Acid mine drainage AMD is a major environmental problem caused by the exposure of sulfide minerals, such as pyrite FeS2 , to water and oxygen during mining activities. This exposure leads to the formation of sulfuric acid and dissolved metals, which can significantly lower the pH of water, making it highly acidic and toxic to aquatic life.To restore the pH of water affected by acid mine drainage to a safe level for aquatic life, several treatment methods can be employed:1. Neutralization: The most common method is to add alkaline materials, such as limestone CaCO3 , hydrated lime Ca OH 2 , or soda ash Na2CO3 , to the acidic water. These materials react with the sulfuric acid, neutralizing it and raising the pH of the water. The choice of neutralizing agent depends on the specific conditions and requirements of the treatment site.2. Passive treatment systems: These systems use natural processes to treat AMD without the need for chemicals or energy inputs. Examples include constructed wetlands, anoxic limestone drains, and open limestone channels. These systems rely on the natural buffering capacity of limestone and the ability of certain microorganisms to remove metals from the water.3. Active treatment systems: These systems involve the use of engineered structures and require regular maintenance and energy inputs. Examples include lime dosing systems, where lime is continuously added to the water to maintain a neutral pH, and high-density sludge treatment, where lime is added to precipitate metals as sludge, which is then removed from the water.4. Prevention and source control: This involves preventing the formation of AMD in the first place by sealing off mine entrances, isolating pyrite-containing materials from water and oxygen, and diverting clean water away from mine waste materials.5. Bioremediation: This method uses microorganisms, such as sulfate-reducing bacteria, to convert sulfate ions into sulfide ions, which can then react with dissolved metals to form insoluble metal sulfides. This process not only removes metals from the water but also helps to raise the pH by consuming acidity.The choice of treatment method depends on factors such as the volume and acidity of the AMD, the concentration of dissolved metals, the available resources, and the desired level of water quality. In many cases, a combination of treatment methods may be required to effectively restore the pH of water affected by acid mine drainage to a safe level for aquatic life.