Heavy Metals in Coal Fire Sponges in the Wuda Coalfield, Inner Mongolia

Coal fire sponge (CFS) is a sponge-like pollutant commonly found in areas affected by coal fires. This study investigated heavy metals in CFSs and soil samples in the vicinity of the Wuda Coalfield, Inner Mongolia. Studies of the sources and functions of heavy metals in CFSs and their impacts on the surroundings were conducted. The concentrations of Hg, Cd, As, Cr, and Zn were determined using inductively coupled plasma mass spectrometryand atomic fluorescence spectrometry. The results revealed that the concentrations of Hg (16,831.39 ng/g), As (30.32 μg/g), Cd (11.7 μg/g), Cr (185.99 μg/g), and Zn (795.06 μg/g) in CFSs were significantly higher than those in background samples, which indicates that CFSs have become a significant point source of heavy metal pollution in the region. The positive matrix factorization model identified four sources of heavy metals in CFSs: the accumulation of coal gangue, underground coal fires, industrial and traffic composite sources, and natural sources. Underground coal fires were the primary contributors, accounting for 43.6% of the total heavy metals. The ecological risk assessment demonstrated that Cd and Hg were the dominant potential risk factors, with CFSs exhibitinglevels ranging from heavily to extremely contaminated. CFSs not only effectively capture heavy metals from coal-fire emissions, but also serve as natural collectors of coal-fire pollutants, providing a convenient resource to study the chemical composition of coal-fire smoke.