Optimization of biological sulfuric acid generation by Acidithiobacillus thiooxidans in a stirred-tank reactor system

Conventional sulfuric acid production is tied heavily to the fossil fuel industry, where it is typically obtained from the desulfurization of fuels. Here, the biological production of sulfuric acid by Acidithiobacillus thiooxidans in a reactor setup was optimized using design of experiments methodology. The effects of initial sulfur dosage, phosphate (PO₄^3-) concentration, and inoculum size were evaluated with respect to sulfate (SO₄^2-) concentration, optical density, and pH. A significant effect was observed for sulfur dosage; the highest sulfur supplementation, 35 g L^-1, yielded the highest SO₄^2- concentration and lowest pH after 14 days of cultivation. A mean sulfur-to-SO₄^2- conversion rate of 81 ± 9% was determined across all trials conducted with 35 g L^- 1 of sulfur. Among all trials, a maximum SO₄^2- molarity of 1.08 M was achieved, with a corresponding pH of 0.18. No significant effects were determined for PO₄^3- and inoculum volume. The feasibility of using an industrial by-product as a sulfur source was demonstrated, in addition to establishing a strong correlation between the measured SO₄^2- concentration and conductivity of the biological lixiviant. KEY POINTS: • Sulfur dosage correlated positively with SO₄^2- concentration. • DoE revealed no significant effect for PO₄^3- concentration and inoculum size. • Strong correlation between SO₄^2- concentration and conductivity was established.