Modes of occurrence of elements in coal: A critical evaluation

Coal, containing all the elements that are present in nature and more than 200 minerals, has a complex chemical structure, making it one of the most complex geological materials. Inorganic matter in coal includes minerals (in which element concentration may vary from trace to major), non-crystalline mineraloids, and elements with non-mineral associations such as those occurring in pore waters, organically bound, or in an organic association. Understanding the modes of occurrence of elements in coal is important because, theoretically, they provide useful information on peat deposition, diagenesis and epigenesis of coal, coal-hosted basin formation, and the regional geological background or evolution. Practically, the modes of occurrence of elements play a significant role in affecting coal mining, coal preparation, coal combustion, and coal utilization, and in exerting adverse effects on both the environment and human health. The modes of occurrence of critical elements in coal and coal ash are key factors for designing the method and technology required for extracting critical metals from coal or coal ash. In this paper, the following aspects are reviewed, including the modes of occurrence of 73 elements and rare gases that occur in coal (with the exceptions of organically associated C, H, O, and N), the definition of modes of occurrence and their practical and academic significance, analytical methods for determining modes of occurrence of elements in coal and their advantages and limitations, and reported modes of occurrence of elements in coal and their likely associations. Overall, the modes of occurrence of elements in coal are classified into inorganic, organic, and intimate organic associations. Although there are common modes of occurrence of many elements in coal, there are many exceptions and most, if not all, elements have multiple modes of occurrence. Each mode of occurrence of an element may also show different levels of confidence, namely, certain, probable, possible, doubtful, unlikely, and may occur in coal with different frequencies, namely abundant, common, uncommon, rare, and unlikely. For each element, the authors present concluding comments viewpoints on the modes of occurrence of almost each element in coal that is listed in the old literature. The different modes of occurrence for each element in different coals depend on the geological conditions of coal formation, and do not necessarily indicate inconsistency in the reported resul