Assessment of effective factors on bacterial oxidation of ferrous iron by focusing on sweetening natural gas

In this study, the effects of some factors on bacterial growth and ferrous oxidation rates were investigated by Acidithiobacillus ferrooxidan in 250 ml shake flasks. One factor at a time (OFAT) design approach was used for preliminary evaluation of various factors affecting the process, such as pH, initial ferrous and elemental sulfur concentrations, shaker agitation rate, and liquid to flask volume ratio. After that, optimal levels of effective last three factors to achieve high oxidation rate and cell growth rate were investigated using a full factorial design. It was obtained that agitation rate and liquid to flask volume, as well as their binary interaction, are significant factors on ferrous iron bio-oxidation rate. In contrast, initial high ferrous iron concentration was the only effective factor on the cell growth rate. Maximum bio-oxidation rate of 0.417 g/L was achieved at the media with Fe2+ ion concentration of 30 g/l, agitation rate of 200 rpm, and liquid to flask volume ratio of 20% by full factorial optimization, which is an about 40% increase compared to the result obtained in OFAT method. Then, the effect of step-wise adaptation of A. ferrooxidans to in high Fe2+ concentration was studied, and about 40% reduction in bacterial lag phase time, and 36 and 86% increase in bacterial growth rate and bio-oxidation rate were acquired, respectively.