Bio-oxidation Of Pyrite, Chalcopyrite, Pyrrhotite And Galena By Acidithiobacillus Ferrooxidans

The interaction between mineral and microorganism, especially the bio-oxidization of sulfide by Acidithiobacillus ferrooxidans has been one of the focuses in geochemical studies for decades. However, there are still some debates or questions about mineral oxidation mechanism remained for confirmation. The study of interaction between sulfide and Acidithiobacillus ferrooxidans is very important to explain the role of microorganism in mineral oxidization. In addition, this bacterium has great prospective meanings on environmental protection, hydrometallurgy and bioprocessing.A strain of Acidithiobacillus ferrooxidans from acid mine drainage in Yunfu pyrite mine, Guangdong province, China is isolated in this study. This strain was used to study the bio-oxidation mechanism of pyrite, chalcopyrite, pyrrhotite and galena in laboratory. A series of experiments were designed and conducted to compare the differences between Fe³⁺ oxidation and bio-oxidation of the four kinds of sulfide minerals. The changes of chemical parameters in time series such as pH, redox potential, and the concentration of suspending bacterium, cation and sulfate irons in solution during the experiment were monitored. Meanwhile, micro morphologic and chemical changes on the surface of the four kinds of sulfide minerals were also investigated through fluorescence microscope, transmission electron microscope (TEM), scanning electron microscope (SEM) and energy spectrum. The "solution-bacterium-mineral" was studied as one uniform system, and the role of bacterium in bio-oxidation of minerals was analyzed systematically.It is observed that the oxidation rates of pyrite, chalcopyrite and pyrrhotite by Fe³⁺ are slower than their bio-oxidization, while the galena shows opposite trends to the former three kinds of sulfides. This indicates that the concentration of total iron ion in the solution has great influence on the bio-oxidization rate of sulfide. In the presence of Acidithiobacillus ferrooxidans, the bio-oxidization rate of the four kinds of sulfide minerals decreases in the order as: pyrrhotite, chalcopyrite, pyrite and galena. 88.64% of the intermediate product (elemental sulfur) of chalcopyrite bio-oxidization has been oxidized to sulfate. The adherence of cell to the mineral surface exists in the process of the bio-oxidation of the four kinds of sulfides. The SEM imaging shows that a large number of erosion pits with certain orientations appeared on the face of pyrite and galena crystal in the bio-oxidation environment. These erosion pits or micro-perforations which arranged regularly are similar to the bacterium in shape and length, which indicates that the adherence of cell to mineral surfaces may result in the formation of erosion pits. The SEM and TEM images indicate the biofilm exists on the surface of chalcopyrite and pyrrhotite. The bacterial biofilm as a structured community of Acidithiobacillus ferrooxidans cells enclosed in the extracellular polymeric substances (EPS) and adherent to the mineral surface. Along with the process of bio-oxidization of chalcopyrite and pyrrhotite, the biofilm has been covered with the jarosite deposition, so the mineral surface looks smoothly. It is a dynamic procedure that the biofilm comes to being and grows, and the biofilm doesn't restrain the bio-oxidization of chalcopyrite and pyrrhotite. At last, from the perspective of energy utilization by Acidithiobacillus ferrooxidans, this paper provides the evidence that Acidithiobacillus ferrooxidans can directly oxidize galena as the source of energy.