Fe~(2+) oxidation by Acidithiobacillus ferrooxidans (At. ferrooxidans) under different solid contents by adding inert Al_2O_3 powder was examined in rotating-drum and stirred-tank reactors. The results show that the bioactivity of At. ferrooxidans in the stirred-tank is higher than that in the rotating-drum in the absence of Al_2O_3 powder, but the biooxidation rate of Fe~(2+) decreases markedly from 0.23 g/(L·h) to 0.025 g/(L·h) with increasing the content of Al_2O_3 powder from 0 to 50% (mass fraction) in the stirred- tank probably due to the deactivation of At. ferrooxidans resulting from the collision and friction of solid particles. The increase in Al_2O_3 content has a little adverse effect on the bioactivity of At. ferrooxidans in the rotating-drum due to different mixing mechanisms of the two reactors. The biooxidation rate of Fe~(2+) in the rotating-drum is higher than that in the stirred-tank at the same content of Al_2O_3 powder, especially at high solid content. The higher bioactivity of At. ferrooxidans can be maintained for allowing high solid content in the rotating-drum reactor, but its application potential still needs to be verified further by the sulfide bioleaching for the property differences of Al_2O_3 powder and sulfide minerals
