Effect of ionic strength on iron oxidation with batch cultures of chemolithotrophic bacteria

Predicting the growth and activity of chemolithotrophic bacteria in process operations as a function of process water quality is beyond the capability of current models. The models in usage at present, based on substrate limitation of enzyme kinetics, are typically expressions of the Monod form. We argue that the ionic nature of bio-oxidation process solutions is a pre-determined variable that renders these models invalid to describe the solution matrix impact on cell growth. To test this proposal, a series of experiments were conducted to determine whether ionic strength of the growth media is a significant variable. Subsequently, a simple model was developed with parameters for the cell specific growth rate Kd (h−1) and the specific maximum rate of iron oxidation rmax (mol s−1 cell−1). This model was applied successfully to the exponential phase of culture growth. The derived values for these parameters in media of minimal ionic strength were 0.16 h−1 and 5×10−17–2×10−18 mol s−1 cell−1, respectively. By increasing the ionic strength whilst maintaining a constant total iron concentration both Kd and rmax were observed to be progressively reduced, particularly the Kd. Reducing the iron concentration while holding the calculated value for the ionic strength constant produced insignificant changes in the values of Kd and rmax. After several growth cycles at elevated ionic strength, the values of Kd and rmax were 0.14 h−1 and 5×10−17–2×10−18 mol s−1 cell−1, respectively, demonstrating an adaptation process. We conclude that ionic strength is a significant variable in the growth of chemolithotrophic cultures.