Acid mine drainage AMD is a major environmental issue associated with mining activities. It occurs when sulfide minerals, such as pyrite FeS2 , are exposed to water and oxygen, resulting in the formation of sulfuric acid and the release of heavy metals into the environment. The effect of AMD on the concentration of heavy metals in surface water and groundwater in mining regions can be significant, leading to severe water pollution and negative impacts on aquatic ecosystems and human health.The increased concentration of heavy metals in surface water and groundwater is primarily due to the following chemical processes involved in their mobilization and transport:1. Oxidation of sulfide minerals: When sulfide minerals are exposed to water and oxygen, they undergo oxidation, producing sulfuric acid and metal ions. For example, the oxidation of pyrite can be represented by the following reaction:4FeS2 s + 15O2 g + 14H2O l 4Fe OH 3 s + 8H2SO4 aq The sulfuric acid produced in this reaction can further react with other minerals, releasing additional heavy metals into the environment.2. Acidification of water: The sulfuric acid generated by the oxidation of sulfide minerals lowers the pH of the surrounding water, making it more acidic. This acidic environment enhances the dissolution of heavy metals from minerals, increasing their concentration in surface water and groundwater.3. Formation of soluble metal complexes: In acidic conditions, heavy metals can form soluble complexes with sulfate, chloride, and other anions, which increases their mobility and transport in water. For example, copper can form soluble complexes with sulfate ions:Cu2+ aq + SO42- aq CuSO4 aq 4. Adsorption and desorption processes: Heavy metals can adsorb onto the surfaces of minerals, organic matter, and other particles in the environment. Changes in pH, redox conditions, and the presence of competing ions can cause these metals to desorb from these surfaces and become more mobile in water.5. Colloidal transport: Heavy metals can also be transported in the form of colloidal particles, which are small, suspended particles that can move through water. These colloids can consist of metal oxides, hydroxides, or other compounds, and their transport can be influenced by factors such as pH, ionic strength, and the presence of organic matter.Overall, acid mine drainage can significantly increase the concentration of heavy metals in surface water and groundwater in mining regions, posing risks to aquatic life, human health, and the environment. Effective management and remediation strategies, such as the prevention of sulfide mineral oxidation, neutralization of acidic waters, and removal of heavy metals, are crucial to mitigate the impacts of AMD.