage it.[1]
While most natural rains are weakly acidic, alkaline rain can also occur in natural conditions without the significant impact of pollutants.[2] Natural alkaline rains from semiarid areas carry a substantial amount ofmineral dust lifted from desert soilconvection and transported by winds. After mixing with water vapor, they are carried by clouds and deposited on the ground in the form ofrain dust.
The principal cause of alkaline rain are emissions from factories and waste deposits.Mineral dust containing large amounts of alkaline compounds such ascalcium carbonate can also increase the pH of precipitation and contribute to basic rain.[3] Alkaline rain can be viewed as opposite toacid rain. Industrial processes such ascoal combustion,limestone, chromium ore, alumina extraction, iron, and steel manufacture can cause pollution by producing alkaline residue.[1] These residues are significant and increasing in the global flux and are composed ofsodium,calcium, ormagnesium oxides that are hydrated to produce solublehydroxides.[1] Other sources include the surfaces of unpaved roads and soils that are covered in major alkalineelements (e.g.sodium,calcium,magnesium, andpotassium).[4]
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 whichammonia is dispelled, which leads to accumulation in organisms.[1] The pH change in the water can cause precipitation of calcite from alkalineleachates that suffocatebenthic andlittoral aquatic habitats, along with reducing light penetration.[1]
Air pollution control yields two management practices of recyclingceramic material orlandfill after treatment.[1] These practices are made possible by several treatment methods of solidification/stabilization, thermal, and combined.[1] The most commonly used method for dealing with these types of waste such asbauxite is solidification / stabilization.[1] Remediation for alkalineleachate requires activeaeration in order to promotecarbonation, recirculation of drainage waters over stockpiled orlagooned residues, and acid dosing. Strong acids (e.g.hydrochloric acid andsulfuric 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.[1]Wetlands are also a low-cost remedy for alkalineleachates.[1]