Modes of occurrence of organically-associated arsenic in Ge-rich coal deposits

A significant proportion of germanium (Ge), a critical metal for manufacturing chips and other high-tech equipment, is globally extracted from Ge-rich coal deposits. The hazardous trace element arsenic (As) is also abnormally enriched in the world-class Wulantuga and Lincang coal-hosted Ge deposits. However, the modes of occurrence of As in the Ge-rich coal remain unclear, although it is mainly incorporated in crystalline structures in other Ge deposits. In this study, the coordination and local structure of organically-associated As in coals are systematically investigated by applying the density functional theory (DFT) calculations. This approach provides an insightful comprehension of the modes of occurrence of As and benefits the cleaner utilization of the germanium coal deposits. Based on experimental indications, model compounds of hydroxyl- (phenol, catechol, ethanol, acetic acid, and benzoic acid) and sulfydryl- (methyl mercaptan, ethanethiol, phenyl mercaptan, benzene-1,2-dithiol, and thiobenzoic acid) containing groups are investigated. The data indicate that As can exist in coal organics in various forms of As-O (hydroxyls) and As-S (thiols) complexes. To achieve best stabilization, arsenic can bind with five molecules of phenol (binding energy −7.19 eV), four molecules of ethanol/benzoic acid (−8.45 eV/−9.70 eV), or three molecules of catechol/acetic acid (−5.79 eV/−8.91 eV). Similarly, arsenic can bind with three sulfydryl-containing molecules to form most stable structures (−6.18 eV to −7.44 eV). Additionally, organic sulfur structures in coal that lack S–H bond are less likely to be the bonding sites for As due to significantly lower binding energies (−0.90 eV to −1.77 eV when As is bonded to one molecule of model compound). Hence, destroying O/S-bridged complexes is a promising method to remove organically-associated As from the coals in the Ge ore deposits, as well as other coals enriched in organic As.