The Characteristics of Heavy Metal Pollution in Open-pit Coal Mining Wasteland in Southwest Guizhou

Taking a typical open-pit coal mine in Qianxinan as the research object, collecting soil samples from the waste slag area, the external soil cover area, the phytoremediation area and the unmined control area around the mining area, and determining the pH value, heavy metal content and morphology of soil. At the same time, analyzing the heavy metal pollution diffusion process by standing leaching method. The results showed that the pH values of soil in the waste slag area, the external soil cover area and the phytoremediation area were 2.86, 3.49 and 4.90, respectively, which were lower than those in the unmined control area(pH=6.48), and the soil in the waste land was severely acidified. The contents of heavy metals Cu and Cd in the waste slag area exceeded the soil background values of Guizhou Province by 4.57 times and 3 times, and exceeded the soil pollution risk screening values by 1.86 times and 0.73 times, respectively. The Cr content of heavy metals in the waste slag area exceeded the soil background value of Guizhou Province by 1.14 times and the soil pollution risk screening value by 0.37 times. The heavy metals Cu and Cd in the waste slag area were moderately polluted or above, the heavy metals Cr and Pb were moderately polluted, and the pollution degrees of Pb, Zn and Cr in the phytoremediation area were not significantly different from the control. The nemero pollution index, soil pollution load index and soil comprehensive potential ecological hazard index showed that the pollution level of the phytoremediation area and the external soil cover area was mild pollution, and the waste slag area was severely polluted. The proportion of weak acid extractable state of Cu, Cr and Cd in the waste slag area and the external soil cover area was significantly higher than that in the phytoremediation area and the control area. The release rate and amount of heavy metal leaching in soil in the waste slag area were higher than those in other areas. The correlation analysis showed that there was a significant correlation between soil pH and the morphological characteristics of heavy metals Cr, Cu and Pb, and the decrease in pH significantly increased the proportion of extractable state of heavy metal weak acid. The soil acidification obviously promoted the migration and release of heavy metal elements and the improvement of bioavailability, and leaching intensified soil acidification and the diffusion of heavy metal pollution.