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.
Highlights
He et al (2016) hypothesized that oxygen-containing functional groups on the surface of humic substances, the main organic of low-rank coal, played the role of adsorption site in transporting trace element cation ions, and in subsequent polymerization, trace element cation ions entered into the fluids, precipitated with authigenic minerals or were exported by fluids
Due to the high ionic potential, high field strength elements (HFSEs) tend to form tridentate with the B site of benzoquinonetetracarboxylic acid where two carboxylates on ortho position chelate HFSE
The effect of local structure on the ability of main oxygen-containing groups including –COOÀ and C6H5OÀ 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
Summary
Enrichment of trace element relative to world average coal has been extensively reported (Dai et al, 2017: Lauer et al, 2017; Long and Luo, 2017; Qin et al, 2015; Saikia et al, 2009, 2015, 2016; Sun et al, 2016; Sun et al, 2017; Xiao et al, 2016; Yi et al, 2015; Zhao et al, 2014, 2017). Based on field investigations, He et al (2016) hypothesized that oxygen-containing functional groups on the surface of humic substances, the main organic of low-rank coal, played the role of adsorption site in transporting trace element cation ions, and in subsequent polymerization, trace element cation ions entered into the fluids, precipitated with authigenic minerals or were exported by fluids. Our previous work has qualitatively revealed that singly deprotonated alkyl/aryl carboxylic acids, analog of humic substance, are able to effectively transport trace element cation ions (He et al, 2016). Intensive coalification is shown to be unable to stabilize carboxylates To decipher these scenarios, the ability to transport and adsorb trace element cation ions by oxygen-containing functional groups on the coal surfaces must be explored as well as the effect of coal structure. Besides p-benzoquinonyl and carbonyl, more typical electron-withdrawing groups including C=C, CC, and C=C-C=C are selected to further probe the effect of electron re-distribution on the stability of Li carboxylate
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