Organic modes of occurrence and evolution mechanism of germanium and lithium in coal: Insights from density functional theory

Abstract

Critical elements in coal, such as Ge and Li, recently have attracted attention due to their economic significance. Modes of occurrence of these critical elements are academically and practically important, because they can not only provide evidence for sources of elements and minerals in coal and regional geological background information, but also help with design of recovery methods. However, conventional analytical methods are unable to observe precisely the organic binding sites of Ge and Li in coal, and this limits the understanding of their enrichment mechanism and the improvements for recovery techniques. In this study, organic modes of occurrence and evolution mechanism of Ge4+ and Li+ were investigated at the atom level by constructing molecular models of Ge- and Li-rich coals combined with density functional theory (DFT) methods. The solid-state 13C nuclear magnetic resonance spectroscopy (NMR), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and helium pycnometry analyses indicate that the molecular formulas of Ge- and Li-rich coals are C166H162N2O32 and C153H174N2O24S, respectively. The DFT analysis reveals that the binding sites for Ge4+ in Ge-rich coal are located near the carboxyl group (-COOH) and the pyrrole ring, while those for Li+ in Li-rich coal are near the carbonyl group (-C=O) and the pyrrole ring. The Ge4+ is immobilized in the Ge-rich coal molecular model through coordination bonds with the O atom in the -COOH and the C atom in the pyrrole ring, while being away from the N atom in the pyrrole ring. Li+ forms a coordination bond with the O atom in the -C=O and additional coordination bond with the nearby hydroxyl group during binding to the pyrrole ring. The impact of coal rank on the organic modes of occurrence of Ge4+ and Li+ was investigated using Wender coals of different ranks, which were one of the earliest proposed coal models. At the lignite stage, oxygen-containing functional groups and aromatic rings show a strong binding ability to Ge4+ and Li+, facilitating their enrichment in coals. Along with coal rank advance to bituminous coal, the reduction of oxygen-containing functional groups (e.g., -COOH and -C=O) and the relatively low condensation of aromatic rings decrease binding sites for Ge4+ and Li+, and the binding ability also decline, resulting in a decrease in their concentration. In anthracite stage, highly condensed aromatic rings provide binding sites for Ge4+ and Li+. The strong binding ability of aromatic rings to Ge4+ indicates that it is probably enriched in anthracite, whereas Li+ is difficult to enrich owing to its relatively weak binding ability to aromatic rings. The low content of oxygen-containing functional groups in anthracite reduces their effect on the organic modes of occurrence of Ge4+ and Li+. This study elucidates the organic modes of occurrence and evolution mechanism of Ge4+ and Li+ from the perspective of coal structure, providing fundamental insights for future research on the interaction between organic and inorganic matter within coal.