Binding characteristics of metal ions (Li + , Cd 2+ , Zn 2+ , Cu 2+ ) with humic substances (HSs) are investigated by first principle calculation at BP86/SDD level. In the present work, HSs are well represented by singly deprotonated alkyl/aryl carboxylic acid. The results reveal that the bidentate is the strongest complexation form. Moreover, with the increase of ionic potential, the binding energies of HSs to chelate metal ions increase in order, Li + < Cd 2+ < Zn 2+ < Cu 2+ . The nature of the ligand also exerts important influence on the stability of organometallic complexes. Specifically, for Li + , the ionic bond-forming part occurs as electroneutrality. Whatever electrondonating groups or electron-withdrawing groups including alkyl, aryl and its derivatives (salicylic acid and m (p)-substituted gallic acid by hydroxyl or amino) and carbon-carbon double bond connects to the carboxylate group, the electron density of the ionic bond-forming part will be changed resulting in the decrease of binding energy (BE). However, for ions with high ionic potential or high positive charge density (Cd 2+ , Cu 2+ and Zn 2+ ), the ionic bond-forming part is positively charged. From electron-withdrawing groups to electron-donating groups, the extent to deviate from electroneutrality is gradually decreased, although the quantity of electron is possibly insufficient to balance the extra positive charge. As a result, the BE value gradually increases, but does not reach a maximum for studied ions and ligands. Besides, mixed functional groups are able to stabilize organometallic complexes. The optimal mixed functional group is carboxyl and hydroxyl on ortho position, followed by two hydroxyls, hydroxyl and carbonyl, carboxyl and carbonyl, In the end, the geochemical behavior of trace elements in coalification is predicted: small organic acids generated by decomposition of dead plants in relatively oxidized environments will facilitate the enrichment of lithium and this proportion of lithium will be released into pore water in the polymerization of small molecules, possibly accumulating in secondary minerals in low-rank coals; metal ions with high ionic potential bonded by organic molecules will be released in a later stage and accumulate in slightly higher rank coals.

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This work investigated the mineralogy and geochemistry of the No. 3 coal seam in Permian Shanxi Formation in Guotun Mine, Juye Coalfield, north China, in order to understand the genesis of the minerals and the enrichment of trace elements. Approaches used were optical microscopy and electron probe microanalysis for minerals, X-ray fluorescence analysis for major elements and inductively coupled plasma mass spectrometry analysis for trace elements. The coal is comprised of dominant kaolinite and calcite, the claystone is characteristic of major kaolinite, montmorillonite, and proportional illite, and the sandstone contains mainly quartz and chalcedony, and a relative amount of feldspar and kaolinite. These minerals were derived dominantly from the weathered source rocks with volcanic and granitic constituents in the pre-diagenetic period, and minor from the hydrothermal deposit in the epigenetic period. The claystone is relatively enriched in Li, Cs, Be, Nb, Mo, U, Th, V, In, Pb, Bi, and Se, which may probably be controlled by the source rocks and the specific sedimentary environment. The enriched trace elements are mostly associated with minerals. Li may probably occur in montmorillonite and illite, while In, Pb, Bi, and Se occur mainly in selenio-galena.

Trace element enrichment in coals has increasingly been focused in the community of coal geochemistry. To research the enrichment mechanism of trace element in coals on molecular/atomic scale is an important complement of field investigations, but relevant works are scattered. In the present work, the effect of local structure on the ability of main oxygen-containing groups to chelate trace element metal ions including -COO À and C 6 H 5 O À is preliminarily quantified. It is completed by evaluating the Mulliken charge excess n, i.e. the sum of Mulliken charge for metal ions and atoms involved in chelating functional groups. When electron-donating groups like benzene and its derivatives immediately connect to cation carboxylates, excess electron will be transferred to cation carboxylates, pulling down the n values and stabilizing metal-humate complexes. Otherwise, electron-withdrawing groups like C ¼ C, CC, and C ¼ O play a contrary role. Based on calculations, at the vicinity of n % 0, the binding energy is highest. For monovalent cation ions, the most stable metal-humate complex is that chelated by small alkyl carboxylate, while the highest binding energy is not found for high-valent cation ions in the investigation. Meanwhile, both the combination of hydroxyl and carboxyl and hydroxyls have