A study of the adaptation of native iron oxidizing bacteria to thiourea

To improve gold dissolution from a pyrrhotite - arsenopyrite refractory ore with a combined process of bioleaching and thiourea leaching, three iron oxidizing bacteria cultures isolated from Ecuadorian mines were used. The capability of these bacteria to resist high contents of thiourea in solution was studied. The cultures were inoculated in progressively higher concentrations of thiourea, starting from 10 ppm, up to 400 ppm in the TK nutrient media with 7 g/l Fe2+, at pH 1.8, 30 degrees C, and 125 rpm in a reciprocating shaking bath. Bacterial population and ferrous ion concentration were controlled by Thoma plate microscope count and by titration with KMnO4, respectively. Results show that the bacteria were insensitive to lowest concentration of thiourea with good rates of growing population and Fe2+ oxidation. Higher concentration of thiourea in the solution created more difficulties for the bacterial development and required more adaptation steps. One of the cultures was also adapted to pH < 1 in order to decrease jarosite precipitation. The test was carried out to 300 ppm of thiourea. The bacteria survived at these extreme conditions, but needed a longer time to grow and to oxidize the ferrous ion.