Comparative study of external addition of Fe2+ and inoculum on bioleaching of marmatite flotation concentrate using mesophilic and moderate thermophilic bacteria

Bioleaching of marmatite flotation concentrate by Acidithiobacillus ferrooxidans and a moderately thermoacidophilic iron-oxidizing bacterial strain (MLY) was carried out in batch experiments. The effects of external addition of Fe2+ and the inoculum of the bacteria were studied. For A. ferrooxidans, the supplementary addition of Fe2+ as a source of Fe3+ to be generated by the bacteria was necessary in the starting of the bioleaching of marmatite. While for MLY, it was not necessary because the higher temperature improved the dissolution of marmatite in sulphuric acid, and thus acidic leaching happened before ferric ion leaching. Leaching rate was enhanced with the increased inoculum, however, this influence decreased at a higher level of inoculum. The evolution of the redox potential and the total concentration of iron in the bioleaching indicated that Eh was more important than the concentration of Fe3+ during the dissolution of the marmatite for both strains. Different affinities for the Fe2+ and S0 substrates led to different trends of leaching rate for the two bacteria.