Binding characteristics of Cd²⁺, Zn²⁺, Cu²⁺, and Li⁺ with humic substances: Implication to trace element enrichment in low-rank coals

Abstract

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.

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

“…Our previous work effectively optimized the geometry of adsorbed metal ions and calculated the BE for monovalent and divalent cations with BP86/Stuttgart Dresden Dunning ECP method (He et al., 2016). Thus, this method is still selected to perform geometry optimization for HFSEs.…”

“…BE: binding energy. Note: The BE values for Cd, Zn, Li, and Cu cases from He et al (2016). BE: binding energy.…”

“…However, further investigations of the occurrence of trace elements and their origins in coals have been impeded by the state-of-the-art analytical methods and unambiguous enrichment mechanisms, albeit significant potential availability. 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, then 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).…”

“…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, then 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). The less the metal-ligandbonding part deviates from electroneutrality, the more stable the organometal complex is.…”

Mineralogy and geochemistry of the coal seam of Shanxi Formation in Guotun Mine, Juye Coalfield, North China

“…The mode of occurrence of Li in coal is still an unresolved topic. The coalification seems not to be fit for much enrichment of Li (He et al, 2016). A Li-carrier mineral discovered in coal was an intermediate between cookeite and chamosite (Dai et al, 2012).…”