Organic carbon coupling with sulfur reducer boosts sulfur based denitrification by Thiobacillus denitrificans

Sulfur autotrophic denitrification utilizes elemental sulfur as the electron donor for nitrate removal from aquatic environments. Organic carbon could stimulate the conversion of sulfur and facilitates the S0-based denitrification process in the mix-trophic. In this study, the co-cultured system of sulfur reducer (Geobacter sulfurreducens) and Thiobacillus denitrificans was used to investigate that how organic carbon could boost the S0-based denitrification. The results showed that the rate of S0-based denitrification was improved with C/N ratio of 0.13 and this improvement continued even after the acetate was exhausted. Sulfur probe test and Raman analysis suggested that reduced sulfur species (Sx2−) were formed with the addition of organic carbon. The Sx2− could recombine with element sulfur and the bioavailability of S0 would be improved, as a result, the rate of S0-based denitrification increased as well. Nitrate reduction rate could further increase with the C/N ratio of 0.88, but it would decrease significantly when the C/N ratio increased to 1.50 as the high concentration of generated S2−. Our results provided explanations that why organic carbon addition would improve the bioavailability of S0 which could further promote the S0-dominant denitrification process.