Heavy Metal Concentrations of Soil, Rock, and Coal Gangue in the Geological Profile of a Large Open-Pit Coal Mine in China

Xiaoyang Liu,Zhongke Bai,Ming Jing

doi:10.3390/su14021020

Abstract

Risk assessment related to heavy metals in mining areas is crucial to ensuring the sustainable development of regional ecosystems and protecting human health. However, almost all research on the impact of mining activities on environmental quality entails field monitoring of surface soils or soil profiles. Here, to compare the variety of heavy metal concentrations in the geological profile, 39 samples (including soil, rock, and coal gangue) were collected and analyzed from hundreds of meters underground in the Pingshuo coal mine (Anjialing coal mine, Antaibao coal mine, and Donglutian coal mine), which is the largest open-pit coal mine in China. The mean heavy metal concentrations of Cd, Hg, As, Pb, Cr, Cu, Zn, and Ni in soils were 0.15 mg/kg, 0.02 mg/kg, 13.70 mg/kg, 27.12 mg/kg, 70.89 mg/kg, 26.10 mg/kg, 79.88 mg/kg, and 37.68 mg/kg, respectively; most of these metals were more concentrated in the soil samples than in the rock and coal gangue samples. A potential ecological risk assessment indicated that As, Pb, Cr, Cu, Zn, and Ni in all samples showed low risk, while the ecological risk degrees of Cd and Hg were higher and contributed most to the total risk index (RI). The highest E_r^i value (92.94) for Cd was found in rock samples, contributing to approximately 55.18% of the to tal RI. For Hg, four rock sampling sites had the highest E_r^i values (427.70, 270.78, 198.96, and 188.70), contributing approximately 68.36–88.07% of the total RI. Ranked after soil (0.15 mg/kg), limestone and shale showed high mean Cd concentrations (both 0.13 mg/kg). The highest mean concentration was found in coal gangue samples (0.1 mg/kg). Compared with the other types of samples, the mean concentrations of Cr, Cu, and Ni in soils, limestone, and mudstone were relatively higher. At different sites, heavy metal concentrations varied greatly at different depths, owing to diverse sample types and rock types. Correlations between heavy metals and soil nutrients indicated that N and P were closely correlated with heavy metals. Comparing the proportions of different heavy metals in various types of samples showed that heavy metals in soils were extremely similar to those in limestone, mudstone, and shale, implying that soils might indirectly inherit their features from their parent material.

Highlights

Coal is one of the world’s main energy sources, generating 40.4% of the world’s electricity
In contrast with previous studies [47], we found that standard methods described organic mattersoil (SOM) had no significant correlations with heavy metals in this study; this may be attributed to the coal gangue being enriched in soil organic C, which would influence the accurate determination of SOM
This study investigated the concentrations, potential ecological risk, and variations with depth and sample type of heavy metals in a large open-pit coal mine in China

Summary

Introduction

Coal is one of the world’s main energy sources, generating 40.4% of the world’s electricity It has helped greatly with the development of society and the global economy. Heavy metals are commonly found in the soil at various concentrations, owing to the pedogenetic process of weathering of the parent materials. It is accepted that heavy metal concentrations in soils vary depending on the source material, mainly due to the presence of geochemically unique rocks [8]. Rocks and soils are common solid wastes and are produced in large volumes during the process of open-pit coal mining. As they are usually used for backfilling pits and land reclamation, their environmental quality is vital for the regional environment. Toxic substances in coal gangue and rocks might migrate into recultivated soils, which might pollute soil [11,12,13] and threaten ecological environment and human health [14,15]

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Conclusion