Rethinking enrichments of trace element in low-rank coals: Preliminarily quantifying the effectiveness of local structure to stabilize humates

Abstract

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− and C6H5O− is preliminarily quantified. It is completed by evaluating the Mulliken charge excess ξ, 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 ξ values and stabilizing metal–humate complexes. Otherwise, electron-withdrawing groups like C = C, C≡C, and C = O play a contrary role. Based on calculations, at the vicinity of ξ ≈ 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 higher binding energies than the combination of hydroxyl and carbonyl or carboxyls at the same ξ range.