The oxidation of ferrous iron in solution using Acidithiobacillus ferrooxidans has industrial applications in the regeneration of ferric iron as an oxidant agent for the removal of hydrogen sulfide from waste gases, desulphurization of coal, leaching of non-ferrous metallic sulfides and treatment of acid mine drainage. The aim of this attempt was to increase the biooxidation rate of ferrous sulfate by using immobilized cells. Rate of ferrous iron oxidation was determined in a packed-bed reactor configuration with low density polyethylene (LDPE) particles as support material in order to find the most practical system for scale-up. The present work studies the influence of basic parameters on the ferrous iron biooxidation process using an indigenous iron-oxidizing microorganism, namely A. ferrooxidans, in a 2 L packed-bed bioreactor. Effects of several process variables such as initial pH, temperature, dilution rate, initial concentrations of ferrous and ferric ions on oxidation of ferrous sulfate were investigated. Experimental results indicate that in the temperature range of 31-34 degrees C the biooxidation of ferrous ions to ferric ions could be resulted efficiently. A pH range of 2-2.2 was optimum for the growth of the culture and effective bacterial activity for oxidation of ferrous ions to ferric ions. The highest oxidation rate of 2.9 g Fe2+ L-1 h(-1) was obtained using a culture initially containing 25 g L-1 Fe+2 at the dilution rate of 0.4 h(-1). This rate is very high compared to that achieved in other bioreactors found in the literature. In addition the biooxidation of Fe2+ to Fe3+ conversion could be achieved effectively in the presence of the Fe3+ in the concentration range of 0.1-0.7 g/L. (c) 2006 Elsevier Ltd. All rights reserved.