Spatial variation characteristics of soil heavy metal speciation and bioavailability in coal gangue accumulation area—Taking Fengfeng mining area as an example

The surface accumulation of coal gangue is a typical form of ecological environment destruction in the eastern coal mining area of China, and the prevention and control of heavy metal pollution in the soil in the accumulation area has become a hot spot in the current research. Taking the shallow soil (with a vertical depth of 40 cm and below) around a coal gangue mountain (within a radius of 300 m) in use in the Fengfeng mining area of Hebei as the research object, the active state, iron manganese oxidation state, organic state and residue state of four kinds of soil heavy metals, including Cu, Cr, As and Pb, were determined by Tessier continuous extraction method and inductively coupled plasma emission spectrometer. Based on Bioactivity Factor (MF) and Ecological Risk Assessment Coding (RAC) method, the bioavailability and ecological risk of the four soil heavy metals were calculated and evaluated, and the spatial variation characteristics of the morphological composition and bioavailability of the four soil heavy metals were analyzed.The results show that: (1) The storage of coal gangue has the effect of increasing the proportion of Cu, Cr, As and Pb active state content in the surrounding soil and reducing the proportion of residue state content, and is inversely proportional to the horizontal distance from the coal gangue mountain. When the horizontal distance reaches or approaches 300 m, the effect basically disappears. (2) The correlation coefficients between the mass fraction of soil Total Organic Carbon (TOC) and the available mass fractions of the four heavy metals all exceeded 0.6, reaching a significant positive correlation level (p<0.05), which was the main factor affecting the spatial variation characteristics of the soil heavy metal availability content in the study area. (3) The spatial variation characteristics of the bioavailability of the four soil heavy metals can be divided into two types. Type I is “MF value is generally greater than 3 and has significant spatial heterogeneity”, such as Cu, Cr. Type Ⅱ is “MF value is generally less than 3 and has significant spatial homogeneity”, such as As, Pb. (4) When the horizontal distance from the coal gangue mountain is less than 80 m and 40 m, respectively, the average MF of soil heavy metals Cu and Cr exceeds 60% of the medium risk level threshold, which should be taken seriously. It is recommended to comprehensively consider factors such as the existence status of coal gangue mountains, the cumulative effect of heavy metals in the soil and the spatial variation characteristics of bioavailability. Formulate a precise prevention and control and safe utilization strategy for soil heavy metal pollution in coal gangue accumulation areas.