Fe, As and S Speciation Transformation During Arsenopyrite Bioleaching Based on Spectral Analysis Technology

In the present study, the bioleaching of arsenopyrite by the moderately thermoacidophilic strain Sul fobacillus thermosulfidooxidans YN-22 was investigated based on iron, arsenic, and sulfur speciation analysis by synchrotron radiation As/S K-, and Fe L- edge X-ray absorption near edge structure (XANES) spectroscopy, X-ray diffraction (SR-XRD), accompanied by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES) with the determination of the leaching parameters. The results showed that the presence of bacteria significantly promoted the dissolution of arsenopyrite, and dissolved As and Fe in the bioleaching solutions mainly existed as As(V) and Fe3+, while were mainly As (III) and Fe2+ in the chemical leaching. Extracellular polymeric substances (EPS) plays a critical role in the interactions between bacteria and minerals during bioleaching, and the FTIR analysis of the EPS showed that the contents of protein and polysaccharides of the adhered cells were higher than those of the free cells. Results of SEM showed that the surface of the arsenopyrite was gradually corroded, and some secondary products were formed during bioleaching, while for the sterile control experiment, the mineral surface was only slightly corroded, and only a few products were found after 10 days of chemical leaching. The results of SR-XRD showed that elemental sulfur (S-0), jarosite and ferric arsenate were detected after day 4, and gradually developed into the main components of the residues in the bioleaching experiment. Fe Ledge XANES analysis showed that during the bioleaching of arsenopyrite by Sulfobacillus thermosulfidooxidans, the Fe(II) species was gradually converted to Fe(III) species with time. As K-edge XANES analysis showed that valences of arsenic in the bioleached residues included As(-I) As(II) and As(V), the fitted results of the As K-edge XANES spectra showed that the residue composition of arsenic species for the same leached time(10 days) consisted of 18. 6% arsenopyrite, 23. 5% orpiment and 57. 9% ferric arsenate in the bioleaching assay, and of 93. 8% arsenopyrite and 6. 2% ferric arsenate in the chemical leaching. The fitted results of the S K-edge XANES spectra showed that after 10 days bioleaching, the residue composition of sulfur species consisted of 15. 3% arsenopyrite, 23. 7% , 3. 5% thiosulfate, 11. 3% schwertmannite and 46. 2% jarosite, while only a small amount of S-0 (7. 8%) was found as the sulfur intermediate at day 10 in the sterile control experiment. Based on these results, it could be concluded that the chemical speciation transformation of iron, arsenic and sulfur were performed in the path- ways: Fe(II)-Fe(III) As(-I)-As(II)-As(V), and S -> S-0 -> S2O32 -> SO42, respectively. Based on the observations of leac- hing behavior, it was found that as a massive accumulation of S-0 jarosite, ferric arsenate, and As2S3 occurred the dissolution of arsenopyrite was severely hindered. Moreover, the formation of thiosulfate-like species during bioleaching indicated that arsenopyrite was dissolved in the similar way to that of pyrite.