Potential electron donor for nanoiron supported hydrogenotrophic denitrification: H2 gas, Fe0, ferrous oxides, Fe2+(aq), or Fe2+(ad)?

The mechanism of electron transmission in combined nanoiron-bacteria denitrification cannot be explained by the classic model, in which an Fe0H2-nitrate transferring chain is proposed. In this study, we used characteristic techniques and electrochemical analysis to investigate the necessity of molecular hydrogen for the combined denitrifying system using commercial nanoiron with Alcaligenes eutrophus, and to analyze its potential electron donor. Based on our results, nitrate removal and its by-products (NO2- and NH4+) generation was not significantly affected by residual H2 gas, indicating that H2 was not necessary for hydrogenotrophic denitrification. As to the potential electron donor analysis, nanoscale zero-valent iron did not appear to be the electron donor due to its high level of toxicity (83% mortality using nanoiron versus 36% in the control cells). In addition, when iron oxides (Fe3O4, Fe2O3 and FeOOH on the nanoiron surface) and free ferrous ions [Fe2+(aq)] were present alone, they were not utilized by the bacteria to degrade nitrate. According to the results of electrochemical analysis, adsorbed ferrous iron [Fe2+(ad)] on ferric oxides might be the electron donor in this kind of nitrate removal.