Aqueous arsenic and sulfur speciation analysis and solid phase characterization in the dissolution of arsenic trisulfide

Abstract

Arsenic removal from metallurgical waste streams in the form of arsenic trisulfide (As2S3) is promising due to its high arsenic content. However, the major barrier to widely implementing the sulfidization process is the stability of arsenic trisulfide. Our previous study has shown that the release rate of arsenic and sulfur increased with pH, the dissolved oxygen concentration, and temperature. The rate-limiting step was proposed to be surface chemical reaction based on the magnitude of the reaction orders and the activation energy. To further support the proposed mechanism, we analyzed aqueous arsenic and sulfur speciation and examined the changes in the solid surface properties during leaching. The speciation analyses indicate that arsenic was initially released in the form of arsenite, which was subsequently oxidized to arsenate. The extent of oxidation increased with pH, the dissolved oxygen concentration, and temperature. Thiosulfate was the predominant dissolved sulfur species, which was subsequently converted to sulfate via two different pathways. The oxidation pathway was favored at pH higher than 8. The disproportionation pathway was favored at pH 8 and below, where elemental sulfur was generated and precipitated out of solution as a solid. The SEM/EDX and XRD analyses of the solid residues confirmed the formation of elemental sulfur on the solid surfaces at pH 8 but not at pH 9. The formation of elemental sulfur reduced the sulfur concentration in the leachates, resulting in the aqueous As/S ratio being higher than the stoichiometric value in the As2S3 structure. The elemental sulfur produced did not act as a barrier to diffusion of reactants, as evidenced by the leaching being insensitive to the removal of elemental sulfur.