Distribution and leachability of heavy metals from lead-zinc tailings with different degrees of chemical weathering in the Qinling Mountains

The Qinling Mountains in Shaanxi Province are one of China's most significant ecological reserves, but many lead-zinc tailings have been produced during decades of mining development. The heavy metals in these lead-zinc tailings pose a severe pollution risk to the surrounding areas. However, the vertical distribution, leaching behaviour and ecological risk of heavy metals in these tailings remain unclear. A lead-zinc tailings profile from the southeast foot of the Qinling Mountains was studied. The particle size, chemical composition, mineralogy, micromorphology, Nemerow pollution index and leaching behaviour were analysed. The results showed that the lead-zinc tailings profile had an intermediate chemical weathering layer (depth 0–70 cm) and an incipient chemical weathering layer (depth 70–100 cm). Quartz and muscovite were the main minerals. The intermediate chemical weathering layer was more enriched in Cr and Pb, while the incipient chemical weathering layer was more enriched in Cd and Zn. The final leaching concentrations of Cr, Cd, Zn and Pb in the incipient and intermediate chemical weathering layers after 50 days were very similar, at 0.27–0.36, 0.60–0.81, 6.93–9.03 and 7.81–10.47 mg L⁻¹, respectively. However, the average leaching efficiency was the reverse. Calcite and chemical weathering intensity may have been connected to this. The lead-zinc tailings were highly polluted, with Pb, Zn, Cd and As being the main pollutant elements. In summary, this study provides new insights for controlling heavy metal migration and mitigating environmental hazards in existing lead-zinc tailings ponds in the Qinling Mountains.