Characterization of secondary arsenic-bearing precipitates formed in the bioleaching of enargite by Acidithiobacillus ferrooxidans

The purpose of this study was to characterize secondary minerals that were formed in the bioleaching of enargite (Cu3AsS4) by Acidithiobacillus ferrooxidans. Two parallel cultures were used: one was adapted to arsenic in the growth medium and the other was wild-type. The progress of the solubilization of As in A. ferrooxidans cultures was stepwise and different from that observed in the non-adapted culture. In contrast, the bioleaching of Cu and Fe from enargite was not affected by prior adaptation of the culture. Minor presence of jarosite was observed by X-ray diffraction (XRD) in solid residues after the bioleaching, and no other peaks of secondary crystalline minerals were detected. The relative intensities of As 3d and Fe 2p to Cu 2p in X-ray photoelectron spectroscopy (XPS) for the solid residues were at maximum at 46 days after the bioleaching with As-adapted A. ferrooxidans. The results from the examination of solid residues with XPS, transmission electron microscopy with energy-dispersive microprobe (TEM-EDS) and XRD after 46 days of contact with As-adapted A. ferrooxidans showed the presence of metastable, amorphous ferric arsenate as an intermediate on the surface of enargite and minor amounts of jarosite. The amorphous ferric arsenate phase did not appear to have an adverse effect of the dissolution of Cu from enargite.

Research Highlights
► The leaching of As from enargite in arsenic-adapted A. ferrooxidans demonstrated a stepwise pattern, which was different from that in the non-adapted culture.
► The initial arsenic dissolution may be associated with a specific function of the arsenic-adapted A. ferrooxidans.
► In light of the XPS, TEM-EDS, Raman, FTIR and XRD results and the leaching data, amorphous ferric arsenate was formed during the bioleaching of enargite in the arsenic-adapted culture.
► The formation of amorphous ferric arsenate did not seem to interfere with the leaching of copper from enargite.
► If the amorphous ferric arsenate is converted to crystalline scorodite at higher tempertures, arsenic in the solids would be more stable, and the bioleaching of enargite could be environmentally more manageable.