Increasing cell concentration does not affect specific ferrous iron oxidation rate in a continuously stirred tank bioreactor

Microbial oxidation of ferrous to ferric iron allows efficient oxidative processing of sulfide minerals under ambient conditions. This study determined the effect of cell concentration of a mixed mesophilic microbial culture on iron oxidation rate, and evaluated if there was a cell concentration threshold that dictates a maximal volumetric iron oxidation rate. A bioreactor with feedback-loading of ferrous media was operated at 30 °C to maintain a redox potential of +480 mV vs. Ag/AgCl at pH of 1.3. A positive and linear correlation (R² = 0.955) between the cell concentration (6.8 × 10⁷–7.1 × 10⁹ cells mL⁻¹) and volumetric biological iron oxidation (up to 6.9 g L⁻¹ h⁻¹) was observed. The specific iron oxidation was not affected by cell concentration, and no biocatalytic threshold was observed. This indicated that a high cell concentration can be used to achieve a high volumetric iron oxidation rate, enabling the use of a compact reactor size.