Biogas-derived sulfur enhances microbial sulfuric acid production.

Sulfuric acid is one of the most widely used industrial chemicals, yet its conventional production depends on purified sulfur, chemical catalysts, and energy-intensive processes. In contrast, sulfur-oxidizing bacteria can convert reduced inorganic sulfur compounds into sulfuric acid under mild conditions. To optimize biological sulfuric acid production, six Acidithiobacillus thiooxidans strains were first studied on chemical sulfur, with strain DSM 14887 showing the highest sulfur-oxidation activity. This strain, along with intermediate- and low-performing strains, were then investigated on biogas-derived sulfur collected from a digester and external desulfurization units. Remarkably, sulfate production from 1% biogas-derived sulfur reached 0.34 M, 2.5-fold increase compared to chemical sulfur. Increasing the biogas-derived sulfur load to 6% further improved performance, yielding 0.80 M sulfate, a seven-fold increase compared to chemical sulfur. Ultimately, the prolonged incubation led to the complete oxidation of the 6% sulfur, yielding 2 M sulfate. The characterization of the biogas-derived sulfur using scanning electron microscope, water contact angle measurements, and ATR-FTIR revealed several features that explain its enhanced reactivity. The material displayed a more heterogeneous particle size distribution and a more hydrophilic surface, as indicated by a lower contact angle (73 ± 2°) compared to chemical sulfur (100 ± 5°). FT-IR spectra also showed functional groups associated with proteins and polysaccharides. Together, these properties make biogas-derived sulfur a particularly effective substrate for microbial sulfuric acid production and a promising alternative to conventional acid-production processes.