Removal of hydrogen sulfide by sulfate-resistant Acidithiobacillus thiooxidans AZ11

Toxic H₂S gas is an important industrial pollutant that is applied to biofiltration. Here, we examined the effects of factors such as inlet concentration and space velocity on the removal efficiency of a bacterial strain capable of tolerating high sulfate concentrations and low pH conditions. We examined three strains of Acidithiobacillus thiooxidans known to have sulfur-oxidizing activity, and identified strain AZ11 as having the highest tolerance for sulfate. A. thiooxidans AZ11 could grow at pH 0.2 in the presence of 74 g l^-1 sulfate, the final oxidation product of elemental sulfur, in the culture broth. Under these conditions, the specific sulfur oxidation rate was 2.9 g-S g-DCW (dry cell weight)^-1 d^-1. The maximum specific sulfur oxidation rate of A. thiooxidans AZ11 was 21.2 g-S g-DCW^-1 d^-1, which was observed in the presence of 4.2 g-SO₄^2- l^-1 and pH 1.5, in the culture medium. To test the effects of various factors on biofiltration by this strain, A. thiooxidans AZ11 was inoculated into a porous ceramic biofilter. First, a maximum inlet loading of 670 g-S m^-3 h^-1 was applied with a constant space velocity (SV) of 200 h^-1 (residence time, 18 s) and the inlet concentration of H₂S was experimentally increased from 200 ppmv to 2200 ppmv. Under these conditions, less than 0.1 ppmv H₂S was detected at the biofilter outlet. When the inlet H₂S was maintained at a constant concentration of 200 ppmv and the SV was increased from 200 h^-1 to 400 h^-1 (residence time, 9 s), an H₂S removal of 99.9% was obtained. However, H₂S removal efficiencies decreased to 98% and 94% when the SV was set to 500 h^-1 (residence time, 7.2 s) and 600 h^-1 (residence time, 6 s), respectively. The critical elimination capacity guaranteeing 96% removal of the inlet H₂S was determined to be 160 g-S m^-3 h^-1 at a space velocity of 600 h^-1. Collectively, these findings show for the first time that a sulfur oxidizing bacterium has a high sulfate tolerance and a high sulfur oxidizing activity below pH 1.