Long-term acid generation and heavy metal leaching from the tailings of Shimokawa mine, Hokkaido, Japan: Column study under natural condition

The leaching from mine tailings results in contamination of nearby groundwaters and rivers by hazardous metals like copper (Cu), zinc (Zn), and iron (Fe). In this study, unweathered tailings samples at depths of 1 to 3 m were collected from a tailings dam of an abandoned mine located in the north of Hokkaido, Japan. The mechanisms of long-term tailings weathering were assessed through leaching of hazardous metals by three column experiments. Measurements of mineralogical and chemical constituents, observation by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX), sequential extraction of the tailings, and chemical analyses of the leachates were carried out to determine the processes responsible for the leaching of Cu, Zn, Fe, and sulfate ion (SO42- ). The contents of Cu, Zn, and Fe in the tailings were mainly associated with ion exchangeable and sulfide fractions. The pH values of the effluents from the columns were 3.0-3.7 throughout the experiments over 84 weeks, and approximately 15-23% of Cu, 35-45% of Zn, 2.5-4% of Fe and 15-20% of S were leached. Higher concentrations of Cu, Zn, Fe, and SO42- at the beginning of the experiments were observed, which could be attributed to the dissolution of soluble sulfate minerals present in the tailings. This indicates that the formation and dissolution of secondary soluble sulfate minerals contributed to Cu and Zn leaching. The continuous leaching of Cu, Zn, Fe, and SO42- suggests the oxidation of pyrite and other sulfide minerals. During these processes, ferrihydrite, goethite, lepidocrocite, and maghemite were formed and these minerals also acted as a sink for Cu and Zn by adsorption, and/or co-precipitation. These results mean the significance of the long-term behavior of hazardous metals released from mine tailings dams, which could provide helpful information on the management of tailings dams after mine closure.