Modes of Occurrence of Cr, Co, Ni, Cu, Cd, and Pb in the Main Coal Seams of Southwestern China’s Nantong Coalfield

Abstract

The modes of occurrence of trace elements in coal reveal essential information about environmental impact during coal exploitation and utilization. Utilizing improvements on a recently published sequential leaching protocol, this study investigated the modes of occurrence of Cr, Co, Ni, Cu, Cd, and Pb (water-soluble, ion-exchangeable, carbonate-bound, sulfide-bound, organic-bound, and silicate-bound) in coal samples collected from the Nantong coalfield in southwestern China. Results suggest the improved method favorably affected the sequential leaching experiment. The ashing temperature did not exceed 390°C and did not damage the structure of the silicate minerals, facilitating the effective extraction of trace elements within the organic matter. The case study in the Nantong coalfield demonstrated that the water-soluble and ion-exchangeable fractions account for only a small portion of the six elements. Specifically, chromium exhibited affinities with carbonates and silicates. The percentages of different target fractions for cobalt varied significantly in different samples. Nickel was almost evenly distributed in the fractions of carbonates, sulfides, organic matter, and silicates. For cadmium, silicate was the main fraction, followed by sulfides. Lead predominantly appeared in carbonates and sulfides. The correlativity of different elements in the corresponding fractions revealed carbonates and sulfides as the primary carriers for all six elements, followed by silicates and organic matter. Overall, Cr, Co, Ni, and Cu exhibited very close relationships and similar distribution patterns in different coal samples. Organic-bound Cr, Co, and Ni were noticeable in the No. 4 and 6 coal seams, though they were not the dominant fractions. The affinity between Pb and Cd indicated they have similar modes of occurrence.