Kinetics of continuous ferrous ion oxidation by Acidithiobacillus ferrooxidans immobilized in poly(vinyl alcohol) cryogel carriers

Bio-oxidation of ferrous ion by immobilized Acidithiobacillus ferrooxidans is potentially useful in the new technology of two-step bioleaching. The kinetics of continuous oxidation of ferrous ion by PVA-cryogel-immobilized A. ferrooxidans was investigated in a 365-ml packed-bed bioreactor. The effects of ferrous ion concentration and operating dilution rate of the bioreactor on the kinetics of ferrous ion oxidation were examined. Media containing 1–11.5 g/L ferrous ion were tested. For each medium, the kinetics of reaction at different dilution rate, at a constant temperature of 31 °C, were determined. For a medium containing 3.8 g/L or less ferrous ion, the oxidation rate was proportional to the dilution rate of the bioreactor, and the highest oxidation rate of 2.3 g/L h was achieved at the dilution rate of 0.76 1/h with a medium containing 3.8 g/L ferrous iron. In contrast, for a medium containing more ferrous ion, the kinetic performance of the bioreactor showed a plateau region that was independent of volumetric loading or even exhibited a decrease at high dilution rate. Under these circumstances, the increase of concentration of ferrous ion led to a lower dilution rate, at which the maximum oxidation rate was achieved, and a lower maximum oxidation rate. The results indicated an inhibitory effect of high ferrous ion concentration on the activity of immobilized A. ferrooxidans.