Acid mine drainage AMD is a major environmental problem 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 dissolved metal ions. The presence of AMD can significantly affect the pH level and concentration of heavy metals in nearby bodies of water, causing severe ecological and health issues.1. Effect on pH level:The primary chemical process responsible for the formation of AMD is the oxidation of pyrite. This process can be represented by the following reactions:1 Pyrite FeS2 reacts with water and oxygen to form ferrous iron Fe2+ and sulfate ions SO42- :FeS2 + 7/2 O2 + H2O Fe2+ + 2 SO42- + 2 H+2 Ferrous iron Fe2+ reacts with oxygen to form ferric iron Fe3+ :4 Fe2+ + O2 + 4 H+ 4 Fe3+ + 2 H2O3 Ferric iron Fe3+ reacts with pyrite to form ferrous iron Fe2+ and sulfate ions SO42- :FeS2 + 14 Fe3+ + 8 H2O 15 Fe2+ + 2 SO42- + 16 H+These reactions result in the release of hydrogen ions H+ , which lowers the pH of the water, making it more acidic. The pH of AMD-affected water can range from 2 to 5, which is significantly lower than the neutral pH of 7.2. Effect on heavy metal concentrations:The acidic conditions created by AMD promote the dissolution of heavy metals from the surrounding rocks and minerals. The low pH increases the solubility of heavy metals such as lead Pb , copper Cu , zinc Zn , cadmium Cd , and arsenic As , causing them to leach into the water. The presence of these heavy metals in high concentrations can be toxic to aquatic life and pose a risk to human health.Moreover, the formation of ferric hydroxide Fe OH 3 precipitates can also contribute to the release of heavy metals. When the pH of AMD-affected water increases due to natural processes or remediation efforts, ferric iron Fe3+ can react with hydroxide ions OH- to form ferric hydroxide:Fe3+ + 3 OH- Fe OH 3 s These ferric hydroxide precipitates can adsorb heavy metals, but they can also release them back into the water under certain conditions, such as changes in pH or redox potential.In summary, the presence of acid mine drainage affects the pH level and concentration of heavy metals in nearby bodies of water by promoting the oxidation of sulfide minerals, which generates sulfuric acid and releases hydrogen ions. This leads to acidic conditions that increase the solubility and mobility of heavy metals, causing them to leach into the water and pose ecological and health risks.