The effects of Fe²⁺ on sulfur-oxidizing bacteria (SOB) driven autotrophic denitrification

With the short-term exposure to Fe²⁺, the mechanism of autotrophic denitrification and sulfide oxidation and the correlation between microbial community changes and environmental factors have been explored in the ADSOB process. RSM was used to optimize conditions for the maximum nitrate reduction and sulfide oxidation. About 88% of nitrate could be autotrophically denitrified to nitrogen by utilizing sulfide as the electron donor with the molar ratio C/N of 1.14 and S/N of 0.99 at pH 7.1. Lower Fe²⁺ additions can reduce TDS inhibition with dissolved sulfide to form FeS precipitates, while high amount of Fe²⁺ limited the mass transfer of NO₃^- and intermediate products such as S⁰ may be generated. High-throughput sequencing and RDA analysis revealed the correlation between ferrous iron, environmental factors and microorganisms. Sulfurospirillum, Rhodanobacter, Thauera and Thiobacillus were all slightly promoted at NFL level and inhibited at NFH level. And the narrow angles of the arrows indicated that Thauera, Sulfurospirillum and Thiobacillus were positively correlated with SO₄^2- concentrations, while large angles indicated these bacteria were inversely related with TDS and NO₃- arrows, which further confirmed that these bacteria played a dominant role in the ADSOB process, and can reduce NO₃- by the oxidation of TDS. The correlation further indicated that lower Fe²⁺ additions have a promoting effect, while high concentrations have an inhibiting effect.