Mechanism Study Of Water-mineral Surface And Interface Reaction Of Arsenopyrite In Sulphuric Acid Solutions

In an effort to understand the arsenopyrite (Apy) weathering and arsenic (As) releasing process from Apy to aquifers in the natural environment, in this study, much work has been done to identify the composition changes and chemical reactions present at the mineral surface during Apy oxidation. Furthermore, the environmental influence of this process and its products could be assessed accordingly. Based on fully reviewing of the relevant literature and summarizing the published results, we have designed a series of the Apy water-mineral surface and interface reaction experiments. The results obtained have been deeply analyzed, and the reaction mechanism has been discussed.The fifteen arsenopyrite samples used in this study were cut into cubes (length side about2.5mm) and reacted with sulphuric acid under100-300℃in the reaction pots. The pH levels of the acid solutions are0,1, and3respectively.Inductively coupled plasma atomic-emission spectroscopy (ICP-AES) was taken to analyze the remaining solutions after reaction. The products at the residual solid surface were tested by the X-ray photoelectron spectroscopy (XPS), electron probe micro-analysis (EPMA) and X-ray powder diffraction test (XRD). Scanning electron microscopy (SEM) was used to accomplish the morphological analysis.Results from ICP-AES demonstrate that a large number of As move into solution (the valence untested), and the level of total As ion in the solution significantly increased with the rise of temperature and acid concentration. The SEM morphological observation shows that the dissolution of arsenopyrite starts from the edges of the cube and the weak areas such as fractures. Then, the reaction interface gradually migrates into the inner part of the mineral cube. XPS narrow scan of the residual solid surface indicates that Fe (Ⅲ), S8, SO32-, SO42-, As (Ⅲ), As (Ⅴ) and a small amount of As (Ⅰ) are generated at the surface, among which As (Ⅲ) dominates. XPS profile analysis of As2p3/2manifeststhat with the increase of the erosion depth, the concentration of oxidized As grows. It means that the reaction prompts arsenopyrite to oxidize and generates the oxidation state of As. XRD results have confirmed that three new-born mineral phases are present at the solid surface, which are kornelite, ferricopiapite and a small amount of parasymplesite relatively.The products of the surface/interface reaction between arsenopyrite and acidic solution products tend to be transferred into the solution. Consequently, it is hard to form a perfect product layer which may help hinder the reaction to proceed. Due to the oxidized As could be migrated to the solution, the oxidation of arsenopyrite would result in As being released into the superficial water from the As-containing minerals, and participating the groundwater circulation. Thus, it will pose a huge potential threat to the ecological environment.