Gold is mainly present in the form of [Au(CN)₂]⁻ during the cyanide leaching process, and this [Au(CN)₂]⁻ can be adsorbed by graphite in carbonaceous gold ore resulting in preg-robbing gold. In order to clarify the adsorption mechanism between the [Au(CN)₂]⁻ and graphite, the interaction between the [Au(CN)₂]⁻ and graphite (0001) surface was studied using density functional theory (DFT). The distance between [Au(CN)₂]⁻ and graphite (0001) decreased from (4.298–4.440 Å) to (3.123–3.343 Å) after optimization, and the shape of [Au(CN)₂]⁻ and graphite (0001) obviously changed from straight to curved, which indicated that the [Au(CN)₂]⁻ had been adsorbed on the graphite (0001) surface. A partial densities of state (PDOS) analysis revealed that there was little change in the delocalization and locality of the PDOS on the graphite (0001) surface after adsorption. However, the valence bands of the Au 5d orbital, C 2p orbital, and N 2p orbital near the Fermi level moved slightly towards lower energy levels; therefore, the adsorption configuration was stable. An analysis of the Mulliken charge population indicated that the Au, N, and C in [Au(CN)₂]⁻ obtained 0.26, 0.18, 0.04 electrons after adsorption, respectively, while C(surf) lost 0.03 electrons. [Au(CN)₂]⁻ changed to a conductor from an insulator after adsorption. Taking into account the surface electrical properties of [Au(CN)₂]⁻ and graphite (0001), there was still a slight electrostatic adsorption between them. The analysis of adsorption energy, electronic structure, PDOS, electron density, Mulliken charge population, and Mulliken bond population revealed that [Au(CN)₂]⁻ could be adsorbed to the graphite (0001) surface; the adsorption was a type of physical adsorption (including electrostatic adsorption) and mainly occurred on the two C≡N. These results contributed to the understanding of the mechanisms involved in preg-robbing gold formation by graphite and the optimization of this process during cyanide leaching.

Hydrothermal carbon (HTC) was prepared by the one-step hydrothermal method for Cr (VI) removal from wastewater, which was considered a green chemistry method. The specific surface area (S_BET) of HTC was 85m²/g with the pore size in range of 2.024.0nm. FT-IR spectra analysis showed that the HTC had abundant chemical surface functional groups. The influence of adsorption parameters such as pH, HTC dosage, Cr (VI) concentration, and contact time on the removal efficiency of Cr (VI) had been investigated. When the initial concentration was 50mg/L, pH=6, amount of adsorbent was 0.2g/50ml, and adsorption time was 90min; the Cr (VI) absorbed rate of HTC reached 98. Batch adsorption experiments indicated that Cr (VI) adsorption data of HTC fitted the Freundlich isothermal and pseudo-second-order kinetic models. Overall, our findings provide a promising material in treatment of Cr (VI)-rich wastewater and give a clear picture of its application, which is worthy of further study.

The distortion process of kaolinite annealed from 25 °C to 550 °C for different holding times can be characterized using a thermogravimetric/differential scanning calorimeter (TG/DSC) for thermal analysis, X-ray diffraction (XRD) for establishing the crystal structure, the Fourier transform infrared spectrum (FTIR) for identifying the functional groups, and a scanning electron microscope (SEM) for establishing the microstructure. Dehydroxylation is the main reaction during annealing from 25 °C to 550 °C and leads to kaolinite crystal distortion. A stable crystal structure during distortion was obtained by optimizing the bulk phase with quantum chemistry. Then, the crystal structure was studied by using ab initio multiple scattering calculations for X-ray absorption of the fine structure (XAFS). The results of X-ray absorption near the edge structures (XANES) determined that peak shifts and intensity phases slightly increased. The crystal structure distortion of kaolinite during annealing can be explained by the experimental and simulation results. This work provides theoretical support for identifying kaolinite with different degrees of distortion and has the potential for further developments in coal gangue separation.