Numerical modeling of ferrous-ion oxidation rate in Acidithiobacillus ferrooxidans ATCC 23270: optimization of culture conditions through statistically designed experiments

Statistically designed experimental strategy has been performed in order to evaluate and optimize nutritional and environmental parameters that affect ferrous ion oxidation rate in Acidithiobacillus ferrooxidans ATCC 23270. Plackett-Burman design was carried out to evaluate efficiently the biological significance of 10 culture conditions influencing ferrous-ion oxidation rate of A. ferrooxidans grown for 5 days in shake-flask batch mode on the newly modified 9-K media. Among ten fermentation factors examined, the most significant variables influencing ferrous-ion oxidation rate were statistically elucidated to be pH and calcium nitrate as positive contributors, whereas trace metals solution and potassium chloride were the most significant negative contributors. The optimal levels of the most significant three nutritional factors were further predicted from a polynomial model created from the data obtained from three level factorial design, a Box-Behnken design. Predicted optimal ferrous-ion oxidation rate Q(Fe2+) was recorded to be 0.148 (g Fe2+/l/hr). On verifying the predicted value, an experiment was performed under optimal predicted conditions and showed an actual experimental Q(Fe2+) of 0.152 g/l/hr, which was 2.7% over the predicted value. Our optimized medium formula gave overall five folds increase in ferrous-ion oxidation rates over the previously published data of standard 9-K medium on batch culture of A. ferrooxidans ATCC 23270 with higher mu(max) (hr(-1)) of 0.177 which was achieved within 75 h incubation in shake-flask culture.