Role of Ag+ in the bioleaching of arsenopyrite by acidithiobacillus ferrooxidans

Abstract

Arsenopyrite (FeAsS) is often associated with gold, but pre-treatment is necessary prior to gold leaching, mainly due to the gold encapsulation in the matrix of FeAsS. Bio-oxidation is attractive and promising, largely due to its simplicity, low cost and environmental friendliness. A critical problem that still impedes the large-scale applications of this green technology is its slow leaching kinetics. Some metal ions such as Ag+ have previously been found to expedite the bioleaching process. In this paper, the role of Ag+ in the arsenopyrite bioleaching by Acidithiobacillus ferrooxidans was investigated in detail by bioleaching experiments and a series of analyses including thermodynamics, X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Experimental results suggested that addition of 5 mg/L Ag+ to the leaching system could significantly improve the final As leaching efficiency from 30.4% to 47.8% and shorten the bioleaching period from 19 days to 15 days. Thermodynamic analysis indicates that Ag+ destabilises As2 S2, As2 S3 and S0 via forming Ag2 S, which is confirmed by the XRD analysis on the phase transformation during bioleaching. SEM and XPS analyses further showed that Ag+ removed the passivating film consisting mainly of As2 S2, As2 S3 and S0 because Ag2 S formed on the arsenopyrite surface from the start bioleaching of 36 h. In the presence of Fe3+, Ag2 S could easily be dissolved to Ag+ again, likely leading to the establishment of the Ag+ /Ag2 S cycle. The bacteria utilised the two synergistic cycles of Fe3+ /Fe2+ and Ag+ /Ag2 S to catalyse the bioleaching of arsenopyrite.