Chemical Imaging of Coal Speciation within Chinese Outcrop Coals Using Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and Scanning Transmission X-Ray Microscopy (STXM)
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The study of combustion of Chinese outcrop coal has shed light on potential diseases that are found to be at high incidences in villages within the Chinese province of Guizhou. The important endemic disease of interest is skeletal fluorosis. It is believed that outcrop coals used for cooking and heating within homes of poor villagers leads to exposure of fluorine and other toxins that create a higher incidence of disease. Chemical imaging of outcrop coals samples was first performed using time-of-flight secondary ion mass spectrometry (TOF-SIMS) in order to determine speciation within the coals. Coal samples were cut and polished until a mirror-like surface was achieved. The analyses revealed that fluorine is chemically associated with aluminosilicates, specifically muscovite, which is a common aluminosilicate in nature. Reference materials were analyzed to confirm the presence of aluminosilicates in the coal. Sulfur was found to exist in three different forms within the coals: elemental sulfur, organic sulfates, and pyrite. Scanning transmission X-ray microscopy (STXM) was utilized to analyze the different carbon species present with the outcrop coals. The results of the outcrop coals revealed that there were three different forms of carbon detectable in the near edge x-ray absorption fine structure (NEXAFS) spectra: aromatics, carboxylics, and carbonates. The chemical images of the outcrop coals reveal that carboxylics appear to be the dominate species within the coals. This was confirmed using TOF-SIMS. Coal reference materials from Canada and the United States were analyzed for comparative purposes, and they revealed that the dominating carbon species in North American coal were aromatics.