X-Ray photoelectron spectroscopic analysis of surface products on pyrite formed by bacterial leaching

Bacterial leaching is often applied to recover useful metallic elements from low grade ores; an understanding of the leaching mechanism is necessary to improve the efficiency of this process. In the present work, surface analysis by XPS was carried out on pyrite (FeS2) after leaching with Thiobacillus ferrooxidans for 1 to 11 d. The measured complex spectra were analyzed by the concept of differential charging effects; it was found that Fe(II, III), S, SO42-, and probably SO32-, are formed on the pyrite. The accumulation of K ions was also found with more than 6 d of leaching. Although the observed elemental S is considered to be an intermediate product in the indirect leaching mechanism, the formation on pyrite particles during the bacterial leaching has not been directly observed. The assignment of the SO32- peak is still conditional but, as a metabolic intermediate, the formation of SO32- ions in the cell has been proposed by others. The time variations in the mole ratios of species of S or K vs. Fe in the surface products are helpful to understand the leaching mechanism. The accumulation of Fe(III), SO42-, K+ and OH- ions on the surface with leaching suggests the formation of insoluble compounds, such as jarosite (chemical formula: K[Fe3(SO4)2(OH)6]).