Insight into acid generation and the release risk of Cd, Cu and Mo from a high-latitude polymetallic mine in China

Considerable amounts of heavy metals can be released through mining activities, and different heavy metals, such as Cd, Cu and Mo exhibit varying release behaviors from tailings and ores under similar conditions; therefore, the insight to their release risk for tailings, waste rock, and ores is crucial for developing protection strategies for mines. In this study, samples of tailings, ores, and surface water were collected from a high-latitude polymetallic mine in China to investigate the risk of heavy metal release. Relatively high concentrations of heavy metals such as Cd, Cu and Mo were found in tailings and some ores. However, tailings were classified as nonacid generation indicating a low release risk of heavy metals, and only some ore samples (DBS2, DBS3 and DBS5) with high concentrations of Cd, Cu and Mo were classified as acid generation. At low leaching pH, more Cd, Cu, and Fe were leached from tailings and DBS5 because of the dissolution of chalcopyrite. In contrast, elevated Mo concentrations in the leachate were observed under neutral and alkaline conditions (pH = 7 and 9), which can be attributed to the high adsorption capacity of MoO₄^2- on tailings and ores, as well as the easy dissolution of molybdenite under neutral to alkaline conditions. The results suggest that under acidic conditions and in environments with potential for acid generation, increased attention should be directed toward the release risks of cationic heavy metals such as Cd and Cu. In contrast, under neutral or alkaline conditions, the mobilization risks of anionic heavy metals such as Mo require more rigorous monitoring. The surface water samples near the mine contained relatively high concentrations of Cd, Cu, and SO₄^2- with low pH, indicating that the surface water was affected by mining activities. These findings suggest that acid mine drainage (AMD) generated from waste rocks or ores, rather than tailings, is the primary cause of elevated heavy metal concentrations in surface water.