Effects of the oxygen transfer rate on ferrous iron oxidation by Thiobacillus ferrooxidans D. S. Savic ´,* V. B. Veljkovic ´,* M. L. Lazic ´,* M. M. Vrvic ´, ² and J. I. Vuc ˇetic ´ ² *Faculty of Technology, University of Nis ˆ, Leskovac, Yugoslavia, ² Faculty of Chemistry, University of Belgrade, Belgrade, Yugoslavia Ferrous iron oxidation by Thiobacillus ferrooxidans was studied in shake flasks and a bubble column under different aeration conditions. The maximum biooxidation rate constant was affected by oxygen transfer only at low aeration intensities. At oxygen transfer rates higher than 0.03 mmol O 2 l -1 min -1 , the maximum biooxidation rate constant was about 0.050 h -1 in both shake flasks of different size and the bubble column. The oxygen transfer rate could be used as a basis for scaling up bioreactors for ferrous iron biooxidation by T. ferrooxidans. © 1998 Elsevier Science Inc. Keywords: Bioleaching; biohydrometallurgy; Thiobacillus ferrooxidans; ferrous iron biooxidation; oxygen transfer rate Introduction The bacterium Thiobacillus ferrooxidans, an acidophilic chemolithoautotroph, has been used for the leaching of metals and fossil-fuel desulfurization. 1 In these biopro- cesses, two biooxidation reactions are mediated by T. ferrooxidans, namely, the oxidation of sulfur or reduced sulfur compounds to sulfate and the oxidation of ferrous to ferric ions in low pH sulfate solutions under aerobic conditions. 2 The bacterium gains energy for growth from these oxidation reactions. The bacterial growth and the ferrous iron oxidation depend on the physiological properties of the organism, pH, temperature, concentrations of dissolved oxygen, and fer- rous and ferric ions as well as the presence of other essential nutrients. Under aerobic conditions, the biooxidation rate is directly proportional to the concentrations of biomass, ferrous ion, hydrogen ion, and oxygen. 3 The effect of oxygen was proposed on the basis of the generally accepted knowledge of the importance of this parameter on the biological oxidation of ferrous ions. 3 Sufficient aeration is required to ensure an adequate supply of oxygen (and carbon dioxide) for rapid bacterial growth and biooxida- tion. 4 The importance of oxygen for ferrous iron oxidation is well recognized in most studies, but only in a few are aeration conditions defined and connected to the rate of the bioprocess. Some authors 2,5,6 studied iron biooxidation in oxygen-saturated conditions or under forced aeration in order to avoid oxygen limitation, and another 7 performed parallel experiments in shake flasks without and with aeration to prove that the growth conditions were not oxygen limited. An exception is the work of Guay et al. 8 where the effect of the volumetric oxygen mass transfer coefficient on the maximum iron oxidation rate at an initial pH of 2.3 and 32°C in an aerated, agitated fermentor was observed. A fivefold increase in the volumetric oxygen mass transfer coefficient (from 2.4  10 -3 to 1.3  10 -2 s -1 ) causes a moderate increase in the maximum iron oxidation rate (from 9.65 to 12.6 mmol Fe l -1 h -1 ). It was also shown that the optimum aeration for the growth of T. ferrooxidans on the modified 9K medium at pH 2.7 and 30°C in a batch culture was higher than 5  10 -7 mol O 2 ml -1 min -1 atm -1 , corresponding to an oxygen transfer rate of about 0.11 mmol O 2 l -1 min -1 . Ferrous iron oxidation by T. ferrooxidans has been performed by means of different bioreactors from the most common such as shake flasks, bubble columns, and aerated, agitated tanks to more specific ones such as air-lift, fluid- ized-bed, and packed-bed bioreactors. The differences among them are related to different flow conditions due to the specific methods of agitation and aeration. For instance in a comprehensive study with six different contacting devices, better results for ferrous iron oxidation were achieved in bioreactors with more efficient aeration, Address reprint requests to Dr. V. B. Veljkovic ´, University of Nis, Faculty of Technology, Bul. oslobodenja 124, 16000 Leskovac, Yugoslavia. Received 12 August 1997; revised 20 April 1998; accepted 20 April 1998. Enzyme and Microbial Technology 23:427– 431, 1998 © 1998 Elsevier Science Inc. All rights reserved. 0141-0229/98/$19.00 655 Avenue of the Americas, New York, NY 10010 PII S0141-0229(98)00071-4