Atomic chemical environment of tungsten in coal: Implication for the evolution of the germanium coal deposits

Abstract

The Wulantuga, Lincang, and similar germanium deposits are abnormally enriched in organically associated Ge and W. In this study, the Density Functional Theory (DFT) calculation is employed to investigate the structure and bonding nature of W in coal, which helps for a better knowledge of the mode of occurrence of organically bound metals and the evolution of the coal-hosted Ge deposits.Five model compounds (ethanol, phenol, catechol, acetic acid, and benzoic acid) that represent all hydroxyls in coal structure are used to simulate the formation of W-O complexes. According to the binding energies and Natural Population Analysis (NPA) charges, W is preferred to be bonded with O-exposed model compounds to form stable W-O complexes. The phenolic hydroxyls are easier to break OH bond, suggesting that the fragments of catechol and phenol are more reactive. The carbon rings in coal are capable of bonding W, but there is less chance for graphene to be the binding site.The W-O complex can be coordinately saturated if W connects with three (catechol, acetic acid, and benzoic acid) or four (phenol and ethanol) O-exposed molecules. Generally, due to stronger binding energies, W is likely to form more stable complexes with hydroxyls relative to Ge. It is noted that W is preferred to be bonded with both O in each molecule of acetic acid or benzoic acid, and the W-O complex can reach saturation with three molecules in a distorted octahedral structure at the center. In contrast, Ge is tended to be bonded with one O in each molecule, and the Ge-O complex can be saturated with four molecules in a tetrahedral structure.The deposit evolution includes two stages: (1) Formation. The peat/lignite possesses abundant hydroxyls capable of capturing Ge and W from hydrothermal solutions and meteoric waters. Germanium and W can be fixed in the coal structure by O-bridged complexes until coal rank reaching lignite/subbituminous coal. (2) Redistribution. Along with coal rank advance to bituminous coal/anthracite, oxygen as heteroatom is expelled out from coal structure, and the bonding sites for Ge and W disappear as O-containing groups (especially hydroxyls) vanish. Thus, Ge and W can be liberated from coal.