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+, Cd2+, Zn2+, Cu2+) 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+ < Cd2+ < Zn2+ < Cu2+. 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 electron-donating 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 (Cd2+, Cu2+ and Zn2+), 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, two carboxyls. Strong electron-withdrawing effect attributes to the decrease of the BE value for the latter three. 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.