Coordination structures of organically bound paramagnetic metals in coal and their transformation upon solvent extraction

In this paper the organically bound paramagnetic (transition) metals (Fe, Cr, Mn, Co, Ni, V, Zn and Cu) were studied by a combination of sequential acid leaching and electron spin resonance spectroscopy. Additional investigation was also conducted on the occurrence of these metals in coal extracts generated by solvent extraction at 360 °C. The results indicate that, a sequential leaching by acetic acid (1 M), HCl (3 M), HNO3 (2 M) and HF (48%) can remove most the inorganic species in a coal. The remaining elements are mainly transition metals. Of those, Fe is the most prevalent, which is mainly present as octahedral Fe3+ complex associated with phenolic hydroxyl or nitrogen-bearing functional groups. Nanometric ferric oxide/hydroxide and highly ordered Fe-rich crystals were also observed in coal extract. They are finely dispersed in the closed voids in the carbonaceous matrix, which may have been of biological origin, such as ferritin iron in proteins and iron porphyrin complexes that have undergone complex transformation during coalification. The water-derived colloidal iron phase can be one possible source as well. Regarding the other transition metals, they mostly have the hyperfine configurations that co-exist with Fe. The amounts of these organically bound species may depend on coal rank, but their relationship has not been clarified yet. Comparison between acid-washed coal and the respective coal extract also indicates the non-uniform distribution of organically bound transition metals within coal matrix.