Impact of bioavailable Pb2+ on Fe2+ oxidation in the presence of a mixed culture of Acidithiobacillus ferrooxidans

Numerous investigations were conducted on the effects of a variety of metals, including As, Cu, Zn, Cr on the growth of Acidithiobacillus ferrooxidans (an iron oxidizer and indigenous to acidic environment) and Fe2+ oxidation. However, less work was reported concerning the Pb2+ effect due to its quick precipitation as anglesite in SO42--rich solutions. The reported inhibiting concentrations of Pb2+ varied greatly on the oxidizing rate of ferrous in the presence of A. ferrooxidans, and the reasons remain unclear. Comparative studies were conducted between chemical and microbial oxidation of ferrous by a mixed culture of A. ferrooxidans in the presence of different concentration of Pb2+. Eh, pH and Fe2+ concentration were monitored periodically and the final precipitates were analyzed by X-ray diffraction (XRD), scanning electronic microscopy (SEM), and SEM-EDAX (Energy-dispersive X-ray spectroscopy). To check the impact of bioavailable Pb2+ on Fe2+ oxidation, initial precipitation was removed before the microbial inoculation. Our data showed that Pb2+ will exert a remarkable inhibition on microbial oxidation of ferrous when initial Pb2+ concentration reached as high as 5 g/L. However, the bioavailable Pb2+ in this case should be much lower than 5 g/L in the solution due to the precipitation of anglesite (The absolute concentration was under analysis). The threshold of Pb2+ concentrations to inhibit the microbial oxidation varies among the previous studies. This might result from the different microbial strains used or the mistaking of initial concentration as the substantial concentration of bioavailable Pb2+ after precipitation as anglesite. In contrast, Pb2+ does not show any obvious influence on chemical oxidation of ferrous. XRD spectrum of the final precipitates showed that anglesite was the only solid phase detected in chemical systems, while pure jarosite was found in the microbial systems. No lead was detected in jarosite by SEM-EDAX, inferring that Pb was neither adsorbed by jarosite nor incorporated in the crystal structure of jarosite. These results shed light on the understanding of the behavior of bioavailable Pb2+ in SO42--rich environment in the presence of microorganisms and its impact on the geochemical cycles of iron and sulfur. This work is supported by National Basic Research Program of China (973 program, 2007CB815601) and the National Natural Science Foundation of China (grant No. 40672081). Key words: A. ferrooxidans, bioavailability, Pb, jarosite, acid mine drainage (AMD), anglesite