The identification of arsenic species produced in the dissolution of crystalline orpiment under anoxic conditions: XAS evidence

Abstract

The orpiment (As2S3) is an important secondary mineral in the geochemical process of arsenic (As) in the environment. The dissolution of orpiment has a close relationship with the migration and transformation of As. The dissolved species of As2S3 is closely related to sulfide (S−II) in the anoxic and sulfidic environment. This paper focuses on the various As species formed when As2S3 dissolved in the presence and absence of excess S−II under anoxic conditions with simulation tests via X-ray absorption spectroscopy (XAS), liquid chromatography with (hydride generation) atomic fluorescence spectrophotometry, and Raman spectroscopy. The results showed that the As produced when As2S3 dissolved in the excess S−II contained a mixture of arsenite and thioarsenite (ThioAsIII). Based on the linear combination fitting, ThioAsIII is the dominant As species (88.2 %) with arsenite as the leftover component. However, the percentage of ThioAsIII decreased to 43.7 % if As2S3 dissolved in the absence of excess S−II, indicting ThioAsIII favored under sulfidic conditions. The findings may give further insights about the role and formation mechanism of ThioAsIII in the dissolution process of As2S3. Environmental ImplicationThe dissolution of crystallization orpiment has a close relationship with the transport of As in the environment. Qualitatively and quantitatively identification of the dissolved species of As2S3 in the presence and absence of excess S−II may be helpful for a better understanding and predicting the fate of As. The formed trithioarsenite was the dominant dissolved species compared to arsenite in the sulfidic system. It has higher mobility than AsV and AsIII, and has been found in many As-related adsorption/desorption and redox reactions. Therefore, great cautions should be given when choosing technologies to remediate the As contaminated soils and waters.