Relationships between trace elements and organic matter in coals

The modes of occurrence of trace elements (TEs) in coals are important for assessing the potential for economic extraction of valuable TEs (Ge, Ga, Li, REY, PGEs, etc.) and to determine the environmental risk of hazardous TEs (F, Cl, As, Cr, Cd, Pb, etc.). To date, associations of TEs with inorganic components (minerals) are well understood. However, information on the relationships of TEs with organic matter in coals is relatively limited. This review compares indirect and direct methods employed to investigate the modes of occurrence of TEs in coals, focusing on the associations of TEs with organic matter. Those TEs with a strong organic relationship are identified and detailed; they include Ge, U, Ga, REY, Be, B, As, Se, Cl, Br, and W. Other TEs weakly related with organic matter in coals are also detailed in the order of their periodic table groups. Finally, the influence of coal rank on the relationship of TEs with organic matter is discussed, and an inherent geochemical law, i.e., the element periodic law, is proposed to govern the relationship.