Uranium speciation in the Lincang Ge-rich coal deposit, Yunnan Province, China: Theoretical implication for uranyl fixation and U mineralization

The Germanium (Ge)-rich coal deposit in Lincang, Yunnan Province, is an important source for Ge production in China. However, the elevated concentration of U in the Lincang coal limits the green utilization of Ge. This is partially attributed to inadequate knowledge of the modes of occurrence of U, despite its widely recognized organic affinity. In the present study, the density functional theory (DFT) calculation was adopted to explore the uranium species in the Lincang coal, based on existing experimental indications. Results show that uranyl cation (UO22+) in hydrothermal solution can be fixed by hydroxyls, phenols, and carboxyls in coal organics, as well as corresponding functional groups with O substituted by S. The reaction energies of uranyl-O complexes (5.87–7.90 eV) are a bit smaller than those of uranyl-S complexes (6.00–7.95 eV), implying slightly easier fixation of U by O-H containing structures. More specifically, carboxylic acids (5.87–5.96 eV) and thionocarboxylic acids (6.00–6.12 eV) are easier to bind with uranyl cation relative to other O-H and S-H containing structures, respectively. Eighteen model compounds were calculated to simulate the modes of occurrence of U in the Lincang deposit. Results show that U species are present in the forms of various U-O and U-S complexes, and the S-bridged complexes are generally less stable than corresponding O-bridged structures. To achieve best stabilization, uranium is capable of binding with at most six molecules of phenol/ethanol, four molecules of catechol (one H dissociated)/benzoic acid, or three molecules of catechol (both H dissociated)/acetic acid. By comparison, the U-S complexes can achieve a 4-coordinated tetrahedral saturation when U combined with four molecules of methyl mercaptan, ethanethiol, phenyl mercaptan, benzene-1,2-dithiol (one H dissociated), or thiobenzoic acid, as well as a 6-coordinated distorted octahedral saturation for three molecules of benzene-1,2-dithiol (both H dissociated). In addition, there is a slight possibility for U bonded to carbon rings, suggested by weak binding energies (−0.70 eV to −2.20 eV). The disulfide bond (S-S) can react with U to form a stable S-U-S complex, which is probably the cause of retention of enriched U in the Late Permian super-high-organic-sulfur (SHOS) bituminous coals from southern China. This study helps for a better understanding on the modes of occurrence of U in the Lincang Ge-rich coal deposit and benefits the green recovery of critical metals. Further, it perhaps inspires the eco-friendly utilization of other U-rich coals.