The addition of bacteria in a system without pyrite caused a very rapid increase of solution Eh to over 700 mV, which is consistent with the rapid increase of ferric concentration in that case (Fig. 2A). While in bioleaching experiments with pyrite, the Eh values were lower than 500 mV in the first 16 days. However, a rapid increase of Eh was found after that, when the ferrous concentration dropped to a very low level.