In a chemo-biological process, an alternative for the treatment of $H_2S-$containing gases, biological Fe(II) oxidation is usually the rate limiting step. This study was aimed to enhance the Fe(II) oxidation rate using immobilized cells of Thiobacillus ferrooxidans. For this purpose, a medium for the minization of jarosite formation was developed first. Secondly, cell immobilization in celite beads and depth filter was carried out. And then, repeated-batch and continuous operations of Fe(II) oxidation by using the immobilized cells were performed. In a series of flask cultures, three types of media were tested : media with much lower salt concentrations than that of the 9K midium; media which contained different nitrogen sources from that of the 9K medium, that is, $(NH_4)_2HPO_4$, $NH_4Cl$ and $HNO_3$; media which contained $(NH_4)_2HPO_4$ as nitrogen and phosphate source, but without $K_2HPO_4$, the phosphate source in the 9K midium. As a result, the M16 medium which contained 3g/L of $(NH_4)_2HPO_4$ as nitrogen and phosphate source was found to be the optical one. It sustained good cell growth allowing no jarosite formation. Two immobilized bioreactor systems with celite beads in an airlift reactor and a depth filter, respectively, as support material were employed in carrying out repeated-batch and continuous operations. In repeated-batch operation, both system a similar trend to each other. The rate of Fe(II) oxidation gradually increased to reach a maximum value as the batch was repeated. The bioreactor with depth filter displayed a maximum Fe(II) oxidation rate of 2.4 g/L·h whereas the airlift reactor with celite beads showed 2.33 g/L·h. In continuous operations, both system showed a maximum Fe(II) oxidation rate at a dilution rate greater than the maximumspecific growth rate of the bacteria. By using the bioreactor with depth filter, the iron oxidation rate could be increased to 2.6 g.L·h at a dilution rate of 0.3 $h^{-1}$. In the case of the airlift reactor with celite beads, the maximum Fe(II) oxidation rate was 2.14 g/L·h at a dilution rate of 0.25 $h^{-1}$.
