Kinetics and mechanisms of arsenic and sulfur release from crystalline orpiment

Abstract

Arsenic trisulfide (As2S3), in both amorphous and crystalline form, is a significant arsenic-containing compound naturally occurring in mine waste rock or generated as metallurgical waste. It is typically unstable in the enviroment, especially under alkaline and oxidizing conditions, which is a significant barrier to fixation of arsenic present in metallurgical waste streams in the form of arsenic trisulfide. Our previous study has shown the effect of different factors on the kinetics and mechanisms of arsenic and sulfur release from amorphous As2S3. To assess the effect of crystallinity on the stability of arsenic trisulfide, a series of laboratory leaching tests were performed with crystalline orpiment under fully controlled conditions. The experimental results showed that the release of arsenic and sulfur increased with pH, dissolved oxygen concentration, and temperature, but the release rates were much slower than those of amorphous arsenic trisulfude. The speciation analyses indicated that arsenic was present in the leachates as arsenite, which was subsequently oxidized to arsenate to different extents; thiosulfate was the main soluble sulfur species, which was converted to sulfate via oxidation and disproportionation pathways. The kinetic modeling suggested that the arsenic release process is a mixed-control reaction, in which the surface chemical reaction and the diffusion of dissolved oxygen through a product layer control the arsenic release rate. The solid surface characterization supported the presence of an arsenic-deficient phase enriched in elemental sulfur on the solid surfaces. The product layer acted as a barrier to the diffusion of reagents, as evidenced by the leaching being sensitive to the removal of elemental sulfur.