Hydrothermal alteration of arsenopyrite by acidic solutions

- In a very acidic or at a lower T, the leaching process becomes the dominant.
- The CDP mechanism dominated the fluid-mineral reaction at a higher T.
- The mechanism of mineral-fluid interfacial reaction differed at the pH and temperature.

In an effort to understand what occurred at the mineral-solution interface during the arsenopyrite (Apy) oxidation process, one arsenopyrite block was cut into 15 cubes (side length of approximately 2.5 mm), which were reacted with sulphuric acid at 100-300 °C in a reaction autoclave sealed by a metal screw-like vessel. The pH levels of the acid solutions are 0, 1, and 3. After the reaction, various techniques were used to investigate both the chemical, morphological and mineralogical changes. Among these techniques, inductively coupled plasma atomic emission spectroscopy (ICP-AES) was used to analyze the remaining solutions after the reaction. Scanning electron microscopy (SEM) and Transmission Electron Microscopy (TEM) were used for the morphological analysis and mineral analysis of the products. The products at the residual solid surface were examined using X-ray photoelectron spectroscopy (XPS), electron probe microanalysis (EPMA), the X-ray powder diffractometry (XRD) and micro-XRD test. The ICP-AES results indicate that a large amount of As moves into solution and that the total level of As ions in the solution significantly increased as the temperature and acid concentration increased. The secondary electron image (SEI) coupled with the backscattered electron image (BEI) of the morphology shows that the dissolution of arsenopyrite starts from the edges of the cube and fracture. Then, the action interface gradually migrates into the inside of the mineral cube. The XPS narrow scan of the residual solid surface indicates that Fe3+, S8, SO32-, SO42-, As3+, As5+ and a small amount of As1+ are generated at the surface, among which As3+ dominates. The XPS profile analysis of As 2p3/2 indicates that with an increase in the erosion depth from inner to the surface, the concentration of oxidized As increases; the reaction prompts arsenopyrite to oxidize and generates the oxidized state of As. And multiple-layer concentric banded structure may be reached according to the disappearance of every oxidized state of As. Due to the transfer of a large amount of oxidized As to the hydrothermal fluid from the As-containing minerals and rocks, the oxidation of arsenopyrite means that As would be released into hydrothermal fluid as well as to the superficial water and groundwater circulation.

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