Heavy Metal Regularity of Carboniferous Weathered Black Shale in Qiziqiao Area, Central Hunan

Abstract

The Hunan region is a high geological background area of black rock series rich in various metallic elements; accordingly, local heavy metal pollution is severe. Previous studies on black shale have primarily focused on the lower Cambrian strata, while research on Carboniferous black shale remains scarce. To better explore the activity law of heavy metals during Carboniferous black shale weathering, this study analyzed the elemental components of samples through field observations of outcrops in the Qiziqiao area of central Hunan province, China using inductively coupled plasma mass spectrometry and X-ray fluorescence spectrometry. The results showed that the heavy metal content of black shale under different degrees of weathering varied greatly, with different heavy metals maintaining distinct migration and enrichment rules throughout this process. The heavy metal content in Carboniferous black shale and soils of central Hunan was generally less than that of the regional lower Cambrian black shale and soil; however, the Cd content in the black shale soil was slightly higher than background values, while the Sr content was substantially higher than background values. Heavy metals V, Cr, Co, Ni, Cu, and Pb were not generally leached or released during weathering, and may undergo relative enrichment or secondary enrichment. Comparatively, Zn, Cd, and Ba can be more readily leached and released, and may undergo secondary enrichment. The lithophilic element Sr tended to leach out overall and expressed strong activity, whereas the chemical activities of the high-field-strength elements, Zr and Hf, were relatively stable. During soil formation, all heavy metal elements save Sr were significantly enriched. The enrichment factor analysis of different degrees of weathered black shale reveals that the heavy metals Ba, Hf, and Sr in black shale soil originate from the parent rock. V, Cr, Co, Ni, Cu, Zn, Cd, and Pb are influenced by both natural and anthropogenic factors, with Cd being significantly impacted by human activities. The evaluation of soil heavy metals using the geoaccumulation index method indicates that V, Cr, Co, Ni, Cu, Zn, Zr, Ba, Hf, and Pb are non-polluted, Cd exhibits moderate pollution, and Sr shows moderately heavy pollution. From a geochemical perspective, it can be inferred that heavy metals in black shale soil are likely to be secondarily enriched in clay and iron oxide minerals.

Keywords

• black shale
• major elements
• trace elements
• weathering
• heavy metal