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 o xidizing bacteria cultures isolated from Ecuadorian mines were used. The ca pability of these bacteria to resist high contents of thiourea in solution was studied. The cultures were inoculated in progressively higher concentra tions of thiourea, starting from 10 ppm, up to 400 ppm in the TK nutrient m edia with 7 g/l Fe2+, at pH 1.8, 30 degrees C, and 125 rpm in a reciprocati ng 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 concentra tion of thiourea in the solution created more difficulties for the bacteria l development and required more adaptation steps. One of the cultures was also adapted to pH < 1 in order to decrease jarosit e precipitation. The test was carried out to 300 ppm of thiourea. The bacte ria survived at these extreme conditions, but needed a longer time to grow and to oxidize the ferrous ion.