Overall, the modes of occurrence of elements in coal are classified into inorganic, organic, and intimate organic associations.
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. 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. 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. 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. 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. Compared with the single, complete, and centroid, the average-linkage algorithm is indeed the optimum.Ĭoal, 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. WSPC produces more interpretable results than those from pivot coordinates transformed data for these coal elemental data. Results showed that the Pearson correlation produces more meaningful results than the Euclidean distance in clustering rare earth elements and Y. In this paper, we discuss four commonly used hierarchical clustering algorithms utilizing pivot coordinates and weighted symmetric pivot coordinates (WSPC), two types of log-ratio transformations, to infer modes of occurrence of elements in coal, based on published coal elemental data. Different hierarchical clustering algorithms with various data transformations can infer modes of occurrence for coal elements, but which algorithm is optimum deserves to be investigated. This work applied log-ratio transformations in order to overcome this problem. The traditional statistics (e.g., Pearson correlation, Euclidean distance) for the elemental data of coal may lead to misinterpretation because the elemental data of coal are of compositional nature and follow the rules of Aitchison geometry. Hierarchical clustering algorithm has been widely adopted to investigate the modes of occurrence of elements in coal. The modes of occurrence for elements in coal are extremely important for deciphering geological process of coal formation and for anticipating the technological behavior and environmental and health impacts derived from coal utilization. Disadvantages of PIXE/PIGE techniques include: (1) their detection limits are less sensitive than atomic absorption spectroscopy or ICP-MS methods (Whateley, 2002) (2) despite the advantage of examining small amounts of sample material, the small volume of excitation raises questions of sample representativeness and homogeneity, particularly for heterogeneous materials like coal (3) matrix corrections tend to be large, especially for thick samples, and, although initially there were some issues with accurate quantification, these early problems have been overcome (Teesdale et al., 1988 Burnett et al., 1988 Pineda and Peisach, 1988) and (4) they require particle accelerators (Wong and Robertson, 1993). (1977) and Kullerud and Steffen (1979) first reported the PIXE/PIGE data on US coals spread on a thin plastic substrate, with a weight < 2 mg of coal and pulverized to less than 20 μm (2) It is capable of rapidly determining most elements (i.e., 75 elements in the periodic table) simultaneously (3) Little is required in regard to sample preparation and, therefore, there is a reduced possibility for contamination and preparation error and (4) The technique is more sensitive to trace element species than conventional XRF.
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