The concentration of acidity in acid mine drainage AMD plays a significant role in the solubility of heavy metals and other toxic substances, which in turn impacts the water quality in nearby aquatic ecosystems.Acid mine drainage is formed when sulfide minerals, such as pyrite FeS2 , are exposed to water and oxygen during mining activities. This exposure leads to the formation of sulfuric acid H2SO4 and dissolved iron Fe . The resulting acidic water can dissolve heavy metals and other toxic substances present in the surrounding rocks and soil.The solubility of heavy metals, such as lead Pb , copper Cu , zinc Zn , and cadmium Cd , increases with increasing acidity. This is because the acidic conditions promote the formation of soluble metal ions and complexes, which can be more easily transported in water. As the pH of the AMD decreases i.e., becomes more acidic , the solubility of these metals increases, leading to higher concentrations of dissolved metals in the water.When acid mine drainage enters nearby aquatic ecosystems, it can have several negative impacts on water quality:1. Lowered pH: The acidic nature of AMD can lower the pH of the receiving water body, making it inhospitable for many aquatic organisms, including fish, invertebrates, and plants.2. Increased heavy metal concentrations: The increased solubility of heavy metals in AMD can lead to elevated concentrations of these toxic substances in the water, which can be harmful or even lethal to aquatic life. Heavy metals can accumulate in the tissues of aquatic organisms, leading to bioaccumulation and biomagnification in the food chain, ultimately affecting larger predators and even humans who consume these organisms.3. Precipitation of metal hydroxides: When AMD mixes with water that has a higher pH, metal hydroxides can precipitate, forming a visible orange, red, or yellow sludge known as "yellow boy." This sludge can smother aquatic habitats, clog fish gills, and reduce light penetration, negatively affecting photosynthesis and overall ecosystem health.4. Reduced dissolved oxygen levels: The oxidation of sulfide minerals and the precipitation of metal hydroxides can consume dissolved oxygen in the water, leading to hypoxic or anoxic conditions that are harmful to many aquatic organisms.To mitigate the impacts of acid mine drainage on water quality and aquatic ecosystems, various treatment methods can be employed, such as neutralization with alkaline materials e.g., limestone , passive treatment systems e.g., constructed wetlands , and active treatment systems e.g., chemical precipitation . These methods aim to raise the pH of the AMD, reduce the concentrations of dissolved metals, and restore the health of affected aquatic ecosystems.